High Enrichment and Recovery of Dilute Hydrocarbons by Dual-Reflux

During the cycle, the two columns are highly coupled because the evacuation gas from one column is used to pressurize the other column with a diaphrag...
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Ind. Eng. Chem. Res. 2002, 41, 3499-3504

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RESEARCH NOTES High Enrichment and Recovery of Dilute Hydrocarbons by Dual-Reflux Pressure-Swing Adsorption J. A. Mc Intyre, C. E. Holland, and J. A. Ritter* Department of Chemical Engineering, Swearingen Engineering Center, University of South Carolina, Columbia, South Carolina 29208

A twin-bed, dual (stripping and enriching)-reflux pressure-swing adsorption (DR-PSA) system utilizing an intermediate feed position in the columns was designed, constructed, and operated to study the separation and concentration of a dilute feed of ethane in nitrogen using WestVaco BAX-1500 activated carbon. Four runs were performed, changing only the stripping or enriching reflux ratios. In all the runs, the ethane was enriched between 50 and 90 times with a pressure ratio of only 8. In addition, high heavy-component recoveries were obtained, with the purity and recovery of nitrogen also being very high. The best case enriched a 0.75 vol % ethane feed to 68.4 vol % with an ethane recovery of 99.6%. The nitrogen purity was also very high containing only 30 ppm of ethane in the light product stream. This separation was markedly better than that achievable with a conventional stripping reflux PSA process, and it suggests that it should be entirely feasible to produce two very pure products from a dilute feed stream. Future work will consider this exciting possibility with DR-PSA. Introduction Only in the late 1970s and early 1980s did pressureswing adsorption (PSA) begin to gain commercial acceptance.1 Today, PSA is used in a wide variety of applications, like air separation, gas drying, and hydrogen recovery; however, because of increasing environmental concerns, the use of PSA in solvent vapor recovery (SVR) has become very popular in cleaning industrial emissions, including gasoline vapors.2 In this case a very pure light component is produced (air) but with only moderate enrichment of the heavy component(s). This low enrichment is caused by the four-step Skarstrom-type cycle that most PSA-SVR processes utilize, which consists of a high-pressure adsorption or feed step, a countercurrent blowdown step, a countercurrent low-pressure purge step, and a light product or feed pressurization step. The problem associated with this kind of cycle is mainly with the countercurrent lowpressure purge step, which is actually a stripping reflux step. The light product gas that is used as purge effectively dilutes the heavy component,3,4 which in turn lowers the enrichment. The best enrichment a stripping reflux PSA system can attain is the pressure ratio, which is the thermodynamic limit.4 In reality, however, the enrichment is usually much lower than the pressure ratio and typically less than 2, because of not only the dilution effect associated with the purge gas but also the fact that the adsorption isotherm of the heavy component is typically a favorable Langmuir-type isotherm which gives rise to a simple spreading wave during desorption (i.e., during purge regeneration).4 * Corresponding author. E-mail: [email protected]. Tel.: (803) 777-3590. Fax: (803) 777-8265.

The low enrichment is not a major concern for feeds that contain, for example, 40 vol % or more of the heavy component because the most that this stream can be enriched is 2.5. So, concentrated heavy-component product streams can be achieved rather easily for bulkgas separations with moderate pressure ratios, as shown by Yang and co-workers, where a 50/50 vol % mixture of methane and hydrogen was separated into two relatively pure components with decent recoveries using pressure ratios from 6.7 to 34.3.5 The low enrichment of the heavy component associated with dilute to moderately concentrated feed streams is a concern, however, because it limits the PSA process to producing only one highly enriched product, namely, the light product, unless exceedingly high pressure ratios are used. Nevertheless, there are many gaseous or vaporladen process streams where it would be desirable to produce two highly enriched products from a dilute feed stream, e.g., ethylene in nitrogen, hydrogen in air, carbon dioxide in air, solvent vapors in air, etc. Hence, if the enrichment could be increased to well above the thermodynamic limit associated with conventional PSA processes, the cost of the heavy-component recovery could dramatically be reduced. This problem may be resolved by combining the stripping reflux purge step associated with conventional PSA systems with an enriching reflux purge step as shown recently by Hirose and co-workers.6-8 This novel idea of using stripping and enriching refluxes respectively at the top and bottom of a column that are separated by an intermediate feed position, much like distillation, no longer limits the enrichment of the heavy component to the pressure ratio; it is limited simply by the material balance. Hence, a very high enrichment of the heavy component is possible, far

