Feb-2023

To Claus or not to Claus?

Using SWSPlus technology to transform sulphur recovery facilities from a cost centre to positive cash flow.

Martin Taylor and Charles Kimtantas
Bechtel Energy Technologies & Solutions, Inc.

Article Summary

Most refineries have sour water strippers and Claus sulphur recovery units (often referred to as Claus SRU or SRU). Depending on the crude slate, both absolute value and the ratio of sulphur to nitrogen, the refinery may reach a processing limitation with its Claus sulphur recovery unit (SRU) due to the hydrogen sulphide (H2S) and ammonia (NH3) produced. Excessive amounts of ammonia can cause deposition of ammonium polysulphide salts in the unit, leading to higher burner pressure, production loss, and unscheduled shutdowns.

Bechtel Energy Technologies & Solutions (BETS) proprietary SWSPlus technology can unload the SRU by separating the H2S and NH3 in the sour water such that the H2S can go to the SRU, and the NH3 can be recovered and sold as anhydrous NH3 for agricultural or chemical production. The sulphur facilities can go from a cost centre to positive cash flow while allowing the refinery to have the feedstock flexibility to process higher sulphur and higher nitrogen crudes.

Variations in crude costs
The crude slate is getter higher in sulphur and nitrogen in various regions. Due to increased processing costs associated with sulphur and nitrogen, the available low sulphur and low nitrogen crudes tend to be more expensive. In the refinery or chemical plant, the sulphur is ultimately converted into H2S, and the nitrogen is converted into NH3. The NH₃ and a portion of the H2S are usually collected in the refinery sour water system and then removed in a sour water stripper. The H2S and NH₃ are then sent to the SRU, where the H2S is recovered as elemental sulphur, and the converted ammonia is discharged to the atmosphere as nitrogen or as oxides of nitrogen. Extra H₂S and NH3 can cause the SRU to become a bottleneck in processing the cheaper high sulphur/high nitrogen crude oil feeds or processing additional crude into finished products.

For refiners, Bechtel provides the SWSPlus technology with the ability to separate the H2S from NH3 present in refinery sour water. The SWSPlus unit generates an H2S stream and a separate NH3 product stream. The H2S is sent to the SRU, and the recovered NH3 produced is sold, used as a fuel source, or used for NOx control. The NH3 product can be as anhydrous NH3 (a common commodity) or aqueous NH3, each of which is suitable for use as a chemical feedstock or for agricultural uses.

The SRU is a mass-flow limited device. The following chemical equations describe the mass flows resultant in an SRU. When processing H2S and NH3 in the SRU, a portion of the H2S is combusted to sulphur dioxide (SO2), which then reacts with the remaining H2S to make elemental sulphur. The NH3 is combusted into nitrogen and water.
The basic overall chemical reaction in an SRU in the generation of elemental sulphur (S) from H2S is:

H2S + 0.5 O2 ® S + H2O + heat
[Mass = 50 kg per kg-mole of H2S]

However, this basic equation does not truly describe the whole picture. If we include atmospheric nitrogen (N2) and humidity (H2O) present in the combustion air, we have a more complete summary:

H2S + 0.5 O2 + 1.9 N2 + 0.2 H2O ® S + 1.2 H2O + 1.9 N2 + heat
[Mass = 105 kg per kg-mole of H2S]

Note that the chemical reactions above are partially reversible.

The overall reaction for NH3 shows it is combusted to nitrogen and water, consuming oxygen in the process. The simplified equation shown is:

NH3 + 0.75 O2 ® 0.5 N2 + 1.5 H2O + heat
[Mass = 41 kg per kg-mole of NH3]

As before, this is not the whole picture. We should consider atmospheric nitrogen and humidity present in the combustion air. In addition, the sour water stripper acid gas (SWSAG) contains additional water vapour. If we assume a 1:1:1 ratio of NH3:H2S:H2O in the sour water stripper acid gas, we have a more complete summary of NH3 combustion:

NH₃ + 0.75 O2 + 2.8 N2 + 1.2 H2O ® 3.3 N2 + 2.7 H2O + heat
[Mass = 142 kg per kg-mole of NH3]

Combining the above, we get an overall equivalent mass of H2S to NH3:

       142 kg         kg-mole of H2S      kg-mole of NH3        34 kg H2S  = 2.7 kg H2S
kg-mole of NH3           105 kg       x       17 kg NH3       kg-mole of H2S      kg NH3

This indicates that an additional 2.7 kg H2S can be processed for each kg of NH3 removed from the Claus SRU feed. Considering the hydrocarbons typically present in the sour water stripper acid gas, the replacement value of the NH3 removed increases to about 3 kg H2S per kg NH3.

This simplified mass-flow approach shows that NH3 is a very inefficient use of the available space in an SRU. Thus, one way to expand SRU capacity is to remove the NH3, which is how the SWSPlus unit can become a valuable asset in the refinery.

SWSPlus unit description
The SWSPlus process segregates the NH3 from the H2S using a two-column distillation approach. The SWSPlus process consists of four main processing steps: 1) degassing and feed preparation, 2) acid gas stripping, 3) NH3 stripping, and 4) NH3 purification and liquefaction (see Figures 1 and 2).

Å’ Degassing and feed preparation Sour water feeds from a single or multiple sources are cooled and passed through a degasser where dissolved hydrogen, methane, and other light hydrocarbons are removed. The degassed sour water is pumped to a SWSPlus feed preparation tank, which serves to attenuate flow rate and composition changes while also providing the opportunity to settle and remove entrained oil and solids.

Acid gas stripping From the SWSPlus feed preparation tank, the degassed sour water feed is pumped to the feed/product exchanger and fed to the reboiled acid gas stripper. H2S and carbon dioxide (CO2) are stripped to the overheads, and a water wash reduces NH3 contamination in the overhead H2S stream. The resulting acid gas is of high purity and is an excellent feed for an SRU or a sulphuric acid plant. It contains negligible NH3 (less than 50 ppmw) and very few hydrocarbons since the plant feed has been degassed.