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Ứng dụng lớn nhất cho đến nay vẫn là làm mềm nƣớc, khử ion kim loại nặng để sản xuất nƣớc tinh khiết cho nhu cầu sinh hoạt và công nghiệp (đặc biệt là nƣớc có độ tinh khiết cao) Thu hồi ion kim loại nặng trong nƣớc thải Làm giàu các nguyên tố đất hiếm và nguyên tố phóng xạ Làm xúc tác cho các phản ứng hữu cơ Ứng dụng trong kỹ thuật phân tích Ứng dụng trong kỹ thuật phân tách bằng sắc ký và hấp phụ 3 Ứng dụng trong công nghiệp thực phẩm (xử lý nƣớc hoa quả, đƣờng...) Ứng dụng trong công nghiệp dƣợc phẩm
Ion Exchangers Francois de Dardel, Rohm and Haas, Paris, France Thomas V Arden, Cobham, United Kingdom Ullmann's Encyclopedia of Industrial Chemistry Copyright © 2002 by Wiley-VCH Verlag GmbH & Co KGaA All rights reserved DOI: 10.1002/14356007.a14_393 Article Online Posting Date: January 15, 2002 The article contains sections titled: 2.1 2.2 2.2.1 2.2.2 2.2.3 2.3 3.1 3.2 3.3 3.4 3.5 3.6 4.1 4.2 4.3 5.1 5.2 5.3 5.4 6.1 6.2 6.3 6.3.1 6.3.2 6.4 7.1 7.2 7.3 7.4 Introduction Structures of Ion-Exchange Resins Polymer Matrices Functional Groups Cation-Exchange Resins Anion-Exchange Resins Other Types of Ion-Exchange Resins Adsorbent Resins and Inert Polymers Properties Degree of Cross-Linking and Porosity Exchange Capacity Stability and Service Life Density Particle Size Moisture Content Ion-Exchange Reactions Cation Exchange Anion Exchange Cation and Anion Exchange in Water Treatment Ion-Exchange Equilibria Dissociation and pK Value Mono - Monovalent Exchange Mono - Divalent Exchange (Water Softening) General Case Exchange Kinetics Principles Kinetic Curves Strongly Acidic or Strongly Basic Resins Film Diffusion Particle Diffusion Weakly Acidic or Weakly Basic Resins Practical Results of Ion-Exchange Equilibrium and Kinetics Operating Capacity, Regeneration Efficiency, and Regenerant Usage Permanent Leakage Water Analysis Calculations in the Design of Ion-Exchange Plants for Water 7.5 7.5.1 7.5.2 7.5.3 7.5.4 8.1 8.2 9.1 9.2 9.3 9.4 9.5 10 10.1 10.2 10.2.1 10.2.2 10.2.3 10.2.4 10.2.5 10.3 10.4 10.5 11 11.1 11.2 11.3 11.4 11.5 11.5.1 11.5.2 11.6 11.7 12 12.1 12.1.1 12.1.2 12.1.3 12.1.4 12.1.5 12.1.6 12.1.7 12.1.8 12.1.9 Purification Example of Calculation Principle Basic Data Demineralization Unit Polishing Unit Industrial Use of Ion Exchange Description of the Ion-Exchange Cycle Methods for Overcoming Equilibrium Problems Ion-Exchange Resin Combinations Pretreatment Softening Demineralization (Primary System) Polishing Choice of Resin Plant Design General Considerations Fixed-Bed Ion-Exchange Units Column Diameter Small-Scale Units Industrial Co- and Counterflow Plants Mixed Beds Other Ion-Exchange Polishers Continuously Circulated Ion-Exchange Resins External Valves and Pipework Control Systems Special Processes in Water Treatment Removal of Organic Matter Treatment of Potable Water Treatment of Brackish Water Processes Involving Sea Water Treatment of Condensates Conventional Resins Powdered Resins Water Treatment in the Nuclear Industry Production of Ultrapure Water Special Applications of Ion Exchange Processing Steps Purification Ion Substitution Recovery and Concentration Separation Diffusion Catalysis Dehydration Coalescence on Oleophilic Resins Liquid Ion Exchangers 12.1.10 Ion-Exchange Membranes 12.2 Technical Considerations Introduction Definition and Principles In ion exchange, ions of a given charge (either cations or anions) in a solution are adsorbed on a solid material (the ion exchanger) and are replaced by equivalent quantities of other ions of the same charge released by the solid The ion exchanger may be a salt, acid, or base in solid form that is insoluble in water but hydrated Exchange reactions take place in the water, retained by the ion exchanger; this is generally termed swelling water or gel water The water content of the apparently dry material may constitute more than 50 % of its total mass Figure shows the partial structure of a cation exchanger; each positive or negative ion is surrounded by water molecules Figure Structure of a cation exchanger that exchanges H+ for Na+ ions Swelling water is represented in the inset [Full View] Ion exchange forms the basis of a large number of chemical processes which can be divided into three main categories: substitution, separation, and removal of ions Substitution A valuable ion (e.g., copper) can be recovered from solution and replaced by a worthless one Similarly, a toxic ion (e.g., cyanide) can be removed from solution and replaced by a nontoxic ion Separation A solution containing a number of different ions passes through a column containing beads of an ion-exchange resin The ions are separated and emerge in order of their increasing affinity for the resin Removal By using a combination of a cation resin (in the H+ form) and an anion resin (in the OH– form), all ions are removed and replaced by water (H+OH–) The solution is thus demineralized Historical Aspects The discovery of ion exchange dates from the middle of the nineteenth century when THOMSON [1] and WAY [2] noticed that ammonium sulfate was transformed into calcium sulfate after percolation through a tube filled with soil In 1905, GANS [3] softened water for the first time by passing it through a column of sodium aluminosilicate that could be regenerated with sodium chloride solution In 1935, LIEBKNECHT [4] and SMIT [5] discovered that certain types of coal could be sulfonated to give a chemically and mechanically stable cation exchanger In addition, ADAMS and HOLMES [6] produced the first synthetic cation and anion exchangers by polycondensation of phenol with formaldehyde and a polyamine, respectively Demineralization then became possible At present, aluminosilicates and phenol – formaldehyde resins are reserved for special applications and sulfonated coal has been replaced by sulfonated polystyrene Polystyrene Resins.The first polystyrene-based resin was invented by D'ALELIO in 1944 [7] Two years later, MCBURNEY produced polystyrene anion-exchange resins by chloromethylation and amination of the matrix [8] The anion exchangers known until then were weakly basic and took up only strong mineral acids The new resins produced by the McBurney process were stronger bases and could adsorb weak acids such as carbon dioxide or silica, allowing complete demineralization of water with a purity previously obtainable only by multiple distillation in platinum Even today, ion exchange is still the only process capable of producing the water quality needed for high-pressure boilers Reverse osmosis (sự thẩm thấu ngược) and electrodialysis can demineralize solutions with 50 – 90 % efficiency Only ion exchange can “polish” the predemineralized solution with a demineralization