Raman Analyzers

In the renewable energy processes, the applications of Raman analyzers are in the same areas where fiber optics (Section 3.2.9) and IR analyzers (Section 3.2.11) are applicable. In addition, Raman analyzers can also detect H2 and 02, which are very important measurements in renewable energy processes. The relative capabilities of IR, NIR, and Raman analyzers are summarized in Table 3.39. 

Raman analyzers can be considered for applications that IR or NIR detectors are suited for, but the cost of a Raman process analyzer exceeds that of other analyzers. Therefore, as yet, their applications are few and can be found in areas where continuous and fast readings are more important than cost and precision. In the future, the availability of inexpensive, rugged process probes that can work reliably within a high-pressure, high-temperature, and corrosive process environment will increase their use. 

Optical fibers are used to deliver light and collect Raman signals remotely. Silica fibers with core diameters ranging from 50 to 500 pm are used. The larger the core diameter, the less the bending radius (flexibility). Raman spectroscopy uses low-hydroxy fibers to minimize fiber background interference in Raman data and also to minimize light attenuation caused by fiber absorption. 

Invasive and noninvasive Raman probes are used. The requirements for an invasive or insertion probe are that it should be industrially hardened, have a high degree of chemical/corrosion resistance, and be able to withstand very high process temperatures and pressures. Raman probes can withstand up to 350°C and 200 bar pressures, and operate under highly corrosive condi- 

Selection rule Change in dipole moment Change in dipole moment Change in polarizability 

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N. Everall, H. Owen and J. Slater, Performance analysis of an integrated process Raman analyzer using a multiplexed transmission holographic grating, CCD detection, and confocal fiberoptic sampling, Appl. Spectrosc., 49, 610-615 (1995). 

Everall, N. Owen, H. Slater, J. Performance Analysis of an Integrated Process Raman Analyzer Using a Multiplexed Transmission Holographic Grating, CCD Detection, and Confocal Fiberoptic Sampling Appl. Spectrosc. 1995, 49, 610-615. 

Selected Successful Raman Applications, http //www.kosi.com/raman/analyzers/ramanrxn2.html. 

Merrit, R. Intel Applies Chip-Making Prowess to Cancer Research - MPU Giant Hopes Raman Analyzer Speeds Detection Electronic Engineering Times 2003, 10272003 6. 

Figure 3.40 shows the layout of a typical Raman analyzer that uses fiber optics for process application. In a Raman process system, light is filtered and delivered to the sample via excitation fiber. Raman-scattered light is collected by collection fibers in the fiber-optic probe, filtered, and sent to the spectrometer via return fiber-optical cables. A charge-coupled device (CCD) camera detects the signal and provides the Raman spectrum. To take advantage of low-noise CCD cameras and to minimize fluorescence interference, NIR diode lasers are used in process instruments. 

Schematic of fiber-optic Raman analyzer for process measurements.

The decision to use a Raman analyzer depends on the availability and practicality of alternative analyzer technologies. Similar to mid-IR, Raman bands represent fundamental modes of vibration. Bands are narrow and molecule specific, and they provide quantitative chemical analysis. The technology is equally amenable to the analysis of gases, liquids, slurries, emulsions, powders, and solids, including samples with particulates and bubbles. Noninvasive probes can be used and are a key feature in many applications where the process must not be breached. 

The limitations of Raman spectroscopy are its low sensitivity compared to IR absorption and fluorescence interference from impurities in the sample. Raman spectroscopy is a developing technology, and a good amount of research and planning is necessary before deciding whether or not to employ it. The cost of a Raman process analyzer exceeds that of other analyzers. To reduce cost, Raman analyzers often include multichannel capability. Up to four process streams can be analyzed with a single CCD camera by splitting the lasers. 

Kawai, N. Janni, J.A. Chemical identification with a portable raman analyzer and forensic spectral database. Spectroscopy, 2000, 15 (10), 32. 

