Doctor of Philosophy / Department of Chemical Engineering / Peter H. Pfromm / Since its inception, the Haber-Bosch (HB) process for ammonia (NH3) synthesis has allowed for a significant increase in global food production as well as a simultaneous decrease in global hunger and malnutrition. The HB process is estimated to be responsible for the subsistence of 40% of the world population as approximately 85% of the over 182 metric tons of NH3 produced in 2017 was used as fertilizer for crop production. The natural gas consumed (mostly to generate H2) represents approximately 2% of the global energy budget, while the CO2 produced is about 2.5% of all global fossil CO2 emissions. Approximately 40% of food consumed is essentially natural gas transformed by the HB process into agricultural products. However global food production will need to double due to expected increase in world population to 9.6 billion by 2050 and rising demand for protein among developing nations. A novel thermochemical reaction cycle for sustainable NH3 synthesis at atmospheric pressure is explored herein. Both thermochemical and kinetic rationales are discussed regarding choice of Mn as the cycled reactant. The energetic driving force for these reactions is conceptually derived from concentrated solar energy. Mn was reacted with N2 forming Mn-nitride, corrosion of Mn-nitride with steam at 500 °C formed MnO and NH3, and lastly MnO was reduced at 1150 °C in a 4 vol % CH4 – 96 vol % N2 stream to Mn-nitride closing the cycle. Optimum nitridation at 800 °C and 120 min produced a Mn6N2.58-rich Mn-nitride mixture containing 8.7 ± 0.9 wt. % nitrogen. NH3 yield was limited to 0.04 after 120 min during nitride corrosion but addition of a NaOH promotor improved NH3 yield to 0.54. Mn6N2.58 yield was 0.381 ± 0.083 after MnO reduction for 30 min with CO and H2 but no CO2 detected in the product. Mn-nitridation kinetics were investigated at temperatures between 600 and 900 °C for 10 and 44 μm reactant powder particle sizes. That equilibrium conversion decreased with increasing temperature was confirmed. Jander’s rate law, which assumes gaseous reactant diffusion through a solid product layer, described the experimental data reasonably well. The rate constants and initial rates were as much as an order of magnitude greater for the 10 μm Mn reactant particle size. Additionally the activation energy was found to be 44.1 kJ mol-1 less for the 10 μm reactant particle size. Reducing the particle size had a small but positive effect on Mn-nitridation kinetics. Further reducing particle size will likely have a greater impact. A review of relevant classical thermodynamics is discussed with special attention paid to open systems. Confidence issues regarding over-reliance on x-ray diffraction are considered with options suggested for mitigation. Opportunities for future work are assessed.

Because of the high demand for hydrogen in the oil industries, new technologies for hydrogen production are being investigated. The thermochemical splitting cycle is one of them. Among the cycles that have been investigated, sulfur-iodine (S-I) water splitting is the most studied. In the S-I cycle, there are three reactions: H2SO4 decomposition, Bunsen reaction and HI decomposition. A new thermochemical cycle has been developed based on the S-I cycle, which is a H2S splitting cycle. In the H2S cycle, there are also three reactions. The only difference between S-I and H2S cycle is that the H2SO4 decomposition reaction is replaced by a reaction between hydrogen sulfide and sulfuric acid which produces sulfur dioxide, elemental sulfur and water. Research on this reaction has been done for many years, studying thermodynamic, kinetics and mass transfer. This reaction produces sulfur, sulfur dioxide and water. The SO2 produced is the used in the second reaction in the H2S cycle; the Bunsen reaction.<p> The main objective of this research was to find an operating condition to increase the production of SO2 from the reaction between H2S and H2SO4. This study investigated different conditions such as temperature, stirring rate and sulfuric acid concentration to maximize the production of SO2. The temperature and stirring rate range used in the reaction were from 120 to 160°C and from 0 to 400 rpm, respectively. The sulfuric acid concentrations were between 90 and 96 wt%. The results showed that increasing the temperature and the acid concentration in the reaction between H2S and H2SO4, the SO2 produced from this reaction will increase. There is no need to apply stirring in the reaction, because the stirring will increase the surface area which allows the produced sulfur dioxide in the gas phase to be dissolved more in sulfuric acid solution, which favors the unwanted side-reaction between SO2 and H2S. A model that was developed to predict the partial pressure change of SO2 in closed reactor. This model was used to compare the data between experimental and simulation through Matlab software. The simulated data was compared to the experimental data and the results indicated that the model fits the data satisfactorily. Additionally, study on the separation between the remaining sulfuric acid and produced elemental sulfur from the reaction between H2S and H2SO4 were performed. The mixture was placed in an oven at140°C of temperature for two hours. It was found that all small droplets of sulfur produced during the reaction between hydrogen sulfide and sulfuric acid agglomerated and the sulfuric acid solution became clearer.

