PTQ Q4 2025 Issue

loading’. It is nothing but the total moles of acid gas per mole of amine in the stream leaving the absorber. As can be seen from Figure 6 , the typical rich loading that TGTU plants should be operating at is around 0.1 mol/mol, unlike the typical deep acid gas removal unit (seen upstream of SRUs) loadings of 0.4-0.5 mol/mol. x Lean amine temperature The temperature of the lean amine solvent entering a TGTU absorber has a critical impact on its performance. This is because temperature strongly affects H₂S and CO₂ solubil - ity and reaction kinetics in the amine solution. Lean amine is usually routed through a trim cooler before it enters the absorbers and, therefore, is usually controlled by the ambi- ent air temperature. Two important considerations can be concluded from Figure 7 :  During the cooler months of the year, the TGTU operates almost perfectly. MDEA-H₂S reactions are generally known to be exothermic in nature and, therefore, low temperatures tend to favour H₂S absorption into the amine. However, as the temperature of the lean amine increases, H₂S concen - tration in the vent line increases. v The CO₂ slip effect diminishes with increasing tem - perature. This is because CO₂ absorption also follows Arrhenius-style behaviour that increases its absorption rates with temperature. Because of this mechanism, many refiners could feel the pinch of overloaded TGTU amine regenerator reboilers during the hot summer months. One

remedy could be to include supplemental propane chillers on the lean amine line that only turn on during extreme hot weather, to further cool the solvent before it enters the absorber. Conclusion Adding a TGTU at the back end of a SRU will play a crit- ical role in managing the SOx emissions of refineries and gas plants as they look to expand and process more sour hydrocarbons. The performance of TGTUs was examined across various operating scenarios using a process simu- lation model. The analysis trends concluded the following best practices when designing a TGTU:  A mid-height packed amine absorber. Not too short (such that H₂S absorption is diminished) and not too long (such that CO₂ co-absorption is enhanced). v Use of a low flow rate (targeting a rich loading of 0.1 mol/mol) and low-strength MDEA solvent (20-30 wt%), preferably promoted with phosphoric acid (0.1-0.5 wt%). w H₂S absorption almost always decreases with rising lean amine temperatures, whereas the CO₂ absorption rate could increase. Therefore, in the hot summer months of the year, if your trim coolers are limited, consider adding sup- plemental chillers. Ganank Srivastava is Engineering Consultant – GCC & South Asia, at Bryan Research & Engineering LLC. He also holds a degree in chemical engineering from the National University of Singapore and an MBA from Singapore Management University. Email: g.srivastava@bre.com

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PTQ Q4 2025