Ion exchangersPhương pháp trao đổi ion

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:

2.2.3 Other Types of Ion-Exchange Resins

2.3 Adsorbent Resins and Inert Polymers

5.1 Dissociation and pK Value

5.2 Mono - Monovalent Exchange

5.3 Mono - Divalent Exchange (Water Softening)

6.4 Weakly Acidic or Weakly Basic Resins

7 Practical Results of Ion-Exchange Equilibrium and Kinetics7.1 Operating Capacity, Regeneration Efficiency, and Regenerant

Usage7.2 Permanent Leakage

7.3 Water Analysis

7.4 Calculations in the Design of Ion-Exchange Plants for Water

8 Industrial Use of Ion Exchange

8.1 Description of the Ion-Exchange Cycle

8.2 Methods for Overcoming Equilibrium Problems

9 Ion-Exchange Resin Combinations

10.2.5 Other Ion-Exchange Polishers

10.3 Continuously Circulated Ion-Exchange Resins10.4 External Valves and Pipework

10.5 Control Systems

11 Special Processes in Water Treatment

11.1 Removal of Organic Matter

11.2 Treatment of Potable Water

11.3 Treatment of Brackish Water

11.4 Processes Involving Sea Water

11.5 Treatment of Condensates

11.5.1 Conventional Resins

11.5.2 Powdered Resins

11.6 Water Treatment in the Nuclear Industry

11.7 Production of Ultrapure Water

12 Special Applications of Ion Exchange

12.1.8 Coalescence on Oleophilic Resins

12.1.9 Liquid Ion Exchangers

12.1.10 Ion-Exchange Membranes

12.2 Technical Considerations

1 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 1 shows the partial structure of a cation exchanger; each positive or

negative ion is surrounded by water molecules

Figure 1 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

1 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

2 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

3 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

2 Structures of Ion-Exchange Resins

An ion exchanger consists of the polymer matrix and the functional groups that interact ( sự 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 linked 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

1 Phenol – formaldehyde resins (→Phenolic Resins) which show interesting adsorption properties

2 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:

1 Strongly acidic (sulfonic groups)

2 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

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 1 resin) –CH2N+(CH3)2CH2CH2OHCl– e.g., Amberlite IRA410

(type 2 resin) –CH2N(CH3)2 e.g., Amberlite IRA96

Resins with quaternary ammonium groups are strongly basic Those with

benzyltrimethylammonium groups are known as type 1 and are the most strongly basic, whereas those with benzyldimethylethanolammonium groups are known as type 2 and are slightly less basic

Type 1 resins are used when total removal of anions, even those of weak acids (including silica), is essential Type 2 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 1 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 do not affect neutral salts or weak acids.Manufacturers do 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 1 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:

1 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)

2 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)

3 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

3 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 ion- exchange 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 8 % DVB.

Cross-Linking and Affinity The greater the ionic mobility in the resin, the

poorer is the differentiation between the adsorption of ionic species with the same charge Consequently, the degree of cross-linking in the resin must be increased when greater differences in ionic affinity are required.

In water treatment, the sulfonated polystyrene resins usually have a DVB

concentration of ca 8 % Resins with a slightly higher degree of cross-linking (10 – 12 %) are sometimes used to increase the retention of mineral ions when water of very high purity is being produced Resins with slightly lower levels of cross-linking (5 – 7 %) may be chosen when easier desorption and, hence, better regeneration efficiency are required, especially in water softening.

Nonuniformity in the Matrix Cross-linking reduces the retention of water in ion-exchange resins (Section Moisture Content ) The volume occupied by this water is a measure of the resin's porosity Cross-linking is not uniform because the DVB – DVB reaction is more rapid than that between DVB and styrene Polymerization begins to occur around the catalyst molecules, and polymer growth is faster at sites rich in DVB than at those rich in styrene Material with

an average of 8 % DVB may contain local microscopic regions with more than

20 % DVB, whereas other regions may have less than 4 %.

Macroporous resins are made by mixing the monomers with a compound (e.g., heptane, saturated fatty acids, C4 – C10 alcohols or polyalcohols, or low

molecular mass linear polystyrene) which expands the resin The substance does not itself polymerize and, thus, although it acts as a solvent for the monomers, it causes the polymer to precipitate from the liquid.

Channels are formed inside the beads, producing an artificially high porosity

Resins containing such channels are described as macroporous, whereas other resins with natural porosity are known as gel resins (Fig 2 )

Figure 2 Arrangement of structural units in gel (A) and macroporous (B) resins

[Full View]

Macroporous resins have a higher degree of cross-linking than gel resins to strengthen the matrix and compensate for voids left by the added solvent The porosity and mechanical strength of the resin can be modified by varying the degree of cross-linking or the amount of solvent added Therefore, various macroporous resins are available, with different moisture-holding capacities and internal structures

The pore diameter is ca 100 nm in a macroporous resin and ca 1 nm in a gel resin The macropores form a network of channels filled with free water, and large molecules can move freely in the resin into the center of a bead Once inside the resin, ions generally have a much shorter distance to travel before they encounter an active group: ca 100 nm in macroporous

resins and up to 500 µm in gel resins Exchange is thus faster in a macroporous resin.

Macroporous resins are highly resistant to physical stress and generally withstand osmotic shock very well They are therefore used in systems where mechanical and osmotic stress would otherwise cause gel resins to deteriorate rapidly, such as those involving circulation of resin, fluidized beds, high flow rates, oxidizing conditions, concentrated solutions, and short cycles

Finally, macroporous resins are used when reversible uptake of large molecules is necessary, without fouling the resin The adsorbents described in Section Adsorbent Resins and Inert Polymers have a macroporosity that allows selective retention of various molecules

3.2 Exchange Capacity

Total Capacity The total exchange capacity of a resin, expressed in equivalents per unit weight (or per unit volume), represents the number of active sites available In polystyrene exchangers, the maximum number of active sites corresponds to the “grafting” of one active group per benzene ring The capacity is expressed in equivalents (eq) per kilogram of dry

resin (the weight capacity Cp) or equivalents per liter of wet settled resin (the volume capacity

CV) Total capacity values for some of the most common resins are given in Table 2

Equivalent resins of other brands have similar capacity values (see Table 10)

Table 2 Typical capacities of ion-exchange resins *

Strongly acidic gel, 8 % DVB IR120 4.5 (Na) 2.05 (Na)

Weakly acidic gel IRC86 11.0 (H) 4.2 (H)

Strongly basic, type 1 IRA402 3.9 (Cl) 1.3 (Cl)

Strongly basic, type 2 IRA410 3.6 (Cl) 1.3 (Cl)

Strongly basic acrylic IRA458 4.2 (Cl) 1.3 (Cl)

Weakly basic styrene IRA96 4.7 (free base) 1.25 (free base)

Weakly basic acrylic IRA67 5.6 (free base) 1.6 (free base)

* Values depend to some extent on the analytical method used for capacity determination Macroporous strongly acidic resins have a dry weight capacity of 4.3 – 4.5 eq/kg Their volume capacity depends greatly on their porosity, which can be adjusted within relatively broad limits Similarly, macroporous strong base resins have dry weight capacities close to their gel counterparts, but generally lower volume capacities.

