8 replies to this topic

[process101]

My question is regarding Amine flash tank design pressure.
Is this tank should have same design pressure as Amine absorber or depends on operating pressure of flash tank. I have seen in literatures that amine flash drum or tank design pressure is 125 or 150 psig.
But if Level control valve on rich amine line fail opened or something happened that valve cannot drop pressure to 125 or 150 psig. Then the tank will be filled with all amine and gas will be blown down to flash tank and can be damaged.
Can we install a choke valve before the level control valve?
Please let me know if am correct or wrong. OR amine flash tank should have same design pressure as absorber.
Thank you in advance for your help

[Art Montemayor]

The best way to help you with regards to this type of question is to have you go back to the basis of the question and study the application. If you don't know the reason for introducing a flash tank in the amine regeneration cycle of an amine acid gas (or sour gas) process, then you should first find that out.

It serves no purpose (and wastes your time as well as others) to tell you what the pressure in the flash tank should be if you don't know what it is that the flash tank is supposed to accomplish. My question to you is: do you?

Once you answer that basic question we can proceed on to the logic and reasoning in designing a flash tank.

[Suresh Kumar.P]

Hi,
The flash drum need not be designed to the absorber pressure. The flash drum's function is to remove the dissolved/ condensed HC's in the amine solution. The pressure is maintained using PCV's. In case the LV of absorver opens due to some component failure, will lead to ESD of the unit as per the pressure/ level settings on the flash drum.

[process101]

Mr. Art,
I Know the purpose of the amine flash separator that is "degas or remove the entrained gases from the MDEA solution leaving the absorber.Also it helps removing hydrocarbon.
I was just wondering that someone might tell me that why we can not desgin it for high pressure. Mr. Suresh answer makes sense to some extent.
Now would you explain me more about design pressure?

Thank you

[apex]

Hi,
Amine flash drum is designed to remove the absorbed light hydrocarbons. This needs lowering of pressure. It makes no sense to design it for absorber pressure as it will lead to very high cost for the flash drum. To avoid the gas blowby, apart from having safety instrumentation (absorber low level closure of shut down valve at the inlet of flash drum) there is always a PSV on the flash drum to protect it from gas blowby.

[process101]

Thank you all for replying me

[Art Montemayor]

Process101:

You are correct. The flash drum in an Amine process for removing acid or sour gas from a hydrocarbon gaseous stream is used because:

• It removes the majority of dissolved hydrocarbon gas in the Amine solution – especially when the hydrocarbon stream is at a high pressure in the amine absorber;
• It serves to remove any liquid hydrocarbon that is mixed with or dissolved into the Amine solution.

Therefore, the flash drum functions as a 3-phase separator: it separates the dissolved hydrocarbons by expansion down to a lower pressure and it decants the heavier, amine solution from any oily, lighter layer formed in the same drum.

Because the flash drum operates at a lower pressure than the absorber, there is no reason to design it for the same pressure – unless you want to do that to ensure that should you have a “blow-by” through the amine level control valve on the absorber, you can contain the excess pressure there. But this is not usually the case in design because you can supply instrumentation that mitigates such an event from over-pressuring the equipment downstream should the amine level control valve on the absorber fail open.

Some acid (or sour) gas will flash off at the Flash Drum. What you want is to avoid having hydrocarbon gases enter the amine regeneration section as well as hydrocarbon heavies. If that happens, you will have fouling in the amine reboiler and excessive pressure upsets as well. You want to strip out only acid (or sour) gas in the regeneration section and in a controlled manner. The pressure that you maintain the Flash Drum at is determined by that scope of work.

[Behzad Yasini]

Hi,
I'm interested to know whether it is possible for small unit capacities (pilot size) to delete the rich amine flash drum from process. Because simulation shows that the amount of flashed gas is negligible (1-2 Nm3/h). Oil skimming could be done in other vessels of the unit (absorber and/or surge drum).The acid gas from regenerator is routed to flare.
Regards

Edited by Behzad Yasini, 02 January 2013 - 07:59 AM.

[RockDock]

You may omit the flash drum, if your process allows it. As Art alluded to, low pressure amine columns probably don't need flash drums. Also, if you maintain a high Lean Amine T, entering the absorber, you'll minimize hydrocarbons in the rich amine. I have seen perfectly functioning plants with no flash tank.

One more thought, keep an eye on your aromatics. Be sure you have a good model for their phase behavior.