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In our CDU we have stablizer and sipliter columns, stablizer for separating LPG from Naphtha, after the annual maintenance, we have a problem in the boot of the overhead drum of stablizer column we have a Black water and high iron number so what's the problem that makes this black water ?

Feb-2024

Answers

N Sekar Murthy, Ashphil Consultancy, nssvdvr@gmail.com

Water entry into CDU stabilizer is only from upstream either OH of main fractionator of crude unit and or crude pre-opping column OH unstabilized naphtha. The newer designs have GASCON (mainly to maximize LPG yield from Crude unit close to potential as per assay) which provides another separation / retention of water in the HP receiver. As such setting up a coalescer on feed to stabilizer will help to remove most free water getting entrained as well help to restrict water emulsion specially when crude brings in surfactants from upstream chemicals use. Also, a strict follow up on CDU & Pretopping column OH corrosion control programme and effective interface control between unstabilized naphtha and sour water will help to minimize water ingress into stabilizer.

Feb-2024
Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

The black sludge and water observed in the overhead vessel is probably pickering emulsion stabilized by iron particles which is accumulated in the interface between sour water and naphtha.

It's important to check the pH, Iron and Chloride content in the overhead system, a lack of control of these parameters can lead to a severe corrosion process in this system as well as in the atmospheric tower. When the emulsion is formed in the vessel, this residue cant be removed without the shutdown of the processing unit or through draining the overhead vessel totally which requires a special procedure aiming to minimize the safety risks as well as the damage to the pumps of the overhead drum.

Regarding the corrosion control in the overhead systems it's important to analyze that the corrosion control parameters is under an adequate range, especially the operating temperature of the overhead system. There are some correlations in the literature which relates the ammonia and chloride concentration in the sour water to determine salt deposition temperature in the top of the tower and this needs to be considered to define the operating temperature of the system.

Feb-2024