Amine absorber

[eilma]

Hi
I work in gas plant. Newly, we faced with serious problem.
We have two amine pumps. amine circulation is 145m3/h. according design when amine flow reached to 120 m3/h(any reason),due to be active a Alarm, this Alarm to start the stand-by pump. Sufficient time to this work is 10sec.if in this time flow reach to 109.5m3/h, another Alarm to be active and after 30 second if reach flow to above 109.5 ,that due to be unit shutdown.
Now,we would like to change second time to above 30.i don’t know how we must investigate the feasibility it?
Meanwhile, in a same plant, Empirically, Time will reached to 90sec.but No reasonable calculations.

 

[EmmanuelTop]

Reduced flow of Amine to the absorber means a slip-through of acid gases - H[sub]2[/sub]S, CO[sub]2[/sub], etc. from absorber overhead to the equipment downstream. You need to investigate how much time you can afford to have this breakthrough happening in the plant, before some anticipated and unwanted event takes place.

If downstream is a cryogenic section, passing too much CO2 from the feed gas will cause freezing inside the equipment downstream, with possible damages and definitely a log time shutdown for defrosting. As said above, your team should investigate what happens if the flow of Amine is below design value for certain amount of time, and what are the consequences. Only then you can start evaluating effects of different duration of acid gas breakthrough.

Dejan IVANOVIC
Process Engineer, MSChE

 

[eilma]

hi
Thanks you for your answer
Do calculation residence time on any tray, we can find the best time??
We have residence time on tray in design mode???
As you know, residence time is inversely ratio to the fluid flow.
If we can draw performance cure tower, that be good help for we.

 

[georgeverghese]

There is no information provided on why the pumps in the other plant take 90seconds to start and reach flow higher than 109.5m3/hr - you may need to talk to your mechanical and electrical engineers and study each step of the startup sequence - for example, if these pumps are on soft start motors, then it will take longer to accelerate up to full speed. There may be many other reasons and solutions - automated shutdown valves on pump inlet / exit also add to the delay if there are closed during pump stop.

Acid gas analysers on the absorber exit line may help to provide a more realistic logic sequence to trigger the shutdown, but agree that reliance on analysers for trip sequences is not a good idea.

 

[gcastan]

absortion of CO2 and H2S is greatly affected by amine flowrate.
I'd check your downstream specifications for CO2 and H2S to see how much time you give on those pumps to start.
You can also check on the type of pump used, you might find faster to start pumps.

Gabriel Castaneda, P.E.

http://www.gabcheminc.com

 

[georgeverghese]

Do you have a mole sieves unit downstream of the amine absorber to remove residual CO2 and H2S ? May be the plan is to make use of the additional buffer removal capacity in the mole sieves unit while the standby pump is revving up ?