Ind. Eng. Chem. Res. 1991,30, 2333-2340 size distribution occurs in the wake at latter stages of the reaction.
Acknowledgment We acknowledge Professor Dennis Foreman of the Department of Geology and Mineralogy at The Ohio State University for helpful discussion. The work was supported by NSF Grant CTS-8905463.
Nomenclature
L = particle size, pm AL = increment of measured size, pm n = population density defined by eq 1,number/(mm2pm) AN = number of particle in a unit area Registry No. Ca(OHI2, 1305-62-0;C02, 124-38-9;CaCO8, 471-34-1.
Literature Cited Boynton, R. S. Chemistry and Technology of Lime and Limestone; Wiley: New York, 1980;pp 380-482. Fan, L.-S.; Tsuchiya, K. Bubble Wake Dynamics in Liquids and LiquidSolid Sus ensions; Butterworth Stoneham, MA, 1990;pp 295-329. Garside, J.; Davey, R. J. Secondary Contact Nucleation: Kinetics, Growth and Scale-up. Chem. Eng. Commun. 1980,4,393-424. Jean, R. H.; Fan, L A . Rise Velocity and Gas-Liquid Mass Transfer of a Single Large Bubble in Liquids and Liquid-Solid Fluidized Beds. Chem. Eng. Sci. 1990,45,1057-1070.
2333
Juvekar, V. A.; Sharma, M. M. Absorption of C02 in a Suspension of Lime. Chem. Eng. Sci. 1973,28,825-837. Laine, J. Manufacture of Precipitated Calcium Carbonate. Pap. Puu 1980,11,725-735. Maruecak, A.; Baker, C. G. J.; Bergougnou, M. A. Calcium Carbonate Precipitation in a Continuous Stirred Tank Reactor. Can. J . Chem. Eng. 1971,49,819-824. Morris, R. M. The Control of Particle Characteristics in Precipitation and the Inter-Relationship of Size-Analysis Techniques. Analyst 1965,90,657-663. Randolph, A. D.; Lamon, M. A. Theory of Particulate Processes, 2nd ed.; Academic Press: San Diego, CA, 1988;Chapter 1. Sohnel, 0.; Mullin, J. W. Precipitation of Calcium Carbonate. J . Cryst. Growth 1982,60,239-250. Swinney, L. D.; Stevens, J. D.; Peters, R. W. Calcium Carbonate Crystallization Kinetics. Znd. Eng. Chem. Fundam. 1982,21, 31-36. Tsuchiya, K.;Fan, L.-S. Near-Wake Structure of a Single Gas Bubble in a Two-Dimensional Liquid-Solid Fluidized Bed: Vortex Shedding and Wake Size Variation. Chem. Eng. Sci. 1988,43, 1167-1181. Tsuge, H.; Kotaki, Y.; Hibino, S. Fkactive Crystallization of Calcium Carbonate by Liquid-Liquid Reaction. J. Chem. Eng. Jpn. 1987, 20,374-379. Wachi, S.;Jones, A. G. Mass Transfer with Chemical Reaction and Precipitation. Chem. Eng. Sci. 1991,46, 1027-1033. Weber, H.C.; Nilsson, K. T. The Absorption of Gases in Milk of Lime. Znd. Eng. Chem. 1926,18,1070-1075. Wray, J. L.;Daniels, F. Precipitation of Calcite and Aragonite. J. Am. Chem. SOC.1957, 79,2031-2034. Receioed for reuiew November 8, 1990 Revised manuscript received December 3, 1990 Accepted June 20,1991
GENERAL RESEARCH Adsorption of Hydrocarbons and Other Exhaust Components on Silicalite Klaus Otto,* Clifford N. Montreuil, Olimpia Todor,?Robert W. McCabe, and Haren S. Gandhi Research Staff, Scientific Research Laboratories, Ford Motor Company, Dearborn, Michigan 48121-2053
Temporary storage of vehicle emissions, in particular hydrocarbons (HC’s), was evaluated by adsorption on silicalite. Silicalite, a zeolite consisting almost completely of silica, is characterized by a three-dimensional system of intersecting channels of molecular dimensions. The adsorption capacity of the zeolite for individual exhaust components was measured gravimetrically as a function of pressure, temperature, and time. Initial adsorption of a representative HC from simulated car exhaust was measured in a flow reactor. The requirements for practical storage of HC’s during coldstart by adsorption are quantitatively evaluated, and the commanding influence of competitive water adsorption is examined. The volume of silicalite required for temporary adsorption of HC’s emitted before the catalytic converter is hot and operational is found to be too large to be practical. A primary reason for insufficient HC storage is the competitive adsorption of the high concentration of water vapor present in automotive exhaust.
