Continuous flow synthesis of silicon compounds as feedstock for solar-grade silicon production
- Authors: Chigondo, Fidelis
- Date: 2016
- Subjects: Silicon -- Synthesis , Homogeneous catalysis
- Language: English
- Type: Thesis , Doctoral , DTech
- Identifier: http://hdl.handle.net/10948/4529 , vital:20613
- Description: This thesis describes the key steps in the production of high purity (solar-grade) silicon from metallurgical-grade silicon for use in the production of photovoltaic cells as alternative renewable, environmentally benign and cheap energy source. The initial part of the project involves the development and optimization of a small chemical production platform system capable of producing alkoxysilanes from metallurgical-grade silicon as green precursors to solar-grade silicon production. Specifically, the main aim of the study was to synthesize trialkoxysilanes in continuous flow mode, although the synthesis on monosilane was also done in batch mode. The alkoxylation reaction was carried out in a traditional slurry phase batch reactor, packed bed flow tubular reactor and also attempted in a continuous flow falling film tubular reactor. The effect of key parameters which affect the silicon conversion and selectivity for the desired trialkoxysilane were investigated and optimized using ethanol as a reagent model. The synthesis was then extended to the other alcohols namely methanol, n-propanol and n-butanol. Copper catalysts which were tested in the alkoxylation reaction included: CuCl, Cu(OH)2, CuO and CuSO4. CuCl and Cu(OH)2 showed comparable activity in the batch mode but the former was more efficient in the packed bed flow tubular reactor. Cu(OH)2 could be used as a non-halide catalyst but its activity is limited to short reaction cycles (<10 h). The uncatalysed reaction resulted in negligible reaction rates in both types of reactors. High temperature catalyst pre-heating (>500 oC) resulted in a lower rate of reaction and selectivity than when slightly lower temperatures are used (<350 oC) in both reactors, although much difference was noticed in the packed bed flow tubular reactor. Synthesis in the batch reactor needed longer silicon-catalyst activation time, higher pre-heating temperature and higher catalyst amounts as compare to the packed bed flow tubular reactor. Reaction temperature and alcohol flow rate influenced the reaction in both methods. The optimum reaction temperature range and alcohol flow rate was comparable in both reactors (230 to 240 oC) and 0.1mL/min respectively. The effect of alcohol R-group (C1 to C4) on the reaction revealed that conversion and selectivity generally decrease with an increase in carbon chain length in both methods. Ethanol showed highest selectivity (>95% in batch and >97% in flow) and conversion (about 88% in batch and about 64% in flow) as compared to all other alcohols studied showing that it could be the most efficient alkoxylation alcohol for this reaction. Overally, the packed bed flow tubular reactor resulted in higher selectivity to trialkoxysilanes than the batch system. Performing the reaction under pressure resulted in increased conversion but selectivity to the desire trialkoxysilane diminished. Synthesis in a continuous flow falling film tubular reactor was not successful as it resulted in very poor conversion and selectivity. Monosilane was successfully synthesized from the disproportionation of triethoxysilane using homogeneous and heterogeneous catalysts in batch mode. The results obtained from homogeneous catalysis showed that the reaction can be conducted at room temperature. The heterogeneous catalysis method resulted in slow conversion at room temperature but mild heating up to 55 oC greatly improved the reaction. Conducting the reaction under neat conditions produced comparable results to reactions which were carried out using solvents. The disproportionation reaction was best described by the first order kinetic model. The results obtained in this research indicate that the packed bed flow tubular reactor can be utilized with future modifications for continuous flow synthesis of alkoxysilanes as feedstock for the solar-grade silicon production.
