Vacuum Scenario Protection

[Bill3752]

I have a Client with a distillation column. I am designing for vacuum protection. The column has a reboiler. A portion of the OH vapor is fed downstream to a process and a portion is condensed. Assuming the steam to the reboiler stops and the condenser still operates. The default in the facility is to assume that the vacuum relief needs to handle the design rate of flow to the condenser (i.e. condenser keeps "removing" normal vapor rate). However, when this event has actually occurred in the past, the column has never gone to negative pressure (normal operating pressure is a couple of psig). I cannot find any guidance in API. Anyone have any ideas?

 

[The Obturator]

API-2000 Gives various relief scenarios - is this the standard you are referring to?

 

[LittleInch]

You won't find what you're looking for in a design guide as this is very project specific.

Post a sketch or similar, but it would seem that the condenser just doesn't see the steam rate to condense.

You need to work out flows, temperatures and pressures yourself looking at what happens when steam flow stops.

Clearly some steam will condense, but at a much slower rate than when it is being fed into the condenser

 

[Compositepro]

As the pressure drops in the column the contents will boil at a lower temperature. You can't drop lower than the vapor pressure of the liquid. Is it prohibitively costly to make the column rated for full vacuum so accidents don't happen?

 

[Bill3752]

Thanks for the great input.

Littleinch - your general comment about looking at conditions after the steam is cut off is what is really needed, but I don't have the software to model this. Btw, the steam is fed to a reboiler, which in turns vaporizes a mix of water, MeOH and other components. Part of the over head is then condensed and fed as reflux.

Compositepro, thanks for the comment about vapor pressure. As mentioned above, this is a transient problem. As a result of your comment i looked at the vapor pressure of a couple of the key streams (feed to column and bottoms). The column would need to cool pretty significantly to reduce the pressure to less than atmospheric; but this could happen due to continued input of MeOH liquid to the top of the column.

So I still feel that intuitively designing for the full condensing rate is conservative, but to resolve this I likely will need to model this.

 

[georgeverghese]

The lowest the column pressure can go during normal operation is the bubble point pressure of the overheads vapor stream at the lowest normal operating temperature of the overheads condensor. This condensor temp can be conservatively taken to be the lowest normal feed temperature of the condensor's cold utility stream.

On the other hand, in winter, is the lowest ambient temp lower than this min condensor temp ?

And you should use the vapor overheads composition that would exist at the time of this shutdown for this bubble point pressure estimate, not the feed or the bottoms composition.

Conservative estimate would be based on feed stage flash vapor composition as overhead vapor.