Progress and New Perspectives on Integrated Membrane Operations for Sustainable Industrial Growth

Transcription

1 Ind. Eng. Chem. Res. 2001, 40, REVIEWS Progress and New Perspectives on Integrated Membrane Operations for Sustainable Industrial Growth Enrico Drioli* and Maria Romano Institute on Membranes and Modeling of Chemical Reactors, CNR, and Department of Chemical Engineering and Materials, University of Calabria, 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 engineering 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 Membrane science and technology has led to significant innovation in both processes and products, particularly appropriate for sustainable industrial growth, over the past few decades. The purpose here is to present a summary review of the extent to which membrane processes have been integrated into industrial practice. The preparation of asymmetric cellulose acetate membranes in the early 1960s by Loeb and Sourirajan is generally recognized as a pivotal moment for membrane technology. They discovered an effective method for significantly increasing the permeation flux of polymeric membranes without significant changes in selectivity, which made possible the use of membranes in largescale operations for desalting brackish water and seawater by reverse osmosis and for various other molecular separations in different industrial areas. Today, reverse osmosis is a well-recognized basic unit operation, together with ultrafiltration, cross-flow microfiltration, and nanofiltration, all pressure-driven membrane processes. In 1999, the total capacity of reverse osmosis (RO) desalination plants was more than 10 millions m 3 /day, which exceeds the amount produced by the thermal method, 1 and more than m 2 of ultrafiltration membranes were installed for the treatment of whey and milk. Composite polymeric membranes developed in the 1970s made the separation of components from gas * Corresponding author: IRMERC-CNR c/o Department of Chemical Engineering and Materials, via Ponte P. Bucci, Arcavacata di Rende (CS), Italy. Tel.: (39) / Fax: (39) e.drioli@unical.it. streams commercially feasible. Billions of cubic meters of pure gases are now produced via selective permeation in polymeric membranes. The combination of molecular separation with a chemical reaction, or membrane reactors, offers important new opportunities for improving the production efficiency in biotechnology and in the chemical industry. In 1997, five large petrochemical companies announced a research project devoted to the development of inorganic membranes to be used in syngas production. At about the same time, an $84 million project, partly supported by the U.S. Department of Energy (DOE), that has Air Products and Chemical Inc. working together on the same objective has been promoted. The availability of new high-temperature-resistant membranes and of new membrane operations as membrane contactors offers an important tool for the design of alternative production systems appropriate for sustainable growth. The basic properties of membrane operations make them ideal for industrial production: they are generally athermal and do not involve phase changes or chemical additives, they are simple in concept and operation, they are modular and easy to scale-up, and they are low in energy consumption with a potential for more rational utilization of raw materials and recovery and reuse of byproducts. Membrane technologies, compared to those commonly used today, respond efficiently to the requirements of so-called process intensification, because they permit drastic improvements in manufacturing and processing, substantially decreasing the equipment-size/ production-capacity ratio, energy consumption, and/or waste production and resulting in cheaper, sustainable technical solutions. 2 The possibilities of redesigning innovative integrated /ie CCC: $ American Chemical Society Published on Web 02/13/2001

2 1278 Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001 Table 1. Sales of Membranes and Modules in Various Membrane Processes 5 membrane process 1998 sales (millions of U.S. dollars) growth (%/year) microfiltration ultrafiltration reverse osmosis gas separation electrodialysis electrolysis 70 5 pervaporation >10? miscellaneous membrane processes in various industrial sectors characterized by low environmental impacts, low energy consumption, and high quality of final products have been studied and in some cases realized industrially. Interesting examples are in the dairy industry and in the pharmaceutical industry. Research projects are in progress in the leather industry and in the agrofood industry based on the same concept. In this review, 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. Actual possibilities and future perspectives of medical and biomedical applications of membrane technology are not discussed in this work. This theme is the object of another recent paper. 3 The continuous interest and growth of the various new industrial processes related to life sciences, as evidenced also by the strategies and reorganization adopted by large chemical groups worldwide in this area (e.g., Aventis, Novartis, Vivendi Water, etc.) will also require significant contributions from membrane engineering. We will, however, not concentrate our analysis on this subject in this review. Membrane Operations Various membrane operations are available today for a wide spectrum of industrial applications. Most of them can be considered as basic unit operations, particularly the pressure-driven processes such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and RO; electrodialysis (ED) is another example of a mature technology. 4 Their worldwide sales are reported in Table 1. 5 The significant variety of existing membrane operations is based on relatively simple, compact, and largely clarified fundamental mechanisms characterizing transport phenomena in the dense or microporous membrane phases and at the membrane-solution interface. The understanding and prediction of transport phenomena in the membrane phase is today at least qualitatively possible, also theoretically through the newly available tools provided by molecular simulation. 