Introduction air pollutants Catalytic converters designed to from fully operational only at
* Author to whom correspondence should be addressed. Teacher Fellow, summer 1989.
sufficiently high temperatures. Therefore, emissions that exit the engine while the exhaust system is still cold pass over the catalyst intact. These coldstart emissions can comprise an appreciable fraction of the total air pollutants released into the atmosphere. Hydrocarbons (HC’s), in particular, emitted by a car while the catalyst is cold, can account for more than 60% of the integrated HC emis-
0888-5885/91/2630-2333$02.50/00 1991 American Chemical Society
2334 Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 sions, as defined by the Federal Test Procedure (FTP). It would be beneficial to trap these coldstart emissions and oxidize them later on a hot catalyst. Trapping of air pollutants can be achieved, in principle, by adsorption on a suitable solid. The goal of this work is to assess, on the basis of laboratory experiments, the adsorption characteristics and limitations involved in the trapping of HC‘s emitted during coldstart on a thermally stable adsorbent, silicalite. Automotive exhaust consists of many components, including more than 100 different HC’s. In order to be able to predict relative adsorption capacities, it is necessary to evaluate adsorption characteristics of representative HC’s and other exhaust constituents. In particular, it is important to know the adsorption capacity for water vapor, a major exhaust component that has the potential to interfere with the adsorption of HC species present at much smaller concentrations. The empirical evaluation of HC storage by adsorption is guided by several basic considerations critical for the practical design of a temporary storage device. One aspect is the variation in maximum adsorption capacity at equilibrium for different HC’s. Other concerns are the initial rate of adsorption and the maximum molecular size that can be accommodated by the adsorbent pore structure. These aspects require different experimental approaches. A static gravimetxic apparatus and a flow reactor system were used for the acquisition of various adsorption data. On the basis of the results of both experimental methods an estimate is made with respect to the dimensions of a practical adsorption trap to be used with the complex gas mixture of actual car exhaust.
Basis of an Adsorption Trap for Coldstart Emissions 1. Concept of Adsorption Trap. Solid materials that are effective in accommodating substantial amounts of both fuel components and their combustion products by physical adsorption require a relatively large specific surface area. Examples of common adsorbents are active carbons and molecular sieves. Active carbons have the advantage of a larger specific surface area. Molecular sieves, such as zeolites, on the other hand, are thermally more stable. Thus, an adsorptive zeolite trap could be positioned near the engine in front of the catalyst. The zeolite would be self-generating in such a configuration; HC’s adsorbed during coldstart would desorb and react as the catalyst warms up and becomes operational. Activated carbons, in contrast, must be positioned far behind the exhaust catalyst to prevent combustion of the carbon. This arrangement, in turn, requires a complicated valving and piping arrangement to isolate the trap from the exhaust stream during trap regeneration and to purge the adsorbed HC’s into either the intake air or the exhaust stream in front of the catalyst after warm-up. 2. Choice of Adsorbent. Zeolites, a class of molecular sieves, exist in a large variety of natural and synthetic structures (Rabo, 1976). The science of zeolite materials has grown into a vast field, especially in the area of catalysis. For the research reported here, silicalite was selected as an adsorbent for coldstart HC’s mainly because of its thermal stability. Silicalite consists essentially of pure silica (Flanigen and Grose, 1977; Flanigen et al., 1978; Kokotailo et al., 1978; Dessau, 1980; Olson et al., 1981). The silicalite zeolite is a medium-pore 10-member-ring structure that is similar to ZSM-5 zeolite. Both silicalite and ZSM-5 zeolites are classified under the pentasil group (Barer, 1984; Szostak, 1989). The silicalite structure is described by two channel
Table I. Adsorption of Selected Exhaust Components on Silicalite at Room Temwrature capacity at max 1 Torr, kl, capacity, pmol/(g ko, (Torr ko/kl, adsorbent g)/mmol g/mmol mmol/g Torr) Torr 839 0.409 2.44 1.19 2052 CH4 4.11 0.464 2.15 21 8.85 C3H6 C3H8 4.68 0.525 1.90 19 8.91 C4H1o 2.53 0.620 1.61 318 4.08 C6H14 0.66 0.752 1.33 709 0.88 CHSOH 1.90 0.258 3.88 464 7.36 C2H60H 0.68 0.380 2.63 941 1.80 6.70 0.444 2.25 140 15.1 0.738 1.35 2467 0.41 3342 197 0.340 2.94 5.06 580 COZ 3470 0.591 1.69 0.29 5865 NZ 0 2 3175 1.260 0.80 0.32 2530
Et)
systems of molecular dimensions (Flanigen et al., 1978; Kokotailo et al., 1978). Silicalite can be heated to 900 “C without irreversible changes; it converts to silica glass at 1300 “C (Flanigen et al., 1978). In addition, it has been claimed that silicalite adsorbs HC’s preferentially over water (Flanigen et al., 1978). Hydrophobic adsorption is an obvious advantage for the preferential adsorption of HC’s from burnt fuel, as the exhaust gas contains a substantial amount of water. Since HC adsorption is considered to be physisorption, the overriding property governing the adsorption capacity is the available surface area given by the geometry of the silicalite structure. The specific surface area of silicalite is reasonably large. The material tested here had a surface area of 331 m2/g when evaluated according to the BET method by nitrogen adsorption at the temperature of liquid nitrogen. Evaluation based on the Langmuir isotherm yielded a considerably higher adsorption capacity of 490 m2/g, under the same adsorption conditions. The discrepancy between the two values is not surprising, considering that the application of the BET isotherm for measuring the surface area of molecular sieves, which is located predominantly in cages of molecular dimensions, is questionable. The BET isotherm (Brunauer et al., 1938), based on physical adsorption, requires the formation of multiple adsorption layers. The BET value is simply given here for an empirical comparison with other molecular sieves for which specific surfaces are often derived from the BET equation without consideration of the limitation mentioned. Finally, as the experimental results will show, the channels in the silicalite structure, with a free cross section of 0.54 f 0.02 nm (Flanigen et al., 1978) are wide enough to permit diffusion of gas molecules of interest with relative ease. 3. Choice of Adsorbate. Three groups of representative gases were selected to assess adsorption equilibria of typical exhaust components. The first group consisted of HC’s commonly present in automotive exhaust, i.e., methane, propylene, propane, butane, and hexane (Table I). These HC’s tested as prototypes include two limiting cases. Methane is included, as the most volatile HC, and neopentane (not shown in Table I) is included to demonstrate a geometric limitation of the silicalite adsorbent. The second group, methanol and ethanol, are of interest as representatives for accepted alternate fuels. Finally, water vapor, C02,CO, N2,and O2were included as major constituents of automotive exhaust. 