2 Sy Synt nthe heti ticc Me Memb mbra rane ness o orr Me Memb mbra rane ne Pr Proc oces esse sess
2.1 Intro Introduction duction According to Wikipedia [], a membrane is a thin, typically planar structure or material that separates two environments or phases and has a finite volume. It can be reerred to as an interphase rather than an interace. Membranes selectively controll mass tran tro transport sport betwe between en phase phasess or en envir vironme onments nts.. Aga Again, in, acco accordin rdingg to Wi Wikipe kipedia, dia, membranes membra nes can be divided into three groups: () biological membranes, () artificial membranes, membra nes, and () theoretical membra membranes. nes. Biological membranes include: 1. 2. 3. 4.
Cell membranes membranes and intracell intracellular ular membranes membranes Mucous Mu cous membra membranes nes S-layer S-la yer Serous membranes membranes and mesothelia mesothelia that surround organs, organs, including: a) Te peritoneum peritoneum that that lines the abdominal abdominal cavity b) Te pericardium pericardium that that surrounds the heart heart c) Te pleura pleura that surrounds surrounds the lungs lungs d) Te periosteum periosteum that that surrounds bone e) Te meninges meninges that surround the brain (the dura mater, mater, the arachnoid, and the pia mater) Artificial membranes are used in:
1. 2. 3. 4. 5. 6. 7. 8. 9.
Reverse osmo Reverse osmosis sis Filtration Filtra tion (microfiltratio (microfiltration, n, ultrafiltration) ultrafiltration) Pervaporation Perva poration Dialysis Dial ysis Emulsion Emulsio n liquid liquid membranes membranes Membrane-base Mem brane-based d solvent solvent extraction extraction Membrane Mem brane reactors Gas permea permeation tion Supported Sup ported liquid liquid membran membranes es
Tis book is devoted to synthetic, or artificial, membranes. In particular, our ocus will be on poly polymeri mericc synth synthetic etic memb membran ranes, es, sinc sincee most indu industri strial al membr membrane aness bebelong to this categ category ory.. Beore entering entering the main subject o this book, i.e., atomic orce
6
2 Synthetic Membranes for Membrane Processes
microscopy, the current status o synthetic polymeric membranes is outlined. Tus, in the ollowing pages, we will provide some inormation about the preparation o membranes, the properties o membranes, and their applications.
2.2 Membrane Preparation Synthetic membranes are abricated in two main geometries: 1. Flat sheet—utilized in the construction o flat sheet, disc, spirally wound, plate, and rame modules 2. Cylindrical—utilized in tubular and capillary, or hollow fiber, modules Membranes can be prepared rom both ceramic and polymeric materials. Ceramic materials have several advantages over polymeric materials, such as higher chemical and thermal stability. However, the market share o polymeric membranes is ar greater than ceramic membranes as the polymeric materials are easier to process and less expensive. A handul o technical polymers are currently used as membrane materials or % o all practical applications []. Polymeric materials that are used to prepare separation membranes are mostly organic compounds. A number o dierent techniques are available to prepare synthetic membranes. 2.2.1 Membranes with Symmetric Structure
Although most o the practically useul membranes are asymmetric, as explained later, some o the membranes have symmetric structures. Tey are prepared in the ollowing ways: Track etching A sheet o polymeric film moves underneath a radiation source and is irradiated by high-energy particles. Te spots that are subjected to bombardment o the particles are degraded or chemically altered during this process. Ten, the film undergoes an etching process in an alkaline or hydrogen peroxide bath (depending on the material), where the polymer is etched along the path o high-energy particles. Precipitation rom the vapor phase A cast polymer solution that consists o polymer and solvent is brought into a nonsolvent vapor environment saturated with sol vent vapor. Te saturated solvent vapor suppresses the evaporation o solvent rom the film; the nonsolvent molecules diffuse into the film causing polymer coagulation. 2.2.2 Membranes with Asymmetric Structure
Most membranes used in industries have an asymmetric structure. Figure . shows schematically a typical cross-sectional view o an asymmetric membrane []. It consists o two layers: the top one is a very thin dense layer (also called the top skin layer), and the bottom one is a porous sublayer. Te top dense layer governs the perormance (permeation properties) o the membrane; the porous sublayer only provides mechanical strength to the membrane. Te membranes o symmetric structures do not possess a top dense layer. In the asymmetric membrane, when the material o the top