6,7 Much progress has been made in this area in recent years in the design of polymeric materials, such as polyimides, etc., and in the calculation of the diffusion coefficients of simple gases in the dense phase. An interesting agreement can be found between the theoretical and experimental values. 8,9 It is the integration of advanced knowledge about transport phenomena in dense or microporous thin phases, combined with the understanding of interfacial phenomena controlling the adsorption and desorption of penetrants and other species at the membrane surfaces, with the correct flow-dynamic analysis of the tangential flow and concentration profile built up in the bulk solutions upstream and in the membranes downstream and with the reology of often concentrated non- Newtonian fluids, that permits the design of correct membrane separation units. Membrane operations show potential in molecular separations, clarifications, fractionations, concentrations, etc. in the liquid phase, in the gas phase, or in suspensions. They cover practically all existing and requested unit operations used in process engineering. All of the operations are modular, easy to scale-up, and simple to design. Other important aspects are the lack of moving parts; ability to work totally unattended; lower cost; operational flexibility; and, when necessary, portability. Coupling of molecular separations with chemical reactions can be realized in a simple unit efficiently, having ideal reaction surfaces where the products can be continuously removed and the reagents continuously supplied at stoichiometric values. These overall properties make membrane operations ideal for the design of innovative processes where they will carry on the various necessary functions integrated eventually with other traditional unit operations, optimizing their positive synergic effects. It is interesting to mention that statistical analysis carried out by Electricité de France on 174 different membrane installations in France using MF, UF, RO, and ED mainly in small- and medium-sized industries found a normal percentage of satisfaction between 70 and 95%, one of the highest positive responses received in this kind of analysis. This result is, in part, surprising because of the high innovative content of the technology and the lack of education still existing on their basic properties. It is, however, consistent with the important contributions that membrane operations can make in terms of cost reduction, quality improvement, pollution control, etc. Several examples of successful applications of membrane technology as alternatives to traditional processes can be mentioned. Ion-Exchange Membranes. The use of ion-exchange membrane cells in chloro-soda production represents, for example, an interesting case study for analyzing the possibilities of membrane operations and one of the first successes in terms of their electrochemical application in minimizing environmental impacts and energy consumption. The technology is based on the discovery and utilization of fluorinated polymeric membranes stable in a specific environment, such as Nafion. Today, membrane systems in which the anodic and cathodic species are directly produced in separate compartments without mixing and final separation problems permit one to overcome the limitations of traditional mercury cells, related to the need for Hg recovery, and of diaphragm cells, in which the separation and concentration of final products still create difficulties. All new chloro-soda installations are now practically based on this design, which represents a typical rationalization of the process, removing all of the pollution problems that characterized chloro-soda productions in the past. In principle, other molecular halogens could be produced from their respective gases. The direct production

3 Table 2. Worldwide Desalination Production Capacity a country total capacity (m 3 /day) % of world production Saudi Arabia U.S United Arab Emirates Kuwait Japan Libya Qatar Spain Italy Bahrain Oman a Phase-change processes: MSF (multistage flash), ME (multi-effect evaporation), VC (vapor condensation). Single-phase processes: RO (reverse osmosis), ED (electrodialysis). Table 3. Costs Related to Various Sea Water Desalination Processes energy consumption MSF (%) electric energy equivalent (kwh/m 3 ) Ind. Eng. Chem. Res., Vol. 40, No. 5, MEE (%) scale of application VC (%) RO (%) ED (%) cost for 1 m 3 of freshwater produced (ECU) process a maturity MSF very thermal small-large ME partly thermal 6-9 small-medium VC partly mechanical 7-15 small RO yes mechanical 4-8 small-large 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 formation, which allows the reaction to be run at much higher current densities, with much less purification and drying required compared to the chlorine produced by other systems. Reverse Osmosis and Nanofiltration. As already mentioned, desalination of seawater and brackish water has been at the origin of the interest for membrane operations, and the research efforts on reverse osmosis membranes have had an impact on all of the progress in membrane science and technology. Evaporation plants have been substituted with RO systems in different part of the world (Table 2). 10 The relatively low energy consumption is one of the reasons for this success (Table 3). In seawater desalting, in fact, the global energy consumption of RO, with a recovery factor of 30% and energy recovery, has been 5.32 kwh/m 3 corresponding to a primary energy consumption of kj/kg. 11 Costs for brackish water desalination are 60-70% lower than those for seawater desalination. RO desalination is not only devoted to the production of drinkable water but today is also strategic in various industrial sectors and particularly in ultrapure water production for the electronic industry. It is interesting to realize that, in Japan, the largest part of the water produced by RO is for the electronics industry, in which the country has worldwide leadership. Reverse osmosis has not generally been used until recently in the purification, separation, or concentration of chemicals, particularly because of osmotic limitations and the low chemical and thermal resistance of the existing membranes. The recent development of nanofiltration and lowpressure reverse osmosis membranes with interesting selectivities and fluxes, as well as higher chemical and thermal resistances, has been rapidly utilized in the realization of innovative processes in various industrial sectors. An interesting case studied in Italy is represented by the preparation of Iopamidol in the pharmaceutical industry. 