15 ADSORPTION AND ION EXCHANGE

eparation of the components of a fluid can be effected by contacting them with a solid that has a preferential attraction for some of them. Such S processes are quantitatively significant when the

specific surfaces of the solids are measured in hundreds of m*/g. Suitable materials are masses of numerous fine pores that were generated by expulsion of volatile substances. The most important adsorbents are activated carbon, prepared by partial volatilization or combustion of a carbonaceous body, and activated alumina, silica gel, and molecular sieves which are all formed by expulsion of water vapor from a solid. The starting material for silica gel is a coagulated silicic acid and that for molecular sieves is hydrated aluminum silicate crystals that end up as porous crystal structures. Porous glasses made by leaching with alkai have some application in chromatography. Physical properties of common adsorbents are listed in Tables 15.1 and 15.2. Representative manufacturing processes are represented on Figure 15.1.

The amount of adsorption is limited by the available surface and pore volume, and depends also on the chemical natures of the fluid and solid. The rate of adsorption also depends on the amount of exposed surface but, in addition, on the rate of diffusion to the external surface and through the pores of the solid for accessing the internal surface which comprises the bulk of the surface. Diffusion rates depend on temperature and differences in concentration or partial pressures. The smaller the particle size, the greater is the utilization of the internal surface, but also the greater the pressure drop for flow of bulk fluid through a mass of the particles.

In ion exchange equipment, cations or anions from the fluid deposit in the solid and displace equivalent amounts of other ions from the solid. Suitable solids are not necessarily porous; the ions are able to diffuse through the solid material. A typical exchange is that of H + or OH - ions from the solid for some undesirable ions in the solution, such as Ca ++ or SO;-. Eventually all of the ions in the solid are replaced, but the activity is restored by contacting the exhausted solid with a high concentration of the desired ion, for example, a strong acid to replace lost hydrogen ions.

For economic reasons, saturated adsorbents and exhausted ion exchangers must be regenerated. Most commonly, saturation and regeneration are performed alternately and intermittently, but equipment can be devised in which these processes are accomplished continuously by countercurrent movement of the solid and fluid streams. Only a few such operations have proved economically feasible. The UOP and Toray processes for liquid adsorption are not true continuous processes but are effectively such.

temperature, or reducing the pressure, or by washing with a suitable reagent. The desorbed material may be recovered as valuable product in concentrated form or as a waste in easily disposable form. Adsorbent carbons used for water treating often must be regenerated by ignition in a furnace. Relatively small amounts of adsorbents that are difficult to regenerate are simply discarded.

Desorption is accomplished by elevating the

15.1. ADSORPTION EQUILIBRIA

The amount of adsorbate that can be held depends on the concentration or partial pressure and temperature, on the chemical nature of the fluid, and on the nature, specific surface, method of preparation, and regeneration history of the solid. For single adsorbable components of gases, the relations between amount adsorbed and the partial pressure have been classified into the six types shown in Figure 15.2. Many common systems conform to Type I , for example, some of the curves of Figure 15.3. Adsorption data are not highly reproducible because small contents of impurities and the history of the adsorbent have strong influences on their behavior.

One of the simplest equations relating amount of adsorption and pressure with some range of applicability is that of Freundlich,

w =UP" (15.1)

and its generalization for the effect of temperature

w=aP"exp(-b/T). (15.2)

The exponent n usually is less than unity. Both gas and liquid adsorption data are fitted by the Freundlich isotherm. Many liquid data are fitted thus in a compilation of Landolt-Bornstein (11/3, Numerical Data and Functional Relationships in Science and Technology, Springer, New York, 1956, pp. 525-528), but their gas

data are presented in graphical form only (LB IV 4/b, 1972, pp. 121-187). The effect of temperature also is correlated by a theory of Polanyi, whereby all data of a particular system fall on the same curve; Figure 15.4 is an example. For isothermal data, a combination of the Freundlich and Langmuir equations was developed by Yon and Turnock (Chem. Eng. Prog. Symposium Series 117, 67, 1971):

w = kP"/(l+ kP"). (15.3)

Individuals of multicomponent mixtures compete for the limited space on the adsorbent. Equilibrium curves of binary mixtures, when plotted as x vs. y diagrams, resemble those of vapor-liquid mixtures, either for gases (Fig. 15.5) or liquids (Fig. 15.6). The shapes of adsorption curves of binary mixtures, Figure 15.7, are varied; the total adsorptions of the components of the pairs of Figure 15.7 would be more nearly constant over the whole range of compositions in terms of liquid volume fractions rather than the mol fractions shown.

Higher molecular weight members of homologous series adsorb preferentially on some adsorbents. The desorption data of Figure 15.8 attest to this, the hydrogen coming off first and the pentane last. In practical cases it is not always feasible to allow sufficient time for complete removal of heavy constituents so that the capacity of regenerated adsorbent becomes less than that of fresh, as Figure 15.9 indicates. Repeated regeneration causes gradual deterioration

495

TABLE 15.1. Physical Properties of Adsorbents

External Effective Bulk Void External Specific Reactivation

Particle Mesh Diameter Density Fraction Surface Heat Tempera! ure Form* Size D1,, ft. pb, Lb/cu.ft. F, a,, sq.ft. C,, Btullb O F O F Examples

A

Activated Carbon ... P 4 x 6 0.0128 30 0.34 310 0.25 20@-1000 Columbia L I, *I P 6 x 8 0.0092 30 0.34 446

P 8 X 10 0.0064 30 0.34 645 G 4 x 10 0.0110 30 0.40 460 G 6 X 16 0.0062 30 0.40 720 G 4 X 10 0.0105 28 0.44 450

G 6 X 16 0.0062 45 0.35 720 S 4 x 8 0.0130 50 0.36 300 0.25

Activated Alumina.. G 4 x 8 0.0130 52 0.25 380 G 8 X 14 0.0058 52 0.25 480 G 1 4 x 2 8 0.0027 54 0.25 970 S (1/4*) 0.0208 52 0.30 200 S (1/8") 0.0104 54 0.30 400

Molecular Sieves.. . G 14 x 28 0.0027 30 0.25 970 P (1/16') 0.0060 45 0.34 650 P (1/8") 0.0104 45 0.34 400

S 8 X 12 0.0067 45 0.37 565

I* ., Pittsburgh BPL

Witco 256 Davison 03

., I,

, I I.

,I I ,

0.25

250-450

300-450 Mobil Sorbead R

Silica Gel.. . ... . . ... G 3 x 8 0.0127 45 0.35 230 0.22 II 1 ,

Alcoa Type F 0.22 350-600 ,I *I I. ,* I,

I. ,I I , I, I.

0.22 350-1000 Alcoa Type H

300-600 Davison. Linda 0.23 * # I t I. I. ,.

I. .I

P . .. #I .I S 4 x 8 0.0109 45 0.37 347

' P = pellets; G = granules; S = spheroids

I, I.