2.2 Membrane Preparation
7 Fig. 2.1. Cross-sectional view of an asymmetric membrane. Reprinted from [3], with kind permission from the author
layer and porous sublayer are the same, the membrane is called an integrally skinned asymmetric membrane. On the other hand, i the polymer o the top skin layer is dierent rom the polymer o the porous sublayer, the membrane is called a composite membrane. Te advantage o the composite membrane over the integrally skinned asymmetric membrane is that the material or the top skin layer and the porous sublayer can be chosen separately to optimize the overall perormance. Tere are various methods or the preparation o asymmetric membranes, which are described in the sections that ollow. 2.2.2.1 Phase Inversion Technique or Preparation o Integrally Skinned Asymmetric Membranes Dry–wet phase inversion technique (Loeb-Sourirajan method) A number o methods can be used to achieve phase inversion. Among these, the dry–wet phase in version technique and thermally induced phase separation (IPS) are the most commonly used in membrane manuacturing. Te dry–wet phase inversion technique, also called the Loeb-Sourirajan technique, was used by Loeb and Sourirajan in their development o the first cellulose acetate membrane or seawater desalination []. In this method, a polymer solution is prepared by mixing polymer and solvent (sometimes even nonsolvent). Te solution is then cast on a suitable surace by a doctor blade to a precalculated thickness. Afer a partial evaporation o the solvent, the cast film is immersed in a nonsolvent medium called a gelation bath. Due to a sequence o two desolvation steps, i.e., evaporation o the solvent and solvent–nonsolvent exchange in the gelation bath, solidification o the polymer film takes place. It is desirable to choose a solvent o strong dissolving power with a high volatility. During the first step o desolvation by solvent evaporation, a thin skin layer o solid polymer is ormed instantly at the top o the cast film due to the loss o solvent. In the solvent– nonsolvent exchange process that ollows, the nonsolvent diffuses into the polymer solution film through the thin solid layer while the solvent diffuses out. Te change in the composition o the polymer solution film during the solvent–nonsolvent exchange process, ofen called a composition path, is illustrated schematically in Fig. . (lines A, B, and C each represent a composition path). Te top skin layer can also be made porous by lowering the polymer concentration in the casting solution and the solvent evaporation period. Tis is called, hereafer, the porous skin layer. Asymmetric membranes can also be made in a tubular orm using a casting bob assembly and a hollow fiber spinneret [].
8
2 Synthetic Membranes for Membrane Processes Fig. 2.2. Triangular diagram of polymer ( P ), solvent (S), and nonsolvent (N ). Reprinted from [3], with kindpermission from the author
Thermally induced phase separation method In this method, phase inversion is introduced by lowering the temperature o the polymer solution. A polymer is mixed with a substance that acts as a solvent at a high temperature and the polymer solution is cast into a film. When the solution is cooled, it enters into an immiscible region due to the loss o solvent power. Because the solvent is usually nonvolatile, it must be removed with a liquid that is miscible with the solvent but not miscible with the polymer. 2.2.2.2 Preparation o Composite Membranes Dip coating An integrally skinned asymmetric membrane with a porous skin layer (called hereafer a substrate membrane) is prepared rom a polymer solution by applying the dry–wet phase inversion method. Te membrane is then dried according to the method described later, beore it is dipped into a bath containing a dilute solution o another polymer. When the membrane is taken out o the bath, a thin layer o coating solution is deposited on the top o the substrate membrane. Te solvent is then removed by evaporation, leaving a thin layer o the latter polymer on top o the substrate membrane. Interacial polymerization Tis method, developed by Cadotte and the coworkers o Film ech in the s, is currently most widely used to prepare high perormance reverse osmosis and nanofiltration membranes []. A thin selective layer is deposited on top o a porous substrate membrane by interacial in situ polycondensation. Tere are a number o modifications o this method primarily based on the choice o the monomers []. However, or simplicity, the polycondensation procedure is described by a pair o diamine and diacid chloride monomers. A diamine solution in water and a diacid chloride solution in hexane are prepared. A porous substrate membrane is then dipped into the aqueous solution o diamine. Te pores at the top o the porous substrate membrane are filled with the aqueous solution in this process. Te membrane is then immersed in the diacid chloride solution in hexane. Since water and hexane are not miscible, an interace is ormed at the boundary o the two phases. Polycondensation o diamine and diacid chloride