- Full Text:
- Date Issued: 2016
- Authors: Chigondo, Fidelis
- Date: 2016
- Subjects: Silicon -- Synthesis , Homogeneous catalysis
- Language: English
- Type: Thesis , Doctoral , DTech
- Identifier: http://hdl.handle.net/10948/4529 , vital:20613
- Description: This thesis describes the key steps in the production of high purity (solar-grade) silicon from metallurgical-grade silicon for use in the production of photovoltaic cells as alternative renewable, environmentally benign and cheap energy source. The initial part of the project involves the development and optimization of a small chemical production platform system capable of producing alkoxysilanes from metallurgical-grade silicon as green precursors to solar-grade silicon production. Specifically, the main aim of the study was to synthesize trialkoxysilanes in continuous flow mode, although the synthesis on monosilane was also done in batch mode. The alkoxylation reaction was carried out in a traditional slurry phase batch reactor, packed bed flow tubular reactor and also attempted in a continuous flow falling film tubular reactor. The effect of key parameters which affect the silicon conversion and selectivity for the desired trialkoxysilane were investigated and optimized using ethanol as a reagent model. The synthesis was then extended to the other alcohols namely methanol, n-propanol and n-butanol. Copper catalysts which were tested in the alkoxylation reaction included: CuCl, Cu(OH)2, CuO and CuSO4. CuCl and Cu(OH)2 showed comparable activity in the batch mode but the former was more efficient in the packed bed flow tubular reactor. Cu(OH)2 could be used as a non-halide catalyst but its activity is limited to short reaction cycles (<10 h). The uncatalysed reaction resulted in negligible reaction rates in both types of reactors. High temperature catalyst pre-heating (>500 oC) resulted in a lower rate of reaction and selectivity than when slightly lower temperatures are used (<350 oC) in both reactors, although much difference was noticed in the packed bed flow tubular reactor. Synthesis in the batch reactor needed longer silicon-catalyst activation time, higher pre-heating temperature and higher catalyst amounts as compare to the packed bed flow tubular reactor. Reaction temperature and alcohol flow rate influenced the reaction in both methods. The optimum reaction temperature range and alcohol flow rate was comparable in both reactors (230 to 240 oC) and 0.1mL/min respectively. The effect of alcohol R-group (C1 to C4) on the reaction revealed that conversion and selectivity generally decrease with an increase in carbon chain length in both methods. Ethanol showed highest selectivity (>95% in batch and >97% in flow) and conversion (about 88% in batch and about 64% in flow) as compared to all other alcohols studied showing that it could be the most efficient alkoxylation alcohol for this reaction. Overally, the packed bed flow tubular reactor resulted in higher selectivity to trialkoxysilanes than the batch system. Performing the reaction under pressure resulted in increased conversion but selectivity to the desire trialkoxysilane diminished. Synthesis in a continuous flow falling film tubular reactor was not successful as it resulted in very poor conversion and selectivity. Monosilane was successfully synthesized from the disproportionation of triethoxysilane using homogeneous and heterogeneous catalysts in batch mode. The results obtained from homogeneous catalysis showed that the reaction can be conducted at room temperature. The heterogeneous catalysis method resulted in slow conversion at room temperature but mild heating up to 55 oC greatly improved the reaction. Conducting the reaction under neat conditions produced comparable results to reactions which were carried out using solvents. The disproportionation reaction was best described by the first order kinetic model. The results obtained in this research indicate that the packed bed flow tubular reactor can be utilized with future modifications for continuous flow synthesis of alkoxysilanes as feedstock for the solar-grade silicon production.
