FIELD EXPERIENCE WITH A SINGLE NDUV + TCD ANALYZER ON AMINE BASED TAIL GAS TREATING UNITS

The paper describes the initial installation and subsequent improvements of a gas analyzer installed on a SCOT (Shell Claus Off-gas Treating) tail gas treating unit (TGTU) for Claus SRU (Sulfur Recovery Unit) tail gas. The process requires the measurement of H2 (Hydrogen) to control the reduction gas to a CoMo (Cobalt Molybdenum) catalytic reactor and the measurement of H2S + COS (Hydrogen Sulfide and Carbonyl Sulfide) to monitor the efficiency of the amine contactor and CoMo reduction reactor. The analyzer combines a non-dispersive ultraviolet (NDUV) optical bench with a thermal conductivity detector (TCD) for the simultaneous measurement of H2S, COS and H2 from the sample point at the overhead of the amine absorber. This measurement has previously been performed by a gas chromatograph or using discrete cold / dry analyzers. The innovation comes from combining the detection techniques, electronics and sample system into a single integrated continuous analyzer for a specific application. A significant reduction in cost and maintenance is realized. The sample system was given special attention in this application. The toxicity of H2S is well known and therefore the sample system was designed so the sample could be returned to the process at the same point as the sample take off. A heated sample probe was designed which has sample wetted parts fabricated from stainless steel and an aluminum heater for temperature control. The probe can be isolated from the process and back flushed (along with the analyzer) so safety integrity is maintained during service intervention. An integral membrane filter removes entrained liquids, and an aspirator is used to return the sample to the process. The probe, sample line and analytical components of the sample system are heated above the water dew point temperature to avoid water removal from the sample.