Pressure Swing

Ind. Eng. Chem. Res. 2007, 46, 5723-5733

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Gas Separation by a Novel Hybrid Membrane/Pressure Swing Adsorption Process Isabel A. A. C. Esteves and Jose´ P. B. Mota* Requimte/CQFB, Departamento de Qumica, Faculdade de Ciencias e Tecnologia, UniVersidade NoVa de Lisboa, 2829-516 Caparica, Portugal

Novel gas separation processes, coupling pressure swing adsorption (PSA) and membrane technologies, are presented for both cooperative and opposing regions of selectivity for the two units. The membrane works as a prebulk separation unit and is coupled to the intrinsically dynamic periodic operation of the PSA in a way that enhances the separation performance of the hybrid unit with respect to that of the stand-alone PSA. The operating pressure of the PSA unit is used as the driving force for permeation. Unlike in a conventional PSA process, the adsorption beds for the integrated system are fed with a varying-composition gas stream, initially rich in the less adsorbed component, which is progressively enriched in the other component with the opposite behavior. This gives rise to sharper concentration fronts and increased product purity. The integrated system allows for two schemes that are introduced and described: scheme A, in which the least adsorbed component is the more permeable one; and scheme B, where the least adsorbed component is the less permeable one. In the current work, the hybrid process is applied to H2/CH4 separation over activated carbon by coupling the membrane to a five-step PSA cycle using scheme A. The membrane permeances are those commonly expected for a polysulfone membrane, with a typical selectivity of 35 for H2/CH4 separation. The effect of various operating parameters, such as permeation throughput, total feed amount per cycle, purge-to-feed ratio, and adsorption and blowdown pressures, is assessed through detailed process simulation. Depending on the values of the operating parameters and for an equimolar feed mixture at 35 bar, a 7 min integrated cycle produces H2 and CH4 products with purities within the 83-97% and 81-99% ranges, respectively. The product recoveries obtained are in the ranges of 77-99% for H2 and 81-98% for CH4. Introduction Pressure swing adsorption (PSA) and membrane permeation are two widely known technologies, which have been extensively developed and applied in industry for gas separation.1-3 They are frequently considered as alternatives to the conventional cryogenic processes. Their process configurations have been widely studied to either minimize recompression work for reducing final operational costs or give a better reuse to some waste gases that are not usually recovered by conventional methods. Membrane permeation is currently a mature technology that is applied at industrial scale for the separation of gaseous mixtures such as CO2/CH4, H2/CH4, He/CH4, O2/N2, H2/CO, or H2S/CH4.4-7 In addition to the conventional membranes for gas separation, such as those of polysulfone and silicone rubber, there has been an exponential growth in the research and development of other membrane materials, such as polyimides and polyaramides.8-11 Nonetheless, membrane permeation is generally unfavorable when a high-purity product is required and is usually considered to be more suitable for bulk separation. Unless permeate is recycled, higher product purity is usually accompanied by lower product recovery. Often, membranes provide a moderately pure product at low cost that may be inexpensively upgraded by a subsequent process.12 This fact has motivated active research on the integration of membranes with other separation processes.6 Cyclic adsorption processes are well-established separation methods in the chemical and petrochemical industries. Since the pioneering works of Skarstrom13 and Guerin de Montgareuil and Domine14 on PSA, many schemes have been developed and * To whom correspondence should be addressed. Tel.: (351) 212948300 (ext. 10961). Fax: (351) 212948385. E-mail: pmota@ dq.fct.unl.pt.

commercialized to increase energy efficiency, improve product purity, and enhance operation flexibility.15 The available elementary steps to build a PSA cycle are the following: pressurization with feed or raffinate product, high-pressure adsorption with raffinate withdrawal, high-pressure purge with the more retained species, blowdown with possible evacuation, and low-pressure purge with raffinate product.2 Recompression work can be minimized by applying a pressure equalization step to selected pairs of columns. The configurations differ depending on the mixture to be treated and on the desired separation performance. Although there is some published work on the development of hybrid membrane/PSA systems,5,6,16-19 truly synergistically concepts have only been proposed by Feng et al.,12 Esteves and Mota,20 and Esteves.21 An important conclusion drawn from these works is that membrane permeation can be an effective aid in the pressurization and high-pressure adsorption steps of a typical PSA process. The results also indicate the feasibility of incorporating membrane permeation into the blowdown step of the PSA cycle, so that the operating pressure range available from the PSA can be used as the driving force for permeation. Therefore, a complete understanding of these hybrid processes for gas separation is crucial, mainly because benefits, such as product quality, plant minimization, environmental impact, and energetic cost reductions, are arising. This paper describes novel gas separation processes, coupling PSA and membrane technologies, which are applicable to either cooperative or opposing regions of selectivity for the two individual units. In particular, the performance of one of the proposed hybrid schemes is analyzed in detail for H2/CH4 separation under different operating conditions. The effect of various operating parameters on process performance is assessed through numerical simulation. It is shown that the integrated steps of the hybrid systems sharpen the composition wavefronts

