Control Strategy Of A Crude Oil Desalting Unit

Abstract

Large amounts of dissolved salts can affect the crude refining process quite significantly. It can, for instance, foul heat exchangers, block pipe lines and generally affects the performance of other refinery equipments. In order to avoid salts-related problems, salts must be removed from the crude oil. Alfola crude oil, selected as the case study for this research, has a high calcium contents (calcium chloride) which amounts for up to 1600 ppm and must be lowered down to approximately 100 ppm to avoid equipment malfunctions and to obtain sellable quality crude. Calcium salts can be removed by the addition of other chemicals (decalcinizer) to the process. The dose of the decalcinizer is quite critical as it enhances the removal of calcium salts in the crude oil to a maximum attainable level. Further additions of the decalcinizer may be costly and may lead to the deterioration of the decalcinization process. Various decalcinizing materials were studied, acetic acid was found among these materials as an effective and quicker calcium removal compound. The objective of this study is to explore the controllability of the rate of acetic acid dosing in an attempt to keep the decalcinizer concentration at a level that will attain maximum removal of calcium. iii A cascade and a feedback control systems were considered. Three assumptions were explored to account for the possible variations in i) process lag (td) and ii) the process time (τ p). The process feedback and cascade control loops were then tuned and later evaluated for each of these assumptions using Routh-Hurwitz, Bode, Nyquist and Root-locus stability criteria. It was found as a result of this research that the cascade control system consistently gave better results in all cases considered. It may therefore be more appropriately applied for the crude oil decalcinization process studied.