ceramic membranes for the filtration of liquids: an actual ... · for gas diffusion in the...

1. Development and design ofceramic membranes

1.1. Historical background ofceramic membranes

While there is very diverse informationconcerning the time of the first ceramicmembranes on a laboratory scale, the 40smark the birth of ceramic membranescommercially produced on a large scale, as

they were successfully used as membranesfor gas diffusion in the subproject of the uranium concentration during the“Manhattan project”. Shortly after, thedevelopment and modification of thesemembranes went on and membranes formicro- and ultrafiltration have been devel-oped, which consisted already of a porouscarrier and a thin ceramic membrane layer,their shapes have already been in singleand multi-channel designs. Due to theirvarious advantages, which are for exampleimportant for the treatment and filtrationof foods and beverages, most of the ceram-ic membranes were used for microfiltra-

tion of milk as well as (pre-)filtration ofwines and juices.

Next to the development of ceramicmembranes for food and beverages, after the end of the second world war,companies like Carbone Lorraine andDesmarquest did a lot of research anddevelopment in ceramic micro- and ultra-filtration membranes for the enrichment ofuranium. At the same time, also the devel-opment and optimization of polymericmembranes went on; the specific pricingof polymeric membranes (price per mem-brane area) was significantly below thepricing of ceramic membranes, but the

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Ceramic membranes for the filtrationof liquids: An actual overview St. Duscher*

Since nearly one century, ceramic membranes are an established component for the separation of particles out ofliquids. Since the development and rollout of ceramic nanofiltration membranes, they also offered an additional methodin separating dissolved solids or ions out of liquids under rough conditions. Due to constant improvements in materialsand production methods, it is possible to replace established processes like e.g. evaporators by these membraneswith lower investment and running costs without any disadvantages in product quality or efficiency. The following articlegives an overview about the actual state of development of ceramic membranes with an emphasized focus on ceramicnanofiltration membranes. The outlook of the article gives a rough overview about some actual fields of developmentand optimization.

* Dipl.-Ing. Stefan DuscherHead of Sales, Inopor GmbHIndustriestrasse 1, 98669 Veilsdorf / Germanywww.inopor.de

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applications were limited due to the chem-ical and mechanical properties of polymer-ic membranes. Until the beginning of thesecond millennium, microfiltration andultrafiltration was available as polymericor ceramic membranes, but nanofiltrationand reverse osmosis were still technolo-gies which were covered exclusively bypolymeric membranes. Things changedsignificantly, when a new ceramic nanofil-tration membrane with a cut-off of 450Dalton was developed by HermsdorferInstitut für technische Keramik (H.I.T.K.) ,which is now known as Fraunhofer IKTSand located at Dresden. While the methodof reverse osmosis certainly will be oper-ated indefinitely only with polymer mem-branes, a patent-protected ceramic nanofil-tration membrane – based on the results ofFraunhofer IKTS - has been availablesince 2004 and is being sold on an indus-trial scale by Inopor GmbH.

1.2. Geometry of ceramicmembrane carriers

As mentioned at the beginning, ceramicmembranes consist of a carrier - mostlyalso ceramic - on which the actual mem-brane layer is fixed. In terms of the shapeand design of these carriers, there are dif-ferent approaches on the market, like e.g.flat-sheet membranes, tubular membranes,capillaries and some further specialdesigns. Next to Inopor GmbH, typicalsupplier of ceramic membranes on the

European market are companies likeKerafol GmbH, Tami GmbH or AtechInnovations GmbH. Because tubulargeometries dominate on the market, theywill be discussed primarily in the furtherarticle.

Tubular ceramic membranes are made of an extruded carrier (so-called“Support”) which has one or more chan-nels on which the membrane layers arefixed on the cannel surface by some inter-mediate layers. Typically, the support isalso made of a ceramic material, but thereare also some technical alternatives avail-able. Fig. 1 shows some typical single- andmulti-channel geometries of ceramicmembranes. Today a huge number ofmaterial and membrane combinations areavailable on the market, like for exampleSiC, Al2O3, ZrO2 and TiO2.

