simultaneous separation of impurities, concentration and solvent … · chairperson: professor...

Simultaneous separation of impurities, concentration and

solvent exchange of nanolignin particle suspensions using

ultrafiltration

Sofia Faria Capelo

Thesis to obtain the Master of Science Degree in

Chemical Engineering

Supervisors:

Professor Anton Friedl (TU Wien)

Professor Maria Norberta Correia de Pinho (IST)

Examination Committee

Chairperson: Professor João Carlos Bordado

Supervisor: Professor Anton Friedl

Member of the Committee: Luís Miguel Minhalma

June 2019

i

Acknowledgements This master thesis has been performed at the Institute of Chemical, Environmental and

Bioscience Engineering, TU Wien, in Vienna.

I would like to express my gratitude to Professor Maria Norberta de Pinho, who gave me the

opportunity to carry out this work abroad. To professors Anton Friedl and Michael Harasek, as

supervisors, who received me and provided a good working space and a good environment. Also, to Dr

Martin Miltner and Stefan Beisl that supported and conducted me throughout all the work and for all the

dedication and precious advises provided.

I would also like to give a special thanks to Ruben Santos, Péter Adorján, Anja Dakic, Katarina

Knežević, Stefan Beisl and Rita Alves. People who made my time in Vienna wonderful, whom I have to

thank for making me feel at home, and which somehow helped me in the elaboration of this work.

To the ones who stayed in Portugal, specially to my family and friends that enabled me this experience.

iii

Abstract

Lignocellulosic biomass emerged as an alternative to non-renewable resources, since they are

scarce and highly polluting. The biomass is subdivided into several fractions such as lignin,

hemicellulose and cellulose, which defines the concept of biorefinery by the production of several

products.

The goal of the work was to evaluate the performance, the decline in performance and the

potential for regeneration of membrane performance during the ultrafiltration operation of nanolignin

suspension in diafiltration mode. The suspension used was produced from wheat straw using the

Organosolv pre-treatment, where the membrane used for its filtration had a MCWO of 30 kDa. This

membrane was used to concentrate particulate nanolignin in the retentate and to exchange solvent and

remove impurities when operated in diafiltration mode with distilled water. The regeneration of the

membrane was performed at certain points of the filtration by washing with organic solvent, since there

is fouling during ultrafiltration that will reduce the transmembrane flux.

The membranes used showed a removal efficiency for dissolved components of 93.6% and

85.2%, for the experiment using three and two membranes in series, respectively. The ethanol and

impurities were also reduced as intended.

The study of the flux in function of the concentration showed that the flux is affected by the

increase of the concentration of nanolignin particles, therefore the fouling is also affected.

The membrane regeneration revealed to be a good option to improve the performance of the

membrane, the regeneration after the concentration mode revealed to be more effective than

regeneration after the diafiltration step.

Keywords: Biorefinery, Organosolv, lignin, ultrafiltration, diafiltration, membranes.

v

Resumo A biomassa lignocelulósica surge como uma alternativa aos recursos não renováveis, uma vez

que estes são escassos e altamente poluentes. A biomassa é subdividida em várias frações, como

lenhina, hemicelulose e celulose, que define o conceito de biorrefinaria pela produção de vários

produtos.

O objetivo do trabalho foi avaliar o desempenho das membranas, o seu decrescimento e o seu

potencial de regeneração durante a operação de ultrafiltração da suspensão de nanolenhina em modo

de diafiltração. A suspensão utilizada foi produzida a partir de palha de trigo utilizando como pré-

tratamento o Organosolv, no qual a membrana utilizada para a sua filtração apresentava um peso

molecular de corte de 30 kDa. Esta membrana foi utilizada para concentrar nanolenhina em partículas

no concentrado e para alterar o solvente e a remoção de impurezas quando operado em modo de

diafiltração. A regeneração da membrana foi realizada em determinados pontos da filtração por

lavagem com solvente orgânico, uma vez que há acumulação de partículas durante a ultrafiltração, o

que consequentemente reduzirá o fluxo transmembranar.

As membranas utilizadas mostraram uma eficiência de remoção para componentes dissolvidos

de 93,6% e 85,2%, para o procedimento utilizando três e duas membranas em série, respetivamente.

O etanol e as impurezas também foram reduzidos tal como pretendido.

O estudo do fluxo em função da concentração mostrou que o fluxo é afetado pelo aumento da

concentração de partículas de nanolenhina, portanto o fouling também é afetado.

A regeneração da membrana revelou-se uma boa opção para melhorar o desempenho da

membrana, a regeneração após a ultrafiltração revelou-se mais eficaz quando comparado com a

regeneração após a diafiltração.

Palavras-chave: Biorrefinaria, Organosolv, lenhina, ultrafiltração, diafiltração, membranas.

vii

Table of contents Acknowledgements .................................................................................................................................. i

Abstract .................................................................................................................................................... iii

Resumo ................................................................................................................................................... v

List of Acronyms and Nomenclature ...................................................................................................... ix

Introduction .............................................................................................................................................. 1

Literature Review ..................................................................................................................................... 3

Biorefinery ............................................................................................................................................ 3

Biomass ............................................................................................................................................... 3

Cellulose .......................................................................................................................................... 4

Hemicellulose .................................................................................................................................. 5

Lignin ............................................................................................................................................... 5

Nanolignin Particles Production........................................................................................................... 6

Pretreatment of Lignocellulosic material ............................................................................................. 7

Organosolv Pretreatment ................................................................................................................ 9

Membranes ........................................................................................................................................ 10

Membranes Technology ................................................................................................................ 10

Membranes Classification ............................................................................................................. 10

Membranes Material ...................................................................................................................... 11

Membranes Processes .................................................................................................................. 11

Ultrafiltration ................................................................................................................................... 13

Membranes Characterization ........................................................................................................ 14

Diafiltration ..................................................................................................................................... 17

Aim of the thesis ................................................................................................................................ 17

Material and Methods ............................................................................................................................ 19

Experimental Procedure for Nanolignin particles production ............................................................ 19

Extract Production ......................................................................................................................... 19

Precipitation ................................................................................................................................... 20

Membrane Filtration ........................................................................................................................... 20

Ultrafiltration Process Setup .......................................................................................................... 20

Membrane Instructions .................................................................................................................. 22

viii

Membrane Selection ...................................................................................................................... 23

Membrane Stability ........................................................................................................................ 23

Ultrafiltration and Diafiltration of Nanolignin Particles Suspension ............................................... 23

Membrane Regeneration ............................................................................................................... 24

Membrane Fouling ......................................................................................................................... 25

Analytics ........................................................................................................................................ 26

Results and Discussion ......................................................................................................................... 31

Membrane Filtration – 2 Membranes in Series ................................................................................. 31

Membrane stability......................................................................................................................... 31

Transmembrane Flux .................................................................................................................... 32

Ultrafiltration/Diafiltration of Nanolignin Particles Suspension ...................................................... 32


Analytics ........................................................................................................................................ 36

Membrane Filtration – 3 Membranes in Series ................................................................................. 43

Membrane Filtration – Flux and Concentration Experiment .......................................................... 43

Membrane stability......................................................................................................................... 46

Ultrafiltration/Diafiltration of Nanolignin Particles Suspension ...................................................... 48


Analytics ........................................................................................................................................ 53

Membrane Regeneration ................................................................................................................... 58

Conclusions ........................................................................................................................................... 62

References ............................................................................................................................................ 64

Appendix ................................................................................................................................................ 68

ix

List of Acronyms and Nomenclature Acronyms

KL − Kraft Lignin

MF − Microfiltration

MWCO − Molecular weight cut − off

NF − Nanofiltration

OL − Organosolv Lignin

OP − Organosolv Process

PES − Polyethersulphone

PESH − Hydrophilic Polyethersulphone

PSU − Polysulphone

PVDF − Poly(vinylidene fluoride)

RC − Regenerated Cellulose

RO − Reverse osmosis

TF − Transmembrane Flux

UF − Ultrafiltration

DF − Diafiltração

Nomenculature

Am − Membrane Active Area

CAa − Solute Concentration in the feed

CAp − Solute Concentration in the permeate

DM − Dry Matter

DMbefore centrifuge − DM content before centrifuge

DMafter centrifuge − DM content of supernatant

DMPermeates − DM amount in each permeate

DMInitial Suspension − DM amount in the supernatan of initial suspension

DMsamples − DM amount in retentate sample

DMRxW − DM of the final concentrate remaining inside the tank

Lp − Hydraulic Permeability

minitial − mass of sample before oven

mdry sample − mass of sample after the oven

fA − Retention Coefficient

PXMX − Permeate X for Membrane X

RE − Removal Efficiency

TL − Total nanolignin particles in the sample

vp − Permeation Flux

∆π − Average Osmotic Pressure

x

Qa − Feed Flow

Qp − Permeate Flow

Qr − Retentate Flow

xi

List of figures Figure 1 - Structure of lignocellulosic biomass with cellulose, hemicellulose, and basic elements of lignin

represented (Alonso, Wettstein, & Dumesic, 2012). ............................................................................... 4

Figure 2 - Structure of a cellulose molecule (Matsutani, Harada, Ozaki, & Takaoka, 1993). ................. 4

Figure 3 - Hemicellulose backbone of arborescent plants (Matsutani et al., 1993). ............................... 5

Figure 4 - Lignin/Phenolics-carbohydrate complex in wheat straw(Buranov & Mazza, 2008). ............... 6

Figure 5 - Potential and investigated applications of lignin from micro- to nanosize (S Beisl, et al., 2017).

................................................................................................................................................................. 7

Figure 6 - Schematic of goals of pretreatment on lignocellulosic material. ............................................. 8

Figure 7 - Ligno-cellulosic feedstock biorefinery (Gavrilescu, 2014)....................................................... 8

Figure 8 - Schematic diagram of the organosolv fractionation process (Nitsos, et al., 2018). ................ 9

Figure 9 - Symmetrical membranes: (a) Isotropic microporous, (b) Nonporous dense, (c) Electrically

charged. (Rautenbach, R. & Albert, 1989). ........................................................................................... 10

Figure 10 - Anisotropic membranes: (a) Loeb-Sourirajan, (b) Thin-film composite, (c) Supported liquid.

(Rautenbach, R. & Albert, 1989). .......................................................................................................... 11

Figure 11 - Pressure-driven membrane processes. (Gaspar, 2018.).................................................... 12

Figure 12 - Classification of membrane processes based on pore size. (Ultrafiltration, 2018) ............. 13

Figure 13 - Schematic representation of ultrafiltration. 𝐶𝐴𝑎 and 𝐶𝐴𝑝 represent the solute concentration

in the feed and permeate. 𝑄𝑎, 𝑄𝑝 and 𝑄𝑟 represent the flow of the feed, permeate and retentate...... 14

Figure 14 - Gel layer of colloidal material on the surface of an ultrafiltration membrane (Rautenbach et

al., 1989). ............................................................................................................................................... 15

Figure 15 - Permeate flux as a function of time with membrane cleaning (Rautenbach et al., 1989). . 16

Figure 16 - (a) Example of extract, (b) Autoclave in operation, (c) Autoclave ...................................... 20

Figure 17 - Precipitation Setup: (a) T-fitting and static mixer, (b) Syringe Pumps. ............................... 20

Figure 18 - MEMCELL plant for membrane flush, from OSMO Membrane Systems. .......................... 21

Figure 19 - MEMCELL plant for membrane filtration, from OSMO Membrane Systems. ..................... 22

Figure 20 - Calibration curve for lignin content in UV. ........................................................................... 25

Figure 21 - Analysis methodology for different samples. ...................................................................... 26

Figure 22 - Calibration curve for Acetic Acid content. ........................................................................... 28

Figure 23 - Calibration curve for HMF and Furfural content. ................................................................. 28

Figure 24 - Calibration curve for Ethanol content. ................................................................................. 28

Figure 25 – Initial transmembrane flux (g/min) over time (min) of each membrane (1.2 L/min at 4 bar).

............................................................................................................................................................... 31

Figure 26 - Ultrafiltration and diafiltration process for membrane 1 (1.2 L/min at 4 bar). ...................... 33

Figure 27 - Ultrafiltration and diafiltration process for other membranes, 2, 3, 4 and 5 (1.2 L/min at 4

bar). ....................................................................................................................................................... 33

Figure 28 - Transmembrane Flux (g/min) over time (min) for the final flush of each membrane (1.2 L/min

at 4 bar). ................................................................................................................................................ 35

Figure 29 - Ethanol content (mg/L) of the retentate samples for the experiment 2 Membranes in Series.

............................................................................................................................................................... 38

xii

Figure 30 - Acetic Acid, HMF and furfural content (mg/L) of the retentate samples for the experiment 2

Membranes in Series. ............................................................................................................................ 38

Figure 31 - Particle size for all retentate samples for the experiment 2 Membranes in Series. ............ 40

Figure 32 - DryMatter content (%) for all retentate samples, before and after centrifuge for the experiment

2 Membranes in Series. ......................................................................................................................... 41

Figure 33 – Dry Matter content (%) for all permeate samples for each membrane for the experiment 2

Membranes in Series. ............................................................................................................................ 42

Figure 34 - Experiment 1 of Flux and Concentration Experiment, using membrane 1, 6 and 7 (1.2 L/min

at 4 bar). ................................................................................................................................................ 44


at 4 bar). ................................................................................................................................................ 44


at 4 bar). ................................................................................................................................................ 45

Figure 37 - Initial Transmembrane Flux (g/min) over time (min) for membranes 1, 6 and 10 for the

experiment 3 Membranes in Series (1.2 L/min at 4 bar). ...................................................................... 46

Figure 38 - Initial Transmembrane Flux (g/min) over time (min) for membranes 11, 12 and 13 for the


Figure 39 - Transmembrane Flux (g/min) over time (min) for membrane 1 for the experiment 3

Membranes in Series (1.2 L/min at 4 bar). ............................................................................................ 48

Figure 40 - Transmembrane Flux (g/min) over time (min) for the other membranes, 10, 11, 12 and 13

for the experiment 3 Membranes in Series (1.2 L/min at 4 bar). ........................................................... 49

Figure 41 - Transmembrane Flux (g/min) over time (min) for membrane 6 for the experiment 3

Membranes in Series (1.2 L/min at 4 bar). ............................................................................................ 49

Figure 42 - Final Transmembrane Flux (g/min) over time (min) for membrane 1, 6 and 10 for the




Figure 44 - Ethanol content (mg/L) for all retentate samples for the experiment 3 Membranes in Series.