- Full Text:
- Date Issued: 2016
Customer satisfaction with the guesthouse experience in Ghana
- Authors: Amoah, Felix
- Date: 2016
- Subjects: Consumer satisfaction -- Ghana , Hospitality industry -- Ghana -- Management , Tourism -- Ghana -- Marketing
- Language: English
- Type: Thesis , Doctoral , DTech
- Identifier: http://hdl.handle.net/10948/6073 , vital:21036
- Description: Guesthouse accommodation plays an important role in Ghana’s hospitality industry and was therefore the focus of this study as little research dealing with guesthouses in Ghana could be found. The number of guesthouses in Ghana continues to grow and they thus represent an important alternative to hotels. However, these establishments seem to perform poorly and face several challenges such as lack of managerial knowledge, insufficiently skilled employees, poor interaction with customers, and criticisms of the provision of low quality service. The main reason for undertaking this research was to provide guesthouse managers in Ghana with information that might assist them in making decisions about the experience they offer. Such knowledge could make this type of accommodation more competitive and eventually help the hospitality sector in Ghana attract tourists and grow. It is imperative that guesthouses focus on the experience they offer, because contemporary tourism and hospitality literature suggest that successful businesses require a shift from functional and financial interests to a more profound focus on total experiences which embody emotional aspects. It is thus argued that, if guesthouse managers in Ghana do not know how their guests perceive their guesthouse experience, they might make costly mistakes and allocate resources to aspects that already provide quality and value in a functional sense, instead of those elements perceived as providing quality and value in the total experience. Two dominating concepts, namely experience quality and experience value, formed the basis of the examination of guests’ experience with the guesthouses in Ghana. In addition, the relationships between the experience and satisfaction were also investigated. Four experience quality dimensions, namely hedonics, peace of mind, involvement, and recognition, and seven value dimensions were examined. The experience value dimensions included atmospherics, enjoyment, entertainment, efficiency, excellence, escape, and economic value. A proportional stratified random sampling procedure was followed to select the guesthouses for the study. Thereafter, a structured questionnaire was distributed to the guesthouse guests selected by means of convenience sampling. Five hundred and forty one useable questionnaires were received. The guesthouse guests formed the primary sampling unit for this study. The results of the empirical study showed a strong positive correlation between all the dimensions of experience quality and experience value, while the factor analysis confirmed that all these dimensions loaded on a single factor. Therefore, experience quality and experience value cannot be separated, and the resulting single multi-dimensional factor was subsequently renamed, overall experience. The results also indicated a positive relationship between hedonics, peace of mind, involvement, recognition, atmospherics, enjoyment, entertainment, efficiency, excellence, escape, economic value, and overall satisfaction. The inferentially established rank-order of the dimensions contributing to satisfaction can guide managers when allocating resources. Overall, atmospherics was ranked first, followed by economic value. Escape was the lowest ranked dimension. In addition, it was found that, except for escape, all the experience dimensions were perceived as basic satisfiers. This implies that guests will be dissatisfied when provision of these dimensions is inadequate. With regard to escape, listed as a performance factor, guests will be satisfied when performance is improved and dissatisfied when performance is low. A positive relationship was also found between overall experience and overall satisfaction. The latter include the likelihood of return and recommending the guesthouse to others. Finally, structural equation modelling confirmed a model representing the 11 experience dimensions (hedonics, peace of mind, involvement, recognition, atmospherics, enjoyment, entertainment, escape, efficiency, excellence, and economic value) and the relationships between overall experience and overall satisfaction pertaining to guesthouses in Ghana.
- Full Text:
- Date Issued: 2016
- Authors: Amoah, Felix
- Date: 2016
- Subjects: Consumer satisfaction -- Ghana , Hospitality industry -- Ghana -- Management , Tourism -- Ghana -- Marketing
- Language: English
- Type: Thesis , Doctoral , DTech
- Identifier: http://hdl.handle.net/10948/6073 , vital:21036
- Description: Guesthouse accommodation plays an important role in Ghana’s hospitality industry and was therefore the focus of this study as little research dealing with guesthouses in Ghana could be found. The number of guesthouses in Ghana continues to grow and they thus represent an important alternative to hotels. However, these establishments seem to perform poorly and face several challenges such as lack of managerial knowledge, insufficiently skilled employees, poor interaction with customers, and criticisms of the provision of low quality service. The main reason for undertaking this research was to provide guesthouse managers in Ghana with information that might assist them in making decisions about the experience they offer. Such knowledge could make this type of accommodation more competitive and eventually help the hospitality sector in Ghana attract tourists and grow. It is imperative that guesthouses focus on the experience they offer, because contemporary tourism and hospitality literature suggest that successful businesses require a shift from functional and financial interests to a more profound focus on total experiences which embody emotional aspects. It is thus argued that, if guesthouse managers in Ghana do not know how their guests perceive