Fig. 2 shows the design of a typicalmulti-channel membrane, including thefront-side sealing. During operation, themembrane is installed in a housing and thefeed flow / raw medium flows through thechannels of the ceramic carrier. The sur-face of the channels are coated with aceramic membrane layer. The filtrationprocess is done by leading liquid throughthe membrane layer and separating thecomponents out of the feed medium whichare not able to pass the membrane layer.Liquids and components which can passthrough the membrane layer are called“permeate”, while the remaining particles,

which cannot pass through the membranelayer, are called “concentrate”. For a max-imum efficiency, it has to be avoided, thatfeed liquid gets on the permeate side with-out passing through the membrane layer;otherwise, this would mean a contamina-tion of the permeate flow. So, also a front-side sealing is necessary to avoid that feedmedium flows directly through the porousmembrane carrier to the permeate side. Asshown in Fig. 3, the filtration process isregulated by a concentrate valve, whichcreates a ram pressure by throttling thediameter and creating a transmembranepressure (TMP). Concerning the porosityof the materials, the membrane layers havemuch smaller pores than the intermediatelayers and the membrane carriers, so thatnearly all of the hydraulic resistance of theflow through the membrane element isgenerated by the membrane layer(s) butnot by the support material. Mathe -matically, the transport mechanismthrough porous membranes can bedescribed by pore models, depending onpore sizes, driving forces and transportconditions. Nanofiltration with polymericmembranes can be described pretty goodby some solution-diffusion-models whichare suitable for tight membranes. Ceramicmembranes – even if they are in a range ofa nanofiltration membrane – are stillporous membranes, so the transport mech-anism cannot be described by the laws fortight membranes, but by a modification ofthe Nernst-Planck-Equation (NPE), whichcombines the effects of transport throughporous media with the electrical surfaceeffects and potentials.

1.3. Materials of ceramic membranesAs already mentioned, the development

of ceramic membranes partly took place atthe same time in different applications, sothat also different developments in materi-als and shapes were made with similarproperties. In general ceramic membranesare distinguished by the following fea-tures:

Never-the-less, also ceramic mem-branes are only suitable within definedranges of chemical and physical condi-

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Table 1: Properties of ceramic membranes Fig. 2: Ceramic multi-channel membrane

Fig. 3: Separation principle of a multi-channel membrane


tions. A fact, which becomes more andmore important for decreasing pore sizesof the membranes, because decreasingpore sizes mean also decreasing resis-tance. Generally, micro- and ultrafiltrationmembrane layers are formed by filling thechannels of the membrane carrier with aslurry, drain the channels and sinter thecarriers to make the slurry film becominga membrane layer. During this productionprocess, the average pore size of the mem-brane layer is related with the sinter tem-perature: So, higher sinter temperaturesmean coarser pore sizes. Typically, aceramic filter element does not consist ofonly one membrane layer but of severallayers, while the first layers are intermedi-ate layers to fix the membrane layers onthe carrier. Then, one membrane layerafter the other is sintered on the existinglayers with decreasing pore sizes. Thismeans, that during this production process,the sinter temperature decreases duringeach coating process because pore sizesbecome smaller. This means also adecreasing of the maximum operatingtemperature, because the operating tem-perature must not be higher than the sintertemperature of the top membrane layer.While ceramic micro- and ultrafiltrationlayers are sintered by a standard slurry

process, ceramic nanofiltration mem-branes are manufactured by a polymericsol-gel process, which means even lowermax. operating temperatures and a lowerchemical resistance because of a signifi-cantly increased membrane surfacebecause of a multitude of small pores. Inthe end, the thermal and chemical resis-tance of ceramic membranes is still higherthan polymeric membranes, but decreas-ing pore sizes mean decreasing chemicaland thermal resistance.