............................................................................................................................................................... 54

Figure 45 - Acetic Acid content (mg/L) for all retentate samples for the experiment 3 Membranes in

Series. .................................................................................................................................................... 55

Figure 46 - Particle Size measurements for all samples for the experiment 3 Membranes in Series. . 56

Figure 47 - Dry Matter content (%) for all retentate samples, before and after centrifuge for the

experiment 3 Membranes in Series. ...................................................................................................... 56

Figure 48 - DryMatter content (%) for all permeate samples for the experiment 3 Membranes in Series.

............................................................................................................................................................... 58

Figure 49 - Mean transmembrane flux for each regeneration step with 15 %wt ethanol solution. ....... 59

Figure 50 - Initial TF and TF before and after regeneration of membrane 1. ........................................ 60

Figure 51 - Product temperature (°C) and pressure (bar) for one extract production experiment. ....... 68

xiii

Figure 52 - Membranes used in the experiment 2 Membranes in Series: (a) Membrane 1; (b) Membrane

2; (c) Membrane 3; (d) Membrane 4; (e) Membrane 5. ......................................................................... 69

Figure 53 – (a) Membrane 1 (regenerated) at the end, after being used in all the experiments; (b)

Membrane 6 (without regeneration) at the end, after being used in experiments Flux and Concentration

and 3 Membranes in Series ................................................................................................................... 69

Figure 54 - Membranes used in the experiment Flux and Concentration: (a) Membrane 3; (b) Membrane

4; (c) Membrane 5. ................................................................................................................................ 70

Figure 55 - Membranes used in the experiment 3 Membranes in Series: (a) Membrane 6; (b) Membrane

7; (c) Membrane 8; (d) Membrane 9. .................................................................................................... 71

xiv

List of tables Table 1 -Technical data of the MEMCELL plants. ................................................................................. 21

Table 2 - Properties and application areas of NADIR® PESH membrane. .......................................... 23

Table 3 - Initial transmembrane flux for each membrane used in the experiment 2 Membranes in Series.

............................................................................................................................................................... 32

Table 4 - Mean transmembrane fluxes for UF/DF for membrane 1 and other membranes, 2,3, 4 and 5.

............................................................................................................................................................... 34

Table 5 - Mean transmembrane fluxes for each membrane. ................................................................ 34

Table 6 - Initial and final transmembrane flux each membrane and the respective flux decline for the


Table 7 - Samples labeling code for the experiment 2 Membranes in Series. ...................................... 37

Table 8 - Degradation products characterization of straw at 180ºC. ..................................................... 39

Table 9 - DM amount (g) of different samples used for the mass balance of the filtration system for the


Table 10 - Transmembrane flux for all membranes for each experiment. ............................................ 45

Table 11 - Initial TF for each membrane for the experiment 3 Membranes in Series. .......................... 47

Table 12 - Mean TF for each filtration step for the experiment 3 Membranes in Series. ...................... 50

Table 13 - Mean TF of each membrane used in the filtration for the experiment 3 Membranes in Series.

............................................................................................................................................................... 50

Table 14 - Initial and Final transmembrane flux for each membrane for the experiment 3 Membranes in

Series. .................................................................................................................................................... 52

Table 15 - Sample labeling for the experiment 3 Membranes in Series. .............................................. 53



Table 17 - Regeneration steps for membrane 1.................................................................................... 58

Table 18 - Lignin removed from membrane 1 in each step. .................................................................. 59

Table 19 - Membrane recovery (%) for each regeneration step. .......................................................... 60

Table 20 - Characteristics of membrane 1. ........................................................................................... 61

Table 21 - Characteristics of membranes used in experiment 2 Membranes in Series........................ 61

Table 22 - Characteristics of membranes used in experiment 3 Membranes in Series........................ 61

1

Introduction

Lignocellulosic biomass is the most abundant renewable resource in the world and has been

considered with the potential to produce chemicals and biomaterials. The main contents of biomass are

cellulose, hemicellulose, and lignin that is the second most abundant biopolymer in the world (Weinwurm

et al., 2016).

Nowadays there are several industrial sectors that produce waste that can be used as a source

for other processes and the waste produced from the pulp and paper industry has a lot of lignin in it.

Almost half of the lignin retrieved from processes is incinerated and then used for producing energy, the

other half can be transformed (S Beisl et al., 2017). Therefore, the valorisation through transformation

of lignin into high-prize products has gained more interest over the past years to improve the economic

performance of lignocellulosic biorefinery concepts. So, in the past few years, the main goal was to

retrieve as much lignin as possible and the research regarding processes of extraction have gained

interest improving through the years.

Lignin is a highly irregularly branched polyphenolic polyether, consisting of the primary

monolignols, p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, which are connected via

aromatic and aliphatic ether bonds as well as non-aromatic C-C bonds (Stefan Beisl, Miltner, et al.,

2017). The lignin can be separated into three major types: softwood lignins, hardwood lignins, and grass

lignins. Softwood lignins are mainly composed of coniferyl alcohol, hardwood lignins contain coniferyl

and sinapyl alcohol and the grass lignins contain all three types of lignols (S Beisl et al., 2017).

Lignin is present in the biomass and to retrieve the lignin it’s necessary to fractionize the biomass

with the help of different treatments, like cooking it at high temperatures with elevated temperatures

(160-240ºC) under pressure or cooked with diluted acid or in alkaline conditions. The organosolv

treatment (60 wt% ethanol solution and a mass ratio of straw to solvent of 1:11) is widely used and was

developed as an alternative to conventional pulping processes and is quite promising in regards of

achieving high delignification of the biomass, with relatively high purity lignin (Weinwurm et al., 2016).

After the organosolv treatment, a step of precipitation is performed that results in a diluted

nanolignin suspension with some impurities. Therefore, it’s necessary to purify this mixture with the goal

of increasing the concentration of nanolignin particles and exchange the alcoholic solvent used in the

organosolv process with water. Through membrane separation, ultrafiltration, it is possible to achieve

separation with several benefits like, for example, the possibility of withdrawing the solvent at any

position and the possibility to obtain lignin with defined properties (Stefan Beisl, Loidolt, Miltner, Harasek,

& Friedl, 2018).

In the present work, the focus is the ultrafiltration step in diafiltration mode giving high

importance to the membranes used in the process, in order to concentrate the nanoparticles suspension

and cleaning the suspension from impurities (sugars, degradation products and dissolved lignin) without

changing the properties of the lignin particles. The membranes used should be mechanically strong

enough to withstand the pressure applied without rupture or distortion. It’s important that they don’t react

with the mixture passed through them and that they are isoporous preventing that some free occasional

pores be larger than the average. The size of the pores in the membranes should be chosen in a way

2

that they can be large enough to let the components desired in the filtrate pass and small enough to

retain the components desired in the residue (Ferry, 1936).

Each membrane used in the following work has been subjected to different conditions to find

out which conditions are better to improve the filtration step obtaining a product with a high concentration

of nanolignin particles.

3

Literature Review

Biorefinery

The demand for crude oil has been increasing in the past years and the population is starting to

be conscious of the finite nature of the world’s oil supplies. This consciousness has led to an increase

in the price and the fear of the potential use of crude oil as a political weapon (Prasad, Singh, & Joshi,

2007). The raw material used is neither environmentally friendly nor sustainable (Gavrilescu, 2014).

Consequently, there is a need to develop and implement new technologies based on alternative energy

platforms, wind, water, sun, nuclear fission and fusion, and biomass (FitzPatrick et al. , 2010).

“New technologies are being developed that use biomass to make not only low-value products

such as fuels but also high-value materials such as polymers. It is also important to look back at what

happened in the past when crude oil prices surged.” (Aresta et al., 2012)

There are three biorefinery systems being researched and developed (Gavrilescu, 2014) (Kamm

& Kamm, 2004): “The whole crop Biorefinery”, that uses cereals as raw material or maize with the aim

of producing straw and corn. “The Green Biorefinery” that uses biomasses such green grass. The third

and last system is “The Lignocellulose Feedstock (LCF) Biorefinery” which uses straw, reed, grass,

wood, and others as raw material.

Although the biorefinery is a key technology for a more sustainable world, the high needs of

capital for this type of industry leads to a lack of adherence. Previous studies show that biorefineries

should focus on high-value chemicals/materials and use residual waste for energy integration, producing

energy and fuels (Aresta et al., 2012).

Biomass

The carbon neutrality and renewable characteristics of the biomass made it a respectable

alternative as a fundamental resource in the sustainable society, for energy and material resources.

Biomass is a renewable material derived from living or recently living organisms. Biomass is a plant

material derived from the photosynthetic process, where the carbon dioxide that comes from the

atmosphere and water from the plant roots are combined. This reaction produces carbohydrates that

are converted in biomass. This process is a cycle, the existing carbon in biomass combined with the

oxygen from the atmosphere (during biomass combustion) leads to the production of water and carbon

dioxide, being then available again for biomass production (Parmar, 2017). This reverse reaction is due

to the sunlight that is stored in the chemical bonds of the structural components of biomass (McKendry,

2002).

The lignocellulosic biomass’s structure is composed of three main polymeric components, which

are cellulose, lignin, and hemicellulose. Biomass can be transformed into fibers and chemicals when

plants or animal matter are used as a raw material. Besides this, biomass can also be used to produce

energy by using residues and waste matter (Jaya Shankar Tumuluru, et al. 2013). They store chemical

energy, however, the quantity of energy is dependent on the type of plant, and also their proportions

determine the optimum energy conversion (McKendry, 2002).

4

The three components that constitute biomass have different chemical composition and

structure which results in different chemical reactivities. Besides this, carbon, hydrogen, and oxygen in

biomass molecules exhibit complications in the catalytic conversion of biomass to fuels and chemicals

due to their inert chemical structure and compositional ratio (Kohli, Prajapati, & Sharma, 2019). Figure

1 represents the structure of a lignocellulosic biomass, where basic elements of lignin, cellulose and

hemicellulose are represented.

Cellulose

Main component of biomass, it represents half of the organic carbon in the biosphere (Kohli et

al., 2019) and is the main constituent of the plant cell wall, which confer structural support. Cellulose

has linear chains of (1,4)-D-glucopyranose units, where they are connected to 1-4 in β-configuration

(McKendry, 2002). These chains are grouped to form microfibrils thereby forming cellulose fibers. The

microfibrils are connected by covalent bonds, by hydrogen bonding and Van der Waals forces, which

determine the straightness of the chain being able to present crystalline or amorphous structure in the

final form (Agbor, et al., 2011).

Cellulose typically comprises 40-50% of lignocellulosic biomass feedstock and has gained

interest as a source of fuels and chemicals via catalytic processing. Its hydrolyzation degree influences

how soluble it is and the cellulose does not melt at high temperatures, however, it starts to decompose

at 180ºC (Harmsen et al., 2010). The structure of a molecule of cellulose is represented in Figure 2.

Figure 1 - Structure of lignocellulosic biomass with cellulose, hemicellulose, and basic elements of lignin represented (Alonso, Wettstein, & Dumesic, 2012).

Figure 2 - Structure of a cellulose molecule (Matsutani, Harada, Ozaki, & Takaoka, 1993).

5

Hemicellulose

Hemicellulose is also one of the most abundant polymers and represents 20-50% of

lignocellulosic biomass feedstock. It is a low molecular weight heterogeneous polymer composed by

pentoses, hexoses and acetylated sugars. The composition of hemicellulose depends on the type of

biomass. Hemicelluloses from straw and grasses are mainly composed of xylan, whereas hemicellulose

from softwood is mainly composed by glucomannan. Hemicellulose at low temperatures is not soluble,

has a random and amorphous structure when compared with cellulose, this structure makes it less

resistant against hydrolysis. Compared to cellulose, the hydrolysis is conducted at lower temperatures

which makes it soluble at higher temperatures (Matsutani et al., 1993). In Figure 3 is represented the

chemical structure of hemicellulose.

Lignin

Lignin is also amongst the most abundant biopolymers, being the most complex natural

polymer. This polymer waterproofs the cell wall, which improves the transport of solutes and water

through the vascular system. It is decisive in the integrity of the cell wall structure and stiffness, and

strength of the steam. Lignin is an amorphous three-dimensional polymer with phenylpropane units as

the predominant building blocks (Matsutani et al., 1993). It is composed of three main kinds of

monolignols (p-coumaryl, coniferyl and sinapyl alcohols). These monomers are synthesized in the

cytoplasm during lignin deposition, and posteriorly polymerized into lignin (Li & Chapple, 2010).

There are three major groups of lignin, most of the lignin from softwoods is constituted of

coniferyl alcohol and the remaining of p-coumaryl alcohol units. The lignin of hardwoods is composed

by coniferyl and sinapyl alcohol in different ratio variations. The other main group is lignin from grasses.

(Matsutani et al., 1993).

Figure 3 - Hemicellulose backbone of arborescent plants (Matsutani et al., 1993).

6

A new biorefinery focus is to use agricultural residues as raw material, due to the amount of

waste that is produced annually, as reported in (Agricultural, et al., 2016). After harvested, the biomass

must be subjected to pretreatments to convert it into chemical compounds or fuels. Afterwards, post-

treatments can be required for purification or stabilization of the final product (Hu, et al., 2018).

The characteristics of the product depend on the type of straw collected and on its habitat.

Although this change oscillates on a larger scale, the elemental composition on moisture and ash free

basis does not differ much (Tröger, et al., 2013).

Nanolignin Particles Production Due to the structural complexity of the products obtained from lignin it is necessary to obtain

lignin with superior properties, therefore producing nanolignin particles. From the point of view of the

particle size distribution, there are three parameters (pH, temperature and ratio of lignin/solvent) that

can be varied in order to obtain the best condition (Gilca et al., 2015). In comparation with molecules

with larger dimensions, the structure of the nanoparticles (especially in the 1-100 nm range) present

distinct properties due to the increase of the surface area (S Beisl et al., 2018).