Because of the high demand for hydrogen in the oil industries, new technologies for hydrogen production are being investigated. The thermochemical splitting cycle is one of them. Among the cycles that have been investigated, sulfur-iodine (S-I) water splitting is the most studied. In the S-I cycle, there are three reactions: H2SO4 decomposition, Bunsen reaction and HI decomposition. A new thermochemical cycle has been developed based on the S-I cycle, which is a H2S splitting cycle. In the H2S cycle, there are also three reactions. The only difference between S-I and H2S cycle is that the H2SO4 decomposition reaction is replaced by a reaction between hydrogen sulfide and sulfuric acid which produces sulfur dioxide, elemental sulfur and water. Research on this reaction has been done for many years, studying thermodynamic, kinetics and mass transfer. This reaction produces sulfur, sulfur dioxide and water. The SO2 produced is the used in the second reaction in the H2S cycle; the Bunsen reaction.<p> The main objective of this research was to find an operating condition to increase the production of SO2 from the reaction between H2S and H2SO4. This study investigated different conditions such as temperature, stirring rate and sulfuric acid concentration to maximize the production of SO2. The temperature and stirring rate range used in the reaction were from 120 to 160°C and from 0 to 400 rpm, respectively. The sulfuric acid concentrations were between 90 and 96 wt%. The results showed that increasing the temperature and the acid concentration in the reaction between H2S and H2SO4, the SO2 produced from this reaction will increase. There is no need to apply stirring in the reaction, because the stirring will increase the surface area which allows the produced sulfur dioxide in the gas phase to be dissolved more in sulfuric acid solution, which favors the unwanted side-reaction between SO2 and H2S. A model that was developed to predict the partial pressure change of SO2 in closed reactor. This model was used to compare the data between experimental and simulation through Matlab software. The simulated data was compared to the experimental data and the results indicated that the model fits the data satisfactorily. Additionally, study on the separation between the remaining sulfuric acid and produced elemental sulfur from the reaction between H2S and H2SO4 were performed. The mixture was placed in an oven at140°C of temperature for two hours. It was found that all small droplets of sulfur produced during the reaction between hydrogen sulfide and sulfuric acid agglomerated and the sulfuric acid solution became clearer.

The demand for energy is increasing throughout the world, and fossil fuel resources are diminishing. At the same time, the use of fossil fuels is slowly being reduced because it pollutes the environment. Research into alternative energy sources becomes necessary and important. An alternative fuel should not only replace fossil fuels but also address the environmental challenges posed by the use of fossil fuels. Hydrogen is an environmentally friendly substance considering that its product of combustion is water. Hydrogen is perceived to be a major contender to replace fossil fuels. Although hydrogen is not an energy source, it is an energy storage medium and a carrier which can be converted into electrical energy by an electrochemical process such as in fuel cell technology. Current hydrogen production methods, such as steam reforming, derive hydrogen from fossil fuels. As such, these methods still have a negative impact on the environment. Hydrogen can also be produced using thermochemical cycles which avoid the use of fossil fuels. The production of hydrogen through thermochemical cycles is expected to compete with the existing hydrogen production technologies. The sulphur iodine (SI) thermochemical cycle has been identified as a high-efficiency approach to produce hydrogen using either nuclear or solar power. A sound foundation is required to enable future construction and operation of thermochemical cycles. The foundation should consist of laboratory to pilot scale evaluation of the process. The activities involved are experimental verification of reactions, process modelling, conceptual design and pilot plant runs. Based on experimental and pilot plant data presented from previous research, this study presents the simulation of the sulphur iodine thermochemical cycle as applied to the South African context. A conceptual design is presented for the sulphur iodine thermochemical cycle with the aid of a process simulator. The low heating value (LHV) energy efficiency is 18% and an energy efficiency of 24% was achieved. The estimated hydrogen production cost was evaluated at $18/kg. / Thesis (M.Ing. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2012.

