DYNAMIC SORPTION BY HYGROSCOPIC SALTS A Comparative Study 0HA.RAMVIR
P U N W A N I , C. W.
CHI, A N 0
0 . T.
WASAN
Institute of Gas Technology and Department of Chemical Engineering, Illinois Institute of Technology, Chicago, Ill.
60616
Sorption of water vapor by solid desiccants is one of the methods employed for dehumidification of air, Studies have been made on silica gel, activated alumina, molecular sieves, and salts such as calcium chloride. However, no systematic evaluation of sorption rates of hydroscopic salts such as halides of lithium, cesium, rubidium, and their mixtures has been reported. A method has been devised to compare the sorption rates of hygroscopic salts. Sorption rates of lithium chloride, lithium bromide, cesium fluoride, cesium bromide, and their mixtures have been compared at a constant bulk temperature of 40’ C., with an inlet stream flow rate of 138.5 cm. per minute and a humidity of 18,000 p.p.m. on a volumetric basis. Lithium chloride and lithium bromide have been studied more intensively. Their sorption rates have been compared at various humidities, flow rates, and temperatures. For the same surface area, lithium bromide shows a higher sorption rate at all temperatures. A simple empirical model has been developed which can adequately predict the sorption rate data of lithium chloride in the region between anhydrous and saturated solution.
control for comfort and for industrial application Methods employed for reducing the humidity of air (Hougen and Dodge, 1947) include cooling of air below its dew point by the use of surface condensers or of cold water spray, compression of air to a point where the partial pressure of water vapor exceeds its saturation pressure, combined compression and cooling, absorption in spray chambers using organic liquids such as glycerol or aqueous solutions of salts such as lithium chloride, absorption in packed columns using a countercurrent flow of concentrated sulfuric acid, phosphoric acid, or organic liquids, and sorption of water vapor in solid desiccants, such as silica gel, activated alumina, molecular sieves, and hygroscopic salts. A survey of the literature shows intensive studies of silica gel (Bullock, 1965; Bullock and Threlkeld, 1966; Elstonalberg, 1939; Hubard, 1954; Simpson and Cummings, 1964). Molecular sieves (Barrer, 1964; Greismer et al., 1959; Hougen and Dodge, 1947; Nutter and Barnet, 1966), activated alumina (Eagleton and Bliss 1953; Fleming et al., 1964; Getty and Armstrong, 1964), and activated carbon (Allman et al., 1929) have received considerable attention. Work has also been reported on hygroscopic salts such as calcium chloride (Baxter and Starkweather, 1916; Baxter and Warren, 1911), barium oxide (Booth and McIntyre, 1930), and perchlorates (Hammond and Withrow, 1933; Lenher and Taylor, 1930). Relatively little work has been done on the sorption rates of hygroscopic salts such as halides of lithium (Smith, 1943), considering that their potential utilization for dehumidification of air seems very high on the basis that they can easily be coated on materials having a large surface area per unit volume-viz., honeycombed asbestos. O n the other hand, silica gel, activated alumina, and molecular sieves cannot be easily coated on other materials and, hence, must be used in the form of beds and consequently require higher operating pressures. Furthermore, sorption equilibrium curves for silica gel, molecular sieves, activated alumina, lithium chloride, lithium bromide, etc., show that lithium chloride and lithium bromide have more drying capacity than the others (Bichowsky, 1940; Davidson Chemical Division, W. R. Grace Co., 1966; Fleming et al., 1964; Hougen and Dodge, 1947; Smith, 1943). UMIDITY
H is becoming more and more important.
