Optimization Of Heat Exchanger Network In Crude Oil Preheat Train Considering Fouling Problem

Crude oil preheat train(a specified heat exchanger network)of the refinery consumes large amount of energy,and about 60%～70% of the heat in the system is provided by the heat of the downstream products recovered by the heat exchanger network(HEN).Thus,the optimization of HEN is of great significance.Fouling is a long-standing operational problem in refinery,and it can increase pressure drop,reduce heat transfer efficiency and aggravate corrosion of the equipment,and even causes the shutdown of the train.Therefore,it is necessary to consider fouling in HEN optimization.This paper reviewed previous HEN synthesis approach and the development of HEN optimization considering the fouling problem.The advantages of plate heat exchanger(PHE)compared with the shell and tube heat exchanger and the application of PHEs in crude oil preheat train is also introduced.According to the advantages and disadvantages of conventional optimization methods,two new methods are proposed in this paper to promote HEN performance with consideration of pressure drop,heat transfer and fouling simultaneously based on the velocity correlation model.The main work is as follows:Chapter 2 puts forward a method to promote refinery HEN performance by simultaneously optimizing the velocity distribution and flow rate distribution with consideration of crude oil fouling.The velocity distribution of HEN is changed through the detailed structure modification of the heat exchangers and adjusting split ratios.Both static optimization and dynamic optimization are considered.Chapter 3 establishes a velocity correlation model suitable for PHEs,and puts forward a new HEN optimization method through replacing traditional shell and tube heat exchangers by PHEs in crude oil preheat train.Binary variable is introduced to optimize the location for applying PHE,and the velocity distribution of heat exchanger and the heat transfer area of PHE are considered simultaneously.In chapter 4,two case studies are carried out to illustrate the application of the proposed methods.The optimization results show that the proposed methods both can improve the performance of HEN.In the case study of optimizing the velocity distribution and flow rate distribution with consideration of crude oil fouling,the dynamic optimization has better economic efficiency than static optimization.In the case study of applying PHEs in crude oil preheat train,the optimization results show that when PHE is used to optimize the HEN,it will give priority to the shell and tube heat exchanger with higher heat capacity flow rate of hot stream.The optimization process tends to maximize the use of original pump power,and PHEs have priority over other shell and tube heat exchangers to obtain pump power to increase velocities.The application of PHEs can significantly improve the energy saving and economic benefit of HEN,as well as improve fouling of heat exchangers.Considering the dynamic characteristics of fouling problem,the investigated time period is evenly divided into several intervals in the modeling process of the two new optimization methods.In every interval all parameters are reestablished and adjacent intervals are connected by fouling thermal resistance.In order to avoid the tedious iterative process,heat transfer unit method(NTU)is used to calculate the outlet temperature of the streams in each heat exchanger.Simulated annealing algorithm(SA)on the optimization platform is used in the optimization process to solve the mathematical model in this paper,and the optimization objective function is the minimum annual total cost of the HEN.