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Our Delayed Coker Unit Fractionator boot water chloride content is ~240ppm, Iron content ~0.22ppm. We have started injection of DM water at the upstream of condenser. What is the desirable range/ its consequence and how to reduce it ? What is the root cause and contributing factors for high chloride?

Additional info:
Vacuum residue is the feed to DCU and its water content is 1.2-1.4%.
02/03/2011 A: AMIYA LAHIRI, AK Associate, lahiri04k@rediffmail.com
I have come across two types of situation which can result in overhead corrosion in Coker distillation column:
1. High pH in the range of 9.0 – 9.5. Corrosion under this condition in my opinion is related to ammonium bi sulfide corrosion. The temperature being high (about 520C), sulfur and nitrogen compounds in the feed crack to produce H2S and NH3 respectively which in turn produces corrosive alkaline bi sulfide.
2. Presence of chloride in O/H. Chloride is not expected in this case as all Mg and Ca chlorides get dissociated in crude and vacuum units. Feed to coking unit is heavy hydrocarbon from bottom of different units and also includes slop. In one such study the source of chloride in Coker distillation column was identified to be the desalter drain water (containing soluble Mg and Ca chlorides) which was added to the slop.
3. AnalyseNa, Mg and Ca chloride in vacuum residue water
4. I would suggest you additionally measure pH, and bi sulfide content of overhead water and also check if desalter drain water is part of the slop fed to the Coker. Relating corrosion rates to the above parameters will help in taking reliable corrective action.
01/03/2011 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Chlorides in the coker overhead normally come from additional hydrolysis of salt in the feed that did hydrolyze in the crude or vacuum unit furnaces. The only way to lower the mass of chlorides is to improve desalting. Putting in more make up water for the overhead water wash will dilute the chlorides but not change the mass. You should pick the amount of water used to ensure that you excess water injected that will remain liquid after contacting the hot overhead vapors. Typical guidelines for similar systems call for at least 25% of the injected water to remain as liquid after flashing at the injection point.
If your chlorides are too high you can form ammonium chloride that precipitates in the fractionator. The ammonia is a normal byproduct of the thermal cracking reactions of nitrogen containing compounds in the coker feed and is not normally controllable. This salt causes tower DP to increase until a water wash is required to remove the salts. Besides hurting fractionation and possibly capacity, this salt formation and subsequent water washes can also cause corrosion.
To avoid ammonium chloride deposition in the tower, you need to either control desalting to control the mass of chlorides formed for the overhead temperature being operated at. If you are getting salt deposition you need to either improve desalting to reduce the mass of chlorides formed or increase the overhead temperature to move the salt point out of the tower. Your refinery chemical supplier should be able to estimate the salt point temperature based on your actual operation.
01/03/2011 A: Morgan Rodwell, Fluor Canada Limited, morgan.rodwell@fluor.com
The root cause of chlorides in the coker fractionator overhead is most likely chlorides in the feed. Chloride salts that are not removed via desalting will largely pass through the CDU and VDU and end up in vacuum residue. Some crudes also contain small amounts of organic chloride hetero-atoms. The conditions in the delayed coker are sufficient to break down the organic chloride and to hydrolyze many of the chloride salts. The chlorides will then rise up the fractionator as HCl and then react with basic species (e.g. NH3) in the overhead. You may want to check whether you have NH4CL sublimation occurring in the top of the column or in the condenser as this can cause very serious corrosion.
Wash water and amine/ammonia neutralization are often used to reduce the corrosive nature of this water.
Reducing the chlorides in the overhead is really only achievable if you do a better job of desalting the crude upstream of the CDU.
01/03/2011 A: Celso Pajaro, Sulzer Chemtech, celso.pajaro@sulzer.com
Besides chloride you need to analyze ammonia, the corrosion mechanism in the overhead of a delayed coker main Fractionator is ammonia chloride salts deposition which cause under deposit corrosion. Chlorides are coming with the residue and they are release under the Delayed Coker Fired Heater conditions, one way to reduce chlorides is to improve the operation of the crude unit desalter, other method that has been used is adding a small amount of low strength caustic to the crude after desalter, caustic will react with the calcium and Magnesium chlorides producing sodium chloride which is less susceptible to hydrolyze under fired heater conditions, please made sure that sodium content on the vacuum residue is kept below 30 ppm to avoid high coking rate in the delayed coker fire heater tubes.
01/03/2011 A: Christopher Turner, Snamprogetti Ltd, chris.turner@snampro.co.uk
How much water enters the column with the hydrocarbon feed? This water (even after desalting and dehydration) will still contain salts which may be contributing to the chloride.