Article pubs.acs.org/IECR
Reappraisal of the Skarstrom Cycle for CO2 Recovery from Flue Gas Streams: New Results with Potassium-Exchanged Zeolite Adsorbent Anshu Nanoti,* Soumen Dasgupta, Aarti, Nabanita Biswas, Amar N. Goswami, Madhukar O. Garg, Swapnil Divekar, and Chandrasekhar Pendem CSIR-Indian Institute of Petroleum, Dehradun-248005, India S Supporting Information *
ABSTRACT: CO2−N2 separation performance of a simple Skarstrom type pressure vacuum swing adsorption (PVSA) cycle is compared with a heavy reflux cycle with a potassium exchanged X zeolite adsorbent. Contrary to literature reports, the performance of the Skarstrom cycle has been found to be better under different PVSA process conditions. The best CO2 purity achieved was 92 mol % at a recovery as high as 80% from a feed consisting of 15 mol % CO2 in N2 and under an excellent feed throughput of 790 LSTP/h/kg. The power consumption in the Skarstrom cycle is also found to be lower than in the heavy reflux cycle.
1. INTRODUCTION Rising levels of CO2 in the atmosphere due to burning of fossil fuel have been recognized to be the main contributor of global warming and associated climate change phenomenon. The majority of anthropogenic CO2 emission comes from stationary industrial sources like power generation (65%), iron and steel industry (13%), cement plants (10%), petroleum refineries (7%), and chemical industries (5%). Implementing the right kind of technology for CO2 capture from the flue gas as well as from process off gases originating from such stationary sources is therefore considered crucial in the strategy to combat global warming. A number of processes are available for CO2 capture either in the postcombustion or in precombustion environment.1 The preferred option currently is based on absorption which uses an amine for scrubbing out the CO2 in the flue gas. Amine processes for CO2 and hydrogen sulphide capture are well established in the chemical industry. The costs cited for CO2 capture are in the range 45−55 US $/ton CO2 avoided. 2The major concerns with this technology are the problems of amine degradation in the oxidizing environment of flue gas and large energy requirements for regenerating the amines. CO2 can also be recovered by cooling and condensation. For economic operation, cryogenic cooling is applicable only when the gas stream has high CO2 concentration but energy requirements are too large for this process to be attractive for capture from dilute gas streams, as in postcombustion capture. Also the presence of moisture in flue gas creates complications during cooling. Membranes have also been used for CO2 separation. However membranes cannot usually achieve high degrees of separation, require multiple stages, and also require the flue gas to be compressed to high pressures. This increases the energy costs to levels three times higher than amine processes. R&D efforts are now under way to combine membranes with amine absorption in membrane based gas−liquid contacting devices.3 Solid adsorbents like zeolites and activated carbons can be used to recover CO2 from flue gas mixtures by the pressure swing adsorption technique. Pressure swing adsorption (PSA) © 2012 American Chemical Society
technology is well established in the petroleum refining industry for hydrogen production, air separation, and in the natural gas processing industry for CO2 removal. Conventional PSA cycles that are being used for hydrogen production are designed for the recovery and purification of weakly adsorbed “light” components (hydrogen, for example) and are based on the well-known Skarstrom cycle. This cycle generally involves a high pressure feed step followed by countercurrent depressurization step, countercurrent low pressure purge step with light product (a light reflux), and countercurrent pressurization step with light product. Such conventional “stripping” cycles with light reflux are able to produce the light product in high purity but are not suited to produce the strongly adsorbed or “heavy” product in high purity.4,5 The addition of a high pressure cocurrent “rinse” with the heavy product or heavy reflux step has been investigated in several studies in attempts to improve separation of the heavy product at high purities and recoveries.5−9 Such an inclusion of a cocurrent rinse step just after the feed adsorption step recycles part of the heavy product obtained from evacuation/ regeneration back to the high pressure column. This gas, enriched in heavy product, displaces light component from adsorbent and effectively fills void space and adsorbent with heavy product. Subsequent evacuation/regeneration yields a more enriched heavy product stream. Thus Na et al.9 incorporated such improvement and have reported a six step VSA cycle in a three bed unit involving feed pressurization, cocurrent depressurisation, light end equalization, heavy reflux, null, and countercurrent depressurization. From a 15% CO2 feed mixture, the enrichment was up to 99.8% CO2 but recoveries were only around 33%. On comparing with a cycle without heavy reflux as reported by Park et al.10 in a two bed four step VSA wherein CO2 purity obtained was 68% with a recovery of 50%, it is Received: Revised: Accepted: Published: 13765
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Figure 1. Schematic of the PVSA unit used in the present study.