Ceramic membranes have in common,that they are inorganic membranes, butnever the less, depending on the produc-tion process and application, the ceramicmaterials have different physical andchemical properties. Sometimes, it is saidabout ceramic membranes, that they areresistant against acids and basics throughthe whole pH-range; unfortunately, this isnot the whole truth, because differentceramic materials have different chemicalproperties, depending for example on the

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Tab. 1: Properties of ceramic membranes


type of atomic bonding. Last but not least,it must not be forgotten, that the ceramicmembrane layer is only one part of themembrane element or the filtration stage:To make the whole plant run properly, allmaterials have to resist the media, incl. thefront-side sealing, the housing and the o-rings.

Generally, metal oxids are more resis-tant against acids if their chemical bondingis more acidic; an increasing resistanceagainst acids also means a decreasingresistance against basics. Fig. 4 shows hisproperties for some typical ceramic mate-rials. So, for example with SiO2, this is asignificant behaviour, which reduces the

hydro-thermal resistance of SiO2 signifi-cantly, because of its high affinity forwater. Next to the material itself, also theallotrophy / crystal structure of the materi-al plays an important role: For example,for example, α-Al2O3 has a good resis-tance against acids and basics, while γ-Al2O3 with tetragonal crystal structure hasa worse chemical resistance, especiallyagainst basics. Next to the chemical resis-tance, the lattice structure of the materialalso influences the thermal resistance ofthe membrane carrier and membrane layer.A maximum thermal resistance means nochanging of pore sizes, atomic matrix andchemical reactions in defined temperature

range. In case of a phase transformation,the risk of stress cracks is very high; alsothe risk of cracks is given when differentmaterials with different transformationproperties are coated on each other. Thisrisk of cracks is given during operation athigh temperatures as well as during theproduction process. Fig. 5 shows a cross-section through channel of a ceramicmembrane element taken with a SEM,including the support, the intermediatelayer and the membrane layer. Dependingon the different thermal and chemicalproperties of these different materials, themechanical junction between the differentlayers has to compensate different expan-sions and structure conversions. WhileAl2O3 and SiO2 appear here to be very sta-ble, tetragonal zirconium oxide and titani-um oxide, on account of their relativelylow phase transition temperature, haveonly a limited thermal resistance. For theselection of the suitable ceramic material,it is hence not enough to know only themedium to be filtered, also the temperatureand the pH value must be taken intoaccount. Although the prevailing propor-tion in ceramic tubular membranes is cov-ered with the materials aluminium oxideand titanium oxide, reference should bemade at this point also to the very goodchemical resistance of zirconium oxide; asan example here, reference should bemade to the work of van Gestel /2/, whosucceeded in making ceramic nanofiltra-tion membranes on the basis of ZrO2.

2. Operation of ceramicmembranes

2.1. Construction and incoming flowCeramic membranes are primarily used

in the cross flow operation; time by time,there are applications – mainly in the phar-maceutical and chemical processes –where ceramic membranes are operated in“Dead end” mode, but the flow rates arevery low and this are not typical installa-tions.

As far as the fundamental constructionof a membrane system with ceramic mem-branes is concerned, this differs onlyslightly from the structure of a plant withpolymer membranes (see Fig. 6).However, ceramic membranes requirehigher cross-flow velocities and largercross-flows, compared to polymeric mem-branes. The practice showed and showsthat especially with applications in thefield of wastewater treatment, as well asthe filtration of chemicals and pharmaceu-ticals, a cross flow of 4 m/s for ceramicmembranes is an acceptable minimumcross-flow. The processes of the separationof micro-organisms (for example from fer-mentation broths) in this case form anexception here, since, in addition, the cross


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Fig. 6: Schematic structure of a system with ceramic membranes (including CIP cycles)

Fig. 5: Structure of a ceramic membrane support with membrane layer

Fig. 4: Overview of the material resistance properties /1/


flow is to be selected before the back-ground that the organisms are notdestroyed by the appearing shear rates.