The global fossil reserves are dwindling and there is need to find alternative sources of energy. With global warming in mind, some of the most commonly considered suitable alternatives include solar, wind, nuclear, geothermal and hydro energy. A common challenge with use of most alternative energy sources is ensuring continuity of supply, which necessitates the use of energy storage. Hydrogen has properties that make it attractive as an energy carrier. To efficiently store energy from alternative sources in hydrogen, several methods of hydrogen production are under study. Several literature sources show thermochemical cycles as having high potential but requiring further development. Using literature sources, an initial screening of thermochemical cycles was done to select a candidate thermochemical cycle. The copper–chlorine thermochemical cycle was selected due to its relatively low peak operating temperature, which makes it flexible enough to be connected to different energy sources. Once the copper–chlorine cycle was identified, the three main copper–chlorine cycles were simulated in Aspen Plus to examine which is the best configuration. Using experimental data from literature and calculating optimal conditions, flowsheets were developed and simulated in Aspen Plus. The simulation results were then used to determine the configuration with the most favourable energy requirements, cycle efficiency, capital requirements and product cost. Simulation results show that the overall energy requirements increase as the number of steps decrease from five–steps to three–steps. Efficiencies calculated from simulation results show that the four and five–step cycles perform closely with 39% and 42%, respectively. The three–step cycle has a much lower efficiency, even though the theoretical calculations imply that the efficiency should also be close to that of the four and five–step cycles. The five–step reaction cycle has the highest capital requirements at US$370 million due to more equipment and the three–step cycle has the lowest requirement at US$ 275 million. Payback analysis and net present value analysis indicate that the hydrogen costs are highest for the three–step cycle at between US$3.53 per kg for a 5–10yr payback analysis and the five–step cycle US$2.98 per kg for the same payback period. / Thesis (M.Ing. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2012.

The demand for energy is increasing throughout the world, and fossil fuel resources are diminishing. At the same time, the use of fossil fuels is slowly being reduced because it pollutes the environment. Research into alternative energy sources becomes necessary and important. An alternative fuel should not only replace fossil fuels but also address the environmental challenges posed by the use of fossil fuels. Hydrogen is an environmentally friendly substance considering that its product of combustion is water. Hydrogen is perceived to be a major contender to replace fossil fuels. Although hydrogen is not an energy source, it is an energy storage medium and a carrier which can be converted into electrical energy by an electrochemical process such as in fuel cell technology. Current hydrogen production methods, such as steam reforming, derive hydrogen from fossil fuels. As such, these methods still have a negative impact on the environment. Hydrogen can also be produced using thermochemical cycles which avoid the use of fossil fuels. The production of hydrogen through thermochemical cycles is expected to compete with the existing hydrogen production technologies. The sulphur iodine (SI) thermochemical cycle has been identified as a high-efficiency approach to produce hydrogen using either nuclear or solar power. A sound foundation is required to enable future construction and operation of thermochemical cycles. The foundation should consist of laboratory to pilot scale evaluation of the process. The activities involved are experimental verification of reactions, process modelling, conceptual design and pilot plant runs. Based on experimental and pilot plant data presented from previous research, this study presents the simulation of the sulphur iodine thermochemical cycle as applied to the South African context. A conceptual design is presented for the sulphur iodine thermochemical cycle with the aid of a process simulator. The low heating value (LHV) energy efficiency is 18% and an energy efficiency of 24% was achieved. The estimated hydrogen production cost was evaluated at $18/kg. / Thesis (M.Ing. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2012.