10.1021/ie070139j CCC: $37.00 © 2007 American Chemical Society Published on Web 07/17/2007

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Ind. Eng. Chem. Res., Vol. 46, No. 17, 2007

Figure 1. Schematic diagram and scheduling chronogram of elementary steps comprising the cyclic operation of hybrid scheme A. The notation is as follows: PR, pressurization; HPA, high-pressure adsorption; CD, cocurrent blowdown; BD, countercurrent blowdown; PG, low-pressure purge. The product from the HPA step is enriched in the less adsorbed species (A). The gray area of the chronogram identifies periods during which the membrane is working as an empty tube. The step times of the cycle are specified in Table 4.

inside the adsorbent bed, giving rise to a decrease of band broadening and higher product purity, thus enhancing the overall process performance. This is especially evident in the highfeed throughput region. Process Description To explore the synergy obtained by coupling a PSA unit and a membrane module for separating different types of gaseous mixtures, the hybrid concept was formulated to address both cooperative and opposing regions of the selectivity for the two stand-alone units. Thus, two main schemes were developed: • scheme A, in which the more permeable component is the least adsorbed one; • scheme B, in which the more permeable component is the more adsorbed one. In both cases, the regular feed is sent to the membrane unit, typically through the shell-side of a hollow-fiber module. As a result of the pressure difference imposed between the two sides of the membrane, the low-pressure permeate stream becomes enriched in the more rapidly permeating components, whereas the slower diffusing species are concentrated in the residue stream of the membrane, which is recovered at a lower pressure than the feed. Figure 1 shows a schematic diagram for hybrid process A, consisting of a membrane module and a dual-bed PSA unit. In this scheme, the residue is sent directly to the PSA unit but, unlike scheme B, the permeate is temporarily stored in an intermediate tank before being sent to the adsorption process. Also, in scheme A, the residue, instead of the permeate, is used to feed the adsorption step. The cycle for hybrid scheme A starts with an incomplete pressurization (PR1) of one of the PSA beds using the gas stored in the tank, which is enriched in the least adsorbed species A. The gas stored in the tank corresponds to a permeate stream that was obtained during the previous high-pressure adsorption (HPA) step operating on the other PSA bed. During this step, valve V1 is kept opened while V2 and V3 stay closed until pressure equalization between the tank and the PSA bed is established. To complete the pressurization step (PR2), the tank

Figure 2. Schematic of integrated cycle for scheme A. P1 is the intermediate pressure in the tank after permeation at the end of the HPA step; P2 represents the equalization pressure at the end of step PR1. Downward and upward arrows indicate decreasing and increasing pressure changes, respectively.

outlet is closed by shutting valve V1, and valve V3 is opened to pressurize the PSA bed with the residue stream from the membrane unit, which is less rich in species A than the permeate stream employed in step PR1. During PR2, the membrane behaves essentially as an empty tube, since both permeate and residue sides are at feed pressure Ph. Thus, during this step, the residue stream essentially has the same composition as the regular feed. The cycle then follows with the HPA step, which is initiated by opening valve V2 while 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 the 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 high-pressure value Ph, whereas the permeate pressure increases with time due to gas buildup in the tank. This happens first through fast equalization between the tank and the permeate side of the membrane and then slowly as more gas is driven through the membrane while both the permeate side of the membrane and the tank together build up pressure at the same rate. The two pressurization steps, PR1 and PR2, as well as the HPA step, are illustrated schematically in Figure 2. Ultimately, the PSA cycle proceeds with the following steps: cocurrent blowdown (CD) to recover the residual amount of A that was pushed to the end of the bed during the HPA step; countercurrent blowdown (BD) and purge (PG) to recover species B and to regenerate the bed for the next cycle. During these steps, the membrane module is operating with the other PSA bed. Although the operation of each bed is batchwise, the system as a whole is a continuous one that is operated under cyclic steady-state conditions. Unlike conventional membrane operation, in which the pressures are kept constant, here the permeation occurs cyclically due to its coupling to the PSA cycle. Figure 3 shows the schematic diagram of hybrid process B, which is applicable when the more strongly adsorbed species is also the more permeable one. In this scheme, the pressures on both sides of the membrane are kept constant during permeation. This is another major difference with respect to

Ind. Eng. Chem. Res., Vol. 46, No. 17, 2007 5725

Figure 3. Schematic diagram of the elementary steps comprising the cyclic operation of hybrid scheme B. A and B are the less and more strongly adsorbed species, respectively.

