Near Infrared Analysis

Near infrared (NIR) analysis is a new process liquid measurement technology that is growing. NIR spectroscopy is an optical scanning technique that operates in a range of wavelengths between 800 and 2400 nm. The primary advantage of NIR analysis is that because the sample probe is placed directly in the process stream, an extractive sample handling system is not needed. More importantly, process NIR analysis addresses applications that have not been tried by other technologies.

Traditionally, the measurement of liquid components using NIR analysis has been done by analyzers that operate at only one wavelength. Recently, analyzers using gratings, filter wheels, and other moving parts have been developed to vary the infrared wavelengths, making measurements of multiple liquid components possible. However, when these moving parts are in the process environment, they require frequent maintenance.

Traditionally, NIR analysis has been used to measure moisture in the process industry. However, its ability to perform scanning and the advent of fast computers with sophisticated computer software (e.g., chemometric) have expanded its applications, particularly in polymers.

A summary of proven and potential applications of spectroscopy follows. Table 3.12.1 summarizes a proven, closed-loop control application for NIR spectroscopy. Any molecule containing a carbon-hydrogen, hydroxyl (O--H), carboxyl (C=O), or amine (N--H) bond and many inorganic species adsorbs NIR radiation. Water allows NIR radiation to pass through. Thus, NIR analysis can perform water analysis or determine the concentration of the materials that are mixed with it. Table 3.12.2 lists online NIR projects that are in development for processes which have not been amenable to the technique in the past. The NIR analyzer can monitor alkylation, reforming, blending, and isomerization in addition to distillation.

With the advent of fiber optics, online spectroscopy has become safer and more flexible. Fiber-optic cable allows the analyzer and online probes to be separated up to 1000 m. The incident light travels along the cable to the probe, where sample absorption occurs. The reflected signal travels back to the detector through the cable, where it is analyzed (see Figure 3.12.1).

If process safety is a high priority, fiber optics may be the best technique to use because only the probe is in contact with the process material; the analyzer is in a safe location. The tradeoff is that the spectral quality of such systems can suffer and they can be expensive.

The maintenance of online spectrophotometers is equipment- and application-oriented. The maintenance of the equipment is fairly simple. Application maintenance consists of the scientific and engineering work required to validate the system's results and generate the learning set as process formulations and the associated feedstocks change. Application maintenance is often overlooked by users, but it is critical to the reliability of online analysis.

Even today's simplest analyzer has a microprocessor that refines raw data, does calculations, and displays results. Analyzers can have graphic interfaces and can be networked with other analyzers and data systems.

Every analyzer should be linked to a plant's distributed control system (DCS). Thus, process analytical data can

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