Desalting Efficiency and Salt Management in Critical Sour Facility

Abstract One of the largest sour oil and gas processing stations in South of Oman experienced loss of desalting efficiency that led to increase in salt in crude content affecting downstream equipment integrity and oil export specification. The objective of this paper is to present the investigation steps taken to identify the root cause of the efficiency loss, key findings and measures taken to restore the desalting train functionality. Process simulations, fluid analyses, root cause analyses, material sample analysis and performance test trials were the key processes behind the troubleshooting analysis to identify the causes of the problem. Performance test trial was conducted with support of Desalter Vendor and process system adjustments were made to verify the impact on desalter efficiency. Sour crude samples were taken from desalter inlet and outlet streams during the trial and analyzed. Innovative and alternative desalter technologies have been considered and evaluated for process upgrade. Inspection of the equipment during the turnaround has revealed significant findings that helped to reveal root cause of the problem. During the performance trial conducted along with the equipment manufacturer several operating modes were tried to check the impact on the desalter efficiency. To achieve stringent basic sediment and water content of <0.15% at desalter outlet stream, double stage desalter process is deployed consisting of three-phase horizontal separator as first stage and electrostatic coalescer separator as second stage. Fresh wash water is supplied in the inlet of the second stage desalter via conventional mixing valve and static mixer. Water from second stage desalter can be recycled back either into the first stage or to the second stage or by utilizing the two routes simultaneously, allowing to increase the wash water content according to the demand. The lowest salt content was observed when recycle wash water was utilized simultaneously in first stage and in second stage inlet. The performance trial was concluded with recommendation of process upgrade either through higher wash water flowrate upstream the static mixer/desalter to improve desalting efficiency or by deploying alternative mixing equipment with higher mixing efficiency. Utilizing more efficient mixer technology considerable savings could be made in terms of capital costs associated with wash water system upgrade and operating costs of wash water treatment. Inspection of electrostatic desalter took place during major turnaround after the desalter performance trial and revealed significant amounts of deposits inside the vessel. Upon detailed laboratory analysis of the sludge samples, it was found that major constituent was asphaltene content. Desalter efficiency was reinstated with installation of asphaltene dispersant injection. Limited number of crude samples and quantity could be obtained during the investigation process due to high H2S content in the crude. Low water content in the crude was additional challenge as it created uncertainty around water salinity when conventional salt in crude measurement methods were deployed. All together these challenges prevented to identify exact source of salt and tackling the problem at the origin.