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above the pressure ratio. For example, early work on this novel PSA process configuration showed that a feed containing 10 vol % of CO2 in air could be enriched eight times with a pressure ratio of 1.64 using a relatively simple twin-bed system containing MS-13X zeolite.7 This was a significant feat because a conventional stripping reflux, twin-bed PSA system would be limited to a trivial enrichment of 1.64, but because of the reasons given earlier, the actual enrichment would be even much less. These works6-8 demonstrated quite clearly that the pressure ratio limitation can be exceeded by coupling stripping and enriching reflux sections together with an intermediate feed in a PSA column. However, can these concepts be applied to other adsorbate systems, like hydrocarbon systems, which tend to exhibit high adsorption affinities with adsorbents such as activated carbon? The answer to this question is addressed in this paper, where the objective is to demonstrate the use of a dual (stripping and enriching)-reflux (DR)-PSA process with an intermediate feed for enriching a dilute feed stream containing a hydrocarbon in nitrogen, in this case ethane. A fully automated twin-bed PSA system utilizing WestVaco BAX-1500 activated carbon was designed and constructed for this purpose. Four feasibility experiments were carried out with this DRPSA system. The results are presented and discussed below, following a brief description of the experimental apparatus and procedure and the DR-PSA cycle. Experimental System and Cycle The DR-PSA system consists of two columns each of 0.87 m in length and 0.02 m in diameter and containing approximately 0.114 kg of WestVaco BAX-1500 activated carbon. The intermediate feed position is located at z/L ) 0.5, with the stripping section located above and the enriching section located below the feed. Nine, centerline, K-type thermocouples are placed equidistant and axially along one of the columns, with sample taps located at the same axial positions. A 10th thermocouple is located at one of the same axial positions in the other column to verify that the columns are balanced during operation. This DR-PSA system is similar to a previously developed PSA-SVR system9,10 in that it utilizes National Instrument’s LabView in house-developed software and hardware for data acquisition and control (DAC). Gas samples obtained automatically from the column and from the feed and product effluents are analyzed with a gas chromatograph equipped with both a thermal conductivity detector (TCD) for concentrated samples and a flame ionization detector (FID) for dilute samples. The nitrogen (UHP grade) and ethane (OP grade) were obtained from National Welders and used as received. More details on this DR-PSA system will be given in a future publication. Figure 1 shows the novel DR-PSA cycle utilized in this study which involves four steps consisting of a lowpressure feed step, a pressurization step, a highpressure purge step, and an evacuation step. During the cycle, the two columns are highly coupled because the evacuation gas from one column is used to pressurize the other column with a diaphragm pump. Also, a portion of the high-pressure light-product effluent from one column is used as a low-pressure stripping reflux purge in the other column, with the remaining portion being the light withdrawn product, and the low-pressure

Figure 1. DR-PSA four-step cycle sequence, which consists of a low-pressure feed step, a pressurization step, a high-pressure purge step, and an evacuation step. LP, HP, QF, QS, QL, QE, and ∆P represent low pressure, high pressure, feed flow rate, stripping flow rate, light-product flow rate, enriched-product flow rate, and pressure change, respectively. Table 1. DR-PSA Operating Parameters and Conditions yF (10-3) QF [cm3(STP) min-1] QF,e [cm3(STP) min-1] QL [cm3(STP) min-1] RS QE [cm3(STP) min-1] RE

