Catalytic processes available today for the treatment of H2S containing gases in refineries and gas plants, are limited to an overall sulphur recovery performance that can't exceed 99.7 %. A new process is proposed to reach 99.9 % min. sulphur recoveries, based on the direct oxidation of H2S below sulphur dew point, using a new catalyst especially designed for this application. A first industrial experience is currently on stream.
Reducing atmospheric pollution by SO2 has become a major environmental concern for natural gas and oil industries.
The so-called "modified Claus" process has known a wide-spread development for some decades, to recover more and more H2S from acid gas fields and crude oils by converting it into the more environmentally friendly sulphur, according to the following reaction sequences:
In the same time, steadily tightening regulations have nourished the parallel development of improved techniques to help to get the best of the Claus process by acting mainly in two fields:
catalyst developments, aimed at improving Claus as well as COS/CS2 hydrolysis reactions using advanced pure aluminas, increasing the life time of alumina-based catalysts downstream by oxygen scavenging, increasing the overall catalytic activity by introducing the performing titanium dioxide.
process developments, not only to improve the basic Claus sulphur recovery unit, but also to further process the Claus plant tail gas in order to get rid of the maximum of residual H2S.
Totally catalytic routes involving the most up to date catalysts from Sulphur Recovery Unit to Tail Gas Treatment make it possible today to industrially reach an overall sulphur recovery as high as - but not exceeding - 99.7%.
The 99.9 % sulphur recovery performance can be presently achieved only by implementing wet Tail Gas Treatment processes involving a solvent absorption step and subsequent recycling of H2S. Although efficient, these processes are highly expensive, both as regards capital investment and operating costs, and moreover, they generate liquid acid wastes to be treated.
It was decided to try to overcome these two disadvantages by reconsidering the approach of the 99.9% performance through a totally catalytic route. This challenge supposed to basically reconsider both process and catalyst philosophies.
A significant breakthrough in tail gas treating was introduced in the 70's with the concept of operating below sulphur dew point. This combined two major advantages:
the enhancement of the formation of sulphur through immediate trapping within the catalyst porous network
the decrease of yield losses in the form of gaseous elemental sulphur.
This concept entails a cyclic operating mode, with the continuous succession of two sequences : an adsorption-reaction phase where the sulphur is formed and remains adsorbed within the catalyst, followed by a regeneration phase where the sulphur is expelled from the catalyst by heating up to 300 C, and recovered by condensing.