Because the necessary cross flow forceramic membranes is clearly higher thanthat of polymer membranes, the higherenergy entry associated with this is anargument against ceramic membranes, butonly in case of that the process can also beoperated with polymeric membranes.That’s why manufacturers of ceramicmembranes also continuously work onreducing this disadvantage, somethingthat, in the end, means to find a geometrywhere the pressure loss, the necessaryoncoming flow, membrane surface and notleast the manufacturability are in an opti-mal relationship with each other. Fig. 7shows the cross section of such a carriergeometry.

2.2. Geometries of ceramicmembrane elements

Ceramic membranes are available invarious geometries and shapes with vari-ous numbers of channels and channeldiameter. Next to tubular membranes, alsoflat-sheet membranes are available with ashape of a disc and diameters between25mm and 90mm, while diameters of25mm, 47mm and 76mm define an inter-nal standard, because the usual test cellsfor flat-sheet membranes are manufac-tured for discs with these diameters. Oneside of the disc is coated with a ceramicmembrane while the carrier is made out ofthe same materials like tubular ceramicmembranes. The advantage of test cells forflat-sheet membranes is, that initial testscan be done with a small amount of liquid:The membrane disc is installed in a hous-ing and a liquid column is filled above themembrane. Then, the liquid column ispressed in “Dead end” mode through themembrane by filling compressed air or gasbetween the column and the housing of thetest cell. Typically, maximum transmem-brane pressures up to 6 bar / 85 PSI can be

realized with standard test-cells. Tohomogenize the boundary layer on themembrane surface, a magnetic stirrer canbe used. Filtration processes with flat-sheet test cells are only suitable for smallamounts of liquids or for initial trials; theoperating pressures of flat-sheet cells aretypically lower than operating pressures ofprocesses with tubular membranes. Never-the less, flat-sheet test cells are invaluablefor trials because they are easy to do andcan be done with a minimum amount ofliquid. Next to test cells for flat-sheetmembranes, depending on the applicationchemically high resistant materials areneeded which means for example metal-free housings, made of PTFE or PVDF.Fig. 8 shows an overview of lab scaleequipment, incl. metal-free housings forthe filtration of high-corrosive liquids.

Regarding tubular geometries, the sin-gle-channel tube (EKR) forms the smallestunit, while – depending on the applica-tions – the number of channels can bemore than hundred per ceramic element.Table 2 shows an overview about typicalgeometries available on the market and thetechnical requirements to design a mem-brane plant with this membranes.

Similar to polymeric membrane ele-ments, ceramic membrane elements can beinstalled in series or in parallel with thesame advantages and disadvantages likeinstalling polymeric membranes. Whilepressure vessels for polymeric membranescan capture spiral-wound elements inseries installation, housings for ceramicmembranes cannot capture ceramic mem-branes in series; to install ceramic mem-branes in series, housings have to beinstalled after each other and be coupledby piping. On the other hand, housings forceramic membranes are available to installceramic membranes in parallel, while spi-ral-wound elements cannot be installed inparallel in one pressure vessel. Byinstalling membranes in parallel, therequired cross-flow and the size of the cir-culation pumps grows linearly to the num-ber of elements. Fig. 9 shows a housing forup to seven tubular ceramic membranes,while the first membrane is alreadyinstalled and sealed with an o-ring againstthe housing. Fig. 10 shows a general cutthrough a membrane housing for ceramicmembranes. Each membrane has to besealed against the housing by an o-ringand a stopper plate is necessary to avoid

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Fig. 7: Optimized geometry of a ceramicmembrane /1/

Fig. 8: Flat-sheet test cell and further lab equipment forceramic membranes /1/

Table 2: Parameter overview of membranes commonly available on the market


that membranes are “shot” out of the hous-ing in case of a pressure hammer.Typically, screwed flange connections areused to connect the housing with the pip-ing, but in pharmaceutical or food andbeverages applications, also tri-clamp con-nections are used.

