Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 361-365
Literature Cited Cruver, R. E. Trans. ASME, Ser. B: J . Eng. Ind. 1974, Paper No. 74ENAS-41. Eykamp, W. Paper presented at the 79th National Meeting, AIChE, Houston, Texas, March 1975. Gildert, 5. R.; Matsuura, T.; Sourlrajan, S. J. Appl. Polym. Sci. 1979, 24, 305
Goldsmith, R. L. Trans. ASME, Ser. 8 : J . Eng. Ind. 1974, Paper No. 74ENAS-26. Goldsmith, R. L.; de Filippi, R. P. I n “Membranes In Separation Processes-A Workshop Symposium“, Case Western Reserve University, Cleveland, Ohlo. 1973. Goldsmith. R. L.: Roberts, D. A.; Burre. D. L. J. Water Pollut. Control Fed. 1974, 46, 2183. Goilan, A.; Grant, D.; Goldsmith, R. L. ASME Reprint No. 75-ENAS-57, 1975. Hockenberry, H. R.; Lieser, J. E. ASLE, Preprint No. 76-AM-2B-2, 1976. Kunst, B.; Sourirajan, S. J. Appl. Polym. Sci. 1974, 18, 3423. Kutowy, 0.; Sourlrajan, S. J. Appl. Polym. Sci. 1975, 19, 1449. Kutowy, 0.;Thayer, W. L.; Sourirajan, S. Desalination 1978, 26, 195. Kutowy, 0.; Thayer, W. L.; Capes, C. E.; Sourirajan, S. J. Sepn. Process Techno/. 1980, 1(3), 28. Markind, J.; Minard, P.; Neri, J.; Stone, R. AIChE Symp. Ser. No. 144 1974, 70, 157. Matsuura, T.; Baxter, A. G.; Sourirajan, S. Ind. Eng. Chem. Process Des. Dev. 1977, 16, 82. Matsuura, T.; Biais, P.; Sourirajan, S. J. Appl. Polym. Sci. 1976, 20, 1515. Matsuura. T.; Sourirajan, S. J. Appl. Polym. Sci. 1972, 16, 1663.
361
Matsuura, T.; SourIraJan, S. J. Appl. Polym. Scl. 1973, 77, 3683. Matsuura, T.; Sourirajan, S. J. Collokj Interface Sclence 1978r, 66, 589. Matsuura, T.; Sourlrajan, S. Ind. Eng. Chem. Process Des. Dev. 1978b, 17, 419. Milstead, C. E.; Loos, J. F. U.S. Nat. Tech. Infor. Serv., AD Rep& No. 758321, 1972. Nordstrom, R. P., Jr. Pollut. Eng. Oct 1974, 46. Priest, W. Wlre Ind. Feb 1978, 106. Sourirajan, S. Pure Appl. Chem. 1978. 50, 593. Sourlrajan, S.; Kunst, B. I n “Reverse Osmosis and Synthetlc Membranes”, Sourlrajan, S., Ed.; Natlonal Research Council Canada: Ottawa, 1977; Chapter 7. Sourlrajan, S.; Matsuura, T. “Proceedlngs of the Symposium on Textlle Industry Technology, U.S. Environmental Protection Agency, EPA-600/2-79104, May 1979, pp 73-106. Sourirajan, S.; Matsuura, T.; Hsieh, F. H., Gildert, G. R. Paper presented at the ACS Symposium on Ultrafiltration Membranes and Applications, Washington, D.C., Sept 1979. Thayer, W. L.; Pageau, L.; Sourirajan, S. Desallnatlon 1977, 21, 209. Tweddle, T. A.; Sourirajan, S. J. Appl. Polym. Sci. 1978, 22, 2265. Young, W. S., Jr. Prod. Finlsh. 1974, 39(1), 62.
Received for review June 10,1980 Accepted December 1, 1980
Issued as N.R.C. No. 18828.
