Langmuir 1986, 2, 173-178 mesitylene as oil, respectively. As expected, the position of the fishes on the temperature scale rises with increasing hydrophobicity of the oil, the fish for benzene being located below the melting point of the mixture. The comparison of these fishes with that for the quinary system HzOtoluene-C,Eo-SDS-NaC1 (represented in a pseudoquaternary phase tetrahedron in Figure 11 in ref 3) demonstrates that the nonionic CloDMPO is even more effective with respect to increasing the mutual solubility between H20and aromatics than that of a combination of butanol and SDS. An addition of NaCl will merely lower the position of the fishes on the temperature scale (see Figure 3). On the other hand, the three-phase bodies of the ternary systems are restricted to a temperature interval of about 10 deg, whereas that of the quinary system (at the particular ratio SDS/C4Eo= studied by Bellocq et
173
al.13) extends from the melting to the boiling point of the mixture. Finally, we note that the phosphorus nucleus 31P is well suited for NMR studies. This work is in progress.
Acknowledgment. This work was carried out in the laboratory of Prof. M. Kahlweit. I am indebted to him and to Dr. R. Strey for suggestion of the problem and advice with the experiments. I am further indebted to D. Luckmann for drawing the figures. Registry No. CloDMPO, 2190-95-6; C,,DMPO, 871-95-4; C14DMP0,2190-96-7; SDS, 151-21-3;NaC1, 7647-14-5; octane, 111-65-9;decane, 124-18-5;dodecane, 112-40-3;toluene, 108-88-3; mesitylene, 108-67-8;xylene, 1330-20-7. (13) Bellocq, A.; Biais, J.; Clin,B.; Gelot, A.;Lalanne, P.; Lemanceau, B. J. Colloid Interface Sci. 1980, 74, 311.
Adsorption Interference in Mixtures of Adsorbate Gases Flowing through Activated Carbon Adsorber Beds Richard Madey,* Panos J. Photinos, and Daniel Rothstein Department of Physics, Kent State University, Kent, Ohio 44242
Robert J. Forsythe Broome Community College, Binghamton, New York 13902
Jan-Chan Huang Department of Plastics Engineering, University of Lowell, Lowell, Massachusetts 01854 Received June 27, 1985. I n Final Form: November 6, 1985 We studied adsorption interference in several binary mixtures and one ternary mixture of adsorbate gases in a helium carrier gas flowing through activated carbon adsorber beds at 25 O C . Interference is manifested by a reduction of the adsorption capacity of each component in the mixture from the value for a pure adsorbate and an outlet concentration greater than the inlet concentration for the weaker adsorbing component until the stronger adsorbing component elutes. No interference effects were seen when both components of the mixture exhibit linear isotherms. The magnitude of the interference depends on the adsorption capacities and relative concentrations of the two components in the mixture.
Introduction The measurement and interpretation of the dynamic behavior of binary mixtures are necessary to the understanding of the adsorption and transport of multicomponent mixtures. Most practical applications of the adsorption process involve the transport of mixtures of several adsorbable gases. The application of gas adsorption on solid adsorbents to separate a component from a gas mixture requires knowledge of two general subjects: (1) the physicochemical properties of the gas-solid interaction, particularly adsorption isotherms of the various components, and (2) the dynamic behavior of the key gas components in the separation device. Three diffusion processes are important in the transport of a gas through a porous medium: longitudinal diffusion in the gas phase, mass transfer through the film a t the gas-solid interface, and solid-phase diffusion within the adsorbent particles. These three processes are characterized by coefficients DL, k F , and D,, respectively. The differential equations for the isothermal adsorption of an absorbate gas in an inert carrier gas are
and
Here C is the gas-phase concentration, u is the interstitial flow velocity, t i s the void fraction of the adsorber bed, q is the adsorbed-phase concentration, and Q is the adsorbed-phase concentration averaged over the volume of the adsorbent particles. For spherical particles, of radius
R, 3
Q = R3 - l R q0( r , z , t ) r 2 dr
The introduction of an average concentration eliminates the radial dependence of the solid-phase concentration from eq 1. The differential equation relating q and C is1
(4) Equations 1-4 contain two dependent variables C and q (1) Lapidus, L.; Amundson, N. R. J. Phys. Chem. 1952,56, 373.
