Amount of Vacuum Created from Condensing Vapor

[anubis512]

Hi,

I'm attempting to calculate the amount of vacuum generated on a distillation column to determine if a vacuum relief device is required. Currently I'm examining two cases - 1) the condenser operating at full load while the reboiler is shut off & 2) a cold top feed collapsing the rising vapor.

How would you calculate the change in pressure from condensing vapor in these scenarios?


Thanks a lot

 

[StoneCold]

I would say that the pressure would drop to the vapor pressure of the feed at either the temperature of the cooling medium for the condenser or the feed temperature, whichever is lower.
You might also want to consider vacuum created during steam out. Often column shells are designed for 50 psig and full vacuum to accommodate these design scenarios, unless the column is very large or of a special alloy that makes it cost prohibitive.

Regards
Stonecold

 

[don1980]

The lowest pressure is the vapor pressure at the minimum temperature the fluid could experience. That minimum temperature could either be the temp of the coolant in the condenser, or it could be the minimum ambient temp.

Consider the range of feed compositions.

 

[MortenA]

Are you considering when in operation, or in connection with a shut-in? It seem a bit unclear to me. If the vessel shall be designed for steam out then its almost full vacuum anyway.

 

[Duwe6]

It will be Full Vacuum plus the absolute vapor pressure of the fluid(s).

NOTE: Condensing/Collapsing vapor is a very common cause of failure for storage tanks. It happens so fast that any reasonable-size PRV or Rupture Disk is too small to allow enough flow during this {almost] instantaneous event. Thus the reason that all prudent engineers include Full Vacuum in their calc's for fractinating columns. Between reducing rates during operation, and steam-out, columns are guaranteed to experience partial to full vacuum during their lifespan.