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.
M ost 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, produc - tion loss, and unscheduled shutdowns. Bechtel Energy Technologies & Solutions (BETS) propri - etary SWSPlus technology can unload the SRU by separat - ing 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 flex - ibility to process higher sulphur and higher nitrogen crudes. Excess 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 Variations in crude costs The crude slate is getter higher in sulphur and nitrogen in various regions. Due to increased processing costs associ - ated 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 con - verted 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 pro - cessing 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 H2 S 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 aque - ous 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 sul - phur. The NH3 is combusted into nitrogen and water. The basic overall chemical reaction in an SRU in the gen - eration of elemental sulphur (S) from H2S is: Advantages of SWSPlus • Integration of SWSPlus into the refinery allows higher nitrogen and higher sulphur crude processing (opportunity crudes) without SRU expansion • Adds NH3 as a revenue stream • SWSPlus can be up to 25% less carbon intensive than on-purpose NH3 production • Increases SRU processing capacity; half of the Capex compared to an additional SRU train (SRU, TGTU, degassing, thermal oxidiser) • No new air emissions permit is required!
H2S + 0.5 O2 → S + H2O + heat [Mass = 50 kg per kg-mole of H2 S]
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 H2 S] 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:
PTQ Q1 2023
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