Chemical process monitoring

Optical. Besides the gem quaHties dependent on optical properties, diamond is very useful as a light-transmitting window for lasers and for simple windows for monitoring chemical processes in corrosive and otherwise hostile environments. 

Fiber optic interfaces are ideal for monitoring chemical processes. The fiberoptic FT-Raman system shown in Fig. 2-15 was used to demonstrate the... 

Yu.Ya. Fialkov, Solvent as a Mean of Monitoring Chemical Processes (Khimiya, Leningrad, 1990) p. 240. 

The Raman spectrum shown in Figure 18-10 illustrates how a fiber-optic probe can be used to monitor chemical processes. In this ca.se a fiber-optic probe w-as used to monitor the hanging drop crystalli/ation of aprotinin (a serine protease inhibitor) and (Nll4)-S04 in aqueous solution. Kaman bands were attributed lo both the protein and the salt. By using chemomet-ric techniques, changes in the spectrum during crystallization were correlated with depletion of both the protein and the salt. The authors were able to determine accurately supersaturation of aprotinin using this technique. 

Single photon emission computed tomography (SPECT) was used by Kantzas et to monitor chemical processes. In SPECT imaging, a... 

Optical fibers are increasingly used for remote Raman probing, for example to monitor chemical processes in-line or inside a reactor. The laser beam is guided to the probe head by an excitation fiber, and the Raman signal is returned to the detector by a collection fiber. Probe heads usually work in the 180° arrangement. There are two different types of fiber probes for common use in Raman spectroscopy the concentric unfiltered fiber bundle and the filtered probe. The fiber bun-... 

The methods most widely in use now for understanding and monitoring chemical processes that affect our environment and the atmosphere are those of TDLAS, and remote absorption/Raman spectroscopy based on lidar (absoiption-Hdar/ Raman-lidar). Application examples of these two techniques are outlined in Sections 28.1—28.3 and Sections 28.4-28.6 respectively. The chapter will conclude with the description of some less-developed techniques, which, however, provide information not easily obtained, or not accessible at all. All of them are based on ionization in one form or other, and include laser-induced breakdown spectroscopy (LIBS), matrix-assisted laser desorption ionization (MALDl) and aerosol TOFMS (ATOFMS). Examples of these are provided in Section 28.7. 

Optical methods have been used to monitor chemical processes for a long time. In a photochemical reaction, a continuous flow of light is used to drive the reaction. In rare cases, light may be produced by a chemical reaction (chemiluminescence). A photochemical reaction may be initiated on the molecular level by a short pulse. By studying the absorption characteristics shortly after absorption, details on what happens on the molecular level can be obtained. We will first go through photokinetics, which is different from ordinary kinetics only in the appearance of radiation as a reactant. 

A clever and very powerful variant of this has been pioneered by Tsien and co-workers. A wide array of fluorescence imaging techniques are now available that allow one to directly monitor chemical processes in cells in real time. A good example of this is the collec-... 

F Hunt, H Ohde, CM Wai. a high pressure fiber-optic reactor with CCD array UV- fis spectrometer for monitoring chemical processes in supercritical fluids. Rev Sci Instrum 70(12) 4661 667, 1999. 

The requirement for information on chemical composition extends into most sectors of industry. The chemical measurement analytical market is extremely diverse. Chemical analysis is a key process in drug discovery, in assessing the quality of feedstock materials and end products, in monitoring chemical processes, in demonstrating regulatory compliance, in disease diagnostics, and in environmental conservation. 

Immunosensors promise to become principal players ia chemical, diagnostic, and environmental analyses by the latter 1990s. Given the practical limits of immunosensors (low ppb or ng/mL to mid-pptr or pg/mL) and their portabiUty, the primary appHcation is expected to be as rapid screening devices ia noncentralized clinical laboratories, ia iatensive care faciUties, and as bedside monitors, ia physicians offices, and ia environmental and iadustrial settings (49—52). Industrial appHcations for immunosensors will also include use as the basis for automated on-line or flow-injection analysis systems to analyze and control pharmaceutical, food, and chemical processing lines (53). Immunosensors are not expected to replace laboratory-based immunoassays, but to open up new appHcations for immunoassay-based technology. 

