The temperature of sulphur

[By our guest columnist, 'Thiophilos']

If any one person in the sulphur business was on top of the matter of measuring and interpreting the temperature of sulphur it was surely Earnie Emery, whose initials became his company name E2T, synonymous with Claus front end reaction furnace measurement in sulphur recovery systems. Earnie passed on late last year and it seems timely that this Sulphur column take the opportunity to both recognise Earnie’s contributions and the critical importance of measuring, understanding and controlling temperatures throughout the sulphur recovery process.

The Claus reaction furnace has been called the ‘heart’ of the SRU, in the thousands of such units that collect sulphur from where Mother Nature put a lot of it, to make it available to mankind where it is needed in this day and age of major commodity economics. The values of “T” in such units are well into the 1,000C plus range; hardly a trivial challenge to measure with confidence and accuracy. To make matters even more challenging, the chemical reactions that generate these temperatures produce conditions that are, to say the least of it, hard on the construction and measurement materials. But Earnie, with his background in rocket propulsion technology, was up to the task with his focus on infra-red radiation probes and their quantifiable sensitivity to the temperature of the generating source. So much of the early approach to Claus front end furnace technology regarded the unit as a waste disposal technology rather than a sophisticated and relatively complex chemical reactor whose efficiency depended very much on the monitoring and control of its operating temperature.

Earnie and others asked: at what temperature did you optimize the combustion product sulphur dioxide/ hydrogen sulphide ratio to feed to the downstream catalytic conversion units? At what temperature did the firebox lining, burner tips and tube sheet walls start to show unacceptable wear and tear? How much hydrogen was being produced in the furnace by direct thermal cracking of hydrogen sulphide for later use as a reducing agent to reduce emissions? How did ammonia and COS production vary with temperature? How valuable would so called “waste heat” from the FEF high temperature operation in the overall thermodynamics of an optimally functioning furnace be?

All of these (and more) factors became very much more relevant as the struggle to improve overall Claus efficiency from a barely adequate 70% sulphur recovery to the 99% target in today’s systems that is required to meet environmental standards. So often we forget that these kinds of targets can only be achieved if pioneers such as Earnie Emery have had the smarts to apply technology that helped lead from the rocket propelled way to the moon landing to the more mundane but, in its own, way equally important eco-friendly production of the stuff that makes the stuff that grows the stuff we eat?

Earnie’s efforts focused on the FEF and its temperature measurement and control, but it also drew attention to the more general importance of the temperature parameter in many other components of the sulphur production system. The efficiency of the catalytic stages of the Claus redox system which react the hydrogen sulphide and sulphur dioxide from the front end furnace to produce elemental sulphur are critically temperature dependent. Earnie’s infra red thermometers might not have such a seminal role inside a catalyst bed, but they sure worked in keeping an IR eye on the external temperature gradients on the outside of the reactor bed. Catalyst activity drops off inside the charge and the heat of reaction drops and this can be detected on the outside shell. We have a problem inside; let’s fix it and keep the recovery up in the high nineties. Stands to reason. Oh that more of us humans had a better handle on this essential of all civilised activities – reasoning.

But there is still more about the role of temperature in the overall sulphur business. There are few if any other chemical elements that go through as many fundamental changes as sulphur in such a readily accessible temperature range. Its orthorhombic solid crystalline form ‘morphs’ to monoclinic before melting to liquid cyclooctasulphur at 119C. As it heats up to 159C its eight-membered ring molecules break up and reform into a variety of so called Sx species, and finally into long chain polymers which hugely affect its viscosity and fluidity. When it cools down again to solidify, all these species have to reverse themselves and do so at a variety of rates leading to very complex solid sulphur mixtures which can effect the handling and storage properties! Who said temperature control wasn’t a critical factor in the sulphur business?

Thank you, Earnie Emery for your contribution.
‘Thiophilos’