Environ. Sci. Technol. 1994, 28, 1561-1564
Reactor Trap To Remove Hydrocarbons from Engine Exhaust during Cold Start Barry L. Yang and Harold H. Kung'
Ipatieff Laboratory and Department of Chemical Engineering, Northwestern University, Evanston, Illinois 60208-3120 The concept and the performance of a device to remove hydrocarbons from engine exhaust during cold start were described. The device consisted of a hydrocarbon adsorbent and a metal oxide that could react with the hydrocarbon by oxidation to form carbon oxide and water. The metal oxide could then be regenerated later in the drive cycle by reaction with oxygen. Using a mixed oxide containing Cr, Co, Fe, and Al; ZSM-5 zeolite as the adsorbent; and propene, propane, or toluene as the hydrocarbon, the efficiency of hydrocarbon removal was measured in the laboratory during the 2 min when the unit was heated from room temperature at a rate of 150 OC/min. The efficiency was found to be suppressed by the presence of water vapor, but not affected by the presence of CO and C02. With a mixture of 10% H20, 2 % CO, 0.6% 02,2000 ppm hydrocarbon, and the balance helium, the efficiencies were 92,78, and 57 % for toluene, propene, and propane, respectively. A conceptual device to remove the hydrocarbon efficiently was described.
catalytic bed with improved efficiency in hydrocarbon removal (1, 4-7). One approach that has not been widely explored is to use a material which would react with the hydrocarbon stoichiometrically to form environmentally harmless products, and the material is then regenerated at a later time in the drive cycle. For example, a metal oxide could react with the hydrocarbon to form carbon dioxide, water, and reduced oxide. The latter is then reoxidized to reform the original oxide by air later such as after the engine is turned off but the trap is still hot. The chemical reactions involved are shown below: CxHy+ n M O ,
-
xC02 + y/2H20 + nMO,-,
Mom-, + 2/20,
-
MO,
(1) (2)
This approach would be successful if a metal oxide can be found that could react with the hydrocarbon effectively during the cold start period. We report here our laboratory investigation of the concept of this approach.
Introduction
Experimental Section
To meet the emission standard on hydrocarbons emission from automobile exhaust in the new EPA regulation would require lowering the emission level quite substantially, and the California requirements are even stricter ( 1 ) . It is known that with the current operation of engines and design of exhaust systems, the majority of the hydrocarbon emission as determined in the Federal Test Procedure occurs in the first 2-4 min of the start of the engine (or commonly known as during cold start), depending on the make of the vehicle ( 1 ) . During this period, the temperature of the three-way catalytic bed is too low for effectiveoperation and cannot convert the hydrocarbon in the exhaust. The fuel-air mixture feeding into the engine is lean in oxygen to enhance drivability, thus there is insufficient oxygen in the exhaust for complete combustion of the hydrocarbons in the fuel. Different approaches have been attempted to solve this problem. One method is to heat the catalytic converter electrically to bring the temperature of the three-way catalyst to the operating regime earlier, instead of relying solely on heating by the exhaust from the engine (2, 3). However, there is a limit to the ultimate efficiency of this method because the exhaust does not contain enough oxygen to oxidize all the hydrocarbon completely. Another method is to use a hydrocarbon trap that adsorbs the emitted hydrocarbon when the trap is at low temperatures and releases them when the trap is heated up by the exhaust gas, hopefully at a temperature sufficiently high so that the three-way catalytic converter has become active. Unfortunately, there are no known adsorbents that could trap the hydrocarbons to high temperatures, and premature release of hydrocarbons is still a problem. Some elaborate systems have been reported that involve a combination of heat exchanger, traps, and multiple
The automobile exhaust during the cold start period was modeled by a rectangular test pulse of 2-min duration of 0.2% propene, 0.6% oxygen, 0 or 2% CO, 2 or 10% water vapor, and the balance helium. Some experiments were performed with propane or toluene instead of propene as the hydrocarbon. Propene, toluene, and propane each represent one of the three major components of hydrocarbon found in the engine exhaust: alkene, aromatics, and alkane (8-10). The mole ratio of hydrocarbon to oxygen, being 1:3, was higher than the stoichiometric ratio of 1:4.5,1:5, and 1:9 for complete combustion of propene, propane, and toluene, respectively. Carbon monoxide, when present, would make the gas mixture even more reducing. In a typical experiment, this pulse of premixed gases was carried by a carrier gas of 2 % H2O in helium into a tubular fused silica reactor containing the metal oxide. The water in the carrier gas was to ensure that the water content in the pulse was not significantly diluted. When a test pulse of 10% water was used, the injection valve and the line to the reactor were heated to about 6OoCto avoid water condensation. For this investigation, 0.070 g of a metal oxide containing a 1:l:l:latomic ratio of Cr:Co:Fe:Al was used. This amount of metal oxide and the flow rate of gases used corresponded to a space velocity of 30 000 h l . This mixed oxide was prepared by coprecipitation of the corresponding nitric salts with ammonium hydroxide. The behavior of this oxide was compared with that of 0.018 g of 0.5 wt % Pd/SiO2 at the same space velocity. In some experiments, an adsorbent of 0.030 g of ZSM-5 (Si/Al = 120) was placed upstream of the metal oxide (or Pd/SiOz). In a typical experiment, the solid was pretreated by first heating in flowing 0 2 at 500 "C for 10 min, cooled in 02, and then purged with a stream of 2 95 H2O in helium
@ 1994 American Chemical Soclety
Environ. Sci. Technol., VoI. 28,No. 8, lg94 1561
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Flgure 2. Efficiency of converting a pulse of 0.01 mol of propene and 0.025 mol of O2by reaction with 0.20 g of mixed (Cr-Co-Fe-AI) oxide at various temperatures. Filled points are for carrier gas containing 2 % water, and open points are those without water. Carrier flow rate was 30 mL/min.