procedure was repeated twice. After final exchange, the solid was washed with distilled water, dried in an air oven at 90 °C overnight and then calcined at 500 °C for 4 h in a muffle furnace. BET surface area and pore volume measurement on the adsorbent sample has been carried out using Micromeritics Tri Star analyzer. 2.2. Equilibrium Isotherm Measurements. Single component adsorption isotherms of CO2 and nitrogen on each adsorbent have been measured in a Hiden IGA gravimetric system at three different temperatures 0, 30, and 60 °C. 2.3. Breakthrough and PSA Cycle Studies. Column breakthrough and PVSA experiments were performed in a two bed PC-PLC based PVSA unit. A schematic diagram of the unit is shown in Figure 1. The unit is equipped with solenoid valves, a pressure controller, a recycle compressor, a vacuum pump, and a heavy product storage vessel. The adsorber columns have a diameter of 20 mm i.d. and 3 mm wall thickness with a packed bed length 100 cm. The feed section is composed of mass flow controllers for delivering CO2 and N2 feed mixtures of desired composition. A separate mass flow controller was used for countercurrent light product purge of the column. After loading of the columns, fresh adsorbent was activated in situ with helium purge at 350 °C for 6 h. One of the columns was used for measuring CO2 breakthrough concentrations during adsorption and countercurrent N2 purge (regeneration). The rinse cycle was implemented through the recycle compressor taking CO2 rich product from the heavy product storage tank and delivering to the adsorber. The sequencing of the opening and closing of solenoid valves during different steps of the PVSA process and data logging of other process parameters such as feed composition, feed flow rate, purge flow rate, column pressure, etc. were made through a SCADA based software system. The concentration of CO2 in the process streams coming out of the column during different steps such adsorption, evacuation, rinse withdrawal, light product purge, etc.,
evident that the heavy reflux improves the purity substantially but recoveries are reduced. On removing the concurrent depressurization step, recovery was improved to 55% but at reduced throughput.8 On including a heavy product pressurization prior to the heavy product rinse, the recovery could be increased to 69% with a throughput11 of 228 NL/(h kg). Thus, inclusion of heavy reflux step is instrumental in achieving high purities for recovered CO2 in a VSA process. However the CO2 recoveries can vary and depend on the way the heavy reflux step is implemented. This inclusion of a heavy product “rinse” also increases the power requirements of the process as a recycle compressor has to be used in its implementation. Recently, Thakur et al.12 and Sivakumar et al.13 have also shown in simulation studies that high purities and recoveries of CO2 are possible in PSA separation from CO2− N2 mixtures using 13X as adsorbent but a dual reflux cycle is required. In the present work, we relook at the two bed Skarstrom cycle for CO2−N2 gas separation using a metal-exchanged zeolite adsorbent. We show that under conditions of moderate temperature, where one would normally expect poor performance in a conventional Skarstrom type VSA, and with an adsorbent which shows a somewhat lower capacity for CO2 than a commercial 13X zeolite, we get surprisingly an improved performance. We show that the simple cycle outperforms even the heavy reflux “rinse cycle” with respect to purities and recoveries of the heavy product.
