What are the pros and cons of adding Caustic at the upstream of Desalter?
Answers
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15/05/2014
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Sunderam Nagar, ALBEMARLE CATALYST CO BV, snagar31@gmail.com
In general, caustic is always added downstream of Desalter, the main reason is that if added upstream Caustic will cause High pH in the desalter due to which a tight emulsion is formed and separation of Oil and Water becomes difficult. This will lead to increase in Oil in brine and in some cases the brine is so much of oil that even the ETP is not able to handle it. Caustic helps in conversion of CaCl2 and MgCl2 into NaCl, but has adverse effects on the downstream units such as FCC and Delayed Coker units, as Sodium levels shoot up dramatically. The NaOH if added should be freshly made and the strength should be 4%-8%. The maximum limit of Na in VGO should be 5 ppm.
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12/05/2014
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Berthold Otzisk, Kurita Europe GmbH, Berthold.Otzisk@kurita-water.com
Most refiners inject caustic in the heat exchanger train downstream of the desalter. It neutralizes the HCl by forming more NaCl, which leaves with the bottom product. The caustic injection there is a very helpful tool to minimize corrosion in the overhead system, but it promotes fouling in the heat exchangers after desalter.
The desalter is typically operated with a pH between 6 and 8. Caustic can be injected upstream of the desalter for pH control, when the wash water pH is too low. A caustic overdosing negatively affects the desalter operation. When caustic and naphthenic acids are present in the crude oil, there is a risk to form sodium naphthenates. This promotes formation of very stable emulsions and reduces the desalter efficiency.
About 90% of the chlorides are already removed with the desalter effluent. It is not the most economical way to injected caustic upstream of the desalter to neutralize the HCl by forming NaCl.
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