2.3. Permeate volume andtransmembrane pressure

Due to its properties as a pore mem-brane, the permeate performance ofceramic membranes within a determinedwindow obeys quasi-linear laws, which iswhy the specific permeate performance ofa process with ceramic membranes is oftendescribed also in the dimension, whichexpresses the fact that a duplication of thepermeate performance entails the duplica-tion of the transmembrane pressure.Within certain windows this can be con-firmed from practise; however, it must notbe concluded therefrom that, for example,the transmembrane pressure can be multi-plied arbitrarily without consequences,since, the laws of transportation throughthe membrane and the behaviour in form-ing a surface layer will become more andmore important with increasing pressuresand potentials. Although ceramic nanofil-tration will be discussed more extensivelyin the following chapters, it must alreadybe mentioned that nanofiltration is espe-cially a process that yields no satisfactorypermeate qualities in the case of low trans-membrane pressure, even if some scientif-ic works argued that ceramic nanofiltrationonly follows the laws of porous mem-branes.

Because the optimum working pointdepends in the end on a huge number offactors, such as for example viscosity,

temperature, solids cargo, particle sizeetc., a blanket statement about the opti-mum operating point to be selected is notpossible. The values listed in Table 3 showsome data typical from practice.

In this case, it must be mentioned that,for example, an increase of the tempera-ture of the inflow medium leads to anincrease of the permeate performance witha concurrent drop of the permeate quality,while a reduction of the media temperatureleads to the fact that the transmembranepressure must be raised in order to keepthe permeate flow constant.

3. Ceramic nanofiltrationmembranes

3.1 Membrane construction andcharacteristic variables

With development and market launch ofceramic nanofiltration membranes with aseparation limit below 1 kDa, a totally newproduct branch of ceramic membranes hitthe market which differs in several aspectsfrom ceramic micro and ultrafiltrationmembranes. Ceramic nanofiltration mem-branes - like ceramic micro and ultrafiltra-tion membranes - are built up in layers, i.e.on a very porous carrier (called“Support”), separation-active membranelayers are applied in several process stepsand these layers have a finer separationlimit with every process step, so that at theend of the manufacturing process, thefinest membrane layer - which also definesthe separation limit of the ceramic mem-brane tube - is applied and this is also thelayer that during operation receives theoncoming flow from the feed medium. Anexample construction is shown in Fig. 5.

It is obvious that maximum membranepermeability is to be aimed without a lossof selectivity and without using a manu-facturing method that would make com-mercialisation impossible for cost reasons.Concerning the membrane permeability L,the law from Hagen-Poiseuille for aporous structure is:

In detail, there are the porosity of themembrane, the pore diameter, the dynam-ic viscosity of the medium and the mem-brane thickness. Essential problemsalready become obvious here, which one isconfronted with due to increasingly small-er pore sizes. Because the pore sizesquarely enters in the permeability of themembrane, an increasing reduction of thepores means a disproportionate loss of per-meability that one must counteract:remaining here as possible manipulatedvariables are the porosity and the thicknessof the membrane layer, i.e. it is alwaysaimed for that a membrane layer is verythin and has very high porosity.Unfortunately, increasing porosity goeshand in hand with a loss of chemical resis-tance, since increasing porosity means asurface enlargement that offers a biggerattack area for the respective chemicals.This circumstance, in turn, can be counter-acted only by having starting materials aspure as possible and the cleanest produc-tion conditions. Besides the application ofadditional membrane layers, these require-ments for cleanliness in the manufacturingprocess and the purities of the startingmaterials constitute the essential influenceon the cost structure of ceramic nanofiltra-tion membranes. In practice the law ofHagen-Poiseuille also means, for example,that the membrane layer should be verythin, but still mechanically stable enoughto resist the forces occurring - for example,through incident flow of solids loadedinflow media. At the same time it can alsobe recognized from the law of Hagen-


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Fig. 10: Membrane housing for ceramic membraneelements /1/

Fig. 9: Membrane housing for up to seven ceramicmembrane elements with one element installed /1/

Tab. 3: Typical permeate performance with ceramic membrane processes


Poiseuille that the hydraulic resistance ofthe ceramic support plays a minor part,since its pore size - as a function of thematerial and production process - is usual-ly in the range of 3 - 5 μm, while the poresizes of ceramic microfiltration mem-branes can lie approx. in the ranges 100 -800 nm and the pore sizes of ceramicnanofiltration membranes can be less than1 nm.

