SULFON IC-TYPE CAT10N-EXCHANG E R E S I N S A S DESICCANTS C. ELMER W Y M O R E Physical Research Laboratory, The Dow Chemical Co., Midland, Mich.
Sulfonic-type cation-exchange resins are excellent desiccants for drying organic liquids. The use of Dowex 50W resin for drying has been extensively investigated using ethanol and 1 , I ,1 -trichloroethane as examples of relatively polar and nonpolar materials, respectively. O f the resin variables studied, the ionic form i s the most important. The potassium form of Dowex 50W-X8 resin (20- to 50-mesh) was the best tested. It will dry nonpolar organic liquids to less than 1 p.p.m. of water and exhibit high capacities. It can be regenerated at the relatively low temperature of 240' to 280" F. The influence of water concentration in the feed, cross linking, flow rate, and temperature on capacity was studied.
HE swelling and osmotic behavior of ion-exchange resins Thave been of both practical and theoretical interest. For this reason the equilibrium water vapor sorption curves of many resins have been studied (7, 3-6, 8 ) , but little work has been reported on the practical use of ion-exchange resins as desiccants in dynamic systems. Two reports ( 2 , 7) concerning the use of the acid form of resins for drying do not give capacity information. A note on the kinetics of water vapor sorption has also appeared ( 9 ) . This paper deals mainly with the performance of resins used to dry organic liquids. Resin variables .such as ionic form and cross linking as well as operating variables such as water in the feed. flow rate. and particle size were studied. For use as desiccants, ion-exchange resins are probably best viewed as a n insoluble matrix to which are attached charged ions Lvhich do the actual drying. Thus, the dry sodium form of a sulfonated cross-linked polystyrene such as Dowex 50LV resin might be likened to an insoluble anhydrous sodium sulfate. [Dowex is a registered trademark of the Dow Chemical Co. Dowex 50 and Dowex 50W are strong acid cation exchange resins prepared by monosulfonation of styrene-divinylbenzene copolymer beads, manufactured and sold by Dow. SuffiYes -X8. -X10>etc., indicate the degree of divinylbenzene (DVB) cross linking.] The acid form is then similar to a solid sulfuric acid. This analogy is supported by the water vapor sorption curves of sulfuric acid and Dowex 50-XIO, which are very similar up to 7053, relative humidity, if the secondary ionization of sulfuric acid is assumed negligible in concentrated solutions (5). One variable not directly related to the drying agent or operating conditions is the nature of the organic liquid. The conditions used to dry any organic liquid with any solid desiccant. including ion-exchange resins, depend upon many characteristics of that liquid. I n a general sense the polarity of a liquid is one approach to describing its facility of drying. Very polar materials such as the lower alcohols are difficult to dry; they show reduced capacities and require long contact rimes. Relatively nonpolar materials such as the hydro-
carbons and their halogenated derivatives display high capacities, and high flow rates are possible. Because of variation with the nature of the organic liquid, most of the variables were checked with two different materials. Ethanol and l , l , l trichloroethane (methyl chloroform) were employed as examples of relatively polar and nonpolar organic liquids: respectively. Experimental
The resins used were production grade and, after conversion to the desired form, were dried a t 110' C. in vacuo. Most of the analyses were performed by the Karl Fischer method. An electrometric "dead stop" back-titration procedure was used. By using a dilute Karl Fischer reagent with good sampling and titration apparatus and techniques, 5 p.p.m. of water may be detected. For some experiments the effluent from the column was continuously monitored by stripping the water from the solvent with dry nitrogen and measuring the moisture in the nitrogen with an electrolytic hygrometer (Beckman Model 17901) for a continuous run. A schematic diagram of the apparatus is shown in Figure 1. Copper tubing was used wherever possible and the lines were kept short and small in diameter. A syringe pump specially modified to hold 1-ml. tuberculin
I
c _ _ -
I
RECORDER