efficiency of 99 – 99.99 % Macroporous Resins.Two of the problems encountered in the use of ion-exchange resins are the fouling of the resin by natural organic acids present in surface waters and the mechanical stress imposed by plants operating at high flow rates To cope with these, three manufacturers [9-11] invented resins with a high degree of cross-linking but containing artificial open pores in the form of channels with diameters up to 150 nm that can adsorb large molecules Resins in which the polymer is artificially expanded by the addition of a nonpolymerizable compound that is soluble in the monomer are known as macroporous or macroreticular resins (see Section Degree of Cross-Linking and Porosity) Other naturally porous resins are known as gel resins Polyacrylic Anion Exchangers.Between 1970 and 1972, a new type of anion-exchange resin with a polyacrylic matrix appeared on the market This possesses exceptional resistance to organic fouling and a very high mechanical stability due to the elasticity of the polymer Uniform Size Resins.In the 1980s and 1990s, several producers developed new manufacturing technologies aimed at producing resins with particles of almost identical size Structures of Ion-Exchange Resins An ion exchanger consists of the polymer matrix and the functional groups that interact ( tương tác) with the ions This article deals only with organic ion exchangers; inorganic ion exchangers are of minor importance and are primarily layer silicates and zeolites (→ Silicates, → Zeolites) 2.1 Polymer Matrices Polystyrene Matrix (→ Polystyrene and Styrene Copolymers) The polymerization of styrene [100-42-5] (vinylbenzene) under the influence of a catalyst (usually an organic peroxide) yields linear polystyrene [900`3-53-6] Linear polystyrene is a clear moldable plastic which is soluble in certain solvents (e.g., styrene or toluene) and has a well-defined softening point If a proportion of divinylbenzene is mixed with styrene, the resultant polymer becomes crosslinked and is then completely insoluble In the manufacture of ion-exchange resins, polymerization generally occurs in suspension Monomer droplets are formed in water and, upon completion of the polymerization process, become hard spherical beads of the polymer Polyacrylic Matrix Matrices for ion exchangers can also be obtained by polymerizing an acrylate, a methacrylate, or an acrylonitrile, any of which can be cross-linked with divinylbenzene [105-06-6], DVB [→ Polyacrylamides and Poly(Acrylic Acids)); → Polyacrylates Other Types of matrix Other types of matrix include Phenol – formaldehyde resins (→ Phenolic Resins) which show interesting adsorption properties Polyalkylamine resins, obtained from polyamines by condensation with epichlorohydrin, which gives an anion exchanger directly in a single step 2.2 Functional Groups 2.2.1 Cation-Exchange Resins Cation-exchange resins in current use can be separated into two classes according to their active groups: Strongly acidic (sulfonic groups) Weakly acidic (carboxylic groups) Strongly Acidic Cation-Exchange Resins Chemically inert polystyrene beads are treated with concentrated sulfuric or chlorosulfonic acid to give cross-linked polystyrene 3-sulfonic acid This material is the most widely used cation-exchange resin and is strongly acidic Examples: Amberlite IR 120, Dowex HCR, Lewatit S 100 Weakly Acidic Carboxylic Cation-Exchange Resins The weakly acidic resins are almost always obtained by hydrolysis of polymethylacrylate or polyacrylonitrile to give a poly(acrylic acid) matrix Examples: Amberlite IRC 86, Lewatit CNP 2.2.2 Anion-Exchange Resins Polystyrene Materials Cross-linked polystyrene beads are treated with chloromethyl methyl ether under anhydrous conditions, with either aluminum chloride or tin(IV) chloride as catalyst Chloromethylated polystyrene is obtained: In a second stage, the chlorine in the chloromethylated group can be replaced by an amine or even by ammonia Depending on the reaction selected, the anion exchanger obtained may be strongly to weakly basic The degree of basicity can be “made to measure” because of the large number of amines available The anion exchangers listed below are arranged in order of decreasing basicity: where R can be –CH2N+(CH3)3Cl– e.g., Amberlite IRA402 (type resin) –CH2N+(CH3)2CH2CH2OHCl– e.g., Amberlite IRA410 (type resin) –CH2N(CH3)2 e.g., Amberlite IRA96 Resins with quaternary ammonium groups are strongly basic Those with benzyltrimethylammonium groups are known as type and are the most strongly basic, whereas those with benzyldimethylethanolammonium groups are known as type and are slightly less basic Type resins are used when total removal of anions, even those of weak acids (including silica), is essential Type resins are also basic enough to remove all anions, but they release the anions more easily during regeneration with caustic soda; as a result, they have a high exchange capacity and a better regeneration efficiency (see Section Operating Capacity, Regeneration Efficiency, and Regenerant Usage) Unfortunately, they are chemically less stable and produce greater silica leakage than type resins Resins whose active group is an amine are generally denoted as weakly basic, although their basicity may vary considerably Tertiary amines are sometimes called medium-base or intermediate-base resins, whereas primary amines are very weakly basic and are rarely used The most widely used weakly basic resins contain tertiary amino groups and adsorb any strong acids present in the solution to be treated but not affect neutral salts or weak acids Manufacturers not always indicate the chemical structure of their exchangers in their literature Care should therefore be taken not to assume that resins are chemically identical merely because they have similar general characteristics Secondary and Tertiary Cross-Linking During chloromethylation, a side reaction may occur in which the chloromethyl group of a chloromethylated benzene ring reacts with an unconverted ring, to yield a methylene bridge These bridges form additional cross-links in the polystyrene matrix: The amount of this secondary cross-linking can be adjusted by varying the conditions (quantity and type of catalyst, temperature) of the chloromethylation reaction Most strongly basic and weakly basic polystyrene resins have some degree of secondary cross-linking Furthermore, during the amination of weakly basic resins, another type of cross-linking may be produced This is called tertiary cross-linking and yields strongly basic quaternary groups in addition to the weakly basic tertiary groups Polyacrylic Resins Polyacrylic resins are manufactured in a manner analogous to that used for polystyrene resins Beads are prepared from an