By an OP, the wheat straw structure fractures at high temperatures and low pressure. By

changing the operational conditions (temperature and pressure) and the organic solvent, the final extract

will acquire different characteristics. There are different ways to produce nanolignin particles from wheat

straw, however most of them have a very high solvent consumption. To reduce the consumption of

solvent, the most appropriate method is the direct precipitation of lignin nanoparticles. This method uses

an Organosolv pretreatment with a 60wt% ethanol solution at 180ºC (S Beisl et al., 2018).

Lignin Nanomaterials Applications

For several years, researchers have been studying practical applications for the lignin that is

obtained from biomass or pulping liquors. The lignin structure depends on the source of the biomass

and the isolation method used, therefore the applications will depend on the type of lignin used. Lignin

may be subjected to chemical treatments in order to make it usable in a particular application, that

treatment can improve the reactivity or even its efficacy (Calvo-Flores, et al., 2015). This is an

Figure 4 - Lignin/Phenolics-carbohydrate complex in wheat straw(Buranov & Mazza, 2008).

7

opportunity to adapt the features of the final product, optimizing the entire process chain. Lignin is known

to have different branches of applications, such as resistance to decay and biological attacks, UV

absorption, the capability to retard and inhibit oxidation reactions and high stiffness. Nanostructured

materials have different properties than molecules of larger dimension (same composition), and their

applications field starts in simple polymer blends with upgraded mechanical properties to capable drug

carriers. In Figure 5 is shown the published and potential applications of lignin micro/nanomaterial

(Stefan Beisl, et al., 2017).

Pretreatment of Lignocellulosic material

Figure 5 - Potential and investigated applications of lignin from micro- to nanosize (S Beisl, et al., 2017).

8

Lignocellulosic biomass is a promise in the renewable sources of carbon since it is available

around the world with low cost, however, the major adversity is the complex chemical composition of

the lignocellulosic biomass. In order to access more easily the components, present in the lignocellulosic

material, it is necessary to alter the structure with a pretreatment. The goals of pretreatment on

lignocellulosic material are represented in Figure 6:

The optimization of pretreatments steps is crucial for an economically reliable biorefinery. The

aim of a pretreatment is to extract the lignin in its natural form and to prepare the materials for enzymatic

degradation since the properties of the natural lignocellulosic turn the material more resistant. To choose

the best pretreatment it is necessary to consider the type of lignocellulose feedstock. The main factors

are the degree of polymerization and degree of acetylation. Figure 7 shows the general lignocellulosic

feedstock biorefinery.

Many pretreatment methods have been studied and are still in development, they can be divided

into four different categories (Agbor et al., 2011):

Figure 7 - Ligno-cellulosic feedstock biorefinery (Gavrilescu, 2014).

Figure 6 - Schematic of goals of pretreatment on lignocellulosic material.

9

a) Physical pretreatment: Based on the principle of particle size reduction by mechanical

stress;

b) Biological pretreatment: Non-energy intensive processes, using fungi and

actinomycetes. Some microbes used in this process consume part of the carbohydrates

in the biomass which will affect the sugar yield. Another negative aspect is that this

process requires longer residence time, which is a limitation in a larger scale biorefinery;

c) Chemical pretreatment: Use of different chemicals to study the effect on the native

structure of lignocellulosic biomass;

d) Physicochemical pretreatment: This pretreatment includes treatments such as steam

explosion, ammonia pretreatments, liquid hot water pretreatment, wet oxidation, among

others. These methods have the advantage of affecting both the physical and chemical

properties of biomass.

Organosolv Pretreatment The Organosolv process (OP) is integrated within the chemical pre-treatments. Comprises the

cooking of lignocellulosic biomass in a mixture of an organic solvent with water that leads to the

deconstruction of lignin and hemicellulose and its dissolution in the liquor. It produces three different

streams, a cellulose-rich pulp, a lignin rich solid precipitate as well as a hemicellulose rich liquid.

Moreover, the solvent can be recovered from the liquid stream by distillation (Nitsos, et al., 2018). Figure

8 represents the process diagram of an ethanol organosolv fractionation.

Initially, this pretreatment was operated in the pulp and paper industry. Even though this process

has minor consequences to the environment, it does not achieve the necessary degree of delignification

when compared to the kraft process.

Seen as a promising alternative to remove practically pure lignin from the biomass with sugars

available for conversion is the Organosolv pretreatment. Moreover, Organosolv pretreatment is the only

pretreatment capable of isolating each component of the biomass, which can then be possibly sold as

a by-product or even transformed into a higher value product. For this pretreatment, it is necessary to

have a recyclable and efficient solvent. From previous studies, a solvent recovery unit is required in

Figure 8 - Schematic diagram of the organosolv fractionation process (Nitsos, et al., 2018).

10

order to make it economically viable in an operation sequence of distillation, neutralization, and

evaporation or membrane processes like nanofiltration and reverse osmosis. (da Silva, Errico, & Rong,

2018)

The Organosolv process (OP), extracts low molecular weight lignin from the biomass, nearly

pure, which presents the minimum amount of carbohydrates and impurities, thus being possible to

convert it into final products of higher value rather than heat and power generation (S. Beisl, et al., 2017).

In this pretreatment, there are some side reaction’s products that inhibit microorganism’s

fermentation. Moreover, the use of volatile organic liquid at high temperatures leads to special care due

to higher pressure and because it also uses high-value chemicals (Agbor et al., 2011).

Membranes

Membranes Technology

Since the 1960s the Loeb-Sourirajan development of reverse osmosis asymmetric cellulose

acetate membranes stands as a milestone on the use of pressure-driven membrane technology at the

industrial large scale. In the last years, membrane separation processes have been used in

pharmaceutical, biological, chemical and food industry. The focus of research has been the fractionation

of spent cooking liquor in the kraft chemical pulping process (Wallberg, Jönsson, & Wimmerstedt, 2003).

With the development of new membranes that present upgraded transport properties and are chemically

and thermally stable, new applications were then identified to these membranes (Strathmann, 2001).

Membranes Classification There is a vast diversity of membranes, depending on the materials and structures. In Figure 9

and Figure 10, the principal types of membranes, symmetric and asymmetric are shown. (Rautenbach

et al., 1989).

Symmetric membrane has identical structural morphology at all positions within it. A

microporous membrane (a), is rigid with interconnected pores distributed randomly, these pores have a

diameter in the order of 0.01-10µm. The separation of solutes is a function of molecular size and pore

size distribution. Membrane type (b) is nonporous and dense, where the permeants are transported by

diffusion under the driving force of pressure, electrical potential gradient or concentration. Membrane

Figure 9 - Symmetrical membranes: (a) Isotropic microporous, (b) Nonporous dense, (c) Electrically charged. (Rautenbach, R. & Albert, 1989).

(a) (b) (c)

11

type (c), is a charged membrane, it works by the exclusion of ions of the same charge as the ions present

on the membrane structure.

An asymmetric membrane is a composite of two or more structural planes of divergent

morphologies. Anisotropic membranes have different permeabilities and structures for each membrane

layer supported on a thicker porous substructure. The Loeb-Sourirajan membrane, type (a), consists of

a single membrane material, yet the pores size and the porosity differ depending on the membrane

layer. Membrane (b), has an ultra-thin top layer which is responsible for separation selectivity, where a

microporous sublayer supports this top layer (Wu, 2015). Membrane type (c), liquid membrane, consists

of a thin film which separates two phases, gas or aqueous solutions mixtures. The porous structure

provides the mechanical strength of the membrane, whereas the selective separation barrier is provided

by the liquid-filled pores (Drioli et al., 2010). The membrane structure for ultrafiltration is an asymmetric

membrane made following the Loeb-Sourirajan method, allowing high permeation fluxes and selectivity.

Membranes Material

Membranes should combine high permeability and high selectivity with enough mechanical

stability. Usually, organic polymers are the most used in pressure-driven membrane processes, the most

common polymers used in membranes include polyethersulfone (PES), regenerated cellulose (RC) and

poly(vinylidene fluoride) (PVDF) (Rheingaustr et al., 2018).

PES membranes are used peculiarly in the pharmaceutical industry and sterilizing filtration.

They are resistant at high temperatures and their performance decreases with the fouling as a

consequence of the hydrophobic character (Wavhal & Fisher, 2002). Materials from regenerated

cellulose are better for the environment since they are a non-toxic material with low cost. These

membranes have an important role in seawater desalination, filtering methanol, ethanol, and urea

(Bhongsuwan & Bhongsuwan, 2008). PVDF membranes also present a high level of hydrophobicity and

are thermodynamically compatibility with other polymers. Poly(vinylidene fluoride) membranes show a

chemical resistance and a high mechanical strength which presents a good option for wastewater

treatment (Liu et al., 2011).

Membranes Processes The separation of molecular and particulate mixtures in membrane reactors, artificial organs,

energy storage, and conversion systems, and also the controlled discharge of active agents are the four

Figure 10 - Anisotropic membranes: (a) Loeb-Sourirajan, (b) Thin-film composite, (c) Supported liquid. (Rautenbach, R. & Albert, 1989).

(a) (b) (c)

12

main areas where membranes are used (Strathmann, 2001). The membrane process is determined

according to the driving force used. The most common and relevant are the pressure-driven processes,

based on the pressure difference between the permeate and the feed. Operations that use this type of

membrane process are reverse osmosis, nano-, ultra-, and microfiltration. Another driving force for the

process is concentration-gradient, being used essentially by processes such as dialysis. Moreover,

partial-pressure-driven processes such as pervaporation and gas permeation, and electrical-potential-

driven processes such as electrodialysis and electrolysis are known (Strathmann, 2001). In this work,

the focus is in the pressure-driven membrane processes, which include reverse osmosis, nano-, ultra-

and microfiltration, the following Figure 11 shows the characteristics for each membrane process.

Membrane Process Membrane Type Transmembrane Pressure

Mechanism

Reverse Osmosis (RO)

Asymmetric composite with homogeneous skin

High (20-100 bar) Solution-diffusion

Nanofiltration Asymmetric composite with homogeneous skin

High (10-40 bar) Solution-diffusion and electrostatic interactions

Ultrafiltration (UF) Asymmetric microporous Low (0.5-8 bar) Size exclusion

Microfiltration (MF) Symmetric and asymmetric microporous

Low (0.1-1 bar) Size exclusion

Figure 11 - Pressure-driven membrane processes. (Gaspar, 2018.)

Membranes can be defined as a barrier that split two phases and have a selection process in

relation to the transport of different components. Membranes can be flat sheets, hollow fiber, tubes or

capillaries installed in a suitable device (membrane module). The nature of the membrane used depends

on the intended application (Strathmann, 2001). The key properties for a good membrane performance

are high selectivity and fluxes, and also the need for thermal, chemical and mechanical stability.

The biggest difference between the processes described, is the average pore diameter of the

membrane applied, where the separation process is distinguished by the size of the particles or

molecules that is possible to retain, as can be seen in Figure 12 (Satyanarayana, Bhattacharya, & De,

2000).

13

Ultrafiltration Ultrafiltration has been used for a long time as a separation method which relies on molecular

size exclusion, however, the need to present a low mechanical and chemical resistance led to the

development of new membranes (Strathmann, 2001). Which happened with the appearance of the

anisotropic cellulose acetate membrane by Loeb-Sourirajan for reverse osmosis, that made this process

a potentially practical method of desalting water. It consisted of an ultrathin and selective surface film

on a thicker but much more permeable microporous support, that granted fluxes much higher than any

membrane available at the time (Baker, 2004).

The separation process used in this work was ultrafiltration, which was chosen based on the

particle size of the nanolignin particles in suspension and on the type of material being processed

(anisotropic membranes). This separation method is usually used for the concentration, clarification,

diafiltration, and fractionation of macromolecules. UF membranes represent almost 40% of the food and

biotechnological industry, however, this is a high-cost market and there is a need to develop cost-

effective and scalable purification processes. Ultrafiltration features high throughput of product, easy to

clean and to sanitize equipment, and it can be easily scaled-up (Ghosh, 2003).

The fractionation of lignin from the black liquor resulting from pulping processes was previously

studied comparing two different methods, selective precipitation, and ultrafiltration. Both methods were

effective although ultrafiltration showed better results since the lignin obtained was less contaminated

with hemicellulose (Toledano, et al., 2010). Furthermore, the final average molecular weight of the

product is controlled by the MCWO of the membrane. Ultrafiltration also showed the advantage of not

needing temperature or pH adjustment, as well as the fact that the concentration of the liquor not being

Figure 12 - Classification of membrane processes based on pore size.

(Ultrafiltration, 2018)

14

crucial, making it possible to improve the pulp quality of bleachability by decreasing the lignin

concentration from the cooking liquor (Jönsson & Wallberg, 2009).

However, the method to extract the lignin has to be chosen according to the desired application,

since the selective precipitation shows less energy consuming which reflects in lower operating costs

(Toledano, et al., 2010).

The module designs depend on the application desired, the most common designs are

distinguished from each other by the hydraulic diameter and the package density. Tubular modules such

as polymeric or ceramic element, plate modules, spiral wound modules, follow fiber modules and

capillary modules are the most common module designs in ultrafiltration (OSMO Membrane Systems,

2019).

Membranes Characterization

Molecular Weight Cut-off The choice of MCWO is as important as any other decisions in a filtration process, this

parameter specifies the size of the molecules that will go through the membrane. The research on the

effects of different MCWO membranes on the permeate flux and membrane rejection was conducted in

a stirred cell module. For pure water, the higher the MCWO of the membrane the higher the flow, since

the pores are bigger (Toledano, et al., 2010). Moreover, if the pore size has increased and the amount

of solute that passes through the membrane increases, that also means that the rejection values will be

lower. It is necessary to evaluate specifically each experimental procedure and select a suitable

membrane, that would be ideal for each situation, depending on the type of suspension used.

Ultrafiltration membranes are usually in the range of 1-500 kDa. In this work the MCWO chosen was

30kDa, because from previous studies it was the one that showed the most promising results to separate

nanolignin particles from the impurities. This membrane MCWO retained the least quantity of dissolved

components, being more efficient at separating and purifying the nanolignin particles (Gaspar, 2018).

Hydraulic Permeability

The feed circulates tangentially to the surface of the membrane, this procedure has as a result

two output currents. The coefficient of hydraulic permeability, 𝐿𝑝, is the capacity of water permeation,

which is obtained by calculating the slope of the variation of the water permeation flux, 𝑣𝑝, with pressure.

