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Publication > Issue > Articles

Processing gas for acid

Summary

Sour natural gas processing is about to arrive for the first time on the UK mainland, and the sales by-product of a new desulphurisation unit will be sulphuric acid.

Abstract

The end of this year will include a significant milestone for gas processing in the UK with the introduction of sour gas processing on a significant scale. The project to sweeten sour natural gas arriving from offshore fields may also say a lot about the balance of the local market for sulphur and sulphuric acid; its end byproduct will be sulphuric acid for local sale rather than elemental sulphur. And the choice has been on commercial grounds – the sheer saleability of acid over brimstone – more than for any other reason.

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Sulphuric acid catalysts shape up

Summary

Monsanto Enviro-Chem Systems has introduced a new range of six-lobed, ribbed-ring shaped sulphuric acid catalysts, which offer significantly lower pressure drop build-up compared to traditional ring type catalyst.

Abstract

In late 2002, Monsanto Enviro- Chem System’s Catalyst Department introduced to the marketplace a six-lobed, ribbed-ring shaped catalyst specifically designed for pressure drop reduction in all beds of the converter. The catalyst is designated as XLP-220 (used specifically in 1st bed applications) and XLP-110 (recommended for 2nd, 3rd, and 4th pass applications). The “X” represents the extended surface area of these rings. The higher void area of the ribbed shape improves its dust handling capabilities, resulting in a lower rate of pressure drop build-up.

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Tough stuff

Summary

Sulphur polymer concrete does not compete economically with Portland cement concrete for most applications in most parts of the world. However, it wins a small but thriving market in tough environments, and may be a solution for local difficulties of over-production of sulphur.

Abstract

The search for a new source of mass demand for sulphur often enough turns to sulphur polymer concrete, in which polymeric sulphur replaces water and Portland cement. Will the first city of sulphur ever rise from the plain?

The answer has to be “unlikely”. First of all, there is barely enough sulphur in production to build an average town, and the world is not so over-filled with brimstone that there is any need, on a massive scale, to panic-dispose of it.

Much more importantly, there is no possibility that sulphur-based concrete can compete economically with more conventional varieties. Brimstone and Portland cement are broadly similarly-priced commodities and, in any case, fractional components, by weight, of any concrete pour.

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A milder approach

Summary

A demonstration of fuels desulphurisation which could save refiners the major direct capital cost of meeting new legislation is up and running and approaching its commercial launch.

Abstract

Another way of producing ultralow sulphur road fuels made its public operating debut in July when a 50 b/d demonstration unit started up at Valero Energy’s Krotz Springs, La refinery. Several weeks later, the process developer, UniPure of Houston, Tx, reported to Sulphur that the Advanced Sulfur Removal (ASR) test unit was “going well” and producing ultra-low sulphur diesel (ULSD) within the specification of future federal law.

With US legislators attempting to develop a love affair between drivers and the diesel engine, whilst forcing down, eventually, the level of sulphur permitted in diesel fuel, was this to be a serious contender for the refinery business’s big spend on desulphurisation of road fuels (estimated in total at somewhere between $16 billion and $19 billion in the US alone)?

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Addressing operating issues

Summary

The iron-redox process has enjoyed commercial success for over 25 years, generally in applications requiring sulphur removal capacity below 20 t/d. Douglas L. Heguy and Gary J. Nagl of Gas Technology Products LLC review the status of the technology, explain how the operating issues are being addressed in commercial practice, and provide a glimpse of improvements that are in the final stages of development.

Abstract

The family of liquid redox processes that has been developed since the 1920s is best represented, currently, by the “iron-redox process” or “chelated-iron” process. This technology has served its clients well for more than 25 years. Units typically achieve 99.9+% H2S removal efficiency, treat a wide variety of gas types over a wide variety of operating conditions, have substantial turndown capability on H2S concentration and gas flow and produce innocuous products and by-products. More than 200 such units have been licensed around the world.

The iron-redox technology is typically applied to gas streams requiring less than 20 t/d sulphur removal capacity, unless operating conditions limit use of other sulphur plant technologies, such as Claus. In such cases, iron redox may still be the best sulphur removal technology. Highly variable gas and low H2S concentration are examples. Iron-redox plants as large as 80 t/d are in commercial use.

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Beyond tail gas H2S/SO2 monitoring

Summary

UV photometry is widely accepted for monitoring H2S and SO2 concentrations, critical for control of the modified Claus process. Recent developments in analytical techniques and sample system designs provide additional measurements for process control of SRU feed streams and TGTUs. Daniel Potter, SRU Product Manager, Ametek Process Instruments, reviews the instrumentation technologies available to provide additional parameters for optimising SRU processes.

Abstract

Sulphur recovery by the modified Claus process is well known and universally practiced. Equally accepted is the method for control of the Claus process, whereby a combination of feed forward and feedback control loops determine the required process air flow rate for the correct modified-Claus process ratio of 2:1. Figure 1 illustrates the feed forward and feed back control loop of a Claus SRU.

It is generally agreed that the tail gas analyser and trim control of the process air account for at least 3 to 5% of the recovery efficiency, and it goes without saying that the tail gas analyser remains the most important analytical tool in Claus SRU operations. Technological advancements in ultraviolet (UV) photometry and sample system design have improved reliability of tail gas analysers even further, resulting in improved operations of Claus SRU plants. However, as environmental regulations continue to mandate lower sulphur emissions, Claus based process enhancements or tail gas treating unit operations have placed even greater emphasis on control and monitoring of critical parameters in sulphur recovery applications. This article will highlight recent analytical improvements made to UV analysers to further enhance control and improve operations of Claus based SRUs, as well as tail gas treating units. Figure 2 shows the analyser locations and preferred sample points for the modified Claus process.

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