(Fair, 1969).

Na Na I Z[(A102)lZ(Si0,)1 21 8-ring 3.8 Desiccant. CO, removal

Ca Ca,N a,[( AlO,) I ,( SOz) I ,] 8-ring 4.4 Linear paraffin (obstructed) from natural gas

(free) separation. Air separation

K K I z[(AIOz)l z(Si0z) I 21 8-ring 2.9 Drying of cracked gas

Age[(A10~)dSi02)41 12-ring 7.0 I and Kr removal from nuclear ~ f f - g a s e s ' ~ ~ - ~ )

Mordenite ( Ag H Hd(A102)dSi0z).d

Silicali te - (SiO,), IO-ring 6.0 Removal of organics

ZSMJ Na Na3[(A102)dSiO~k31 IO-ring 6.0 Xylene separatiod3') from water

"Also K-BaX.

(Ruthven, 1984).

496

15.2. ION EXCHANGE EQUILIBRIA 497

TABLE 15.2-(continued) (b) Typical Properties of Union Carbide Type X Molecular Sieves

Ncniiiiinl piirv h i l k I Irill ld IGpiililiriiiiii Basic diaiiieter, density, adsorption (mu), HtO capacity, Molecules Molecules type angstroms Available form Ib/ft3 B t d h HtO % w t adsorbed excluded Applications

3A 3 Powder 30 1800 23 Molecules with an Molecules with The preferred molemlar sieve $bin pellets 44 20 effective diameter an effective adsorbent for the commerical %-in pellets 44 20 < 3 A, including diameter > 3 .&, dehydration of unsaturated

HIO and NH, e.g., ethane hydrocarbon streams such as cracked gas, propylene, butadiene, and acetylene. It is also used for drying polar liquids such as methanol and ethanol.

4A 4 Powder 30 1800 28.5 Molecules with an Molecules with The preferred molecular sieve $bin pellets 45 22 effective diameter an effedive adsorbent hor static dehydration in %-in pellets 45 22 e 4 A, including diameter > 4 A, a closed gas or liquid system. It is 8 x 12beads 45 22 ethanol, HIS, CQ, e.g., propane used as a static desiccant in 4 ~ 8 b e a d s 45 22 SO,, CH,, G k household refrigeration systems; 1 4 x 30mesh 44 22 and CH, in packaging of drugs, electronic

components and perishable chemicals; and as a water scavenger in paint and plastic systems. Also used commercially in drying saturated hydrocamon ctrparnr.

5A 5 Powder 30 1800 2a Molecules with an Molecules with Separates normal p&s from %-in pellets 43 21.5 effective diameter an effective branchedchain and cyclic %-in pellets 43 21.5 < 5 A, including n- diameter > 5 A, hydrocarbons through a selective

C,H,OH,t n- e.g., is0 adsorption process. C,H,,,t CH. to compounds and GHa R-12 all +carbon rings

lox 8 Powder ' b i n pellets %-in pellets

13X 10 Powder %-in pellets %-in pellets 8 x l2beads 4 x 8 beads 14 x 30 mesh

30 36 36

- 30 38 38 42 42 38

1800 36 Is0 paraffins and 28 olefins, CH,, 28 molecules with an

effective diameter < 8 A

Di-n-butylamine Aromatic hydrocarbon separation. and larger

1800 36 Molecules with an 28.5 effective diameter 28.5 < 10 A 28.5 28.5 28.5

Molecules with an effective diameter > 10 A, e.g., (CF,),N

Used commercially for general gas drying, air plant feed purification (simultaneous removal of H,O and CQ), and liquid hydrocarbon and natural gas sweetening (HIS and mercaptan removal).

(Kovach, 1978).

of adsorbent; Figure 15.10 reports this for a molecular sieve operation.

Representation and generalization of adsorption equilibria of binary and higher mixtures by equation is desirable, but less progress has been made for such systems than for vapor-liquid or liquid-liquid equilibria. The Yon and Turnock equations (1971) applied to components 1 and 2 of binary mixtures are

(15.4) (15.5) (15.6)

They have been found useful as an empirical correlation method for adsorption on molecular sieves [Maurer, Am. Chem. SOC. Symp. Ser. l35, 73 (1980)l. Other attempts at prediction or correlation of multicomponent adsorption data are reviewed by Ruthven (1984). In general, however, multicomponent equilibria are not well correlatable in general form so that design of equipment is best based on direct laboratory data with the exact mixture and the exact adsorbent at anticipated pressure and temperature.

Adsorption processes are sensitive to temperature, as the data of Figures 15.3, 15.5, and 15.11 show. Thus practical adsorption processes are complicated by the substantial heats of adsorption

that necessarily develop. These are of the same order of magnitude as heats of condensation. Some data are in Figure 15.4

15.2. ION EXCHANGE EQUILIBRIA

Ion exchange is a chemical process that can be represented by a stoichiometric equation, for example, when ion A in solution replaces ion B in the solid phase,

A (solution) + B (solid) e A (solid) + B (solution) (15.7)

A + B / A + B, (15.8)

where the overstrike designates a component in the solid phase. The equilibrium constant is called the selectivity, designated by

or

KAB,

= [&]/[A] I-x, 1 - X A '

(15.9) (15.10)

(15.11)

The last equation relates the mol fractions of the ion originally in the solution at equilibrium in the liquid (xA) and solid ( x i ) phases.

Calcium Wash r- chloride r- water

Sodium aluminate

Sodium silicate -

Crystallization tank

Coarse

I Spenrchar I

Dust

Treatment with HCI to remove lime

I I

Washing with water

adsorbed materials

NaOH solution I I , 1 I Boilingwith Na,CO,

or NaOH solution 8

Washing with 1 dilute HCI I

Retort or kiln burning at 750 deg. F with regulated air

supply - selective oxidation of impurities

Cooling

I Screening of revivified char I

Screen

(a)

SDrav

- Hydrogel

(b) (C)

Figure 15.1. Processes for making adsorbents. (a) Flowsketch of a process for making molecular sieve adsorbents. (b) Process for reactivation of bone char. (c) Silica gel by the BASF process. The gel is formed and solidifies in air from sodium silicate and sulfuric acid, then is washed free of sodium sulfate with water (Ullmann, Encyclopedia of Chemical Technology, Verlag Chemie, Weinheim, Germany).

498

15.2. ION EXCHANGE EQUILIBRIA 499

P r e s s u r e

Figure 15.2. Types of adsorption isotherms: (I) monomolecular layer; (I1 and 111) multimolecular layers; (IV and V) multimolecular layers and condensation in pores; (VI) phase transition of a monomolecular layer on the surface (after Brunauer, Physical Adsorption, Princeton Univ. Press, 1945).

The residual mol fraction in the liquid phase corresponding to a given mol fraction or degree of saturation in the solid phase is

Approximate values of the selectivity of various ions are shown in Table 15.3; for a particular pair, K,, is the ratio of tabulated values for each.

