Research On Energy Saving Optimization And Optimal Structure Design Of Heat-Intrgrated Air Separation Columns

Cryogenic air separation equipment,as industrial equipment widely used for large-scale separation of industrial gases such as pure oxygen or pure nitrogen from the air,has important applications in energy,chemical,medicine,smelting industry,etc.On the other hand,cryogenic air separation is also an energy-intensive industrial process.It has been estimated that the energy consumption of cryogenic air separation equipment accounts for about 5%of the country's total energy consumption.In recent years,energy conservation and emission reduction have increasingly become an important starting point for low-carbon development.In particular,high-efficiency and low-emission power generation technologies such as integrated coal gasification combined cycle and oxy-fuel combustion have been rapidly developing,both of which rely on high-purity oxygen generation by air separation equipment.Cryogenic air separation has become one of the bottlenecks in the large-scale promotion of the above advanced technologies due to its low thermal efficiency and high energy consumption.Recently,the heat-integrated air separation column(HIASC)has been proposed as a novel energy-saving implementation for the air separation process.However,due to its strong coupling and strong nonlinear characteristics,energy-saving optimization and structural design are more difficult for this process.Therefore,this thesis aims to explore the energy-saving potential of HIASC.Considering the energy-saving optimization and structural design problems for HIASC,the modeling and optimization solutions of the thermally coupled air separation tower are proposed.Optimization results of the operating parameters and structural parameters,the new heat-integration structure,and the heat-integration configuration have been obtained.The main work and innovations of this paper are as follows:(1)The influence of the operating conditions and basic structural parameters of a fully-coupled HIASC on its energy consumption and cost is analyzed and optimized.The parameters include heat exchange capacity,the pressure of the high-pressure column,and the heat condition of the feed.Four optimization results were obtained,which showed a satisfying energy-saving effect compared to conventional air separation columns.(2)An optimization study was carried out for the key parameter of the thermally coupled air separation tower structure design,namely the number of stages of the distillation columns.The number of stages in HIASC has a different effect from that of its conventional counterpart,which makes the optimal number of stages with the lowest energy consumption available.In order to further tap the energy-saving potential of HIASC,a comprehensive optimization problem involving the number of stages and the heat exchange capacity is proposed.Furthermore,the optimization of the number of trays under different tray pressure drops is discussed,The obtained optimization results further improve the energy-saving effect of the heat-integrated air separation column,and also obtains the number of trays and the corresponding structure and operating parameters that minimize the total annual cost.(3)Two novel heat-integration structures are proposed focusing on the heat transfer efficiency of heat-integrated stages.These two structures are the structure in which the relative displacement occurs between column sections and the structure in which the stages are partially coupled.By re-pairing the stages where heat-integration occurs,the heat-integration effect is improved or the heat exchange driving force is enhanced,which further improves the energy-saving effect and reduces investment costs.Under the constraints of different minimum heat-transfer temperature differences,the partially-coupling structure keeps showing the best energy-saving effect,and therefore is the most potential thermal coupling structure among the above structures.The experimental results of the optimization of an actual industrial facility also show the energy-saving potential of the novel heat-integration structure.(4)The design procedure of the thermal coupling quantity configuration scheme of each stage is proposed,of the diameter of the air separation tower and the maximum heat exchange capacity of the heat exchanger was established through hydraulic analysis.The distribution characteristics of the thermal coupling quantity are analyzed according to the cumulative phase change under different feed thermal conditions.The optimization problem of thermal coupling quantity configuration is proposed,and the optimization result minimizes the equipment cost while maintaining the energy-saving effect.Compared with the fully-coupling structure,the optimized structure effectively reduces the heat exchanger investment and ensures the thermal capacity constraints are satisfied at the same time.