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Methanol Steam Reforming in Pd-Ag Membrane Reactors: Effects of Reaction System Species on Transmembrane Hydrogen Flux Sameer H. Israni and Michael P. Harold* Department of Chemical & Biomolecular Engineering, UniVersity of Houston, Houston, Texas 77204
The objective of this study is to understand and quantify the effects of the reactant and product species of the methanol steam reforming reaction (CH3OH, H2O, CO2, CO) on the H2 flux through a Pd77Ag23 membrane. Various concentrations of said gases along with H2 were fed to a membrane separator apparatus containing a 3.9 µm thick Pd-Ag (23 wt % Ag) “nanopore” membrane. The decrease in H2 flux through the membrane due to the presence of these gases was quantified at different temperatures (225-300 °C) and pressures (3-5 bar). The data show that CO causes the largest drop in H2 flux while H2O has the least effect. A mechanistically based adsorption and reaction model was developed to quantify the fractional surface coverages of the nonH2 species. Estimates of surface parameters such as adsorption equilibrium constants and binding energies are consistent with literature values. The adsorption model was incorporated into a two-dimensional separator model that accounted for concentration polarization (radial transport) effects. The model simulations successfully captured most of the trends in the flux data. The developed flux model is suitable for incorporation into a Pd-Ag membrane reactor model in order to evaluate the potential of a methanol membrane reformer for coupled hydrogen generation and purification. 1. Introduction Methanol steam reforming (MSR) is an important reaction for hydrogen generation1 and has been extensively studied in relation to Pd-based packed bed membrane reactors (PBMRs).2,3 The reaction is typically carried out between 250 and 300 °C on Cu/ZnO catalysts. The selective permeation of H2 through Pd and certain alloys (Cu, Ag, Au) is an activated process so high temperatures give higher flux. This is desirable; however, higher temperatures are to be avoided in MSR due to limitations of the Cu based catalysts that are most widely used.4 The Pd-Ag (∼23 wt % Ag) alloy membrane is a popular choice due to its high permeability and resistance to embrittlement below 300 °C.5 At these low temperatures, the presence of reactants and products of the MSR reaction, namely CH3OH, H2O, CO2, and CO, can result in a decreased H2 flux through Pd-based membranes. Some of this flux decrease is simply due to a reduction in H2 partial pressure difference across the membrane. The remaining decrease can be due to concentration polarization effects,6,7 but a large part is due to surface coverage effects. A number of previous studies have been carried out to determine the effect of CO, CO2 and H2O on the transmembrane H2 flux of Pd-based membranes.8-14 The presence of CO has been shown to significantly reduce H2 flux through Pd-based membranes especially for temperatures lower than 500 °C.10 Not only does CO occupy surface sites, but CO is also known to block hydrogen adsorption onto neighboring sites15 and increase the activation barrier for hydrogen dissociation by altering the local density of states.17 There is some dispute over the effect of CO2 and H2O on H2 flux. Some studies suggest that CO2 and H2O do not significantly reduce H2 flux due to surface adsorption11,16 while some have claimed the opposite.13,14 The effect of CH3OH on transmembrane H2 flux has not been studied to the best of our knowledge. Barbieri et al.12 and Wang et al.18 developed Langmuir adsorption based models that can predict the decrease in H2 flux due to the presence of CO. * To whom correspondence should be addresed. E-mail:
[email protected].
Notwithstanding these previous contributions, there is a need to better understand the effects of the CO, CO2, H2O, and CH3OH individually and as mixtures on the H2 flux through a Pd-Ag membrane in the temperature range of interest for MSR. In addition, a predictive model is needed to accurately simulate the performance of a PBMR for MSR and related reactions. In this study, experiments were carried out in a membrane separator apparatus to determine the effect of CH3OH, H2O, CO2, and CO on the H2 flux. A competitive adsorption model was developed consistent with previous literature and based on experimental observations. The adsorption model was incorporated into a two-dimensional separator model to estimate the adsorption parameters over a range of temperatures. The viability of the model was examined by simulating independent experimental data. 2. Experimental Details A Pd-Ag membrane (∼23 wt % Ag) was used in a custom built separator unit. A 3.7 mm OD porous asymmetric R-Al2O3 hollow fiber supplied by Media & Process Technology Inc. (Pittsburgh PA) was used as the substrate. The Pd-Ag was deposited on the Al2O3 surface using the “nanopore” synthesis method which has been described elsewhere.19,20 Pd was deposited first followed by a layer of Ag. The membrane was then annealed at 500 °C in a 50% H2 in He mixture for 72 h. Energy dispersive X-ray spectroscopy (EDX) of the crosssection of the membrane was used to confirm that the composition of the alloy was ∼23 wt % Ag. The total Pd-Ag thickness was 3.9 µm (including 1 µm of the thickness embedded in the pores of the substrate). The length of the exposed part of the membrane was about 5 cm. The membrane was fitted coaxially within a 3/8 in. OD (0.035 in. wall thickness) stainless steel tube, comprising the membrane separator apparatus (Figure 1). The 3/8 in. diameter was chosen to minimize external concentration gradients due to concentration polarization.6 Swagelok fittings along with Kalrez o-rings were used to connect the membrane with the stainless steel tubing.
