Separation of Concentrated Binary Gases by Hybrid Pressure-Swing

Ind. Eng. Chem. Res. 2009, 48, 4445–4465

4445

Separation of Concentrated Binary Gases by Hybrid Pressure-Swing Adsorption/ Simulated-Moving Bed Processes Kyle P. Kostroski† and Phillip C. Wankat* Forney Hall of Chemical Engineering, School of Chemical Engineering, Purdue UniVersity, 480 Stadium Mall DriVe, West Lafayette, Indiana 47907-1283

Stripping-type PSA (S-PSA) is a commonly used gas separation process for purification of the light component. Rectifying-type PSA (R-PSA), although not commonly used, is a gas separation process for purifying predominately the heavy component. Because S-PSA and R-PSA each tend to favor the production of a single product, these processes are typically not used for complete binary separation. On the other hand, the gas-phase simulated-moving bed (SMB) is capable of achieving complete binary separation; however, its commercial application has been stymied by the need for carrier gas/desorbent recovery and unfavorable economics. In this work, S-PSA and R-PSA are combined with a two-zone SMB to develop S-PSA/SMB and R-PSA/SMB hybrid processes and these processes are integrated into combination-type C-PSA/SMB processes. Combining PSA and SMB eliminates the carrier gas/desorbent by taking advantage of gas expansion and by using both light and heavy purge streams. Separation of H2 and CH4 mixtures with Zeolite 5A was simulated to determine the feasibility of the hybrid processes. The primary products are H2 and CH4 plus an impure offgas may be produced. Complete binary separation can be achieved by recycling the entire offgas stream. The best separation was achieved with an eight-bed combination of the S-PSA/SMB and R-PSA/ SMB processes, the SRC-PSA/SMB. This process separated a 70% H2/30% CH4 feed into 99.99% H2 with 99.6% H2 recovery and 99% CH4 with 99.9% CH4 recovery with productivity of 9.37 × 10-5 mol feed/ (kg · s) and an energy requirement of 260.9 kJ/mol feed. Introduction Several types of adsorption processes are feasible for bulk gas separations, each having its own advantages and disadvantages. The ultimate choice of which adsorption process to use ultimately depends on process economics.1 Stripping-type pressure-swing adsorption (S-PSA) is a widely used process for bulk gas separations when the less adsorbed component is the desired product.2-5 Typical S-PSA processes for bulk gases use pressure equalization steps with several adsorption beds operating in parallel to increase the recovery of the light product. Figure 1 shows a typical six-step S-PSA process with pressure equalization. As shown in the figure, S-PSA involves feed at high pressure and purge (regeneration) at low pressure. During the high-pressure feed, the heavy component of the feed is adsorbed at high pressure while light product is produced at high pressure. Purge is achieved by taking a portion of the light product, dropping its pressure, and using it to regenerate the adsorbent material at low pressure.3,4 S-PSA uses a pressure driving force to achieve gas separation and thus no addition of inert desorbent is necessary. An inherent disadvantage of S-PSA is that it uses a portion of the light product for purge, thereby decreasing the light product recovery. The light product recovery can be especially low if the light product is required at high purity and a small number of adsorbent beds are used in parallel. Low recovery implies that the heavy product is impure. Complete binary separation of light and heavy components is very difficult to achieve because the heavy product, produced during the purge step, is contaminated with the light product used for regeneration of the adsorbent. When the light product * To whom correspondence should be addressed. Tel: 765-494-0814. Fax: 765-494-0805. Email: [email protected]. † Current address: BP Products North America, 150 W. Warrenville Road, MC H6, Naperville, IL, 60563. Tel: 630-420-5906; Fax: 630420-4507; E-mail: [email protected].

purity is of primary importance and the heavy product is considered a waste, S-PSA is favorable. In order to produce high-purity heavy product, a different PSA process has been developed: rectifying-type PSA (R-PSA). R-PSA has been studied for separation of dilute hydrocarbons, nitrogen production, air separation, and carbon dioxide capture for sequestration.6-12 The R-PSA cycle uses a feed step at low pressure with a purge step at high pressure. Figure 2 schematically shows the sequence of four steps used in a typical two-

Figure 1. Schematic of steps in the two-bed, six-step S-PSA cycle.

10.1021/ie801371t CCC: $40.75  2009 American Chemical Society Published on Web 04/06/2009

4446 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009

Figure 2. Schematic of steps in the two-bed R-PSA process.

bed R-PSA process: (i) feed (and production of heavy product) at low pressure, (ii) repressurization, (iii) purge at high pressure with heavy product, and (iv) depressurization. The repressurization and depressurization steps are linked through a compressor: the gas from depressurizing the second bed is used to repressurize the first bed. The purge step is linked with the feed step because a fraction of the low-pressure heavy product is compressed and used for purge of the high-pressure bed; this is analogous to the use of light product for purge in the S-PSA process. R-PSA is essentially the inverse of S-PSA: adsorbent loading occurs during the purge step (at high pressure) while adsorbent regeneration occurs during the feed step (at low pressure). This is most easily realized by analyzing each step in Figure 2 individually. During the depressurization step, previously adsorbed solute (i.e., heavy) gas is desorbed and the bed void volume is enriched in the heavy component. The feed step that follows at low pressure is used to push this heavyenriched gas upward as product and for use as purge. Subsequent repressurization causes the heavy gas to adsorb, loading the adsorbent and leaving the bed void volume enriched in light component. Lastly, during the purge step that follows, heavy enriched gas is introduced at high pressure, thereby causing even more loading of the adsorbent. At this point, the cycle begins again. The use of a heavy purge step in the R-PSA process ensures that a pure heavy product can be produced; however, it hinders production of a pure light product and decreases the recovery of heavy product. Thus, it is very difficult for R-PSA to achieve complete binary separation. Another process studied for bulk gas separation is the gasphase simulated-moving bed (SMB), which is usually isobaric and uses the addition of desorbent to achieve separation. The gas-phase SMB can produce both light and heavy products with high purities and recoveries since none of the products are used for purge. However, downstream separations are required to remove the desorbent and concentrate the light and heavy products. Recent work on gas-phase SMBs has focused on hydrogen/deuterium separation,13 propane/propylene separation,14 methane/carbon dioxide separation,15,16 and enantiomeric enflurane separation.17-19 SMB processes combined with pressure swings have been reported in the literature. LaCava and McKeigue20 developed

a rotary SMB device with more than 20 beds and a continuous pressure difference to separate air into nitrogen and oxygen products. Rothchild21 developed a similar device that uses a pressure swing to induce desorption and gas flow through an SMB. This device was applied to air separation using five zones. Two nonisobaric 4-zone gas-phase SMB processes with multiple pressure levels were developed by Cheng and Wilson22,23 for propane/propylene separation. Rao et al.24 developed a tubular-type SMB process with helical slots that uses a partial pressure swing to achieve propane/propylene separation. This device varies the pressure of one of its four zones by using rotating beds. Most recently, Kostroski and Wankat25 developed several hybrid PSA/SMB processes for the separation of dilute mixtures of enantiomeric enflurane. Of these, the relatively simple 2-zone configuration used the least amount of desorbent. In this work, the dilute, 2-zone PSA/SMB concept25 is extended for the separation of concentrated binary gases. Three new hybrid PSA/SMB processes combine the desorbenteliminating nature of S-PSA and R-PSA and the complete binary separation power of the gas-phase SMB. S-PSA is combined with the gas-phase SMB to form S-PSA/SMB processes, R-PSA is combined with the gas-phase SMB to form R-PSA/SMB processes, and S-PSA/SMB and R-PSA/SMB are combined to form “combination-type” PSA/SMB (C-PSA/SMB) processes. Table 1 summarizes the processes developed. Although the concepts are similar, there are distinct differences between the dilute and concentrated PSA/SMB processes. In the dilute PSA/SMB processes, the feed stream contains a large amount of carrier gas that could be recovered and be used subsequently for purge. Because carrier gas is already present, adding more carrier gas as desorbent does not add to complexity. Since the feed stream to the concentrated PSA/SMB processes contains only the light and heavy components, there is no carrier gas or desorbent readily available to use for purge. To avoid requiring downstream separation, desorbent addition is not used in the PSA/SMB processes for concentrated binary gas separation. Thus, the dilute PSA/SMB uses both mass and energy separation agents (desorbent plus pressure swing), but the concentrated PSA/SMB uses solely an energy agent (pressure swing). S-PSA/SMB S-PSA/SMB Process Description. The most basic of the stripping-type PSA/SMB processes (S-PSA/SMB) combines S-PSA, which uses feed at high pressure and purge at low pressure, with the gas-phase 2-zone SMB, which uses port switching and a circulation step to achieve binary separation (Figure 3). This process has two beds and four steps: (i) feed/ production, (ii) depressurization/production, (iii) purge with product, and (iv) repressurization with product. During the feed step, fresh feed enters Zone I at high pressure (PH) and raffinate is produced as product and for internal use as light purge. Simultaneously, Zone II undergoes a circulation step at PH with heavy product, the effluent of which is fed to Zone I. This circulation pushes any light product remaining at the top of Zone II into Zone I. Next, both beds are depressurized. During this step, Zone I is depressurized cocurrently to moderate pressure (PM) and raffinate is produced for repressurization of Zone II; simultaneously, Zone II is depressurized countercurrently to low pressure (PL) and heavy product (extract) is produced for repressurization of Zone I. In the third step, Zone I undergoes heavy purge at PM with internal extract product to push out the mass

Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 4447 Table 1. Summary of PSA/SMB Processes Developed in This Work process S-PSA/SMB

