98 99 97 100 101
0 20 30 40 10 50 60 70
0.12 0.14 0.16
0.1
0.08
94 93 96 95
0.06 0.04 0.02
27 32 37 42 47 52 57 62 92
0
0
2
4
6
8
10
12
Lean amine temperature (˚C)
L/G r atio
CO Slip %
H S Recovered %
H S in Vent ppm
Figure 6 Rich loading trends in TGTUs
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 tail gas treatment unit (TGTU) at the back end of a sulphur recovery unit (SRU) will play a critical 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 simulation model. The analysis trends concluded the following best practices when designing a TGTU: A mid-height packed amine absorber. Not too short (such that H 2 S absorption is diminished) and not too long (such that CO 2 co-absorption is enhanced). Use of a low flow rate (targeting a rich loading of 0.1-0.15 mol/mol) and low-strength MDEA solvent (20-30 wt%), preferably promoted with phosphoric acid (0.1-0.5 wt%). H 2 S absorption almost always decreases with rising lean amine temperatures, whereas the CO 2 absorption rate could increase. Therefore, in the hot summer months, if trim coolers are limited, consider adding supplemental chillers. Figure 7 Impact of lean amine temperature on TGTU performance
off-spec SOx emissions). Therefore, the ideal zone in most TGTUs is a balanced L/G ratio. Another plant variable that is directly linked to amine circulation rate and a much more popularly tracked and calculated indicator among sulphur/amine operators is known as ‘rich 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. 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 H 2 S and CO 2 solubility and the 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 ambient air temperature. According to Figure 6, two things can be concluded. One, during the cooler months of the year, the TGTU operates almost perfectly. MDEA-H 2 S reactions are generally known to be exothermic in nature and, therefore, low temperatures tend to favour H 2 S absorption into the amine. However, as the temperature of the lean amine increases, the H 2 S concentration in the vent line increases. Two, the CO 2 slip effect diminishes with increasing temperature. This is because CO 2 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
Ganank Srivastava G.Srivastava@BRE.com
Refining India
33
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