Design,Identification Of Process Reengineering And Operating Re-optimization For Heat And Mass Exchanger Networks

In the chemical process,like as the ethylene complex usually contains several different kinds of equipment to achieve the separation of the cracking gas.The performance of some equipment changes a lot and directly affects the separation and utilization of the energy of the entire system,and becomes the bottleneck in energy savings of the system.For the separating plant in the chemical process,due to the fouling of the devices or the changes of compositions and flow rate of the feed,the core process distillation column may show the abnormal heat and mass transfer after the equipment has been running for a long time,and the tray efficiency lows down.In order to weaken the influence of inappropriate feed positions,the identification of the feed positions which result into the abnormal heat and mass transfer on the tray is urgent.At the same time,it is more convenient to conduct the adjustment of the feed positions than the retrofit of the internal trays.Therefore,this paper focuses on the identification of abnormal heat and mass transfer trays caused by the improper feed positions,and investigates methods to remove the feed bottleneck of the distillation system.For the feed bottleneck,the process reengineering method is discussed by adjusting the feed positions.Finally,the control and re-optimization strategies for the system after process reengineering is designed.(1)Considering the inconvenience in the chemical plant to carry out the research work on above subject,the dynamic model of the whole demethanizer plant is proposed and verified based on the characteristics and difficulties of various parts in the demethanizer.Aiming at laws of heat and mass transfer inside the distillation column,this paper presents a new method to construct the heat and mass transfer curve of the distillation column,and identifies the heat and mass transfer bottlenecks of the distillation column by graphical method.Meanwhile,this study associates the internal bottleneck with the feed locations,and proposes a new method to identify the bottleneck of feed locations.Besides,a case study of the multi-feed demethanizer in ethylene complex is selected as a good example to find out the feed bottleneck of the system.According to the analysis of the characteristic and influence of switching feed positions on the heat and mass transfer inside the column,the process reengineering strategy of adjusting feed positions of the multi-feed demethanizer is suggested and conducted.The simulation of the demethanizer verifies the accuracy of the plant.And by the application of the process reengineering and feed bottleneck identification,the proposed method can reduce the energy consumption of the multi-feed demethanizer system effectively for either the product quality or the amount of throughput.(2)In terms of operating conditions of the multi-feed demethanizer plant after the process reengineering,it is possible that the operating conditions of the new system is different from the old one.And after process reengineering,the original operating conditions maybe not optimal,which result in the large loss of the system energy and resources,so the new operating conditions need to be re-optimized.To achieve the above goals,optimization of single column on the core process in demethanizer plant and the optimization of the compound column are proposed and investigated.Firstly,the univariate analysis method is used to find the core and optimal operational variables of the energy consumption on the optimization of single and compound column.Secondly,considering the energy utilization of different systems,the quantity of cooling energy of the demethanizer and the power of the multi-compressor are chosen as the objective function of single column and compound column,respectively.Finally,the orthogonal test method is employed to acquire the optimal operating variables.It is proved that the method of selecting the core and optimal operating variables is easy to be realized.And by the comparison of two schemes,the optimization of compound column is more efficient in energy savings than the optimization of single column,so the optimization of compound column should be selected.(3)The multi-feed demethanizer is the core operating unit among all operating devices,whose control strategy has important influence on the separation of the final products and energy consumption.The process control and coordination strategy are introduced to realize the control of key products and optimal utilization of the energy.At the same time,considering the process reengineering of the feed positions,the problem of switching feed positions and control index is also discussed.Taking the two-input-two-output system as an example,one variable is used to control the target value through the controller,and the other variable adopts the simultaneous optimization method to obtain the optimal variable control strategy under the minimum energy consumption.The results of case study on multi-feed demethanizer show that the control of the top and simultaneous optimization of the bottom can reduce the energy consumption of the system effectively.For the switch of the feed position,the control effect at the top and bottom is analyzed under the conventional control strategy.The switch of the set point of the controller and feed positions is implemented through step by step and simultaneous methods,respectively.The simulation results show that the original control strategy can realize the adjustment of the switching feed positions,and simultaneous switch strategy is better than step by step method when changing the main control index and feed positions of the multi-feed demethanizer.