4. Adsorption Capacity. As mentioned above, description of the physical adsorption of HC’s in molecular sieve structures by the BET equation is based on a wrong assumption. An expedient description is obtained by using the Langmuir isotherm (Hayward and Trapnell, 1964), in
Ind. Eng. Chem. Res., Vol. 30, No. 10,1991 2335 agreement with earlier studies of methane adsorption on zeolites (Otto, 1982) at higher pressures. A priori it is expected that the physical adsorption of HC's at lower pressures in the silicalite channels conforms to the principles of the Langmuir isotherm, as long as the adsorbate layer is not influenced by lateral forces. A derivation of the Langmuir isotherm and the respective coordinate transformation employed to describe the experimental data reported here are given in the Appendix. Since, as will be shown, the adsorption data of typical HC's in the pressure range of interest are fitted well by the Langmuir isotherm, there is obviously no need to invoke a more complex adsorption isotherm, such as the Temkin or Freundlich isotherms, which are basically the Langmuir isotherm modified by assuming a distribution of adsorption energies rather than a single one. 5. Experimental Strategy. Adsorption characteristics of an individual gas component were measured accurately and conveniently by using a static microbalance system. On the other hand, adsorption of a multicomponent mixture requires a more specific analysis. Therefore, the adsorption of a typical HC in the presence of other exhaust gas components, especially water vapor, was measured in a flow reactor. This apparatus is useful in measuring the initial adsorption kinetics that limits the trapping of coldstart emissions. Materials and Experimental Procedures The silicalite powder was manufactured by Union Carbide and deposited onto a monolithic honeycomb by Corning Inc. The adsorption was studied by two methods. The first technique was used to measure adsorption equilibria and kinetics of individual exhaust components on the silicalite powder. The second method examined the adsorption of a selected HC from a gas mixture simulating car exhaust during the initial adsorption process on silicalite deposited onto a honeycomb support. 1. Adsorption Equilibria. One of the adsorption studies was carried out with a Cahn microbalance, which can be used to measure mass changes in a controlled atmosphere, including vacuum. The silicalite powder was contained in a bucket made of quartz glass. This method is suited for accurately measuring the adsorption capacity and kinetics of a single gas as a function of pressure, temperature, and time. Obvious limitations exist in measuring simultaneously more than one adsorbate, as only the total change in sample mass is recorded. Some information for multiple adsorption, however, can be gained by consecutive experiments within an error limit caused by a possible displacement of the first adsorbate by the second one. Adsorption isotherms of a silicalite powder were measured for a series of gases and vapors at two or more temperatures, to obtain general information on the pressure and temperature dependence of adsorption equilibria. The study included some kinetic measurements as well. The adsorbed gas was removed after the completion of an isotherm by evacuation at 250 "C until a constant weight was established. Adsorption equilibria were reached typically within 20-60 min and were confirmed by the agreement of points measured after adsorption and desorption steps. 2. Adsorption Transients of Simulated Car Exhaust. The second series of experiments was carried out in a flow reactor with a synthetic gas mixture containing the major components found in automotive exhaust. The reactor system consisted of a reactor tube, made of quartz glass, and an analytical train. Desired gas compositions, produced via a mixing manifold, passed either through the sample or through a bypass line into the analyzers. Gases
were blended by calibrated mass-flow controllers to ensure constant and accurate flows. The analytical train included a CO detector (NDIR), a flame-ionization detector (FID) to measure total HC concentration, a zirconia sensor for measuring the concentration of oxygen, and a quadrupole mass spectrometer. A monolithic core sample (diameter 1 in., length 1.5 in.), similar to the honeycomb substrate conventionally used to fabricate automotive catalysts, coated with 19.5 w t % silicalite, was employed for the adsorption tests. Automotive exhaust was simulated by a gas mixture containing 12% C02,2.5% H20, 1.0% CO, 0.33% Ha, 1500 ppm C3H6(propylene), lo00 ppm NO, and 20 ppm SOz. The components and concentrations of this mixture are typical for engine exhaust and are commonly used in our laboratory to test the performance of automotive catalysts. Propylene was selected as a typical HC. Usually, for the evaluation of catalyst performance, a mixture of propylene and propane is employed to represent HC's of different reactivity. Because of the similar adsorption characteristics of propane and propylene (see Table I), the adsorption studies were carried out with propylene as the only HC. It should be pointed out that the water vapor concentration of 2.5% was lower than the concentration found in hot automotive exhaust (lo%), since excess water condenses under coldstart conditions. At the start of the adsorption experiments, the reactor tube containing the adsorbent sample was flushed with nitrogen and sealed off; the desired gas mixture was then blended, routed through the bypass line, and, after