Figure 13 - Schematic representation of ultrafiltration. 𝐶𝐴𝑎 and 𝐶𝐴𝑝 represent the solute concentration

in the feed and permeate. 𝑄𝑎, 𝑄𝑝 and 𝑄𝑟 represent the flow of the feed, permeate and retentate.

15

The variation of the permeation flux with the coefficient of hydraulic permeability is described in the

equation 1.

𝑣𝑝 = 𝐿𝑝. ∆𝑃 (1)

If the components get rejected and retained on the membrane surface, it will increase the

resistance to mass transfer. The permeation flux variations are represented by equation 2 due to

difference osmotic pressures between the permeate and the feed.

𝑣𝑝 = 𝐿𝑝. (∆𝑃 − 𝜋) (2)

To characterize the membranes performance the rejection coefficient is used, which indicates

the quantity of solute retained by the membrane. This coefficient is based on equation 3.

𝑓𝐴 =𝐶𝐴𝑎−𝐶𝐴𝑝

𝐶𝐴𝑎 (3)

Concentration Polarization and Fouling During the ultrafiltration process the membrane begins to retain material, creating a layer on the

surface of the membrane. The major problem in this membrane separation process is the decrease of

the permeate flux over time. Initially, the flux reduction is due to the build-up of osmotic pressure of the

solution, however, the gradual decline is caused by some consolidation on the membrane surface and

some in the membrane pores, formed by concentration polarization (Satyanarayana et al., 2000).

In ultrafiltration, the suspension is carried in the direction of the membrane surface by the

solution permeating the membrane, there is an accumulation of the larger molecules while the solvent

molecules permeate the membrane. In the course of the filtration, the solutes retained on the membrane

surface get so concentrated that they form a gel layer which is characterized as a second layer of the

membrane. Figure 14 illustrates a model of the gel layer (Rautenbach et al., 1989).

Once a gel layer is formed, the increase of the pressure will not increase the flux, however, the

gel thickness increases (Bhattacharjee et al., 1992). There are three factors that affect concentration

polarization which are boundary layer thickness, volume flow and diffusion coefficient.

Figure 14 - Gel layer of colloidal material on the surface of an ultrafiltration membrane (Rautenbach et al., 1989).

16

The boundary layer thickness can be minimized with the increase of the turbulence at the

membrane surface, through the increase of the flow velocity of the fluid, the use of membrane modules

or pulsing the feed fluid flow through the membrane. However, the turbulence must be increased

cautiously due to the energy needed for this achievement.

The total volume flow is also an aspect to consider, when the volume increases also the

concentration polarization increases. This is an aspect that can be improved by changing the operation

conditions. The consequences of high fluxes are one of the critical aspects in ultrafiltration.

The third factor is the diffusion coefficient that affects the concentration polarization, in

comparison with reverse osmosis, ultrafiltration filters colloids and macromolecules which have diffusion

coefficients about 100 times smaller than solutes in reverse osmosis. This is an important factor for

concentration polarization which explains why the size of the solute diffusion coefficient is such important

in ultrafiltration (Rautenbach et al., 1989).

Membrane Cleaning

To remove or decrease the layer created on the surface of the membrane it is necessary to

have repeated membrane cleaning, allowing the restoring of the membrane capacity. Figure 15 shows

the effects of membrane cleaning.

A different approach to control membrane fouling was studied, applying a pretreatment with

chlorination (Yu, et al., 2014). One of the major contributors to membrane fouling in typical ultrafiltration

processes are microorganisms called biological fouling. From laboratory tests it was concluded that

when conducting this type of pretreatment, the membrane fouling decreases substantially, which was

subsequently confirmed on a pilot scale. The addition of chlorination compounds resulted also in a lower

production of substances that cause fouling, proteins and polysaccharides, resulting ultimately in a

thinner cake layer.

Figure 15 - Permeate flux as a function of time with

membrane cleaning (Rautenbach et al., 1989).

17

Diafiltration Diafiltration is a well-recognized technique used in membrane separation process, with many

applications in the biotechnological, pharmaceutical, and food industries (Kovács et al., 2008). In this

process, the retentate is diluted with a solvent and further ultrafiltered in order to obtain selective removal

of lower molecular weight components. While working with kraft black liquor, the goal is not only to

obtain pure lignin but also to obtain the chemicals present in the permeate, which represent a significant

value. The addition of deionized water during diafiltration affects the viscosity. The reduction of the

viscosity increases the flux (inversely proportional) and reduces the thickness of the layer as a result of

the increase of Reynolds number in the tubes, which results in a decrease of the mass transfer

resistance (flux increase). (Wallberg et al., 2003)

Diafiltration can be performed in batch or continuous mode, the mode used is usually

determined by the rest of the process. For the same volume reduction before diafiltration, the final

product present higher purity in the continuous system (Wallberg et al., 2003).

Aim of the thesis

The aim of the work is to contribute to the development of the state of the art on the separation

and purification of wheat straw nanolignin particles from its impurities. To achieve that an ultrafiltration

membrane process ultrafiltration of nanolignin suspensions in diafiltration mode is used, recurring to a

commercial UF membrane with a MWCO of 30kDa will be used which has been analyzed in preliminary

works (Gaspar, 2018).

The following steps were based on previous experiments (Beisl et al., 2018):

1. Organosolv-Extraction of wheat straw at 180°C for 1h to prepare extracts for further usage (this

is the standard extract);

2. Filtering and centrifugation of the extract to remove particulate matter;

3. Precipitation of nanolignin with given operational conditions to produce nanolignin suspensions;

4. Ultrafiltration of nanolignin suspensions:

a) Pre-condition fresh membranes by flushing a given time;

b) Increase the concentration of the suspension in UF mode;

c) Remove impurities and ethanol in DF mode;

d) Trace the membrane performance decline as a function of permeate mass and stop

experiment at a given time;

e) Regenerate used membranes by flushing or backflushing;

f) Repeat these experiments several times with the regenerated membrane and compare

the performance with a completely fresh membrane to elaborate a ‘long-term’ stability

of the regenerated membrane;

5. Determine particle size of nanolignin suspension from time to time to check for particle size

stability;

6. Analyze important parameters in final nanolignin suspension.

19

Material and Methods

Experimental Procedure for Nanolignin particles production

There are 2 essential steps to obtain the nanolignin suspension used in current work. Firstly,

the wheat straw is subjected to a Pretreatment/Extraction in order to separate the lignin, which is then

followed by a Precipitation. The extract production, the choice of the antisolvent and setup was based

on previous experiments (Stefan Beisl et al., 2018).

Extract Production

The wheat straw used was harvested in 2015 in Margarethen am Moos, region in Lower Austria.

The composition of the straw was 16.1 %(w/w) of lignin and 63.1 %(w/w) carbohydrates, which consists

in arabinose, glucose, mannose, xylose and galactose (S Beisl et al., 2018) .

A stirred autoclave of 1L (Zirbus, HAD 9/16, Bad Grund, Germany) was used for the Organosolv

extraction, in order to obtain approximately 400mL of extract in each extraction. The necessary steps

to conduct the experiment are shown below:

1. Measure the humidity of the straw in order to determine the amount of water required.

2. An aqueous solution of 60wt% aqueous ethanol mixture was mixed with 8.3 %(w/w) of wheat

straw inside the reactor.

3. The reactor was set to a mantle and product temperature of 210°C and 180°C respectively, the

temperature and pressure are continuously recorded in the program LabVIEW.

4. When the product reaches the desired temperature, the mantle temperature is changed to

190°C, so that the product temperature does not exceed the set temperature.

5. After 1h of extraction, the cooling system is started so that the mixture is cooled down and

reaches room temperature after approximately 1h.

6. To take all the liquid from the solid part, the mixture was placed in the hydraulic press (Hapa,

HPH 2.5, Achern, Germany) at 200bar.

7. The solid part is stored in the freezer for future analyses, sugars, degradation products, and

lignin, and the liquid part is centrifugated, Sigma 4K15, Germany at 24000g for 20min.

To achieve the goal of the thesis, approximately 3L of extract were produced for that 8

extractions were conducted. The following Figure 16, shows a portion of the extract produced at 180°C

and the equipment used to obtain the extract.

20

Precipitation

For the precipitation step an antisolvent composed of pure water at 25°C was used

(Jääskeläinen, Liitiä, Mikkelson, & Tamminen, 2017) to dilute the amount of ethanol in the mixture. This

procedure results in the reduction of the solubility of lignin in the solution, forcing it to precipitate. The

precipitation occurs in a T-fitting followed by a static mixer, using 2 Syringe Pumps, Figure 17, with a

volume ratio of extract to antisolvent of 1:5. The use of a T-fitting followed by a static mixer was based

on (Stefan Beisl et al., 2018), this setup showed a faster mixing and a smaller particle size when

compared to other setups.

Membrane Filtration

Ultrafiltration Process Setup

Figure 16 - (a) Example of extract, (b) Autoclave in operation, (c) Autoclave

(a) (b) (c)

(a)

Figure 17 - Precipitation Setup: (a) T-fitting and static mixer, (b) Syringe Pumps.

(b)

21

Two MEMCELL plants, from OSMO Membrane Systems, were used to perform the membrane

separation experiments. The systems present a flat-membrane, where the ultrafiltration is operated in

cross-flow.

Table 1 -Technical data of the MEMCELL plants.

A smaller plant, Figure 18, was used to flush the membranes and regenerate them. In this

montage, the concentrate stream returns to the feed tank, which is connected to a gear pump from

Liquiflo.

For the filtration experiments another setup was used, Figure 19, this plant was adapted for a

5L tank, with 2 gear pumps in parallel, from Liquiflo, connected to the feed tank. For the last filtration

experiment, the two pumps used had to be replaced by another pump, Liquiflo, due to technical issues.

A stirrer was used in the feed tank to ensure good homogenization of the suspension and to avoid

aggregation of the particles. In both montages, global valves are installed to control the flow rate

additionally a pressure valve.

Technical Data

Working Pressure (bar) 64 (standard)

Material Stainless steel (1.4571 standard)

Feed Tank Volume (L) 0.5-2

Active Membrane Area (cm2) 80

Figure 18 - MEMCELL plant for membrane flush, from OSMO Membrane Systems.

22

Both systems consist of flat modules with an active membrane area of 80 cm2 each, the first

setup with only one membrane module while the second setup has 4 membranes modules. The

modules can be operated in parallel or in series and the suspension flows tangentially to the membrane

surface area. Particles smaller than the pores of the membrane penetrates the membrane, whereas

particles larger than the pores are retained and flow along the membrane surface. Thus, there are two

outflows streams, a permeate stream that flows through the membrane, and a concentrated stream that

passes along the membrane surface and returns to the feed tank. It was used the program RsCom

which was connected to a scale to record the permeate mass over time, this program was used during

the filtration of nanolignin particles suspension and membranes flush.

Membrane Instructions

When handling membranes, some precautions have to be taken into account. The main

concerns are:

1. Store the membrane in distilled water when not in use, between 5-50°C.

2. Before installation, flush the system with water to remove any existing residue. Flush the

membrane for 15 minutes to remove the preservative.

3. For a good filtration performance, the permeate must be drained without pressure on the system

and the valves should be opened gradually.

4. For a system shutdown of more than 24 hours, the plant must be flushed and cleaned.

Figure 19 - MEMCELL plant for membrane filtration, from OSMO Membrane Systems.

23

Membrane Selection

In ultrafiltration, membranes separate lignin based on MCWO that restrict passage based in

molecular size. The membrane material chosen was polyerthersulfone (PESH), a chemical compatible

with ethanol, since the suspension used had an ethanol content of 15wt%.

Table 2 shows the properties and application areas of NADIR® PESH membrane. The

membrane chosen was operated with a MCWO of 30kDa since it was the membrane that retained less

quantity of dissolved lignin at 4 bar and 1.2 L/min (Miltner et al., 2019).

Table 2 - Properties and application areas of NADIR® PESH membrane.

Membrane Stability

Fresh membranes need to be conditioned by flushing them with 15 %w/w hydroalcoholic

solution for a certain time, so the membrane maintains its stability for longer periods. From the program

data, it is possible to trace the membrane performance decline as a function of permeate mass per unit

of membrane surface area and time, thus obtaining the transmembrane flux from the slope of the linear

regression. The definition of transmembrane flux and the calculation of this factor is given in the next

equation 6:

𝑇𝑟𝑎𝑛𝑠𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒 𝐹𝑙𝑢𝑥 (𝑔

𝑐𝑚2.𝑚𝑖𝑛) =

𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒 𝑚𝑎𝑠𝑠 (𝑔)

𝐴𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒(𝑐𝑚2)×𝑇𝑖𝑚𝑒 (min) (6)

Ultrafiltration and Diafiltration of Nanolignin Particles Suspension

The goal of this thesis is, firstly to increase the concentration of the suspension in ultrafiltration

mode and remove impurities and ethanol in diafiltration mode. Several experiments were done. In the

first experiment two membranes in series were used, where the first membrane is regenerated, and the

second membrane is repeatedly changed for a new one. On the second filtration experiment, three

membranes were used, the first membrane was the same as in the first experiment, with regeneration,

a second membrane that is neither changed nor regenerated and a third membrane that is repeatedly

swapped. The experimental work of the membrane filtration follows the following steps:

Material Properties Range of

pH

Max.

Temperature

Line of business, industrial

sector

Permanently hydrophilic

polyethersulfone

(PESH)

Hydrophilic (low

fouling potential)

High chemical

stability

0 – 14

95°C

Environmental protection

Metal processing

Textile manufacture

Paper manufacture

Food industry/dairy

Pharma/biotechnology

Chemical industry

24

1. Initial membrane flush with a 15wt%ethanol solution for 2h15 for all membranes that will be

used in the procedure;

2. Lay the membranes in series and start filtration. Set the pressure and flow rate to 4 bar and 1.2

L / min, respectively.

3. The number of output currents depends on the number of membranes used since the

experiments were conducted using two membranes and three membranes in different runs,

therefore at the most 3 output streams were obtained and collected separately in beakers placed

on scales. When the first beaker reached 400g the filtration is stopped and a sample of all

permeates and the retentate was taken for future analyses. The beakers were changed, and

filtration is restarted;

4. When the suspension volume is reduced to approximately half, for the first experiment (2

membranes in series) the membrane 1 is regenerated and the second membrane is changed

for a new one. For the second experiment (3 membranes in series), membrane 1 is regenerated,

membrane 2 is held equal and the third membrane is replaced by a new one;

5. After the membranes have passed the required procedure written in point 4, the filtration is

continued in order to obtain in the end between 5 and 10% of the initial volume of suspension

in the feed tank.