run 1

run 2

run 3

run 4

7.9 555 4.38 538 0.56 11.7 72

8.0 580 4.64 548 0.82 11.8 85

7.8 544 4.24 538 0.56 6.8 123

7.5 572 4.29 537 0.84 6.5 152

heavy-product effluent from this column is compressed and a portion is used as a high-pressure enriching reflux purge, with the remaining portion being the heavy withdrawn product. During the cycle, feed is delivered continuously to a column only when it is undergoing the low-pressure feed step. Four experimental runs were carried out to the periodic state, which was verified (1) by repeated sampling of the column profiles and effluents and noting constancy over many cycles and (2) by the periodic state mass balance on the total system and ethane closing typically with less than 3% and 5% errors, respectively. Considering the small amount of ethane being fed and recovered from this system, these errors were considered to be quite acceptable. This set of runs was initiated with the columns regenerated at 423 K while purging with nitrogen at atmospheric pressure and back-filling with the same nitrogen to just above atmospheric pressure after cooling to room temperature. These runs were carried out with all operating parameters and conditions kept as constant as possible; only the stripping and/or enriching reflux ratios were changed from run to run. In all cases, the low-pressure feed and highpressure purge times were equal at 95 s, and the pressurization and evacuation times were equal at 10 s. The pressure ratio was also kept the same for each run with PH ) 210.5 kPa, PL ) 26.8 kPa, and PH/PL ∼ 8. The other operating conditions for each run are given in Table 1. Note that once a periodic state was reached for one of the runs, a condition was changed and the system was allowed to progress to the new periodic state. In general, thousands of cycles were required to reach the periodic state whether starting from clean beds filled only with nitrogen or starting from a previous periodic state condition. The stripping (RS) and enriching (RE) reflux ratios given in Table 1 are defined in terms of the flow rates entering and leaving the columns:6

RS ) QS/QL

(1)

RE ) (1 + QS/QL)(QL/QE)

(2)

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Figure 2. Typical periodic state pressure histories during one cycle of run 3. The low-pressure feed and high-pressure purge times are equal at 95 s, and the pressurization/evacuation times are equal at 10 s.

At the periodic state the mass balance for the heavy component is given by

yFQF ) yjEQE + yjLQL

(3)

In a run with no breakthrough of the heavy component into the light-product stream, yL) 0 in eq 3, and this gives the maximum enrichment (Emax) as

Emax ) yjE/yF ) QF/QE

(4)

which corresponds to 100% recovery of the heavy component in the enriched heavy-product stream. Notice that the mass balance and not the pressure ratio limits the maximum enrichment. Also notice that based on eq 3, if the feed flow rate of just the heavy component is greater than the enriched product flow rate, then breakthrough of the heavy component into the lightproduct stream is imminent. In other words, a necessary but not sufficient condition to prevent breakthrough of the heavy component into the light-product stream is to ensure that the heavy (enriched)-product flow rate (QE) is greater than or equal to the heavy-component flow rate in the feed (QF,e). In all four of these experiments, QE > QF,e, in an attempt to prevent breakthrough of ethane in the light-product stream. However, this necessarily limited the maximum enrichment and hence the ethane purity in the heavy-component product stream, according to eqs 3 and 4. Process Characteristics and Performance A typical periodic state pressure history from run 3 is shown in Figure 2. Notice that the pressurization/ evacuation step ceases long before the columns attain the high and low pressures. In fact, for this particular cycle time and sequencing, the pressures in the columns continuously change during the low-pressure feed and high-pressure purge steps, attaining PH and PL only at the very ends of these steps. Nevertheless, the columns are balanced and operate very well with this cycle. This balanced operation is also observed from the periodic state temperature histories corresponding to this run, which are displayed in Figure 3. Thermocouples (TCs) 7 and 10, which are located at the same axial position in each column, show very clearly the balanced behavior

Figure 3. Typical periodic state temperature histories during one cycle of run 3. Each number corresponds to an axially positioned centerline thermocouple (TC), with TC 1 located near the top of the column in the stripping section, TC 9 located near the bottom of the column in the enriching section, and TC 10 located in the other column at the same axial position as TC 7. Table 2. DR-PSA Process Performance yL (10-6) Rec(N2) yE Rec(C2H6) Emax(C2H6) Eexp(C2H6)

run 1

run 2

run 3

run 4

0.0 0.999 0.414 1.00 47.4 52.4

0.0 0.999 0.410 1.00 49.2 51.4

7.0 0.999 0.639 0.999 80.0 81.9

30.3 0.999 0.684 0.996 88.0 91.4

of the two columns operating 180° out of phase with each other. The temperature histories in Figure 3 also reveal some interesting and typical trends that are associated with this DR-PSA cycle. This column is undergoing 2535 K temperature swings over much of the bed, with the maximum swing occurring at TC 5. These swings give a very good indication of where the ethane wave front is located. Notice that TCs 1 and 2 experience only moderate swings of about 3 K. These are caused by nitrogen adsorption and desorption in the stripping section of the column that is relatively free of ethane.9,10 These profiles are also very similar to those observed in a PSA-SVR process, with a feed stream consisting of 20-60 vol % butane in nitrogen.9,10 Hence, it is quite clear that once the ethane builds up in the enriching section of the column, which takes thousands of cycles when starting from columns containing only nitrogen and a feed containing less than 1 vol % ethane, the DRPSA process behaves very similarly to a stripping reflux PSA process separating a bulk gas stream. The main difference, of course, and the essential feature of this work is that a stripping reflux PSA process operating at a pressure ratio of 8 could never concentrate a feed stream containing 1 vol % ethane in nitrogen beyond 8 vol %, and it would typically be much less due to dilution with the low-pressure purge.9,10 The process performance results shown in Table 2 substantiate this unique ability of a DR-PSA process and reveal a striking contrast to that expected from a stripping reflux PSA process. In general, the experimental enrichments of the heavy component far exceed the pressure ratio of 8, with values ranging between 51.4 and 91.4. Note that, at these particular process conditions and parameters, the experimental enrichments are slightly above the mass