3.2 Manufacture of ceramicnanofiltration membranes

As shown in Fig. 11, the supports ofceramic nanofiltration membranes areextruded - just like the supports of ceram-ic micro and ultrafiltration membranes - inorder to implement tubular geometry. Inthis case, different oxide ceramics are usedas a material, which in each case, onaccount of their physical and chemicalproperties, are of advantage or disadvan-tage for corresponding applications.Typical materials are, for example, alu-minium oxide (Al2O3), titanium oxide(TiO2) and zirconium oxide (ZrO2), andhere, aluminium oxide is used nearlyexclusively as α-Al2O3, since γ-Al2O3 isnot stable at the temperatures at which therespective membrane layers are sintered.The influencing of the pore size of ceram-ic supports is a subject of a huge numberof publications and, hence, should not bediscussed at this point in depth. Typicallythe pore size of ceramic supports can beinfluenced by admixing of excipients thatincinerate during the combustion processand then leave behind respective pores inthe matrix. A typical excipients for this isstarch and also here, as a function of thestarch, the pore size varies: While withpotato starch pore sizes of up to 50 μm canbe achieved, the pore sizes when usingwheat starch lie in the range of 20 μm andwith rice starch in the range of 15 μm. Inaddition, a crucial role is also played bythe combustion temperature and burningperiod: While burning for too short a timeor burning at too low a temperature entailan inadequate formation of the ceramicstructure and therefore a poor mechanical

stability, too much time or overheatingcauses the loss of porosity and a decreaseof the number of pores.

While ceramic micro and ultrafiltrationmembranes are usually applied and sin-tered in layers with a slurry method, poresizes for nanofiltration in a range of a porediameter of 1 nm cannot be realized withthese processes anymore. Here, a Sol gelprocess is used, in which either a coarse-porous carrier, or a carrier that has beeninitially pre-coated with an ultrafiltrationmembrane, is coated with liquid Sol. Thismethod - in connection with polymericSols - was particularly promoted byPuhlfürß et al. and optimised from thepoint of view that it can be used beyondjust on laboratory scales /3/. The coatingof the carrier can be done in differentways, for example, by means of Spray-Coating (spraying on) or Dip-Coating(immersion), wherein Spray-Coating issuitable primarily for flat structures and,hence, is not applicable for using with atubular geometry. The liquid gel is trans-ferred into a solid gel when the gel point isexceeded. In general with this Sol-gelprocess there are two different methodsthat differ in the kind of the Sol. The pre-vailing part of the ceramic membranesavailable on the market that are producedby means of the Sol gel process are pro-duced by means of a colloidal sol in whicha metal alkoxide is hydrolyzed in an envi-ronment with a surplus of water; in thiscase nanodisperse particles are thenformed, which can be peptized by treat-ment with respective electrolytes. Theseaqueous suspensions - which either consistof colloidal or aqueous sols - are admixedwith corresponding substances whichimplement mechanical bonding on the sur-face and then are applied directly to acoarse-porous support. The adjustment ofthe pore size occurs during hydrolysis byvarying the temperature and concentrationaccordingly.

Because with a colloidal sol no porescan be mapped that are smaller than 2 nm,these pore sizes are implemented by

means of a Sol gel method with a poly-meric sol. In this case, pore sizes can bemapped down to 0.9 nm. The method isdistinguished by the fact that they areworking with an alcoholic solution wherea partial hydrolysis of the metal alkoxidesis triggered by addition of water and themetal alkoxides then polycondense in thesolution. If the dilution is corresponding,the forming oligomers will remain in solu-tion and are allowed to gel only whenapplied to the membrane support, sincehere, a suction effect results due to capil-lary forces. With this method, ceramicnanofiltration membranes can be producedfrom titanium dioxide, whose separationlimit in aqueous media is 450 Dalton.

