Does overhead water wash play any role to achieve dew point early?
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27/01/2014
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Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Yes, water wash will typically force the water dew point by quenching the overhead temperature. When that happens the amount of hydrocarbon in the vapor phase decreases while the amount of water vapor increases thus increasing the water dew point. Particularly in cold climates, without a water wash you can get water condensation on cold spots that form on uninsulated sections of overhead line. Forcing the water dew point with water wash, ensures that only diluted acids contact the vessel wall. As Mr. Ragsdale indicated, water wash alone is not effective at stopping corrosion. Neutralizing amines and corrosion inhibitors are also added to further improve pH control and to provide a protective barrier on metal surfaces. In addition to forcing the dew point and diluting acids, the water wash helps prevent formation of corrosive amine chloride and ammonium chloride salts in the overhead system.
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27/01/2014
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
The issue is that chlorides and H2S become more corrosive in the presence of liquid water. So, if the dew point is upstream of the wash water injection point, the location of the dew point will not be affected by the wash water. If the overhead vapor temperature is significantly above the dew point, the corrosive mixture will not be in the pipe leading to the point of water injection. The purpose of the wash water is to dilute a low pH mixture to increase the pH and retard the corrosive action.
Many believe that neutralizing amines and filming amines are more effective than water washing. The design is important. Assuming amine injection is used, if the point of condensation is in a shell and tube exchanger, with the overhead vapor on the tube side, the tube bundle will experience whatever corrosion occurs. It is usually easier to plan to replace the bundle than any other repair or remedial steps required if corrosion is experienced at any other location in the overhead system.
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