Paul Maina - Academia.edu (original) (raw)
Papers by Paul Maina
Lime reactivity was improved by blending it with either fly ash, bottom ash or waste activated sl... more Lime reactivity was improved by blending it with either fly ash, bottom ash or waste activated sludge (W.A.S). The reactivity was tested using a pH-stat apparatus which simulates wet flue gas desulfurization (FGD). Temperature, solid to liquid ratio and stirring speed were varied in addition to lime to additive ratio in optimization tests. Design expert’s central cubic design was used in the design of experiments to aid with regression analysis. Temperature had the highest effect whereas W.A.S was the best additive then fly ash and finally bottom ash. The results were confirmed by the use of fixed bed apparatus where the best sorbents from each additive was tested in dry FGD. In a bid to explain the results, BET surface area analysis was used where the sorbent from the blend of lime and W.A.S had the highest improvement in surface area.
South African Journal of Science, 2015
Tough environmental laws and stringent government policies have revolutionised the refrigeration ... more Tough environmental laws and stringent government policies have revolutionised the refrigeration sector, especially concerning the cycle fluid known as the refrigerant. It has been observed that only natural refrigerants are environmentally benign. When other refrigerant qualities are considered, especially those relating to toxicity and flammability, carbon dioxide emerges as the best among the natural refrigerants. However, carbon dioxide based refrigerants are not without drawbacks. Even though the use of R744 – a carbon dioxide based refrigerant gas – has solved the direct effect of emissions on the environment, studies to investigate the indirect effects of these systems are needed. Improvement in existing technical solutions and the formulation of additional solutions to existing R744 refrigeration problems is paramount if this technology is to be accepted by all, especially in areas with warm climates. National policies geared to green technologies are important to clear ...
A CO2 water to water heat pump system was used to investigate the effects of throttle valve openi... more A CO2 water to water heat pump system was used to investigate the effects of throttle valve opening and water flow rates at higher charge capacity (149% of the full charge). It was observed that the throttle opening affects the gas cooler pressure (and inlet temperature) where the less the opening the higher the pressure. Although the increase in gas cooler pressure was improving the gas cooler heat output and therefore the system efficiencies, there is an optimum pressure which when exceeded, the system performance deteriorates. This optimum pressure also is related to a certain throttle opening which is approximately at 20% in this study. Similarly, as the throttle valve is reduced, the overall system becomes more ideal as indicated by Lorentz efficiency. On the other hand, the higher the flow rates the higher the performance although it's better to keep the chilling water (evaporator) flow rate low and cooling water (gas cooler) flow rates high than vice versa for the best he...
A CO2 water to water heat pump system was used to investigate<br> the effects of throttle v... more A CO2 water to water heat pump system was used to investigate<br> the effects of throttle valve opening and water flow rates at higher<br> charge capacity (149% of the full charge). It was observed that the<br> throttle opening affects the gas cooler pressure (and inlet<br> temperature) where the less the opening the higher the pressure.<br> Although the increase in gas cooler pressure was improving the<br> gas cooler heat output and therefore the system efficiencies, there<br> is an optimum pressure which when exceeded, the system<br> performance deteriorates. This optimum pressure also is related to<br> a certain throttle opening which is approximately at 20% in this<br> study. Similarly, as the throttle valve is reduced, the overall system becomes more ideal as indicated by Lorentz efficiency.<br> On the other hand, the higher the flow rates the higher the performance although it's better to keep the chilling w...
2013 Proceedings of the 10th Industrial and Commercial Use of Energy Conference, 2013
In order to make CO2 as a preferred refrigerant, its efficiency of operation and/or heat output v... more In order to make CO2 as a preferred refrigerant, its efficiency of operation and/or heat output values should be higher than those achieved by convectional systems. It is in this context that this study was conducted. A CO2 trans-critical water to water test bed was used to study the output of the heat pump. In a bid to improve the output of this system, the effect of throttle valve opening and water flow rates were investigated by adjusting them relevantly. The output of the system was in terms of the components energies and system efficiencies. It was observed that as the throttle valve opening is reduced, the performance increased until around 20% opening where the performance started to reduce. Also, the higher the flow rate, the better the performance.