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T e l l o n 51oocock Flawrole 1 Bccim,n
Figure 1. analyzer
Schematic diagram of continuous liquid moisture
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syringes was used to pump the stream to be analyzed against the 10-p.s.i.g. nitrogen pressure needed to operate the hygrometer. I t was equipped with a Teflon stopcock weighted down with a 1-kg. weight. The flow rate was approximately 1.8 ml. per minute. The stripping column was a glass tube of 1/2-inch i.d. and 18 inches long containing two concentric 1/8-inch diameter glass rod spirals (six turns per 4 inches). Approximately 12 inches of the column between the nitrogen inlet and outlet actually was used. The liquid dropped from a tip onto the connected V-shaped top of the two glass spirals and spread out in a thin film over the glass. The water was stripped from the liquid by nitrogen and the remainder of the solvent escaped through a capillary tube a t the bottom of the column. The length and inside diameter of the capillary were adjusted to allow all of the solvent and only a small amount of nitrogen to escape. The nitrogen purge gas (dried just before the stripping column) was regulated at 10 p.s.i.g. and a flow of about 26 cc. per second (measured at room temperature and 1 atm.) was maintained. The flow through the electrolytic cell was 100 cc. per minute and the remainder of the gas passed through the bypass system of the hygrometer into the atmosphere. The solvent passed through the system and registered on the recorder in approximately 4 minutes. This time was small when compared with the total time of a run, so no correction was made except for very short runs. The apparatus was calibrated for each liquid used. A blank was determined by running the stripping column with no liquid flowing until the hygrometer reading leveled out (approximately 24 hours). The continuous analyzer was not too reliable above 150 to 200 p.p.m.; therefore the water concentration of the organic liquids before drying was determined by the Karl Fischer procedure. Since part of the solvent is evaporated in the stripping column, only organic liquids which do not react with phosphorus pentoxide (active component of the electrolytic cell) may be used with the continuous analyzer.
Ionic Form
For the alkali metal ion forms of sulfonic-type ion-exchange resins, the equilibrium water vapor sorption isotherms show that the lithium form sorbs more water a t all relative humidities (7, 5, 8 ) . The amount sorbed decreases with increasing atomic weight: Li+ > N a + > K + % NH4+ ^v Cs+. This order can be explained by the relative sizes of the ions. The small lithium ion has the highest charge density and therefore attracts more water molecules and holds them more tightly. Its size also shows up in its larger heat of hydration. As the atomic weight increases, so does the size of the ion, which causes the charge density to decrease. This leads to the binding of fewer water molecules and hence a decrease in the capacity. The hydrogen ion, being the smallest monopositive ion, sorbs more water than even the lithium ion when in equilibrium with water vapor. The order with the alkaline earth metal ion forms is the same as with the alkali metals, magnesium ion being best. Each alkaline earth metal form picks up more water than its neighboring alkali metal form because it is doubly charged and as a result has a high charge density. Figure 2 shows the results for the dynamic or column drying of methyl chloroform with the alkali metal and hydrogen forms of Dowex 50W-X8 (20- to 50-mesh) resin. The water a t the beginning of the run is due to “tramp” moisture in the lines of the analytical apparatus. The amount of moisture in the dry portion of the runs is less than 1 p.p.m. Of the alkali metal forms, the potassium ion has the lowest capacity, followed by sodium; the lithium form tends to have the highest capacity if the leakage is ignored. The ammonium ion (not shown) is about equivalent to the potassium form and the cesium form is poorer on a weight basis but almost equal to the potassium 174
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
Resin. Dowex 5 0 W - X 8 (20-50) in 3 0 “ x 5/16”I.D. Bed Feed.
3 LL LL W
75
200 R R M . H20 i n CHsCCI,
Flow R o t e . 5 G.RM./Sq.Ft.
