By Edo Becker, Process Analytical Consultant for Specac Ltd
When specifying a spectroscopic online monitoring system, there are many stages of design and review to proceed through. Choosing a suitable Process Cell for installation in-line with the process will be one of the considerations, among several others. Due to variable compositions, high pressures, temperatures and corrosivity, a robust technique like NIR technology is ideally suited to carry out on-line analysis in-situ.
Interaction of engineering and laboratory
The following graphics illustrates the evolvement of an on-line installation from laboratory concept to final installation at a production facility.
Ideally the life cycle of a successful on-line NIR installation can be divided into three stages, the laboratory feasibility stage, the pilot plant testing stage, and the final installation stage.
Laboratory feasibility studies
These should always be conducted on samples resembling the process conditions as closely as possible (composition, pressure, temperature, particle contamination) to understand whether NIR technology is viable for the robust on-line analysis of the process stream. If not all components are required or not all of them are visible in the NIR spectral region the calibration could potentially be simplified.
If process samples are available for calibration the off-line analysis must be accurate and complete enough to build a reliable calibration. Any error in the off-line analysis will be replicated in the calibration of the NIR system. Temperature has got a strong influence on NIR absorption spectra and should match the expected process temperature range as closely as possible. A liquid cell might also have to be pressurised at high temperatures using an inert gas like Helium or nitrogen to avoid boiling of light components at higher temperatures. For a gas cell calibration the pressure also needs to be taken in account for the calibration.
Therefore, in the laboratory facilities should be available to mimic the process conditions in a safe manner, including gas and liquid mixing, heating of the process cell and pressurising the NIR cell using a gas supply and mixing system. Nowadays companies are available which manufacture that equipment, as they are also used for calibrating mandatory emissions monitoring systems.
If samples are from a production process expect low variability, as many processes run at the optimum for most of the time. In that case process samples can be spiked with additional materials to break any co-linearities between the components and to build a robust model. The spiking can be limited though as mixtures with water can have undesired effects like emulsions forming or phase separation.
The calibration robustness and representation of the final installation will also increase if the measurements are carried out using the optical cell with the same optical fibres which are going to be used in the pilot plant or the production facility (optical response and wavelength range limitation).
Pilot plant stage
If the chemical process is in the early development stage or the main plant has just started up companies normally keep a pilot plant running in parallel to the main plant to improve the understanding of the behaviour of the larger chemical unit.
This is an ideal opportunity to install a NIR cell with the initial calibration of the laboratory, as it will give the pilot plant operators valuable insight into the behaviour of the different reaction and separation systems. In our experience a first laboratory calibration will provide the pilot plant operator already with trending information. The addition of a few process samples will often be enough to finalise the initial pilot plant calibration.
However, it is also the time to move from a purely analytical project into the engineering area, as issues like metallurgy, filtering, phase separation, corrosion, sampling, sealing and many others can be spotted early in the development cycle and fixed for the main plant installation.
Main plant stage
After a few months of pilot plant operation, the PAT team should have gained enough confidence for a main plant installation that will be successful and improve the control and optimisation of the chemical plant. Engineering issues will have been identified and rectified.
After the installation further in-situ samples should be taken to verify and expand the chemometric calibration, sometimes other analytical methods on the same process stream with a less frequent sampling and testing regime might be available to add more calibration points to the analysis.
If the configuration of the main plant changes an update of the NIR calibration is also required. In the last few years companies have developed software to link off-line LIMS systems (Laboratory Information Management Systems) with the spectra and calibration of the NIR system to implement automatic updates should these be necessary.
At this stage the integration of the NIR cell into the overall control scheme of the DCS (Distributed Control System) becomes the major focus of the installation. This also includes HAZOP studies (Hazard and Operability studies), ATEX or Div. Class certification and the communication protocol with the DCS.
In terms of cost and complexity there is a big increase between the different stages, however the financial gain for the company becomes more and more important during the overall life cycle of the NIR installation as well. Typically pay-back times can be anything between 6 months and 1 year for a NIR installation.
Specac PAT support team
At Specac we are happy to help you by evaluating your specific application with our dedicated team of mechanical engineers, chemists, and process experts. We have a wide range of contacts within the chemical industry, spectrometer manufacturers and fibre-optic producers to help and turn your initial idea into reality.
Related Posts
GET IN TOUCH