148 (2002) 275-280 www.elsevier.com/locate/desal
Simulation of a new hybrid membrane/pressure adsorption process for gas separation
I.A.A.C. Esteves, J.P.B. Mota* Departamento
de Quimica, Centro de Q&mica Fina e Biotecnologia, Faculdade de Ck?ncias e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal Tel. +351 (21) 2948300 (ext. 0961); Fax +351 (21) 2948385; e-mails: [email protected]
, [email protected]
1 February 2002; accepted 25 March 2002
Abstract A new hybrid gas separation process combining membrane permeation and pressure swing adsorption (PSA) is presented. An integrated model was formulated which successfully predicts all process characteristics. Our modeling work shows that the coupled process increases the efficiency of the pressurization and high-pressure adsorption steps, thereby improving the separation performance as compared to a standalone PSA. The new process has been applied successfully to the bulk separation of a mixture of 50/50 H,/CH, and preliminary results have been obtained for CO,,CH, and HJCO,/CH, mixtures. Keywords:
Pressure swing adsorption;
Membrane permeation and PSA are frequently considered as alternatives to the conventional cryogenic gas separation processes. With the present generation of membrane processes, the PSA system still maintains an advantage in the highpurity region while membranes become clearly advantageous when product purity requirements *Corresponding
Presented at the International Julv 7-12, 2002.
Congress on Membranes
are less severe [l]. Therefore, it is reasonable to expect that an optimized gas separation process integrating membrane and PSA improve product purity and/or recovery as compared to the standalone systems. Although coupled membrane-PSA processes have been proposed in the literature for various applications [2-6], most of them are essentially a simple combination of the two units and not a truly synergistically integrated process: either the and Membrane
001 l-9 164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved PII:SOOl
(ICOM), Toulouse, France,
I.A.A.C. Esteves, J.P.B. Mota /Desalination
membrane is followed by PSA or the reverse case . When the membrane is followed by PSA, the former is used for bulk separation and its product stream is fed to the PSA for higher purity, while the residue gas is used to purge the adsorber during the desorption step. When the PSA precedes the membrane, the latter is used for residual recovery and its waste gas is used as purge to the PSA for adsorbent regeneration . The present work is the first step in the development of a hybrid gas separation process, designed and optimized for cyclic steady-state operation in terms of product purity and recovery. The feasibility of the integrated process presented here is assessed through the effect of the various operating parameters on separation performance. 2. Process description Fig. 1 shows a schematic diagram of the hybrid cyclic process developed for bulk separation of a binary ([email protected]
mixture using a membrane module and a dual-bed PSAunit. The membrane performs most of the bulk separation operating in countercurrent flow mode to maximize the average driving force and therefore providing the most efficient
Fig. 1. Cycle sequence of hybrid membrane-PSA process.
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arrangement. Permeate and residue streams are fed to the PSA at different steps for higher purity and enhanced recovery. Although the residue stream is sent directly to the PSA, the permeate stream is stored in an intermediate tank before being sent to the adsorbent unit. The integrated process, as presented here, operates under the assumption that component A permeates faster and is the least adsorbed. The cycle starts with an incomplete pressurization (PR,) of one the PSA beds with the permeate stream, which is stored in the intermediate tank and is enriched in component A. This stream was obtained during the previous highpressure adsorption step operating on the other bed. In step PR, valve V, is kept opened and valves VZand V3 stay closed until pressure equalization between the tank and the PSA bed is established. Then the tank outlet is closed by shutting valve V,. To complete the pressurization step (PR,), valve V3 is opened and the adsorbent bed is pressurized with regular feed gas, which is less rich in component A than the permeate stream employed in step PR, . During step PR, the membrane behaves as an empty tube, since both permeate and residue sides are at feed pressure P,. The high-pressure adsorption step (HPA) is initiated by opening valve V, and feeding the PSA with the residue stream from the membrane at a prescribed flow rate. The residue stream is enriched in the strongly adsorbed component B, while permeate is stored in the intermediate tank to be employed in the next cycle. During the HPA step the residue pressure is kept constant at the highpressure value P,, whereas the permeate pres- sure increases with time due to gas build-up in the tank. The PSA cycle proceeds with the following steps [8,9]: co-current blowdown (CD) to recover the residual amount of component A, which was pushed to the end of the bed during the HPA step; counter-current blowdown (BD) and purge (PG) to recover component B and to regenerate the bed for the next cycle. During these steps the membrane module is operating with the other PSA bed in order to provide continuity of flow (Fig. 1).
EC. Ferreira et al. /Desalination
Although the operation of each bed is batchwise, the system as a whole is a continuous one that is operated in cyclic steady state. The membrane module behaves similarly because of its coupling to the PSA cycle. 3. Theoretical
c pvar T
_n,c:(_~~,~=~~_~(T-T~), w For a binary mixture ([email protected]
the pore-diffusion model can be written as 
ART = (2k - 3)*1;;,,,, k
where 0 c z c L, A = (1 - E)E I&, and n, = p,,RgAP. The global mate/;ial balance and the energy equation read, respectively:
For simplicity we assume that there is negligible pressure drop on both sides of the membrane. Except for constant permeances and ideal gas behavior, this, is the only approximation made regarding the membrane module. The PSA bed is modeled using the usual non-isothermal and variable-velocity axially dispersed plug-flow model with negligible pressure drop . A pore-diffusion model is assumed to govern mass transfer inside the adsorbent particles. Given these assumptions, the individual (i = 1,. . .,iV-I ) and global material balances in the residue (k = 1) and permeate (k = 2) sides of the membrane are:
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where De is the intraparticle effective diffusivity and 7, is the particle radius.
