4 replies to this topic

[manish.iitr]

Dear All

I am trying to build a model to optimize the operation of Crude Desalter in Crude Distillation Unit and 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

[Technical Bard]

The individual salts do not behave differently in the desalter - a desalter is really just about adding water and removing brine to reduce the salt content of the entrained water droplets.

The different salts behave differently in the crude column furnace and flash zone, because the the temperatures and steam in the column with hydrolyze some of the salts, converting some of the CaCl2 and MgCl2 to HCl. The fraction that is converted is dependent on the temperature and partial pressure of water. Sodium chloride is generally not hydrolyzed at crude or vacuum column conditions - however it will partially hydrolyze in a delayed coker.

I have never seen a "typical" salt breakdown, as they can vary from crude to crude depending on the geological source.

[ankur2061]

Manish,

The important issue in desalting is that the crude pH in the desalter preferably be maintained below 7. At pH values of 8 and above emulsions of brine are formed with the naphthenic and sulfur compounds (sodium naphthenate and sodium sulfide) present in the crude oil which are difficult to remove. Better dehydration is obtained in electric desalters when they are operated at a pH of 6.The pH can be controlled by addition of acid to the wash water make-up or recycle water.

Regards,
Ankur.

Edited by ankur2061, 04 July 2012 - 02:54 PM.

[manish.iitr]

Thanks Bard & Ankit for your knowlegeable inputs!

@ Ankit
Ours is two stage desalter; and for each stage we have pH analyser for desalter outlet brine. The first stage brine pH is about 6.9 and the second stage brine pH is about 7.8. Currently, we do not add any acid.
As you said "Better dehydration is obtained in electric desalters when they are operated at a pH of 6". This require addition of acid, which can pose additional risk of corrosion at desalter downstream. Is there any way to find out trade-off between desalter acid imposed corrosion V/s maintaining right pH for better desalter performance.

[ankur2061]

Manish,

I do not know about any trade-off study between acid corrosion due to pH of 6 and better performance of electric desalters. You will need to perform your own study to evaluate this. Another information that may be useful is that contrary to electric desalters chemical desalters perform better at a pH of 8 to 9.

Regards,
Ankur.