Energy Fuels 2009, 23, 5073–5076 Published on Web 08/25/2009
: DOI:10.1021/ef900382u
Pd-Based Membrane Reactor for Syngas Upgrading Adele Brunetti,† Giuseppe Barbieri,*,† and Enrico Drioli†,‡ †
National Research Council, Institute on Membrane Technology (ITM-CNR), Via Pietro BUCCI, c/o The University of Calabria, cubo 17C, 87030 Rende CS, Italy, and ‡Department of Chemical Engineering, c/o The University of Calabria, Via P. Bucci, cubo 44/A, 87030 Rende CS, Italy Received April 30, 2009. Revised Manuscript Received August 10, 2009
In H2 production, the reformer downstream upgrading is a fundamental step for CO (ca. 10%) reduction. H2 (ca. 50%) presence limits CO conversion (ca. 25%) significantly (by thermodynamics) in a traditional reactor (TR). A Pd-Ag membrane (60 μm thick) removing H2 from the reaction side shifted the water-gas shift reaction toward a further product formation, and a very high (ca. 90%) CO conversion was measured, significantly exceeding the TR equilibrium value. In the meantime, a pure H2 stream, suitable for a protonexchange membrane fuel cell (PEMFC), is recovered on the permeate side because no sweep gas is used. The H2 permeation was driven by feed pressure.
adsorption (PSA) unit for the H2 separation from the other gases. The substitution of these three units with a single membrane reactor (MR), combining the reaction and H2 separation by means of a selective membrane, proved to be a promising approach for this important upgrading step. The MR technology produces a pure H2 stream as permeate and allows for a higher CO conversion to be obtained with respect to the traditional processes. As a consequence, a significant reduction of the reaction volume is also observed because of the positive effect of the feed pressure on the CO conversion, acting positively on H2 permeation.3-5 Palladium-alloy membranes6-10 were successfully used for H2 production/separation also for the WGS reaction because of the infinite H2 selectivity, which allows for a pure H2 stream to be obtained without requiring a further separation. In many of these studies, the H2 permeation was promoted also by means of a sweep gas, which reduces the partial pressure of H2 in the permeate side, increasing the driving force. However, the use of a sweep gas (generally nitrogen or argon) produces a diluted H2 stream, requiring a further concentration, particularly if storage is required. The upgrading of a syngas mixture containing not only the reactant but also the reaction products, i.e., CO2 and H2 (H2O, 33%; CO, 33%; H2, 29%; CO2, 4%; N2, rest), was investigated experimentally in the Pd-Ag MR, and significant advantages in terms of higher CO conversion with respect to TR and high H2 purity were obtained.
Introduction The increasing effort being made to reduce environmental problems has recently led to the development of clean technologies, designed to enhance both the efficiency and environmental acceptability of energy production, storage, and use, in particular, for power generation.1 In the last few decades, a great deal of attention has been attracted to the use of H2 as a carrier to be employed for clean energy production by means of new technologies, such as polymer electrolyte fuel cells (PEMFCs). The new use of H2 as feed in fuel cells for mobile power sources requires a CO concentration lower than 10-20 ppm2 in the anode inlet gas. Otherwise, the anode is poisoned, and the cell efficiency suddenly drops. If H2 is produced from hydrocarbon or alcohol reforming (carbon-containing species), purification is required to reduce the CO level down to cell requirements. In an integrated membrane plant for H2 production by natural gas, the reforming stream contains CO2, N2, H2O, H2, CH4, and CO. In the meantime, the latter can also be processed to reduce CO content, producing more H2. The water-gas shift (WGS) reaction is exothermic (ΔH = -41.165 kJ mol-1) and favored by a low temperature. COþH2 O ¼ CO2 þH2
ΔH 0 298 K ¼ -41 kJ mol-1