R-PSA/SMB

description stripping-type PSA/SMB: feed at PH, purge at PL

rectifying-type PSA/SMB: feed at PL, purge at PH

configuration single-train (two beds total): one light product, one heavy product dual-train (four beds total): one light product and one heavy product dual-train (four beds total): one light product, one heavy product, one offgas dual-train (four beds total): one light product, one heavy product, recycled offgas single-train (two beds total): one light product, one heavy product

dual-train (four beds total): one light product, one heavy product dual-train (four beds total): one light product, one heavy product, one offgas dual-train (four beds total): one light product, one heavy product, recycled offgas SRC-PSA/SMB combination PSA/SMB with S-PSA/SMB and single-train (two unit operations, eight beds total): one light product, one heavy product, R-PSA/SMB units connected in series one offgas with optional recycle SSC-PSA/SMB combination PSA/SMB with two single-train (two unit operations, eight beds total): one light product, one heavy product, S-PSA/SMB units connected in series one offgas with optional recycle

transfer zone while Zone II undergoes light purge at PL with internal raffinate product; during this step, Zone II produces extract both as product and for internal use. Finally, in the fourth step, Zones I and II are repressurized to PH with internal extract and raffinate products, respectively. A port switch follows the fourth step (Bed B takes the place of Bed A and vice versa). Figure 3 illustrates several key aspects of the S-PSA/SMB process. First, different regeneration pressure levels are used. Zone I is regenerated at PM and Zone II is regenerated at PL while feed occurs at PH. Typically, PH is set by the process stream one wishes to separate; however, PM and PL can be set by design. PL is selected to adequately remove the strongly adsorbed heavy component from Zone II during the light purge step. A low value of PL has the added benefit of requiring very little light product as purge on a molar basis and therefore increasing the light product recovery. PM should be small enough to allow a significant portion of the light product to be pushed out of Zone I during depressurization and heavy purge but large enough that it does not cause large amounts of the heavy product to desorb during Zone I depressurization and heavy purge which would cause contamination of the raffinate product.

Figure 3. Schematic of steps in the S-PSA/SMB process with one train and no pressure equalization. As shown, process is discontinuous; tanks not shown. Zone I steps are italicized.

figure 3 4 5 6 11 12 13 14 18 19

Second, the circulation and port switch characteristics of the S-PSA/SMB process in Figure 3 are key aspects of the process. The circulation step is a high pressure, heavy rinse of Zone II that pushes remaining raffinate product in Zone II into Zone I, ensuring that the extract gas produced in subsequent steps will be pure. Port switching retains the mass transfer zones in the adsorption beds and makes more productive use of the adsorbent. A third key aspect of the S-PSA/SMB process is the use of both heavy and light purge steps for regeneration. The heavy purge of Zone I during the regeneration step serves to push out raffinate product and introduce heavy gas to the adsorbent bed, which will produce extract after the subsequent port switch. Similarly, the light purge of Zone II during the regeneration step serves to push out extract product and introduce light gas to the adsorption bed; this bed will produce raffinate after the subsequent port switch. The S-PSA/SMB process in Figure 3 is discontinuous and requires two tanks (one for light product, one for heavy product) to accomplish the purge steps. The final key aspect of the S-PSA/SMB process in Figure 3 is its pressurization scheme. Zones I and II are repressurized with heavy and light products, respectively. These gases would need to come from tanks for the discontinuous process in Figure 3. The need for tanks can be eliminated with the nearly continuous 4-bed, S-PSA/SMB process with eight steps per cycle (Figure 4). Two S-PSA/SMB trains in parallel but 180° out of phase are integrated such that light and heavy products are available for purge and for circulation. Only small buffer tanks (not shown) would be required to control pressure surges. In addition, a pressure equalization step can easily be included. During the first part of the repressurization step, the beds are allowed to equilibrate to some intermediate pressure (as in a conventional pressure equalization step). After equilibration, the compressors between trains 1 and 2 provide the remainder of the compression and vacuum needed. PSA technology for hydrogen purification has matured from three-bed units26 into the relatively complex Polybed processes used today.2,27,28 Continuous improvement of these processes has led to large production capacities for pure hydrogen.29,30 The Polybed processes used commercially today typically have 12-16 interacting beds that operate in parallel to continuously produce light product (hydrogen).28 To enhance the light product recovery and minimize blowdown losses, several pressure equalization steps between the different beds are used. In contrast, PSA/SMB uses four interacting beds (two trains) and a single pressure equalization step to achieve binary separation and production of pure light and pure heavy products. It does so by taking advantage of PSA and SMB-type steps, as described previously.


Figure 4. Schematic of steps in the S-PSA/SMB process with two trains and pressure equalization. Zone I steps are italicized. Dotted lines indicate port switches.

Membrane processes have also received considerable research attention for hydrogen purification.28 Typically, because of their relatively drastic yield vs purity tradeoff, membrane units have to be staged in order to achieve the required throughput. This type of staged design is often more expensive than PSA technology and, thus, has seen less commercial use. The feed to a typical H2 PSA process is often a process stream containing concentrated hydrogen as well as a number of hydrocarbons (either trace or concentrated). Hydrogen recovery from coke oven offgas, which contains mostly hydrogen and methane, is one common application.31,32 A zeolite adsorbent with selectivity toward methane is typically used.33 When large concentrations of other hydrocarbons (besides methane) are present, layered adsorbent beds are often used.34-37 Because this work focuses on investigating different PSA process configurations, multicomponent hydrocarbon feeds were not studied in order to reduce simulation complexity and make convergence easier. Instead, a relatively simple binary feed of hydrogen/methane (with a concentration resembling coke oven offgas) was studied. In evaluating the S-PSA/SMB processes, a feed stream composed of 70% hydrogen and 30% methane on a molar basis was separated with Zeolite 5A. Table 2 lists the parameters for hydrogen and methane adsorption on Zeolite 5A.32,33 The work of Yang et al.,33 who studied this system with a two-bed S-PSA process, is used to benchmark the S-PSA/SMB processes. The operating conditions used by Yang et al. were adopted as the design bases for the S-PSA/SMB processes. A pressure of 11 bar for PH was used; this value appears to be typical for hydrogen purification from coke oven offgas. Operation at a higher PH is not very beneficial.37,40 Ambient pressure (1.0 bar) was chosen for PM initially; however, PM ) 1.6 was found to

be optimal when PH ) 11 bar and PL ) 0.05 bar for the production of H2 with a purity of 99.99%.40 A vacuum of 0.05 bar was chosen for PL to provide enough desorption of methane during regeneration; other values of PL were also investigated.40 A PL ) 0.05 bar is somewhat low compared to other PSA processes and the use of low vacuum may pose several practical issues from a commercial perspective such as in-leakage, high cost, and difficult large-scale implementation. However, unlike other PSA processes, PSA/SMB was developed to achieve complete binary separation; thus, more complete regeneration and lower vacuum are required. Although a full economic evaluation of PSA/SMB is beyond the scope of this work, in some cases the simultaneous production of two pure products may justify the use of low vacuum. Performance of S-PSA/SMB processes was evaluated with six measures: H2 purity, H2 recovery, CH4 purity, CH4 recovery, productivity, and energy use. The purity and recovery measures refer to the light and heavy product streams and recoveries are calculated relative to the feed. Productivity is based on the moles of fresh feed processed per unit time (nfeedtfeed/tcycle) per kilogram adsorbent (mads col ): nfeedtfeed mads col tcycle

) P′S-SMB/PSA ) productivity (mol/(kg·s))

(1)

The productivities of the various S-PSA/SMB processes (P′S-PSA/SMB) studied can be compared to the productivity of the two-bed S-PSA process (P′S-PSA ) 5.51 × 10-4 mol/(kg · s)).33 The power required to perform the separation is calculated from38

[( )

0.371Taγq0 Pb η(γ - 1) Pa

1-(1/γ)

]

- 1 ) P¨ ) power (kW)

Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 4449

(2)

where η ) 0.8 is the compressor efficiency, γ (Cp/CV) is the ratio of specific heats (assumed to be 1.5 for ideal gases), q0 is the volumetric flow rate of the stream being processed (standard m3/s), and Ta is the temperature of the stream being processed (K). The energy per mole of fresh feed is P¨ ) E¨ ) energy (kJ/mol feed) tfeed nfeed tcycle

(3)

The hydrogen/methane feed stream is assumed to available at 1 atm. Since many applications that utilize hydrogen require an H2 purity of at least 99.99%, purities at or above this threshold are required for the production of “high-purity” H2.28 Production of a moderate purity H2 product with 99% purity is also considered. A CH4 purity target of 99% is used. The challenge is to meet these targets with sufficiently high productivity and sufficiently low energy. The trade-offs between productivity, energy, purities, and recoveries will be examined and discussed in detail. Several operating variables are defined for the S-PSA/SMB process. The light purge ratio (LPRS) for Zone II is the ratio of the light purge flow rate (nlight,purge) to the fresh feed flow rate (nfeed) on a molar basis:

nlight,purge ) LPRS ) light purge ratio for S-PSA/SMB (4) nfeed Similarly, the heavy purge ratio (HPRS) for Zone I is the ratio of the heavy purge flow rate (nheavy,purge) to the fresh feed flow rate (nfeed) on a molar basis: nheavy,purge ) HPRS ) heavy purge ratio for S-PSA/SMB nfeed (5) Analogously, the effects of circulation are manifested in terms of the recycle ratio (RRS), defined as the ratio of the flow rate in Zone II during feed (ncirc) to the fresh feed flow rate (nfeed): ncirc ) RRS ) recycle ratio for S-PSA/SMB nfeed

(6)

The S-PSA/SMB process in Figure 4 produces one raffinate product and one extract product. Figure 5 shows an alternative process that produces one raffinate product, one extract product, and one offgas. Offgas (fuel gas) production is used by the Gemini-8 and Gemini-9 processes for H2/CO2 separation.2 Another difference between the processes is in Figure 5; the heavy product is produced during depressurization of Zone II. The effluent from Zone II during the light purge step is offgas rather than heavy product, as in Figure 4. Because the extract product is removed before purge gas is introduced, it has a higher purity than the offgas taken off during light purge. Because the offgas produced by the

Figure 5. Schematic of steps in the two-train S-PSA/SMB process producing one light, one heavy, and one offgas. Zone I steps are italicized. Dotted lines indicate port switches.

