Prediction of Removal of Vapors from Air by Adsorption on Activated Carbon Eric B. Sansone*, Yadu B. Tewari, and Leonard A. Jonas Environmental Control and Research Laboratory, Frederick Cancer Research Center, Frederick, Md. 2 170 1
w Carbon granules, packed to a reproducible bulk density in beds of uniform cross-sectional area, were subjected to constant inlet vapor concentrations, volume flow rate, and temperature. The breakthrough time of the vapor from the bed ( t ~ , a) t, an exit concentration equal to 1%of the inlet concentration, was determined for several bed weights ( W ) .Using carbon tetrachloride as the reference vapor, removal performance of the carbon was determined from the adsorption capacity ( W e )and adsorption rate constant (k,) for the vapor, as calculated from the straight line curves obtained when t b was plotted vs. W . From these properties, the kinetic adsorption equations for chloroform, benzene, p-dioxane, 1,2dichloroethane, sec- butylamine, chlorobenzene, and acrylonitrile were predicted. Comparisons of predicted with experimental parameters revealed variations of from -1.8 to +7.7% for W eand from -12.9 to +14.4% for K,. This technique yields reliable predictions of the adsorption behavior of carcinogenic, or otherwise toxic vapors, on carbons. Use of activated carbons for the removal of vapor phase potentially hazardous materials in air streams, e.g., in hood exhaust systems and respirator cartridges, raises an important risk-benefit problem: if the carbon is disposed of or regenerated too soon, the full benefit of the system is not realized; if its capacity is exceeded, risk to personnel and/or the environment may ensue. I t is, of course, possible to test the capacity of a carbon for a particular adsorbate; however, not only does this require special equipment and precautions, but also it is hardly practical in view of the multitude of candidate adsorbates. In this paper, it is demonstrated that the adsorption capacity and the adsorption rate constant of a carbon for an adsorbate can be predicted using current adsorption theory and the data obtained from characterization of the carbon with a reference adsorbate. With this information it is possible accurately to predict the capacity of an activated carbon for a particular adsorbate.
sorption by beds of activated carbon under dynamic flow conditions, was originally derived by Wheeler ( 4 ) from a continuity equation of mass balance between the vapor entering an adsorbent bed and the sum of the vapor adsorbed by plus that penetrating the bed. The Wheeler equation can be written ( 5 ) :
t ~=, (W,/CoQ)[W- PRQ In (Co/C,)/k,l
where t h = the vapor breakthrough time (min) a t which the concentration C, (g/cm") appears in the exit stream, Co = the inlet concentration (g/cm"), Q = the volumetric flow rate (cmii/min),PB = the bulk density of the packed bed (g/cm3), W = the absorbent weight (g), W e = the kinetic adsorption capacity (g/g) a t the arbitrarily chosen ratio of C,/Co, and k , = the pseudo-first-order adsorption rate constant (min-l). In Equation 1 the parameters Co, C,, and Q are set by the conditions of test, and PB is measured. When breakthrough time ( t h ) is plotted vs. carbon weight ( W ) ,a straight-line curve results, from whose slope and x-axis intercept the properties W,,and h , can be calculated, respectively. Equilibrium Adsorption The Dubinin-Radushkevich (D-R) equation (6):
W , = WOexp[-B(T/PI2 log2 (PoIP)]
Kinetic Adsorption The kinetic equation, applicable to the study of vapor ad0013-936X/79/0913-1511$01.00/0
(2)
describes the adsorption properties of fine-grained activated carbons for vapors under equilibrium conditions. W, = the volume of condensed adsorbate (cm7/g), Wo = the maximum space available for condensed adsorbate (cm3/g), P = the equilibrium pressure (atm) of the adsorbate vapor a t T (K), Po = the saturated vapor pressure (atm) of liquid adsorbate a t T, B = the structural constant of the adsorbent (K-2), and @ = the affinity coefficient (unitless), which compares the strength of the adsorptive interaction of the adsorbate to some reference adsorbate. By taking the logarithm of both sides of Equation 2 and by setting:
