Page 200 - Petroleum and Gas Field Processing

6.5 DETERMINING DILUTION WATER REQUIREMENT

From the operational point of view, it has been reported that the amount
of water of dilution W D added in the desalting of crude oils is in the range
5–10% by volume, based on the amount of remnant water and its salinity.
The following correlation has been developed [3] for determining W D
as a function of W R , S R , S D , (salinity of the dilution water) and the
efficiency of mixing between the two phases, E.

3 0:01533
2:5 10 ðW R Þ
W D ¼ 0:2606 0:0758 0:6305 ð3Þ
ðS D Þ ðS R Þ E
where S D and S R are in parts per million. On the other hand, the following
analytical relationship could set the acceptable limits on the salt content in
crude oil. A component material balance for the salt gives

EW D S D þ W R S R ¼ S B ðW D þ W R Þ
or
EW D S D þ W R S R
S ¼ ð4Þ
B
W D þ W R
where S B refers to some average salinity in the bulk of the homogeneous
phase as a result of mixing the remnant water with the fresh water.

6.6 EFFECT OF OPERATING PARAMETERS

The efficiency of desalting is dependent on the following parameters [6,7]:
1. Water–crude interface level. This level should be kept constant;
any changes will change electrical field and perturbs electrical
coalescence.
2. Desalting temperature. Temperature affects water droplet settling
through its effect on oil viscosity; therefore, heavier crude oils
require higher desalting temperatures.
3. Wash water ratio. Heavy crudes require a high wash water ratio to
increase electrical coalescence. A high wash ratio acts similarly to
raise temperatures, as illustrated in Table 4.
4. Pressure drop in the mixing valve. A high-pressure-drop operation
results in the formation of a fine stable emulsion and better
washing. However, if the pressure drop is excessive, the emulsion
might be difficult to break. The optimum pressure drop is 1.5 bar
for light crudes and 0.5 bar for heavy crudes.

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.