Refining India March 2026 Issue

105

-500 1000 2000 1500 500 0 3500 2500 3000 4000

120 140 160 100 180

100

80 60 40 20

0.2

0.4

0.6

0.8

1.2

1.4

L/G r atio on mass basis

Phosphoric acid addition to MDEA (wt%)

CO Slip %

HS Recovered %

HS in Vent ppm

monoethanolamine (MEA). MDEA is also less corrosive than DEA and MEA, so it can be used in concentrations up to 50 wt%. However, in TGTU applications where one is trying to prevent CO 2 co-absorption, lower MDEA strengths may be preferred. As seen in Figure 3 , a 20-30 wt% MDEA gives the best CO 2 slippage and optimum H 2 S absorption. Going lower than 20 wt% can result in the unit being too close to a ‘cliff’, leading to insufficient H2 S removal (due to too little amine available) and a sudden spike in H 2 S concentration in the vent line (thereby resulting in failure to meet the SOx emission specifications). Figure 4 Trends in H 2 S concentration in vent as acid is added to MDEA A common additive to MDEA-based amine solvents that increases H 2 S absorption is phosphoric acid. The acids are purposely added at low concentrations to improve H 2 S removal, as proven 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, right. According to Bryan Research & Engineering LLC’s BRE231 Training Manual , in the absorber, the addition of acid is usually considered bad due to the acid neutralising the amine (Rxn. 2). In the regenerator, however, the presence of the acid is beneficial, since the regenerator “ With better removal of the H2 S in the regenerator, it is possible to get better absorption in the absorber and achieve lower sweet gas concentrations of H2 S ”

is pushing Rxn. 1 backwards. At high acid concentrations (for example, >0.8 wt%), the detrimental effects of the acid in the absorber are dominant in the system performance. At low acid concentrations, the beneficial effects in the regenerator are dominant (for example, <0.5 wt%). With better removal of the H 2 S in the regenerator, it is possible to get better absorption in the absorber and achieve lower sweet gas concentrations of H2 S. Figure 5 Impact of lean amine flow rate on TGTU performance

( Rxn. 1 ) ( Rxn. 2 )

H 2 S ↔ H + + HS −

H + + RH 2 N ↔ RH 2 NH +

In this case, a 25 wt% MDEA amine blended with around 0.5 wt% acid is the ideal choice, as it not only gives optimum results but also stays well clear of the H 2 S breakthrough points.  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, H2S pick-up improves. However, the excess solvent also increases contact with CO 2, and the CO 2 slip trends start decreasing, eventually going below 90%, which is not desirable. Also, it goes without saying that higher solvent flow rates also increase the solvent regeneration costs. On the contrary, as the L/G ratio is decreased to below three, there is a risk of H 2 S breakthrough (and, of course,

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