Yi Hua Ma

Dependence of hydrogen flux on the pore size and plating surface topology of asymmetric Pd-porous stainless steel membranes

The influence of the support properties on the characteristics of Pd/PSS composite membranes has been evaluated for a large group of membrane samples prepared by electroless plating. The H2 permeation in the membranes was found to follow Sieverts law between 325–500°C. The steady state H2 flux was stable for over 200 h. The hydrogen permeance was measured at 350°C with a 1-atm pressure difference and ranged from 20.0 to 5.1 m3/(m2·h·atm0.5) for a Pd layer thickness of 11.7 and 33.8 μm, respectively. The thickness of the membranes and therefore the permeance obtained was dependent on the size of the largest pores present in the support. The thickness of the Pd layer was approximately three times the dimension of the largest pores in the support, consistent with the previous theoretical results of Ma et al. [3].

Adsorption of small organic pollutants from aqueous streams by aluminosilicate-based microporous materials

Organic pollution in industrial waste streams is of growing environmental concern. Adsorption has been applied to remove organics from aqueous solutions. Activated carbon and polymer resin are the most commonly used adsorbents. In this work, a novel class of aluminosilicate-based microporous materials with good adsorption capacity and high selectivity are investigated. In order to adsorb organic molecules selectively from aqueous solution, the adsorbents must be hydrophobic. Phenol and chlorinated phenols were adsorbed by three different adsorbents: pillared clays, silicalite and zeolite beta. Pillared clays were modified by incorporating a non-ionic surfactant of the general formula C2−14H25−290O (CH2CH20)5H (Tergitol 15S-5). Also, high SiAl ratio zeolites were used for this purpose. Factors which are important in determining the selectivity and adsorption capacity of these adsorbents are the hydrophobicity of the adsorbent, the size of the organic, and the diameter of channels which are accessible to the adsorbate.

Oxygen-permeable dense membrane reactor for the oxidative coupling of methane

A perovskite material (BaCe0.8Gd0.2O3) in powder form, with both electronic and ionic conductivity, was synthesized by the ethylene glycol method. A dense membrane tube was fabricated using a plastic extrusion technique. The oxygen permeances of the dense membrane tube were measured as functions of temperature and oxygen partial pressure on the feed side. In the temperature range of 688–955°C, the oxygen flux showed an approximately exponential dependence on temperature. The oxygen flux increased proportionally to the natural logarithm of the ratio of oxygen partial pressures across the membrane. Experimental results for the oxidative coupling of methane (OCM) to C2 hydrocarbons, in the absence of additional catalyst, showed that this material has fairly good catalytic activity for the OCM reaction. The maximum yield to C2 hydrocarbons that was obtained was 16%, which compares favorably to prior dense membrane studies.

Thin Composite Palladium and Palladium/Alloy Membranes for Hydrogen Separation

Abstract: Dense composite Pd and Pd/alloy membranes are currently being extensively investigated. The synthesis and characterization of these membranes, with a special emphasis on Pd/alloy membranes, are reviewed in this paper. Experimental results on Pd/Cu membranes supported on porous stainless steel exhibited good thermal stability and reasonable hydrogen flux. Furthermore, optical micrographs showed the formation of the dense palladium layer was unaffected by the topological features of the porous stainless steel, although the surface of the support directs the topology of the final Pd layer.

Oxidative coupling of methane using oxygen-permeable dense membrane reactors

Abstract Oxidative coupling of methane was studied with La/MgO catalyst and a distributed oxygen feed through mixed-conducting dense membrane tubes in a shell-and-tube reactor configuration. A SrFeCo0.5O3 membrane was tested, and a blank run confirmed that it acted as a total oxidation catalyst, with no C2 products. Attempts to coat the inside of the membrane tube with a non-combustion material BaCe0.6Sm0.4O3 were only partially successful, giving 7% yield to C2 products. The oxygen flux through the coated tube was reduced to 30% of its original value. A membrane tube was fabricated from a non-combustion oxygen-permeating material, BaCe0.8Gd0.2O3, and it was confirmed that this was not a total oxidation catalyst. Yields to C2 products of up to 16.5% were obtained, higher than those in comparable fixed bed studies. The C2 yield obtained is the highest reported in the literature for oxidative coupling of methane in dense membrane reactors. The flux of oxygen through both dense membranes increased under reaction conditions, by a factor of four, over the non-reaction flux measured in permeation experiments. Changes in surface morphology were observed for the side of the membrane in contact with the reducing atmosphere. Similar phenomena have been observed in previous studies.