2.2 Membrane Preparation
9
Fig. 2.3. Steps in the formation of a composite membrane via interfacial polymerization. Reprinted from [3], with kind permission from the author
takes place at the interace, resulting in a very thin layer o polyamide. Te preparation o composite membranes by interacial in situ polycondensation is schematically presented in Fig. .. 2.2.2.3 Membrane Surace Modifcation As mentioned above, the top skin layer governs the perormance o a separation membrane. Te surace deposition o contaminants rom solutions or rom gas mixtures is also affected by the surace properties o the membrane. Tis is particularly important when decline in the membrane flux with a prolonged operating period is observed, since it is ofen caused by the contaminant deposition. Hence, many attempts have been made to modiy the membrane surace, aimed at prevention o contaminant deposition and maintenance o high flux. Several methods o surace modification are described below. Chemical modifcation Te surace o a membrane can be modified by chemical reactions. For example, when the surace o a polyamide composite membrane is brought into contact with a strong hydrofluoric acid solution, the top polyamide layer becomes slightly thinner by a chemical reaction with hydrofluoric acid. As a result, the flux increases considerably while the rejection o sodium chloride is unchanged or slightly increased []. Plasma polymerization When a vacuum is maintained inside a tubular reactor and a high requency electric field is applied outside, a glow discharge is generated inside the reactor (Fig. .). Plasma that consists o various ions, radicals, electrons, and molecules is ormed in the glow discharge. When a porous substrate membrane is placed in the plasma, the surace o the membrane is subjected to various changes corresponding to the property o plasma. Te substrate surace can be etched and/or chemically active sites can be introduced to the surace, and, upon contact with organic compounds, an irregular polymerization can occur at the substrate surace. Tis is called plasma polymerization [].
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2 Synthetic Membranes for Membrane Processes
Fig. 2.4. Reactor for plasma treatment. Reprinted from [3], with kind permission from the author
Grat polymerization Te surace o a porous substrate membrane is irradiated with γ-rays, which causes the generation o radicals on the membrane surace. Ten, the membrane is immersed in a monomer solution. Te graf polymerization o the monomers is initiated at the membrane surace. By choosing a very hydrophilic monomer, the hydrophilicity o the surace is increased considerably. Surace modifcation by surace modiying macromolecules(SMMs) Inapolymer blend, thermodynamic incompatibility between polymers usually causes demixing o polymers. I the polymer is equilibrated in air, the polymer with the lowest surace energy (the hydrophobic polymer) will concentrate at the air interace and reduce the system’s interacial tension as a consequence. Te preerential adsorption o a polymer o lower surace tension at the surace was confirmed by a number o researchers or the miscible blend o two different polymers. Based on this concept, surace modiying macromolecules as surace-active additives were synthesized and blended into polymer solutions o poly(ether sulone) (PES). Depending on the hydrophobic [, ] or hydrophilic [] nature o the SMMs, the membrane surace becomes either more hydrophobic or more hydrophilic than the base polymeric material. 2.2.3 Membrane Drying
Te wet cellulose acetate membranes prepared or reverse osmosis purposes can be used or gas separation when they are dried. Te water in the cellulose acetate membrane cannot be evaporated in air, however, since the asymmetric structure o the membrane will collapse. Instead, the multi-stage solvent exchange and evaporation method is applied. In this method, a water-miscible solvent such as ethanol first replaces the water in the membrane. Ten, a second volatile solvent such as hexane replaces the first solvent. Te second solvent is subsequently air-evaporated to obtain a dry membrane [, ]. Te reason or replacing water with hexane is to reduce the capillary orce inside the pore so that it will not collapse during the drying process.
