Economic Feasibility of Crude Desalting With Multistage Agitated Extractors
Authors: Alatiqi I.M. , Gasmelseed G.A. 
Published: December 2007

Desalting of crude oil for processing is related to environment protection and corrosion problems. Spherical, vertical and horizontal desaltors and electrodehydrators have been used for desalting of crudes, with a considerable processing cost. The crude oil contains fine globules of brine that remain in the oil. Natural surfactants are absorbed on the surfaces of these globules. These surfactants are emulsifiers that form strong hydrophobic films which can be broken up by mechanical agitation and counter-current flow of the crude oil and water. Crude oil desalting is particularly difficult when the oil contains large amount of resins and asphaltenes, which gives stable emulsion and persists gravity separation. However, counter-current liquid extraction columns make gravity separation much easier. Agitated extraction columns require smaller vessel size, operate at lower temperature, can handle viscos crude, operate counter-currently with high mass transfer rate, high efficiency, and large throughput with reduced operational cost. The process is much feasible and desirable with substantial reductions in the quantity of fresh water and thermal energy.

It is strongly recommended that the concerned parties start experimentation on these columns in parties start experimentation on these columns in order to establish the operating range for an efficient alternative to the existing desalting technology.

Introduction

The presence of sediments and salts causes plugging, abrasion, erosion and corrosion problems in plugging, abrasion, erosion and corrosion problems in storage vessels as well as fouling of heat exchanges and coking of furnaces. The salt is normally present in the crude oil as crystalline suspensions and as entrained solution with water. These salts break down during processing and form acid that causes corrosion. The salts also deposit in heat exchanges and tubes still hindering flow, cause hot spots and scaling. On the other hand, the metallic constituents act as poisons to catalysts. The salts are bound to the crude by an organic film. They can be separated by liquid extraction in presence of specific chemicals depending on the type of salt present and the nature of crude oil. Caustic and acid are part of this chemical mixture and sometimes they may be added to adjust the pH of the solution. The extraction process is carried out at temperatures in the range of (40-50 degrees C), to lower the viscosity of the crude oil, the surface tension, increase the mass transfer rate and enhance the flow process.

PROCESS DESCRIPTION PROCESS DESCRIPTION Crude oil is heated to the required temperature (40-50 degrees C) to improve its fluid properties. The crude oil is pumped to the bottom distributor and flows up the extractor counter-currently with fresh water which is the continuous heavy phase. The fresh water dissolves and extracts impurities and salts from the crude oil. To secure intimate mixing and high mass transfer rate, agitation is provided and the drop size is reduced with subsequent increase in interfacial area.

THE SCHEIBEL COLUMN

The schematic diagram of a typical Scheibel column is shown in Figure 2, Scheibel E.G., 1948. The column consists of alternate calming and mixing sections with a centrally located shaft upon which are mounted the agitators. The calming section generally consists of a woven wire mesh which acts as an entrainment separator for the two liquids. The heavier phase is introduced at the top of the column and flows downwards through the packing at the mixing chamber where it is brought into intimate contact with the lighter liquid by agitation in the chamber.

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