Operating Capacity The operating capacity is defined as the proportion of total capacity used during the exchange process It can amount to a large or small proportion of the total capacity and depends on a number of process variables including

1 Concentration and type of ions to be absorbed

2 Rate of percolation

3 Temperature

4 Depth of resin bed

5 Type, concentration, and quantity of regenerant

In a packed column, reaction between the ions in solution and those in the resin

occurs over a well-defined region of the resin bed known as the reaction zone

When the selectivity of a resin for a dissolved ion is high (see Chap

Ion-Exchange Equilibria ), a sharp exchange wavefront is formed which moves toward the column outlet, making maximum use of the resin The depth of the reaction zone depends on factors such as flow rate (kinetics) and ionic

concentration, but is independent of column length [ 12 ].

When selectivity is low, a diffuse wavefront develops The depth of the reaction zone then depends on the selectivity coefficient (Section Mono – Monovalent Exchange ) and also on the bed depth The longer the column, the deeper is the reaction zone and the greater is the operating capacity of the resin Figure 3

illustrates this: the top of the column contains the completely exhausted resins (a), whereas the reaction zone (b) contains partially exhausted resin The time at which the lowest point of this zone reaches the bottom of the column (i.e., when the adsorbed ions break through the bottom of the bed) is generally taken to be the time at which the service phase is complete A proportion of the resin is still not exhausted at the time of breakthrough

Figure 3 Reaction zone in a resin column during percolation

a) Exhausted resin; b) Reaction zone; c) Regenerated resin [Full View]

In practice, strongly acidic and strongly basic resins are never 100 % regenerated at the beginning of a cycle The operating capacity thus represents the difference between the available capacity at the beginning of a cycle and that remaining at the end point The most important factor is the amount of regenerant used to convert the resin to the regenerated form required at the beginning of the service cycle (c) The calculation of capacity is described in Chapter Practical Results of Ion-Exchange Equilibrium and Kinetics

3.3 Stability and Service Life

Because ion-exchange resins must give several years of service, their stability over long periods of time is of prime importance.

Chemical Stability of the Matrix Industrially available resins have a degree of cross-linking high enough to make them insoluble A new resin may release minute quantities of short-chain polymers or other soluble substances, but this effect is short-lived.

Highly oxidizing conditions (presence of chlorine or chromic acid) can attack the matrix and destroy cross-linking A sulfonated polystyrene cation-exchange resin with 8 % DVB cross-linking withstands 0.2 mg/kg of chlorine at ambient temperature for several years and is also completely stable at 120 °C in the absence of oxidants However, 1 mg/kg of chlorine oxidizes the polymer at a rate dependent on temperature; this breaks down the cross-linking, releases sulfonated organic compounds and causes the resin to swell until it softens, resulting in excessive head loss (sloughage) [ 13 ] When oxidizing agents are present, highly cross-linked resins with a greater resistance to oxidation, such as the macroporous resins, should be used.

Degradation products from a cation-exchange resin may foul anion resins [ 14 ], [ 15 ] This is particularly critical in processes designed to produce ultrapure water (Fig 4 ) Oxidants break the cross-links to produce soluble, short-chain oligomers that can be measured as the total organic carbon (TOC) in the treated water (Fig 4 A) The oligomers bear ionized active groups and thus decrease the resistivity of the treated water (Fig 4 B) Under normal conditions of water treatment, resins can operate continuously for many years (sometimes up to 20 years) without deterioration of their physical and chemical properties

Figure 4 Suitability of resins for producing ultrapure water in mixed beds [14]

Resins were tested at ambient temperature during first chlorine exposure by using an influent with 0.30 ppm active chlorine dosed as NaOCl and 0.02 – 0.03 ppm TOC Resins: a) 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 resins, on the other hand, are

temperature-sensitive When heated, Hofmann degradation may transform

quaternary ammonium groups (strongly basic) into tertiary amines (weakly basic) or even destroy the active group completely Because this reaction occurs under alkaline conditions, anion exchangers are more stable in the form of a salt than as a base.

Strongly basic type 1 resins are the most stable; the Hofmann degradation

reaction becomes significant only above 50 °C (Fig 5 ) At ambient temperature, these resins can last for five to seven years or more

Figure 5 Half-life of Amberlite IRA 402 as a function of temperature

Half-life is the contact time of resin with hot solution required to reduce total capacity by

capacity because the weakly basic groups are fully capable of taking up mineral acids)

Mechanical Stability Polycondensation-type resins that are manufactured in bulk and broken

up into irregular grains are comparatively fragile and used only in fixed beds (Section Bed Ion-Exchange Units) Polystyrene and polyacrylic resins made by suspension

Fixed-polymerization are perfect spheres and suffer little damage when used in continuous bed ion-exchange plants However, mechanical strength can vary considerably from one product to another, and resin beads which are seen to have many internal cracks under the microscope are more likely to break under mechanical stress than crack-free products

moving-Gel-type anion resins are generally weaker than cation materials and are particularly poor at

withstanding compression However, new polymerization techniques produce a more

uniformly structured polystyrene matrix As a result, gel resins with high physical stability are now available To prevent fragile resins from breaking, it is important to keep the bed clean

by frequent backwashing because the water pressure on a layer of fine debris can be as much

as 60 kPa

Acrylic resins are more elastic than polystyrene materials and can normally withstand any mechanical stress encountered in practice Macroporous resins are often the strongest of all and are used widely for the most severe stress conditions