analysis, was passed over the silicalite sample at a space velocity of 50000 h-l. The decrease in HC concentration from the baseline bypass value was recorded and integrated over time to yield HC adsorption as a function of time. All values were corrected for the contributions from the substrate by comparison with an uncoated monolith. The adsorption was monitored for not more than 2 min, judged to be an appropriate time for coldstart simulation on a vehicle before warm-up triggers desorption and catalytic combustion. Although true adsorption equilibrium was not reached in these experiments, it was noted that after 2 min of adsorption the HC level had almost returned to the baseline value, indicating that the values measured at the end of the runs did not greatly differ from equilibrium values. The sample was purged for 10 min by heating in flowing nitrogen at 250 "C to complete desorption before the sample was cooled to room temperature for the next run. Results and Discussion I. Gravimetric Measurements. 1. Single-Component Adsorption. Depending on the purpose, it may be desirable to express adsorption capacity either in terms of moles or grams of adsorbate per gram of adsorbent. Automotive emissions are usually reported in grams. On the other hand, when measuring surface coverage, the number of moles adsorbed per unit surface provide a more immediate understanding of spatial requirements. Examples for the two representations are given in Figures 1and 2. When viewed superficially, a qualitative comparison of the two figures provides a different impression of the adsorption. For example, when adsorption is based on the number of adsorbed molecules, at 40 Torr, butane adsorption exceeds hexane adsorption, since the butane molecule is somewhat smaller. In contrast, plotted in the coordinates of Figure 2,hexane adsorption is about 40% larger than butane adsorption because of the difference in molecular weight. The adsorption capacity of silicalite for water is relatively small, compared to that of butane, ethanol, or hexane. This holds especially when the ad-
2336 Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991
E
f
300
Carbon Dioxide
L.
;1.5
-t $
Butane
a
Hexane
f
200
.-c
E 0 100
.L
I
0
40
80
120
160 200
240
o - - I n "
280
0
Pressure (Torr)
Figure 1. Adsorption capacity of silicalite at 23 'C expressed in mmol/g.
i
a
401
-
100
150
200
300
250
350
Pressure (Torr)
Figure 3. Adsorption expressed in the coordinates of the Langmuir isotherm.
-
Butane
i
50
c
I
.-b
a
-
n 0
n
Carbon Dioxide
AI
40
80
160 200 Pressure (Torr)
120
n Methone
A
240 280
Figure 2. Adsorption capacity of silicalite at 23 "C expressed in mg/g.
sorption is expressed in milligrams per gram because of the low molecular weight of water (Figure 2). The degree to which HC's are preferentially adsorbed relative to water, known to increase with the Si/A1 ratio of the ZSM-5 structure, is of obvious relevance for the adsorption of small concentrations of HC's from exhaust gas containing a considerable amount of water. It should be noted that coadsorption of water with other adsorbates is not necessarily a matter of a simple linear addition of the individual surface layers. Considering that surface coverage 6, i.e, the amount adsorbed, at zero pressure is always zero, it follows that the isotherms for butane, hexane, and ethanol, as plotted in Figures 1 and 2, are strongly curved at pressures below 10 Torr. Thus,interpolation to the small partial pressures characteristic for air pollutants in automotive exhaust is difficult in the linear coordinates of Figures 1 and 2. Interpolation to lower pressures can be greatly facilitated by plotting the adsorption isotherms in different coordinates. A good linear relationship of the adsorption isotherm is found, if the data are plotted in terms of the Langmuir isotherm, after the appropriate ordinate transformation (see the Appendix). A t room temperature, the lines representing fuel vapors, i.e., butane, hexane, and ethanol, intersect the ordinate axis in close proximity of the coordinate origin, while adsorption at the same temperature for such gases as COz (Figure 3) and CHI (shown in Figure 4 for two temperatures) is characterized by a substantial ordinate intercept ko. The ko values at room temperature are listed in Table I. It should be noted that because of the reciprocal nature of the ordinate unit, the adsorbed amount, taken at a given pressure, decreases as the ordinate increases. Thus, for the compounds shown in Figure 3, ethanol represents the largest quantity adsorbed and COZ the smallest one. The samples did not indicate a decrease in adsorption capacity by irreversible aging. Thus, the measurements are reproduced satisfactorily as
250:
,.-
c
r
c
0
-
r_
2
70 60
L
$
50
40 30
.
0
.- 20
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'
o
~
0
50
100
I
Il
I
I
I
I
,
I
I
I
,
150 200 250 300 350 400 Pressure (Torr)
Figure 5. Carbon monoxide adsorption on silicalite at liquid nitrogen temperature: fresh (0) and thermally aged ( 0 )silicalite powder.
shown for adsorption of CO on two different silicalite samples (Figure 5), one of which was used after a series of adsorption measurements; it had never been heated above 350 "C.The second sample was as received, but had been heated to 800 "C before the isotherm was measured. The Langmuir isotherm is completely described by two parameters. The reciprocal slope of the line l / k , = m,, as given in the Appendix, describes the maximum adsorption capacity of silicalite for a given compound. In Table I, column 4, these maxima, measured at room temperature, are given in millimoles per gram for 12 adsorbates. As expected, with the increase in HC size from propane to butane to hexane, the maximum capacity decreases accordingly. The ratio k , / k , can be used as the second parameter with a physical meaning, as it is a measure of the volatility of an adsorbate molecule. These values are listed in the last column of Table I, showing a change over 4 orders of magnitude for the molecules examined. An alternate expression for the second adsorption parameter is the adsorption capacity at a fixed pressure, selected as 1 Torr in the fifth column of Table I. The data
Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 2337 15.0 L
t
1
I
0 Bulane Corrected for Water
g
2.51
0 Butane, Dry Silicalile
/
o0 l d
"""""' 125
250
Preadrorbed Water
25 Q
I
1 ' ' ' ' ' ' ' ' ' 1 ' ' ' '
375 500 Pressure (Torr)
625
750
Figure 6. Methane adsorption on silicalite.