their guesthouse experience, they might make costly mistakes and allocate resources to aspects that already provide quality and value in a functional sense, instead of those elements perceived as providing quality and value in the total experience. Two dominating concepts, namely experience quality and experience value, formed the basis of the examination of guests’ experience with the guesthouses in Ghana. In addition, the relationships between the experience and satisfaction were also investigated. Four experience quality dimensions, namely hedonics, peace of mind, involvement, and recognition, and seven value dimensions were examined. The experience value dimensions included atmospherics, enjoyment, entertainment, efficiency, excellence, escape, and economic value. A proportional stratified random sampling procedure was followed to select the guesthouses for the study. Thereafter, a structured questionnaire was distributed to the guesthouse guests selected by means of convenience sampling. Five hundred and forty one useable questionnaires were received. The guesthouse guests formed the primary sampling unit for this study. The results of the empirical study showed a strong positive correlation between all the dimensions of experience quality and experience value, while the factor analysis confirmed that all these dimensions loaded on a single factor. Therefore, experience quality and experience value cannot be separated, and the resulting single multi-dimensional factor was subsequently renamed, overall experience. The results also indicated a positive relationship between hedonics, peace of mind, involvement, recognition, atmospherics, enjoyment, entertainment, efficiency, excellence, escape, economic value, and overall satisfaction. The inferentially established rank-order of the dimensions contributing to satisfaction can guide managers when allocating resources. Overall, atmospherics was ranked first, followed by economic value. Escape was the lowest ranked dimension. In addition, it was found that, except for escape, all the experience dimensions were perceived as basic satisfiers. This implies that guests will be dissatisfied when provision of these dimensions is inadequate. With regard to escape, listed as a performance factor, guests will be satisfied when performance is improved and dissatisfied when performance is low. A positive relationship was also found between overall experience and overall satisfaction. The latter include the likelihood of return and recommending the guesthouse to others. Finally, structural equation modelling confirmed a model representing the 11 experience dimensions (hedonics, peace of mind, involvement, recognition, atmospherics, enjoyment, entertainment, escape, efficiency, excellence, and economic value) and the relationships between overall experience and overall satisfaction pertaining to guesthouses in Ghana.
- Full Text:
- Date Issued: 2016
The development and evaluation of a new manufacturing process for β-sitosterol-D glucoside
- Mtyopo, Mthetheleli Bethwell
- Authors: Mtyopo, Mthetheleli Bethwell
- Date: 2016
- Subjects: Pharmaceutical chemistry Chemistry, Organic
- Language: English
- Type: Thesis , Doctoral , DTech
- Identifier: http://hdl.handle.net/10948/45920 , vital:39320
- Description: The existing production sequence of β-sitosterol-D-glucoside, a glucoside used in an “over-the-counter” (OTC) preparation under the brand name of Moducare® comprises of three process steps with an overall yield of less than 20%. The low yield is partly due to the instability of intermediates at reaction temperatures > 0oC, and partly due to the thermodynamic equilibrium between two stereoisomers. An economically alternative process was developed, evaluated and scaled-up in a 2l reactor. The project was initiated with a specific limitation in terms of the starting material that comprised a mixture of plant sterols, which necessitated a study of the isolation and purification of the desired product from a rather complex reaction mixture. The use of silver as halide acceptor for the Koenigs-Knorr synthesis did not give statistically significant different results from the same approach but using cadmium as halide acceptor instead. However, using the direct O-glucosylation approach not only gave statistically significant higher results, but also resulted in a much more convenient procedure. Under optimum conditions, a yield of approximately 83% (isolated) of 2,3,4,6- tetra-О-acetyl-β-sitosterol-D-glucoside could be achieved, which was substantially higher than that achieved with the traditional Koenigs-Knorr methodology and above reported yields in the literature (60-80%) for direct glycosylation. Separation of 2,3,4,6-tetra-О-acetyl-β-sitosterol-D-glucoside (BSSGT) from a reaction mixture that contains 2,3,4,6-tetra-О-acetyl-campesterol-D-glucoside (CSGT), 2,3,4,6-tetra-О-acetyl campestanol-glucoside (CSSGT), and 2,3,4,6-tetra-О-acetyl-sitostanol-Dglucoside (SSGT) was investigated using column chromatography. When using silica gel particles, very good separation efficiency and product recovery could be achieved using hexane/ethyl hexane as eluent. The isolated 2,3,4,6-tetra-О-acetyl-β-sitosterol-Dglucoside was easily hydrolysed to β-sitosterol-D-glucoside in high yields (79%) using methanolic KOH. The process for the production of β-sitosterol-D-glucoside was scaled-up from the laboratory bench scale (250 cm3) to a laboratory scale of 2 l using the direct Oglycosylation method. The overall yields of the scaled reaction for β-sitosterol-D-glucoside was slightly above the literature reported values (59%, 8/92) for the KnoenigsKnorr synthesis and compares well above (62%, 0/100) the current production process (less than 20% yields). When using catalogue prices, the material costs (without recycling) for the direct Oglucosylation route is approximately 57% less for the synthesis of 1kg of β-sitosterol-Dglucoside compared to the Koenigs-Knorr route. Given further savings for recycling, the direct O-glucosylation route provides an attractive alternative route for the synthesis ofthe target compound.
