98 100 102
1000 2000 1500 500 0 2500 3000
102
115
98 100
110
92 90 88 96 94
105
96
94
100
92
95
90
86
-500
88
90
0
10
20
30
40
50
2
4
6
8
10
12
MDEA strength (wt%)
Random packing height (m)
CO Slip %
H S Recovered %
H S in Vent ppm
CO Slip %
H S Recovered %
H S in Vent ppm
identify opportunities to drive performance improvements. The variables selected for the evaluation of a TGTU amine absorber will be the following: Absorber dimensions Amine chemistry For this study, a characteristic tail gas stream that has undergone hydrogenation and cooling consistent with some of the most common sulphur recovery facilities around the world will be used, as shown in Table 1 . The focus will be on refinery sulphur blocks using air co- firing rather than gas plants (where the CO2 content will be higher in the feed). The goal while comparing various operating scenarios will be to meet the following objectives: • Recover more than 99% H 2 S from this stream (to recycle back to the SRU). • Maintain <100 ppm of H 2 S in the absorber vent line routed for the incinerator (minimising eventual SOx emissions). • Slip >90% of the CO2 (to ensure SRE is not affected). A process simulator suitable for accurately modelling sour gases and based on operating data from hundreds of sulphur facilities units globally will be utilised. Process simulation Lean amine circulation rate Lean amine temperature models will be built for all the variables above, and their responsiveness to various cases will be tracked and reported. This should give readers deep insight into how a TGTU responds to changes in key process conditions and provide a technical conclusion Figure 2 Impact of absorber height on TGTU performance
Figure 3 Impact of MDEA strength on TGTU performance
on the best design and operational boundaries of the unit. Analysis and discussion of results The fixed variables across all the scenarios, unless stated otherwise, will be a maximum flooding percentage of 70% in the towers (letting the diameter float), an L/G solvent-to- gas flow ratio of three (on a mass basis), an amine regenerator reboiler steam rate of 130 kg/m3 of amine, and a selective methyldiethanolamine (MDEA)-based solvent at a temperature matching the feed. Absorber dimensions As can be inferred from Figure 2 , as the height of the random packing in the absorber increases, H 2S removal efficiency improves. This is because of more mass transfer surface area and contact time, which can be seen from the downward trend of the H 2 S ppm in the absorber vent line. However, this comes at the cost of more CO2 also being co-absorbed, as indicated by the CO2 slip trend also moving downwards. Therefore, a tower that is ideally not too short and not too tall is preferred for such a unique enrichment application. In this case, a trade-off of around 6-8m gives the best results. Amine chemistry MDEA is a commonly available tertiary amine that offers many advantages over other alkanolamines. Additionally, as a slow-reactive amine, MDEA is effective at selectively removing H 2S over CO2 compared to faster-reactive alternatives such as diethanolamine (DEA) and
Refining India
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