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Figure 4. Periodic state ethane mole fraction column profiles obtained at the end of the high-pressure purge step of each run.

balance limitation because of the slight but acceptable errors in the mass balance mentioned earlier. Notwithstanding any attempt to optimize this system, the best run so far (run 4) enriched a feed stream containing 0.75 vol % ethane in nitrogen 91.4 times, which corresponds to an average ethane mole fraction of 0.684 in the heavyproduct stream, an ethane recovery of 99.6%, and an average ethane concentration of 30 ppm in the lightproduct stream. On the basis of these very exciting results, it seems entirely plausible that this system can be improved to achieve the ultimate separation, i.e., to produce two very pure products from a dilute feed stream. In fact, the feasibility of such a separation has been realized recently from a equilibrium theory analysis of a new rectifying (only) PSA process designed to produce a pure heavy component at one end and a mixture of the light and heavy components at the other end.11 This mode of separation is exactly the opposite of a conventional stripping PSA process, which is designed to produce a pure light component at one end and a mixture of light and heavy components at the other end. The ethane mole fraction column profiles in Figure 4 reveal potential reasons why these runs do not give rise to the ultimate separation and enrichment. At the end of the high-pressure purge step, high ethane mole fractions penetrate into the stripping section for runs 3 and 4 and cover 60-80% of the column, with a fairly broad mass-transfer zone covering the remaining 2040%. This suggests that the feed location at z/L ) 0.5 is not optimum for these conditions and that the columns are loaded with too much ethane. The ethane mole fraction histories in the heavy-product effluent shown in Figure 5 also suggest that the process conditions are not optimum but from the point of view of too little ethane loaded in the columns. It is highly desirable and most likely feasible to produce a constant concentration of ethane in the heavy-product stream throughout the cycle.11 Clearly, this is not the case in any of the four runs because the ethane concentration decreases essentially linearly during the low-pressure feed step. These results suggest one or more of the following deficiencies: the feed position is not optimum, the columns are too small, or the stripping reflux ratios are too large. Nevertheless, it is surmised that more optimized conditions exist and that a DR-PSA can be designed to produce two pure products from a dilute feed stream at relatively low pressure ratios. This will be

Figure 5. Periodic state ethane mole fraction histories in the heavy-product effluent obtained during the low-pressure feed step of each run.

Figure 6. Periodic state ethane mole fraction histories in the light-product effluent obtained during the low-pressure feed step of each run. Runs 1 and 2 have no detectable levels of ethane in the light-product effluent (100 ppb FID lower detector limit).

considered in future studies, both experimentally and theoretically. The ethane column profiles in Figure 4 and the heavyand light-product effluent histories in Figures 5 and 6 reveal some interesting trends associated with changing the reflux ratios in this system, which are the only parameters varied in this feasibility study. The major difference in the operating conditions between runs 1 and 3 is the decreased heavy-product flow rate for run 3 (Table 1). This causes an increase in the enriching reflux ratio of run 3 because the stripping reflux ratios are the same for both runs. This increase in the enriching reflux ratio, with the stripping reflux ratio held constant, causes an increase of ethane in the light product from 0 to 7 ppm, as shown in Table 2 and Figure 6. This in turn causes the ethane recovery to decrease slightly. Figure 4 shows that the mass-transfer zone also increased remarkably, as might be expected, but the substantial increase in the heavy-component enrichment as shown in Table 2 and Figure 5, might not be expected. Apparently, the higher enriching reflux flow rate at a constant stripping reflux ratio (and hence stripping flow rate) pushes the mass-transfer zone further down the bed, resulting in more of the heavy component being loaded into the columns. This increase in loading, occurring without an increase in the strip-

