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Analysis: Brownfield options in bottom-of-the-barrel sulphur processing

by Guest on Nov 12, 2017

With impending marine sulphur specifications likely to slash the value of high-sulphur fuel oil, the Shell Claus off-gas treating (SCOT1) ULTRA process offers an ever-more attractive brownfield solution for increasing sulphur-processing capacity, especially in the Middle East climates.

Bunker C marine fuel (residual fuel oil No 6) has long been a reliable bottom-of-the-barrel revenue stream. But this outlet is about to close. The International Maritime Organisation (IMO) has already reduced the sulphur content of marine fuel oil from 1 to <0.1wt% in emission control areas. By 2020, marine fuel oil sulphur content will be cut from 3.5 to 0.5wt% around the globe.

This is a major challenge for refiners. Five possible responses would enable refiners to meet the new IMO specifications: (i) process more-expensive, lower-sulphur crudes; (ii) blend in high-value distillates; (iii) make a loss on high-sulphur fuel oil, which would be worth less than the crude oil feed; (v) burn high-sulphur fuel oil on-site in utility boilers; and (v) invest in residue conversion.

The economic penalties of the first three options are clear and the fourth option creates local emission challenges. That leaves residue conversion. The question is how can we take sulphur out (i.e., increase sulphur processing) in an affordable manner?

Adding or upgrading?

The simplest solution would be to add new sulphur recovery unit (SRU) trains. With the IMO specifications coming into play in just two years, any new trains would need to be copies of the existing train and implemented immediately. However, the low oil price makes justifying capital expenditure difficult and refineries may not have plot space near the SRU trains.

So, what is the alternative? Can we make better use of the existing facilities? Capacity in the existing SRUs can be enhanced by increasing the front-end pressure or through increased oxygen enrichment or a combination of both (this article considers only the SRU and assumes design margins in the upstream amine unit). Let us consider an example.

Brownfield case study

A case study looking at brownfield SRU capacity increase options has been developed using the composition and parameters of the amine and sour water stripper acid gases that feed an SRU at a Shell refinery (Table 1).

In this example, the SRU line-up includes indirect heating (steam reheating) with two Claus catalytic reactors. Formulated methyl diethanolamine (MDEA) solvent is used in the SCOT unit. The unit is equipped with a thermal incinerator that operates at 650°C and there is a Shell sulphur degassing system to achieve a <10-ppmw H2S specification in liquid sulphur.

The brownfield modifications considered assume that the solvent flow system has an additional 10% capacity and that oxygen supply for low (up to 28%) oxygen enrichment is achievable.

Table 2 compares three options, assuming a high (250-mg/Nm3) sulphur dioxide (SO2) emissions regime. Option 1: Higher front-end pressure, would increase capacity to 110% of the base case and requires 110% solvent flow, which is feasible within the assumed boundaries. Option 2: Low-level oxygen enrichment, would give 120% capacity with 102% solvent flow, which is also feasible. The modest solvent flow increase is because the oxygen enrichment also means less volumetric flow through the SCOT absorber with lower nitrogen content and consequently only a minor H2S column load increase. Option 3: However, combining increased front-end pressure and low-level oxygen enrichment would be unfeasible, as it would require 115% solvent flow – 5% more than the assumed maximum.

For a low (150-mg/Nm3) SO2 emissions regime, the solvent flow required for options 1 and 3 are 113 and 118% respectively, which leaves low-level oxygen enrichment as the feasible option for 120% SRU capacity with 105% solvent flow. However, now there is a fourth option with the potential to deliver up to 130% SRU capacity with substantially lower operating costs and without hardware changes.

Increasing SRU capacity

The Shell SCOT ULTRA process offers a step-change in the performance of the well-established SCOT process. It features Shell and Huntsman Corporation’s jointly developed highly selective JEFFTREAT2 ULTRA family of solvents, which can achieve deep decreases in H2S emissions while maximising carbon dioxide (CO2) slippage.

It also absorbs H2S at higher temperatures compared with MDEA, which eliminates the need for a solvent refrigeration system – an additional cost in hot climates such as in the Middle East.

The process provides robust performance compared with formulated MDEA, even with line burner/fuel gas co-firing designs, as it handles CO2 better.

It also features Criterion Catalysts & Technologies’ C-834 high-activity, low-temperature SCOT catalyst, which adds further value by increasing the destruction of organic sulphur compounds at low operating temperatures.

The process potentially offers an easy and cost-effective upgrade of existing units. In most cases, a simple solvent and catalyst swap is required without additional hardware changes.


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