Research On Integrated Optimization Of Production System And Energy System Of Petrochemical Enterprise

Oil refining industries account for an important part of global energy consumption and the main part of the energy of oil refining is utility generation. In the current global market, the pressure for the improvement of energy utilization efficiency and more strict environmental restrictions have united to force the enterprise to enhance the existing optimization approach for the enterprise-wide planning. Due to significant interactions between the production system and utility system, it is imperative to consider the simultaneous optimization of the two systems for the global optimal solution. In this thesis, after reviewing recent researches on the planning optimization for the production system and utility system in both petroleum refinery and the ethylene plant, we propose several novel models integrating the production and the utility system for the petrochemical complex. These models includes the production planning model for refinery, energy operational optimization model for the petrochemical enterprise, the integrated optimization model of the production and utility system in the refinery, the production planning model of the ethylene plant integrating energy consumption, and the integrated model for the refinery and the ethylene plant. The details are listed as follows:1. A refinery-wide production planning model integrating process operations constraints which include the cut points of CDU (the Crude oil Distillation Unit) and the conversion of FCC (the Fluid Catalytic Cracker) was proposed to obtain the optimal refinery production plan and to achieve the optimization plan of the unit operating conditions simultaneously. The mass balance of the materials, product quality constraints and the major process models with operation condition were incorporated in the proposed mathematical model. The model was illustrated by a case study of a real refinery. The optimization results of production planning proved an effective guidance to the real industrial production due to its process operation integration.2. A general modeling framework for operation optimization of energy system in petrochemical enterprise is proposed as a multi-periods mathematical model incorporating the detailed semi-kinetic model of key equipment for energy consumption and generation. Piecewise linear approximation method is used for accurate characterization of the load condition of boiler and turbines. The proposed optimization approach offers an effective guidance for industrial operation and a basis for following integrated optimization.3. A novel integrated optimization approach which couples the production system and the utility system of a refinery is proposed to explore the potential of increasing the overall refinery margin and reducing the economic losses of energy waste compared with the traditional approach where the two systems are optimized in a sequential procedure. A multi-period refinery-wide mixed-integer nonlinear programming (MINLP) model is formulated to optimize the production planning of the processing unit and the operational planning of the utility equipment simultaneously. The energy consumption and generation model of processing unit is introduced to correlate the two systems in the energy aspect by linking the amount of energy consumption and generation with unit throughput, product property and operation mode. Material balance of fuel oil and fuel gas is considered to integrate the two systems in the material aspect for improving energy utilization in multi-period planning site. An industrial example shows that the integrated approach not only obtains the optimal unit operations and gains improvement in overall profit of the refinery production, but also leads to significant performance in energy saving and emission reduction compared with the sequential approach.4. A solution strategy based on heuristics to solve the integrated model of the refinery’s two systems (the production system and the utility system) to deal with inconsistency between solution quality and time resulting from the bilinear terms for the correlation of the blending process and gas emission in the original MINLP model. The integrated model is decomposed into a mixed-integer linear programming (MILP) model and a nonlinear programming (NLP) model which are then solved iteratively through variables transferring to further reduce the solution time. The solution of the traditional sequential method is incorporated to generate better initial estimates for the decomposed model to gain better solution quality and efficiency.5. A novel short-term planning model of the ethylene plant that incorporates the operating variables and energy utilization in both the thermal cracking and the down-stream process is proposed to explore the potential for increasing the production margin and reducing the energy losses. A multi-period mixed-integer nonlinear programming model is formulated to attain the scheduling of parallel furnaces and the energy distribution of the overall plant, along with the determination of the key process operation involving the coil outlet temperature (COT) and coke deposition. The behavior of the product yields and coke formation in terms of varying COT profiles is investigated to enhance the profitability of the whole plant. A real industrial example is investigated to exploit the performance of the proposed model. The results show that the integrated approach attains an improvement in overall profit and achieves significant enhancement in energy savings, compared with the original optimization approach.6. A novel integrated optimization approach which couples the up-stream refinery and the down-stream ethylene plant is proposed to explore the potential of increasing the overall margin and reducing the economic losses of intermediate products compared with the traditional approach where the two complexes are optimized in a sequential procedure. A multi-period enterprise-wide mixed-integer nonlinear programming model is formulated to optimize the production planning of the processing units and the operation of the energy system in the refinery and the ethylene plant simultaneously. Material balance of raw material to the ethylene cracking produced by the processing unit and the intermediate products transported from the ethylene plant to the refinery is considered to integrate the two complexes in the material aspect in multi-period planning site. Energy balance of various utilities between the energy system and the production system in the refinery and the ethylene plant is considered to integrate the material and energy flows. The Lagrangian algorithm is applied to decompose the integrated mathematical model into an MILP problem for the production planning model of refinery and a small-scale MINLP problem for cracking furnaces’ scheduling, along with the heuristic and model relaxation strategy, to produce better solutions for the integrated planning problem. Results of the case show that the integrated approach obtains the optimal unit operations and gains improvement in overall profit of the refinery production compared with the traditional sequential approach.