- Full Text:
- Date Issued: 2016
- Authors: Mtyopo, Mthetheleli Bethwell
- Date: 2016
- Subjects: Pharmaceutical chemistry Chemistry, Organic
- Language: English
- Type: Thesis , Doctoral , DTech
- Identifier: http://hdl.handle.net/10948/45920 , vital:39320
- Description: The existing production sequence of β-sitosterol-D-glucoside, a glucoside used in an “over-the-counter” (OTC) preparation under the brand name of Moducare® comprises of three process steps with an overall yield of less than 20%. The low yield is partly due to the instability of intermediates at reaction temperatures > 0oC, and partly due to the thermodynamic equilibrium between two stereoisomers. An economically alternative process was developed, evaluated and scaled-up in a 2l reactor. The project was initiated with a specific limitation in terms of the starting material that comprised a mixture of plant sterols, which necessitated a study of the isolation and purification of the desired product from a rather complex reaction mixture. The use of silver as halide acceptor for the Koenigs-Knorr synthesis did not give statistically significant different results from the same approach but using cadmium as halide acceptor instead. However, using the direct O-glucosylation approach not only gave statistically significant higher results, but also resulted in a much more convenient procedure. Under optimum conditions, a yield of approximately 83% (isolated) of 2,3,4,6- tetra-О-acetyl-β-sitosterol-D-glucoside could be achieved, which was substantially higher than that achieved with the traditional Koenigs-Knorr methodology and above reported yields in the literature (60-80%) for direct glycosylation. Separation of 2,3,4,6-tetra-О-acetyl-β-sitosterol-D-glucoside (BSSGT) from a reaction mixture that contains 2,3,4,6-tetra-О-acetyl-campesterol-D-glucoside (CSGT), 2,3,4,6-tetra-О-acetyl campestanol-glucoside (CSSGT), and 2,3,4,6-tetra-О-acetyl-sitostanol-Dglucoside (SSGT) was investigated using column chromatography. When using silica gel particles, very good separation efficiency and product recovery could be achieved using hexane/ethyl hexane as eluent. The isolated 2,3,4,6-tetra-О-acetyl-β-sitosterol-Dglucoside was easily hydrolysed to β-sitosterol-D-glucoside in high yields (79%) using methanolic KOH. The process for the production of β-sitosterol-D-glucoside was scaled-up from the laboratory bench scale (250 cm3) to a laboratory scale of 2 l using the direct Oglycosylation method. The overall yields of the scaled reaction for β-sitosterol-D-glucoside was slightly above the literature reported values (59%, 8/92) for the KnoenigsKnorr synthesis and compares well above (62%, 0/100) the current production process (less than 20% yields). When using catalogue prices, the material costs (without recycling) for the direct Oglucosylation route is approximately 57% less for the synthesis of 1kg of β-sitosterol-Dglucoside compared to the Koenigs-Knorr route. Given further savings for recycling, the direct O-glucosylation route provides an attractive alternative route for the synthesis ofthe target compound.
- Full Text:
- Date Issued: 2016
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