Q & A > Question Details
We are facing problems with the main fractionator reflux drum bootwater high chlorides(120 ppm)
There is prefractionator ahead of main fractionator but we are getting zero chlorides in overhead boot water.
does the inorganic chlorides dissociate more at temperatures greater than 250 degC which is prefractionator temperature?
11/04/2011 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
For conventional crude oils, the vast majority of HCl, aqueous in the overhead accumulator of CDU+VDU atmospheric tower derive from thermal cracking of residual salts left in "desalted crude oil". Residual salts comprise a mixture of various salts besides NaCl. When the crude oil is heated in hot encon exchangers and the crude heater, most of residual MgCl2 thermally cracks; good share of CaCl2 thermally cracks; and a small but significant amount of NaCl thermally cracks.
Temperatures in the preflash drum or preflash tower is often not quite hot enough to drive these thermal cracking reaction, especially if residual MgCl2 is minimal. A small proportion of HCl may derive from incidental or partial thermal cracking of organic chlorine species.
Mitigation of HCl in overhead water is achievable by using best possible desalting of the crude oils, i.e. double desalting in appropriate size desalter vessels using "electrics" and "internal piping" provided by highly qualified desalter technology suppliers and chemicals treatments and auxiliaries. Target performance for conventional crudes [API = 28 to 42] is residual salt =0.5 ptb [one-half pound mixed inorganic salts per thousand barrels of desalted crude oil].
11/04/2011 A: AMIYA LAHIRI, AK Associate, lahiri04k@rediffmail.com
Overhead chloride in processing of crude is due to hydrolysis of MgCl2 and CaCl2 to form HCl. The temperature range of hydrolysis of the two salts is however different. While starting temperatures of hydrolysis of MgCl2 is about 120C, that of CaCl2 is about 210C. The fact that you have problem with high chloride in main fractionator and not the pre flash column (Temperature – 250C), indicates that nature of chloride coming with the crude consists of Ca and Na chlorides and almost no MgCl2. Thus HCl is formed only downstream of pre flash column.
Suggest you analyze the crude tank drain water for nature of soluble chlorides. This can be done also with desalter water provided fresh water added for de salter operation is process water.
09/04/2011 A: Celso Pajaro, Sulzer Chemtech, celso.pajaro@sulzer.com
Inorganic chlorides will dissociate at higher temperatures. 95% of Magnesium chloride will dissociate around 375 C, while only 15% of Calcium Chloride will dissociate, and 5% of sodium chloride will dissociate around 430 C