12 Recently, in X-ray diagnosis, the use as contrast media of new nonionic iodinated compounds as opacifying agents was studied and introduced to the markets as a substitution for the traditional iodinated ionic compounds. However, the preparation and, particularly, the final purification of these products were much more complex and expensive than for those previously used. In particular, the neutral iodinated agents cannot be isolated by precipitation in water because of their high solubilities. The problems to be solved were particularly the removal of ionic species, usually inorganic salts present in the final reaction mixture and the recovery of valuable reagents present in excess and of the water-soluble reaction media. A technique was developed based on the treatment of the raw solutions of the contrast media with a complex series of operations such as removal of the solvent (DMAC or DMF) by evaporation; extraction of the residual reaction medium by a chlorinated solvent; elution of the aqueous phase on a system of cationic and anionic ion-exchange resins; concentration by evaporation; and crystallization of the crude residue to remove the last impurities. Various drawbacks are present in this system. A much better system has recently been realized based on the use of two nanofiltration stages operating on highly concentrated raw solutions containing the contrast media, inorganic salts, organic compounds with a relatively low mass (about 200), and the solvents (Figure 1). The first NF unit operates in diafiltration mode and the retentate, partially concentrated and purified with respect to contrast media, is recycled at the first stage after dilution with a small amount of deionized makeup water; the permeate (water, inorganic salts, solvents, etc.), which still contains small amounts of the iodinated compound, proceeds toward the second NF unit. The permeate from this second step is completely contrast agent free. The degree of purification that can be reached is such that the total amount of residual impurities in the final

4 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 amount and is generally on the order of 5%. The process is simple, economical, and environmentally acceptable; it permits the elimination of acid and basic reactants necessary for the regeneration of the resins; and it avoids the use of toxic organic solvents, etc. Also, the integrated membrane processes proposed for chromium recovery in the leather industry 13 and for the treatment of secondary textile effluents for their direct reuse, 14 which will be described and discussed later in this work, show efficient applications of nanofiltration and low-pressure reverse osmosis operations. Microfiltration and Ultrafiltration. Recently, in the food industry, membrane technology made realistic the possibility of cold sterilization. Tetra Pak (Bacto- Catch System) developed a cross-flow microfiltration system that debacterizes fresh milk, avoiding any thermal treatment and taste alteration. An industrial process using this technology is already in operation at Villefranche (Lyon) producing 2000 L/day of fresh milk registered with the trade name Marguerite (Figure 2). The skimmed milk, obtained by whole milk centrifugation, is sterilized at low temperature by microfiltration. Then, it is mixed with pasteurized cream. After homogenization and cooling, a debacterized whole milk is obtained using a process alternative to the classical UHT (ultrahigh-temperature) treatment. Similar products have also recently been commercialized in Italy by Parmalat S.p.A. The current systems for cleaning oil-water streams via cross-flow microfiltration or ultrafiltration are very reliable and compact. They can decrease the oil content of water from mass % to less than 5 ppm. Nitrogen blanketing helps to prevent oxidation of oils during mechanical oilseed pressing, while also reducing explosion risks in extraction and during desolventizing. New possibilities exist, however. Solvent recovery, dehydration of solvents, use of membrane reactors, winterization, and fractionation of fats are interesting cases. More than two million tons of extraction solvents, mainly hexane, is used in the U.S. alone. Its recovery is by distillation and condensation. It is estimated that, also in the most modern units, 0.7 kg of hexane per ton of seed is still released into the environment. The possibilities of recovering solvents from the oil-micelle mixture and from air exist today with membrane operations that might significantly reduce these losses. A reduction of the solvent content of the oil-micelle mixture from 70 to 40% has been demonstrated, with an energy saving of about 50%. An important aspect of the utilization of membrane operations in this area will be the possibility of using other solvents such as alcohols. Their higher evaporation heats make them unattractive in traditional evaporation units. Better solvent-resistant membranes, eventually inorganic ones, however, will be necessary for large-scale applications in this area. Cross-flow microfiltration can also be used successfully for the removal of long-chain traces of saturated fat that are present in, e.g., sunflower oil. 15 Considerable advances in UF and MF technologies in water purification processes for drinking water production have been achieved to such a point that, presently, more than m 3 /day of water are treated using these membrane operations. 