0743-7463/86/2402-0173$01.50/0
(3)
0 1986 American Chemical Society
174 Langmuir, Vol. 2, No. 2, 1986
Madey et al.
AE
I-
r
-
c. Y)
v)
E
I
v)
FCB
FCC
z 4
az t
0 T I M E ,
F i g u r e 2. Single-component transmission curve.
F i g u r e 1. Schematic diagram of the flow system.
which are related at the gas-solid interface by the adsorption isotherm: q =
F(C)
(5)
A mathematical difficulty exists when the adsorption isotherm is nonlinear; however, analytical solutions were derived on the basis of simplifying assumptions. Systems with linear isotherms were solved by Lapidus and Amundson' who assumed negligible resistance to solidphase diffusion and by Masamune and Smith2and Rosen3 who assumed negligible diffusion in the gas phase. GlueckauP presented an analytical solution for a Langmuir system in equilibrium. Solutions for systems with nonlinear isotherms require either numerical integration or simplifying assumptions such as neglecting one or more of the diffusion processes. Experimental Technique We generated experimental data on the isothermal time-dependent transmission of several binary mixtures and one ternary mixture of organic adsorbates in a helium carrier gas flowing through activated carbon adsorber beds. The transmission of each component of the mixture is the ratio of the outlet concentration to the inlet concentration. The cylindrical adsorber beds were packed with "Columbia" type 4LXC 12/28 activated carbon. The Brunauer-Emmett-Teller (BET) surface area5 for this carbon is 1130 m2/g. The mass of the activated carbon in each adsorber bed was measured after 36 h of desorption a t 200 "C with helium flowing through the adsorber a t a rate of 200 cm3/min. The diameter of the adsorber bed was kept small in order to avoid excessively long experiments and t o keep the pressure drop, the Reynolds number, and the length-to-diameter ratio within reasonable values. Repeated experiments verified the reproducibility of the results. The adsorber bed temperature was 25 "C and was controlled to A20 mdeg by means of a dual-bath constant-temperature system. The flow system, shown schematically in Figure 1,is constructed of shinless-steel tubing and valves with Teflon seats and gaskets. Flow controllers FCA, FCB, and FCC enable setting of the gas flow rate from cylinders A, B, and He (usually helium), while (rotameter) flow indicators RA, RB, RC, and RD give an approximate value of the flow rate. Three-way ball valves B1, B2, B3, and B4 can divert the flow to the electronic dual-channel mass-flow controller, which controlls the flow rate to * l % . The flow meter FM measures the flow rate with an accuracy of *0.5%. Three-way ball valves B5, B6, and B7 enable mixing of the three gas streams, while similar valves B8 and B9 permit two modes, namely, flow through the adsorber bed and flow bypassing the adsorber bed. The latter configuration is useful for calibration and normalization purposes. The pressure drop across the bed (2) Masamune, S.; Smith, J. M. AIChE J. 1965, 11, 34. (3) Rosen, J. B. J. Chem. Phys. 1952, 20, 387. (4) Glueckauf, E.; Barker, K. H.; Kitt, G. P. Discuss. Faraday SOC. 1950, 7, 199.
(5) Manes, M., private communication.
t
was measured by using the manometer M and by appropriate manipulation of toggle valves T 3 and T4. The automatic dataacquisition system, located downstream from the adsorber bed, consists of an automated sampling valve, a (Varian Model 3700) gas chromatograph with a flame ionization detector, and a digital integrator. The data-acquisition system records the outlet concentration of each gas a t regular time intervals. The gas mixtures were prepared by specialty gas suppliers, and were checked against primary standards. The concentrations were known to within 1%. In view of the small concentration of the adsorbates in the mixtures, the flow rate a t the inlet is equal to the flow rate a t the outlet. Because the volumetric flow rate is constant, one can convert the concentration vs. time plots into concentration vs. eluted volume plots.
