Impact of American Petroleum Institute Standards Variation on Crude Distillation Unit

Abstract

This work aimed to study the effects of crude oil variation on the performance of a crude distillation unit (CDU). CDUs are generally designed for specific type of crude oil. However, the crude oil varies with time as a natural result of continuous reservoir depletion. The CDU of Khartoum refinery was taken as a case study. Data covering the period 2002 to 2012 was collected. The collected data include crude oil assay, feed and products flow rate, and CDU design and operation data. The software HYSYS simulation program was used to simulate refinery processes. The main parameters that are considered include (1) design parameters of trays numbers and feed tray position and (2) adaptation measures of reflux ratio. The CDU performance was measured by yield percentage; the percentage of the product relative to maximum product that could be obtained in accordance with crude assay. The HYSYS program was validated by an example from the literature. Regarding design parameters, the results showed that, when the crude oil becomes heavier than the initial one (API decreases from 34.81 to 30.58) and under the same CDU number of trays and feed position the performance decreases. Under these conditions it yields less light product Naphtha and more of the heavy product residue. The Results showed that the liquid volume percent decreased with number of trays for light products, whereas the percentage increased for heavy product. The efficiency of producing naphtha was found to be decreasing with increasing number of trays. And the efficiency of minimizing the production of ATM Residue was found to be decreasing with increasing number of tray. It is recommended to reduce number of trays to maximize the volume percentage of naphtha, and to minimize the volume percentage of ATM Residue. The results of the effect of feed tray position showed that the liquid volume percentage of the light products increases with the feed tray position decreases, whereas the yield decreases for heavy product. The adaptation measures of reflux ratio need to be decreased to obtain high light product. The study recommends the development of flexible CDU design by which the number of trays and feed position can be changed. The current design does allow for such changes. Flexible design however, calls for flexible pipe work and heat exchanger network as well.