Retrofit of Existing Heat-Integrated Crude Oil Distillation Systems for Energy Recovery

Heat integration is implemented in crude oil distillation systems to reduce energy consumption. In recent years, increasing fuel prices, tighter margins and stricter environmental regulations have motivated the development of retrofit projects to further improve the energy-efficiency of existing distillation systems (i.e. distillation process and associated heat exchanger network).

To support this goal, this paper presents a new optimisation-based approach to retrofit existing crude oil distillation systems. In this approach, the distillation process and the heat exchanger network (HEN) are considered together to find the operating conditions (e.g. furnace inlet and outlet temperatures, pump-around flow rates and temperature drops) and HEN retrofit modifications (e.g. add or remove a heat exchanger or stream splitter) that reduce energy consumption.

In this new approach, it is possible to monitor and constrain the heat transfer area (existing and additional area) of individual heat exchangers during optimisation. Thus, practical constraints related to heat transfer area (e.g. lower and upper bounds on additional heat transfer area, plant layout restrictions, etc.) can be easily captured and implemented in the optimisation framework. Other important features are the consideration of additional practical constraints (e.g. product quality, column flooding, maximum number of HEN retrofit modifications) and the consideration of temperature-dependent heat capacities.

The proposed methodology is applied to an industrial distillation system consisting of a prefractionator, an atmospheric distillation unit, a vacuum distillation unit and the associated HEN. The methodology provides industrially-applicable designs with decreased energy consumption and relatively few equipment modifications.