Air Separation Process Based On The Expanded Nitrogen Refrigeration Of LNG Cold Energy

With the growth of domestic natural gas consumption,to import liquefied natural gas(LNG)has become an important way to supplement domestic gas gap.However,due to the difference between the pricing mode of LNG purchase and sale,the price of imported LNG is seriously upside down.The utilization of cold energy is significant for the promotion of the economic and industrial development of the LNG receiving station.With the rapid development of technology and economy in the world,the demand for oxygen and nitrogen has increased sharply,which greatly promoted the air separation technology in the direction of energy saving and high efficiency.Therefore,the study of cryogenic air separation technology by taking advantage of LNG cold energy is meaningful for reducing the production cost and improving the economic benefits of the receiving station.Based on the theory of chemical process design,optimization and improved exergy analysis,the objective is to optimize the process of cryogenic air separation with the LNG cold energy nitrogen expansion refrigeration.The following contents are included:(1)The analysis of properties of the stream of LNG and air in the process,including:the release of LNG gasification process under different conditions of energy and cold;the property of nitrogen,oxygen,air;the T-H curves under different pressures of liquefaction process;the phase equilibrium of air in the process of distillation;The trend of the key parameters in the air separation process,which is the fundamental of the the design and optimization of the LNG cold energy air separation.(2)The cooling effect and energy consumption of five refrigeration mode in different LNG gasification pressure are investigated,including:the phase transition,the single-stage nitrogen expansion,the two-stage nitrogen expansion,the one and a half stage nitrogen expansion and the precooling expansion.It is found that phase transition refrigeration is suitable for the receiving stations with the low gasification pressure and the poor adaptability to the fluctuation of gasification pressure.The adaptability of nitrogen expansion refrigeration is strong,and it has good cooling effect under different LNG gasification pressure.The optimal mode of refrigeration under different LNG gasification pressure was determined.(3)Based on the data of Dalian LNG receiving station,the precold nitrogen expansion refrigeration is determined and a whole process of LNG cold energy nitrogen expansion air separation is proposed.Then,the pressure and circulation of the nitrogen cycle and the pressure of the air circulation liquefaction are analyzed in the steady state model of HYSYS.To minimize the energy consumption,the optimization model is established,and the parameter optimization is completed.After optimization,the energy consumption per unit liquid product of the whole process is 0.4229kWh,which is 10.9%less than that of the same process,indicating that the process has better energy saving effect.(4)To avoid the limitations of the traditional energy analysis methods and to find the reasons for the energy losses of the major equipment,the improvced exergy analysis method is applied,by which the equipments are divided into four type:the internal exergy losses is avoidable,the external exergy losses is avoidable,the internal exergy losses is unavoidable,the external exergy losses is unavoidable.The improved exergy model is established and the calculation is simplified.Then,the energy flow diagram of nitrogen circulation system is drawed up and the overall system exergy analysis model is established.Finally,the optimization method based on improved exergy analysis is proposed.(5)Based on the improved energy analysis method,the parameters of the major equipment and the whole sysyem of the optimized precold nitrogen expansion process are calculated and analyzed.The main cause of energy loss id found and the optimization is finished by comparing the energy consumption of per unit of liquid product and the exergy efficiency of the system.Optimization of exergy efficiency compared with the original scheme increased nearly 30%,energy consumption per unit of liquid products decreased by 6.6%,which proves the feasibility of the improved optimization method based on exergy analysis.The adaptability of the optimized scheme on the condition of the fluctuation in the LNG gasification volume and the LNG gasification pressure is studied,and the direction of parameter adjustment is proposed.Additionly,it is found that the performance of the optimization scheme is better than that of the original process under different working conditions.