2. EXPERIMENTAL SECTION 2.1. Adsorbent Preparation and Characterization. A commercial 13X zeolite Z10-04 from ZEOCHEM AG was modified by room temperature ion exchange with potassium. The ion exchange was carried out by adding 50 g zeolite to 1260 mL of 0.5 M aqueous solution of potassium oxalate and stirring up to 24 h followed by filtration. The ion exchange 13766
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Table 1. Step Sequence and Timings of VSA Cycles for CO2 Recovery heavy reflux VSA cycle column 1 step
Skarstrom type VSA cycle column 2
timing (s)
feed pressurization 20 adsorption
30−60% of CO2 breakthrough time
depressurisation
10
step
column 1 timing (s)
step
column 2 timing (s)
cocurrent heavy product 20 feed pressurization 10 pressurization cocurrent heavy product 29% of adsorption adsorption 30−60% of CO2 reflux time breakthrough time evacuation 63% of adsorption time evacuation with light 8% of adsorption depressurisation 10 product purge time pressure equalization 10
step blow down
timing (s) 10
evacuation
92% of adsorption time evacuation with light 8% of adsorption product purge time pressure equalization 10
Table 2. Surface Characterisation of Adsorbents
were measured with the help of an online CO2 analyzer based on IR based detection principle. Gas flows of different effluent streams were measured by a Ritter make digital wet gas meter placed on the respective exit lines. Repeatability in CO2 concentrations were observed to be within ±3% while component material balances fell within ±4%. This study compares the CO2 separation performance of two types of VSA cycles: one a conventional Skarstrom cycle and the other a heavy reflux cycle with incorporation of the CO2 rinse step. Binary feed mixtures of varying concentrations of CO2 in nitrogen have been used in these experiments. The total adsorption cycle time for both cycles has been chosen on the basis of observed CO2 breakthrough (BT) times with each feed mixture and varies from 30% to 70% of the breakthrough time. The step sequence and timings of both cycles are given in Table 1. For the Skarstrom type cycle, the evacuation time was taken as 92% of the selected adsorption time for the feed mixture under study while the remaining 8% time was used for evacuation coupled with countercurrent light product purge. For the heavy reflux cycle, evacuation time was 63% of the adsorption time for the feed mixture under study, 8% of adsorption time was used for evacuation coupled with light product purge and the remaining 29% was used for heavy product reflux. Further for both adsorbents, the Skarstrom and heavy reflux cycles were compared under conditions of same evacuation time. A feed flow rate of 2 NLPM was maintained during all PVSA experiments. In each experiment, flow and CO2 concentrations in different product streams were measured after the column steady state was achieved as observed from constancy in measured CO2 concentration levels in the exit streams (generally after about 15 cycles). The adsorption pressure was fixed at 2 bar (abs), desorption (vacuum) was at 0.03 bar (abs), and temperature was varied from 25 to 60 °C. 2.4. TPD Measurements on Spent Adsorbents. TPD measurements have been carried out on both CO2 saturated adsorbents in a Chemisorption Analyzer Auto Chem II 2920 from Micromeritics, USA. The analysis was carried out under helium flow of 30 mL/min with identical temperature ramp of 5 °C/min carried out up to 600 °C.
property 2
BET surface area [m /g] micropore area [m2/g] external area [m2/g] total pore volume [cm3/g] a
K-Z-10-04
Z-10-04a
301.7 239.4 62.3 0.1955
576
0.332
Values from supplier M/s Zeochem AG.
Figure 2. Adsorption isotherms on K+-exchanged zeolite.
Figure 3. Adsorption isotherms on commercial Z10-04.
