MDEA absorber flash gas

[prq123]

We have an MDEA absorber to remove CO2/H2S in a natural gas stream. The MDEA co-absorbs a small amount of hydrocarbons in the absorber that is mostly flashed off from the rich solvent in the flash column. The normal flash gas from the flash column is, say, 1000 Nm3/h, based on a natural gas feed flowrate of 1000 kNm3/h.

The question if is natural gas feed stream is reduced to zero, does the flash gas flowrate stay about the same (1000 Nm3/h), assuming the lean solvent rate to the absorber stays the same? According to our process licensor, the flash gas rate should stay about the same (ie. the lean solvent will constinue to abosrb the same amount of hydrocarbons from the feed gas). However, I have my doubts as I would expect the mass transfer rate to decrease substantially if the feed gas rate is reduced to zero.

 

[djack77494]

I would tend to agree with your licensor. Let's make a little assumption for purposes of illustration. Let's say your natural gas is 100% methane, except for small quantities of CO2 and H2S that are normally removed by your process. When you stop the flow of natural gas to your Absorber, the gas that is "trapped" in the Absorber will rapidly be depleted of CO2 and H2S. Now you know that a small quantity of methane is normally absorbed by the lean solvent. Why would this quantity change? The gas in the Absorber is unchanged except for the removal of trace components, and this should not affect the absorption of methane. So the methane in your rich sorbent should not change, nor should the amount of methane flashed from the rich solvent.

 

[maddocks]

Flash gas rate may in fact go up ever so slightly. Since the co-absorption of hydrocarbons is a function of partial pressure and solubility, rate of the primary gas is not a factor. It is directly proportional to the circulation of the amine. If the flow of gas is zero, we can assume that the solution has reached full equilibrium - hence the potential for a slight rise in flash gas volumes.

 

[prq123]

Djack, maddocks - thanks for the confirmation.

 

[maddocks]

I've re-thought this and am thinking that once the CO2 and H2S has been removed, the flash gas rate will drop - the only remaining flash gas will be whatever is co-absorbed in the MDEA mixture.

 

[prq123]

We just recently put the unit in operation and compared the flash gas flow at zero feed gas flow verus ~30% feed gas flow (with same MDEA flow). The flow of flash gas from the flash column overhead showed little difference. So process licensor was correct.

 

[RPilch]

To clear up some misconceptions, the main reason bulk gas from the absorber ends up in the regenerator/stripper is due to physical entrapment in the solvent. Amine solutions are notoriously foamy and gas pockets in the foamy liquid of at the bottom of the absorber will carry under to the regenerator/stripper. If the inlet gas distributor has downward facing slots, there may be additional agitation of the bottom holdup leading to even more carry-under.

 

[chemicalengineerthai]

1) Your licensor say correct but it can not stay for last long, it happen only inteval of the residence time or retention time.
2) After the feed gas stop longer than its retention time , the mass transfer will change ( in flow can not equal to out flow) , then your understanding is correct.
3) Normally eventhough thae gas stop but lean amine still doing absorbtion since their still has gas in the column, but acid gas H2S/CO@ already reduce but metahne or orther gas can co-absorb and produce its off gas, the the rate/composition will change as your understanding.