the rest of the permeate obtained from the membrane at a prescribed flow rate. During this stage, the permeate pressure is kept constant at pressure Ph. • If the adsorption pressure is attained during permeation and there is residue left from the membrane (case B.2 in Figure 3), the cycle proceeds with a high-pressure adsorption step (HPA1) using that stream enriched in the lighter component A. After HPA1, the membrane is no longer operational for this bed. The bed pressure is kept constant at Ph, and the high-pressure adsorption step is then completed using the permeate enriched in component B (step HPA2). As in scheme A, the PSA cycle proceeds with co- and countercurrent blowdowns, in which the bed is depressurized cocurrently to the feed from pressure Ph to an intermediate value Pd and then down to Pg countercurrently to the feed. Finally, a low-pressure (P ) Pg) purge (PG) with part of the HPA product may occur to recover species B and to regenerate the bed for the next cycle. Hybrid scheme A is detailed and analyzed in the present work using the H2/CH4 separation as a case study; an extensive analysis of scheme B will be reported elsewhere.22 The latter scheme is suitable for CO2(B)/CH4(A) and CO2(B)/N2(A) separations. Theoretical Model

Figure 4. Schematic of integrated cycle for scheme B. P1 is the intermediate pressure in the adsorption column after permeation at the end of step PR1. Upward arrows indicate increasing pressure changes.

hybrid scheme A. Although the residue stream is sent directly to the PSA, the permeate stream is temporarily stored and is either used to complete the pressurization step or is fed to the bed during the adsorption step. This depends on the total feed amount admitted per cycle. For a more comprehensive understanding of this process, a schematic diagram of its cyclic operating principle is given in Figure 4. The integrated cycle for scheme B starts with a permeation step during which the residue side of the membrane is kept at Ph, where Ph is the pressure value in the adsorption bed during the high-pressure adsorption (HPA) step, and the permeate side of the membrane is at a lower pressure Pm. The value of Pm must be appropriately selected for each separation because, unlike in process A, the gas coming from the permeate must be repressurized to feed the HPA step. Simultaneously, an initial pressurization stage (PR1) takes place with the residue effluent stream from the membrane, enriched in A, until pressure equalization between the membrane and the bed is established. Subsequently, two situations can occur: • If the pressure in the bed is lower than the adsorption pressure, Ph, the residue was insufficient to complete the pressurization step (case B.1 in Figure 3) and the bed is pressurized with permeate gas enriched in the more strongly adsorbed species B (step PR2). After PR2, the membrane is no longer operational for this PSA bed. Then, a high-pressure adsorption step (HPA2) is carried out by feeding the PSA with

Prior to performing parametric studies and assessing the performance of the hybrid schemes, individual simulation models of the two stand-alone units were implemented in gPROMS and successfully validated21,24,25 against published work by other authors.23,26,27 gPROMS is a well-known software package for the modeling and simulation of lumped- and distributed-parameter process models with combined discrete and continuous characteristics.28,29 The model for the membrane module was validated for H2 separation from a gas mixture containing low molecular-weight hydrocarbons using a polymeric hollow-fiber contactor operating in countercurrent mode.23 For this process, the H2-enriched product is obtained in the permeate stream. The stand-alone PSA model was validated for bulk separation of a 50/50% H2/CH4 mixture over activated carbon.26,27 As previously indicated, the PSA cycle comprises the following elementary steps: pressurization (PR), high-pressure adsorption (HPA), cocurrent blowdown (CD), countercurrent blowdown (BD), and purge (PG). The weakly adsorbed component, H2, is collected during HPA and CD steps, and the strongly adsorbed species, CH4, is recovered during the BD and PG steps. A dualbed process is assumed with continuous feed as depicted in the chronogram of Figure 1. The simulation results obtained with our models of the two stand-alone units are in agreement with those reported by the aforementioned authors.23,26,27 The hybrid concepts were then implemented by coupling the models for the two stand-alone units through appropriate boundary conditions. The simulation model for the hybrid scheme A is summarized in Table 1; the corresponding boundary conditions for the PSA and membrane units are given in Tables 2 and 3, respectively. Variables with no subscript refer to gas in the interparticle void space of the packed bed, subscript “p” denotes either adsorbed phase or adsorbent properties, “f” denotes feed conditions, “v” is associated to the intermediate tank, and subscripts “m” and “s” refer to the permeate and residue sides of the membrane, respectively. Symbol Vh denotes the inlet gas velocity in the HPA step, Vg stands for the purge gas velocity, and yig represents the mole fraction composition of the purge gas, which is taken

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Table 1. Model Equations for PSA and Membrane Units

a

Reference 26. b Reference 2. Λp ) p(1 - )/, Ωp ) ΛpFpR/p.