4450 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 Table 2. Data for Hydrogen/Methane Model System32-34 parameter

hydrogen/methane model system

adsorbent L D εi εp Fb Rp kMTC,solid mass Dax thermal Dax isotherm parameters

Zeolite 5A 100 cm 4.4 cm 0.315 0.65 0.746 g/cm3 0.157 cm H2 0.700 s-1; CH4 0.147 s-1 H2 and CH4 6.13 cm2/s H2 and CH4 4.52 × 10-3 J/(K · cm · s) k1,H2 ) 4.314 mmol/g k1,CH4 ) 5.833 mmol/g k2,H2 × 102 ) -1.060 mmol/(g · K) k2,CH4 × 102 ) -1.192 mmol/(g · K) k3,H2 × 104 ) 25.15 atm-1 k3,CH4 × 104 ) 6.507 atm-1 k4,H2 ) 458 K k4,CH4 ) 1731 K k5,H2 ) 0.986 k5,CH4 ) 0.820 k6,H2 ) 43.03 K k6,CH4 ) 53.15 K 0.22 cal/(g · K) H2 -2800 cal/mol CH4 -5300 cal/mol estimated with kez ) 0.026 W/(m · K) 1910 m-1 25 °C

Cp,solid ∆Hads hHTC ap Tfeed

Theory and Simulation. The equations that are used to describe nonisothermal fixed bed adsorption are the mass and energy balances, the mass and energy transfer equations, and the equilibrium isotherm. The mass balance assumes that radial gradients are negligible, no chemical reactions (other than adsorption) take place, and mass transfer follows a linear lumped parameter driving force model.39 εe

∂ci ∂cji,pore ∂qji + Kdi(1 - εe)εp + Fs(1 - εe)(1 - εp) + ∂t ∂t ∂t ∂(Vfci) ∂2ci εe - εeDmass ) 0 (7) ax ∂z ∂z2

The variables in eq 7 are defined in the Nomenclature. The adsorption beds were assumed to be initially clean and were charged with 100% hydrogen at the start of the dynamic simulation. The boundary and initial conditions are presented in Table 3.2 The assumptions for the energy balance are radial gradients are negligible, heat transfer follows a lumped parameter linear driving force model, and the bed is adiabatic.39 FfCpfεe

Table 3. Boundary and Initial Conditions During blowdown, the appropriate boundary conditions for concentration and velocity are:2 (∂ci/∂z)|z)0 ) 0 (∂ci/∂z)|z)L ) 0 Vf|z)0 ) 0 During the pressurization, adsorption, and purge steps, the appropriate boundary conditions on concentration are:2 mass Dax (∂ci/∂z)|z)0 ) -Vf|z)0(ci|z)0- - ci|z)0) (∂ci/∂z)|z)L ) 0 (ci|z)0-)purge ) (PL/PH)(ci|z)L)adsorption The initial conditions for a clean adsorbent bed and a saturated adsorbent bed are respectively:2 ci(z,0) ) 0, qji(z,0) ) 0, ci(z,0) ) ci0, qji(z,0) ) qi0 During blowdown, the appropriate boundary conditions for temperature are:2 (∂T/∂z)|z)0 ) 0 (∂T/∂z)|z)L ) 0 During the pressurization, adsorption, and purge steps, the appropriate boundary conditions on temperature are:2 thermal Dax (∂T/∂z)|z)0 ) -Vf|z)0FfCpf(T|z)0- - T|z)0) (∂T/∂z)|z)L ) 0 (T|z)0-)purge ) (T|z)L)adsorption The initial condition for temperature is:2 T(z,0) ) Tfeed

process in Figure 5 contains a significant portion of the H2 fed to the process, an obvious possibility is to recycle it (Figure 6). However, recycling the offgas is expected to reduce the productivity. In the context of H2/CH4 separation, a full economic analysis of PSA/SMB (although interesting) is beyond the scope of this work for several reasons. First, the lack of complete data in the open literature makes it impossible to directly compare PSA/ SMB performance to existing commercial technologies (such as Polybed PSA). Second, it would be difficult to scale-up and calculate the costs required to implement PSA/SMB with any reasonable degree of accuracy because of the lack of data in the open literature. Lastly, because the overarching goal of this work is the development of PSA/SMB processes for the separation of concentrated binary gases, we are more concerned with the technical feasibility of PSA/SMB than with its economic feasibility.

∂T ∂T* + FfCpf(1 - εe)εp + ∂t ∂t ∂Ts ∂(VfT) + FfCpfεe FsCps(1 - εe)(1 - εp) ∂t ∂z 2 ∂ ci FfCpfεeDthermal ) 0 (8) ax ∂z2

The boundary and initial conditions are presented in Table 3.2 Yang et al. reported the equilibrium parameters for adsorption of methane and hydrogen on Zeolite 5A (Table 2).33,34 The adsorption of these components on Zeolite 5A follows the temperature-dependent Langmuir-Freundlich isotherm:33,34 qi )

qmiBi(Pyi)ni 1+

∑ B (Py )

nj

j

(9)

j

j)1

where qmi ) k1i + k2iT

()

Bi ) k3i exp ni ) k5i +

k4i T

k6i T

(10)

(11)

(12)

The sorption rate into the adsorbent can be described by the linear driving force (LDF) mass transfer model with solid-phase driving force:39 ∂qji ) kMTCsolid(q*i - qji) ∂t

(13)

and kMTCsolid )

15De,i Rp2

(14)

where kMTC,solid is a lumped mass transfer coefficient inside the adsorbent and De is the effective diffusivity. The LDF model is


Figure 6. Schematic of steps in the two-train S-PSA/SMB process producing one light, one heavy, and recycled offgas. Zone I steps are italicized. Dotted lines indicate port switches.

widely accepted and the parameters used in this work were taken directly from the source literature.33,34,39 Mass axial dispersion was included with a constant mass axial dispersion coefficient.33,34 The linear lumped parameter heat transfer model is39 ∆Hads ∂q ∂Ts ap ) hHTC (Tf - Ts) ∂t FfCpf Cpf ∂t

(15)

Thermal axial dispersion was included with a constant thermal axial dispersion coefficient.33,33 The Colburn j-factor correlation was used to determine hHTC:41 hHTC ) jCpfVf FfPr-2/3

(16)

where j ) 1.66Re-0.51 if Re < 190 and j ) 0.93Re-0.41 otherwise. The kinetic parameters are provided in Table 2. The temperature of the hydrogen/methane feed stream was set at 25 °C to match the source articles.32-34 The adsorption beds were initialized at the feed temperature of 25 °C before the start of the dynamic simulation. At cyclic steady state, a maximum temperature swing of about 7 °C occurred as a result of adsorption. Figure 7 shows the adsorption isotherms for hydrogen and methane at the feed temperature of 25 °C on Zeolite 5A. In the range of pressures investigated in this work (PL ) 0.05 bar to PH ) 11 bar), the adsorption of methane is relatively linear and hydrogen adsorbs to a very small extent compared to methane. Thus, the separation behavior is similar to that of a feed with a single adsorbing component in an inert carrier. Because Zeolite 5A has high selectivity toward methane, the separation of these two components should be

relatively easy if an adequate regeneration step is used. The effects of feed concentration are discussed later. ADSIM, from Aspen Technology, Inc., was used to dynamically simulate the S-PSA/SMB processes. ADSIM uses numerical integration to solve the governing algebraic-differential equations numerically via the method of lines. Each adsorption bed in the S-PSA/SMB process was simulated with 50 nodes in the axial direction; discretization of the nodes was accomplished by using the first-order upwind differencing scheme (UDS1). The implicit Euler technique with a variable time step of 0.01-5 s was used for time integration. The Karman-Kozeny equation was used to relate the gas velocity to the specified pressure drop across the adsorption beds (0.1 bar). Blowers were included on the flowsheet in the circulation loop between Zones I and II to overcome the small pressure drop between the zones. Table 4 summarizes the base operating conditions for the S-PSA/ SMB processes. Simulation was continued until cyclic steady state was reached and the material and energy balances converged to within a tolerance of 1 × 10-5. Due to the concentrated nature of the model system, the processes typically reached cyclic steady-state with less than 50 cycles. However, due to the relatively large number of beds being simulated simultaneously and the interconnected nature of the S-PSA/SMB process, the simulations were considerably slower than those for a two-bed S-PSA or R-PSA process. The theory and use of ADSIM were identical for all processes studied (S-PSA, R-PSA, S-PSA/SMB, R-PSA/SMB, and C-PSA/SMB). In order to determine the base operating conditions and the effects of several parameters, a detailed parametric study was completed. Several other processes which proved to be less successful were also examined. The detailed results of these studies are documented elsewhere.40


Figure 7. Adsorption isotherms of hydrogen and methane on Zeolite 5A at 25 °C.

Figure 8. Hydrogen purity and process productivity vs hydrogen recovery for S-PSA and S-PSA/SMB (one light product, one heavy product). Refer to Table 4 for operating conditions.

