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
A model for supporting environmental awereness in higher education using social media
- Authors: Tlebere, Thabo Eugene
- Date: 2013
- Subjects: Social media , Environmental education , Universities and colleges
- Language: English
- Type: Thesis , Masters , MCom
- Identifier: vital:10496 , http://hdl.handle.net/10948/d1020820
- Description: University sustainability is a field of research that has been gaining increased interest in recent years. The reduction of environmental impact has become a strategic objective of universities globally. Universities have been prompted to take necessary action to ensure that their environmental impact is at a minimum. The environmental component of sustainability deals with the current conservation of the earth’s natural resources so that future generations can also have access to them. Human beings, due to their increasing needs, are accountable for the exploitation of natural resources. They are regarded as the main contributors to imbalances in the natural systems. Environmental concerns such as global warming, deforestations, disposal of wastes, and ozone reduction are the outcomes of the damage caused by humans on the environment. The aim of environmental education is to acquire remediation of the environment by making individuals aware of the environment and by educating them about how to live a more sustainable lifestyle. Environmental awareness is perceived as knowledge of the factors that affect the environment and having sensitivity towards the environment. Higher Education Institutions (HEIs) bear the responsibility of educating individuals about environmental issues since they provide education to future leaders in society who may have an influence on future conditions in the environment. Social media are capable of delivering information to a large spectrum of audiences at a low cost. The Pew Internet American Life Project reported that the number of adults who utilise social media has increased by 57 percent from 2005 to 2011. Several environmental activist organisations utilise social media to carry out environmental awareness campaigns. In this study two environmental awareness campaigns which were powered by social media were conducted to improve environmental awareness of individuals in a higher education environment. A Social media Model for ENvironmental Awareness (SMENA) was developed to facilitate the environmental awareness campaigns. The SMENA includes a website, social media as well as theoretical guidelines for creating environmental awareness campaigns, and for using social media for environmental awareness campaigns. A case study at the Nelson Mandela Metropolitan University (NMMU) was used to empirically evaluate SMENA. Students at the Department of Computer Sciences of NMMU were exposed to information about environmental issues through social media with the intention of improving their environmental knowledge and awareness. The SMENA website usability was rated positively and students enjoyed the blogs and information distributed by means of social media. The results of the study intervention were positive and showed that social media can be used to improve the environmental knowledge of students. This study provides a valuable contribution to both the field of environmental education and social media usage and acceptance. The guidelines and requirements for using social media to improve environmental awareness provided in this study can be used to assist educators and university management with addressing the problems of reducing environmental impact.
- Full Text:
- Date Issued: 2013
- Authors: Tlebere, Thabo Eugene
- Date: 2013
- Subjects: Social media , Environmental education , Universities and colleges
- Language: English
- Type: Thesis , Masters , MCom
- Identifier: vital:10496 , http://hdl.handle.net/10948/d1020820
- Description: University sustainability is a field of research that has been gaining increased interest in recent years. The reduction of environmental impact has become a strategic objective of universities globally. Universities have been prompted to take necessary action to ensure that their environmental impact is at a minimum. The environmental component of sustainability deals with the current conservation of the earth’s natural resources so that future generations can also have access to them. Human beings, due to their increasing needs, are accountable for the exploitation of natural resources. They are regarded as the main contributors to imbalances in the natural systems. Environmental concerns such as global warming, deforestations, disposal of wastes, and ozone reduction are the outcomes of the damage caused by humans on the environment. The aim of environmental education is to acquire remediation of the environment by making individuals aware of the environment and by educating them about how to live a more sustainable lifestyle. Environmental awareness is perceived as knowledge of the factors that affect the environment and having sensitivity towards the environment. Higher Education Institutions (HEIs) bear the responsibility of educating individuals about environmental issues since they provide education to future leaders in society who may have an influence on future conditions in the environment. Social media are capable of delivering information to a large spectrum of audiences at a low cost. The Pew Internet American Life Project reported that the number of adults who utilise social media has increased by 57 percent from 2005 to 2011. Several environmental activist organisations utilise social media to carry out environmental awareness campaigns. In this study two environmental awareness campaigns which were powered by social media were conducted to improve environmental awareness of individuals in a higher education environment. A Social media Model for ENvironmental Awareness (SMENA) was developed to facilitate the environmental awareness campaigns. The SMENA includes a website, social media as well as theoretical guidelines for creating environmental awareness campaigns, and for using social media for environmental awareness campaigns. A case study at the Nelson Mandela Metropolitan University (NMMU) was used to empirically evaluate SMENA. Students at the Department of Computer Sciences of NMMU were exposed to information about environmental issues through social media with the intention of improving their environmental knowledge and awareness. The SMENA website usability was rated positively and students enjoyed the blogs and information distributed by means of social media. The results of the study intervention were positive and showed that social media can be used to improve the environmental knowledge of students. This study provides a valuable contribution to both the field of environmental education and social media usage and acceptance. The guidelines and requirements for using social media to improve environmental awareness provided in this study can be used to assist educators and university management with addressing the problems of reducing environmental impact.