When the exchanged ion D is divalent, the reaction is

D + 2B S D + 2B, (15.13)

Pressure, Torr

Figure 15.3. Effects of temperature, pressure, and kind of adsorbent on the amount of ethane adsorbed: (1) activated carbon at 25°C; (2) type 4A molecular sieve (MS) at 0°C; (3) type 5A MS at 25°C; (4) type 4A MS at 25°C; (5) type 4A MS at 75°C; (6) silica gel at 25°C; (7) type 4A MS at 150°C. (Data from Union Carbide Corp.)

0.02 0.0 m 1 .o 2.0

RT In (Po/P) (kcal/gmol)

Figure 15.4. Polanyi characteristic curve for effect of temperature on adsorption of n-butane on silica gel [AI-Sahhat et al., Ind. Eng. Chem. Process. Des. Dev. 20, 658 (1981)l.

and the equilibrium constant or selectivity is given by

(C/C)KDB = x;xfi/x&~~ (15.14)

= (xfi/xD)[(l - xD)/(1 - Xfi)]', (15.15)

are the total concentrations of the two kinds of ions where C and in the solution and in the solid, respectively.

Mol fraction of first-named in adsorbate

Figure 15.5. Adsorption of binary mixtures: (1) ethane + ethylene. Type 4A MS 25"C, 250 Torr; (2) ethane + ethylene. Type 4A MS, 25T, 730Torr; (3) ethane+ethylene. Type 4A MS, 75"C, 730 Torr; (4) carbon dioxide + hydrogen sulfide. Type 5A MS, 27"C, 760 Torr; (5) n-pentane + n-hexane, type 5A MS, 100°C, 760 Torr; (6) ethane + ethylene, silica gel, 25"C, 760 Torr; (7) ethane + ethylene, Columbia G carbon, 25T, 760 Torr; (8) acetylene + ethylene. Type 4A MS, 31T, 740 Torr. (Data from Union Carbide Corp.)

500 ADSORPTION AND ION EXCHANGE

5 -

I Mol fraction of first-named in liquid 0

Figure 15.6. Binary liquid adsorption equilibria on X-Y diagrams: (1) toluene+iso-octane on silica gel (Eagle and Scott, 1950); (2) toluene + iso-octane on charcoal (Eagle and Scott, 1950); (3) ethylene dichloride + benzene on boehmite (Kipling); (4) ethylene dichloride + benzene on charcoal (Kipling). (Kidina in Proceedings ~- - , \ . of the Second International Congress of Surface Activity. (19575, Vol. III, p . 462.)

Mol fraction of chloroform or benzene in liquid

Figure 15.7. Adsorption of liquid mixtures on charcoal. Chloro- form + acetone and benzene + ethanol. The ordinate gives the amount of each individual substance that is adsorbed, the abscissa the mol fraction of chloroform (mixed with acetone) or the mol fraction of benzene (mixed with ethanol). (Data gathered by Kipling. Adsorption from Solutions of Non- Electrolytes, 1965).

I 1 1 1 1

c (mmolelg anhydrous zeolite) 0 1 0 2 0 3.0 4 0 50

Figure 15.8. Variation of isosteric heat of adsorption with coverage showing the difference in trends between polar and nonpolar sorbates. nC4H,,-5A (data of Schirmer et al.); CF4-NaX, SF,-NaX (data of Barrer and Reucroft); C0,-NaX, (data of Huang and Zwiebel), NH,-SA (data of Schirmer et al.); H,O-LiX, NaX, and CsX, (data of Avgul et al.). (Ruthven, Sep Purification Methods 5(2), 189 (1976))

Example 15.1 is concerned with such an exchange and regeneration process.

15.3. ADSORPTION BEHAVIOR IN PACKED BEDS

Adsorption is performed most commonly in fixed vertical beds of porous granular adsorbents. Flow of adsorbing fluid usually is down through the bed, that of regenerant usually is upward. Moving and fluidized beds have only a limited application in the field.

If the time is sufficient, the adsorbent nearer the inlet of the fluid becomes saturated at the prevailing inlet fluid concentration but a concentration gradient develops beyond the saturation zone. Figure 5.12 depicts this behavior. The region of falling concen- tration is called the mass transfer zone (MTZ). The gradient is called the adsorption wave front and is usually S-shaped. When its leading edge reaches the exit, breakthrough is said to have been attained. Practically, the breakthrough is not regarded as necessarily at zero concentration but at some low value such as 1% or 5% of the inlet that is acceptable in the effluent. A hypothetical position, to the left of which in Figure 15.12(b) the average adsorbate content equals the saturation value, is called the stoichiometric front. The distance between this position and the exit of the bed is called the length of unused bed (LUB). The exhaustion time is attained when the effluent concentration becomes the same as that of the inlet, or some practical high percentage of it, such as 95 or 99%.

The shape of the adsorption front, the width of the MTZ, and the profile of the effluent concentration depend on the nature of the adsorption isotherm and the rate of mass transfer. Practical bed depths may be expressed as multiples of MTZ, values of 5-10 multiples being economically feasible. Systems that have linear adsorption isotherms develop constant MTZs whereas MTZs of convex ones (such as Type I of Figure 15.1) become narrower, and those of concave systems become wider as they progress through

15.3. ADSORPTION BEHAVIOR IN PACKED BEDS 501

I I

Y

Time on stream

- c

1 c al 3

al C

- L - z

8 n C

‘ 8 .-

3 3

3 3 . .

0 lime after breakthrough 0

0

Mass transfer front Stoichiometric front

(b)

Figure 15.9. Concentrations in adsorption beds as a function of position and of effluent as a function of time. (a) Progress of a stable mass transfer front through an adsorption bed and of the effluent concentration (Lukchis, 1973). (b) The mass transfer zone (MTZ), the length of unused bed (LUB), stoichiometric front, and profile of effluent concentration after breakthrough.

the bed. The last types are called unfavorable isotherms; separations in such cases usually are accomplished more economically by some other kind of process. The narrower the MTZ, the greater the degree of utilization of the bed.

The rate of mass transfer from fluid to solid in a bed of porous granular adsorbent is made up of several factors in series:

1. Diffusion to the external surface. 2. Deposition on the surface. 3. Diffusion in the pores. 4. Diffusion along the surface.

Various combination of shapes of isotherms and mass transfer factors have been taken into account by solutions of the problem in the literature. One of the simpler cases was adopted by Hougen and Marshall (1947, see Figure 15.13), who took a linear isotherm and diffusion to the external surface as controlling the rate. They developed the solution in analytical form, of which several approximations that are easier to use are mentioned for instance by Vermeulen et al. (1984, p. 16.28). A graphical form of the solution appears in Figure 15.13. This shows the effluent concentration ratio,

C/C, , in terms of a time parameter T, at a number of values of a parameter Z‘, which involves the bed length Z. In Example 15.2, this chart is used to find the concentration profile of the effluent, the break and exhaustion times, and the % utilization of the adsor- bent bed. In this case, the model affords a fair comparison with experimental data.