Besides ceramic nanofiltration mem-branes from TiO2 /2/ ceramic nanofiltra-tion membranes with a separation limit ofapprox. 300 Dalton were implemented byvan Gestel on the basis of ZrO2 / TiO2; forthis purpose, the tetragonal zirconiumoxide was stabilised with yttrium. Agoudilet al. /4/ have implemented ceramicnanofiltration membranes from a mixtureof ZrO2 and SiO2 by using tetraethoxysi-lanes (Si(C2H5O)4) and zirconium tetra -propoxide (Zr(C3H7O)4); the transforma-tion temperature of zirconia from thetetragonal to the monoclinic phase wasable to be shifted to higher temperaturesby addition of SiO2.

In general, different materials and pro-duction approaches are currently to befound on the market for ceramic nanofil-tration membranes, which result in corre-sponding advantages and disadvantages;for instance, the system SiO2 permits agood monitoring of the pore size in themanufacturing process, but at neutral andalkaline pH values has detrimental proper-ties in the stability. However, the system γaluminium oxide is at low pH values (pH<2) only conditionally stable while thesystems ZrO2 and TiO2 have a good toacceptable stability across the whole pHvalue range.

3.3 Surface load of ceramicmembranes

Besides the pore size, the surface loadplays an important role with ceramicmembranes. In general the surface loadcan form itself with ceramic membraneson account of different mechanisms:

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Fig. 11: Extrusion of ceramic membrane carriers /1/

Tab. 4: Isoelectric points of different oxideceramics


- Dissolution / absorption of ions into / from the surrounding liquid- Dissociation of molecule groups on the surface- Adsorption of ions / electrolytes on the surface

Because ceramic membranes typically have amphoteric behav-iour, they can absorb and/or emit positive or negative ions / mole-cule groups as a function of the pH value. While ceramic mem-branes with low pH values have positive electric potential, it isnegative with high pH values. This electric potential is called “Zetapotential” and can be measured by means of suitable measuringcells and electrodes. The point at which the electric potential isequal to zero is called isoelectric point. Typical values of oxideceramics for this are listed in Table 4.

As a function of the pH value of the medium to be filtered andthe isoelectric point of the membrane material used, the retentionof the membrane is also influenced and in general it has to be saidthat the retention near the isoelectric points is worse because there,the repulsion or attraction effects, which otherwise can influencethe separation process positively, cancel each other out. Fig. 12shows how the retention of a ceramic nanofiltration membranewith a pore size of 0.9 nm from TiO2 changes as a function of pres-sure and pH value.

3.4 Material transport throughceramic nanofiltrationmembranes

An essential difference between ceram-ic and polymeric nanofiltration mem-branes is that even the ceramic nanofiltra-tion membrane – like the micro andultrafiltration membranes - is a pore mem-brane while polymer membranes aredense membranes. Therefore, the solu-tion-diffusion model that is used in thecase of dense diffusion membranes, can-not be modified for ceramic nanofiltrationmembranes. According to the solution-dif-fusion model, inter alia, it arises that withrising water flow rate and/or pressure, theretention increases and theoreticallyapproaches a value of 100%. This isbecause the back diffusional proportion,with rising water flow rate, has increas-ingly less influence on the deterioration ofthe permeate; consequently it is not advis-able to operate polymeric systems withwound modules with insufficient pressure,since the permeate quality noticeablydecreases. The higher the proportion ofthe ingredients to be separated, the moreclearly this effect is detected.

However, with ceramic nanofiltrationmembranes the substance transport can bedescribed sufficiently well for practicewith the expanded Nernst-Planck equationaccording to Dresner. The expansion ofthe Nernst Planck equation according toDresner consists of the fact that, beside thesolutes and the solvent, he also includedthe membrane and its material in the ther-modynamic consideration. To this end,Dresner introduced an Interaction parame-ter β /5/. The surface-related molar trans-port of a component j according toDresner consists of the following terms:

Convective Transport:

Diffusive Transport:

Electromigrative Transport:

Therefore the following equation arises for the surface-relatedmolar transport of a component:

