Hydrogen sulfide, H2S, is a very poisonous, corrosive, and flammable gas. As such, it needs to be carefully monitored and controlled in industrial processes where it is generated or used. At relatively low concentrations, it has an odour of rotten eggs which effectively works as a warning, but at higher concentrations where it becomes acutely toxic and even lethal, it can no longer be smelled and the awareness of danger disappears.
Hydrogen sulfide is produced naturally in the breakdown of organic matter by microbes, in absence of oxygen. For example, it can often be smelled in swamps and close to sewage treatment plants. However, the vast majority of man-made H2S originates from refineries, where the sulfur in the crude oil is removed by reaction with hydrogen. The result is low-sulfur oil and desulfurized petroleum, but also huge amounts of H2S. Hydrogen sulfide can also be found in very high concentrations in raw natural gas and biogas (“sour gas”), and has to be removed before the remaining gas (“sweet gas”) can be distributed and further used.
Although hydrogen sulfide is a by-product from refineries, it is also the primary ingredient for production of elementary sulfur in the industry-standard Claus process. In a first combustion step, H2S reacts with oxygen to form sulfur dioxide and water. In one or more following catalytic steps, H2S and SO2 react and form elementary sulfur and additional water. The resulting sulfur is then typically used to produce sulfuric acid, which in its turn is one of the base substances in the chemical industry.
It is important to be able to monitor concentrations of hydrogen sulfide, both from a process control perspective and for safety reasons. Starting with the latter, there are several types of hand-held or wearable gas monitors used for personal protection. Such devices can often detect both H2S and other gases of concern like CO. The devices are often based on electrochemical detection methods. They may lack in precision but still serve well at relatively low concentrations of H2S, typically in the range 0-50 ppm (75 mg/m3, the maximum personal safety limit for short-term exposure) or lower. It can then provide warnings if the H2S concentration increases, before the situation becomes dangerous to the person carrying the device.
However, for emissions monitoring and also for process control purposes, other types of devices are needed. Compared to the monitors for personal use, they have to meet quite different requirements on for example measurement ranges, detection limits, and ability to operate continuously with aggressive gas mixtures in harsh industrial environments.
The method to measure H2S concentrations in industrial processes has traditionally been to extract a gas sample, dilute it, remove any SO2 by means of a scrubber, then convert H2S to SO2, and finally measure the SO2 concentration which effectively yields the H2S concentration. However, this method requires a lot of maintenance of the dilution system, scrubber, and converter, and it is prone to errors. This can be a problem since both availability and reliability of measurement results ca be crucial not only for the production process but also for safety reasons.
For continuous emissions monitoring as well as process monitoring of H2S, OPSIS offers a solution based on the proven DOAS open-path technology, without dilution or scrubbing. The extracted gas is led to a durable H2S converter, which is followed by the direct monitoring of the converter product in a gas analyser. The system has a very low detection limit and works well both at low and moderate measurement ranges, from 0-20 mg/m3 up to 0-1,000 mg/m3. OPSIS also offers gas analyser systems for higher measurement ranges, up to 100%, suitable for process control purposes. The same single monitoring system can also be configured to measure concentrations of many other gaseous substances such as NO, NO2, CO, CO2, NH3, H2O, HF, and HCl, both in-process and for continuous emissions monitoring.
About the Author
Bengt Löfstedt
Operative Support, OPSIS AB