The less elastic resins, i.e., those with the highest degree of cross-linking (gel resins with

> 8 % DVB and macroporous resins with >15 % DVB), have the disadvantage that, when they do break, they explode into minute fragments, whereas other resins break into two or three usable pieces

Osmotic Stability During ion exchange, the configuration around each active group in the resin changes: the adsorbed ion generally has a different size and, more important, a different hydration layer than the displaced ion The resin bead may therefore swell or contract

appreciably during the reaction The stresses to which the resin is subjected during these volume changes are known as osmotic forces They are very intense and can produce local pressures of several thousand kilopascals — much greater than purely mechanical stress.Resins for industrial use must be able to withstand hundreds of cycles of exhaustion and regeneration The nature and, hence, the strength of osmotic shock vary according to the ionic species in solution and their concentration

Higher mechanical and osmotic strengths are obtained with resins whose matrix is sufficiently strong to withstand physical shock (attrition) but sufficiently flexible and porous to deform without breaking under the effect of osmotic shock Some macroporous resins combine both

of these qualities

Resistance to Drying Repeated drying and rewetting produce stresses analogous to those due

to osmotic shock and can lead to fragmentation of most gel resins Resins must therefore be kept permanently moist

3.4 Density

Resin density is an important property because it determines the hydrodynamic behavior in counterflow systems Resin density normally lies in the following ranges (figures in

parentheses are the most common values for standard resins):

Strongly acidic cation exchangers 1.18 – 1.38 (1.28)Weakly acidic cation exchangers 1.13 – 1.20 (1.18)Strongly basic anion exchangers 1.07 – 1.12 (1.10)Weakly basic anion exchangers 1.02 – 1.10 (1.05)

By choosing suitable particle sizes, several different types of resin can be used in the same column If necessary, they can be kept separate by an upflow of water This is used mainly in layered beds (see Section Demineralization (Primary System))

Resin density can be increased artificially, for example, by attaching chlorine or bromine atoms to the matrix This yields high-density anion exchangers, which are useful when

fluidized-bed operation is required However, such “high-density” resins are not available commercially.

3.5 Particle Size

For industrial use, particle size is a compromise between the speed of the exchange reaction (which is greater with small beads) and high flow rates (which require coarse particles to minimize the head loss)

The size of the polymer droplets formed during polymerization, and hence the size of the resin beads, is determined by the polymerization technology, the suspension medium and, the monomer concentration

Traditional polymerizations are carried out in batches in a stirred reactor The beads produced

in this way have a range of particle sizes rather than a uniform size (Table 3) The population

of a resin sample (i.e., the number of beads classified according to bead diameter) has an approximately Gaussian distribution Because the volume of a fraction is a logarithmic

function of the number of beads, the particle-size distribution is often represented by a straight line on probability graph paper (Fig 6)

Table 3 Particle-size distribution of ion-exchange resins *

Sieve aperture, mm Noncumulative total Cumulative total

(between sieves), % (passing through), %

* Effective particle size 0.50 mm, mean diameter 0.68 mm, uniformity coefficient 0.73/0.50 = 1.46

Figure 6 Typical particle-size distribution in a normal resin

[Full View]

For Gaussian distribution, the particle-size distribution is defined by

1 The mean diameter (corresponding to the mesh size of a sieve allowing 50 % of the beads

to pass)

2 The uniformity coefficient, U.C (given by the ratio between the aperture of a sieve allowing

60 % of the beads to pass and that of a sieve allowing 10 % to pass The latter theoretical

sieve is known as the effective size

The closer the uniformity coefficient is to unity, the narrower is the Gaussian curve and hence the smaller is the range of particle size Resins produced with the traditional stirred-reactor process usually have a uniformity coefficient of 1.5 to 1.9

Several producers now offer ion-exchange resins with a very uniform particle size

distribution, which are produced with different polymerization technologies In one technique, the monomers are injected into the suspension medium through a plate perforated with

thousands of very small holes Droplets of the monomer mixture with an almost identical volume are expelled The uniformity coefficient of resins produced in this way is always less than 1.2, often in the range of 1.05 – 1.10 Examples of such resins are Amberjet (Rohm and Haas), Dowex Monosphere (Dow Chemical), and Lewatit Monoplus (Bayer)

3.6 Moisture Content

Ion-exchange resins carry both fixed and mobile ions which are always surrounded by water molecules located in the interior of the resin beads The water retention capacity governs the kinetics, exchange capacity, and mechanical strength of ion-exchange resins

The moisture content or moisture-holding capacity (MHC) is defined as

where PHydr is the weight of the hydrated resin sample and PDry the weight of the

same sample after drying.

The MHC of an ion-exchange resin is an inverse function of the degree of linking unless the porosity or degree of cross-linking in the polymer is

cross-artificially increased (as in macroporous resins) Figure 7 shows how the

moisture content varies with the proportion of DVB for gel-type sulfonic

polystyrene resins

Figure 7 Variation of moisture content (A) and total capacity (B) with the degree of

cross-linking in a sulfonated polystyrene resin in sodium form

[Full View]

Another useful quantity is the dry matter (DM) defined as

where PDry is the weight of the dry resin sample and VHydr the volume of the sample before drying Although they are closely connected, no 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 form is usually preferred because the resin has a relatively low affinity for sodium, which facilitates the adsorption of other metals Furthermore, sodium chloride is an

inexpensive regenerant

The following reaction is used to treat wine (R denotes the resin):

The reaction used in water softening is

In each case, the resin is regenerated by reversal of the reaction with sodium chloride solution.Hydrogen Exchange in Strongly Acidic Resins The replacement of metal ions with hydrogen ions leads to a reduction of the total dissolved solids in solution and the production of free acid:

This reaction is used as the first stage in the demineralization of water and other solutions It

is sometimes called salt splitting Regeneration is carried out with a mineral acid

Hydrogen Exchange in Weakly Acidic Resins Carboxylic resins are such weak acids that they ionize only slightly under acid conditions However, they have such a high affinity for divalent metals that in the presence of these metals, they are forced to ionize and therefore remain active under slightly acidic conditions down to ca pH 4.5

