27 Feb 2020

Application Note AN18-09 Specac Process Cells

The fundamental advantage of using NIR analysis as the basis of industrial on-line process monitoring is that high quality spectra can be obtained remotely using optical fibres.

Near infrared (NIR) has bought many benefits to process plant operators as a wide variety of sampling protocols have been developed. Gas and liquid flow cells can be directly inserted into a process stream or inserted into a fast loop by-pass and monitored using a single remote spectrometer coupled to a fibre multiplier.
The benefits of in-line analysis are:
  • Real time information
  • Better process control
  • Enhanced product quality
  • Optimised plant throughout and efficiency
  • Plant data for quality management
  • Monitoring of plant safety
  • Reduced amount of process samples for off-line analysis resulting in reduced exposure of plant personnel sampling equipment

Transmission flow cells have been developed which are capable of collecting both liquid and vapor phase NIR spectra over fibre optic runs many hundreds of metres in length.
Historically, the most important measurements have been for water at ppm or % levels in various solvents using the overtone/combination bands at 7150 and 5260 cm. A 1 mm pathlength enables detection at up to 5000 ppm weight.
An important parameter in the batch manufacture of polyols is the hydroxyl number of the product. Polyol products from a wide variety of different precursors and with varying hydroxyl numbers can be correlated using the band at 4830 cm.
More recently FT-NIR spectroscopy has been successfully used for in-line monitoring of the transesterification reactions for biodiesel production [1].
NIR has also been used to monitor the concentration of NaOH for the control of scrubbing process gases prior to stack discharge to ensure that acid gas surges do not break through.
Cell Design
The transmission cells used in this project have a high efficiency coupling into fibres of only 200 µm diameter. A commercially available NIR fibre optic spectrometer was used to obtain the spectra shown. Sapphire optics in these cells have the benefit of low fouling in harsh environments. For liquid cells, flat windows are desirable in order to avoid refraction effects at the liquid-window interface. Similarly, with vapour streams at pressures of ~8 bar the linear increase in vapour density with increasing pressures has shown that flat windows are preferable in order to prevent refraction losses.
High performance process cells are ruggedized transmission units that must be designed for use in process main stream pipelines or in fast loop by-pass systems. For coupling to the fibre optic system, industry standard SMA connectors are used which allow a wide choice of fibre diameter.

The design of a process cell cannot simply be based upon the modification of a laboratory cell. These cell types have entirely different requirements due to the unique demands on them in a process installation such as continuous exposure to harsh conditions over periods of months or even years.
Seal design, particularly around the optical components, is crucial to ensure leak free operation under all conditions. This may even include extreme thermal shock such as when superheated steam is introduced for cleaning. The principal factors that must be modelled are the thermal expansion coefficients of the components, the manufacturing tolerances (and their build-up), as well as the swell, chemical compatibility and compression set characteristics of the seal materials

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