Analytical Instrumentation & Environmental Monitoring


Photo-transmission meters

September 2008 Analytical Instrumentation & Environmental Monitoring

Online water measurement in bio-ethanol.

Bio-ethanol is produced from the starch component in renewable plant materials such as sugar cane, maize and sorghum. The advantage of bio-ethanol is that it is a growable energy source and by virtue of its oxygen component allows for better combustion of the fuel, resulting in less residue from the burning process.

Hygroscopic problems

The production processes, however, cannot achieve 100% pure ethanol as it forms an azeotropic mixture with water – the best azeotropic ratio achievable is 95,4% ethanol/4,5% water. To achieve a higher purity, pressure swing distillation or drying agents have to be employed. A further problem for motor vehicle owners using this new fuel blend, is that atmospheric water and ethanol can mix very easily, which then causes other problems such as a lowering of the boiling point of the resultant mixture or a change of the evaporation pressure. Corrosion in fuel tanks is also accelerated; in New Zealand it is recommended that ethanol blended bio-fuel is stored for less than two months so as to not absorb too much atmospheric moisture.

It is thus very important to monitor the water content during the bio-ethanol production process in order to minimise these problems. To this effect photo-transmission meters lend themselves perfectly.

Photo-transmission principles

The bio-ethanol can be piped directly through a measuring cell with transparent glass walls which prevents contamination of the fluid and allows containment of dangerous vapours inside the production piping. A near IR light source of known wavelength is then transmitted through the sample cell. On the other side a detector compares the received wavelength to a standard; the difference is then used to calculate the amount of water present in the fluid sample. The set-up is also available as an explosion proof version.

The maintenance for the process gauges is minimal with the measuring cell being easily accessible, the light source of the long life variety and the operating procedures optimised towards ease of use.

These gauges can also be used to measure water in ammonia, butane, ethyl chloride, propane, propylene oxide and vinyl acetate.



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