Q & A > Question Details
I am trying to build a model to optimize the operation of Crude Desalter and study its effect on Crude Column Overhead Corrosion.
The major salts present in Crude are NaCl, MgCl2 & CaCl2; but in our laboratory we measure only Total Salt Content of Crude (before and after Desalter); we do not measure individual salt.
My queries are:
1) How the individual salt affect Desalter performance and Crude Overhead corrosion
2) Is it required to measure the individual salt's content in Crude?
3) Can I assume some typical break-up of individual salt (Note that the type of crude we process changes very often).
05/07/2012 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Typically the relative concentrations of chloride salts in crude oil mimic that of sea water. My rule of thumb is that 70% of the chlorides are in the form of NaCl, 20% MgCl2 and 10% CaCl2. I don't believe there is a significant difference in the desaltability of the different salts, although I have never seen data to prove or disprove this. MgCl2 is the primary contributor to overhead corrosion because it almost all hydrolyzes to form HCl. CaCl2 hydrolyzes but to a much lesser extent than Mg. NaCl does not hydrolyze to any appreciable extent. If you look at some old hydrolysis vs. temperature curves available in the literature, you will find that at typical crude unit conditions that about 20% of the salts in the desalted crude could be expected to hydrolyze in the absence of caustic. Some literature indicates that besides temperature, nap acid can increase the % hydrolysis.
While there can be differences in the salt composition for crudes, the effort to try and figure it out is probably not worth it. From a practical stand point you can't speciate the salts present in crude oil. You can do elemental analysis or ion chromatography, but in any case you have to make assumptions about what the actual salts present are.
One handy thing you can do is track the % hydrolysis of the salts leaving your desalter. Calculate the lb/d of chlorides in the overhead system from the chloride concentration and the total water. Divide this by the lb/d of chlorides in the desalted crude to get the fraction of the salts that hydrolyzed.