Many of the refrigerants currently being used in heating, ventilation, air conditioning and refri... more Many of the refrigerants currently being used in heating, ventilation, air conditioning and refrigeration systems have high global warming potential. One potential, environmentally friendly replacement refrigerant is carbon dioxide (CO2). In this study, a CO2 trans-critical water to water test bed was used to study the output of a typical heat pump. Initial experimental results and thermo-physical properties were analyzed by NIST REFROP and plotted in a temperature-entropy (T-S). The energy output in the gas cooler were compared to the energy input in the compressor and the efficiency of the system in terms of coefficient of performance (COP) was observed to vary from 3.7 to 3.9. It was observed that there was a wide difference between the theoretical and experimental results when the same state conditions are considered. This was related to change on theoretical processes due to efficiency consideration of the equipment involved and flow process variables like friction which affect...
Jatropha curcas oil was extracted from jatropha seeds of Kenyan and Tanzanian origin. Biodiesel w... more Jatropha curcas oil was extracted from jatropha seeds of Kenyan and Tanzanian origin. Biodiesel was then produced by two step acid-base catalytic transesterification process due to the amount of free fatty acids (FFA) in the oil. Physicochemical properties of the produced biodiesel were tested according to international standards and were found to be different according to the source of biodiesel, though both were still within the requirements. Engine performance tests were carried out in an Audi, 1.9 litres, turbocharged direct injection, compression ignition engine at different loads. The biodiesels from different regions were found to have different engine performance behaviours, thus it was concluded that different regions produce fuel of different characteristics.
The increasing cost of energy has caused the energy intensive industries to examine means of redu... more The increasing cost of energy has caused the energy intensive industries to examine means of reducing energy consumption in processing in order to remain competitive both in local and global markets. This paper presents a method for modeling and optimizing energy use in textile manufacturing using linear programming (LP). A linear programming model has been developed which meets the finished product requirements at a minimum cost of energy used in the process subject to different operational constraints. To develop the model, data required were collected through energy audit of the plant. It is through energy audit that constraints and products manufactured were identified and used in the development of the model. Constraints come in the form of system material balance equations and output production demands. Material balance equations and energy balance are the main features of the model. The model will determine optimum values for the process design variables, so as to achieve min...
South African Journal of Science, 2015
A lot of work has been done on the enhancement of lime reactivity by materials containing alumina... more A lot of work has been done on the enhancement of lime reactivity by materials containing alumina silicates through pozzolanic reaction. This has lead to investigation of new sources CaO of alumina silicates. Waste Activated Sludge (W.A.S) was found to be enhancing limestone reactivity but to a certain optimum amount, after which it becomes detrimental. Shrinking Core Model (SCM) with mass transfer through the fluid film as the rate-limiting step was compared with chemical reaction as the rate-limiting step. Central cubic design was used for regression analysis of the effects of temperature, lime to W.A.S ratio, liquid to solid ratio and stirring speed in linear, quadratic and interactive aspect. The effects were more pronounced for temperature and least pronounced for stirring speed. Index Terms — Lime, Waste activated sludge, Reactivity constant, Pozzolanic reaction.
ABC Journal of Advanced Research
Lime reactivity was improved by blending it with either fly ash, bottom ash or waste activated sl... more Lime reactivity was improved by blending it with either fly ash, bottom ash or waste activated sludge (W.A.S). The reactivity was tested using a pH-stat apparatus which simulates wet flue gas desulfurization (FGD). Temperature, solid to liquid ratio and stirring speed were varied in addition to lime to additive ratio in optimization tests. Design expert's central cubic design was used in the design of experiments to aid with regression analysis. Temperature had the highest effect whereas W.A.S was the best additive then fly ash and finally bottom ash. The results were confirmed by the use of fixed bed apparatus where the best sorbents from each additive was tested in dry FGD. In a bid to explain the results, BET surface area analysis was used where the sorbent from the blend of lime and W.A.S had the highest improvement in surface area.
