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Serge Vessel – Level Control

S.R. Bhat (Technical Director, COMFIT)

Many times, a product produced in a process equipment is used as feed to another equipment. In these cases, it is necessary to make the feed flow to the second equipment steady, i.e. any variations in the feed should be very slow. This must be achieved even if the product out coming put of the first equipment is not steady. E.g. Naphtha coming out of the crude distillation unit of a refinery is unstable, i.e. it contains a lot of volatile components. These volatile constitute Liquefied Petroleum Gas (LPG), which is very valuable. This stream is fed to another distillation column called Naphtha Stabilizer. Purpose of the Naphtha Stabilizer are two. They are:

1. Recover the LPG, allowing minimum slippage of LPG to Naphtha.

2. Remove LPG from Naphtha with minimum slippage of Naphtha to LPG. To achieve this, a serge vessel is introduced in between the Crude Distillation Column and the Naphtha Stabilizer. Feed flow into the stabilizer is controlled by a control valve introduced between the serge tank and stabilizer. Process variable for this control loop is the liquid level in the serge tank. This level keeps varing around 50%. If this controller is well tuned, level will always mainted tightly near the 50 % mark. This would defeat the purpose of surge vessel by making the vessel outlet flow closely follow the badly varing inlet flow. On the other hand, by keeping the controller sluggish (i.e. making proportionality gain low), the surge tank serves its purpose. When the input flow to the serge vessel increses, sluggish level controller acts very slowly, resulting in the slow and steady increase of outlet flow from the vessel. This increses the level in the serge tank. On the other hand, when the inlet flow to the serge vessel reduces, exactly the reverse happens.

This is a strange case of attuned controller gives the desire result. This has been successfully done in the Naphtha Stabilizer of refinery, which resulted in substantial increase in of the production of the valuable LPG, but also improved its weathering.