16,17 The employment of integrated membrane systems in the production of drinking water is growing rapidly with excellent results. The reliability of the reverse osmosis membrane is greatly increased when UF or MF operationsswhich emerged in the past decade as an efficient way to remove suspended solids and organic and microbiological contaminantssare used in the pretreatment step. Furthermore, economical considerations have shown that multiple membrane systems are more competitive than conventional processes, resulting in the reduction of capital and operating costs. In addition to the already-mentioned membrane operations, gas separation, pervaporation, and some others membrane processes, which have recently shown significant possibilities for their application in various industrial areas, must be cited; among these, a class of membrane-based unit operations identified as membrane contactors, membrane bioreactors, and catalytic membrane reactors will be discussed. Gas Separation. Membrane processes for gaseous mixture separation are, today, technically well-consolidated and apt to substitute for traditional techniques. 18 Separation of air components, natural gas dehumidification, and separation and recovery of CO 2 from biogas

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

6 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 Separation a permeability coefficient, barrer selectivity (ideal) (-) CO 2 O 2 N 2 CO 2/N 2 O 2/N 2 poly[1-(trimethylsilyl)-1-propyne] poly(dimethylsiloxane) poly(dimethylsilmethylene) poly(cis-isoprene) poly(butadiene-styrene) natural rubber (at 25 c) ethyl cellulose polystyrene butyl rubber poly(ethyl methacrylate) poly(phenylene oxide) (at 25 c) bisphenol A polycarbonate cellulose acetate bisphenol A polysulfone PMDA-4,4 -ODA polymide poly(methyl methacrylate) poly(vinyl chloride) (at 25 C) PEEK-WC (at 25 C) polyphosphazeny (at 25 C) 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 circuit to provide the driving force for the separation, with a significant reduction in cost. The driving force for the separation process is the partial pressure difference across the membrane. High-pressure air ( psia) is required to achieve a significant flux of O 2 across the membrane. The oxygen flux is directly proportional to the pressure gradient and inversely proportional to the membrane thickness. The pressure of the oxygen product is typically only a fraction of an atmosphere. These dense inorganic perovskite type membranes, today manufactured in tubular configurations, transport oxygen as lattice ions at elevated temperatures with infinite selectivity ratios in O 2 separations. The ionic conductivity of the material studied is mainly equal to the electronic conductivity. Because this oxygen-ion-conducting membrane must operate at temperatures above 700 C, an effective means of recovering the energy contained in the nonpermeate, oxygen-depleted stream is required. An efficient and cost-effective means to accomplish this is to integrate the membrane system with a gas turbine (Figure 4). 28 A technology known as OTM syngas (oxygen transport membrane synthesis gas) utilizes these ionconducting membranes able to separate oxygen from air with a high flux in the same temperature region required for the reforming of natural gas. This technology was presented in 1997 by an alliance of five

7 Ind. Eng. Chem. Res., Vol. 40, No. 5, Figure 5. Scheme of a plant for H 2 recovery from ammonia synthesis. international companies including AMOCO, BP Chemicals, PRAXAIR, SASOL, and STATOIL. Philippe Petroleum joined the alliance in The new process, still under development, integrates the separation of oxygen from air, steam reforming, and natural gas oxidation into one step, eliminating the need for a separate oxygen plant. The new technology offers the possibility of reducing the energy and capital costs of syngas production. Considering that 60% of the cost for manufacturing any product from natural gas is related to synthesis gas production, the interest of this innovation technology is evident. Various plants for the recovery of hydrogen from the purge of the synthesis of ammonia have been realized today. 29 The unit modules are in general arranged in a one-stage-two-unit form. One of the first plants of this type has been realized by Permea in Louisiana (Figure 5). 30 The first unit, consisting of eight hollow-fiber modules [total feed capacity about 3800 m 3 (stp)/h] is operated with a transmembrane pressure difference of 60 bar, the permeate leaving at a pressure of about 70 bar. At this pressure, the permeate can be fed to the second stage of the synthesis feed compressor. The retentate of the first unit is fed to the second unit where the permeate leaving the modules at 25 bar is mixed with fresh feed (suction side of the first stage of the compressor). The retentate is utilized for heating purposes. Gas pretreatment consist of conventional scrubbing to reduce the ammonia content of the bleed from 2% (molar) to less than 200 ppm in order to avoid membrane swelling and, as a consequence, damage of the membrane. The economical and technical advantages related to this membrane system for the recovery of hydrogen are shown in Table 5. Methanol synthesis is another process based on a gaseous feed; in purge recovery, a water scrubber is also used with a similar purpose, and it pays for itself in terms of the recovered methanol. The methanol/water mixture is simply sent to the existing crude methanol distillation column. Hydrogen recovered from this purge can result in energy savings, and if additional carbon oxide is available, it can be used to obtain increased methanol production. PRISM separators operate on Table 5. Economic and Technical Advantages for a 1000 ton/day Ammonia Plant 30 ammonia recovery (scrubbing) heat saving additional ammonia production increase in ammonia production (at constant natural gas consumption) reduction in natural gas production (at constant production rate) 4 ton/day kj/ton of NH 3 produced ton/day ton/day stoichiometric as well as nonstoichiometric H 2 /(CO) x ratio methanol plants at differential pressures up to 70 bar. Figure 6 shows the flow diagram of such a hydrogen recovery unit installed for demonstration purposes. 