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SUMMARYThis work deals with a thermodynamic analysis of crude oil distillation systems to study energy and exergy efficiencies for system analysis, performance evaluation and optimization. In this regard, a new analysis methodology is proposed, and the SimSci/PRO II (2000) is employed as a simulation package for the required parametric studies, particularly to study the effects of the distillation column temperatures and pressures on the efficiencies. The simulations are carried for the operating conditions of the model. It is found that the total irreversibility losses are 608 MW for a flow rate of 507 kg s À1 [e.g. 56% by the Atmospheric Distillation Unit (ADU), 26% by the Vacuum Distillation Unit (VDU) and 18% by the heaters]. The highest irreversibility losses occur in the ADU as the main separation takes place there. Of the losses, 6.2% are due to chemical exergy losses associated with the separation process itself. The rest of the losses are due to the physical exergy losses mainly because of the temperature difference. The energy efficiencies are 0.497 for the ADU, 0.579 for the VDU and 0.519 for the overall system. The exergy efficiencies are 0.433 for the ADU, 0.501 for the VDU, 0.821 for Heater 1, 0.956 for Heater 2 and 0.233 for the overall system. The overall efficiency is not equal to the product of the components efficiencies.

In a refinery distillation plant, there are many components of interest to be analyzed thermodynamically, e.g., the crude oil heating furnace, the distillation column and a network of heat exchangers. Previous studies showed that the highest exergy losses occur when there is a heat transfer process especially in the crude oil heating furnace where high quality fuel is used to heat the crude oil, which is a low quality duty, beside the high temperature difference. Therefore, it is proposed in this work to perform distillation in two stages rather than one to reduce heat duty of the heating furnace and thus reducing irreversible losses. In this paper, energy and exergy analyses of a traditional one-stage crude oil distillation unit and a newly proposed two-stage crude oil distillation unit are conducted to study energy and exergy efficiencies of these units and determine the exergy losses. The results are compared for both one- and two-stage distillation units. In this regard, a commercial software package, SimSci/PRO II program is used to carry out both energy and exergy calculations. It is found that the overall exergy efficiencies for single- and two-stage distillation units are 14.0% and 31.5%, respectively. The proposed two-stage distillation unit shows 43.8% decrease in the overall exergy losses and 125% increase in the overall exergy efficiency.

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