PhD, Faculty of Science, University of the Witwatersrand

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Thesis (M.S.)--Missouri University of Science and Technology, 2008. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed June 9, 2009) Includes bibliographical references (p. 64-67).

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The goal of this work was to establish the suitable and limiting concentrations of Zn, Cu and Mn compounds during syngas fermentation. The results showed that cells encased in polyvinylidene difluoride (PVDF) membranes had a faster accumulation of methane in reactors containing fermentation medium dosed with 5 mg/L of each heavy metal compared to free cells. It was also revealed that total inhibition of biohydrogen production occurred in medium containing 5 mg/L Cu, 30 mg/L Zn and 140 mg/L Mn while the most suitable metal concentration level was 0.1 mg/L Cu, 0.6 mg/L and 2.8 mg/L Mn. In addition, a comparison test showed that for the most suitable metal concentration in the medium, rate of performance at pH 6 and 7 was higher than at pH 5.

Atmospheric transport and air-water exchange of organochlorine pesticides (OCPs) were investigated in temperate North America and the Arctic. OCPs studied were hexachlorocyclohexanes (HCHs, a-, b- and g-isomers), components of technical chlordane (trans- and cis-chlordane, trans-nonachlor), dieldrin, heptachlor exo-epoxide and toxaphene. Air and water samples were taken on cruises in the Great Lakes and Arctic to determine concentrations and gas exchange flux direction and magnitude. The Henry’s law constant, which describes the equilibrium distribution of a chemical between air and water, was determined for several OCPs as a function of temperature and used to assess the net direction of air-water exchange. Air samples were collected in Alabama to investigate southern U.S. sources of OCPs. Chemical markers (isomers, and enantiomers of chiral OCPs) were employed to infer sources and trace gas exchange. Elevated air concentrations of toxaphene and chlordanes were found in Alabama relative to the Great Lakes, indicating a southern U.S. source. Profiles of toxaphene compounds in air were similar to those in soil by being depleted in easily degraded species, suggesting that soil emissions control air concentrations. Gas exchange fluxes in the Great Lakes indicated near-equilibrium between air and water with excursions to net volatilization or deposition. Net volatilization of a-HCH from the Arctic Ocean was traced by evasion of non-racemic a-HCH into the atmosphere.

This work describes the application of porous metal supported Pd-membranes to the water-gas shift catalytic membrane reactor in the context of its potential application to the Integrated Gasification Combined Cycle (IGCC) process. The objective of this work was to develop a better understanding of Pd-membrane fabrication techniques, water-gas shift catalytic membrane reactor operation, and long-term behavior of the Pd-membranes under water-gas shift conditions. Thin (1.5 - 16 um) Pd-membranes were prepared by electroless deposition techniques on porous metal supports by previously developed methods. Pd-membranes were installed into stainless steel modules and utilized for mixed gas separation (H2/inert, H2/H2O, dry syngas, and wet syngas) at 350 - 450C and 14.5 atma to investigate boundary layer mass transfer resistance and surface inhibition. Pd-membranes were also installed into stainless steel modules with iron-chrome oxide catalyst and tested under water-gas shift conditions to investigate membrane reactor operation in the high pressure (5.0 - 14.6 atma) and high temperature (300 - 500C) regime. After the establishment of appropriate operating conditions, long-term testing was conducted to determine the membrane stability through He leak growth analysis and characterization by SEM and XRD. Pd and Pd/Au-alloy membranes were also investigated for their tolerance to 1 - 20 ppmv of H2S in syngas over extended periods at 400C and 14.0 atma. Water-gas shift catalytic membrane reactor operating parameters were investigated with a focus on high pressure conditions such that high H2 recovery was possible without a sweep gas. With regard to the feed composition, it was desirable to operate at a low H2O/CO ratio for higher H2 recovery, but restrained by the potential for coke formation on the membrane surface, which occurred at a H2O/CO ratio lower than 2.6 at 400C. The application of the Pd-membranes resulted in high CO conversion and H2 recovery for the high temperature (400 - 500C) water-gas shift reaction which then enabled high throughput. Operating at high temperature also resulted in higher membrane permeance and less Pd-surface inhibition by CO and H2O. The water-gas shift catalytic membrane reactor was capable of stable CO conversion and H2 recovery (96% and 88% respectively) at 400C over 900 hours of reaction testing, and 2,500 hours of overall testing of the Pd-membrane. When 2 ppmv H2S was introduced into the membrane reactor, a stable CO conversion of 96% and H2 recovery of 78% were observed over 230 hours. Furthermore, a Pd90Au10-membrane was effective for mixed gas separation with up to 20 ppmv H2S present, achieving a stable H2 flux of 7.8 m3/m2-h with a moderate H2 recovery of 44%. The long-term stability under high pressure reaction conditions represents a breakthrough in Pd-membrane utilization.