31,30 Before entering the gas permeators, the feed is scrubbed in order to reduce the methanol content to levels below 100 ppm. From a bleed stream of 4000 mol/h, for example, a recovery of 2000 mol/h of hydrogen has been achieved. Gas mixture dehumidification is a process of great industrial interest, especially for natural gas purification and air dehumidification. An efficient membrane system for air dehumidification called the Cactus Membrane Air Dryer, developed in the late 1980s, has been commercialized by Permea. 32 When the Cactus dryer is fed with compressed air, water vapor and a small amount of oxygen pass through the walls of the hollow fiber, while nitrogen, argon, and most of the oxygen continue through the hollow core of the fibers to the end of the separator. A small amount of the slower gases passes through the fiber, and this is used to sweep the water vapor through the separator. Cactus membranes work on the principle of dew point depression. For example, a membrane might be sized for inlet conditions of 100 psig and 100 F inlet dew point to achieve a0 F pressure dew point. If inlet conditions change, e.g., compressed air with a lower inlet dew point is supplied, the separator will provide dry air at an even lower dew point. The removal of hydrogen sulfide (H 2 S) and carbon dioxide (CO 2 ) from natural gas is an ideal application for membranes (Figure 7); both H 2 S and CO 2 permeate through membranes at a much higher rate than meth-

8 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 H 2S and CO 2 from natural gas. Figure 8. Recovery of CO 2 from exhaust gas and reuse to produce chemicals by hydrogenation. 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 diethanolamine (DEA) gas-absorption process, it was found that the membrane process is more economical than the DEA gas-absorption process in the range of CO 2 concentrations in the feed between 5 and 40 mol %. When the feed also contains H 2 S, the cost for reducing the CO 2 and H 2 S concentrations in the feed to pipeline specifications increases with increasing H 2 S concentration (1000 to ppm). If membrane processes are not economically competitive because of the high H 2 S concentration in the feed, the separation cost could be significantly lowered by using hybrid membrane processes. In such processes, the bulk of CO 2 and H 2 S is separated from sour natural gas with membranes, and the final purification to pipeline quality gas is performed by means of suitable gas-absorption processes. 33 Despite the high levels of H 2 S in the feed, membrane selectivities are maintained. 34 The possibility of utilizing membrane technology in solving problems such as the greenhouse effect related to CO 2 production has also been suggested. Membranes able to remove CO 2 from air, having a high CO 2 /N 2 selectivity, might be used at any large-scale industrial CO 2 source as a power station in petrochemical plants. The CO 2 separated might be converted by reacting it with H 2 in methanol, starting a C 1 chemistry cycle. As schematized in Figure 8, a membrane reactor might be ideally used to carry out hydrogenation reactions for chemical production using CO 2 recovered from exhaust gases by membrane separation. The separation and recovery of organic solvents from gas stream is also rapidly growing at the industrial level. Polymeric rubbery membranes that selectively permeate organic compounds (VOC) from air or nitrogen have been used. Such systems typically achieve greater than 99% removal of VOC from the feed gas and reduce the VOC content of the stream to 100 ppm or less. The technology has been applied to the recovery of highvalue organic vapors such as vinyl chloride monomer, methyl chloride, and methyl formate. Membrane systems are competitive with carbon ad-

9 Ind. Eng. Chem. Res., Vol. 40, No. 5, 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 than 5000 ppm, particularly if high VOC recovery is required. The typical industrial applications of vapor recovery are off-gas treatment in gasoline tank farms, gasoline station vapor return, and end of pipe solvent recovery in the chemical and pharmaceutical industries. Another interesting example of an industrial application is VOC recovery by the compression-condensation and vapor permeation method, presented schematically in Figure 9. This is a scheme of the process developed in Anwil (Wloclawed, Poland), which has been built by MTR (U.S.) for the recovery of monovinyl chloride (MVC). The recovery of ethylene and propylene from nitrogen in polyolefin plant vent streams has been suggested and realized at the industrial level by DSM in Geleen, The Netherlands. To recover the unreacted monomer (up to 25%) and other volatile hydrocarbons from the nitrogen used during polymer particles degassing, MTR 35 developed a two-stage operation in which the mixture of N 2 and propylene is first compressed and later directed into a membrane vapor separation unit, as shown in Figure 10. The spiral-wound membrane modules (8 in. diameter, 20 m 2 surface area) used are times more permeable to organic vapors than to air or nitrogen. In 1989, the first vapor recovery unit (VRU) based on membrane technology was commissioned for off-gas treatment in a gasoline tank farm. At present, various membrane VRU s are in operation or under construction. The capacity of these units ranges from 100 to 2000 m 3 /h. These are single-membrane stages of hybrid systems of a membrane stage combined with a posttreatment facility, e.g., a catalytic incinerator, gas engine, or pressure swing adsorption unit. These plants are equipped with a modified plate and frame configuration. 36 A case of a vapor recovery unit based on membrane technology is that commissioned in a gasoline tank farm in Munich for the treatment of the off-gasses generated from the storage, handling, and distribution of gasoline. The plant capacity was 300 m 3 /h. The only external available energy source was the electrical power supply. This was planned in the framework of a pilot project for the reduction of emissions at the BP tank farm Hamburg-Finkenwerder. The VRU has a capacity of 1500 m 3 /h and a hybrid system of a membrane stage and a gas engine. Two gas engines coupled with a generator are permanently in operation to supply the basic electrical power of the side. The gas engines are designed to switch the fuel feed from natural gas to retentate of the membrane stage over a period of VRU operating time. A commercially successful application is a hybrid system of a membrane stage with pressure swing adsorption (PSA) (Figure 11). The liquid ring compressor operating with gasoline as the service liquid sucks the hydrocarbon (HC) contaminated air from the gasometer. The off-gases are compressed and fed into a scrubber. Gasoline from the tank farm is used as a lean absorbent. The HC concentration of the feed gas leaving the scrubber depends on the operating temperature and pressure. The layout of the membrane stage (membrane area and permeate pressure) is governed by the permissible HC intake concentration of the PSA unit. Two parallel PSA units are installed and operated alter-