However, H+ from carboxylic resins cannot remove significant amounts of metals from

solutions of mineral acid salts because the acid produced quickly lowers the pH and prevents further ion exchange In this case, ion exchange is controlled by the basicity of the anion in solution A strongly nucleophilic anion “tears off” hydrogen from the carboxylic group, which takes up the associated cation in exchange, whereas a stable anion does not react significantly.Because the resin has a very high affinity for divalent ions (the effect of chelation), but only moderate affinity for monovalent ions, it has a high capacity for removing calcium and

magnesium from bicarbonate solution but takes up only a small amount of sodium This occurs because sufficient carbonic acid is formed to suppress the exchange of monovalent ions:

Carboxylic exchangers (weakly acidic) are selective: they preferentially remove divalent or trivalent cations until competition arises from alkaline anions present in solution

Selective Exchange in Weakly Acidic and Complexing Resins Because carboxylic resins take

up multivalent cations in preference to monovalent ones and are regenerated very easily, they can be used for selective removal of divalent and trivalent metals, even when high

concentrations of alkaline cations are present To achieve this, the exchanger is first converted

to the monovalent sodium form:

General Anion Exchange The most widely used resin for general anion exchange is a

strongly basic exchanger in chloride form:

This process is used to remove natural organic acids (e.g., humic acid, see Section Water Analysis) and nitrate from water and in hydrometallurgy to selectively adsorb metals that form anionic complexes:

The resins are regenerated by reversal of the reactions with sodium chloride solution

Acid Absorption in Strongly Basic Resins is the most widely used form of anion exchange When it follows hydrogen exchange in strongly acidic resins (Section Cation Exchange), it completes the demineralizing process:

Regeneration of the bicarbonate form of the resin requires two equivalents of OH– ions per equivalent of taken up, because half the OH– neutralizes the bicarbonate and converts it

to carbonate The same applies to silica

Acid Absorption in Weakly Basic Resins Weakly basic resins, in which the active groups are usually amines, do not have a true hydroxide form They ionize only under acidic conditions:

Under alkaline conditions, they exist as free bases and can therefore adsorb acids in the same way that free ammonia reacts with hydrochloric acid to form ammonium chloride In practice, they are called the “chloride form” Weakly basic resins can only adsorb strong acids Neutral salts are not “split”:

This is because no H+ ion exists to which the nucleophilic base can give its electrons and thus balance the anion Similarly, weak acids do not have dissociated H+ ions, so they are taken up only in very small amounts, if at all:

4.3 Cation and Anion Exchange in Water Treatment

Figure 8 summarizes the way in which the various forms of ion exchange

described in Sections Cation Exchange and Anion Exchange are used in water treatment The composition of raw water is 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 by the equilibrium reaction

where R– is the co-ion fixed in the matrix structure The acidity of the resin is defined by its degree of dissociation at equilibrium

where K is the equilibrium constant; the quantities in square brackets represent

concentrations, and underlines indicate the resin phase rather than the aqueous phase The pK

is defined as

Cation-exchange resins can be titrated (Fig 9) with sodium hydroxide in the presence of sodium chloride 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 (sulfonic) resin Amberlite IR 120 exchanges sodium ions from sodium chloride, and released H+ ions lower the pH of the aqueous phase (Fig 9, curve a) The pH remains low as long as the resin releases H+ ions to replace the H+

ions picked up from the added sodium hydroxide solution As soon as all H+ ions have been replaced in the resin, further addition of sodium hydroxide raises the pH sharply

The weakly acidic (carboxylic) resin Amberlite IRC 86 behaves differently Because its acidic groups are weakly dissociated, the exchange reaction (Eq 1) remains incomplete, and the pH

of the solution increases progressively even during the early stages of titration (Fig 9, curve b) A sharp rise is noticed, however, when the resin is converted completely to the Na+ form

In both cases, the total capacity of the resin can be read from the titration curves and

corresponds to the amount of sodium hydroxide that produces the sharp rise in pH In

Figure 9, Amberlite IR 120 has a total capacity of 2 meq/mL, whereas Amberlite IRC 86 has a capacity > 4 meq/mL

Similar considerations apply to anion-exchange resins

5.2 Mono – Monovalent Exchange

The law of mass action applied to the reaction

gives

(2)

where [Na+] and [H+] are the equivalent concentrations in the liquid phase and Na and

H are the corresponding activity coefficients The concentrations and coefficients for the resin are indicated by underlines Thus,

The activity coefficients may be assumed constant, so that

(the + signs are omitted for simplicity) The parameter is known as the selectivity

coefficient for the Na+/H+ exchange

Selectivity coefficients differ for each pair of ions because the affinity of the resin for an ion

is governed by the size of its hydrated form The larger the ion, the more the resin bead must expand to accommodate it Expansion is opposed by the restraining cross-links, so that large ions require a greater force to penetrate the resin than small ones

Table 4 lists the relative selectivities of sulfonic resins for mono- and divalent cations, and Table 5 gives the selectivities of strongly basic type 1 and type 2 resins for monovalent

anions Values increase with the degree of cross-linking and tend to unity as the cross-linking tends to zero Data are only approximate and merely demonstrate the scale of selectivities

Table 4 Relative selectivities of sulfonic resins for cations

Cation Degree of cross-linking, % DVB

Ag 6.0 7.6 12.0 17.0Divalent

Acetate 3.2 0.5Propionate 2.6 0.3Fluoride 1.6 0.3

* Reference value

The degree of cross-linking in resins is not uniform, so that an ion entering a resin first seeks the most favorable regions All exchange reactions become less favorable as they progress toward completion The practical effects of this general phenomenon on the exchange rate, exchange capacity, and leakage from resins are considered in Chapter Practical Results of Ion-Exchange Equilibrium and Kinetics

5.3 Mono – Divalent Exchange (Water Softening)

In the exchange reaction

In exchange involving only two ions (e.g., Na+ and Ca2+), the following equations apply to the solution:

where [ C ] is constant and is approximately constant at low concentrations Equation (3)

shows that as the solution becomes more dilute (decreasing C ) with a constant fraction of calcium (X Ca constant), X Ca increases The resin takes up more calcium as the total cation concentration of the solution decreases This effect is illustrated in

Figure 10 , which shows the equilibrium curves for a given resin at various total concentrations of the solution Each point on a curve corresponds to the

equivalent fraction of calcium in the resin at equilibrium, i.e., the curve gives the proportion of active sites in the resin in Ca2+ form as a function of the proportion

of calcium in solution These curves are called ion-exchange isotherms

Figure 10 Mono – divalent equilibrium curves for Na+ – Ca 2+ solutions of different total concentration