Properties of the Amphoteric, Thermally Regenerable Ion-Exchange Resins Amberlite XD-2 and Amberlite XD-5 Tah-Ben Hsu and Robert L. Plgford’ Depaflment of Chemical Engineering, University of Delaware, Newark, Delaware 1971 1
The amphoteric ion-exchange reaction is endothermic in both Amberlite XD-2 and Amberllte XD-5 resins. Both show significant capacity decrease for several solutes when temperature increases owing to the increase of water ionization with increasing temperature. Amberllte XD-5 has about 50% higher resln capacity than Amberllte XD2. Amberlite XD-2 is more porous and stiffer in structure. Bath resins swell with an increase of equilibrium llquld concentration up to a certain point beyond which swelling stops. In the process off swelling, which is reverslble, approximately 80% of the volume change is due to the expansion of the pore volume; the rest Is due to the increase in volume of the solid resin phase. Temperature is not a critlcal factor for swelling. The lntrapartlcle porosity is increased by swelling. The solid resin densities for both resins are also reported.
Introduction Different from the conventional anionic or cationic ion-exchangeresins, Amberlite XD-2 and Amberlite XD-5 (products of the Rohm and Haas Co., Philadelphia, Pa.) are amphoteric. Both resins are synthesized by introducing a gelular “guest copolymer” into a matrix of macroreticular “host copolymer”, such as styrene-divinylbenzene or POlyvinylbenzyl chloride (Barrett and Clemens, 1976). After proper procedures of chloromethylation, aminolysis, and hydrolysis, both carboxylic groups and amine groups offer the amphoteric functionality. Typically, the “guest copolymer” contains divinylbenzene (DVB) which is also a cross-linking agent. The major difference between Amberlite XD-2 and Amberlite XD-5 lies in their DVB content which changes the extent of cross-linking (Helfferich, 1962). These two resins virtually possess highly porous structure with reasonably good mechanical strength. The thermally regenerable property of the two resins may make them superior to the common chemically regenerable ion-exchange resins from the viewpoint of cost in pollution prevention appplications. Using weakly acidic carboxylic groups and basic secondary amine groups to 0196-4321/81/1220-0361$01.25/0
exchange with sodium and chloride ions as an example, the chemistry of these ion-exchange resins (Ackerman et al., 1976) can be described as follows. cation exchange R-COO--H+
+ Na+ F? R-COO-Na+ + H+
(1)
anion exchange RiNH
+ C1- + HzO F? R,’NH,+Cl-+
OH-
(2)
water ionization
HzO F? H+ + OH-
(3) As temperature increases, the degree of water ionization also increases, driving the equilibrium of (1)and (2) from the loaded forin (right-hand side) to the regenerated form (left-hand side). Mass transfer phenomena in Amberlite XD-2 have been studied by Knaebel et al. (1979a). Processes using Amberlite XD-2 for salt removal have also been developed (Dabby et al., 1976; Knaebel and Pigford, 1979b). Little is published for the newer Amberlite XD-5 resin. 0 1981 American Chemical Society
362 Ind. Eng. Chem. Prod. Res. Dev., Vol. 20,No. 2, 1981
XD-2
(y6’ -l------
c
~
-31
OOL c
513
CONCENTRATION
-
iC‘.
(34
-
_c
I-
meqlml
Figure 1. Equilibrium isotherms for Amberlite XD-2; solute, NaC1. I:
E
E
----
,.
80 A m b e r 1 te X D - 5
60 -. E 40
-
,
:1 2 -
.,
=
L~
20
m
O
C
02
04 06
08
IO
12
I4
I6
I8
q , meq NaCI/g dry r e s n
Figure 3. Derivation of Q and K from isotherms. The common intersection of the straight lines on the horizontal axis represents the maximum number of sites for adsorption, Q; the slope of each line yields the equilibrium constant, using the dissociation constant of water.
c
-t i
C O N C E N T R A T I O N , meq/ml
Figure 2. Equilibrium isotherms for Amberlite XD-5; solute, NaC1.
It is essential for investigations of transport phenomena and for process design to understand first the resin properties thoroughly. In this paper properties for both Amberlite XD-2 and Amberlite XD-5 are investigated, including resin capacity, density, porosity, and swelling.