0
8 12 16 20 24 28 32 LE. WATER S O R B E D PER 100 L E . DRY R E S I N
4
36
Figure 2. 1 , I ,1 -Trichloroethane drying with Dowex 50WX8 in alkali metal ion forms
form on an equivalent basis. The capacity of the hydrogen form is greater than 34 pounds of water sorbed per 100 pounds of dry resin. Up to this point, 32 grams of resin had dried 10 gallons of methyl chloroform. These results are in line with what might be anticipated from the equilibrium water vapor sorption curves, in that the smaller ions tend to have the larger capacities. Since the slope of the breakthrough curve is an indication of the kinetics of a resin, the potassium form appears to have the fastest kinetics of the alkali metal forms, followed by the sodium and lithium forms. This has been shown to be true with kinetic data obtained by the limited bath technique using methyl chloroform (Figure 3). The potassium form has faster kinetics than the sodium form, which is considerably faster than the lithium form. Westermark ( 9 ) found the same trends and orders for the sorption of water vapor. Thus, there is a reversal of the order for kinetics compared to the order of capacities. An explanation of the slow kinetics makes use of the high energy of hydration of small ions such as‘ the lithium ion. The lithium ion has a high charge density
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Resin. 1.5 G. Dowex 45 Mesh
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Temp. 30’C.
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Resin. Dowex S O W - X 8 (20-50) in 61"x 1/2* I.D. Bed Feed. - 2 . 5 % H20 in E t O H
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1.5 1.0
z ~
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0
4
8
12
16
20
24
28
32
36
L B . W A T E R S O R B E D P E R 100 LB. D R Y R E S I N
Figure 4. drying
Use of mixed resins in 1,1,1-trichloroethane
and attracts more water molecules, which gives it good equilibrium characteristics, but it also coordinates so tightly to water molecules that it is reluctant to transfer the water to another lithium ion located further on the inside of the resin. Such strong coordination increases the activation energy for transfer and decreases the rate of water sorption. According to this theory, the hydrogen form should have the highest capacity (which it appears to have) and also the slowest kinetics. Yet, all available evidence indicates that it has fast kinetics. Such behavior may be due to its small size and high mobility or the possibility of a different mechanism. The same trend in ca.paciries is observed when the alkaline earth metal ion forms are used to dry methyl chloroform under dynamic conditions. The magnesium form has a very slow breakthrough and starts to leak very quickly. The capacity decreases from the calcium to the barium form. The divalent ions have a higher chare;e density, which leads to stronger coordination and resultant slower kinetics of water sorption. Because of this they have low capacities in situations in ivhich equilibrium conditions are not approached. All the metal ion forms tried (Ag+, Zn+*. Cr+S)dried methyl chloroform a t slow flow rates. Mixtures of resins, containing one ion with good equilibrium characteristics and one ion Tvith good kinetic properties, were tried. Figure 4 shows the results with methyl chloroform and a resin containing equivalent amounts of potassium and lithium ions compared with the regular resins. The properties of the mixed resin are intermediate between those of the two normal resins. This leads one to postulate that the ions with slow7 kinetics which bind water tightly are again slowing down the kinetics of the mixed resin, even though they occupy only half of the sites in the resin. A mixed magnesium-hydrogen resin gave the same results. The results of dynamic drying with the more polar organic liquid, ethanol, are shown in Figure 5. With rhe lithium and sodium forms, breakthrough occurs immediately. The potassium form gives alcohol containing about 20 p.p,m. of water and the capacity is much lower than that obtained in drying methyl chloroform. IVith ethanol the performance of the resins is in the order of their relative kinetics of water sorption, because ethanol is difficult to dry, and the kinetics become more important in the over-all performance of the resin. The difference between the two organic liquids can be illustrated by the dissimilarity of activity of water in the two solvents. Saturation, which is often taken as unit activity, is
0
2 4 6 8 1 0 1 2 1 4 L B . WATER SORBED PER 100 L B . DRY R E S I N
Figure 5. Ethanol drying with Dowex 50W-X8 in the alkali metal icn forms
