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Can we inject caustic at upstream of De-Salter instead of downstream? What will be the consequences?
 
Answers
01/11/2017 A: Pannalal Chakraborty, Indian Oil Corpn Ltd, chakrabortyp2@indianoil.in
Yes, many refiners do inject caustic upstream of Deslater. But the reason & consequence should be understood clearly.
Crude contains chloride salts. NaCl - approx 70-75%, MgCl2 and CaCl2 - both approx 15-20%, making it 100%. The chloride in CDU overhead comes from decomposition of these chlorides. But all chlorides do not get equally hydrolysed. Degree of dissociation - MgCl2 - approx 95%, CaCl2- 20% or so, NaCl - approx 5%. From that angle, if we can convert the MgCl2 to NaCl, it will be better as far as CDU is concerned. That's why we add caustic pre or post Deslater.
Now comes effective salt removal by Deslater. Again, depending on the salt composition, the removal varies. Norman Lieberman reports that in single stage desalter, following is the degree of removal
• 90% NaCl removal
• 50% CaCl2 removal
• 40% MgCl2 removal.
So, it is much easier to remove NaCl. So it is better to convert MgCl2 in Crude to NaCl before it enters Desalter.
Hence, some people do inject caustic upstream of Desalter, provided other conditions (pH of desalter, pH of wash water, not a naphthenic crude, no strong emulsification problem etc.) are being met. One has to review the operating conditions of all parameters before arriving to a decision. We do inject such caustic without much apparent problem.
11/09/2009 A: Luis Marques, Galpenergia, luis.marques@galpenergia.com
The rule is to inject downstream of desalters the same amount of OHNA as the salts of outlet stream at a maximum of 21 PPM of OHNA to avoid caustic embrittlement downstream of caustic injection. The quills condition of OHNA injection must be checked to be sure of a good injection mixture.
07/02/2009 A: AMIYA LAHIRI, AK Associate, lahiri04k@rediffmail.com
In crude distillation caustic addition has two objectives:
1. pH of desalter is to be maintained between 8.0 to 6.0. Depending on the pH of water used, either caustic or acid is added upstream of desalter. The caustic added is drained out from the desalter. Overdosing of caustic upstream, adversely affects desalter operation.
2. Caustic is also added to control overhead corrosion of distillation column. Here caustic helps in converting MgCl2 & CaCl2 to NaCl, thus reducing amount of HCL in the column over head stream. Caustic for this purpose has to be added downstream of the desalter. Caustic added upstream of desalter would not be available for reacting with Ca / Mg - chlorides.
18/12/2008 A: Berthold Otzisk, Kurita Europe GmbH, Berthold.Otzisk@kurita.eu
If caustic is injected in front of the desalter the potential is very high to stabilise the temporary emulsion in the desalter. This can create significant desalting problems. Very high amounts of caustic would be needed to form more NaCl salts, leaving with the desalter effluent.
Most of the chlorides are already removed with the desalter effluent. As a rule of thumb a single stage desalter should give a performance of >90% salt efficiency or <10ppm chloride salts after desalting. These salts are a mixture of NaCl, CaCl2 and MgCl2. For economic reasons it makes more sense to inject the caustic after desalting to form more NaCl.
The ACF technology is an alternative to using caustic. The liquid ACF product can be injected after the desalter and will reduce the chloride concentration in the crude overhead accumulator significantly.
18/11/2008 A: Egbert van Hoorn, Hocon B V, Egbertvh@hotmail.com
I suppose that you mean upstream in relation to the crude oil.
The major problems have already been outlined in the two previous answers.
The water used in the desalter is often partly coming from a sour water stripper, particularly if an FCC unit is present in your refinery. The aim is to remove phenols from the SWS effluent.
Some refineries benefit from injecting caustic in the SWS feed. This can help to liberated fixed amonia. Amonia in the SWS can be fixed by strong acids present in the water.
In those cases, some caustic injection in the SWS feed will help.
The target pH of the effluent is however 8. This caustic dosing should be very moderate, else you encounter the emulsion problems already indicated in the desalter.
17/11/2008 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Caustic upstream of the desalter can react with naphthenic acids to form sodium naphthenates. These naphthenates will promote emulsion stability and make desalter operation more difficult. The salt form of the naphthenic acid will be more water soluble than the acid itself, leading to increased COD loading on the waste water plant. Unreacted caustic will go out with the desalter brine, making it unavailable to suppress HCl formation like caustic injected downstream of the desalter does. At least one chemical supplier claims to like desalting in a high pH environment like would be created by injecting caustic upstream of the desalter, but most do not like to operate in this fashion due to the desalting problems it causes.
17/11/2008 A: keith bowers, B and B Consulting, kebowers47@gmail.com
One can possibly add caustic to the crude ahead of the desalter, but it might cause emulsion problems. If your crude has naphthenic acid or other organic acids, the caustic will react with them to make SOAP. I suggest you consult with the vendor of your desalting chemicals and other antifoulants and corrosion inhibitors in your operation before 'trying' the caustic upstream of the desalter.