Modified Duplex PSA. 1. Sharp Separation and Process Intensification

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Modified Duplex PSA. 1. Sharp Separation and Process Intensification for CO2-N2-13X Zeolite System S. V. Sivakumar† and D. P. Rao*,‡ Department of Chemical Engineering, IIT Kanpur, Kanpur 208016, India ABSTRACT: A modified duplex PSA has been proposed, which could yield sharp separation and has the potential for process intensification. The performances for the original and the modified duplex PSA have been evaluated for two types of systems: the CO2-N2-13X zeolite with single component adsorption and the N2-O2-5A zeolite with competing adsorption. Part 1 of the companion papers presents the simulation studies on the separation of the CO2-N2 mixture. The purities of both products were in excess of 99 mol % with the modified duplex PSA against 78 mol % CO2 and 95 mol % N2 with the original duplex PSA for the same conditions. The bed sizes and energy requirements for the modified duplex PSA were one-half of those for the original duplex PSA. Part 2 presents studies on the separation of the N2-O2 mixture.

’ INTRODUCTION Consider the separation of a binary gas mixture of strongly adsorbed (heavy) component A and weakly adsorbed or nonadsorbing, (light) component B by pressure swing adsorption (PSA). The PSA based on the Skarstrom cycle, referred to as stripping PSA, yields a raffinate product of high purity and another extract product as a mixture of A and B.1,2 Diagne et al.1 and Ebner and Ritter3 have shown that the enrichment of component A in the extract product is constrained by the thermodynamics of adsorption. Wilson4 proposed an “inverse” of the Skarstrom cycle to yield an extract product of high purity and a raffinate product as a mixture of both A and B. This is called enriching PSA. The enrichment of B in the raffinate product is similarly constrained by thermodynamics. The enriching PSA has been studied by Ebner and Ritter,3 Reynolds et al.,5,6 and Yoshida et al.7 for bulk as well as trace component separations. To overcome the thermodynamic constraints, Hirose8 in 1991 and Leavitt9 (patent application filed in 1990) have independently proposed a simple PSA with two beds based on a cycle, which is a combination of stripping and enriching PSA. It yields both products of high purity at a moderate pressure ratio of high pressure, PH, and low pressure, PL. Leavitt called it duplex PSA, and Hirose named it dual reflux PSA. In this work, it is referred to as duplex PSA. The duplex PSA was proposed to yield sharp separation, which we arbitrarily set as both product purities in excess of 99 mol %. Leavitt9 claims that air can be fractionated in a duplex PSA employing PH of 105 kPa and PL of 70 kPa using 13X zeolite to yield a raffinate product of oxygen and argon of 99.9 mol % and extract product of 99.9 mol % nitrogen. Indeed, this is a remarkable achievement! Unfortunately, only limited details are disclosed in the patent. Diagne et al.1,10,11 have carried out experimental and simulation studies on the separation of CO2 from air. However, the sharp separation was not observed. They show that an extract product in excess of 94 mol % CO2 and raffinate product less than of 4 mol % CO2 are feasible from a feed of 20 mol % CO2 with PH = 1 atm and PL = 0.08 atm. Ebner and Ritter have shown, using the equilibrium theory, the feasibility of “perfect” separation of a mixture into pure components over a wide range of parameters for coadsorption with linear r 2011 American Chemical Society

