Angelo Basile

An analysis of the performance of membrane reactors for the water–gas shift reaction using gas feed mixtures

Abstract The water–gas shift (WGS) reaction in membrane reactors has been widely studied by several authors. From these works, the increase of the CO conversion above the equilibrium values appears to be possible when hydrogen is removed through the membrane. However, to date, this feasibility has been verified mostly when feeding pure reagents to the reactor, although in an industrial context the feed normally contains several other compounds. The objective of this work has been to analyse the effect of the feed composition on the membrane reactor efficiency in order to determine the best conditions in terms of CO conversion. At this purpose, experimental tests with mixtures of different compositions have been carried out in three different systems of reaction: (1) traditional fixed-bed reactor; (2) membrane reactor with mesoporous ceramic membrane; (3) membrane reactor with palladium membrane. The experiments included permeation (for the membrane reactors) and reaction tests. The experimental results obtained with the various systems of reaction have been compared. A mathematical model has been also formulated for the different type of reactors used in order to verify the experimental results obtained. From the work carried out it can be concluded that by using the palladium membrane reactor it is possible to overcome the equilibrium conversion. Moreover, a complete conversion has been achieved for one of the mixtures fed to the reactor.

Experimental and simulation of both Pd and Pd/Ag for a water gas shift membrane reactor

Abstract The study of the water gas shift reaction performance in terms of complete conversions is presented. The behaviour of a membrane reactor (MR) consisting of a tubular microporous ceramic within a thin palladium membrane was compared with a membrane reactor using a palladium/silver membrane. Membranes were developed in order to obtain a metallic layer thick enough to avoid any defects of the metallic layer and ensure infinite hydrogen selectivity with respect to other gases. The lumen of both membrane reactors was filled with the catalyst. The experiments were carried out by using nitrogen as inert gas in the streep having a flow rate ranging between 1×10−4 and 4×10−4 mol s−1 in co-current and counter-current mode in the temperature range 331–350°C and in the feed molar flow range 3.05×10−5–7.1×10−5 mol s−1. Hydrogen was the only one gas passing through both membranes. A complete separation of hydrogen from the other gases of the reaction system was obtained. The water gas shift reaction conversion was close to 100% by using the Pd/Ag membrane. A mathematical model was developed to interpret the experimental data. It described the system under isothermal conditions and considered an axial differential mass balance in terms of partial pressure for each chemical species. The simulation study and the experimental results show a satisfactory agreement and both highlight the possibility to shift towards 100% the conversion of the considered reaction.

Synthesis, characterization, and applications of palladium membranes

Publisher Summary This chapter discusses the synthesis, characterization, and applications of palladium membranes. The main role of the membrane film is to control the exchange of materials between two adjacent fluid phases. A membrane is able to act as a selective barrier, which separates different species either by sieving or by controlling their relative rate of transport through itself. Transport processes across the membrane are the result of a driving force associated with a gradient of concentration, pressure, temperature, and electric potential. The synthesis of stable microporous or dense inorganic materials for the preparation of membranes is the key factor for increasing the application of membrane-based reactive separations in the catalysis field. An important step before using a membrane in a separation or a reaction system is its characterization in terms of permeation as well as morphology.

Membrane reactor for water gas shift reaction

In this experimental study the water gas shift (WGS) reaction is considered as a particular application of a catalytic membrane reactor (CMR). Experiments on the WGS reaction were carried out using a composite palladium membrane obtained by coating an ultrathin double-layer palladium film on the inner surface of the support of a commercial tubular ceramic membrane by a so-called co-condensation technique. The best operating conditions were determined at various H2OCO molar ratios, temperature, PIumen, gas feed flow, and with and without nitrogen sweep gas. For a non-porous stainless steel tube and for the commercial ceramic membrane having the same geometrical dimensions, the conversion results are always lower than the equilibrium value. For the composite palladium membrane, the conversion also depends on the flow of the sweep gas utilized. For example, using a nitrogen sweep gas flow of 28.2 cm3/min, the maximum conversion value reaches 99.89%. The study of the effect of temperature on conversion of carbon monoxide in the WGS reaction shows that at higher reaction temperature, the thermodynamic equilibrium conversion of CO decreases. In contrast for the CMR considered in this work, there is a maximum conversion value around 600 K. This value is a compromise between the kinetic rate of the reaction (which increases with increasing temperature) and thermodynamic considerations for the WGS reaction. The effect of the time factor (WF) on conversion of CO, with and without sweep gas at three different temperatures (595, 615 and 633 K) shows that at greater WF there are correspondingly higher values of the CO conversion for each temperature considered. For each temperature there is a slight effect of the sweep gas, and this is higher at 595 K. The good performance of the composite ceramic-palladium membrane is confirmed by a comparison with experimental results recently presented in the literature for the same reaction. Reaction tests have been carried out for a feed mixture also. In this case, however, the resulting values are always below the equilibrium ones.

Hysteresis in soil water characteristics as a key to interpreting comparisons of laboratory and field measured hydraulic properties

[1] This study is mainly aimed at assessing the adequacy of laboratory-based soil hydraulic characterization carried out on undisturbed cores for reproducing the in situ soil hydraulic behavior. Assuming that the laboratory sample size is a representative elementary volume (REV) for a single horizon of the soil profiles examined, the analysis of the hysteretic features of the soil water retention function is used to explain the differences observed in the constitutive soil water relationships as obtained from either laboratory or in situ experiments. Because the wetting procedures differ in laboratory and in field experiments, it is argued that the difference in the obtained hydraulic characteristics is the result of different hysteretic paths. From the comparison of measured hydraulic conductivities we deduce that field and laboratory retention curves are a part of the same hysteresis loop. In such a context the field hydraulic functions can be derived from the laboratory functions using the maximum water content (i.e., at zero pressure head) and air entry value measured in the field. It is shown that the field parameters identified from the measurements in the early phases of the field test are sufficient to reliably describe field hydrological behavior.