Results and Discussion for S-PSA/SMB Processes. Single-Train S-PSA/SMB Process (Figure 3). The process in Figure 3 is discontinuous and requires buffer tanks for holding the light- and heavy-enriched gases for subsequent internal use. In addition, the single-train S-PSA/SMB process cannot employ an explicit pressure equalization step since depressurization and repressurization are out of phase. Lack of a pressure equalization step drastically reduces the recovery of the light product and the purity of the heavy product. Preliminary studies showed that

a 99.99% H2 could be produced with about 70% recovery; the heavy product was about 60% CH4 with ∼100% CH4 recovery. This performance is less favorable than that of the S-PSA process, which produced 99.99% H2 with ∼80% recovery and ∼70% CH4 with ∼100% recovery at the same productivity as the single-train S-PSA/SMB process.33 Because significant improvements in H2 recovery and CH4 purity can be achieved by utilizing a pressure equalization step, the single-train S-PSA/ SMB processes were not studied further.

Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 4453 Table 4. Base Operating Conditions for PSA/SMB Processes variable

base value

4-Bed S-PSA/SMB (Figures 4-6) 4.66 × 10-3 mol/s 62 s 12 s 148 s 0.08 0.04 0.1 11 bar 0.05 bar 1.0 bar

feed flow rate tfeed ) tpurge tdpe ) trpe tcycle LPRS HPRS RRS PH PL PM

4-Bed R-PSA/SMB (Figures 12-14) 8.17 × 10-4 mol/s 12 s 12 s 48 s 0.3 0.7 0.3 11 bar 1 bar

feed flow rate tfeed ) tpurge tdpe ) trpe tcycle LPRR HPRR RRR PH PL

8-Bed C-PSA/SMB (Figures 18 and 19) feed flow rate tfeed ) tpurge tdpe ) trpe tcycle LPRj HPRj RRj PH PL PM (S-PSA/SMB unit only)

S-PSA/SMB unit, R-PSA/SMB unit 4.66 × 10-3 mol/s, N/A 62 s, 12 s 12 s, 12 s 148 s, 48 s 0.08, 0.3 0.04, 0.7 0.1, 0.3 11 bar, 11 bar 0.05 bar, 1 bar 1.0 bar, N/A

Dual-Train S-PSA/SMB Process with Light and Heavy Products (Figure 4). The two trains in the S-PSA/SMB process in Figure 4 are configured out of phase so that pressure equalization can be easily achieved. In addition, the two-train S-PSA/SMB process is continuous, with production of light and heavy products ceasing only during the brief nonisobaric steps. At the base operating conditions shown in Table 4 (PH ) 11.0 bar, PM ) 1.6 bar, and PL ) 0.05 bar), this process produces 99.99% H2 with 85.4% H2 recovery and 74.6% CH4 with 99.9% CH4 recovery, an energy requirement of 56.6 kJ/mol feed, and productivity of 5.51 × 10-4 mol feed/(kg · s) (see Case 1, Table 5). Clearly, the light product (H2) is the dominant product and is easier to produce. Comparing the results of Case 1 to the results of the S-PSA process studied by Yang et al. (Case * in Table 5),33 the S-PSA process achieves the same H2 purity with lower H2 recovery at the same productivity. However, the S-PSA process requires less energy: 16.6 kJ/mol feed. For S-PSA, energy is expended to compress the feed from 1 to 11 bar for the feed and partial repressurization steps. The S-PSA/SMB requires energy to compress the feed from 1 to 11 bar, for the compressors between the trains, and for the vacuum pump to regenerate Zone II. For less pure H2, Table 5 (Case 7) shows that S-PSA/SMB can produce 99% H2 with 89.6% H2 recovery and 80.1% CH4 with 97.9% CH4 recovery with a productivity of 5.74 × 10-4 mol/(kg · s) and an energy requirement of 53.9 kJ/mol. This performance is achieved by increasing the feed time (tfeed) from the base value of 60 to 90 s. The analogous S-PSA process achieves 99% H2 with 84.9% H2 recovery and 73.6% CH4 with 98.0% CH4 recovery with the same productivity but at a lower energy requirement of 14.9 kJ/mol (Case ** in Table 5). Figure 8 illustrates the trade-offs in H2 purity, H2 recovery, and

productivity for S-PSA/SMB. As H2 purity increases, H2 recovery decreases and process productivity decreases slightly. Dual-Train S-PSA/SMB Process with Light and Heavy Products and Offgas (Figure 5). The process in Figure 5 produces light product, heavy product, and offgas. For the base operating conditions shown in Table 3, the process in Figure 5 can produce 99.99% H2 with 85.2% H2 recovery, 91.1% CH4 with 88.4% CH4 recovery, and an offgas that is 69.1% H2 (see Table 5, Case 2). Since both light and heavy products are at least ∼90% pure, some degree of binary separation is being achieved. However, about 13% of the feed on a molar basis is offgas (i.e., noffgas/nfeed ) 0.13). Comparing Cases 1 and 2 in Table 5, the H2 purity and recovery are essentially the same, as are productivity and energy requirements; however, when the CH4 product is taken off during depressurization, it is purer than when it is taken off during light purge (91.1% CH4 for Case 2 vs 74.6% CH4 for Case 1). The disadvantage is that offgas is produced. Dual-Train S-PSA/SMB Process with Light and Heavy Products and Offgas Recycle (Figure 6). Cases 3-5 in Table 5 show the results for the process in Figure 6 that recycles offgas. The productivity, nRaff/nfeed, and nExt/nfeed parameters were adjusted to maintain a 99.99% H2 purity product. In Case 5, at a productivity of 4.53 × 10-4 mol/(kg · s) with energy requirements of 68.8 kJ/mol, 99.99% purity H2 with 89.8% H2 recovery was produced simultaneously with a 80.8% CH4 purity with 99.9% recovery. Comparing the base case (Case 1, Figure 4), which also produces no offgas, to Case 5, shows that use of offgas recycle stream increased H2 recovery nearly 5 percentage points and CH4 purity by about 6 percentage points while the H2 purity and CH4 recovery remained nearly the same. The increased separation of H2 and CH4 comes at the costs of productivity and energy. The productivity of Case 5 is ∼18% less than Case 1 while the energy requirements of Case 5 are ∼22% higher than Case 1. These energy, productivity, recovery, and purity trade-offs are shown in Figures 9 and 10. The productivity and energy penalties become very steep as binary separation is approached, and the results did not conveniently fit on Figures 9 and 10. Lastly, the productivity, recovery, and purity trade-offs were examined for the production of 99% purity H2. For this case, the optimal PM value is ∼2.0 bar.40 Table 5 (Cases 8-11) and Figures 9 and 10 show these results. Comparing Case 11 to Case 7 in Table 5, offgas recycle increased H2 recovery by about 3 percentage points and CH4 purity by about 6 percentage points; however, productivity decreased by ∼18% and energy requirements increased by ∼22%. Not surprisingly, at the same productivity, higher recoveries of H2 are possible with lower energy requirements when 99% H2 is produced compared to when 99.99% H2 is produced. Case 6 shows production of 99.99% H2 (99.6% H2 recovery) and 99% CH4 (99.9% CH4 recovery) at a productivity of 2.69 × 10-5 mol/(kg · s) with an energy requirement of 1161.4 kJ/ mol. Case 12 shows analogous results for the production of 99% H2. In both of these cases, the striking decrease in productivity and increase in energy requirements are direct results of the offgas recycle stream because less fresh feed is processed per unit time. R-PSA/SMB R-PSA/SMB Process Description. The previous results showed that S-PSA/SMB works well when pure light product is of primary interest and a somewhat impure heavy product is tolerable. When a pure heavy product is of primary interest,

4454 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 Table 5. Results for Two-Train S-PSA/SMB Processesa offgas productivity PM H2 H2 CH4 CH4 offgas purity energy case figure recycle (mol/(kg · s)) (bar) purity (%) recovery (%) nRaff/nfeed purity (%) recovery (%) nExt/nfeed (H2) noffgas/nfeed (kJ/mol) * 1 2 3 4 5 6 ** 7 8 9 10 11 12

1 4 5 6 6 6 6 1 4 5 6 6 6 6

N/A N/A no yes yes yes yes N/A N/A No yes yes yes yes

5.51 × 10-4 5.51 × 10-4 5.51 × 10-4 4.98 × 10-4 4.76 × 10-4 4.53 × 10-4 2.69 × 10-5 5.74 × 10-4 5.74 × 10-4 5.74 × 10-4 5.18 × 10-4 4.94 × 10-4 4.72 × 10-4 2.81 × 10-5

N/A 1.6 1.6 1.6 1.6 1.6 1.6 N/A 2.0 2.0 2.0 2.0 2.0 2.0

99.99% 99.99 99.99 99.99 99.99 99.99 99.99 99 99 99 99 99 99 99

80.4 85.4 85.2 88.4 89.3 89.8 99.6 84.9 89.6 89.2 91.6 92.7 93.2 99.6

0.562 0.598 0.597 0.619 0.625 0.629 0.697 0.601 0.634 0.630 0.648 0.655 0.659 0.704

68.6 74.6 91.1 78.7 80.0 80.8 99 73.6 80.1 92.3 83.3 85.2 86.0 99

99.9 99.9 88.4 99.9 99.9 99.9 99.9 98.0 97.9 81.9 97.9 97.9 97.9 97.9

0.437 0.402 0.291 0.381 0.375 0.371 0.303 0.399 0.366 0.267 0.352 0.345 0.341 0.296

N/A N/A 69.1% recycled recycled recycled N/A N/A N/A 53.5% recycled recycled recycled N/A

N/A N/A 0.112 0 0 0 0 N/A N/A 0.103 0 0 0 N/A

16.6 56.6 56.6 62.6 65.4 68.8 1161.4 14.9 53.9 53.9 59.7 62.7 65.6 1102.0

a Refer to Table 4 for base operating conditions. Cases correspond to the figures listed. Cases * and ** refer to two-bed PSA processes. Productivity and energy are calculated based on moles of fresh feed processed.