In industrial applications, the upgrading of reformate streams is performed by a two-stage CO-shift process followed by a final CO2 removal unit. The first stage, operating at a high temperature (about 350-400 °C), converts a large portion of carbon monoxide, giving H2 and CO2. The second, operating at a low temperature (around 200-250 °C), where the thermodynamic conversion is higher, refines the carbon monoxide conversion, allowing for a low CO final concentration to be achieved (around 1%, molar). The H2-rich stream coming out from the last reactor is sent to a pressure swing
(3) Kikuchi, E.; Uemiya, S.; Sato, N.; Inoue, H.; Ando, H.; Matsuda, T. Chem. Lett. 1989, 489–496. (4) Barbieri, G.; Marigliano, G.; Perri, G.; Drioli, E. Ind. Eng. Chem. Res. 2001, 40, 2017–2026. (5) Barbieri, G.; Brunetti, A.; Granato, T.; Bernardo, P.; Drioli, E. Ind. Eng. Chem. Res. 2005, 44, 7676–7683. (6) Shu, J.; Grandjean, A.; Van Neste, A.; Kaliaguinem, S. Can. J. Chem. Eng. 1991, 69, 1036–1048. (7) Uemiya, S.; Sato, N.; Inoue, H.; Ando, H.; Kikuchi, E. Ind. Eng. Chem. Res. 1991, 30, 585–589. (8) Dittmeyer, R.; Hollein, V.; Daub, K. J. Mol. Catal. A: Chem. 2001, 135–184. (9) Paglieri, S. N.; Way, J. D. Sep. Purif. Methods 2002, 31, 1–169. (10) Seok, D. R.; Hwang, S. T. In Future Opportunities in Catalytic and Separation Technology; Iwamoto, M., Mison, M., Kimura, S., Morooka, Y., Eds.; Elsevier: Amsterdam, The Netherlands, 1990; pp 248-267.
*To whom correspondence should be addressed. Telephone: þ390984-492029. Fax: þ39-0984-402103. E-mail:
[email protected]. (1) Wadhwani, S.; Wadhwani, A. K.; Agarwal, R. B. Clean coal technologies;Recent advances. First International Conference on Clean Coal Technologies for Our Future, Sardinia, Italy, 2002. (2) Barbir, F. Sol. Energy 2005, 661–669. r 2009 American Chemical Society
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Energy Fuels 2009, 23, 5073–5076
: DOI:10.1021/ef900382u
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In addition, the permeation driving force was promoted by feed pressure instead of a sweep gas. Materials and Methods The MR used consists of two concentric tubes (Figure 1): the outer tube is a stainless-steel shell, and the inner tube is the Pd-alloy self-supported membrane. It is blind and has only one exit; therefore, the sweep gas for promoting the H2 permeation cannot pass. The catalyst was packed in the annular volume. The MR characteristics are summarized in Table 1. The experimental apparatus used is shown in Figure 2. The MR was placed in a temperature-controlled electric furnace. The gaseous mixture was fed by a mass flow controller (5850S, Brooks Instrument), while a high-performance liquid chromatography (HPLC) pump (P680A, Dionex Corp.) was used to feed the water, which was vaporized using a heating coil put into the furnace before meeting the syngas mixture. The flow rates of the outlet streams were measured by means of bubble soap flow meters. The chemical analyses on the retentate and permeate streams were performed by means of a gas chromatograph (6890N, Agilent Technologies, Inc.) with two parallel analytical lines, to analyze retentate and permeate composition at the same time. The temperature was measured using a thermocouple positioned in the middle of the reactor shell. To describe the H2 permeating flux through the Pd-Ag membrane, Sieverts’ law (eq 1) is used worldwide, when, as in this case, the temperature range in which the diffusion in the metal bulk is the rate-determining step. qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi side 0 -E=RT side JHpermeating ¼ permeance e Preaction - Ppermeate H2 H2 H2 2
Figure 1. MR configuration. Table 1. MR Characteristics membrane thickness (μm) OD membrane (mm) membrane length (cm) superficial area (cm2) ID shell (mm)
Pd-Ag commercial (Johnsonn-Matthey) self-supported 60 1 9.5 2.9 ca. 4
