New Design Washtank for Dehydration and Desalting of Large Volumes of Crude Oil
Authors: de Wit H.P. 
Published: December 2007

This paper was prepared for the SPE-European Spring Meeting 1974 of the Society of Petroleum Engineers of AIME, held in Amsterdam, the Netherlands, May 29–30, 1974. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made.
Discussion of this paper is invited. Three copies of any discussion should be sent to the Netherland Section of the Society of Petroleum Engineers, P. O. Box 228, The Hague, the Netherlands. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
A new dehydration-desalting wash-tank design for treating large volumes of light crude oil is proposed. This wash tank has a perfect degassing system, a good coalescing area, and an ideal flow distribution. The effect of diurnal temperature changes is minimized and an undisturbed settling zone is assured.
The design criteria of this new wash tank are explained. The various dehydration-desalting methods are reviewed and limitations discussed.
Relatively light crude oil (above 30 degrees API) could be dehydrated with this new design wash tank to a sufficiently low final water content such that the desalting stage, using wash water, would not be required. This should result in considerable saving in capital investment, considerable saving in operating cost, and alleviation of problems in areas where the supply of wash water is scarce.

Introduction
With time, many oil fields will start producing formation water. The formation water can producing formation water. The formation water can be either fresh or salty. Salinities of 200 gm/liter NaCl are quite normal for many oil fields.
The salt content of the crude oil depends on the salinity of the formation water and the amount of emulsified water. The generally accepted limit of salt in crudes shipped from the Middle East fields is 10 ptb (pounds per thousand barrels) at loading site. It should be mentioned that this crude, when received at European refineries, could have a salt content of up to 60 ptb due to contamination with sea water during transport (loading-on-top system, etc.).
U.S. oil fields normally apply a limit to the water percentage, but not on the salt content. Any desalting is done at the refinery.
If wet wells must remain on production, dehydration-desalting facilities have to be installed in the field. These facilities could be very costly.
Dehydration (separation of formation water from the crude) and desalting (washing with relatively fresh water) can be done either chemically or electrically, depending on the quantity of the crudes the viscosity, the specific gravity, the temperature, the water content, and the cost of the demulsifier.