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TIME (MIN) Flgure 1. Propene breakthrough curve for (a) an empty reactor: (b) a reactor containing 0.070 g of mixed (Cr-Co-Fe-AI) oxlde at room temperature; and (c) the same as panel b, but the reactor was heated at 150 "Wmin. Pulse contained 2 % H20, 0.8% 02, 0.2% propene, and the balance helium.
at room temperature for 30 min. A test pulse was then passed into the reactor, and the mixed oxide (or Pd/SiO2) and the ZSM-5 adsorbent were heated at a rate of 150 OC/min to 350 "C. The heating commenced when the pulse of mixed gases was estimated to reach the reactor. The hydrocarbon that passed through the reactor without being converted was quantified by a flame ionization detector. For the measurement of COz, the reactor effluent was first directed to a cold trap of Chromosorb W AW at -197 OC for 10 min. The trap was then switched to a GC carrier stream of helium and was warmed to room temperature to release C02 and propene. They were separated by a Porapak T column at 60 "C and quantified by a thermal conductivity detector (TCD).
Results and Discussion Figure l a shows the propene concentration as a function of time at the detector when an empty reactor was used. It shows that the test pulse was roughly rectangular in shape. Figure l b shows the propene concentration profile when the reactor contained 0.070 g of mixed (Cr-Co-FeAl) oxide at room temperature. The area under this profile and that in Figure l a were nearly the same. There was no significant delay in the propene breakthrough curve by the mixed oxide. Thus, at room temperature, the mixed oxide was inactive in removing the hydrocarbon in the presence of water vapor and did not adsorb propene significantly. If the temperature of the mixed oxide was 1562
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increase at 150 "C/min when the test pulse was passed over it, the propene profile shown in Figure 1c was obtained. The figure shows that the breakthrough profile became shorter in duration, and the propene concentration dropped to zero in about 1.5min. That is, the oxide became active in removing propene when its temperature reached about 150-200 "C. From the area under the propene concentration profile, it was estimated that about 56% of the propene was removed. That a temperature higher than 150 OC was needed for the mixed oxide to become active in removing propene was due to the inhibition by water at the lower temperatures. This was shown by the following experiment. A pulse containing 0.01 mol of propene and 0.025 mol of oxygen was passed over 0.2 g of mixed oxide kept isothermally at various temperatures. The carrier gas used was helium, either dry or containing 2% water. Any hydrocarbon passed through the metal oxide bed was detected by gas chromatography using a flame ionization detector. The oxide was then heated rapidly to about 500 "C, at the maximum rate of the system, in 2-3 min. Any hydrocarbon emitted during heating was also quantified by gas chromatography. Figure 2 shows a summary of the results. It shows the hydrocarbon conversion efficiency as a function of the temperature of the metal oxide when the pulse of propene and oxygen was passed over it. The hydrocarbon conversion efficiency is the percent of the propene in the pulse removed by the metal oxide (and converted to CO and C02). The data show that this efficiency was suppressed appreciably by water vapor below 200 "C. The use of an adsorbent for propene was examined by using 0.030 g of the ZSM-5 zeolite in the reactor. Figure 3a shows the propene breakthrough curve for a test pulse that was passed over the ZSM-5 when the reactor was being heated. Adsorption of propene on ZSM-5 was indicated by a delay of the propene front compared to the case of an empty reactor (Figure la). However, instead
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Table 1. Efficiencies of Propene Removal by Mixed (CrCo-Fe-Al) Oxide and by ZSM-&Mixed Oxide Combination from Pulses with Different CO and HsO Contents' 0070 ofmixed
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TIME (MIN) Figure3. Propene breakthroughcurve for (a) reactor containing0.030 g of ZSM-5, heated at 150 'Clmin; (b) same as panel a, but the heating commenced after a 2-min delay; (c) same as panel a, but the reactor contained 0.030 g of ZSM-5 and 0.070 g of mixed (Cr-Co-Fe-AI) oxide; and (d) same as panel a, but the reactor contained 0.030 g of ZSM-5 and 0.018 g of Pd/SiOz. Pulse contained 10% HzO, 2% CO, 0.6% 02, and 0.2% propene.
