Fresh catalyst
48 months, no poisoning
48 months, with RGG burner
2.50
2.50
2.50
2.00
2.00
2.00
1.50
1.50
1.50
1.00
1.00
1.00
0.50
0.50
0.50
0.00
0.00
0.00
20.00
20.00
20.00
0.00
80.00 60.00 40.00
100.00
0.00
80.00 60.00 40.00
100.00
0.00
80.00 60.00 40.00
100.00
0.03 0.02 0.05 0.04 0.06
0.03 0.02 0.05 0.04 0.06
0.03 0.02 0.05 0.04 0.06
0.01
0.01
0.01
0.00
0.00
0.00
20.00
20.00
20.00
0.00
80.00 60.00 40.00
100.00
0.00
80.00 60.00 40.00
100.00
0.00
80.00 60.00 40.00
100.00
% catalyst bed depth
% catalyst bed depth
% catalyst bed depth
Hydrogen
Hydrogen sulphide
Carbonyl sulphide
Carbon disulphide
Carbon monoxide
Sulphur dioxide
Methyl mercaptan
Methane
Figure 7 Reactor composition profiles
water-gas shift reaction. In addition to providing a hydro- gen source for the reduction of trace sulphur species, this ensures carbon monoxide emissions are reduced. The mercaptan concentration initially increases with depth into the bed because it is an intermediate product in the CS 2 reaction. However, it is later hydrogenated, driv- ing down the outlet concentration. However, as activity decreases from ageing and poisoning, mercaptan concen- tration increases as it is still formed from CS 2 (which is still almost completely converted), but the degree of intermedi- ate RSH destruction declines as the active portion of the Modelling with appropriate ageing and poisoning factors provides the opportunity to use the knowledge presented by the model to decide whether to remove the poison or plan for catalyst replacement bed shrinks. Total sulphur slippage (represented by the total of all sulphur atoms in COS, CS 2 x2, and RSH) typically slips past the amine system into thermal oxidiser. Both age- ing and poisoning affect sulphur slip, as seen for fresh and 48-month conditions: • Fresh ~26 ppmv • Ageing only at 48 months ~83 ppmv • Refinery + RGG at 48 months ~153 ppmv Note that this analysis does not consider additional plant-specific factors. Firstly, the analysis is limited to a
typical TGU feed with first-generation Co/Mo hydrogena - tion catalyst. Improved catalysts may be available at some sites. Secondly, a robust TGU design will allow operators to increase the reactor inlet temperature to compensate for some slower reactions. The penalties for this flexibility are incremental fuel or energy usage (cost) and increased baseline hydrothermal ageing of the catalyst. Both factors can be studied with the use of the OGT SulphurPro model. Conclusions A hydrogenation reactor that accounts only for ageing shows most of the temperature rise in the top zone even at the end of catalyst life. Some degree of poisoning is indi- cated for most units as they exhibit operating temperature profiles shifting into middle and bottom zones across the life cycle. The temperature profile in a TGU hydrogenation reactor provides useful insight into performance and catalyst health. Catalyst deactivation is caused by ageing and poisoning – it is inevitable and generally irreversible. Deactivation and poisoning lower conversion and increase slip of non-H 2 S compounds; in severe cases, SO 2 may slip, causing corro- sion, fouling, and equipment damage. The kinetic model presented here captures the detailed reaction kinetics and accounts for deactivation from age- ing (depending on temperature, humidity, and time) and poisoning (related to operational stresses, such as BTEX or O 2 in the feed). Hydrothermal ageing affects the rela- tive activity of the entire bed, whereas poisoning affects the bed along the flow path, starting at the inlet and mov - ing toward the outlet. Poisoning accelerates performance decline, related to loss of conversion of sulphur species that slip through the TGU into the thermal oxidiser.
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PTQ Q2 2023
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