Adsorption Model Physical adsorption has been considered to be second order in kinetics, involving the reaction between an active site and a free vapor molecule ( I , 2 ) . However, it has been shown ( 3 ) that the sigmoid curve, resulting from a plot of vapor concentration exiting a packed bed of activated carbon granules vs. time, exhibits second-order kinetics only in the linear or mid-portion of the curve where the exit concentration rises rapidly with time. The first portion of the sigmoid curve, with low concentrations of vapor penetrating the bed, is convex to the time axis; this portion of the curve represents an excess of active sites over free vapor molecules and displays pseudo-first-order kinetics with respect to vapor molecules. Analogously, the final portion of the curve, where the number o f vacant sites is rapidly being depleted, and which is concave to the time axis, represents an excess of free vapor molecules over active sites, and therefore displays pseudo-first-order kinetics with respect to active sites. This study deals only with the adsorption characteristics of the carbon bed a t low gas penetrations (Il%),and is therefore characterized by pseudo-first-order kinetics with respect to gas molecules.
(1 1
B = (2.303)2R2k
(3)
(Po/P)
(4)
and E
= RT In
the D-R equation can be written: In ( W,) = In ( W o )- K E ~ / P *
(5)
where R = the gas constant (1.987 cal/(mol.K)), h = a constant of the adsorbent (cal/mol)-2, and E = the adsorption potential (cal/mol). The affinity coefficient @ can be obtained (7) from of the test adsorthe ratio of the electronic polarization (P,) bate to that of the reference adsorbate. P, is related t o n , the refractive index determined a t the sodium D wavelength, by: n2
-1
P , = -(Midi) (6) n2 2 where M = the molecular weight (g/mol) and dl = the liquid density (g/cm:'). By characterizing an adsorbent with a reference adsorbate, setting p equal to unity, and calculating the parameters B and W Ofrom Equation 2 or k and W ofrom Equation 5, the adsorption space W, can be calculated for untested vapors from
@ 1979 American Chemical Society
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Volume 13, Number 12, December 1979 1511
~~
the above equations. T o convert Wv (cm3/g) to adsorption capacity (g/g), the concept of volume pore filling (8), wherein the adsorbed gas condenses to a liquid and fills the adsorption space of the micropore, can be used: We = W,d
1
(7)
where dl is the liquid density of the condensed test gas. I t has been shown (9) that the adsorption capacity determined under kinetic conditions in a well-packed bed of carbon granules closely approximated that calculated under equilibrium conditions from the D-R equations. The only constraint was that the carbon had first to be characterized with a reference vapor before the equilibrium relationships could be used to calculate the carbon's adsorption capacity for other vapors. Adsorption Rate Constant The adsorption rate constant k , has been found t o be a function of temperature, linear flow velocity, adsorbent grain size, and the molecular weight of the adsorbate vapor (10-1.2). In this study, however, temperature, flow velocity, and adsorbent grain size were held constant. The only variable that affected h , was the molecular weight of the vapors under test. After characterization of the carbon with the reference vapor, the rate constants for other vapors were predicted from the relationship (12):
where ref indicates the reference vapor (i.e,, carbon tetrachloride) and i is a test vapor. Experimental