Table 2. Boundary Conditions and Pressure Dynamics Applicable to the PSA Model for Each Step of Hybrid Scheme Aa step

z)0

z)L

dP/dt

PR1 PR2 HPA CD BD PG

T ) Tf, yi ) yiv T ) Tf, yi ) yis|z)Lm T ) Tf, yi ) yis|z)Lm T′ ) 0, y′i ) 0, V ) 0 T′ ) 0, y′i ) 0 T′ ) 0, y′i ) 0

V ) 0, T′ ) 0, y′i ) 0 V ) 0, T′ ) 0, y′i ) 0 V ) Vh, T′ ) 0, y′i ) 0 T′ ) 0, y′i ) 0 V ) 0, T′ ) 0, y′i ) 0 V ) -Vg, T ) Tf, yi ) yig

eq 1 eq 1 0 eq 2 eq 2 0

a Primed variables denote partial derivatives with respect to the axial coordinate along the packed bed, e.g., T′ ≡ ∂T/∂z. Note: inlet conditions (z ) 0) for PR1, PR2, HPA, and (z ) L) PG were actually implemented using Robin-type boundary conditions (equality of mass or heat fluxes at boundary).

Table 3. Boundary Conditions Applicable to the Membrane Module for Each Step of Hybrid Scheme Aa step PR1 PR2 HPA CD, BD, PG

stream residue permeate residue permeate residue permeate

z)0

z ) Lm

dPs/dt ) 0, yis ) yif Vs ) 0, y′is ) 0 Vm ) 0, y′im ) 0 Vm ) 0, y′im ) 0 dPs/dt ) 0, yis ) yif AsVsPs ) AV|z)0P, y′is ) 0 Vm ) 0, y′im ) 0 Vm ) 0, y′im ) 0 AsVsPs ) AV|z)0P, y′is ) 0 dPs/dt ) 0, yis ) yif dPm/dt ) dPv/dt, y′im ) 0 Vm ) 0, y′im ) 0 state variables are frozen until next cycle

a Primed variables denote partial derivatives with respect to the axial coordinate along the membrane unit, e.g., y′im ≡ ∂yim/∂z.

as the average value of the product composition obtained during the final fraction of the HPA step. Amem is the permeation area, Lm is the membrane length, and AmLm and AsLm are the volumes

of the permeate and residue sides of the membrane, respectively. Fm is used as a scaling parameter for the membrane that represents Amem in dimensionless form, by establishing a ratio between a hypothetical molar flow of a pure stream of species A through the membrane unit, when the pressure differential is Ph - Pg, and FHPA is the molar flow through the PSA unit during the HPA step. The reader is referred to the notation section for the definition of the other symbols in Tables 1-3. To simplify the computational model, the following assumptions have been considered for the membrane module: isothermal operation and the ideal gas law; constant permeances; shellside feed and countercurrent operation with negligible pressure drop on both sides of the membrane. The dynamics of the PSA unit is modeled using a nonisothermal and variable-velocity axially dispersed plug-flow model, with negligible pressure loss inside the PSA bed. A pore-diffusion model governs mass transfer inside the adsorbent particles.26,27 Transport and thermodynamic properties are assumed constant and temperature independent. Multicomponent adsorption equilibrium is modeled by the Sips isotherm model (cf. Table 1). For simplicity, the pressurization of the PSA bed with permeate stream (PR1) and regular feed gas (PR2) is assumed to be governed by a linear pressure dynamics, i.e.

dP Ph - Pg (0 < t < ∆tPR) ) dt ∆tPR

(1)

where ∆tPR ) ∆tPR1 + ∆tPR2 is the duration of the pressurization step, Ph is the pressure value at the HPA step, and Pg is the

Ind. Eng. Chem. Res., Vol. 46, No. 17, 2007 5727 Table 4. Simulation Parameters for H2/CH4 Separation Using Hybrid Process Aa Physical Properties for PSA Unit 0.028 column radius, Rc (cm) 0.61 column length, L (cm) 0.850 wall 0.498 thickness, ew (m) 10 density, Fw (g cm-3) 0.43 heat capacity, Cpw (cal g-1 K-1) 0.250 conductivity, kw (W m-1 K-1) 0.106 Heat transfer coef, hw (cal m -2 K-1 s-1)

particle diameter, dp (cm) intraparticle porosity, p particle density, Fp (g cm-3) bulk density, Fb (g cm-3) particle tortuosity, τ interparticle porosity, bed heat capacity, Cps (cal g-1 K-1) bed conductivity, Doh (W m-1 K-1)