3. RESULTS AND DISCUSSIONS BET surface area and pore volume of both the K+-exchanged adsorbent and commercial Z10-04 are given in Table-2 . CO2 and nitrogen isotherms on both adsorbents at three different temperatures are shown in Figures 2 and 3. From this data, heats of adsorption of CO2 on these adsorbents have been calculated as 22.08 kJ/mol for the K+-exchanged and 30.45 kJ/mol for the commercial Z10-04, respectively. It is seen from this data that adsorption capacities and heats of adsorption for
CO2 are much higher for commercial Z10-04 compared to the potassium exchanged zeolite while the capacities for nitrogen are lower. Thus selectivities for CO2/N2 separation will be higher with the commercial zeolite, while CO2 recovery from this adsorbent could be more difficult. Figures 4 and 5 show the breakthrough curves obtained with the binary feed mixtures of CO2 and nitrogen at different temperatures and composition, respectively, with K-Z-10-04 adsorbent. 13767
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Table 3. Effect of Feed Concentration and Temperature on Breakthrough Time with K-Z-10-04 Adsorbent CO2 in feed [vol %]
pressure [bar]
feed flow rate [NL/min]
temperature [°C]
breakthrough time [s]
9.0 15.5 24.0 15.5 15.5 15.5
2 2 2 2 2 2
2 2 2 2 2 2
25 25 25 25 45 60
980 700 495 700 525 480
Figure 6. Regeneration curves at 25 °C with N2 purge flow rate 2 NLPM.
was measured at the same purge flow rate and pressure with each adsorbent. It is seen that the K-Z-10-04 zeolite regenerates faster than the commercial zeolite Z-10-04. Figure 7 shows the transient fractional uptake curves for CO2 from the gravimetric measurements in the low pressure range
Figure 4. (a) CO2 breakthrough curves at 25 °C with commercial Z10-04 and K-Z-10-04. (b) CO2 breakthrough curves at different temperatures with K-Z-10-04.
Figure 5. CO2 breakthrough curves with K+-Z10-04 adsorbent with different feed compositions at 25 °C.
Figure 7. Comparison of fractional uptake curves for CO2.
Feed compositions varied from 9.0% CO2 in nitrogen to 24% CO2 in nitrogen while the adsorber was maintained at 2 bar pressure at a feed gas flow rate of 2 NL/min. Table 3 gives the breakthrough times obtained with each feed mixture in these experiments. With increase in CO2 concentration in the feed or with increase in temperature, the breakthrough time observed decreases. In Figure 4, we also show the breakthrough curve obtained with the commercial zeolite at 25 °C and 2 bar. The breakthrough time is higher with this adsorbent due to its higher CO2 capacity as also seen from isotherm measurements Figure 6 shows the regeneration curves obtained during countercurrent purge of the adsorbent column with nitrogen after a typical breakthrough experiment. The regeneration curve
for each adsorbent. Here again the K-Z-10-04 zeolite shows faster diffusivities compared to the commercial zeolite. Figures 8 and 9 show the TPD curves obtained with each adsorbent. It is seen that for the K+-exchanged zeolite, the desorption of CO2 occurs at a lower temperature than the commercial zeolite, which appears to substantiate our observations reported above that K+ exchange leads to adsorbent with better regenerability during CO2 separation The effect of adsorption step time on PVSA performance in terms of CO2 purity and recovery was studied for both cycles. Typical pressure variation in the adsorber with each type of cycle is reported in Figures S1 and S2 (Supporting Information). In the first series of experiments, the feed mixture contained 15% CO2 in nitrogen. The adsorption step time was varied 13768
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tendency of the light product (nitrogen) purity to decrease. Zhang et al.14 also report similar behavior of increase in purity at higher step times with more CO2 in waste stream at larger feed step time. This behavior of bed performance is exhibited with both types of cycles in the present study. However it is noteworthy that the purities and recoveries obtained with the VSA cycle incorporating the simple Skarstrom type are distinctly better than the one with the heavy product reflux. Table 4 reports data on bed performance taken with both adsorbents under conditions of same evacuation time of 263 s for the K exchanged zeolite and 380 s for the commercial zeolite. The corresponding adsorption times are also given in the same table. It is seen that here again the Skarstrom type cycle performs better with the K-exchanged zeolite once again showing superior CO2 purities and recoveries. The power consumption has been calculated for each step in both cycles using the following equation15 and values are reported in Table 5.