Resin Capacity Resin capacity not only represents the capability of resin utilization in ion-exchange operations, but also is needed for evaluating the driving force for mass transfer. The capacities of Amberlite XD-2 and Amberlite XD-5 were studied for various inorganic solutes in isothermal equilibrium batch experiments. The resin beads used in these experiments were first regenerated fully with hot distilled water at about 95 “C. Usually this regeneration procedure lasted for at least 5 days at the regenerant flow rate of 100 mL/min for each 2000-mL sample of raw resin beads. Presumably, all solute was removed after the effluent from the regeneration became virtually distilled water. The fully regenerated resin beads then were dried overnight in an oven at 100 “C. After being cooled to room temperature, a measured sample (about 10 g) of dry resin was introduced into a flask which contained a fixed amount (about 200 mL) of solution with a known initial concentration of solute. The flask sat in an isothermal bath for 4 days t o guarantee equilibrium. Then, the bulk concentration of solution in the flask was found by titration using Mohr’s method (Skoog and West, 1969). From a mass balance on the ions, the amount of solute adsorbed by the resin phase could be calculated to find the resin capacity based on dry resin mass (Hsu, 1979). Figures 1 and 2 show the resin capacities of Amberlite XD-2 and XD-5 for NaCl at four different temperatures. These data can be represented by the Langmuir-type isotherms. Ac q==
(4)
Since Mohr’s method requires that the titrated sample be cooled to room temperature, the pH of each sample was measured immediately before titration. The pH values
t
K
Figure 4. Temperature dependence of the equilibrium constants.
measured for Amberlite XD-2 equilibrated samples were always 5.5, while those for Amberlite XD-5 gave pH about 5.8. To study these isotherms from the viewpoint of chemical equilibrium, we can combine (1) and (2) to form the following expression.
+
RCOO-H+ + R i N H + Na+ C1- + H 2 0 RCOO-Na+ + RiNH2+Cl- H+ + OH- (5)
+
A shift of equilibrium from the left side to right side is called “loading”; “regeneration” happens when the equilibrium shifts from right to left. Assuming that the amphoteric resin has the same number of basic and acidic exchange groups, an “amphoteric equilibrium constant”, K , can be defined for reaction 5. q2
K=
x
1o-pKw
(Q - q P C 2
(6)
where Q is the maximum number of active site pairs per gram of resin, q is the equilibrium resin capacity, c is the equilibrium liquid concentration, and pK, = pH + pOH = -log K,. K and Q can be found from the plot of q / c vs. q as shown in Figure 3. Q is found from the point on the q axis to which all straight lines converge. For Amberlite XD-2, Q is 1.13 mequiv/g of dry resin for both carboxylic groups and amine groups. Amberlite XD-5 has a Q of 1.72 mequiv/g of dry resin, which is about 50% higher than that of Amberlite XD-2. Also, from the slope of the lines
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 2, 1981
363
Table I. Langmuir Isotherm resin
solute
Amberlite XD-2
NaCl
Amberlite
NaCl
MgCI,
CaCI,
temp, "C
A, d / g dry resin
B , mL/ mequiv solute
D, d / g dry resin
E, mequiv solute/g dry resin
25.8 31.7 49.5 69.0 25.9 47.4 68.8 89.7 24.8 49.8 79.3 24.8 49.8 79.3
131.956 124.308 69.132 45.437 72.429 41.595 20.233 12.202 315.264 177.775 124.71 2 224.167 148.742 115.406
116.762 113.707 61.124 50.700 42.579 22.406 7.919 3.866 422.193 242.009 178.287 252.582 154.910 129.320
1.7225 1.7163 1.5185
0.6763 0.6455 0.6132
a q = AC/(l t BC). If C < Cbomd, 4 = A c / ( l -t BC). If C > CboWd, 4 = DC t E. temperatures: a t 24.8 OC, 0.015; a t 49.8 'C, 0.015; at 79.3 'C, 0.020.
0.901
0.45
I m
f 030 0
0151
1
-&Li
VdUeS Of % o m d a t Various
PH
24 80C
49 8*C 79 3'C
Q--
0 05
0 10 0 15 c , meq MgCIz m l
j
0 20
0 18
0
002
004
006 008 010 012 c . meq MgClz mi
014
016
Figure 5. Equilibrium isotherms for Amberlite XD-5; solute, MgC12 24 8'C 49 8'C
79 3oc 4' 0
I
0 05
, 0 10
I
I
0 15
0 20
c , meq CoCIz/ml
, D
oi
#
0 45
h
F
030
'OI5[,,
'0
l
002
004
I
006 008 010 012 c, meq CoC12 ml
014
I
,
016
Figure 7. pH Changes for the equilibrium samples of Amberlite XD-5 using MgCll and CaCl,.