The global fossil reserves are dwindling and there is need to find alternative sources of energy. With global warming in mind, some of the most commonly considered suitable alternatives include solar, wind, nuclear, geothermal and hydro energy. A common challenge with use of most alternative energy sources is ensuring continuity of supply, which necessitates the use of energy storage. Hydrogen has properties that make it attractive as an energy carrier. To efficiently store energy from alternative sources in hydrogen, several methods of hydrogen production are under study. Several literature sources show thermochemical cycles as having high potential but requiring further development. Using literature sources, an initial screening of thermochemical cycles was done to select a candidate thermochemical cycle. The copper–chlorine thermochemical cycle was selected due to its relatively low peak operating temperature, which makes it flexible enough to be connected to different energy sources. Once the copper–chlorine cycle was identified, the three main copper–chlorine cycles were simulated in Aspen Plus to examine which is the best configuration. Using experimental data from literature and calculating optimal conditions, flowsheets were developed and simulated in Aspen Plus. The simulation results were then used to determine the configuration with the most favourable energy requirements, cycle efficiency, capital requirements and product cost. Simulation results show that the overall energy requirements increase as the number of steps decrease from five–steps to three–steps. Efficiencies calculated from simulation results show that the four and five–step cycles perform closely with 39% and 42%, respectively. The three–step cycle has a much lower efficiency, even though the theoretical calculations imply that the efficiency should also be close to that of the four and five–step cycles. The five–step reaction cycle has the highest capital requirements at US$370 million due to more equipment and the three–step cycle has the lowest requirement at US$ 275 million. Payback analysis and net present value analysis indicate that the hydrogen costs are highest for the three–step cycle at between US$3.53 per kg for a 5–10yr payback analysis and the five–step cycle US$2.98 per kg for the same payback period. / Thesis (M.Ing. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2012.

Neste trabalho, foi estudado a oxidação do ferro com vapor d\'água em forno elétrico, para a produção de gás hidrogênio. Partindo-se da revisão bibliográfica, escolheu-se o ferro devido suas propriedades e por apresentar um bom rendimento, além disso o ferro é um material barato e abundante. Na estudo experimental foi três experimentos diferentes. No primeiro, o ferro foi oxidado em forno elétrico em temperaturas de 600 a 1000ºC, variando a cada 100ºC, e tempo fixado em 3 horas. Na segunda série de experimento, foi fixado a temperatura em 800ºC e variou a duração do processo de oxidação de 1 a 4 horas, com variação de 1 hora. E na terceira série de experimentos foi realizado a análise termogravimétrica para avaliação da cinética química do processo de oxidação. Os resultados dos experimentos indicaram a produção de gás hidrogênio em quantidades maiores em temperatura de 1000ºC. Além disso foi possível observar que a taxa de oxidação do ferro é maior durante a primeira hora de ensaio. A estimativa de hidrogênio produzido é de 0,9549 g/min -m2 em oxidação a 1000ºC. Já nos resultados da termogravimetria foi obtido a energia de ativação de 147 kJ/mol. / In this work was studied the oxidation of iron by steam in the electric furnace to produce hydrogen. The first step was the literature review and iron oxide was chose to be oxidized, due to its characteristics and good yield. Furthermore, the iron is a cheap and abundant in the earth. In the experimental studies was conducted three different experiments. The First one, the iron was oxidized in the electric furnace in the temperature range of 600 - 1000ºC with a variation of 100ºC and the oxidation time was fixed in 3 hours. The second experiment was conducted with fixed temperature of 800ºC and varied the oxidation time, the range of time was from 1 to 4 hours with a variation of 1 hour. The third experiment was the thermogravimetric analysis to study the chemical kinetics, with three different temperature, 600, 800 and 1000ºC. The result of studies showed that a high temperature the hydrogen production increased and decreased with low temperature. Furthermore, the high oxidation rate was observed in the first hour of the experiment. The hydrogen production was estimated in 0.9549 g/min - m2 at 1000ºC. Another result was the activation energy Ea= 147 kJ/mol.

