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

REVIEWS

Progress and New Perspectives on Integrated Operations for Sustainable Industrial Growth

Enrico Drioli* and Maria Romano Institute on and Modeling of Chemical Reactors, CNR, and Department of and Materials, University of Calabria, 87030 Arcavacata di Rende (CS), Italy

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

Introduction streams commercially feasible. Billions of cubic meters of pure gases are now produced via selective Membrane science and technology has led to signifi- in polymeric membranes. cant innovation in both processes and products, par- ticularly appropriate for sustainable industrial growth, The combination of molecular separation with a over the past few decades. chemical reaction, or membrane reactors, offers impor- tant new opportunities for improving the production The purpose here is to present a summary review of efficiency in and in the chemical industry. the extent to which membrane processes have been In 1997, five large petrochemical companies announced integrated into industrial practice. a research project devoted to the development of inor- The preparation of asymmetric cellulose acetate mem- ganic membranes to be used in syngas production. At branes in the early 1960s by Loeb and Sourirajan is about the same time, an $84 million project, partly generally recognized as a pivotal moment for membrane supported by the U.S. Department of Energy (DOE), technology. They discovered an effective method for that has Air Products and Chemical Inc. working significantly increasing the permeation flux of polymeric together on the same objective has been promoted. The membranes without significant changes in selectivity, availability of new high-temperature-resistant mem- which made possible the use of membranes in large- branes and of new membrane operations as membrane scale operations for desalting brackish water and sea- contactors offers an important tool for the design of water by and for various other molec- alternative production systems appropriate for sustain- ular separations in different industrial areas. Today, able growth. reverse osmosis is a well-recognized basic unit opera- tion, together with , cross-flow microfil- The basic properties of membrane operations make tration, and , all pressure-driven mem- them ideal for industrial production: they are generally brane processes. In 1999, the total capacity of reverse athermal and do not involve phase changes or chemical osmosis (RO) plants was more than 10 additives, they are simple in concept and operation, they millions m3/day, which exceeds the amount produced by are modular and easy to scale-up, and they are low in the thermal method,1 and more than 250 000 m2 of energy consumption with a potential for more rational ultrafiltration membranes were installed for the treat- utilization of raw materials and recovery and reuse of ment of whey and milk. byproducts. Membrane technologies, compared to those Composite polymeric membranes developed in the commonly used today, respond efficiently to the require- 1970s made the separation of components from gas ments of so-called “process intensification”, because they permit drastic improvements in manufacturing and - processing, substantially decreasing the equipment-size/ * Corresponding author: IRMERC CNR c/o Department of production-capacity ratio, energy consumption, and/or Chemical Engineering and Materials, via Ponte P. Bucci, 87030 Arcavacata di Rende (CS), Italy. Tel.: (39) 0984- waste production and resulting in cheaper, sustainable 2 492039/492025. Fax: (39) 0984-402103. E-mail: technical . [email protected]. The possibilities of redesigning innovative integrated

10.1021/ie0006209 CCC: $20.00 © 2001 American Chemical Society Published on Web 02/13/2001 1278 Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001

Table 1. Sales of Membranes and Modules in Various phenomena controlling the adsorption and desorption Membrane Processes5 of penetrants and other species at the membrane 1998 sales surfaces, with the correct flow-dynamic analysis of the (millions of growth tangential flow and concentration profile built up in the membrane process U.S. dollars) (%/year) bulk solutions upstream and in the membranes down- 900 8 stream and with the reology of often concentrated non- ultrafiltration 500 10 Newtonian fluids, that permits the design of correct reverse osmosis 400 10 membrane separation units. gas separation 230 15 110 5 Membrane operations show potential in molecular 70 5 separations, clarifications, fractionations, concentra- pervaporation >10 ? tions, etc. in the liquid phase, in the gas phase, or in miscellaneous 30 10 suspensions. They cover practically all existing and requested unit membrane processes in various industrial sectors char- operations used in process engineering. All of the acterized by low environmental impacts, low energy operations are modular, easy to scale-up, and simple to consumption, and high quality of final products have design. Other important aspects are the lack of moving been studied and in some cases realized industrially. parts; ability to work totally unattended; lower cost; Interesting examples are in the dairy industry and operational flexibility; and, when necessary, portability. in the . Research projects are Coupling of molecular separations with chemical in progress in the leather industry and in the agrofood reactions can be realized in a simple unit efficiently, industry based on the same concept. having ideal reaction surfaces where the products can In this review, some of the most interesting results be continuously removed and the reagents continuously already achieved in membrane engineering will be supplied at stoichiometric values. presented, and predictions about future developments These overall properties make membrane operations and the possible impact of new membrane science and ideal for the design of innovative processes where they technology on sustainable industrial growth will be will carry on the various necessary functions integrated analyzed. eventually with other traditional unit operations, opti- Actual possibilities and future perspectives of medical mizing their positive synergic effects. and biomedical applications of membrane technology are It is interesting to mention that statistical analysis not discussed in this work. This theme is the object of 3 carried out by Electricite´ de France on 174 different another recent paper. membrane installations in France using MF, UF, RO, The continuous interest and growth of the various and ED mainly in small- and medium-sized industries new industrial processes related to life sciences, as found a normal percentage of satisfaction between 70 evidenced also by the strategies and reorganization and 95%, one of the highest positive responses received adopted by large chemical groups worldwide in this area in this kind of analysis. This result is, in part, surprising (e.g., Aventis, Novartis, Vivendi Water, etc.) will also because of the high innovative content of the technology require significant contributions from membrane engi- and the lack of education still existing on their basic neering. We will, however, not concentrate our analysis properties. It is, however, consistent with the important on this subject in this review. contributions that membrane operations can make in terms of cost reduction, quality improvement, pollution Membrane Operations control, etc. Various membrane operations are available today for Several examples of successful applications of mem- a wide spectrum of industrial applications. Most of them brane technology as alternatives to traditional processes can be considered as basic unit operations, particularly can be mentioned. the pressure-driven processes such as microfiltration Ion-Exchange Membranes. The use of ion-ex- (MF), ultrafiltration (UF), nanofiltration (NF), and RO; change membrane cells in chloro-soda production rep- electrodialysis (ED) is another example of a mature resents, for example, an interesting case study for technology.4 Their worldwide sales are reported in Table analyzing the possibilities of membrane operations and 1.5 one of the first successes in terms of their electrochemi- The significant variety of existing membrane opera- cal application in minimizing environmental impacts tions is based on relatively simple, compact, and largely and energy consumption. The technology is based on the clarified fundamental mechanisms characterizing trans- discovery and utilization of fluorinated polymeric mem- port phenomena in the dense or microporous membrane branes stable in a specific environment, such as Nafion. phases and at the membrane- interface. The Today, membrane systems in which the anodic and understanding and prediction of transport phenomena cathodic species are directly produced in separate in the membrane phase is today at least qualitatively compartments without mixing and final separation possible, also theoretically through the newly available problems permit one to overcome the limitations of tools provided by molecular simulation.6,7 traditional mercury cells, related to the need for Hg Much progress has been made in this area in recent recovery, and of diaphragm cells, in which the separa- years in the design of polymeric materials, such as tion and concentration of final products still create polyimides, etc., and in the calculation of the difficulties. All new chloro-soda installations are now coefficients of simple gases in the dense phase. An practically based on this design, which represents a interesting agreement can be found between the theo- typical rationalization of the process, removing all of the retical and experimental values.8,9 pollution problems that characterized chloro-soda pro- It is the integration of advanced knowledge about ductions in the past. transport phenomena in dense or microporous thin In principle, other molecular halogens could be pro- phases, combined with the understanding of interfacial duced from their respective gases. The direct production Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001 1279

Table 2. Worldwide Desalination Production Capacitya total capacity % of world MSF MEE VC RO ED country (m3/day) production (%) (%) (%) (%) (%) Saudi Arabia 5 250 000 25.9 67.5 0.3 1.2 31.0 1.9 U.S. 3 100 00 15.2 1.7 1.8 4.5 78.0 11.4 United Arab Emirates 2 200 00 10.7 89.8 0.4 3.0 6.5 0.2 Kuwait 1 500 00 7.6 95.5 0.7 0.0 3.4 0.3 Japan 746 000 3.7 4.7 2.0 0.0 86.4 6.8 Libya 685 000 3.4 67.7 0.9 1.8 19.6 9.8 Qatar 570 000 2.8 94.4 0.6 3.3 0.0 0.0 Spain 530 000 2.6 10.6 0.9 15.1 20.4 19.2 Italy 520 000 2.6 43.2 1.9 15.1 20.4 19.2 Bahrain 310 000 1.5 52.0 0.0 1.5 41.7 4.5 Oman 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 ( condensation). Single-phase processes: RO (reverse osmosis), ED (electrodialysis).

Table 3. Costs Related to Various Sea Water Desalination Processes electric energy cost for 1 m3 energy equivalent scale of of freshwater processa maturity consumption (kWh/m3) application produced (ECU) MSF very thermal 10-14.5 small-large 0.6-1.9 ME partly thermal 6-9 small-medium 0.5-2.0 VC partly mechanical 7-15 small 0.6-2.4 RO 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). of essentially dry chlorine gas would also reduce oxygen industry.12 Recently, in X-ray diagnosis, the use as formation, which allows the reaction to be run at much contrast media of new nonionic iodinated compounds as higher current densities, with much less purification opacifying agents was studied and introduced to the and drying required compared to the chlorine produced markets as a substitution for the traditional iodinated by other systems. ionic compounds. However, the preparation and, par- Reverse Osmosis and Nanofiltration. As already ticularly, the final purification of these products were mentioned, desalination of seawater and brackish water much more complex and expensive than for those has been at the origin of the interest for membrane previously used. In particular, the neutral iodinated operations, and the research efforts on reverse osmosis agents cannot be isolated by precipitation in water membranes have had an impact on all of the progress because of their high solubilities. The problems to be in membrane science and technology. Evaporation solved were particularly the removal of ionic species, plants have been substituted with RO systems in usually inorganic salts present in the final reaction different part of the world (Table 2).10 mixture and the recovery of valuable reagents present The relatively low energy consumption is one of the in excess and of the water-soluble reaction media. A reasons for this success (Table 3). In seawater desalting, technique was developed based on the treatment of the in fact, the global energy consumption of RO, with a raw solutions of the contrast media with a complex recovery factor of 30% and energy recovery, has been series of operations such as removal of the solvent 3 5.32 kWh/m corresponding to a primary energy con- (DMAC or DMF) by evaporation; extraction of the 11 sumption of 59.94 kJ/kg. residual reaction medium by a chlorinated solvent; - Costs for brackish water desalination are 60 70% elution of the aqueous phase on a system of cationic and lower than those for seawater desalination. anionic ion-exchange resins; concentration by evapora- RO desalination is not only devoted to the production tion; and of the crude residue to remove of drinkable water but today is also strategic in various the last impurities. Various drawbacks are present in industrial sectors and particularly in ultrapure water this system. A much better system has recently been production for the electronic industry. It is interesting realized based on the use of two nanofiltration stages to realize that, in Japan, the largest part of the water operating on highly concentrated raw solutions contain- produced by RO is for the electronics industry, in which ing the contrast media, inorganic salts, organic com- the country has worldwide leadership. pounds with a relatively low mass (about 200), and the Reverse osmosis has not generally been used until solvents (Figure 1). recently in the purification, separation, or concentration of chemicals, particularly because of osmotic limitations The first NF unit operates in diafiltration mode and and the low chemical and thermal resistance of the the retentate, partially concentrated and purified with existing membranes. respect to contrast media, is recycled at the first stage The recent development of nanofiltration and low- after dilution with a small amount of deionized makeup pressure reverse osmosis membranes with interesting water; the permeate (water, inorganic salts, solvents, selectivities and fluxes, as well as higher chemical and etc.), which still contains small amounts of the iodinated thermal resistances, has been rapidly utilized in the compound, proceeds toward the second NF unit. The realization of innovative processes in various industrial permeate from this second step is completely contrast sectors. agent free. An interesting case studied in Italy is represented by The degree of purification that can be reached is such the preparation of Iopamidol in the pharmaceutical that the total amount of residual impurities in the final 1280 Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001

