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Layered Vacuum Pressure-Swing Adsorption for Biogas Upgrading Carlos A. Grande* and Alı´rio E. Rodrigues Laboratory of Separation and Reaction Engineering (LSRE), Associate Laboratory LSRE/LCM, Department of Chemical Engineering, Faculty of Engineering, UniVersity of Porto Rua Dr. Roberto Frias, s/n 4200-465, Porto, Portugal
Biogas is an important source of renewable raw methane that can be upgraded and applied as fuel for vehicles. One of the economic limitations of the upgrading relies in the bulk separation of CO2. In a previous communication (Grande, C. A.; Rodrigues, A. E. Ind. Eng. Chem. Res. 2006, 46, 4595), we have compared the performance of equilibrium- and kinetic-based adsorbents for application in vacuum pressure-swing adsorption (VPSA) processes for biogas upgrading. The main disadvantage of using carbon molecular sieves (CMSs) is that less than 40% of the total capacity to remove CO2 is employed to satisfy purity requirements of methane product (>98%) with direct impact in process productivity. In this work, we report a new adsorbent layering strategy to improve the total productivity of CO2 removal, resulting in a kinetic VPSA process with size reductions up to 60%. 1. Introduction Biogas is a very important source of renewable methane. For upgrading of biogas for applications as high-quality fuel, several contaminants (sulfur compounds, CO2, and H2O) and other dangerous compounds (siloxanes) should be removed. The amount of these contaminants strongly depends on the source of the biogas. Carbon dioxide is the major biogas contaminant, and the economics of its removal is the most critical step in biogas upgrading. Water washing, amine scrubbing, and also vacuum pressure-swing adsorption (VPSA) are being employed in industry.2 Amine scrubbing is the most economical upgrading process, but only for high flowrates. In Sweden, the country with the more intensive application of biogas as a source of renewable fuel, water washing is the most often employed technique.2 To employ water washing with high productivity, a source of cold water has to be available, which is not the case in warm countries. For these countries, VPSA seems to be an interesting process to extend to locations with small and medium flowrates and with mild temperatures, particularly to be employed within the Clean Development Mechanism of the Kyoto protocol. In our previous communication,1 we have studied two vacuum pressure-swing adsorption (VPSA) processes, operated with adsorbents that allow us to operate under equilibrium- or kinetic-control regimes. The adsorbents studied were zeolite 13X, where CO2 is much more adsorbed than CH4, and carbon molecular sieve (CMS-3K), where CO2 is adsorbed much faster than CH4, as examples of equilibrium and kinetic adsorbents, respectively. We have observed that the kinetic adsorbent CMS-3K could produce fuelgrade methane (purity > 98%) with recovery close to 80% with unit productivity of 3.83 (mol of CH4)/(kg‚h). In the case of zeolite 13X, the recovery of methane was not higher than 60% and the unit productivity was 3.20 (mol of CH4)/(kg‚h). The VPSA processes employing these adsorbents have several advantages as well as disadvantages; a list of the most important ones for both processes is shown in Table 1. According to the previously obtained results, the kinetic adsorbent CMS-3K showed a good tradeoff between purity and recovery and also proved to consume less power than zeolite * To whom correspondence should be addressed. Phone: +351 22 508 1618. Fax: +351 22 508 1674. E-mail:
[email protected].
Figure 1. VPSA cycle scheme employed in this work and resulting pressure history in cyclic steady state. Steps are as follows: (1) pressurization, 70 s; (2) feed, 130 s; (3) intermediate depressurization, 10 s; (4) blowdown, 140 s; and (5) purge, 50 s.
Figure 2. Carbon dioxide adsorbed-phase concentration profiles at the end of each step in cyclic steady state for a VPSA unit using CMS-3K (operating conditions in ref 1). Numbers correspond to (1) pressurization; (2) feed; (3) intermediate depressurization; (4) blowdown; and (5) purge.
