Pressure control with flooded condensers

[lgmarti]

Our depropanizer has flooded condensers. Over the years we have tried to improve the pressure conrol in this tower without success. The last proposed control scheme will control the accumulator pressure with the level in the drum. That is, when the pressure increases, the overhead release will increase which will lower the accumulator level to expose more surface area in the ovh condensers. When the pressure decreases, less will be released which will increase the level in the accumulator, to reduce the surface area in the overhead condensers. Is this effective? Can I control the tower overhead temperature with this configuration? Will I ever overflow or empty the accumulator drum completely?

 

[MaNaTMoS]

The accumulator vessel should be filled with liquid all the time; the only thing that changes is liquid level in the upstream condenser. If liquid level shows in the accumulator, that means you're lacking of condensing capacity and therefore gas must be vented off from the reflux (accumulator) drum.

This control scheme is appliable only in systems with no noncondensibles in the overhead gas. As compared to other schemes it's the most effective and reliable, but requires total overhead condensation possibility. If you have C1 or C2 in the overhead product, it presents bad pressure control solution.

Hope this helps.
MaNaTMoS

 

[25362]

I know of two books by F.G. Shinskey: DISTILLATION CONTROL for Productivity and Energy Conservation, and Process Control Systems, both edited by Mc-Graw Hill, which are good reading material for precisely this case.

 

[pmureiko]

Once the condenser if drained, it won't matter what happens in the accumulator. Draining the accumulator increases deadband and slows response if the tower pressure drops.

Draining the drum can be used to drop pressure by venting vapor and non-condensables to a fuel gas / vent system.