Revamps 2024 Issue

in two steps: from oxidation state -2 first to oxidation state +4 and finally to oxidation state +6. The reactions involved are also exothermal, but with an almost four times higher enthalpy jump, in total yielding approximately 804 kJ per mole of H₂S processed. The contrasting reaction enthalpies of the Claus and WSA processes, as depicted in Figure 2 , result in a sub - stantially higher energy output from a WSA plant compared to a Claus plant. By harnessing this exothermic energy released during the process in the form of high-pressure (HP) steam, refineries can significantly reduce their overall CO₂ footprint. Sour crude advantage Refineries processing sour crudes, characterised by higher sulphur content, stand to gain the most significant benefits from transitioning to the WSA process. Table 1 provides a breakdown of the theoretical energy recovery potential and associated CO₂ equivalent reductions between the Claus and WSA processes, assuming complete H2S conversion to either elemental sulphur or sulphuric acid, respectively. While the theoretical energy yield of the acid route is substantial, practical limitations exist. Complete energy capture from H₂S conversion into sulphur or acid is unat - tainable. And the wholesale replacement of existing Claus plants with WSA units seems logistically and financially impractical. Nevertheless, recovering even 90% of the energy released during sulphuric acid production presents a compelling argument for a gradual transition. Modern Claus processes necessitate the use of amine-based tail gas treatment unit (TGTU) regenerator reboilers, which consume a signifi - cant quantity of low-pressure (LP) steam. This LP steam consumption hinders the decarbonisation potential of the sulphur route as the LP steam produced within the Claus process is insufficient to meet the demands of the TGTU. Replacing or revamping older Claus plants with new WSA units emerges as a strategically sound approach to reducing carbon emissions compared to constructing entirely new Claus units. A progressive shift from sulphur to acid production affords refineries valuable experience in acid handling and facilitates the establishment of robust long-term off-take agreements. As refineries incrementally increase acid gas allocation from energy-inefficient sulphur production to the more efficient acid production, the overall carbon footprint reduces.

222

Sulphur: 222 kJ/mole

HSO (gas)

HSO (liquid)

101

519

Sulphuric acid: 804 kJ/mole

-2 0 Oxidation state of sulphur

Case study To underscore the benefits of this approach, Topsoe con - ducted a case study comparing the environmental perfor - mance of the WSA process to a modified Claus process and amine-based TGTU configuration. Both systems were designed to achieve 99.9% sulphur recovery (see Figure 3 ). The case study focused on greenfield oil refinery SRUs processing 90 mol% H₂S amine acid gas and sour water stripper (SWS) off-gas, producing 270 tons per day (TPD) of sulphur and 800 TPD of sulphuric acid. The analysis compared direct CO₂ emissions from flue gas and indi - rect CO₂ emissions associated with utility production and consumption. The utility consumption and production data for the Claus SRU and the WSA SRU exhibit significant dispari - ties attributable to distinct process designs and equipment configurations. A detailed examination of these differences follows: • Electric power: The WSA process has a higher electrical consumption than the Claus process, mainly because of the higher gas flow rate and the use of an air-cooled acid condenser. The volumetric gas flow rates in Claus and WSA processes diverge primarily due to combustion methods. The Claus process employs partial oxidation, resulting in a spe - cific gas volume. In contrast, the WSA process utilises com - plete combustion, leading to higher process gas volumes. • Natural gas: Unlike the Claus process, which necessitates natural gas consumption in the incinerator to eliminate residual sulphur compounds, the WSA process does not Figure 2 Standard reaction enthalpies of sulphur and sul - phuric acid production from H₂S

Theoretical energy recovery potential and associated CO₂ equivalent reductions

Sulphur route

Sulphuric acid route

Wt% of sulphur in the crude oil Theoretical energy recovery, MWD

0.5

1.0

3.0

0.5

1.0

3.0

275

551

1,652

997

1,994

5,982

Theoretical scope 1 CO₂eq 4 emission reduction, TPA³ Theoretical scope 2 CO₂eq emission reduction, TPA⁴

23,588

47,175

141,525

85,425

170,850

512,550

19,271

38,542

115,625

69,792

139,583

418,750

Table 1

Revamps 2024