13X. The main disadvantage of this unit is that the columns operate with less than half of the equilibrium capacity because the process is entirely kinetically controlled and the length of the mass transfer zone is comparable or even larger than the
10.1021/ie070942d CCC: $37.00 © 2007 American Chemical Society Published on Web 10/09/2007
Ind. Eng. Chem. Res., Vol. 46, No. 23, 2007 7845 Table 1. Advantages and Disadvantages of Equilibrium and Kinetic Adsorbents for Biogas Upgrading zeolite 13X
CMS-3K
high capacity of CO2 fast CO2 diffusion f high productivity steepness of CO2 isotherms f require high vacuum for adsorbent regeneration low tolerance to trace humidity large heat of CO2 adsorption/desorption; smaller fluctuations due to CH4 intermediate depressurization to increase CH4 recovery affects purity high adsorption of CH4 between pressurization and feed steps CH4 diffuses in the micropores displacing partially the CO2 molecules
smaller CO2 capacity than zeolite 13X slow diffusion of CO2 f low productivity steepness of CO2 isotherms is not large f vacuum can be adjusted
Table 2. Definition of Product Purity, Recovery, and Unit Productivity of a Five-Step Cycle Comprising Feed, Depressurization, Blowdown, Purge, and Pressurization
purity )
∫
tfeed+tdepres
∫ ∫
tfeed+tdepres
0
recovery )
tfeed+tdepres
CCH4 u|z ) LC
CCH4 u|z ) LC dt -
∫
a bi-LDF model). For the case of the CMS-3K material, the micropore LDF constant (Km,i) is composed by the micropore diffusion resistance (Dm,i) and also by a surface barrier at the mouth of the micropores (kb,i)4 described by
∫
tpress
0
Kµ,i )
tfeed+tdepres
CCO2 u|z ) LC dt)
0
tpurge
0
CCH4 u|z ) LC dt
tfeed
0
productivity )
∫ dt - ∫
CCH4 u|z ) LC dt +
0
almost no net adsorption of CH4 per cycle CH4 does not displace CO2 in the purge step (CH4 cannot diffuse in micropores)
CCH4u|z ) LC dt
0
(
can withstand trace humidity temperature variations are not large due to CO2 heat of adsorption only allows fast depressurization to increase CH4 recovery
CCH4 u|z)0 dt
yCH4,feed n˘ feed‚recovery nωadsorbent‚purity
length of the columns. The length of the CO2 mass transfer zone cannot be modified in this adsorbent without affecting the kinetic selectivity. An alternative to modify the properties of the bed without changing the properties of a solid adsorbent is to employ two or several layers of different adsorbents that will produce a column with a more-targeted capacity. Different layers of adsorbents are normally employed for removal of different kinds of contaminants.6-11 Another example of successive layers of adsorbents is to modify the properties of a column to remove a single gas. This concept was applied for water removal from a saturated stream that should be reduced to a few ppm and for oxygen purification to reduce product costs.12,13 In this work, we propose a new arrangement of adsorbents within the VPSA process in order to improve the unit productivity and power consumption for biogas upgrading.3 In particular, we discuss the effect of a second layer of zeolite 13X (equilibrium-based adsorbent), after a layer of kinetic adsorbent CMS-3K. We have simulated VPSA cycles comprising five steps: pressurization, feed, intermediate cocurrent depressurization, countercurrent blowdown, and countercurrent purge with product. The final target is to have methane purity over 98% starting from a biogas stream with methane content of 55% balanced by carbon dioxide. 2. Modeling and Process Performance The simulations of CH4-CO2 separation by VPSA process rely on a mathematical model previously tested for binary separations and discussed in our previous communication.1 In this model, the adsorption equilibrium of pure components is described by the multisite Langmuir model, and parameters are employed to predict binary adsorption equilibrium in fixed-bed columns. The resistances to diffusion are located in the film between the fluid and the extrudates, within the macropores, and also within the micropores of the adsorbent (described by
1
(1)
rc2 1 + kb,i 15Dµ,i
The cycle employed in this work has five steps comprising the following: countercurrent pressurization with product, feed, intermediate depressurization, countercurrent blowdown, and countercurrent purge with product. The column scheme as well as a typical pressure history of a simulation in cyclic steady state (CSS) is shown in Figure 1. In all the simulations presented in this work, the dimensions of the column were fixed (4.667 m height and 0.4667 m radius with column porosity of 0.36). Step duration was also fixed as follows: pressurization (70 s), feed (130 s), depressurization (10 s), blowdown (140 s), and purge (50 s). The pressure in the different steps was also fixed as follows: 800 kPa for feed, intermediate depressurization until 250 kPa, and blowdown at 10 kPa. Purge was performed at low pressure. The feed flowrate was modified in the different simulations to obtain a final product purity of 98%. The performance of the VPSA process was evaluated according to four different parameters: purity of CH4-rich stream, product recovery defined as the amount of methane obtained with fuel grade divided by the amount of methane introduced in the feed step, the unit productivity, and the total power consumption of the process. The product purity, the product recovery, and the unit productivity are defined in Table 2. For the calculation of VPSA productivity, we have considered that the process is composed of two columns (n ) 2). For the calculations of power consumption, we have assumed that all compression/decompression steps are performed by devices that operate under an adiabatic regime such that power consumption can be calculated by power