4. Results and discussion
(2) where 0 < z c L,,, and A, and An3are the cross-section and permeation areas, respectively. If Q, is the permeance of species i, then its molar flux can be written as: F ,.I-+?=
(Q,la,) )h,.,- pZv,,z)
where y,, = Q,/Q, and A is the reference species. The individual material balances (i = 1, . , .,Nc1) in the adsorbent bed are:
The first case study selected for assessment of process performance is the bulk separation of a 50/50 H,(A)/CH,(B) gas mixture using a PSA employing activated carbon as the adsorbent coupled to a membrane with selectivity aAB= 35 [lo]. The values of the parameters of the PSA model are given in [8,9] and are not reproduced here to save space. The standalone models for the membrane module and PSA unit were validated by comparing numerical results with experimental data reported in the literature [8-lo]. For the separation under study the integrated system attained the cyclic steady state after the 10th cycle from startup for all runs. Unless otherwise stated, the following operating conditions were considered: P, = 1.2 bar, P, = 35 bar, V, = 2500 cm3, F (total feed per cycle) = 48 L STP, Q, = 100 GPU, Fn, = 17.
I.A.A. C. Esteves, J.P.B. Mota / Desalination 148 (2002) 275-280
Parameter F,,, is a dimensionless defined as :
Fig. 2 shows the effect of feed flow rate on product purity and recovery for the PSA unit operating alone and for the integrated system. The recoveries are defined as: Recovery,
A orrr(llPA+CD) ,
The results show that for the same separation performance, the integrated system has a higher feed throughput than the PSA unit operating alone. Increasing the feed flow rate results in longer bed coverage at the end of the HPA step and lower product purity of the strongly adsorbed component, CH,. The purge step regenerates the bed for the next cycle and increases purity of the product stream enriched in CH, by striping it out of the bed. As seen in Fig. 3, increasing the purge to feed ratio, P’F = 4nW)lAinW4~’ increases H, purity and CH, recovery steadily, but with loss of H, recovery and dilution of the CH, product stream. The H, recovery and CH, purity for the integrated system 100 ,
8 3 s
* I 50
100 f 96
Fig. 2. Impact of feed flow rate on I-J product purity and recovery for standalone PSA (A, P, = 35 bar; A, P, = 2.5bar) and for integrated system ( q , P, = 35 bar; n , P, = 25 bar).
Fig. 3. Impact of P/F ratio on product purity and recovery q , recovery) and for integrated system (A, purity; n , recovery).
for standalone PSA (A, purity;
F.C. Ferreira et al. /Desalination
are higher than those for the PSA unit operating alone. Fig. 4 shows CH, concentration profiles in the adsorbent bed for the PSA unit alone and for the integrated system. It is clear that the concentration profiles for the hybrid process are sharper than those for the PSA alone. This enhances the separation performance of the combined process. Unlike in a conventional PSA process, the adsorbent beds in the integrated system are fed with a varyingcomposition gas stream, initially rich in the more permeable but less adsorbed component, which is progressively enriched in the other component having opposite behavior. During the HPA step, permeation occurs driven by the pressure difference between the residue side of the membrane and the storage tank. The permeation rate decreases as permeate pressure increases, because of gas build up in the tank. The H, mole fraction in the permeate stream decreases with time, approaching the value in the feed gas. As seen in Fig. 5, as the tank volume is increased, the final permeate pressure is decreased and the mole fraction of H2 in the permeate stream is increased. Increasing the dimensionless permeation flow F,,, increases the permeate pressure and 0.8 ,
Fig. 4. CH, concentration profiles in PSA bed at the end of each step for standalone PSA (open symbols) and for the integrated system (filled symbols). O/W, PRI (9 s); ON, PR2 (30 s); Lx/A, HPA (210 s).
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0.55 1 30
Fig. 5. Pressure and H, concentration histories in the permeate stream as a function of tank volume Vc (0, 250; & 500; +, 2500 cm’) and dimensionless permeation flow F, (‘k, 5; n , 17;& 51).
decreases the H, mole fraction in the permeate stream. The H&H, mixture was selected because it is a challenging separation to show the benefits of the hybrid process since the selectivity between CH,/H, on activated carbon is high, making the PSA alone a very efficient separation process for this mixture. Nevertheless, even in this unfavorable case the membrane module enhances the separation performance. We are currently applying the same process to CO,/CH, and H,/CH,/CO, mixtures. High-purity products of H, and CH, can be obtained using a standalone PSA, but the product
I.A.A.C. Esteves, J.P.B. Mota /Desalination
purity of CO, only reaches 60% due to the low selectivity between CO&H, on activated carbon. Our preliminary results suggest that a properly selected membrane can significantly increase the CO, product purity. Using the operating pressure of the PSA bed as the driving force for the membrane permeation minimizes the need for recompression work and enhances productivity. It is expected that this will make the integration of the two processes economically feasible.
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Bed porosity Heat of adsorption, J.mol-I Particle density, kg.m-3
Acknowledgements Financial support from FCTLWT under Grant PRAXIS XXI/BD/19832/99 is gratefully acknowledged.
Heat capacity of gas, J.mol-‘.K-’ Particle diameter, m Dispersion coefficient, m2.s-’ Adsorbent bed and membrane lengths, m Number of components Pressure, bar Concentration of adsorbed phase, mol .kgm3 Gas constant, 8.3143 J.mol-‘.K-’ Temperature, K Time, s Mole fraction in gas phase, mole fraction in intraparticle gas phase Gas velocity, m.s-’
d 0” L, Lm Y P 4 R T”
yj y,, V
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