isotherms.12 Later, Kearns and Webley have presented, for different configurations of duplex PSA, the composition profiles along the beds loaded with the minimum adsorbent required for perfect separation.13,14 In addition, they have studied the productivity and energy requirements for different configurations. McIntyre et al.15 have reported the enrichment of a dilute mixture of 0.75 mol % C2H6 in nitrogen up to 68.4 mol % C2H6 with a recovery of 99.6% using duplex PSA. Experimental studies by Wakasugi et al.16 show that it is possible to recover ethanol as liquid from a dilute vapor-gas mixture of ethanol (1.2 vol %) and air using duplex PSA with recovery close to 100%. Distillation of a ternary liquid mixture into three products, each rich in one component, using two heat-integrated columns is well-known as Petlyuk distillation.17 Dong et al.18 proposed a Petlyuk PSA process for separating ternary mixtures. They have reported experimental studies on the separation of a ternary mixture of CO2, CH4, and N2. The objective of this work is to devise a modified duplex PSA, which could yield sharp separation, and study its potential for process intensification (PI), which aims at orders of magnitude volume reduction of beds. We have chosen two systems with widely differing adsorption equilibrium characteristics, CO2-N2-13X zeolite and N2-O2-5A zeolite, to evaluate the relative performances of the original and modified duplex PSA. Part 1 presents the studies on separation of the CO2-N2 mixture. The companion paper, Part 2, presents the studies on the separation of the N2-O2 mixture.

’ DUPLEX PSA A brief description of the duplex PSA is given below. It has two beds and operates in a cycle consisting of four steps. Figure 1 shows the configurations in all four steps. The streams represented as the broken lines are inactive in that step. Step 1: Bed-1 is at high pressure, PH, and bed-2 is at low pressure, PL. The feed is introduced into bed-1 at an intermediate position along the length of the bed. A part Received: May 28, 2010 Accepted: January 28, 2011 Revised: January 25, 2011 Published: February 18, 2011 3426

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Industrial & Engineering Chemistry Research

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Figure 1. Schematic of four steps of duplex PSA.

of the gas drawn from bed-1 is recycled to bed-2, and the rest is drawn as a raffinate product. Likewise, a part of the effluent from bed-2 is drawn as an extract product, and the rest is recycled to bed-1. This is the feed-purge step. We could also look upon bed-1 as undergoing a feed step and bed-2 a countercurrent purge step, the terminology used in dealing with conventional PSA. Step 2: The pressures in the beds are equalized by connecting bottom ends of the beds. Further, the gas drawn from bed-1 is compressed and is used to pressurize bed-2 to PH. It is the pressure-resetting step. It may be viewed as cocurrent blowdown of bed-1 and countercurrent pressurization of bed-2. Step 3: This step is the reverse of step 1. The feed is introduced into bed-2 with the end streams recycled and products drawn as shown in the figure. Step 4: The pressure-resetting in the beds is similarly done as in step 2. Several other duplex PSA configurations are possible. The feed could also be introduced into the bed at PL or into both beds in steps 1 and 3.9 Further, the pressure-resetting of beds can be done by connecting (1) the top ends, (2) the bottom ends, (3) both ends, or (4) the top ends after step 1 up to the pressure equalization and bottom ends thereafter or vice versa, or (5) along the various positions along the lengths of the beds, parallel equalization.15 Different combinations of the feed-purge and pressure-resetting steps lead to different modes of operation of duplex PSA. Kearns and Webley analyzed some of the pressure-resetting modes.13 Here, we refer to the pressure resetting from the raffinate end (one shown in Figure 1) as mode-R. Likewise, when the pressure-resetting is done from the extract end, we refer to it as mode-E. Note only one compressor is needed in mode-E, as the same compressor can be used for the recycling and pressure resetting. The dual beds are intricately linked unlike in conventional PSA, where, for instance, blowdown could be independent of other beds. Sivakumar showed that the mechanism of separation of duplex PSA is entirely different from the PSA based on the Skarstrom cycle.19

Figure 2. Variation in mole fraction of CO2 of extract and raffinate of duplex PSA (F = 100 mmol/half cycle, PH = 1 atm, PL = 0.3 atm, RR = 0.2).

