Claus process

The Claus process serves as the benchmark technology for extracting elemental sulphur from gas streams rich in hydrogen sulphide (H2S). The underlying principle involves controlled partial oxidation: a portion of the H2S is converted to sulphur dioxide (SO2) through combustion, after which this SO2 reacts with the remaining H2S over a catalyst to yield sulphur that subsequently condenses. When combined with downstream tail gas treatment, overall recovery rates ranging from 98.66% to 99.99% can be achieved^[1].

Fields of application

The Claus process is universally deployed wherever industrial streams contain significant quantities of H2S. It represents the standard method in petroleum refining and sour gas processing, where throughput volumes far exceed those encountered in the fertiliser sector. Within the ammonia industry, it is particularly integrated into partial oxidation units processing heavy residues or coal. The sulphur present in the feedstock — potentially accounting for up to 7% thereof — emerges as H2S in the raw gas leaving the gasifier^[2]. Following cooling and heat recovery, refrigerated methanol wash enables separation of a CO2/H2S fraction directed to Claus treatment^[2]. According to best available technique references for ammonia production via partial oxidation, 87% to 95% of the sulphur contained in the gasifier feed is recovered in the Claus unit^[3]. The sulphur thus obtained can supply sulphuric acid manufacturing plants^[4].

Process variants and catalysis

Two principal configurations coexist for synthesis gas purification. In the conventional route, H2S is removed prior to the water-gas shift conversion through selective absorption, then processed in the Claus unit^[2][5]. In an alternative arrangement, the raw gas proceeds directly to shift conversion without prior sulphur removal; the H2S is then eliminated after this step, together with the CO2 formed during conversion^[2]. During regeneration of the loaded solvent, two distinct fractions are obtained: a pure CO2 fraction, usable for urea or other product manufacture, and an H2S/CO2 fraction routed to the Claus unit^[2]. This second pathway employs cobalt- and molybdenum-based shift catalysts, tolerant to sulphur compounds, whose activity operates across a broad temperature range. For these catalysts, the presence of sulphur in the gas is not a drawback but rather a prerequisite for their performance^[2].

Energy and environmental performance

Claus units require an SO2 emission abatement system downstream of the catalytic reactor^[2]. Tail gas treatment constitutes the additional step that enables extraction of the remaining sulphur after the main process, accounting for the high yields cited^[1].

From an energy standpoint, partial oxidation installations equipped with a Claus unit report a specific consumption of 38.7 GJ (NCV) per tonne of ammonia produced^[6]. This production route, more energy-intensive than steam reforming, remains relevant for heavy carbonaceous feeds or coal.

Commercial sulphur recovered via the Claus route is delivered to sulphuric acid units preferably in liquid form, or as a solid at temperatures of 140 to 150°C; filtration may be required prior to combustion^[7].