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I&EC PROCESS D E S I G N A N D DEVELOPMENT
This paper deals with the dehumidification of air by hygroscopic salts. A method has been devised for comparing the sorption rates of various hygroscopic salts. The effect of several variables-viz., bulk temperature, flow rate, and humidity-on sorption rates of lithium chloride has been systematically evaluated, and a mathematical model which can predict sorption data adequately is presented. Apparatus and Procedure
A flow diagram of the apparatus is shown in Figure 1. The heart of the apparatus consists of an electronic microbalance and a hygrometer. Gilbert and Barker Manufacturing Co.’s Gilbarco sorption hygrometer Model SHL-100 was used to measure the humidity in the inlet stream to the sample. The instrument compares the frequencies of two quartz crystal oscillators and utilizes the difference between them to measure moisture content. Accuracy of the instrument is 5% of the maximum scale. I t can read only steady-state humidity levels from 1 to 25,000 p .p .m (y01 /vol .) Sartorium Products’ Electrona 11, an electronic microbalance, was used to measure continuously the weight changes in the sample. A change of weight produces a torque moment which in ordinary torsion balances would cause the beam to deflect. This movement, in the balance, is inhibited electronically by a current. The equilibrium flow of this current is automatically established in the bridge circuit and is strictly proportional to the change in weight. Beam oscillation is stopped by an incidental damping effect of the compensating current. The result can be read instantly on the indicating or recording instrument. The entire mechanism is located inside a glass body. The beam is accessible from both sides after removal of the groundglass caps. The beam is 20.0 cm. long and carries hooks on both ends. The sample hangs inside a glass tube of 2.5-cm. diameter. Over-all accuracy of the instrument is h0.570 of the range, but not better than 1 fig, T h e details of the apparatus are available (Punwani, 1967). Thin sheets of asbestos measuring about 2 X 0.3 cm. are cut and placed in an oven at 120’ to 150’ C., for a minimum of 12 hours, The zero point on the microbalance is determined by adjusting the counterweights. The asbestos is removed from the oven and hooked to one end of the beam and its weight is recorded. It is then removed from the balance and dipped in a prepared solution of known composition. This wet asbestos sheet is placed between two filter
.
.
.
WEIGHT CHANCE RECORDER
I
SIGNAL CONVERTER
I
i j
i
MOISTURE ANALYZER
i I
I
I
I
THERMAL BALANCE
I
A
m
c
-
I
*
-
r
COOLER
REMOVABLE FURNACE
SATURATOR
I
!
I
I
! VARIAC
@ PRESSURE GAGE
&
PRESSURE REGULATOR
a 0
TOGGLE VALVE THROTTLE VALVE
ROTAMETER
U Figure 1.
Schematic diagram of apparatus
papers until no excess solution is visible. This process of dipping and drying is repeated four times. The sample thus prepared is hooked onto the balance and its weight is recorded. A stream of dry nitrogen is then passed over the sample and the furnace is switched on. The temperature of the furnace is controlled at 150’ C. The sample starts losing weight as it dries. When it has been completely dehydrated, the weight-recorded reading becomes constant and shows the total weight of the asbestos plus the salt coated on it. T h e weight of the asbestos alone had been recorded earlier. Therefore, the weight of the salt coating can be calculated by difference. I t always took less than 10 minutes to regenerate the sample. For nonisothermal cycle of operation after 30 minutes from the starting time the furnace was switched o f f ,but the flow of dry nitrogen was kept on. Ten minutes later the flow was switched from dry to wet nitrogen (18,000 p.p.m.). The sample began to gain weight. The meter reading was recorded at regular intervals for 120 minutes. The sample was regenerated by returning to the first step, and the process was repeated to obtain a check run. For isothermal operation after the sample has been regenerated and while the dry nitrogen is still flowing, the furnace is switched off. T h e heating tape wrapped around the tube enclosing the sample is switched on through a precalibrated Variac to obtain a desired bulk temperature. After a steady bulk temperature has been obtained, the flow can be switched from the dry nitrogen to the stream of known humidity and the weight gained by the sample recorded continuously. Results and Discussion
Sorption rates of plain asbestos, lithium chloride, lithium bromide, cesium fluoride, cesium chloride, cesium bromide, rubidium fluoride, rubidium chloride, and rubidium bromide were compared a t a c’onstant inlet humidity (18,000 p.p.m.) and flow rate (138.5 cm. per minute) following the nonisothermal cycle of operation d.escribed previously.