0
500
600
Figure 8. Effect of preadsorbed water on butane adsorption. 120 F
f 4 ll
2E 300-0
300 400 Pressure (Torr)
200
100
-
+ Water
P /
" / 9
LNel Melhonol
Methanol Water
0 0
50
100 150 200 250 3 0 0 350 400 450 500 Pressure (Torr)
Figure 7. Butane adsorption on silicalite.
in Table I and Figures 1and 2 show that N2,COO,and CO, which are major components in automotive exhaust, are adsorbed only very sparsely because of their low boiling points. Note that for the same reason methane adsorption from automotive exhaust is highly inefficient. Another advantage of analyzing adsorption in the linear representation of the Langmuir isotherm is the fact that, in general, isotherms measured for a selected adsorbate at different temperatures yield the same maximum adsorption, measured by the reciprocal slope, as long as the space requirement of the adsorbate does not change with temperature. An example of such parallel isotherms is shown for methane in Figure 4. In Figure 6, the same absorption data are plotted simply in milligrams per gram vs pressure. At the higher temperature (23 "C, filled circles) the data fall completely within the linear portion of the Langmuir isotherm. On the other hand, at -78 "C (open circles) considerable curvature is evident. Nevertheless, when plotted in the coordinates of Figure 4, the two sets of data fall on parallel straight lines. The parallelism is useful for extrapolating adsorption capacity to other temperatures. However, sometimes the space required per adsorbate molecule may increase significantly, as shown in Figure 7 for butane adsorption at 23 and 104 "C,where the change in slope corresponds to a decrease in maximum adsorption capacity by 12% in the given temperature range. The total decrease in adsorption capacity with temperature is described by adsorption isobars. For example, taken at 1 Torr, the equilibrium amount of butane adsorbed at 23 and 104 "C decreases substantially from 18.4 to 3.0 mg/g. For the intended purpose of this paper it is useful to know about the change in adsorption characteristics with temperature. The temperature coefficient can be evaluated, on the basis of fundamental thermodynamics, in accordance with the Clausius-Clapeyron equation. The heat of adsorption q at a fixed surface coverage is given by two absolute temperatures T1and T2 and the two
5
IO
15 20 25 Pressure (Torr)
30
35
40
Figure 9. Effect of preadsorbed water on methanol adsorption.
pressures p1and p2,respectively, which are associated with the given coverage:
(;, b) ;:
-q=R---
log-
where R is the gas constant. Since the heat of HC adsorption, q, is of a physical nature, described by van der Waals forces, it is similar for HC's of comparable size and not expected to depend significantly on the chemical bond. The heat of adsorption is q = 3.8 f 1.0 kcal/mol(16 f 4 kJ/mol) for methane (Otto, 1982; Reich et al., 1980) and increases with the length of the HC chain as more atoms per adsorbate molecule interact with the surface. 2. Adsorption under Wet Conditions. A few experiments were carried out to assess the influence of water on HC and alcohol adsorption. Preadsorption of water vapor was limited to maximal 20 Torr, dictated by the H20 equilibrium pressure at room temperature. In one experiment, 14.4 mg/g water was adsorbed at 8.37 Torr (the filled square in Figure 8). The adsorption of butane on this wet silicalite is represented by the filled diamonds and shows that the total adsorption is greater than that of butane alone, given by the circles. If the amount of water is subtracted, the net butane adsorption (dashed line) indicates an adsorption loss of 9% compared to dry silicalite. Methanol adsorption is affected by water in a similar way. The results, shown in Figure 9, indicate a 20% decrease in methanol adsorption at 25 Torr caused by the preadsorption of 11.8 mg of water. These examples are merely meant to demonstrate qualitatively that competitive water adsorption can substantially lower the adsorption of HC molecules. A quantitative prediction of the water adsorption effect is difficult as a comparison with the flow reactor results, described below, shows. 3. Adsorption Kinetics. The adsorption for the molecules of interest is reasonably fast, as exemplified for butane adsorption at room temperature in Figure loa. At 20 Torr, 75% of the equilibrium amount is adsorbed within 1 min. The adsorbed percentages at 2,5, and 10 min are
2338 Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991
- too I
1
P
I
LOLA------
Butane (a)
Table 11. Adsorption Results in the Flow Experiments propylene adsorbed, mg/g gas components 1 min 2 min 8.52 10.02 1500 ppm C3H6in N2 7.77 9.30 + 12% C02added 7.79 9.35 I + 1% CO added 7.46 9.11 + lo00 ppm NO added 7.54 8.90 + 0.33% H2added 7.80 9.15 + 20 ppm SO2 added 3.13 3.46 + 2.5% H20added 1.90 9.37 1500 ppm C& in N2only a
Initial condition.