Figure 1. Recovery of Iopamidol by membrane process.12 recycled retentate does not exceed 10% of the initial possibilities of recovering solvents from the oil-micelle amount and is generally on the order of 5%. mixture and from air exist today with membrane The process is simple, economical, and environmen- operations that might significantly reduce these losses. tally acceptable; it permits the elimination of acid and A reduction of the solvent content of the oil-micelle basic reactants necessary for the regeneration of the mixture from 70 to 40% has been demonstrated, with resins; and it avoids the use of toxic organic solvents, an energy saving of about 50%. An important aspect of etc. the utilization of membrane operations in this area will Also, the integrated membrane processes proposed for be the possibility of using other solvents such as chromium recovery in the leather industry13 and for the alcohols. Their higher evaporation heats make them treatment of secondary textile effluents for their direct unattractive in traditional evaporation units. reuse,14 which will be described and discussed later in Better solvent-resistant membranes, eventually in- this work, show efficient applications of nanofiltration organic ones, however, will be necessary for large-scale and low-pressure reverse osmosis operations. applications in this area. Microfiltration and Ultrafiltration. Recently, in Cross-flow microfiltration can also be used success- the , membrane technology made realistic fully for the removal of long-chain traces of saturated the possibility of cold sterilization. Tetra Pak (Bacto- fat that are present in, e.g., sunflower oil.15 Catch System) developed a cross-flow microfiltration Considerable advances in UF and MF technologies in system that debacterizes fresh milk, avoiding any processes for drinking water produc- thermal treatment and taste alteration. An industrial tion have been achieved to such a point that, presently, process using this technology is already in operation at more than 1 000 000 m3/day of water are treated using Villefranche (Lyon) producing 2000 L/day of fresh milk these membrane operations.16,17 The employment of registered with the trade name Marguerite (Figure 2). integrated membrane systems in the production of The skimmed milk, obtained by whole milk centrifuga- drinking water is growing rapidly with excellent results. tion, is sterilized at low temperature by microfiltration. The reliability of the reverse osmosis membrane is Then, it is mixed with pasteurized cream. After homog- greatly increased when UF or MF operationsswhich enization and cooling, a debacterized whole milk is emerged in the past decade as an efficient way to obtained using a process alternative to the classical remove suspended solids and organic and microbiologi- UHT (ultrahigh-temperature) treatment. cal contaminantssare used in the pretreatment step. Similar products have also recently been commercial- Furthermore, economical considerations have shown ized in Italy by Parmalat S.p.A. that multiple membrane systems are more competitive The current systems for cleaning oil-water streams than conventional processes, resulting in the reduction via cross-flow microfiltration or ultrafiltration are very of capital and operating costs. reliable and compact. They can decrease the oil content In addition to the already-mentioned membrane of water from 10-30 mass % to less than 5 ppm. operations, gas separation, pervaporation, and some Nitrogen blanketing helps to prevent oxidation of oils others membrane processes, which have recently shown during mechanical oilseed pressing, while also reducing significant possibilities for their application in various explosion risks in extraction and during desolventizing. industrial areas, must be cited; among these, a class of New possibilities exist, however. Solvent recovery, membrane-based unit operations identified as mem- dehydration of solvents, use of membrane reactors, brane contactors, membrane bioreactors, and catalytic winterization, and fractionation of fats are interesting membrane reactors will be discussed. cases. More than two million tons of extraction solvents, Gas Separation. Membrane processes for gaseous mainly hexane, is used in the U.S. alone. Its recovery mixture separation are, today, technically well-consoli- is by and condensation. It is estimated that, dated and apt to substitute for traditional techniques.18 also in the most modern units, 0.7 kg of hexane per ton Separation of air components, natural gas dehumidi- of seed is still released into the environment. The fication, and separation and recovery of CO2 from biogas Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001 1281

Figure 2. Flow sheet of an industrial system for the debacterization of fresh milk by cross-flow microfiltration (Villefranche, Lyon, France). and of H2 from industrial gases are some examples in relatively low purity (0.5-5% O2). Single-stage operation which membrane technology is applied at the industrial is preferred. Oxygen is the third largest commodity level. chemical in the U.S. with annual sales in excess of $2 The gas separation business was evaluated in 199619 billion. Whereas nitrogen membrane separation has at $85 million in the U.S., with growth of about 8% per been a great success, oxygen separation using mem- year. branes is still underdeveloped. The major reason for this Asymmetric polymeric membranes, used for gas mix- is that most of the industrial oxygen applications require ture separations, are made either as plane sheets and purity higher than 90%, which is easily achieved by assembled in spiral-wound modules or as hollow fibers. adsorption or cryogenic technologies but not by mem- These modules are made and commercialized by various branes. Today’s limited application of membrane-based companies all over the world. Although the kind of oxygen generation systems operate either under feed module used is declared, the type of polymer is still compression or permeate vacuum mode (Figure 3). Both protected as industrial know-how. methods of separating oxygen are inferior to the adsorp- In Table 4, some permeability and selectivity data for tion separation processes using various zeolites. the various polymers used in the manufacture of the New materials are being developed that could possibly most commercial membranes are reported.20,22 have higher permeabilities than conventional solid The separation of air components or oxygen enrich- electrolytes, in which ionized atoms are transported ment has advanced substantially during the past 10 through the crystalline lattice under a driving force years. The oxygen-enriched air produced by membranes provided by partial pressure differences over the mem- has been used in various fields, including chemical and brane (pressure-driven process) or by electrical potential related industries, the medical field, food packaging, etc. gradients (electrochemical pumping). In industrial furnaces and burners, for example, injec- Mixed conductors with high electronic and oxygen ion tion of oxygen-enriched air (25-35% oxygen) leads to conductivities could be used as a membrane alternative higher flame temperatures and reduces the volume of to solid electrolytes for oxygen separation. In such ma- parasite nitrogen to be heated; this means lower energy terials, both oxygen ions and electronic defects are consumption. Mixtures containing more than 40% v/v transported in an internal circuit in the membrane ma- of O2 or 95% v/v of N2 from the air can be obtained. terial. Promising oxygen permeation fluxes have been Industrial nitrogen is used in the chemical industry to obtained in many perovskite systems, e.g., La-Sr-Co- protect fuels and oxygen-sensitive materials. Fe-O,23,24 Sr-Fe-Co-O,25,26 and Y-Be-Co-O.27 Membranes today dominate the fraction of the nitro- In particular, in the ITM-oxygen systems, simulta- gen market for applications less than 50 tons/day and neous conduction of ions and electrons in the same 1282 Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001

Table 4. Permeability and Selectivity Data of Some Polymers Used in the Manufacture of Commercial Membranes for Gas Separationa permeability coefficient, selectivity barrer (ideal) (-)

CO2 O2 N2 CO2/N2 O2/N2 poly[1-(trimethylsilyl)-1-propyne] 28 000 7730 4970 5.60 1.55 poly(dimethylsiloxane) 4550 781 351 13.0 2.22 poly(dimethylsilmethylene) 520 91.0 35.9 14.5 2.53 poly(cis-isoprene) 191 37.5 14.5 13.2 2.60 poly(butadiene-styrene) 171 32.9 10.3 16.6 3.19 natural rubber (at 25 °c) 153.0 - 9.43 16.2 - ethyl cellulose 5.0 12.4 3.4 22.1 3.65 polystyrene 12.4 2.9 0.52 23.8 5.58 butyl rubber 5.18 - 0.324 16.0 - poly(ethyl methacrylate) 7.01 1.9 0.33 21.2 5.76 poly(phenylene oxide) (at 25 °c) 75.7 - 3.81 19.9 - bisphenol A polycarbonate 6.8 1.6 0.38 17.9 4.21 cellulose acetate 4.75 0.82 0.15 31.7 5.47 bisphenol A polysulfone 4.6 1.2 0.19 24.2 6.32 PMDA-4,4′-ODA polymide 2.7 0.61 0.1 27.0 6.10 poly(methyl methacrylate) 0.62 0.14 0.02 31.0 7.00 poly(vinyl chloride) (at 25 °C) 0.157 - 0.0118 13.3 - PEEK-WC (at 25 °C) 2.75 0.5 0.1 27.5 5.00 polyphosphazeny (at 25 °C) 5.76-10.2 0.955-1.72 - 21.2-30.5 3.71-5.05 a At 35 °C unless otherwise specified.

Figure 3. Oxygen production systems.

Figure 4. Integrated oxygen and power production. material obviates the need for an external electrical conductivity of the material studied is mainly equal to circuit to provide the driving force for the separation, the electronic conductivity. with a significant reduction in cost. The driving force Because this oxygen-ion-conducting membrane must for the is the partial pressure dif- operate at temperatures above 700 °C, an effective ference across the membrane. High-pressure air (100- means of recovering the energy contained in the non- 300 psia) is required to achieve a significant flux of O2 permeate, oxygen-depleted stream is required. An ef- across the membrane. The oxygen flux is directly ficient and cost-effective means to accomplish this is to proportional to the pressure gradient and inversely integrate the membrane system with a gas turbine proportional to the membrane thickness. The pressure (Figure 4).28 of the oxygen product is typically only a fraction of an A technology known as OTM syngas (oxygen trans- atmosphere. port membrane synthesis gas) utilizes these ion- These dense inorganic perovskite type membranes, conducting membranes able to separate oxygen from air today manufactured in tubular configurations, transport with a high flux in the same temperature region oxygen as lattice ions at elevated temperatures with required for the reforming of natural gas. This technol- infinite selectivity ratios in O2 separations. The ionic ogy was presented in 1997 by an alliance of five Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001 1283

Figure 5. Scheme of a plant for H2 recovery from ammonia synthesis. international companies including AMOCO, BP Chemi- Table 5. Economic and Technical Advantages for a 1000 cals, PRAXAIR, SASOL, and STATOIL. Philippe Pe- ton/day Ammonia Plant30 troleum joined the alliance in 1998. The new process, ammonia recovery (scrubbing) 4 ton/day still under development, integrates the separation of heat saving 522-836 kJ/ton of oxygen from air, steam reforming, and natural gas NH3 produced oxidation into one step, eliminating the need for a additional ammonia production 50-55 ton/day separate oxygen plant. The new technology offers the increase in ammonia production 20-50 ton/day possibility of reducing the energy and capital costs of (at constant natural gas consumption) reduction in natural gas production syngas production. Considering that 60% of the cost for (at constant production rate) manufacturing any product from natural gas is related to synthesis gas production, the interest of this innova- stoichiometric as well as nonstoichiometric H2/(CO)x tion technology is evident. ratio methanol plants at differential pressures up to 70 Various plants for the recovery of hydrogen from the bar. Figure 6 shows the flow diagram of such a hydrogen purge of the synthesis of ammonia have been realized recovery unit installed for demonstration purposes.31,30 today.29 The unit modules are in general arranged in a Before entering the gas permeators, the feed is scrubbed “one-stage-two-unit” form. One of the first plants of this in order to reduce the methanol content to levels below type has been realized by Permea in Louisiana (Figure 100 ppm. From a bleed stream of 4000 mol/h, for 5).30 The first unit, consisting of eight hollow-fiber example, a recovery of 2000 mol/h of hydrogen has been modules [total feed capacity about 3800 m3(stp)/h] is achieved. operated with a transmembrane pressure difference of Gas mixture dehumidification is a process of great 60 bar, the permeate leaving at a pressure of about 70 industrial interest, especially for natural gas purifica- bar. At this pressure, the permeate can be fed to the tion and air dehumidification. An efficient membrane second stage of the synthesis feed compressor. The system for air dehumidification called the Cactus Mem- retentate of the first unit is fed to the second unit where brane Air Dryer, developed in the late 1980s, has been the permeate leaving the modules at 25 bar is mixed commercialized by Permea.32 When the Cactus dryer is with fresh feed (suction side of the first stage of the fed with compressed air, water vapor and a small compressor). The retentate is utilized for heating pur- amount of oxygen pass through the walls of the hollow poses. Gas pretreatment consist of conventional scrub- fiber, while nitrogen, argon, and most of the oxygen bing to reduce the ammonia content of the bleed from continue through the hollow core of the fibers to the end 2% (molar) to less than 200 ppm in order to avoid of the separator. A small amount of the slower gases membrane swelling and, as a consequence, damage of passes through the fiber, and this is used to sweep the the membrane. The economical and technical advan- water vapor through the separator. Cactus membranes tages related to this membrane system for the recovery work on the principle of dew point depression. For of hydrogen are shown in Table 5. example, a membrane might be sized for inlet conditions Methanol synthesis is another process based on a of 100 psig and 100 °F inlet dew point to achievea0°F gaseous feed; in purge recovery, a water scrubber is also pressure dew point. If inlet conditions change, e.g., used with a similar purpose, and it pays for itself in compressed air with a lower inlet dew point is supplied, terms of the recovered methanol. The methanol/water the separator will provide dry air at an even lower dew mixture is simply sent to the existing crude methanol point. distillation column. Hydrogen recovered from this purge The removal of hydrogen sulfide (H2S) and carbon can result in energy savings, and if additional carbon dioxide (CO2) from natural gas is an ideal application oxide is available, it can be used to obtain increased for membranes (Figure 7); both H2S and CO2 permeate methanol production. PRISM separators operate on through membranes at a much higher rate than meth- 1284 Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001

