Study On The Design And Optimization Of Multiple Interconnected Utility Systems In An Integrated Refinery And Petrochemical Complex

The utility system is the major contributor to fuel consumption,design and operation of which will directly affect the fuel efficiency and economic benefits of the enterprises.In an integrated refinery and petrochemical complex,there is a centralized utility system(CUS)to provide or collect steam from the production plants and also two sub-utility systems(SUSs)locating inside alkene and refinery plants respectively to satisfy the shaft demands.Obviously,the complex utility system is typical multiple interconnected utility systems.Although a large volume of researches on the design and optimization of a single utility system in alkene or refinery plants has been published,there is little research related to the multiple interconnected ones.Thus,the research in this field is very urgent.Compared with a single one,the design and optimization of multiple interconnected utility systems need not only to determine the optimal structure and operating conditions of every single one but also to consider the interactions between CUS and SUSs.Therefore,mixed-integer nonlinear programming(MINLP)model was established for the design and optimization of multiple interconnected utility systems in this paper.Then this model is applied to the integrated refinery and petrochemical complex.The main work of this paper is as follows:(1)In order to determine the optimal configuration of the utility system,a superstructure of multiple interconnected utility systems was extended from the one of single utility system.As important equipment in the utility system,the complex steam turbine(steam turbines with extractions)model was modified to improve the accuracy.(2)A simultaneous optimization model for the design and optimization of multiple interconnected utility systems was built according to the proposed superstructure.To study the effect of the steam main temperature on the utility system structure and the corresponding operation conditions,two different scenarios were investigated in this paper.The results indicate that the steam flows through the letdown valves were substantially reduced by 86.2% in the scenario concerning the optimized main temperatures.(3)In order to reduce the size and complexity of the model,a multistep optimization model was established according to the characteristics of multiple interconnected utility systems.Then this model was applied to the complex utility system.Compared with the simultaneous optimization model concerning the optimized temperature,the steam flows through the letdown valves and condensing turbines were reduced by 30.8% and 13.9% respectively by adopting the multistep optimization model.(4)There is a large amount of steam entering condensing turbines in the SUSs locating inside alkene and refinery plants separately so that the thermal efficiency is decreased.To tackle this problem,the selection of direct-drive gas turbines is also allowable besides direct-drive steam turbines and electric motors in SUSs.The multistep optimization model concerning direct-drive gas turbines was applied to the complex utility system.The results indicate that the steam flow through the condensing turbines in the SUS locating inside the refinery plant is reduced by 60.9%because the direct-drive gas turbine was selected.