Journal of Energy in Southern Africa
Biodiesel was produced from jatropha curcas oil of Kenyan origin through a two-step acid-base cat... more Biodiesel was produced from jatropha curcas oil of Kenyan origin through a two-step acid-base catalytic transesterification process. The relevant physicochemical properties of the produced biodiesel were tested according to appropriate standards and were found to be within the requirements. Engine tests were carried out in an Audi, 1.9 litre, turbocharged direct injection, compression ignition engine at different loads. Emissions were measured by a Horiba emission analyser system while combustion data was collected by a data acquisition system, from which, cylinder pressure and rate of heat release of the test engine in every crank angle were calculated. Though the biodiesel had slightly higher brake specific fuel consumption when compared to fossil diesel, its emission behaviour was significantly better. The combustion characteristics were also slightly higher as compared to fossil diesel. This study therefore concluded that biodiesel derived from jatropha curcas of Kenyan origin c...
2013 Proceedings of the 10th Industrial and Commercial Use of Energy Conference, Aug 1, 2013
2012 Proceedings of the 9th Industrial and Commercial Use of Energy Conference, Aug 1, 2012
2015 International Conference on the Industrial and Commercial Use of Energy (ICUE), 2015
Journal of Thermal Engineering, 2015
The influence of variables such as; refrigerant amount, chilling and cooling water temperature, t... more The influence of variables such as; refrigerant amount, chilling and cooling water temperature, throttle valve opening, cooling and chilling water flow rates, on the efficiency (coefficient of performance -COP) of a water to water carbon dioxide heat pump was investigated. Design of experiments was done using design-expert® 6 software for regression analysis. A response surface method known as central cubic design was used to provide optimum results with minimum experiments. Through multiple regression analysis, an empirical equation relating the COP to the variables was derived. Analysis of variance revealed that these regressions are statistically significant at 95% confidence level compounded with a very low standard deviation and a high adequate precision. The close relationship between the predicted COP values and the actual values further proves the worthiness of the empirical equation. It was observed that cooling water temperature had the highest influence followed by the chilling water temperature. Surprisingly, the amount of the refrigerant was third followed by the throttle valve opening. Understandably, chilling water flow rate had the least effect on the COP. Through response surface diagrams, the interactive influence of the variables were also observed. The COP values arrived at varied from 1.545 to 6.914 although if the variables were optimized fully within the scope of this study, a value of up to 11.8 could be achieved. Still, if the variables range is increased further, higher COP could be achieved. Finally, a discussion was done in a bid to explain these results.
African Journal of Science, Technology, Innovation and Development, 2013
Water, Air, & Soil Pollution, 2012
Lime was physically blended with Waste Activated Sludge (W.A.S) in various proportions. These ble... more Lime was physically blended with Waste Activated Sludge (W.A.S) in various proportions. These blends were hydrated, dried, ground, and sieved to size of utmost 200 μm. The sorbents were then used in fixed-bed reactor for dry desulfurization tests. It was found that the blends had higher sorption capacities than lime alone which means that W.A.S augmented lime's sorption capacity. Higher surface area and porosity of the blended sorbents compared to lime was the main cause of the improved sorption capacity, a conclusion supported by Brunauer-Emmett-Teller surface area analysis (surface area increment with sorption capacity) and scanning electron microscopy imaging (rough morphology being formed). This higher surface area and porosity was caused by pozzolanic reaction between lime and the alumina silicate constituents of W.A.S. The products of this reaction are high surface area, complex calcium alumina silicates.
South African Journal of Science, 2014
Fuel, 2012
ABSTRACT Lime and iron waste were blended in various ratios and the resulting sorbents tested in ... more ABSTRACT Lime and iron waste were blended in various ratios and the resulting sorbents tested in two equipments namely; pH-stat apparatus simulating wet flue gas desulfurization (FGD) and fixed bed reactor for dry FGD, to investigate the effect of the blends in the FGD process. The best blend was found at a ratio of 1:2 (lime to iron waste) by mass. Brunauer, Emmett and Teller (BET) surface area analysis and Scanning Electron Microscopy (SEM) were used to assist in characterizing the resulting sorbents. Temperature, solid to liquid ratio and stirring speed were varied in addition to lime to iron waste ratio in optimization tests that followed. Design expert’s central cubic design was used in the design of experiments to aid with regression analysis. Temperature had the highest effect, followed by lime to iron waste ratio. Solid to liquid ratio had the least effect on the overall reactivity of the sorbents. The effects were more pronounced at higher levels of these variables.
