,
460
INDUSTRIAL AND ENGINEERING CHEMISTRY
ETACIDNl :d
HEAVY LINES WIRIN,G
POTENTIOMETER
GHOST VIEW
BASIC DIAGRAM
I
RESISTANCE
T
h S T GAS INLET
"\REFERENCE HLET
GAS
FIGURE 2. THERMAL CONDUCTIVITY CELL
CONSTANT FLOW
FROM TANKS
Vol. 14, No. 6
I n actual runs the variable pressure drop through the apparatus produces effects upon the cell readings which require correction. These corrections become increasingly important at the higher rates of flow. However, precise values are possible because increased pressure does not alter the linear nature of the millivolt-composition relation. Therefore, it is only necessary to consider each reading taken during a given run as a fraction of the final, saturation reading. This fraction multiplied by the composition obtained from the calibration curve for an open system millivolt value is the true composition.
Adsorption Studies The method of conducting adsorption studies is diagrammatically indicated by Figure 5. A metered stream of the test gas is analyzed by cell 1 in order to check its composition continuously while the remainder is bypassed. The two streams reunite and flow through a carbonpacked tube containing the desired amount of activated carbon. Continuous analysis of the exit gas is made by cell 2. A second carbon tube is inserted to ensure the passage of pure air through the flowmeter and wet-test meter. The indicated valve system CELL MANOMETER (pressure-reducing valves and needle valve) makes possible a constant flow rate. The entire setup is enclosed in an air thermostat.
f LOWM E T E R
The agreement between the actualamount of vapor adsorbed as determined CARBON TUBE by direct weighing of the NO. I carbon-packed tube, before and after the run, and the amount of adsorption calculated from the TO effluent gas analysis has been used as an over-all measure of the success of CONDUCI'IVITY CELL6 FIGURE 3. APPmTua FOR CALIBUTION OF THERMAL the experimental method. The-data obtained in a typical run, in which chloroform vapor was adsorbed from mixtures then entered a tube packed with bone-dr active carbon (Figure 3). The amount of material adsorged was a chloroform-air stream, are recorded in Table I. This run determined by direct weighing; a second carbon-packed tube, was carried out at a rate of flow of 1.98 standard liters of weighed before and after passage of the gas, was used as a check air per minute and a composition of 104 mg. of chloroform on completeness of adsorption in tube 1. The volume of the pure air in the total amount of gas was directly measured by a per standard liter of dry air. precision wet-test meter. The relation: c = -W
1
L
(1)
where C
= composition in milligrams of vapor per liter of air, L = liters of air passed, and W = mg. of vapor adsorbed, allowed ready computation of the desired composition.
Mixtures containing air and benzene, dichloromethane, chloroform, or carbon tetrachloride all showed a simple linear relation between millivolts and composition. The calibration curves for cells 1 and 2 for chloroform-air mixtures are shown in Figure 4. These data were obtained for a cell current of 200 milliamperes and a cell-block temperature of 30' C.
YILLIQRAYS VAPOR PER STD LITER AIR
ANALYTICAL EDITION
June 15, 1942
46 1
c
ADSORPTION STUDIES APPA R AT U$ FIG. 5
BASIC C E L L DIAGRAM 4LL CELLS IN SERIES WITH BATTERY; IN PARALLEL WITH POTENTIOMETER
The amount of vapor adsorbed a t the break point and a t saturation is readily calculated. I n any interval of time, dt, the amount of absorbate entering the tower is: CORdt (2) where CO= feed composition in weight of vapor per unit volume of dry air and R = volume rate of passage of pure air, and will be constant for any given run
A good approximation of this area can often be made by treating the total area as a sum of the areas of a rectangle and a right triangle, as indicated by the dotted line in Figure 6. The calculation is indicated herewith for the run repre-
The amount of vapor which passes from the tower without being adsorbed in the same time interval is CRdt, where C = the effluent gas composition. Therefore, the amount of vapor adsorbed is (C, - C) Rdt (3) Integrating this expression from t = 0 to any time, t ,
gives the total amount adsorbed-i.
x"
e.,
(Co - C) Rdt.
Graphically, this integral is evaluated by determining the area under the curve (CO - C) us. t , and multiplying by R.
TABLE I. ADSORPTION DATAFOR Time Min. 0
5
6 8 9 10 11 12 13 14 15 16
17 18 19 20 21 22 23
Removal
Ma. 0
Break point 0.05 0.10 0.22 0.41 0.61 1.02 1.30 1.78 2.29 2.98 3.50 3.95 4.25 4.40 4.50 4.59 4.68
% 100
...
99 98 95.5 91.5 87.5 78.8 73.0 63.0 52.5 38.1 27.4 18.0 11.8 8.7 6.6 4.8 2.9
A
TYPICAL RUN C
MQ./1.
