Hydrogen sulfide (H₂S) is one of the most closely monitored contaminants in sulfur recovery and handling operations.
Naturally generated during crude oil and natural gas processing, H₂S is a toxic, corrosive, flammable gas that commonly remains dissolved in liquid sulfur streams after the recovery process is complete. Understanding where it comes from, and why it is so difficult to fully remove, is the first step toward managing the risk it poses.
In an industrial context, H₂S is generated as a byproduct of desulfurization and hydrocarbon refining processes. In sulfur recovery units, H₂S is converted into elemental sulfur (S₈) through the Claus process: a combination of thermal oxidation and catalytic conversion reactions.
Modern Claus units can achieve sulfur recovery efficiencies above 99.5%; however, even at that level of performance, residual H₂S concentrations between 100 and 400 ppmw may remain dissolved in the liquid sulfur produced.
At low concentrations, H2S has a characteristic rotten-egg odor, but at higher concentrations it rapidly paralyzes the olfactory nerve, making the gas undetectable by smell.
There's also a flammability risk. Liquid Sulfur is typically stored and transported at temperatures above 135°C, and at that temperature, H₂S released into the headspace of a tank or container can approach or exceed the Lower Explosive Limit (LEL) in air (3.4 vol%). In confined spaces like storage tanks and transport vessels, this is a real operational hazard, not a theoretical one.
This is why degassing liquid sulfur, reducing dissolved H₂S before it is stored, shipped, or solidified, is a critical step in sulfur handling.
What makes accurate monitoring difficult is that dissolved H₂S doesn't exist in isolation.
Infrared spectroscopy studies show that H₂S coexists in liquid sulfur alongside a family of polysulfides, H₂Sₓ, and the two are in a temperature-dependent equilibrium.
Elemental sulfur exists as S₈ rings held together by a relatively weak S-S bond. Above approximately 150°C, this bond readily undergoes homolysis, forming diradicals that favor the formation of H₂Sₓ. Below that threshold, the process slows, and any H₂Sₓ present tends to revert back toward the more thermodynamically stable S₈ form, releasing free H₂S in the process.
Research conducted by Alberta Sulfur Research Ltd. (ASRL) has quantified this relationship using FT-IR spectroscopy: the H₂S/H₂Sₓ ratio shifts from approximately 1.5 ppmw at 155°C to around 10 ppmw at 125°C. In practical terms, this means liquid sulfur drawn from different points in a Claus unit can have very different compositions; sulfur from the high-temperature condenser (above 200°C) tends to be richer in H₂Sₓ, while sulfur from lower-temperature converter stages tends to retain more free dissolved H₂S.
This equilibrium has a direct, practical consequence: degassing isn't simply a matter of stripping out as much H₂S as possible at any temperature. Free H₂S dissolved in liquid sulfur behaves like any gas dissolved in a solvent: it can be displaced by air or steam injection. However, H₂Sₓ due to its higher molecular weight, is non-volatile and cannot be removed the same way. To take advantage of the equilibrium shift, degassing must occur below roughly 155°C, ideally closer to 130°C, otherwise free H₂S simply recombines with sulfur diradicals to re-form H₂Sₓ before it has a chance to escape.
There is also a structural reason not to over-degas. Solid sulfur, that is free of polymeric sulfur, is mechanically weaker than sulfur containing roughly 2–5% polymeric material by weight. That polymeric fraction is closely tied to the same H₂Sₓ chemistry, meaning degassing systems need to be carefully tuned, not just maximized.
Even after solidification, this chemistry doesn't fully settle. While a significant fraction of dissolved H₂S, typically 20–80%, depending on cooling rate, surface exposure, and agitation, is lost during solidification, H₂Sₓ is more stable and can remain detectable in solid sulfur samples for months afterward.
Because H₂S and H₂Sₓ are in constant, temperature-sensitive equilibrium, a single spot measurement at the wrong temperature can give a misleading picture of how much H₂S a sulfur stream will actually release downstream, during storage, transport, or handling.
Reliable monitoring requires more than a generic gas check. It requires an analytical approach that can hold liquid sulfur at a controlled, stable temperature while measuring both species accurately. This is the analytical challenge that the next article in this series will address, and the reason a heated, FT-IR-based measurement approach has become the standard tool for getting this right.
This is the first in a series of articles on H₂S in liquid sulfur.
The next article looks at the current state of industry guidance, and why there is still no single standardized test method for dissolved H₂S in liquid sulfur today.