,/ 018
Figure 6. Equilibrium isotherms for Amberlite XD-5; solute, CaC12.
in Figure 3, K can be calculated if one knows pK, at different temperatures (Dean, 1979). A plot of log K vs. 1/T is given in Figure 4. The data are very closely represented by a straight line, the van't Hoff isochore AH0 l n K = - - RT
+ constant
(7)
The heat of reaction, AHo, is about +2.6 kcal/g-mol for both resins, indicating that reaction 5 is endothermic. Two divalent cations, Mg2+and Ca2+,also were studied in resin capacity experiments for Amberlite XD-5. Figure 5 shows the resin equilibrium capacity of Amberlite XD-5 for MgC12. Langmuir isotherm behavior is still observed, but the equilibrium capacity of CaC12,shown in Figure 6, has different behavior. It is difficult to represent the whole range of data by a Langmuir-type expression since q tends to increase linearly with respect to concentration in the high concentration region. Unlike the equilibrium experiment with NaC1, a decrease in pH with increasing salt concentration was observed for both MgClz and CaClz as
shown in Figure 7. This indicates that more protons are released when more double-valent cations are adsorbed. Ca2+causes more protons to be released than Mg2+at the same temperature. It is believed that the functional groups in both resins are monovalent and are separated from each other. Table I summarizes the empirical parameters needed to fit the equilibrium capacity data in Figures 1, 2, 5, and 6. Solid Resin Density Fully regenerated resin beads were used to determine ps after soaking in deaerated distilled water for several weeks. The wet resin sample was put in a Buchner funnel which was connected to a water ejector to remove the interstitial water. The suction lasted at least 5 min. Then a sample of wet resin without interstitial water was added to a graduated cylinder which contained a known volume of distilled water sufficient to cover the resin sample. From the volume change in the cylinder we knew V,, the volume of the wet beads in the sample. Also, we measured Mwet, the mass of the wet beads in the sample, by weighing the cylinder before and after the addition of the resin. Then the sample was dried in an oven at 100 "C for 1day and weighed to find MdW,the dry resin mass. Since and since the first term is the observed dry resin mass, e
Ind Eng. Chem Prod Res Dev, Vol 20, No 2, 1981
364
6"-
-
5
,~--
i'-
;
3c
4
-
A
"6
^.i
-1
-
31 4
-
L
3-6 -08
1
i
02
- e a Yc:
Figure 8. Swelling data for the resins. The data were collected at successively greater salt concentrations
t
03
L
04
05
-
76
4
01
08
i m
Figure 10. Effect of concentration on intraparticle porosity. The left-most point on the Amberlite XD-5 curve was derived from the measurement of ps The data were collected at successively greater ,sit concentrations.
F
Figure 9. Typical size distribution of resin particles.
(the internal porosity of a resin particle') and pi can be calculated simultaneously by JKWt -E
Mdrv .~_
19;
vbbP€I:9 Mdrv
,osof Amberlite XD-2 is 1.252 g/mL (Knaebel, 1978) and
that of Amberlite XD-5 is 1.195 g/mL. Swelling of Resins The effects of concentration and temperature on resin swelling are discussed in this section. In the study of swelling due to the adsorption of salt, 10-mL ( 3 / 8 in i.d.1 graduated cylinders were utilized. First, a certain volume of distilled water was poured into the cylinder. Then fully regenerated wet resin beads (with interstitial water removed by suction as described before) were added to the cylinder. Vibration was applied to the cylinder to allow the resin beads to settle into a compact fixed bed, which had the initial volume V,. NaCl solutions of different concentration were poured into each graduated cylinder. The resin beds expanded as salt was taken up by the resin. After 4 days each sample was titrated to find the equi librium concentration. Intermittent vibration continued during this period of time. Before recording the final expanded volue, V, vibration again was applied to settle the resin beads into a compact mass. The results ot swelling experiments