Sulphur is an undesirable constituent of fossil fuels that is found in several forms in a wide variety of compounds at differing concentrations. Of the many sulphur‑containing compounds that may be present, the volatile components such as sulphides, mercaptans (thiols) and thiophenes are of most interest.
Hydrogen sulphide (H2S) is most commonly the predominant volatile species found in produced hydrocarbons whilst mercaptans (RSH) are typically the second most prevalent species. When present, these sulphur compounds can cause issues for gas and oil processing, thus it is often important to identify which species are present and quantify the respective concentrations in order to establish an effective management plan. The main issues resulting from the presence of these compounds are;
- Toxicity – these compounds are highly toxic even at low concentrations.
- Corrosion – H2S has the potential to cause corrosion issues if the incorrect grade of steel is used.
- Poisoning – of refinery catalysts.
It is common for processing plants that receive gas and/or oil which has significant concentrations of volatile sulphur compounds to incorporate bespoke systems designed to remove these unwanted contaminants. Most such plants have an amine or metal oxide based H2S removal capability whilst some also include an RSH and/or COS removal facility.
Sampling for Sulphur Analysis
The sampling of pressurised oils (ASTM D4057) and gases (ISO 10715) for subsequent determination of these sulphur species can often be problematic since reduced sulphur species have a strong affinity for the surfaces of steel sampling vessels. Sulphur species, in particular H2S, may adsorb onto the interior walls of uncoated steel sample vessels during transit and storage. Thus, any delay between sampling and analysis in the laboratory may result in losses of the volatile sulphur and underestimations when quantifying these components.
For this reason, sampling vessels used to capture liquids and gases for the subsequent determination of volatile sulphur species must incorporate an inert coating designed to minimise chemisorption of sulphur species (e.g. SilcoTek®) applied to the internal surface.
In addition to employing sample vessels with an inert internal coating, Qa3 utilise a passivation protocol in order to block all available active sites on the interior surface of the vessel. Calibration gases are then routinely used to ensure no losses are occurring. This is of particular importance as the inert coatings may deteriorate with repeated use, reducing their effectiveness significantly. In addition all sampliung equipment used by Qa3 that comes into contact with the sample have an inert coating to help minimise any chemisorption during sampling
For gases, when sampling to obtain a non-pressurised sample, gas bags (Tedlar® or equivalent) fitted with an inert valve are used.
For more comprehensive sulphur speciation, depending on geographical location and concentration of sulphur compounds, Qa3 can perform on-site sampling of gases and liquids for subsequent full sulphur speciation in a UK laboratory facility.
- On-site determination of H2S, volatile RSH and COS for immediate data.
- On-site determination of concentrations ranging from trace (ppm) up to high (%).
- On-site evaluation of sulphur removal unit performance.
- Mass balance of sulphur compound distribution across a processing facility (gas plant / refinery).
- Detailed sulphur speciation (via off-site analysis) applicable when higher concentrations of several species are present or when quantification of higher MW sulphur compounds is required.