10-3 8.22 0.110 43.3 30

Parameters of Sips Adsorption Isotherm Model26 1.795/T qm,CH4 (mol kg-1) 1.00 nCH4 3.80 × 10-5 b0,CH4 (psi-n) 1.73 × 103 QCH4 (J mol-1)

qm,H2 (mol kg-1) nH2 b0,H2 (psi-n) QH2 (J mol-1) tank volume, Vv (L) membrane volumes, Vs ) Vm

feed pressure, Ph (bar) purge pressure, Pg (bar) P/F ratio blowdown pressure, Pd (bar) feed amount per cycle (L STP) (STP: 0 °C, 1 atm)

0.308T-0.59 0.96 4.23 × 10-6 1.23 × 103

Physical Properties for Membrane Unit 2.5 (0.25-2.5) membrane selectivity, RH2/CH4 0.02Vv permeation flow parameter, Fm

composition, yif (%) temperature, Tf (K) heat capacity, Cpg (cal mol-1 K-1)

a

2.05 60.0

50/50 293.15 7.647

35 17.18 (5.15-51.5)

Feed Mixture intraparticle diffusivity, De (m 2 s-1) molecular diffusion, PDm (m 2 bar s-1)

Operating Parameters 35.5 (25.5-35.5) 1.21 0.09 (0.06-0.22) 12.73 (4.73-14.73) 47.7 (47.7-80)

2 × 10-8 6.85 × 10-5

pressurization, ∆tPR (s) high-pressure adsorption, tHPA (s) cocurrent blowdown, tCD (s) countercurrent blowdown, tBD (s) purge, tPG (s) cycle time, tcyc (min)

30 180 90 90 30 7.0

Numbers in parentheses represent ranges spanned in parametric study.

pressure value during the purge step. During the two blowdown steps (CD and BD), the pressure follows an exponentially decaying curve30 which can be defined as

dP ) R(β - P) dt

{

βCD ) βBD )

Pd - Ph exp(-RCD∆tCD)

dyiv )0 dt

1 - exp(-RCD∆tCD) Pg - Pd exp(-RBD∆tBD) 1 - exp(-RBD∆tBD) (tCD < t < tPG) (2)

The parameters RCD and RBD are equal to 0.01 and 0.05, respectively. The interaction between the two units when they are interconnected as a hybrid process is taken into account by modifying the usual boundary conditions for the two standalone models. This is detailed in Tables 2 and 3. As stated above, scheme A requires a tank for temporary storage of the permeate stream (Figure 1). The global material balances for this piece of equipment during the PR1 and HPA steps are (see Figure 2)

Vv

{

dPv A ∈ Pυ|z)0 (PR1) )AmPmυm|z)0 (HPA) dt

(3)

where A ) πRc2 is the cross-sectional area of the PSA column. During the other steps, the tank is closed, i.e.,

dPv )0 dt

(4)

During HPA, the individual material balance for the ith component can be written as

dyiv ) AmυmPm(yiv - yim|z)0) VvPv dt

where yiv is the mole fraction of component i in the tank and yim|z)0 is the corresponding mole fraction at the permeate outlet. During the other steps, the gas composition in the tank remains constant, because it is either closed or being depressurized, i.e.,

(5)

(6)

Note that when the HPA step is initiated valve V2 is opened in order to store permeate in the intermediate tank (Figure 2). At this instant, an initial short transient occurs during which the permeate side of the membrane at Pm ) Ph is quickly depressurized until pressure equalization with the tank is attained. The decrease of the permeate pressure is assumed to be much faster than the permeation rate, so that it can be modeled as a step change in the initial conditions for both the tank and permeate side of the membrane at the start of the HPA step. Essentially, this corresponds to decreasing Pm down to the equalization pressure, which is very close to that in the tank, because the tank volume is much larger than the volume of the permeate side of the membrane. However, we do this more rigorously by satisfying the material balances across the step change. At the end of the pressurization step, t ) tPR the tank is at an whereas the permeate side of the memintermediate pressure Pv brane is at Pm ) Ph. From a global material balance for the two contacting volumes, the equalized pressure at instant t ) t+ PR is + P+ v ) Pm )

VvPv + AmLmPh Vv + AmLm

(7)

+ ) is also slightly changed The gas composition in the tank (yiv with respect to that before the step change (yiv):

(yivPv)+ ) (yivPv)- +

P+ v - Pv Lm

∫0L

m

yim dz

(8)

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Figure 5. Simulated pressure histories in both PSA and permeate side of the membrane for the reference case, over the first 11 cycles of operation (bottom) and under CSS conditions (top). Gray and dashed lines denote intermediate tank volumes Vv of 0.25 and 2.5 L, respectively.

Figure 7. Simulated H2 concentration (top) and pressure (bottom) histories in the permeate stream, under CSS conditions, as a function of dimensionless permeation flow Fm. All other operating parameters are fixed at their reference values.