Total concentration of Na + and Ca 2+ , eq/L: a) 0.005; b) 0.01; c) 0.1; d) 1; e) 5 [Full View]

Water Softening For water with a total salinity of 5 meq/L, i.e [C ] = 0.005 N, Figure 10

shows that water containing only 5 % Ca2+ (XCa = 0.05 or [Ca] = 0.25 meq/L) is in equilibrium

with a resin loaded with 95 % calcium (X Ca = 0.95) In other words, the resin is capable of removing calcium from the water even if the concentration accounts for only 5 % of total

cations (XCa = 0.05), as long as the resin contains more than 5 % sodium ( X Ca < 95 %) At high concentrations, the affinity of the resin for calcium over sodium decreases until sodium becomes favored This is the effect that makes regeneration of the resin possible

Figure 11 shows the complete softening cycle and regeneration with 12 % brine (ca 2 N NaCl) using curves representing successive equilibrium states of the system At the starting point O, all of the resin is in the sodium form The water, with a total sodium and calcium content of 5 meq/L (0.005 N), comes in contact with the resin, and establishment of an

equilibrium leads to the uptake of calcium by the resin and its removal from the water The first upward-pointing arrow on the curve indicates the displacement of the equilibrium A new equilibrium is then established at P′ where the resin loaded with 20 % calcium is in

equilibrium with completely softened water The equilibrium then shifts to P, where resin

loaded with 40 % calcium is still in equilibrium with completely softened water, and finally to

Q where the resin loaded with 90 % calcium is in equilibrium with water which is about 98 %

softened (XCa = 0.02) and therefore contains 0.02 × 5 = 0.1 meq/L of calcium At this point, known as the end point, a new stage of the process is initiated to regenerate the resin (to continue as before would lead to the removal of less calcium) The system is displaced to point R on the other curve as the sodium chloride concentration changes from 0.005 to 2 N

At R, the resin is in equilibrium with a solution containing about 95 % calcium and therefore tends to release calcium ions so as to produce this concentration in the liquid In this

regeneration process, the resin is converted continuously into the sodium form by exchanging the calcium ions adsorbed during the first stage for sodium ions from the regenerating brine

As the system moves toward the bottom of the curve, the equilibrium becomes less favorable:

at R, 95 % of the sodium chloride in the solution is used up in regeneration of the resin

(because XCa = 0.95), whereas at point S the proportion is already less than 60 % and at point

T, 25 % At the latter point, the resin is 60 % regenerated (X Ca = 0.4) and the process stops because any attempt to reach point O would need increasing amounts of brine, only a small proportion of which would be used in actual regeneration The resin is therefore rinsed at point T and another new stage is initiated by displacing the system to P and beginning a second cycle in which the water is again almost completely softened All subsequent cycles follow the path PQRT

Figure 11 Successive equilibria in the water-softening cycle

5.4 General Case

In the general exchange reaction between ions A and B of ionic valences a and b,

respectively, the selectivity coefficient K is defined as

depends on experimental conditions such as concentration and temperature

Another useful quantity is the separation factor, defined as

For mono – monovalent exchange, the separation factor and selectivity coefficient K are

identical and practically independent of the total salt content For mono – divalent exchange,

is a function of the term K · [ C ]/[C] A separation factor greater than unity means that the resin will take up ion B in preference to ion A.

Figure 12 shows ion-exchange isotherms for two monovalent ions A and B The

upper curve ( = 2) represents a case in which the resin has a higher affinity for B than for A; the lower curve ( = 0.2) represents the opposite case For any point on a given isotherm, the ratio between the two hatched areas is equal

to the separation factor

Figure 12 Ion-exchange isotherms for mono – monovalent ions

Mass action equations apply only to systems in equilibrium In industrial

practice where a solution flows through the resin, equilibrium is not necessarily reached and the results are 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 greater than that in

solution However, any cations diffusing out of the resin into the dilute solution create a net negative charge in the solid phase and a net positive charge in

solution The resulting potential difference is called the Donnan potential; it

prevents anions from penetrating the (negatively charged) resin This

phenomenon is called Donnan exclusion Therefore, the co-ion (i.e., the anion in

the case of cation exchange) does not participate in the ion-exchange process.

Figure 13 illustrates the uptake of Na+ ions by a bead of H+-form resin The bulk solution contains a large excess of available Na+ ions, with an effectively

constant concentration A static layer of solution, known as the Nernst film,

surrounds the bead This film is defined such that it is unaffected by convection (i.e., flow) around the bead; ion transport takes place by diffusion only Strong convection (i.e., high flow rate) decreases film thickness The ion concentration

is practically constant outside the Nernst film, and a concentration gradient occurs within it (Fig 14 )

Figure 13 Diffusion through a film and inside a particle

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

H = Helfferich constant

= thickness of Nernst film

r = radius of ion-exchange bead

= separation factor (defined in Section General Case)

When H 1, film diffusion is rate-limiting; when H 1, particle diffusion is limiting

rate-The definition of H shows that film diffusion is favored by the following:

High resin capacity C

Thick Nernst film (i.e., low flow rate)

Low concentration C in solution

Small resin beads

High selectivity (high )

In general, forward and reverse exchange rates differ (Fig 15) In this example

of H+/Na+ exchange in a conventional strongly acidic cation-exchange resin, forward exchange (uptake of Na+ ions) is faster than reverse exchange

(regeneration) Because the H+ ion generally is more mobile, exchange is faster when the faster ion is in the resin initially The rates of sodium – calcium

exchange are several times slower

Figure 15 Forward and reverse exchange rates in a polystyrene sulfonic resin

a) 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 coefficient (cm/s) given by D/ Measurement of the transfer coefficient of new and used resins can provide useful information about their ability

mass-to operate under critical conditions [21]

Diffusion in the resin phase is always slower than that in solution, due to the obstruction created by the resin matrix Highly cross-linked materials have smaller diffusion coefficients.For a comprehensive treatment of diffusion and rate laws in ion exchange, the reader is referred to [19], [20] The remainder of this chapter deals with practical consequences of the above kinetic principles

6.2 Kinetic Curves

As with ionic equilibria, exchange kinetics are difficult to appreciate from a purely mathematical point of view However, exchange rates for different resins can be compared under the same conditions.