acryclic ester copolymerized with divinylbenzene by using suspension polymerization and free-radical catalysis The polyacrylate formed is then given active groups by reaction with a polyfunctional amine containing at least one primary amino group and one secondary or, more frequently, tertiary amino group The primary amino group reacts with the polyester to form an amide, whereas the secondary or tertiary amino group forms the active group of the anion exchanger This method always yields a weakly basic exchanger, which can be further treated with chloromethane or dimethyl sulfate to give a quaternary strongly basic resin: In principle, a wide range of anion-exchange resins can be obtained by varying the type of ester chosen as the starting material and the polyamine used for activation In practice, the range is limited by the availability and cost of raw materials 2.2.3 Other Types of Ion-Exchange Resins By using polymerization and activation methods analogous to those described above, a wide variety of functional groups can be grafted onto a given polymer Some of these groups can be used for selective uptake of ions, principally metals (Table 1) Table Principal active groups of ion exchangers used for selective uptake of metals The thiol group forms very stable bonds with certain metals, particularly mercury The iminodiacetic, aminophosphonic, and amidoxime groups form metal complexes whose stability depends mainly on the pH of the solution Selective adsorption of certain metals can thus be achieved by varying the pH These types of material are known as chelating or complexing resins The N-methylglucamino group is used to make resins specific for boric acid, which is taken up as a complex 2.3 Adsorbent Resins and Inert Polymers Strictly speaking, adsorbent resins are not ion exchangers but resemble them very closely They have a high porosity and are used for the adsorption of nonionic or weakly ionized species as a complement to ion exchange They may have cation- or anion-exchange groups or no ion-exchange groups at all The latter are ionically inert In order of decreasing polarity, adsorbent resins can be classified in the following manner: Ionized adsorbents are strongly basic exchangers used in chloride form for color removal from sugar juices or as “organic scavengers” (see Section Removal of Organic Matter, e.g., Amberlite IRA958) Phenolic adsorbents contain weakly basic amine and phenolic groups or phenolic groups, only They are used to remove color bodies (colored impurities) from solutions of organic acids and food-processing streams (e.g., Duolite A561, XAD761) Inert adsorbents are macroporous copolymers of styrene and divinylbenzene with a very high degree of cross-linking and a large surface-to-volume ratio These resins are used to remove organic, weakly ionized, or nonionic substances, such as phenols, chlorinated solvents, antibiotics, and complexing agents, from aqueous or organic solutions (e.g., Amberlite XAD4, Diaion HP20) Inert polymers without measurable porosity and without active groups can be used either to separate two resin layers or to keep a resin separate from a collector system Properties 3.1 Degree of Cross-Linking and Porosity An increase in the degree of cross-linking (i.e., the weight percentage of DVB related to the total amount of monomer prior to polymerization) produces harder, less elastic resins Resins with higher degrees of cross-linking show more resistance to oxidizing conditions that tend to de-cross-link the polymer Above 10 – 12 % DVB, however, the structure becomes too hard and dense Activation (i.e., chemical transformation of the inert copolymer into an ionexchange resin) becomes difficult because access to the interior of the bead is hindered by the high density of the matrix In addition, osmotic stress cannot be absorbed by the elasticity of the structure, which causes the bead to shatter Finally, the rate of exchange increases in proportion to the mobility of the ions inside the exchanger bead: if the structure is too dense, ionic motion is slowed down, thus reducing the operating capacity of the resin For sulfonic resins, maximum operating capacity (Section Exchange Capacity) is obtained with approximately % DVB 252RF H SP112 MSC Ambersep Monoplus Marathon 252 H SP112 MSC Imac C16NS Weakly basic anion resins Amberlite VPOC1072 IRA67 Amberlite IRA67RF Amberlite IRA95 Amberlite MP64 MWA1 IRA96 Amberlite Monoplus Monosphere IRA96RF MP64 WB500 Amberlite MWA1LB IRA96SB Strongly basic anion resins Gel types Amberjet Monoplus Marathon A, 4200 Cl M500 Monosphere A625 Amberjet Marathon 4400 Cl ALB Amberjet Monoplus Marathon 4600 Cl M600 A2, Monosphere A2 500 Amberlite M500/M511 SBRP IRA402 Cl Amberlite IRA402 OH Amberlite IRA404 Cl Amberlite M600, SAR IRA410 Cl M610 Amberlite VPOC1071 IRA458 Cl Amberlite IRA458RF Cl Amberlite VPOC1073 IRA478RF Cl Macroporous Amberlite MP500 MSA1 IRA900 Cl C150TL WA10 A845, A847 A845FL WA30 WA20 A329 A100 A328 A100FL A100DL PFA400 PFA300 SA10A A400 Impact AG1P UPS Impact AG1 UPS Impact AG2 UPS ASB1P A400MB OH SA11A SA20A A420S, A444 A200, A300 A850 ASB2 A850FL A870 PA308, PA312 A500, A500PS A641 Amberlite Monoplus Marathon IRA900RF MP500 MSA Cl Amberlite MP600 MSA2 IRA910 Cl Amberlite VPOC1074 IRA958 Cl Ambersep Marathon 900 SO4 MSA Imac SN36 HP555 Mixed resins (ready for use) Amberlite MB6113 Amberlite SM92 MB50 MB20 Amberlite MB9L Nuclear-grade resins Amberlite S100KR H HGR NG IRN77 Amberlite M500KR SBR NG IRN78 OH Amberlite Monosphere IRN97 H 500C NG Amberlite MR3LC NG IRN150 Amberlite Monosphere IRN160 MR500LC NG Semiconductor-grade resins Amberjet Ultrapure 650CUPW UP1400 1212 Amberjet 550AUPW UP4000 Amberjet Ultrapure MR450UPW UP6040 1293 Amberjet MR3UPW UP6150 Inert spacer resins Amberlite IN42 IF59 RF14 Ambersep Monosphere 359 600BB Chelating resins Amberlite TP207, IRC748 TP208 Duolite TP260 C467 A500FL PA412, PA416 A510 A651 A860 A500TL A642 A520E SR7 MB500 IND MB400 NM65 MB46 SKN1 NRW100 NC10 SAN1 NRW400, NA38 NRW600 SMN1 NRW37 NM60 EXMB NM201SG SKNUP SANUP IP4 IP5 CR10, CR11 S930 SR5 S940 SR12 Duolite TP214 GT73 Polymeric adsorbents Amberlite XAD16 Amberlite XAD4 a b S920 SR4 HP20 HP10 Only the most current resins are listed Correspondence of resins is only approximate For detailed advice, consult the resin manufacturers Table 11 Principal producers of ion-exchange resins Company Location Principal trademarks Bayer AG Dow Chemical Company Mitsubishi Chemical Purolite Company Leverkusen, FRG Midland, Michigan, USA Lewatit Dowex, Kastel Tokyo, Japan Bala Cynwyd, Pennsylvania, USA Philadelphia, Pennsylvania, USA Birmingham, New Jersey, USA Diaion, Relite Purolite Rohm and Haas Company Sybron Corporation Amberlite, Duolite, Imac Ionac 12.1 Processing Steps