| Energy and environment are the two main problems that humans face with ongoing sustainable development.Increasing energy efficiency has become an important measure to alleviate the energy crisis and global climate change.In refineries,huge amounts of low-temperature waste heat are cooled down by cold utility,effectively wasting this energy.If low-temperature energy is utilized,the efficiency of the system will be increased dramatically and thus also greenhouse gas emissions will be reduced.Organic Rankine Cycles(ORCs)have become a promising technology to recover low-temperature waste heat due to their relatively high thermal efficiency,simple operation,low maintenance and capital cost.Based on this background,this thesis focuses on ORC technology,in particular :(1)the study of working fluid selection based on pinch technology,(2)the study of process integration and optimization of ORC with a background process based on mathematical programming,(3)the study of systematically customizing an efficient ORC to a background process with hot water as the intermediate using pinch technology,and(4)the study of simultaneous heat integration and techno-economic optimization of ORC using mathematical programming.For an ORC,the working fluid is a very crucial factor to the system.Working fluid affects the thermodynamic performance,environmental effects and the profitability of the system.With the increasingly severe problems concerning pollution and climate change,the environmental effects also should be taken into account along with the thermodynamic properties(thermal efficiency and the ability to recover heat)when selecting an appropriate working fluid.In refineries,there are many types of waste heat sources characterized according to their temperature-enthalpy(T-H)diagrams.The pinch point between organic working fluid and waste heat carrier may be different for different types of heat sources.Working fluid selection should consider the characteristics of the T-H profile of the waste heat sources.In this thesis,further research has been done on working fluid selection considering the pinch limitation and the characteristics of waste heat source.For each type of waste heat source,a criterion for selecting the optimal working fluid has been derived,which may give useful instructions for industrial practice.For sensible heat sources,which are the most common type,a new method for simultaneous working fluid selection and operating condition determination is proposed.The addition of an ORC will exert effects on the heat integration of the background process.The evaporation and condensation process of an ORC may be regarded as cold and hot process streams.Because more streams are added to the background process,there may be more opportunities for integration and efficient energy utilization.The best integration scheme can be obtained by varying the configuration and operating conditions of an ORC.To increase the thermal efficiency,regeneration and turbine bleeding are proposed.For an ORC incorporating regeneration and turbine bleeding,determination of the optimal configurations and operating conditions and which waste heat streams will be recovered are key problem for the system.The pinch based method can integrate an ORC into a background process,but this kind of method cannot deal with too many degrees of freedom.Therefore the results are obtained by trial and error.The method based on pinch technology lack the ability to handle many degrees of freedom,and thus the results are not very satisfactory.In this thesis,a mathematical programming method is proposed including a rigorous thermodynamic model and the Duran-Grossmann model for heat integration and the model is used to determine the optimal configuration and operating conditions of an ORC.After the configuration and operating conditions of the ORC are determined,the final heat exchanger network can be derived based on expanded transshipment model.In refineries,specifying that an organic working fluid will recover the waste heat directly may lead to the safety and operability problems.Therefore,a new scheme,in which the waste heat is recovered by hot water and the hot water releases heat to the organic working fluid,is proposed.For this scheme,the optimal hot water flowrate and the waste heat recovery network are the key variables to be determined.For a heat exchanger network,a new concept-waste heat composite curve(WHCC)is defined in this thesis,which is a one to one correspondence with the composite curve from heat exchanger networks.The grand composite curve(GCC)is irrelevant to the heat exchanger network configuration.The inefficiency of heat exchanger network can be diagnosed via WHCC and GCC,and the corresponding measures can be taken to improve the heat exchanger network.This thesis proposes a method for heat exchanger network improvement considering the waste heat utilization.This method can reduce the utilities consumption and/or improve the waste heat grade to enhance the energy efficiency.The optimal mass flow rate of hot water and the waste heat recovery network can be determined in an integrated manner based on WHCC.An ORC with hot water as the intermediate can be integrated into a background process via pinch technology.However,this method can just generate an energy efficient scheme,in which the capital cost cannot be considered.The capital cost of ORC system is strongly related to the operating conditions.Therefore,in this thesis,a mathematical model considering the heat integration and techno-economic optimization simultaneously is proposed.This model can determine the optimal hot water flowrate and operating conditions of an ORC.The effects of heat recovery approach temperature on the design of the system are explored.The model can determine the optimal heat recovery approach temperature(HRAT),the optimal mass flowrate of hot water,and the optimal operating conditions of ORC simultaneously.Sensitivity of the optimal solution to the key parameters(electricity price,utility cost)is quantified. |
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