Many investigations have been conducted of the mass transfer coefficient at the external surfaces of particles and of other diffusional mechanisms. Some of the correlations are discussed in Chapters 13 and 17. A model developed by Rosen [Znd. Eng. Chem. 46, 1590 (1954)l takes into account both external film and pore diffusional resistance to mass transfer together with a linear isotherm. A numerical example is worked out by Hines and Maddox (1985, p. 485).

In the model developed by Thomas [ J . Am. Chem. SOC. 66, 1664 (1944)], the controlling mechanism is the surface kinetics represented by the Langmuir isotherm. Extensions of this work by Vermeulen et al. (1984) incorporate external surface and pore diffusional resistances.

No comprehensive comparisons of the several models with each other and with experimental data appear to have been published.

502 ADSORPTION AND ION EXCHANGE

8 3 3 .

Distance (1) At end of adsorption.

r

Distance (2) At end of regeneration.

Distance (3) Useful capacity of cycle.

Figure 15.10. Incomplete regeneration of adsorbent bed by a thermal-swing cycle.

Temperature, "C

Figure 15.11. Effect of temperature on molecular sieve type SA, silica gel and activated alumina at water vapor pressure, 13.3 kPa (100 mm Hg). A, molecular sieve type SA; B, silica-type adsorbent; C, alumina-type adsorbent. (Chi nnd Cummings, 1978).

TABLE 15.3. Gas Phase Adsorption Cycles, Steam Requirements, and Operating Parameters

(a) Typical Cycle Times in Hours for Adsorber Operation

High Pressure Organic Solvent Gas Dryer Recovery Unit

~~ ~~~ ~~ ~~~

A B A B Onstrearn ................. 24 24 2.00 1.00 Depressure/purge.. ....... 2 1 . . . . . . . . . . Hotgas .................... 10 13 .......... Steam.. ...................... ... 0.75 0.67 Hot gas ....................... ... 0.33 ..... Cold gas ................... 5 8 0.42 0.33 Pressure/standby.. ....... 7 2 0.50 .....

24 24 2.00 1.00 - - - -

(Fair, 1969).

(b) Steam/Solvent Ratios and Amount of Adsorbate for a Coconut-Shell Carbon 6-12 Mesh, 1200 m2/g

24

22

20

18

16 s z l4 oi tij 12 2 2 10 2

8

6

4

2

0 I 2 3 4 5 0 Pounds of steam per pound of solvent

(Kovach, 1979).

(c) Typical Operating Parameters for Gas Phase Adsorption

Range Design ~~ ~~~ ~~

Superficial gas velocity 20 to 50 c d s 40 c d s (80 Wmin) (40 to 100 fUmin)

Adsorbent bed depth 3 to 10 MTZ 5 MTZ Adsorption time 0.5 to 8 h 4h Temperature -200 to 50°C Inlet concentration

Adsorption base LEL base 40%

100 to 5OOO vppm

0.5 to 10 mm 4 t o 8 mm Adsorbent particle size Working charge 5 to 20% wt 10% Steam solvent ratio 2:l to 8:l 4: 1 Adsorbent void volume 38 to 50% 45% Steam regeneration temperature Inert gas regenerant termperature Regeneration time 'h adsorption time 1 Number of adsorbers 1 to 6 2 to 3

(Kovach, 1979).

105 to 110°C 100 to 300°C

15.3. ADSORPTION BEHAVIOR IN PACKED BEDS 503

EXAMPLE 15.1 Application of Ion Exchange Selectivity Data

The SO; ion of an aqueous solution containing C-= 0.018 eq/L is to be replaced with C1- ion from a resin with C = 1.2eq/L. The reaction is

SO;(solution) + 2Cl-(resin) Ft SOb(resin) + 2Cl-(solution), D + 2B FtD + 2B.

From Table 15.3, the selectivity ratio K,, = 0.15/1.0 = 0.15, and

KDBC/C = 0.15(1.2)/0.018 = 10.

Then Eq. (15.15) becomes

XD/(l - X 6 ) 2 = lOX,/(l - XD)2.

For several values of mol fraction corresponding mol fractions x, in tabulated:

XSOZ

x, of SO; in solution, the the resin are calculated and

In Solution In Resin

1 1 0.1 0.418 0.05 0.284 0.01 0.0853

For regeneration of the resin, a 12% solution of NaCl will be used;

its ion concentration is 2.23eq/L. Other values for the system remain at C = 1.2 eq/L and K,, = 0.15. Accordingly,

KDBC/C = 0.15(1.2)/2.23 = 0.0807

and Eq. (15.15) becomes

~ 6 / ( 1 - ~6)' = 0.0807~,/(1- x ~ ) ' .

The values of x,,? in the liquid phase will be calculated for several values in the resin. Those results will be used to find the minimum amount of regenerant solution needed for each degree of regeneration

xso?j L regenerant/

In Resin In Solution L resin

0.1 0.455 1.06 0.05 0.319 1.60 0.01 0.102 5.22

Sample calculation for the last entry of the table: The equivalents of SOT transferred from the resin to the solution are

0.99(1.2) = 1.188 eq/L.

The minimum amount of solution needed for this regeneration is

""' = 5.22L solution/Liter. 0.102(2.23)

2 ethoxyethyl acetate

0 -

Proponone

0 5 10 15 20 25 L, in

(a)

Desorption time, min

(b)

Figure 15.12. Multicomponent mixtures, adsorption, and desorption. (a) Concentrations of the components of a ternary mixture in continuous adsorption, as in a moving bed unit (Kovach, 1979). (b) Composition of a desorbed stream consisting of several components as a function of time.

504 ADSORPTION AND ION EXCHANGE

0.999 0.898 0.995 0.99 0.98

0.95

0.9

0.8 0.7 0.6 0.5

0 0.3 2 0.4

0.2

0.1

0.05

0.02 0.01

0.005 o.m2 0.001

Figure 15.13. Dependence of the concentration ratio, C / C , of the effluent from an adsorber on parameters of bed length and time; for the case of a linear isotherm, zero initial adsorbate content and constant inlet composition C,

2’ = ( K p / c u i ) Z ,

K D = q/C,

bed length parameter, z = ( K p / K D p b ) ( t - Z/u,),

ui = interstitial velocity in the bed, E = voidage of the bed, 2 = length of the bed,

time parameter, coefficient of linear adsorption isotherm,

Kp = mass transfer coefficient, (L3 fluid)/(L3 bed)(time).

(Hougen and Watson, Chemical Process Principles, Wiley, New York, 1947, p . 1086; Hougen and Marshall, Chem. Eng. Prog. 43, 197 (1947); Vermeulen et al., Chemical Engineers’ Handbook, McGraw-Hill, New York, 1984, p . 16.29.)

