no job name · 2016-02-02 · years in the design of polymeric materials, such as polyimides, etc.,...

REVIEWS

Progress and New Perspectives on Integrated Membrane Operationsfor Sustainable Industrial Growth

Enrico Drioli* and Maria Romano

Institute on Membranes and Modeling of Chemical Reactors, CNR, and Department of Chemical Engineeringand Materials, University of Calabria, 87030 Arcavacata di Rende (CS), Italy

Membrane science and technology has led to significant innovation in both processes and productsover the last few decades, offering interesting opportunities in the design, rationalization, andoptimization of innovative productions. The most interesting developments for industrialmembrane technologies are related to the possibility of integrating various of these membraneoperations in the same industrial cycle, with overall important benefits in product quality, plantcompactness, environmental impact, and energetic aspects. Possibilities for membrane engineer-ing might also be of importance in new areas. The case of transportation technologies is ofparticular interest. The purpose here is to present a summary review of the extent to whichmembrane processes have been integrated into industrial practice. Some of the most interestingresults already achieved in membrane engineering will be presented, and predictions about futuredevelopments and the possible impact of new membrane science and technology on sustainableindustrial growth will be analyzed.

Introduction

Membrane science and technology has led to signifi-cant innovation in both processes and products, par-ticularly appropriate for sustainable industrial growth,over the past few decades.

The purpose here is to present a summary review ofthe extent to which membrane processes have beenintegrated into industrial practice.

The preparation of asymmetric cellulose acetate mem-branes in the early 1960s by Loeb and Sourirajan isgenerally recognized as a pivotal moment for membranetechnology. They discovered an effective method forsignificantly increasing the permeation flux of polymericmembranes without significant changes in selectivity,which made possible the use of membranes in large-scale operations for desalting brackish water and sea-water by reverse osmosis and for various other molec-ular separations in different industrial areas. Today,reverse osmosis is a well-recognized basic unit opera-tion, together with ultrafiltration, cross-flow microfil-tration, and nanofiltration, all pressure-driven mem-brane processes. In 1999, the total capacity of reverseosmosis (RO) desalination plants was more than 10millions m3/day, which exceeds the amount produced bythe thermal method,1 and more than 250 000 m2 ofultrafiltration membranes were installed for the treat-ment of whey and milk.

Composite polymeric membranes developed in the1970s made the separation of components from gas

streams commercially feasible. Billions of cubic metersof pure gases are now produced via selective permeationin polymeric membranes.

The combination of molecular separation with achemical reaction, or membrane reactors, offers impor-tant new opportunities for improving the productionefficiency in biotechnology and in the chemical industry.In 1997, five large petrochemical companies announceda research project devoted to the development of inor-ganic membranes to be used in syngas production. Atabout the same time, an $84 million project, partlysupported by the U.S. Department of Energy (DOE),that has Air Products and Chemical Inc. workingtogether on the same objective has been promoted. Theavailability of new high-temperature-resistant mem-branes and of new membrane operations as membranecontactors offers an important tool for the design ofalternative production systems appropriate for sustain-able growth.

The basic properties of membrane operations makethem ideal for industrial production: they are generallyathermal and do not involve phase changes or chemicaladditives, they are simple in concept and operation, theyare modular and easy to scale-up, and they are low inenergy consumption with a potential for more rationalutilization of raw materials and recovery and reuse ofbyproducts. Membrane technologies, compared to thosecommonly used today, respond efficiently to the require-ments of so-called “process intensification”, because theypermit drastic improvements in manufacturing andprocessing, substantially decreasing the equipment-size/production-capacity ratio, energy consumption, and/orwaste production and resulting in cheaper, sustainabletechnical solutions.2

The possibilities of redesigning innovative integrated

* Corresponding author: IRMERC-CNR c/o Department ofChemical Engineering and Materials, via Ponte P. Bucci,87030 Arcavacata di Rende (CS), Italy. Tel.: (39) 0984-492039/492025. Fax: (39) 0984-402103. E-mail:[email protected].

1277Ind. Eng. Chem. Res. 2001, 40, 1277-1300

10.1021/ie0006209 CCC: $20.00 © 2001 American Chemical SocietyPublished on Web 02/13/2001

membrane processes in various industrial sectors char-acterized by low environmental impacts, low energyconsumption, and high quality of final products havebeen studied and in some cases realized industrially.

Interesting examples are in the dairy industry andin the pharmaceutical industry. Research projects arein progress in the leather industry and in the agrofoodindustry based on the same concept.

In this review, some of the most interesting resultsalready achieved in membrane engineering will bepresented, and predictions about future developmentsand the possible impact of new membrane science andtechnology on sustainable industrial growth will beanalyzed.

Actual possibilities and future perspectives of medicaland biomedical applications of membrane technology arenot discussed in this work. This theme is the object ofanother recent paper.3

The continuous interest and growth of the variousnew industrial processes related to life sciences, asevidenced also by the strategies and reorganizationadopted by large chemical groups worldwide in this area(e.g., Aventis, Novartis, Vivendi Water, etc.) will alsorequire significant contributions from membrane engi-neering. We will, however, not concentrate our analysison this subject in this review.

Membrane Operations

Various membrane operations are available today fora wide spectrum of industrial applications. Most of themcan be considered as basic unit operations, particularlythe pressure-driven processes such as microfiltration(MF), ultrafiltration (UF), nanofiltration (NF), and RO;electrodialysis (ED) is another example of a maturetechnology.4 Their worldwide sales are reported in Table1.5

The significant variety of existing membrane opera-tions is based on relatively simple, compact, and largelyclarified fundamental mechanisms characterizing trans-port phenomena in the dense or microporous membranephases and at the membrane-solution interface. Theunderstanding and prediction of transport phenomenain the membrane phase is today at least qualitativelypossible, also theoretically through the newly availabletools provided by molecular simulation.6,7

Much progress has been made in this area in recentyears in the design of polymeric materials, such aspolyimides, etc., and in the calculation of the diffusioncoefficients of simple gases in the dense phase. Aninteresting agreement can be found between the theo-retical and experimental values.8,9

It is the integration of advanced knowledge abouttransport phenomena in dense or microporous thinphases, combined with the understanding of interfacial

phenomena controlling the adsorption and desorptionof penetrants and other species at the membranesurfaces, with the correct flow-dynamic analysis of thetangential flow and concentration profile built up in thebulk solutions upstream and in the membranes down-stream and with the reology of often concentrated non-Newtonian fluids, that permits the design of correctmembrane separation units.

Membrane operations show potential in molecularseparations, clarifications, fractionations, concentra-tions, etc. in the liquid phase, in the gas phase, or insuspensions.

They cover practically all existing and requested unitoperations used in process engineering. All of theoperations are modular, easy to scale-up, and simple todesign. Other important aspects are the lack of movingparts; ability to work totally unattended; lower cost;operational flexibility; and, when necessary, portability.

Coupling of molecular separations with chemicalreactions can be realized in a simple unit efficiently,having ideal reaction surfaces where the products canbe continuously removed and the reagents continuouslysupplied at stoichiometric values.

These overall properties make membrane operationsideal for the design of innovative processes where theywill carry on the various necessary functions integratedeventually with other traditional unit operations, opti-mizing their positive synergic effects.

It is interesting to mention that statistical analysiscarried out by Electricite de France on 174 differentmembrane installations in France using MF, UF, RO,and ED mainly in small- and medium-sized industriesfound a normal percentage of satisfaction between 70and 95%, one of the highest positive responses receivedin this kind of analysis. This result is, in part, surprisingbecause of the high innovative content of the technologyand the lack of education still existing on their basicproperties. It is, however, consistent with the importantcontributions that membrane operations can make interms of cost reduction, quality improvement, pollutioncontrol, etc.

Several examples of successful applications of mem-brane technology as alternatives to traditional processescan be mentioned.

Ion-Exchange Membranes. The use of ion-ex-change membrane cells in chloro-soda production rep-resents, for example, an interesting case study foranalyzing the possibilities of membrane operations andone of the first successes in terms of their electrochemi-cal application in minimizing environmental impactsand energy consumption. The technology is based on thediscovery and utilization of fluorinated polymeric mem-branes stable in a specific environment, such as Nafion.Today, membrane systems in which the anodic andcathodic species are directly produced in separatecompartments without mixing and final separationproblems permit one to overcome the limitations oftraditional mercury cells, related to the need for Hgrecovery, and of diaphragm cells, in which the separa-tion and concentration of final products still createdifficulties. All new chloro-soda installations are nowpractically based on this design, which represents atypical rationalization of the process, removing all of thepollution problems that characterized chloro-soda pro-ductions in the past.

In principle, other molecular halogens could be pro-duced from their respective gases. The direct production

Table 1. Sales of Membranes and Modules in VariousMembrane Processes5

membrane process

1998 sales(millions of

U.S. dollars)growth(%/year)

microfiltration 900 8ultrafiltration 500 10reverse osmosis 400 10gas separation 230 15electrodialysis 110 5electrolysis 70 5pervaporation >10 ?miscellaneous 30 10

1278 Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001

of essentially dry chlorine gas would also reduce oxygenformation, which allows the reaction to be run at muchhigher current densities, with much less purificationand drying required compared to the chlorine producedby other systems.

Reverse Osmosis and Nanofiltration. As alreadymentioned, desalination of seawater and brackish waterhas been at the origin of the interest for membraneoperations, and the research efforts on reverse osmosismembranes have had an impact on all of the progressin membrane science and technology. Evaporationplants have been substituted with RO systems indifferent part of the world (Table 2).10

The relatively low energy consumption is one of thereasons for this success (Table 3). In seawater desalting,in fact, the global energy consumption of RO, with arecovery factor of 30% and energy recovery, has been5.32 kWh/m3 corresponding to a primary energy con-sumption of 59.94 kJ/kg.11

Costs for brackish water desalination are 60-70%lower than those for seawater desalination.

RO desalination is not only devoted to the productionof drinkable water but today is also strategic in variousindustrial sectors and particularly in ultrapure waterproduction for the electronic industry. It is interestingto realize that, in Japan, the largest part of the waterproduced by RO is for the electronics industry, in whichthe country has worldwide leadership.

Reverse osmosis has not generally been used untilrecently in the purification, separation, or concentrationof chemicals, particularly because of osmotic limitationsand the low chemical and thermal resistance of theexisting membranes.

The recent development of nanofiltration and low-pressure reverse osmosis membranes with interestingselectivities and fluxes, as well as higher chemical andthermal resistances, has been rapidly utilized in therealization of innovative processes in various industrialsectors.

