Q & A > Question Details
I have following questions on desalter:
What is typical salt content at crude out from 2 stage desalter?
Does mixing of crudes result in more salt slip from desalter than design?
Does wash water salt content affect the salt removal efficiency (more salt comes out from desalter)?
Does inadequate/inefficient demulsifier result in more salt slip from desalter than design?
 
Answers
03/04/2012 A: Bruce Carr, Suncor Inc, matukaze1@hotmail.com
Here is a recent article on DESALTER OPERATION from Oil & Gas:
A single-stage crude desalter should remove 90% of the salt content of the crude oil. A double-desalter operation should extract 99% of the salt.
These statements create the comforting but misleading impression that double desalting removes troublesome salts before they can cause downstream problems. It is unfortunately far more difficult to remove MgCl2 than NaCl.
For example, the following chloride-salt removal efficiencies were observed in one single-stage desalter:
•90% NaCl removal
•50% CaCl2 removal
•40% MgCl2 removal.
Because the hydrolysis of MgCl2 proceeds far more readily than that of NaCl, its potentially lower removal efficiency in the desalter will create a disproportionate downstream corrosion problem.
Experience also indicates that the operations of desalters are, in many refineries, rather erratic. One client operating a two-stage desalter.
Experienced salt-removal efficiencies varying between 65 and 95% and averaging 90%. Another refiner experienced intermittent desalter water carry-over caused by interface level-control problems.
Of course, during such upset conditions, the operators did not sample the desalted crude because they realized the desalter efficiency would be low.
Based on observations, the principal problems with desalter efficiency are:
•Level-control problems caused by plugging of level taps
•Failure caused by shorting of electrical grids
•Improper control of desalting chemicals and lack of pH control
•Poor-quality desalter makeup water
•Low mix-valve delta pressure
•Low desalter-water flow rate
•Vaporization in the desalter
•Erratic desalter back-pressure control
•Sludge buildup in the bottom of the desalter.
Don't know if that answers your question but it is an interesting article
02/04/2012 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
There are a lot of factors that influence desalting efficiency - Crudes being processed and the rate, size of desalters, number of stages, size of transformers, temperature, and % wash water are all important variables. Two stage desalters can typically achieve 90 to 99+% desalting efficiency with low end salt contents of .5 to 1 ptb (1.5 to 3ppm NaCl).
Mixing crudes does not necessarily hurt desalter performance. If the blending of the crudes produces a blend with low asphaltene stability, then destabilized asphaltenes can create rag layer problems, which hurts desalter performance. If the blend is heavier than the normal slate that is also likely to hurt performance.
On most desalters the bulk of the salt in the desalted crude comes from chlorides in the entrained water leaving with the desalted crude; therefore, if the wash water has more chlorides in it, the chlorides in the desalted crude will also increase. If your water entrainment is low and most of the salt in the desalted crude is from crystalline salts, then increased chlorides in the wash water may be more of a theoretical problem than a practical one.
Inadequate demulisifier (rate or effectiveness) will definitely hurt desalter performance. You will not be able to run your mix valve with as high of delta P, which reduces mixing efficiency and thus the mass transfer efficiency of salts from the crude to the wash water is decreased.
02/04/2012 A: sam lordo, Becht Engineering, salordo@comcast.net
1)Typical salt out of a second stage desalter is based on the incoming salt content and under design conditions you figure 90% removal per stage
PTB Inlet PTB Outlet
Dual Stage 50 or less 0.5
50.1 to 100 1.0
above 100 99% removal
2)If you are referring to incompatibility then yes it is possible to slip some salt primarily due to emulsion stability increase and potential water carry over
3)Only if it is extremely high (percent levels); nominal Cl level found in most wash water are 25-300 ppm and cause no issues; if you suspect an impact or want to quantify the impact, one can calculate it
4)Yes it can contribute (other parameters have bigger impacts) to higher salts out only with respect to water carry over; which could come from too low of mixing due to inadequacies in demulsifer to resolve emulsion at higher mix energies; or too low of dosage is used
02/04/2012 A: sam lordo, Becht Engineering, salordo@comcast.net
Typical salt out of a second stage desalter is based on the incoming salt content and under design conditions you figure 90% removal per stage
Dual Stage
50 or less: 0.5
50.1 to 100: 1.0
above 100: 99% removal