6. The membranes were again subjected to regeneration/alteration as explained in step 4. After

this, the tank was filled with water to the initial volume, but the initial volume was reduced by the

sample amount taken previously;

7. Restart the filtration, in diafiltration mode, and the suspension was again filtered until

approximately half of the initial volume;

8. The membranes were again subjected to regeneration/alteration as explained in step 4, and

filtration was restarted and done until the 5 or 10% of the initial volume of suspension is reached.

9. In the end, membranes were flushed with 15wt%ethanol for 2h15 in order to obtain the final flux

of the membranes.

It is necessary to check that all the pressures stay constant during all experiments.

After each experiment, the setup needs to be cleaned with solutions of 50wt% acetone and

15wt% ethanol.

Membrane Regeneration

The experimental work of the membrane regeneration follows the following steps:

1. Membrane 1 is flushed for 1h with a solution of 15wt%ethanol without pressure with a flow rate

of 1.2L/min;

2. After 1 h a sample of concentrate is withdrawn and analyzed in UV-spectrometer. After 30 more

minutes, repeat the analyses and if the value is constant finish the flush, if not, continue flushing

for another 30minutes and repeat until the value is constant, that is, until the membrane can no

longer be cleaned. (for the analyses of the samples in the UV it is necessary to add ethanol until

the mixture has 60wt% of ethanol);

25

3. After this, membrane 1 is again flushed with a 15wt%ethanol solution for 2h15 at 4 bar and

1.2L/min in order to understand how the regeneration, step 2, improved the membrane

performance.

For the calculation of the amount of lignin removed from the membrane and for the membrane

recovery, a calibration curve was prepared using different values of lignin concentration and the

respective absorbances for a wavelength of 280 nm, shown below in Figure 20. These experiments

were repeated to elaborate long-term stability of the regenerated membrane.

Membrane Fouling

After ultrafiltration/diafiltration of nanolignin particles suspension, the membranes are also

flushed to understand how the fouling affects the membranes. The last membrane flush is made with

the same type of solution as used initially, 15wt% ethanol solution. The final transmembrane flux was

then compared with the initial TF to understand how the performance decreases with nanolignin particles

suspension, and whether all membranes behave similarly.

Although the initial TF calculations were obtained with the linear regression slope, for the final

flush, the transmembrane flux was determined based on the first and last 10% of the derivative of the

curve because it does not show linear behavior.

Figure 20 - Calibration curve for lignin content in UV.

26

Analytics

As described before, samples were taken when the first permeate outlet stream reached 400g.

In every system shutdown, samples of the permeate and the concentrate were taken for future analyses.

The diagram shown below, Figure 21, illustrates which analyses were done on each sample.

Dry Matter Content

Dry matter refers to material remaining after removal of liquid. This is a simple method which

allows to compare membranes and how efficient they are. To realize this method, a certain amount (at

least 10g) of sample is collected in a container that is left in an oven at 110ºC for at least 24 hours, being

weighed at the end in order to determine the quantity of solid components in the sample. This is done

for all the permeate samples and for the retentate samples, before centrifugation (dissolved components

with nanolignin particles) and after centrifugation (dissolved components), and the difference gives lignin

particles. Therefore, there is the possibility of having a mass balance of the dissolved components. The

drymatter concentration is calculated based on equation 7:

%𝐷𝑀 =𝑚𝑑𝑟𝑦 𝑠𝑎𝑚𝑝𝑙𝑒

𝑚𝑖𝑛𝑖𝑡𝑖𝑎𝑙

(7)

%𝐷𝑀 − % 𝑜𝑓 𝐷𝑀 𝑐𝑜𝑛𝑡𝑒𝑛𝑡,𝑔𝑙𝑖𝑔𝑛𝑖𝑛+𝑑𝑖𝑠𝑠𝑜𝑙𝑣𝑑 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡𝑠

𝑔𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛

𝑚𝑖𝑛𝑖𝑡𝑖𝑎𝑙 − 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒 𝑏𝑒𝑓𝑜𝑟𝑒 𝑜𝑣𝑒𝑛, 𝑔

𝑚𝑑𝑟𝑦 𝑠𝑎𝑚𝑝𝑙𝑒 − 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒 𝑎𝑓𝑡𝑒𝑟 𝑡ℎ𝑒 𝑜𝑣𝑒𝑛, 𝑔

𝑇𝐿 = 𝐷𝑀𝑏𝑒𝑓𝑜𝑟𝑒 𝑐𝑒𝑛𝑡𝑟𝑖𝑓𝑢𝑔𝑒 − 𝐷𝑀𝑠𝑢𝑝𝑒𝑟𝑛𝑎𝑡𝑎𝑛𝑡 (8)

Figure 21 - Analysis methodology for different samples.

27

𝑇𝐿 − 𝑇𝑜𝑡𝑎𝑙 𝑛𝑎𝑛𝑜𝑙𝑖𝑔𝑛𝑖𝑛 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒𝑠 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑎𝑚𝑝𝑙𝑒,𝑔𝑛𝑎𝑛𝑜𝑙𝑖𝑔𝑛𝑖𝑛 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒𝑠


𝐷𝑀𝑏𝑒𝑓𝑜𝑟𝑒 𝑐𝑒𝑛𝑡𝑟𝑖𝑓𝑢𝑔𝑒 − 𝐷𝑀 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑏𝑒𝑓𝑜𝑟𝑒 𝑐𝑒𝑛𝑡𝑟𝑖𝑓𝑢𝑔𝑒,𝑔𝐷𝑀


𝐷𝑀𝑎𝑓𝑡𝑒𝑟 𝑐𝑒𝑛𝑡𝑟𝑖𝑓𝑢𝑔𝑒 − 𝐷𝑀 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑜𝑓 𝑠𝑢𝑝𝑒𝑟𝑛𝑎𝑡𝑎𝑛𝑡,𝑔𝐷𝑀


Equation 8 is the result of the difference between the DM before and after centrifugation, is the

solid amount of lignin in the sample. The sample before centrifugation corresponds to the total

components in the suspension, and the sample after centrifugation, from which the solid part is

withdrawn, corresponds to the dissolved components.

From the DM of the permeates and the samples after centrifugation, it is possible to make a

mass balance of the dissolved components, calculated based on the equation 9:

(9) 𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠 − 𝐷𝑀 𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝑒𝑎𝑐ℎ 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒, 𝑔.

𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛 − 𝐷𝑀 𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑢𝑝𝑒𝑟𝑛𝑎𝑡𝑎𝑛 𝑜𝑓 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑠𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛, 𝑔.

𝐷𝑀𝑠𝑎𝑚𝑝𝑙𝑒𝑠 − 𝐷𝑀 𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝑟𝑒𝑡𝑒𝑛𝑡𝑎𝑡𝑒 𝑠𝑎𝑚𝑝𝑙𝑒, 𝑔.

𝐷𝑀𝑅13𝑊 − 𝐷𝑀 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑛𝑎𝑙 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑒 𝑟𝑒𝑚𝑎𝑖𝑛𝑖𝑛𝑔 𝑖𝑛𝑠𝑖𝑑𝑒 𝑡ℎ𝑒 𝑡𝑎𝑛𝑘, 𝑔.

Where:

𝐷𝑀𝑥 = ∑ (𝐷𝑀𝑥 × 𝑚𝑥 )𝑖

To find the removal efficiency of the dissolved components from the initial nanolignin particle

suspension, the ratio of DM amount from the permeates and DM amount of dissolved components in

the initial suspension was calculated, equation 10.

𝑅𝐸 = ∑ 𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠

𝑥𝑖=1

𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛 (10)

𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠 − 𝐷𝑀 𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝑒𝑎𝑐ℎ 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒, 𝑔.

𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛 − 𝐷𝑀 𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑢𝑝𝑒𝑟𝑛𝑎𝑡𝑎𝑛 𝑜𝑓 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑠𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛, 𝑔.

Degradation Product

The solutions of nanolignin particles over the experiments present different amounts of

degradation products, it is possible to find ethanol in a large amount and other components such as

𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛 = ∑ 𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠

𝑥

𝑖=1

+ ∑ 𝐷𝑀𝑠𝑎𝑚𝑝𝑙𝑒𝑠 + 𝐷𝑀𝑙𝑎𝑠𝑡 𝑠𝑎𝑚𝑝𝑙𝑒

𝑦

𝑗=1

28

acetic acid, HMF and furfural. One of the goals of this work was to remove impurities and ethanol from

the initial nanolignin particles suspension. To access this information, the samples have been taken

during filtration, permeate and retentate were analyzed in an HPLC, which is capable of identifying

compounds present in any sample that can be dissolved in a liquid in very low concentrations. The

equipment used was from Shimadzu Corporation, that pumps a sample in a solvent (mobile phase) at

high pressure through a column with chromatographic packing material (stationary phase). The

components of the column used were Sugar-SH1011 (Shodex), Guard Column: Sugar SH-G (Shodex),

the detector was the refractive Index, using 0.005M of sulfuric acid as eluent with a flow rate of 0.6

mL/min. The calibration curves obtained are shown in the Figures 22, 23 and 24.

Figure 24 - Calibration curve for Ethanol content.

Figure 23 - Calibration curve for HMF and Furfural content. Figure 22 - Calibration curve for Acetic Acid content.

29

Particle Size

One of the main points of this work was to purify the main product without particle agglomeration.

The particle size measurement was done for all retentate samples and was immediately measured after

the withdrawn sample. The equipment used to measure the particle size was ZetaPals from Brookhaven

Instruments Corporation. Each sample was analyzed twice, one first analysis with the original sample,

and another one using a sample diluted in deionized water with a volume ratio of 1:100. The diluted

samples are more reliable since it is not necessary to resort to correlations, due to the fact that the

viscosity and the refractive index are well known.

31

Results and Discussion

As explained previously, the goal was to increase the concentration of the suspension of

nanolignin particles and remove ethanol and impurities using membrane filtration, ultrafiltration followed

by diafiltration. Also, to compare and characterize membranes behavior and understand if it is possible

to regenerate a membrane to the initial condition. Previously was made membrane filtrations with the

same type of membrane and setup with the difference of the initial amount of feed, 1.2 L, and the amount

to concentrate, 50% concentration. These experiments were made with an initial feed of 5 L with the

goal to concentrate to 5-10 % of the initial suspension.

This chapter is divided into two parts, a first one where the concentration is increased using two

membranes, one of those with regeneration. And the other part of the chapter shows the results using

three membranes in series, with one of the membranes being regenerated, a second membrane used

since the beginning and a third one changed every 50% concentration. Is important to note that

membrane 1 was used previously in another experiment before being used in the next filtrations.

Membrane Filtration – 2 Membranes in Series

Membrane stability

The procedure outlined in earlier section Material and Methods was adopted in this work, all

membranes need to be flushed for a certain time so they can be stable for longer periods. Membranes

are also flushed after being used to compare their performance. It was used solution with 15wt% ethanol

since the suspension of nanolignin particles has a maximum ethanol concentration of 15wt%.

All membranes were flushed for 2h15, three parts of 45 min with a refill of the permeate inside

the feed tank, at 4 bar and 1.2 L/min of flow-rate. Figure 25 represents the mass of permeate over time

for all the membranes used in the first experiment, flushed with different 15 %wt ethanol solutions.

Figure 25 – Initial transmembrane flux (g/min) over time (min) of each membrane (1.2 L/min at 4 bar).

32

Transmembrane Flux

Initially the membranes performance decreases and stabilizes after a certain time, maintaining

that stability for the rest of the operational time. The initial TF was calculated based on the last 20

minutes of the initial flushing within an uncertainty range of 10%. In Table 3, is shown the initial TF

obtained for membranes 1, 2, 3, 4 and 5.

Table 3 - Initial transmembrane flux for each membrane used in the experiment 2 Membranes in

Series.

Membrane Initial TF

(L/(m2.h))

Membrane 1 78.8

Membrane 2 82.3

Membrane 3 98.4

Membrane 4 98.3

Membrane 5 91.5

As expected, the transmembrane flux of membrane 1 is lower since this membrane was used

in a previous experiment. However, the other membranes were all new and yet membrane 2 has a

transmembrane flux lower than the other membranes, being closer to the TF of membrane 1. It’s

expected that membrane 2 has a lower performance compared to the other new membranes, since it

has a similar TF to membrane 1. However, this TF difference is not so significant.

Ultrafiltration/Diafiltration of Nanolignin Particles Suspension

Initially, there was 5L of nanolignin particles suspension inside the feed tank, the first step was

to increase the concentration in ultrafiltration mode followed by diafiltration, to remove impurities and

ethanol. For the diafiltration, the feed tank was filled with water to the initial volume, but the initial volume

was reduced by the amount of the sample so that the lignin concentration be identical to the initial one

in the suspension.

In both modes 2 membranes in series were used, membrane 1 was regenerated and the other

membrane was changed when the feed suspension had a volume reduction of approximately 50%. For

future analysis, it was taken, for every 400g of one of the permeates, samples of the retentate.

Figure 26 and Figure 27, show the transmembrane flux over time for all the membranes. Figure

26 represents the filtration of membrane 1 and the vertical lines represent when the membrane was

regenerated, MR. Figure 27 shows the TF for the other membranes and the vertical lines represent

when there was a change of membrane, MC. The blue color represents the concentration mode and the

orange the diafiltration mode.

33

Figure 26 - Ultrafiltration and diafiltration process for membrane 1 (1.2 L/min at 4 bar).

Figure 27 - Ultrafiltration and diafiltration process for other membranes, 2, 3, 4 and 5 (1.2 L/min at 4 bar).

34

From the graph it is possible to notice that the permeate flux does not happen constantly with

time, the more pronounced variations occur when the pumps are restarted and system pressure

variation occurs, taking some time for the system pressure to remain constant. The pressure increased

even though there was made precautions, like opening the valves slowly.

As explained before, the vertical lines in Figure 27 represent the change of membrane.