- 9
p 8 01
Butane
5 7 w a 6
(b)
0.I
I
IO
io0
Time ( m i d
Figure 10. Butane adsorption at 23 ‘C and 18 Torr, plotted on (a) linear and (b) logarithmic time scales.
c $
5
2
4
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2 2
e
n
l
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0
0.5
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I .5
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2.5
Time (min.)
Figure 12. Propylene adsorption during flow experiment. Two runs illustrate the reproducibility of the adsorption data.
Time”‘ (min.)”‘
Figure 11. Time dependence of neopentane adsorption at 23 ‘C and 20 Torr.
82, 92, and 97%, respectively. The time dependence of the total amount adsorbed can be conveniently extrapolated, since the adsorption is described by the Elovich equation, characterized by a logarithmic time scale, which yields a straight line up to a fairly high coverage of the silicalite (Figure lob). The Elovich equation is used frequently to describe empirically the kinetics governing physisorption as well as chemisorption (Aharoni and Tompkins, 1970). A logarithmic law to describe adsorption and desorption kinetics is not unique and is followed “by a number of different processes, such as bulk or surface diffusion, activation or deactivation of catalytic surfaces, and chemisorption” (Low, 1960). Molecules that are too large to fit into the silicalite channels, of course, cannot be adsorbed. A limiting size is given by the neopentane molecule, C(CH3)4,which is adsorbed very slowly. As shown in Figure 11, after 60 min only 5.0 mg/g neopentane has been adsorbed. It is estimated that this amount is less than 6% of the equilibrium amount under the given experimental conditions. On the basis of the results obtained with butane and hexane, a neopentane equilibrium value of about 95 mg/g was expected. The neopentane adsorption does not follow the Elovich characteristics of the smaller HC molecules. In this case a linear relationship is obtained by plotting the adsorbed amount as a function of t1/2,shown in Figure 11. The linear plot together with the sparse coverage of less than 6% obtained within 1 h is consistent with an ad-
sorption step that is completely diffusion controlled. 11. Flow Experiments. Table I1 shows the results of the flow experiments. The initial adsorbate mixture consisted of 1500 ppm propylene in nitrogen. Additional components were added to the gas mixture one by one, in separate experiments, carried out after heating of the silicalite to remove adsorbed species. As other gases were added in the flow experiments, i.e., C02,CO, NO, H2, and SO2, the adsorption of the HC decreased gradually by a modest 8%. However, a larger drop in the adsorption capacity was found when water was added, decreasing the adsorbed amount of propylene by 60%. To understand the massive drop in propylene adsorption, one has to realize that silicalite is hydrophobic only t~ a limited degree. At comparable pressures of butane and water, for example, as shown in Figure 8, water adsorption is relatively small. A water concentration of 2.5% corresponds to 20 Torr. At this point water adsorption still increases steeply with pressure (Figures 1 and 2). It should be noted that the heat of water liquefaction (9.7 kcal/mol) exceeds the isosteric heat of water adsorption in the silicalite (6 kcal/mol) (Flanigen et al., 1978). Thus, water condensation outside of the silicalite channels is energetically more favorable than adsorption on the silicalite. When the gas was switched back to the initial mixture, 1500 ppm C3H6 in N2, after purging in nitrogen at 250 OC, the initial propylene adsorption capacity was recovered within 7%. Propylene adsorption kinetics of the flow experiment is shown in Figure 12 at the time when hydrogen was added to the gas mixture (see Table 11). A substantial fraction (about 80% of the equilibrium value) is adsorbed within 1 min. Two sets of data are plotted to illustrate the reproducibility of the measurements.
Conclusions for Practical Applications As shown by a series of experiments with various adsorbates, the adsorption of individual exhaust gas components on silicalite can be measured reproducibly and
Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 2339
s
2.00
0
.g E + E .-s a
.E
w
I
1.75
1.50 1.25 1.00 0.75
0.50 0.25
0 0
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100 150 200 250 300 350 400 450 500
Time
(5)
Figure 13. HC tailpipe emissions of 3-LTaurus collected in bag 1 of FTP.