The data so obtained are shown in Figures 2, 3, and 4. Comparison of these figures shows clearly that lithium chloride has a larger moisture retention capacity, while lithium bromide has a faster initial sorption rate than the other salts. As Figure 2 indicates, in the first 10 minutes lithium bromide removes almost twice as much moisture as lithium chloride. The first 10 minutes of sorption is the only region of importance for the present study, because within this time lithium chloride and lithium bromide are hydrated beyond the saturated solution and in actual practice the salt will not be allowed to be hydrated beyond the saturated solution region because of dripping. Of all the cesium and rubidium salts which were studied, only cesium floride and rubidium fluoride were promising. To facilitate the data analysis and to study the effects of various parameters-temperature, humidity, and flow rate-it was decided to take isothermal sorption rate data following the procedure explained above. The authors observed that the heat of a solution of cesium bromide is positive while the heat of solutions of lithium chloride and lithium bromide is negative. McBain (1909) stated that “true adsorption is nearly instantaneous. Any lag, at present, can be accounted for by the time required for the dissipation of the heat evolved, or the comparative inaccessibility of a position of the surface of an adsorbing medium.” Brunauer (1942) concurred with this statement. With McBain’s statement in view it was thought worthwhile to investigate mixtures of lithium bromide and cesium bromide in various proportions, letting cesium bromide serve mainly as a heat sink and possibly to increase the sorption rate. Known interactions of cesium and rubidium with lithium salts have inVOL 7
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TIME, MIN 20
0
40
60
80
100
TIME. MIN
Figure 5. Weight gain under isothermal conditions plotted against time for mixtures of LiBr and CsBr
Figure 2. Weight gain under nonisothermal conditions plotted against time for plain asbestos, lithium chloride, and lithium bromide
0
2
4
6
8
10
TIME, MIN TIME, MIN
Figure 3. Weight gain under nonisothermal conditions plotted against time for rubidium salts
Figure 6. Weight gain under isothermal conditions plotted against time for mixtures of RbF and CsF
0 0
20
40
60
BO
100
TIME, M I N
2
4
6
TIME, MIN
8
IO
Figure 4. Weight gain under nonisothermal conditions plotted against time for cesium salts
Figure 7. Weight gain under isothermal conditions plotted against time for mixtures of LiBr and LiCl
dicated striking departures from ideality when considered as crystals in equilibrium with liquids. Therefore, a similar influence upon crystal water-vapor equilibrium is possible. The same may also be true for mixtures of other salts. Sorption rate data of salt mixtures taken under standard con18,000 p.p.m., 138.5 cm. per ditions of operation (4Oo,C., minute) are presented in Figures 5, 6, and 7, wherein weight gained us. time is plotted for the first 10 minutes of sorption. Table I lists the amount of dry salt used in each experimental run.
Temperature Effect. To study the effect of bulk temperature on sorption rates of lithium chloride, runs were made a t a constant inlet humidity of 18,000 p.p.m. and a flow rate of 138.5 cm. per minute but at various temperatures40°, 50°, and 60' C. Figure 8 shows that the sorption rate of lithium chloride at constant water content decreases when the bulk temperature increases. A similar effect was observed with lithium bromide. The sorption rate of lithium bromide is higher than that of lithium chloride a t all the temperatures investigated. Except
412
l&EC PROCESS DESIGN A N D DEVELOPMENT
Table 1.
Weights of Dry Salts Used in Runs
Wt. Salt
of Dry Salt, Mg.
Figure
LiBr LiCl RbBr RbCl RbF CsBr CSCl CsF LiBr: CsBr: : 1 : 1 LiBr: CsBr: :5.5 : 1 LiCl RbF: CsF: : 1 : 1 RbF:CsF: : 3 : 1 LiBr:LiCl: : 3 : 1 LiBr:LiCl: : 5 : 1 LiBr LiCl
3.58 2.34 3.52 7.30 7.68 7.12 8.40 9.14 3.18 5.20 2.02 5.66 6.20 3.12 3.06 3.20 1.94
2 2, 3, 4 3 3 3 4 4 4 5 5 5, 6, 7, 8, 9, 12, 1 4 6 6 7 7 8 10, 13
a t 4OoC., lithium bromide sorbs more moisture than lithium chloride during the first 10 minutes. Effect of Inlet Humidity. TO study the effect of inlet humidity or sorption rates of lithium chloride, runs were made a t a constant flow rate (700 cc. per minute) and bulk temperature (40' C.) but a t humidity levels of 12,000, 18,000, and 24,000 p.p.m. T h e data, weight gained us. time, are presented in Figure 9. Figure 9 shows that the sorption rate of lithium chloride a t constant water content increases when the moisture content of the inlet stream increases. A similar effect was observed in the case of lithium bromide. Effect of Flow Rate. To study the effect of the flow rate on sorption rates of lithium chloride, data were taken a t a constant inlet humidity (18,000 p.p.m.) and a bulk temperature (40' C.) but a t various flow rates-700 (138.5), 333 (65.8), and 200 cc. per minute (39.6 cm. per minute), Weight gained us. time was plotted as shown in Figure 10. This plot shows that the sorption rate of lithium chloride increases as the flow rate of the inlet stream is increased.