’ MOTIVATION FOR MODIFICATION The experimental and theoretical studies on the separation of the CO2-N2 mixture by duplex PSA by Hirose and co-workers10,11 show that the purities of both products are in the range of 95 mol %, which fell short of the sharp separation. We explored the possibilities of obtaining both products of purities in excess of 99 mol %. The composition of the products varied with time considerably, indicating the possibility of obtaining high purities, if they are drawn appropriately during the steps. Figure 2 shows a typical variation of the mole fraction of CO2 of the raffinate and extract streams in the feed-purge step. The extract was almost pure CO2 in the beginning; however, its mole fraction dropped to 0.7 by the end of the step. This indicates that it is possible to obtain extract product of high purity, if it was drawn in the beginning of step 1. This prompted us to devise ways to enhance the purities by modifying the withdrawal of products. Several modifications are possible. Kumar20 explored the possible improvement in purities by drawing extract product from bed-2 at the beginning of step 1 3427

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Figure 3. Schematic showing the different steps of the cycle of modified duplex PSA.

and recycling the rest during the remaining period. The purities were higher than those of the original PSA, but the improvement was insignificant. Consider a half-cycle of the duplex PSA, and let us examine the process in bed-1. During step 1, as the extract reflux is entering at the top end, the mole fraction of heavy component in the adsorbate would be the highest at this end and would decrease along the bed length. During step 2, the gas rich in the light component is drawn out of the bed from the bottom during the blowdown. This leads to enrichment of the adsorbate throughout the bed. Therefore, one would expect that further evacuation from top end would yield an extract product of higher purity. We proposed a “modified” duplex PSA based on this premise.

’ DESCRIPTION OF MODIFIED DUPLEX PSA Figure 3 shows a schematic diagram of a modified duplex PSA (patent pending 1567/DEL/2006). It differs from the original duplex PSA only in its mode of operation. Its dual beds operate in a cycle consisting of six steps. Step 1: Step 1 is the same as in original duplex PSA except for the withdrawal of extract product. The entire effluent stream from bed-2 is recycled to bed-1 unlike in the original duplex. Step 2: It is the same as step 2 of the original duplex PSA. As described earlier, the pressure-resetting step can be carried either from the extract (top) end or from the raffinate (bottom) end. Step 3: Bed-1 is further evacuated to pressure PL to draw the extract product from the top of bed-1. This is referred to as blowdown-II. During this period, bed-2 is idle. Step 4: Feed is introduced into bed-2, and the end streams are recycled and raffinate product is drawn as in step 1. Step 5: The pressure-resetting is done as in step 2. Step 6: Extract product is drawn from bed-2 by further evacuation to PL as in step 3.

Table 1. Parameters Used for the Simulation Studies Physical Properties of the Adsorbent11,21 bulk density of adsorbent, kg/m3

590

bed length, m bed diameter, m

1.0 0.025

bed void fraction

0.4

tortuosity factor

2.2

particle diameter, mm

0.707

particle porosity

0.33 Langmuir Isotherm Model Parameters11 CO2: 0.4405 [17.77 bar-1]

b, m3/mol 3

qs, mol/m

CO2: 3411 [3.47 mol/kg] LDF Parameters

k, s-1

CO2: 0.808 Operating Variables

feed composition

20% CO2

temperature, K

298.15

cycle time, s

original duplex: 50 modified duplex: 54

feed, s intermediate blowdown, s

20 5

purge, s

20

pressurization, s

5

final blowdown, s

2

’ SIMULATION OF ORIGINAL AND MODIFIED DUPLEX PSA The mathematical model and method of simulation of original and modified duplex PSA processes are given in the Appendix of Part 2. The parameters employed in the simulations of the separation of the CO2-N2 mixture are given in Table 1. 3428



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Figure 4. Mass-balance sheet of original duplex for CO2-N2 separation (mode-E).

Figure 5. Mass-balance sheet of original duplex PSA for CO2-N2 separation (mode-R).