Figure 8. Impact of total feed amount per cycle, F, on CH4 and H2 purities and recoveries, for both a stand-alone PSA and integrated process, at Ph ) 25.5 bar. All other operating parameters are fixed at their reference values. Lines represent simulated results.

Figure 6. Simulated H2 concentration (top) and pressure (bottom) histories in the permeate stream, under CSS conditions, as a function of tank volume Vv. All other operating parameters are fixed at their reference values.

It is further assumed that the composition profile in the permeate side of the membrane is not affected by the pressure equalization during the initial short transient. This is equivalent to assuming that yim(z) at t ) t+ PR is the same as that at t ) tPR. The bulk of the pressure equalization step then proceeds slowly

as more gas is driven through the membrane, while both the permeate side of the membrane and the tank together build up pressure at the same rate. The initial conditions (t ) 0) are

Ps ) Pm ) Ph; yis ) yim ) yif ∀ z ∈ [0, Lm]

(9)

Pv ) Ph, yiv ) yif

(10)

P ) Pg; T ) Tf, yH2 ) 1, yCH4 ) 0, qi ) q/i (Tf, Pg, yH2, yCH4) ∀ z ∈ [0, Lm] (11)


Figure 9. Impact of total feed amount per cycle, F, on CH4 and H2 purities and recoveries, for both a stand-alone PSA and integrated process, at Ph ) 35.5 bar. All other operating parameters are fixed at their reference values. Lines represent simulated results.

Figure 11. Effect of intermediate blowdown pressure, Pd, on CH4 and H2 separation performance for the integrated system. Operating parameters are fixed at their reference values. Lines represent simulated results.

The performance of the separation process is measured in terms of purity (Pur) and recovery (Rec) of each product, which are defined as follows:

PurH2 )

Figure 10. Impact of P/F ratio on product purity and recovery, for both a stand-alone PSA and integrated process. Operating parameters are fixed at their reference values. Lines represent simulated results.

It is worth mentioning that due to the intrinsic periodic operation of the PSA unit, the hybrid process approaches a cyclic steady-state (CSS) after a sufficiently long period of time. For process design, the initial transient behavior is of little interest, but only the fully established periodic state. The latter is independent of the particular choice of initial conditions, which only affect the number of cycles necessary to obtain the steady periodic solution.

NH2,out in (HPA + CD) - NH2,in in PG NH2+CH4,out in (HPA + CD) - NH2+CH2,in in PG (12)

RecH2 )

NH2,out in (HPA + CD) - NH2,in in PG NH2,in in (PR + HPA)

NCH4,out in (BD + PG) PurCH4 ) NH2+CH4,out in (BD + PG) RecCH4 )

NCH4,out in (BD + PG) NCH4,in in (PR + HPA)

(13)

(14)

(15)

where, e.g., “Ni,out in HPA” denotes the accumulated amount of component i withdrawn from the PSA during the HPA step

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Figure 12. Simulated CH4 (left) and H2 (right) axial composition profiles in the gas phase along the packed bed at the end of each PSA step, under CSS conditions, for both a stand-alone PSA and integrated process. Operating parameters are fixed at their reference values, except for the feed amount which is F ) 47.7 L STP.

and “Ni,in in PG” is the amount of i consumed in the purge step. The purge-to-feed ratio, P/F, is defined as

P/F )

NH2,in in PG NH2,out in HPA

(16)

where the numerator is the amount of H2 from the feed that is consumed in the purge step. The spatial derivatives in the model equations are discretized using a second-order control-volume formulation.31 In order to prevent nonphysical oscillations of the solution, the convective terms are spatially discretized using the van Leer harmonic fluxlimited scheme implemented in the form advocated by Waterson and Deconinck.32 The large, sparse system of differentialalgebraic equations (DAEs) obtained upon spatial discretization is implemented and solved in gPROMS. Results and Discussion The performance of hybrid scheme A is assessed and compared to that of a stand-alone PSA unit for the separation of a 50/50% (v/v) H2/CH4 mixture. The PSA employs activated carbon as the adsorbent,26 whereas the membrane permeances are those commonly expected for a polysulfone membrane with a 0.1 mm thick separation layer.23 It has a typical gas selectivity RH2/CH4 of 35; the H2 and CH4 permeances are assumed to be 100 and 2.86 GPU, respectively. Process and operating parameters are listed in Table 4. The values of the main operating parameters for the reference case are: Vv ) 2.5 dm3, Fm ) 17.2, Ph ) 35.5 bar, Pd ) 12.7 bar, Pg ) 1.21 bar, P/F ) 0.09,