Lime reactivity was improved by blending it with either fly ash, bottom ash or waste activated sl... more Lime reactivity was improved by blending it with either fly ash, bottom ash or waste activated sludge (W.A.S). The reactivity was tested using a pH-stat apparatus which simulates wet flue gas desulfurization (FGD). Temperature, solid to liquid ratio and stirring speed were varied in addition to lime to additive ratio in optimization tests. Design expert’s central cubic design was used in the design of experiments to aid with regression analysis. Temperature had the highest effect whereas W.A.S was the best additive then fly ash and finally bottom ash. The results were confirmed by the use of fixed bed apparatus where the best sorbents from each additive was tested in dry FGD. In a bid to explain the results, BET surface area analysis was used where the sorbent from the blend of lime and W.A.S had the highest improvement in surface area.
South African Journal of Science, 2015
Tough environmental laws and stringent government policies have revolutionised the refrigeration ... more Tough environmental laws and stringent government policies have revolutionised the refrigeration sector, especially concerning the cycle fluid known as the refrigerant. It has been observed that only natural refrigerants are environmentally benign. When other refrigerant qualities are considered, especially those relating to toxicity and flammability, carbon dioxide emerges as the best among the natural refrigerants. However, carbon dioxide based refrigerants are not without drawbacks. Even though the use of R744 – a carbon dioxide based refrigerant gas – has solved the direct effect of emissions on the environment, studies to investigate the indirect effects of these systems are needed. Improvement in existing technical solutions and the formulation of additional solutions to existing R744 refrigeration problems is paramount if this technology is to be accepted by all, especially in areas with warm climates. National policies geared to green technologies are important to clear ...
A CO2 water to water heat pump system was used to investigate the effects of throttle valve openi... more A CO2 water to water heat pump system was used to investigate the effects of throttle valve opening and water flow rates at higher charge capacity (149% of the full charge). It was observed that the throttle opening affects the gas cooler pressure (and inlet temperature) where the less the opening the higher the pressure. Although the increase in gas cooler pressure was improving the gas cooler heat output and therefore the system efficiencies, there is an optimum pressure which when exceeded, the system performance deteriorates. This optimum pressure also is related to a certain throttle opening which is approximately at 20% in this study. Similarly, as the throttle valve is reduced, the overall system becomes more ideal as indicated by Lorentz efficiency. On the other hand, the higher the flow rates the higher the performance although it's better to keep the chilling water (evaporator) flow rate low and cooling water (gas cooler) flow rates high than vice versa for the best he...
A CO2 water to water heat pump system was used to investigate<br> the effects of throttle v... more A CO2 water to water heat pump system was used to investigate<br> the effects of throttle valve opening and water flow rates at higher<br> charge capacity (149% of the full charge). It was observed that the<br> throttle opening affects the gas cooler pressure (and inlet<br> temperature) where the less the opening the higher the pressure.<br> Although the increase in gas cooler pressure was improving the<br> gas cooler heat output and therefore the system efficiencies, there<br> is an optimum pressure which when exceeded, the system<br> performance deteriorates. This optimum pressure also is related to<br> a certain throttle opening which is approximately at 20% in this<br> study. Similarly, as the throttle valve is reduced, the overall system becomes more ideal as indicated by Lorentz efficiency.<br> On the other hand, the higher the flow rates the higher the performance although it's better to keep the chilling w...
2013 Proceedings of the 10th Industrial and Commercial Use of Energy Conference, 2013
In order to make CO2 as a preferred refrigerant, its efficiency of operation and/or heat output v... more In order to make CO2 as a preferred refrigerant, its efficiency of operation and/or heat output values should be higher than those achieved by convectional systems. It is in this context that this study was conducted. A CO2 trans-critical water to water test bed was used to study the output of the heat pump. In a bid to improve the output of this system, the effect of throttle valve opening and water flow rates were investigated by adjusting them relevantly. The output of the system was in terms of the components energies and system efficiencies. It was observed that as the throttle valve opening is reduced, the performance increased until around 20% opening where the performance started to reduce. Also, the higher the flow rate, the better the performance.