Figure 8. TPD curve for CO2 loaded K+-exchanged Z10-04 zeolite.
power = (γ /(γ − 1))RT[(Phigh /Plow)(γ − 1)/ γ − 1]B /(1000η)
Here γ is the ratio of specific heats, R is the gas constant, T is the experimental temperature, Phigh is the discharge pressure, Plow is the suction pressure, B is the molar flow rate of the stream, and η is the mechanical efficiency (assumed to be 80%). We have used a γ value of 1.3. The total power consumed in each type of cycle has been plotted in Figure 11. It is seen that the power consumed in the heavy reflux cycle exceeds that in the Skarstrom cycle The effect of adsorption cycle time on performance of the Skarstrom cycle was also examined at a higher temperature level of 45 °C, and the results are given in Table 6. The trend is similar, in that with increase in adsorption step time and the average CO2 purity as well as recovery increases. The average purity of nitrogen in the adsorption product also showed a slight decrease. At very high adsorption cycle time corresponding to 70% of the breakthrough time, there is a drop in bed performance with respect to CO2 recovery and purity. At the optimum adsorption cycle time (which corresponds to 60% of the breakthrough time), when temperature is increased further to 60 °C, data reported in Table 7 show that there is a noticeable deterioration in bed performance with the Skarstrom
Figure 9. TPD curve for CO2 loaded commercial Z10-04 zeolite.
from 30% to 60% with respect to the CO2 breakthrough time which is 700 s at 25 °C for the feed mixture, while keeping other step times fixed as stated in Table 1. The results obtained with each type of cycle are shown in Table S1 (Supporting Information) and plotted in Figure 10. The results show that with increase in adsorption time the average CO2 purity as well as recovery increases. The average purity of nitrogen in the adsorption product also showed a slight decrease. With increasing adsorption step time, the CO2 mass transfer front progresses further into the column and the increase in the column loading results in higher purities of CO2 product obtained in evacuation. The progress of the mass transfer front into the column could also result in leakage of CO2 in the effluent during the adsorption cycle so that there will be a
Figure 10. Comparison of performance of Skarstrom cycle with heavy reflux cycle [error margin CO2: ± 3%]. 13769
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Table 4. Comparison of Skarstrom Cycle with Heavy Reflux Cycle under Conditions of Same Adsorption and Evacuation Time feed CO2 (mol %)
column 1 step description
cycle type
15.3
Skarstrom
15.3
heavy reflux
15.0
Skarstrom
15.0
heavy reflux
step time (s)
column 2 step description
feed pressurization adsorption adsorption adsorption depressurization feed pressurization
blow down null countercurrent evacuation evacuation with light product purge pressure equalization cocurrent heavy product pressurization cocurrent heavy product reflux countercurrent evacuation evacuation with light product purge pressure equalization blow down null countercurrent evacuation evacuation with light product purge pressure equalization cocurrent heavy product pressurization cocurrent heavy product reflux countercurrent evacuation evacuation with light product purge pressure equalization
adsorption adsorption adsorption depressurization feed pressurization adsorption adsorption adsorption depressurization feed pressurization adsorption adsorption adsorption depressurization
10 120 265 35 10 10 120 265 35 10 10 182 380 51 10 10
adsorbent
CO2 purity (vol %)
CO2 recovery (vol %)
K-Z10-04
86.8
75.0
K-Z10-04
80.9
62.7
Z10-04
66.1
45.8
Z10-04
82.0
59.7
182 380 51 10
Table 5. Comparison of Performance of Skarstrom Cycle with Heavy Reflux Cycle % of total power consumed cycle type
adsorption
evacuation
evacuation + purge
rinse
total power consumed [kW]
total power consumption per mole CO2 recovered [kW/mol CO2]
Skarstrom heavy reflux
25.2 18.9
61.5 74.1
13.3 6.7
0.3
0.01536 0.0216
0.4653 0.7244
Figure 11. Comparison of power consumption of Skarstrom cycle with heavy reflux cycle.