The uses of ion-exchange resins for various processing steps are described below Resin manufacturers can provide more detailed information about specific processes and the corresponding ion exchange and adsorbent materials 12.1.1 Purification Ion exchange is used for the removal of impurities from an aqueous or organic solution The term impurities covers here a large spectrum of chemical species Deacidification A solution containing acids is passed through a strongly basic anion exchanger in OH– form or a weakly basic exchanger in free base form, depending on whether all acids or only strong acids are to be removed Since this process is a neutralization reaction, the product of reaction with a strongly basic resin is water, and with a weakly basic resin the acid is taken up as a whole, so that the solution is effectively purified: Examples: Removal of formic acid from formaldehyde, deacidification of fruit juice, and removal of mineral acids from alcohols Demineralization All cations and anions may be removed (total demineralization) or only some of them, generally the strongly dissociated ions (partial demineralization) This is achieved by passing a solution successively through a cation-exchange resin in H+ form and an anion-exchange resin either in OH– or free base form Non-ionized substances in the solution are largely unaffected by this treatment This type of demineralization is sometimes called deashing Examples: Treatment of sugar syrups, glucose, antibiotics, glycerin, cheese whey, alcohols, polyols (e.g., sorbitol), and glycols In a variation of this process, organic acids are purified by using strongly acidic and weakly basic resins Inorganic salts are removed, whereas weakly dissociated organic acids are not taken up by the anion-exchange resins If the organic acid has a pK value lower than that of the resin, it is taken up during the first part of the run but then displaced by the stronger mineral acids, so that losses of the valuable organic acid are negligible or minimal As weakly basic resins with different structures and active groups have different pK values, proper choice of resin will greatly enhance separation Examples: Demineralization of citric and lactic acids Selective Removal of Impurities Harmful ions must be removed, while other constituents of the solution are affected as little as possible Various sorts of selective resins are available Examples: The commonest applications are Selective removal of toxic metals from industrial effluents by using complexing (chelating) resins (see Section Other Types of Ion-Exchange Resins) such as Amberlite IRC748 for transition metals and Duolite GT73 for mercury, cadmium and other heavy metals Nitrate removal from drinking water by using the Cl– or form of anion-exchange resins (see Section Treatment of Potable Water) Calcium removal from saturated brine with an aminophosphonic resin (e.g., Duolite C467) that can remove Ca2+ in a concentration of 10–4 g/L from brine containing >102 g/L of Na+ Removal of borate from drinking water with a resin having glucamino groups (e.g., Amberlite IRA743) Decolorizing differs from selective removal in that it normally combines ion exchange with adsorption Resins have partially replaced bone char or activated carbon for this application, as they are easier to handle and to regenerate Decolorizing is applied mainly to solutions of sugar or other organic compounds Examples of the color bodies to be removed from solution are sugar degradation products, flavonoids, polyphenols, melanoidins, tannins, and anthocyanines Because impurities responsible for the color of these solutions are often acidic, the resins used are normally highly porous anion exchangers, sometimes in combination with carbon, inorganic adsorbents, or nonionic resins Examples: Strongly basic polystyrene materials in chloride form, strongly basic polyacrylic materials in chloride form are used for color removal from cane sugar, and regenerated with a sodium chloride brine Weakly basic or nonionic phenol – formaldehyde resins are used for citric or tartaric acid Acrylic weakly basic resins simultaneously decolorize and deacidify antibiotics Aromatic polymeric adsorbents are used for the removal of polyphenols or anthocyanines Adsorption of Impurities True adsorption is not strictly speaking an ion-exchange process However, adsorbent resins are so similar to ion-exchange resins that the process can be mentioned here The adsorbent materials used are macroporous polystyrene such as the Amberlite XAD series or phenol – formaldehyde resins without active groups Examples: Current applications include the removal of impurities from effluents containing phenol derivatives, pesticides, chlorinated solvents, or dyes Macroporous phenol – formaldehyde resins have also been used in cigarette filters to remove tar, aldehydes, and heavy nitriles 12.1.2 Ion Substitution All ion-exchange processes are ion substitutions However, a distinction should be made between processes that remove ions while producing water or a pure solution of nonionic substances and those that leave the total ion concentration unchanged, merely replacing an undesirable ion by an acceptable one Softening The principle of softening is the same as that used in water treatment: Ca 2+, Mg2+, and possibly Fe2+ and Mn2+, are replaced by Na+ ions Example: The most widespread application is in the beet sugar industry, where the crude juice containing a high concentration of calcium is softened to prevent scaling in the evaporators Variations on the basic softening process have been developed to save regenerant and reduce waste One of the most efficient is the NRS process, in which regeneration of the cation-exchange resin is carried out with a mixture of the concentrated softened juice and a little caustic soda, so that no additional salt is required and the process produces no waste Desodation is a kind of reverse softening: the sodium in a solution is replaced by calcium, magnesium, or potassium, and the cation resin used is regenerated by salts of one or more of the latter metals Examples: Sodium removal from milk (Na – Ca, Na – K exchange) and the Quentin process in sugar refining (Na – Mg, K – Mg exchange) Production of Organic Acids and Salts Organic acids can be prepared from their salts simply by passage through a cation-exchange resin in H+ form This process has an excellent yield and is used widely to manufacture modified, water-soluble ethylenediaminetetraacetic acids and some amino acids from their sodium salts Conversely, an organic acid can be converted into its salt One organic salt can also be converted into another Example: In the pharmaceutical industry, cephalosporin C is converted into its potassium salt by