An interesting case studied in Italy is represented bythe preparation of Iopamidol in the pharmaceutical

industry.12 Recently, in X-ray diagnosis, the use ascontrast media of new nonionic iodinated compounds asopacifying agents was studied and introduced to themarkets as a substitution for the traditional iodinatedionic compounds. However, the preparation and, par-ticularly, the final purification of these products weremuch more complex and expensive than for thosepreviously used. In particular, the neutral iodinatedagents cannot be isolated by precipitation in waterbecause of their high solubilities. The problems to besolved were particularly the removal of ionic species,usually inorganic salts present in the final reactionmixture and the recovery of valuable reagents presentin excess and of the water-soluble reaction media. Atechnique was developed based on the treatment of theraw solutions of the contrast media with a complexseries of operations such as removal of the solvent(DMAC or DMF) by evaporation; extraction of theresidual reaction medium by a chlorinated solvent;elution of the aqueous phase on a system of cationic andanionic ion-exchange resins; concentration by evapora-tion; and crystallization of the crude residue to removethe last impurities. Various drawbacks are present inthis system. A much better system has recently beenrealized based on the use of two nanofiltration stagesoperating on highly concentrated raw solutions contain-ing the contrast media, inorganic salts, organic com-pounds with a relatively low mass (about 200), and thesolvents (Figure 1).

The first NF unit operates in diafiltration mode andthe retentate, partially concentrated and purified withrespect to contrast media, is recycled at the first stageafter dilution with a small amount of deionized makeupwater; the permeate (water, inorganic salts, solvents,etc.), which still contains small amounts of the iodinatedcompound, proceeds toward the second NF unit. Thepermeate from this second step is completely contrastagent free.

The degree of purification that can be reached is suchthat the total amount of residual impurities in the final

Table 2. Worldwide Desalination Production Capacitya

countrytotal capacity

(m3/day)% of worldproduction

MSF(%)

MEE(%)

VC(%)

RO(%)

ED(%)

Saudi Arabia 5 250 000 25.9 67.5 0.3 1.2 31.0 1.9U.S. 3 100 00 15.2 1.7 1.8 4.5 78.0 11.4United Arab Emirates 2 200 00 10.7 89.8 0.4 3.0 6.5 0.2Kuwait 1 500 00 7.6 95.5 0.7 0.0 3.4 0.3Japan 746 000 3.7 4.7 2.0 0.0 86.4 6.8Libya 685 000 3.4 67.7 0.9 1.8 19.6 9.8Qatar 570 000 2.8 94.4 0.6 3.3 0.0 0.0Spain 530 000 2.6 10.6 0.9 15.1 20.4 19.2Italy 520 000 2.6 43.2 1.9 15.1 20.4 19.2Bahrain 310 000 1.5 52.0 0.0 1.5 41.7 4.5Oman 190 000 0.9 84.1 2.2 0.0 11.7 0.0

a Phase-change processes: MSF (multistage flash), ME (multi-effect evaporation), VC (vapor condensation). Single-phase processes:RO (reverse osmosis), ED (electrodialysis).

Table 3. Costs Related to Various Sea Water Desalination Processes

processa maturityenergy

consumption

electric energyequivalent(kWh/m3)

scale ofapplication

cost for 1 m3

of freshwaterproduced (ECU)

MSF very thermal 10-14.5 small-large 0.6-1.9ME partly thermal 6-9 small-medium 0.5-2.0VC partly mechanical 7-15 small 0.6-2.4RO yes mechanical 4-8 small-large 0.4-1.4

a Phase-change processes: MSF (multistage flash), ME (multi-effect evaporation), VC (vapor condensation). Single-phase processes:RO (reverse osmosis), ED (electrodialysis).

Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001 1279

recycled retentate does not exceed 10% of the initialamount and is generally on the order of 5%.

The process is simple, economical, and environmen-tally acceptable; it permits the elimination of acid andbasic reactants necessary for the regeneration of theresins; and it avoids the use of toxic organic solvents,etc.

Also, the integrated membrane processes proposed forchromium recovery in the leather industry13 and for thetreatment of secondary textile effluents for their directreuse,14 which will be described and discussed later inthis work, show efficient applications of nanofiltrationand low-pressure reverse osmosis operations.

Microfiltration and Ultrafiltration. Recently, inthe food industry, membrane technology made realisticthe possibility of cold sterilization. Tetra Pak (Bacto-Catch System) developed a cross-flow microfiltrationsystem that debacterizes fresh milk, avoiding anythermal treatment and taste alteration. An industrialprocess using this technology is already in operation atVillefranche (Lyon) producing 2000 L/day of fresh milkregistered with the trade name Marguerite (Figure 2).The skimmed milk, obtained by whole milk centrifuga-tion, is sterilized at low temperature by microfiltration.Then, it is mixed with pasteurized cream. After homog-enization and cooling, a debacterized whole milk isobtained using a process alternative to the classicalUHT (ultrahigh-temperature) treatment.

Similar products have also recently been commercial-ized in Italy by Parmalat S.p.A.

The current systems for cleaning oil-water streamsvia cross-flow microfiltration or ultrafiltration are veryreliable and compact. They can decrease the oil contentof water from 10-30 mass % to less than 5 ppm.Nitrogen blanketing helps to prevent oxidation of oilsduring mechanical oilseed pressing, while also reducingexplosion risks in extraction and during desolventizing.New possibilities exist, however. Solvent recovery,dehydration of solvents, use of membrane reactors,winterization, and fractionation of fats are interestingcases. More than two million tons of extraction solvents,mainly hexane, is used in the U.S. alone. Its recoveryis by distillation and condensation. It is estimated that,also in the most modern units, 0.7 kg of hexane per tonof seed is still released into the environment. The

possibilities of recovering solvents from the oil-micellemixture and from air exist today with membraneoperations that might significantly reduce these losses.A reduction of the solvent content of the oil-micellemixture from 70 to 40% has been demonstrated, withan energy saving of about 50%. An important aspect ofthe utilization of membrane operations in this area willbe the possibility of using other solvents such asalcohols. Their higher evaporation heats make themunattractive in traditional evaporation units.

Better solvent-resistant membranes, eventually in-organic ones, however, will be necessary for large-scaleapplications in this area.

Cross-flow microfiltration can also be used success-fully for the removal of long-chain traces of saturatedfat that are present in, e.g., sunflower oil.15

Considerable advances in UF and MF technologies inwater purification processes for drinking water produc-tion have been achieved to such a point that, presently,more than 1 000 000 m3/day of water are treated usingthese membrane operations.16,17 The employment ofintegrated membrane systems in the production ofdrinking water is growing rapidly with excellent results.The reliability of the reverse osmosis membrane isgreatly increased when UF or MF operationsswhichemerged in the past decade as an efficient way toremove suspended solids and organic and microbiologi-cal contaminantssare used in the pretreatment step.Furthermore, economical considerations have shownthat multiple membrane systems are more competitivethan conventional processes, resulting in the reductionof capital and operating costs.

In addition to the already-mentioned membraneoperations, gas separation, pervaporation, and someothers membrane processes, which have recently shownsignificant possibilities for their application in variousindustrial areas, must be cited; among these, a class ofmembrane-based unit operations identified as mem-brane contactors, membrane bioreactors, and catalyticmembrane reactors will be discussed.

Gas Separation. Membrane processes for gaseousmixture separation are, today, technically well-consoli-dated and apt to substitute for traditional techniques.18

Separation of air components, natural gas dehumidi-fication, and separation and recovery of CO2 from biogas

Figure 1. Recovery of Iopamidol by membrane process.12


and of H2 from industrial gases are some examples inwhich membrane technology is applied at the industriallevel.

The gas separation business was evaluated in 199619

at $85 million in the U.S., with growth of about 8% peryear.

Asymmetric polymeric membranes, used for gas mix-ture separations, are made either as plane sheets andassembled in spiral-wound modules or as hollow fibers.These modules are made and commercialized by variouscompanies all over the world. Although the kind ofmodule used is declared, the type of polymer is stillprotected as industrial know-how.

In Table 4, some permeability and selectivity data forthe various polymers used in the manufacture of themost commercial membranes are reported.20,22

The separation of air components or oxygen enrich-ment has advanced substantially during the past 10years. The oxygen-enriched air produced by membraneshas been used in various fields, including chemical andrelated industries, the medical field, food packaging, etc.In industrial furnaces and burners, for example, injec-tion of oxygen-enriched air (25-35% oxygen) leads tohigher flame temperatures and reduces the volume ofparasite nitrogen to be heated; this means lower energyconsumption. Mixtures containing more than 40% v/vof O2 or 95% v/v of N2 from the air can be obtained.Industrial nitrogen is used in the chemical industry toprotect fuels and oxygen-sensitive materials.

Membranes today dominate the fraction of the nitro-gen market for applications less than 50 tons/day and

relatively low purity (0.5-5% O2). Single-stage operationis preferred. Oxygen is the third largest commoditychemical in the U.S. with annual sales in excess of $2billion. Whereas nitrogen membrane separation hasbeen a great success, oxygen separation using mem-branes is still underdeveloped. The major reason for thisis that most of the industrial oxygen applications requirepurity higher than 90%, which is easily achieved byadsorption or cryogenic technologies but not by mem-branes. Today’s limited application of membrane-basedoxygen generation systems operate either under feedcompression or permeate vacuum mode (Figure 3). Bothmethods of separating oxygen are inferior to the adsorp-tion separation processes using various zeolites.

New materials are being developed that could possiblyhave higher permeabilities than conventional solidelectrolytes, in which ionized atoms are transportedthrough the crystalline lattice under a driving forceprovided by partial pressure differences over the mem-brane (pressure-driven process) or by electrical potentialgradients (electrochemical pumping).

Mixed conductors with high electronic and oxygen ionconductivities could be used as a membrane alternativeto solid electrolytes for oxygen separation. In such ma-terials, both oxygen ions and electronic defects aretransported in an internal circuit in the membrane ma-terial. Promising oxygen permeation fluxes have beenobtained in many perovskite systems, e.g., La-Sr-Co-Fe-O,23,24 Sr-Fe-Co-O,25,26 and Y-Be-Co-O.27

In particular, in the ITM-oxygen systems, simulta-neous conduction of ions and electrons in the same

Figure 2. Flow sheet of an industrial system for the debacterization of fresh milk by cross-flow microfiltration (Villefranche, Lyon,France).


material obviates the need for an external electricalcircuit to provide the driving force for the separation,with a significant reduction in cost. The driving forcefor the separation process is the partial pressure dif-ference across the membrane. High-pressure air (100-300 psia) is required to achieve a significant flux of O2across the membrane. The oxygen flux is directlyproportional to the pressure gradient and inverselyproportional to the membrane thickness. The pressureof the oxygen product is typically only a fraction of anatmosphere.

These dense inorganic perovskite type membranes,today manufactured in tubular configurations, transportoxygen as lattice ions at elevated temperatures withinfinite selectivity ratios in O2 separations. The ionic

conductivity of the material studied is mainly equal tothe electronic conductivity.

