Multi-plant Heat Integration Research Using Mathematical Programming Methodology

Energy crisis and environment pollution are two serious challenges for human life in the 21 st century.Because petrochemical processing plants consume a large amount of energy,it is particularly necessary to implement heat recovery within plants to decrease energy consumption and increase energy efficiency in industries.Energy is also the critical fundamental in industry economies that are often significant in the drive of profit.With development of studies,heat exchanger network synthesis for intra-plant heat integration has been extended to multiple heat exchanger networks synthesis for multi-plant heat integration.Firstly,early studies mostly focus on sequential methods which cannot target intra-plant and inter-plant heat integration holistically.Secondly,different processing plants always have different operation time hence it is necessary to design network to meet the heating and cooling demand in different operating scenarios.Finally,most previous studies are based on graphic methods which cannot make the trade-offs between capital investment and energy expense simultaneously.Given these research backgrounds,this dissertation researches multi-plant heat integration by using mathematical programming methodology.For instance,(1)direct multi-plant heat integration using process streams,(2)an energy hub approach for direct multi-plant heat integration,(3)indirect multi-plant heat integration using thermal oil circles,(4)indirect multi-plant waste heat integration using hot water circles and(5)multi-plant heat integration combining direct and indirect methods.This study implements multi-plant heat integration via transporting process streams to directly transfer heat between individual plants.A novel mathematical programming methodology is proposed for such energy conservation approach.The methodology can target both intra-plant and inter-plant heat integration simultaneously with the objective of minimizing the total annualized capital investment.Using the proposed methodology,process designers obtain process stream transportation networks,inter-plant connections and detailed heat exchanger networks within plants.This work proposes an energy hub approach for direct multi-plant heat integration.Inter-plant heat transfers are performed only in the hub plant.In this manner,the overall network system is more flexible and reliable.Also,different processing plants may have different industrial operating time.During different shutdown scenarios,some efficient valves are added and installed to turn on or off to control both intra-plant and inter-plant heat exchanger networks which can meet the heat transfer demands in all conditions.Thermal oil and hot water are good heat transfer medium so this work selects them as the intermediate fluids to study indirect multi-plant heat integration.A mathematical programming methodology makes use of a superstructure representation to design both intra-plant and inter-plant heat recovery networks simultaneously.Using the introduced methodology,process designers can obtain interconnections among plants,the flow rate of intermediate fluids and detailed networks within each plant automatically.This work also develops an efficient solution strategy to overcome the nonlinear term in the model.Thus,the computational difficulties are reduced and better solutions are obtained.This research introduces a combined approach to make good use of both direct and indirect heat integration methods to target multi-plant heat integration network.Multiple kinds of energy carriers are used as the intermediate fluids to transfer latent and sensible heat among different plants such as steam,thermal oil and hot water.The corresponding mathematical programming method is based on a new representative superstructure that covers most possible networks for heat integration in multi-plant sites.Both intra-plant and inter-plant heat integration are optimized synchronously.