1 reply to this topic

[ramnathsharma]

Dear Friends,

We have a distillation column operating under vacuum 700 mmHG (thru water ring vacuum pump)
System volume is 8000 liters.

Problem intro:

We used to charge our product crude to the reboiler.
Then evacuation starts, using a 120 m3/hr water ring vacuum pump.
Intially vacuum at the pump side would be 710 mmHG and at the system 700 mmHG.
Steam heating is started , there will be a dip in vacuum ie., 680-690 mmHG.
After 20 minutes vacuum of 700 mmHG is established in the system.
Our product crude is organic and highly corrosive in nature.

Question:

Last week due to problems in vacuum pump, we couldnt get 700 mmHG in the system.
Also vacuum pump failed frequently, in the meantime reboiler was heated and cooled repeatedly.
Now after changing the vacuum pump , we couldnt get 700 mmHG in the system ie., after heating .
Pump side it is 710 mmHG. We have observed one peculiar thing , ie., more fumes were emanating from the vacuum pump delivery side, which was not so .

What could have gone wrong

1. Whether the crude has been charred because of repeated heating under vacuum?
2. Any problems in vacuum pump, in capacity wise.


Kindly give your inputs.

Thanks & Regards
ramnathsharma

[Art Montemayor]

ramnathsharma:

Your problem should have a quick and easy solution. This type of vacuum problem is easy to resolve. One of two things must have happened at one point or another:

1. There is an excessive pressure drop between the system and your vacuum device; or,
2. Your vacuum device (Liquid Ring Pump - LRP) has lost its capacity.

Of course, in the above assertions, some assumptions have been made: air leaks have not increased, the LRP has the required internal impellers working adequately, and the process has stayed the same.

The excessive pressure drop can be caused by plugging within the system.

The LRP can loose capacity by one of various ways: it can mechanically break down. It is "vapor locking". Vapor locking is one of the principle (& few ) drawbacks of an LRP. It happens when the vapor pressure of the seal liquid is similar to or equal to the vacuum being drawn. When that happens, the LRP simply quits drawing a partial vacuum - it can displace any vapor because it is generating its own.

The LRP can easily vapor lock if you get a low boiler dissolved in your seal liquid, if you get gases included in the seal liquid, or if the temperature of the seal liquid is high enough to raise its vapor pressure to that of the set partial vacuum - 700 mm HG.

Other than the above, I can't come up with any specific reasons without being there and physically seeing any evidence at site.