Apr-2010

Enhanced sulphur recovery from lean acid gases containing COS and mercaptans

Claus configurations are available for more effective sulphur recovery from lean acid gas containing significant concentrations of organic sulphur compounds

Angela Slavens, Justin Lamar, Sara O’Dell and Laura Francoviglia
Black & Veatch

Article Summary

Claus sulphur recovery from lean acid gas can be problematic. Low H2S concentration in acid gas creates a cool thermal reactor temperature and a tendency for flame instability, reducing the reliability of conventional operation of a modified Claus sulphur recovery unit (SRU). The acid gas enrichment (AGE) process is commonly employed to provide greater reliability, flexibility and improved Claus unit operations. AGE achieves these results by increasing lean acid gas H2S concentration, producing an enriched acid gas feed stream with significantly reduced volumetric flow, for processing in the Claus SRU.

Lean acid gas containing significant concentrations of mercaptans and/or carbonyl sulphide (COS) creates difficulty for AGE to achieve high sulphur recovery. The selective treating solvents commonly utilised for AGE absorb H2S from the acid gas, but do not absorb mercaptan or COS components. These organic sulphur species remain in the AGE absorber overhead stream and flow to the incinerator without being recovered, thereby negatively affecting the efficiency of sulphur recovery. With acid gas having high organic sulphur content, 1% or more of the sulphur in lean acid gas can be present as mercaptan or COS sulphur. Recovery of this sulphur is required to achieve high sulphur recovery efficiency.

This article compares the performance of several SRU schemes processing a lean acid gas containing COS and mercaptans. A conventional SRU/tail gas unit (TGU), a conventional SRU/TGU with acid gas enrichment, a SRU/TGU scheme bypassing acid gas to the TGU, and a SRU/TGU scheme that entails routing the AGE absorber overhead to the TGU (based on a concept originally patented by Shell in 1982) are compared on the basis of processing a lean acid gas containing 25% H2S and having significant COS and mercaptan concentrations.

Lean acid gas test cases
The lean acid gas stream used for comparison of the various process configuration test cases is shown in Table 1. Acid gas sulphur content is 100 t/d. About 4% of the acid gas sulphur is present as COS and/or mercaptan; therefore, recovery of these sulphur species is important if high sulphur recovery is to be achieved. Acid gas such as this could be produced from sweetening natural gas with high COS/mercaptan content, where removal of these components is necessary to achieve a low sales gas sulphur specification. Organic sulphur removal from natural gas is becoming increasingly important in today’s gas market.

Table 2 describes the seven 
process configuration test cases compared here, each of which is illustrated in Figures 1 to 7.

The Case 1 process configuration is shown in Figure 1. Acid gas from the upstream sour gas treatment unit is processed in a conventional two-bed Claus SRU. SRU tail gas flows to a methyl diethanolamine (MDEA)-based TGU, where residual sulphur species in the SRU tail gas are converted to H2S, which 
is subsequently removed in the 
downstream TGU absorber. The TGU regenerator regenerates the rich MDEA solvent, and the overhead from this tower is routed to the front of the SRU for recovery of sulphur in this stream. The combined acid gas H2S concentration is higher than in the inlet acid gas because the TGU regenerator produces a recycle acid gas stream that is more concentrated than the inlet acid gas.

Case 1 is a conventional SRU/TGU arrangement commonly applied to achieve good sulphur recovery efficiency, typically above 99.7% of the inlet sulphur. Most COS and mercaptan sulphur can be recovered with this approach. Hence, it would be the preferred process configuration for most plants, except when acid gas H2S concentration falls below a level that enables sustainable Claus unit performance. Below approximately 25% H2S, low thermal reactor temperature and flame instability make operation difficult, and another flow scheme should be considered to improve operation.

A split-flow configuration, where some acid gas bypasses the thermal stage, can improve operation of the acid gas burner and thermal reactor. However, split-flow exposes the catalytic section to acid gas hydrocarbons, which can foul the Claus catalyst. Acid gas enrichment, where acid gas is first fed to an amine absorber to absorb H2S and slip some CO2, produces an acid gas feed stream with a higher H2S concentration that is a more suitable feed stream for a Claus SRU. In an effort to improve Claus plant operation with lean acid gas, an additional six cases have been developed for comparison, all of which employ AGE schemes.

The Case 2 process configuration is shown in Figure 2. Acid gas from the upstream sour gas treatment unit is first processed in an AGE absorber, which absorbs H2S from the acid gas using MDEA solvent. The AGE absorber’s overhead gas, containing most of the CO2 in the acid gas and other components not absorbed by the amine, flows directly to the tail gas incinerator. Concentrated acid gas from the combined AGE/TGU regenerator, containing all of the sulphur to be recovered, plus the TGU’s recycled sulphur, flows to the SRU/TGU for sulphur recovery. The regenerator acid gas has a significantly higher H2S concentration than the inlet acid gas feed stream to the AGE absorber.

AGE re-absorbs the acid gas H2S in MDEA solvent, which must then be regenerated. The increased solvent regeneration energy and larger solvent system are significant cost penalties when compared with the conventional Case 1 approach. However, greatly improved Claus SRU operation with enriched acid gas makes smooth, reliable operation possible for lean acid gases that cannot be effectively processed using Case 1. Additionally, AGE significantly reduces the acid gas volumetric flow rate to the SRU, leading to significant reductions in equipment sizes and corresponding capital costs. Another advantage to using AGE is that acid gas hydrocarbons are, for the most part, slipped to the incinerator by the enrichment absorber. Removal of these hydrocarbon components from the Claus feed stream reduces Claus air control problems, which are often caused by fluctuations in hydrocarbon concentrations.

AGE in Case 2 does not recover sulphur from acid gas COS or mercaptans. These components are not absorbed significantly by amine-based selective treating solvents; therefore, they flow in 
the enrichment absorber overhead to the incinerator and are emitted to the atmosphere as SO2, negatively affecting sulphur recovery efficiency.