Single-Component Transmission (or Breakthrough) Curve Figure 2 is a typical transmission (or breakthrough) curve for a single-component adsorbate. The adsorbate gas in a nonadsorbing or weakly adsorbing carrier gas (such as helium or nitrogen) flows through an adsorber bed at a volumetric flow rate Q. The cylindrical bed has length L, cross-sectional area A , and volume V (=AL). The interstitial flow velocity u is the volumetric flow velocity Q divided by the product EA. The dimensionless adsorption capacity6 of the adsorbent for the adsorbate is denoted by
K, where qo is the solid-phase concentration of the adsorbate that is in equilibrium with the gas-phase concentration Co of the adsorbate a t the inlet to the bed. Four adsorbent parameters govern the transmission of the adsorbate through the bed (viz., the dimensionless adsorption capacity K and the three diffusion coefficients DL,D,, and kF). If the bed consists of an empty pipe or tank devoid of adsorbents, then the holdup time is simply V/Q. Now if we pack the bed with adsorbent, and if we consider the ideal case of no diffusion (DL= 0) and instantaneous equilibrium (D,= k p = a),then the holdup time is amplified by a factor B , called the effective adsorptivity:6 B = t + K(1- t) (7)
If there is no diffusion, a step function increase in adsorbate concentration a t the input to the bed will propogate through the bed without distortion and appear at the output of the bed a t a propagation time t,: t, = BV/Q (8) The only property of the adsorbent that affects t , is the (6) Huang, J.-C.; Forsythe, R.; Madey, R. Sep. Sci. Technol. 1981, 16, 475.
Langmuir, VoE. 2, No. 2, 1986 175
Adsorption Interference of Adsorbate Gases
Table I. Solutions for the Transport of Binary Systems through an Adsorber Bed model of binary system isotherm solution Langmuir (binary) two single-component systems equilibrium (nondispersive) Langmuir (binary) algebraic equilibrium (nondispersive) Langmuir (binary) two single-component systems nonequilibrium (constant pattern) sequential single nonequilibrium (pore-diffusion controlled) Langmuir (one component), Langmuir (binary) for other component nonequilibrium Freudlich (binary) numerical (finite difference)
dimensionless adsorption capacity K. In the actual case with diffusion, the concentration of the adsorbate gas at the bed output “breaks through” a t a time earlier than the propagation time t , and rises along an S-shaped curve until the outlet concentration equals the inlet concentration Co. The transmission T of the adsorbate is given by the ratio of the adsorbate concentration C ( L ) at the bed outlet to the concentration Co at the inlet.
T ( t ) = C(L,t)/Co
(9)
Adsorption Isotherms Adsorption isotherms of adsorbates can be determined by a chromatographic method, which requires numerical integration of a time-dependent transmission curve and calculation of the solid-phase concentration from a mass-balance equation,6 namely, qo(1 - E)L+EC& = suCoJm[l - T ( t ) ]dt
(10)
The right-hand side represents the moles of adsorbate retained in the adsorber bed. The first term on the lefthand side is the moles of adsorbate retained on the granules of the solid adsorbent; the second term is that in the interstitial regions. Now the propagation time t, is defined by the time-integral of one minus the transmission:
t , = Jm(l- T ) dt
(11)
Thus, the mass balance eq 10 can be rewritten:6
Thus, by measuring the time-dependent transmission curve, we can obtain the dimensionless adsorption capacity from eq 12. Since eq 12 is the result of mass balance, it is model-independent and valid for any gas-solid isotherm. The dynamic technique is much more precise than conventional gravimetric methods for measuring small adsorption capacities in the region of low concentration^.'^^ Single-component isotherms commonly used in the literature can be represented by the following expression: 40
=
a0coa
b
+ alCoo
When b = a = 6 = 1,eq 13 yields the Langmuir isotherm: aoco 4o = 1 + q C o
If b = 0, eq 13 becomes the Freundlich isotherm: qo = ACoa-O (7)Lee, K.B.;Madey, R. Nucl. Sci. Eng. 1971,43,27. (8)Madey, R.; Photinos, P. J. Carbon 1979,17,93.
