Vapor Line Restriction

Coke that builds up in the coke drum overhead vapor line is responsible for most of the back-pressure incidents. Operators find that tearing the insulation off these lines slows the rate at which coke deposits. A better method is to inject a heavy slop oil quench, as shown in Figure 3-4, into the vapor line to retard coke formation.

The quench rate is about 3% on resid feed, or enough to lower the drum vapor temperature by around 15°F. In addition to reducing coke buildup, this is also an energy-free method to dispose of refinery slop oil. An added bonus is that shift operators report that slop injection makes the overhead vapor valves easier to turn.

Other causes of excessive coke drum back pressure are badly fouled combination tower overhead condensers, partially plugged trays, or insuffi­cient tower pumparound heat removal. Fouled condensers and lack of pumparound heat removal overload the wet gas compressor. Plugged trays are best identified with a pressure drop survey.


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