On-line process analysis

The environmental appHcations of infrared spectrometry are many and varied. Many appHcations at industrial sites are analogous to those for on-line process analysis waste streams and recycling processes can be monitored in the same way. Commercial infrared stack-gas monitors are based on either an extractive probe attached to a long-path gas ceU or an open-path (across stack) configuration (69). Stack plume and flare monitoring can be done externally... 

In each of the preceding studies, one major advantage of the near-IR technique is the rapid evaluation of the sample without the need for traditional extraction techniques and subsequent chromatographic or colorimetric analysis. For this reason, near-IR is an ideal form of analysis in a quality control, or on-line, process analysis environment. In a complementary mid- and near-IR... 

Develop real-time and on-line process analysis and monitoring methods. 

On-line investigation methods for statistical analysis are used when the performances of a continuous process carried out in a pilot unit or in an apparatus, have to be improved. The Evolutionary Operation Process (EVOP) method [5.7, 5.27, 5.28, 5.31] is the most famous method for on-line process analysis. The name of this method comes from its analogy with biological evolution. This analogy is based on the observation of the natural selection process in which a small variation in independent life factors is responsible for genetic mutations and thus for the evolution of species. 

The Raman technique has been readily adapted for on-line process analysis, especially in the pharmaceutical industry ". It has the benefits of mid IR, e.g. the ability to identify compounds from the vibrational fundamentals, without the constraints of mid IR, e.g. the limitations of the optical materials that can be used. Its popularity is also due in part to the excellent throughput of optical fibres for the radiation required for Raman, i.e. in the Vis and NIR regions. This use of optical fibre probes (Figure 9.14) facilitates easy in-line analysis because the sample can be remote from the instrumentation, even to hundreds of metres in distance. Fibre optic multiplexers are also available, allowing many samples to be analysed sequentially. Small laser diode sources and CCD detectors can be attached to the optical fibres and changed as required, rendering the overall device small and flexible. Radiation from the laser diode light source is transmitted to the sample by optical fibre... 

By far the most common use of mid-infrared radiation for process analysis is in the non-dispersive infrared analysers that are discussed below. The widespread use of FTIR spectrometers in the mid-lR has yet to be fully realized in process analytical apphcations. The requirements for the optical components and the wavelength sta-bihty of the instraments available have, until recently, detracted from the use of this region of the spectrum in on-line process analysis. Optical fibers that provide such a benefit to the apphcations of NIR (see below) are not available for the mid-IR in robust forms or forms that are capable of transmitting over more than a few tens of metres. Improvements and developments to sample cells, particularly designs of attenuated total reflectance (ATR) cells, for use with mid-lR are being made and will influence the application of the technique. An impressive list of apphcations including both FTIR and the NDIR approaches has been compiled (2, 3]. 

Table 17.1 Examples of the application of NIR spectroscopy to on-line process analysis...

In this section, some of the successful field applications along with some laboratory applications, which have shown potential for scale-up to on-line process analysis, have been reviewed. 

In on-line control, an automated sampling system attached to a reactor or by-pass system is used to extract the sample, if needed conditioned, and presented to an analytical instrument or probe for measurement. Sampling delays can be significant in on-line installations, because of the transit line and gear pumps. On the other hand, on-Une devices are isolated from the main stream by the use of a gear pump, and the temperature and pressure of the polymer sampling flow can thus be controlled. Their maintenance can therefore be done without a complete process shutdown. Many of the apparent disagreements between results from split side stream on-line analysers and results from the laboratory can be traced back to differences which occur because samples differ in acquisition time, location and stability. The main characteristics of on-line process analysis are summarised in Table 7.4. 

Choosing a suitable Raman spectrometer for on-line process analysis requires different criteria from laboratory analysis. Some key considerations are laser safety, ruggedness, repeatability, long-term and environmental stability, high uptime, calibration ttansferability, ease of operation and maintenance, smart diagnostics for analyser performance, and industry-standard communication. Many processes require the analysis to be performed... 

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