Figure 9. Process productivity and methane product purity vs hydrogen recovery for two-train S-PSA/SMB process with one light product, one heavy product, and one offgas with recycle.

S-PSA/SMB is probably not the most favorable process design. To produce heavy product, R-PSA and the two-zone SMB were combined to form the hybrid R-PSA cycle shown in Figure 11. The basic process utilizes two beds and four steps: (i) feed/production (at low pressure), (ii) repressurization, (iii) purge with product (at high pressure), and (iv) depressurization. During the feed step, fresh feed enters Zone I (Bed A) at low pressure (PL) and pushes the heavy-enriched gas in the column void space upward, producing heavyenriched gas as product (extract) and for internal use as heavy purge. At the same time, Zone II (Bed B) undergoes a circulation step at PL with light product, the effluent of which is fed to Zone I. During the circulation step, heavy product remaining at the top of Zone II (Bed B) is pushed into Zone I (Bed A). Next, Zone I (Bed A) is repressurized cocurrently with light-enriched gas while Zone II (Bed B) is repressurized cocurrently with heavy-enriched gas. In the third step, Zone I (Bed A) undergoes light purge at PH with raffinate to push out the extract mass transfer zone while Zone II (Bed B) undergoes heavy purge at PH with extract to reload the adsorbent. Finally, in the fourth step, Zone I (Bed A) and Zone II (Bed B) are depressurized to PL, producing the gases

required for the repressurization step. A port switch follows the fourth step: Bed B becomes Zone I and Bed A becomes Zone II. Both S-PSA/SMB and R-PSA/SMB processes use a circulation step and port switch, remnants of their SMB parent. In the S-PSA/ SMB process heavy product is circulated to Zone II to push any light product still in the bed into Zone I; during this step, both zones are at high pressure. Similarly, the R-PSA/SMB process uses a circulation step to link Zones I and II; however, both zones are at low pressure and light product is circulated to Zone II to push any heavy product left in the bed into Zone I. Both circulation steps serve the same purpose (clean-out of Zone II) but operate differently (high pressure vs low pressure, light product circulation vs heavy product circulation). The use of the port switch in the R-PSA/SMB is identical to its use in S-PSA/SMB: a port switch occurs after every four steps to retain the raffinate and extract mass transfer zones in the columns. S-PSA/SMB uses light product to countercurrently purge Zone II at low pressure while simultaneously using heavy product to cocurrently purge Zone I at low pressure. R-PSA/ SMB reverses the roles of the purge steps: heavy product is used to countercurrently purge Zone II at high pressure while


Figure 10. Process productivity and energy purity vs hydrogen recovery for two-train S-PSA/SMB process with one light product, one heavy product, and one offgas with recycle.

Figure 11. Schematic of steps in the R-PSA/SMB process with one train and no pressure equalization. As shown, process is discontinuous; tanks not shown. Zone I steps are italicized.

simultaneously using light product to cocurrently purge Zone I. In S-PSA/SMB, the cocurrent purge of Zone I ensures that the mass transfer zone is pushed sufficiently out of the product end of the bed; at the same time, the countercurrent purge of Zone II ensures that the product end of the bed is not contaminated with extract. In the R-PSA/SMB, the cocurrent

purge of Zone I ensures that the mass transfer zone is pushed sufficiently out of the product end of the bed; simultaneously, the countercurrent purge of Zone II ensures that the product end of the bed is not contaminated with raffinate. Because the results for S-PSA/SMB showed that pressure equalization is favorable, a two-train R-PSA/SMB process with


Figure 12. Schematic of steps in the two-train R-PSA/SMB process. Zone I steps are italicized. Dotted lines indicate port switches.

pressure equalization was developed (Figure 12). In the R-PSA/ SMB, Zone I is repressurized cocurrently with light-enriched gas from the countercurrent depressurization of Zone II, and Zone II is repressurized countercurrently with heavy-enriched gas from cocurrent depressurization of Zone I. This is essentially the opposite of the pressurization/depressurization scheme used for S-PSA/SMB. Compressors and vacuum pumps are placed as needed. Only two pressures, PL (feed pressure) and PH (purge pressure), are used in the R-PSA/SMB process compared to the three pressures used for S-PSA/SMB. In the R-PSA/SMB, regeneration of the adsorbent material is never complete because feed is done at low pressure and purge (reloading of the adsorbent) is done at high pressure; thus, a third pressure level is not necessary. One could, conceivably, use a second high pressure (P′H) for the purge of Zone I; this is analogous to the use of PM for purge of Zone I. The use of P′H in this case would cause less adsorption of extract and thus better clean out of the heavy component; however, this does not appear to be necessary for adequate removal of extract. First, a methane-rich feed (71% CH4, 29% H2) was used to study the R-PSA/SMB. This feed is the same concentration as the offgas from the S-PSA/SMB process shown earlier, which will allow easy coupling of these processes (discussed later). For the sake of continuity, a pressure of PH ) 11 bar was used; ambient pressure (1.0 bar) was chosen for PL. Target purities of 99% were set for both H2 and CH4. The necessary parameters are again listed in Table 2 and the operating conditions are summarized in Table 4. Because no data was available for hydrogen/methane separation by two-bed R-PSA, this process was simulated to determine the productivity P′R-PSA for benchmarking the R-PSA/SMB.

From initial studies on R-PSA, reasonable values of the feed velocity, feed time, and heavy purge velocity were determined and subsequently used in the R-PSA/SMB process as were the values of PH and PL. The light purge and circulation flow rates used in the R-PSA/SMB process were adjusted to provide adequate removal of the extract mass transfer zone. The light and heavy purge ratios for R-PSA/SMB are nlight,purge ) LPRR ) light purge ratio for R-PSA/SMB nfeed (17) nheavy,purge ) HPRR ) heavy purge ratio for R-PSA/SMB nfeed (18) The recycle ratio (RRR) for R-PSA/SMB is ncirc ) RR ) recycle ratio for R-PSA/SMB nfeed

(19)

The first R-PSA/SMB process studied produces one extract product and one raffinate product (Figure 12). Figure 13 shows an alternative process that produces one extract product, one raffinate product, and an offgas recovered during the repressurization of Zone II. During this step, the heavy gas is adsorbed rapidly and relatively pure light gas is pushed to the end of the bed where a portion of it is collected. The process in Figure 13 recycles offgas to the feed stream. These processes are analogous to those developed for S-PSA/SMB (Figures 5 and 6). Recycle


Figure 13. R-PSA/SMB process in two-train configuration. Zone I steps are italicized. Dashed line indicates separate trains; port switching not shown. Table 6. Results for R-PSA and R-PSA/SMB Processesa case

process

† 13 14 15 16 17 18 19 20 21 22 23 24 25 26

R-PSA R-PSA/SMB R-PSA/SMB R-PSA/SMB R-PSA/SMB R-PSA/SMB R-PSA/SMB R-PSA/SMB R-PSA/SMB R-PSA/SMB R-PSA/SMB R-PSA/SMB R-PSA/SMB S-PSA/SMB S-PSA/SMB

a

offgas fresh feed productivity CH4 CH4 nExt/ H2 H2 offgas purity noffgas/ energy figure recycle (H2, CH4) (mol/(kg · s)) purity (%) recovery (%) nfeed purity (%) recovery (%) nRaff/nfeed (H2) nfeed (kJ/mol) 2 12 13 14 14 14 14 14 14 14 12 12 14 4 6

N/A N/A no yes yes yes yes yes yes yes N/A N/A yes N/A yes

0.29, 0.71 0.29, 0.71 0.29, 0.71 0.29,0.71 0.29, 0.71 0.29,0.71 0.29, 0.71 0.29,0.71 0.29, 0.71 0.29,0.71 0.70, 0.30 0.10, 0.90 0.10, 0.90 0.10, 0.90 0.10, 0.90

1.16 × 10-4 1.16 × 10-4 1.16 × 10-4 1.09 × 10-4 9.61 × 10-5 8.10 × 10-5 6.83 × 10-5 5.91 × 10-5 4.63 × 10-5 3.83 × 10-6 1.16 × 10-4 1.16 × 10-4 5.39 × 10-6 5.74 × 10-4 8.99 × 10-6

99 99 99 99 99 99 99 99 99 99 99 99 99 94.2 99

50.3 53.1 53.0 56.1 61.1 69.7 81.7 86.7 89.5 99.6 19.4 62.7 99.9 99.9 99.9

0.356 0.381 0.380 0.402 0.438 0.500 0.586 0.621 0.642 0.714 0.059 0.569 0.908 0.045 0.092

44.8 46.2 90.5 47.8 50.8 57.0 68.6 75.0 79.1 99 74.3 21.9 99 99 99

98.8 98.8 30.6 98.6 98.5 98.3 98.0 97.9 97.9 97.5 99.99 94.3 90.9 44.3 90.9

0.644 0.619 0.101 0.598 0.562 0.500 0.414 0.379 0.358 0.286 0.941 0.431 0.092 0.955 0.908

N/A N/A 39.0% recycled recycled recycled recycled recycled recycled recycled N/A N/A recycled N/A recycled

N/A N/A 0.519 0 0 0 0 0 0 0 N/A N/A 0 N/A 0

278.0 284.8 284.8 302.7 342.8 406.5 482.2 556.9 712.0 8590.1 290.3 272.6 5866.7 71.5 4565.2

Refer to Table 4 for base operating conditions. † denotes two-bed R-PSA cycle. Cases correspond to the figures listed.

is again expected to increase the product recoveries at the cost of lower productivity and increased energy requirements. Results and Discussion R-PSA (Figure 2) and Dual-Train R-PSA/SMB Process with Heavy and Light Products (Figure 12). The two-bed R-PSA process (Figure 2) was simulated at the base operating conditions listed in Table 4. Figure 15 and Table 6 (Case †) show that R-PSA can produce 99% CH4 with 50.3% CH4 recovery at a productivity of 1.16 × 10-4 mol/(kg · s) with energy requirements of 278.0 kJ/mol; simultaneously, it produces a “light” product that is 44.8% H2 with 98.8% H2 recovery. Clearly, R-PSA does not achieve binary separation of CH4 and H2 to any appreciable degree and the process favors production of the heavy product.