10.1021/ie1005178 2010 American Chemical Society Published on Web 07/21/2010
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anisms not involving surface adsorption. In the third part of the experimental studies H2 along with mixtures of the other species were fed to the separator. These results were used to determine any interaction effects between the various species and to verify previously estimated parameters. During the experiments, the H2 stream (565 sccm) was continuously fed to the separator. Once the stream containing the other species was switched on, at least 30 min were allowed to elapse before any readings were taken (to achieve steady state). Multiple measurements (at least three) were taken for each case in order to ensure that steady state had been reached and to reduce errors. Once the required readings were taken, the non-H2 stream(s) were shut off and pure H2 was fed until the permeate H2 flow rate reached a steady value. Usually about 20 min were required to reach this steady state. In the experiments, no permanent drop in pure H2 permeate flow rate was observed due to membrane exposure to the non-H2 species. The propagation of error formula20,21 was used to estimate a total possible error of (3% in the reported values of percentage decrease in H2 flux. 3. Model Development
Figure 1. Hydrogen separator apparatus containing the Pd77Ag23 membrane.
The entire apparatus along with an inlet gas preheater and vaporizer (for CH3OH) was kept in a tube furnace which was set to maintain the inlet at a desired temperature between 225 and 300 °C. Temperatures were monitored by type-K thermocouples (1/16 in. OD, stainless steel sheathed) positioned at several points in the separator apparatus including the gas inlet tubing. The various gases were metered through individual Brooks mass flow controllers. All the gases were ultrahigh purity grade supplied by Matheson Tri-Gas. The CH3OH was supplied by EMD (HPLC grade). The H2 flow rate through the separator was fixed at 565 sccm for all the experiments. The inlet H2 flow rate and length of the membrane was chosen so as to minimize change in gas composition from inlet to outlet, while maintaining a measurable H2 flow rate through the membrane at all conditions. A Swagelok back pressure regulator was located on the outlet retentate gas line and was used to set the separator retentate side at a desired pressure (between 3 and 5 bar absolute). The permeate side was always open to atmosphere and no sweep gas was employed. The retentate and permeate streams were fed to a 6890C Agilent gas chromatograph equipped with a thermal conductivity detector (TCD) to measure the stream compositions. The flow rates of permeate and retentate streams were measured using a bubble flow meter. In the first part of the experimental study pure hydrogen or pure helium were fed to the retentate side of the separator (at specified temperatures and pressures) in order to determine the pure gas permeation properties of the membrane. In the second part of the experimental study H2 along with variable flow rates of a second component (CH3OH, H2O, CO2, CO) were fed to the retentate side. The decrease in H2 flux as compared to under pure H2 conditions indicated the effect of the second component on the H2 transmembrane flux. These data were used to determine the parameters of the model which is described in Section 3. Different concentrations of H2 and He were also fed to the retentate side. Since He does not adsorb at high temperatures, these experiments indicated the effect of mech-
There are three primary reasons for the H2 flux to decrease due to the presence of species other than H2: these are (i) reduced partial pressure of H2, (ii) surface adsorption of the other species, and (iii) concentration polarization. Each reason is expanded on below. The first effect, i, was accounted for by simply adjusting the partial pressure of retentate H2 in the membrane flux expression, which in the absence of effects ii and iii is the Sieverts flux expression. JH2 ) Q(PH2,ret0.5 - PH2,perm0.5)
(1)
where the permeance Q is a function of temperature. The surface adsorption effect ii was modeled by including a term (θV + θH) in the flux expression. θV represents the fraction of vacant (and potential H occupied) surface adsorption sites, and θH represents the fraction of surface sites already occupied by H atoms. (θV + θH) has a value of unity in the case where the membrane is exposed to pure H2. When the membrane is exposed to a mixture of gases, as is the case in the membrane reactor, (θV + θH) decreases with an increase in adsorption of non-H2 species. JH2 ) (θV + θH)Q(PH2,ret0.5 - PH2,perm0.5)
(2)
On the basis of the experimental observations from this study and previously published studies, surface mechanisms are proposed to relate (θV + θH) to measurable quantities such as partial pressures and temperature. These are described next. In the results section, we describe our approach of including these effects in the hydrogen flux expression. For Pd-Ag alloy membranes under vacuum, the Ag atoms tend to occupy the surface layer, but in the presence of H2 the Pd atoms segregate to the surface layer.22 H2 dissociatively adsorbs on the Pd-Ag surface and prefers adsorbing onto the Pd sites,23 according to H2 + 2S T 2 H-S
(3)
where “S” represents an unoccupied adsorption site on the Pd-Ag membrane surface.
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On Pd(111) the type of CO binding is a strong function of adsorbed CO coverage.24 Below 0.5 monolayer (ML) coverage, CO prefers threefold hollow adsorption sites. Above 0.5 ML, twofold bridge site adsorption takes over, and above 0.6 ML coverage top-site adsorption is observed. Even on Pd-Ag alloy surfaces, CO has been shown to adsorb in a number of different configurations depending on surface Pd concentration. For high surface Pd concentrations (which would be the case for H2 exposed membranes), threefold site adsorption has been observed on Pd-Ag alloy surfaces.24 The following adsorption mechanism was proposed for CO on Pd-Ag membrane surface. CO + nS T CO-Sn
(4)
COsSn in eq 4 denotes that one CO molecule makes “n” surface sites unavailable for hydrogen adsorption. This could be by actual CO adsorption onto those sites or by the inhibition of adsorption onto neighboring sites: a site exclusion effect.15 The distinction is not important for this study. On the basis of the previous literature15,24 and the best model fits for the experimental observations in this work (see section 4 below), n ) 3 was chosen. It should be noted that even for the case when threefold hollow site adsorption dominates, kinetically the CO molecule likely goes through the top-site and twofold bridge site adsorption transition states. H2O has been shown to adsorb on single sites on Pd surfaces.15 Here we assumed that this was also true for Pd-Ag membranes: H2O + S T H2O-S
(5)
When CO2 was fed along with H2 into the separator in this study, small amounts of CO and H2O (