Membrane 2 was used since the beginning until 50 % concentration, about 2.5 L. Afterwards membrane

2 was changed for a new one, membrane 3, which was used until the end of the ultrafiltration when the

10% of the suspension was reached. Membranes 4 and 5 were used in diafiltration mode under the

same conditions as membranes 2 and 3, respectively. The membrane regeneration to membrane

occurred at the same time as membrane change, as can been seen in the figures above.

Table 4 - Mean transmembrane fluxes for UF/DF for membrane 1 and other membranes, 2,3, 4 and 5.

UF/DF Step Mean Transmembrane Flux (𝑳

𝒎𝟐.𝒉)

Ultrafiltration (Membrane 1) 39.7

Diafiltration (Membrane 1) 49.1

Ultrafiltration (Membranes 2 and 3) 32.5

Diafiltration (Membranes 4 and 5) 72.3

Diafiltration step for membrane 1 and other membranes, Figure 26 and Table 4, shows a linear

trend contrary to what happens in the concentration step. That means that the amount of dissolved

components that passes through the membrane are constant with the time unlike what happens in

ultrafiltration. For all membranes the transmembrane flux increases when water is added, which is

explained by the fact that when water is added the membrane is flushed, getting cleaner, decreasing

the fouling.

Except for membrane 1 all the other membranes used in the procedure were new but for the

concentration mode the new membranes show a lower transmembrane flux, the difference can be

explained by natural variations of the membrane samples. However, for the diafiltration step membranes

show a higher transmembrane flux, which means the membranes are more susceptible to fouling in UF

mode. In Table 5 is shown the mean TF for each membrane used in experiment with 2 membranes in

series.

Table 5 - Mean transmembrane fluxes for each membrane.


𝒎𝟐.𝒉)

Membrane 1 38.5

Membrane 2 31.6

Membrane 3 33.3

Membrane 4 86.2

Membrane 5 65.5

35

Membrane 1 is the regenerated membrane which presents a higher TF value than the

membranes that are used for the first time (membranes 2 and 3). Even though the regeneration is to

improve the performance of a used membrane, it was expected to have a lower transmembrane flux

than a new membrane.

Membranes 2 and 3 were new despite having a lower TF when compared to membrane 1.

Nevertheless, when comparing the initial flush it was already expected that the performance of

membrane 2 should be similar to membrane 1. This membrane was probably damaged by the process

system.

In the diafiltration mode, membranes 4 and 5 show an increase of TF which indicates that the

membrane gets cleaner, decreasing the fouling. Also, when the suspension gets more concentrated,

the transmembrane flux decreases which means the concentration affects the membrane performance.

In the concentration mode it was not possible to understand if the concentration affects the

transmembrane flux.

Membrane Fouling

Fouling results in a decrease in permeate flux and an increase in hydraulic resistance that is

inversely proportional to the flux. As explained in Material and Methods, fouling can occur due to the

absorption of feed components, accumulation of particles in the pores of the membranes, formation of

a layer on the membrane surface due to the continuous deposition of particles and a gel layer formed

because of concentration polarization. As described, membranes were also flushed with 15wt% ethanol

after the experiments with the suspension in order to be able to evaluate the fouling effect.

Figure 28 - Transmembrane Flux (g/min) over time (min) for the final flush of each membrane (1.2 L/min at 4 bar).

36

It is perceptible from Figure 28 that membranes 4 and 5 have higher fluxes when compared with

the other two membranes (membranes 1 and 2). When compared with the other membranes, membrane

5 shows a different performance in the flushing, during the first and last part of the flush (approximately

the first and last 45 minutes) this membrane showed a higher increase of the permeate mass.

Contrary to what happens to the initial flush (during this flush), the membrane will change again,

particles or precipitated lignin might be washed away. The TF was calculated based on the first minutes

of the final flushing with ethanol/water. In Table 6 is shown different values obtained for the

transmembrane flux of each membrane, and the respective flux decline.

As expected, membranes 4 and 5 are the ones with higher final transmembrane flux, since they

were used only in the diafiltration mode. The other membranes have a similar final transmembrane flux,

which means that the filtration of the nanoparticle suspension similarly affects the membranes in

concentration mode.

It is expected that the membranes behave the same way since they present the same material

and MCWO. For each filtration mode (ultrafiltration and diafiltration) the membranes have the same

MCWO, so the amount of dissolved components crossing the membranes should be the same and the

polarization effect should affect them equally. For all membranes, the transmembrane flux is decreasing

over time mainly because of the increase of the layer thickness on the membrane’s surfaces. The

concentration of the particles gradually increases on the surface of the membrane, due to solute

accumulation from the convective flow.

DF leads to significantly lower flux decline. As particle concentrations are similar in UF and DF,

the flux decline is mainly influenced by dissolved components which are reduced in DF.

Analytics

Previously, it was said that samples of retentate and permeates were taken. It was decided that

when the first permeate stream reached approximately 400g, the membrane filtration was stopped, and

the samples were taken and labeled. The code of each sample is shown in Table 7.

Table 6 - Initial and final transmembrane flux each membrane and the respective flux decline for the

experiment 2 Membranes in Series.

Membranes

Initial TF (𝑳

𝒎𝟐.𝒉) Final TF (

𝑳

𝒎𝟐.𝒉) TF decline (%)

Membrane 1 78.8 53.2 32.5

Membrane 2 82.3 60.3 26.7

Membrane 3 98.4 59.4 39.6

Membrane 4 98.3 96.5 1.8

Membrane 5 91.5 73.1 20.1

37

Table 7 - Samples labeling code for the experiment 2 Membranes in Series.

Code Name Sample Step of filtration

S Initial Suspension Suspension after precipitation

R1 1st retentate 1st sample after 764.3g filtred

P1M1 1st permeate 1st sample after 400.1g of permeate membrane 1


R2 2nd retentate 2nd sample after more 764.8g filtred

P2M1 2nd permeate 2nd sample after 399.9g of permeate membrane 1


R3 3rd retentate 3rd sample after more 917.9g filtred

P3M1 3rd permeate 3rd sample after 489.4g of permeate membrane 1


R4 4th retentate 4th sample after more 737.1g filtred

P4M1 4th permeate 4th sample after 336.9g of permeate membrane 1


R5 5th retentate 5th sample after more 734.7g filtred



R6 6th retentate 6th sample after more 480g filtred



Addition of 4687g of Water (Initial suspension mass reduced by the amount of samples)

R7W 7th retentate 7th sample after 810.8g filtred after water addition

P7M1W 7th permeate 7th sample after 295.1g of permeate membrane 1


R8W 8th retentate 8th sample after more 621.3g filtred





P9M4W 9th permeate 9th sample after 401g of permeate membrane 4













38

Degradation Products

Degradation components like ethanol, acetic acid, HMF, and furfural were analyzed since one

of the goals is to remove ethanol and impurities from the nanoparticle’s suspension. For these products’

analysis HPLC (High-Performance Liquid Chromatography) was used.

Figure 29 - Ethanol content (mg/L) of the retentate samples for the experiment 2 Membranes in Series.

Figure 30 - Acetic Acid, HMF and furfural content (mg/L) of the retentate samples for the experiment 2 Membranes in Series.

Diafiltration Ultrafiltration


39

Figure 29 shows the ethanol content whereas, Figure 30 shows the HMF, Furfural, and Acetic

Acid content. It is expected that at the end only 10-15% of the concentration of the initial suspension

remain. The concentration of the components was not expected to vary greatly for each filtration step.

During volume reduction the ethanol concentration is constant and only drops when refilling with water.

The ethanol content in sample R4 is an error, as samples R5 and R6 show high and relatively similar

ethanol contents like samples R1, R2 and R3.

After the addition of water, the concentration of ethanol decreases as expected since for the

same concentration of ethanol a large volume of water was added, resulting in a decrease in ethanol

content. In the first three samples there are no major changes except for the ethanol content not being

constant, which is explained by the fact that the samples that are being taken are not being considered,

also possible evaporation of ethanol may occur and even errors associated with the analysis.

The ethanol content decreased significantly in sample R10W when compared to R9W, the

difference between them was when the samples were measured, since the HPLC has a limited space

sample.

The same way as the ethanol content, the other degradation products content drops after the

addition of water. However, for sample R7W only acetic acid remains in the samples which disappears

after sample R10W, where there is a membrane change (membrane 4 to membrane 5). Even though

the membrane changes this is not a reason for such decline, since the content should maintain constant

during the filtration step. However, the last samples where measured in a different batch which may

explain the nonexistence of content in the last samples due to evaporation.

To conclude, it’s necessary to make sure how much of the degradation products are being

removed from the nanoparticle’s suspension. From previous experiments made in the laboratory, it was

concluded the amount of each component that was present in straw at 180ºC, the values are presented

in Table 8.

Table 8 - Degradation products characterization of straw at 180ºC.

Component Quantity (𝒎𝒈

𝑳)

Acid Acetic 1282.5

Ethanol 545250

HMF 4.4

2-Furaldehyde 34.1

For this experiment (2 Membranes in Series), it’s not possible to conclude how good is the

removal in relation to the HMF and furfural, since after the water addition these components do not

present any concentration value. Ethanol has a removal percentage of 99.8% and the acid acetic of

86.8%. As explained before, after sample R9W there are some errors associated and the decrease of

the content is not anticipated, which can explain such high removal percentage for the ethanol. If the

last samples were ignored, and for this calculation was used the last content value credible (R9W), the

removal of ethanol would be 97% (closer to the assumption value).

40

Particle Sizing

One of the most important parts of the work was to ensure that there was no particle

agglomeration, this process would only be feasible if this did not happen. For this purpose, particle size

measurement was performed on all samples using ZetaPals from Brookhaven Instruments Corporation.

In Figure 31 is shown the different measurements made during the filtration process for each

sample. Each sample was measured twice, one was diluted and the other one was concentrated. For

the diluted sample, the properties were already defined while for the concentrated samples it was

necessary to measure the density and refractive index of each sample, since the ethanol content after

water addition is lower and therefore, the viscosity changes.

To evaluate the particle size, it is more reliable to analyze the values of the diluted samples,

since the viscosity and refractive index of water are well known, while for the concentrate it is necessary

to resort to correlations. However, it is possible to determine that the particle size is approximately

constant over time.

Dry Matter Content

The Dry Matter method was used for all samples despite being a method with many errors. The

method was applied for all samples, retentate and permeate. For the retentate samples, the method

was applied before and after centrifugation, and the results are shown in figure 32.

Figure 31 - Particle size for all retentate samples for the experiment 2 Membranes in Series.

41

Samples before the centrifuge (dissolved components with lignin particulates), should always

have higher values of DM than the samples after centrifugation (dissolved components), since the

nanolignin particles are removed, leaving only the supernatant. The difference between them gives

particulate lignin. Therefore, sample R7W must be discarded, the error was probably caused by the fact

that a small sample was used for the method.

Samples after centrifugation show that the dissolved components content is approximately

constant during UF mode, which means there is no retention of dissolved components, while in

diafiltration, the content of dissolved components is rising slightly, which means there is retention of

dissolved components.

Samples before centrifugation show an increase of lignin particles during each of the filtration

steps and the drop after adding water.

Removal efficiency for Dry Matter method

To understand how efficient the membranes were, the Dry Matter method was also executed

for the permeate samples since the biggest error of this method is due to the small amount used from

the retentate samples, as explained in Material and Methods. For this method it is needed at least 10g

of sample to obtain credible values.

Figure 32 - DryMatter content (%) for all retentate samples, before and after centrifuge for the experiment 2 Membranes in Series.

42

To calculate the removal efficiency of dissolved components following equation 10, the DM of

the permeates were used, Figure 33, and also the DM of the initial suspension, after centrifugation.

All the membranes show an increase in DM during UF, but during DF this is not the case. It is

possible to conclude that the retention of DM is decreasing during UF, maybe due to an increase in DM

content in the retentate samples. However, in DF the DM content in the retentate samples is

approximately constant.

In Figure 33 is shown the variation of Dry Matter content (%) for the different membranes

permeate.

Where,

𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠 = ∑ (𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒 𝑖 × 𝑚𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒 𝑖)𝑖

𝐷𝑀𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛 = 𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛 × 𝑚𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛

The DM amount was based on the sample amount multiplied per the percentage of DM content

(𝑔𝑙𝑖𝑔𝑛𝑖𝑛+𝑑𝑖𝑠𝑠𝑜𝑙𝑣𝑒𝑑 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡𝑠

𝑔𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛). The membranes removed 85% of the dissolved components from the

nanolignin particle suspension, a result very close to the one intended (90-95%). From the dry matter,

it is also possible to perform a mass balance to the filtration system, based on the equation 9.

𝑅𝐸(%) =∑ 𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠

26𝑖=1

𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛 = 85,2%

𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛 = ∑ 𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠

26

𝑖=1

+ ∑ 𝐷𝑀𝑠𝑎𝑚𝑝𝑙𝑒𝑠 + 𝐷𝑀𝑅13𝑊

12

𝑗=1

Figure 33 – Dry Matter content (%) for all permeate samples for each membrane for the experiment 2 Membranes in Series.

43

𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠 − 𝐷𝑀 𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝑒𝑎𝑐ℎ 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒, 𝑔.

𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛 − 𝐷𝑀 𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑢𝑝𝑒𝑟𝑛𝑎𝑡𝑎𝑛𝑡 𝑜𝑓 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑠𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛, 𝑔.

𝐷𝑀𝑠𝑎𝑚𝑝𝑙𝑒𝑠 − 𝐷𝑀 𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝑟𝑒𝑡𝑒𝑛𝑡𝑎𝑡𝑒 𝑠𝑎𝑚𝑝𝑙𝑒, 𝑔.

𝐷𝑀𝑅13𝑊 − 𝐷𝑀 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑛𝑎𝑙 𝑠𝑎𝑚𝑝𝑙𝑒 𝑟𝑒𝑚𝑎𝑖𝑛𝑖𝑛𝑔 𝑖𝑛𝑠𝑖𝑑𝑒 𝑡ℎ𝑒 𝑡𝑎𝑛𝑘, 𝑔.

The mass balance is made to the dissolved components, as such, the values used are relative

to those of the supernatant after centrifugation. The values obtained for each part of the equation 9 are

represented in Table 9.


experiment 2 Membranes in Series.

𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛 (𝒈) 7.39

𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠 (𝑔) 6.29

𝐷𝑀𝑠𝑎𝑚𝑝𝑙𝑒𝑠 (𝑔) 0.73

𝐷𝑀𝑅13𝑊 (𝑔) 0.12

𝑀𝑎𝑠𝑠 𝐵𝑎𝑙𝑎𝑛𝑐𝑒 (𝑔) 0.25

𝑆𝑐𝑎𝑙𝑒 𝐸𝑟𝑟𝑜𝑟 (𝑔) 3.4

Since the mass balance to the dissolved components does not close, this means that there are

losses in the system, possibly deposition on the membranes.

Membrane Filtration – 3 Membranes in Series

Membrane Filtration – Flux and Concentration Experiment

The next step was membrane filtration with 3 membranes in series. When compared with the

previous experiment with 2 membranes in series, the difference relies in the addition of a membrane

that is never altered or regenerated. However, prior to filtration, there was the need to understand if the

flow was decreasing due to increased concentration. As in the first experiment the transmembrane flux

was decreasing, and so the question of whether it was decreasing because of the increase in scale or

increase in concentration remained.

Three experiments (experiment 1, 2 and 3) were conducted with an initial suspension of 5L,

which were filtered to 50% of the initial volume, approximately 2.5L. This was repeated three times, with

refilling of the permeate into the feed tank so that the lignin concentration did not change. The results

obtained are shown below, from Figure 34 to Figure 36, together with the mean transmembrane flux

44

values for each membrane, in each experiment, Table 10. Membrane 1 is regenerated, membrane 6 is

held equal and the third membrane is repeatedly replaced by a new one.

Experiment 1 of the flux and concentration experiments, there is a gap in membrane 6 due to

some complications with the scale used. The scale stopped recording at 74g and restarted at 510g, that

is why there is a difference in the time of this membrane compared to the others. The TF was calculated

based on the permeate mass and time after the scale restarted, the first values were ignored.

Figure 34 - Experiment 1 of Flux and Concentration Experiment, using membrane 1, 6 and 7 (1.2 L/min at 4 bar).

Experiment 1


Experiment 2

45

In experiment 2 the program crushed, and the setup was immediately stopped. The peaks are a result

of stopping the pumps and turning them on again. In Figure 36 is represented experiment 3, where the

same problem occurred.

Table 10 - Transmembrane flux for all membranes for each experiment.

The three filtration sets lasted approximately 20 hours where 2.1L of volume filtrated in each

filtration step. From the results, it was verified that the transmembrane flux decreases with the increase

of the suspension’s concentration, since there is no decrease in the transmembrane flux while the lignin

concentration stays constant. In fact, the fouling of the membrane did not result in flux decline since

none of the membranes reveals a large change in the transmembrane flux from experiment to

experiment. In fact, membrane 6 has a gradual but not very significant increase while membrane 1 has

Experiment 1 Mean TF (𝑳

𝒎𝟐.𝒉) Experiment 2 Mean TF (

𝑳

𝒎𝟐.𝒉) Experiment 3 Mean TF (

𝑳

𝒎𝟐.𝒉)

Membrane 1 9.8 Membrane 1 8.8 Membrane 1 11.9




Experiment 3

46

an oscillating flow value and the third membrane module (membranes 7, 8 and 9) have different values.

However, the third module is not relevant because it is known that two membranes under the same

conditions do not behave in the same way.

After these experiments to confirm the reason for the decrease of the flux, ultrafiltration and

diafiltration were again applied, to concentrate the suspension of nanoparticles and remove the

impurities and ethanol. For this, three membranes were used in series. A first membrane, membrane 1,

that was the same as the one used in the previous experiments (regenerated membrane). Membrane

6, which was initially a new membrane but was then used throughout the procedure without any change

and a third membrane that is replaced by a new one whenever 50% of the suspension is filtered.

Membrane stability

As was done for the other procedure, the membranes used were subjected to an initial flush

with a solution of 15wt% ethanol. For better understanding, the membranes curves were divided in two

different diagrams, Figure 37 and Figure 38. These figures show the curves obtained for each

membrane and Table 11 shows the mean initial transmembrane flux for each membrane.

Figure 37 - Initial Transmembrane Flux (g/min) over time (min) for membranes 1, 6 and 10 for the experiment 3 Membranes in Series (1.2 L/min at 4 bar).

47

The behavior of membranes 6, 10, 11 and 12 is physically unexpected, since the operating

conditions of these membranes were identical when compared to the others. They were measurement

errors, maybe due to not having constant pressure or to accumulation of permeate in the permeate hose

connected to the scales. For these membranes, the initial TF was calculated based on the average over

the whole time. However, the TF of membrane 1 and 13 was calculated based on the last few minutes

of flushing. Table 10 shows the linear regression of each membrane and the respective initial TF within

an uncertainty range of 10%.

Table 11 - Initial TF for each membrane for the experiment 3 Membranes in Series.

Membrane Initial TF (L/(m2.h))

Membrane 1 14.9

Membrane 6 225.8

Membrane 10 335.3

Membrane 11 358.6

Membrane 12 418.6

Membrane 13 134.7

The new membranes used, 10, 11, 12 and 13 were from a different pack of membranes, which

may explain this significant difference of the initial flux to the membranes used before. However, this

deviation is critical for future work since membranes may exhibit such dissimilar capacities. The

experiment should be repeated several times with different membrane samples in order to be possible

Figure 38 - Initial Transmembrane Flux (g/min) over time (min) for membranes 11, 12 and 13 for the experiment 3 Membranes in Series (1.2 L/min at 4 bar).

48

to calculate mean values to reach statistically relevant conclusions, since the membrane performance

parameters are highly fluctuant.

Membrane 1 presents a higher value of initial TF than the mean value obtained in experiment 3

of the flux and concentration experiment. This is due to the fact that that membrane has been

regenerated between experiments.

Ultrafiltration/Diafiltration of Nanolignin Particles Suspension

The initial suspension for this experiment was the same as used in the flux and concentration

experiment. The initial volume was 5L as in the first experiment with 2 membranes in series, and the

goal was to increase the concentration in ultrafiltration mode and remove ethanol and impurities in

diafiltration mode. The procedure was the same, filter up to 10% volume of suspension with membrane

change and membrane regeneration every time the volume is reduced by 50%.

The difference now is the number of membranes used, it was used 3 membranes in series, a

first membrane (membrane 1) with regeneration. A second membrane (membrane 6) that is never

changed and a third one that it is changed every time membrane 1 is regenerated. The graphs below,

Figure 39 to Figure 40 show the performance of the membranes for each step, ultrafiltration (UF) and

diafiltration (DF). Figure 39 represents the fitration of membrane 1, and the vertical line (MR) represents

when the membrane was regenerated. This membrane regeneration occurs at the same time as the

membrane is changed in the third module, which is represented in Figure 40. Figure 41 represents

membrane 6 which is never changed or regenerated.

Figure 39 - Transmembrane Flux (g/min) over time (min) for membrane 1 for the experiment 3 Membranes in Series (1.2 L/min at 4 bar).

49

Figure 40 - Transmembrane Flux (g/min) over time (min) for the other membranes, 10, 11, 12 and 13 for the experiment 3 Membranes in Series (1.2 L/min at 4 bar).

Figure 41 - Transmembrane Flux (g/min) over time (min) for membrane 6 for the experiment 3 Membranes in Series (1.2 L/min at 4 bar).

50

The pressure was always being controlled, however there was a certain fluctuation specially

after the system was restarted, which results in peaks from turning on the pump after temporary

shutdown for sampling. After some time, the membrane returns to its initial behaviour, before the

withdrawn sample due to pressure peaks during pump startup.

From the graphs it is possible to perceive that when there is membrane change there is also the

regeneration of membrane 1. It is possible to see the effects of the regeneration, where the first

regeneration in this filtration happens at minute 400, having as a consequence the increase of the

transmembrane flux almost till the initial flux. Table 12 shows the means TF for each filtration step.

Table 12 - Mean TF for each filtration step for the experiment 3 Membranes in Series.

UF/DF Step Mean Transmembrane

Flux (𝑳

𝒎𝟐.𝒉)

Ultrafiltration Membrane 1 10.0

Diafiltration Membrane 1 18.1

Ultrafiltration Membrane 6 17.0

Diafiltration Membrane 6 24.3

Ultrafiltration Other Membranes (Membranes 10 and 11) 27.3

Diafiltration Other Membranes (Membranes 12 and 13) 70.5

For this filtration, the flow varies the same way as in the first filtration with 2 membranes in series.

After the water addition, the transmembrane fluxes increase for all membranes which can be explained

by the dissolved components being reduced in DF.

The graph represents the way the filtration worked, a first step, where 3 membranes were used

in series. Membranes 1, 6 and 10 were used up to minute 400. Between minute 400 and approximately

minute 700, membrane 1 (after regeneration), 6 and 11 were used. Membrane 1 (after regeneration), 6

and 12 were handled from minute 700 to 900, and until the end membrane 1, 6 and 13 were used.

Table 13 - Mean TF of each membrane used in the filtration for the experiment 3 Membranes in Series.


𝒎𝟐.𝒉)

Membrane 1 14.3

Membrane 6 21.7

Membrane 10 35.5

Membrane 11 32.0

Membrane 12 85.9

Membrane 13 44.2

51

In Table 13 is shown the transmembrane flux of all membranes used in this experiment.

Membranes 10, 11, 12 and 13 have the highest fluxes because they are new.

When comparing all the membranes used, membrane 6 is the one without regeneration and has

a higher mean TF than membrane 1, as can be seen in Table 13. So, this filtration process is not enough

to create a thick layer as dense as the one on the surface of membrane 1. Although membrane 6 is not

regenerated, it had better fluxes right from the start. From Figure 40 and Table 13 membrane 1 had a

TF of 90 (L/m2.h) while membrane 6 had a TF of 345 (L/m2.h).

During the filtration in UF mode (membranes 1, 6, 10 and 11) it is possible to see that while the

filtration occurs the TF reduce, since the concentration of nanolignin particles is increasing and because

of membrane fouling. When the filtration changes from UF mode to DF (membranes 1, 6, 12 and 13),

the flux increases considerably due to the lower concentration of dissolved components.

Membrane Fouling

At the end of the filtration, the membranes were again subjected to a flush with a 15wt% ethanol

solution. The TF of the membranes was divided in two graphs for a better viewing.


experiment 3 Membranes in Series (1.2 L/min at 4 bar).

52

In Figure 42 is represented the final transmembrane flux of membrane 1, 6 and 10. As was

previously said, membrane 1 is the one with regeneration, membrane 6 is never changed and a new

membrane, membrane 10, used during UF. Figure 43 represent membrane 11, 12 and 13, while these

last two were used in diafiltration, membrane 11 was used during UF.

Table 14 - Initial and Final transmembrane flux for each membrane for the experiment 3

Membranes in Series.

Membranes

Initial TF (𝑳

𝒎𝟐.𝒉) Final TF (

𝑳

𝒎𝟐.𝒉) TF decline (%)

Membrane 1 14.9 12.9 10.8

Membrane 6 225.8 16.7 95.1

Membrane 10 335.3 34.0 89.4

Membrane 11 358.6 70.6 84.6

Membrane 12 418.6 61.4 88.1

Membrane 13 134.7 46.6 82.5

Figure 43 - Final Transmembrane Flux (g/min) over time (min) for membrane 11, 12 and 13 for the experiment 3 Membranes in Series (1.2 L/min at 4 bar).

53

In Table 14 is shown the different transmembrane fluxes for each membrane and the respective

flux reduction. The last 20 minutes of the final flushing with ethanol/water were used for the final TF

calculation due to particles or precipitated lignin that might be washed away.

Except for membrane 11, the final flux is significantly similar when compared to the mean TF.

As what happened in the experiment using two membranes, the diafiltration mode has higher final

transmembrane flux than membranes used in concentration mode. Membrane 1 no longer shows a

significant decrease of the flow, what can mean that a membrane can be regenerated several times and

after a certain point it will be stable, but this is only possible to prove if more experiments are done with

the same membrane.

Analytics

Table 15 describes each sample and where each membrane is used. The code of each sample

is similar, with the difference that for each round of samples there is one more permeate because there

is one more membrane module.

Table 15 - Sample labeling for the experiment 3 Membranes in Series.

Code Name Sample Step of filtration

S Initial Suspension Suspension after precipitation

R1 1st retentate 1st sample after 816.8g filtred




R2 2nd retentate 2nd sample after 816.9g filtred




R3 3rd retentate 3rd sample after 784.6g filtred




R4 4th retentate 4th sample after 769.2g filtred




R5W 5th retentate 5th sample after 1031.5g filtred




Addition of 4797g of Water (Initial suspension mass reduced by the amount of samples)







54

Table 16 - (continuation) Sample labeling for the experiment 3 Membranes in Series.







R9W 9th retentate 9th sample after 781g filtred








Degradation Products

The method of High-Performance Liquid Chromatography was once more used for the

determination of the degradation products, like, ethanol, acetic acid, HMF, and furfural. Figure 44 and

Figure 45 show the results obtained for each product in each step of filtration.


Figure 44 - Ethanol content (mg/L) for all retentate samples for the experiment 3 Membranes in Series.

55

Figure 44 and Figure 45 show the acetic acid and ethanol content in the retentate samples. As

explained before, the goal is to reduce the amount of degrading compounds at the end. For all of them,

it is expected that at the end there is 10% of the initial concentration of the initial suspension. For each

filtration step it is not expected that the concentration of ethanol varies. During volume reduction the

ethanol concentration is constant and only drops when the water is added.

For permeate and retentate samples, it is expected that for all components the concentration

decreases after the water addition, which only occurs after sample number 5. As the total amount of

ethanol and acetic acid remain the same for a bigger volume, the ethanol concentration in the

suspension decreases. However, between sample 6 and 7 there is no difference in the system that can

explain such a high decrease of the ethanol content. One possible explanation is the evaporation of

ethanol and some errors associated with the analysis.

The retentate sample number R7W shows a sudden drop in the ethanol content. From that

sample on, the ethanol content is not measured except for samples R8W and R10W, which can be

explained by some errors in the equipment such as not flushing the samplings line sufficiently. Whereas

in the permeates samples only sample R9W has no value. However, the last measurement compared

with the ones before (R6W to R8W) presents a higher content of ethanol, which should be like the

samples before since its expected that the total amount of ethanol stays constant for each filtration step.

