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Feb-2025

Revamping sulphur recovery units with high-level oxygen enrichment

Two case studies demonstrate how high-level oxygen enrichment can address two distinct issues that existing SRUs are likely to face in the future of refining.

Debopam Chaudhuri, Theresa Flood, Denny Li, and Jyoti Bist
Fluor

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Article Summary

All crude oil contains some sulphur compounds. These compounds range from the simplest form, namely hydrogen sulphide (H₂S) found in natural gas, through simple mercaptans compounds (R-S-H) to very complex molecules. These sulphur compounds pass through to the various distillate products in various degrees of concentration. If allowed to remain in the distillates, they have adverse effects on the environment, are corrosive to equipment, and deactivate high-value catalysts of downstream processes. Therefore, sulphur treatment and recovery become inevitable.

The sulphur compounds present in the crude get displaced from the hydrocarbon phase as H₂S. This H₂S is either captured by an amine solvent in the amine treatment units or dissolved in the process condensate. The amine circulates in the refinery in a closed circuit, capturing the H₂S from the hydrocarbon phase in various amine treaters. This ‘rich’ amine (rich in H₂S) is then regenerated in the amine regeneration unit (ARU) to liberate the H₂S gas and regenerate lean amine (lean in H₂S) to be circulated back to the various amine treaters.

Similarly, the sour water generated from the various process units in the refinery is treated in sour water stripping unit(s) (SWSU) to liberate the H₂S and other gases. The combined stream of the H₂S-rich gases from the ARU and SWSU is then sent to the sulphur recovery unit (SRU). Figure 1 demonstrates how sulphur travels through the various process units and hydrocarbon in a refinery, starting from the crude oil until recovered as elemental sulphur.

The H₂S in acid gas streams is converted to elemental sulphur utilising the modified Claus process. The typical feed gas streams to an SRU, originating from the amine regeneration and the SWSUs, contain varying amounts of H₂S as the sulphur source. The process involves burning the acid feed gas with a sub-stoichiometric amount of air, typically just enough to combust approximately a third of the H₂S to SO₂. The SO₂ formed then reacts with the unconverted H₂S to produce elemental sulphur.
The main reactions involved are:

H₂S + 3/₂ O2 ® SO2+H2O + Heat
2 H₂S + SO₂ « 3 S + 2 H₂O – Heat
3 H₂S + 3/₂ O₂ ® 3 S + 3 H₂O + Heat

Reaction 1 is highly exothermic, while Reaction 2 is endothermic, with a net effect of exothermicity for the net conversion described by Reaction 3. Another reaction of importance and of special interest to this discussion is the destruction of ammonia in the presence of air:

2 NH₃ + O₂ ® 3/₂ N₂ + 3 H₂O

The Claus reaction furnace needs to be hot enough to ensure near-complete destruction of ammonia to nitrogen, with the target typically 1,260°C (2,300°F) for a standard Claus.

Traditional sulphur plants employing the modified Claus process utilise air as the source of oxygen in the thermal reaction furnace. The major drawback of using air as the oxygen source is the large amount of nitrogen that comes with the oxygen supply. The nitrogen from air adds to the hydraulic load of the unit, thus ‘eating up’ capacity. The nitrogen also adds thermal inertia, lowering furnace temperatures and increasing the duties of sulphur condensers and reheaters.

Oxygen enrichment has been implemented in many sulphur plants to debottleneck the process and reclaim SRU capacity. Oxygen enrichment is the process where part or all of the oxygen needed for the modified Claus reaction is replaced by pure oxygen. Conventionally, oxygen enrichment is classified as low-level, mid-level, and high-level. This corresponds to the amount of pure oxygen mixed into the air-oxygen mixture sent to the furnace. Low-level oxygen enrichment typically limits the overall oxygen concentration to 28 vol% O₂ in the final mixture of air and oxygen, whereas high-level oxygen enrichment involves a concentration greater than 45 vol% O₂. The in-between concentration is categorised as mid-level enrichment.

Case study 1: Capacity enhancement
The first case study involves a sulphur plant consisting of three identical trains designed to process refinery acid gas equivalent to a total of 450 MTPD (3 x 150 MTPD) of sulphur production. The existing

SRU consisted of the following systems:
υThree identical Claus sulphur recovery sections, including the thermal and catalytic stages.
ϖDedicated sulphur pit and ejector for each SRU train.
ωA common tail-gas incinerator for all three trains.

The refinery needed to increase its sulphur handling capability to meet the present and future crude operating trends, as well as to increase its overall flexibility of operation. The owner proceeded to implement Fluor’s proprietary Claus Oxygen-based Process Expansion (CopeII) high-level oxygen enrichment to increase the sulphur handling capability by 50%. This technology would increase the amount of acid gas equivalent each Claus train can process to 225 MTPD of sulphur production while maintaining the same sulphur recovery efficiency. Oxygen enrichment reduces the volumetric flow of process gas and tail gas by reducing the quantity of nitrogen that enters with the combustion air. This reduction in volumetric flow rate allows for a corresponding increase in SRU acid gas feed rate and a subsequent increase in sulphur production with the same main equipment in the SRU downstream of the Claus furnace.

The typical composition of the feed gases to the Claus section for the sulphur plant is summarised in Table 1. The feed gas composition is the same for both the base case (air only operation) and the revamp case (oxygen enrichment) operations. The only differential between the two cases for the feed gas is quantity.

Table 2 provides a summary of the flow rates for the various streams marked in the simplified flow diagram shown in Figure 1 for the two operating cases: base case – air only operation; revamp case – oxygen enrichment.

As Table 2 clearly shows, when the unit operates with oxygen enrichment and processes 150% of the base plant capacity, the process gas flow from the first sulphur condenser is still less than the base case. Hence, there is no impact on the hydraulic load of the unit downstream of the first sulphur condenser even while it processes more acid gas.

The primary drawback of implementing high-level oxygen enrichment is that with so little nitrogen diluent in the furnace, the bulk gas temperature achieved via combustion exceeds the allowable limits for any available refractory. Hence, special design modifications are needed to provide adequate temperature control in the furnace.


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