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

Figure 7. Removal of H2S and CO2 from natural gas. ane, enabling a high recovery of the acid gases without significant loss of pressure in the methane pipeline product gases. These membrane processes are going to substitute for the more traditional methods of hydrocarbon stream purification. Through a comparison of the separation cost for the membrane process with that for the dietha- nolamine (DEA) gas-absorption process, it was found Figure 8. Recovery of CO2 from exhaust gas and reuse to produce that the membrane process is more economical than the chemicals by hydrogenation. DEA gas-absorption process in the range of CO2 con- centrations in the feed between 5 and 40 mol %. When the feed also contains H2S, the cost for reducing the CO2 cal plants. The CO2 separated might be converted by and H2S concentrations in the feed to pipeline specifica- reacting it with H2 in methanol, starting a C1 chemistry tions increases with increasing H2S concentration (1000 cycle. to 10 000 ppm). If membrane processes are not economi- As schematized in Figure 8, a membrane reactor cally competitive because of the high H2S concentration might be ideally used to carry out hydrogenation reac- in the feed, the separation cost could be significantly tions for chemical production using CO2 recovered from lowered by using hybrid membrane processes. In such exhaust gases by membrane separation. processes, the bulk of CO2 and H2S is separated from The separation and recovery of organic solvents from sour natural gas with membranes, and the final puri- gas stream is also rapidly growing at the industrial fication to pipeline quality gas is performed by means level. 33 of suitable gas-absorption processes. Despite the high Polymeric rubbery membranes that selectively per- levels of H2S in the feed, membrane selectivities are 34 meate organic compounds (VOC) from air or nitrogen maintained. have been used. Such systems typically achieve greater The possibility of utilizing membrane technology in than 99% removal of VOC from the feed gas and reduce solving problems such as the greenhouse effect related the VOC content of the stream to 100 ppm or less. The to CO2 production has also been suggested. technology has been applied to the recovery of high-

Membranes able to remove CO2 from air, having a value organic such as vinyl chloride monomer, high CO2/N2 selectivity, might be used at any large-scale methyl chloride, and methyl formate. industrial CO2 source as a power station in petrochemi- Membrane systems are competitive with carbon ad- Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001 1285

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 used during polymer particle degassing (MTR). sorption or condensation for streams containing more treatment facility, e.g., a catalytic incinerator, gas than 5000 ppm, particularly if high VOC recovery is engine, or pressure swing adsorption unit. These plants required. are equipped with a modified plate and frame configu- The typical industrial applications of vapor recovery ration.36 are off-gas treatment in gasoline tank farms, gasoline A case of a vapor recovery unit based on membrane station vapor return, and end of pipe solvent recovery technology is that commissioned in a gasoline tank farm in the chemical and pharmaceutical industries. Another in Munich for the treatment of the off-gasses generated interesting example of an industrial application is VOC from the storage, handling, and distribution of gasoline. recovery by the compression-condensation and vapor The plant capacity was 300 m3/h. The only external permeation method, presented schematically in Figure available energy source was the electrical power supply. 9. This is a scheme of the process developed in Anwil This was planned in the framework of a pilot project (Wloclawed, Poland), which has been built by MTR for the reduction of emissions at the BP tank farm (U.S.) for the recovery of monovinyl chloride (MVC). Hamburg-Finkenwerder. The VRU has a capacity of The recovery of ethylene and propylene from nitrogen 1500 m3/h and a hybrid system of a membrane stage in polyolefin plant vent streams has been suggested and and a gas engine. Two gas engines coupled with a realized at the industrial level by DSM in Geleen, The generator are permanently in operation to supply the Netherlands. basic electrical power of the side. The gas engines are To recover the unreacted monomer (up to 25%) and designed to switch the fuel feed from natural gas to other volatile hydrocarbons from the nitrogen used retentate of the membrane stage over a period of VRU during polymer particles degassing, MTR35 developed operating time. a two-stage operation in which the mixture of N2 and A commercially successful application is a hybrid propylene is first compressed and later directed into a system of a membrane stage with pressure swing membrane vapor separation unit, as shown in Figure adsorption (PSA) (Figure 11). The liquid ring compres- 10. The spiral-wound membrane modules (8 in. diam- sor operating with gasoline as the service liquid sucks eter, 20 m2 surface area) used are 10-100 times more the hydrocarbon (HC) contaminated air from the gas- permeable to organic vapors than to air or nitrogen. ometer. The off-gases are compressed and fed into a In 1989, the first vapor recovery unit (VRU) based scrubber. Gasoline from the tank farm is used as a lean on membrane technology was commissioned for off-gas absorbent. The HC concentration of the feed gas leaving treatment in a gasoline tank farm. At present, various the scrubber depends on the operating temperature and membrane VRU’s are in operation or under construc- pressure. The layout of the membrane stage (membrane tion. The capacity of these units ranges from 100 to 2000 area and permeate pressure) is governed by the permis- m3/h. These are single-membrane stages of hybrid sible HC intake concentration of the PSA unit. Two systems of a membrane stage combined with a post- parallel PSA units are installed and operated alter- 1286 Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001

Figure 11. Membrane stage with integrated pressure swing adsorption.

Table 6. Practical Applications of Pervaporation application details separation and/or dehydration of water/organic (water/ethanol, water/2-propanol, water/pyridine) separation of water from organic/aqueous mixtures dehydration of organic solvents shifting of the reaction equilibrium (e.g., esterification) removal of chlorinated hydrocarbons separation of organics from the fermentation broth removal of volatile compounds from aqueous and separation of aroma compounds gas streams wine and beer dealcoholization removal of VOCs from air separation of azeotropes (e.g., ethanol/cyclohexane, separation of organic/organic mixtures 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 vapor entrainer molecular sieve permeation pervaporation distillation adsorption utilities ($/ton) ($/ton) ($/ton) ($/ton) vapor - 12.80 120.00 80.00 electricity 40.00 17.60 8.00 5.20 cooling water 4.00 4.00 15.0 10.00 entrainer -- 9.60 - replacement of 19.00 30.60 - 50.00 membranes and molecular sieves total costs 63.00 64.0 152.60 145.20 nately. One is in the adsorption phase while the other Pervaporation. Some applications of pervaporation is in the desorption and regeneration phase. A bypass processes are listed in Table 6. of the clean stream is used as a purge gas for regenera- Dehydratation of ethanol by PV was the first indus- tion. To maintain a low vacuum, the vacuum pump at trial-scale application proposed by GFT in the 1980s. the downstream side of the membrane stage can be a Today, more than 40 industrial pervaporation plants liquid ring pump with mineral oil as the service liquid built by Sulzer Chemtech Membrantechnik (former or a rotary vane vacuum pump. This vacuum pump is GFT) are in operation worldwide. They are used for the also used to support the desorption of the PSA column. dehydration of different solvents and/or solvent mix- The adsorber material is activated carbon, a carbon tures. molecular sieve, or an inorganic molecular sieve. A In many practical applications, it might be more typical VRU combined with a PSA is installed at Shell economical to use pervaporation or vapor permeation in Ludwigshafen.36 only to break the and to concentrate the Other interesting applications of the technology might retentate further by the above-azeotropic distillation be in the separation of light hydrocarbons from refinery (Table 7). waste gas streams, the recovery of natural gas liquids Another successful example of PV is its application and hydrogen, or the separation of propane, butane, and in the enhancement of chemical reaction efficiency. higher hydrocarbons from methane in the processing of Examples of such reactions are esterification or phenol- natural gas for dew point control. acetone condensation. The first industrial plant for the Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001 1287

Figure 12. Pervaporation-enhanced MTBE production.

Figure 13. Membrane system for CO2 recovery from fermentation broth. pervaporation-enhanced ester synthesis was built in Membrane Contactors. In these systems the mem- 1991 by GFT for BASF. brane function is to facilitate diffusive mass transfer A possible application of the removal of organic between two contacting phases, which can be liquid- solutes could be the treatment of industrial and mu- liquid, gas-liquid, gas-gas, etc.39 The traditional strip- nicipal water supplies contaminated with carcinogenic ping, scrubbing, absorption, and liquid/liquid extraction halogenated organic compounds. Such a process would processes can be carried out in this new configuration. also be attractive for the extraction of organics. With respect to conventional systems, membrane The possibility of recovering volatile organic com- contactors can guarantee some advantages such as pounds from gases by pervaporation has been demon- nondispersion of the phases in contact, independently strated and applied recently at the industrial level.37 variable flow rates without flooding limitations, lack of The elimination of volatile solutes from dilute aqueous phase-density difference limitations, lack of phase sepa- solutions might be possible by pervaporation. ration requirements, higher surface area/volume ratios, Separation of organic/organic mixtures represents the and direct scale-up due to a modular design. least-developed application and the largest potential Traditional liquid-supported membranes in which a commercial impact of pervaporation, but considerable carrier is immobilized in the microporous hydrophobic developments in membrane materials and processes structure of the polymeric membranes are the most remains to be done. The first industrial application of traditional and well-developed example of a membrane PV to organic/organic separation was the separation of contactor system. methanol from a methyl tert-butyl ether (MTBE) stream Other applications, however, have been studied and in the production of octane enhancer for fuel blends realized today or are under investigations. Interesting (Figure 12). examples include the removal of trace of oxygen (at Flexibility with respect to part-load performance and levels of <10 ppb) from water for ultrapure water changing product and feed concentrations is one of the preparation for the electronics industry,40 the removal advantages of pervaporation over other separation of CO2 from fermentation broth (Figure 13), and the processes. This is especially useful in the production of supply of CO2 as a gas to liquid phases (carbonation of fine chemicals and in the pharmaceutical industry, soft drinks).41 The flow sheet of a water carbonation where solvents are used and almost no single waste process is presented in Figure 14. Additional examples solvent is generated continuously. include the removal of alcohol from wine and beer, the Pervaporation-based hybrid processes offer significant concentration of juice via osmotic or membrane distil- potential for new economical and efficient solutions to lation,41 the nitrogenation of beer,42 the degassing of some classical separation problems.38 organic solutions, and water ozonation.43 1288 Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001