The R-PSA/SMB process was studied to see if it could achieve better separation for the same operating conditions. The R-PSA/SMB process in Figure 12 produces 99% CH4 with 53.1% CH4 recovery and 46.2% H2 with 98.7% H2 recovery at a productivity of 1.16 × 10-4 mol/(kg · s) with an energy requirement of 284.8 kJ/mol (Figure 15 and Case 13 in Table 6). At the same productivity (compare Case † to Case 13 in Table 6), the R-PSA/SMB achieves higher recovery of CH4 at a CH4 purity of 99% and produces a purer H2 product than R-PSA. However, this enhanced separation comes at the expense of higher energy requirements. Dual-Train R-PSA/SMB Process with Light and Heavy Products and Offgas (Figure 13). Although the results above indicate that the R-PSA/SMB process in Figure 12 achieves better separation that R-PSA, neither process achieves binary


Figure 14. Schematic of steps in the two-train R-PSA/SMB process producing one light, one heavy, and recycled offgas. Zone I steps are italicized. Dotted lines indicate port switches.

separation. The advantage of taking off H2 product during the pressurization step of Zone I (Figure 13) is that it will be pure: during pressurization, large amounts of CH4 are adsorbing and relatively pure H2 left in the bed void volume is pushed toward the opposite end of the column where it is collected as product. The disadvantage is that only a small amount of pure H2 can be collected since the bed must still reach PH during the step time allotted. Case 14 in Table 6 shows that the process in Figure 13 produced 99% CH4 with 53.0% CH4 recovery while 90.5% H2 is produced with 30.6% H2 recovery at a productivity of 1.16 × 10-4 mol/(kg · s) with an energy requirement of 284.8 kJ/mol. Clearly, better separation is achieved; however, there is a large amount of impure offgas (noffgas/nfeed ) 0.519, 39% H2). Dual-Train R-PSA/SMB Process with Light and Heavy Products and Recycled Offgas (Figure 14). Table 6 (Cases 15-21) shows the results for the R-PSA/SMB process with offgas recycle (Figure 14). The productivity and the nExt/nfeed and nRaff/nfeed parameters were adjusted to maintain a 99% purity CH4 product. Improvement can be seen by comparing Cases 14 and 20 in Table 6. In Case 20, all of the offgas is recycled and 99% CH4 is produced with 89.5% CH4 recovery and 79.1% H2 is produced with 97.9% H2 recovery. In Case 13, which produces only light and heavy products (no offgas and thus no offgas recycle), 99% CH4 is produced with 53.1% CH4 recovery and 46.2% H2 is produced with 98.7% H2 recovery. Offgas recycle greatly increases CH4 recovery and H2 purity; however, these improvements come at the cost of decreased productivity and increased energy requirements. In Case 13, the productivity is 1.16 × 10-4 mol/(kg · s) and the energy requirement is 284.8 kJ/mol; Case 20 has a productivity of 4.63 × 10-5 mol/(kg · s) and an energy requirement of 712.0 kJ/mol. The strikingly low

productivity and drastically high energy requirements are direct results of the offgas recycle stream. Clearly, R-PSA/SMB has a purity/recovery/productivity/energy tradeoff similar to that seen with the S-PSA/SMB process. Figures 16 and 17 illustrate these trade-offs. If one wishes to continue sacrificing productivity, it is possible to achieve binary separation with R-PSA/SMB. Case 21 in Table 6 shows that 99% CH4 can be produced with 99.6% CH4 recovery while producing 99% H2 with 97.5% H2 recovery. Unfortunately, this involves a sharp decrease in productivity to 3.83 × 10-6 mol/(kg · s) and a drastic increase in the energy requirement to 8590.1 kJ/mol. The effect of feed concentration was also examined. As discussed earlier, Figure 7 showed that the adsorption of methane is relatively linear while the adsorption of hydrogen is essentially negligible at the operating conditions of interest in this work. For heavy, methane-rich feeds, the adsorbent becomes saturated much faster than with light, hydrogen-rich feeds; thus, the feed time had to be significantly shortened. Case 22 in Table 6 examines the performance of R-PSA/SMB when the feed is the same as that fed to the S-PSA/SMB process: 70% H2 and 30% CH4. The R-PSA/SMB with no offgas produces 99% CH4 with 19.4% CH4 recovery and 74.3% H2 is produced with 99.99% H2 recovery; the productivity is 1.16 × 10-4 mol/(kg · s) and the energy requirement is 290.3 kJ/mol. Comparison of Case 22 to Case 13 shows that the dilution of the feed with H2 significantly reduces the CH4 recovery and increases the energy requirement slightly at the same productivity. The opposite trend occurs if the feed concentration is changed to 10% H2 and 90% CH4 (Case 23 in Table 6). In this case, at the same productivity as Cases 13 and 22, 99% CH4 is produced with 62.7% CH4 recovery and 21.9% H2 is produced with 94.3% H2 recovery; the energy requirement has decreased


Figure 15. Methane purity and process productivity vs methane recovery for R-PSA and R-PSA/SMB (one light product, one heavy product). Refer to Table 4 for operating conditions.

Figure 16. Process productivity and hydrogen product purity vs methane recovery for R-PSA/SMB process with one light product, one heavy product, and one offgas with recycle. Feed concentration is 71% CH4/29% H2 and CH4 product purity is 99%. Refer to Table 4 for base operating conditions.

slightly to 272.6 kJ/mol. Case 24 shows that binary separation of the 10% H2/90% CH4 feed can be achieved with a productivity of 5.39 × 10-6 mol/(kg · s) and an energy requirement of 5866.7 kJ/mol. This productivity and energy requirement are more favorable than those of Case 21 which uses a feed of 71% CH4/29% H2. These results show that R-PSA/SMB favors the production of the heavy product and, to that end, has a preference for heavy-rich feeds. Comparison of R-PSA/SMB to S-PSA/SMB. The results for R-PSA/SMB indicate that achieving binary separation of H2 and CH4 requires reductions in productivity and increased energy requirements. These effects are more severe with R-PSA/ SMB than with S-PSA/SMB. For example, compare Cases 23 and 24 (R-PSA/SMB) to Cases 25 and 26 (S-PSA/SMB) for the separation of the same 90% CH4/10% H2 feed. Case 24 shows that binary separation of that feed could be achieved with

R-PSA/SMB at a productivity of 5.39 × 10-6 mol/(kg · s) with an energy requirement of 5866.7 kJ/mol. However, Case 26 achieves the same degree of binary separation with a productivity of 8.99 × 10-6 mol/(kg · s) with an energy requirement of 4565.2 kJ/mol. Even with a very heavy-rich feed, these results show that the S-PSA/SMB process is more favorable than the R-PSA/SMB process in terms of productivity and energy; further, the degree to which S-PSA/SMB is more favorable than R-PSA/SMB increases as the feed becomes more light-rich (compare Case 22 in Table 6 to Case 7 in Table 5). A significant reason for the productivity and energy incentives associated with S-PSA/SMB is its use of a high pressure (and hence high density) feed step: this means that S-PSA/SMB processes more feed on a molar basis per unit time per unit adsorbent than R-PSA/SMB (which feeds at low pressure).


Figure 17. Process productivity and energy requirement vs methane recovery for two-train R-PSA/SMB process with one light product, one heavy product, and one offgas with recycle. Feed concentration is 71% CH4/29% H2 and CH4 product purity is 99%. Refer to Table 4 for base operating conditions.

Clearly, given the choice between these two processes and regardless of the feed concentration (within the range examined), one would use S-PSA/SMB rather than R-PSA/SMB to achieve binary separation of H2 and CH4 due to its higher productivity and lower energy requirements. However, as discussed earlier, it is possible to combine S-PSA/SMB and R-PSA/SMB to achieve the same degree of binary separation with a higher productivity. C-PSA/SMB S-PSA/SMB and R-PSA/SMB are both capable of achieving binary separation. S-PSA/SMB can produce pure light product (H2) simultaneously with a heavy-enriched (but still impure) CH4 product at a much higher productivity than if binary separation is required. Similarly, R-PSA/SMB can produce pure heavy product (CH4) simultaneously with a light-enriched (but still impure) H2 product at a much higher productivity than if binary separation is required. By combining S-PSA/SMB with R-PSA/SMB, yet another hybrid PSA/SMB process is formed: combination-type PSA/SMB (C-PSA/SMB). The combination of both processes into C-PSA/SMB allows for the production of light product by the S-PSA/SMB and heavy product by the R-PSA/SMB; each process therefore produces its dominant product. Process Description. Eight-Bed Combination SRC-PSA/ SMB (Figure 18) and SSC-PSA/SMB (Figure 19) Processes. If we place S-PSA/SMB first and R-PSA/SMB second, the SRCPSA/SMB process is formed (Figure 18). In this combination the S-PSA/SMB process receives fresh feed and produces a raffinate product and an extract product that is fed to the R-PSA/ SMB unit. Downstream, the R-PSA/SMB unit produces an extract product and a raffinate product that is recycled to the fresh feed to the S-PSA/SMB. Recycling the raffinate from the R-PSA/SMB unit is convenient because it is already at high pressure and compression is not required. Because the S-PSA/ SMB and R-PSA/SMB units are quasi-independent, they can be operated out of phase with respect to each other. Small buffer tanks can be installed as necessary to accommodate this. Recycling the raffinate to the fresh feed to the S-PSA/SMB unit is expected to decreases the productivity and increase the energy