(15)
ref 14 15 16 17
18
When b = 1, p = a , and al = ao, then eq 13 becomes an isotherm introduced by Chakravarti and Dharg for chemisorption and used by SipslO and Koble and Corrigan:ll
Isotherms that are commonly used for the prediction of the equilibrium behavior of binary mixtures are extensions of single-componentisotherms. Most binary isotherms can be derived from the following generalization12of eq 13 for each component i: aOiCOiab 4Oi =
bi +
2
i=1,2
(17)
ai,CojoLJ j=l
For bi = ai= pij = 1, the binary isotherm reduces to a modified Langmuir isotherm first used by G1~eckauf.l~ For bi = 0, eq 17 yields a modified Freundlich isotherm used by Fritz and Schleunder.12J4
Transmission of Binary Mixtures Most practical applications involve the transport of gaseous mixtures rather than only a single gas. Two coupled differential equations are required to describe the dynamic behavior of the transport of a binary mixture. In some special cases, certain simplifying assumptions permit the differential equations to be uncoupled and solved as two noninteracting single-component systems. Special cases that were reported in the literature are listed in Table I. Glueckauf13showed that a binary nondispersive system in local equilibrium with Langmuir isotherms separates into a “pseudobinary” single-component system. By assuming that local equilibrium is attained a t all points in the bed, Helfferich and Klein15 reduced the coupled differential equations to algebraic equations and predicted the qualitative behavior of multicomponent mixtures. Cooney and Strusi16 showed that a binary system with Langmuir isotherms uncouples under constant pattern conditions. If the isotherm of only one of the components in a binary mixture depends on the second component, Carter and Husain17showed that a sequential solution is possible in terms of single-component isotherms. For the general case, only numerical solutions were pursued. Monsour et a1.18 surveyed the models used to predict the dynamic behavior of multicomponent mixtures. (9) Chakravarti, D. N.; Dhar, N. R. Kolloid-2. 1907,43,377. (10)Sips, R. J. Chem. Phys. 1948,16,490. (11)Koble, R.; Corrigan, T. E. Ind. Eng. Chem. 1952,44,383. (12)Fritz, W.;Schleunder, E. U. Chem. Eng. Sci. 1974, 29, 1279. (13)Glueckauf, E.h o c . R . SOC.London, Serl A 1946,186,35. Schleunder, E. U. C h e n . Eng. Sci. 1981,36,721. (14)Fritz, W.;
(15)Helfferich, F.; Klein, G. ‘Multicomponent ChromatographyTheory of Interference”; Marcel Dekker: New York, 1970. (16)Cooney, D. 0.; Strusi, R. P. Ind. Eng. Chem. Fundam. 1972,11, 123. (17)Carter, J. W.;Husain, H. Chem. Eng. Sci. 1974,29, 267. (18)Monsour, A.;von Rosenberg, D. U.; Sylvester, N. D. AIChE J. 1982,28, 765.
176 Langmuir, Vol. 2, No. 2, 1986
Madey et al.
Table 11. Binary Systems Studied Experimentally by Prior Investigators binary system adsorbent temp, "C carrier gas carbon dioxide (COP),ethane (CZH6) carbon 25 N2 ethylene (C2H4), ethane (C2H6) benzene (CsH6),toluene (C,H,) methane (CH,), propane (C3H8) methane (CHI), butane (C4Hio) methane (CH,), hexane (C6H14j pentane (C5H12),hexane (C,Hl,)
carbon carbon carbon carbon carbon silica gel
25 150 25 25 25 36
CH, CH4 CH4
hexane (C6H14),benzene (CsH6) water vapor (HzO) carbon dioxide (eoz), methane (CHI), ethane (C2H6)
silica gel molecular sieve (type 5A) molecular sieve (type 5A)
70, 130 0 20
He He He
Table 111. Transmission of Binary Systems in a Helium Carrier Gas through Activated Carbon" Beds at 25 "C applicable single-component binary system concn, ppm isotherm type methane 53 linear (8) acetylene (25) 57 linear (8) Freundlich (30) acetone 100 (unavailable) Freon 113 (25) 100 acetaldehyde 90, 66, 49, 33 Freundlich (30) propane (25, 26) 12, 34, 52, 67 (unavailable) acetylene 67, 49, 33 linear (8) ethane (26) 167, 250, 332 Freundlich (30) ethane 2410, 3150, 5100, 6660 Chakravarti-Dhar (29) propane (27) 7780, 7000, 4860, 3440 Chakravarti-Dhar (29) "Columbia 4LXC 12/28 Activated Carbon, Union Carbide Corporation.