F ) 47.7 dm3 (STP). These values are subsequently changed in a parametric study in order to study the influence of each operating parameter. The hybrid scheme requires 11 cycles of operation to attain the cyclic steady state, which corresponds to approximately 110 s of computational time on an 500 MHz Intel Pentium III PC. The bottom graph of Figure 5 shows the pressure histories, for the PSA and permeate side of the membrane, over the first 11 cycles of operation for two different tank volumes. The top plot gives an enlarged and more detailed graph of the steady periodic pressure histories over a single cycle. Unlike conventional membrane operation where the two pressures Ps and Pm are kept constant, here Pm swings cyclically due to its coupling to the PSA unit. As expected, the larger the volume of intermediate tank, the higher its equalization pressure and the lower the value of Pm at the end of the HPA step. During the HPA step, permeation occurs driven by the pressure difference, Ps - Pv, between the residue side of the membrane and the storage tank. Since the permeation rate decreases as Pm increases, because of gas buildup in the tank, the mole fraction of H2 in the permeate stream decreases with time, approaching its value in the feed gas. As seen in Figure 6, this effect is more pronounced for smaller storage tanks. Membrane productivity is governed by parameter Fm, which is proportional to the product of membrane area and H2 permeance. Figure 7 shows that by increasing the value of Fm, the permeate pressure increases and the H2 mole fraction in the permeate stream decreases. It is clear from the analysis of Figures 5-7 that both Vv and Fm should be correctly selected for optimum operation of the hybrid process. It is worth


Figure 13. Simulated CH4 (left) and H2 (right) axial composition profiles in the gas phase along the packed bed at the end of each PSA step, under CSS conditions, for both a stand-alone PSA and integrated system. Operating parameters are fixed at their reference values, except for the feed amount which is F ) 70.0 L STP.

mentioning that the values used in this study were not subjected to a rigorous optimization analysis and that better performances are expected if state-of-the-art optimization methods are employed.33-35 Figures 8 and 9 show the effect of the total feed amount admitted per cycle on product purity and recovery for both the stand-alone PSA unit and integrated system. The results obtained for two different values of adsorption pressure, Ph ) 25.5 bar and Ph ) 35.5 bar, show that, for the same separation performance, the hybrid system withstands a higher feed throughput than the PSA operating alone. Increasing the feed flow rate results in longer bed coverage at the end of the HPA step, improving both purity and recovery. If the total feed amount admitted per cycle exceeds 65 L (STP), the H2 product is diluted with CH4 and both H2 purity and CH4 recovery deteriorate significantly. Nevertheless, in this region, it is clearly seen that the hybrid process performs better than the conventional PSA unit for the same amount of feed. The purge step regenerates the bed for the next cycle and recovers the product stream enriched in CH4 by striping it out of the bed. As seen in Figure 10, increasing the purge-to-feed ratio, P/F, steadily increases H2 purity and CH4 recovery, but with loss in H2 recovery and dilution of the CH4 product stream. It is worth noting that the purge step is performed at the expense of consuming part of the H2 product obtained during the HPA step. It is also observed that the H2 recovery and CH4 purity for the integrated system are higher than those for the conventional PSA unit operating alone. Figure 11 shows the effect of the intermediate blowdown pressure, Pd, on separation performance for the integrated

system, under CSS conditions. As expected, by increasing Pd, which corresponds to the bed pressure at the end of the first depressurization step, the purity of the less adsorbed species, H2, increases significantly, but less product is recovered. The opposite trend occurs for CH4, where the product is more diluted as Pd is increased. Figures 12 and 13 compare the simulated H2 and CH4 concentration profiles along the packed bed for the stand-alone PSA unit with those for the hybrid process, considering two different total feed amounts: F ) 47.7 and F ) 70.0 L STP. As seen in Figure 9, these amounts correspond to the lowest feed throughput considered in the parametric study, as well as the feed throughput which exhibits the maximum process performance for the given operating conditions. The comparison between the profiles for the stand-alone PSA and the ones for the integrated process is carried out at the end of each step of the cycle. It is seen that the concentration profiles for the hybrid process are sharper than those for the PSA alone. This results in an enhanced separation performance for the hybrid process. Unlike in a conventional PSA process, the adsorbent bed in the integrated system is fed with a varying-composition gas stream, initially rich in the more permeable but less adsorbed component, which is progressively enriched in the other component having opposite behavior. This is clearly seen in Table 5, which lists the average composition of the various gaseous streams fed to the PSA during the PR and HPA steps for the reference case analyzed in this work. The membrane performs a prebulk separation of the feed, and simultaneously works with the intrinsically dynamic periodic operation of the PSA, to enhance separation performance when compared to the

5732


Table 5. Average Composition (and Recovery When Applicable) of the Various Streams Fed to the PSA during the PR and HPA Steps for the Reference Case Analyzed in This Work stream

usage

% CH4 (v/v)

