Page 190 - Petroleum and Gas Field Processing

6

Desalting of Crude Oil

6.1 INTRODUCTION

The removal of salt from crude oil for refinery feed stocks has been and
still is a mandatory step. This is particularly true if the salt content exceeds
20 PTB (pounds of salt, expressed as equivalent sodium chloride, per
thousand barrels of oil).
The most economical place for the desalting process is usually in the
refinery. However, when marketing or pipeline requirements are imposed,
field plants are needed for processing the salty oil prior to shipping. The
principles involved are the same whether desalting takes place at the
refinery or in the field. Salt in crude oil is, in most cases, found dissolved
in the remnant brine within the oil.
The remnant brine is that part of the salty water that cannot be
further reduced by any of the dehydration methods described in the
previous chapter. It is commonly reported as basic sediments and water
(B.S.&W.). It is understood that this remnant water exists in the crude oil
as a dispersion of very fine droplets highly emulsified in the bulk of oil.
The mineral salts of this brine consist mainly of chlorides of sodium,
calcium and magnesium. A summary of the properties of crude oil as
received at the refinery is given in Table 1. Nelson [1] compiled the data
given in Table 2 on the amount of salts found in oils for various regions in
the world.
The amount of salt in the crude oil is a function of the amount of the
brine that remains in the oil W R (% B.S.&W.) and of its salinity S R in
parts per million (ppm). In other words, this relationship could be written
in the following functional form (after Manning and Thompson [2]):

1000W R S R
PTB ¼ 350
Brine 6 ð1Þ
100 W R 10

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