Moreover, they are all based on isothermal behavior and approximations of adsorption isotherms and have not been applied to multicomponent mixtures. The greatest value of these calculation methods may lie in the prediction of effects of changes in basic data such as flow rates and slopes of adsorption isotherms after experimental data have been measured of breakthroughs and effluent concentration profiles. In a multicomponent system, each substance has a different breakthrough which is affected by the presence of the other substances. Experimental curves such as those of Figure 15.14 must be the basis for sizing an adsorber.

Since taking samples of adsorbent from various positions in the bed for analysis is difficult, it is usual to deduce the shape of the adsorption front and the width of the MTZ from the effluent concentration profile which may be monitored with a continuous analyzer-recorder or by sampling. The overall width of the MTZ, for instance, is given in terms of the exhaustion and breakthrough times and the superficial velocity as

width = us(& - tb)/&.

REGENERATION

Adsorbents are restored to essentially their original condition for reuse by desorption. Many hundreds of cycles usually are feasible, but eventually some degradation occurs, as in Figure 15.15 for instance, and the adsorbent must be discarded.

The most common method of regeneration is by purging the bed with a hot gas. Operating temperatures are characteristic of the adsorbent; suitable values at atmospheric pressure are shown in Table 15.1. The exit temperature of the gas usually is about 50°F higher than that of the end of the bed. Typical cycle times for adsorption and regeneration and steam/adsorbent ratios are given in Table 15.4. Effluent composition traces of a multicomponent system are in Figure 15.9. Complete removal of adsorbate is not always economically feasible, as suggested by Table 15.4(b). The effect of incomplete removal on capacity is shown schematically by Figure 15.10. Sufficient heat must be supplied to warm up the adsorbent and the vessel, to provide heat of desorption and enthalpy absorption of the adsorbate, and to provide for heat losses to the surroundings. Table 15.4(c) suggests that regeneration times be about one-half the adsorption times. For large vessels, it may be worthwhile to make the unsteady heating calculation by the general methods applicable to regenerators, as presented, for instance, by Hausen (Heat Transfer in Counterjlow, Parallel Flow and Cross- flow, McGraw-Hill, New York, 1983).

Purging of the adsorbate with an inert gas at much reduced pressure is feasible in high pressure adsorption plants. The adsorption of Example 15.2, for instance, is conducted at 55 atm, so that regeneration could be accomplished at a pressure of only a few atmospheres without heating. If the adsorbate is valuable, some provision must be made for recovering it from the desorbing gas.

Ignition of adsorbents in external furnaces is practiced to remove some high molecular weight materials that are difficult to volatilize. This is done, for example, for reactivation of carbon from water treating for trace removal of impurities such as phenol. Caustic solution can convert the phenol into soluble sodium phenate in readily disposable concentrated form as an alternate process for regeneration.

Displacement of the adsorbate with another substance that is in turn displaced in process is practiced, for instance, in liquid phase recovery of paraxylene from other C, aromatics. In the Sorbex process, suitable desorbents are toluene and paradiethylbenzene. This process is described later.

15.4. ADSORPTION DESIGN AND OPERATING PRACTICES

When continuous operation is necessary, at least two adsorbers are employed, one on adsorption and the other alternately on regeneration and cooling. In cases where breakthrough is especially harmful, three vessels are used, one being regenerated, the other two onstream with the more recently regenerated vessel down- stream, as in Figure 15.16.

Beds usually are vertical; adsorbers 45 ft high and 8-10 ft dia are in use. When pressure drop must be minimized, as in the recovery of solvents from atmospheric air, horizontal vessels with shallow beds are in common use. Process gas flow most often is downward and regenerant gas flow is upward to take advantage of counterflow effects. Upflow rates are at most about one-half the fluidizing velocity of the particles. Vertical and horizontal types are represented on Figure 15.17.

A major feature of adsorber design is the support for the granular adsorbent, preferably one with a low pressure drop. The combination of Figure 15.18(a) of grid, screens, and support beams is inexpensive to fabricate and maintain, has a low heat capacity and a low pressure drop. The construction of Figure 15.18(b) is suited to adsorbers that must be dumped frequently. Supports of layers of ceramic balls or gravel or anthracite, resting on the bottom of the vessel, are suited to large vessels and when corrosion-resistant construction is required. Typical arrangements are shown in Figures 17.26 and 17.27. The successive layers increase in diameter by factors of 2-4 up to l in . or so. Holddown balls also may be

15.4. ADSORPTION DESIGN AND OPERATING PRACTICES 505

EXAMPLE 15.2 Adsorption of n-Hexane from a Natural Gas with STca Gel

Hexane is'to be recovered from a natural gas with silica gel. Molecular weight of the gas is 17.85, the pressure is 55.4atm, temperature is 94"F, and the content of n-hexane is 0.853 mol % or 0.0982 Ib/cuft. The bed is 43 in. deep and the superficial velocity is 11.4ft/min. Other data are shown with the sketch:

0.0982 Ibkuft I

3.58 f l

ft/rnin

Z = 3.58 ft, bed depth, us = 11.4 ft/min, superficial velocity, 0, = 0.01 ft, particle diameter,

a = 284 sqft/cuft, packing external surface, pb = 52 lb/cuft, bed density,

E = 0.35 bed voidage.

From these and physical property data, the Schmidt and Reynolds numbers are calculated as

Sc = 1.87, Re = 644.

The equation of Dwivedi and Upadhyay, Eq. (13.148), is applicable:

lL4 0.35( 1. 87y3

(0.0038 + 0.0301) = 0.7268 ft/min, :. kg =

k,a = 0.7268(284) = 206.4cuft gas/(cuft solid)(min).

Saturation content of adsorbate is 0.17 Ib/lb solid. Accordingly, the coefficient of the linear adsorption isotherm is

0.17 Ib hexane/lb solid - 0.0982 - 1'731 lb hexane/cuft gas'

k

Use the Hougen-Marshall chart (Fig. 15.13):

kdp, z 1.731(52) 3.58 t = - t + - = - t+-

kga U, /E 206.4 11.4/0.35 = 0.4362 + 0.11 min.

Values of t are read off Figure 15.13 and converted into values of t:

Cf c, 0.01 0.05 0.1 0.2 0.4 0.6 0.8 0.9 0.95 0.99

The total amount adsorbed sqft of bed cross section is

c t(min)

40 17.56 45 19.74 50 21.92 53 23.23 60 26.28 65 28.46 73 31.95 79 34.57 82 35.87 92 40.24

to the breakpoint, at C/CO = 0.01, per

0.0982(11.4)(17.56) = 19.66 Ib/sqft cross section.

The saturation amount for the whole bed is

3.58(0.17)(52) = 31.65 lb/sqft cross section.

Accordingly,

utilization of bed = (19.66/31.65)(100%) = 62.1%.