Rate of Ion Uptake (Exhaustion Rate) The exhaustion rate of a resin through which a given solution flows is measured under standard conditions In the

general case of a single ion species, the exhausted fraction Xi of the resin is

plotted vertically against time (Fig 16 ) The example illustrates the recovery of uranium from a sulfuric acid leaching solution in a stirred tank with strongly basic exchangers in the sulfate form Duolite A 101U is a high-capacity

In the example shown in Figure 17, new and used macroporous strongly acidic cation-exchange resins were tested for their ability to remove sodium from

dilute sodium chloride solutions percolated through a mixed bed made of the cation resin and of a new, highly regenerated nuclear-grade anion resin The conductivity of effluent water was measured Both mixtures give identical low leakage for the more dilute sodium chloride solution (1.7 mg/L, curve c)

However, the mixture containing used cation resin is more sensitive to the flow rate than that with new cation resin when the sodium chloride concentration is increased to 16 mg/L (curves a and b); both mixtures then become strongly sensitive to flow rate

Figure 17 Kinetic leakage: conductivity vs flow rate for two resin mixtures at two

concentrations a) Used cation resin, NaCl 16 mg/L; b) New cation resin, NaCl 16 mg/L; c) Used and new resins, NaCl 1.7 mg/L

process exhibits film-controlled kinetics When sodium chloride solution passes through a column of resin, originally in H+ form, the concentration of the ion under consideration in the emergent liquor exhibits variations of the form shown

in Figure 18

Figure 18 Film kinetics: exhaustion curves and volumes passed at maximum leakage

For explanation see text.

[Full View]

If the flow rate is slow enough, equilibrium is established as the solution reaches a new layer

of the resin The concentration in the emergent liquor is represented by the curve OFP At F, the breakthrough point, the concentration reaches its maximum permitted value and flow stops If it were continued up to P where the concentration in the emergent liquor equals that

of the raw solution, the resin would be 100 % exhausted

In industrial practice, flow is stopped when the concentration of the ion under consideration in the emergent liquor reaches a small fraction (e.g., 1 %) of the concentration in raw solution

At F, a volume A of the solution has flown through the column The capacity used is given by the area OFA′Z, which differs very little from the area OFPZ representing the total capacity of the resin

Figure 18 can also be taken to represent the progress of the exhaustion wavefront through a resin column In the slow flow described above, the wavefront is only slightly diffuse, each successive layer of resin being almost completely exhausted before leakage occurs As flow rate increases, equilibrium is no longer reached and the exhaustion curve OGQ is obtained If the process is continued beyond the breakthrough point G as far as Q, the area OGQZ gives another measure of the total capacity of the resin If, however, the process is stopped at G, the capacity used is only that given by OGB′Z, which is less than the total capacity

If the flow rate is increased further, a curve such as OHR is obtained, which has a still smaller operating capacity at the new breakthrough point H

Because the Nernst film thickness is an inverse function of flow rate, the film becomes

thinner as the flow rate increases Thus, doubling the flow rate does not mean that the leakage curve is spread out over twice the distance In film-controlled kinetics, capacity depends very little on the rate of presentation of ions or kinetic load (i.e., the product of flow rate and concentration in the solution to be treated) In the complete absence of kinetic effects, the operating capacity is entirely independent of flow rate; therefore, the volume of the

exhaustion stage is inversely proportional to the ionic concentration As described in

Section Mono – Divalent Exchange (Water Softening), the operating capacity of a strongly acidic resin depends mainly on the amount of regenerant For a given quantity of regenerant, a corresponding capacity is available which is less than the total capacity

If the exhaustion time is too short (i.e., if resin volume is small and flow rate per unit volume

is high), the operating capacity is less than the available capacity This reduction in operating capacity (which is mainly confined to strongly and weakly basic anion resins) can increase as the resin ages due to partial blocking of its pores with organic matter absorbed during several years of service Fouling of this type reduces ion diffusion rates In designing a

demineralizing plant, the anion-exchange column should have sufficient capacity to allow the plant to run for at least 8 h at the maximum loading rate.

6.3.2 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 1 and 3 N

Breakthrough curves are similar to those in Figure 18, except that the length of the wavefront

is a linear function of flow rate Because of the high concentration gradient through the resin, the whole process is much faster than the exhaustion stage Virtually complete equilibrium can be achieved in 15 min, but if a shorter regeneration time is used, operating capacity can be significantly reduced Nevertheless, small automatic water softeners sometimes have brine injection times as short as 5 min

6.4 Weakly Acidic or Weakly Basic Resins

Weakly acidic or weakly basic resins are not fully ionized in the regenerated form As a result, the adsorption process involves not only diffusion through the resin but also the formation or destruction of covalent bonds, which are slower processes The exchange rate of these resins is therefore controlled by slow particle diffusion and the resin cannot be exhausted as quickly as with strongly ionized materials Weakly acidic and weakly basic resins are said to have slow kinetics: their capacity depends strictly on the rate of presentation of ions.

As shown in Figure 19, the carboxylic resin Amberlite IRC 86 requires a cycle time of over 24 h to utilize its full capacity (in this example, the ions to be

removed from solution have a concentration of 5 meq/L) In practice, usually less than half the total capacity of weakly acidic resins is utilized The kinetic effect is not as dramatic for weakly basic resins, which are usually operated at

ca 75 % of their total capacity

Figure 19 Effect of cycle time on usable capacity for a weakly acidic cation resin (a)

and two weakly basic anion resins (b, c) a) Amberlite IRC 86 (weakly acidic); b) Amberlite IRA 67 (weakly basic, acrylic); c) Amberlite IRA 96 (weakly basic, styrenic)

[Full View]

7 Practical Results of Ion-Exchange Equilibrium and Kinetics

As shown in Chapter Exchange Kinetics, under normal conditions the exchange process is always incomplete in both the service and the regeneration stages, which means that the total capacity of the resin can never be fully used

7.1 Operating Capacity, Regeneration Efficiency, and Regenerant Usage

Calculation of operating capacity must take into account the following:

Raw water analysis

Required quality of treated water (acceptable leakage)

Service flow rate

Temperature of water to be treated

Type and amount of regenerant

Regeneration flow rate

Regenerant temperature

Required duration of cycle

The concept of operating capacity has been introduced in Section Exchange Capacity and developed in Section Mono – Divalent Exchange (Water

Softening) for the special case of water softening (Na+ – Ca2+ equilibrium), in both instances starting from 100 % regenerated resin In practice, this is seldom the situation because an enormous amount of regenerant would be required to remove all the ions from the resin that were taken up during the previous cycle The variable that has the greatest effect on operating capacity is therefore the amount of regenerant used.