As for the experiment with 2 Membranes in Series, it is necessary to calculate the removal

percentage of the degradation products. Taking in account the same characteristics of the suspension

analyzed in the laboratory (Table 8), the values obtained for the ethanol and acetic acid are 97.9% and

87.7%. The deviation from the expected value (90-95%) can be answered by the errors that happened

during the measurements, as explained.


Figure 45 - Acetic Acid content (mg/L) for all retentate samples for the experiment 3 Membranes in Series.

56

Particle Sizing

The particle size was measured again for every different retentate sample, the values obtained

for diluted samples vary between 138.2±3.3 nm and 212.7±11 nm. In this case, the difference between

the nanoparticle’s diameter is higher but is still reliable. This increase may be related to the longer time

interval between taking the sample and analyzing it.

Dry Matter Content

The Dry Matter method was repeated at the same conditions, and the results are shown in

Figure 47.

Figure 46 - Particle Size measurements for all samples for the experiment 3 Membranes in Series.

Figure 47 - Dry Matter content (%) for all retentate samples, before and after centrifuge for the experiment 3 Membranes in Series.

57

In this filtration, the method functioned without errors because the DM of dissolved components

present higher values than the DM of nanolignin particles. Considering the DM before centrifugation,

(nanolignin particles) an increase of the particles during each step of filtration is observed for UF and

DF. In the DF there is a drop in the DM of the nanolignin particles, which is expected due to the addition

of water. The dissolved components are show a slight increase during UF and DF, which means that

some of the dissolved components are being retained in the system.

Removal efficiency for Dry Matter method

To calculate the efficiency of the removal of dissolved components, the mass balance

calculation was repeated. The mass of the samples and the percentage of DM (𝑔𝑙𝑖𝑔𝑛𝑖𝑛+𝑑𝑖𝑠𝑠𝑜𝑙𝑣𝑒𝑑 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡𝑠

𝑔𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛)

were once again used for those calculations. the mass balance was calculated based on equation 9,

with the slight difference that the number of samples is now higher, and the last remaining sample in the

tank is 𝐷𝑀𝑅10𝑊. In Table 17 is shown the value of each parcel of the equation for the mass balance

calculation.

Table 17 - DM amount (g) of different samples used for the mass balance of the filtration system for the experiment 3 Membranes in Series.

𝑫𝑴𝑺𝒖𝒑𝒆𝒓𝒏𝒂𝒕𝒂𝒏𝒕 𝒐𝒇 𝒊𝒏𝒊𝒕𝒊𝒂𝒍 𝒔𝒖𝒔𝒑𝒆𝒏𝒔𝒊𝒐𝒏 (𝒈) 7.14

𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠 (𝑔) 6.68

𝐷𝑀𝑠𝑢𝑝𝑒𝑟𝑛𝑎𝑡𝑎𝑛𝑡 𝑠𝑎𝑚𝑝𝑙𝑒𝑠 (𝑔) 0.47

𝐷𝑀𝑃13𝑊 (𝑔) 0.24

𝑀𝑎𝑠𝑠 𝐵𝑎𝑙𝑎𝑛𝑐𝑒 (𝑔) -0.26

𝑀𝑎𝑠𝑠 𝐵𝑎𝑙𝑎𝑛𝑐𝑒 𝑒𝑟𝑟𝑜𝑟 (%) 3.7

Once more the mass balance is made for the dissolved components and it does not close. This

error is negative, which means more DM is leaving the system than contained in the initial suspension.

For the removal efficiency of dissolved components, equation 10, the DM of the permeates was used to

get more accurate values. In Figure 48 is shown how the Dry Matter content varies in the whole filtration

process.

58

All the membranes show an increase in DM during ultrafiltration. However, this increase is not

so pronounced during DF. The retention of DM is decreasing in both filtration modes, which can be due

to an increase of DM content in the retentate.

When using equation 10, the removal efficiency of dissolved components from the initial

nanolignin particle suspension was 93.6%. Comparing with the 85.2% obtained in the experiment with

2 membranes, the removal efficiency now is higher (using 3 membranes), which means it is even closer

to the goal.

Membrane Regeneration

Membrane 1 was regenerated ten times, and in Table 18 is shown in each step when the

regeneration happened.

Table 18 - Regeneration steps for membrane 1.

Regeneration Step of Regeneration

1st Middle of ultrafiltration step in the filtration with 2 membranes in series

2nd End of ultrafiltration step in the filtration with 2 membranes in series

3rd Middle of diafiltration step in the filtration with 2 membranes in series

4th After membrane filtration with 2 membranes in series

5th Between Experiment 1 and 2 of Flux and Concentration experiment

6th Between Experiment 2 and 3 of Flux and Concentration experiment

7th Before filtration with 3 membranes in series

8th Middle of ultrafiltration step in the filtration with 3 membranes in series

9th End of ultrafiltration step in the filtration with 3 membranes in series

10th Middle of diafiltration step in the filtration with 3 membranes in series

Figure 48 - DryMatter content (%) for all permeate samples for the experiment 3 Membranes in Series.

59

As explained in Material and Methods, after each experiment, membrane 1 was cleaned with a

15wt% ethanol solution to be regenerated. Samples were taken to analyze in the UV-spectrometer,

when the value was constant, the regeneration reached an end, which means that it is not possible to

remove more lignin from the membrane. The amount of lignin removed in each regeneration steps is

shown in Table 19.

Table 19 - Lignin removed from membrane 1 in each step.

Figure 49 shows the mean TF of each flush after membrane regeneration. From the figure, it is

perceived that until the fourth regeneration (referring to the first experiment with 2 membranes in series)

there is a decrease in the transmembrane flux. The flux and concentration experiment was the one that

revealed to have quite a significant change in membrane 1. After this, the transmembrane flux remains

Regeneration

Step

UV

Lignin removed from

the membrane (mg)

1st 0.041 4.3

2nd 0.054 5.5

3rd 0.028 3.1

4th 0.02 2.3

5th 0.032 3.5

6th 0.04 4.3

7th 0.029 3.2

8th 0.042 4.5

9th 0.029 3.2

10th 0.025 2.9

Figure 49 - Mean transmembrane flux for each regeneration step with 15 %wt ethanol solution.

60

approximately constant, so it is difficult to conclude whether the membrane was affected by this

intermediate experiment or if the membrane reached its stabilization and conditioning characteristics.

To better understand how each regeneration affects the membrane, the mean flux value after

regeneration was compared to the initial value of TF. The comparison is shown in Figure 50 and the

percentage of membrane recovery in Table 20.

Table 20 - Membrane recovery (%) for each regeneration step.

Regeneration

Step

1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th

% Membrane

Recovery

66.0 66.3 57.7 54.3 20.8 21.4 18.9 22.0 18.2 22.3

Comparing the average flow value of the last filtering hour before its regeneration (Before

Regeneration) with the mean flux value after regeneration (After Regeneration), the third, fourth and

tenth regenerations present the lowest cleaning efficacy. These three regenerations take place after the

diafiltration steps, where the flux decline is low and consequently the regeneration is less effective.

Therefore, it will be necessary to make a cost analysis in order to confirm if the regeneration of the

membranes in the diafiltration step is worthwhile.

Regarding the membrane recovery, the TF after each regeneration was compared to the initial

TF of membrane 1. The highest recovery obtained was approximately 66% of the initial capacity of the

membrane, and the lowest recovery was 18%. It is also significant to mention that the membrane

recovery is roughly constant, although after the fourth regeneration the recovery percentage decreases

significantly. This drop was mentioned previously, and it was due to an experiment that affected the

membrane or the membrane that reached its stabilization and conditioning characteristics. Finally, the

Figure 50 - Initial TF and TF before and after regeneration of membrane 1.

61

working time and degeneration of the membranes were calculated, these values are shown in Table 21,

21 and 22.

Table 21 - Characteristics of membrane 1.

Initial Flux (𝑳

𝒎𝟐.𝒉) 89.4

Final Flux (𝑳

𝒎𝟐.𝒉) 13.9

Work time (min) 3615.3

Work time (h) 60.3

% Reduction 84.4

Flux decrease per hour (𝑳

𝒎𝟐.𝒉𝟐) 1.25

Table 22 - Characteristics of membranes used in experiment 2 Membranes in Series.

Table 23 - Characteristics of membranes used in experiment 3 Membranes in Series.

Membrane 2 Membrane 3 Membrane 4 Membrane 5

Initial Flux (𝑳

𝒎𝟐.𝒉) 82.3 98.4 98.3 91.5

Final Flux (𝑳

𝒎𝟐.𝒉) 60.3 59.4 96.5 73.1

Work time (min) 320.2 253.5 141.3 146.8

Work time (h) 5.3 4.2 2.4 2.4

% Reduction 26.7 39.6 1.8 20.1

Flux decrease per

hour (𝑳

𝒎𝟐.𝒉𝟐)

4.1

9.2

0.8

7.5

Membrane 6 Membrane 10 Membrane 11 Membrane 12 Membrane 13

Initial Flux (𝑳

𝒎𝟐.𝒉) 225.8 335.3 368.6 418.6 134.7

Final Flux (𝑳

𝒎𝟐.𝒉) 16.7 34.0 70.6 61.4 46.6

Work time (min) 2022.7 368.8 302.2 194.0 158.9

Work time (h) 33.7 6.1 5.0 3.2 2.6

% Reduction 92.6 89.9 80.8 85.3 65.4

Flux decrease per

hour (𝑳

𝒎𝟐.𝒉𝟐)

6.2

49.0

59.2

110.5

33.3

63

Conclusions

The aim of this work was to evaluate the performance and the potential of regeneration of

membrane performance during ultrafiltration of nangolignin suspensions in diafiltration mode. For this

study only one type of straw was used (wheat straw), and as the literature explains, the results are

dependent on the type of feedstock, as well as the pretreatment used (ethanol Organosolv). The

precipitation method also influences the type of particles that are obtained, specific shapes and particle

sizes. For the precipitation, water was used as an anti-solvent at an exact volume ratio.

This study allowed the successful analysis of the variation of the flux over time, which allowed

the understanding of how the membranes are affected by the suspension used. To implement

successfully a membrane filtration setup, it was important to select optimal operation conditions (1.2

L/min and 4 bar) at room temperature and use an ideal solvent to clean the setup (50wt% acetone and

15wt% ethanol).

Regarding the viability of this procedure, it was imperative that the particle size remained

constant. The particle size for both experiments showed that the nanoparticles of lignin were

approximately constant over time, which confirms that there is no agglomeration of the particles through

the process. However, the time between the sample collection and the measurements should be shorter

in order to minimize the possibility of errors. An additional goal was to remove the ethanol and the

impurities from the nanoparticle’s suspension, which was analyzed using HPLC. The results showed a

removal of ethanol of approximately 97% and acetic acid of approximately 87%. For the HMF and

Furfural there were no valid values. Some errors in the measurements of the initial suspension occurred,

therefore the values used for the suspension correspond to another experiment made in the laboratory.

To evaluate the performance of the ultrafiltration it was necessary to appraise the percentage

of the solute that was retained by the membranes. The removal efficiency of dissolved components was

calculated based on the Dry Matter content (measured in the permeate, the feed/retentate is depleted

in these dissolved components). Both experiments showed a high value of efficiency, close to the

desired value (90-95%). However, the experiment where 3 membranes were used in series showed

higher efficiency (93.6%) than when using 2 membranes (85.2%), which means that the more

membranes used, the less the components are retained by the membranes.

Fouling is one of the main aspects when it comes to ultrafiltration, and a consequence of it, is

the decline of the membrane flux. On the other hand, the influence of the concentration of the suspension

of nanoparticles was also studied in order to understand if this was also a factor in the decrease of the

flux. The results showed that the flux decreases due to the increase of the concentration of the

suspension, which means that the flux decline occurs mainly because of the accumulation of the

particles deposited on the membrane surface and the increase of the suspension of nanoparticles

concentration. Concerning the fouling, one of the membranes (membrane 1) was chosen to be

regenerated, in order to elaborate a “long-term” stability and a performance comparation with new

membranes. The results showed that the regeneration after the diafiltration step is less effective when

compared to the values of regeneration after concentration mode. The regeneration not only removes

Nanolignin particles from the membrane surface, but also the dissolved components that could block

the membrane. These dissolved components are higher in UF and lower in DF steps.

64

For further work, membrane 1 should keep being used in other filtrations in order to understand

if the membrane reached its stabilization and conditioning characteristics, since after the Flux and

Concentration Experiment this membrane does not show a significant variance in its behavior. In

addition to that, due to the divergent capacities of the membranes, it is essential to repeat the flushing

quite a few times with different membrane materials, to reach statistically relevant values.

Being that fouling is one of the biggest problems when it comes to ultrafiltration, it’s important to

focus on its improvement. It is essential to find the best cleaning method to clear the cross-flow system,

ensuring that there are no particles left in the system. The membrane regeneration method should also

be optimized. Several experiments should be realized using different operating conditions in order to

determine the most promising results to regenerate a membrane. Another aspect that should be worked

in the future, is to check if a different technique to store the membrane will affect its capacity. Since the

membrane is regenerated using a solution of ethanol, it should be tested if this being stored in a solution

of water instead of being stored in ethanol affects the membrane.

65

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Appendix

The temperature of the product inside the reactor and the pressure was measured for all the

experiments. Figure 51 describes the temperature and pressure versus time of one of the extractions

done in this work.

Figure 51 - Product temperature (°C) and pressure (bar) for one extract production experiment.

Figure 52 - Membranes used in the experiment 2 Membranes in Series: (a) Membrane 1; (b) Membrane 2; (c) Membrane 3; (d) Membrane 4; (e) Membrane 5.

(a) (b) (c) (d) (e)

70

Figure 53 – (a) Membrane 1 (regenerated) at the end, after being used in all the experiments; (b) Membrane 6 (without regeneration) at the end, after being used in experiments Flux and

Concentration and 3 Membranes in Series

Figure 54 - Membranes used in the experiment Flux and Concentration:

(a) Membrane 3; (b) Membrane 4; (c) Membrane 5.

(a) (b) (c)

(a) (b)

71

Figure 55 - Membranes used in the experiment 3 Membranes in Series: (a) Membrane 6; (b) Membrane 7; (c) Membrane 8; (d) Membrane 9.

(a) (b) (c) (d)

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