Figure 18. Combination eight-bed SRC-PSA/SMB process with optional recycle of offgas. Stream numbers are as follows: (1) FinS-PSA/SMB, (2) in recycle offgas F R-PSA/SMB , (3) F R-PSA/SMB , (4) F R-PSA/SMB . Fresh feed concentration is 70% H2/30% CH4.

requirements, but increase the recovery of both the heavy and light products. It is also possible to place the R-PSA/SMB process first in the sequence followed by S-PSA/SMB. However, this implementation is unfavorable since it places the lowproductivity unit operation first. The eight-bed SRC-PSA/SMB process shown in Figure 18 was studied in detail. In terms of operation of the process, both the S-PSA/SMB and R-PSA/SMB units function quasiindependently; thus, the S-PSA/SMB and R-PSA/SMB are each characterized by their own operating parameters (LPRS, LPRR, HPRS, HPRR, etc). However, the two PSA/SMB units are interconnected with an intermediate feed stream (connecting S-PSA/SMB to R-PSA/SMB) and a recycle stream (connecting R-PSA/SMB to S-PSA/SMB). The following dimensionless ratios are used to track the flow of material: in FR-SMB/PSA in FS-SMB/PSA

)

amount of feed processed by R-PSA/SMB amount of feed processed by S-PSA/SMB (20)

Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 4461 recycle FR-SMB/PSA in FS-SMB/PSA

offgas FR-SMB/PSA in FS-SMB/PSA

)

)

amount of gas recycled by R-PSA/SMB amount of feed processed by S-PSA/SMB (21) amount of offgas produced by R-PSA/SMB amount of feed processed by S-PSA/SMB (22)

We could also produce extract during the vacuum pull step of Zone II in S-PSA/SMB and raffinate during the repressurization step of Zone II in R-PSA/SMB. However, production of these additional products is unfavorable because recovering them pure in appreciable amounts sacrifices productivity and requires significant energy increases. The earlier results for the stand-alone S-PSA/SMB and R-PSA/SMB units showed that the former outperformed the latter in terms of productivity and energy within the feed concentration range examined. Therefore, it is reasonable to ask if coupling two S-PSA/SMB units (Figure 19) may outperform the SRC-PSA/SMB. Comparison of Figure 19 to Figure 18 shows that coupling two S-PSA/SMB units to form the SSCPSA/SMB is somewhat more difficult than the coupling of S-PSA/SMB and R-PSA/SMB to form the SRC-PSA/SMB. In SRC-PSA/SMB, the high-pressure offgas from the R-PSA/SMB unit is directly recycled to the fresh feed and the light and heavy products from the S-PSA/SMB and R-PSA/SMB units, respectively, are taken off during the feed steps to each of these units (Figure 18). SSC-PSA/SMB, on the other hand, involves a more complicated flowsheet. The first S-PSA/SMB unit in Figure 19 operates in the same way as the S-PSA/SMB unit in SRC-PSA/ SMB except the effluent from the light purge step must be compressed from PL to PH before being sent to the second unit. The second S-PSA/SMB unit in Figure 19 produces heavy product (CH4) during the depressurization step because previous results for S-PSA/SMB showed that this is the best way to produce heavy product. The light purge effluent from the second S-PSA/SMB unit is compressed from PL to PH, combined with the high pressure product produced during the feed step of the second unit, and recycled to the fresh feed. An inherent disadvantage of SSC-PSA/SMB is that compression is required for recycle; this was not the case with SRC-PSA/SMB. Four-bed PSA/SMB processes utilizing two feeds (one highpressure feed and one low-pressure feed) were also developed; although these processes had higher productivities, they did not

Figure 19. Combination eight-bed SSC-PSA/SMB process with optional recycle of offgas. Stream numbers are as follows: (1) FinS-PSA/SMB, (2) in recycle offgas F R-PSA/SMB , (3) F R-PSA/SMB , (4) F R-PSA/SMB . Fresh feed concentration is 70% H2/30% CH4.

achieve complete binary separation. A six-bed process composed of a four-bed S-PSA/SMB unit and a two-bed R-PSA unit was also developed, but it had a lower productivity and higher energy requirements at complete binary separation than the SRC-PSA/ SMB. These processes are discussed in detail elsewhere.40 Theory and Simulation. Because of their complexity, the eight-bed SRC- and SCC-PSA/SMB processes required manual convergence of the ADSIM software. A recycle stream composition was assumed and the upstream four-bed S-PSA/SMB unit operation was simulated. Then, its results were used to simulate the downstream unit and these results were used to correct the recycle stream composition. Resimulation of both units was continued until cyclic steady state was reached. Results and Discussion Eight-Bed Combination SRC-PSA/SMB (Figure 18) and SSC-PSA/SMB (Figure 19) Processes. Table 7 shows the results for the combination SRC-PSA/SMB and SSC-PSA/SMB processes with different amounts of offgas production and recycle. The first comparison to be made is the performance of SRC-PSA/SMB to that of a combination of S-PSA and R-PSA (compare Case 27 to Case #). The SRC-PSA/SMB process produces 99.99% H2 with 83.1% H2 recovery and 99% CH4 with 53.1% CH4 recovery; the amount of offgas produced is about 26% of the total feed to the process and the energy requirements are 88.5 kJ/mol. At the same productivity, a linked S-PSA/R-PSA process produces 99.99% H2 with 80.4% H2 recovery and 99% CH4 with 49.6% CH4 recovery; here, in in ) 0.29. The energy requirement of the F R-PSA/SMB /F S-PSA/SMB linked S-PSA/R-PSA process is 41.2 kJ/mol. Clearly, this comparison shows that the SRC-PSA/SMB achieves better binary separation of H2 and CH4 than the S-PSA/R-PSA combination at the same productivity at the cost of increased energy requirements. This finding is analogous to those found for S-PSA/SMB compared to S-PSA and R-PSA/SMB compared to R-PSA. As one begins to recycle the offgas in the SRC-PSA/SMB (Figure 18), the H2 recovery and CH4 purity improve but productivity decreases and energy use increases (Table 7 and Figures 20 and 21). Table 7 (Case 31) shows that binary separation is achieved by the SRC-PSA/SMB process at a productivity of 9.37 × 10-5 mol/(kg · s) with an energy requirement of 260.9 kJ/mol: 99.99% H2 is produced with 99.6% H2 recovery and 99% CH4 is produced with 99.9% CH4 recovery. For the production of 99% H2, binary separation of feed into 99% H2 and 99% CH4 is achieved at a productivity of 1.78 × 10-4 mol/(kg · s) with an energy requirement of 134.7 kJ/mol (Case 36 in Table 7). If one can tolerate a H2 product with lower purity, productivity increases and the energy required decreases (compare Cases 31 and 36 in Table 7). The productivity incentive is also obvious in Figures 17 and 20: for binary separation, 99% H2 and 99% CH4 are produced by SRC-PSA/ SMB with almost twice the productivity and half the recovery of the SRC-PSA/SMB producing 99.99% H2 and 99% CH4. The results for the SSC-PSA/SMB (Figure 19) are also shown in Table 7. Cases 37 and 38 show the performance of the SSCPSA/SMB process for production of 99.99% H2 and 99% CH4: these results can be compared to those for SRC-PSA/SMB under the same conditions (Cases 27 and 31). For binary separation of the feed into 99.99% H2 and 99% CH4, SRC-PSA/SMB (Case 31) has a productivity of 9.37 × 10-5 mol/(kg · s) and an energy requirement of 260.9 kJ/mol; for the same degree of separation, SSC-PSA/SMB (Case 38) has a productivity of 4.11 × 10-5 mol/(kg · s) and an energy requirement of 790.5 kJ/mol. Clearly

4462 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 Table 7. Results for Eight-Bed C-PSA/SMB Processes with and without Recyclea case figure process # 27 28 29 30 31 ## 32 33 34 35 36 37 38 39 40

none 18 18 18 18 18 none 18 18 18 18 18 19 19 19 19

2-PSA SRC SRC SRC SRC SRC 2-PSA SRC SRC SRC SRC SRC SSC SSC SSC SSC

F recycleR-PSA/SMB/ F inR-PSA/SMB/ F offgasR-PSA/SMB/ H2 H2 CH4 CH4 offgas purity productivity energy F inS-PSA/SMB F inS-PSA/SMB F inS-PSA/SMB purity (%) recovery (%) purity (%) recovery (%) (H2) (%) (mol/(kg · s)) (kJ/mol) 0 0 0.07 0.13 0.20 0.26 0 0 0.09 0.15 0.17 0.19 0 0.64 0 0.46

0.44 0.42 0.44 0.45 0.47 0.48 0.40 0.37 0.40 0.42 0.42 0.43 0.42 0.75 0.37 0.62

0.29 0.26 0.20 0.13 0.07 0 0.24 0.19 0.17 0.15 0.09 0 0.29 0 0.21 0

99.99 99.99 99.99 99.99 99.99 99.99 99 99 99 99 99 99 99.99 99.99 99 99

80.4 83.1 85.8 89.6 94.1 99.6 84.9 89.1 93.2 96.5 98.3 99.6 83.1 99.6 89.1 99.6

99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99

49.6 53.1 62.8 73.5 85.8 99.9 49.6 59.3 77.3 87.9 93.6 97.7 41.3 99.9 51.6 97.7

47.3 44.9 46.6 47.0 47.4 N/A 41.3 39.0 42.0 42.2 42.4 N/A 39.9 N/A 32.1 N/A

2.76 × 10-4 2.76 × 10-4 2.53 × 10-4 2.20 × 10-4 1.71 × 10-4 9.37 × 10-5 2.92 × 10-4 2.92 × 10-4 2.37 × 10-4 2.13 × 10-4 1.94 × 10-4 1.78 × 10-4 2.76 × 10-4 4.11 × 10-5 2.92 × 10-4 9.13 × 10-5

41.2 88.5 96.7 111.0 143.3 260.9 40.0 82.4 101.2 113.0 123.9 134.7 117.7 790.5 107.8 344.8

a Note: in the process column, SRC-PSA/SMB and SSC-PSA/SMB are abbreviated by SRC and SSC, respectively. Cases # and ## refer to combinations of two PSA units: S-PSA and R-PSA. Fresh feed concentration is 70% H2, 30% CH4. Refer to Table 4 for base operating conditions.