Because this oxygen-ion-conducting membrane mustoperate at temperatures above 700 °C, an effectivemeans of recovering the energy contained in the non-permeate, oxygen-depleted stream is required. An ef-ficient and cost-effective means to accomplish this is tointegrate the membrane system with a gas turbine(Figure 4).28

A technology known as OTM syngas (oxygen trans-port membrane synthesis gas) utilizes these ion-conducting membranes able to separate oxygen from airwith a high flux in the same temperature regionrequired for the reforming of natural gas. This technol-ogy was presented in 1997 by an alliance of five

Table 4. Permeability and Selectivity Data of Some Polymers Used in the Manufacture of Commercial Membranes forGas Separationa

permeability coefficient,barrer

selectivity(ideal) (-)

CO2 O2 N2 CO2/N2 O2/N2

poly[1-(trimethylsilyl)-1-propyne] 28 000 7730 4970 5.60 1.55poly(dimethylsiloxane) 4550 781 351 13.0 2.22poly(dimethylsilmethylene) 520 91.0 35.9 14.5 2.53poly(cis-isoprene) 191 37.5 14.5 13.2 2.60poly(butadiene-styrene) 171 32.9 10.3 16.6 3.19natural rubber (at 25 °c) 153.0 - 9.43 16.2 -ethyl cellulose 5.0 12.4 3.4 22.1 3.65polystyrene 12.4 2.9 0.52 23.8 5.58butyl rubber 5.18 - 0.324 16.0 -poly(ethyl methacrylate) 7.01 1.9 0.33 21.2 5.76poly(phenylene oxide) (at 25 °c) 75.7 - 3.81 19.9 -bisphenol A polycarbonate 6.8 1.6 0.38 17.9 4.21cellulose acetate 4.75 0.82 0.15 31.7 5.47bisphenol A polysulfone 4.6 1.2 0.19 24.2 6.32PMDA-4,4′-ODA polymide 2.7 0.61 0.1 27.0 6.10poly(methyl methacrylate) 0.62 0.14 0.02 31.0 7.00poly(vinyl chloride) (at 25 °C) 0.157 - 0.0118 13.3 -PEEK-WC (at 25 °C) 2.75 0.5 0.1 27.5 5.00polyphosphazeny (at 25 °C) 5.76-10.2 0.955-1.72 - 21.2-30.5 3.71-5.05a At 35 °C unless otherwise specified.

Figure 3. Oxygen production systems.

Figure 4. Integrated oxygen and power production.


international companies including AMOCO, BP Chemi-cals, PRAXAIR, SASOL, and STATOIL. Philippe Pe-troleum joined the alliance in 1998. The new process,still under development, integrates the separation ofoxygen from air, steam reforming, and natural gasoxidation into one step, eliminating the need for aseparate oxygen plant. The new technology offers thepossibility of reducing the energy and capital costs ofsyngas production. Considering that 60% of the cost formanufacturing any product from natural gas is relatedto synthesis gas production, the interest of this innova-tion technology is evident.

Various plants for the recovery of hydrogen from thepurge of the synthesis of ammonia have been realizedtoday.29 The unit modules are in general arranged in a“one-stage-two-unit” form. One of the first plants of thistype has been realized by Permea in Louisiana (Figure5).30 The first unit, consisting of eight hollow-fibermodules [total feed capacity about 3800 m3(stp)/h] isoperated with a transmembrane pressure difference of60 bar, the permeate leaving at a pressure of about 70bar. At this pressure, the permeate can be fed to thesecond stage of the synthesis feed compressor. Theretentate of the first unit is fed to the second unit wherethe permeate leaving the modules at 25 bar is mixedwith fresh feed (suction side of the first stage of thecompressor). The retentate is utilized for heating pur-poses. Gas pretreatment consist of conventional scrub-bing to reduce the ammonia content of the bleed from2% (molar) to less than 200 ppm in order to avoidmembrane swelling and, as a consequence, damage ofthe membrane. The economical and technical advan-tages related to this membrane system for the recoveryof hydrogen are shown in Table 5.

Methanol synthesis is another process based on agaseous feed; in purge recovery, a water scrubber is alsoused with a similar purpose, and it pays for itself interms of the recovered methanol. The methanol/watermixture is simply sent to the existing crude methanoldistillation column. Hydrogen recovered from this purgecan result in energy savings, and if additional carbonoxide is available, it can be used to obtain increasedmethanol production. PRISM separators operate on

stoichiometric as well as nonstoichiometric H2/(CO)xratio methanol plants at differential pressures up to 70bar. Figure 6 shows the flow diagram of such a hydrogenrecovery unit installed for demonstration purposes.31,30

Before entering the gas permeators, the feed is scrubbedin order to reduce the methanol content to levels below100 ppm. From a bleed stream of 4000 mol/h, forexample, a recovery of 2000 mol/h of hydrogen has beenachieved.

Gas mixture dehumidification is a process of greatindustrial interest, especially for natural gas purifica-tion and air dehumidification. An efficient membranesystem for air dehumidification called the Cactus Mem-brane Air Dryer, developed in the late 1980s, has beencommercialized by Permea.32 When the Cactus dryer isfed with compressed air, water vapor and a smallamount of oxygen pass through the walls of the hollowfiber, while nitrogen, argon, and most of the oxygencontinue through the hollow core of the fibers to the endof the separator. A small amount of the slower gasespasses through the fiber, and this is used to sweep thewater vapor through the separator. Cactus membraneswork on the principle of dew point depression. Forexample, a membrane might be sized for inlet conditionsof 100 psig and 100 °F inlet dew point to achieve a 0 °Fpressure dew point. If inlet conditions change, e.g.,compressed air with a lower inlet dew point is supplied,the separator will provide dry air at an even lower dewpoint.

The removal of hydrogen sulfide (H2S) and carbondioxide (CO2) from natural gas is an ideal applicationfor membranes (Figure 7); both H2S and CO2 permeatethrough membranes at a much higher rate than meth-

Figure 5. Scheme of a plant for H2 recovery from ammonia synthesis.

Table 5. Economic and Technical Advantages for a 1000ton/day Ammonia Plant30

ammonia recovery (scrubbing) 4 ton/dayheat saving 522-836 kJ/ton of

NH3 producedadditional ammonia production 50-55 ton/dayincrease in ammonia production(at constant natural gas consumption)

20-50 ton/day

reduction in natural gas production(at constant production rate)


ane, enabling a high recovery of the acid gases withoutsignificant loss of pressure in the methane pipelineproduct gases.

These membrane processes are going to substitute forthe more traditional methods of hydrocarbon streampurification. Through a comparison of the separationcost for the membrane process with that for the dietha-nolamine (DEA) gas-absorption process, it was foundthat the membrane process is more economical than theDEA gas-absorption process in the range of CO2 con-centrations in the feed between 5 and 40 mol %. Whenthe feed also contains H2S, the cost for reducing the CO2and H2S concentrations in the feed to pipeline specifica-tions increases with increasing H2S concentration (1000to 10 000 ppm). If membrane processes are not economi-cally competitive because of the high H2S concentrationin the feed, the separation cost could be significantlylowered by using hybrid membrane processes. In suchprocesses, the bulk of CO2 and H2S is separated fromsour natural gas with membranes, and the final puri-fication to pipeline quality gas is performed by meansof suitable gas-absorption processes.33 Despite the highlevels of H2S in the feed, membrane selectivities aremaintained.34

The possibility of utilizing membrane technology insolving problems such as the greenhouse effect relatedto CO2 production has also been suggested.

Membranes able to remove CO2 from air, having ahigh CO2/N2 selectivity, might be used at any large-scaleindustrial CO2 source as a power station in petrochemi-

cal plants. The CO2 separated might be converted byreacting it with H2 in methanol, starting a C1 chemistrycycle.

As schematized in Figure 8, a membrane reactormight be ideally used to carry out hydrogenation reac-tions for chemical production using CO2 recovered fromexhaust gases by membrane separation.

The separation and recovery of organic solvents fromgas stream is also rapidly growing at the industriallevel.

Polymeric rubbery membranes that selectively per-meate organic compounds (VOC) from air or nitrogenhave been used. Such systems typically achieve greaterthan 99% removal of VOC from the feed gas and reducethe VOC content of the stream to 100 ppm or less. Thetechnology has been applied to the recovery of high-value organic vapors such as vinyl chloride monomer,methyl chloride, and methyl formate.

Membrane systems are competitive with carbon ad-

Figure 6. Hydrogen recovery from the bleed of a methanol synthesis.

Figure 7. Removal of H2S and CO2 from natural gas.

Figure 8. Recovery of CO2 from exhaust gas and reuse to producechemicals by hydrogenation.


sorption or condensation for streams containing morethan 5000 ppm, particularly if high VOC recovery isrequired.

The typical industrial applications of vapor recoveryare off-gas treatment in gasoline tank farms, gasolinestation vapor return, and end of pipe solvent recoveryin the chemical and pharmaceutical industries. Anotherinteresting example of an industrial application is VOCrecovery by the compression-condensation and vaporpermeation method, presented schematically in Figure9. This is a scheme of the process developed in Anwil(Wloclawed, Poland), which has been built by MTR(U.S.) for the recovery of monovinyl chloride (MVC).

The recovery of ethylene and propylene from nitrogenin polyolefin plant vent streams has been suggested andrealized at the industrial level by DSM in Geleen, TheNetherlands.

To recover the unreacted monomer (up to 25%) andother volatile hydrocarbons from the nitrogen usedduring polymer particles degassing, MTR35 developeda two-stage operation in which the mixture of N2 andpropylene is first compressed and later directed into amembrane vapor separation unit, as shown in Figure10. The spiral-wound membrane modules (8 in. diam-eter, 20 m2 surface area) used are 10-100 times morepermeable to organic vapors than to air or nitrogen.

In 1989, the first vapor recovery unit (VRU) basedon membrane technology was commissioned for off-gastreatment in a gasoline tank farm. At present, variousmembrane VRU’s are in operation or under construc-tion. The capacity of these units ranges from 100 to 2000m3/h. These are single-membrane stages of hybridsystems of a membrane stage combined with a post-

treatment facility, e.g., a catalytic incinerator, gasengine, or pressure swing adsorption unit. These plantsare equipped with a modified plate and frame configu-ration.36

A case of a vapor recovery unit based on membranetechnology is that commissioned in a gasoline tank farmin Munich for the treatment of the off-gasses generatedfrom the storage, handling, and distribution of gasoline.The plant capacity was 300 m3/h. The only externalavailable energy source was the electrical power supply.This was planned in the framework of a pilot projectfor the reduction of emissions at the BP tank farmHamburg-Finkenwerder. The VRU has a capacity of1500 m3/h and a hybrid system of a membrane stageand a gas engine. Two gas engines coupled with agenerator are permanently in operation to supply thebasic electrical power of the side. The gas engines aredesigned to switch the fuel feed from natural gas toretentate of the membrane stage over a period of VRUoperating time.