2.3 Membranes for Separation Processes
11
2.3 Membranes or Separation Processes 2.3.1 Membranes or the Separation o Solutions and Solvent Mixtures
Membranes or the separation o solutions and liquid mixtures may be distinguished on the basis o pore sizes as reverse osmosis (RO, below nm), ultrafiltration (UF, – nm), and microfiltration (MF, nm to μm), although this classification is very arbitrary. Pore sizes o nanofiltration (NF) membranes are between RO and UF membranes. 2.3.1.1 Reverse Osmosis Membranes An RO membrane acts as a barrier to flow, allowing selective passage o a particular species (solvent) while other species (solutes) are retained partially or completely. Solute separation and permeate solvent (water in most cases) flux depend on the material selection, the preparation procedures, and the structure o the membrane barrier layer [, ]. Cellulose acetate (CA) is the material or the first generation reverse osmosis membrane. Te announcement o CA membranes or sea water desalination by Loeb and Sourirajan in triggered the applications o membrane separation processes in many industrial sectors. CA membranes are prepared by the dry–wet phase inversion technique. Another polymeric material or RO is aromatic polyamide []. In aromatic polyamide polymers, aromatic rings are connected by an amide linkage, –CONH–. While the aromatic ring attached to –NH– is metasubstituted, the ring attached to –CO– is the mixture o meta- and parasubstitutions, which gives more flexibility to the polymeric material. Aromatic polyamide remains one o the most important materials or reverse osmosis membranes since the thin selective layer o composite membranes is aromatic polyamide synthesized by interacial in situ polymerization. 2.3.1.2 Nanofltration Membranes Most NF membranes are negatively charged. In interacial polycondensation, trimesoyl (triacid) chloride is ofen mixed with phthaloyl (diacid) chloride in the acidic component o the polycondensation reaction. Although most carboxylic groups are consumed to orm amide linkage, a small portion o the carboxylic groups do not participate in the reaction, becoming the source o the electric charge. Since –COOH becomes –COO upon dissociation, the membranes are negatively charged. Because o the negative charge, anions are preerentially rejected by nanofiltration membranes. Another method o preparing nanofiltration membranes is to dip-coat a thin layer o sulonated poly(phenylene oxide) (SPPO) [], sulonated polysulone (SPS) [], or carboxylated polysulone [] on a porous substrate membrane. Te sulonic acid groups in SPPO and SPS also become negatively charged with –SO groups upon dissociation. Sulonic acid is a stronger acid than carboxylic acid. −
−
2.3.1.3 Ultrafltration Membranes Ultrafiltration is primarily a size-exclusion-based, pressure-driven membrane separation process. UF membranes typically have pore sizes in the range o – nm and retain species in the molecular range rom to Da [], while sol-
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2 Synthetic Membranes for Membrane Processes
vent (water) passes through the membrane. UF membranes have a porous skin layer. Te most important UF membrane properties are the membrane productivity (flux) and extent o separation (rejection o various eed components). In contrast to the polymeric materials or reverse osmosis and nanofiltration membranes, or which the macromolecular structures have much to do with permeation properties such as salt rejection characteristics, the choice o membrane material or ultrafiltration does not depend on the material’s influence on the permeation properties. Membrane permeation properties are largely governed by the pore sizes and the pore size distributions o UF membranes. Rather, thermal, chemical, mechanical, and biological stability are considered o greater importance. ypical UF membrane materials are polysulone (PS), poly(ether sulone), poly(ether ether ketone) (PEEK), cellulose acetate and other cellulose esters, polyacrylonitrile (PAN), poly(vinylidene fluoride) (PVDF), polyimide (PI), poly(etherimide) (PEI), and aliphatic polyamide (PA). All these polymers have a T g higher than C except or cellulose esters. Tey are also stable chemically and mechanically, and their biodegradability is low. Te membranes are made by the dry–wet phase inversion technique.