passage through a cation-exchange resin in K+ form 12.1.3 Recovery and Concentration In recovery and concentration, the principle is the same as in selective purification, but the aim is different Here, the objective is to recover a valuable substance from solution, so that the stage during which elution from the resin takes place is particularly important In most cases, the substance recovered from the dilute solution is simultaneously concentrated during the regeneration stage Examples: Recovery of traces of precious metals, most often in the form of metal complexes, in the metal plating industry (jewelry) and silver recovery from photographic baths Isolation of pharmaceutical compounds during the manufacturing process (extraction and purification) Recovery of ammonium nitrate in fertilizer plants Recovery of sugar and amino acids from molasses in sugar refining by ion exclusion Recovery and recycling of water by demineralizing the rinse water from surface treatment plants Recovery of uranium or gold in the mining industry 12.1.4 Separation A special type of ion-exchange resin (with fine, uniform particles and an accurately defined moisture content) is used in columns several meters tall for various industrial separation methods that are similar to those in chromatography This technique allows separation of a mixture of ions, an electrolyte from a nonelectrolyte, and a mixture of nonelectrolytes (see also → Process-Scale Chromatography; → Basic Principles of Chromatography) For separations involving nonelectrolytes, a special type of ion-exchange resin with fine, uniform particles and an accurately defined moisture content is used in columns several meters tall Cation exchange resins with a mean diameter of 250 to 350 µm and a uniformity coefficient of less than 1.1 are used in many processes, in the appropriate ionic form, usually Ca, K, or Na In the pharmaceutical field, chromatographic separations are carried out with polymeric adsorbents of similarly uniform particle size As shown in Tables and 5, ions can be arranged in order of their relative affinity for a resin For a sulfonated polystyrene exchange material, the following monovalent ions are listed as examples in order of increasing affinity: Separation by Displacement A small amount of the mixture to be separated is passed through a column (e.g., a mixture containing K+ and Cs+ is passed through a resin in Na+ form) The ions taken up are then displaced by means of another ion with a greater affinity (e.g., Ag+) The emergent liquor is thus fractionated because the silver ions, in turn, displace the sodium, potassium, and cesium ions Separation by selective displacement is a variation of the last process A small amount of the mixture to be separated (e.g., Li+ and Cs+) is passed through a column containing a resin in a form with intermediate affinity (e.g., ) This is followed by a solution containing which displaces only the lithium Lithium is therefore completely separated from cesium Cesium can then be displaced by silver (Fig 56) Figure 56 Separation of Li+ and Cs+ by selective displacement in a resin initially in form [Full View] This technique is called displacement chromatography It can be applied to the separation of amino acids which are taken up by an H+ form of cation-exchange resin column and displaced with sodium hydroxide or ammonium hydroxide Separation by Elution A column is prepared as above but, instead of ions being displaced by an ion with greater affinity than those taken up, they are now eluted with an ion of lower affinity This is normally the ion with which the resin was initially loaded (e.g., H+) The ions to be separated are displaced toward the bottom of the column by the eluant (HCl in the example shown in Fig 57) The ions always emerge in order of their increasing affinity for the resin and are usually separated well Figure 57 Separation of Na+, K+, Rb+, and Cs+ in a solution by elution with 0.1 N HCl through a resin initially in H+ form [Full View] Separation by Ion Exclusion An electrolyte can be separated from an electrolyte in an ion-exchange column by a sorption process (Fig 58) The ion with which the resin column is loaded is normally the same as one of those in the electrolyte to be separated For instance, an aqueous solution of a mixture of sodium chloride and glycerol can be passed through a column containing a cationexchange resin in Na+ form Cation exchange does not take place because the column and the solution contain the same cation The Cl– anion cannot penetrate the resin because it encounters the Donnan potential which guarantees electrical neutrality within the resin (see Section Principles ) The electrolyte is thus excluded from the resin The nonelectrolyte, on the other hand, can penetrate the resin by means of adsorption until its concentration is the same inside the beads as outside When this equilibrium state has been reached, pure water is passed down the column and drives out the electrolyte more quickly than the glycerol, which must diffuse out of the beads This process of alternating input of the mixture and displacement by water is repeated without the need to regenerate the resin, and successive fractions containing pure glycerol are obtained Figure 58 Separation of an electrolyte from an organic product by ion exclusion a) Electrolyte; b) Organic compound [Full View] Examples: An exclusion process is used industrially to recover sugar from sugar beet molasses [59] Figure 59 shows the successive fractions obtained, alternately rich in sugar and nonsugars The same process can be applied to the separation of strong and weak electrolytes (e.g., hydrochloric acid from organic acids) Figure 59 Recovery of sugar from molasses by ion exclusion a) Conductivity; b) Sugar concentration Fractions: AB nonsugars (residual molasses); BC recycled; CD sugar produced; DA recycled [Full View] Chromatography of Nonelectrolytes Another property of resins becomes evident during separation by ion exclusion: if different nonelectrolytes are present in solution, the process not only separates out the mineral salts but also the nonelectrolytes from each other because of their different rate of uptake and release by the resin Example: Separation of glucose from fructose is carried out commercially with fine bead sulfonic resins, generally in the calcium form [60] A typical elution profile is shown in Figure 60 Figure 60 Separation of glucose from fructose in high-fructose corn syrup a) Glucose; b) Fructose; c) Oligosaccharides [Full View] Separation by Acid Retardation Somewhat surprisingly, an acid can be separated from its salts by using a column containing a strongly basic anionexchange resin of suitable porosity and particle size [61] This occurs