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

In particular, in the carbonation process, hollow fibers using this new concept, and important phenomena such have generally been used for industrial units. During as oil combustion might be optimized. operation, an aqueous liquid flows over the shell side (outside) of the hollow fiber. A strip gas or vacuum, Membrane Reactors either separately or in combination, is applied on the lumen side of the hollow fiber and flows counter current. The possibility of combining molecular separation and Because of its hydrophobic character, the membrane chemical transformations in a single unit soon attracted acts as an inert support to allow intimate contact the interest of membrane engineers.49 The first studies between the gas and liquid phases without . on such reactors were devoted to the immobilization of The interface is immobilized at the pores by applying a biocatalysts on polymeric membranes. Recently, high- higher pressure to the aqueous stream than the gas temperature reactions have been the objective of im- stream. The result is fast diffusive transfer of dissolved portant studies. Both areas will be analyzed in the gases from or to the liquid phase. following pages. Since 1993, a bubble-free membrane-based carbon- Membrane Bioreactors. Biocatalytic membrane ation line has processed about 112 gal/min of beverage reactors are interesting with respect to conventional by membrane contactors having a total interfacial area membranes as they combine selective mass transport of 193 m2 (Pepsi bottling plant in West Virginia).40 with chemical reactions. The selective removal of prod- Permea commercializes beer dispensing systems known ucts from the reaction site increases conversion of as CELLARSTREAM Dispense Systems using PULSAR product-inhibited or thermodynamically unfavorable gas/liquid contactors, which increase or decrease the reactions. amount of carbon dioxide and nitrogen in draft beer for Biocatalysts can be used suspended in solution and optimal presentation.42 compartmentalized by a membrane in a reaction vessel and osmotic distillation can be or immobilized within the membrane matrix itself.50 considered examples of membrane contactors for real- Since the advent of what has been called solid-phase izing the concentration of aqueous solutions with non- biochemistry, the advantages of immobilized biocatalytic volatile solutes as salts and sugars.44-47 In the mem- preparations over homogeneous-phase enzymatic/cel- brane distillation process, two liquids or solutions at lular reactions have been exploited to develop new and different temperatures are separated by a porous mem- less-expensive processes. brane acting as a barrier between the two phases, which Synthetic membranes provide an ideal support for must not wet the membrane (this implies that hydro- biocatalyst immobilization because of a wide available phobic membranes must be used in the case of aqueous surface area per unit volume and the possibility for the solutions). Because of the temperature gradient, a vapor development of new immobilization procedures. En- pressure difference exists across the membrane, and it zymes are retained in the reaction side, do not pollute is the driving force inducing vapor transport the products, and can be continuously reused. Im- through the pores from the high-vapor-pressure side to mobilization has also been shown to enhance enzyme the permeate side. In the case of osmotic distillation, stability. Moreover, provided that membranes of suit- the vapor molecule transport is due to a vapor pressure able molecular weight cutoff are used, chemical reaction driving force provided by having a low-vapor-pressure and physical separation of biocatalysts (and/or sub- solution on the permeate side of the membrane, e.g., a strates) from the products can take place in the same concentrated salt solution. unit. Substrate partition at the membrane/fluid inter- The formation of emulsions or dispersions character- face can be used to improve the selectivity of the ized by very uniform dimensions of droplets or micro- catalytic reaction toward the derived products with bubbles can be realized using the same technology. minimal side reactions. Membranes are also attractive The membrane emulsification process is applied for retaining in the reaction volume the expensive mainly in the preparation of food emulsions. Moreover, cofactors that are often required to carry out some microbubble formation increases the stability of the enzymatic reactions. system by minimizing coalescence phenomena. An At the 1997 Achema conference in Frankfurt, Ger- interesting study evidenced the relationship existing many, statements on the impact that innovative biore- between membrane pore diameter and droplet size.48 actors, and particularly those based on the hollow-fiber The formulation of various products might be realized design, have in setting new performance standards were Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001 1289 clearly presented. For example, hollow-fiber bioreactors processes is relatively high, but the investments are in which cells attach into a capillary-type space have substantial. Pure hydrogen can be produced at signifi- been designed to mimic biological processes more closely cantly lower temperatures by integrating into the reac- than any other reactor system. Through the fibers, tor a membrane that selectively removes hydrogen nutrients such as glucose and oxygenase are fed to the during conversion. Potential savings in membrane cells, and wastes such as CO2 and H2O are removed. reformer and downstream processing costs compared to Roche Diagnostic declared the use of such reactors to conventional steam reforming apparatuses must, in produce monoclonal antibodies for diagnostic tests. many cases, be weighed against additional costs associ- technology can also be applied ated with recompression of the hydrogen permeate to produce pure enantiomers, in that a membrane stream. separation process can be combined with an enantiospe- Ag membranes were initially suggested for their H2 cific reaction to obtain a so-called “enantiospecific permeability. Howevere, they present the same prob- membrane reactor”. As for general membrane reactors, lems that characterize Pd membranes, also having a the result is a more compact system with higher much lower permeability. conversion. This technology can respond to the strongly Solid oxide membranes have recently been suggested increasing demand for pharmaceuticals, food additives, for large-scale applications in syngas production.57 feeds, flavors, fragrances, agrochemicals, etc., as opti- Studies carried out in the U.S. showed the possibility cally pure isomers.51 of preparing membranes with improved mechanical and Recently, the results achieved in the production of a thermal characteristics, able to operate, for example, at chiral intermediate used for the preparation of an 900 °C for over 21 days. important calcium channel blocker, diltiazem, were discussed in the open literature,52 confirming the pos- Integrated Membrane Processes sibilities of membrane reactors also in the large-scale Traditionally, the various membrane operations (RO, production of biotechnological products. UF, MF, etc.) have been introduced in industrial pro- Phase-transfer catalysis can also be realized in mem- duction lines as an alternative to other existing units. brane reactor configurations, immobilizing the ap- Reverse osmosis instead of distillation and ultrafiltra- propriate catalysts in the microporous structure of the tion in place of centrifugation are typical examples. hydrophobic membranes. The possibility of redesigning overall industrial pro- Biphasic membrane reactors have been extensively duction by the integration of various already developed studied with lipases entrapped or bonded on the mem- membrane operations is becoming of particular interest, brane surface, which confirms the possibilities of the because of the synergic effects that can be reached, the 53,54 approach, as already discussed. simplicity of the units, and the possibility of advanced Catalytic Membrane Reactors. The development levels of automatization and remote control that can be of catalytic membrane reactors for high-temperature realized. The rationalization of industrial production by applications became realistic only in the last few years use of these technologies permits low environmental with the development of high-temperature-resistant impacts, low energy consumption, and higher quality membranes. In particular, the earlier applications of final products. New products also often become involved mainly dehydrogenation reactions, where the available. role of the membrane was simply hydrogen removal. These results are related to the introduction of new The earlier studies carried out, particularly in the technologies from the very early stages of the same Soviet Union on palladium and palladium alloys, con- material transformations and not at the end of the pipe, firmed the existence of membranes able to permeate H2 as was often done in the past. with high selectivity. Both capital and operative cost The leather industry might be an interesting case savings were anticipated, as units for hydrocarbon study because of (1) the large environmental problems separation from the streams were avoided and the related to its operation, (2) the low technological content possibility of operating at lower temperatures because of its traditional operations, and (3) the tendency to of reactor yield enhancement was realized. The fact that concentrate a large number of small-medium industries the membranes separate intermediates and products in specific districts. More than 2000 companies are in from the reacting zone, avoiding possible catalyst de- operation in Italy, which is recognized as a world leather activation or secondary reactions, is also of practical leader for the quality of the leather produced. The interest. The kinetic mechanisms might be modified or traditional flow sheet of the tanning process in its humid controlled by the presence of appropriate membrane phase consists of about 20 steps operating in a discon- systems, which can also act only on a reactive interface tinuous cascade system. The possibility of rationalizing with no permselectivity, optimizing phase-transfer ca- the overall process by introducing advanced molecular talysis.55 separation systems such as ultrafiltration, cross-flow Palladium membrane costs and availability, their , microfiltration, nanofiltration, and reverse mechanical and thermal stability, and poisoning and osmosis was suggested and has recently also become the carbon deposition problems are still obstacle to the objective of an Italian National Research Program large-scale industrial application of these dense metal coordinated and carried out by a consortium represent- membranes, also when prepared in a composite config- ing most of the companies in the country. In Figure 15 uration.56 is presented an ideal process based on integrated Hydrogen can be produced by steam reforming and membrane operations.58,59 shift conversion of natural gas or other hydrocarbons. The innovative integrated scheme suggested in Figure In conventional steam reformers, high conversions of 15 allows the pollution problems of the leather industry natural gas, on the order of 85-90% or even higher, are to be faced by solving or minimizing them one by one obtained at reformer outlet temperatures of around where they originate, thereby avoiding the need for huge 850-900 °C. The energy efficiency of steam reforming wastewater treatments at the end of the overall produc- 1290 Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001

Figure 15. Flow sheet of some humid phases of the tanning process integrated with membrane operations. tion line. The fact that membrane operations act by the recovered chromium solution can be further con- physical mechanisms without modification of the chemi- centrated by traditional techniques. These recovered cal procedure at the origin of the final high quality of solutions were used in sheepskin retanning and tanning the leather should also be mentioned. operations; the skins showed improved physical and The possibility of also introducing an enzyme mem- chemical characteristics compared to those obtained brane reactor as an alternative to the traditional chemi- with the traditional chromium solutions. The permeate cal dehairing process and for the optimization of the from the nanofiltration unit, considering its high content degreasing step has also been considered. The recovery of chlorides, might be used in the pickling phase, of the proteins produced in dehairing in the retentate realizing an interesting closed-loop process. In Figure of an ultrafiltration unit and the recovery and reuse in 16, the schematic flow sheet of chromium recovery is the same process of the excess of sodium sulfide used shown. and separated in the permeate became realistic when The possibility of a membrane-based posttreatment UF tubular membranes able to operate at the high pH (>12) characterizing the dehairing bath were prepared of secondary textile wastewater for the direct reuse of polished effluent within the dyeing process was also commercially. It is interesting to consider that around 14 40% of the overall pollution in leather processing recently verified by tests at the pilot scale. A first originates in the dehairing step, where only 10% of the treatment scheme examined requires two filtration overall liquid stream is generated. The problem of steps: membrane microfiltration followed by nanofil- chromium used in the tanning step has always been tration. The preliminary filtration on ceramic MF crucial for its negative environmental impact. The modules reduces the fractions of pollutants (suspended exhaust chromium coming from the tanning bath can solids and ) that can induce a rapid in be recovered and concentrated by a two-stage process the nanofiltration membrane. An addition of high based on MF or UF as a pretreatment and nanofiltration concentrations of aluminum polychloride (10-70 mg/ as a concentration technique.13 The concentrated chro- L) is necessary to obtain satisfactory performance of the mium solutions obtained by NF have an improved treatment system. Permeate quality confirms the pos- quality with respect to those obtained by the conven- sibility of reusing the secondary effluent for textile tional recovery process of chemical precipitation because industry purposes, but approximate preliminary calcu- of the optimally low ratio of organic lipolytic component/ lations on this coupled membrane process indicate that, chromium characterizing the new product. If necessary, at present, this process cannot be transferred to a full- Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001 1291

Figure 16. Scheme of the proposed process for reuse of exhausted chromium solutions from tanning operations. scale plant, because of the high price of the ceramic MF distillation are integrated has been realized. It consists membranes and the need for high dosing of coagulants. of UF and RO pretreatment stages, an osmotic distil- A second treatment scheme studied requires a clari- lation unit, and a single-stage brine evaporator. This fication-flocculation step followed by multimedia filtra- plant concentrates fresh juices up to 65-70°Brix and tion prior to a low-pressure reverse osmosis operation. has a capacity of 50 L/h. Being athermal, osmotic The clarification-flocculation/filtration is aimed at re- distillation allows for concentration of the juices without moving the colloidal fraction that promotes fouling of product deterioration or loss of flavors.60 RO membranes. A polished effluent of high quality Hogan et al.61 reported a total process cost of osmotic (COD < 0.10 mg/L; conductivity < 40 mS; negligible distillation concentration on the order of $1.00/L of residual color) that can be reused in textile mills is concentrate. From1Loffresh juice, it is possible to obtained. Costs for the complete polishing by low- achieve about 200 mL of 70°Brix concentrate. The value pressure RO are comparable to the costs of conventional of this concentrate is between $2.50 and $7.50/L. From secondary and are quite afford- these data, the economical advantages of the integrated able (on the order of 0.20-0.25 Euro) even for the Italian membrane process seem evident. situation, where price of water is much lower than in The coupling of RO and membrane distillation for most industrialized countries.14 obtaining high recovery factors has been also tested in Interesting cases of integrated membrane processes fruit juice concentration.62 can also be found in the agrofood industry, in water The potential for osmotic distillation flux enhance- desalination, in biotechnological production, etc. ment in grape juice concentration by ultrafiltration In the dairy industry, single-membrane operations pretreatment has recently been investigated.63 such as UF in the treatment of wheys, cross-flow Today, the integration concept finds interesting suc- microfiltration in the stabilization of milk, and RO in cess in the use of membrane operations for brackish the concentration of milk or in lactose concentration water treatment. Large-scale applications after many have been widely applied in the past year. As already years of trials with other membranes have recently been mentioned, an overall quantity of about 250 000 m2 of successfully realized. UF membranes were already installed in 1999 and For instance, at the end of 1999, the world’s largest around 165 000 m2 of RO membranes. integrated membrane system was put into operation by Successful application of integrated membrane opera- PWN Water Supply Company North Holland in The tions in fruit juice concentration (in an osmotic distil- Netherlands for drinking water production from lake lation process) has been developed by the Australian water. Ultrafiltration and reverse osmosis are the most company The Wingara Wine Group (Melbourne, NSW). essential process elements of this treatment plant, A hybrid pilot plant in which UF/RO and osmotic having a capacity of 18 000 000 m3/year.64 1292 Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001