The calculated concentration profile is compared in the figure with experimental data, Run 117, of McLeod and Campbell, SOC. Pet. Eng. J . , 166 (June 1966):

1 1 1 I I I ,. Y 1 -

- I

-

0.8 -

-

0.6 -

t - 0" . 0 0.4 -

-

0.2 - -

0 I I I I I I I 10 20 30 40 50

t, min -

506 ADSORPTION AND ION EXCHANGE

n-Pontono concontrot ion in food

-Hrxono concontrotion in food

n-Pentono LUB 0.75

tone rtoichiomotric front

0.50 n-tbxono rtoichiomotric

0.25 -Pentane t n-hoxono odrorpvien section adsorption soction

0 20 25 30 35 40 45 50

Adsorption l i m e , Minutor

F w e 15.14. Breakthrough curves in the adsorption of a mixture of hydrocarbons with composition n-butane 0.4 mol %, n-pentane 25.9, n-hexane 23.9 is0 and cyclic hydrocarbons 49.8 mol % (Lee, in Recent Advances in Separation Science, CRC Press, Boca Raton, FL, 1972, Vol. II, pp. 75-110).

provided at the top to prevent disturbance of the top layer of adsorbent by incoming high velocity gas or entrainment by upflowing gases. When regeneration is by heating, a drawback of the ball support arrangement is their substantial heat capacity, which slows up the heating rate and subsequent cooling to process temperature.

Representative values and ranges of operating parameters are summarized in Table 15.3. Cycle times for some adsorptions are adjusted to work shift length, usually multiples of 8 hr, with valve adjustments made by hand. When cycle times are short, as for solvent recovery, automatic opening and closing of valves is necessary.

I I I I I I 0 50 100 1 50 200 250

Cycle¶ I

Figure 15.15. Capacity decline with service of a molecular sieve (plant data, Davbon Sieve 562). Flow, 8150 kg mol; pressure, 3600 kPa (36 atm); temperature, 15°C; water content, 96 kg/hr; minimum cycle time, 24 hr. (Chi and Cummings, 1978).

Steam rates for regeneration of a particular adsorbent carbon are shown in Table 15.3(b). Steam/solvent ratios as high as 8 sometimes are necessary.

Data for liquid phase adsorption are typified by water treating for removal of small but harmful amounts of impurities. Some conditions are stated by Bernardin [Chern. Eng., (18 Oct. 1976)l. Water flow rates are 5-l0gpm/sqft. When suspended solids are present, the accumulation on the top of the bed is backwashed at 15-20 gpm/sqft for 10-20 min/day. The adsorbent usually is not regenerated in place but is removed and treated in a furnace. Accordingly, a continuous operation is desirable, and one is simulated by periodic removal of spent adsorbent from the bottom of the vessel with a design like that of Figure 15.18(b) and replenishing of fresh adsorbent at the top. The pulses of spent and fresh carbon are 2-10% of the total bed. Height to diameter ratio in such units is about 3.

A carbon adsorber for handling 100,OOO gal/day of water consists of two vessels in series, each 10 ft dia by 11 ft sidewall and containing 20,OOOIb of activated carbon. Total organic carbon is reduced from 650 mg/L to 25 mg/L, and phenol from 130 mg/L to less than 0.1 mg/L.

The capacity of regeneration furnaces is selected so that they operate 8O-W% of the time. In multiple-hearth furnaces the loading is 70-80 Ib/(sqft)(day). In countercurrent direct fired rotary kilns, a 6% volumetric loading is used with 45min at activation temperature.

Details of the design and performance of other liquid phase adsorptions such as the Sorbex processes are proprietary.

15.5. ION EXCHANGE DESIGN AND OPERATING PRACTICES

Ion exchange processes function by replacing undesirable ions of a liquid with ions such as H+ or OH- from a solid material in which the ions are sufficiently mobile, usually some synthetic resin. Eventually the resin becomes exhausted and may be regenerated by contact with a small amount of solution with a high content of the desired ion. Resins can be tailored to have selective affinities for

TABLE 15.4. Properties of Ion-Exchange Materials (a) Physical Properties

I- 8 NO; 4 Br- 3

Material

HCO; 0.4 CH,COO- 0.2 F- 0.1

Crtion cachmgcn: strongly acidic

Homogencow (gel) resin 4% nos-linked 6% cross-linked aim cmr-linked 12% cross-linked 16% nos-linked eCm cross-linked

Mocmpaour structure 10-12% cross-linked

Sulfauted phenolic rnln Sulfauted coal

Pdys~ymm sulfonate

Cation eichan en weakly acidic

Homogeneous (gel) resin Acrylic ( p u g ) or methacrylic (pK 6)

phcndic resin Polystyrene phosphormte Polystyrene aminodiacetate Polystyrene amidoaime P d yrene thid

M.CropoW

C d L S C PhmptKmate Methylene carboxylate

Crmtrasnd (Fe silicate) Zeolite (AI silicate) Zirconium tungstate

Anion eachangm: strongly basic P d yst y r r n c h x d

Trimethyl benzyl ammonium (type 1) Homogeneous, 8% CL

;our. 11% CL D E h T y d r o a yeth yl ammonium (type 11)

Homogemour. 8% CL Macroporous. 10% CL

Acrylic-bued Hnnoarneour (gel) Mac+ -

C ~ l l U b b P u d Ethyl trimethyl ammonium Triethyl hydroaypropyl ammonium

Anion eachnngen: intermediite~y tasic (pK 11)

Pol ystyrme-based Epky-pol yamine

Anion eachangen: weakly basic (pK 9) Aminopdystyrene

Hanoge- (gel) Macrocarour

Arrylic-b;wd amine Hmagcnau (gel) M.cro~+our ~

Crllulou-bued Aminoethyl Diethyl amimthy l

shr' particles

S

S G G

S S C c. s S S S

F F, P. C G c G

S S

S S

S S

F

S S

S S

S S

P P

'Shapes: C. cylindrical pellets; G. g ram P. powder, S. I When two temperatures are shown. th st applies to H NOTE To convert kilograms per liter to pounds per cubic

Bulk wet density

(drained). k/L

0 75-0 85 0 7&0 86 0 77-0 87

0 79-0 89 0 80-0 90

o 78-0 as

0 81 0 74-0 85

0 70-0 75 0 67-0 74 0 70-0 80

0 74 0 75

-0 75 -0 75

1 3 0 85-0 95 115-1 25

0 70 0 67

071 0 67

0 72 0 67

0 75 0 72

0 67 0 61

0 72 0 72

Moisture content

(d?P). wei&

64-70 58-65 48-60 44-48 42-46 40-45

50-55 50-60

45-50 50-55 -50

50-70 68-75

58 45-50

1-5 40-45 -5

46-50 57-60

-42 -55 - 70 -60

- 50 -64

- 45 55-60

-63 -68

Swelling due to

eachange. 5%

10-12 8-10 6-8 5 4 3

4-6 7

20-80 10-100 10-25 (40

(100 10

0 0 0

-20 15-20

15-20 12-15

-15 -12

15-25 8-10

8-12 -25

8-10 12-15

Maaimum operating

temperature. f OC

120-190

120-150 50-90

120 120

6-65 120 75 50 60

60 60

> I50

60-80 60-80

40-80 40-0-80

40-80 40-80

I 0 0 io0

65 75

100 100

80 60

icres Irm for cation or OH form for anion. eachanger. the m n d . to salt ion d. multiply b; 6238 X I O t , O F = X "C + 32