The partial oxidation of methane to syngas in a palladium membrane reactor : simulation and experimental studies

Abstract Among numerous potential applications of inorganic membrane reactors, the partial oxidation of methane (POM) may offer an alternative route, with respect to steam reforming of methane, for producing synthesis gas. Inorganic membrane reactors are considered to be multifunctional reactors because they are able to combine catalytic reactions with membrane separation properties. In particular, dense palladium membranes are characterised by the fact that: (1) only hydrogen might permeate through them; (2) both Arrhenius and Sievert laws are followed. In this investigation, a dense palladium membrane reactor (PMR) concept is analysed referring both to experimental data and to simulation study. The partial oxidation of methane (POM) reaction to produce synthesis gas was chosen as a model reaction to be investigated. A membrane reactor model that includes the membrane, the gas phase and the catalyst activity is proposed. The experimental results in terms of methane conversion obtained by using a pin-hole free palladium membrane permeable to hydrogen only were compared with model predictions. The effect of reaction temperature on methane conversion at different time factors and sweep gas flow rates was considered. In particular, the effects of temperature profiles on the methane conversion are taken into account in the kinetic model.

A study on catalytic membrane reactors for water gas shift reaction

In the present work the water gas shift reaction is considered as a particular application of a catalytic membrane reactor. Three different methods to deposit a thin film of palladium on a porous ceramic tubular membrane have been studied: the magnetron sputtering technique, the physical vapour deposition technique, and the co-condensation technique or solvated metal atom deposition method. For each composite membrane, characterization in terms of pore distribution, thickness of the film, percentage of Pd deposited along the thickness of the membrane and CO conversion versus feed flow rate and versus different H2OCO molar ratio are presented.

Soil hydraulic behaviour of a selected benchmark soil involved in the landslide of Sarno 1998

Abstract A multidisciplinary approach was adopted in this paper in order to study the very complex and unpredictable phenomenon of rainfall-triggered landslides. Physical and chemical measurements were made on a selected benchmark soil involved in the landslide of Sarno (Southern Italy) in May 1998. Water behaviour inside the soil, covering a typical slope of the area, was simulated using a two-dimensional simulation model. Sensitivity analyses of the model showed that a few uncertainties regarding input data (climatic and physical) can be accepted without substantially changing output data, namely soil water storage values. The effects of the interruption of the pedological continuity of the slope were simulated in terms of total weight encumbering the bottom of the soil stratification just behind a section of discontinuity. There resulted an increase of more than 30% in soil water storage with respect to the same section of the undisturbed slope. It causes an overloading which produces the same values of soil tangential pressure as those of peak strength measured by direct shear tests. Notwithstanding the simplifications introduced to carry out this study, the proposed approach demonstrates the possibility of quantifying the consequences of some human or natural changes regarding soil-covered slopes and hence the potential of improving landslide risk assessment.

Temporal stability of spatial patterns of soil water storage in a cultivated Vesuvian soil

Abstract Spatial and temporal variability of soil water storage were studied on a sandy soil in Ponticelli (NA), Italy. Aluminium tubes were installed in a plot (0.3 ha and cultivated with barley) at a depth of 150 cm. A neutron probe moisture meter was used to measure the water content at intervals of 15 cm along the vertical soil profiles. The spatial dependence of the soil water storage along a 50 m transect was examined by variograms at different times. These experimental variograms were found to be similar and a combined variogram could be used. On the basis of the spatial correlation a grid of 20 sites were selected with sufficient spatial separation to be uncorrelated. Measurements were performed from November 1986 to April 1987 at six different dates. The temporal stability of spatial patterns of soil water storage was tested in two ways: (1) by ranking the storage values for a given time and comparing the ranks of the sites; (2) by analyzing the rank correlations between storage values at different times. Moreover, contour maps of soil water storage were drawn by punctual kriging. It has been demonstrated that some measuring locations show extreme values of water storage continuously. However, it was not possible to identify a limited number of locations representative of the average field water storage, due to the extreme homogeneity of the soil. Therefore this method offers better possibilities with more heterogeneous soils. Correlation analysis showed only a moderate loss of information at rank-order level during the growing season.

Characterization of thin wall Pd–Ag rolled membranes

Abstract A thin wall Pd–Ag tube (thickness 50 μm ) obtained by a procedure of cold-rolling and annealing of thin metal foils has been characterized in long-term tests for determining the hydrogen permeability under operating condition of a temperature range from 300°C to 400°C and transmembrane differential pressure from 50 to 100 kPa . During testing, the physical and mechanical stability of the rolled membranes have been observed and high hydrogen fluxes have been measured. The long-term tests have also shown the modification of the surface structure of the membrane due to the hydrogen–metal interaction and the thermal cycling: as a consequence, an increase of the mass transfer properties of the hydrogen through the material (diffusivity and solubility) has been observed and the membrane tube has attained very high performances in about 3 months of operation in terms of hydrogen permeability. Furthermore, the tests have demonstrated that these Pd–Ag membranes have the capability to separate hydrogen from gas mixtures with a complete hydrogen selectivity and can be used to produce ultra-pure hydrogen for applications in energetic fields in membrane reactors by molecular reforming in membrane reactors (i.e. isotopic hydrogen separation, fuel cell, etc.).