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Publication > Issue > Articles

ASRL Review

Summary

Refinery residues: handling ammonia, NOx and sulphur emissions

Abstract

Utilisation of high sulphur and nitrogen content crude oil heavy fractions will require new strategies for dealing with SO2, NH3 and NOx. Peter Clark describes how the chemistry learned from understanding ammonia destruction in the Claus furnace can be applied to the design of new processes for ammonia conversion and for emissions-free cogeneration options for bottom of the barrel residues.

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Handling impurities in Claus feedstocks

Summary

Several strategies are available to achieve the best Claus conversions with high impurity acid gas feedstocks. Options include different measures to increase the flame temperature of the Claus reaction furnace to ensure complete combustion of impurites and an improved catalyst configuration specially designed for Claus catalysis in the presence of BTX. Lisa Connock reports.

Abstract

The acid gas streams produced in the amine treatment of natural gas and the treatment of gas associated with oil production are often lean in H2S with a low H2S/CO2 ratio and contain impurities such as mercaptans, BTX (benzene, toluene and xylene) and hydrocarbons. The processing of these lean acid gas streams in a modified Claus sulphur recovery unit present a number of operating and design challenges (see Sulphur No. 282).

The concentration of H2S and the presence of impurities in the feedstocks to Claus units play an important role in the selection of the appropriate plant configuration. The main issue is the flame temperature during combustion of the acid gas in the reaction furnace, which has to be high enough to ensure flame stability and to allow the complete combustion of the hydrocarbon-based compounds.

If the flame is not stable (i.e. when the temperature is too low) the conversion of H2S to sulphur may be affected resulting in dramatic operating problems in the downstream Claus reactors, which may be exposed to process gas containing unreacted oxygen from the reaction furnace.

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Incinerator optimisation in North America

Summary

Upgrading an existing natural draft incinerator to a high efficiency forced draft incinerator will result in less fuel consumption, reduced greenhouse gas emissions, lower operator attendance and a lower life time cost.

Abstract

The primary function of an incinerator after a sulphur recovery unit (SRU) is to oxidise the H2S, COS and CS2 to comply with the maximum allowable emission levels as imposed by legislation. SRU thermal incinerators in the province of Alberta, Canada, have been regulated using two main parameters: stack top temperature and Total Reduced Sulphur (TRS) compounds. The rationale for this was:

  • Plume dispersion – the stack effluent had to be of sufficient temperature so that a limit on ground level SO2 concentration was being met.
  • Total Reduced Sulphur destruction – the incinerator temperature had to be sufficient for the destruction of H2S, COS and CS2 in the SRU tail gas.

The maximum allowable emission level in Alberta is 300 ppm TRS (sum of H2S, COS and CS2). Historically, mainly natural draft incinerators have been applied to comply with these regulations, mainly because the capital cost is lower than for forced draft incinerators and fuel was considered as free. Natural draft thermal incinerators are designed to effectively oxidise all reduced sulphur species to SO2 and then emit the SO2 to the atmosphere at a prescribed temperature to ensure adequate dispersion. They consume significant quantities of fuel and emit tonnes of greenhouse gases (GHG).

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Improving the quality of lean acid gases

Summary

A common approach for handling problematic lean acid gases is to use an acid gas enrichment step. This method works well but requires significant additional capital and operating expenses. The new approach described in this article* focuses on the selective treating operation and avoids big increases in capital and operating costs.

Abstract

A new method for improving the H2S/CO2 selectivity in high CO2 content, high CO2:H2S ratio gas treating applications using MDEA has been developed. This work shows that significant improvements in the selectivity of the MDEA solvent can be attained by removing heat from the absorber via interstage cooling (patent pending). The interstage cooling consists of two stages whereby the semi-lean solvent is withdrawn from an absorber chimney tray, pumped through a cooler and returned to the absorber (Fig 1). This interstage cooling, combined with additional lean solvent cooling, allows enough improvement in the H2S/CO2 selectivity so that the required heat removal can be accomplished with a conventional cooling water medium. In contrast, if only the lean solvent is cooled, an equivalent improvement in performance will require a much colder cooling medium, such as a refrigerant.

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Smelting output trails metals recovery

Summary

Recovering base metal prices are not a short-term fix for worldwide tightness in smelter acid supply. Supplies of ore in particular are suffering from a downturn in the metals industry in recent years. Chris Cunningham reports.

Abstract

With base metal prices on the rise and smelter capacity picking up, at least on balance there appears reason to conclude that the continuing tight market in sulphuric acid is about to ease.

The problem with this argument is that the chief source of ‘fatal’ acid, the copper smelting industry, is beset by severe shortages of its principal raw material, ore concentrates from the world’s copper mines. In one or two notable cases these upstream shortages are a result of technical difficulties which are, eventually, capable of solution by their owners.

However, combined with a closures, under-performance and technical difficulties of their own among the acid-producing smelter community, the immediate outlook is for something less than a flood of sulphuric acid in the market place.

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