Atmospheric transport and air-water exchange of organochlorine pesticides (OCPs) were investigated in temperate North America and the Arctic. OCPs studied were hexachlorocyclohexanes (HCHs, a-, b- and g-isomers), components of technical chlordane (trans- and cis-chlordane, trans-nonachlor), dieldrin, heptachlor exo-epoxide and toxaphene. Air and water samples were taken on cruises in the Great Lakes and Arctic to determine concentrations and gas exchange flux direction and magnitude. The Henry’s law constant, which describes the equilibrium distribution of a chemical between air and water, was determined for several OCPs as a function of temperature and used to assess the net direction of air-water exchange. Air samples were collected in Alabama to investigate southern U.S. sources of OCPs. Chemical markers (isomers, and enantiomers of chiral OCPs) were employed to infer sources and trace gas exchange. Elevated air concentrations of toxaphene and chlordanes were found in Alabama relative to the Great Lakes, indicating a southern U.S. source. Profiles of toxaphene compounds in air were similar to those in soil by being depleted in easily degraded species, suggesting that soil emissions control air concentrations. Gas exchange fluxes in the Great Lakes indicated near-equilibrium between air and water with excursions to net volatilization or deposition. Net volatilization of a-HCH from the Arctic Ocean was traced by evasion of non-racemic a-HCH into the atmosphere.

Coal gasification is currently being developed as a cleaner alternative to conventional combustion technology. To optimise H2 production in this process, a water gas shift reaction is utilised to convert all CO with H2O to produce CO2 and H2. Typically industrial processes involve a two-step reaction system followed by a downstream H2 purification system, though attracting significant inefficiencies and high capital costs. Replacing a conventional unit process with a membrane reactor in this application is foreseen to provide major advantages: • Removing H2 from the reaction in-situ, a membrane reactor can minimise downstream processing and associated capital and operational costs. • Shift the reaction to higher conversions, improving efficiencies and reducing CO in the outlet. • Provide a purified H2 stream for use in PEM fuel cells, while concentrating the CO2 stream at high pressure for possible sequestration. If the concept of membrane reactor is to be adopted in coal gasification, important material improvements and operational challenges must be overcome before commercialisation can be realised. In addition, the water gas shift reaction has only recently gained interest for membrane reactors and is currently lacking comprehensive research on the effects of operating conditions on both the conversion and separation found within the unit. To this end, these are strong motivations of this work to contribute with knowledge in this field of research. This thesis examines the effects of operating conditions such as temperature, pressure, space velocity, sweep gas rate and feed water ratio on the performance of a water gas shift membrane reactor as compared with a conventional reactor. Novel cobalt silica molecular sieve membranes were used with conventional low temperature water gas shift reaction CuZnAl2O3 catalysts. Two type of membrane reactor configuration were investigated: a small flat template with catalyst on the feed side, and a scale up tube membrane with catalyst placed also in feed stream, the inner shell of the tube membrane. The cobalt silica membranes complied with activated transport, following a flux dependency gas permeation, where He and H2 permeance increased with temperature whilst N2, CO and CO2 showed the opposite effect. Best single gas selectivities were very high, with values of 4500 (He/N2) and 1100 (H2/CO2). In addition, the energy of activation for He and H2 was also very high, in excess of 9-10 kJ.mol-1, clearly indicating the high quality of the membranes employed in this study. It was found that the MR improved CO conversions for a range of space velocities as a function of temperature, which was attributed to both activate transport property of the membrane and increased conversion. Below equilibrium limits this provided an improved H2 production of 5 – 12% at 200-250oC as the removal of H2 through the membrane allowed enhanced conversion. With a set feed rate, the optimum advantage of the MR was seen at a water ratio of 1 as the lower equilibrium limits allowed greater potential for conversion enhancement. With increasing excess water this advantage decreased from 7% down to 0.5% at 300oC. The use of pressure and sweep rate was used to optimise the membranes permeation rate and selectivity. While pressure (or driving force) provided the highest potential for increasing permeation (or flow rate), temperature in tandem with pressure provided the greatest improvement in membrane selectivity, thus increasing H2 concentration from 95 – 99% in the permeate stream. Detailed study of permeate concentrations with changing conditions was undertaken to provide an understanding of the transport properties of silica membranes. It was observed that membrane selectivity and permeation decreased with the gas composition (ie Single>Binary>Ternary). Nevertheless, for separation of a ternary mixture at increased temperatures (250oC) the membrane could provide up to 99% purified H2 while reducing CO down to 700ppm. Competitive gas permeation regimes are an industrial reality which is seldom addressed in membranes for high temperature gas separation. The effect of gas mixtures on permeation and selectivity was attributed to several factors: chemical potential (or driving force) of the feed gas mixture, blockage of micropores by large molecules (CO2 and CO) which in turn affects the percolation of H2. As a result, gas separation was reduced for higher CO and CO2 feed concentrations, leading to a significant reduction in the H2 flow rate. Temperature played a vital role in this competitive process, as H2 diffusivity and CO, CO2 adsorption followed an inverse trend. Thus, increasing temperature led to higher H2 pore diffusivity, while decreasing the competitive effect of CO and CO2 adsorption. The use of cobalt modified silica to improve the hydrothermal stability of the membranes was investigated for use in the water gas shift reaction. It was found that the addition of cobalt stabilised the silica pore network, maintaining microporosity after exposure to steam. This is validated with long term stability testing in a water gas shift membrane reactor, where it was seen that the membrane could provide up to 95% H2 concentration in the permeate for over 200hrs of MR operation. This provided novel work, establishing the feasibility of these membranes for long term testing and operation in an industrial WGS MR.