are shown in Figure 8 for both resins. All these swelling experiments were conducted at 20 "C Both Amberlite XD-2 and Amberlite XD-5 stop swelling when the equilibrium liquid concentration reaches some value. That Amberlite XD-5 has much higher swelling ability than its counterpart indicates that the latter has a much stiffer structure than the former. The resin samples used in these experiments had a typical size distribution as shown in Figure 9. Even with fully swollen samples of Amberlite XD-5, the interparticle porosity of the packed bed does not change significantly (Leva, 1947). Thus, the observed increase in bed volume is primarily due to the expansion of the resin bead> themselves. To investigate the effect of temperature on resin swelling, a sample about 1700 mL of raw Amberlite XD-5
was added to a large graduated glass cylinder. Continuous hot distilled water (about 95 " C ) ran through the column to regenerate the resin. After 24 h the packed bed of the resin beads fell to 1270 mL. Then cold distilled water (about 15 "C) replaced the previous hot water. After 1h the resin bed had expanded to 1330 mL. Again, hot distilled water was introduced and 18 h later the bed was observed to have decreased in volume to 1250 mL. Then, once more, after being cooled down to cold distilled water, the packed bed volume reached 1300 mL. The initial ieduction of volume from 1700 to 1270 mL and the difference between 1270 and 1250 mL were mostly due to the loss of salt from the resin beads. The volume change after treatment with cold distilled water indicated that temperature does not affect resin volume significantly. This .onclusion should be valid also for Amberlite XD-2, which has a stiffer structure than Amberlite XD-5. Kntraparticle Porosity The intraparticle porosity is one of the resin properties which has to be known in the design of processes which use ion-exchange resins. Since equilibrium concentration afrects resin swelling, it is good to know whether concentration influences c as well. The equilibrated resin samples from the swelling experiments were used for this purpose. The same techriiques were applied to find Mwetand V , as in the study of' (1 However, we have here .fq,, = l'hf/)H20+ v b j l -- c)Ps + v b ( 1 - 6)PsqMNaCl (11) since wet resin mass comprises intraparticle water mass, drv resin mass and mass of salt adsorbed. From eq 11we get
P s ( l + qMNaCl) t =
P,(l
+ qMNaC1)
- Pp -
PH20
(12)
where pp = M,,,/ v b . q can be calculated from the Langmuir isotherm parameters as long as the salt concentration i i known. Figure 10 gives the results of this work. Both resins have macroreticular structures of which t increases with concentration. But Amberlite XD-2 is more porous. For it, the concentration change has less influence on E than for Amberlite XD-5. This is consistent with the swelling observation that Amberlite XD-2 is structurally stiffer. Reversibility of Swelling Phenomena It is interesting to know whether the swelling of the particles as a whole and the increase in porosity are reversible with respect to the concentration change. The solid curves in Figure 11 are transferred from Figures 8 and 10. On each curve there are two pairs of data points. For each pair, the right one was taken first by means of the methods discussed before. Then the concentrated liquid solution was discarded, and pure distilled water was
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 2, 1981 365 05
I Amberl11e X D - 5
v.
02
Imberllte X D - 2
O I L
0
02
008
004
,
07,
,
,
04
06
08
mrq NoCI/mI
L,
,
,
Amberlife X D - 5
i
0 3 t Q 2 t O I t 01 0
'
"
004
"
008
"
02
'
04
1
"
06
I
OB
c mea N o C l I m l
Figure 11. Study o f swelling reversibility. Pairs o f data are shown; each pair i s designated by a d i f f e r e n t symbol. For each p a i r t h e point a t higher concentration was observed first, followed by the one a t lower concentration. N o t e t h a t the lower points fall o n t h e curves, i n d i c a t i n g very l i t t l e irreversible effects.
I GO1
0 01
0 10
,
,
,
1
1
i
4
I O0 -0 0 001 c meq NaCI/ml
Figure 12.