Following Diagne et al.11 and Chue et al.,22 we considered the N2 as the non-adsorbing component. The LDF parameters reported in the literature for CO2 over 13X zeolite varied over a wide range, for instance, 0.33 s-1 (Chue at al.22), 0.202 s-1 (Cavenati et al.23), 0.002 s-1 (Diagne et al.11), and 1.1 s-1 (by Farooq et al.24 for activated carbon). There is also a trend to enhance the LDF parameters to achieve process intensification using structured adsorbent and small

particles.24-26 We have chosen one-half the particle size used by Diagne et al.11 and estimated the LDF parameter for CO2 (0.808 s-1), as outlined by Suzuki using adsorbent properties given in Table 1.27 The cyclic steady state was attained after 150 cycles. However, we carried out simulations up to 300 cycles. The bed length has been divided into equispaced grids of 105 grid points, and feed was introduced at the 52nd grid (52/105th grid) from the extract 3429



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Figure 6. Step 2 profiles in bed-1 and bed-2 for mode-E and mode-R.

recycle end of the bed. A run took about 1 h on a Compaq P-IV PC (512 MB RAM and 2.79 GHz) for original duplex PSA and modified duplex PSA.

’ RESULTS AND DISCUSSION Figure 4 presents the mass-balance sheet for the original duplex PSA with mode-E configuration. The extract recycle ratio (RE) and raffinate recycle ratio (RR) shown in the following figures are defined as: RE ¼

amount of extract recycle amount of extract product

RR ¼

amount of raffinate recycle amount of raffinate product

Figure 4 shows the product purities and productivities, the amount held in the beds at the beginning and end of the steps, the amount recycled, specific energy requirement, and other details. The details of the calculation are given in the Appendix of Part 2. The extract and raffinate purities were 78.69 mol % CO2 and 94.84 mol % N2, respectively. These purities were nearly in the same range as those reported by Diagne et al.10 The amount of CO2 held in the beds was very large as compared to the overall changes that took place during the steps. Because N2 was considered as non-adsorbing gas, it was held only in the gas phase, and hence the amount held in the bed was less than 1% of the total amount. There was a small amount of surplus gas available after the pressure-resetting between the beds. It was considered to have been recycled as shown by the broken line in the figure. Figure 5 shows the mass-balance sheet for the original duplex PSA with mode-R configuration. The product purities and the amounts recycled in the feed-purge step were nearly the same for both modes. In the pressure-resetting step, the gas required fell short by a small amount to reset the pressures, which was in

excess in mode-E. It is attributed to the nature of the composition profiles in the bed. Therefore, a part of the gas drawn during the feed-purge step was used to reset the pressures in the beds (shown as a broken line). The amount of gas transferred in the pressure-resetting step for mode-E was about 7 times the amount for mode-R. As a consequence, the energy requirement for the former was larger as compared to the latter. Therefore, mode-R was employed in the later studies. However, it may be noted that this advantage as compared to mode-E comes at the cost of an additional compressor. Kearns and Webley14 also observed the energy requirement to be higher for mode-E as compared to that required for mode-R for the case of coadsorption. As the CO2-rich gas was transferred from bed-1 to bed-2 in mode-E, it led to desorption of CO2 in bed-1 and its adsorption in bed-2 with zone formation as seen in Figure 6. As a result, a significant amount of gas was transferred. In mode-R, the gas transfer was at the N2-rich ends. This led to redistribution of the CO2 by desorption and adsorption within the upper half of bed-1. In bed-2, the entry of a small amount of N2 nudged the gas upward while simultaneously pressurizing the bed without zone formation. Mode-E led to an accumulation of CO2 as seen in Figure 6a, whereas mode-R led to displacement of CO2 (see Figure 6b) to where its equilibrium partial pressure was higher, thus effectively preventing the adsorption of CO2. Figure 7 shows the mass-balance sheet for mode-R of modified duplex PSA. The adsorption pressure, PH, has been fixed at 1 atm, and the intermediate-blowdown pressure, PL0 , is at 0.5 atm. The final-blowdown pressure, PL (0.34 atm), is governed by the amount of extract product drawn and the intermediate-blowdown pressure. In the pressure-resetting step, the amount drawn from bed-1 was not sufficient to pressurize bed-2 to PH, and this deficient amount of 1.4 mmol was met from the effluent collected from the bottom of bed-1 during the feed step as shown in the figure. The extract and raffinate purities were 99.5 and 99.9 mol % for modified duplex PSA as compared to 77 and 94 mol % for 3430



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Figure 7. Mass-balance sheet of modified duplex PSA for CO2-N2 separation (mode-R).

