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Part 4: Addressing the Impacts of Changing Regulations and Feedstocks in Sulfur Recovery Units

Part Four of a five part series.

Tail gas treatment (AT4, AT5, AT6)

For sulfur recovery units (SRUs) equipped with tail gas treating units (TGTUs), proper analyzer selection and installation can result in high measurement availability (high uptime) and prevention of significant operational expenses when the unexpected occurs. Not every SRU contains a TGTU, as super-efficient conversion and condensing of elemental sulfur (SX) may not require final treating prior to the thermal oxidizer at the emission stack. For those SRUs that do have a TGTU, there are two locations that could most benefit from some type of gas analysis.

TGTUs almost always include a reduction reactor, catalyst – cobalt molybdenum (CoMo) or other proprietary active material, a quench tower, an absorber, and a regenerator. Gases enter the TGTU after having nearly all of the SX removed and, after moving through the TGTU, the gases are sent to a thermal oxidizer before being emitted through the stack with as little hydrogen sulfide (H2S) and sulfur dioxide (SO2) as possible – exact limits or emission rates are often defined by local environmental requirements.

Between the reduction reactor and the quench tower, gas from the final condenser is heated and mixed with an excess amount of hydrogen – often injected, but sometimes already present in the tail gas – to convert any remaining sulfur components (that are not H2S) to H2S. Formulas 1 – 5 indicate the reactions that take place in the CoMo reactor:

SO2 + 3H2 ---> H2S + 2H2O (1)

S + H2 ---> H2S (2)

H2O + CO ---> H2 + CO2 (3)

COS + H2O ---> CO2 + H2S (4)

CS2 + 2H2O ---> CO2 + 2H2S (5)

Some users have chosen to measure SO2 and/or hydrogen (H2) at this location (AT4) to ensure that SO2 is not entering the absorber where it is known to damage amine, as mentioned in Part Three. However, for most operations, this location is not ideal. SO2 concentration usually is at a very low ppm level, leading users to question whether the analyzer is reading correctly, the sample often has a high dew point requiring water removal, and is highly toxic. It is known, however, that if a proper amount of excess H2 is maintained elsewhere in the TGTU, SO2 will be converted (formula 1). H2 concentrations do not change much from the inlet of the quench tower to the thermal oxidizer, so users have moved to measuring H2 before and after the absorber instead.

In the quench tower, the H2S in the gas stream is cooled and removed from the TGTU, with the resulting ‘sour water’ often returned to the reaction furnace at the beginning of the SRU. At the top of the quench tower, commonly referred to as the quench tower outlet, H2 is measured, as little to none should be swept away by the cooling of the quench tower. This is a much more reliable measurement point for H2, as the dew point is usually much lower, making sample handling much easier. Engineers may also measure H2S at this location for material balance purposes or to measure the efficiency of the absorber (absorber outlet H2S/absorber inlet H2S). AMETEK has found that H2 analysis at this location is required for confirmation that the TGTU is operating correctly and that the amine in the absorber is protected from SO2 upsets.

In the absorber, any remaining H2S is removed by amine or other stripper. The stripper is then recycled in the regenerator and the H2S that is removed in the regenerator is again returned to the reaction furnace at the start of the SRU. Gas exiting the absorber ‘overhead’ is then directed to the thermal oxidizer. H2S measurement is universal at this point, as it directs sample and air flow rates at the thermal oxidizer prior to final release of the completely treated gas to the stack. The company’s experience indicates users are also measuring H2, carbonyl sulfide (COS) and carbon disulfide (CS2) at this point:

  • The H2 measurement at this point is a back-up to H2 analysis made after the quench tower. This redundancy is driven by a desire to confirm that excess H2 has been present, resulting in complete conversion of SO2 to H2S prior to the absorber.
  • COS and CS2 measurements help complete the plant sulfur balance calculations, but more importantly indicate an issue with the catalyst found in the CoMo reactor. As the catalyst ages, the COS and CS2 levels will rise (Figure 1).

Figure 1 details two different issues with the TGTU operations. The increase in COS readings indicated a problem with the CoMo catalyst properly converting sulfur species to H2S. After the catalyst was replaced – note one week of no analyzer readings – the TGTU was started back up and the H2S readings were much higher than expected. The end user subsequently made corrections to their amine regenerator. The net result was that the user was able to properly repair their TGTU, and not shut down the entire SRU to troubleshoot the entire system. This saved time and considerable expense.

Unpredicted upsets in the TGTU, captured by a gas analyzer
Figure 1. Unpredicted upsets in the TGTU, captured by a gas analyzer

One final point to note about the quench tower outlet and absorber outlet measurements is that the sample at these points is extremely hazardous to human life. Use of a heated sample probe that integrates a single inlet and outlet tap and double block mechanism – such as AMETEK’s HAG probe – is recognized as a requirement and not an option.

Part One - Introduction
Part Two - Feed Gas Analysis 
Part Three - Tail Gas/Air Demand Analyzer
Part Five - Continuous Emission Monitoring Systems

Learn more about AMETEK's solutions for tail gas treatment.

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