Tail gas treatment units (TGTUs), and their ability to improve overall sulphur recovery, have made them a standard component in high-efficiency sulphur recovery units (SRU).

Their inclusion in the SRU process enables sulphur recovery rates exceeding 99 per cent, with some facilities achieving upwards of 99.99 per cent recovery.

With tightening emission standards for sulphur dioxide (SO2) emissions and other pollutants, maintaining effective TGTU operation is essential for both production and regulatory compliance.

To optimise long-term TGTU performance, operators must maximise sulphur conversion within the Claus unit catalytic beds.

This necessitates careful control of the stochiometric ratio between hydrogen sulphide (H2S) and SO2, while also managing the amount of hydrogen (H2) required in the TGTU to complete sulphur conversion.

Understanding how H2 functions in the TGTU and monitoring its concentration are, therefore, critical to efficient operation.

Through aluminium, or titanium-dioxide reactor catalysts in the SRU Claus plant, it is possible to convert both H2S and SO2 into elemental sulphur.

Without this reaction, sulphur conversion efficiency drops to a mere 50-55 per cent.

This is notably poor for plant performance, and it also presents obvious economic, environmental, and compliance concerns.

As such, these catalysts operate to achieve a stochiometric balance, between H2S and SO2, at 2:1, as shown in Equation 1. 

Figure 2 ... a TGTU illustrating three critical sampling locations (AT1, AT2, and AT3), with specific gas components measured at each point

By achieving this 2:1 ratio, and utilising 2-3 catalytic reactors, sulphur recovery efficiency jumps to 93-96 per cent.

Stockpiles of elemental sulphur can be produced from this reaction, as showcased in Figure 1.

It is at this point where SRU operators rely on TGTU for acquiring recovery yields of over 99 per cent.

A key facilitator in the further reduction of SO2 in the TGTU is using H2 injection in the reduction gas generator (RGG).

H2 is an important reaction element within the TGTU, and regulating its concentration in the injection process serves multiple purposes.

Much like the Claus unit, feed gas into the TGTU is fed into a catalyst bed consisting of cobalt- molybdenum, or nickel-molybdenum.

Under temperatures of 240-320 deg C, hydrogenation reactions occur across multiple sulphur-bearing species, converting them into H2S, as seen in Equation 2, for one reaction example.

This H2S is then captured and sent to the inlet of the sulphur recovery unit, prior to the thermal oxidiser located at the emissions point.

The management of H2 injection during this reaction is critical, driving the use of process analysers to provide concentration measurements in the TGTU.

For example, if H2 injection is not supplied at excess concentrations, then SO2, carbonyl sulphide (COS), and carbon disulphide (CS2) conversion may be incomplete and slip downstream.

The SO2 presence in most H2S scrubber media will be selectively bound and enable H2S and SO2 to be emitted at the end of the SRU train.

Because moisture is a common byproduct of hydrogenation, the presence of these sulphur compounds can contribute to sulphuric acid formation and severe reactor corrosion.

This corrosion damage can cost SRU owners millions in costly repairs.

For these reasons, SRU users are encouraged to install process analysers at their TGTU feed gas sampling points for the monitoring of H2 and H2S, and in some applications, SO2.

Figure 2 helps in illustrating these sampling points in the TGTU:

• H2 measurement can reveal whether sufficient reducing gas is available for sulphur conversion. These measurements can take place at the quench inlet, absorber inlet or absorber outlet.

• H2S measurements at the absorber inlet and outlet can provide insights on removal efficiency and scrubber regeneration efficiency.

• SO2 measurements can indicate whether the conversions in the RGG and cobalt-molybdenum (Co-Mo) reactor are occurring as expected.

Well designed, operated, calibrated and maintained process analysers enable operators to optimise reducing gas generation, minimise fuel consumption, improve sulphur recovery efficiency, reduce SO2 emissions, and identity reactor and catalyst performance woes.

Owners of these process analysers will attest that these instruments return their money back by several orders of magnitude.