Research On Refinery Production Scheduling

Scheduling is the key of Manufacturing Executing System (MES), which links process control system and business management system in process industry. It determines whether production process is running smoothly, influences production cost and reasonable utilization of resources. It is oil refinery that is one of the typical representatives of process industry. The refineries have to make great efforts to increase profits while decrease costs under market competition. The optimal solution of refinery production scheduling can reduce the costs of purchase, transportation, storage, and make the best of production capacity, improve efficiency of resource utlization and capacity ratio, meet demands and maximize profits, the refinery production scheduling problem receives significant attention in recent year. However, the refinery scheduling problem is difficult to solve for its complexity, uncertainty, nonlinearity, multi-objectives and multi-contraints. In many literatures, scheduling object was one section of production process in refinery such as tank farm storage, production unit process, crude oil or products blending, but entire refinery was not considered as the scheduling target. It is obvious that local production optimization does not achieve global refinery optimization. The main works of this dissertation are as follows:(1) A general modeling framework and modeling approaches for scheduling in refineries are proposed. Modeling frameworks of processing units, storage tanks, and pipelines, which are introduced for operational planning (Neiro and Pinto), are extended for operational scheduling. The most important extension is that operation modes for these entities are considered. And at the same time, another framework for perimeter unit is introduced to control raw material supplies and product demands. A complex production topology can be built by these grouped elementary entities that are interconnected by intermediate streams.(2) We reveal that many modeling problems in refineries can be understood from a logic and qualitative view. This work will shed light on the logic essential of many mathematical formulations presented in the literature. And, how to convert logic expressions to mathematical formulations is also presented. The application of the approach to some modeling problems is elaborated. These modeling problems include: modeling some important operation rules in refineries, decomposing large- index binary variables to small-index binary variables and linearization of some nonlinear formulations.( 3 ) Based on the modeling framework and the logic modeling methods, a scheduling optimization model for refineries is built. This example shows how to build scheduling models by using the modeling framework, and how to model operation rules by using the logic inference. The computational results show that the operation rules, such as a minimum amount of time to allow the settle of tanks after receiving materials, that just one tank receives the same kind of material at the same time and a tank cannot receive and send material at the same time, that one mode of a tank can only hold one kind of material, will lead to a large number of 0-1 variables that would make the model solution unattainable within reasonable time.( 4 ) A hierarchical approach with two decision levels for short-term scheduling problems in refineries is proposed. This approach includes two levels: the optimization model at the upper level and the heuristics and rules adopted in simulation system at the lower level. The optimization model is used to decide sequencing and timing of the operation modes of processing units and pipelines, and to determine the quantities of materials consumed/produced by each operation mode of a unit. Only aggregate tanks are used at the upper level, and the simulation system at the lower level uses a heuristic to adjust the operation modes of some actual multipurpose tanks within aggregate tanks. The iteration procedure between the upper level and the lower level is used to find optimal solution under new aggregate storage capacities. The simulation system also uses some heuristics and rules to arrange actual tanks to receive/send materials with logic operation constraints (operation rules) on tanks respected.( 5 ) A methodology for the integration of production planning and scheduling in refineries is proposed. This method also relies on the two-level hierarchical approach. In this paper, uncertainties are also considered, and, two robust optimization approaches presented for optimization problems under uncertainty are integrated to cope with uncertain parameters with continuous and discrete probability distribution respectively.