The progress in fuel cell technology has resulted in an increased interest towards hydrogen fuel. Consequently, water gas shift reaction has found a renewed significance. Even though iron- and copper-based catalysts have been used for water gas shift reaction for decades, the catalysts are not strong enough to bring carbon monoxide concentration to a level tolerable for a fuel cell working at low temperatures. This study is focused on hydrogen production from water gas shift reaction using a nickel catalyst. Literature review revealed that nickel is one of the promising catalysts for water gas shift reaction. A thermodynamic analysis proved that exothermic water gas shift reaction is thermodynamically favorable at low temperatures but kinetically limited, and vice versa at higher temperatures. Initial experiments using 12 catalysts supported over monolith alumina revealed that nickel supported on ceria-promoted monolith alumina (Ni/CeO2-Al2O3) performed best, especially at 500oC. At this temperature and steam flowrates of 0.1-0.5 ml/min, the nickel catalyst had an activity of 94-99%, H2 yield of 55-61 vol.%, and H2 selectivity of 77-99%. A second set of experiments examined nine nickel based catalysts using different supports (mostly in powder form) which also demonstrated that Ni/CeO2-Al2O3 is the most promising catalyst for high temperature (450oC) water gas shift reaction. When nickel loading was varied from 1 to 8% (w/w), it was apparent that the catalyst performance increased with the nickel loading. Powder alumina resulted in better catalysis than monolith alumina. In this experiment, it was evident that the presence of minor amounts (1% (w/w) of the nickel loading) of a dopant material that included cobalt, chromium, molybdenum, or ruthenium affected the catalytic activity of the primary catalyst. The addition of cobalt or chromium resulted in positive effect on the performance of Ni/CeO2-Al2O3 catalyst. There was no appreciable effect due to the addition of ruthenium, and there was negative effect owing to the presence of molybdenum. Undoped, cobalt-doped, or chromium-doped Ni/CeO2-Al2O3 catalyst performed much better for water gas shift reaction at 450oC than that of a commercial (control) catalyst. A kinetic study revealed that the activation energy of water gas shift reaction over Ni/CeO2-Al2O3 was to be 104.5 kJ/mol.