| Summary: | Heat-integrated crude oil distillation systems – the atmospheric and vacuum distillation towers and associated heat recovery system – are energy and capital intensive. The structures of the distillation columns are very complex and the distillation columns interact strongly with the preheat train. There are many degrees of freedom in this system, most of which are interlinked with each other and cannot be considered separately. A systematic design approach is necessary to exploit these design issues for increasing the efficiency with which energy and capital are employed in the overall system. This thesis develops an optimisation-based methodology for the simultaneous design of crude oil distillation systems. Both new design and retrofit scenarios are considered. This design approach considers some significant design issues and generates design solutions that are realisable and industrially practicable. Robust and more accurate models have been developed to represent the distillation columns 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 atmospheric distillation unit and the vacuum unit. This work extends and modifies these simplified models to account more accurately for the effect of pump-around location on the separation performance in atmospheric units. Moreover, the simplified model has been extended to consider an atmospheric distillation column with a pump-around located above the top side-stripper. This work also proposes a new methodology to incorporate product specifications following refining conventional in the simplified models. The proposed approach enables systematic identification of key components and associated recoveries to match specified boiling temperature profiles, as these are normally used as indicators of separation performance in the refining industry. The new simplified models are validated by comparison with rigorous simulation results of atmospheric distillation columns. Multi-segmented stream data are implemented in the design and analysis of heat exchanger networks, in which the thermal properties of streams are temperature dependent and cannot be assumed constant. Two existing promising HEN design approaches, the simulated annealing optimisation-based approach (Rodriguez, 2005)and the network pinch approach (Asante and Zhu, 1996), are modified and extended to apply to the HEN design with multi-segmented stream data. In the modified network pinch approach, the bottleneck of an existing HEN configuration is better overcome by varying stream split fractions and heat exchanger loads at the same time, rather than simply redistributing heat loads. The modified network pinch approach also combines structural modifications and cost optimisation in a single step to avoid missing cost-effective design solutions. An optimisation framework, applying a stochastic optimisation method – multiple simulated annealing runs – is developed to generate grassroots and retrofit designs of the heat-integrated crude oil distillation systems. The heat integration of the system is accounted for more accurately than previously by using multi-segmented stream data. Operating conditions and pump-around locations of distillation columns are optimised, together with structural options and continuous variables of heat exchanger networks as appropriate, in a single optimisation framework. The new degrees of freedom considered in this work include key components and associated recoveries (used in simplified models of distillation columns to express the separation of products) and operating pressures of distillation columns. The optimisation of key components and recoveries allow the systematic exploitation of product distributions and product slate in order to maximise net profit. Including operating pressures in the optimisation facilitates creation of heat recovery opportunities in configuration studies. Product specification constraints are imposed in the optimisation so that product quality is not compromised during design. A novel distillation configuration, with a liquid side-draw prefractionator column upstream of an atmospheric distillation column, is proposed in this work. The case study shows a very promising performance with respect to energy efficiency. Case studies illustrate the beneficial application of the proposed approach in both grassroots and retrofit design of crude oil distillation systems, with respect to energy demand and net profit improvement. Comparisons are made between different configurations, and results are given as proof of principle.
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