conversion of a TR (TREC) is a widely consolidated concept, the equilibrium of a MR requires also the equilibrium of permeation in addition to the reaction equilibrium typical of a TR. Therefore, the MR equilibrium conversion (MREC) is a function of the thermodynamic variables and initial compositions on both sides of the Pd-alloy membranes. The presence of a membrane modifies the reaction conversion (shift effect), and correctly, both reaction and permeation affect each other. As a result, the presence of a membrane permselective to hydrogen permits attaining in a membrane reactor conversion values higher than a traditional reactor. The equilibrium of a MR is the temperaturedependent condition in which the concentration (partial pressure, in the present case) of the chemical species present in the system (on both membrane sides) does not change in time. This happens when the chemical reaction and, specifically, the permeation have the same rate of the forward and reverse processes. Also, the condition about the permeation is reversible. For instance, if for any reason (e.g., reaction or pressure variation) the species concentration (e.g., hydrogen) on one side changes, the related concentration on the other side changes appropriately as well, to minimize the generated effect. The capability of the system to recover H2 as permeate was quantified in terms of the recovery index,5 defined as (eq 3)
ð1Þ The H2 permeating flux is a linear function of the driving force that is the difference of the square root of the H2 partial pressure on both membrane sides. A commercial Pd-Ag-dense and self-supported membrane was used in the experiments and showed infinite hydrogen selectivity.11 A linear dependence of the H2 flux as a function of the driving force was observed at all of the temperatures investigated (Figure 3), confirming that hydrogen flux follows Sieverts’ law; thus, a constant permeance value can be assumed for each temperature. Furthermore, no permeation was observed feeding N2, CO, etc. The membrane was used for several months in analogous operating conditions for different experiments, undergoing many start-ups and shut-downs. No performance differences in terms of permeance or selectivity were observer, confirming its stability. The CO conversion of a TR and MR was calculated using eq 2, including the CO and CO2 present in the outlet streams. 2 ! !3 retentate feed retentate -FCO FCO 1 4 FCO 2 2 5 CO conversion ¼ þ 1feed feed 2 FCO FCO
H2 recovery index ¼ RIH2 ¼
FHpermeate 2 permeate F H2 þ FHretentate 2
ð3Þ
It is the H2 fraction permeated through the membrane with respect to the total produced by the reaction and fed to the MR as a syngas mixture. In this work, the WGS reaction was investigated by feeding a syngas mixture (H2O, 33%; CO, 33%; H2, 29%; CO2, 4%; N2, balance) in a Pd-Ag MR packed with 3.4 g of a commercial CuO/ CeO2-based catalyst (Engelhard Corp.). This MR was used for several months in analogous operating conditions undergoing many start-ups and shut-downs. The reported results reflect this very long use in the laboratory. No performance reduction in terms of reaction confirms the stability of the catalyst used. The operating conditions used in the experiments are reported in Table 2.
ð2Þ The conversion is calculated as the arithmetic average of the two values: the first calculated by CO2 yield (lower limit) and the other calculated on CO present in the outlet stream with respect to the fed CO. The difference of the two values is the carbon balance and experimental error. In any case, this difference was always lower than 3%, as shown by the error bars. During the reaction tests, carbon deposition was not investigated; however, a performance decrease was not observed. The upper limit of a chemical reaction is given by the thermodynamic equilibrium conversion. Whereas the equilibrium
Results and Discussion The enhancement of CO conversion in a MR strongly depends upon the feed pressure that has a positive effect on the permeation and pushes the reaction toward further product formation, even though the reaction is independent of the pressure from a thermodynamic point of view in a traditional reactor (TR). Figure 4 shows MR CO conversion at 300 °C as a function of the feed pressure, for two GHSVs. Both of the curves follow an increasing trend with the feed pressure. Also, the
(11) Barbieri, G.; Brunetti, A.; Tricoli, G.; Drioli, E. J. Power Sources 2008, 182, 160–167.