[ ] ( ) [( ) Phigh kW γ ) RgTfeed mol γ-1 Plow
γ-1/γ
-1
]
B˙ (2) 1000ηNCH4
where γ ) Cp/Cv (equal to 3/2 for ideal gases), Rg is the universal constant of gases, Phigh is the high pressure, Plow is the suction or blowdown pressure, B˙ is the molar flowrate to be compressed, NCH4 is the number of moles of methane obtained as product, and η is the mechanical efficiency, which typically assumes the value of 0.8. Power consumption calculations of the process involve the compression of the biogas stream considered to be available at 101 kPa up to the feed pressure (800 kPa), the compression of the product (from 800 to 20 000 kPa), and the power required to keep vacuum in the blowdown
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Figure 3. Carbon dioxide (a,b) and methane (c,d) gas and adsorbed-phase concentration profiles at the end of each step in cyclic steady state for a VPSA unit using a layer of CMS-3K followed by a second layer of zeolite 13X: ratio of 0.25 (see operating conditions in Table 3). Numbers correspond to (1) pressurization; (2) feed; (3) intermediate depressurization; (4) blowdown; and (5) purge. Table 3. Simulations of CO2 Removal for Biogas Upgrading by Layered VPSA Employing CMS-3K Followed by Zeolite 13X in a Five-Step Cycle
a
run
Z13X/CMS3K
Qpres, SLPM
Qfeed, SLPM
Qfeed, SLPM
purity, CH4 %
recovery, CH4 %
productivity, (mol of CH4)/h‚kg
1a 5 6 60 7 78 79 80 81 90
0 (CMS) 0.111 0.111 0.111 0.25 0.25 0.25 0.429 0.429 1.000
8 000 10 000 16 000 16 000 16 000 16 000 16 000 18 000 18 000 23 000
16 000 16 000 22 000 22 700 22 000 30 000 27 600 30 000 31 600 40 000
500 700 700 700 850 850 850 1 000 1 000 3 000
98.1 99.2 98.3 98.0 99.8 96.7 98.0 98.7 98.0 98.0
79.7 48.8 76.8 78.5 50.1 83.7 79.1 74.8 80.3 60.1
3.83 2.31 5.03 5.37 3.22 7.55 6.57 6.58 7.65 7.24
Data from run 5 of Table 6 in ref 1.
and purge steps. For the compression of methane to 20 000 kPa, γ ) 1.31 was employed. Numerical simulations were carried out using gPROMS (PSE Enterprise, U.K.). 3. Layered Vacuum Pressure-Swing Adsorption The main disadvantage of kinetic adsorbents is that, when kinetics is slow, only a small amount of the total equilibrium capacity of the column is used.5 Although the separation can be well-performed by kinetic adsorbents, large units should be built and unit productivity values are much lower than what they should be based on equilibrium capacities. To establish a comparison between the kinetic behavior of VPSA using CMS-3K and the new adsorbent arrangement we are proposing, we will use a reference simulation with the following operating conditions: a stream of 16 000 Standard Liters per Minute (SLPM) being treated by a two-column VPSA unit (4.667 m height and 0.4667 m radius with column porosity of 0.36). The five-step cycle is as follows: feed (130 s), depressurization (10 s), blowdown (140 s), purge (50 s), and pressurization (70 s). These operating conditions correspond to run 5 in Table 6 of our previous publication.1 The amount of
CO2 at the end of each step for cyclic steady state is shown in Figure 1. Using only CMS-3K in the column, the purity obtained was >98% with a CH4 recovery of 79.7% and a unit productivity of 3.83 (mol of CH4)/h‚kg. This value of productivity corresponds to ∼35% of the unit productivity that can be achieved based on the equilibrium capacity. The reason for this low unit productivity is the broadness of the mass transfer zone. After the initial 50% of the column, the adsorbent is almost not employed for bulk separation. By replacing part of the CMS-3K adsorbent at the end of the column with another material with different characteristics, the column behavior will change. The material at the end of the column should have faster kinetics of CO2 diffusion. By assuming that the material will have a faster CO2 diffusion, we are assuming that CH4 will also penetrate into the micropores. This is the reason why higher equilibrium selectivity is required. If CH4 penetrates into the micropores, the purge step will also be able to displace CO2 from this second layer, restoring the capacity to remove CO2 in the next cycle. Another fact is that the heat effects will not be very important because this adsorbent will receive only small concentrations of CO2.
