Energy Analysis And Engineering Optimization Of Separation Process Of Methanol To Olefin Production

The separation process of methanol-to-olefin products is mainly distillation operation,including seven distillation units,such as high and low pressure de-propane tower and de-butane tower.The distillation process has high thermodynamic irreversibility,high energy consumption and great energy saving potential.In this paper,the separation process of methanol-to-olefin products is taken as the research object.On the basis of fully understanding the technological route of the separation process of methanol-to-olefin products,the thermodynamic data of the whole process is obtained by Aspen plus process simulation software.The energy efficiency of the process is evaluated by the method of exergy analysis and the three-link model of energy use,and the weak link of energy efficiency in the process is found out.Some effective energy-saving measures are put forward.The main contents are as follows:(1)Using Aspen plus software,the whole process of MTO product separation in a plant was simulated by using RK-SOAVE physical property method and RadFrac,a rigorous calculation module of distillation column.By comparing the simulated value with the real value,it was found that the product flow rate,component concentration and temperature of the top and kettle of each distillation separation unit were in good agreement with the actual situation,which ensured the reliability of the simulation results of the whole process.On this basis,the thermodynamic data needed for the "three-link" model are obtained.(2)A "three-link" exergy balance structure model,which was suitable for MTO product separation process was established,and the energy consumption analysis and evaluation of the process were completed.The analysis shows that in the energy conversion process,because of the low energy quality of hot steam,heat dissipation of equipment and flue gas discarding,the exergy loss is larger,the energy conversion efficiency and exergy conversion efficiency are lower;in the energy utilization process,exergy loss of distillation unit is the main source of exergy loss.Among which,the condenser load of propylene two-tower distillation unit is very large,the heat transfer efficiency is only 48%,while the exergy loss of ethylene distillation unit is up to 887 MJ/hr because of the low feed temperature.Thus,it is necessary to optimize the operation conditions of the ethylene distillation column and the propylene distillation columns.In the energy recovery process,there are many heat exchangers and low heat transfer efficiency in the separation process of MTO products,and the exergy recovery rate is only 56.72%.In addition,in practice,the solid holdup of quench water has a great influence on the heat transfer efficiency of heat exchanger.(3)According to the energy consumption analysis and evaluation results of MTO product separation process,several energy-saving optimization projects were proposed.Firstly,the sensitivity analysis of key operating parameters of the ethylene distillation column and the propylene distillation unit in energy utilization were carried out by Aspen plus software.For ethylene distillation column,on the premise of guaranteeing separation effect,when feed temperature rises to-26 ?,energy-saving efficiency of heat load of the pre-cooler and the reboiler reach 96.81% and 27.43%,respectively.When reflux ratio drops to 1.27,heat load of top condenser decreases by 67.28%,and heat load of reboiler decreases by 66.71%.For propylene distillation column,when the reflux ratio is reduced to 10,the separation effect is not affected,while the heat load of the top condenser decreases by 41.98%,and that of the reactor reboiler decreases by 47.49%.Secondly,two Potter filters are added after the original bag filter,which can reduce the solid content of quench water to 10 mg/L,thus effectively alleviating the problem of excessive reduction of heat exchanger efficiency.The actual engineering data show that after the application of Porter filter,the cut-out cleaning cycle of the bottom quench water reboiler of the low-pressure de-propane tower prolongs to more than three times of the original,and the average consumption of water vapor in the bottom stream reboiler saves about 12.5%.The cleaning time of the quench water reboiler at the bottom of 2# propylene distillation column is prolonged to about 100 days,and the average consumption of steam in the stream reboiler at the bottom of 1# propylene distillation column is saved about 23.8%.The energy-saving effect is remarkable..