Vacuum Column Overhead Design

[petrotrinsmerv]

We have a vacuum column designed with a three stage overhead (ejector/condenser) system. From inception the pressure obtained in the column was above design (40 mmHgabs compared to design 20 mmHgabs). This was so whether the first stage ejectors were on or not. We had three first stage ejectors with three first stage condensers of equal capacity in paralell. So the first stage ejectors were decommisioned and remained that way for many years. We have been told that the original design first stage condenser pressure of 64 mmHgabs was too low for 86 degF cooling water (seawater). Uncondensed steam would load up the second stage condensers. Does this claim seem reasonable. For a three stage system, what would people normally design for the first condenser pressure to ensure adequate steam condensation.

 

[25362]

Assume a vapor molar composition to the first condenser as follows:
Steam, 0.932; air equivalents, 0.062; light hydrocarbons, 0.006.

If the total pressure was 64 mm Hg and the partial pressure (ie, mol fraction) of steam corresponds to 59.6 mm Hg, a shell-side condensing temperature of 107 deg F could be comfortably attained with cooling water at 86 deg F.

The general assumption is that the first condensers would be capable of condensing about 90% of the steam coming from the first-stage ejectors ("dry vacuum tower").

 

[petrotrinsmerv]

Thanks for the reply. Actually, the designers assumed 95% of the steam condensed with a vapour outlet temp of 99 degF from the condenser. This was actually a 'wet' column so the steam load was quite high. I don't know but its possible that the load on the 2nd stage ejectors may have been above design if more uncondensed steam remained than anticipated. In any case the original design did not work. When the plant starts back I will have to check the 1st stage condensers performance, though I'm not sure I have the proper instrumentation in place for that.

 

[25362]

I'd recommend Chapter 16 in A Working Guide to Process Equipment by Lieberman and Lieberman (McGraw-Hill) ISBN 0-07-038075-9, for an excellent rundown on ejector systems in vacuum units.