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In the refinery sour water stripper, ammonia content is 90 ppm vs deisgn of 50ppm. ph of stripped water is high @9 and this is being reused in desalters as wash water. Due to this, in our CDU overhead system, the boot water pH is reamaining high in the range of 7-7.5 without any injction of neurtraliser. Will this affect the CDU over head system from the point of view of corrosion? And is it required to dose any neutraliser to CDU overhead in this case? Also, is it ok if the desalter wash water has 9 ph?
 
Answers
22/03/2012 A: Berthold Otzisk, Kurita Europe GmbH, Berthold.Otzisk@kurita-water.com
A high pH of 7 to 7.5 will give negative impacts. The recommended pH for CDU overhead systems is in the 5.5 to 6.5 pH range, where minimum carbon steel corrosion occurs. Above pH 7 corrosion rapidly increases with increasing pH. Above pH 8.5 a stable iron sulphide protection film is formed. Corrosion products will also leave with the hydrocarbon phase. Less iron is detected in the accumulator sour water, giving doubtful results.

In the past ammonia was often used as a neutralizing amine for CDU overhead systems. Ammonia is a very volatile gas. Acidic species are not neutralized at the beginning of the condensation. Just at the end of the condensation process ammonia shifts the pH to a higher level to form corrosive NH4Cl salts. The high pH of 7 to 7.5 in the accumulator sour water is an indication, that there is excess ammonia present.

The high pH of the desalter wash water may create emulsion problems in the desalter vessels. The temporary emulsion phase can be stabilized, resulting in water-carryover into the desalted crude oil. Some refiners add acids like diluted sulphuric acid to control the pH of the desalter wash water.
20/03/2012 A: Stefan van der Wal, Mettler-Toledo Process Analytics, stefan.vanderwal@mt.com
All seems ideal except for the ammonia value in the sour water. You'll easily reduce ammonia when you slightly increase the sour water pH (add caustic).
20/03/2012 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
It is advisable to maintain the overhead boot water pH in the range of 5.5 - 6.5. Since this is in Acidic range acid attack is prevented by having protective film thru Filmer Amine injection. High pH (>6.5) is not advisable as this can lead to sludge formation and will lead to Caustic embrittlement (alkaline corrosion). Both corrosions are detrimental to the helath and relaibility of the system. However, as the Alkaline corrosion is more expensive to handle compared to that of Acid corrosion it is better that the system pH is maintained in the acidic range.

High pH of Desalting water will lead to the formation of tough emulsions that make the water and oil separation more difficult. In that situation one has to compromise either on desalting efficiency or on deydration efficiency of desalters. If the deydration is poor you will have more oil under carry over than desirable to Waste water treatment / slop generation. Also this poor dehydration at times causes brine carry along with desalted crude to the down stream equipment. Since the brine is rich with salt that will aggravate corrosion of the down stream equipment.