A commercially successful application is a hybridsystem of a membrane stage with pressure swingadsorption (PSA) (Figure 11). The liquid ring compres-sor operating with gasoline as the service liquid sucksthe hydrocarbon (HC) contaminated air from the gas-ometer. The off-gases are compressed and fed into ascrubber. Gasoline from the tank farm is used as a leanabsorbent. The HC concentration of the feed gas leavingthe scrubber depends on the operating temperature andpressure. The layout of the membrane stage (membranearea and permeate pressure) is governed by the permis-sible HC intake concentration of the PSA unit. Twoparallel PSA units are installed and operated alter-

Figure 9. Flow diagram of compression/condensation and membrane separation for MVC recovery.

Figure 10. Flow sheet of two-stage recovery system of unreacted monomer and other volatile hydrocarbons from the nitrogen usedduring polymer particle degassing (MTR).


nately. One is in the adsorption phase while the otheris in the desorption and regeneration phase. A bypassof the clean stream is used as a purge gas for regenera-tion. To maintain a low vacuum, the vacuum pump atthe downstream side of the membrane stage can be aliquid ring pump with mineral oil as the service liquidor a rotary vane vacuum pump. This vacuum pump isalso used to support the desorption of the PSA column.The adsorber material is activated carbon, a carbonmolecular sieve, or an inorganic molecular sieve. Atypical VRU combined with a PSA is installed at Shellin Ludwigshafen.36

Other interesting applications of the technology mightbe in the separation of light hydrocarbons from refinerywaste gas streams, the recovery of natural gas liquidsand hydrogen, or the separation of propane, butane, andhigher hydrocarbons from methane in the processing ofnatural gas for dew point control.

Pervaporation. Some applications of pervaporationprocesses are listed in Table 6.

Dehydratation of ethanol by PV was the first indus-trial-scale application proposed by GFT in the 1980s.Today, more than 40 industrial pervaporation plantsbuilt by Sulzer Chemtech Membrantechnik (formerGFT) are in operation worldwide. They are used for thedehydration of different solvents and/or solvent mix-tures.

In many practical applications, it might be moreeconomical to use pervaporation or vapor permeationonly to break the azeotrope and to concentrate theretentate further by the above-azeotropic distillation(Table 7).

Another successful example of PV is its applicationin the enhancement of chemical reaction efficiency.Examples of such reactions are esterification or phenol-acetone condensation. The first industrial plant for the

Figure 11. Membrane stage with integrated pressure swing adsorption.

Table 6. Practical Applications of Pervaporation

application details

separation of water from organic/aqueous mixtures

separation and/or dehydration of water/organic azeotropes(water/ethanol, water/2-propanol, water/pyridine)dehydration of organic solventsshifting of the reaction equilibrium (e.g., esterification)

removal of volatile compounds from aqueous andgas streams

removal of chlorinated hydrocarbonsseparation of organics from the fermentation brothseparation of aroma compoundswine and beer dealcoholizationremoval of VOCs from air

separation of organic/organic mixturesseparation of azeotropes (e.g., ethanol/cyclohexane,methanol/MTBE, ethanol-ETBE)separation of isomers (e.g., xylenes)

Table 7. Comparison of the Dehydration Costs of Ethanol from 99.4 to 99.9 vol % by Different Techniques

utilities

vaporpermeation

($/ton)pervaporation

($/ton)

entrainerdistillation

($/ton)

molecular sieveadsorption

($/ton)

vapor - 12.80 120.00 80.00electricity 40.00 17.60 8.00 5.20cooling water 4.00 4.00 15.0 10.00entrainer - - 9.60 -replacement ofmembranes andmolecular sieves

19.00 30.60 - 50.00

total costs 63.00 64.0 152.60 145.20


pervaporation-enhanced ester synthesis was built in1991 by GFT for BASF.

A possible application of the removal of organicsolutes could be the treatment of industrial and mu-nicipal water supplies contaminated with carcinogenichalogenated organic compounds. Such a process wouldalso be attractive for the extraction of organics.

The possibility of recovering volatile organic com-pounds from gases by pervaporation has been demon-strated and applied recently at the industrial level.37

The elimination of volatile solutes from dilute aqueoussolutions might be possible by pervaporation.

Separation of organic/organic mixtures represents theleast-developed application and the largest potentialcommercial impact of pervaporation, but considerabledevelopments in membrane materials and processesremains to be done. The first industrial application ofPV to organic/organic separation was the separation ofmethanol from a methyl tert-butyl ether (MTBE) streamin the production of octane enhancer for fuel blends(Figure 12).

Flexibility with respect to part-load performance andchanging product and feed concentrations is one of theadvantages of pervaporation over other separationprocesses. This is especially useful in the production offine chemicals and in the pharmaceutical industry,where solvents are used and almost no single wastesolvent is generated continuously.

Pervaporation-based hybrid processes offer significantpotential for new economical and efficient solutions tosome classical separation problems.38

Membrane Contactors. In these systems the mem-brane function is to facilitate diffusive mass transferbetween two contacting phases, which can be liquid-liquid, gas-liquid, gas-gas, etc.39 The traditional strip-ping, scrubbing, absorption, and liquid/liquid extractionprocesses can be carried out in this new configuration.

With respect to conventional systems, membranecontactors can guarantee some advantages such asnondispersion of the phases in contact, independentlyvariable flow rates without flooding limitations, lack ofphase-density difference limitations, lack of phase sepa-ration requirements, higher surface area/volume ratios,and direct scale-up due to a modular design.

Traditional liquid-supported membranes in which acarrier is immobilized in the microporous hydrophobicstructure of the polymeric membranes are the mosttraditional and well-developed example of a membranecontactor system.

Other applications, however, have been studied andrealized today or are under investigations. Interestingexamples include the removal of trace of oxygen (atlevels of <10 ppb) from water for ultrapure waterpreparation for the electronics industry,40 the removalof CO2 from fermentation broth (Figure 13), and thesupply of CO2 as a gas to liquid phases (carbonation ofsoft drinks).41 The flow sheet of a water carbonationprocess is presented in Figure 14. Additional examplesinclude the removal of alcohol from wine and beer, theconcentration of juice via osmotic or membrane distil-lation,41 the nitrogenation of beer,42 the degassing oforganic solutions, and water ozonation.43

Figure 12. Pervaporation-enhanced MTBE production.

Figure 13. Membrane system for CO2 recovery from fermentation broth.


In particular, in the carbonation process, hollow fibershave generally been used for industrial units. Duringoperation, an aqueous liquid flows over the shell side(outside) of the hollow fiber. A strip gas or vacuum,either separately or in combination, is applied on thelumen side of the hollow fiber and flows counter current.Because of its hydrophobic character, the membraneacts as an inert support to allow intimate contactbetween the gas and liquid phases without dispersion.The interface is immobilized at the pores by applying ahigher pressure to the aqueous stream than the gasstream. The result is fast diffusive transfer of dissolvedgases from or to the liquid phase.

Since 1993, a bubble-free membrane-based carbon-ation line has processed about 112 gal/min of beverageby membrane contactors having a total interfacial areaof 193 m2 (Pepsi bottling plant in West Virginia).40

Permea commercializes beer dispensing systems knownas CELLARSTREAM Dispense Systems using PULSARgas/liquid contactors, which increase or decrease theamount of carbon dioxide and nitrogen in draft beer foroptimal presentation.42

Membrane distillation and osmotic distillation can beconsidered examples of membrane contactors for real-izing the concentration of aqueous solutions with non-volatile solutes as salts and sugars.44-47 In the mem-brane distillation process, two liquids or solutions atdifferent temperatures are separated by a porous mem-brane acting as a barrier between the two phases, whichmust not wet the membrane (this implies that hydro-phobic membranes must be used in the case of aqueoussolutions). Because of the temperature gradient, a vaporpressure difference exists across the membrane, and itis the driving force inducing vapor molecule transportthrough the pores from the high-vapor-pressure side tothe permeate side. In the case of osmotic distillation,the vapor molecule transport is due to a vapor pressuredriving force provided by having a low-vapor-pressuresolution on the permeate side of the membrane, e.g., aconcentrated salt solution.

The formation of emulsions or dispersions character-ized by very uniform dimensions of droplets or micro-bubbles can be realized using the same technology.

The membrane emulsification process is appliedmainly in the preparation of food emulsions. Moreover,microbubble formation increases the stability of thesystem by minimizing coalescence phenomena. Aninteresting study evidenced the relationship existingbetween membrane pore diameter and droplet size.48

The formulation of various products might be realized

using this new concept, and important phenomena suchas oil combustion might be optimized.

Membrane Reactors

The possibility of combining molecular separation andchemical transformations in a single unit soon attractedthe interest of membrane engineers.49 The first studieson such reactors were devoted to the immobilization ofbiocatalysts on polymeric membranes. Recently, high-temperature reactions have been the objective of im-portant studies. Both areas will be analyzed in thefollowing pages.

Membrane Bioreactors. Biocatalytic membranereactors are interesting with respect to conventionalmembranes as they combine selective mass transportwith chemical reactions. The selective removal of prod-ucts from the reaction site increases conversion ofproduct-inhibited or thermodynamically unfavorablereactions.

Biocatalysts can be used suspended in solution andcompartmentalized by a membrane in a reaction vesselor immobilized within the membrane matrix itself.50

Since the advent of what has been called solid-phasebiochemistry, the advantages of immobilized biocatalyticpreparations over homogeneous-phase enzymatic/cel-lular reactions have been exploited to develop new andless-expensive processes.

Synthetic membranes provide an ideal support forbiocatalyst immobilization because of a wide availablesurface area per unit volume and the possibility for thedevelopment of new immobilization procedures. En-zymes are retained in the reaction side, do not pollutethe products, and can be continuously reused. Im-mobilization has also been shown to enhance enzymestability. Moreover, provided that membranes of suit-able molecular weight cutoff are used, chemical reactionand physical separation of biocatalysts (and/or sub-strates) from the products can take place in the sameunit. Substrate partition at the membrane/fluid inter-face can be used to improve the selectivity of thecatalytic reaction toward the derived products withminimal side reactions. Membranes are also attractivefor retaining in the reaction volume the expensivecofactors that are often required to carry out someenzymatic reactions.

At the 1997 Achema conference in Frankfurt, Ger-many, statements on the impact that innovative biore-actors, and particularly those based on the hollow-fiberdesign, have in setting new performance standards were

Figure 14. Simplified flow sheet for the water carbonation process.


clearly presented. For example, hollow-fiber bioreactorsin which cells attach into a capillary-type space havebeen designed to mimic biological processes more closelythan any other reactor system. Through the fibers,nutrients such as glucose and oxygenase are fed to thecells, and wastes such as CO2 and H2O are removed.