Many of the refrigerants currently being used in heating, ventilation, air conditioning and refri... more Many of the refrigerants currently being used in heating, ventilation, air conditioning and refrigeration systems have high global warming potential. One potential, environmentally friendly replacement refrigerant is carbon dioxide (CO2). In this study, a CO2 trans-critical water to water test bed was used to study the output of a typical heat pump. Initial experimental results and thermo-physical properties were analyzed by NIST REFROP and plotted in a temperature-entropy (T-S). The energy output in the gas cooler were compared to the energy input in the compressor and the efficiency of the system in terms of coefficient of performance (COP) was observed to vary from 3.7 to 3.9. It was observed that there was a wide difference between the theoretical and experimental results when the same state conditions are considered. This was related to change on theoretical processes due to efficiency consideration of the equipment involved and flow process variables like friction which affect...
Jatropha curcas oil was extracted from jatropha seeds of Kenyan and Tanzanian origin. Biodiesel w... more Jatropha curcas oil was extracted from jatropha seeds of Kenyan and Tanzanian origin. Biodiesel was then produced by two step acid-base catalytic transesterification process due to the amount of free fatty acids (FFA) in the oil. Physicochemical properties of the produced biodiesel were tested according to international standards and were found to be different according to the source of biodiesel, though both were still within the requirements. Engine performance tests were carried out in an Audi, 1.9 litres, turbocharged direct injection, compression ignition engine at different loads. The biodiesels from different regions were found to have different engine performance behaviours, thus it was concluded that different regions produce fuel of different characteristics.
The increasing cost of energy has caused the energy intensive industries to examine means of redu... more The increasing cost of energy has caused the energy intensive industries to examine means of reducing energy consumption in processing in order to remain competitive both in local and global markets. This paper presents a method for modeling and optimizing energy use in textile manufacturing using linear programming (LP). A linear programming model has been developed which meets the finished product requirements at a minimum cost of energy used in the process subject to different operational constraints. To develop the model, data required were collected through energy audit of the plant. It is through energy audit that constraints and products manufactured were identified and used in the development of the model. Constraints come in the form of system material balance equations and output production demands. Material balance equations and energy balance are the main features of the model. The model will determine optimum values for the process design variables, so as to achieve min...
South African Journal of Science, 2015
A lot of work has been done on the enhancement of lime reactivity by materials containing alumina... more A lot of work has been done on the enhancement of lime reactivity by materials containing alumina silicates through pozzolanic reaction. This has lead to investigation of new sources CaO of alumina silicates. Waste Activated Sludge (W.A.S) was found to be enhancing limestone reactivity but to a certain optimum amount, after which it becomes detrimental. Shrinking Core Model (SCM) with mass transfer through the fluid film as the rate-limiting step was compared with chemical reaction as the rate-limiting step. Central cubic design was used for regression analysis of the effects of temperature, lime to W.A.S ratio, liquid to solid ratio and stirring speed in linear, quadratic and interactive aspect. The effects were more pronounced for temperature and least pronounced for stirring speed. Index Terms — Lime, Waste activated sludge, Reactivity constant, Pozzolanic reaction.
ABC Journal of Advanced Research
Lime reactivity was improved by blending it with either fly ash, bottom ash or waste activated sl... more Lime reactivity was improved by blending it with either fly ash, bottom ash or waste activated sludge (W.A.S). The reactivity was tested using a pH-stat apparatus which simulates wet flue gas desulfurization (FGD). Temperature, solid to liquid ratio and stirring speed were varied in addition to lime to additive ratio in optimization tests. Design expert's central cubic design was used in the design of experiments to aid with regression analysis. Temperature had the highest effect whereas W.A.S was the best additive then fly ash and finally bottom ash. The results were confirmed by the use of fixed bed apparatus where the best sorbents from each additive was tested in dry FGD. In a bid to explain the results, BET surface area analysis was used where the sorbent from the blend of lime and W.A.S had the highest improvement in surface area.