2.3.1.4 Microfltration Membranes Polymeric materials or MF membranes cover a very wide range, rom relatively hydrophilic to very hydrophobic materials. ypical hydrophilic materials are polysulone, poly(ether sulone), cellulose (CE) and cellulose acetate, polyamide, polyimide, poly(etherimide) and polycarbonate (PC). ypical hydrophobic materials are polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PFE, eflon) and poly(vinylidene fluoride). Hydrophilic MF membranes can be made by the dry–wet phase inversion technique, which can also be used to make PVDF membranes. On the other hand, other hydrophobic microfiltration membranes are made by the thermally induced phase separation technique. In particular, semicrystalline PE, PP, and PFE are stretched parallel to the direction o film extrusion so that the crystalline regions are aligned in the direction o stretch, while the noncrystalline region is ruptured, orming long and narrow pores. Hydrophobic membranes do not allow penetration o water into the pore until the transmembrane pressure drop reaches a threshold called the liquid entry pressure o water (LEPw). Tese membranes can thereore be used or membrane distillation. Te track-etching method is applied to make microfiltration membranes rom PC. An especially important characteristic o a microfiltration membrane is uniorm pores with as many o them per unit area as possible, and with the thinnest possible layer where these pores are at their smallest. Te use o MF membranes is the quantitative separation o suspended matter in the .– μm size range rom liquids and gases. 2.3.2 Membranes or Gas and Vapor Separation
Te concept o separating gases with polymeric membranes is more than years old, but the widespread use o gas separation membranes has occurred only within
2.3 Membranes for Separation Processes
13
the last – years. Separation is achieved because o differences in the relative transport rates o eed components. Components that diffuse more rapidly become enriched in the low pressure permeate stream, while the slower components are concentrated in the retentate, or residue, stream. Te membrane process that separates components based on their relative rates o permeation distinguishes it rom equilibrium processes such as distillation or extraction. Gas and vapor separation membranes are classified into two categories. In the first, rubbery polymers such as silicone rubber, natural rubber, and poly(-methyl-pentene) are used to take advantage o their high permeabilities, even though selectivities are rather moderate. Production o enriched oxygen or medical purposes is perormed by this type o membrane with an oxygen/nitrogen selectivity o about two. Asymmetric membranes made rom glassy polymers such as cellulose acetate and other cellulose derivatives, polycarbonate, aromatic polyamide, aromatic polyimides, and poly(phenylene oxide) (PPO) and its derivatives belong to the second category. Tese asymmetric membranes are made by the dry–wet phase inversion technique. Membranes must be dried beore being used. Solvent exchange is necessary to dry cellulose acetate membranes. Tese membranes take advantage o the high selectivity o glassy polymers. Te selective dense layer at the top o the membrane must be very thin so that a high flux can be achieved. Tey are used in a wide range o industrial gas separation processes such as hydrogen recovery rom various chemical syntheses, sour gas removal rom natural gas and production o nitrogen-enriched air. For the asymmetric membranes to be effective in gas separation, the thin selective layer at the top o the membrane should be perect. Tis requirement is more stringent in gas separation membranes than liquid separation membranes since deective pores cannot be automatically closed when the surace is in contact with dry gas. In contrast, deective pores o RO and pervaporation (PV) membranes can be closed by the swelling o the top skin layer when it is brought into contact with eed liquid. Since it is difficult to make a selective skin layer deect-ree, a method was proposed by Henis and ripodi to seal deective pores. Teir method was applied to asymmetric polysulone membranes, which led to the production o the commercial Prism membrane []. According to the method, a relatively thick silicone rubber layer is coated on a thin selective layer o an asymmetric polysulone membrane. Te thickness o silicone rubber is about μm while the effective thickness o the selective polysulone layer is one tenth o μm. While being coated, silicone rubber penetrates into the pores to plug them. Tus, eed gas is not allowed to leak through the deective pores. Te selectivity o the membrane approaches that o the deect-ree polysulone layer. Moreover, since the permeabilities o silicone rubber or gases are orders o magnitudes higher than those o polysulone, the permeation rate is not affected very much even when a relatively thick silicone rubber layer is coated. Membranes or vapor removal rom air have a structure similar to the Prism membrane, but they are prepared on a different principle []. Aromatic poly(etherimide) is used to produce a porous substrate membrane by the dry–wet phase inversion method. Tis polymer was chosen over polysulone/poly(ether sul-
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2 Synthetic Membranes for Membrane Processes