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

In Bahrain, the Addur SWRO desalination plant is exergetic analysis of the overall integrated production planned for rehabilitation utilizing ultrafiltration mem- lines, if not of the complete system, is recommended.66 branes instead of the traditional flocculation-clarifica- The total energy requirements can be estimated on tion process65 in water pretreatment. the basis of the overall supply of electrical energy of The traditional seawater or brackish water desalina- pump engines and external equipments and the thermal tion process can be reconsidered by optimizing the energy supplied. pretreatment by MF and/or UF and by adding NF before The energy analysis must be elaborated in order to the RO step. The introduction of a membrane distilla- include all real involved variables, which generally are tion stage operating on the RO retentate might permit very numerous and variously aggregated. It is necessary recovery factors on the order of 87-88% (while the RO to establish the exact size of the unit operations and of unit alone worked with around a 40% of recovery factor) all of the flows of mass and energy of the process. The at costs that might be acceptable in various situations. block diagram of the operating phase of interest, con- Typically, the process systems for wastewater treat- nected to the recovery operation (Figure 18), or the block ment are designed with reverse osmosis operation as diagram of all of the productive process, integrated with the final treatment step and require several process the new operation, can help to report, in a compact way, steps, a large land area, and high capital investment all of the pertinent information for the elaboration of and operating costs. Microfiltration and ultrafiltration the estimation. membranes simplify the conventional water reuse pro- The layout of the “traditional” process and the layout cess by treating effluent directly from the secondary of the alternative process, both completed with all clarifier, with a simpler process that is easy to operate, information relative to the fluxes of matter and energy, requires less land area, and is less vulnerable to process can then be compared. disruption (Figure 17). Because membrane operations utilize primarily elec- There are also some advantages in combining a trical energy, the benefit estimate can be done using membrane bioreactor with a reverse osmosis step. This the “substitution coefficient” introduced by Electricite´ process solution increases the life of the RO membrane de France;67 this coefficient compares the primary and overall facility productivity. A new commercial energy saved to the electrical energy consumed in cycles membrane bioreactor for wastewater treatment already that utilize electricity-consuming operations in substitu- used in this type of integrated membrane system is tion for conventional thermal operations. The substitu- known as the ZenoGem (ZENON Inc.) process. This unit tion coefficient is defined as the ratio between the consists of a biological reactor integrated with immersed primary energy (thermal) saved in the new process with membranes that form an absolute barrier to solids and respect to the conventional process and the amount of and retain them in the process tank. electrical energy consumed, relative to the conventional The benefit of using biocatalytic membrane reactors process: CS ) C - C /E - E , where CS is the combined with other membrane processes, such as 1 2 2 1 substitution coefficient, C is the consumption of thermal microfiltration, ultrafiltration, reverse osmosis, mem- primary energy (MJ or Mcal), E is the consumption of brane extraction, etc., for the production and processing electrical energy (kWh), and 1 and 2 are the relative of bioreactive compounds is also apparent. This integra- indexes of the conventional and innovating process, tion is particularly important for products obtained by respectively. fermentation processes, such as organic acids, antibiot- ics, etc., and in the processing of food and beverages, Taking into account that 1 kWh of electrical energy, such as wine, fruit juices, milk, etc. available at the utilization site, requires to burn, in a power station, 10.5 MJ of primary energy from a combustible source (oil, gas, coal, etc.), the substitution Energy Requirements (or process innovation) results are advantageous when One of the significant and recognized benefits of CS is greater than 10.5 MJ/kWh (2.5 Mcal/kWh). membrane operations is their low direct energy con- Other than energy, recovered and recycled materials sumption (in general electricity) because of the absence are also involved; therefore, it is necessary to evaluate of phase transformations. An important possibility for their indirect energy content. For example, one should reducing indirect energy consumption through the also consider the energy consumed for the production recycling and reuse of raw materials or secondary of a material and, therefore, intrinsically associated with products and minimization of the formation of wastes it; the energy used for disposal of a material in dumping; also characterizes these techniques. For a correct evalu- the energy used for an inertization treatment of a ation of the total energy involved, an energetic and material, if required; etc. Considering that a substance, Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001 1293

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 CS membrane process analyzed (MJ/kWh) recovery and recycling of water in the textile industry 35.3 recovery and recycling of a monomer in the chemical industry 30.8 recovery and recycling of the sulfide in the tanning industry 36.0 saving of thermal energy and fat substance recovery in the dairy industry 21.7 thermal energy savings in tomato juice concentration 137.9 to be produced, needs a certain amount of energy, Table 9. Some New Investigated Membranes and recycling a substance means, in addition to an economic Membrane Materials saving, also an indirect energy saving corresponding to thermostable polymeric membranes the amount of primary energy that would be utilized (PEEK, PPS, PEEK-WC, PEEK-WC functionalized, etc.) for its production. polymeric membranes resistant to hostile environments The results summarized in Table 8 indicate clearly (HYFLON AD, etc.) H2 permselective dense membranes the energetic advantages of some suggested membrane (Pd-based, dense SiO , etc.) 68 2 operations. O2 permselective dense membranes (Brownmillerite, solid oxides, etc.) New Membranes porous infiltrated composite membranes for MRs (dense silica/γAl2O3 composite membranes, More and more complicated and special separation VMgO/ZrO2/RAl2O3, perovskites/RAl2O3, R R problems of liquid and gaseous mixtures in industry and VmgO/ Al2O3, VPO/ Al2O3, etc.) biomedical or medical technology require tailored fin- inorganic nanofiltration membranes (ZrO2 on ceramic support) ished products made from potential membrane materi- hollow-fiber ceramic membranes als available on the market. The range of application fields involves widely spread uses in micro-, ultra-, and by Bayer). It is characterized by a maximum long-term nanofiltration, , membrane electrolysis, or re- stability of 190 °C. Hollow fibers produced from PEEK verse osmosis, as well as in fields such as high- and PPS show textile-like properties, so that flexible temperature gas separation, hydrogen recovery from modules with large membrane areas and small volume syngases, and also oxygen-enriched air. Accordingly, the filling can be realized for the purposes of gas separation. number of investigated and established membrane Regarding separation performance, permeability, and materials has also simultaneously grown. Some ex- thermal and mechanical stability, PEEK membranes amples are listed in Table 9. are better than the PPS ones.69 Among the variety of new thermoplastics developed Modified PEEK (PEEK-WC), an amorphous polymer up to now are some special polymers that have been exhibiting mechanical and electrical properties equal to shown to be particularly suitable for membrane produc- or better than those shown by traditional PEEK, is also tion. For example, the aromatic poly(etheretherketone) soluble in DMA, DMF, chlorohydrocarbons, etc., which called Vitrex (PEEK produced by ICI) shows a remark- makes possible its use for asymmetric membrane for- able long-term temperature stability of 250 °C, where mation with the phase inversion procedure.70 Interest- the modulus of elasticity remains sufficient even at 150 ing results have been obtained with PEEK-WC dense °C. Because of its mechanical and chemical stability, membranes showing high O2/N2 selectivity and good Vitrex represents a suitable material for the production permselectivity to water in the pervaporation of water/ of hollow fibers. Another polymer tested for this purpose methanol mixtures, as well as in acetic acid aqueous is called Tedur (polyphenyle sulfide, PPS, manufactured solutions. 1294 Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001

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

chemical vapor deposition/infiltration synthesis of dense silica/γAl2O3 composite membranes highly selective to H2 (CVD/CVI) (supplied by MPT)82 83 direct impregnation of a porous support synthesis of VMgO/ZrO2/RAl2O3 (contactor for ODHP) 84 with salt solutions synthesis of perovskites/RAl2O3 (VOC combustion) 85 86 - synthesis of meso- and microporous inert or catalytically active membranes sol gel/infiltration 77 synthesis of mixed oxides composite membranes (VMgO/RAl2O3, VPO/RAl2O3 ) 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 catalytically active sites87

PEEK-WC modified by the introduction of NO2 thickness of the separating layer and, to a lesser extent, groups or by sulfunation are also studied for gas changes in the material composition of the separating separation applications.71,72 layer and optimization of the fiber size. - The use of PEEK WC membranes functionalized The good H2 permselectivity and permeability of the with o-octyloxycarbonyl â-cyclodextrin derivatives to recently developed dense (Pd-based) and almost dense carry out the hydrolysis reaction of p-nitrophenyl ace- SiO2 membranes were successfully exploited for a tate to p-nitrophenol in phosphate buffer enhances the number of H2-consuming or -generating reactions. For reaction rate with an enzyme-like behavior, improving some applications, the thermochemical instability of Pd 73 productivity and stability and decreasing costs. membranes and the hydrothermal instability of silica Much research on the synthesis of more selective, are the main problems to solve. Concerning O2-generat- permeable, and stable membrane materials for gas ing or -consuming reactions, the development of O2 separation has been done and is still ongoing all over permselective membranes with good fluxes in the range the world. For this purpose, some interesting results of 400-700 °C is still needed. Promising Brownmillerite have been presented in the recent literature. dense membranes were recently developed by Eltron Novel silicone-coated hollow-fiber membrane modules Research Inc.77 Most of the membrane research for for the removal of toluene and methanol from N2 have membrane reactors aims at the development of thin been tested.74 This novel membrane offers lower per- films on porous supports for obtaining high fluxes. meation resistance than other silicone-based mem- Because of a strict synthesis protocol, large-scale pro- branes because the selective barrier is ultrathin (1 µm) duction of such membranes with consistent quality and the porosity of the polypropylene substrate is high. induces high-cost membranes and limits the range of The bond between the substrate and the coating layer industrial applications. Porous infiltrated composite has been obtained by plasma polymerization. High membranes (in which the membrane material is depos- separation factors have been obtained (toluene/nitrogen ited in the pores of the support) are attractive candi- )10-55; methanol/nitrogen )15-125; more than 96% 3 dates, with good thermochemical resistance (barrier of VOCs removed from a feed stream of 60 cm /min effect) and easy reproducibility (see Table 10). Further- when the permeate side was subjected to a high 74 more, in the case of catalytic membranes, a high vacuum). quantity of catalyst can be deposited in such a mem- New 6FDA-DAF polyimide membranes have been brane configuration, which provides for easy diffusion obtained by simultaneous suppression of intersegmental of the reactants to the catalyst.78,79 In particular, zeolite packing and inhibition of intrasegmental motion with membranes,80 mainly used for gas and vapor separa- a significant increase in both permeability and selectiv- 75 tions, have scarcely been used as O2 distributors in ity. These membranes have been tested with mixtures membrane reactors81 or for their catalytic properties. of He-CH and CO -CH . Permselectivities of helium 4 2 4 The insertion (postsynthesis or in situ) of catalytically and carbon dioxide over methane are improved with active sites (e.g., Pd, Pt, V, etc.) might extend the respect to the other polymers: the permeability of He/ possibilities of these membranes for membrane reactor CH is 2.6 times higher in these membranes than in 4 applications.82,83 polysulfone. Roman76 recently presented new fiber spinning and Recent studies on membranes made with perfluori- processing technology and streamlined/automated nated polymers show the possibility of their application bundle-forming processes to reduce manufacturing costs in the field of separation processes performed in hostile and enable greatly increased production volumes. An environments, i.e., high temperatures or aggressive important innovation has been the development of a nonaqueous media, such as chemicals and solvents. proprietary co-extruded sheath/core fiber construction, Perfluoropolymers are polymers designed for high de- effectively a thin asymmetric layer coated on a rugged manding applications in hostile environments. The porous support. The mechanical support function is presence of fluorine in the polymer backbone imparts uncoupled from the permeation function, so both func- to the structure an ability to withstand very high tions can be optimized and a large fraction of the fiber temperatures and a very high resistance to chemical wall, the core in this case, can be made of an inexpensive attack. Copolymers of tetrafluoroethylene (TFE) and polymer to save on material costs. An additional ad- 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD), com- vantage of the sheath/core construction is that it helps mercially known as HYFLON AD, are amorphous form a thinner skin in the sheath layer by allowing the perfluoropolymers with glass transition temperatures use of sheath spinning solution with low polymer (Tg) higher than room temperature. They show a content and low (lower than could be used for thermal decomposition temperature exceeding 400 °C. a self-supporting monolithic membrane). The O2 and N2 An important peculiarity of these polymer systems is flow rates in Air Liquide’s N2 membrane have been that they are highly soluble in fluorinated solvents, with increased 2-fold since 1990,77 via reduction of the low solution . This aspect allows for the Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001 1295