% H2 (v/v)

product recovery

feed residue permeate

PR2 HPA PR1

50.0 57.9 18.5

50.0 42.1 81.5

92.1 32.4

two individual units. This sequence of progressively heavier feed steps has the net effect of sharpening the composition wavefronts inside the adsorbent bed, giving rise to a decrease of band broadening and higher product purity. As shown in previous figures, this effect is more pronounced in the highfeed throughput region. Conclusions Novel membrane/PSA schemes were developed for gas separation, which cover both cooperative and opposing regions of the selectivity for the two stand-alone units. Hybrid scheme A, is applicable when the more permeable component is the least adsorbed one, whereas hybrid process B is suitable when the more permeable component is also the more adsorbed one. The former scheme was thoroughly evaluated for H2/CH4 separation on activated carbon and a polysulfone membrane with a typical selectivity RH2/CH4 ) 35. The membrane works as a prebulk separation unit and is coupled to the intrinsically dynamic periodic operation of the PSA in a way that the operating pressure of the PSA unit is used as the driving force for permeation. Unlike in a conventional PSA process, the adsorption beds for the integrated system are fed with a varying-composition gas stream, initially rich in the more permeable but less adsorbed component, which is progressively enriched in the other component having opposite behavior. This gives rise to sharper concentration fronts and increased product purity. The hybrid process behavior was analyzed and compared with the conventional PSA unit. The simulation results show that the inclusion of a membrane module into the cyclic adsorption process improves the separation performance when compared with the stand-alone PSA. Depending on the values of the operating parameters and for an equimolar feed mixture at 35 bar, a 7 min integrated cycle gives H2 and CH4 products with purities within the ranges of 83-97% and 81-99%, respectively. The product recoveries obtained are in the ranges of 7799% for H2 and 81-98% for CH4. The results presented here suggest that a pre-established PSA process, already in operation, can be advantageously coupled with a suitable membrane module, according to one of the proposed schemes, to enhance product purity and recovery. Whether the obtained improvements are large enough to overcome capital and extra operating costs for a given separation, must be assessed on a case per case basis. Acknowledgment This work was partly supported by FCT/MCTES through Project I&DT POCTI/EQU/45102/02 and EU through contract ENK6-CT-200-00053. I.A.A.C.E. acknowledges FCT/MTCES for funding (PRAXIS XXI/BD/19832/99 and SFRH/BPD/ 14910/2004). Notation A ) cross-sectional area of permeate and residue sides of membrane, m2

Amem ) permeation area, m2 b0 ) parameter of Sips isotherm model, bar-n Cpg ) gas heat capacity, J mol-1 K-1 Cps ) adsorbent heat capacity, J kg-1 K-1 Cpw ) wall heat capacity, J kg-1 K-1 dp ) particle diameter, m DL ) axial dispersion coefficient, m2 s-1 Doh ) stagnant bed conductivity, W m-1 K-1 Dh ) effective heat dispersion coefficient, W m-1 K-1 ew ) wall thickness, m F ) feed amount per cycle, L STP FHPA ) reference total molar flow admitted in HPA step, mol s-1 Fm ) dimensionless permeation flow parameter hw ) wall heat-transfer coefficient, W m2 K-1 kw ) wall conductivity, W m-1 K-1 L ) column length, m Lm ) membrane length, m n ) parameter of Sips isotherm model N ) molar flux, mol m-2 s-1 P ) pressure, bar P/F ) purge-to-feed ratio Pur ) purity q ) equilibrium solid loading, mol kg-1 qm ) loading at saturation, mol kg-1 Q ) heat of adsorption, J mol-1 R ) universal gas constant, J mol-1 K-1 Rc ) column radius, m Rec ) recovery t ) time, s or min tcyc ) cycle time, min T ) temperature, K V ) interstitial fluid velocity, m s-1 Vv ) tank volume, m3 y ) mole fraction z ) axial coordinate in either PSA bed or membrane module, m Subscripts BD ) countercurrent blowdown CD ) cocurrent blowdown d ) blowdown conditions f ) feed conditions g ) purge conditions h ) high-pressure adsorption conditions HPA ) high-pressure adsorption i ) component m ) permeate p ) adsorbent PR ) pressurization PG ) purge s ) residue v ) intermediate tank Greek Letters RH2/CH4 ) membrane selectivity R,β ) parameters of the pressure profiles in the blowdown steps ) interparticle porosity p ) intraparticle porosity Fb ) bulk density, kg m-3 Fp ) particle density, kg m-3 Fw ) wall density, kg m-3 τ ) particle tortuosity


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ReceiVed for reView January 23, 2007 ReVised manuscript receiVed May 25, 2007 Accepted June 7, 2007 IE070139J

Pressure Swing

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