one) due to the higher durability o poly(etherimide) to organic vapors. Unlike an asymmetric polysulone substrate or the Prism membrane, the top layer o the asymmetric poly(etherimide) membrane has a large number o pores, the size o which is equivalent to those o ultrafiltration membranes. When a layer o silicone rubber is coated on the top layer o the porous substrate membrane, the silicone rubber layer will govern the selectivity, and the porous support will provide only mechanical strength to the composite membrane. Since the permeabilities o water and organic vapors through the silicone rubber layer are much greater than those o oxygen and nitrogen, these membranes are effective in dehumidification o air and removal o organic vapors rom air. 2.3.3 Membranes or Pervaporation and Membrane Distillation
Pervaporation and membrane distillation (MD) are distinguished rom the above membrane separation processes since phase change, rom liquid to vapor, takes place in the process. 2.3.3.1 Pervaporation Pervaporation is characterized by the imposition o a barrier (membrane) layer between a liquid and a vaporous phase, with a mass transer occurring selectively across the barrier to the vapor side. Separation occurs with the efficacy o the separation eect being determined by the physiochemical structure o the membrane. Pervaporation membranes were developed or the dehydration o ethanol and other organic solvents. Tereore, the dense selective layer is made o polyvinyl alcohol that is one o the most hydrophilic materials. Water is preerentially sorbed to polyvinyl alcohol and also preerentially transported. o suppress the excessive swelling o polymer in water, polyvinyl alcohol is partially cross-linked by dialdehydes such as glutaraldehyde []. Te dense polyvinyl alcohol layer is supported by a porous PAN substrate membrane. Polyelectrolyte material [] and chitosan [], a natural product, are also potentially useul or dehydration by pervaporation. Silicone rubber membranes developed or the removal o organic vapors rom air can also be used or the removal o volatile organic compounds (VOCs) rom water by pervaporation []. Because o the high hydrophobic nature o silicone rubber, VOCs are preerentially sorbed and transported through the membrane. 2.3.3.2 Membrane Distillation Membrane distillation is similar to pervaporation since phase change is involved in the process. When eed liquid (usually water) is in contact with a nonwetted porous hydrophobic membrane, water does not enter into the pores because the eed liquid is maintained below a threshold pressure, the liquid penetration pressure o water. Only water vapor permeates through the pores rom the eed to the permeate side. Te driving orce is the vapor pressure drop rom the eed to the permeate side, since the permeate temperature is maintained below the eed temperature. Commercial hydrophobic membranes made o polypropylene, poly(vinylidene fluoride) and poly-
2.4 Membrane Applications
15
tetrafluoroethylene, either in capillary or flat-sheet orm, are used or MD, although these membranes were primarily prepared or microfiltration purposes. With a salt solution, or example NaCl in water, only water has a vapor pressure, i.e., the vapor pressure o NaCl can be neglected, which means that only water will permeate through the membrane, and consequently very high selectivities are obtained. 2.3.4 Membranes or Other Separation Processes
While all the abovementioned membrane separation processes utilize the transmembrane pressure drop as the driving orce, there are other membrane separation processes based on different driving orces. 2.3.4.1 Electrodialysis Membranes or electrodialysis (ED) are either positively or negatively charged. Ionic species in the solution are transported through the membrane by the electrical potential difference between the two sides o the membrane. When a membrane is positively charged, it is called an anion exchange membrane since only anions are allowed to permeate through the membrane. A negatively charged membrane is called cationic since only cations are allowed to permeate through the membrane. Te base polymeric material is polystyrene cross-linked by divinylbenzene. Quaternary ammonium cations are attached to some aromatic rings o anionic membranes, while sulonic groups or carboxylic groups are attached to some aromatic rings o cationic membranes []. 2.3.4.2 Dialysis Dialysis is the separation o smaller molecules rom larger molecules, or dissolved substances rom colloidal particles, in a solution by selective diffusion through a semipermeable membrane. Dialysis is a rate-governed membrane process in which a microsolute is driven across a semipermeable membrane by means o a concentration gradient. Te microsolute diffuses through the membrane at a greater rate than macrosolutes also present in the eed solution. Ordinary dialysis is reerred to as diusion o neutral molecules. I electrolytes are separated with neutral membranes, or with charged membranes, then the Donnan effects arising rom the unequal distribution o ions interere with the normal dialysis process. Tis type o dialysis is called Donnan dialysis. In the medical field, it is the process used or cleaning blood, artificially, with special equipment. Hemodialysis membranes have ultrafiltration capacities ranging rom to mL h m mmHg . Donnan dialysis makes use o ion selective membranes to provide improved selectivity. −