10 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 separation of water from organic/aqueous mixtures removal of volatile compounds from aqueous and gas streams separation of organic/organic mixtures details separation and/or dehydration of water/organic azeotropes (water/ethanol, water/2-propanol, water/pyridine) dehydration of organic solvents shifting of the reaction equilibrium (e.g., esterification) removal of chlorinated hydrocarbons separation of organics from the fermentation broth separation of aroma compounds wine and beer dealcoholization removal of VOCs from air separation of azeotropes (e.g., ethanol/cyclohexane, methanol/mtbe, ethanol-etbe) separation of isomers (e.g., xylenes) Table 7. Comparison of the Dehydration Costs of Ethanol from 99.4 to 99.9 vol % by Different Techniques utilities vapor permeation ($/ton) pervaporation ($/ton) entrainer distillation ($/ton) molecular sieve adsorption ($/ton) vapor electricity cooling water entrainer replacement of membranes and molecular sieves total costs nately. One is in the adsorption phase while the other is in the desorption and regeneration phase. A bypass of the clean stream is used as a purge gas for regeneration. To maintain a low vacuum, the vacuum pump at the downstream side of the membrane stage can be a liquid ring pump with mineral oil as the service liquid or a rotary vane vacuum pump. This vacuum pump is also used to support the desorption of the PSA column. The adsorber material is activated carbon, a carbon molecular sieve, or an inorganic molecular sieve. A typical VRU combined with a PSA is installed at Shell in Ludwigshafen. 36 Other interesting applications of the technology might be in the separation of light hydrocarbons from refinery waste gas streams, the recovery of natural gas liquids and hydrogen, or the separation of propane, butane, and higher hydrocarbons from methane in the processing of natural gas for dew point control. Pervaporation. Some applications of pervaporation processes are listed in Table 6. Dehydratation of ethanol by PV was the first industrial-scale application proposed by GFT in the 1980s. Today, more than 40 industrial pervaporation plants built by Sulzer Chemtech Membrantechnik (former GFT) are in operation worldwide. They are used for the dehydration of different solvents and/or solvent mixtures. In many practical applications, it might be more economical to use pervaporation or vapor permeation only to break the azeotrope and to concentrate the retentate further by the above-azeotropic distillation (Table 7). Another successful example of PV is its application in the enhancement of chemical reaction efficiency. Examples of such reactions are esterification or phenolacetone condensation. The first industrial plant for the

11 Ind. Eng. Chem. Res., Vol. 40, No. 5, Figure 12. Pervaporation-enhanced MTBE production. Figure 13. Membrane system for CO 2 recovery from fermentation broth. pervaporation-enhanced ester synthesis was built in 1991 by GFT for BASF. A possible application of the removal of organic solutes could be the treatment of industrial and municipal water supplies contaminated with carcinogenic halogenated organic compounds. Such a process would also be attractive for the extraction of organics. The possibility of recovering volatile organic compounds from gases by pervaporation has been demonstrated and applied recently at the industrial level. 37 The elimination of volatile solutes from dilute aqueous solutions might be possible by pervaporation. Separation of organic/organic mixtures represents the least-developed application and the largest potential commercial impact of pervaporation, but considerable developments in membrane materials and processes remains to be done. The first industrial application of PV to organic/organic separation was the separation of methanol from a methyl tert-butyl ether (MTBE) stream in the production of octane enhancer for fuel blends (Figure 12). Flexibility with respect to part-load performance and changing product and feed concentrations is one of the advantages of pervaporation over other separation processes. This is especially useful in the production of fine chemicals and in the pharmaceutical industry, where solvents are used and almost no single waste solvent is generated continuously. Pervaporation-based hybrid processes offer significant potential for new economical and efficient solutions to some classical separation problems. 38 Membrane Contactors. In these systems the membrane function is to facilitate diffusive mass transfer between two contacting phases, which can be liquidliquid, gas-liquid, gas-gas, etc. 39 The traditional stripping, scrubbing, absorption, and liquid/liquid extraction processes can be carried out in this new configuration. With respect to conventional systems, membrane contactors can guarantee some advantages such as nondispersion of the phases in contact, independently variable flow rates without flooding limitations, lack of phase-density difference limitations, lack of phase separation requirements, higher surface area/volume ratios, and direct scale-up due to a modular design. Traditional liquid-supported membranes in which a carrier is immobilized in the microporous hydrophobic structure of the polymeric membranes are the most traditional and well-developed example of a membrane contactor system. Other applications, however, have been studied and realized today or are under investigations. Interesting examples include the removal of trace of oxygen (at levels of <10 ppb) from water for ultrapure water preparation for the electronics industry, 40 the removal of CO 2 from fermentation broth (Figure 13), and the supply of CO 2 as a gas to liquid phases (carbonation of soft drinks). 41 The flow sheet of a water carbonation process is presented in Figure 14. Additional examples include the removal of alcohol from wine and beer, the concentration of juice via osmotic or membrane distillation, 41 the nitrogenation of beer, 42 the degassing of organic solutions, and water ozonation. 43

