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Heat Integrated Crude Oil Distillation System design
Lu Chen
[Thesis].University of Manchester School of Chemical Engineering and Analytical Science;2008.
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Abstract
Heat-integrated crude oil distillation systems – the atmospheric and vacuumdistillation towers and associated heat recovery system – are energy and capitalintensive. The structures of the distillation columns are very complex and thedistillation columns interact strongly with the preheat train. There are many degreesof freedom in this system, most of which are interlinked with each other and cannotbe considered separately. A systematic design approach is necessary to exploitthese design issues for increasing the efficiency with which energy and capital areemployed in the overall system.This thesis develops an optimisation-based methodology for the simultaneousdesign of crude oil distillation systems. Both new design and retrofit scenarios areconsidered. This design approach considers some significant design issues andgenerates design solutions that are realisable and industrially practicable. Robustand more accurate models have been developed to represent the distillationcolumns and heat exchanger networks (HENs) within an optimisation framework,compared with previous work.Facilitated by the decomposition approach (Liebmann, 1996), simplified models(Suphanit, 1999; Gadalla et al., 2003a; Rastogi, 2006), based on the Fenske-Underwood-Gilliland method, were developed previously to model the atmosphericdistillation unit and the vacuum unit. This work extends and modifies these simplifiedmodels to account more accurately for the effect of pump-around location on theseparation performance in atmospheric units. Moreover, the simplified model hasbeen extended to consider an atmospheric distillation column with a pump-aroundlocated above the top side-stripper.This work also proposes a new methodology to incorporate product specificationsfollowing refining conventional in the simplified models. The proposed approachenables systematic identification of key components and associated recoveries tomatch specified boiling temperature profiles, as these are normally used asindicators of separation performance in the refining industry. The new simplified17models are validated by comparison with rigorous simulation results of atmosphericdistillation columns.Multi-segmented stream data are implemented in the design and analysis of heatexchanger networks, in which the thermal properties of streams are temperaturedependentand cannot be assumed constant. Two existing promising HEN designapproaches, the simulated annealing optimisation-based approach (Rodriguez, 2005)and the network pinch approach (Asante and Zhu, 1996), are modified and extendedto apply to the HEN design with multi-segmented stream data. In the modifiednetwork pinch approach, the bottleneck of an existing HEN configuration is betterovercome by varying stream split fractions and heat exchanger loads at the sametime, rather than simply redistributing heat loads. The modified network pinchapproach also combines structural modifications and cost optimisation in a singlestep to avoid mi ssing cost-effective design solutions.An optimisation framework, applying a stochastic optimisation method – multiplesimulated annealing runs – is developed to generate grassroots and retrofit designsof the heat-integrated crude oil distillation systems. The heat integration of thesystem is accounted for more accurately than previously by using multi-segmentedstream data. Operating conditions and pump-around locations of distillation columnsare optimised, together with structural options and continuous variables of heatexchanger networks as appropriate, in a single optimisation framework.The new degrees of freedom considered in this work include key components andassociated recoveries (used in simplified models of distillation columns to expressthe separation of products) and operating pressures of distillation columns. Theoptimisation of key components and recoveries allow the systematic exploitation ofproduct distributions and product slate in order to maximise net profit. Includingoperating pressures in the optimisation facilitates creation of heat recoveryopportunities in configuration studies. Product specification constraints are imposedin the optimisation so that product quality is not compromised during design.A novel distillation configuration, with a liquid side-draw prefractionator columnupstream of an atmospheric distillation column, is proposed in this work. The casestudy shows a ver y promising performance with respect to energy efficiency.18Case studies illustrate the beneficial application of the proposed approach in bothgrassroots and retrofit design of crude oil distillation systems, with respect to energydemand and net profit improvement. Comparisons are made between differentconfigurations, and results are given as proof of principle.