approximately 325 p.p.m. for methyl chloroform and 100% for ethanol, since water and ethanol are miscible in all proportions. Now assume that a t some particular place in the drying column the resin has dried each solvent down to 30 p.p.m. of water. The activity of water in methyl chloroform is now approximately 0.1 while it is only 3 X 10-5 for ethanola difference of about four powers of 10. In both cases, the ratio of water concentration to the saturation concentration has been used as a rough measure of activity. The hydrogen form has not been mentioned above because it swells about 100% in dry ethanol-almosr as much as in water-while the other dry metal ion forms shoivn do not swell a t all in dry ethanol. The alcohol which has invaded the resin appears to compete with the water for the hydrogen ions. The water left in the ethanol dried with the hydrogen form, approximately O.l%,, is evidence of such competition. \\:bile ethanol is not as polar as water, more of it is present. The ethanol in the swollen resin allows an alternative mechanism for water sorption. IVater can enter the resin Xvith rhe erhanol and then diffuse to the hydrogen ions. I2:hile the metal forms of resins do not sirell in dry ethanol, as soon as \rater is sorbed, some ethanol penetrates the resin also. The concentration of water in ethanol in equilibrium with resins (8%, DVB) containing approximately 4% water is 0.32, 0.55, and 0.84% for the lithium, potassium, and hydrogen forms, respectivel>-. The better performance of the lithium form under equilibrium conditions again points out that kinetics are very imporrant in determining ethanol drying ability under dynamic conditions. The hydrogen form is the poorest in the equilibrium experiments, presumab1)- because of competition from the ethanol. When the alkaline earth metal forms are used to dry ethanol, breakthrough starts immediately but the slopes decrease as the molecular weight increases. Other metal ion forms such as the Ag+, Zn+e, Cr+3, and Y+3forms also give immediate breakthrough under the conditions shown in Figure 5. The two mixed resins (Mg+?-H+ and LiT-K4) showed no drying ability with ethanol. A partially sulfonated resin (26% of theoretical monosulfonation) in the potassium form swelled 47% in ethanol but showed no capaciry. This seems to indicate that the mechanism of water sorption via diffusion through the ethanol in the beads is of minor importance. This partially sulfonated resin did dry methyl chloroform, but the capacity was lolver than a normal resin even on an equivalent basis (2.2 us. 2.8 mmoles per meq. of resin). VOL.
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1
28 I
Cross Linking
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Resin. Dowex 50W K t Form
(20-50)
The equilibrium water vapor sorption curves of the potassium form of a sulfonic-type cation-exchange resin show some variation with the amount of cross linking (5, 8 ) . .4t low relative humidities the low cross-linked resins sorb less water than those of higher cross linking (up to approximately 20% DVB). while a t higher relative humidities the low DVB resins sorb more water because of fewer restrictions to swelling. This means that the equilibrium curve for a resin containing a cer. tain amount of divinylbenzene must at some point cross the curve for a resin differing from it only in the amount of divinylbenzene. The variation in performance of resins containing different amounts of DVB is shown in Figure 6. The data are on the potassium form of Dowex 50W (20- to 50-mesh) resins Ivhen used to dry methyl chloroform. Capacities are to a breakthrough of 2.5 p.p.m. of water in the effluent. Each point on the curve represents two runs and the values have been corrected to 200 p.p.m. of water in the feed by using the slope shown in Figure 7 for the potassium form of an 8% divinylbenzene resin. Admittedly, this is not completely valid, but it does help reduce the effect of water in the feed on the results. The data were obtained at 10 gallons per minute per square foot, at which flow rate it is thought that conditions are approaching those which exist at equilibrium and the results are not due to kinetics. The maximum: at about 497, divinylbenzene, may be due to the fact that the two trends of (1) high capacity for lower cross-linked resins at high relative humidity and (2) low capacity for low cross-linked resins at low relative humidity are such that the capacity is at a maximum. Data on the potassium form with ethanol indicate that the maximum is with resins containing 8% divinylbenzene. The capacity of the hydrogen form when used to dry ethanol increases with decreasing cross linking. Four, 8, and 12 pounds of water are sorbed per 100 pounds of dry resin for 16, 8, and 2% cross-linked resins at 0.5 gallon per minute per square foot. In each case the ethanol is not completely dry. Such behavior is in line with that anticipated on the basis of the fully water-swollen forms which these resins resemble, since they swelled 42.5, 98, and 228%,, respectively. in dry ethanol.
1
S o l v e n t , M e t h y l Chloroform
$ 24
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8 12 16 DIVINYLBENZENE
4 PERCENT
Figure 6.
