This article describes a study in which the various stagewise and interval-based superstructures ... more This article describes a study in which the various stagewise and interval-based superstructures for the mixed integer non-linear programming (MINLP) optimisation of heat exchanger networks were systematically compared, using exactly the same basis. The effect of using different logarithmic mean temperature difference (LMTD) approximations on the Total Annual Cost of the network and the network structure were examined. The accuracy of the approximations was also analyzed over a wide range of temperature difference ratios. It is concluded that the best approximation, which is the Underwood-Chen approximation, should be used in future mathematical optimisation of Heat Exchanger Network Synthesis problems.
This paper presents a methodology for integrating various energy sources such as solar, wind, fos... more This paper presents a methodology for integrating various energy sources such as solar, wind, fossil fuel and biomass for energy generation into the synthesis of multi-period heat exchanger networks (HEN). The integration solution is imperative due to the rising cost of fossil based energy sources and their attendant potential environmental impact. Furthermore, since chemical plant operations are in reality multi-period in nature, the demand for energy at various periods of operation can be efficiently satisfied not only in a cost-efficient manner but also in an environmentally sustainable way as well. This is accomplished in this paper by adopting existing multi-period superstructure model for heat exchanger networks and extending it to handle multiple options of utilities which may be available at various times of the day. The objective function entails a simultaneous minimisation of costs and environmental impact (EI). Both renewable and nonrenewable energy sources are included, however, since all renewable energy sources may not be available at all times in a day, a hybrid energy system (a system using two or more energy sources) was considered depending on the availability of the energy source. Pareto curves for these different hybrid systems were then generated. Based on the results obtained, it was found that the windbiomass hybrid energy source gave the most favourable HEN system with regards to cost and EI considerations. This was followed by wind-coal, solar-biomass and solar-coal respectively, also in terms of cost and EI considerations. There were several limitations in this study and recommendations have also been proposed for future work improvement.
Lignocellulosic biomass such as sugarcane bagasse is non-food biomass that can be used to produce... more Lignocellulosic biomass such as sugarcane bagasse is non-food biomass that can be used to produce ethanol. Lignocellulose is a complex network of cellulose, hemicellulose and lignin, which requires pre-treatment to improve access to cellulose for hydrolysis which produces glucose for fermentation. Lignin prevents access to cellulose thus delignification using alkaline is often included before hydrolysis. A variety of pre-treatment methods exist requiring different raw materials and operating conditions thus having different economics and environmental impacts. This paper aims to use computer modelling in an optimisation environment called GAMS (General Algebraic Modelling System) to screen a host of pre-treatment options of sugarcane bagasse for bio-ethanol production. The criteria to determine the best pre-treatment option evaluates both economic and environmental objectives. Pre-treatment options included steam explosion, with and without acid catalysis, and acid pre-treatment. Methane was produced from xylose formed in pre-treatment in all options. Delignification using NaOH was included in some investigated pre-treatment flowsheets. The delignification was included in these flowsheets prior to the hydrolysis stage which used either acid or enzymes. The solution space was used to evaluate possible flowsheets in terms of the two aforementioned objectives. For a scenario where methane is the only desired product, steam explosion would be recommended. Adding acid hydrolysis to steam explosion (SA) to produce bio-ethanol increases profitability and reduces environmental impact however the glucose flowrate from this flowsheet is low. For a scenario where higher glucose flowrate is desired, steam explosion with enzymatic hydrolysis pre-treatment flowsheet is recommended however the environmental impact of this flowsheet may be large depending on the energy efficiency of enzyme production.
This article describes a study in which the various stagewise and interval-based superstructures ... more This article describes a study in which the various stagewise and interval-based superstructures for the mixed integer non-linear programming (MINLP) optimisation of heat exchanger networks were systematically compared, using exactly the same basis. The effect of using different logarithmic mean temperature difference (LMTD) approximations on the Total Annual Cost of the network and the network structure were examined. The accuracy of the approximations was also analyzed over a wide range of temperature difference ratios. It is concluded that the best approximation, which is the Underwood-Chen approximation, should be used in future mathematical optimisation of Heat Exchanger Network Synthesis problems.
This paper presents a methodology for integrating various energy sources such as solar, wind, fos... more This paper presents a methodology for integrating various energy sources such as solar, wind, fossil fuel and biomass for energy generation into the synthesis of multi-period heat exchanger networks (HEN). The integration solution is imperative due to the rising cost of fossil based energy sources and their attendant potential environmental impact. Furthermore, since chemical plant operations are in reality multi-period in nature, the demand for energy at various periods of operation can be efficiently satisfied not only in a cost-efficient manner but also in an environmentally sustainable way as well. This is accomplished in this paper by adopting existing multi-period superstructure model for heat exchanger networks and extending it to handle multiple options of utilities which may be available at various times of the day. The objective function entails a simultaneous minimisation of costs and environmental impact (EI). Both renewable and nonrenewable energy sources are included, however, since all renewable energy sources may not be available at all times in a day, a hybrid energy system (a system using two or more energy sources) was considered depending on the availability of the energy source. Pareto curves for these different hybrid systems were then generated. Based on the results obtained, it was found that the windbiomass hybrid energy source gave the most favourable HEN system with regards to cost and EI considerations. This was followed by wind-coal, solar-biomass and solar-coal respectively, also in terms of cost and EI considerations. There were several limitations in this study and recommendations have also been proposed for future work improvement.
Lignocellulosic biomass such as sugarcane bagasse is non-food biomass that can be used to produce... more Lignocellulosic biomass such as sugarcane bagasse is non-food biomass that can be used to produce ethanol. Lignocellulose is a complex network of cellulose, hemicellulose and lignin, which requires pre-treatment to improve access to cellulose for hydrolysis which produces glucose for fermentation. Lignin prevents access to cellulose thus delignification using alkaline is often included before hydrolysis. A variety of pre-treatment methods exist requiring different raw materials and operating conditions thus having different economics and environmental impacts. This paper aims to use computer modelling in an optimisation environment called GAMS (General Algebraic Modelling System) to screen a host of pre-treatment options of sugarcane bagasse for bio-ethanol production. The criteria to determine the best pre-treatment option evaluates both economic and environmental objectives. Pre-treatment options included steam explosion, with and without acid catalysis, and acid pre-treatment. Methane was produced from xylose formed in pre-treatment in all options. Delignification using NaOH was included in some investigated pre-treatment flowsheets. The delignification was included in these flowsheets prior to the hydrolysis stage which used either acid or enzymes. The solution space was used to evaluate possible flowsheets in terms of the two aforementioned objectives. For a scenario where methane is the only desired product, steam explosion would be recommended. Adding acid hydrolysis to steam explosion (SA) to produce bio-ethanol increases profitability and reduces environmental impact however the glucose flowrate from this flowsheet is low. For a scenario where higher glucose flowrate is desired, steam explosion with enzymatic hydrolysis pre-treatment flowsheet is recommended however the environmental impact of this flowsheet may be large depending on the energy efficiency of enzyme production.
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