of a rectangular shape, the breakthrough profile showed first a sharp peak followedby a dispersed rectangular peak. The sharp peak was due to desorption of adsorbed propene when the zeolite was heated above 80 OC. This was confirmed by the following experiment. A test pulse was passed over the zeolite at room temperature. After 2 min, the reactor was heated at 150 "Urnin. The desorbed propene was detected as a sharp peak as shown in Figure 3b that appeared at about the same time lapse after commencement of heating as that in Figure 3a. During desorption of adsorbed propene that accounted for the sharp peak in Figure 3a, adsorbed water was also desorbed from ZSM-5. Desorption of water exposed sites in the zeolite that adsorb propene more strongly. Thus, the portion of the propene pulse that passed over the zeolite after this time was adsorbed. With increasingtemperature, these molecules were also desorbed and passed through the zeolite bed with the remaining portion of the pulse. This then accounted for the appearance of the second dispersed rectangular peak. The total area under both peaks corresponded exactly to the total amount of propene in the input test pulse. If 0.070 g of the mixed (Cr-Co-Fe-Al) oxide was placed immediately after the bed of ZSM-5 and a test pulse was passed over it in a temperature ramp experiment, the propene breakthrough curve shown in Figure 3c was observed. The first sharp peak was observed but with a smaller area than that in Figure 3a, and the second dispersed rectangular peak disappeared. This indicates that the portion of the propene that passed over the oxide
at the higher temperatures was removed by reaction with the mixed oxide, in agreement with the data shown in Figure IC that the mixed oxide began to be active when its temperature reached about 150-200 "C. From the area of the curve, it was estimated that 78% of the propene in the test pulse was removed. If 0.018 g of Pd/SiOZ catalyst was used instead of the mixed oxide in the same experiment, the propene breakthrough curve shown in Figure 3d was obtained. Again, the sharp peak of desorbed propene from the zeolite was observed. The dispersed rectangular peak was smaller than the one in Figure 3a. In particular, the hightemperature portion of that peak disappeared. Thus the Pd/SiOz catalyst became active in removing propene above approximately 300 OC. For the experiment shown, about 30% of the propene in the test pulse was removed. The behavior of the ZSM-&mixed oxide combination did not change if the solids were exposed to CO2 at room temperature for 10 min before the admission of the test pulse. I t did not change either if the pretreatment of heating in 02 was omitted. The effect of CO and H2O content in the test pulse was investigated, and the results are shown in Table 1. The efficiency of removing propene was not affected whether the pulse contained no CO or 2 % CO. This indicated that the efficiencywas not limited by the oxygen capacity of the oxide, but by the kinetics of oxidation of the hydrocarbon. On the other hand, increasing the water content from 2 to 10% lowered the propene removal efficiency from 84 to 78 7%. When the products of propene conversion in a pulse of 0.2% propene, 0.6% 0 2 , and 2% HzO were collected in a trap and analyzed, propene, COZ,and water were detected. Carbon monoxide and 0 2 were not collected by the trap. The amount of unreacted propene agreed well with that measured without using the trap, indicating that no other hydrocarbons were formed. The amount of COZformed accounted for 75% of the propene oxidized, suggesting that the remaining 25 % was converted to CO. If 2% CO was also present in the pulse, half of it was also oxidized to con. Figure 4 compares the breakthrough curves for pulses containing propane, propene, and toluene over the ZSM5-mixed oxide combination. The pulses contained 10% H20, 2 % CO, 0.6% 0 2 , and 0.2% hydrocarbon, with a duration of 2 min. The efficiencies of hydrocarbon removal were 92,78, and 57 % for toluene, propene, and propane, respectively. The fact that toluene can be effectively converted again indicates that the availability of lattice oxygen was not a limiting factor for the lighter hydrocarbons and that the lower conversions of propene and propane were due to the kinetics of oxidation by the oxide. I t should be noted that the breakthrough curve of propene consisted of only one peak, while that of propane Environ. Scl. Technol., Vol. 28, No. 8 , 1984 1563
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Figure 5. Schematic drawing of a passive device, comprlsing of a heat exchanger containing mixed metal oxide and an adsorbent, to remove hydrocarbons during englne cold start.