Table 11. New Modules and Strategies for Concentration Polarization and Fouling Control module/strategy for fouling reduction effect applicationsa countercurrent transverse flow mass-transfer coefficient increase RO hollow-fiber module with baffles spirally wound feed flow channels Dean vortex formation RO, UF, MF coiled modules (tubular/hollow fiber) Dean vortex formation UF, MF, PV, MC gas sparging secondary flow and local mixing generation near membrane UF, MF, MBR surface fluidized bed turbulence, continuous mechanical erosion of particle deposits MF at wall of membrane negative TMP pulsing (back-pulsing) periodic removal of particle cakes from membrane surface UF, MF (with ceramic membranes) dynamic filtration generation of high shear rates in fluid near membrane UF, MF rotary disk modules vortex flow filtration vibratory shear-enhanced processor immersed membrane with aeration system contaminants not forced into membrane pores under high UF, MF, RO pressure, aeration minimizes settling of solids and both agitates and scrubs the membrane surface a UF ) ultrafiltration; MF ) microfiltration; PV ) pervaporation; MC ) membrane contactor; MBR ) membrane bioreactor; TMP ) transmembrane pressure. preparation of self-supported and composite membranes purification of biological materials for hemodialysis and with desired membrane thicknesses.84,85 virus filtration. During the 1990s Techsep (Orelis) initiated an ambi- In the meantime, Praxair, Inc. and the University of tious R&D project to design inorganic nanofiltration New Mexico are studying a new gas separation technol- membranes. These membranes were first developed in ogy consisting of porous membranes containing special collaboration with the nuclear industry (Commissariat sites designed to temporarily bind to particular gas a` l’Energie Atomique and its subsidiary SFEC, France). , promoting their transport through the mem- The membrane is a pure inorganic ZrO2 layer obtained brane (facilitated transport membranes). In particular, by sol-gel technology and deposited on a Kerasep TM polymeric systems, compatible with current manufac- ceramic support. New possibilities have been opened by turing methods, and mixed inorganic-polymer coatings, nanofiltration ceramic membranes now available on the offering better pore-size control, are being studied. market for several years: they can be used in a very broad range of operating conditions (pHs from 0 to 14, New Module Design and Strategies for severe oxidizing or reducing conditions, thermal resis- Concentration Polarization and Fouling tance from 0 to 350 °C, high-pressure resistance, inert- Control ness toward radiation, etc.), which means that new membrane applications can be examined. Although involving less novel scientific principles, Recently, engineers at the TNO Institute of Applied module technology is also absolutely crucial to the Physics, Materials Research and Technology Division, successful implementation of membrane technology. in Eindhoven, The Netherlands, believe they have Seals, assembly methods, flow distribution, and pres- solved several of the common problems with ceramic sure-drop minimization require careful attention, and membranes by developing and commercializing them in this trend will intensify as the field matures and the shape of hollow fibers instead of tubes. These competition intensifies. patented hollow-fiber ceramic membranes have a high In the past few years, additional innovations in surface-to-volume ratio (more than 1000 m2/m3) and are module design and new strategies and techniques have easy to scale-up. TNO also combines this technology been explored, particularly in the reduction of concen- with highly selective top layers. Compared to existing tration polarization and fouling problems in pressure- flat and tubular membranes, ceramic hollow-fiber mem- driven membrane operations. Some examples are listed branes have the advantage that they are compact and in Table 11. up to 10 times less expensive to produce. These mem- A countercurrent transverse-flow hollow-fiber RO branes are now being used in slurry reactors. There are module with baffles has been designed for low feed flow also applications in wastewater treatment to selectively velocities transverse to the hollow fibers to achieve high remove pollutants and in gas separation. One ceramic mass transfer coefficients in an overall countercurrent membrane nearing commercialization is being devel- flow (on the module scale) to the permeate flow through oped by Pall Corp., East Hills, NY, based on DOE the tube side.87 classified technology through a Cooperative Research Spirally wound feed flow channels with membrane and Development Agreement (CRADA). Pall also re- walls will allow the formation of Dean vortices, which cently commercialized a stainless steel membrane with will mix the bulk with the wall layers and reduce this technology.86 concentration polarization without moving the mem- Recently (October 1998), in the U.S., a research brane or the module or having flow reversal in the feed project has been partially funded by the DOE to develop stream via inserts or otherwise.88 Such a concept is a new fabrication process for ultrafiltration membranes useful for UF and MF as well (see Figure 19). Although based on thermally induced phase separation (TIPS) to Dean flows satisfy the exigencies of an efficient mem- produce membranes with more uniform microscopic brane process, such as high permeate flux by increased pore sizes in an appropriate range (down to the 10-50 wall shearing, radial mixing, and low concentration nm size range), enabling more efficient separation and polarization, the presently employed modules still have 1296 Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001

Figure 19. Schematic representation of a spiral tubular membrane module. the problem of low packing density and increased The fluidized solids ensure a significant reduction in pressure drop, which renders the process operating costs concentration polarization as well as a continuous relatively higher than conventional processes.89-91 In mechanical erosion of the particles deposited at the wall laminar conditions, the limiting flux obtained in ultra- of the membrane. The improved permeate flux that is and microfiltration coiled modules is higher than that achieved is due to the combined action of turbulence and obtained in straight ones. The enhancement can reach particle motion.97 a factor of up to 5 depending on module characteristics, Negative rapid transmembrane pressure (TMP) puls- hydrodynamic conditions, and suspension properties. ing has been increasingly adopted to control fouling in The energy analysis shows that, for the same energy conventional and ceramic membranes to restore mem- dissipation, the limiting flow reached in a helical module brane productivity and increase solvant flux.98 is still far greater than that reached in a straight An other strategy explored to enhance filtration module. performances in UF and MF is so-called “dynamic There are also potential advantages of the Dean flow filtration”, which consists of creating high shear rates in some other membrane operations, e.g., membrane in the fluid near the membrane by relative motion oxygenators or pervaporation. between a fixed membrane and a moving wall or vice In the laminar flow regime, the mass transfer coef- versa. The main advantages of this technique are that ficients obtained in gas-liquid contactors (water oxy- these high shear rates can be generated independently genation and VOC removal) are higher than those of the feed flow and that, because of the small head loss obtained using modules with straight fibers.92,93 in the system, transmembrane pressure can be kept It is known that, in ultrafiltration, concentration lower than in the classical cross-flow filtration. It is, polarization affects the permeate flux (the productivity) therefore, advantageous for filtrating highly charged and membrane rejection characteristics (the separation fluids or for some biotechnological applications99 when efficiency). Injection of gas bubbles to generate second- it is necessary to use low TMPs for solute recovery. The ary flow and to promote local mixing near the mem- system is very efficient in terms of permeate flux, for brane surface proves effective in overcoming concentra- example, with highly concentrated mineral suspensions. tion polarization. When applied to protein fractionation, A rotary disk module was developed based on the gas sparging can also improve the selectivity signifi- patent of NIMIC and Hitachi Plant Engineering and cantly. One application of such a process might be in Construction Co. for the low-power separation of highly enzyme membrane bioreactors. For vertical membrane concentrated liquids. A conventional membrane module systems, the observed enhancement, in terms of per- circulates the treated liquid at a high flow rate so as to centage increase in permeate flux, depends on the obtain a rapid flow at membrane surface and a high flux severity of the concentration polarization and membrane through the surface. On the other hand, rotating the surface shear. When concentration polarization is se- membrane would allow a high flow rate at the mem- vere, for example, at low cross-flow velocities, at high brane surface to be maintained without having to transmembrane pressures, and at high feed concentra- maintain high pressure. Therefore, relatively low power tions, the observed flux enhancement is higher (up to would be required to run the system.100 320%). With high shear systems, including hollow-fiber An alternative widely commercialized technique de- module and spiral-wound membranes, or even for highly veloped by Membrex Inc. is called the Vortex Flow turbulent flow, injecting gas at low flow can only achieve Filtration (VFF) system wherein the feed is introduced a marginal flux enhancement (20-36%).94,95 into the annular gap between two cylinders, one of Two-phase flow in ultrafiltration hollow fibers is very which is rotating. The membrane can be placed on either efficient in enhancing mass transfer when it is limited the inner or the outer cylinder. Taylor vortices are by particle deposition. Air sparging seems to expand the generated between the two curved surfaces, creating particle cake, as it increases both cake porosity and high shear at the membrane surface, but the feed thickness, thus allowing higher water fluxes. This effect pumping is low. The VFF technique is employed for can be explained by the mixing and turbulence created smaller systems. by the slug flow. In some cases, intermittence also For feed streams having high solids fractions, new UF affects the cake structure.96 techniques are used commercially. The VSEP system The use of a fluidized bed during the microfiltration employs membrane leaf elements as parallel disks of suspensions on ceramic membranes results in a separated by gaskets in a disk stack, which is spun in significant increase in permeate flux in comparison with a torsional oscillation like the agitator in a washing results that can be obtained with an empty-tube system. machine at a fast rate to produce shear rates as high This phenomenon is especially pronounced during the as 15000 s-1 and, therefore, a much higher flux. The microfiltration of oil emulsions when the permeate flux cost per square meter is high compared to that of in a fluidized-bed system is almost three times higher. commercial cross-flow systems. A second high solid Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001 1297 commercialized UF system called Discover achieves sociated with water production and high fluid shear at a flat membrane plate surface by (the contaminants are not forced into the membrane spinning a grooved disk between adjacent membrane pores under high pressure). An aeration system permits plates; fluxes are 5-6 times larger than those in a consistent flux to be maintained because of the competing units with sludges containing oil and sus- generation of a recirculation pattern in the process tank pended solids. that minimizes the settling of solids and both agitates The PallSep VMF system101 is another vibrating and scrubs the membrane fiber surface to prevent membrane system commercialized by Pall Co. used as plugging and fouling. an efficacious and economical alternative to rotary vacuum filters, centrifuges, and cross-flow systems for New Areas of Interest for Membrane the treatment of a wide variety of difficult-to-filter Engineering process streams in pharmaceutical and bioprocess ap- plications. The significant results already reached in the devel- opment of membrane operations as discussed in the A characteristic of these UF techniques is the decou- previous pages suggest other areas in which the overall pling of the wall shear from the bulk liquid flow rate possibilities of membrane engineering might be of (and sometimes pressure). This is radically different importance. from fluid management techniques used in conventional The case of transportation technologies is of particular membrane devices for controlling polarization, fouling, 102 interest. gel layer formation, etc. Transportation technologies, for example, will go Drinking water and industrial water on Hokkaido through an important revolution in the next few years, island (Japan) are produced with conventional water mainly because of concerns about the poor air quality production technology from natural river water that in many of the cities of industrialized countries, increas- contains organic components such as humic substances, ing levels of greenhouses gases, and problems with the but the conventional membrane separation system (UF) oil supply. is not suitable for treating river water in the summer Various interesting projects are in progress worldwide when turbidity and color become high. To solve the trying to accelerate the solutions to these problems. problem mentioned above, vibratory shear-enhanced The Exploratory Technology Research Program in the processing is used. The system has a unique vibration U.S., which seeks to identify new batteries and fuel- mechanism that generates shear rates on the surface cell systems with higher performance and lower life- of the membrane so that it is resistant to fouling caused cycle costs than those available today, is an important by the PAC [poly(aluminum chloride] used to facilitate example of these actions. Membrane systems represent coagulation of such natural organic matters for easy a significant aspect of these efforts. removal. The commercial production facility achieves a For example, proton-exchange-membrane (PEM) fuel fairly high permeate flux compared with that of con- cells are, in principle, capable of high power density and ventional membrane operation technology and has been of changing their power output more quickly than other able to produce the required water quality. The opera- types, making them candidates for replacing tion with vibration increases flux by about 1.5 times internal combustion engines in transportation applica- 103 when a relatively high pressure is applied. tions, particularly in the automotive industry. Methanol, An inorganic membrane module of the external pres- ethanol, hydrogen, natural gas, and gasoline are being sure type was developed by Kubota Corporation. The evaluated as fuels. The technical barriers that must be main feature of the equipment is that some tubular overcome include size, weight, and cost reductions; fuel membrane modules without casing are submerged in storage, conditioning and delivery; durability; reliability; the anaerobic tank and row water is directly filtered etc. through the membrane. The results obtained were At Argonne National Laboratory, a new 10-kW partial succeeded by the Japanese national project Membrane oxidation methanol reformer with 50% H2, less than 4% Aqua-Century 21 by the Ministry of Health and Welfare, CO, thermal efficiency of 88-95%, excellent dynamic which was aimed at the development of membrane response, and rapid start-up (<100 s) has already been technology for the purification of drinking water.104 The demonstrated. equipment consists of a coagulation tank, an aeration Direct methanol fuel cells (DMFC), which eliminate tank equipped with modules inside, an air blower, a the need for an external reformer, reducing the system suction tank, and an air compressor for back-washing. weight and cost, are under investigation at Los Alamos The driving force of filtration is hydraulic pressure and National Laboratories (LANL). Technical problems the suction force of a pump to keep water flow constant. include methanol permeation through the membrane. The system is now commercially available for water By NMR spectroscopy, the diffusion coefficient of metha- purification. nol in Nafion membranes has been measured. It is only Recently, ZENON Environmental Systems Inc.105 a factor of 2-3 smaller than the diffusion coefficient in developed a new generation of membranes for water and aqueous solutions, and this is an important factor wastewater treatment known as ZeeWeed. This new contributing to the sizable methanol crossover rates membrane is of the immersed hollow-fiber type and is observed. able to operate in high solids environments (10 000- Solid polymer electrolyte membranes play a vital role 15 000 ppm). Unlike conventional membranes that are in these fuel cell systems. Unfortunately, costs (U.S. $ housed in pressure vessels and require a positive 70-150/ft2) appear to be still too high.106 pressure, the immersed membrane operates in an open In the U.S., an interesting coordinated national tank environment under a slight vacuum (-2to-8 psi). program was launched in 1998 for the promotion of The lower operating pressure, which provides increased industrial research in this area. membrane life and reduced replacement costs, also The goal is to develop a totally new fuel cell system permits the reduction of the energy requirements as- with improved CO tolerance (raising permissible levels 1298 Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001 by 50 ppm to 3000 ppm), by using a high-temperature, In fact, as already evidenced, the most interesting ion-conductive solid polymer membrane electrode as- developments for industrial membrane technologies are sembly and new bipolar separator plates. Other impor- related to the possibility of integrating various of these tant projects are in progress on fuel cell technologies membrane operations in the same industrial cycle, with that are going to depend mainly on the membrane overall important benefits in terms of product quality, concept design. plant compactness, environmental impact, and energetic aspects. Conclusions It is known that existing non-membrane-based equi- librium-driven separation technologies (e.g., absorption, Not many years have passed from the days when Loeb adsorption, distillation, extraction, ion exchange, strip- and Sourirajan, with their preparation of asymmetric ping), which represent the core of the traditional chemi- membranes, made the reverse osmosis process of in- cal and petrochemical industry, have significant short- dustrial interest. comings: inherent operational difficulties, lack of The early membranologists have always been opti- flexibility and modularity, slower rates, need for haz- mistic about the possibilities of membrane operations, ardous chemicals, high capital costs, higher energy but the scientific and technical results reached today requirements, and need for large equipment volume. are even superior to the expectations. These shortcomings are exacerbated by new separation The intrinsic multidisciplinary character of mem- demands (for example, environmental pollution control brane science has been and is still today one of the major laws). New membrane-based separation concepts and obstacles to the further exploitation of its possibilities. technologies (e.g., vapor permeation, osmotic distillation, The new logic of membrane engineering based on a facilitated transport, supported liquid membranes, mem- drastic rationalization of the existing process design and brane-based extractors, membrane-based absorption, not on the more traditional approach of adding one and stripping in contactors) do not suffer from many more, eventually innovative, unit at the end of the such deficiencies and are poised to invade more and existing pipe, which has been another obstacle to the more the domain of traditional separation technologies. growth of membrane units, will also contribute to the It is, then, realistic to affirm that new wide perspec- exploitation of these technologies. tives of membrane technologies and integrated mem- A variety of technical challenges must be overcome brane solutions for sustainable industrial growth are to permit the successful industrial application of new possible. membrane solutions. For example, the development of It is also important to recall that the most recent catalytic membranes will depend on material advances legislation and standardization are finally starting to and increases in module reliability under extreme- identify the role of membrane operations in various temperature cycling. The development of affinity mem- areas (production of pure water for pharmaceutical uses, branes will require research on electron-beam grafting production of wine, of drinkable water or of ultrapure and other approaches to the modification of membrane water for electronic industry, etc.). In Japan and in the chemistry. The development of tunable membranes will U.S., the introduction of official standards for character- require extensive research on materials (e.g., conducting izing the membranes used in various processes is polymers) and assembly processes (e.g., chemical vapor already in progress. deposition). In general, advanced membrane and module materials need to be matched with appropriate, eco- nomical manufacturing processes. Acknowledgment The limitations still existing today to the large-scale We have to acknowledge various colleagues from the industrial applicability of some membrane operations IRMERC-CNR for their data and information on the can be attributed only in part to inadequate intrinsic various processes of their specific interest and, in membrane properties (low permeability and selectivity, particular, Dr. L. Giorno, Dr. G. Clarizia, Ms. A. low thermal and chemical resistance, etc.) but probably Gordano, and Mr. A. Cassano. more to inadequate module design, hydrodynamic stud- ies, and, in general, engineering analysis. As already evidenced in this work, significant progress Literature Cited has been made in the study and realization of new (1) Furukawa, D. H. Conference on Reverse Osmosis Process organic and inorganic membranes, and many academic Status, 1999. Proceedings of ICOM ‘99, Toronto, Canada, June and industrial research projects in this area are also in 1999. progress. (2) Stankiewicz, A. I.; Moulijn, J. A. Process Intensification: Many efforts on new module configuration designs Transforming Chemical Engineering. Chem. Eng. Prog. 2000, 96, and on the individuation of more efficient strategies for 22. concentration polarization and fouling control are show- (3) De Bartolo, L.; Drioli, E. Membranes in artificial organs. Biomed. Health Res. 1998, 16, 167. ing growing possibilities. (4) Baker, R. W.; Cussler, E. L.; Eykamp, W.; Koros, W. J.; A continuous research effort on fundamental aspects Riley, R. L.; Strathmann, H. Membrane separation systems; Noyes of transport phenomena in the various membrane Data Corp.: Park Ridge, NJ, 1991. operations already existing and in the new ones under (5) Strathmann, H. Membrane processes for a sustainable investigation is evident. industrial growth. New frontiers for catalytic membrane reactors However, these efforts need to be combined with new and other membrane systems. Ravello, Italy, May 1999. research works in the process dynamics of these pro- (6) Hofmann, D.; Fritz, L.; Ulbrich, J.; Schepers, C.; Bo¨hning, cesses and in the study of advanced control systems M. Detailed-atomistic molecular modeling of small molecule dif- fusion and solution processes in polymeric membrane materials. applied to integrated multimembrane operations. These Macromol. Theory Simul. 2000, 9, 293-327. multidisciplinary studies will offer interesting oppor- (7) Laciak, D. V.; Robeson, L. M.; Smith, C. D. Group Contribu- tunities for the design, rationalization, and optimization tion Modeling for Gas Transport in Polymeric Membranes; Ameri- of innovative productions. can Chemical Society: Washington, D.C., 1999; Chapter 12. Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001 1299

