The ‘sourness’ of crude oil technically refers to its hydrogen sulfide (H2S) content before processing. Crude can naturally contain up to 14% sulfur content by weight, but this percentage is comprised of myriad sulfur compounds; only a small ratio is H2S. Unfortunately, even very low levels of H2S in crude can cause excessive corrosion and degrade catalysts in the refinery.
The reasons why one source of crude would be sour (e.g. Venezuelan crude) and another source would be sweet (e.g. Libyan crude) are complex. The sulfurization of crude occurred during its initial formation, when ancient kerogen (decomposed organic matter which has polymerized) was cooked into oil by subterranean heat; the sulfur content of the living matter in that region was thus transferred to the oil reserve. Another sulfur enrichment factor is the presence of special hydrocarbon-degrading bacteria known as OHCB which reduce the hydrocarbon:sulfur ratio. The concentration of H2S in the crude rises with overall sulfur content, and thermal reactions (both geologic and during refining) can produce H2S from reactions with elemental sulfur and decomposition of unstable sulfur compounds.
Regardless of the complex reasons why, the trend is clear. The crude oil coming out of our world’s reserves is growing increasingly sour:
While the trend in the US is offset by the sweet, light crude being produced from shale plays, American oil refineries have spent decades optimizing their operations for the inexorable rise of sour, heavy crude, making it highly impractical for these refineries to start inputting sweet crude. This is why US oil companies are pushing for the export ban to be lifted—they can make more money selling the high-quality light crude to overseas refiners while importing cheaper sour crude.
In aggregate, and especially noticeable in countries that have historically refined sour crude, the oil input is containing increasing levels of H2S. The H2S concentration in crude oil is of great concern to anyone transporting that oil in their pipelines, buying oil of a certain sourness specification from a producer, or receiving that oil as feed to a refinery. While the oil industry is driven by incredibly advanced technology and innovation, this critical metric is often handled by the archaic paper tape technology developed in the 1950’s. This method requires a lot of user maintenance, from replacing the tape drives to diluting the sample, and stands stubbornly against the growing automation of petroleum processing.
The OMA H2S Analyzer is a totally solid state, automated solution for real-time H2S-in-crude monitoring. This system uses a high-resolution UV-Vis spectrophotometer to measure the absorbance curve of hydrogen sulfide in the 200-300nm range; since crude oil is too opaque to transmit a light signal, Applied Analytics’ unique ‘headspace’ sample conditioner heats the crude oil and flows in a carrier gas to produce a vapor-phase sample. In accordance with Henry’s Law, the measured H2S concentration in the vapor is correlated to the concentration in the crude oil, providing an extremely accurate, fast analysis.