Figure 8. Solid-phase concentration profiles in different steps of original duplex PSA (mode-R).

original duplex PSA. The improvement in the purities was dramatic. However, the energy requirement for the modified duplex was nearly double that of the original duplex. Figures 8 and 9 present the solid-phase concentration profiles of a bed in various steps for the original and modified duplex PSA for mode-R. The shifts in profiles were large for step 1 (feed-purge step) and were small for step 2 (pressure-resetting step). The mass

transfer zone formation and its movement could be seen in bed-1. This is due to the uniform composition profile at the top of the bed. Figure 10 presents the shifts in both gas-phase and solid-phase concentration profiles in a bed for all the steps in a cycle for the modified duplex PSA. The qCO2 at the bottom of bed (see the qCO2 profile at 20 s) was nearly zero, and then it increased exponentially toward the top of the bed reaching a uniform value 3431



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Figure 9. Solid-phase concentration profiles in different steps of modified duplex PSA (mode-R).

Figure 10. Solid-phase and gas-phase concentration profiles in a bed for all steps in a cycle for modified duplex PSA.

corresponding to the saturation of CO2 at PH. The qCO2 profile became steeper in blowdown-I, and steepest in blowdown-II. It can be seen in the gas-phase CO2 profiles that the upper part of bed-1 was almost free from N2 on cocurrent blowdown. However, the N2 moved up on countercurrent blowdown (blowdown-II). It suggests that the product purity can be further increased, if the lower half of the bed can be isolated, which is feasible if the bed is sectioned into two at the feed location. However, the energy requirement would increase as PL would be much lower. High purity may not be required in some cases. In these cases, there is a possibility to enhance the productivity by increasing the feed rate for given purities as shown later.

In the following, we present the effect of feed flow rate, raffinate recycle ratio, and desorption pressure on the performance of modified duplex PSA. Effect of Feed Flow Rate. Figure 11 shows the effect of feed flow rate on purities and energy requirements for original and modified duplex PSA. The performance of original duplex PSA deteriorated as the feed rate increased. In contrast, the modified duplex PSA yielded very high purities, and its performance was nearly constant over a wide range of feed rate. The energy requirements increased with feed flow rate due to lower PL that was needed to draw more extract product as the RR was held constant. 3432



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Figure 11. Effect of feed with feed location in the middle of the bed: (a) original duplex PSA, PH = 1 atm, PL = 0.3 atm, RR = 0.2, half-cycle time = 50 s; (b) modified duplex PSA, PH = 1 atm, P0L = 0.5 atm, RR = 0.6.

Figure 12. Effect of raffinate recycle ratio for F = 100 mmol/half cycle: (a) original duplex PSA, PH = 1 atm, PL = 0.3 atm; (b) modified duplex PSA, PH = 1 atm, P0L = 0.5 atm.

Effect of Recycle Ratio. Figure 12 shows the effect of RR on purities and energy requirements for modified and original

duplex PSA. Note that RR and RE are interdependent, and the specification of one fixes the other. At low values of RR (= 0.2), 3433



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the purities are relatively low. On increasing the recycle ratio, the purities improved and reached maxima. Thereafter, the purities decreased with increasing the recycle ratio for the original, whereas it remained nearly constant over a wide range for modified duplex PSA. On increasing RR, the amount of gas circulating between the beds increased, and hence the energy requirement increased. The energy requirement was higher for modified duplex PSA as compared to the other. Figure 13 shows the gas-phase concentration profiles in the bed undergoing intermediate-blowdown for a recycle ratio of 0.2, 0.6, and 1.8, respectively. For an RR of 0.2, the mole fraction of N2 in the gas phase was high up to the middle of the bed as the