.In this case, cj,M is the concentration of the component j in the

membrane, vW is the velocity of the water, D(j,M) is the diffusioncoefficient of the component j in the membrane, zj is the ion valencyof the component j, F is the Faraday constant and is the

displacement-dependent change in membrane potential. The intro-introduction of the interaction parameter β changes the convectiveterm to the effect that interactions with the solid ions in the matrixof the membrane may be considered, similar to the interaction of


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Fig. 12: Retention of a ceramic NF membrane as a function of pressure and pH value

Fig. 13: Retention of an improved NF membrane of Inopor GmbH


solid ions in the matrix of an ion exchanger membrane. In particu-lar, it is considered with this parameter that the convective iontransport is not necessarily carried out with the speed of the con-vective transport of water. Dresner looks at the parameter β, as wellas the diffusion coefficient of the water, as being independent ofthe concentration. Tests with higher pressures and concentrationshave shown that the independence from the concentration is thennot totally present anymore, but for low and middle concentrations,the equation according to Dresner has still proved to be very accu-rate. In particular, it applies also for ceramic nanofiltration mem-branes that a too low pressure and therefore a too low water flowrate have a negative effect on the permeate quality.

3.5 The future of ceramic nanofiltration membranes

As already described, ceramic nanofiltration membranes with acut-off of 450 Dalton can be produced in large scale with processreliability; in laboratory and prototype scales, membranes with sig-nificant lower cut-off rates were already able to be implemented.An essential challenge will lie in the near future in also mappingthese membranes with a separation limit of 300 Dalton or less withprocess reliability and in pipe geometry.

Fig. 13 shows an example of the retention characteristics of amembrane prototype of the company Inopor GmbH, in which themembrane potential was changed specifically by trying to mapelectro-chemical mechanisms from ion exchangers with modifiedceramic membranes. In this case, a separation limit of smaller than300 Dalton was able to be implemented with constant porosity andconstant flow rate, but currently, the long time stability of thismembrane is not optimized yet.

Alternatively to the optimisation of ceramic membranes, thecomposite of ceramic and polymeric membranes will certainlygain importance in the near future, since, here, the advantages ofboth membrane models can be combined in one product. In theideal case a ceramic nanofiltration membrane would be conceiv-able on which there is a solidly bonded reverse osmosis membrane.In spite of intensive researches in this area - in particular in theUSA - only prototypes on a laboratory scale are currently avail-able.

Beside the optimisation and improvement of the ceramic mem-branes as a product looked at in isolation, the best possible place-ment and combination of appropriate products in increasinglycomplex process and production chains will also increasingly begaining in importance. This assumes, for the design and imple-mentation of a process, that knowledge about the most differentmethods and products exists, in order to be able to select fromamong them the optimum method with the best possible suitablemembranes. As an example for this, it should be mentioned thatInopor GmbH, with acetic acid produced by fermentation, couldcombine several methods in such a manner that the acetic acid wasinitially purified and then, by means of a special membraneprocess, was able to be concentrated to a concentration of 30-32%.

Literature: /1/ Inopor GmbH, Veilsdorf/2/ T. van Gestel, H. Kruidhof, D. H.A. Blank, H. J.M. Bouwmeester: ZrO2 and TiO2 mem-

branes for nanofiltration and pervaporation: Part 1. Preparation and characterization of acorrosion-resistant ZrO2 nanofiltration membrane with a MWCO < 300. Journal ofMembrane Science 284 (2006) , No. 1–2, Pages 128-136

/3/ P. Puhlfürß, A. Voigt, R. Weber, M. Morbé: Microporous TiO2 membranes with a cut-off <500 Da. Journal of Membrane Science 174 (2000), No. 1, Pages 123 – 133

/4/ N. Agoudjil, N. Benmouhoub, A. Larbot: Synthesis and characterization of inorganic mem-branes and applications. Desalination 184 (2005), No. 1-2, Pages 65 - 69

/5/ L, Dresner: Some remarks on the integration of the extended Nernst-Planck-Equations inthe hyperfiltration of multicomponent solutions. Desalination 10 (1972), No. 1, Pages27 - 46

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