Roche Diagnostic declared the use of such reactors toproduce monoclonal antibodies for diagnostic tests.

Membrane bioreactor technology can also be appliedto produce pure enantiomers, in that a membraneseparation process can be combined with an enantiospe-cific reaction to obtain a so-called “enantiospecificmembrane reactor”. As for general membrane reactors,the result is a more compact system with higherconversion. This technology can respond to the stronglyincreasing demand for pharmaceuticals, food additives,feeds, flavors, fragrances, agrochemicals, etc., as opti-cally pure isomers.51

Recently, the results achieved in the production of achiral intermediate used for the preparation of animportant calcium channel blocker, diltiazem, werediscussed in the open literature,52 confirming the pos-sibilities of membrane reactors also in the large-scaleproduction of biotechnological products.

Phase-transfer catalysis can also be realized in mem-brane reactor configurations, immobilizing the ap-propriate catalysts in the microporous structure of thehydrophobic membranes.

Biphasic membrane reactors have been extensivelystudied with lipases entrapped or bonded on the mem-brane surface, which confirms the possibilities of theapproach,53,54 as already discussed.

Catalytic Membrane Reactors. The developmentof catalytic membrane reactors for high-temperatureapplications became realistic only in the last few yearswith the development of high-temperature-resistantmembranes. In particular, the earlier applicationsinvolved mainly dehydrogenation reactions, where therole of the membrane was simply hydrogen removal.

The earlier studies carried out, particularly in theSoviet Union on palladium and palladium alloys, con-firmed the existence of membranes able to permeate H2with high selectivity. Both capital and operative costsavings were anticipated, as units for hydrocarbonseparation from the streams were avoided and thepossibility of operating at lower temperatures becauseof reactor yield enhancement was realized. The fact thatthe membranes separate intermediates and productsfrom the reacting zone, avoiding possible catalyst de-activation or secondary reactions, is also of practicalinterest. The kinetic mechanisms might be modified orcontrolled by the presence of appropriate membranesystems, which can also act only on a reactive interfacewith no permselectivity, optimizing phase-transfer ca-talysis.55

Palladium membrane costs and availability, theirmechanical and thermal stability, and poisoning andcarbon deposition problems are still obstacle to thelarge-scale industrial application of these dense metalmembranes, also when prepared in a composite config-uration.56

Hydrogen can be produced by steam reforming andshift conversion of natural gas or other hydrocarbons.In conventional steam reformers, high conversions ofnatural gas, on the order of 85-90% or even higher, areobtained at reformer outlet temperatures of around850-900 °C. The energy efficiency of steam reforming

processes is relatively high, but the investments aresubstantial. Pure hydrogen can be produced at signifi-cantly lower temperatures by integrating into the reac-tor a membrane that selectively removes hydrogenduring conversion. Potential savings in membranereformer and downstream processing costs compared toconventional steam reforming apparatuses must, inmany cases, be weighed against additional costs associ-ated with recompression of the hydrogen permeatestream.

Ag membranes were initially suggested for their H2permeability. Howevere, they present the same prob-lems that characterize Pd membranes, also having amuch lower permeability.

Solid oxide membranes have recently been suggestedfor large-scale applications in syngas production.57

Studies carried out in the U.S. showed the possibilityof preparing membranes with improved mechanical andthermal characteristics, able to operate, for example, at900 °C for over 21 days.

Integrated Membrane Processes

Traditionally, the various membrane operations (RO,UF, MF, etc.) have been introduced in industrial pro-duction lines as an alternative to other existing units.Reverse osmosis instead of distillation and ultrafiltra-tion in place of centrifugation are typical examples.

The possibility of redesigning overall industrial pro-duction by the integration of various already developedmembrane operations is becoming of particular interest,because of the synergic effects that can be reached, thesimplicity of the units, and the possibility of advancedlevels of automatization and remote control that can berealized. The rationalization of industrial production byuse of these technologies permits low environmentalimpacts, low energy consumption, and higher qualityof final products. New products also often becomeavailable.

These results are related to the introduction of newtechnologies from the very early stages of the samematerial transformations and not at the end of the pipe,as was often done in the past.

The leather industry might be an interesting casestudy because of (1) the large environmental problemsrelated to its operation, (2) the low technological contentof its traditional operations, and (3) the tendency toconcentrate a large number of small-medium industriesin specific districts. More than 2000 companies are inoperation in Italy, which is recognized as a world leatherleader for the quality of the leather produced. Thetraditional flow sheet of the tanning process in its humidphase consists of about 20 steps operating in a discon-tinuous cascade system. The possibility of rationalizingthe overall process by introducing advanced molecularseparation systems such as ultrafiltration, cross-flowfiltration, microfiltration, nanofiltration, and reverseosmosis was suggested and has recently also become theobjective of an Italian National Research Programcoordinated and carried out by a consortium represent-ing most of the companies in the country. In Figure 15is presented an ideal process based on integratedmembrane operations.58,59

The innovative integrated scheme suggested in Figure15 allows the pollution problems of the leather industryto be faced by solving or minimizing them one by onewhere they originate, thereby avoiding the need for hugewastewater treatments at the end of the overall produc-


tion line. The fact that membrane operations act byphysical mechanisms without modification of the chemi-cal procedure at the origin of the final high quality ofthe leather should also be mentioned.

The possibility of also introducing an enzyme mem-brane reactor as an alternative to the traditional chemi-cal dehairing process and for the optimization of thedegreasing step has also been considered. The recoveryof the proteins produced in dehairing in the retentateof an ultrafiltration unit and the recovery and reuse inthe same process of the excess of sodium sulfide usedand separated in the permeate became realistic whenUF tubular membranes able to operate at the high pH(>12) characterizing the dehairing bath were preparedcommercially. It is interesting to consider that around40% of the overall pollution in leather processingoriginates in the dehairing step, where only 10% of theoverall liquid stream is generated. The problem ofchromium used in the tanning step has always beencrucial for its negative environmental impact. Theexhaust chromium coming from the tanning bath canbe recovered and concentrated by a two-stage processbased on MF or UF as a pretreatment and nanofiltrationas a concentration technique.13 The concentrated chro-mium solutions obtained by NF have an improvedquality with respect to those obtained by the conven-tional recovery process of chemical precipitation becauseof the optimally low ratio of organic lipolytic component/chromium characterizing the new product. If necessary,

the recovered chromium solution can be further con-centrated by traditional techniques. These recoveredsolutions were used in sheepskin retanning and tanningoperations; the skins showed improved physical andchemical characteristics compared to those obtainedwith the traditional chromium solutions. The permeatefrom the nanofiltration unit, considering its high contentof chlorides, might be used in the pickling phase,realizing an interesting closed-loop process. In Figure16, the schematic flow sheet of chromium recovery isshown.

The possibility of a membrane-based posttreatmentof secondary textile wastewater for the direct reuse ofpolished effluent within the dyeing process was alsorecently verified by tests at the pilot scale.14 A firsttreatment scheme examined requires two filtrationsteps: membrane microfiltration followed by nanofil-tration. The preliminary filtration on ceramic MFmodules reduces the fractions of pollutants (suspendedsolids and colloids) that can induce a rapid fouling inthe nanofiltration membrane. An addition of highconcentrations of aluminum polychloride (10-70 mg/L) is necessary to obtain satisfactory performance of thetreatment system. Permeate quality confirms the pos-sibility of reusing the secondary effluent for textileindustry purposes, but approximate preliminary calcu-lations on this coupled membrane process indicate that,at present, this process cannot be transferred to a full-

Figure 15. Flow sheet of some humid phases of the tanning process integrated with membrane operations.


scale plant, because of the high price of the ceramic MFmembranes and the need for high dosing of coagulants.

A second treatment scheme studied requires a clari-fication-flocculation step followed by multimedia filtra-tion prior to a low-pressure reverse osmosis operation.The clarification-flocculation/filtration is aimed at re-moving the colloidal fraction that promotes fouling ofRO membranes. A polished effluent of high quality(COD < 0.10 mg/L; conductivity < 40 mS; negligibleresidual color) that can be reused in textile mills isobtained. Costs for the complete polishing by low-pressure RO are comparable to the costs of conventionalsecondary wastewater treatment and are quite afford-able (on the order of 0.20-0.25 Euro) even for the Italiansituation, where price of water is much lower than inmost industrialized countries.14

Interesting cases of integrated membrane processescan also be found in the agrofood industry, in waterdesalination, in biotechnological production, etc.

In the dairy industry, single-membrane operationssuch as UF in the treatment of wheys, cross-flowmicrofiltration in the stabilization of milk, and RO inthe concentration of milk or in lactose concentrationhave been widely applied in the past year. As alreadymentioned, an overall quantity of about 250 000 m2 ofUF membranes were already installed in 1999 andaround 165 000 m2 of RO membranes.

Successful application of integrated membrane opera-tions in fruit juice concentration (in an osmotic distil-lation process) has been developed by the Australiancompany The Wingara Wine Group (Melbourne, NSW).A hybrid pilot plant in which UF/RO and osmotic

distillation are integrated has been realized. It consistsof UF and RO pretreatment stages, an osmotic distil-lation unit, and a single-stage brine evaporator. Thisplant concentrates fresh juices up to 65-70°Brix andhas a capacity of 50 L/h. Being athermal, osmoticdistillation allows for concentration of the juices withoutproduct deterioration or loss of flavors.60

Hogan et al.61 reported a total process cost of osmoticdistillation concentration on the order of $1.00/L ofconcentrate. From 1 L of fresh juice, it is possible toachieve about 200 mL of 70°Brix concentrate. The valueof this concentrate is between $2.50 and $7.50/L. Fromthese data, the economical advantages of the integratedmembrane process seem evident.

The coupling of RO and membrane distillation forobtaining high recovery factors has been also tested infruit juice concentration.62

The potential for osmotic distillation flux enhance-ment in grape juice concentration by ultrafiltrationpretreatment has recently been investigated.63

Today, the integration concept finds interesting suc-cess in the use of membrane operations for brackishwater treatment. Large-scale applications after manyyears of trials with other membranes have recently beensuccessfully realized.

For instance, at the end of 1999, the world’s largestintegrated membrane system was put into operation byPWN Water Supply Company North Holland in TheNetherlands for drinking water production from lakewater. Ultrafiltration and reverse osmosis are the mostessential process elements of this treatment plant,having a capacity of 18 000 000 m3/year.64

Figure 16. Scheme of the proposed process for reuse of exhausted chromium solutions from tanning operations.


In Bahrain, the Addur SWRO desalination plant isplanned for rehabilitation utilizing ultrafiltration mem-branes instead of the traditional flocculation-clarifica-tion process65 in water pretreatment.