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: DOI:10.1021/ef900382u
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Figure 2. Scheme of the experimental laboratory-scale plant: MFC, mass flow controller; GC, gas chromatograph.
Figure 5. MR and TR CO conversion at 500 kPa as a function of the temperature. Figure 3. Pd-Ag membrane module permeation tests. Hydrogen permeating flux as a function of the driving force of Sieverts’ law at different temperatures. Symbols, measured data; lines, linear regression through the axes origin (reprinted with permission from ref 11).
Relevant gains were shown in comparison to the TREC and, thus, with the TR at both the GHSVs considered. A CO conversion of ca. 80% at the highest pressure was achieved; it greatly exceeding 25%, the maximum value allowed in a TR by the thermodynamics. The difference observed for the MR at the two GHSVs is much higher than that measured for the TR. This is owing to the hydrogen removal, because of the membrane presence, in addition to the higher residence time between the reactant and the catalyst. In any case, it is relevant to point out that CO conversion is much higher than that of TREC also at the higher GHSV. This highlights the possibility offered by a MR of operating with a higher feed flow rate with a similar catalyst amount used in a TR. The analysis was extended in a temperature range of 275-330 °C, and at each temperature, the highest CO conversion was achieved at the highest feed pressure operated. Figure 5 shows the CO conversion, for both the GHSVs, for TR and MR. It follows an increasing trend up to the highest value reached at 325 °C, which is explained considering that both the reaction kinetics and the permeation are favored by temperature having Arrhenius behavior. Therefore, in a Pd-Ag MR, a temperature increase, acting positively on both mechanisms, implies a higher CO conversion, which, in all cases, is significantly higher than the TR one, reaching 84% at 325 °C and 2600 h-1. The CO conversion is always higher at the lower GHSV for the higher contact time between reactants and the catalyst, with respect to the other GHSV. However, at equilibrium, at higher temperature, the thermodynamic conversion reduces and such an effect overcomes the permeation increase. Accordingly, in a MR at higher temperature, once the permeation equilibrium has been reached, the CO conversion (MREC) decreases. For the evaluation of the gain reached in terms of conversion in a MR with respect to a TR, the conversion index (CI)
Table 2. Reaction Tests Operating Conditions temperature (°C) feed pressure (kPa) permeate pressure (kPa) H2O/CO feed molar ratio GHSV (h-1) no sweep
270-330 300-600 100 1 2600 and 3200
Figure 4. MR and TR CO conversion at 300 °C as a function of the feed pressure.
membrane reactor equilibrium conversion (MREC), upper (thermodynamic) limit of a MR conversion depending upon the temperature and pressure on reaction and permeate sides, follows the same trend. TR equilibrium conversion (TREC), is also plotted, on the same figure, as a horizontal line because it does not depend upon the reaction pressure. The same trend is observed for the CO conversion of TR, higher at lower GHSV. 5075
Energy Fuels 2009, 23, 5073–5076
: DOI:10.1021/ef900382u
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because of the higher CO conversion (higher H2 production) with respect to the case at 3200 h-1. Furthermore, at the same GHSV, RI increases with the temperature, with the kinetics and the permeation being favored. Better results in terms of RI were reached at 325 °C and 600 kPa for GHSV = 2600 h-1, recovering 82% of the total H2 produced or fed (Table 3). However, also at 3200 h-1, the MR allows ca. 75% of the H2 produced to be recovered and this adds to the possibility of processing a higher feed flow rate with the same catalyst amount. In addition, the produced H2 as a pure permeate stream can be directly fed to a PEMFC. Conclusions
Figure 6. Conversion index at 300 °C as a function of the feed pressure at different GHSVs.