- Full Text:
- Date Issued: 2013
A longitudinal study of the occupational aspirations and perceptions of twelve to fourteen year-old South African children
- Authors: Hunter, Carrie
- Date: 2009
- Subjects: Vocational interests -- Research -- South Africa , Children -- South Africa -- Attitudes , Personality and occupation
- Language: English
- Type: Thesis , Masters , MA
- Identifier: vital:9906 , http://hdl.handle.net/10948/968 , Vocational interests -- Research -- South Africa , Children -- South Africa -- Attitudes , Personality and occupation
- Description: While career development has been acknowledged as a lifelong process, little research attention has been given to the developmental stage of childhood. In addition, there has been a lack of longitudinal research into career development despite consistent calls for research of this nature. The lack of research and theory pertaining to childhood career development prompted the initiation of the current longitudinal project of which the present study forms a part. The longitudinal project was designed to provide information about the occupational aspirations and perceptions of a cohort of South African children from six years of age onwards, with the present studying examining the occupational aspiration development of 12 to 14 year old South African children. South African career research to date has focused primarily on adolescent and adult career development. The present longitudinal study aimed to explore and describe possible changes over a three year period in the interest typology, occupational status level and occupational gender stereotypes of the 44 participants. In addition, the study aimed to explore and describe the sample’s reflections on their own occupational aspiration development. The results of this study will provide much needed baseline information on the development of South African children’s occupational aspirations and will provide useful recommendations for the development of career education programmes. The present study was contextualised within both child and career development theories. This study was quantitative in nature as it made use of semi-structured interviews and biographical questionnaires to record verbal data which was transposed into nominal data for analysis. The semi-structured interview consisted of four broad areas that included the participants’ occupational aspirations, how many occupations they knew about, how much they knew about the identified occupations xiii and the extent to which they held gender stereotypes regarding fourteen different occupations. The data captured was coded according to Holland’s (1985) classification system of interest types. The occupational aspirations identified by participants were also coded according to their status levels. The coded data was then analysed using descriptive statistics in the form of frequency counts and percentages. In addition, content analysis was performed on the participants’ reflections on their own career development in order to elicit themes. Results from the study supported child and career development theories, emphasising childhood as a critical phase of career development. The study found that the majority of children aspired to Social type occupations across all three years of the study and most children consistently aspired to high status occupations. Furthermore, occupational gender stereotyping decreased over time. Lastly, it was found that most children were able to reflect on their career development with the majority attributing changes in their occupational aspirations to changes in their interests. The present study has provided valuable insight into the occupational aspiration development of a group of South African children, which can be used as a foundation on which to base further research and on which to develop career education programmes.
- Full Text:
- Date Issued: 2009
- Authors: Hunter, Carrie
- Date: 2009
- Subjects: Vocational interests -- Research -- South Africa , Children -- South Africa -- Attitudes , Personality and occupation
- Language: English
- Type: Thesis , Masters , MA
- Identifier: vital:9906 , http://hdl.handle.net/10948/968 , Vocational interests -- Research -- South Africa , Children -- South Africa -- Attitudes , Personality and occupation
- Description: While career development has been acknowledged as a lifelong process, little research attention has been given to the developmental stage of childhood. In addition, there has been a lack of longitudinal research into career development despite consistent calls for research of this nature. The lack of research and theory pertaining to childhood career development prompted the initiation of the current longitudinal project of which the present study forms a part. The longitudinal project was designed to provide information about the occupational aspirations and perceptions of a cohort of South African children from six years of age onwards, with the present studying examining the occupational aspiration development of 12 to 14 year old South African children. South African career research to date has focused primarily on adolescent and adult career development. The present longitudinal study aimed to explore and describe possible changes over a three year period in the interest typology, occupational status level and occupational gender stereotypes of the 44 participants. In addition, the study aimed to explore and describe the sample’s reflections on their own occupational aspiration development. The results of this study will provide much needed baseline information on the development of South African children’s occupational aspirations and will provide useful recommendations for the development of career education programmes. The present study was contextualised within both child and career development theories. This study was quantitative in nature as it made use of semi-structured interviews and biographical questionnaires to record verbal data which was transposed into nominal data for analysis. The semi-structured interview consisted of four broad areas that included the participants’ occupational aspirations, how many occupations they knew about, how much they knew about the identified occupations xiii and the extent to which they held gender stereotypes regarding fourteen different occupations. The data captured was coded according to Holland’s (1985) classification system of interest types. The occupational aspirations identified by participants were also coded according to their status levels. The coded data was then analysed using descriptive statistics in the form of frequency counts and percentages. In addition, content analysis was performed on the participants’ reflections on their own career development in order to elicit themes. Results from the study supported child and career development theories, emphasising childhood as a critical phase of career development. The study found that the majority of children aspired to Social type occupations across all three years of the study and most children consistently aspired to high status occupations. Furthermore, occupational gender stereotyping decreased over time. Lastly, it was found that most children were able to reflect on their career development with the majority attributing changes in their occupational aspirations to changes in their interests. The present study has provided valuable insight into the occupational aspiration development of a group of South African children, which can be used as a foundation on which to base further research and on which to develop career education programmes.
- Full Text:
- Date Issued: 2009
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