Showing papers by "Giuseppe Barbieri published in 2010"

Sieverts law empirical exponent for Pd-based membranes: critical analysis in pure H2 permeation.

Abstract: In this paper, the physical meaning of the Sieverts-type driving force exponent n is analyzed for hydrogen permeation through Pd-based membranes by considering a complex model involving several elementary permeation steps (adsorption on the membrane surface on the feed side, desorption from the surface on the permeate side, diffusion through the metal lattice, and the two transition phenomena surface-to-bulk and bulk-to-surface). First, the characteristic driving force of each step is evaluated, showing that adsorption and desorption singularly considered and the adsorption and desorption considered at the same time are characterized by driving forces depending on the ratio of feed and permeate hydrogen pressure. On the contrary, the diffusion step is found to present a driving force that is composed of two terms, one which corresponds to the original Sieverts law (with an exponent of 0.5) and the other which is the product of the pressure difference and a temperature-dependent factor. Then, the characteristic n is evaluated by applying the multistep model to two different membranes from the literature in several cases, (a) considering each permeation step as the only limiting one and (b) considering the overall effect of all steps. The results of the analysis show that for a low temperature and thin membrane thickness, the effect of the surface phenomena is, in general, a decrease of the overall exponent n toward values lower than 0.5, even though, under particular operating conditions, the n theoretical value of the surface phenomena is equal to unity. At a higher temperature and thickness (diffusion-controlled permeation), n tends to 0.5, even though the rapidity of this tendency depends strictly on the membrane diffusional parameters. In this frame, the expression developed for the diffusion step provides a theoretical reason why n values higher than 0.5 are found even for thick membranes and high temperature, where diffusion is the only rate-determining step.

H2 Separation From H2/N2 and H2/CO Mixtures with Co-Polyimide Hollow Fiber Module

Abstract: A lab-scale membrane module, packed with more than 150 hollow fibers of P84® co-polyimide, was used for the separation of hydrogen mixtures. The ideal membrane performance was analysed with pure gases (H2, N2, CO, CO2, CH4) and H2/N2 and H2/CO mixtures at 50°C and up to 6 bar. Significant differences were observed between ideal selectivities and separation factors of mixtures. In gas mixture experiments, no variation of hydrogen flux was observed among the different feeds, whereas the permeance of the less permeating species, i.e., N2 and CO, was significantly higher than that measured with pure gases. A linear dependence of H2 recovery on the stage cut was observed in the whole feed pressure range investigated. No differences in the behavior of the membrane versus the two different mixtures were observed. A higher separation factor was obtained when H2 was mixed with N2 rather than CO, in agreement with the trend followed by ideal selectivity values, since the one of H2/N2 was 78, a bit higher than that ...

Inhibition by CO and polarization in Pd-based membranes: a novel permeation reduction coefficient.

Abstract: In this Article, a novel permeation reduction coefficient (PRC) is defined and used to take into account the presence of both inhibition by CO and concentration polarization in hydrogen permeation through Pd-based membranes. The usefulness of this coefficient consists in the possibility of describing simply, but at the same time powerfully, the behavior of the membrane subject to the combined effect of inhibition and polarization. According to this approach, the effective permeance, which is generally unknown because it depends on these two phenomena, can be directly evaluated by multiplying the “clean” intrinsic membrane Sieverts permeance (measurable by simple pure hydrogen permeation tests) by a PRC function, that is, [effective permeance] = (1 − PRC) [clean Sieverts permeance]. The values of PRC are evaluated by means of a complex model that takes into account the several elementary permeation steps, in which the inhibitory effect of CO is also considered as well as the concentration polarization. The...