Operating pH range

0-14

0-14 0-14

4-14

0-14 3-14 3-14 1-11 1-13

6-8 6-8 2-10

0-14 0-14

0-14 0-14

0-14 0-14

4-10 4-10

0-10 0-7

0-7 0-9

0-7 0-9

Eachange capcity

Dry. equinknt /kg

50-55

46-52 4 4-4 I3 4 2-4 6 30-42

4 5 5 0 2 0 - 2 5

4 a 5 4

8 3-10 -8 0

2 5 6 6 2 9 2 8

-5

-7 0 -0 7

0 14 1 4 1 2

3 4-3 8 3 4

3 8-4 0 3 8

-5 0 30-33

0 62 0 57

4 8 6 5

5 5 4 9

6 5 5 0

10 -0 9

(Chemical Engineers' Handbook, McGraw-Hill, New York, 1984; a larger table complete with trade names is in the 5th edition, 1973)

(b) Selectivity Scale for Cations on 8% Crosslinked Resin

Li+ H+ Na+ NH: K+ Rb+ cs+ Ag+ UOf+ MgZ+

1 .o 1.3 2.0 2.6 2.9

3.3 II

ZnZ+ coz+ cuz+ Cd2+ BeZ+ Mn2+ Niz+ c a z + s1z+ Pb2+ Baz+

3.5 3.7 3.8 3.9 4.0 4.1 3.9 5.2 6.5 9.9 11.5

wet. equivaknt/L

I 2-1 6 13-1 8 14-1 9 15-20 17-2 1 1 8-20

15-19 0 7-0 9

33-40 2 5-3 5 10-14 3 0 0 7

2 0 o a o 9

0 18 0 75 IO

13-1 5 IO

1 2 I 1

10-12 0 8-0 9

18 17

1 8 1 2

17 I 1

HSOT NO; CN-

OH-(Type I)

SO:- ::: / / 1.3

CI- c0:- BrO; k: 1 1 HP0:- OH-(Type 11) 0.65

0.05-0.07

0.15 0.03 0.01

(Bonner and Smith, J. Phys. Chem. 61,1957, p. 326). (Bonner and Smith, J. Phys. Chem. 61,1957, p. 326).

507

508 ADSORPTION AND ION EXCHANGE

Feed in;^ = 10 psi

~

Process gas

r--- --- 7

450

Knockout drum

Water

FI

1 oil L

I I 3 vessels, each 14 f t dia by 12.5

f t TT and 100 I cuft of dessicant I on an 8 hour I cycle.

---J

Dry gas, -90 F dewpoint 450 Ibmol/hr

20 atm

Figure 15.16. A three-vessel drying system for a cracked light hydrocarbon stream. Valve operation usually is on automatic timer control. Recycled process gas serves as regenerant.

particular kinds of ions, for instance, mercury, boron, ferrous iron, or copper in the presence of iron. Physical properties of some commercial ion exchange resins are listed in Table 15.3 together with their ion exchange capacities. The most commonly used sizes are -20+50 mesh (0.8-0.3mm) and -40+80 mesh (0.4- 0.18 mm).

Rates of ion exchange processes are affected by diffusional resistances of ions into and out of the solid particles as well as resistance to external surface diffusion. The particles are not really solid since their volume expands by 50% or more by imbibition of water. For monovalent exchanges in strongly ionized resins, half times with intraparticle diffusion controlling are measured in seconds or minutes. For film diffusion, half times range from a few minutes with 0.1N solutions up to several hours with 0.001N solutions. Film diffusion rates also vary inversely with particle diameter. A rough rule is that film diffusion is the controlling mechanism when concentrations are below 0.1-l.ON, which is the situation in many commercial instances. Then the design methods can be same as for conventional adsorbers.

Ion exchange materials have equilibrium exchange capacities of about 5 meq/g or 2.27 g eq/lb. The percentage of equilibrium exchange that can be achieved practically depends on contact time, the concentration of the solution, and the selectivity or equilibrium constant of the particular system. The latter factor is discussed in Section 15.2 with a numerical example.

Commercial columns range up to 6 m dia and bed heights from 1 to 6 m, most commonly 1-3 m. Freeboard of 50-100% is provided to accommodate bed expansion when regenerant flow is upward. The liquid must be distributed and withdrawn uniformly over the cross section. Perforated spiders like those of Figure 15.19 are suitable. The usual support for the bed of resin is a bed of gravel or layers of ceramic balls of graded sizes as in Figure 17.27. Balls sometimes are placed on top of the bed to aid in distribution or to prevent disturbance of the top level. Since the specific volume of the material can change 50% or more as a result of water absorption and ion-ion exchange, the distributor must be located well above the initial charge level of fresh resin.

Liquid flow rates may range from 1 to 12 gpm/sqft, commonly 6-8gpm/sqft. When the concentration of the exchange ion is less than 50meq/L, flow rates are in the range of 15-80 bed volumes (BV)/hr. For demineralizing water with low mineral content, rates as high as 400 BV/hr are used. Regenerant flow rates are kept low, in the range of 0.5-5.0BV/hr, in order to allow attainment of equilibrium with minimum amounts of solution.

The ranges of possible operating conditions that have been stated are very broad, and averages cannot be depended upon. If the proposed process is similar to known commercial technology, a new design can be made with confidence. Otherwise laboratory work must be performed. Experts claim that tests on columns 2.5 cm dia and 1 m bed depth can be scaled up safely to commercial diameters. The laboratory work preferably is done with the same bed depth as in the commercial unit, but since the active exchange zone occupies only a small part of a normal column height, the exchange capacity will be roughly proportional to the bed height, and tests with columns 1 m high can be dependably scaled up. The laboratory work will establish process flow rates, regenerant quantities and flow rates, rinsing operations, and even deterioration of performance with repeated cycles.

Operating cycles for liquid contacting processes such as ion exchange are somewhat more complex than those for gas adsorp- tion. They consist of these steps:

1. Process stream flow for a proper period. 2. A rinse for recovering possibly valuable occluded process

solution. 3. A backwash to remove accumulated foreign solids from the top

of the bed and possibly to reclassify the particle size distribution. 4. The flow of regenerant for a proper period. 5. Rinse to remove occluded regenerant.

