Gas 2023 Issue

9 7 3 5 1

COS

DEA MDEA

15 17 19 13 11

1 , 500

50 , 000

100 , 000

150 , 000

500

1 , 000

Partial pressure , Pa

Figure 2 CO 2 and COS partial pressure in absorbers

2% of the COS is removed by DEA and 3.3% by MDEA, whereas the Kinetic Model predicts the removal of 65% and 23% by DEA and MDEA, respectively. Figure 2 shows how the Kinetic Model predicts that CO 2 and COS approach final outlet values in the DEA absorber. The gentler decrease in COS partial pressure reflects the much slower reaction kinetics of COS compared with CO2 . In DEA, CO 2 falls rapidly from 5 mol% to a few ppmv, whereas the same 20 trays only take COS from 500 ppmv to 189 ppmv. However, relative to MDEA, both CO 2 and COS decrease rap- idly simply because DEA reacts with both. MDEA does not. COS is severely mass transfer rate limited in a typical amine absorber. It cannot be properly simulated using only its physical (Henry’s Law) solubility in the amine, even aug - mented with salting-in and salting-out corrections. COS reacts with primary and secondary amines at rates that greatly affect its absorption and therefore affect the ability of any absorber to remove it from the inlet gas. Mercaptans The deprotonation of mercaptans into mercaptide ion is known to be instantaneous, leading to huge enhancement factors and hardly any liquid-side resistance to absorption. Therefore, unlike COS, absorption of mercaptans in aqueous amines is almost always VLE limited. However, being a much

weaker acid than CO2 and H 2S, mercaptans have much lower chemical solubility in the basic amine solutions. So, even any low to moderate amounts of H2 S and CO 2 stronger acids dis- solved in the solvent severely impair the ability of the solvent to remove the mercaptans from feed gases efficiently. Figure 3 shows the vapour phase ethyl mercaptan con - tent along with the total acid gas loading of the solvent in a typical absorber that treats a feed containing 10 mol% CO 2 , 2 mol% H 2S and 100 ppmv of ethyl mercaptan using 30 wt% DEA. The absorption profile of the mercaptan in vapour can be split into two zones. The top part of the absorber is actively removing mercaptans from the gas, but as we go down the column, the vapour phase mercaptan content decreases. This indicates that the absorbed mer- captan in the liquid phase is getting stripped back into the vapour. This reversal in the profile can be attributed to the increased bulk acid gas loading in the liquid. As previously discussed, the bulk acid gases, being stronger than mer - captans, start neutralising the mercaptide ions back into free mercaptans, which get salted out of the vapour. The plot shows that with the increase in the solvent circulation rate, as seen from the increasing L/G ratio, the bulge in the vapour phase mercaptan content moves down the column corresponding to the shift in the loading profile. In all three cases, the critical point of mercaptan removal occurs around

9 7 3 5 1

20 L/G 30 L/G 25 L/G

15 17 19 13 11

0.1

0.2

0.3

0.4

0.5

0.2

0.3

0.4

0.5

0.6

0.7

Liquid loading CO + HS

EtSH Vapour content , lbmol/hr

Figure 3 Vapour phase ethyl mercaptan content and liquid phase total acid gas loading in absorber trays. The different curves denote varying liquid to gas ratio in the units of US gal/min of solvent and MMSCFD of feed gas

Gas 2023