The traditional seawater or brackish water desalina-tion process can be reconsidered by optimizing thepretreatment by MF and/or UF and by adding NF beforethe RO step. The introduction of a membrane distilla-tion stage operating on the RO retentate might permitrecovery factors on the order of 87-88% (while the ROunit alone worked with around a 40% of recovery factor)at costs that might be acceptable in various situations.

Typically, the process systems for wastewater treat-ment are designed with reverse osmosis operation asthe final treatment step and require several processsteps, a large land area, and high capital investmentand operating costs. Microfiltration and ultrafiltrationmembranes simplify the conventional water reuse pro-cess by treating effluent directly from the secondaryclarifier, with a simpler process that is easy to operate,requires less land area, and is less vulnerable to processdisruption (Figure 17).

There are also some advantages in combining amembrane bioreactor with a reverse osmosis step. Thisprocess solution increases the life of the RO membraneand overall facility productivity. A new commercialmembrane bioreactor for wastewater treatment alreadyused in this type of integrated membrane system isknown as the ZenoGem (ZENON Inc.) process. This unitconsists of a biological reactor integrated with immersedmembranes that form an absolute barrier to solids andbacteria and retain them in the process tank.

The benefit of using biocatalytic membrane reactorscombined with other membrane processes, such asmicrofiltration, ultrafiltration, reverse osmosis, mem-brane extraction, etc., for the production and processingof bioreactive compounds is also apparent. This integra-tion is particularly important for products obtained byfermentation processes, such as organic acids, antibiot-ics, etc., and in the processing of food and beverages,such as wine, fruit juices, milk, etc.

Energy Requirements

One of the significant and recognized benefits ofmembrane operations is their low direct energy con-sumption (in general electricity) because of the absenceof phase transformations. An important possibility forreducing indirect energy consumption through therecycling and reuse of raw materials or secondaryproducts and minimization of the formation of wastesalso characterizes these techniques. For a correct evalu-ation of the total energy involved, an energetic and

exergetic analysis of the overall integrated productionlines, if not of the complete system, is recommended.66

The total energy requirements can be estimated onthe basis of the overall supply of electrical energy ofpump engines and external equipments and the thermalenergy supplied.

The energy analysis must be elaborated in order toinclude all real involved variables, which generally arevery numerous and variously aggregated. It is necessaryto establish the exact size of the unit operations and ofall of the flows of mass and energy of the process. Theblock diagram of the operating phase of interest, con-nected to the recovery operation (Figure 18), or the blockdiagram of all of the productive process, integrated withthe new operation, can help to report, in a compact way,all of the pertinent information for the elaboration ofthe estimation.

The layout of the “traditional” process and the layoutof the alternative process, both completed with allinformation relative to the fluxes of matter and energy,can then be compared.

Because membrane operations utilize primarily elec-trical energy, the benefit estimate can be done usingthe “substitution coefficient” introduced by Electricitede France;67 this coefficient compares the primaryenergy saved to the electrical energy consumed in cyclesthat utilize electricity-consuming operations in substitu-tion for conventional thermal operations. The substitu-tion coefficient is defined as the ratio between theprimary energy (thermal) saved in the new process withrespect to the conventional process and the amount ofelectrical energy consumed, relative to the conventionalprocess: CS ) C1 - C2/E2 - E1, where CS is thesubstitution coefficient, C is the consumption of thermalprimary energy (MJ or Mcal), E is the consumption ofelectrical energy (kWh), and 1 and 2 are the relativeindexes of the conventional and innovating process,respectively.

Taking into account that 1 kWh of electrical energy,available at the utilization site, requires to burn, in apower station, 10.5 MJ of primary energy from acombustible source (oil, gas, coal, etc.), the substitution(or process innovation) results are advantageous whenCS is greater than 10.5 MJ/kWh (2.5 Mcal/kWh).

Other than energy, recovered and recycled materialsare also involved; therefore, it is necessary to evaluatetheir indirect energy content. For example, one shouldalso consider the energy consumed for the productionof a material and, therefore, intrinsically associated withit; the energy used for disposal of a material in dumping;the energy used for an inertization treatment of amaterial, if required; etc. Considering that a substance,

Figure 17. Water reuse using MF/UF pretreatment for RO.


to be produced, needs a certain amount of energy,recycling a substance means, in addition to an economicsaving, also an indirect energy saving corresponding tothe amount of primary energy that would be utilizedfor its production.

The results summarized in Table 8 indicate clearlythe energetic advantages of some suggested membraneoperations.68

New Membranes

More and more complicated and special separationproblems of liquid and gaseous mixtures in industry andbiomedical or medical technology require tailored fin-ished products made from potential membrane materi-als available on the market. The range of applicationfields involves widely spread uses in micro-, ultra-, andnanofiltration, dialysis, membrane electrolysis, or re-verse osmosis, as well as in fields such as high-temperature gas separation, hydrogen recovery fromsyngases, and also oxygen-enriched air. Accordingly, thenumber of investigated and established membranematerials has also simultaneously grown. Some ex-amples are listed in Table 9.

Among the variety of new thermoplastics developedup to now are some special polymers that have beenshown to be particularly suitable for membrane produc-tion. For example, the aromatic poly(etheretherketone)called Vitrex (PEEK produced by ICI) shows a remark-able long-term temperature stability of 250 °C, wherethe modulus of elasticity remains sufficient even at 150°C. Because of its mechanical and chemical stability,Vitrex represents a suitable material for the productionof hollow fibers. Another polymer tested for this purposeis called Tedur (polyphenyle sulfide, PPS, manufactured

by Bayer). It is characterized by a maximum long-termstability of 190 °C. Hollow fibers produced from PEEKand PPS show textile-like properties, so that flexiblemodules with large membrane areas and small volumefilling can be realized for the purposes of gas separation.Regarding separation performance, permeability, andthermal and mechanical stability, PEEK membranesare better than the PPS ones.69

Modified PEEK (PEEK-WC), an amorphous polymerexhibiting mechanical and electrical properties equal toor better than those shown by traditional PEEK, is alsosoluble in DMA, DMF, chlorohydrocarbons, etc., whichmakes possible its use for asymmetric membrane for-mation with the phase inversion procedure.70 Interest-ing results have been obtained with PEEK-WC densemembranes showing high O2/N2 selectivity and goodpermselectivity to water in the pervaporation of water/methanol mixtures, as well as in acetic acid aqueoussolutions.

Figure 18. General scheme of a system on which an energy balance can be planned.

Table 8. Total Substitution Coefficients Evaluated for Some Processes Integrated with Membrane Operations

membrane process analyzedCS

(MJ/kWh)

recovery and recycling of water in the textile industry 35.3recovery and recycling of a monomer in the chemical industry 30.8recovery and recycling of the sulfide in the tanning industry 36.0saving of thermal energy and fat substance recovery in the dairy industry 21.7thermal energy savings in tomato juice concentration 137.9

Table 9. Some New Investigated Membranes andMembrane Materials

thermostable polymeric membranes(PEEK, PPS, PEEK-WC, PEEK-WC functionalized, etc.)

polymeric membranes resistant to hostile environments(HYFLON AD, etc.)

H2 permselective dense membranes(Pd-based, dense SiO2, etc.)

O2 permselective dense membranes(Brownmillerite, solid oxides, etc.)

porous infiltrated composite membranes for MRs(dense silica/γAl2O3 composite membranes,VMgO/ZrO2/RAl2O3, perovskites/RAl2O3,VmgO/RAl2O3, VPO/RAl2O3, etc.)

inorganic nanofiltration membranes(ZrO2 on ceramic support)

hollow-fiber ceramic membranes


PEEK-WC modified by the introduction of NO2groups or by sulfunation are also studied for gasseparation applications.71,72

The use of PEEK-WC membranes functionalizedwith o-octyloxycarbonyl â-cyclodextrin derivatives tocarry out the hydrolysis reaction of p-nitrophenyl ace-tate to p-nitrophenol in phosphate buffer enhances thereaction rate with an enzyme-like behavior, improvingproductivity and stability and decreasing costs.73

Much research on the synthesis of more selective,permeable, and stable membrane materials for gasseparation has been done and is still ongoing all overthe world. For this purpose, some interesting resultshave been presented in the recent literature.

Novel silicone-coated hollow-fiber membrane modulesfor the removal of toluene and methanol from N2 havebeen tested.74 This novel membrane offers lower per-meation resistance than other silicone-based mem-branes because the selective barrier is ultrathin (1 µm)and the porosity of the polypropylene substrate is high.The bond between the substrate and the coating layerhas been obtained by plasma polymerization. Highseparation factors have been obtained (toluene/nitrogen)10-55; methanol/nitrogen )15-125; more than 96%of VOCs removed from a feed stream of 60 cm3/minwhen the permeate side was subjected to a highvacuum).74

New 6FDA-DAF polyimide membranes have beenobtained by simultaneous suppression of intersegmentalpacking and inhibition of intrasegmental motion witha significant increase in both permeability and selectiv-ity.75 These membranes have been tested with mixturesof He-CH4 and CO2-CH4. Permselectivities of heliumand carbon dioxide over methane are improved withrespect to the other polymers: the permeability of He/CH4 is 2.6 times higher in these membranes than inpolysulfone.

Roman76 recently presented new fiber spinning andprocessing technology and streamlined/automatedbundle-forming processes to reduce manufacturing costsand enable greatly increased production volumes. Animportant innovation has been the development of aproprietary co-extruded sheath/core fiber construction,effectively a thin asymmetric layer coated on a ruggedporous support. The mechanical support function isuncoupled from the permeation function, so both func-tions can be optimized and a large fraction of the fiberwall, the core in this case, can be made of an inexpensivepolymer to save on material costs. An additional ad-vantage of the sheath/core construction is that it helpsform a thinner skin in the sheath layer by allowing theuse of sheath spinning solution with low polymercontent and low viscosity (lower than could be used fora self-supporting monolithic membrane). The O2 and N2flow rates in Air Liquide’s N2 membrane have beenincreased 2-fold since 1990,77 via reduction of the

thickness of the separating layer and, to a lesser extent,changes in the material composition of the separatinglayer and optimization of the fiber size.

