Use of Caustic to Improve Stripping

I was working on a smaller crude unit in Alabama to improve desalter efficiency. Their problem was the periodic carry-over of emulsified brine. The depth of the emulsion between the crude and water phases in their desalter was excessive. The pH of the washwa-ter from the sour water stripper was to 9, even though the NH3 content of the stripper bottoms was zero. Talking to the operators, I discovered that they added NaOH to the sour water stripper feed to diminish the NHj content of the stripped water. 

Caustic contamination of the desalter washwater is known to promote high desalter vessel emulsion levels. There are dozens of references to this problem in my favorite source of technical information. 

That is, the old back issues of the NPRA Q A (i.e.. The National Petroleum Refiners Associated, Question and Answer) sessions. With this authoritative source as support, 1 asked the operator to stop the NaOH addition to his sour water stripper feed. Two hours later, at the same reboiler duty, the NHj content of the stripped sour water had gone from 0 to about 5 ppm. And the emulsion layer in the crude unit desalter had been reduced enough so that brine carry-over was no longer a problem. The 5 ppm of ammonia was quite acceptable. 

Caustic addition to the sour water stripper feed will increase NHj stripping efficiency by reducing the solubility of NHj in water. This may be a good practice if the stripper bottoms is flowing into the refinery effluent treatment basin. But 1 would rather have 10-20 ppm NH in the sour water stripper bottoms than contaminate my desalter with NaOH, which promotes the formation of brine-crude oil emulsions. 

---