Materials. The vapors used in this study were: carbon tetrachloride, 99+%, Baker Chemical Co., Phillipsburg, N.J.; chloroform and benzene, Fisher certified, Fisher Chemical Co., Waltham, Mass.; p-dioxane and acrylonitrile, 99+%, and sec- butylamine, chlorobenzene, and 1,2-dichloroethane,99%, Aldrich Chemical Co., Milwaukee, Wis. The activated carbon adsorbent used was 6-10 mesh, BC-AC Lot 0993 granular carbon from Barnebey-Cheney Co., Columbus, Ohio. The mean diameter of the carbon granules was 0.268 cm. Equipment. The kinetic adsorption tests were carried out on a vapor adsorption test apparatus connected with standard taper and ball and socket joints. The apparatus had three functional sections, one for vapor generation, another for vapor adsorption by the carbon, and the third for the detection of vapor penetration of the carbon bed. T h e apparatus has been described in detail (3). The vapor penetrating the bed was passed into the 0.29-cm" gas sampling value of an H P 5830A gas chromatograph, then into the chromatographic column, and finally into the flame ionization detector. The response was recorded on an HP 18850A terminal. The column consisted of 4 ft of l/s in. (0.d.) stainless steel tubing packed with 3% silicone S E 30 on a Chromosorb W HP, 80-100 mesh support for use with the vapors of carbon tetrachloride, chloroform, benzene, chlorobenzene, and 1,2-dichloroethane. For p-dioxane the column consisted of 6 ft of in. (0.d.) stainless steel tubing packed with 5% silicone DV 225 on a Chromosorb W H P , 100-120 mesh support. For sec- butylamine the column consisted of 6 ft of l/8 in. (0.d.) copper tubing packed with 5% Carbowax 20M on a Chromosorb W H P , 80-100 mesh support. For acrylonitrile the column consisted of 6 ft of */a in. (0.d.) copper tubing packed with 5% DEGS on a Chromosorb W HP, 80-100 mesh support. Procedure. The air-flow rates of the apparatus were Calibrated by a wet test meter. The adsorbate vapor was generated 1512
Environmental Science & Technology
Table 1. Coefficients of the Regression Equation tb = a -k bW, and the Correlation Coefficient of the Data, r vapor
a
b
r
chloroform benzene pdioxane sec-butylamine 1,2-dichIoroethane chlorobenzene acrylonitrile
-27.41 -19.29 -24.17 -61.13 - 13.58 -36.24 -69.16 -33.10
31.78 23.73 41.55 104.48 19.90 59.88 87.19 60.49
0.987 0.989 0.992 0.981 0.987 0.982 0.982 0.992
cc14
by the controlled passage of dried nitrogen over the liquid surface; the desired vapor concentration was obtained by diluting the vapor-laden nitrogen with an auxiliary source of dried nitrogen. Nitrogen was dried by passage through deep-bed columns of Drierite. A flow of 350 cm"/min of the vapor-nitrogen mixture was directed into the reservoir of the apparatus, and samples were analyzed until the desired vapor concentration was obtained. The activated carbon was oven dried a t 120 " C and stored in a desiccator until used. Adsorbent beds were prepared by gravity settling of the carbon granules in cylindrical glass sample holders (this procedure yielded uniform bulk densities). Eight to ten different adsorbent bed weights, ranging from 1 to 2.5 g, were exposed to the established vapor concentration drawn into the carbon bed at a volumetric flow rate of 285 cm"/min, corresponding to a superficial linear velocity of 323 cm/min, a t 23 "C. The inlet vapor concentrations represented a relative pressure (23 "C) of 0.0936 for carbon tetrachloride, chloroform, p -dioxane, benzene, see-butylamine, and acrylonitrile. The relative pressure was 0.3448 for chlorobenzene. (The relative pressures were calculated assuming ideal gas behavior for each vapor.) The exit air stream from the carbon bed was monitored continually by passage into a gas sampling valve. The breakthrough time tb was the time in minutes when the exit stream vapor concentration was 1%of the inlet concentration (C,/CO = 0.01). Results and Discussion Experimental values of gas breakthrough time tb as a function of carbon bed weight W for the vapors tested were obtained. These data were subjected to linear regression analysis in the form of the polynominal tb = a bW. The results of these analyses, a and b , and the correlation coefficients of the data are shown in Table I. An examination of Equation 1 will show that:
