Amine sump pump

[A1nee]

I have a question on amine sump pump design.

This is a acid gas recovery unit with a typical absorber followed by regenerator configuration. If during an upset scenario or a regular maintenance operation, rich or lean amines have to be drained to a underground sump (that is connected to a flare).

The largest inventory for the sump will be lean amine from the bottom of the regenerator. Considering an upset scenario, it if all the volume at the bottom of the regenerator has to be drained to the sump pump. If I consider the pumping temperature same as the max. operating fluid temperature of the regenerator bottoms (i.e 124 deg.C). The simulator actually gives me a high vapor pressure (2.1 kg/cm2A) at the pumping temperature which results in a very low NPSH available.

If I intend to start the sump pump at high liquid level automatically, I cannot avoid selecting the pumping temperature of 124 deg.C.

Are there any ways to get around this problem?

Appreciate your views and suggestions.

Thanks.

 

[sshep]

A1nee,

It isn't clear what pressure you run your sump at, just that the sump is vented to the flare.

Most likely this is boiling liquid (vapor pressure= approx 2 atm abs) which will flash off the vapor into the flare. After flashing Pvap = Psystem and cancels out, so depth is the easiest fix to the NPSHa problem, otherwise you must add some cooling via spillback cooler or external supply.

best wishes,
sshep

 

[A1nee]

Sshep,

Thanks for the suggestion.

The amine sump has an operating pressure of 0.1 kg/cm2g. It is provided with a pressure controlled valve set to open to flare at higher set pressure (3.5 kg/cm2g).Are you suggesting to reset the pressure controller to 2.1 kg/cm2A to cancel out the effect of vapor pressure on the NPSHa?

 

[sshep]

Hey A1nee,

No, don't reset the pressure controller to flare lower. The post just wasn't clear on the pressure in your sump. Now it is clear you have options.

If atmospheric = 1.033 kg/cm2 abs, your vent to flare is set at 1.033 + 3.5 = 4.533 kg/cm2 abs vs a pressure at dump of 1.033 + 0.1 + 2.1 = 3.233 kg/cm2 abs. This is good news for you in a dump situation because Psystem > Pvap by 1.133 kg/cm2. If you run the sump pressure say 0.5 kg/cm2 higher (probably this is set by an N2 pad), you will have even more difference to work with in a dump, and more NPSHa. This is sort of simplistic, because the liquid will cool when it flashes into the sump, and the starting vapor in the drum will exert more pressure when it gets hot. To fully check it out, you should consider these effects based on the sump volume.

In summary, maybe you have more NPSHa than you think.

best wishes,
sshep

 

[A1nee]

Thanks Sshep,

Appreciate your insightful reply.
The available NPSH did improve when the operating pressure is set at 0.5 kg/cm2g with N2 padding. This will indeed help us to retain the auto-start function of the pump at high level in the sump incase the amine solution is being dumped at higher temperature (124 deg.C).