Table 6. Performance of Skarstrom Cycle: Effect of Adsorption Time at 45 °C cycle type
CO2 in feed [vol %]
feed flow [NL/min]
temp [°C]
Skarstrom Skarstrom Skarstrom Skarstrom Skarstrom
15 15 15 15 15
2 2 2 2 2
45 45 45 45 45
ads time [s] 158 210 263 315 368
(30% (40% (50% (60% (70%
of of of of of
BT) BT) BT) BT) BT)
evac time [s]
evac + purge time [s]
CO2 recovery [vol %]
CO2 purity [vol %]
N2 purity in adsorption product [vol %]
145 193 242 290 339
13 17 21 25 29
65 74 81 80 75
67 78 85 90 91
100 100 100 99 98
cycle with CO2 purities dropping to 87% and recovery dropping to 78%. We studied the effect of feed composition on bed performance using both the Skarstrom cycle as well as the heavy
reflux cycle. The CO2 in the feed was varied from 10% to 25%. Adsorption cycle times were taken to be 60% of the corresponding breakthrough times with each feed mixture and other step timings were fixed according to Table 1. 13770
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Table 7. Effect of Temperature on Bed Performance with Skarstrom Cycle CO2 in feed [vol %]
feed flow [NL/min]
temp [°C]
ads time (60% of BT) [s]
evac time [s]
evac + purge time [s]
CO2 recovery [vol %]
CO2 purity [vol %]
N2 purity in adsorption product [vol %]
15 15 15
2 2 2
25 45 60
420 315 288
386 290 265
34 25 23
85 80 78
92 90 87
98 99 100
Table 8. Comparison of Skarstrom Cycle with Heavy Reflux Cycle: Effect of Feed Compositiona cycle type Skarstrom cycle heavy reflux cycle Skarstrom heavy reflux Skarstrom heavy reflux a
CO2 in feed [vol %]
feed flow [NL/min]
ads time [s]
evac time [s]
evac + purge time [s]
24
2
297
273
24
24
2
297
187
15 15 9 9
2 2 2 2
420 420 588 588
386 265 541 370
rinse time [s]
CO2 recovery [vol %]
CO2 purity [vol %]
N2 purity in adsorption product [vol %]
0
86
92
99
24
86
85
89
95
34 34 47 47
0 121 0 171
85 82 75 63
92 83 89 76
98 96 97 97
Pressure 2 bar, temperature 25 °C.
Table 9. Comparison of Present Study with the Dual/Multibed PVSA Processes Reported in Literature for CO2−N2 Separation cycle configuration
adsorbent
PH
temp
PH/PL
CO2 in feed (mol %)
CO2 in heavy product (mol %)
% CO2 recovery
feed throughput (LSTP/ (h kg))
2 2 2 3 4 3 2
13X 13X 13X 13X act. carbon 13X K-13X
1.1 1.1 5.5
17.2 17.2 110
10 10 20
1.2 1.5 2.0
ambient ambient ambient 45 °C ambient ambient 25 °C
12 30 60
20 13 15
68 82 48 92 99 99.5 92
50 57 94 75 68 69 85
507 477 426 1818 33 228 790
K-13X
2.0
45 °C
60
15
90
80
790
bed 4 step bed 6 step bed 4 step bed 9 step bed 4 step bed 8 step bed 4 step
2 bed 4 step
ref 10 10 14 15 7 11 present study present study
was 75% and the cycle involved nine steps with a very low vacuum level required. The results with potassium exchanged 13X (Z-10-04) zeolite in the present study is also considerably better than Skarstrom type VSA cycle reported recently for CO2−N2 separation with 13X zeolite.18
Bed performance is given in Table 8. As the CO2 concentration in feed mixture increases, the CO2 product purity increases and the recovery also increases. This trend is observed with both types of cycles, but here again, the Skarstrom cycle outperforms the heavy reflux cycle with each feed mixture. A comparison of the present study with results of dual/ multibed PSA cycle configurations for CO2−N2 separations reported in literature is given Table 9. The comparison clearly shows that the performance of the simple two bed four step cycle in the present study is better than what is reported in literature for a dual/multibed process using heavy reflux both in terms of CO2 purities and recoveries. Although the reports of studies indicate better CO2 purity (>99%), these are multibed (3−4 bed) processes and the observed recoveries are much lower than our