As complex a cyclic process as this may demand cycle times of more than a few hours. Very high ion concentrations or high volumetric rates may require batteries of vessels and automatic switching of the several streams, or continuously operating equip- ment. Several continuous ion exchange plants are being operated successfully. The equipment of Figure 15.20 employs pulsed transfer of solid between exchange and regenerant zones as often as every 4min to every 20 or 30min. Attrition of the resin may require replacement of as much as 30% of the resin each year in water conditioning applications.

Fluidized bed units such as the multistage unit of Figure 15.20 suffer from some loss of efficiency because the intense mixing eliminates axial concentration gradients. They do have the merit, however, of not being bothered by the presence of foreign solid particles.

The economic break between fixed bed and continuous operation has been estimated as ion concentrations of 0.5N, or flow rates above 300gpm, or when three or more parallel beds are required to maintain continuous operation. The original application of continuous ion exchange was to treatment of radioactive wastes, but some installations of ordinary water treating have been made.

Resin requirements for two extremes of ion concentration are analyzed in Example 15.3. The high concentration stream clearly is a candidate for continuous ion exchange.

ELECTRODIALYSIS

In this process, dissolved electrolytes are removed by application of electromotive force across a battery of semipermeable membranes constructed from cation and anion exchange resins. The cation membrane passes only cations and the anion membrane only anions. The two kinds of membranes are stacked alternately and separated about 1 mm by sheets of plastic mesh that are still provided with flow passages. When the membranes and spacers are compressed together, holes in the corners form appropriate conduits for inflow and outflow. Membranes are 0.15-0.6 mm thick. A commercial stack may contain several hundred compartments or pairs of membranes in parallel. A schematic of a stack assembly is

Spent regeneration

"T

Regeneration

-Product

Hinged head (a)

Gas out A

(b) Figure 15.17. Two designs of fixed bed gas adsorbers. (a) Vertical bed with balls on top for hold-down and distribution of feed (Johnson, Chem. Eng. 79, 87 (27 Now 1972)]. (b) Horizontal fixed bed for low pressure drop operation [Treybal, Mass Transfer Operations, McGraw-Hill, New York, 1980; Logan, U.S. Pat. 2,180,712 (1939)l.

509

510 ADSORPTION AND ION EXCHANGE

Feed, ’ \- , Screw clamps ,/‘,Segmental

1‘ 1’ ’Compressible

:’2--4-mesh screen

/ /’ clamp

.’ ,/ /’ gasket

t Product

Feed,

I

Detail

(a)

‘- Hinged head Detail

(b) Figure 15.18. Two types of supports for adsorbent beds [Johnston, Chem. Eng., (27Nou. 1972)]. See also Figures 17.23 and 17.24. (a) Common type of flat screen support. (b) Conical-type of support suited to frequent removal of adsorbent.

in Figure 15.21. Properties of commercially produced membranes are in Table 15.5 and performance data are in Table 15.6.

Membranes may be manufactured by mixing powdered ion exchange resin with a solution of binder polymer and pouring the heated mixture under pressure onto a plastic mesh or cloth. The concentration of the ion exchanger is normally 50-70%. They are chiefly copolymers of styrene and divinylbenzene, sulfonated with sulfuric acid for introduction of the cation exchange group.

Standard cell sizes are up to 30 by 45in. In an individual stack the compartments are in parallel, but several stacks in series are employed to achieve a high degree of ion exchange. The ion exchange membrane is not depleted and does not need regeneration. The mechanism is that an entering cation under the influence of an emf replaces an H+ ion from the resin and H+ from solution on the opposite face of membrane replaces the migrating cation.

Table 15.6 shows that pressures drops may be as high as 9OOpsi. Flow rates in a single stage are about 1 gal/(hr)(sqft of available membrane surface). The process is distinguished by very low power requirements: the desalination of sea water, for instance, consumes 11-12 kWh/1000 gal. One stage effects a reduction of about 50% in salt content, so several stages in series are used for high performance. A flow sketch of a three-stage electrodialysis plant is in Figure 15.21(c).

Like many other specialities, electrodialysis plants are purchased as complete packages from a few available suppliers. Membrane replacement is about 10% per year. Even with prefiltering the feed, cleaning of membranes may be required at intervals of a few months. The comparative economics of electrodialysis for desalting brackish waters is discussed by Belfort (1984): for lower salinities, electrodialysis and reverse osmosis are competitive, but for higher ones electrodialysis is inferior. Electrodialysis has a number of important unique applications, for removal of high contents of minerals from foods and pharmaceuti- cals, for recovery of radioactive and other substances from dilute solutions, in electro-oxidation reduction processes and others.

15.6. PRODUCTION SCALE CHROMATOGRAPHY

When a mixture of two substances is charged to a chromatographic column, one of them may be held more strongly than the other. Elution with an inert fluid will remove the more lightly held substance first, then the other. Separations even between very similar substances can be very sharp. Figure 15.22(a) is an example of a chromatogram. Only fluid-solid chromatography is an adsorptive process, but gas-liquid and liquid-liquid are used more frequently since liquids with suitable absorption properties are easier to find than solid adsorbents. The active sorbent is a high-boiling solvent deposited on a finely divided inert solid carrier. The process is one of absorption, but the behavior is much like that of adsorption. The principal application is to chemical analysis. Relative retention times on various sorbents are key data which are extensively tabulated, for instance in Meites (Handbook of Analytical Chemistry, McGraw-Hill, New York, 1963).

Chromatographic separations are necessarily intermittent with alternate injections and elutions, although a measure of continuity can be achieved with an assembly of several units, or with suitably sized surge tanks. A process flowsketch appears in Figure 15.22(b). Information on production scale chromatography is provided by Conder (1973). Only separations difficult to achieve by other means are economical with chromatography.

Individual drums are provided for each product fraction. A detector monitors the separation and provides signals for controlling the injection and collection sequence. The operation of partial condensers for the dilute eluted streams presents challenges because of aerosol formations. When a valuable carrier such as nitrogen is used, it must be cleaned up and recycled.

A 1968 estimate of the cost breakdown for a plant with a column 4 ft dia by 15 ft high and a throughput of 200-920 tons/yr has been converted to a percentage basis in Table 15.7 because of its age. The costs are said to not vary greatly with throughput or the nature of the separation, although this analysis has been made specifically for the separation of CY- and /3-pinenes. The temperature was 165°C and the solvent was Carbowax 20M. The design was based on data in a 4in. dia column which had a capacity of 200-1500 mL/hr.

Some of the materials for which chromatographic separation should be considered are essential oils, terpenoids, steroids, alka- loids, pharmaceuticals, metal chelates, isotopes, and close-boiling isomers. For easy separations, vacuum distillation, liquid-liquid extraction, and fractional crystallization are less expensive.

15.7. EQUIPMENT AND PROCESSES

Adsorbents are made in pellet form by extrusion or pressing or in granular form by crushing and classification of larger masses or in spherical or globular form by precipitation in an inert gaseous or liquid medium. Typical processes for some adsorbent preparations are represented in Figure 15.1. The BASF process of Figure 15.l(c)