Stack gas emissions monitoring SO , NOx (mass flow)
AT7
Quench column o -gas H
AT5
AT6
Absorber outlet H , H S plus COS & CS
Cobalt molybdenum reduction reactor
Quench column pH meter
ATx
Thermal oxidiser
Absorber & stripper columns
H S COS CS
H S recycle to Claus inlet
AT4 Reduction reactor off-gas low SO x
Quench column
H
Figure 4 TGTU with sample points
that the ‘sulphur compounds to H₂S’ reaction is complete, an excess of H₂ is required after the reduction reactor. In the second step, the H₂S needs to get separated and returned to the inlet of the modified Claus reaction 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 SO₂ at the exhaust of the final thermal reactor or ‘stack’. Sample point description details include (see Figure 4 ): • AT4: At the reduction reactor outlet, there is always an uncertainty of SO₂ slip. We know that according to the manufacturer’s specification for a new, fresh catalyst, the SO₂ concentration should be below 10 ppmv and not more than 100 ppmv for a used, aged catalyst. We need to keep in mind that under all circumstances, SO₂ breakthrough into the amine absorber must be avoided. Driven by new instrument developments, this measurement can now be considered as feasible as a direct measurement. • ATx: Quench tower water pH measurements to prove that no SO₂ is entering and changing the water into ‘sour’. • AT5 and AT6: Quench tower and absorber tower outlet measurements and control. • AT5: Quench tower outlet measurement H₂ and H₂S. Since the TGTU process was first introduced as the Shell Claus Off-Gas Treating or SCOT Process, H2 measurement has been expected at this point and included in the original system design. The purpose of measuring H2, as previously mentioned, 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 entering the absorber. Sample gas measure- ments at this stage of the process are easier to handle vs AT4 measurement 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. AT6 absorber outlet H₂ and H₂S and COS/CS₂: The pri- mary measurement at this point was defined as a single H₂S measurement to ensure the performance of the amine absorber. By gaining knowledge about the application and
the availability of multicomponent instruments, 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 meas- urement should be added at this point. By adding the H₂ measurement, redundancy can be achieved without signif- icant extra cost. 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 val- ues are increasing, the CoMo catalyst needs to be replaced or another operational variable such as flow rate or temper- ature needs to be adjusted, or the modified Claus unit has some operational concern that needs to be addressed. When looking into the combination of the two previously described sample points, redundant H₂ measurement will ensure optimal performance of the TGTU. It should reduce the cost of replacing contaminated amine and minimise downtime. If H₂S is measured at the quench tower outlet and absorber outlet, it will be possible to measure and control absorber effi- ciency online on a 24/7 basis. Process control of the amine recycling/regeneration is possible based on the H₂S IN and OUT measurement. The final quality control parameter is the SO₂ mass emission to be measured at the thermal oxidiser outlet. Only mass emissions can tell the true sulphur recovery rate. Knowing the H₂S entering the process and the amount of SO₂ leaving the oxidiser will provide an accurate value. Summary Beyond technological novelty, the focus remains on practical and tangible benefits. This ensures innovative solutions not only meet theoretical expectations but also enhance real-world performance. From application-specific analyser designs to tailored sampling systems and strategic technology choices, this holistic approach propels SRU operations towards opti- mal efficiency. Knowledge, understanding, and awareness training are essential to maintaining the instruments, enabling the achievement of required optimisation targets. Jochen Geiger is Business Development Manager at AMETEK Process Instruments. Email: jochen.geiger@ametek.com
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PTQ Q1 2025
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