In Chapter 2, the historical impact of these devices have been reviewed. These individual components have been assembled to produce three popular and fundamental types of Raman spectrometers the laboratory Fourier transform (FT)-Raman system [1,2], the CCD-based microprobe [3], and the fiber-coupled CCD-based process Raman analyzer... 

Gervasio and Pelletier [22] reported a comparison of near-infrared (NIR) dispersive and FT-Raman analyzers for on-line studies of phosphorus trichloride, where the effects of multiplex noise causes by bubbling, particulate movement are discussed. For these reasons, the remainder of this chapter will center on Raman analyzers that use the dispersive approach. Despite the drawbacks mentioned, there have been a number of reports of the use of FT-Raman spectrometers for on-line analysis [23-32],... 

At this juncture, it is necessary to define what will be considered under the title of process Raman spectroscopy in the industrial environment. Quality assurance/quality control (QA/QC) applications and failure analysis are important areas of industrial interest however, these areas, in general, can be adequately addressed with standard laboratory Raman spectrometers including FT-Raman spectrometers and dispersive microprobes. For the purpose of the remainder of this chapter, the industrial environment and the process Raman analyzer will be restricted to instrumentation and protocols usable for on-line measurements. At this point, it is necessary to outline a definition of what the requirements for an on-line process Raman analyzer are. A process Raman analyzer should be composed of components which have the following properties ... 

Figure 26 Diagrammatic representation of the most important input and output communications for a control computer for use with a Raman analyzer.

Data collection and analysis can be broken up into three primary functions. Instrument control includes control of the three primary components of a Raman analyzer (1) the laser, (2) the spectrometer, and (3) the CCD camera. Spectral data collection and preprocessing encompasses collection of the CCD data and subsequent processing of that data to produce a calibrated, resampled Raman spectrum. Data analysis, which is the primary task of the Raman system, is ultimately generated from the collected Raman data. 

A Raman analyzer can be a valuable tool for application in a wide variety of on-line industrial settings. It can provide high information content via well-separated spectral... 

Before embarking in detail on these subjects, it is beneficial to review the basic components of a Raman analyzer. At the most basic level, the system consists of a central... 

These concerns dictate that a process Raman analyzer must be designed and applied with these factors in mind. The following measures merit consideration when designing a process Raman system ... 

Currently, the most common classified environment in which industrial Raman analyzers are being installed are rated Class 1 Division II for North America or Class 1 Zone 2 for Europe. Although the stringent requirements associated with Division 1 are widely recognized throughout the world, not all countries, Japan for example, accept the more relaxed Division 2 concept [243]. 

Figure 32 An installed analyzer shed used in conjunction with a process Raman analyzer.

Figure 33 A wall-mounted process enclosure used to house a process Raman analyzer (a) analyzer cover on (b) analyzer cover off, major analyzer components identified.

It is, therefore, expected that these types of installation may become more common as new process Raman analyzers and applications come on-line. 

As was alluded to in the previous subsection, proper configuration of the analyzer package is a crucial factor in the system s function and acceptance in the process environment. Raman analyzers have several key components, which include the laser, detector, and spectrograph, that must be appropriately packaged in a reliable on-line analyzer. 

Figure 34 A typical NEMA 4X process enclosure used for a Raman analyzer picture of the enclosure showing the location of the control PC, a vortex cooler, and an easy serviceable analyzer utilizing a two-removal-tray construction. (Reproduced with permission from Kaiser Optical Systems, Inc.)...

There are several strategies for enclosure housing used in classified environments. The two most applicable to Raman analyzers are explosion-proof and purged. 

Early Raman analyzers were installed into sample loops where the sample was brought to them [145,252] but the future of on-line process Raman analyzers [253-256] clearly rests with fiber-coupled systems and the remainder of this chapter will assume this configuration. For more information on successful on-line applications of Raman spectroscopy, the reader is referred to Chapter 23. 

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