optimum results using two beds and the feed throughput were also lower than what has been kept in the present study (790 LSTP/h/kg). Thus for example with 13X zeolite adsorbent, Choi et al.11 had achieved 99.5 mol % CO2 purity in the heavy product at a recovery of 69% from a feed composed of 13 mol % CO2 in N2 when the feed throughput was 228 LSTP/(h kg) of adsorbent. Similarly with activated carbon Kikkinides et al.7 achieved 99.9% CO2 at a recovery of 68% but with a much lower feed throughput of 33 LSTP/ (h kg) of adsorbent. With 13X adsorbent and a dual bed process Chou et al.16 however achieved a CO2 purity of 48% with a recovery of 94% at a feed throughput of 426 LSTP/ (h kg) of adsorbent. Xiao et al.17 could get 92% CO2 purity at very high feed throughputs (1818 LSTP/(h kg)), but recovery
4. CONCLUSIONS In conclusion we have demonstrated that a simple Skarstrom type cycle performs better than a heavy reflux cycle in CO2 recovery from CO2/N2 mixtures at temperature range of 25− 60 °C through use of a K+-exchanged zeolite X adsorbent. Gravimetric isotherm, TPD, and regeneration curve measurements with this adsorbent show that though adsorption capacity for CO2 is lower than the commercial X zeolite, heats of adsorption are lower, and CO2 regenerability is better so that very high purity CO2 is recovered in the simple vacuum swing cycle and a heavy product rinse becomes unnecessary. The CO2 purity achieved was >92 mol % at a recovery as high as 82% from a feed consisting of 15 mol % CO2 in N2 and under an excellent feed throughput of 790 LSTP/(h kg). These results are considerably better than Skarstrom type VSA cycle reported for CO2−N2 separation with commercial 13X zeolite. Adsorbent bed performance is still better than reported for heavy reflux systems when temperature is raised to 45 °C. The power consumption in the Skarstrom cycle is lower than in the heavy reflux cycle. As the simple Skarstrom cycle does not require a recycle compressor, capital costs will reduce. We expect the results of this study will have important implications in pointing 13771
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(15) Grande, C. A.; Rodrigues, A. E. Biogas to fuel by vacuum pressure swing adsorption I. behaviour of equilibrium and kineticbased adsorbents. Ind. Eng. Chem. Res. 2007, 46, 4595−4605. (16) Chou, C.; Chen, C. Carbon dioxide recovery by vacuum swing adsorption. Sep. Purif. Tech. 2004, 39, 51−65. (17) Xiao, P.; Zhang, J.; Webley, P. A.; Li, G.; Singh, R.; Todd, R. Capture of CO2 from flue gas streams with zeolite 13X by vacuum− pressure swing adsorption. Adsorption 2008, 14, 575−582. (18) Dantas, T. L. P.; Luna, F. M. T.; Silva, I. J.., Jr; Torres, A. E. B.; de Azevedo, D. C. S.; Rodrigues, A. E.; Moreira, R. F. P. M. Carbon dioxide−nitrogen separation through pressure swing adsorption. Chem. Eng. J. 2011, 172, 698−704.
to an opportunity to further reduce the costs of CO2 capture through adsorption.
■
ASSOCIATED CONTENT
S Supporting Information *
Comparison of performance of Skarstrom and heavy reflux cycles: effect of adsorption time at 25 °C (Table-S1), pressure history in adsorber during skarstrom cycle (Figure S1), and pressure history in adsorber during heavy reflux cycle (Figure S2). This information is available free of charge via the Internet at http://pubs.acs.org/.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors acknowledge the support obtained from Dr. Manoj Kumar, CCP Division, Indian Institute of Petroleum, Dehradun, for characterizing surface area and pore volume of one of the adsorbents.
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dx.doi.org/10.1021/ie300982d | Ind. Eng. Chem. Res. 2012, 51, 13765−13772