pressure extrapolation is possible by describing the adsorption in terms of the Langmuir isotherm. The adsorption capacity that is derived in this way at a given temperature and pressure is a maximum value for a practical, time-limited application. The degree of complexity increases greatly for multicomponent adsorption, especially if the time dependence of the adsorption process is important. This point is illustrated by the adsorption data of propylene-water mixtures, where the water suppressed propylene adsorption by about 20% in the gravimetric experiment, compared to 60% in the flow experiment. Thus, the predictability of competitive adsorption from the static adsorption experiments is very limited. To estimate the amount of silicalite required for an adsorption trap, it is assumed here that the HC’s emitted during coldstart can be approximately described by a single HC, such as butane or propylene, and that a typical HC concentration is 1500 ppm (1.1Torr). The amount of propylene, carried in nitrogen, adsorbed within 1min on a fresh sample of silicalite in the flow experiment was 8.5 mg/g* At room temperature and 1.1Torr, 10.4 mg of propylene was adsorbed per gram of silicalite in the balance system under equilibrium conditions. Equilibrium was reached in about 20 min. Only 75% of the equilibrium amount was adsorbed within 1 min. Thus, the 10.4 mg/g propylene measured in the balance experiment has to be lowered by 25% to 7.8 mg/g for a comparison with the flow experiment. The two results, 7.8 and 8.5 mg/g, are in reasonable agreement. To understand the massive drop in propylene adsorption in the presence of water, as shown in Table 11, one has to realize that the term “hydrophobic” or “organophilic” adsorbent does not mean that water adsorption is excluded. The term only indicates that, at comparable pressures of HC and water vapor, the HC is preferentially adsorbed. However, at a sufficiently low surface coverage, if the water pressure is much higher than the HC pressure (about 20 times in the flow experiment), a large fraction of the total adsorption capacity is taken up by water. Calculations based on the data in Table I show that 2.5% water vapor under these conditions occupies 56% of the available silicalite area. The remaining 44% left for propylene adsorption in the flow experiment is estimated to correspond to 0.44 X 8.5 = 3.74 mg/g. The actually measured value (Table 11) is 3.13 mg/g. In the balance experiment, a decrease in silicalite surface by 56% because of water adsorption yields 0.44 X 7.8 = 3.4 mg/g. The agreement between the estimated values is reasonable considering the experimental error and the assumptions involved. The results place the adsorption capacity of silicalite for 1 Torr of HC, taken at 1 min, in the range from 3 (wet
system) to 10 mg/g (dry system). Coldstart emissions collected during the first minute in the first bag of the FTp typically amount to 0.9 g of HC and constitute about 45% of the total HC’s in bag 1. Cumulative HC tailpipe emissions representative for a 3-L Taurus are shown in Figure 13. Let us assume that these HC’s, on the average, have the adsorption characteristics of propylene. Thus, to adsorb 0.9 g of HC, the calculated amount of silicalite needed falls in the range from 90 to 300 g, depending on the H20 concentration. In terms of monolith volume the requirement is in the 52-170-ir1.~ (1-3-L) range. This is roughly 0.5-2 times the current catalyst volume on the Ford Taurus (3.0 L). The adsorbent would be required in addition to the catalyst. The amount of silicalite under wet conditions seems to be too large to be practical, especially since the actual water concentration in automotive exhaust is almost 4 times larger than that used in the flow experiment. Furthermore, the adsorption capacity decreases as the temperature increases. A practical device would require an adsorbent that would reject water more completely without sacrificing the HC adsorption capacity. The pressure dependence from one HC to another can be predicted from the size of the molecule and its volatility as described by the parameters in Table I. The temperature dependence follows from the change in the ordinate intercept of Langmuir isotherms measured at two temperatures. Since the HC concentrations in automotive exhaust are small, typically less than lo00 ppm, adsorption of a HC mixture can be approximated, within certain limits, by superimposing the adsorption capacities of the individual components (Holbrow and Loughlin, 1977). While the physical adsorption of HC’s on different adsorbents is largely characterized by pore size and surface area of the adsorbent, adsorption of polar molecules, such as water or alcohols, is subject to other properties of the adsorbent. For example, it is generally accepted that water adsorption in zeolites decreases as the Si/Al ratio increases. Thus, prediction of the competitive adsorption of HC’s and water on an untested adsorbent is difficult and should be supported by experimental data, preferably measured in a flow experiment.
Acknowledgment We thank Ms. Carolyn L. Parks for her assistance in setting up the Cahn balance system and for carrying out the literature search for this project.
Appendix. Basis of Langmuir Isotherm The Langmuir isotherm is based on the assumption that all adsorbed molecules are located on identical adsorption sites. This assumption holds as the adsorbate molecules line up in the channels of the silicalite structure. In particular, repulsion between adsorbed molecules found on many adsorbents at higher surface coverage is not indicated by the experimental data obtained here. In the derivation of the Langmuir isotherm it is assumed that the adsorption rate is given by dn, _- c g ( 1 - e) exp( dt where p is the pressure and 0 the fractional surface coverage. Multilayer coverage is excluded; thus 0 i 0 i 1. The coefficient c, combines a pressure conversion unit and a sticking coefficient. The activation energy E and the gas constant R define the dependence on the absolute temperature T. The desorption rate is expressed by
g)
-dnd- - cdO ex,( dt
-
E)
(3)
Ind. Eng. Chem. Res. 1991,30,2340-2344
2340
where cd is a desorption constant and E' the activation energy of the desorption step. The difference in the activation energies defines a positive heat of adsorption q.
E'-E=q (4) Physical adsorption is in general a nonactivated process, i.e., E = 0. The adsorption equilibrium is defined by dn, dnd -=(5) dt dt and it follows at equilibrium p/o = p + c where
2
c = c, erp(
R LT )
(7)
The fractional coverage 8 can be expressed as the ratio of the adsorbed moles m a t pressure p and the maximum adsorption m, at saturation coverage
8 = m/m,
(8)
This yields the linear relationship -P = - +P - C m ma ma or p / m = kg
+ ko
(8b)
used in the evaluation of the adsorption data. Registry No. CHI, 74-82-8; CBHB,115-07-1; C3Hs, 74-98-6; CIHlo, 106-97-8; CBHI4,110-54-3; CHaOH, 67-56-1; C2H60H, 6417-5; H20, 7732-18-5; CO, 430-08-0; C02, 124-38-9; N2,7727-37-9; 02,7782-44-7; neopentane, 463-82-1.
