Monitoring Hydrogen Sulphide (H2S) In Chemical Processes

Louise Smyth

Hydrogen sulphide (H2S) is an important intermediate compound in many chemical processes, but it is also a very poisonous, corrosive and flammable gas.

As such, it needs to be carefully monitored and controlled wherever it is generated or used. It has an odour of rotten eggs at very low concentrations, which effectively works as a warning, but it can no longer be smelled and the awareness of danger disappears at higher concentrations where it becomes acutely toxic and even lethal.

The vast majority of man-made H2S originates from refineries. It is a by-product from desulphurisation processes, but also the primary ingredient for production of elementary sulphur in industry-standard Claus processes.

In a first combustion step, H2S reacts with oxygen to form sulphur dioxide (SO2) and water. In one or more following catalytic steps, H2S and SO2 react and form elementary sulphur and additional water. The resulting sulphur is then normally used to produce sulphuric acid, which in its turn is one of the base substances in the chemical industry.

The Importance Of Monitoring H2S

It is important to be able to monitor concentrations of H2S, both from a process control perspective, for emissions monitoring, 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, such as 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 0-75mg/m3 (50ppm, 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.

Continuous Emissions Monitoring (CEM)

However, for continuous emissions (CEM) monitoring and for process control purposes, other types of devices are needed. They have to meet quite different requirements compared to monitors for personal use, for example on measurement ranges, detection limits, and ability to operate uninterrupted with aggressive gas mixtures in harsh industrial environments.

Such devices in general often use some optical property of the molecule of the specie being monitored. However, the H2S molecule does not always have any such properties that are easy to use in the sought-after concentration ranges found in industrial processes.

Applying Chemical Scrubbing & Conversion

The solution to the H2S monitoring challenge has for many years been to apply chemical scrubbing and conversion. A gas sample is extracted from a monitoring point. The sample is then diluted and any SO2 is removed by means of a scrubber. A converter then transforms any H2S (flowing unaffected through the scrubber) into SO2, and the SO2 concentration is measured. The latter is relatively easy thanks to the optical properties of the SO2 molecule. The result is effectively the H2S concentration in the original sample, although detected as SO2.

This monitoring method is widespread, but it requires a lot of maintenance of the dilution system, the scrubber and the converter and it is prone to errors. This is costly and can be a problem since both availability and reliability of measurement results can be crucial not only for the production process but also for safety reasons.

An Alternative Approach

As an alternative requiring far less maintenance, Opsis offers a solution based on a proven 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 concentration of the converter product by means of a DOAS gas analyser.

The system has a very low detection limit and works well both at low and moderate measurement ranges, from 0-20mg/m3 up to 0-1,000mg/m3. Based on the same DOAS technology, Opsis also offers gas analyser systems for higher measurement ranges, up to 100% H2S, suitable for process control purposes. Further, the same single monitoring system can also be configured to measure concentrations of many other gaseous substances such as NO, NO2, CO, CO2, NH3, and H2O, both in-process and for continuous emissions monitoring.

Bengt Löfstedt is with Opsis