Pd/Ag-based membrane reactors on small scale: Assessment of the feed pressure and design parameters effect on the performance

Abstract: The antagonist effect of pressure on the thermodynamic equilibrium and the H 2 permeation rate has been studied in a small-scale membrane reactor (MR) with no sweep gas The influence of some design parameters as membrane surface area ( A m ), catalyst volume ( V cat ), methane load in feed ( L s ) and gas hourly space velocity (GHSV) on the MR performance has also been evaluated In the MR, a twofold increase of the methane conversion has been obtained by enhancing the ratio A m / V cat from 042 to 21 cm 2 cm −3 A methane conversion about three-times as much as thermodynamic equilibrium limit of a traditional reactor has been obtained when the L s / A m ratio was reduced from 116 to 84 cm 3 ( STP ) cm −2 min −1 Ultimately, a methane conversion of the 68% and a hydrogen recovery approximately of 43% have been reached for a ratio A m / V cat = 21 cm 2 cm −3 and L s / A m = 84 cm 3 ( STP ) cm −2 min −1 Removal of hydrogen increases the tendency to carbon deposition These effects can be reduced by operating at higher temperatures and/or pressures, but necessarily, in the presence of a reagents molar ratio higher than three

Ceramic Membranes in Carbon Dioxide Capture: Applications and Potentialities

Abstract: Today, CO2 capture from e.g. flue gas has becoming an emerging opportunity for membrane gas separation. The flue gas coming out from power plants contains about 10-15% CO2, which should be separated before its sequestration. The most used membranes for this application are polymeric but they cannot be used at a high temperature. The flue gas exits at ca. 200°C, depending on the specific locations in the plant and, thus, it is highly desirable to separate it at high temperature. An alternative class to polymeric membranes is represented by the ceramic one which comprises zeolites, carbons, silica, perovskites membranes, that exhibit high fluxes and thermal resistance. However, a great challenge is to fabricate them as thin layers, avoiding formation of cracks that compromise the separation. Today, new solutions are in progress for the production of ceramic membrane able to overcome these limitations. For example, hybrid membranes able to combine the properties of different materials are proposed. Moreover, new works are done on mixed-matrix membranes, comprising of a molecular sieve guest phase dispersed in a polymer host matrix [3] which combines the advantage offered by the two materials. This work proposes an overview on the main applications of ceramic membranes in CO2 capture processes.

Estimating limit conversion for methane steam reforming in a palladium membrane reactor using countercurrent sweep gas

Abstract: Generally speaking, conversion increases with increasing reactor length to approach a certain value: so-called limit conversion. Limit conversion for a membrane reactor with a cocurrent sweep gas has been studied extensively. However, characteristics of limit conversion for the countercurrent, which are examined in this study, remain unclear. First, conversions of MRs with different reactor lengths were calculated using a conventional mode of integrating differential equations. Results confirmed that limit conversion is not always 100%, even for the countercurrent. Long MRs were found to have a pseudo-constant state inside, characterized by an extremely low apparent reaction rate and permeation rate. Secondly, a novel approach to estimate limit conversion was developed: reaction equilibrium constants and hydrogen partial pressure balance between both sides were applied not for the end of the reactor but for the pseudo-constant state. This new approach suggests that limit conversion for the countercurrent depends on the reaction temperature, feed-side and permeation-side pressures, feed-gas composition, and the sweep rate. It is fundamentally independent of reaction kinetic equations and hydrogen permeation-rate properties. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd.

Modelling and Simulation of Catalytic Membrane Reactors

Abstract: Membrane reactors and catalytic membranes were modeled taking into account the separation provided by the membranes. One-dimensional (1D) mathematical models were presented for tubular reactors with cylindrical symmetry and first- or second-order differential equations were considered depending on the importance of the axial diffusion. The permeation through the membrane was described by the specific transport model. The models presented include both mass and energy balances because the simulation discussed is related to energy-intensive reactions. Simulations for reactions producing hydrogen, such as methane steam reforming and water gas shift, were discussed. When the reaction takes place inside the membrane pores or on its surface, the mathematical modeling has to consider the orthogonal direction as the main one to the membrane surface. Therefore, 1D second-order models and related simulations are presented. The models presented are relative to enzymatic reactions characterized by low kinetics and low reaction heat. This means that the energy balance is useless.