Ind. Eng. Chem. Res., Vol. 46, No. 23, 2007 7847
Figure 4. Methane recovery, unit productivity, and power consumption of a VPSA process to produce methane with purity of 98% using different layers of CMS-3K and zeolite 13X adsorbents.
In order to test this layered VPSA concept, we have started by introducing a small amount of zeolite 13X at the end of the column and performing a simulation employing the same operating conditions. The same operating conditions were employed as in the reference run to simulate the behavior of a layered VPSA using zeolite 13X-to-CMS-3K ratios of 0.111, 0.25, and 0.429 (0.466, 0.933, and 1.401 m of zeolite in the column, respectively). For comparison purposes, we have performed simulations only employing zeolite 13X in the column. The results of the simulation for the different cases are shown in Table 3. Note that, in the simulations reported in Table 3 when more zeolite is introduced in the system, more gas should be employed in the pressurization to take into account the CH4 adsorption in the material. Note that, when a layer of zeolite 13X is employed as a second adsorbent layer after the kinetic adsorbent, much higher productivities can be obtained. This result is mainly due to the fact that the unit can process more feed: more CO2 is retained in a column having the same dimensions. The reason for the improvement of the unit productivity can be explained by observing the internal profiles of the gas concentration in the adsorbed phase. In Figure 3, we show the profiles of amount adsorbed of CH4 and CO2 at the end of each step when cyclic steady state was reached for simulation 78. The area between the end of the intermediate depressurization (3) and the end of the pressurization (1) corresponds to the amount of CO2 removed per cycle in cyclic steady state. Note that, for the case of pure CMS-3K (see Figure 2), only the adsorbent located in the initial 2 m of the columns has a significant contribution to the unit productivity. When the layered adsorbent is used in a VPSA process, the utilization of the CMS-3K material can be more intensive: in this case, after the CMS-3K material, there is a second region with a significant contribution to CO2 adsorption. The layer of zeolite 13X acts as an additional trap for CO2 that passed through the CMS-3K layer and that, without this second layer, would have contaminated the product. The layer of zeolite 13X is more sensitive to the purge step, decreasing the amount adsorbed of CO2 more significantly than the kinetic adsorbent. In run 78 (see Table 3), the whole layer of the kinetic adsorbent has a significant contribution to the unit productivity. In cyclic steady state, the temperature changes within the zeolite 13X are not very large because the amount of carbon dioxide to be removed is much smaller than in the feed step. The temperature changes within the bed are much more important in the case when only zeolite 13X was employed ((30 K from feed to blowdown). It is important to mention that, in the case of the layered unit, when
we increase the amount of zeolite in the second layer, more gas should be employed in the purge and pressurization steps. The comparison of the performance of the different simulations is shown in Figure 4. Note that only the runs with constant purity of 98% are displayed in this figure. We are also presenting the power consumption of the different configurations. From this image, it can be observed that the power consumption of the process employing only zeolite 13X is very high and also the methane recovery is much lower than in the kinetic and layered cases. The reasons are as follows: much methane is recycled to the column, high steepness of the CO2 isotherm, and important thermal effects. By comparing the performance of the CMS-3K and the layered arrangement, it can be observed that only small differences can be observed in terms of methane recovery and power consumption. It should be mentioned that power consumption was not