PROCESS EQUIPMENT UPDATE


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. Te vast majority of man-made H2


H ydrogen sulphide (H2 S


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, H2


and form elementary sulphur and additional water. Te resulting sulphur is then normally used to produce sulphuric 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 H2


following catalytic steps, H2 dioxide (SO2 S and SO2 S, 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 H2


S and other gases of concern, such typically in the range 0-75mg/m3


as CO. Te devices are often based on electrochemical detection methods. Tey may lack in precision but still serve well at relatively low concentrations of H2


S, (50ppm,


the maximum personal safety limit for short- term exposure) or lower. It can then provide warnings if the H2


S concentration increases,


before the situation becomes dangerous to the person carrying the device. However, for continuous emissions (CEM) monitoring and for process control purposes,


Opsis gas analyser technology


S reacts with oxygen to form sulphur ) and water. In one or more


react


S) is an


S is used, for example, in Claus processes


H2 H MONITORING


Bengt Löfstedt discusses the use and monitoring of hydrogen sulphide in chemical and other industries


2S


not always have any such properties that are easy to use in the sought-after concentration ranges found in industrial processes. Te solution to the H2


S monitoring


challenge has for many years been to apply chemical scrubbing and conversion. A gas sample is extracted from a monitoring point. Te sample is then diluted and any SO2


is


removed by means of a scrubber. A converter then transforms any H2


through the scrubber) into SO2 SO2


S (flowing unaffected , and the


concentration is measured. Te latter is relatively easy thanks to the optical properties


other types of devices are needed. Tey 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 H2


S molecule does


of the SO2 the H2


S concentration in the original sample, although detected as SO2


molecule. Te result is effectively .


Tis monitoring method is widespread,


by the direct monitoring of the concentration of the converter product by means of a DOAS gas analyser. Te 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


control purposes. Further, the same single monitoring system can also be configured to measure concentrations of many other gaseous substances such as NO, NO2


DOAS technology, Opsis also offers gas analyser systems for higher measurement ranges, up to 100% H2


and H2O, both in-process and for continuous emissions monitoring.


, CO, CO2


Bengt Löfstedt is with Opsis. www.opsis.se


www.engineerlive.com 23 , NH3 S, suitable for process ,


but it requires a lot of maintenance of the dilution system, the scrubber and the converter and it is prone to errors. Tis is costly and 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. As an alternative requiring far less maintenance, Opsis offers a solution based on a proven open-path technology, without dilution or scrubbing. Te extracted gas is led to a durable H2


S converter, which is followed


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