Oxidative coupling of methane in a modified γ-alumina membrane reactor

Methane oxidative coupling experiments were conducted in a porous γ-alumina membrane reactor using Mn–W–Na/SiO2 catalyst, and its performance was compared with a packed reactor operated at similar conditions. The γ-alumina membrane tube was thermally stabilized by treating the membrane tube with La(NO3)3 aqueous solution and calcining at 900°C. Non-uniform oxygen permeation was achieved by partially coating the outer surface with a high-temperature glaze. C2 yields up to 27.5% were obtained in the membrane reactor. The experimental results clearly demonstrated that it was beneficial to distribute the feed of oxygen along the reactor length for the methane oxidative coupling reactions. Although the membrane reactor showed lower methane conversion at the same reaction conditions, it gave higher C2 selectivity and C2 yield at similar methane conversions. The helium flow rate was varied in the membrane and co-feed reactors to keep the temperature, methane flow rate, and oxygen flow rate constant. At the same methane conversion the membrane reactor gave 10% higher C2 yield and 30% higher C2 selectivity than the co-feed reactor. At similar C2 yield and C2 selectivity, the methane conversion of the membrane reactor was 15% lower than that of a co-feed reactor. The oxygen flow rate was varied in the membrane and co-feed reactors to keep conversion constant at the same temperature, methane flow rate, and helium flow rates. At the same methane conversion, the membrane reactor gave about 3% higher C2 yield and selectivity than the co-feed reactor. Higher helium flow rate gave higher C2 selectivity and yield, whereas changing methane flow rate did not significantly affect the reactor performance.

The synthesis and characterization of zeolite A membranes

Abstract The application of dry gel synthesis procedures in the preparation of zeolite A membranes on α-alumina supports has been explored. With the dry gel method, the amount of initial gel applied to the support as well as the dilution of the initial gel may be used to directly control the amount of zeolite formed, and thus the thickness of the membrane. The results of these experiments show that the combination of dilute aqueous initial gel (1.35SiO 2 :Al 2 O 3 :5Na 2 O:1000H 2 O) with low temperature drying (298 K) yielded zeolite A membranes with small uniform crystals of size ∼0.2 μm. These membranes were characterized by single gas permeation and for gas separations with a mixture of butanes and neo-pentane. Neo-pentane, iso-butane and n-butane could not be separated directly. However, neo-pentane was completely excluded from the pore system by C6 adsorption (hexane or cyclohexane) prior to the permeation test and an n-butane/iso-butane separation factor of 2.3 was obtained.

Deactivation kinetics of ferric molybdate catalysts

Abstract The deactivation of ferric molybdate catalysts was investigated in a differential reactor. Commercial catalysts were pretreated in air at 600 to 700 °C for between 1 and 2.5 h and characterized for catalytic activity in formaldehyde synthesis and for physical and chemical properties. A kinetic model for activity decay was developed for analysis. The results indicate that on heat treatment, the active catalytic component, ferric molybdate, decomposes to an inactive phase, ferrous molybdate.

Isomerization of cyclopropane on synthetic faujasite by pulse technique: I. Mathematical model

Abstract A mathematical model is derived to describe the transient behavior of a continuous stirred tank reactor containing catalyst particles with a bipore distribution when a stimulus is introduced. The reaction taking place inside the catalyst is assumed to be isothermal, first order, and irreversible. The expressions for the zeroth-, first-, and second-order moments of the response curve are given in terms of the adsorption equilibrium constant, the intercrystalline and intracrystalline diffusion coefficients, and the first-order reaction rate constant. A method for the determination of these parameters is presented. The same method also leads to the determination of micro- and macropore effectiveness factors.

Integration of Methane Steam Reforming and Water Gas Shift Reaction in a Pd/Au/Pd-Based Catalytic Membrane Reactor for Process Intensification.

Palladium-based catalytic membrane reactors (CMRs) effectively remove H2 to induce higher conversions in methane steam reforming (MSR) and water-gas-shift reactions (WGS). Within such a context, this work evaluates the technical performance of a novel CMR, which utilizes two catalysts in series, rather than one. In the process system under consideration, the first catalyst, confined within the shell side of the reactor, reforms methane with water yielding H2, CO and CO2. After reforming is completed, a second catalyst, positioned in series, reacts with CO and water through the WGS reaction yielding pure H2O, CO2 and H2. A tubular composite asymmetric Pd/Au/Pd membrane is situated throughout the reactor to continuously remove the produced H2 and induce higher methane and CO conversions while yielding ultrapure H2 and compressed CO2 ready for dehydration. Experimental results involving (i) a conventional packed bed reactor packed (PBR) for MSR, (ii) a PBR with five layers of two catalysts in series and (iii) a CMR with two layers of two catalysts in series are comparatively assessed and thoroughly characterized. Furthermore, a comprehensive 2D computational fluid dynamics (CFD) model was developed to explore further the features of the proposed configuration. The reaction was studied at different process intensification-relevant conditions, such as space velocities, temperatures, pressures and initial feed gas composition. Finally, it is demonstrated that the above CMR module, which was operated for 600 h, displays quite high H2 permeance and purity, high CH4 conversion levels and reduced CO yields.