kelma

Description of application

The analyser is located on a Coke-oven plant of a large steel factory. After the transformation of carbon in coke, gasses are generated, recovered and treated. Water with high NH4 concentration is misted to clean these gases.This water is recovered and distilled before to be ripped of pollution by a biological treatment. Our analyser, located between distillation column and biological treatment, allows monitoring the good performance of columns in order to preserve bacteria of biological treatment. Ammonia is crucial for the existence of bacteria in the plant.

Problematic

The difficulty of this application is a high salt concentration. In this case, methods such as colorimetric or specific electrode are not appropriate because of risk of interference. But the UV400 is suitable because the used method is not sensitive to such interference.

Principle

The measuring principle is based on the analysis of the absorption spectrum of ammonia NH3 in gas phase.
The UV400 measures the concentration of the ammoniac NH3 in the gas phase which is proportional to the concentration of dissolved ammoniac in water according to the principel of Henry's law:[NH3aq]/[NH3g] = KH.RT
But previously, the analyser transforms NH4 to NH3 increasing the pH above 11,5.
The gas absorption spectrum analysis is performed using concave grating spectrograph coupled with a 256 pixel linear photodiode array, scanning the 190 to 300 nm spectral band.
The diffracted spectrum is  treated using Fast Fourier Transform (FFT) in order to evaluate the signal corresponding to the selective absorption peaks of NH3 gas.
The sample temperature is systematically measured in order to carry out automatic compensation of the measurement.

Technical charcteristics

- Component and range of measurement: NH4 0-1000 ppm
- Temperature of sample: 35°C
- Location of analyser: air -conditioned cabinet
- Sampling flow : less than 1 bar
- Dimension and weight:L x D x H = 390 x 285 x 550 mm-28 kg
- Measurement periodicity: 30 minutes
- Cleaning cycle: 48 hours

Results:

After calibration on  site measurements of UV400 have been compared with laboratory results. Correlation is excellent.
Data are recovered by 4-20mA output and integrated to a control software on site. When a peak is detected sample is sent to the retention bassin.

Advantages of UV400

The continues mode allows monitoring of the sample in real time. The 4-20 mA output collects the data in the same time. A very low maintenance is required for this analyser.