Column with condenser

[Bill3752]

I have a MeOH stripping column, with reboiler heated by steam. Overhead vapor is condensed in downstream condenser. At shutdown the feeds and steam are shutoff.

Question is how to handle inbreathing if the condenser is left on. At first glance, it would appear that I should treat the column as a "steam out case" and assume the condenser is not working. I am questioning the impact of the condenser. On one hand it could increase thermal inbreathing by reducing the volume of vapor in the column; on the other hand, removing the vapor might be a good thing.

The Plant ran a trial late last week, taking the system down and no change in pressure was observed.

Any thoughts?

 

[Latexman]

Vacuum rating of components, column and column side of reboiler and condenser?

Good Luck,
Latexman

 

[Bill3752]

Latexman, all designed for at least -8 psig.

 

[Latexman]

If the column is "bottled up" and in total reflux, i.e. "stand-by mode", then if steam is lost, the condenser should pull the column into a vacuum. External cooling (cold rain) will do the same. Also, have you looked at API 521 7th Ed. Sect. 4.5 (and possibly 4.6) Guidance on Vacuum Relief? That should help you. If you take the larger of the max. reboiler or max condenser dutys (winter), to be conservative, and calculate the volumetric flow rate imbalance of vapor using the heat of vaporization in the reboiler/condenser respectively, you can get the volumetric flow rate needed to re-balance the column with air/N2.

If that is too large to be practical, vacuum can be relieved by reducing or stopping condensation in the condenser. This is accomplished by blanketing the condenser with a non-condensable gas. The required relief rate is the gas flow required to blanket the condenser before the system pressure drops below the minimum allowable pressure (vacuum rating).

Good Luck,
Latexman

 

[georgeverghese]

Worst case if if the column and condensor were filled with steam condensate - you'd need full vacuum lower design pressure to avoid having to cater for inbreathing if coolant at condensor is still running. Also the column would leak out heat to ambient through the insulation. -8psig = 7psia which is most likely much higher than water vap pressure at min ambient temp at your location.
Introducing air through a VRV is most likely not a good idea with methanol in the column top. Suggest connecting the VRV to an N2 or inert gas tank and size the VRV for combined heat loss through condensor plus insulation loss through the rest of the column. Awkward.
Else see if mech vessel engineers can somehow rerate this column for FV.

 

[shvet]

Any thoughts?

Just to remind
A conventional spring pressure safety valve can be installed not only to relief excess vapors but to inbreathe also. Design this kind of is not widely spread but takes place. And it will be still a pressure safety device.
Seat pressure difference is the only factor a spring safety valve actually does. It is not relevant upstream or downstream pressure/vacuum.
If you consider cannot avoid formation of explosive mixture then inert gas or steam can be used for vacuum relief. (it is a disputable point as you have only water in bottom of your column as I understood)

Vacuum design is an expensive option which would be hard to argue and can be avoided by simple cheap means.

 

[Bill3752]

I appreciate all your comments. Latexman, I had done exactly what you suggested (computed the amount of vapor condensed based on the maximum condenser, then converted to scfh). My problem is that Plant trials (shut off flows including steam to reboiler, but leave CW on condenser) indicate no change in pressure - doesn't go to vacuum). I am afraid I may be spending a LOT of money to fix a problem on paper that may not make system safer - replacing existing small conservation vent with much larger one, which requires moving it to a "problem" location.

George, column contains MeOH and water. Agree on air issue (actually have flame arrestor on current conservation vent). Rather than add N2 will likely replace RD on column with one that protects for both pressure and vacuum, and keep conservation vent for normal stuff.

shvet, current conservation vent is set at very low P/V. Will likely keep those low set points, but yes I have installed spring and weight loaded P and V vents. As you suggested, replacing equipment with those having FV is not an option.

After your emails I ran a calculation to estimate the evaporative flux from the liquid MeOH still inside the column, but this account for about 10% of the conservative relief rate. Again, thanks!

 

[georgeverghese]

Vapor pressure for pure MeOH at min condensor temp of say 10degC is -13psig.
Column pressure not going down much, with condensor coolant stopped, most likely due to good heat insulation. Another alternative, is to implement a higher integrity trip on the coolant supply to the condensor. Then calculate the residual inbreathing relief load for heat loss through insulation only. Better than not doing anything.