+
b = We/CoQ
(10)
From the known values of COand Q, the kinetic adsorption capacity, We, can be determined from Equation 10. With this , and C,, the value of W e and the known values of p ~ Co, pseudo-first-order adsorption rate constant can be determined from Equation 9. By setting tb = 0 in the regression equations of Table I, and solving for W, the values for W,, the critical weight of the carbon bed, were obtained, since W W, when tt, = 0 and the inlet concentration is instantaneously reduced to the exit concentration by the carbon bed. Considering carbon tetrachloride as the primary reference vapor, it was possible to calculate the adsorption capacity for vapors after initial characterization of the carbon, by using the set of equations 2 through 7. The characteristic properties of the BC-AC Lot 0993 activated carbon, derived from Equation 5, were a maximum adsorption space WOof 0.481
Table II. Comparison of Experimentally Determined with Calculated Adsorption Parameters vapor
cc14 chloroform benzene pdioxane sec-butylamine 1,2-dichIoroethane chlorobenzene acrylonitrile
adsorption capacity, We ( W g ) exptl calcd % dev
0.741 0.728 0.404 0.476 0.331 0.616 0.545 0.404
0.693 0.409 0.483 0.337 0.583 0.527 0.375
+5.1 -1.2 -1.4 -1.8 $5.7 -3.4 4-7.7
adsorption rate constant, kV (rnin-') exptl caicd % dev
735 780 1029 1083 928 1048 799 1160
834 1031 971 1066 916 859 1251
-6.5 -0.2 +11.5 -12.9 +14.4 -7.0 -7.3
(cal/mol)-*. cm.i/gand a structural constant k of 1.5 X Thus, a t a relative pressure, PIP", of 0.0936 the adsorption space W , was 0.467 cm;'/g, and a t a PIP0 of 0.3448, W, was 0.478 cm:'/g. Values for the kinetic adsorption capacity W ewere calculated for the various vapors by using Equation 7, inserting into it the known liquid densities and the W , value for the relative pressure used in the test. Comparisons of experimental and calculated adsorption capacity values for the various vapors are shown in Table 11. The experimental values deviated from those calculated over B range of from -1.8 to +7.7%, with a mean deviation of 3.8%. Values for the adsorption rate constant k , were calculated for the various vapors by using Equation 8, inserting into it the square root of the known molecular weight for the vapor. Comparisons of experimental and calculated adsorption rate constant values are also shown in Table 11. The experimental values deviated from those calculated over a range of from
-12.9 to +14.4%, with a mean deviation of8.,5%. In summary, we have characterized a Rarnebey-Cheney activated carbon using carbon tetrachloride as the reference vapor. Using these data, and applying the theories detailed above, the two basic adsorption parameters of the carbon were obtained. From these basic parameters the expected adsorption behavior of the carbon for seven other vapors was predicted. The mean deviation of the experimental from the predicted values was 8.5%. Insertion of the predicted values into the Wheeler adsorption equation (Equation 1)provides an estimate of the length of time an adsorbent can be expected to operate effectively in removing contaminant vapors from an air stream. On the basis of these results, we believe that the methodology described can be extended to other activated carbons and other vapors.
Literature Cited ( 1 1 Hirster, i1952 1,
N. K., Vermeulen, T., Chcm. Eng. Progr., 48, 5Oij-16
( 2 ) Masamune, S.. Smith, J. M., AIChE J . , 10, 246-52 (1964) (:1) .Jonas. I,. A,. Svirbely, LV. J., J . Catai., 21,446-59 i1972i. (4) LVheeler, A,, Robell, A. J., J . C'atai., 13, 299-305 11969). (.-I) .Jonas, I,. A , , Rehrmann, J . A,. Carbon. 10, 657-63 11972). (6) Dubinin, M.M., Prog. S u r f . M e m b r . Sci., 9, 1-70 (1975). ( 7 ) Reucroft, P. J . , Simpson. W.,J.,