Figure 13. Effect of RR on CO2 profiles after intermediate blowdown.

recycle was not sufficient to purge out the N2 from the bed. Even at the end of intermediate blowdown, a significant amount of N2 was left in the bed, which contaminated the extract product. On the other hand, at high RR of 1.8, the amount of CO2 in the recycle was high and it penetrated down the bed, which contaminated the raffinate product. The RR of 0.6 appears to be the optimum. Effect of Intermediate-Desorption Pressure. Figure 14 shows the effect of P0L on the product purities and energy requirement. At high P0L (> 0.4 atm), the purities were poor. As the difference between PH and P0L was small, the amount of CO2 that could be adsorbed in the bed was smaller than what was entering in the bed, and hence it emerged along with the raffinate lowering itspurity. On decreasing the intermediate-desorption pressure, the purities of both products improved and reached 99.9 mol % at a P0L of 0.3 atm. As the extract has to be compressed from PL to PH for recycling the gas, the energy requirement increased with decrease in P0L.

’ PROCESS INTENSIFICATION Process intensification in adsorption processes aims at the reduction in the volume of beds and energy requirement by an order of magnitude. The ratio of the volumes of beds (that is, the inverse of the ratio of productivities) and the ratio of energy requirements can be viewed as the measures of PI of two competing process alternatives with the same production objectives. The increase in the productivity leads to the reduction of the capital costs, whereas the reduction in the energy requirement leads to the reduction of the operating costs. Table 2 presents the performance of the processes for the CO2N2 separation by duplex PSA and the conventional PSA. If we consider the original duplex PSA as a benchmark to assess the PI of

Figure 14. Effect of intermediate-desorption pressure: F = 60 mmol/half cycle, PH = 1 atm, RR = 0.4.

Table 2. Performance of Processes for CO2-N2 Separation feed (mmol/ half-cycle) original duplex PSA modified duplex PSA conventional PSA (Chue et al.22) conventional PSA (Cho et al.29)

100.0 260.0

purity (mol %) SLPM

xF, CO2

5.4 12.9 39.9 135.0

0.20 0.20 0.26 0.10

pressure ratio, PH/PL (PH-PL) 20.0 (1-0.05) 5.7 (1-0.17) 16.6 (1.1-0.06) first stage 17.2 (1.13-0.07) second stage 8.6 (1.13-0.13) 3434

CO2

N2

CO2 productivity (mol/kg 3 h)

E (kWh/ ton CO2)

97.2 97.5 >99.9 98.9

99.2 99.2 73.9 97.6

4.8 10.6 7.5 0.8

378.4 207.1 60.1 117.1


Industrial & Engineering Chemistry Research the modified duplex PSA, it can be seen that the reduction of the bed volume as well as the reduction of the energy requirement are roughly by a factor 2 for the modified duplex PSA. Recently, Reynolds et al.28 reported an extensive survey of stripping cycles for the CO2-N2 separation. They observed that the performance of a seven-step three-bed PSA cycle reported by Chue et al.23 (third row in the table) is very “impressive” as compared to the other cycles. Its performance data are given in Table 1. The adsorbent used by in Chue et al.23 had a loading of 4.2 mmol/g, whereas the one considered in the present work had 2.8 mmol/g. The performances are not comparable as the recovery of CO2 is low as compared to the modified duplex. We have computed the energy requirement assuming the bed to be saturated with pure carbon dioxide before blowdown, and the effluent is compressed to 1.1 bar. Yet the bed had to be evacuated up to 0.025 atm to match the amount that was drawn during blowdown in the study reported by Yang and his co-workers. Cho et al.29 reported a two-stage PSA process with two beds per stage for capturing CO2 from flue gas with lower CO2 content. The first stage PSA was used to enrich the CO2 from 10 to 63.2 mol %, which was further enriched to 99 mol % in the second stage PSA. The performances are comparable to the modified duplex PSA. The energy for the feed blower was not included. The productivity was calculated by using the bed dimension reported in their previous study.30 The productivity calculated was roughly 13 times lower than that for modified duplex PSA. However, the energy requirement was roughly one-half as compared to modified duplex PSA. Hence, it appears that the PI for the CO2-N2 separation would be modest based on our initial assessment. However, there is a possibility to improve the scope for PI by reducing the energy requirement by optimizing the operating conditions such that sharp separation is obtained at lower recycle ratios. Both duplex PSA suffer from a deficiency as the feed is introduced at an intermediate point along the bed. If the feed has heavy trace components, like water in case of flue gas, they would accumulate in the beds leading to deterioration of the performance and eventual failure. Therefore, the heavy trace components have to be removed unlike in conventional PSA where the guard bed at one end can effectively prevent such deterioration.