12 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 have generally been used for industrial units. During operation, an aqueous liquid flows over the shell side (outside) of the hollow fiber. A strip gas or vacuum, either separately or in combination, is applied on the lumen side of the hollow fiber and flows counter current. Because of its hydrophobic character, the membrane acts as an inert support to allow intimate contact between the gas and liquid phases without dispersion. The interface is immobilized at the pores by applying a higher pressure to the aqueous stream than the gas stream. The result is fast diffusive transfer of dissolved gases from or to the liquid phase. Since 1993, a bubble-free membrane-based carbonation line has processed about 112 gal/min of beverage by membrane contactors having a total interfacial area of 193 m 2 (Pepsi bottling plant in West Virginia). 40 Permea commercializes beer dispensing systems known as CELLARSTREAM Dispense Systems using PULSAR gas/liquid contactors, which increase or decrease the amount of carbon dioxide and nitrogen in draft beer for optimal presentation. 42 Membrane distillation and osmotic distillation can be considered examples of membrane contactors for realizing the concentration of aqueous solutions with nonvolatile solutes as salts and sugars In the membrane distillation process, two liquids or solutions at different temperatures are separated by a porous membrane acting as a barrier between the two phases, which must not wet the membrane (this implies that hydrophobic membranes must be used in the case of aqueous solutions). Because of the temperature gradient, a vapor pressure difference exists across the membrane, and it is the driving force inducing vapor molecule transport through the pores from the high-vapor-pressure side to the permeate side. In the case of osmotic distillation, the vapor molecule transport is due to a vapor pressure driving force provided by having a low-vapor-pressure solution on the permeate side of the membrane, e.g., a concentrated salt solution. The formation of emulsions or dispersions characterized by very uniform dimensions of droplets or microbubbles can be realized using the same technology. The membrane emulsification process is applied mainly in the preparation of food emulsions. Moreover, microbubble formation increases the stability of the system by minimizing coalescence phenomena. An interesting study evidenced the relationship existing between membrane pore diameter and droplet size. 48 The formulation of various products might be realized using this new concept, and important phenomena such as oil combustion might be optimized. Membrane Reactors The possibility of combining molecular separation and chemical transformations in a single unit soon attracted the interest of membrane engineers. 49 The first studies on such reactors were devoted to the immobilization of biocatalysts on polymeric membranes. Recently, hightemperature reactions have been the objective of important studies. Both areas will be analyzed in the following pages. Membrane Bioreactors. Biocatalytic membrane reactors are interesting with respect to conventional membranes as they combine selective mass transport with chemical reactions. The selective removal of products from the reaction site increases conversion of product-inhibited or thermodynamically unfavorable reactions. Biocatalysts can be used suspended in solution and compartmentalized by a membrane in a reaction vessel or immobilized within the membrane matrix itself. 50 Since the advent of what has been called solid-phase biochemistry, the advantages of immobilized biocatalytic preparations over homogeneous-phase enzymatic/cellular reactions have been exploited to develop new and less-expensive processes. Synthetic membranes provide an ideal support for biocatalyst immobilization because of a wide available surface area per unit volume and the possibility for the development of new immobilization procedures. Enzymes are retained in the reaction side, do not pollute the products, and can be continuously reused. Immobilization has also been shown to enhance enzyme stability. Moreover, provided that membranes of suitable molecular weight cutoff are used, chemical reaction and physical separation of biocatalysts (and/or substrates) from the products can take place in the same unit. Substrate partition at the membrane/fluid interface can be used to improve the selectivity of the catalytic reaction toward the derived products with minimal side reactions. Membranes are also attractive for retaining in the reaction volume the expensive cofactors that are often required to carry out some enzymatic reactions. At the 1997 Achema conference in Frankfurt, Germany, statements on the impact that innovative bioreactors, and particularly those based on the hollow-fiber design, have in setting new performance standards were