The good H2 permselectivity and permeability of therecently developed dense (Pd-based) and almost denseSiO2 membranes were successfully exploited for anumber of H2-consuming or -generating reactions. Forsome applications, the thermochemical instability of Pdmembranes and the hydrothermal instability of silicaare the main problems to solve. Concerning O2-generat-ing or -consuming reactions, the development of O2permselective membranes with good fluxes in the rangeof 400-700 °C is still needed. Promising Brownmilleritedense membranes were recently developed by EltronResearch Inc.77 Most of the membrane research formembrane reactors aims at the development of thinfilms on porous supports for obtaining high fluxes.Because of a strict synthesis protocol, large-scale pro-duction of such membranes with consistent qualityinduces high-cost membranes and limits the range ofindustrial applications. Porous infiltrated compositemembranes (in which the membrane material is depos-ited in the pores of the support) are attractive candi-dates, with good thermochemical resistance (barriereffect) and easy reproducibility (see Table 10). Further-more, in the case of catalytic membranes, a highquantity of catalyst can be deposited in such a mem-brane configuration, which provides for easy diffusionof the reactants to the catalyst.78,79 In particular, zeolitemembranes,80 mainly used for gas and vapor separa-tions, have scarcely been used as O2 distributors inmembrane reactors81 or for their catalytic properties.The insertion (postsynthesis or in situ) of catalyticallyactive sites (e.g., Pd, Pt, V, etc.) might extend thepossibilities of these membranes for membrane reactorapplications.82,83

Recent studies on membranes made with perfluori-nated polymers show the possibility of their applicationin the field of separation processes performed in hostileenvironments, i.e., high temperatures or aggressivenonaqueous media, such as chemicals and solvents.Perfluoropolymers are polymers designed for high de-manding applications in hostile environments. Thepresence of fluorine in the polymer backbone impartsto the structure an ability to withstand very hightemperatures and a very high resistance to chemicalattack. Copolymers of tetrafluoroethylene (TFE) and2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD), com-mercially known as HYFLON AD, are amorphousperfluoropolymers with glass transition temperatures(Tg) higher than room temperature. They show athermal decomposition temperature exceeding 400 °C.An important peculiarity of these polymer systems isthat they are highly soluble in fluorinated solvents, withlow solution viscosities. This aspect allows for the

Table 10. Afforded Methods for the Synthesis of Porous Infiltrated Composite Membranes

method applications

chemical vapor deposition/infiltration(CVD/CVI)

synthesis of dense silica/γAl2O3 composite membranes highly selective to H2(supplied by MPT)82

direct impregnation of a porous supportwith salt solutions

synthesis of VMgO/ZrO2/RAl2O383 (contactor for ODHP)

synthesis of perovskites/RAl2O384 (VOC combustion)

sol-gel/infiltration synthesis of meso- and microporous inert85 or catalytically active86 membranessynthesis of mixed oxides composite membranes (VMgO/RAl2O3, VPO/RAl2O3

77)solvothermal (hydrothermal/infiltration) synthesis of supported zeolite membranes (silicalite-1, ZSM-5, A-type, mordenite,

zeolite Y, ferrierite, AlPO4-5, zeolite L, SAPO, etc.) with insertion of catalyticallyactive sites87


preparation of self-supported and composite membraneswith desired membrane thicknesses.84,85

During the 1990s Techsep (Orelis) initiated an ambi-tious R&D project to design inorganic nanofiltrationmembranes. These membranes were first developed incollaboration with the nuclear industry (Commissariata l’Energie Atomique and its subsidiary SFEC, France).The membrane is a pure inorganic ZrO2 layer obtainedby sol-gel technology and deposited on a Kerasep TMceramic support. New possibilities have been opened bynanofiltration ceramic membranes now available on themarket for several years: they can be used in a verybroad range of operating conditions (pHs from 0 to 14,severe oxidizing or reducing conditions, thermal resis-tance from 0 to 350 °C, high-pressure resistance, inert-ness toward radiation, etc.), which means that newmembrane applications can be examined.

Recently, engineers at the TNO Institute of AppliedPhysics, Materials Research and Technology Division,in Eindhoven, The Netherlands, believe they havesolved several of the common problems with ceramicmembranes by developing and commercializing them inthe shape of hollow fibers instead of tubes. Thesepatented hollow-fiber ceramic membranes have a highsurface-to-volume ratio (more than 1000 m2/m3) and areeasy to scale-up. TNO also combines this technologywith highly selective top layers. Compared to existingflat and tubular membranes, ceramic hollow-fiber mem-branes have the advantage that they are compact andup to 10 times less expensive to produce. These mem-branes are now being used in slurry reactors. There arealso applications in wastewater treatment to selectivelyremove pollutants and in gas separation. One ceramicmembrane nearing commercialization is being devel-oped by Pall Corp., East Hills, NY, based on DOEclassified technology through a Cooperative Researchand Development Agreement (CRADA). Pall also re-cently commercialized a stainless steel membrane withthis technology.86

Recently (October 1998), in the U.S., a researchproject has been partially funded by the DOE to developa new fabrication process for ultrafiltration membranesbased on thermally induced phase separation (TIPS) toproduce membranes with more uniform microscopicpore sizes in an appropriate range (down to the 10-50nm size range), enabling more efficient separation and

purification of biological materials for hemodialysis andvirus filtration.

In the meantime, Praxair, Inc. and the University ofNew Mexico are studying a new gas separation technol-ogy consisting of porous membranes containing specialsites designed to temporarily bind to particular gasmolecules, promoting their transport through the mem-brane (facilitated transport membranes). In particular,polymeric systems, compatible with current manufac-turing methods, and mixed inorganic-polymer coatings,offering better pore-size control, are being studied.

New Module Design and Strategies forConcentration Polarization and FoulingControl

Although involving less novel scientific principles,module technology is also absolutely crucial to thesuccessful implementation of membrane technology.Seals, assembly methods, flow distribution, and pres-sure-drop minimization require careful attention, andthis trend will intensify as the field matures andcompetition intensifies.

In the past few years, additional innovations inmodule design and new strategies and techniques havebeen explored, particularly in the reduction of concen-tration polarization and fouling problems in pressure-driven membrane operations. Some examples are listedin Table 11.

A countercurrent transverse-flow hollow-fiber ROmodule with baffles has been designed for low feed flowvelocities transverse to the hollow fibers to achieve highmass transfer coefficients in an overall countercurrentflow (on the module scale) to the permeate flow throughthe tube side.87

Spirally wound feed flow channels with membranewalls will allow the formation of Dean vortices, whichwill mix the bulk with the wall layers and reduceconcentration polarization without moving the mem-brane or the module or having flow reversal in the feedstream via inserts or otherwise.88 Such a concept isuseful for UF and MF as well (see Figure 19). AlthoughDean flows satisfy the exigencies of an efficient mem-brane process, such as high permeate flux by increasedwall shearing, radial mixing, and low concentrationpolarization, the presently employed modules still have

Table 11. New Modules and Strategies for Concentration Polarization and Fouling Control

module/strategy for fouling reduction effect applicationsa

countercurrent transverse flowhollow-fiber module with baffles

mass-transfer coefficient increase RO

spirally wound feed flow channels Dean vortex formation RO, UF, MFcoiled modules (tubular/hollow fiber) Dean vortex formation UF, MF, PV, MCgas sparging secondary flow and local mixing generation near membrane

surfaceUF, MF, MBR

fluidized bed turbulence, continuous mechanical erosion of particle depositsat wall of membrane

MF

negative TMP pulsing (back-pulsing) periodic removal of particle cakes from membrane surface UF, MF(with ceramicmembranes)

dynamic filtrationrotary disk modulesvortex flow filtrationvibratory shear-enhanced processor

generation of high shear rates in fluid near membrane UF, MF

immersed membrane with aeration system contaminants not forced into membrane pores under highpressure, aeration minimizes settling of solids and bothagitates and scrubs the membrane surface

UF, MF, RO

a UF ) ultrafiltration; MF ) microfiltration; PV ) pervaporation; MC ) membrane contactor; MBR ) membrane bioreactor; TMP )transmembrane pressure.


the problem of low packing density and increasedpressure drop, which renders the process operating costsrelatively higher than conventional processes.89-91 Inlaminar conditions, the limiting flux obtained in ultra-and microfiltration coiled modules is higher than thatobtained in straight ones. The enhancement can reacha factor of up to 5 depending on module characteristics,hydrodynamic conditions, and suspension properties.The energy analysis shows that, for the same energydissipation, the limiting flow reached in a helical moduleis still far greater than that reached in a straightmodule.

There are also potential advantages of the Dean flowin some other membrane operations, e.g., membraneoxygenators or pervaporation.

In the laminar flow regime, the mass transfer coef-ficients obtained in gas-liquid contactors (water oxy-genation and VOC removal) are higher than thoseobtained using modules with straight fibers.92,93

It is known that, in ultrafiltration, concentrationpolarization affects the permeate flux (the productivity)and membrane rejection characteristics (the separationefficiency). Injection of gas bubbles to generate second-ary flow and to promote local mixing near the mem-brane surface proves effective in overcoming concentra-tion polarization. When applied to protein fractionation,gas sparging can also improve the selectivity signifi-cantly. One application of such a process might be inenzyme membrane bioreactors. For vertical membranesystems, the observed enhancement, in terms of per-centage increase in permeate flux, depends on theseverity of the concentration polarization and membranesurface shear. When concentration polarization is se-vere, for example, at low cross-flow velocities, at hightransmembrane pressures, and at high feed concentra-tions, the observed flux enhancement is higher (up to320%). With high shear systems, including hollow-fibermodule and spiral-wound membranes, or even for highlyturbulent flow, injecting gas at low flow can only achievea marginal flux enhancement (20-36%).94,95

Two-phase flow in ultrafiltration hollow fibers is veryefficient in enhancing mass transfer when it is limitedby particle deposition. Air sparging seems to expand theparticle cake, as it increases both cake porosity andthickness, thus allowing higher water fluxes. This effectcan be explained by the mixing and turbulence createdby the slug flow. In some cases, intermittence alsoaffects the cake structure.96

The use of a fluidized bed during the microfiltrationof suspensions on ceramic membranes results in asignificant increase in permeate flux in comparison withresults that can be obtained with an empty-tube system.This phenomenon is especially pronounced during themicrofiltration of oil emulsions when the permeate fluxin a fluidized-bed system is almost three times higher.

The fluidized solids ensure a significant reduction inconcentration polarization as well as a continuousmechanical erosion of the particles deposited at the wallof the membrane. The improved permeate flux that isachieved is due to the combined action of turbulence andparticle motion.97

Negative rapid transmembrane pressure (TMP) puls-ing has been increasingly adopted to control fouling inconventional and ceramic membranes to restore mem-brane productivity and increase solvant flux.98

An other strategy explored to enhance filtrationperformances in UF and MF is so-called “dynamicfiltration”, which consists of creating high shear ratesin the fluid near the membrane by relative motionbetween a fixed membrane and a moving wall or viceversa. The main advantages of this technique are thatthese high shear rates can be generated independentlyof the feed flow and that, because of the small head lossin the system, transmembrane pressure can be keptlower than in the classical cross-flow filtration. It is,therefore, advantageous for filtrating highly chargedfluids or for some biotechnological applications99 whenit is necessary to use low TMPs for solute recovery. Thesystem is very efficient in terms of permeate flux, forexample, with highly concentrated mineral suspensions.