because, at high concentration, the acid crosses the Donnan potential barrier (Donnan invasion) and is taken up by the resin, whereas the salts are excluded from it The acid is thus “retarded”, and the salts are allowed through Pure fractions can be obtained at the output by passing concentrated electrolyte solution and water alternately down the column Example: Acid retardation is now used industrially for purifying and recycling acids that have been used for metal pickling An elution curve is shown in Figure 61 Figure 61 Acid retardation by a strong anion resin [Full View] 12.1.5 Diffusion An ion-exchange resin already loaded with ions releases them slowly into a solution This property is used in two special applications Hydroponics (Soilless Cultivation) A resin loaded with fertilizing elements can be used for the hydroculture of plants instead of liquid fertilizers [62] The elements are released gradually into the water as needed by the plant (Fig 62) Figure 62 Hydroculture a) Hydroponic resin; b) Beads of blown clay; c) Water level; d) Inner perforated pot; e) Outer pot [Full View] Delayed-Action Drugs Instead of administering a pure drug directly, it can be absorbed onto a suitable resin and taken in this form The active substance is then released more slowly in the stomach 12.1.6 Catalysis (→ Heterogeneous Catalysis and Solid Catalysts) Acid or Alkaline Catalysis [63] Because standard ion-exchange resins are insoluble acids or bases, they can be used with advantage in many organic reactions where an acidic or basic catalyst is required Special macroporous sulfonic resins are used, for example, in esterification (e.g., the production of butyl acetate), synthesis (e.g., of methyl tert-butyl ether), or the hydrolysis of sucrose to glucose and fructose Catalysis with ion-exchange resins has the following advantages over the use of sulfuric acid: A higher local concentration of H+ (or OH–) ions No corrosion Possibility of use in continuous processes Fewer secondary reactions Easy separation from the reaction medium (by simple filtration) Catalyst Carrier Some resins can be loaded with a metallic catalyst (palladium, silver, nickel, etc.), which is otherwise difficult to use because of its high solubility Enzyme Immobilization Some ion-exchange resins can be loaded with enzymes to enable enzymatic reactions to take place continuously Glucoisomerase or -galactosidase, for example, can be taken up by weakly basic phenol – formaldehyde resins (see also → Immobilized Biocatalysts) 12.1.7 Dehydration Ion-exchange resins are strongly hydrophilic When dried, they tend to take up their quota of swelling water This property is exploited in the two following applications Drying of Gases or Solvents Previously dried sulfonic resins can be used to lower the humidity of air, gases, or organic solvents The resins take up their own weight of water Dehydration of Alcohols The affinity for water is so great that sulfonic resins can be used to dehydrate alcohols, (e.g., in converting ethylene glycol into dioxane or cyclohexanol into cyclohexene 12.1.8 Coalescence on Oleophilic Resins An oleophilic resin can be obtained by loading a resin permanently with an organic substance consisting of long-chain molecules [64] In practice, a sulfonic resin loaded with an organic cation is used (e.g., an aromatic quaternary ammonium ion): or a pyridinium ion: The cationic part of the oleophilic molecule is taken up almost irreversibly by the cation-exchange resin An oleophilic resin bead of this type is illustrated in Figure 63 Figure 63 Schematic representation of an oleophilic resin bead a) Sulfonic resin bead; b) Ionic end of oleophilic molecule; c) Oleophilic end of molecule [Full View] Water containing oils or fats passes through a column containing an oleophilic resin The oil particles are taken up by the resin beads in the form of microscopic droplets The droplets coalesce and grow in size as more oil is taken up Beyond a certain size, the drops become detached from the resin and are then decanted in the normal way This process has been developed on an industrial scale as the Elf – ANVAR process [65] A typical coalescing deoiling unit is shown in Figure 64 Figure 64 Elf coalescing deoiler a) Oil – water mixture; b) Oleophilic resin; c) Decanted oil; d) Level controller operating the oil outlet valve [Full View] 12.1.9 Liquid Ion Exchangers (→ Liquid–Liquid Extraction) Liquid ion exchangers are high molecular mass, water-insoluble acids and bases that are soluble in oil and in organic solvents The compounds have a hydrophobic structure and an ionogenic group They are usually phosphonic acids and amines substituted with long-chain (C8 – C24) aliphatic groups They are used for liquid – liquid extraction of electrolytes from aqueous solutions An example of cation exchange is the following reaction with a polyalkyl phosphonic acid: An example of anion exchange is the following reaction with a polyalkylamine in salt form: where M+ represents a metal cation, A– and B– represent two different anions, R is an alkyl substituent, and the subscript org denotes the organic phase Recovery of the extracted component from the organic phase does not require distillation as is often the case in processes based on solubility differences Instead, the liquid exchanger is “stripped” in a second liquid – liquid extraction, similar to the regeneration of solid ion exchangers The regenerant (stripping agent) is an acid, base, or salt in aqueous solution In practice, the liquid ion exchanger is used as a solution in an organic solvent (frequently kerosene), with an organic : aqueous phase ratio of between : and : In the exchange process, both immiscible phases are thoroughly mixed mechanically in conventional liquid – liquid extraction equipment (mixer – settlers, centrifugal contactors, and pulsed and nonpulsed extraction columns) The advantages of liquid ion exchangers are the relative simplicity of their production, the possibility to adjust their concentration in the organic phase according to the concentration of the aqueous solution, and the possibility of continuous countercurrent operation Their main disadvantage is the inevitable losses of solvent and ion exchanger during the extraction process The main industrial application of liquid ion exchangers is hydrometallurgy Large plants are in operation for the recovery of uranium from sulfuric acid leach liquors The uranyl sulfate anion is extracted with a dialkylamine and subsequently stripped with sodium carbonate, resulting in a water-soluble uranyl carbonate complex which can be precipitated (e.g., by addition of sodium hydroxide) to produce a “yellow cake” of uranium oxide Examples: Amberlite LA-1 and LA-2 liquid anion exchangers