Process Simulation And Thermodynamic Analysis Of Multi-stage Crude Oil Distillation Units

Crude oil distillation unit (CDU) is the first major processing unit in the refinery.All oil entering a refinery runs through the CDU system, hence CDU’s size andoperating cost are the largest in the refinery. CDU is an energy-intensive process thatusually consumes total20-30%of the energy used within a refinery. Since CDU issuch a large energy user, the utilization of thermodynamic analysis to reduce energyconsumption in CDU is a significant task for engineers. Multi-stage CDU is theprocess that adds one or more vacuum distillation units between atmosphericdistillation unit (ADU) and vacuum distillation unit (VDU) in a conventionalthree-stage CDU. Multi-stage CDU is a cascade distillation process that transferspartial processing load of ADU and VDU to the new additional distillation units.There is a controversy that whether four or more-stage CDU could achieveenergy conservation compared to conventional three-stage CDU. So, it appears to be asignificant work to investigate multi-stage effect in CDU process in this study toclarify the intrinsic variances, especially the variances in thermodynamics betweenthree-stage CDU and four or more-stage CDU and discuss whether multi-stage CDUis a feasible and energy-saving scheme for CDU.The PRO/II simulation software (Invensys, Simsci) is employed to perform thisstudy. Braun K10is selected for the thermodynamic data. In this paper, rigorous four-,five-and six-stage CDU processes based on a conventional three-stage CDU processare proposed. The variables including the design guidelines, the operation methods,the quality and distillate yield of crude oil, the quantity and composition of each kindof product and the quantity of steam injected are controlled strictly to be the samebetween four processes.As the results of energy and exergy analyses show, heat demand of CDUdecrease as the number of additional stages increases for the reason that low boilingpoint products could avoid unnecessary overheating with the additional distillationunits. When taking no account of the exergy losses occurring in heaters and coolers,three-and four-stage CDU have lower exergy losses than five-and six-stage CDU.And when considering the overall exergy losses, four-stage CDU has the lowest exergy losses among four processes as φ≤0.306and six-stage CDU is the optimumdesign from the viewpoint of exergy loss as φ>0.306.As the result of pinch analysis shows, the variances especially those in energyflows lead to the variances in the ability to make use of the heat transferred from cycleslurry oil of FCC among three-, four-, five-and six-stage CDU. When there is aplenteous supply of cycle slurry oil, the CDU process with more stages would bebetter design scheme with lower energy consumption. For example, when12MWcycle slurry oil is introduced into heat exchange networks, four-, five-and six-stageCDU would consume energy10.5%,17.4%and22.2%lower than three-stage CDUrespectively.