105
1000 0 2000 1500 3000 3500 2500 4000
100 80 120 160 140 180
100
95
90
0 40 20 60
85
-500
80
0
2
4
6
8
10
12
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
L/G ratio on mass basis
Phosphoric acid addition to MDEA (wt%)
CO slip %
H S recovered %
H S in vent ppm
A common additive to MDEA-based amine solvents that increases H₂S absorption is phosphoric acid. The acids are purposely added at low concentrations to improve H 2S removal, as shown in Figure 4 . This addition primarily works by helping the amine regenerator produce a leaner amine in a phenomenon known as ‘acid-assisted amine regeneration’. The relevant reactions are shown in Rxn. 1 and Rxn. 2. According to Bryan Research & Engineering LLC’s BRE231 Training Manual, the addition of acid in the absorber is usually considered detrimental because it neutralises the amine (Rxn. 2). In the regenerator, however, the presence of the acid is beneficial since the regenerator is pushing Rxn. 1 backwards. At high acid concentrations (>0.8 wt%), the negative effects of the acid in the absorber are domi - nant in the system performance; at low acid concentrations the beneficial effects in the regenerator are dominant (<0.5 wt%). Improved removal of H₂S in the regenerator allows for better absorption in the absorber and achieves lower sweet gas concentrations of H₂S. Figure 4 Trends in H₂S concentration in vent as acid is added to MDEA
For this case, a 25 wt% MDEA amine blended with around 0.5 wt% acid is the ideal choice, which not only gives optimum results but also stays well and clear from the H 2S breakthrough points. w Lean amine flow rate Optimising the amine flow rate is essential to strike the right balance between meeting sulphur recovery targets and minimising unnecessary CO₂ absorption, which increases energy and operating costs. As shown in Figure 5 , as the L/G solvent-to-gas ratio (on a mass basis) increases, we get better H₂S pick-up. However, the excess solvent also increases contact with CO₂, and the CO₂ slip trends start decreasing, eventually going below 90%, which is undesirable. It also goes with - out saying that higher solvent flow rates increase the sol - vent regeneration costs. On the contrary, as the L/G ratio is reduced to less than 3, there is a risk of H₂S breakthrough (and, of course, off-spec SOx emissions). Therefore, the ideal zone in most TGTUs is a balanced L/G ratio. Another plant variable directly linked to amine circulation rate and a much more popularly tracked and calculated indicator among sulphur/amine operators is known as ‘rich Figure 5 Impact of lean amine flow rate on TGTU performance
H₂S ↔ H + + HS -
(Rxn. 1)
H + + RH₂N ↔ RH₂NH +
(Rxn. 2)
100 98 97 99 101
70
60
0 0.04 0.02 0.06 0.08 0.1 0.14 0.12 0.16
50
40
92 93 94 95 96
30
20
10
0
27
32
37
42
47
52 57
62
Lean amine temperature (˚C)
0
2
4
6
8
10
12
CO slip %
H S recovered %
H S in vent ppm
L/G ratio
Figure 7 Impact of lean amine temperature on TGTU performance
Figure 6 Rich loading trends in TGTUs
90
PTQ Q4 2025
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