−
−
2.4 Membrane Applications Te major applications o membranes or membrane separation processes are summarized in able ..
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2 Synthetic Membranes for Membrane Processes
Table 2.1. Applications of synthetic membranes
Membranes
Applications
Reverse osmosis
1. Sea water and brackish water desalination 2. Waste water treatment (industrial and municipal, pulp and paper, textile waste water) 3. Production o boiler quality water or steam generation 4. Petroleum industry 5. Recovery o plating chemicals rom wastewaters and process waters in the electroplating and metal-finishing industry
Nanofiltration
1. 2. 3. 4. 5.
Water treatment Product and chemical recovery Concentration/dewatering Fractionation o monovalent and divalent cations Water sofening
Ultrafiltration
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
Electrodialysis pretreatment Electrophoretic paint Cheese whey treatment Juice clarification Recovery o textile sizing agents Wine clarification Separation o oil/water emulsion Concentration o latex emulsion rom wastewater Dewaxing Deasphalting Egg-white preconcentration Kaolin concentration Water treatment Affinity membranes Reverse osmosis pretreatment
Microfiltration
1. 2. 3. 4.
Gas separation
1. Hydrogen recovery
Purification o fluids in semiconductor manuacturing industry Clarification and biological stabilization in the beverage industry Sterilization (in the ood and pharmaceutical industries) Analysis a) Synthesis gas ratio adjustment (H2 /CO) b) H2 recovery rom hydroprocessing purge streams c) H2 recovery rom ammonia plant purge streams and other petrochemical plant streams
2. 3. 4. 5. 6. Pervaporation
Oxygen/nitrogen separation Helium recovery Removal o acid gases rom light hydrocarbons Biogas processing Separation o organic vapors rom air
1. Removal o organics rom water 2. Water removal rom liquid organics 3. Organic/organic separation
2.5 Membrane Characterization
17
Table 2.1. continued
Membranes
Applications
Vapor permeation Electrodialysis
Removal o organics rom air 1. Desalination o brackish water 2. Production o table salt 3. Waste water treatment 4. Concentration o RO brines 5. Applications in the chemical, ood, and drug industries
Dialysis
1. Hemofiltration and hemodiafiltration 2. Donnan dialysis 3. Alcohol reduction o beverages
2.5 Membrane Characterization Te perormance o membranes depends on their properties, which may be quantified by membrane characterization. Te methods or membrane characterization are listed below. Characterization o the bulk membrane polymer Durability o the membrane in the operational environment depends on the thermal, mechanical, and chemical properties o the membrane polymer. Tey are characterized by differential scanning calorimetry (DSC), tensile strength measurement by contacting the membrane with solutions, and the gases to be treated. Wide angle X-ray spectroscopy (WAXS) is also used to measure the crystallinity o the polymer, on which many other polymeric properties depend. Characterization o the membrane surace It should be emphasized that the properties o the membrane surace strongly affect membrane perormance. Contact angle is ofen used as a measure o surace hydrophilicity or hydrophobicity. X-ray photoelectron spectroscopy (XPS) provides the data on atomic compositions at the membrane surace. Recently, attentions have been ocused on the nodular structure as well as the roughness at the membrane surace that can be measured by atomic orce microscopy (AFM). Pore size and pore size distribution It is obvious that the pore size and the pore size distribution o the membrane affect membrane perormance. A number o methods can be used to determine the pore size and the pore size distribution. Conventional methods include bubble point method, mercury porometry, thermporometry, permporometry, and gas adsorption. ransport data o gases and solutions with solute probes can also be used to determine the pore size and the pore size distribution. Pores can also be observed by scanning electron microscope (SEM) and transmission electron microscope (EM). Atomic orce microscope can observe the pores only on the membrane surace.