Fiber-Optic Probes. Fiber-optic probes provide remote sampling capabilities to Raman instmmentation, are stable, and give reproducible signals. Their historical niche has been in environmental monitoring. More recently these probes have been used in chemical process control and related areas such as incoming materials inspection. 

Analytical methods aie utilised by all branches of the chemical iadustry. Sometimes the goal is the quaUtative deterniiaation of elemental and molecular constituents of a selected specimen of matter othertimes the goal is the quantitative measurement of the fractional distribution of those constituents and sometimes it is to monitor a process stream or a static system. Information concerning the various iadividual analytical methods may be found ia separate articles dispersed alphabetically throughout the Eniyclopedia. The articles ate iatroductions to topics each of which is the subject of numerous books and other pubhcations. 

Precipitated Calcium Carbonate. Precipitated calcium carbonate can be produced by several methods but only the carbonation process is commercially used in the United States. Limestone is calcined in a kiln to obtain carbon dioxide and quicklime. The quicklime is mixed with water to produce a milk-of-lime. Dry hydrated lime can also be used as a feedstock. Carbon dioxide gas is bubbled through the milk-of-lime in a reactor known as a carbonator. Gassing continues until the calcium hydroxide has been converted to the carbonate. The end point can be monitored chemically or by pH measurements. Reaction conditions determine the type of crystal, the size of particles, and the size distribution produced. 

The second area, the implementation of a modem process monitoring and control system, is the most dramatic current appHcation of CAD/CAM technology to the chemical process industry. The state of the art is the use of computer graphics to display the process flow diagram for sections of the process, current operating conditions, and controUer-set points. The process operator can interact directly with the control algorithms through the... 

Fire and uncontroUed polymerization are a concern in the handling of chloroprene monomer. The refined monomer is ordinarily stored refrigerated under nitrogen and inhibited. This is supported by routine monitoring for polymer formation and vessel temperature. Tanks and polymerization vessels are equipped for emergency inhibitor addition. Formalized process hazard studies, which look beyond the plant fence to potential for community involvement, are routine for most chemical processes. 

The safe operation of a chemical process requires continuous monitoring of the operation to stabilize the system, prevent deviations, and optimize system performance. This can be accomplished through the use of instrumentation/control systems, and through human intervention. The human element is discussed in Chapter 6. Proper operation requires a close interaction between the operators and the instrumentation/control system. To a large extent, batch operations have simple control systems and are frequently operated in the manual mode. The instrumentation system is the main source of information about the state of the process. Some of the typical functions of the instrumentation/control system are... 

Laboratoiy procedures may need to be evaluated against the sampling techniques and materials involved in the toll. There may be new laboratoiy chemicals and hazards to be considered. This work may have been identified in the evaluation of special analytical techniques required for the process. A good practice is to ensure that the lab technicians have the necessaiy guidance and types of equipment on hand to monitor the process and waste streams accurately and safely. 

As mentioned earlier, toxic releases may consist of continuous releases or instantaneous emissions. Continuous releases usually involve low levels of to.xic emissions, wiiich are regularly monitored and/or controlled. Such releases include conlinuous slack emissions and open or aerated chemical processes in wliich certain volatile compounds are allowed to be stripped off into the atmosphere tliroiigh aeration or agitation. Mathematical models for these releases to tlie enviroiuncnt are covered in detail in Part III. 

Due to the nature of the SMB process, in-process samples of the unwanted enantiomer and the enantiopure drug substance can be sampled at controlled times during the continuous process to assess the enantiomeric and chemical purity. One can monitor the process without system shutdown by diverting either the extract or the raffinate streams. Further monitoring of the receiving tanks can also be accomplished. 

Neufeld, P. and Queenan, E. D., Frequency Dependence of Polarisation Resistance Measured with Square Wave Alternating Potential , Br. Corros. J., 5, 72-75, March (1970) Fontana, M. G., Corrosion Engineering, 3rd edn., McGraw-Hill, pp 194-8 (1986) Dawson, J. L., Callow, L. M., Hlady, K. and Richardson, J. A., Corrosion Rate Determination By Electrochemical Impedance Measurement , Conf. On-Line Surveillance and Monitoring of Process Plant, London, Society of Chemical Industry (1977)... 

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