In this work, the upgrading of a syngas mixture for pure H2 production was investigated experimentally by means of the WGS reaction carried out in a MR with a Pd-Ag membrane. The conversion of a syngas mixture, in a TR, is strongly depleted by limitations imposed by thermodynamics, reaching values lower than 22%. CO conversion in the Pd-Ag MR was (ca. 90%) significantly higher that that achieved in a TR, and also, the TREC was exceeded. In fact, a conversion index ranging from 3-4 was calculated from the experimental measurements, which means the MR achieves, in the same reaction volume, a CO conversion (ca. 90%) much higher also than the highest value possible in a TR (TREC ca. 25%). The Pd-Ag membrane, in fact, having infinite H2 selectivity, allowed for the selective H2 removal from the reaction volume, shifting the reaction toward further product formation. Meanwhile, ca. 80% of the total H2 produced and fed to the MR was recovered as pure permeate, also at the higher space velocity. This means the possibility of processing a higher feed flow rate with the same catalyst volume, achieving, however, a good H2 recovery and high CO conversion. The gain in CO conversion and H2 recovery was more relevant at a higher feed pressure. The feed pressure, being responsible for the driving force promoting the H2 permeation, allows for the recovery in the permeate side of a pure H2 stream; no further separation is required. As a consequence, the retentate stream is at a high pressure and concentration in CO2 that can be more easily captured.
Table 3. Recovery Index at 2600 and 3200 h-1 for Different Temperatures and Feed Pressures recovery index (%) -1
GHSV
3200 h-1
2600 h
temperature (°C)
400 kPa
600 kPa
400 kPa
600 kPa
275 300 325
39 42 51
67 77 82
38 42 44
66 71 73
was previously12 defined as the ratio of conversion (eq 4) achieved in a MR and a TR, for a set reaction volume. This index is particularly suitable for analyzing the MR performance when a feed mixture also contains products, e.g., H2, such as in the present case. conversion index ¼ CI ðconversionÞMR ð4Þ ¼ ðconversionÞTR set reaction volume Considering, for instance, the case at 300 °C, the CI shows an increasing trend with the feed pressure, ranging from 3 to 5 (Figure 6). This means that, with the same reaction volume, the CO conversion achieved in the MR is 3-4 times higher than that of a TR. As can be easily deduced, CI is favored by a high feed pressure that, favoring the H2 removal from the reaction volume, increases the product formation. Moreover, CI is quite similar at both GHSVs investigated (even slightly higher at 2600 h-1), with the improvement in CO conversion offered by a MR being quite constant with respect to a TR, in the same operating conditions. A CO conversion significantly higher than that achievable in TR means that a larger feed flow rate can be processed with the same catalyst weight, converting much more reactant and thus improving the overall efficiency of the process. A high H2 production is a consequence of a high CO conversion. In a MR, this is traduced into a high permeation driving force that allows for more H2 to be recovered in the permeate as a pure component. The recovery index (eq 3), generally used for indicating the H2 fraction recovered in the permeate with respect to that globally fed and produced in a MR, is positively influenced by the feed pressure, in the whole temperature range, owing to its positive effect on the permeation (Table 3). In particular, it is always higher at 2600 h-1
Acknowledgment. Project FIRB-CAMERE (RBNE03JCR5) “Repubblica Italiana, Ministero dell’Universit a e della Ricerca (MUR)” is gratefully acknowledged for co-funding this research. Johnsonn Matthey (U.K.) and Oleg M. Ilinitch (Engelhard Corp., Iselin, NJ) are gratefully acknowledged for supplying the membrane and catalyst, respectively.
Nomenclature CI = conversion index F = molar flow rate (mol s-1) GHSV = gas hourly space velocity (h-1) M = feed molar ratio, H2O/CO MR = membrane reactor MREC = MR equilibrium conversion P = pressure (kPa) RI = recovery index (%) T = temperature (°C) TR = traditional reactor TREC = TR equilibrium conversion V = reaction volume (m3) WGS = water-gas shift
(12) Brunetti, A.; Caravella, C.; Barbieri, G.; Drioli, E. J. Membr. Sci. 2007, 306, 329–340.
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