24
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20
Capacity vs. cross linking
) F l o w R o t e , G.RM./Sq.Ft.
160
140
180
200
220
240
260
F E E D CONCENTRATION, P P M . HO ,
Figure 7. Capacity vs. feed concentration when drying 1,1,1 -trichloroethane with Dowex 50W-X8 (20-50) K+form
Resin. Dowex 5 0 W - X 8 ( 2 0 - 5 0 ) Feed. Methyl Chloroform contoining 200 RPM. H,O.
-
26
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Resin Feed.
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Water Feed Concentration and Flow Rate
G,P,M/Sq.Ft.
Capacity vs. flow rate
Dowex 5 0 W - X 8 ,
K+ Form in 5-11, B e d
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SORBED PER 100 LB.DRY
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14
RESIN
9. Ethanol drying with two mesh sizes of Dowex
50W-X8 176
l&EC
PRODUCT RESEARCH A N D DEVELOPMENT
A series of runs was made with both the potassium and sodium forms of Dowex 50W-X8 (20- to 50-mesh) resin in which the flow rate of methyl chloroform was increased from 2 to 10 gallons per minute per square foot. The amount of water in the feed also increased with increasing flow rate in these series of runs. For the two sets of data the only significant plots were the feed concentration us. capacity for the potassium form and the flow rate us. capacity for the sodium form. Figure 7 shows the data for the potassium form with the capacity to a breakthrough of 2.5 p.p.m. of water plotted as a function of water concentration in the methyl chloroform. The correlation between the two is good. The numbers in parentheses are the flow rates in gallons per minute per square foot and seem to show that the capacities are independent of the flow rate up to the maximum used, although the curve might be somewhat steeper if the same flow rate were used. An increase in capacity is generally found with an increase in water concentration in the organic liquid, although the amount of the increase varies from one liquid to another. The capacity of the sodium form as a function of flow rate is shown in Figure 8. The capacity decreases as the flow rate is
1.2 ,
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Table I. c
z 1.0 .
Dynamic Drying of Methylene Dowex 50W-X8 (20-50)
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Resin. Dowex 5 0 W - X 8 , K + ( 2 0 - 5 0 ) in 5-Ft. Bed Feed.
-2.5% C,H,OH
HpO in
Chloride with
Capacity,. Lb. H 2 0 Sorbed/ 100 Lb. Dry Resin 13.6 13.5 13.6 10.6
Resin Flow Rate, Form G.P.M./Sq. Ft. Potassium 5 Sodium 5 Potassium 10 Sodium 10 a To 10 p.p.m. breakthrough.
Flow R o t e . 0.5 G.RM./Sq.Ft.
a
2
0
4
L B . WATER
6 8 IO 12 14 I6 SORBED PER I O O L B . D R Y R E S I N
Temperature
Figure 10. Ethanol drying a t two temperatures with Dowex 50W-X8
increased, even though the water in the feed also increases at the same time. T h e decrease is no doubt due to the slower kinetics of the sodium form. Although it cannot be stated with certainty, the capacity of the potassium form is probably fairly constant within the limits and conditions used, and can be best represented by the dashed line in Figure 8. From Figure 7 the capacity for a feed containing 200 p.p.m. of water is 19.7 pounds of water sorbed per 100 pounds of dry resin. The potassium form will also show a decrease in capacity with increasing flow rate, particularly when drying a more polar organic liquid. Particle Size
Decreasing the particle size will increase the effective contact time and improve the performance of a resin. This effect will show up more with organic liquids which are difficult to dry. Figure 9 shows a comparison of the potassium form in nominal U. S. Standard 20- to 50- and 50- to 100-mesh resins when used to dry ethanol. The smaller mesh resin shows a higher capacity but reduces the water to the same level (20 p,p.m,). The higher water in the feed for 50- to 100mesh resin tends to make its capacity slightly greater than normal for 2.5% water in the feed.