’ CONCLUSIONS We have proposed a modification to the original duplex PSA to improve the performance. The simulation studies were carried out on the performance of original and modified duplex PSA for the separation of the CO2-N2 mixture. During the pressureresetting step, the transfer of gas between the beds from the raffinate end was found to be superior to that done from the extract end. It was shown that modified duplex PSA yields sharp separation and the scope for PI was modest. Further studies are needed to evaluate the efficacy of modified duplex PSA as compared to the conventional PSA cycle. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: 91 512 2597432. Fax: þ91 512 2590104. E-mail: [email protected]. Present Addresses †

Shell Projects and Technology, Shell Technology Centre Amsterdam, Grasweg 31, 1031 HW Amsterdam. ‡ Process Intensification Consultants, 201, Varshini Mansion, Deepthisri Nagar, Miyapur, Hyderabad 500049, India.

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’ REFERENCES (1) Diagne, D.; Goto, M.; Hirose, T. New PSA process with intermediate feed inlet position operated with dual refluxes: Application to carbon dioxide removal and enrichment. J. Chem. Eng. Jpn. 1994, 27, 85. (2) Reynolds, S. P.; Ebner, A. D.; Ritter, J. A. Stripping PSA cycles for CO2 recovery from flue gas at high temperature using a hydrotalcite-like adsorbent. Ind. Eng. Chem. Res. 2002, 41, 4122. (3) Ebner, A. D.; Ritter, J. A. Equilibrium theory analysis of rectifying PSA for heavy component production. AIChE J. 2002, 48, 1679. (4) Wilson, P. H. Inverted pressure swing adsorption process. U.S. Patent 4,359,328, 1982. (5) Reynolds, S. P.; Mehrotra, A.; Ebner, A. D.; Ritter, J. A. Heavy reflux PSA cycles for CO2 recovery from flue gas, Part I. Performance evaluation. Adsorption 2008, 14, 399. (6) Reynolds, S. P.; Ebner, A. D.; Ritter, J. A. Enriching PSA cycle for the production of nitrogen from air. Ind. Eng. Chem. Res. 2006, 45, 3256. (7) Yoshida, M.; Ritter, J. A.; Kodama, A.; Goto, M.; Hirose, T. Enriching reflux and parallel equalization PSA process for concentrating trace components in air. Ind. Eng. Chem. Res. 2003, 42, 1795. (8) Hirose, T. A simple design method of a new PSA process consisting of both rectifying and stripping sections. Proceedings of the 2nd China-Japan-USA Symposium on Adsorption, 1991; p 123. (9) Leavitt, F. W. Duplex adsorption process. U.S. Patent 5,085,674, 1992. (10) Diagne, D.; Goto, M.; Hirose, T. Parametric studies on CO2 separation and recovery by a dual reflux PSA process consisting of both rectifying and stripping sections. Ind. Eng. Chem. Res. 1995, 34, 3083. (11) Diagne, D.; Goto, M.; Hirose, T. Numerical analysis of a dual refluxed PSA process during simultaneous removal and concentration of carbon dioxide dilute gases from air. J. Chem. Technol. Biotechnol. 