Process control, automatic measuring elements

Automatization of all stages of the analytical process is a trend that can be discerned in the development of modern analytical methods for chemical manufacture, to various extents depending on reliability and cost-benefit considerations. Among the elements of reliability one counts conformity of the accuracy and precision of the method to the specifications of the manufacturing process, stability of the analytical system and closeness to real-time analysis. The latter is a requirement for feedback into automatic process-control systems. Since the investment in equipment for automatic online analysis may be high, this is frequently replaced by monitoring a property that is easy and inexpensive to measure and correlating that property with the analyte of interest. Such compromise is usually accompanied by a collection of samples that are sent to the analytical laboratory for determination, possibly at a lower cost. 

Figure 13.15 shows the operational scheme of this automatic tltrator. The heart of the unit Is an INTEL 8080 microprocessor mounted on the central processing unit (CPU) board. The rotary reaction cell assembly can accommodate up to three different sensors for multiple measurements on the same processed sample. Each stepper burette board controls up to two burette dispensing assemblies. Function boards such as the colorimeter board, air burette board, E/I output board and RS-232 printer Interface boards are available optionally. The optional D/A and E/I board is used for closed-loop applications where the tltrator controls the final element such as a control valve. The RS-232 printer Interface board Is useful for troubleshooting the equipment and editing user-defined programs. The Instrument accuracy, repeatability and response time vary widely and depend on the particular type of measurement concerned. The system requires a.c. power, a 75-psl air supply and a dilution water supply for proper operation. The air flow-rate required is of about 50 cm3/mln... 

Fluorescence spectrometers are widely used in the metal industry. Frequently, parallel spectrometers are employed. Such an instrument actually consists of a number of crystal spectrometers, each set for a particular emission line. The spectrometers are arranged around the sample, which is irradiated by an X-ray tube. One of the spectrometers is set for a standard sample that is contained in the sample holder. In this way the intensity of the X-ray tube can be monitored. Frequently, a measurement is terminated when a preset number of comits for the reference sample has been obtained. The corresponding number of counts from the other detectors can then be directly used for a pai allel assessment of the elemental composition of the sample. With a sequential spectrometer, a number of selected elements are measured sequentially by turning the crystal and the detector to preset positions. With computer steering the measurement process is automatic. This type of instrument is well suited for varying types of analysis, whereas parallel spectrometers are more suited to continuous control operation of, for example, a steel mill in near-real time. 

Fig. 9-20, Classification of automatic controllers (a) self-operated controller, using energy only from controlled medium through primary element (6) relay-operated controller with self-operated measuring means and relay-operated controlling means (c) relay-operated controller. with relay-operated measuring means and relay-operated controlling means. [G. A. Hall, Jr. by permission from Process Instruments and Controls Handbook, Douglas M. Considine (ed.), McQraw-Hill Book Company, Inc., New York, 1957.]...

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