An increase in temperature should increase the rate of water sorption. Figure 10, again using ethanol, shows the effect with the potassium form of Dowex 50W-X8. The ordinate has been magnified to show the bottom portion of the curves more clearly. The sharper breakthough a t 65' C. indicates faster kinetics of water sorption. Organic liquids
Many other organic liquids have been studied and almost all of them are dried under some conditions. Table I shows the results for drying methylene chloride a t 5 and 10 gallons per minute per square foot. Other conditions are a 3-foot deep bed and about 900 p.p.m. of water in the feed. The capacity of the sodium form is affected by the flow rate. Another indication of its slower kinetics is that it has a much more sloping breakthrough curve. The curves for drying o-dichlorobenzene with both the sodium and potassium forms of Dowex 50W-X8 (20- to 50mesh) resin are sho\vn on Figure 11. Even a t a flow rate of 30 gallons per minute per square foot the sodium form is somewhat better than the potassium form, yet its sloping breakthrough shou s slower kinetics. Apparently o-dichlorobenzene is very easy to dry and the kinetics of even the sodium form are fast enough a t the flow rates used so that they do not influence the capacity. In this case, the greater inherent water capacity of the sodium form shows up. These curves were obtained using the Karl Fischer titration method, but other experiments with the continuous analyzer have shown less than 1 p.p.m. of water in the dried material.
Resin. Dowex 5 O W - X 8 ( 2 0 - 5 0 ) K + F o r m Bed Depth. 2 F t .
Resin. Dowex 5 0 W - X 8 ( 2 0 - 5 0 ) B e d D e p t h . 36 Inches LL W
5 80
c 0
Feed. - 2 5 0
P P M . HO ,
Feed.
W
0 . 0 6 8 % Woter
Z 50 a W
4 8 I2 16 20 24 20 L B . WATER SORBED PER 100 LB. DRY RESIN
Figure 1 1.
Dynamic drying of o-dichlorobenzene
32
0
4
8
12
16
20
28
24
LB. WATER SORBED P E R 100 L E . D R Y R E S I N
Figure 12.
Dynamic drying of Dowtherm A
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Table II. Water level Remaining in Organic liquids Dried with Dowex 50W-X8, Potassium Form Less Than 5 P.P.M. Water Less Than 50 P.P.M. Water
n-Hexane Triethylene glycol Toluene (Na+) Ethanol Benzene (Hi) Butvlene oxide ( N a + ) Propylene 2-~ropanol Dowtherm 4 a o-Chlorophenol MonofluorotrichloroAcrylonitrile methane Dimethylformamide Carbon tetrachloride Less Than 200 P.P..z.I. W a f e r Methyl chloroform Methylene chloride Acetic acid (H--) o-Dichlorobenzene Diisobutvl ketone Nitromethane Aniline Dowtherm A: heat transfer media? is 73.5% diphenyl oxide and 26.5y0 diphenyl. ~I
Q
Table 111.
Properties of Potassium Form Dowex 50W-X8
Bead size, mesh Bulk density (activated), lb./cu. ft. Density (activated), g./cE. Specific heat, B.t.u./lb./ F. pH in water Regeneration temp., ' F. Heat of regeneration, B.t.u./lb. H20
20-60 ( V . S. Std.) 60 Approx. 1.55 0.28 6 5-9 240-280 Approx. 1800
Figure 12 shows the results of drying Dowtherm A heat transfer medium with the potassium form of Dowex 50W-X8. Dowtherm A is a mixture of 26.57, diphenyl and 73.57, diphenyl oxide. T h e high capacity of about 22 pounds of water sorbed per 100 pounds of dry resin indicates that the oxygen in the diphenyl ether is well shielded and that the mixture is essentially nonpolar in nature. Table I1 shows a list of other organic liquids which have been dried with ion-exchange resins. generally the potassium form. They are divided into groups depending upon the residual water. For several of the liquids shown the results are based upon one experiment and d o not represent optimum performance. Most organic functional groups are present. including a ketone, amide, acid, ester, and phenol. Air and other gases can also be dried. Regeneration
