Ten things to consider for a successful FTIR installation

FTIR process analyzers have been used in the Petrochemical and Refinery Industry for several decades and like other high-end complex systems, quite often need regular maintenance and looking after.

For some of the tougher applications, in particular gasoline and diesel blender stream analysis, the failure rate of this technology is higher than you might expect. Flashing forward ten years on an installation often shows that the analyzers have been neglected and are no longer in use.

This blog is an attempt to examine why such complex systems can fail and the ten steps you can perform to improve the chances of a successful long-term installation.

It’s worth mentioning the differences between ‘FTIR’ which is usually referred to as Fourier Transform Infrared in the Mid-Infrared part of the spectrum (300-4000cm-1) and ‘FTNIR’ which is ‘’Fourier Transform Near-Infrared’’ spectroscopy (4000-13000cm-1), see figure 1.

Figure 1 : Electromagnetic spectrum Infrared

While measurements in the Mid-IR can be far more sensitive than in the Near Infrared (NIR), FTNIR is more commonly used in process applications because it has a number of practical advantages.

Firstly the optical materials used are more robust and suitable for process applications; the insitu probes or transmission cells that are most commonly used need to have optical windows that transmit the light through into the sample and back into the fiber optic. Just like a regular window in your house has to be naturally transparent to sunlight, so does the cell window have to be transparent to Infrared light. Mid-IR transparent materials are quite often brittle and hygroscopic salts like Barium Chloride. For Near Infrared you can use quartz or even sapphire which are pretty tough.

A second reason is that cell pathlengths for Mid-IR are typically 200-500 micron size and more susceptible to fouling. Near-IR uses path lengths between 0.5-20mm making it more practical.

Finally, cheap silica-based fiber optics can be used to great effect – having the spectrometer located hundreds of meters away from the measurement point, and even having multiple streams at different places in the plant-feeding to just one spectrometer.

While the Mid-IR can show sharp well-defined peaks for various components the Near-Infrared shows us the overtone absorbances of hydrocarbons and chemometric modeling needs to be used on the spectra to obtain property values from the samples.

That one word ‘’chemometrics’’ is enough to put some people off, and for good reason. Instead of classifying peaks and counting the area underneath it, chemometrics can be a world of voodoo statistics. Despite the elegant software tools available, it is still too easy to massage the data any way you want and make models that in the field, just don't work.

But it gets worse. The central premise behind using a full range spectrometer to make predictions of properties of certain liquid streams is that the spectrometer can fully measure and differentiate the chemistry within the sample.

In many cases it, the spectroscopic method alone can be woefully inadequate, and the models built for such properties can have arbitrary and non-existent correlations.

Online FTIR/FT-NIR analyzers are very powerful and can be used to great effect if done correctly.

So here it is, ten things to consider. Many are hardware-based reasons, but some are modeling-based.

Ten things to consider for a successful FTIR installation :