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2 Synthetic Membranes for Membrane Processes
Reerences 1. Membrane. () Wikimedia Foundation Wikipedia. http://en.wikipedia.org/wiki/Membrane 2. Peinemann K () Next generation membrane materials. In: Abstracts o the th annual meeting o the NAMS, Honolulu, – June 3. Matsuura () Synthetic membranes and membrane separation processes. CRC, Boca Raton, p 4. Loeb S, Sourirajan S () Adv Chem Ser : 5. Sourirajan S, Matsuura () Reverse osmosis/ultrailtration process principles. National Research Council o C anada, Ottawa, p 6. Rozelle L, Cadotte JE, Cobian KE, Kopp CVJr () Nonpolysaccharide membrane or reverse osmosis: NS- membranes. In: Sourirajan S (ed) Reverse osmosis and synthetic membranes: theory, technolog y, engineering. National Research C ouncil o Canada, Ottawa, p 7. Peterson RJ () J Membr Sci : 8. Kulkarni A, Mukherjee D, Gill WN () Chem Eng Commun : 9. Hirotsu () Ind Eng Chem Res : 10. Suk DE, Chowdhury G, Narbaitz RM, Santerre JP, Matsuura , Glazier G, Deslandes Y () Macromolecules : 11. Khayet M, Suk DE, Narbaitz RM, Santerre JP, Matsuura () J Appl Polym Sci : 12. Hester JF, Banerje e P, Won YY, Akthakul A, Acar MH, Mayes AM () Macromolecule s : 13. Lui A, albot FDF, Sourirajan S, Fouda AE, Matsuura () Sep Sci echnol : 14. Gantzel PK, Merten U () Ind Eng Chem Process Des Dev : 15. Lloyd D () Membrane materials science: an overview. In: Lloyd DR (ed) Materials science o synthetic membranes. ACS Symposium Series . American Chemical Society, Washington, DC, p 16. Hoehn HH () Aromatic polyamide membranes. In: Lloyd DR (ed) Materials science o synthetic membranes. ACS Symposium Series . American Chemical Society, Washington, DC, p 17. Matsuura () Reverse osmosis and nanoiltration by composite polyphenylene oxide membranes. In: Chowdhury G, Kruczek B, Matsuura (eds) Polyphenylene oxide and modiied polyphenylene oxide membranes. Kluwer, Dordrecht, p 18. Allegrezza AEJr, Parekh BS, Parise PL, Swiniarski EJ, White JL () Desalination : 19. Guiver MD, remblay AY, am CM () Reverse osmosis membrane rom novel hydrophilic polysulone. In: Sourirajan S, Matsuura (eds) Advances in reverse osmosis and ultrailtration. National Research C ouncil o Canada, Ottawa, p 20. Kulkarni SS, Funk EW, Li N () Ultrailtration. In : Ho WSW, Sirkar KK (eds) Membrane handbook. Van Nostrand, New York, p 21. Henis JMS, ripodi MK () J Membr Sci : 22. Behling RD, Ohlrogge K, Peinemann KV () he separation o hydrocarbons rom waste vapor streams. In: Fouda AE, Hazlett JD, Matsuura , Johnson J (eds) Membrane separations in chemical engineering. AIChE Symposium Series , New York, p 23. Koops GH, Smolders CA () Estimation and evaluation o polymeric materials or pervaporation membranes. In: Huang RYM (ed) Pervaporation membrane separation processes. Elsevier, New York, p 24. suyumoto M, Karakane H, Maeda Y, sugaya H () Desalination : 25. Feng XS, Huang RYM () J Membr Sci : 26. Strathmann H () Electrodialysis. In: Ho WSW, Sirkar KK (eds) Membrane handbook. Van Nostrand, New York, p