Regeneration efficiency is defined as the ratio of the operating capacity to the amount of regenerant used; both expressed in equivalents per liter The

reciprocal of this efficiency is known as the regenerant usage or regenerant

ratio and is always >1 In practice, these quantities are often expressed as

percentages.

With most strongly acidic or strongly basic resins more than 60 % of their total capacity is seldom used As an example, the total capacity of the strongly basic type 1 resin shown in Figure 20 is ca 1.2 eq/L A basic operating capacity of 0.46 eq/L can be obtained by using 60 g of caustic soda per liter of resin

(1.5 eq/L) In this case, the regeneration efficiency (0.46/1.5) barely exceeds

30 % By doubling the amount of regenerant, a basic operating capacity of

0.615 eq/L can be achieved: this is only one-third greater than before, and

regeneration efficiency falls to 21 % Thus, such a resin is seldom operated at more than 50 % of its total capacity However, these very strongly basic type 1 materials are less easily regenerated than type 2 resins and therefore yield a lower operating capacity for the same amount of regenerant than the latter

Weakly acidic and weakly basic resins, on the other hand, can be completely regenerated, but their operating capacity is limited by their poor kinetics

(Section Weakly Acidic or Weakly Basic Resins )

Figure 20 Operating capacity of a strongly basic type 1 resin as a function of regenerant

dosage (caustic soda) Numbers on the curves indicate percentage of regenerant efficiency.

[Full View]

7.2 Permanent Leakage

Strongly Acidic and Strongly Basic Resins When strongly basic or strongly acidic resins are returned to service after regeneration, they are only partially regenerated When coflow operation is used (i.e., when flow is in the same

direction in the service and regeneration stages), the most poorly regenerated layer is usually the lower one through which the treated solution emerges This affects the quality of the treated solution due to self-regeneration In practice, the bed does not consist of two distinct (completely regenerated, completely

exhausted) regions, but has a continuously varying level of regeneration along the column Figure 21 shows the exchange reactions that occur in the column during the service stage, in which salts from the solution (represented here by NaCl) are retained by the regenerated resin layers Each Na+ ion that is taken up ejects an H+ ion from the resin The H+ ions move down the column and reach a region where the resin is only partially regenerated Reverse exchange then takes place (self-regeneration), leading to leakage of Na+ ions into the treated water

Figure 21 Ion leakage due to self-regeneration in a coflow regenerated column

For explanation see text.

[Full View]

Figure 22 shows the operating capacity (A) and sodium leakage (B) of

Amberlite IR 120 (a standard, gel-type, strongly acidic cation-exchange resin) as

a function of regenerant level

Figure 22 Capacity (A) and sodium leakage (B) of a standard strongly acidic cation

resin (Amberlite IR 120) regenerated in coflow mode.

Sodium (NaCl) content with respect to total cation concentration: a) 100 %; b) 50 %; c)

25 %Flow rate, 10 BV/h; total salinity of raw water, 10 meq/L; acid dosage expressed as

100 % HCl per liter of resin; water temperature, 20 °C.

[Full View]

In raw water, the main cations are usually sodium, calcium, and magnesium The selectivity

of strongly acidic resins for these cations increases in the order Na+< Mg2+< Ca2+

(Section Mono – Monovalent Exchange) Therefore, chromatographic separation occurs in the resin column, and the sodium ion is the first to emerge in the treated solution, both other cations being more strongly held by the resin This explains the high leakage obtained for water having only sodium as a cation (Fig 22 B, curve a)

Because the quality of demineralized water is generally expressed in terms of its electrical conductivity, sodium leakage from the cation-exchange column is the main contributor to the conductivity of treated water A sodium leakage of 1 mg/L corresponds to 9 µS/cm

conductivity (as NaOH) In coflow regenerated systems, the level of regenerant is chosen

according to the desired conductivity For high-sodium water, the only way to obtain low leakage is to use counterflow regeneration (see Section Methods for Overcoming Equilibrium Problems).

Weakly Acidic and Weakly Basic Resins Weakly acidic and weakly basic resins are almost

100 % regenerated at the start of the service stage, but their exchange rate is relatively low so that some ions are not taken up and pass through the bed (i.e., kinetic leakage; see

Section Kinetic Curves)

the French degree: 1° f = 0.2 meq/L

the German degree: 1° dH = 0.357 meq/L (which corresponds to 10 mg of CaO per liter)

The following units are used in Great Britain and other English-speaking countries:

1 g of CaCO3 per liter = 0.02 eq/L

1 mg of CaCO3 per liter = 0.02 meq/L

Although resin capacities are internationally expressed in equivalents per liter,

in the United States capacity values are widely given in kilograins (kgr) where

1 kgr as calcium carbonate per cubic foot corresponds to 0.0458 eq/L.

Water Composition For ion-exchange calculations, the exact composition of the solution to be treated must be known The composition of a typical water is given in Figure 23 The total concentration of all the anions and cations in

solution is known as the total dissolved solids (TDS) Ions normally encountered

in water treatment are the cations Na+, Ca2+, Mg2+, and the anions OH–, , , Cl–, , Other ions may be present (K+, , Mn2+, Fe2+), but their concentration in natural water is usually very low For ionic equilibrium, the total cation and anion concentrations measured as equivalents per liter must be equal

Figure 23 Composition of water

[Full View]

Both Ca2+ and Mg2+ ions (and, possibly, Fe2+ and Mn2+) are classified as ions producing

hardness, and their concentration gives the total hardness (TH) measured in milliequivalents

per liter In water softening, Ca2+ and Mg2+ are exchanged for Na+ The Cl–, , and

ions are grouped together, and their total concentration is called the equivalent mineral

acidity (EMA), or free mineral acidity (FMA) after cation exchange.

