Gas 2024 Issue

clean-up units are therefore required. The most installed these days are amine-based TGTUs. The purpose of the TGTU is to convert all remaining sulphur components carried over from the modified Claus unit into H₂S. This happens in the catalytic section of the TGTU process. The reduction reactor utilises a cobalt-mo - lybdenum catalyst (also mentioned as CoMo bed). The cobalt-molybdenum catalysed reactions are shown in Figure 3 . The first step of the reaction does require hydrogen (H₂). H2 can be present as a byproduct of the modified Claus reaction but can also be generated by an inline reduction burner or provided from an external source. To ensure that the ‘sulphur compounds to H₂S’ reaction is complete, an excess of H2 is required after the reduction reactor, regard - less of the H2 source. In the second process step, the H₂S needs to be sepa - rated and returned to the inlet of the modified Claus reac - tion furnace. This step is based on an amine absorber/ regenerator system. The overall sulphur recovery efficiency (modified Claus reaction and TGTU) is required and predicted to be at 99.9+%. This is confirmed by measuring the total mass emission of sulphur dioxide (SO₂) at the exhaust of the final thermal reactor or ‘stack’. Two points of interest for close process control include the quench column and absorber outlet. Considering the AT5 quench tower outlet measurement (H₂ and H₂S), since the TGTU process was first intro - duced as the Shell Claus Off-Gas Treating (SCOT) process, H2 measurement has been expected at this point. It was included in the original system design. As previously men - tioned, the purpose of measuring the H₂ here is to ensure that excess H₂ is coming out of the CoMo reactor. H₂S is also measured at this point so that operators and the automated control system understand the amount of H₂S that will be entering the absorber. Sample gas measurements at this point of the process are easier to handle than AT4 measurements at the quench tower inlet because of the lower temperature of the process gas. Any particulates will also have been removed in the quench tower. The primary measurement at the AT6 absorber outlet H₂ and H₂S and carbonyl sulphide (COS)/CS₂ point was defined as a single H₂S measurement to ensure the perfor - mance of the amine absorber. By gaining knowledge about the application and availability of multicomponent instru - ments, additional measurements became interesting. Knowing the importance of excess H2 in the TGTU and recognising that an additional measured component does not add significant cost to an analyser, a redundant H₂ measurement should be added at this point. By adding the H₂ measurement, redundancy can be achieved without sig - nificant extra costs. The same can be said about adding a COS and/or CS₂ measurement; both can be used to determine the condition of the CoMo bed catalyst. If the COS and CS₂ values are increasing, the CoMo catalyst needs to be replaced, or other operational variables such as flow rate or temperature need

SO + 3H

HS + 2HO

S + H

HO + CO

H + CO

COS + HO CS + 2HO

CO + HS CO + 2HS

Figure 3 Cobalt-molybdenum catalysed reactions

of a modified Claus unit look like? The process consists of two chemical reaction steps: 3H₂S + 3/2O₂ → SO₂ + 2H₂S + H₂O SO₂ + 2 H₂S ↔ 3Sx + 2H₂O First is a thermal reaction (Claus furnace) followed by a catalytical reaction. The key parameter is the air (oxygen) flow to the thermal reactor. This setup will result in a spe - cific H₂S-to-SO₂ ratio. For a modified Claus unit, the ratio is described as 2:1. Depending on the tail gas treater down - stream of the Claus unit, this ratio can be different. As shown in Figure 2 , the tail gas analyser controls the trim air valve, which typically represents 10% of the air - flow to the reaction furnace. The main air valve is set by the acid gas-to-air ratio calculated on the best available infor - mation about the gas entering the reactor. This brings us back to the previous arguments: improved process control by getting a reliable feed gas composition analysis. The two measurements (feed and tail gas) in combination will result in better process control. Significant sulphur recovery rate improvements in the modified Claus plant are possible by using modern process instruments and combining them into a full process control scheme Modified Claus unit control It is essential to understand the potential impact on the process control scheme for each of the installed measure - ments. ‘Nice to have’ does not make the difference; relia - bility and safety are the key. Significant sulphur recovery rate improvements in the modified Claus plant are possible by using modern process instruments and combining them into a full process control scheme. What is important to consider with any addition of pro - cess instrumentation, besides safety, is that all instrumen - tation must provide data quickly and reliably. Downtime is not acceptable. Today’s environmental regulations do not allow sulphur recovery rates <99%. Sulphur emissions will still be too high. A standalone modified Claus unit will not achieve the required sulphur conversion rates. Downstream tail gas

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