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High recovery tail gas treating

Summary

Nowadays, overall sulphur recovery efficiencies of 99.9+% are regularly achieved with selective amines. Lisa Connock discusses amine based Claus tail gas treating. Comparisons of different selective amines are presented, and case studies demonstrating the benefits of switching to a selective amine are reported.

Abstract

In an ideal world, Claus units would recover all the sulphur in the feed, and there would be no pollution from the tail gas leaving the plant. Unfortunately, reaction equilibrium prevents this from happening, and Claus units are only able to achieve 93 to 98% recovery efficiency. To achieve higher recoveries the tail gas is treated in a tail gas treating unit (TGTU).

For recoveries of 99.8% and above the type of process chosen by the vast majority of operators is one where all sulphur compounds in the Claus unit tail gas are hydrogenated to H2S, which is then removed in a selective amine unit.1

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SCOT boosted by low temperature catalyst

Summary

New low temperature SCOT technology that combines the latest catalyst technology with optimised plant layouts provides the highest sulphur recovery levels at lower costs than ever before.

Abstract

In the early seventies Shell Research Ltd introduced the Shell Claus Off-gas Treating (SCOT) process to reach high sulphur recovery levels in sulphur plants. This process combines the reduction of sulphur species in the Claus tail gas to H2S, followed by cooling of the gas and condensing the bulk of the process water and the subsequent washing of the process gas by an alkanol-amine solvent, such as di-isopropanol amine (DIPA) or methyl-diethanol amine (MDEA), similar to gas treating processes.

The standard SCOT process is easily capable of meeting 250ppmv total sulphur in the SCOT off-gas from the absorber, which corresponds to an overall sulphur recovery of 99.8-99.9% (for Claus + SCOT).

In recent years, the demand for higher sulphur recovery efficiencies gave the incentive to improve the SCOT process by lowering the total sulphur content in the off-gas from the SCOT absorber to less than 50ppmv total sulphur and thereby maintaining low operating costs or even decreasing the operating costs. This resulted in a number of improvements and a couple of new versions of the process.

Although the combination of unit operations gives a good performance and recoveries of 99.9 % and higher are easily met, little effort was spent to optimise the SCOT plant; energy was abundant in the early days of SCOT and conservation of energy or reducing CO2 emissions was not an issue.

The SCOT process does not produce any secondary waste, which in combination with the high recovery, were two of the main targets in the development of the process.

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The cost of Claus tail gas clean-up

Summary

In a recent study, a selection of some of the most popular tail gas clean-up processes were compared based on their sulphur recovery yield (%) and the recovery cost per tonne of SO2 ($/t). Technip France performed the work for the French energy conservation agency ADEME. A summary of the results is presented.

Abstract

In the past, the tail gas leaving a modified Claus plant was burned to convert the hydrogen sulphide (H2S), which is lethal even at low levels (lethal concentration: 1000 ppm after one minute), to sulphur dioxide (SO2), which has a much toxic limit (lethal concentration: 2500 ppm after one minute). The off gas stream was then vented to the atmosphere.

Currently, all Western countries and many developing countries limit the amount of sulphur that can be emitted to the atmosphere. All new Claus units must be equipped with a tail gas treating unit and there is also regulatory pressure to add a TGTU to existing units.

TGTUs can be very expensive, often representing an investment of the same order of magnitude as the original Claus plant. The proper selection of tail gas clean-up process is therefore of importance to operators, but the choice is made complicated by the different performance levels and life cycle costs.

This article compares a selection of the most popular tail gas clean-up processes based on their sulphur recovery yield (%) and most importantly in terms of the recovery cost per tonne of SO2 ($/t).Technical evaluations, capital and operating costs are compared.

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Steps to retirement for grandfather

Summary

The first deadline is approaching within proposals to phase out gas processing plants in Alberta that do not comply with rules governing emissions limits. Meanwhile, the province's energy legislators have been checking progress in making the proposals work.

Abstract

It can be a long trip to visit your nearest neighbour on the prairieland of Alberta. When that neighbour is relatively close and happens to be a gas plant that flares sour or acid gas then relations may be as sour as the gas that feeds the flare. But the authorities are on your side. In particular, the Alberta Energy and Utilities Board (EUB) has been squeezing the time-honoured freedom of older plants that are not equipped with modern sulphur recovery installations to release oxides of sulphur and carbon to the environment.

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The considered view

Summary

As the end approached of a year that has been steady for business and short of alarms, Sulphur 2004 promised an opportunity for a considered view of future prospects in sulphur and sulphuric acid trade. And so it proved. Chris Cunningham reports on this year's festival of sulphur in Barcelona.

Abstract

With more than 320 delegates from around 40 nations in Barcelona in late October, attendance alone guaranteed a successful international event for the industry. The lineup of industry experts assembled for the opening ‘markets’ day of Sulphur 2004 of course ensured that this was a significant gathering for all concerned in the business of sulphur and sulphuric acid supply and demand.

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