A rotary disk module was developed based on thepatent of NIMIC and Hitachi Plant Engineering andConstruction Co. for the low-power separation of highlyconcentrated liquids. A conventional membrane modulecirculates the treated liquid at a high flow rate so as toobtain a rapid flow at membrane surface and a high fluxthrough the surface. On the other hand, rotating themembrane would allow a high flow rate at the mem-brane surface to be maintained without having tomaintain high pressure. Therefore, relatively low powerwould be required to run the system.100

An alternative widely commercialized technique de-veloped by Membrex Inc. is called the Vortex FlowFiltration (VFF) system wherein the feed is introducedinto the annular gap between two cylinders, one ofwhich is rotating. The membrane can be placed on eitherthe inner or the outer cylinder. Taylor vortices aregenerated between the two curved surfaces, creatinghigh shear at the membrane surface, but the feedpumping is low. The VFF technique is employed forsmaller systems.

For feed streams having high solids fractions, new UFtechniques are used commercially. The VSEP systememploys membrane leaf elements as parallel disksseparated by gaskets in a disk stack, which is spun ina torsional oscillation like the agitator in a washingmachine at a fast rate to produce shear rates as highas 15000 s-1 and, therefore, a much higher flux. Thecost per square meter is high compared to that ofcommercial cross-flow systems. A second high solid

Figure 19. Schematic representation of a spiral tubular membrane module.


commercialized UF system called Discover achieveshigh fluid shear at a flat membrane plate surface byspinning a grooved disk between adjacent membraneplates; fluxes are 5-6 times larger than those incompeting units with sludges containing oil and sus-pended solids.

The PallSep VMF system101 is another vibratingmembrane system commercialized by Pall Co. used asan efficacious and economical alternative to rotaryvacuum filters, centrifuges, and cross-flow systems forthe treatment of a wide variety of difficult-to-filterprocess streams in pharmaceutical and bioprocess ap-plications.

A characteristic of these UF techniques is the decou-pling of the wall shear from the bulk liquid flow rate(and sometimes pressure). This is radically differentfrom fluid management techniques used in conventionalmembrane devices for controlling polarization, fouling,gel layer formation, etc.102

Drinking water and industrial water on Hokkaidoisland (Japan) are produced with conventional waterproduction technology from natural river water thatcontains organic components such as humic substances,but the conventional membrane separation system (UF)is not suitable for treating river water in the summerwhen turbidity and color become high. To solve theproblem mentioned above, vibratory shear-enhancedprocessing is used. The system has a unique vibrationmechanism that generates shear rates on the surfaceof the membrane so that it is resistant to fouling causedby the PAC [poly(aluminum chloride] used to facilitatecoagulation of such natural organic matters for easyremoval. The commercial production facility achieves afairly high permeate flux compared with that of con-ventional membrane operation technology and has beenable to produce the required water quality. The opera-tion with vibration increases flux by about 1.5 timeswhen a relatively high pressure is applied.103

An inorganic membrane module of the external pres-sure type was developed by Kubota Corporation. Themain feature of the equipment is that some tubularmembrane modules without casing are submerged inthe anaerobic tank and row water is directly filteredthrough the membrane. The results obtained weresucceeded by the Japanese national project MembraneAqua-Century 21 by the Ministry of Health and Welfare,which was aimed at the development of membranetechnology for the purification of drinking water.104 Theequipment consists of a coagulation tank, an aerationtank equipped with modules inside, an air blower, asuction tank, and an air compressor for back-washing.The driving force of filtration is hydraulic pressure andthe suction force of a pump to keep water flow constant.The system is now commercially available for waterpurification.

Recently, ZENON Environmental Systems Inc.105

developed a new generation of membranes for water andwastewater treatment known as ZeeWeed. This newmembrane is of the immersed hollow-fiber type and isable to operate in high solids environments (10 000-15 000 ppm). Unlike conventional membranes that arehoused in pressure vessels and require a positivepressure, the immersed membrane operates in an opentank environment under a slight vacuum (-2 to -8 psi).The lower operating pressure, which provides increasedmembrane life and reduced replacement costs, alsopermits the reduction of the energy requirements as-

sociated with water production and membrane fouling(the contaminants are not forced into the membranepores under high pressure). An aeration system permitsa consistent flux to be maintained because of thegeneration of a recirculation pattern in the process tankthat minimizes the settling of solids and both agitatesand scrubs the membrane fiber surface to preventplugging and fouling.

New Areas of Interest for MembraneEngineering

The significant results already reached in the devel-opment of membrane operations as discussed in theprevious pages suggest other areas in which the overallpossibilities of membrane engineering might be ofimportance.

The case of transportation technologies is of particularinterest.

Transportation technologies, for example, will gothrough an important revolution in the next few years,mainly because of concerns about the poor air qualityin many of the cities of industrialized countries, increas-ing levels of greenhouses gases, and problems with theoil supply.

Various interesting projects are in progress worldwidetrying to accelerate the solutions to these problems.

The Exploratory Technology Research Program in theU.S., which seeks to identify new batteries and fuel-cell systems with higher performance and lower life-cycle costs than those available today, is an importantexample of these actions. Membrane systems representa significant aspect of these efforts.

For example, proton-exchange-membrane (PEM) fuelcells are, in principle, capable of high power density andof changing their power output more quickly than otherfuel cell types, making them candidates for replacinginternal combustion engines in transportation applica-tions, particularly in the automotive industry. Methanol,ethanol, hydrogen, natural gas, and gasoline are beingevaluated as fuels. The technical barriers that must beovercome include size, weight, and cost reductions; fuelstorage, conditioning and delivery; durability; reliability;etc.

At Argonne National Laboratory, a new 10-kW partialoxidation methanol reformer with 50% H2, less than 4%CO, thermal efficiency of 88-95%, excellent dynamicresponse, and rapid start-up (<100 s) has already beendemonstrated.

Direct methanol fuel cells (DMFC), which eliminatethe need for an external reformer, reducing the systemweight and cost, are under investigation at Los AlamosNational Laboratories (LANL). Technical problemsinclude methanol permeation through the membrane.By NMR spectroscopy, the diffusion coefficient of metha-nol in Nafion membranes has been measured. It is onlya factor of 2-3 smaller than the diffusion coefficient inaqueous solutions, and this is an important factorcontributing to the sizable methanol crossover ratesobserved.

Solid polymer electrolyte membranes play a vital rolein these fuel cell systems. Unfortunately, costs (U.S. $70-150/ft2) appear to be still too high.106

In the U.S., an interesting coordinated nationalprogram was launched in 1998 for the promotion ofindustrial research in this area.

The goal is to develop a totally new fuel cell systemwith improved CO tolerance (raising permissible levels


by 50 ppm to 3000 ppm), by using a high-temperature,ion-conductive solid polymer membrane electrode as-sembly and new bipolar separator plates. Other impor-tant projects are in progress on fuel cell technologiesthat are going to depend mainly on the membraneconcept design.

Conclusions

Not many years have passed from the days when Loeband Sourirajan, with their preparation of asymmetricmembranes, made the reverse osmosis process of in-dustrial interest.

The early membranologists have always been opti-mistic about the possibilities of membrane operations,but the scientific and technical results reached todayare even superior to the expectations.

The intrinsic multidisciplinary character of mem-brane science has been and is still today one of the majorobstacles to the further exploitation of its possibilities.The new logic of membrane engineering based on adrastic rationalization of the existing process design andnot on the more traditional approach of adding onemore, eventually innovative, unit at the end of theexisting pipe, which has been another obstacle to thegrowth of membrane units, will also contribute to theexploitation of these technologies.

A variety of technical challenges must be overcometo permit the successful industrial application of newmembrane solutions. For example, the development ofcatalytic membranes will depend on material advancesand increases in module reliability under extreme-temperature cycling. The development of affinity mem-branes will require research on electron-beam graftingand other approaches to the modification of membranechemistry. The development of tunable membranes willrequire extensive research on materials (e.g., conductingpolymers) and assembly processes (e.g., chemical vapordeposition). In general, advanced membrane and modulematerials need to be matched with appropriate, eco-nomical manufacturing processes.

The limitations still existing today to the large-scaleindustrial applicability of some membrane operationscan be attributed only in part to inadequate intrinsicmembrane properties (low permeability and selectivity,low thermal and chemical resistance, etc.) but probablymore to inadequate module design, hydrodynamic stud-ies, and, in general, engineering analysis.

As already evidenced in this work, significant progresshas been made in the study and realization of neworganic and inorganic membranes, and many academicand industrial research projects in this area are also inprogress.

Many efforts on new module configuration designsand on the individuation of more efficient strategies forconcentration polarization and fouling control are show-ing growing possibilities.

A continuous research effort on fundamental aspectsof transport phenomena in the various membraneoperations already existing and in the new ones underinvestigation is evident.

However, these efforts need to be combined with newresearch works in the process dynamics of these pro-cesses and in the study of advanced control systemsapplied to integrated multimembrane operations. Thesemultidisciplinary studies will offer interesting oppor-tunities for the design, rationalization, and optimizationof innovative productions.

In fact, as already evidenced, the most interestingdevelopments for industrial membrane technologies arerelated to the possibility of integrating various of thesemembrane operations in the same industrial cycle, withoverall important benefits in terms of product quality,plant compactness, environmental impact, and energeticaspects.

It is known that existing non-membrane-based equi-librium-driven separation technologies (e.g., absorption,adsorption, distillation, extraction, ion exchange, strip-ping), which represent the core of the traditional chemi-cal and petrochemical industry, have significant short-comings: inherent operational difficulties, lack offlexibility and modularity, slower rates, need for haz-ardous chemicals, high capital costs, higher energyrequirements, and need for large equipment volume.These shortcomings are exacerbated by new separationdemands (for example, environmental pollution controllaws). New membrane-based separation concepts andtechnologies (e.g., vapor permeation, osmotic distillation,facilitated transport, supported liquid membranes, mem-brane-based extractors, membrane-based absorption,and stripping in contactors) do not suffer from manysuch deficiencies and are poised to invade more andmore the domain of traditional separation technologies.

It is, then, realistic to affirm that new wide perspec-tives of membrane technologies and integrated mem-brane solutions for sustainable industrial growth arepossible.

It is also important to recall that the most recentlegislation and standardization are finally starting toidentify the role of membrane operations in variousareas (production of pure water for pharmaceutical uses,production of wine, of drinkable water or of ultrapurewater for electronic industry, etc.). In Japan and in theU.S., the introduction of official standards for character-izing the membranes used in various processes isalready in progress.

Acknowledgment

We have to acknowledge various colleagues from theIRMERC-CNR for their data and information on thevarious processes of their specific interest and, inparticular, Dr. L. Giorno, Dr. G. Clarizia, Ms. A.Gordano, and Mr. A. Cassano.

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Received for review June 27, 2000Revised manuscript received November 16, 2000

Accepted November 17, 2000

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