Journal of Energy in Southern Africa
Biodiesel was produced from jatropha curcas oil of Kenyan origin through a two-step acid-base cat... more Biodiesel was produced from jatropha curcas oil of Kenyan origin through a two-step acid-base catalytic transesterification process. The relevant physicochemical properties of the produced biodiesel were tested according to appropriate standards and were found to be within the requirements. Engine tests were carried out in an Audi, 1.9 litre, turbocharged direct injection, compression ignition engine at different loads. Emissions were measured by a Horiba emission analyser system while combustion data was collected by a data acquisition system, from which, cylinder pressure and rate of heat release of the test engine in every crank angle were calculated. Though the biodiesel had slightly higher brake specific fuel consumption when compared to fossil diesel, its emission behaviour was significantly better. The combustion characteristics were also slightly higher as compared to fossil diesel. This study therefore concluded that biodiesel derived from jatropha curcas of Kenyan origin c...
2013 Proceedings of the 10th Industrial and Commercial Use of Energy Conference, Aug 1, 2013
2012 Proceedings of the 9th Industrial and Commercial Use of Energy Conference, Aug 1, 2012
2015 International Conference on the Industrial and Commercial Use of Energy (ICUE), 2015
Journal of Thermal Engineering, 2015
The influence of variables such as; refrigerant amount, chilling and cooling water temperature, t... more The influence of variables such as; refrigerant amount, chilling and cooling water temperature, throttle valve opening, cooling and chilling water flow rates, on the efficiency (coefficient of performance -COP) of a water to water carbon dioxide heat pump was investigated. Design of experiments was done using design-expert® 6 software for regression analysis. A response surface method known as central cubic design was used to provide optimum results with minimum experiments. Through multiple regression analysis, an empirical equation relating the COP to the variables was derived. Analysis of variance revealed that these regressions are statistically significant at 95% confidence level compounded with a very low standard deviation and a high adequate precision. The close relationship between the predicted COP values and the actual values further proves the worthiness of the empirical equation. It was observed that cooling water temperature had the highest influence followed by the chilling water temperature. Surprisingly, the amount of the refrigerant was third followed by the throttle valve opening. Understandably, chilling water flow rate had the least effect on the COP. Through response surface diagrams, the interactive influence of the variables were also observed. The COP values arrived at varied from 1.545 to 6.914 although if the variables were optimized fully within the scope of this study, a value of up to 11.8 could be achieved. Still, if the variables range is increased further, higher COP could be achieved. Finally, a discussion was done in a bid to explain these results.
African Journal of Science, Technology, Innovation and Development, 2013
Water, Air, & Soil Pollution, 2012
Lime was physically blended with Waste Activated Sludge (W.A.S) in various proportions. These ble... more Lime was physically blended with Waste Activated Sludge (W.A.S) in various proportions. These blends were hydrated, dried, ground, and sieved to size of utmost 200 μm. The sorbents were then used in fixed-bed reactor for dry desulfurization tests. It was found that the blends had higher sorption capacities than lime alone which means that W.A.S augmented lime's sorption capacity. Higher surface area and porosity of the blended sorbents compared to lime was the main cause of the improved sorption capacity, a conclusion supported by Brunauer-Emmett-Teller surface area analysis (surface area increment with sorption capacity) and scanning electron microscopy imaging (rough morphology being formed). This higher surface area and porosity was caused by pozzolanic reaction between lime and the alumina silicate constituents of W.A.S. The products of this reaction are high surface area, complex calcium alumina silicates.
South African Journal of Science, 2014
Fuel, 2012
ABSTRACT Lime and iron waste were blended in various ratios and the resulting sorbents tested in ... more ABSTRACT Lime and iron waste were blended in various ratios and the resulting sorbents tested in two equipments namely; pH-stat apparatus simulating wet flue gas desulfurization (FGD) and fixed bed reactor for dry FGD, to investigate the effect of the blends in the FGD process. The best blend was found at a ratio of 1:2 (lime to iron waste) by mass. Brunauer, Emmett and Teller (BET) surface area analysis and Scanning Electron Microscopy (SEM) were used to assist in characterizing the resulting sorbents. Temperature, solid to liquid ratio and stirring speed were varied in addition to lime to iron waste ratio in optimization tests that followed. Design expert’s central cubic design was used in the design of experiments to aid with regression analysis. Temperature had the highest effect, followed by lime to iron waste ratio. Solid to liquid ratio had the least effect on the overall reactivity of the sorbents. The effects were more pronounced at higher levels of these variables.