13 Ind. Eng. Chem. Res., Vol. 40, No. 5, clearly presented. For example, hollow-fiber bioreactors in which cells attach into a capillary-type space have been designed to mimic biological processes more closely than any other reactor system. Through the fibers, nutrients such as glucose and oxygenase are fed to the cells, and wastes such as CO 2 and H 2 O are removed. Roche Diagnostic declared the use of such reactors to produce monoclonal antibodies for diagnostic tests. Membrane bioreactor technology can also be applied to produce pure enantiomers, in that a membrane separation process can be combined with an enantiospecific reaction to obtain a so-called enantiospecific membrane reactor. As for general membrane reactors, the result is a more compact system with higher conversion. This technology can respond to the strongly increasing demand for pharmaceuticals, food additives, feeds, flavors, fragrances, agrochemicals, etc., as optically pure isomers. 51 Recently, the results achieved in the production of a chiral intermediate used for the preparation of an important calcium channel blocker, diltiazem, were discussed in the open literature, 52 confirming the possibilities of membrane reactors also in the large-scale production of biotechnological products. Phase-transfer catalysis can also be realized in membrane reactor configurations, immobilizing the appropriate catalysts in the microporous structure of the hydrophobic membranes. Biphasic membrane reactors have been extensively studied with lipases entrapped or bonded on the membrane surface, which confirms the possibilities of the approach, 53,54 as already discussed. Catalytic Membrane Reactors. The development of catalytic membrane reactors for high-temperature applications became realistic only in the last few years with the development of high-temperature-resistant membranes. In particular, the earlier applications involved mainly dehydrogenation reactions, where the role of the membrane was simply hydrogen removal. The earlier studies carried out, particularly in the Soviet Union on palladium and palladium alloys, confirmed the existence of membranes able to permeate H 2 with high selectivity. Both capital and operative cost savings were anticipated, as units for hydrocarbon separation from the streams were avoided and the possibility of operating at lower temperatures because of reactor yield enhancement was realized. The fact that the membranes separate intermediates and products from the reacting zone, avoiding possible catalyst deactivation or secondary reactions, is also of practical interest. The kinetic mechanisms might be modified or controlled by the presence of appropriate membrane systems, which can also act only on a reactive interface with no permselectivity, optimizing phase-transfer catalysis. 55 Palladium membrane costs and availability, their mechanical and thermal stability, and poisoning and carbon deposition problems are still obstacle to the large-scale industrial application of these dense metal membranes, also when prepared in a composite configuration. 56 Hydrogen can be produced by steam reforming and shift conversion of natural gas or other hydrocarbons. In conventional steam reformers, high conversions of natural gas, on the order of 85-90% or even higher, are obtained at reformer outlet temperatures of around C. The energy efficiency of steam reforming processes is relatively high, but the investments are substantial. Pure hydrogen can be produced at significantly lower temperatures by integrating into the reactor a membrane that selectively removes hydrogen during conversion. Potential savings in membrane reformer and downstream processing costs compared to conventional steam reforming apparatuses must, in many cases, be weighed against additional costs associated with recompression of the hydrogen permeate stream. Ag membranes were initially suggested for their H 2 permeability. Howevere, they present the same problems that characterize Pd membranes, also having a much lower permeability. Solid oxide membranes have recently been suggested for large-scale applications in syngas production. 57 Studies carried out in the U.S. showed the possibility of preparing membranes with improved mechanical and thermal characteristics, able to operate, for example, at 900 C for over 21 days. Integrated Membrane Processes Traditionally, the various membrane operations (RO, UF, MF, etc.) have been introduced in industrial production lines as an alternative to other existing units. Reverse osmosis instead of distillation and ultrafiltration in place of centrifugation are typical examples. The possibility of redesigning overall industrial production by the integration of various already developed membrane operations is becoming of particular interest, because of the synergic effects that can be reached, the simplicity of the units, and the possibility of advanced levels of automatization and remote control that can be realized. The rationalization of industrial production by use of these technologies permits low environmental impacts, low energy consumption, and higher quality of final products. New products also often become available. These results are related to the introduction of new technologies from the very early stages of the same material transformations and not at the end of the pipe, as was often done in the past. The leather industry might be an interesting case study because of (1) the large environmental problems related to its operation, (2) the low technological content of its traditional operations, and (3) the tendency to concentrate a large number of small-medium industries in specific districts. More than 2000 companies are in operation in Italy, which is recognized as a world leather leader for the quality of the leather produced. The traditional flow sheet of the tanning process in its humid phase consists of about 20 steps operating in a discontinuous cascade system. The possibility of rationalizing the overall process by introducing advanced molecular separation systems such as ultrafiltration, cross-flow filtration, microfiltration, nanofiltration, and reverse osmosis was suggested and has recently also become the objective of an Italian National Research Program coordinated and carried out by a consortium representing most of the companies in the country. In Figure 15 is presented an ideal process based on integrated membrane operations. 58,59 The innovative integrated scheme suggested in Figure 15 allows the pollution problems of the leather industry to be faced by solving or minimizing them one by one where they originate, thereby avoiding the need for huge wastewater treatments at the end of the overall produc-