I n the laboratory, resins are easily dried in a n oven a t 115' C., but they also can be dried in place in a column. By drying the resins in place, the amount of water in the effluent can often be decreased in spite of precautions taken in completely drying the resin and quickly transferring it into the column. T h e recommended regeneration temperature is between 115" and 140' C. The purge gas may be heated to provide the energy requirement for regeneration or used to help sweep the water vapor out of the bed. T h e amount of heat required to regenerate Dowex 50\V (potassium or sodium form) is approximately 1800 B.t.u. per pound of water sorbed on the resin. T h e exact figure will vary with the amount of water on the resin and the degree to which the bed is regenerated. To determine the heat stability of sulfonic-type cationexchange resins. they were heated in air above the recommended regeneration temperature. After the potassium form
178
I&EC P R O D U C T RESEARCH A N D DEVELOPMENT
of Dowex 50\V-X8 had been heated 5 days a t 195' C., no detectable decrease in dry weight capacity was found and the resin had only slightly darkened. Upon reswelling the heated resin with water, its uptake of water was the same as that of the original resin. These tests indicate that the resin did not depolymerize or desulfonate. Experiments with other alkali and alkaline earth metal ion forms gave similar results. Under the same conditions the dry weight capacity of the hydrogen form decreased almost 4576, indicating desulfonation. Advantages and Choice of Resin
T h e potassium form of Dowex 50W-X8 (20-50) with its fast kinetics of water sorption is the best resin to use; some of its properties are shown in Table 111. T h e sodium form has the highest capacity with a few organic liquids, but its capacity is more sensitive to increasing flow rate because of slow kinetics and cannot be used with the very polar liquids. The hydrogen form shows high capacities in many systems and must be used when drying very acidic materials. Its disadvantages are that it degrades when heated for regeneration and swells excessively with polar materials such as the lower alcohols. \.C'ith many organic liquids, ion-exchange resins have higher capacities than conventional solid desiccants. They particularly excel in the drying of relatively nonpolar organic liquids. With such materials, completely dry resins will reduce the water to below 1 p.p,m. of water. The level of water in the dried organic liquid tends to increase as the polarity of the material being dried increases. Resins swell as they sorb water and space for such expansion must be provided. Alkali metal forms of Dowex 50\4'-X8 swell slightly less than 1% for every pound of water sorbed per 100 pounds of dry resin. Dowex 50W is very resistant to bead breakage during the slow expansion and contraction ivhich the resin undergoes during a complete drying cycle. O n e important advantage of cation-exchange resins is their relatively low regeneration temperature of 240' to 280' F. compared with 400' to 600' F. recommended for synthetic zeolite materials. Other desirable properties of the alkali metal salt forms are that they d o not introduce impurities into the liquid being dried. They are relatively inert-Le., best compared with hl2SO4-and will not react with most organic liquids or polymerize monomers. T h e alkali salt forms generally remove only water from most organic liquids, which is important when drying solvents containing inhibitors. Acknowledgment
T h e author thanks D . C. Pinney. who performed many of the Karl Fischer titrations so necessary for this work. literature Cited
(1) Boyd, G. E., Soldano, B. A , , Z. Blektrochem. 57, 162 (1953). (2) Chiras, S. J., Brit. Patent 806,769 (Dec. 31, 1958). ( 3 ) Dickel, G., Hartman, J. LV., Z . physik. Chem. 23, 1 (1960). (4) Glueckauf, E., Proc. Roy. Soc. (London) A214, 207 (1952). (5) Gregor, H. P., Sundheim, B. R.. Held, K. M.. M'axman. M. H.. J . ColloidSci. 7. 51 1 (1952). (6) Pepper, K. W., J . A,@[. Chcrn. 1; 124 (1951). (7) Solomon. P. W, (to Phillius Prtroleum Co.). U. S. Patent 2,909,572 (Oct. 20, i959). (8) Sundheim, B. R., LVaxman. M. H.. Gregor, H. P.. J . Phys. Chem. 57, 974 (1953). (9) Westermark, T.. Ada Chem. .Stand. 14, 1858 (1960). \
I
RECEIVED for review March 30, 1962 .4CCEPTED July 16, 1962