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Figure 7. Ethane and nitrogen adsorption isotherms on WestVaco BAX-1500 activated carbon at 25 °C.14

ping reflux flow rate, clearly results in far less dilution of the ethane in the enriched product stream during the low-pressure feed step, as noted in Figure 5. Runs 3 and 4 are operated at approximately constant heavy- and light-product flow rates. However, both the enriching and stripping reflux ratios are significantly higher for run 4; hence, these two runs illuminate the effect of changing the internal flow rates in the columns. Figure 4 shows that in this case an increase in the internal flow rates pushes the mass-transfer zone further down the bed, causing the breakthrough of ethane in the light product to increase substantially from 7 to 30 ppm, as shown in Table 2 and Figure 6. The corresponding increase in the ethane loading in the column causes the enrichment to also increase by about 12%, similarly to that exhibited between runs 1 and 3. So, the effect of increasing the enriching reflux ratio by 23% in this particular case seems to dominate over the effect of increasing the stripping reflux ratio by 50%, an interesting and unexpected result. Apparently, the columns are loaded with so much ethane that the effect of the increase in the stripping reflux flow rate does not penetrate very far into the enriching section, thereby resulting in a high heavy-component enrichment. The ultimate cause of this behavior, as seen elsewhere,12,13 is most likely related to the nonlinear, Langmuir-type shape of the ethane adsorption isotherm, which is shown in Figure 7 along with the nitrogen adsorption isotherm on BAX-1500 activated carbon at 25 °C.14 A comparison between runs 1 and 4 exemplifies this point even further because in this case the change in the enriching reflux flow rate is even greater, with everything else being the same except the enriched product flow rate, which is about half. However, the effect of changing the enriched product flow rate is thought to be minimal because this flow rate is so low compared to the other flow rates. The only set of runs with easily interpretable performance differences, based on changes in the operating conditions, is runs 2 and 3. The enriching reflux ratio is 45% larger for run 3, whereas the stripping reflux ratio is 46% larger for run 2. So, it is expected that run 2 should exhibit a much cleaner column than run 3, and this is indeed the case, as shown in Figure 4. Based on previous discussions, it is also expected that run 3 should exhibit a marked increase in the enrichment with a corresponding increase in the average ethane mole fraction in the enriched product because of the increase in the ethane loading in the column. Again, this

is indeed the situation, as shown in Figure 4 and Table 2. In fact, the loading increases to such an extent that it causes trace levels of ethane to be detected in the light-product effluent, as shown in Table 2 and Figure 6. What is not clear at this time is the effect of the enriched product flow rate being a factor of 2 smaller for run 3. Again, however, it is surmised that the effect is minimal, because this flow rate is very small. The final comparison made between runs 1 and 2 reveals essentially no effect of changing the process conditions on the process performance, even though the enriching and stripping reflux ratios are 18% and 46% larger for run 2. Apparently, in this case, the conditions are such that the opposing effects of these flow rates cancel each other, thereby leaving the ethane concentration wave front and the other process performance indicators essentially unchanged, as shown in Figure 4 and Table 2. Conclusions This feasibility study demonstrated quite convincingly that a twin-bed DR-PSA system equipped with both stripping and enriching sections located above and below an intermediate feed position in the PSA columns can be used to concentrate dilute feed streams containing hydrocarbons. With this DR-PSA process, the heavy component enrichments can far exceed the pressure ratio, which is the thermodynamic limitation for conventional PSA processes that effectively utilize only a stripping section. The four runs, carried out in a twinbed, bench-scale DR-PSA system containing BAX-1500 activated carbon, revealed that, in all cases, a dilute feed stream containing about 0.75 vol % ethane in nitrogen could be enriched at least 50 times and even up to 90 times with a pressure ratio of only 8. However, the experimental conditions were not optimized, so some breakthrough of ethane into the light product occurred in some cases but never more than 30 ppm, which resulted in ethane recoveries of over 99.6%. Only the effect of changing the stripping and enriching reflux ratios was studied in this preliminary work. In most cases, the trends were as expected, but more research needs to be done to understand the coupled nature of these internal flows and how they relate to the other operating conditions and column dimensions. This study also showed that a DR-PSA system operates in a manner similar to that of a two-cascade distillation column equipped with both a condenser and a reboiler in that the DR-PSA columns are highly coupled at both ends, especially the enriching section during simultaneous evacuation and blowdown of the columns. Hence, when analogies are drawn between these two separation systems, much can be learned about DR-PSA, including operating under total reflux. To corroborate these and future experimental findings, simple and complex mathematical models are being developed to aid in understanding the unique behavior of a DR-PSA process over a wide range of conditions, including models that describe just the enriching (rectifying) section of the process. Acknowledgment The authors gratefully acknowledge financial support provided by the WestVaco Charleston Research Center and the Separations Research Program at the Univer-