There exists a limited amount of experimental data on the transmission of binary gaseous mixtures through adsorber beds. A review of the literature reveals that previous investigators studied six binary systems on activated carbon a d ~ o r b e n t : l ~two - ~ ~on silica ge123-25and two on molecular ~ i e v e . ' The ~ ~ ~binary ~ systems studied by these prior investigators are shown in Table 11. In this paper, we report the transmission of five binary gaseous system^^'-^^ through an adsorber bed packed with (Columbia type 4LXC 12/28) activated carbon a t 25 "C. Also we studied one ternary mixture through activated carbon adsorbent.27 As indicated in Table 111, the single-component isotherms of the seven gases used in this study are either linear, Freundlich, or Chakravarti-Dhar. Isotherms are not available either for propane below 100 ppm or for Freon 113, and the isotherm for acetone is generated from adsorption potential data.30 Noninterfering Binary Mixtures: Methane-Acetylene Shown in Figure 3 are time-dependent transmission (19) Zwiebel, I.; Kralik, C . M.; Schnitzer, J. J. AiChE J . 1974,20, 915. (20) Gariepy, R. L.; Zwiebel, I. AiChE Symp. Ser. 1971, 67, 17. (21) Thomas, W. J.;Lombardi, J. L. Trans. Inst. Chem. Eng. 1971,49, 240. (22) Grant, R. J.; Manes, M. Ind. Eng. Chem. Fundam. 1966,5,490. (23) Needham, R. B.; Campbell, J. M.; McLeod, H. 0. Ind. Eng. Chem. Process Des. Dev. 1966, 5, 122. (24) Collins, H. W., Jr.; Chao, K.-C. AIChE Symp. Ser. 1973, 69, 9. (25) Shen, J.; Smith, J. M. Ind. Eng. Chem. Fund. 1968, 7, 106. (26) Robinson, K. S.;Thomas, W. J. Trans. Inst. Chem. Eng. 1980,58, 219. (27) Madey, R.; Photinos, P. Mech. Eng. 1980, publication 80-ENAs17.
(28) Madey, R.; Photinos, P. J. AIP Coni. h o c . 1980, No. 61, 333. (29) Madey, R.; Forsythe, R.; Huang, J.-C. "Proceedings of the IX Meeting on Flow through Porous Media", Salvador, Brazil, October, 1981. (30) Photinos, P. J.; Nordstrom, A.; Madey, R. Carbon 1979,17, 506.
1.2 z
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0.8
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l
l
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l
l
l
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l
l
l
i
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23 24 25 17 26 l
l
l
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32
z
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0.0 0.00
0.01
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E L U T E D VOLUME,
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Figure 3. Transmission of a binary mixture consisting of 51 ppm acetylene and 53 ppm methane in helium at a volumetric flow rate of 100 cm3(STP)/min through a (30 cm long and 1.716 cm i.d.) adsorber bed at 25 "C packed with 26.14 g of "Columbia" 4LXC 12/28 activated carbon.
Table IV. Dimensionless Adsorption Capacities on Columbia 4LXC 12/28 Activated Carbon at 25 "C for Four Gas Mixtures" dimensionless adsorption capacity re1 capacity pas mixture C,, m m K K* KIK*,% freon 113 52 47 57 82 48 9.8 24 41 acetone 3.5 11 acetaldehyde 49 0.38 100 58 64 91 freon 113 100 14 34 41 acetone propane 52 3.3 3.8 87 acetaldehyde 49 2.7 3.5 77 0.050 0.050 100 acetylene 51 methane 53 0.0040 0.0040 100 K* refers to the corresponding single-component values.
curves for a noninterfering binary mixture of methane and acetylene with approximately equal concentrations, viz., 53 ppm of methane and 51 ppm of acetylene. Plotted on the abscissa is the eluted volume (in cubic meters at STP). The eluted volume is the product of the time and the volumetric flow rate Q, which in these measurements was 100 cm3(STP)/min. For these measurements, the bed was a cylinder (1.716 cm i.d. X 30 cm long) packed with 26.14 g of "Columbia" 4LXC 12/28 activated carbon. The adsorption capacity of each component'in the binary system was obtained from eq 11 and 1 2 and found to have the same values (listed in Table IV) as that obtained in a single-component measurement (with the same adsorber bed under the same operating conditions of flow rate and temperature). The transmission curves for each of these two gases in the mixture are the same as those obtained in single-component measurements. Both methane and acetylene show linear isotherms on carbon in this con-
Langmuir, Vol. 2, No. 2, 1986 177
Adsorption Interference of Adsorbate Gases
Table V. Dimensionless Adsorption Capacities on Columbia 4LXC 12/28 Activated Carbon at 25 "C for Four Ethane 6600 ppm
t
t
AA
Om8
0.4
4
" 1
1
t t
Propane
A
run 1 2
3
3440 p p m 1
.-
4
%
17 100 22 98 32 94 37 95
1.6 1.11 1.2
carbon.