Literature Cited Aharoni, C.; Tompkins, F. C. Kinetics of Adsorption and Desorption and the Elovich Equation. In Advances in Catalysis and Related
Subjects; Eley, D. D., Pines, H., Weisz, P., Eds.; Academic Press: New York, 1970; pp 1-49. Barrer, R. M. Zeolite Structures. In Zeolites: Science and Technology; Martinus Nijhoff: Dordrecht, 1984. Brunauer, S.; Emmett, P. H.; Teller, J. Adsorption of Gases in Multimolecular Layers. J. Am. Chem. SOC. 1938, 60, 309-319. Dessau, R. M. Selective Sorption Properties of Zeolites. In Adsorption and Ion Exchange with Synthetic Zeolites-Principles and Practice; Flank, W. H., Ed.; ACS Symposium Series 135; American Chemical Society: Washington, DC, 1980, pp 123-135. Flanigen, E. M.; Grose, R. W. Crystalline Silica Adsorbent. US Patent 4,061,724, 1977. Flanigen, E. M.; Bennett, J. M.; Grose, R. W.; Cohen, J. P.; Patton, R. L.; Kirchner, R. M.; Smith, J. V. Silicalite, a New Hydrophobic Crystalline Silica Molecular Sieve. Nature 1978,271, 512. Hayward, D. 0.;Trapnell, B. M. W. Chemisorption; Butterworths: London, 1964. Holborow, K. A.; Loughlin, K. F. Multicomponent Sorption Equilibria of Hydrocarbon Ga~esin 5A Zeolite, Paper 32. In Molecular Sieves-II; Katzer, J. R., Ed.; ACS Symposium Series 40; American Chemical Society: Washington, DC, 1977; pp 379-392. Kokotailo, G. T.; Lawton, S. L.; Olson, D. H.; Meier, W. M. J. Structure of Synthetic Zeolite ZSM-5. Nature 1978,272,437-438. Low, M. J. D. Kinetics of Chemisorption of Gases on Solids. Chem. Rev. 1960,60, 267-312. Olson, D. H.; Kokotailo, G. T.; Lawton, S. L.; Meier, W. M. J. Crystal Structure and Structure-Related Properties of ZSM-5. J. Phys. Chem. 1981,85, 2238-2248. Otto, K. Adsorption of Methane on Active Carbons and Zeolites. In Hydrocarbon Technology Environment-Alternative Energy Sources IV; Veziroglu, T. N., Ed.; Ann Arbor Science Publishers, Butterworths: Stoneham, MA, 1982; Vol. 6, pp 241-260. Rabo, J. A. Zeolite Chemistry and Catalysis; American Chemical Society: Washington DC, 1976. Reich, R.; Ziegler, W. T.; Rogers, K. A. Adsorption of Methane, Ethane, and Ethylene Gases and Their Binary and Ternary Mixtures and Carbon Oxide on Activated Carbon at 212-301 K and Pressures to 35 Atmospheres. Ind. Eng. Chem. Process Des. Dev. 1980, 19, 336-344. Szostak, R. Molecular Sieves, Principles of Synthesis and Identification; Van Nostrand Reinhold New York, 1989. Received for review August 14, 1990 Revised manuscript received May 29, 1991 Accepted June 10,1991
Adsorption Behavior of Chlorofluorocarbons in Zeolitic Pores. 1. Adsorption Isotherm Satoru Kobayashi,* Koichi Mizuno, Satoshi Kushiyama, Reiji Aizawa, Yutaka Koinuma, and Hideo Ohuchi National Research Institute for Pollution and Resources, 16-3 Onogawa, Tsukuba, Ibaraki, 305 J a p a n
The adsorption of CFC-12 on Nay, KY, and CsY zeolites was carried out by use of a conventional static adsorption apparatus, and the data were discussed in terms of the best fit adsorption isotherms. An inflection point was observed on each isotherm a t the adsorption amount of ca. 10 molecules per unit cell. The heat of adsorption calculated by Clausius-Clapeyron equation between 0 and 15 "C on NaY zeolite increased with increasing adsorption amount of CFC-12. These data were explained in terms of the nonlocalized equation proposed by Hill.
Introduction Chlorofluorocarbons (CFCs) are widely used in many fields as solvents, refrigerants, foam rubber blowing agents, and propellants. However, the emission of CFCs is considered to result in the destruction of the stratospheric ozone layer (Molina and Rowland, 1974). The removal from gas emission by adsorption is one of the desirable methods for resolving this problem, and a variety of adsorption plants for the recycling have been already commercialized. In contrast to the progress in the practical application, fundamental studies concerning the adsorption of CFCs
on microporous adsorbents have been very scarce. Urano et al. (Urano and Yamamoto,1985) reported the adsorption and desorption of CFC-11 in waste gas from a polyurethane form factory using activated carbons. However, the study placed an emphasis upon the engineering and economic aspect of the recovery rather than the kinetics and mechanism of the adsorption. Therefore, in this series of studies, we aimed at elucidating the adsorption behavior of CFCs in zeolitic pores through adsorption experiments with a variety of CFCs and zeolites. By the way, many investigators have studied adsorption isotherms for the gas phase, and various isotherm equa-
0888-5885/91/2630-2340$02.50/0 0 1991 American Chemical Society