1996, 65, 29. (12) Ebner, A. D.; Ritter, J. A. Equilibrium theory analysis of dual reflux PSA for separation of a binary mixture. AIChE J. 2004, 50, 2418. (13) Kearns, D. T.; Webley, P. A. Modelling and evaluation of dualreflux pressure swing adsorption cycles: Part I. Mathematical models. Chem. Eng. Sci. 2006, 61, 7223. (14) Kearns, D. T.; Webley, P. A. Modelling and evaluation of dual reflux pressure swing adsorption cycles: Part II. Productivity and energy consumption. Chem. Eng. Sci. 2006, 61, 7234. (15) McIntyre, J. A.; Holland, C. E.; Ritter, J. A. High enrichment and recovery of dilute hydrocarbons by dual-reflux pressure-swing adsorption. Ind. Eng. Chem. Res. 2002, 41, 3499. (16) Wakasugi, R.; Kodama, A.; Goto, M.; Hirose, T. Dual reflux PSA process applied to VOC recovery as liquid condensate. Adsorption 2005, 11, 561. (17) Smith, R. Chemical Process Design; McGraw-Hill: New York, 1995. (18) Dong, F.; Lou, H.; Kodama, A.; Goto, M.; Hirose, T. The Petlyuk PSA process for the separation of ternary gas mixtures: exemplification by separating a mixture of CO2-N2-CH4. Sep. Purif. Technol. 1999, 16, 159. (19) Sivakumar, S. V. Sharp separation and process intensification in adsorptive separation processes. Ph.D. Thesis, Indian Institute of Technology Kanpur, India, 2007. (20) Kumar, P. Studies on conventional PSA and Duplex PSA. M. Tech Thesis, Indian Institute of Technology Kanpur, India, 2005. (21) Da Silva, A. F.; Rodrigues, A. E. Adsorption equilibria and kinetics for propylene and propane over 13X and 4A zeolite pellets. Ind. Eng. Chem. Res. 1999, 38, 2051. (22) Chue, K. T.; Kim, J. N.; Yoo, Y. J.; Cho, S. H.; Yang, R. T. Comparison of activated carbon and zeolite 13X for CO2 recovery from flue gas by pressure swing adsorption. Ind. Eng. Chem. Res. 1995, 34, 591. (23) Cavenati, S.; Grande, C. A.; Rodrigues, A. E. Separation of CH4/CO2/N2 mixtures by layered pressure swing adsorption for upgrade of natural gas. Chem. Eng. Sci. 2006, 61, 3893. (24) Farooq, S.; Thaeron, C.; Ruthven, D. M. Numerical simulation of a piston driven PSA unit. Sep. Purif. Technol. 1998, 13, 181. 3435



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(25) Arvind, R.; Farooq, S.; Ruthven, D. M. Analysis of a piston PSA process for air separation. Chem. Eng. Sci. 2002, 57, 419. (26) Babicki, M.; Hall, A. PSA technology hits the fast lane. Chem. Process. Mag. 2003, (August) (http://www.chemicalprocessing.com/ articles/2003/322.html). (27) Suzuki, M. Adsorption Engineering; Elsevier Science Publishers: Amsterdam, 1990. (28) Reynolds, S. P.; Ebner, A. D.; Ritter, J. A. Stripping cycles for CO2 recovery from flue gas at high temperature using a hydrotalcite-like adsorbent. Ind. Eng. Chem. Res. 2006, 45, 4278. (29) Cho, S.-H.; Park, J.-H.; Beum, H.-T.; Han, S.-S.; Kim, J.-N. A 2-stage PSA process for the recovery of CO2 from flue gas and its power consumption. Stud. Surf. Sci. Catal. 2004, 153, 405. (30) Park, J.-H.; Beum, H.-T.; Kim, J.-N.; Cho, S.-H. Numerical analysis on the power consumption of the PSA Process for recovering CO2 from flue gas. Ind. Eng. Chem. Res. 2002, 41, 4122.

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Modified Duplex PSA. 1. Sharp Separation and Process Intensification

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