1.0
: : :
L
1.o
0.8 0.6 0.4 0.2 0.0
1 X
0.10
0.8
0.15
ELUTED VOLUME,
.0.6
L
0.0 0
Ill
1 2 3 4
15 Eluted Volume
7780 7000 4860 3440
0.20
M3
CSTP;
Figure 6. Transmission of a binary mixture consisting of 100 ppm acetone and 100 ppm Freon 113 in helium at a volumetric flow rate of 100 cm3(STP)/minthrough a (10 cm long and 0.46 cm i.d.) adsorber bed at 25 "C packed with 0.596 g of "Columbia" 4LXC 12/28 activated carbon.
Concentration (ppm) R u n Propane Ethane
C
0.4
KIK*,
K* refers to the corresponding single-component values.
Eluted Volume (10'stp cc 1 Figure 4. Transmission of a binary mixture of 6600 ppm ethane and 3440 ppm propane in helium at a volumetric flow rate of 0.874 cm3(STP)/minthrough a (10 cm long and 0.46 cm i.d.) adsorber bed packed with 0.588 g of "Columbia" 4LXC 12/28 activated
E
Mixtures of Ethane and Propanea dimensionless adsorption capacity COP component ppm K K* ethane 2410 2113 358 propane 7780 7245 7280 ethane 3150 2011 443 propane 7727 7000 7560 1798 ethane 5100 581 propane 9585 4860 9050 ethane 6660 1673 615 propane 3440 11120 11680
2410 3150
5100 6600
30
45
( lo2 stp cc 1
Figure 5. Transmission of ethane for several ethane-propane mixtures. centration range;* accordingly, this binary mixture should act as two noninterfering single-component systems, which is the case.
Interfering Binary Mixtures Figure 4 illustrates the transmission of an interfering binary mixture of ethane and propane. For these measurements, the bed was a cylinder (0.46 cm i.d. X 10 cm long) packed with 0.588 g of "Columbia" 4LXC 12/28 activated carbon. Note that ethane breaks through first. The transmission of ethane rises above one and decreases again to one when propane elutes. Single-component adsorption isotherms of ethane and propane obey Chakravarti-Dhar-type isotherms on carbon a t 25 0C.29 In Figure 5, we compare the transmissions of four different concentrations of ethane in a binary mixture of ethane and propane in helium. The total concentration of the binary mixture in helium was approximately constant. In the four mixtures, the heavier propane displaces ethane from the adsorber. The adsorption capacities of both binary components are reduced from their singlecomponent values, as shown in Table V. Note that the overshoot increases with decreasing ethane concentration; for example, the maximum overshoot is about 0.09 for 6660 ppm ethane (run 4) and is about 0.35 for 2410 ppm ethane
(run 1). The observation that the overshoot increases with lower ethane concentration is related to a reduction of the adsorption capacity of ethane in the presence of propane. Since the adsorption capacity for ethane decreases with a higher concentration of propane, the highest overshoot is observed for the highest concentration of propane. Note also in this figure that ethane elutes earlier when its input concentration is higher. Since ethane moves more quickly than propane, the initial concentration front is almost pure ethane, and the elution speed depends only on the ethane concentration; therefore, the higher the ethane concentration, the sooner it elutes. This result is a consequence of the convex nature of Chakravarti-Dhar isotherm obeyed by ethane. Since the dimensionless adsorption capacity K decreases with increasing concentration for a convex isotherm, then the propagation time t, must decrease also as seen from eq 12. Shown in Figure 6 is the transmission of a binary gas mixture of 100 ppm acetone and 100 ppm Freon 113 in helium at a volumetric flow rate of 100 cm3(STP)/min through a (10 cm long and 0.454 cm id.) adsorber bed at 25 "C packed with 0.596 g of "Columbia" 4LXC 12/28 activated carbon. The dimensionless adsorption capacities are listed in Table IV. The adsorption capacity of acetone in the binary mixture is 41 % of its single-component value, while that of Freon 113 is reduced to 91%. Shown in Figure 7 is the transmission of a binary gas mixture of 49 ppm acetaldehyde and 52 ppm propane in helium a t a volumetric flow rate of 200 cm3(STP)/min through a (10 cm long 0.454 cm i.d.) adsorber bed a t 25 "C packed with 0.596 g of "Columbia" 4LXC 12/28 activated carbon. As seen from the dimensionless adsorption capacities listed in Table IV, the relative capacities are
178 Langmuir, Vol. 2, No. 2, 1986
? I
Madey et al.