Rohm and Haas, USA 12.1.10 Ion-Exchange Membranes (→ Membranes and Membrane Separation Processes) Ion-exchange membranes are materials with ion-exchange properties that can be used as a separation wall between two solutions They show pronounced differences in permeability toward counterions, co-ions, and neutral molecules The membranes are permselective Cationic membranes are permeable to cations and impermeable to anions Anionic membranes are permeable to anions and impermeable to cations Ion-exchange membranes are classified as hetero- or homogeneous: Heterogeneous membranes are made of finely ground ion-exchange resin particles embedded in an inert porous binder [polyethylene, poly(vinyl chloride)] in the form of flat sheets The resin is usually sulfonated polystyrene for cationic membranes and polystyrene with quaternary ammonium groups for anionic membranes To increase dimensional stability, the mixture is often reinforced with a woven fabric In homogeneous membranes the ion-exchange groups are grafted directly to the membrane polymer structure Their manufacture is similar to that of bead-form ion-exchange resins They are somewhat less stable than heterogeneous membranes, but have a more uniform structure In the absence of electric current, the membrane is slightly permeable to nonelectrolytes and impermeable to electrolytes because the Donnan potential prevents co-ions from passing through it Furthermore, the principle of electroneutrality makes it impossible for counterions to concentrate on one side of the membrane When an electric potential is applied, however, cations are attracted by the cathode and anions by the anode In the case of a cation-exchange membrane, only cations can wander through it An example of application is the electrolysis of brine using a cation-exchange membrane (chlor-alkali process) to produce chlorine and caustic soda (→ Chlorine – Membrane Process) Sodium chloride and water are introduced into the anode chamber where oxidation of the chloride ions to chlorine takes place Water in the cathode chamber is reduced to hydrogen and hydroxyl ions Sodium ions wander through the cation-exchange membrane into the cathode compartment and combine with the hydroxyl ions to form sodium hydroxide Another important example is electrodialysis, where both cation- and anionexchange membranes are used for demineralization of water (Fig 65) If all three chambers are filled with sodium chloride solution, sodium passes through the cation-exchange membrane and produces NaOH in the cathode chamber, chloride goes through the anion-exchange membrane and produces HCl and the central chamber is progressively demineralized Figure 65 Electrodialysis for demineralization of water [Full View] In industrial practice, a large number of alternating cation- and anion-exchange membranes are combined for demineralization of water, especially brackish or sea water Electrodialysis is also used for the demineralization of cheese whey to recover valuable proteins and lactose Membrane electrodes operating according to the principle of permselectivity can be used for analytical purposes Ion-specific membranes are now available for measuring Important ion-exchange membrane manufacturers are Asahi Chemical (Japan), Asahi Glass (Japan), Du Pont (USA), Ionics (USA), Rhône-Poulenc (France), Sybron (USA), and Tokuyama Soda (Japan) 12.2 Technical Considerations For special applications in softening, purification, demineralization, or recovery, the plants are often similar to those used in water treatment Counterflow regeneration is sometimes used to improve regeneration efficiency and reduce leakage Continuous systems (see Section Continuously Circulated IonExchange Resins ) can be employed, e.g., in the softening of sugar syrup or the treatment of rinse water in electroplating However, many special applications require special operating conditions Flow Rates Concentrated solutions must frequently be dealt with, which requires relatively large amounts of resin The specific flow rate is often low: 0.2 – 20 BV/h instead of the 10 –100 BV/h used in water treatment Regenerant Recycling Regenerant consumption can be reduced by recycling The tails, which are relatively uncontaminated, can be used at the head of the next regeneration stage Merry-go-round Systems To increase the treatment capacity of a plant and improve the quality of the treated liquid, a so-called merry-go-round system can be used This involves passing the solution through several identical columns in series (at least two, but usually more), so that the one at the head is exhausted as completely as possible When this state of exhaustion is reached, the next column is placed at the head of the series, and the one just regenerated is used for polishing while the exhausted column is regenerated The cycle shifts by one column as each column at the head of the chain is exhausted In this way, the product is obtained in the most concentrated form possible (Fig 66) Figure 66 Extraction of streptomycin with weakly acidic resins (three columns in series) [Full View] Sweetening Off and On In most of the purification processes described in Section Purification, the liquid to be treated must be removed from the resin before the latter is regenerated This involves inserting a rinsing stage before regeneration, a stage generally known as sweetening off because of its similarity to the treatment of sugar syrup In the same way, after this stage and a rinse of the water from the exchangers, the water filling the column is replaced by the liquid to be treated, a stage known as sweetening on The complete cycle thus consists of sweetening on, the service stage, sweetening off, decompaction, regeneration (or elution), and rinsing Decompaction is sometimes carried out prior to sweetening off, with the liquid to be treated Multistage Regeneration The resin must sometimes be regenerated in several steps In the first step, the substances taken up in one ionic form are removed or recovered In the next step, the resin is converted into another ionic form more suitable for the service stage In the selective uptake of metals, for example, a carboxylic or chelating resin (see Section Other Types of Ion-Exchange Resins) is often used in Na+ form in order not to lower the pH of the solution to be treated However, the resin must be regenerated with acid The H+ form must then be converted to the Na+ form with sodium hydroxide: Service: Regeneration: Conversion to Na+ form: Temperature Control Some viscous liquids, particularly concentrated organic solutions (e.g., heavy sugar syrups), must be decolorized when hot (70 – 80 °C) so that they remain fluid and not crystallize Other liquids, such as light sugar syrups, must be