Comparison of Cooling Effects Produced by LiCl and LiBr. Cooling effects that can be obtained by using lithium bromide and lithium chloride for Lizenzia air conditioning cycle (Munters, et al., 1960) have been compared (Punwani, 1967). This comparison shows that lithium bromide produces larger cooling effects and provides more flexible operation than lithium chloride. Consider typical Florida summer weather (91" F. dry bulb, 81" F. wet bulb). Assume that the air leaving the drying wheel is in equilibrium with the anhydrous monohydrate of the salt coated on the asbestos. Referring to Figure 11, the outside humid air, 0, can be dried to A or B, depending upon whether 3
g
2
f
A 333cucm/ml
3 0
2
6
4
8
IO
TIME, MIN
I-
r 0
Figure 8. Comparison of weight gained by LiCl and LiBr at various temperatures
si' 0 0
4
I2
8
20
16
TIME, MIN
Figure 10.
0
!5
10
I5
Effect of flow rate on weight gain by LiCl
20
TIME, MIN Figure 9 . Effect gain by LiCl
of
humidity
on
DRY BULB fEhtPERATURE,*F
weight Figure 1 1.
Cooling effects produced by LiCl and LiBr VOL 7
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Table II. Values of rmax under Various Conditions Temp ., Humidity, OC. P.P.M. rmax Figures 27 18,000 3.830 12 40 18,000 1.920 8, 9, 10, 12, 13, 14 60 18.000 0.664 8, 12 40 12;ooo 1.740 9 ; 14 40 24,000 2.385 9, 14 Temperature Dependence of kl (At constant flow rate of 138.5 cm./min. and! humidity of
Table IV.
(At constant temperature of 40' C., and humidity of 18,000 P.PJn.) Flow Rate Cc. /min, Cm./min. ki 700 138.5 0.1380 65.8 333 0.lo75 200 39.6 0.0936 Table V.
Table 111.
18,000 p.p.m.) Temp., OC. 27 40 60
ki 0.1110 0.1459 0.237
Flow Rate Dependence of k,
Humidity Dependence of
kl
(At constant temperature of 40' C., and flow rate of 138.5 cc./min.) Humidity, P.P.M. ki 12,000 0.096 18,000 0,148 24,000 0.205
the drying wheel is coated with lithiun chloride or lithium bromide, respectively. Further, the dehumidified air is cooled by the heat exchange wheel (assuming 90% efficiency of the wheel) to C or C' and subsequently is saturated to give a comfort level of humidity (SOo F. DB; 50% RH) when it is mixed with the room air. The cooling effect is the vertical distance between the isoenthalpy line of the room air, R,and that of the cooled dry air, Cor C'. Lithium chloride gives a cooling effect of 1.7 B.t.u. per pound of dry air, while lithium bromide gives a cooling effect of 7.7 B.t.u. per pound of dry air. Model, An empirical model which describes the effects of various parameters such as temperature, inlet humidity, and flow rate on the dynamic sorption of water vapor by hygroscopic salts was developed. The details of this model are available (Punwani, 1967). According to this model the rate of water sorbed per unit weight of the anhydrous salt is given by the following relationship : dl? - = k l ,?I( - I?) kzr (1) dt
-
where kl and kz are reaction constants, rmsx is the maximum weight of water sorbed per unit weight of the salt, and t is the time in minutes. If the first term on the right-hand side of the equation is much greater than the second term, Equation 1 can be simply integrated to yield
r =1 rma.
- e--lclt
(2)
By making plots of In [(I'mm - l?)/l?mx] us. 1 for various experimental runs for LiCl it was found that kl varies with temperature, humidity, and flow rate. Table I1 lists the values of r, under various conditions. For fixed values of two of the three variables, variation of kl with the third was established. Tables 111, IV, and V tabulate various values of k l . k l was found to follow the relation:
kl = ~ , H , u O exp .~~
(3)
where Ha is the humidity of the inlet stream in parts per million of HzO in dry air and T is the temperature in OK. Hence, the final result is
-I' - - 1 - e x ~ [ - 7 . 1 9 X
rmm
H,~
exp
' 3 . ~ ~
(4) 414
I&EC PROCESS DESIGN A N D DEVELOPMENT
.TIME, MIN Figure 12. Experimental and predicted various temperatures
data
at
Figures 12, 13, and 14 compare the experimental and predicted data at various temperatures, flow rates, and humidity levels. A statistical analysis of the experimental data was made on digital computer; the standard deviation of kl was 3.3854 X while the standard deviation ofk, was 3.9303 X 10-6. All the values of the constant given in Equation 4 are for the lithium chloride data alone. Conclusions
A simple method of comparing the sorption rates and capacities of various salts and mixtures has been devised. Lithium bromide has, by far, the highest sorption rate among the various salts and salt mixtures a t all the temperatures considered. The next best salt appears to be lithium chloride.