Figure 20. Productivity vs methane recovery for eight-bed SRC-PSA/SMB process at various purity levels. Refer to Table 4 for base operating conditions. Fresh feed concentration is 70% H2/30% CH4.

the SRC-PSA/SMB outperforms the SSC-PSA/SMB in terms of productivity and energy for the binary separation of H2 and CH4. The higher energy requirement of SSC-PSA/SMB compared to SRC-PSA/SMB is not unexpected because it requires more compression. The lower productivity of the SSC-PSA/ SMB can be explained by looking more closely at the processes. The SRC-PSA/SMB and SSC-PSA/SMB processes as a whole achieve binary separation: the individual units within them do not. The feeds to the individual units within the SRC- and SSCPSA/SMB processes have different concentrations. In addition, the SRC-PSA/SMB (Figure 18) produces the heavy CH4 product during the low-pressure feed step. Since a relatively large amount of pure CH4 can be collected during this step, a relatively small amount of CH4 is recycled to the fresh feed, which reduces productivity to a relatively small degree (compare Cases 27 and 31 in Table 7). In contrast, SSC-PSA/SMB produces CH4 during the depressurization step. Depressurization occurs relatively rapidly and, in order to maintain the 99% CH4 purity, only a relatively small amount of CH4 can be collected as product. The relatively large amount of CH4 that is not collected as

product is recycled to the fresh feed, which reduces the productivity (compare Cases 37 and 38 in Table 7). Analogous results can be seen for the production of 99% H2 and 99% CH4 with SRC- and SSC-PSA/SMB (compare Cases 39 and 40 to Cases 32 and 36). Earlier results indicated that because the stand-alone R-PSA/SMB processes feed at low pressure, it was outperformed by S-PSA/SMB under a wide range of feed concentrations. Yet, the SRC-PSA/SMB process outperformed the SSC-PSA/SMB process. For this separation, R-PSA/SMB is more useful when it is coupled with and placed downstream of S-PSA/SMB (as is done in SRC-PSA/SMB in Figure 18) than it is in stand-alone operation. In addition, the SRC-PSA/ SMB has marked productivity and energy incentives over the stand-alone S-PSA/SMB for binary separation. For the same degree of separation of a 70% H2/30% CH4 feed, the stand-alone S-PSA/SMB has a productivity of 2.81 × 10-5 mol/(kg · s) with an energy requirement of 1102.0 kJ/mol while the SRC-PSA/SMB has a productivity of 1.78 × 10-4 mol/(kg · s) with an energy requirement of 134.7 kJ/mol


Figure 21. Productivity and energy vs methane recovery for eight-bed SRC-PSA/SMB process at various purity levels. Refer to Table 4 for base operating conditions. Fresh feed concentration is 70% H2/30% CH4.

(compare Case 12 in Table 5 to Case 36 in Table 7). These advantages arise because SRC-PSA/SMB does not require that its S- and R-PSA/SMB subunits achieve binary separation. We have shown that PSA/SMB will probably work for any single component adsorbing with an inert carrier or any high selectivity problem where the adsorption of the less selective component is very weak. Of course, the cycle timing would have to be tuned for the specific separation. The case where adsorption of the light component is competitive with the heavy component adsorption is also of interest. The lower selectivity and competitive adsorption makes this a more difficult separation. The wave interactions that result will require a detailed study for every different feed. Because the separation of hydrogen and methane on zeolite 5A does not show this behavior, this additional study was beyond the scope of this research. Summary and Conclusions The hybrid separation processes developed for the separation of concentrated binary gases are summarized in Table 1. The hybridization of these processes has eliminated the need for a carrier gas/desorbent by taking advantage of gas expansion at low pressure as well as use of both light and heavy purge streams. Among the several S-PSA/SMB and R-PSA/SMB processes developed, the two-train configurations appeared to be the most favorable for achieving binary separation. These processes use two trains (four beds) operating out of phase to produce light and heavy products, and they can be configured to produce impure offgas streams which can be recycled to the feed for further purification. The S-PSA/SMB process favors the production of light product and the R-PSA/SMB process favors the production of the heavy product. Despite the R-PSA/SMB favoring the heavy product, S-PSA/SMB was found to be superior for a broad range of feed concentrations. The SRC-PSA/SMB process combines the light product dominant S-PSA/SMB process with the heavy product

dominant R-PSA/SMB process. The eight-bed SRC-PSA/ SMB separated a feed of 70% H2/30% CH4 into 99.99% H2 with 99.6% H2 recovery and 99% CH4 with 99.9% CH4 recovery at a productivity of 1.71 × 10-4 mol/(kg · s) with an energy requirement of 260.9 kJ/mol. This is an improvement upon the four-bed S-PSA/SMB process, and both of these processes greatly outperformed the stand-alone fourbed R-PSA/SMB process. Although the S-PSA/SMB consistently outperformed the R-PSA/SMB, the SRC-PSA/SMB process outperformed the SSC-PSA/SMB process. Among all of the processes studied, SRC-PSA/SMB appears to be the best choice for complete binary separation since it has the highest productivity and lowest energy requirements. For H2 and CH4 separation, the S-, R-, SRC-, and SSCPSA/SMB hybrid processes have been shown to be technically feasible alternatives to the conventional gas-phase SMB and PSA. The economic feasibility of these processes and their potential as replacement technologies remain to be fully shown. As mentioned earlier, comparison of these processes with Polybed PSA would be particularly interesting, but the necessary Polybed data is not available in the open literature. Acknowledgment The authors gratefully acknowledge the technical support staff at Aspen Technology, Inc. for their assistance with ADSIM. Purdue University and the National Science Foundation (grants CTS-0327089 and CTS-0754906) are acknowledged for providing funding for this research. Nomenclature English Symbols

ap ci ci,pore cf

external surface area per volume, m2/m3 solute i concentration of fluid, kmol/m3 average solute concentration i in pore concentration (or molar density) of feed, mol/m3


Cp,f Cp,s ∆Hads Dmass ax Dthermal ax E in FR-PSA/SMB offgas FR-PSA/SMB recycle FR-PSA/SMB in FS-PSA/SMB hHTC HPRR

HPRS kez Kdi kMTC,solid ks L LPRR LPRS ncirc nfeed nheavy,purge nHP,feed nlight,purge nLP,feed nTot,feed mads col PH PM PL qi q qi* qi R RRR RRS tcycle tfeed tpr T* Ts Ts yi

fluid phase heat capacity, J/kg/K solid phase heat capacity, J/kg/K heat of adsorption, kJ/kmol mass axial dispersion coefficient, cm2/s thermal axial dispersion coefficient, cm2/s activation energy, kJ/mol amount of gas processed by R-PSA/SMB unit, moles amount of offgas produced by R-PSA/SMB unit, moles amount of gas recycled from R-PSA/SMB to S-PSA/ SMB unit, moles amount of feed processed by S-PSA/SMB unit, moles heat transfer coefficient, W/m2/K heavy purge ratio of R-PSA/SMB process, dimensionless heavy purge ratio of S-PSA/SMB process, dimensionless thermal conductivity of gas phase (W/(m · K)) fraction of interparticle volume species i can penetrate linear lumped parameter mass transfer coefficient, 1/s thermal conductivity of solid phase (W/(m · K)) bed length, m light purge ratio of R-PSA/SMB process light purge ratio of S-PSA/SMB process molar flow rate of circulation stream, mol/s molar flow rate of fresh feed, mol/s molar flow rate of heavy purge stream, mol/s molar flow rate of high pressure feed, mol/s molar flow rate of light purge stream, mol/s molar flow rate of low pressure feed, mol/s total molar feed flow rate (nTot,feed ) nHP,feed + nLP,feed), mol/s total mass of adsorbent in process, kg high pressure, bar intermediate pressure, bar low pressure, bar amount of solute i adsorbed, kmol/kg adsorbent average amount of solute adsorbed equilibrium amount adsorbed of species i average amount of solute i adsorbed universal gas constant recycle ratio of R-PSA/SMB process, dimensionless recycle ratio of S-PSA/SMB process, dimensionless cycle time, s time of feed step, s time of pressurization step, s average equilibrium temperature, K solid-phase temperature, K average solid-phase temperature, K mole fraction of solute i

Greek Symbols

εe εp η

external porosity, m3 void/m3 bed internal porosity, m3 pore/m3 particle compressor efficiency, dimensionless

Fs Ff γ

solid-phase density, kg/m3 fluid-phase density, kg/m3 ratio of specific heats (Cp/CV)

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ReceiVed for reView September 11, 2008 ReVised manuscript receiVed January 30, 2009 Accepted February 12, 2009 IE801371T

Separation of Concentrated Binary Gases by Hybrid Pressure-Swing

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