3.32
3 33
E L U T E D VOLUME,
M3
c4
1,
ISTP!
Figure 7. Transmission of a binary mixture consisting of 49 ppm acetaldehyde and 52 ppm propane in helium a t a volumetric flow rate of 200 cm3(STP)/min through an adsorber bed (10 cm long and 0.454 cm i d . ) a t 25 "C packed with 0.569 g of "Columbia" 4LXC 12/28 activated carbon.
:.s
- c .,.Y
.
c.
G. 2
1
E L L T E E VOLLIKE,
M3
:STPI
Figure 8. Transmission of a three-component gas mixture consisting of 49 ppm acetaldehyde, 48 ppm acetone, and 55 ppm Freon 113 in helium a t a volumetric flow rate of 353 cm3(STP)/min through a (5.0 cm long and 0.455 cm i d . ) adsorber bed a t 25 "C packed with 0.289 g of "Columbia" 4LXC 1 2 / 2 8 activated carbon.
86% for propane and 77% for acetaldehyde.
Interfering Ternary Mixture Shown in Figure 8 is the transmission of a three-component gas mixture of 49 ppm acetaldehyde, 48 ppm acetone, and 52 ppm Freon 113 in helium at a volumetric flow rate of 353 cm3(STP)/min through a (5.0 cm long and 0.455 cm i.d.) adsorber bed at 25 "C packed with 0.289 g of "Columbia" 4LXC 12/28 activated carbon. The dimensionless adsorption capacities are listed in Table IV. The adsorption capacity of acetaldehyde is reduced to nearly one-tenth of the single-component value, the adsorption capacity of acetone is about one-quarter of its single-component value, and the adsorption capacity of freon-113 is reduced to about 83% of its single-component value.
Discussion An important feature of Figures 4-8 is that the transmission value of some of the components exceeds unity over some range of the eluted volume. In terms of concentrations, this fact indicates that the outlet concentration exceeds the inlet concentration for the corresponding adsorbate. This phenomenon should be a t t r i b ~ t e dto~ the ~,~~ displacement from the adsorbing surface of the fast-eluting component by the slow-eluting component of the mixture; for instance, in Figure 8, acetaldehyde is displaced from the adsorbed phase into the gaseous phase by acetone, and acetone, in turn, is displaced by the even slower eluting Freon 113. The displacement of acetaldehyde by Freon 113 is almost insignificant, since most of the displacement of acetaldehyde was carried out by acetone already. Strong displacement is observed also in Figure 6 for the mixture of acetone and Freon 113;here the transmission of acetone reaches the value of 1.5. The extensive displacement mentioned above can be explained in part by the affinity to the adsorbing surface of the different single-component gases. For equal concentrations, the overall affinity is reflected in the values of the single-component adsorption capacities listed in Tables IV and V. The less pronounced overshoot observed in the propane-acetaldehyde mixture should be attributed t o the almost equal adsorption capacities of these two compounds. No overshoot was observed in a study31 of adsorption interference on polystyrene of two components (viz., butane and 1,3-butadiene) with nearly the same adsorption capacities. One should note also from Tables IV and V that the adsorption capacities for the components of the binary and ternary mixtures are reduced substantially in comparison with the corresponding single-component values. This reduction results form the competition between the components of the mixture for adsorption on the available surface. Further interpretation of the transmission curves for a binary mixture along the lines of the models listed in Table I requires a knowledge of the binary isotherm for that system. Additional research is needed to establish the binary isotherms. The potential of pronounced displacement phenomenon deserves special consideration in the design of hardware for the removal of trace contaminants from breathing atmospheres in closed ecological systems. Acknowledgment. This work was supported in part by the Division of Chemical Sciences, Office of Basic Energy Sciences, Department of Energy, and by the National Aeronautics and Space Administration. Registry No. Carbon, 7440-44-0; ethane, 74-84-0; propane, 74-98-6; acetone, 67-64-1; Freon-113, 76-13-1; acetaldehyde, 7507-0. (31) Madey, R.; Rothstein, D.; Wu, B.-G.; Huang, J.-C Society of Automotive Engineers (SAE), Technical Paper No. 831121, 1983.