optimized. However, important changes are observed in the unit productivity if a second layer of zeolite 13X is used in the bed. By employing a ratio of zeolite 13X/CMS-3K of 0.429, the productivity is almost twice that of the case when only CMS-3K is employed. The results obtained in this work showed a strategy that can be employed in this particular case of biogas upgrading but that can also be extended to other kinetic VPSA units. It can also be observed in Figure 2 that zeolite 13X may not be the ideal adsorbent to use in a second layer of a kinetic adsorbent like CMS-3K, but other adsorbents with easier regeneration procedures, like activated alumina or silica gel, can be employed.3 4. Conclusions A new layered vacuum pressure-swing adsorption (VPSA) configuration for biogas upgrading was studied. The layers consist of a first kinetic adsorbent CMS-3K followed by a second layer of equilibrium-based adsorbent, in this case, zeolite 13X. For a model mixture composed of 55% of CH4 and 45% of CO2, the layered arrangement proved to have much higher unit productivity than the process having only CMS-3K: with a ratio of CMS-3K to zeolite 13X of 0.429, the unit productivity was 7.65 (mol of CH4)/h‚kg when compared to 3.83 (mol of CH4)/h‚kg using only CMS-3K. The important size reduction of the VPSA process will allow reducing the initial investment of a biogas upgrading plant. The concept of layers of kinetic adsorbent followed by equilibrium-based adsorbent can be applied to other separations. In particular, for biogas upgrading, the use of other equilibriumbased adsorbents with easier regeneration should be more convenient. Acknowledgment The authors would like to thank financial support from Foundation for Science and Technology (FCT) through the projects POCI/EQU/59330/2004 and POCI/N001/2005. Literature Cited (1) Grande, C. A.; Rodrigues, A. E. Biogas to Fuel by Vacuum Pressure Swing Adsorption. I. Behavior of Equilibrium and Kinetic Adsorbents. Ind. Eng. Chem. Res. 2006, 46, 4595-4605. (2) Hagen, M.; Polman, E.; Jensen, J. K.; Myken, A.; Jo¨nsson, O.; Dahl, A. Adding gas from biomass to the gas grid; Report SGC 118; Swedish Gas Centre: Scheelegatan, Sweden, 2001. (3) Grande, C. A.; Cavenati, S.; Rodrigues, A. E. Separation Column and Pressure Swing Adsorption Process for Gas Purification. Portuguese Patent Appl. 103615, 2006 (in Portuguese). (4) Srinivasan, R.; Auvil, S. R.; Schork, J. M. Mass transfer in carbon molecular sievessAn interpretation of Langmuir kinetics. Chem. Eng. J. 1995, 57, 137-144.
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(5) Humphrey, J. L.; Keller, G. E., II Separation Process Technology; McGraw-Hill: New York, 1997. (6) Chlendi, M.; Tondeur, D. Dynamic Behaviour of Layered Columns in Pressure Swing Adsorption. Gas Sep. Purif. 1995, 9, 231-242. (7) Lee, C-H.; Yang, J.; Ahn, H. Effects of Carbon-to-Zeolite Ratio on Layered Bed H2 PSA for Coke Oven Gas. AIChE J. 1999, 45, 535-545. (8) 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, 3893-3906. (9) Rege, S. U.; Yang, R. T.; Qian, K.; Buzanowski, M. A. Air Prepurification by Pressure Swing Adsorption Using Single/Layered Beds. Chem. Eng. Sci. 2001, 56, 2745-2759. (10) Wilson, S. J.; Webley, P. A. Cyclic Steady-State Axial Temperature Profiles in Multilayer, Bulk Gas PSAsThe Case of Oxygen VSA. Ind. Eng. Chem. Res. 2002, 41, 2753-2765.
(11) Warmuzinski, K.; Tanczyk, M. Multicomponent Pressure Swing Adsorption: Part I. Modelling of of Large-Scale PSA Installations. Chem. Eng. Process. 1997, 36, 89-99. (12) Lu¨, Y.; Doong, S-J.; Bu¨low, M. Pressure-Swing Adsorption Using Layered Adsorbent Beds with Different Adsorption Properties. I. Results of Process Simulations. Adsorption 2003, 9, 337-347. (13) Watson, C. F.; Whitley, R. D.; Meyer, M. L. Multiple Zeolite Adsorbent Layers in Oxygen Separation. U.S. Patent 5,529,610, June 25, 1996.
ReceiVed for reView July 11, 2007 ReVised manuscript receiVed September 3, 2007 Accepted September 20, 2007 IE070942D
