Low Level Sulphur Measurement in Oil Refinery Products, The Model HSCA, High Sensitivity Sulphur Analyser

The model HSCA is an on-line process sulphur analyser. It uses a measurement system based on the energy dispersive X-ray fluorescence method and utilises a high sensitivity semi-conductor detector. The analyser has provided accurate and reliable operation for more than two years during rigorous field trails conducted at one of our client’s refineries. 

In order to achieve the required sensitivity and minimise noise, the semi-conductor detector is cooled using liquid N₂. An automatic supply and control system handles the supply of the liquid N₂ to the detector from a commercially available pressurised cylinder located close to the analyser. 

X-ray analysis techniques have been used in the laboratory for many years. In order to meet the increasing demand for the measurement of trace sulphur content in oil refineries, DKK began to apply these techniques and produce on-line analysers using Si(Li) detectors. Our clients’ requirements for greater sensitivity and lower detection limits increased but the conventional Si(Li) semiconductor was already at the limits of its performance. 

In order to overcome these performance limitations, DKK developed a cooling system for the Si(Li) detector using liquid N₂. This simple but extremely effective system employs a 10 litre dewer vessel which is fed with liquid N₂. As the liquid N₂ is consumed by the detector, it needs to be replenished. This is controlled automatically by the analyser using a level sensor suitable for cryogenic service. The only routine work required by the plant technician is to check the liquid level of the N₂ cylinder adjacent to the analyser house on a regular basis. When the cylinder becomes empty, it can be replaced in the same way as compressed gases for other types of on-line analysers are changed (process gas chromatographs for example). If  the maintenance technician does not replace the N₂cylinder when required, then a diagnostic alarm is tripped. The analyser can still function for 3 more days after this alarm has been generated allowing plenty of time to change the cylinder and keep the analyser on-line. 

HSCA analysers are providing excellent performance under field conditions with repeatability's of +/- 1 ppm for 0~20 ppmS ranges. The reliability of the measurement results has allowed our clients to reduce the number of laboratory checks carried out, particularly during night shifts. One client has reported to us that he has been able to reduce the number of cross checks from 17 per month down to 10 per month. 

Typical applications for this analyser are process and quality control at hydro desulphurisation plants for kerosene and gas oil.