I ,o
0,s 0,s
as
087.
0.8
46
0.7
015 x
1
6 Y
E
0.6
L. \
014
a3
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0.5
k
0b4b
1
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I
b 082 A 333 cc/mln 0 700cc/mln PRESENT MODliL
-
0.30
01I
Figure 14. Experimental and various humidity levels
Experimental and predicted data at various
- Present model
2
4
6
TIME, MIN
6
Lithium bromide, for the same surface area, has more water retention capacity than lithium chloride for temperatures above 40’ C. Sorption rates of 1ith.ium chloride and lithium bromide at constant water content decrease with increase in bulk temperature and increase with increase in the inlet humidity. Sorption rates of lithium chloride a t constant water content increase with increase in flow rate of the stream. Lithium bromide can produce a better cooling effect than lithium chloride. The model adequately predicts the adsorption data for hygroscopic salts coated on asbestos sheet. Ac knowledgrnent
The authors express their appreciation to R. A. Macriss, W. F. Rush, and S. A. MIeil for their helpful suggestions and discussion. literature Cited
Allman, A. J., Hand, P. G. T., Manning, J. E., Shiels, D. O., J . Phys. Chem. 33, 1682 (1’329). Barrer, R. M., Endeavour 23, 122-30 (1964). Baxter, G. P., Starkweather, H. W., J . Am. Chem. SOC. 38, 3028 (1916).
4TIME. min 6
IO
0
Figure 13. flow rates
2
Baxter, G. P., Warren, R. D., J . A m . Chem. Soc. 33, 340 (1911). Bichowsky, F. R., Heating, Piping Air Condztioning 12, No. 10, 627 (1940). Booth, H. S., McIntyre, L. H., Znd. Eng. Chem., Anal. Ed. 2, 12 (1930). Brunauer, Stephen, “Adsorption of Gases and Vapors,” Vol. 1, Princeton University Press, Princeton, N. J., 1942.
IO
8
predicted
data
at
Bullock, C. E., Ph.D. thesis, University Microfilms, Inc., Ann Arbor, Mich., 1965. Bullock, C. E., Threlkeld, J. L., Preprint, ASARAE Transactions, February 1966. Davidson Chemical Division, Technical Service Department, W. R. Grace Co., Baltimore, Md., “Fluid Processing Handbook,” 1966. Eagleton, L. C., Bliss, Harding, Chem. Eng. Progr. 49, 534-48 (1953). Elstonalberg, J., Znd. Eng. Chem. 31, 988-92 (1939). Fleming, J. B., Getty, R. J., Townsend, F. .M., Chem. Eng. 71, 69-76 (Aug. 21, 1964). Getty, R. J., Armstrong, W. P., IND.ENG.CHEM.3, 60-5 (1964). Greismer, G. J., Jones, R. A., Sautensack, M., Chem. Eng. Progr. Symp. Ser. (24) 55, 45-50 (1959). Hammond, W. A,, Withrow, J. R., Znd. Eng. Chem. 25, 653 (1933). Hersch, C. K., “Molecular Sieves,” Reinhold, New York, 1961. Hougen, 0. A,, Dodge, F. W., “Drying of Gases,” J. W. Edwards, Ann Arbor. Mich.. 1947. Hubard, S. S.’, Znd. E&. Chem. 46, 356-8 (1954). Lenher, S., Taylor, G. B., Znd. Eng. Chem., Anal. Ed. 2, 58 (1930). McBain, J. W., Z . Physik. Chem. 68, 471 (1909). Munters, C. G;, Stocksund, Norback, P. G., U. S. Patent 2,296,502 (March 1, 1960). Nutter, J. A., Barnet, George, Jr., IND. END. CHEM.PROCESS DESIGN DEVELOP., 5 , 1-5 (1966). Punwani, D., M. S. thesis, Illinois Institute of Technology, January 1967. Simpson, E. A,, Cummings, W.P., Chem. Eng. Progr. 60, 57 (1964). Smith, F. G., “Dehydration Studies Using Anhydrous Magnesium Perchlorate,” G. Frederick Smith Chemical Co., Columbus, Ohio, 1943. RECEIVED for review August 18, 1967 ACCEPTEDFebruary 2, 1968 Financial assistance provided by the American Gas Association through the Institute of Gas Technology to one of the authors, D. Punwani.
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