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How to prevent fouling in feed/effuent exchanger of sour water stripper unit?

Aug-2024

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

Unfortunately, fouling is a common operational issue in sour water stripping units. The first step to understand and solve the fouling issues is to understand the origin of the fouling, in the literature is widely known that are the following types of fouling:
1 - Particulates 
2 - Hydrocarbons
3 - Salts 
4 - Elemental sulphur 
5 - Polymers

The particulates are normally related to corrosion products from upstream sections or coke fines (if the sour water from delayed coking units are being treated), this can be solved using an adequate filtering system to the feed of the sour water treating unit as well as identifying adequately the most critical regions of corrosion and take actions to control this phenomena.

The hydrocarbons are the most common source of fouling in sour water stripping units and the best way to control this is to manage the separation between the hydrocarbon and water phases in the feed, it is necessary to control adequate residence time to allow this separation. The best design practice is to install a feed tank upstream to the sour water stripping unit to allow adequate residence time with interface level control to minimise the hydrocarbon dragging to the sour water stripping unit. Among the operational variables, the pH can be controlled aiming to avoid the emulsion stabilisation, the pH tends to raise along the stripping column considering that the H2S is easily stripped than NH3 causing fouling issues in the bottom section of the column (and than in the feed/effluent exchanger).

The fouling caused by salts is normally related to excess of calcium or magnesium in the sour water, this issue can be avoided by controlling the hardness of the washing water of the upstream processing units like distillation and hydrotreaters through an adequate water treating process. The fouling caused by elemental sulphur can be faced by refiners dosing agents to control cyanide in the sour water (like ammonium polysulphide), a overdosage can reduce the pH and cause precipitation, in this case, it's necessary to define and control the adequate amount of polysulphide. The polymer fouling is caused normally by reasons like hydrogen cyanide polymerisation and amine degradation. The hydrogen cyanide can be neutralised using ammonium polysulphide, but it's necessary to control the correct amount as previously quoted to avoid elemental sulfur precipitation and the amine degradation fouling requires a deep analysis of the upstream process of amine treating units aiming to identify foaming and another operating issues.

Another design strategies like the use of fouling resistant trays in the stripping columns and the installation of by-passes in the feed/effluent exchangers can help to improve the lifecycle of the sour water stripping units. In summary, the first step to define a strategy of solution of fouling in sour water stripping units is identify the main source of fouling among the quoted fouling origin and then define the best strategy for this case. A very good source which can help to understand and solve this issue is the article from Mr. Phillip Le Grange which was published by PTQ Magazine in the Q2 2019 issue, Operational challenges in sour water stripping, which you can READ HERE.

Sep-2024