Flow Velocity Optimization In Fixed Heat Exchanger Network Structure

Fouling is an inevitable problem for heat exchanger operation.It will severely decrease the efficiency of heat transfer and increase pressure drop,resulting in the rise of operation cost,even the shutdown of facility.Fouling rate can be calculated by threshold fouling model,which describes the competition between deposition and depression terms.Flow velocity and wall temperature are the two key factors in this expression.Low velocity and high temperature will lead to severe fouling.Compared with wall temperature,flow velocity is a more important variable,because it can correlate heat transfer coefficient,pressure drop and fouling rate.Obviously,high velocity is benefitical for fouling mitigation,but at a risk of insufficient pump capacity.For a case with constant throughput,there are two ways to change flow velcociy in a heat exchanger,modifying heat exchanegr and adjusting split ratio in branch.In this thesis,flow velocity is emphasized as a key variable to optimize heat exchanger networks.Fouling is a dynamic process,during which the layer is getting thicker with fouling deposition,leading to an increase in velocity.Heat transfer coefficient,pressure drop and fouling rate can interact with each other.To describe the dynamic nature,the investigated time horizon is divided into some intervals.In each interval,all parameters related to fouling are assumed to be constant,and fouling resistance is used as a link between the two adjacent intervals,then the performance can be obtained using NTU method.The optimization algorithm is simulated annealing,and by searching solution with this algorithm,the optimal velocity distribution can be obtained.Crude oil preheat train is a major energy consumer in a refinery.In this thesis,a preheat train is considered as a case study.The results show that the proposed model is efficient in heat exchanger networks with fixed structure.