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TIME (MIN) Flgure 4. Breakthrough curve for (a) propane, (b) propene, and (c) toluene over 0.030 g of ZSM-5 and 0.070 g of mixed (Cr-Co-Fe-AI) oxide. Pulse contained 10% H20, 2 % CO, 0.6% 02, and 0.2% hydrocarbon. Reactor was heated at 150 OC/min.
and toluene consisted of two distinct peaks each. The first peak in the breakthrough curve represents the hydrocarbon desorbed from ZSM-5 when the temperature of the oxide was too low for reaction. The delay of the first peak of toluene compared with the other hydrocarbons suggests that its adsorption on ZSM-5 was much stronger than that of propane and propene. The lower efficiency in converting propane is consistent with the expectation that it is less reactive than the other hydrocarbons. The small higher temperature peaks in the case of propane and toluene indicate incomplete combustion. Since the nature of the hydrocarbons in these peaks not determined, it is not known with certainty whether they were propane and toluene. However, it is more likely that they were partial oxidation or cracking products. In the case of propane, cracking to form methane or ethane would occur at the high temperatures that are less easily oxidized. The data presented in this study show that it is possible to remove hydrocarbon by reacting it with an oxide under conditions that simulated the gas exhaust of an engine during cold start. The oxide oxidizes the hydrocarbon to carbon oxides and water. With the mixed oxide tested that contained Cr, Co, Fe and A1 oxides, about 44% of the propene in the test was removed (Table 1). The oxide appeared t o be stable under the operating conditions, showing no indication of activity loss after 10 cycles of ramp and regeneration. The removal efficiency was improved to 78% by using a zeolite adsorbent for propene 1584
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in front of the bed of mixed oxide. The zeolite delayed the arrival of propene to the mixed oxide such that a larger fraction of the hydrocarbon came into contact with the mixed oxide at a higher temperature, which resulted in a higher operation efficiency. It was also found that the efficiencies of the device for the three hydrocarbons tested were in the order toluene > propene > propane. In order to remove hydrocarbon more efficiently with a passive device, however, it is necessary to use either a mixed oxide that becomes active at a lower temperature or an adsorbent that can adsorb hydrocarbon to a higher temperature. Otherwise a device needs to be used in which the mixed oxide catalyst can be heated much more rapidly than the adsorbent. A device comprising a heat exchanger module containing a catalyst and an adsorbent trap described by Hochmuth et al. (7) could serve the purpose (Figure 5). The exhaust of the engine is first passed through a bed of mixed oxide catalyst, then to a hydrocarbon trap, and then to another bed of mixed oxide catalyst that is in thermal contact with the first bed. Thus, both catalyst beds are heated more rapidly than the adsorbent, and the second bed would be able to remove the hydrocarbon desorbed from the adsorbent when it reaches the desorption temperature.
Acknowledgments Financial support by the US.Department of Energy, Office of Basic Energy Sciences and the GM Corp. and many helpful discussions with Dr. Se Oh of GM are gratefully acknowledged.
Literature Cited (1) Engler, B. H.; Lindner, D.;Lox, E. S.; Ostgathe, K.; SchiiferSindlinger, A,; Muller, W. SAE Tech. Pap. Ser. 1993, No. 930738. (2) Hurley, R. G. SAE Tech. Pap. Ser. 1991, No. 912384. (3) Socha, L. S.; Thompson, D. F. SAE Tech. Pap. Ser. 1992, No. 920093. (4) Minami, T.; Nagase, T. European Patent 424966, 1990. (5) Minami, T. U.S. Patent 4,985,210, 1991. (6) Dunne, S. R.; Reber, R. A. U.S.Patent 5,051,244, 1991. (7) Hochmuth, J. K.; Burk, P. L.; Tolentino, C.; Mignano, M. J. SAE Tech. Pap. Ser. 1993, No. 930739. (8) Kaiser, E. W.; Siegl, W. 0.;Cotton, D. F.; Anderson, R. W. Environ. Sei. Technol. 1993, 27, 1440. (9) Kaiser, E. W.; Siegl, W. 0.; Cotton, D. F.; Anderson, R. W. Environ. Sei. Technol. 1992, 26, 1581. (10) Kaiser, E. W.; Siegl, W. 0.;Henig, Y. I., Anderson, R. W.; Trinker, F. H. Environ. Sei. Technol. 1991,25, 2005.
Received for review April 18, 1994. Accepted April 29, 1994. *
* Abstract published in Advance ACS Abstracts, June 1, 1994.