Comparison o f bead volume change a n d pore volume change based o n t h e volume o f salt-free, solid resin phase a t different e q u i l i b r i u m concentrations.
added. The resin beads desorbed salt to reach another equilibrium from which the left point of each pair was derived with the same techniques. From these results, it appears that the swelling phenomena are reversible. Discussion Using the original data, one can compute the increase in pore volume per unit of original volume of the solid phase, as given on the left of the equation ve - Voeo =-- e V - V n C - C ~ (13) (1- ~0)Vo 1- €0 Vo 1 - €0
+-
Each term on the right can be computed from the data in figures 8 and 10. In Figure 12, the spherical points on the upper curves for both resins represent the beads' total volume change divided by the original volume of the salt-free, solid resin phase. The triangular points on the lower curves represent the change in pore volume alone on the same basis. Fully swollen beads of Amberlite XD-5 increase in volume up to 64% of the volume of their salt-free solid resin phase. For Amberlite XD-2, the increase is only 29%. For both resins, approximately 80% of the volume change in swelling is due to the expansion of the pore volume; the remainder is due to the expansion of the solid resin phase. In resin particles containing both carboxylic and amine groups near each other, it is likely that some of these unlike pairs may associate with each other in the salt-free state, something like a closed "zipper". In the presence of salt, however, some such pairs
may dissociate, permitting the resin backbone to relax and the pores to grow larger. Conclusions Amberlite XD-5 shows higher resin capacity for NaCl than Amberlite XD-2. Divalent cations, such as Mg2+and Ca2+,were also investigated for Amberlite XD-5 in capacity experiments. Based on the observed equilibria, the amphoteric ion-exchange reaction, eq 5, is endothermic. Other resin properties, such as ps and e, are also presented. A volume increase as great as 40% may occur if the beads of Amberlite XD-5 are nearly saturated with salt, but only 13% for Amberlite XD-2. Compared with the stiffer Amberlite XD-2, Amberlite XD-5 is more spongelike, though less porous. Swelling also increases the intraparticle porosity. Influenced very little by the temperature change, swelling of both resins is reversible. Acknowledgment The resins studied in this work were obtained from the Rohm and Haas Company. The financial support from the National Science Foundation (Grant ENG 77-19951) is gratefully acknowledged. Nomenclature A = Langmuir isotherm parameter, mL/g of dry resin B = Langmuir isotherm parameter, mL/mequiv of solute c = equilibrium liquid concentration, mequiv of solute/mL D = linear isotherm parameter, mL/g of dry resin E = linear isotherm parameter, mequiv of solute/g of dry resin AH" = heat of reaction (25 "C, 1 atm), kcal/g-mol K = amphoteric equilibrium constant for ion-exchange reaction A4 = mass, g A ~ N =~ equivalent c ~ weight of NaCl, 0.0585 g/mequiv Q = maximum number of pairs of active sites, mequiv/g of dry resin q = equilibrium resin capacity, mequiv solute/g of dry resin R = universal gas constant, 1.987 cal/g-mol K T = temperature, "C or K V = volume, mL p = density, g/mL = intraparticle porosity Subscripts
b = bead or bed 0 = initial or solid free p = resin particle s = solid resin phase Literature Cited Ackerman, G. R.; Barrett, J. H.; Bossler, J. F.; Dabby, S. S. Water-1976, AIChE Symp. Ser. 1976. Barrett, J. H.; Clemens, D. H. U.S. Patent 3991 017, 1976. Dabby, S. S.;Fior, F. H., Jr.; Fischer, W. F.; Kosaka, K.; Iwatsuka, T.; Shindo, I.; Hotogi, A.; Kunin, R. 37th Annual Meeting, International Water Fonference, Pittsburgh, Pa., 1976. Dean, J. A. "Lange's Handbook of Chemistry", 12th ed.;McGraw-Hill: New York, 1979; pp 5-7. Helfferich, F. "Ion Exchange", McGraw-Hili: New York, 1962. Hsu, T.-B. M.Ch.E. Thesis, University of Delaware, Newark, Del., 1979. Knaebel, K. S. M.Ch.E. Thesis, University of Delaware, Newark, Dei., 1978. Knaebei, K. S.; Cobb, D. D.; Shih, T . T . ; Pigford, R. L. Ind. Eng. Chem. Fundam. 1979a, 18, 175-180. Knaebel, K. S.; Pigford, R. L. AIChE Meeting, San Francisco, Calif., 1979b. Leva, M. Chem. Eng. Prog. 1947, 43, 549. Skoog, D. A,; West, D. M. "Fundamentals of Analytical Chemistry". 2nd ed.: Holt, Rineharl and Winston Inc.: New York, 1969. R e c e i v e d for r e v i e w A u g u s t 1, 1980 A c c e p t e d N o v e m b e r 18, 1980
