Design of Heat Integrated Low Temperature Distillation Systems

This work addresses the challenges in design of heat integrated low-temperatureseparation processes. A novel, systematic and robust methodology is developed,which contributes to the design practice of heat-integrated separation sequenceand the refrigeration system in the context of low-temperature separationprocesses. Moreover, the methodology exploits the interactions between theseparation and refrigeration systems systematically in an integrated designcontext.The synthesis and optimisation of heat-integrated separation processes iscomplex due to the large number of design options. In this thesis, taskrepresentation is applied to the separation system to accommodate both simpleand complex distillation columns. The stream conditioning processes aresimulated and their associated costs are included in the overall cost of theprocess. Important design variables in separation systems, such as theseparation sequence, type and operating conditions of the separation units (e.g.the operating pressure, feed quality and condenser type) are optimised.Various refrigeration provision strategies, such as expansion of a processstream, pure and mixed multistage refrigeration systems and cascades ofmultistage refrigeration cycles, are considered in the present work. A novelapproach based on refrigeration system database is proposed, which overcomesthe complexities and challenges of synthesis and optimisation of refrigerationsystems in the context of low-temperature separation processes. Themethodology optimises the key design variables in the refrigeration system,including the refrigerant composition, the number of compression stages, therefrigeration and rejection temperature levels, cascading strategy and thepartition temperature in multistage cascaded refrigeration systems.The present approach has selected a matrix based approach for assessing theheat integration potentials of separation and refrigeration systems in thescreening procedure. Non-isothermal streams are not considered isothermal andstream splitting and heat exchangers in series are taken into account. Moreover,heat integration of reboiler and condenser of a distillation column through anopen loop heat pump system can be considered in this work.This work combines an enhanced simulated annealing algorithm with MILPoptimisation method and develops a framework for simultaneously optimisingdifferent degrees of freedom in the heat integrated separation and refrigerationprocesses.Case studies extend the approach to the design of heat integrated separationsequences in above ambient temperature processes. The robustness of thedeveloped framework is further demonstrated when it is utilised to design theLNG and ethylene plant fractionation trains.