Multi-objective Operation Optimization Of Dual-vacuum Crude Distillation Unit Based-simulation

In recent years, a new crude oil distillation process, dual vacuum distillation columns, has been taken in some revamping projects of enlarging production capacity. Compared to the traditional process,part load of atmospheric column and the second vacuum column are partly transferred to the first vacuum column.And during the expansion revamping of distillation units, the fist vacuum column and vacuum furnace are just added.This new process has drawn much attention due to low investment and high operating flexibility. It is such a change that makes its operation process become more complex. So it is much necessary to study the process optimization.As PRO/II has already been applied to the design and operation diagnosis of crude oil distillation successfully, resource can be saved as mathematical remodeling is unnecessary. Therefofe, in this dissertation, process simulation is based on PRO/II. However, optimization algorithms in the software can not meet the requirement of difficult problems that dual vacumm distillation process is one of them. Thus, following work has been done, 1)wirte the interface program in Excel VBA to control PRO/II and Matlab through Excellink and COM server; 2)VBA read PRO/II simulation results and deliver data to MATLAB,where algorithm are running automatically to get the optimal operating parameters under particular target.Non-dominated sorting genetic algorithm(NSGA-II)is used as the optimization method because of diversity targets of the dual vacuum distillation process. Maximum light oil yield and minimum energy consumption, maximum light oil product income and minimum energy consumption, maximum profit and minimum exergy loss are taken as the targets, respectively. Finally, the Pareto optimal solution sets are obtained by optimizing calculation automatically.In addition, during the automatic simulation process. some calculation results can’t satisfy some production conditions such as hydraulics and product specification limits owing that bounds of decision variables can’t be decided precisely. In this paper, a set of search rules are embedded in the interface program to avoid providing impractical answers. Once against the rules,the procedure will stop and step into the next cycle automatically, which would reduce the optimization time greatly and ensure the reasonableness of results.At last, 6Mt/a dual vocuum columns crude oil distillation unit can be optimiazed according to the method above. Sensitivity analysis was done to identify numbers of decision variables corresponding to three double objective function which were 6, 7 and 10 respectively. Take double objective function of maximum light oil yield and minimum energy consumption as example, decision variables are atmospheric furnace outlet temperature, first-vacuum furnace outlet temperature, atmospheric column bottom steam flow rates, second-side production of atmospheric column, third-side production of atmospheric column, third-side Engler distillation 95% point temperature of the first-vacuum column.Compared to existing operations, energy consumption of atmospheric furnace and the first-vacuum furnace decrease by 8.2%. and save 2507 yuan/ hour(21.05 million yuan /year).This paper don’t make any restrictions in study subjects. So the multi-objective optimization method based on process simulation software is suitable for all objects simulated by PRO /II(also adapted to the objects simulated by ASPEN after appropriate modifications).