(8) Tocci, E.; Hofmann, D.; Paul, D.; Russo, N.; Drioli, E. Separation and Purification; Crespo, J. G., Boddeker, K. W., Eds.; Polymer 2001, 42, 521. Kluwer Academic Publishers: London, U.K., 1994; p 357. (9) Hofmann, D.; Fritz, L.; Ulbrich, J.; Paul, D. Molecular (37) Ne´el, J. Pervaporation; Lavoisier Tec. & Doc.: London, modelling of amorphous membrane polymers. Polymer 1997, 38 U.K., 1997. (25), 6145. (38) Lipnizki, F.; Fiel, R. W.; Ten, P.-K. Pervaporation-based (10) Ettouney, H. M.; El-Dessouky, H. T.; Alatiqi, I. Understand hybrid process: A review of process design, applications and thermal desalination. Chem. Eng. Prog. 1999, 95 (9), 43. economics. J. Membr. Sci. 1999, 153, 183. (11) Satone, H. Comparison between MSF distillation and RO. (39) Reed, B. W.; Semmens, M. J., Cussler, E. L. Membrane Technol. Proc. 9th Annu. Conf. NWSIA 1981, Vol. I, Session II. Contactors. In Membrane Separations Technology. Principles and (12) Viscardi, C. F.; Piva, R. European Patent EP 0575360, Applications; Noble, R. D., Stern, S. A., Eds.; Elsevier: New York, 1991. 1995; Chapter 10. (13) Cassano, A.; Drioli, E.; Molinari, R.; Bertolutti, C. Quality (40) Sengupta, A.; Peterson, P. A.; Miller, B. D.; Schneider, J.; improvement of recycled chromium in the tanning operation by Fulk, C. W., Jr. Large-scale application of membrane contactors membrane processes. Desalination 1996, 108, 193. for gas transfer from or to ultrapure water. Sep. Purif. Technol. (14) Rozzi, A.; Antonelli, M.; Arcari, M. Membrane treatment 1998, 14, 189-200. of secondary textile effluents for direct reuse. Water Sci. Technol. (41) Sirkar, K. K. Membrane Separations: Newer Concepts and 1999, Vol. 40, No. 4-5, 409. Applications for the Food Industry. In Bioseparation Processes in (15) Cuperus, F. P.; Nijhuis, H. H. Membrane Technology Foods; Singh, R. K., Rizvi, S. S. H., Eds.; Marcel Dekker: New Applied in the Food Industry. Trends Food Sci. Technol. 1993, 4, York; Chapter 10. 277. (42) Permea (Air Products). Private communication. (16) Macroric, C.; Freoman, S. Design and operation of mem- (43) Wikol et al. ICCS 14th International Symposium on brane filtration plants for water treatment. World Filtration Contamination Control, 14th Annual Technology Meeting, Phoe- Congress, Brighton, U.K., April 2000. In Proceedings, Vol. 1, pp nix, AZ, April 26-May 1, 1998. 525-528. (44) Lawson, K. W.; Lloyd, D. R. Membrane Distillation: A (17) Applying membrane technology to drinking water and Review. J. Membr. Sci. 1996, 124 (1), 1. wastewater treatment. Membr. Technol. 2000, 122,4. (45) Calabro`, V.; Drioli, E.; Matera, F. Membrane distillation (18) Koros, W. J.; Chern, R. T. In Handbook of Separation in the textile wastewater treatment. Desalination 1991, 83, 209. Process Technology; Rousseau, R., Ed.; John Wiley & Sons: New (46) Johnson, R. A.; Valks, R. H.; Lefevre, M. S. Osmotic York, 1987. distillationsA low-temperature concentration technique. Aust. J. (19) Puri, P. Membrane gas separations: An opportunity for Biotechnol. 1989, 3, 206. gas industry or just a niche market. Preprints of the International (47) Mengual, J. I.; Ortiz De Zarate, J. M.; Pena, L.; Velazquez, Conference on Membrane Science and Technology (ICMST ‘98), A. Osmotic distillation through porous hydrophobic membranes. June 9-13, 1998, Beijing, China. J. Membr. Sci. 1993, 82 (1-2), 129. (20) Stern, S. A. Polymers for gas separationssThe next decade. (48) Furuya, A.; Asano, Y.; Katoh, R.; Sotoyama, K.; Tomi, M. J. Membr. Sci. 1994, 94,1. Preparation of food emulsions using a membrane emulsification (21) Toi, K.; More, G.; Paul, D. R. Gas sorption and transport system. ICOM ‘96, August 1996, Yokohama, Japan. in poly(phenylene oxide)/polystyrene blends. J. Appl. Polym. Sci. (49) Michaels, A. S. In Separation for Biotechnology 2; Pyle, 1982, 27, 2997. D. L., Ed.; Elsevier Applied Science: Cambridge, U.K., 1990; p 3. (22) Drioli, E.; Zhang, S. M.; Basile, A.; Golemme, G.; Gaeta, (50) Giorno, L.; Drioli, E. Biocatalytic membrane reactors: S. N.; Zhang, H.-C. Gas Permeability of Polyphosphazene Mem- Applications and perspectives. Trends Biotechnol. 2000, 18, 339. branes. Gas Sep. Purif. 1991, 5, 252. (51) Giorno, L. Enantiospecific membrane processes. Korean (23) Tai, L. W.; Nasrallah, M. M.; Anderson, H. U.; Sparlin, D. Membr. J. 1999, 1 (1), 38. M.; Sehlin, S. R. Structure and electrical properties of LA1- (52) Lopez, J. L.; Matson, S. L. A multiphase/extractive mem- XSRXCO1-YFEYO3.1. The system LA0.8SR0.2CO1-YFEYO3. brane reactor for production of diltiazem chiral intermediate. J. Solid State Ionics 1995, 76, 259. Membr. Sci. 1997, 125 (1), 189 (Alan Michaels Special Issue). (24) Teraoka, Y.; Nobunaga, T.; Yamazoe, N. Chem. Lett. 1988, (53) Giorno, L.; Molinari, R.; Natoli, M.; Drioli, E. Hydrolysis 503. and regioselective transesterification catalised by immobilised (25) Qiu, L.; Lee, T. H.; Liu, L.-M.; Yang, Y. L.; Jacobson, A. J. lipases in membrane bioreactors. J. Membr. Sci. 1997, 125, 177. Oxygen permeation studies of SRCO0.8FE0.2O3-DELTA. Solid (54) Giorno, L.; Molinari, R.; Drioli, E.; Bianchi, D.; Cesti, P. State Ionics 1995, 76, 321. Performance of biphasic organic/aqueous hollow fibre reactor using (26) Ma, B.; Balachandran, U.; Park, J.-H.; Segre, C. U. immobilised lipase. J. Chem. Technol. Biotechnol. 1995, 64, 345. Determination of chemical diffusion coefficient of SrFeCo0.5Ox by (55) Falconer, J. L.; Noble, R. D.; Sperry, D. P. Catalitic the conductivity relaxation method. Solid State Ionics 1996, 83, Membrane Reactors. In Membrane Separations Technology. Prin- 65. ciples and Applications; Noble, R. D., Stern, S. A., Eds.; Elsevier: (27) Brinkman, H. W.; Kruidof, H.; Burggraaf, A. Mixed Amsterdam, 1995; p 669. conducting yttrium barium cobalt oxide for high oxygen perme- (56) Dixon, A. G. Innovations in Catalytic Inorganic Membrane ation. Solid State Ionics 1994, 68, 173. Reactors. In Catalysis; Spivey, J. J., Ed.; Royal Society of Chem- (28) Stiegel, G. J. Mixed conductiong ceramic membranes: a istry: London, U.K., 1999; Vol. 14, p 40. new paradigm for gas separation and reaction. Proceedings of the (57) Frost, L. J.; Foster, E. P. T.; Russek, S. L.; Rowley, D. R. Annual Membranes Technologies/Separation Planning Conference, Use of ceramic membranes for oxygen separation and syngas December 1998, Newton, MA. production: Syngas for fuel cells. International Business Com- (29) Drioli, E.; Santella, F.; Molinari, R. Industrial membrane munications (IBC) Floating Platform Situation Offshore (FPSO)/ operations. Ninetieth International Symposium on “Large Chemi- Remote Gas Utilisation Conference, London, U.K., December 1997. cal PlantssFrom 1995 to the Next Decennium”, Antwerp, Belgium, (58) Cassano, A.; Drioli, E.; Molinari, R. Recovery and reuse of October 1995. chemicals in unhairing, degreasing and chromium tanning pro- (30) Rautenbach, R.; Albrecht, R. Membrane Processes; John cesses by membranes. Desalination 1997, 113, 251. Wiley & Sons: New York, 1989. (59) Cassano, A.; Molinari, R.; Romano, M.; Drioli, E. Treat- (31) Porter, M. C. Handbook of Industrial Membrane Technol- ment of aqueous effluents of the leather industry by membrane ogy; Noyes Data Corp.: Park Ridge, NJ, 1990. processes. A review. J. Membr. Sci. 2000, 181, 111. (32) Permea Inc. Private communication. (60) Barbe, A. M.; Bartley, J. P.; Jacobs, A. L.; Jonhson, R. A. (33) Bhide, B. D.; Stern, S. A. Membrane processes for the Retention of volatile organic flavour/fragrance components in the removal of acid gases from natural gas. J. Membr. Sci. 1993, 81, concentration of liquid foods by osmotic distillation. J. Membr. Sci. 209. 1998, 145, 67. (34) Winston, W. S.; Sirkar, K. K. Membrane Handbook; Van (61) Hogan, P. A.; Canning, R. P.; Peterson, P. A.; Johnson, R. Nostrand Reinhold: New York, 1992. A.; Michaelis, A. S. A New Option: Osmotic Distillation. Chem. (35) Baker, R. W.; Jacobs, M. Improve monomer recovery from Eng. Prog. 1998, 94, 49. polyolefin resin degassing. Hydrocarbon Process. 1996, 75, 49. (62) Calabro`, V.; Jiao, B. L.; Drioli, E. Theoretical and experi- (36) Peinemann, K. V.; Ohlrogge, K. Separation of Organic mental study on membrane distillation in the concentration of Vapors from Air with Membranes. In Membrane Processes in orange juice. Ind. Eng. Chem. Res. 1994, 33, 1803. 1300 Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001