demineralized when cool (12 °C) so that they are not hydrolyzed when they come in contact with the cation exchanger in H+ form Temperature control thus plays a major role in many applications Special Plants Some applications require special types of plant, e.g., Very long columns up to 10 m tall for sugar chromatography Columns in the shape of truncated cones widening at the top to enable resins to swell during service or regeneration Fluidized-bed systems, sometimes using high-density resins to treat liquids containing suspended matter Simulated moving beds (SMBs) are progressively replacing the tall columns of the first industrial plants Whilst the old plants had to be operated in a sequential, discontinuous way, alternating the feed solution to be separated and an eluant, the new SMB technology allows the separation of two or more compounds in a quasicontinuous, stepwise manner: the columns (typically 30 of them) are arranged in a carousel The major system of this type is called ISEP [71] and is provided by the U.S company AST (Advanced Separation Technologies) The feed and elution solutions are connected to a stationary upper distributor fitted with typically 20 ports, and the raffinate and the extract are connected to a lower stationary connector fitted with the same number of ports The columns themselves are mounted on a rotating frame The carousel rotates continuously at a speed of 0.1 to 1.5 revolutions per hour, and the ports are thus successively connected to all columns A simplified scheme is shown in Fig 67 with only eight resin columns and six inlet and outlet ports Figure 68 shows the principle of operation in two dimensions Figure 67 ISEP carousel (simplified) a) Rotating resin columns; b) Stationary inlet ports; c) Stationary outlet ports [Full View] Figure 68 ISEP: two-dimensional view [Full View] Simulated moving beds can be used for purification of fermentation broth, sugar syrup deashing, decolorization of various solutions, separation of metals and other applications [...]... Gel cation – porous gel anion; b) Gel cation – standard gel anion; c) Macroporous cation – porous gel anion; d) Macroporous cation – standard gel anion; e) Macroporous cation – macroporous anion; f) Macroporous cation – developmental anion; g) Gel cation – developmental anion [Full View] Thermal Stability of Active Groups [16] The sulfonic group of cation-exchange resins is extremely stable Anion-exchange... influenced by kinetic considerations In fully ionized systems, the rate-determining step of ion exchange is the diffusion of the mobile ions toward, from, and in the resin phase, rather than the chemical reaction between fixed ions of the resin and mobile counterions If a cation-exchange resin with negative fixed ions (e.g., sulfonate ions) is used as an example, the cation concentration in the resin is much... rate-determining: 1 Diffusion of ions within the resin (particle diffusion) 2 Diffusion in the Nernst film (film diffusion) The slower step controls the overall ion- exchange rate A criterion has been established by Helfferich [19] to determine which process is rate-determining: where C = total ion concentration in solution C = total ion concentration in the solid phase (total capacity) D, D = diffusion coefficients... exchange reaction the law of mass action gives If the activity coefficients are constant, then The equivalent fraction Xi of an ion i in a solution with a total concentration of ions [C ] is defined by Similarly, the equivalent fraction of ion i in the resin is The total concentration of ions in the resin [ C ] (in equivalents per liter) is the same as the total capacity (defined in Section Exchange... simple arithmetical relationship exists between dry matter and moisture content In all cases, the ionic form of the resin at the time of measurement should be quoted 4 Ion- Exchange Reactions 4.1 Cation Exchange General cation exchange is used widely to remove undesirable ions from a solution without changing the total ionic concentration or pH The resin can be used in many ionic forms, but the sodium... described in detail in Section Water Analysis Figure 8 Summary of the kinds of ion exchange used in water treatment SAC = strongly acidic cation exchangers; SBA = strongly basic anion exchangers; WAC = weakly acidic cation exchangers; WBA = weakly basic anion exchangers [Full View] 5 Ion- Exchange Equilibria 5.1 Dissociation and pK Value Dissociation of the acid group in a cation-exchange resin is described... Conversion from H+ to Na+ ; b) Conversion from Na+ to H+ [Full View] For a given ion, mass-transfer flux through the film can be described by Fick's law: where J = ion flux, mol s–1 cm–2 grad C = ion concentration gradient, mol/cm3 D = diffusion coefficient cm2/s When D is constant, the ion concentration independent of C, and the concentration gradient linear, the relationship becomes where k is the mass-transfer... Particle Diffusion As the concentration of ions in solution increases, the mass-transfer rate through the film rises until it exceeds the diffusion rate through the resin beads Diffusion through the resin then becomes the controlling factor, and the system is said to exhibit particle-controlled kinetics This condition occurs mainly during regeneration of resins with solutions having concentrations between... areas is equal to the separation factor Figure 12 Ion- exchange isotherms for mono – monovalent ions [Full View] In many cases, simple selectivity calculations show whether proposed ion- exchange processes will function [17], [18] 6 Exchange Kinetics 6.1 Principles [19], [20] Mass action equations apply only to systems in equilibrium In industrial practice where a solution flows through the resin, equilibrium... Uptake of Na+ by the cation-exchange resin in H+ form occurs: Figure 9 Titration curves of cation-exchange resins a) Amberlite IR 120, sulfonic acid resin; b) Amberlite IRC 86 carboxylic acid resin [Full View] (1) The H+ ions released by the resin combine immediately with the OH– ions from the alkali and drive the reaction shown in Equation (1) to completion At the beginning of titration, the strongly acidic ... cation – porous gel anion; b) Gel cation – standard gel anion; c) Macroporous cation – porous gel anion; d) Macroporous cation – standard gel anion; e) Macroporous cation – macroporous anion;... Concentration Separation Diffusion Catalysis Dehydration Coalescence on Oleophilic Resins Liquid Ion Exchangers 12.1.10 Ion- Exchange Membranes 12.2 Technical Considerations Introduction Definition... equivalent fraction Xi of an ion i in a solution with a total concentration of ions [C ] is defined by Similarly, the equivalent fraction of ion i in the resin is The total concentration of ions in the