Neste trabalho, foi estudado a oxidação do ferro com vapor d\'água em forno elétrico, para a produção de gás hidrogênio. Partindo-se da revisão bibliográfica, escolheu-se o ferro devido suas propriedades e por apresentar um bom rendimento, além disso o ferro é um material barato e abundante. Na estudo experimental foi três experimentos diferentes. No primeiro, o ferro foi oxidado em forno elétrico em temperaturas de 600 a 1000ºC, variando a cada 100ºC, e tempo fixado em 3 horas. Na segunda série de experimento, foi fixado a temperatura em 800ºC e variou a duração do processo de oxidação de 1 a 4 horas, com variação de 1 hora. E na terceira série de experimentos foi realizado a análise termogravimétrica para avaliação da cinética química do processo de oxidação. Os resultados dos experimentos indicaram a produção de gás hidrogênio em quantidades maiores em temperatura de 1000ºC. Além disso foi possível observar que a taxa de oxidação do ferro é maior durante a primeira hora de ensaio. A estimativa de hidrogênio produzido é de 0,9549 g/min -m2 em oxidação a 1000ºC. Já nos resultados da termogravimetria foi obtido a energia de ativação de 147 kJ/mol. / In this work was studied the oxidation of iron by steam in the electric furnace to produce hydrogen. The first step was the literature review and iron oxide was chose to be oxidized, due to its characteristics and good yield. Furthermore, the iron is a cheap and abundant in the earth. In the experimental studies was conducted three different experiments. The First one, the iron was oxidized in the electric furnace in the temperature range of 600 - 1000ºC with a variation of 100ºC and the oxidation time was fixed in 3 hours. The second experiment was conducted with fixed temperature of 800ºC and varied the oxidation time, the range of time was from 1 to 4 hours with a variation of 1 hour. The third experiment was the thermogravimetric analysis to study the chemical kinetics, with three different temperature, 600, 800 and 1000ºC. The result of studies showed that a high temperature the hydrogen production increased and decreased with low temperature. Furthermore, the high oxidation rate was observed in the first hour of the experiment. The hydrogen production was estimated in 0.9549 g/min - m2 at 1000ºC. Another result was the activation energy Ea= 147 kJ/mol.

The hydrogen economy is a possible component of an energy future based on use of alternative and renewable energy sources, deemed desirable from the general consensus of the worldwide community that we do not want to further exacerbate the climate problems that we have introduced over the last two centuries from burning fossil fuels. The burning of fossil fuels emits toxic pollutants into the air, such as sulfur compounds and oxidized forms of nitrogen (NOx) but also emit copious amounts of the inert carbon dioxide. The latter is widely recognized as the major cause of the global warming phenomenon. For a hydrogen economy to develop, efficient means of hydrogen generation are required. Thermochemical cycles were conceived in the 1960s but only one operating pilot plant and no commercial installations based on the processes have been built. In the present work the use of a membrane reactor to enable the newly conceived Sulfur-Sulfur cycle, based on equations 1 - 3 is modeled. / 4H₂O+4SO₂ -> H₂S + 3H₂SO₄ Eq. 1 / H₂SO₄ -> SO₂ + H₂O + 1/2O₂ Eq. 2 / H₂S + 2H₂O -> SO₂ + 3H₂ Eq. 3 / The rationale for the use of a membrane reactor to enable the cycle is based on enhancing extent of reaction beyond its predicted equilibrium point due to the severely unfavorable thermochemical parameters for the steam reforming of hydrogen sulfide reaction (Eq. 3 above) which has a low equilibrium concentration of products. The membrane reactor will employ a molybdenum sulfide catalyst driving the steam reformation of hydrogen sulfide reaction and simultaneous extraction of hydrogen (one of the products) will allowing for the reaction to occur to higher extent. A computational model of a catalytic membrane reactor was constructed using the well-known finite element model package Comsol v4.1 in which a catalytic microchannel reactor separated from a sweep gas by a thin hydrogen permeable membrane is built and parametric sweeps to evaluate the effect of membrane transport parameters, pressure and gas feed velocities are calculated. Though the steam reforming of hydrogen sulfide reaction has a competing thermal cracking reaction, the present work focuses on modeling one reaction only (the steam reformation reaction) for simplicity. Fully dense metallic membranes with chemselective permeability to hydrogen are modeled with transport parameters derived from reported literature values for similar applications. The results show that employing a membrane reactor does significantly affect the completeness of the reaction by product extraction (if you do run the model with membrane transport set to zero, compare the extent at zero with extent at 3.6x10⁻⁶ mol.s⁻¹.m⁻²). The effect of changing sweep gas velocity is contingent on membrane properties, and membranes with small diffusion coefficients severely limit the ability of extraction of hydrogen from the feed. Therefore, it is more important that membranes with very high hydrogen permeability be employed in designing a reactor to implement this process, allowing for effective hydrogen separation and high conversion of the reactants in the process. Reactor pressure has minimal effect on the extent of reaction and therefore reactors designed to implement the process may be designed to operate at close to ambient pressure. / Graduation date: 2012