Crystallization process instrumentation and control

Process equipment function changes with different steps in process sequence (e.g., same vessel used as feed tank, reactor, crystallizer pump used to pump in/out). Instrumentation and controls not kept in phase with the current process step (e.g., control set points, interlocks etc.). 

It should be noted that development of the crystallization processes in most of the examples presented in later chapters occurred before the availability of many of the online measurement and control methods that are now available. Utilization of these methods would have aided both the process development and the manufacturing operations. The literature that describes these methods—for example, feedback control of supersaturation for crystallization (Nonoyama et al. 2006 Zhou et al. 2006)—is now extensive, and the instrumentation to carry out the measurements and control continues to be improved. 

The instrument has been evaluated by Luster, Whitman, and Fauth (Ref 20). They selected atomized Al, AP and NGu as materials for study that would be representative of proplnt ingredients. They found that only 2000 particles could be counted in 2 hours, a time arbitrarily chosen as feasible for control work. This number is not considered sufficient, as 18,000 particles are required for a 95% confidence level. Statistical analysis of results obtained for AP was impossible because of discrepancies In the data resulting from crystal growth and particle agglomeration. The sample of NGu could not be handled by the instrument because it consisted of a mixt of needles and chunky particles. They concluded that for dimensionally stable materials such as Al or carborundum, excellent agreement was found with other methods such as the Micromerograph or visual microscopic count. But because of the properties peculiar to AP and NGu, the Flying Spot Particle Resolver was not believed suitable for process control of these materials... 

One problem associated with implementing this technology is the need to build an interface between the process and the measuring instrument. This often requires a dilution step that may alter the size of the particles. In the case of crystallizer control, for example, it may be necessary to remove two streams from the crystallizer and filter one so that the mother liquor can be used as the diluent. 

Although crystal structure analysis was once a very time consuming and very expensive process, this has not been the case for a number of years. Structural results are usually available within two weeks of when a crystal is placed on the diffractometer instrument. This change has been due to a number of factors. Computer-controlled diffractometers have made data collection more accurate and much easier. The process of obtaining a trial structure has been much facilitated by the use of computerized direct methods and by computer graphics. 

The instrumentation for EM uses the same type of X-ray spectrometers discussed in detail in Chapter 8, with an electron beam as the source and a UHV system that includes the sample compartment. An ED X-ray spectrometer allows the simultaneous collection and display of the X-ray spectrum of all elements from boron to uranium. The ED spectrometer is used for rapid qualitative survey scans of sample surfaces. The wavelength dispersive spectrometer has much better resolution and is used for quantitative analysis of elements. The WD spectrometer is usually equipped with several diffracting crystals to optimize resolution and to cover the entire spectral range. The electron beam, sample stage, spectrometer, data collection, and processing are all under computer control. 

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