(63) Bailey, A. F. G.; Barbe, A. M.; Hogan, P. A.; Johnson, R. (87) Futselaar, H.; Reith, T.; Racz, I. G. The countercurrent A.; Sheng, J. The effect of ultrafiltration on the subsequent transverse flow module for the separation concentration of grape juice by osmotic distillation. J. Membr. Sci. of liquid streams. Engineering of Membrane Processes, Gramish- 2000, 164, 195. Partenkirchent, Bavaria, Germany, May 1992. (64) Schippers, J. C. Kiwa NV Research and Consultancy, EDS (88) Brewster, M. E.; Chung, K.-Y.; Belfort, G. Dean vortices Newsletters. February 2000. with wall flux in a curved channel membrane system. A new (65) Ammar Ali, H. Ministry of Electricity & Water, Bahrain. approach to membrane module design. J. Membr. Sci. 1993, 81, Personal communication, 1998. 127. (66) Romano, M.; Drioli, E. Analisi energetica ed exergetica nei (89) Manno, P.; Moulin, P.; Rouch, J. C.; Clifton, M.; Aptel, P. processi a membrana. ICP Riv. Ind. Chim. 2000, 3, 76. Mass transfer improvement in helically wound hollow fibre (67) Electricite´ de France. Le coefficient de substitution, le gain ultrafiltration modules. Yeast suspensionsSep. Purif. Technol. net de petrole. SEPAC: Paris, 1981. 1998, 14, 175. (68) Molinari, R.; Gagliardi, R.; Drioli, E. Methodology for (90) Moulin, P.; Rouch, J. C.; Serra, C.; Clifton, M. J.; Aptel, P. estimating saving of primary energy with membrane operations Mass-transfer improvement by secondary flowssDean vortices in in industrial processes. Desalination 1995, 100, 125. coiled tubular membranes J. Membr. Sci. 1996, 114, 235. (69) Vaorbach, D.; Schulze, Th.; Taeger, E. Thermostable hollow (91) Belfort, G. Coiled Membrane Filtration System. U.S. membranes for separation processes. Chem. Fibers Int. 1999, 49, Patent 5,626,758, May 6, 1997. 133. (92) Guigui, C.; Manno, P.; Moulin, P.; Clifton, M. J.; Rouch, (70) Lufrano, F.; Drioli, E.; Golemme, G.; Di Giorgio, L. J. C.; Aptel, P.; Laine´, J. M. The use of Dean vortices in coiled Transport parameters of carbon dioxide in poly(etheretherketone) hollow-fibre ultrafiltration membranes for water and wastewater membranes. J. Membr. Sci. 1996, 113, 121. treatment. Desalination 1998, 118, 73. (71) Trotta, F.; Drioli, E.; Gordano, A. Nitro derivates of PEEK- (93) Schnabel, S.; Moulin, P.; Nguyen, Qt.; Roizard, D.; Aptel, WC. J. Appl. Polym. Sci., submitted for publication. P. Removal of volatile organic components (vocs) from water by (72) Trotta, F.; Drioli, E.; Moraglio, G.; Baima Poma, E. pervaporationsSeparation improvement by Dean vortices. J. Sulfonation of polyetheretherketone by chlorosulfuric acid. J. Appl. Membr. Sci. 1998, 142, 129. Polymer Sci. 1998, 70 (3), 477. (94) Ghosh, R.; Cui, Z. F. Fractionation of BSA and lysozyme (73) Gordano, A.; Trotta, F.; Drioli, E. â-cyclodextrins immo- using ultrafiltrationsEffect of gas sparging. AIChE J. 1998, 44, bilised in PEEK-WC membranes: Kinetic behaviour and cataly- 61. sis, unpublished results. (95) Ghosh, R.; Cui, Z. F. Fractionation of BSA and lysozyme (74) Cha, J. S. Removal/recovery of VOCs using a rubbery using ultrafiltration: Effect of pH and membrane pretreatment. polymeric membrane. Membr. J. 1996, 6 (3), 173. J. Membr. Sci. 1998, 139, 17. (75) Kim, T.-H.; Koros, W. J.; Husk, G. R. Advanced Gas (96) Laborie, S.; Cabassud, C.; Durand-Bourlier, L.; Laine´, J. Separation Membrane Materials: Rigid Aromatic Polyimides. Sep. M. Flux enhancement by a continuous tangential gas flow in Sci. Technol. 1988, 23, 1611. ultrafiltration hollow fibres for drinking water production: effects (76) Roman, I. C. How Do You Coax 99+% Nitrogen from of slug flow on cake structure. Filtr. Sep. 1997, 34, 887. Membranes? Membrane Technology/Planning Conference, New- (97) Mikulasek, P.; Hrdy, J. Permeate flux enhancement using ton, MA, 1995. a fluidized bed in microfiltration with ceramic membranes. Chem. (77) Sammels, A. F.; Schwartz, M. 3rd International Conference Biochem. Eng. 1999, 13 (3), 133. on “Catalysis in Membrane Reactors”, Copenhagen, Denmark, (98) Ramirez, J. A.; Davis, R. H. Application of cross-flow September 1998. microfiltration with rapid backpulsing to wastewater treatment. (78) Julbe, A.; Farrusseng, D.; Guizard, C. 9th CIMTEC World J. Hazard. Mater. 1998, 63, 179. Ceramic Congress, Firenze, Italy, June 1998. In Advances in (99) Frenander, U.; Jonsson, A. S. Cell harvesting by cross- Science and Technology; Vicenzini, P., Ed.; Techna Pub. Srl: flow microfiltration using shear-enhanced module. Biotechnol. Faenza, Italy, 1998. Bioeng. 1996, 52, 397. (79) Julbe, A.; Guizard, C.; Larbot, A.; Cot, L.; Giroir-Fendler, (100) Ohkuma, N.; Shinoda, T.; Aoi, T., Okaniwa, Y.; Magara, A. The sol-gel approach to prepare candidate microporous inor- Y. Performance of rotary disk modules in a collected human ganic membranes for membrane reactors. J. Membr. Sci. 1993, excreta treatment plant. Water Sci. Technol. 1994, Vol. 30, No. 4, 77 (2-3), 137. 141. (80) Tavolaro, A.; Drioli, E. State of the art on zeolite mem- (101) Pall Corp. Private communication, 2000. branes: Preparations and applications. Adv. Mater. 1999, 11 (12), (102) Sirkar, K. K. Membrane separation technologies: Current 975. developments. Chem. Eng. Commun. 1997, Vol. 157, 145. (81) Pantazidis, A.; Dalmon, J. A.; Mirodatos, C. Oxidative (103) Takata, K.; Yamamoto, K.; Bianc, R.; Watanabe, Y. dehydrogenation of propane on catalytic membrane reactors. Catal. Removal of humic substances with vibratory shear enhanced Today. 1995, 25, 403. processing membrane filtration. Desalination 1998, 117, 273. (82) Wu, J. C. S.; Sabol, H.; Smith, G. W.; Flowers, D. L.; Liu, (104) Kubota Corporation. Private communication, 2000. P. K. T. Characterization of hydrogen-permselective microporous (105) Mourato, D. Water reuse with the immersed membrane ceramic membranes. J. Membr. Sci. 1994, 96, 275. & the membrane bioreactor. Int. Desalination Water Reuse Q. (83) Burggraaf, A. J. Fundamentals of inorganic membrane 2000, 9/4, 27. science and technology. In Membrane Science and Technology (106) Savogado, O. Energing membranes for electrochemical Series, 4; Burggraaf, A. J., Cot, L., Eds.; Elsevier: Amsterdam, systems: solid polymer electrolyte membranes for fuel cell sys- 1996; Chapter 8. tems. J. New Mater. Electrochem. Syst. 1988, 1, 47. (84) Colaianna, P.; Brinati, G.; Arcella, V. European Patent EP 97106156, 1997. Received for review June 27, 2000 (85) Drioli, E. et al. Gas permeability of polyphosphazene Revised manuscript received November 16, 2000 membranes. Gas Sep. Purif. 1991, 5, 252. Accepted November 17, 2000 (86) Caruana, C. M. Oxygen Separation Sparks New Ceramics Membranes. Chem. Eng. Prog. 1999, 95 (10), 11. IE0006209