The total concentration of OH–, , and ions, measured in equivalents per liter gives

the total alkalinity (TAlk) This is often referred to as m-Alk or the m value (alkalinity to

methyl orange):

When TH exceeds TAlk, the difference (TH–TAlk) is called the permanent hardness,

whereas TAlk is known as the temporary (or bicarbonate) hardness When TH is lower than

TAlk, only temporary hardness (no permanent hardness) exists For water with alkaline pH

values, the caustic alkalinity, usually described as p-Alk or the p value (alkalinity to

phenolphthalein) is measured separately

Carbonate Equilibrium The p value includes all anions with a pK > 8.3, i.e., in practice the

OH– ions and a value arithmetically equal to half the total carbonate present because is first neutralized to , which itself has a pK of only 6.7:

In a demineralization system, where the first step is usually cation exchange, ions of p

alkalinity must not be considered in the anion-exchange balance, because they are neutralized

by the H+ ions produced in the cation-exchange part of the process:

Because in the series OH–, , , and , only two adjacent species can coexist in

water, raw water does not contain free carbon dioxide when p is positive.

In practice, the equivalent concentration of carbon dioxide to be considered after cation exchange (and before the degasifier, if any) is

The case in which p = 0 is the most common one, except when the raw water is

lime-decarbonated Besides, free carbon dioxide exists only with a pH close to or lower than neutral

The concentrations of nonionized substances must also be known; these include free silicon dioxide and carbon dioxide The amount of organic substances present in the raw water

should be considered on the anion side, because they are mainly acidic Concentration is usually expressed in terms of the quantity of potassium permanganate needed to oxidize these substances (milligrams of KMnO4 per liter of solution) The amount of organic matter

determines the choice of anion-exchange resin (see Section General Considerations) The permanganate value obtained in the oxidizing test depends on the test method Here, oxidation

is assumed to be performed with 0.0125 N potassium permanganate in the presence of sulfuric acid, the solution being boiled for 10 min The composition of a typical water is given in Figure 23

Fouling with Organic Material The most common organic constitutents in natural waters are the high molecular mass carboxylic acids, humic and fulvic acid These large molecules enter the ion-exchange resins and are trapped in the most highly cross-linked regions, their chains becoming tangled with the resin matrix This eventually reduces the capacity of anion

exchangers Rinsing becomes more difficult, and problems arise with the quality of treated water because the carboxylic character of the organic matter means that, as the pH varies, caustic soda is initially taken up and then leached out The problem is aggravated by synthetic ionic surfactants

Although qualitative and quantitative assessment of organic materials is difficult, their

concentration is usually expressed in milligrams of potassium permanganate required per liter

of water to oxidize them under the given conditions The fouling factor N is the quantity of

organic matter (in milligrams per liter of KMnO4) divided by the total anion concentration (in milliequivalents per liter)

Anion resins can be listed in order of their resistance to fouling, starting with the least

resistant :

Polystyrene, gel type 1 2Polystyrene, macroporous type 1 4Polyamine, weakly basic 4Polystyrene, gel type 2 6Polystyrene, macroporous type 2 8Polyacrylic, strongly basic 20Polystyrene, weakly basic 20Polyacrylic, weakly basic 40

These values merely indicate the fouling risk of each individual resin and give no information about the amount of organic material taken up Anion resins, even those with high fouling resistance, do not remove all organic matter from the influent solution Reversible uptake can range from 20 to 80 %; a reasonably safe figure is 50 %

If N is too large (e.g., > 8 for a strongly basic polystyrene material), the risk of fouling can be

reduced by increasing the volume of resin The working capacity of the resin will be less than its potential operating capacity Weakly basic resins can also protect strongly basic resins against fouling

7.4 Calculations in the Design of Ion-Exchange Plants for Water Purification

General Method The various types of combination process, the criteria used in choice of resin, and the recommended operational conditions are considered in Chapter Ion-Exchange Resin Combinations The operating capacity for water softening is greater when the amount

of regenerant is large, the sodium ion (self-regenerant) concentration is low, and the flow rate

is low

Resin manufacturers provide standard charts and curves for each type of resin, enabling calculations to be made of the volume of each resin, amount of regenerant, operating capacity, and leakage The starting point is normally provided by a curve that applies to standard

conditions, and corrections are made for each condition that differs from the set standard The method of calculation is too complicated to be described here; only the main factors are considered

Weakly acidic carboxylic resins have a very high capacity for divalent ions (Ca2+ and Mg2+): they remove temporary hardness (see Section Water Analysis) and are normally used only when the temporary hardness is high and the total hardness of the water is even greater

(TH > TAlk) Their relatively slow exchange rate means that they are very sensitive to the specific flow rate

Regeneration is carried out with an amount of acid calculated to be slightly in excess of the design capacity, with a regenerant ratio on the order of 105 – 110 % These resins are

therefore very efficient

Strongly Acidic Sulfonic Resins In hydrogen exchange, capacity depends on the type of acid used for regeneration: hydrochloric acid is the most efficient because sulfuric acid is not completely dissociated at the concentration normally used In addition (this also applies to carboxylic resins), if the resin has taken up a lot of calcium, the sulfuric acid must be highly diluted to avoid precipitation of calcium sulfate during regeneration The amount of

regenerant is chosen according to the permitted ion leakage: in other words, it depends on the permissible level of electrical conductivity in the treated water (see Section Permanent

Leakage) The greater the amount of regenerant, the lower is the electrical conductivity and the greater is the operating capacity (see Fig 22) The operating capacity of the resin depends

on the cation composition of the water, more in relation to proportions than to absolute

values Reference should be made to manufacturers' curves

Weakly basic resins also have slow kinetics and are rate-sensitive In addition, their capacity depends on the composition of the water, increasing as the concentration of strong acids decreases Only small quantities of weak acids are removed However, carbon dioxide

improves the capacity of weakly basic styrene resins so that, if an atmospheric degasser is used it should be located between the weakly basic and strongly basic exchange steps Weakly

basic acrylic resins have a higher pK value, so that they effectively remove carbon dioxide

from water, and the benefit of the degasifier is lost if it is located downstream

The quantity of regenerant normally used is on the order of 130 % of the operating capacity of the resin

Strongly basic resins remove strong and weak acids Because the uptake of silica is poorer than that of other anions, it is the first to “leak” The regenerant level thus depends on the acceptable silica leakage In addition, because silica has a tendency to polymerize on the resin, regenerating with hot sodium hydroxide is sometimes worthwhile However, only type 1 materials withstand high temperature (see Section Stability and Service Life)