0 i:o4 2.08 4.68 8.84 13.0 21.1 28.0 38.4 49.5 64.3 75.6 85.2 91.6 95.0 97.0 99.0 101
co-c Mg./l. 104 104 103 101.9 99.3 95.2 91.0 81.9 76 65.6 54.5 39.7 28.4 18.8 12.4 9.0 7.0 5.0 3.0
TIME
(MINUTES)
INDUSTRIAL AND ENGINEERING CHEMISTRY
462
sented by Table I and Figure 6.
99 9 99 8
Vol. 14, No. 6
A more complete and extensive treatment of adsorption data mill be presented in a later paper.
99 5
Further Applications
99
Ca = 104 R = 198 Area A B C D = 520 units Area ABCGF 1580 units Mg. of CHCls adsorbed a t break point = 1030 mg. per 5.0 grams of carbon Jlg. of C H C h adsorbed a t saturation = 3120 mg. per 5.0 grams of carbon
98
-
95
$ 90 Y
E w
80 70 60
2
50 40
6
30
eo
/ I : I
,
!
I I
I \ I id1
I I
The method described is readily adapted to the investigation of the unsteady state conditions which exist in an adsorption tower. The variables of interest, composition of feed gas, rate of gas flow, mesh size of carbon, height of carbon bed, etc., can all be conveniently studied. Work along these lines will be reported from this laboratory at a later date.
.4t the point of inflection y t = a.
1
t - a "-
=
I
_1
50. Therefore,
b is readily evaluated because b =
-
t2
t--a ~
b
= 0 and
- t where t z corre-
sponds t o y2 = 92.3 and ti corresponds t o yl = 7.70. For the data of Figure 6, a = 15.0 and b = 5 . 2 ; the data reduce to:
I
'DEADSORPTION OF CHCI,
1
I5
25
I
0 ' 5 0
a'
9
4o
I- 1 0
~
,-
FIG 9
!
10
.
I
.4 satisfactory agreement has been found be1 I I I 1 1 l'\>Pl tlveen the calculated amount of vapor adsorbed and the amount found bv direct FIGURE8. ADSORPTIOXEFFICIENCY weighings; a large us. TIME number of runs Feed composition,liter 104ofmg have demonairof chloroform per Feed rate, 1 9 8 standard liters of air per strated that these minute two values agree Ivithin 5 per cent. A number of duplicate runs have shown that this method makes possible a n accurate reproduction of results. In Figure 7 the effluent analysis us. time curves for two check runs are shown. These data were obtained for 5-gram samples of a coconut-shell carbon, a feed-gas composition of 32 mg. of chloroform per standard liter of dry air, and a gas rate of 1.83 standard liters of air per minute. The carbon bed was 1.8 cm. in diameter and 5.0 cm. in height. Very fair agreement is apparent. If the efficiency of removal of chloroform from an airchloroform mixture is plotted us. time on cumulative probability paper, as shown in Figure 8, a very fair straight line results. Although deviations a t the two ends of the line are apparent, this plot may in many cases be useful in making comparisons between different carbons. The efficiency-time relation after the break point can be approximated by the relation:
r
I
-"
I
3 LO W L
b IO 0
0
5
IO
10
30
15
TIME MINUTES
Adsorption phenomena involving a multicomponent gas mixture can be investigated by this technique, provided that the components can be quantitatively and individually removed by specific absorbents or adsorbents. Deadsorption studies, where an inert gas is used to remove an adsorbed substance from an adsorbent, can be readily carried out. Figure 9 indicates the progress of deadsorption of a carbon which was first saturated with chloroform vapor from a chloroform-air mixture containing 80.0 mg. of chloroform per liter of air a t 30' C., 760-mm. pressure. In this case, R
K
Cdt is the amount of vapor removed from the
carbon up to time t .
Summary Batch-mixing of vapor and gas facilitates reproduction of use conditions in adsorbent testing. Use of the thermal-conductivity cell for analysis of the effluent gases from a carbon-packed tower makes possible accurate determination of the stoichiometry of the adsorption process. This method is especially useful in large-scale equipment for indicating the extent of adsorption. An empirical equation which can be employed to represent efficiency us. time relations in adsorption studies has been presented. Some further applications of the method are in the study of desorption of adsorbed vapors and in the testing or investigation of adsorption of vapors from multicomponent systems.
Literature Cited (1) Brinker, W. E., Ph.D. thesis, University of Pittsburgh, 1940.
This equation is ertremely convenient; the constants can be evaluated almost by inspection. For any value of t , the t--a
corresponding value of --
/"
4, 0
in tables of the probability integral.
e-"*dz is readily found
(2) Daynes, H. A., "Gas Analysis By Measurement of Thermal Conductivity", London, Cambridge University Press, 1933. (3) Palmer, P. E., and Weaver, E. R., U. S. Bur. Standards, Tech. Paper 249 (1924). (4) Perry, J. H., "Chemical Engineers' Handbook", 1st ed., p . 1090, New York, McGraw-Hill Book Co., 1934. (A) Robinson, C. S., "Recovery of Volatile Solvents", p. 48. New York, Chemical Catalog Co., 1922. PARTof this work submitted in partial fulfillment of the requirements for the degree of M.S. in chemical engineering.
