10 replies to this topic

[Propacket]

Dear All,

In amine sweetening of natural gas, fraction of acid gases stripped in amine flash drum is very small (less than 0.1% of rich amine molar flow). What will be the effect on downstream equipments (Lean Rich Exchanger Plate and Frame Type,Regenerator Trayed Type)if we eliminate flash drum from process?

[Art Montemayor]

A lot depends on how your CO2+H2S Stripper is built.

If you have the initial, hot pressurized rich amine being flashed on the very top tray of the stripper, you will naturally be flashing more gas than usual and you will have to deal with that flow rate in the overheads gas cooler and downstream equipment.

If you have some of the top trays designed to "water-wash" the exiting CO2+H2S top vapors in order to conserve amine entrainment & losses, then you may wind up with more liquid entrainment from the trays if they can't handle the excess gas flow and velocity.

In either case, you will be handling a more flammable gas overheads out of the amine stripper and this may affect anything downstream of the LP gas cooler/condenser.

We can't give a specific answer or meaningful response when you don't contribute any details or your own vested input in the form of sketches and P&IDs.

Put more personal effort into your query and we will respond with more meaningful responses.

[Propacket]

Dear Art,

Actually I am designing an MDEA plant with Hysys. While simulating the plant, I noted that flow rate of acid gases from flash drum was very small. Eliminating the flash drum, I noted small change in duties of downstream equipment. However, flow rate of acid gases from regenerator increased as expected. I have attached the Hysy PFDs for both cases (with and without flash drum) for your review. I have sized the exchangers and tower for both cases and there is no significant difference in equipment sizes. Therefore, from optimization point of view, flash drum is not required. What do you suggest?

I have also reviewed some previous designs for the elimination of flash drum. After performing the simulation and sizing the equipments, I compared the sizes with those of existing design using flash drum in the process. I was surprised to see that there was no difference in equipment sizes. Then, what is the rational of using flash drum in the process. I also want to know the operating problems that could occur if we remove flash drum from process. Because i am not sure how lean-rich exchanger(plate and frame type)behaves if the MDEA solution contains vapours.

Thanks

Attached Files

•  With Flash Drum.pdf   18.14KB   132 downloads
•  Without Flash Drum.pdf   16.01KB   91 downloads

[Art Montemayor]

Pengr:

You should have stated that you are merely simulating, and not dealing with real life. In my opinion this post belongs in the Simulation Forum.

Simulation results are only as valid as the experience and the know-how of the person running the program. If you are a student and not a professional process engineer with at least 10 to 15 years of experience under your belt, you are only guessing with a simulation program - especially HYSYS on an MDEA application. HYSYS is not the "best in the west" when it comes to acid gas removal simulation - or TEG dehydration. There are better and more accurate simulation programs in the market - and even these have their inaccuracies and deviations.

I have addressed all the issues that you brought up and you should read my post again. In my opinion, you are placing excess and undue reliance on HYSYS to do YOUR PROCESS DECISIONS. A computer program is only a TOOL - no different from a slide rule. It cannot make decisions for you. You (thank God!) are the designated one to make all the decisions - including the main one of determining to what extent you can trust a computer program to spit out all the truth (inspite of not knowing the algorithm that the simulation program employs).

I stand by what I replied previously.

[K.K.]

Dear P.Engr.

You are dealing with a really small unit. Please don't scaleup it.

Eliminating flash drum might not have much impact on downstream equipment sizing as far as vapor is concerned, but it does a lot as far as hydrocarbon contents are concerned. A flash drum is put into the scheme due to following reasons:

1. Removing any potential hydrocarbon liquid
2. removing dissolved HC gases
3. Unburdened the Regenerator and exchangers
4. Giving sufficient residence time for stable operation

The most important reason is the potential hydrocarbon liquid contents. HC liquid is poision for amine system. It just destroy the effeciency of the system. It is of prime importance to flash the rich solvent in the flash drum and give enough residence time (15-20 minutes, in some cases even more)for amine-hc phase separation. Otherwise HC will cause severe foaming which has several negative effects on unit performance.

Flashing the vapors in flash drum is very advantegous if you have a sulfur recovery unit downstream of amine unit. HC gases are not good for SRU.

I have dealt with at least 10 amint units in different plants and I never see any scheme without a flash drum, not even in literature. Its good for thought, but may be impractical.

Sufyan Khan K.K.

[Dacs]

I agree.

I must imagine the impact of HC carryover in the downstream equipment when you remove the flash drum.

Amine is very sensitive to HC carryover. The flash drum is the best place to remove any HC in your amine stream.

I think that the purpose of flash drums is on the HC removal, much more so than flashing of acid gases.

If you're really adamant on removing the flash drum, then you have to consider installing HC skimmers in (1) regenerator (2) reflux drum (3) storage tanks. I think this is generally good practice even with the flash drum installed.

[K.K.]

Also, the flash drum can only be removed in low pressure systems, such as Scot Process. Its mendatory in high pressure units.

[Shahine]

Main purpose for flash drum is to decrease the poisin possibility of amine with HC, Also to remove dissolved HC gases from amine,already some sour gas removed but not more as you show.

but these flashed gas are send to fuel gas supply not to SRU, Because the present of HC gases is not required at SRU inlet stream.
So, the benefit of flash drum in this case to remove dissolved HC gas and to skim the liquid hydrocarbon.

Also, there is a rule of thum to decrease Plate HE duty (rich/solvent amine heat Exchanger)to decrease rate of sour gas that flashed from rich amine to protect HE and inlet regenerator rich amine line from corrosion.

[Propacket]

Shahine

You are right in the sense that flash drum is mandatory to protect the downstream equipment/s from foaming as well as from corrosion.If you read my earlier posts, i had a misconception that eliminating the flash drum will save lots of dollars of a designer with little change in duties.But, after posting this topic,i have come to conclusion that rationale of flash drum is not to optimize the equipment/s duties.

[GasPro]

I agree with Art regarding his post. I believe that HYSYS is in no way reliable in designing this sort of unit.

However I have worked on units that do not have a Rich Amine Flash Drum. With this said, in the absence of a RAFD there is a HP column and and a LP column prior to entering into the regenerator. With these columns there are HC skimmers in each to remove any entrained HC. As someone stated earlier HC is the enemy of amine.
Operationally, you would want to design your unit to remove any amount of HC that might present itself.

The RAFD, is also there to recover any usable gas that will be entrained in the rich solvent. This gas has benifits and could be use in operations such as adding the plants fuel gas system to run boilers, heater etc.

Taking all these things into concideration I bet that who ever has the final word on this project will go for a RAFD.

I hope that this was informative and in no way am I attacking.

GP

[Propacket]

GasPro,

You won the bet.

We convinced our client that flash drum is mandatory for the process .