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sity of Texas at Austin and by the U. S. National Science Foundation under Grant No. GER-9554556. Nomenclature Eexp ) experimental enrichment Emax ) maximum enrichment QE ) enriched product flow rate, cm3(STP) min-1 QF ) volumetric feed rate, cm3(STP) min-1 QF,e ) volumetric feed rate of ethane, cm3(STP) min-1 QL ) light product flow rate, cm3(STP) min-1 QS ) stripping flow rate, cm3(STP) min-1 RE ) enriching reflux ratio RS ) stripping reflux ratio Rec ) recovery t ) time, s tf ) low-pressure feed step time, s yF ) feed mole fraction of ethane yL ) average light product mole fraction of ethane yE ) average heavy product mole fraction of ethane

Literature Cited (1) Ruthven, D.; Farooq, S.; Knaebel, K. Pressure Swing Adsorption; VCH: New York, 1994. (2) Pezolt, D. J.; Xollick, S. J.; Johnson, H. A.; Robbins, L. A. Pressure Swing Adsorption for VOC Recovery at Gasoline Loading Terminals. Environ. Prog. 1997, 16, 16-19. (3) Liu, Y.; Ritter, J. A. Pressure Swing Adsorption-Solvent Vapor Recovery: Process Dynamics and Parametric Study. Ind. Eng. Chem. Res. 1996, 35, 2299-2312. (4) Subramanian, D.; Ritter, J. A. Equilibrium Theory for Solvent Vapor Recovery by Pressure Swing Adsorption: Analytical Solution for Process Performance. Chem. Eng. Sci. 1997, 52, 31473160. (5) Yang, R. T.; Doong, S. J. Gas Separation by Pressure Swing

Adsorption: A Pore-Diffusion Model for Bulk Separation. AIChE J. 1985, 31, 1829-1842. (6) Diagne, D.; Goto, M.; Hirose, T. New PSA Process with Intermediate Feed Inlet Position and Operated with Dual Refluxes: Application to Carbon Dioxide Removal and Enrichment. J. Chem. Eng. Jpn. 1994, 27, 85-89. (7) Diagne, D.; Goto, M.; Hirose, T. Parametric Studies on CO2 Separation and Recovery by Dual Reflux PSA Process Consisting of Both Rectifying and Stripping Sections. Ind. Eng. Chem. Res. 1995, 34, 3083-3089. (8) Diagne, D.; Goto, M.; Hirose, T. Numerical Analysis of a Dual Refluxed PSA Process During Simultaneous Removal and Concentration of Carbon Dioxide Dilute Gas from Air. J. Chem. Technol. Biotechnol. 1996, 65, 29-38. (9) Liu, Y.; Holland, C. E.; Ritter, J. A. Solvent Vapor Recovery by Pressure Swing Adsorption. I. Experimental Transient and Periodic Dynamics of the Butane-Activated Carbon System. Sep. Sci. Technol. 1998, 33, 2311-2334. (10) Liu, Y.; Holland, C. E.; Ritter, J. A. Solvent Vapor Recovery by Pressure Swing Adsorption. II. Experimental Periodic Performance of the Butane-Activated Carbon System. Sep. Sci. Technol. 1998, 33, 2431-2463. (11) Ebner, A. D.; Ritter, J. A. Equilibrium Theory Analysis of Rectifying PSA for Heavy Component Production. AIChE J. 2002, in press. (12) Liu, Y.; Ritter, J. A. Periodic State Heat Effects in Pressure Swing Adsorption-Solvent Vapor Recovery. Adsorption 1998, 4, 159-172. (13) Liu, Y.; Ritter, J. A.; Kaul, B. Simulation of Gasoline Vapor Recovery by Pressure Swing Adsorption. Sep. Purif. Technol. 2000, 20, 111-127. (14) Ritter, J. A. University of South Carolina, Columbia, SC, unpublished results.

Received for review March 1, 2002 Revised manuscript received May 14, 2002 Accepted May 17, 2002 IE020139D