Unit operations, control dryers

Hydraulic pressure nozzles cannot combine the capability for fine atomization with high feed capacity in one single unit. Many spray dryer applications, where pressure nozzles are applied, therefore require multinozzle systems with the consequence that start-up, operational control, and shutdown procedures become more complicated. 

Manual versus Automatic Control Dryers can be controlled either manually or automatically. Generally lab-, pilot-, and small-scale production units are controlled manually. These operations are usualy batch systems, and manual operation provides lower cost and greater flexibility. The preferred mode for large-scale, continuous dryers is automatic. 

It must also be noted that multiple PAT analyzers can be used on a single unit operation to understand the process. For example, Aaltonen et used in-line Raman spectroscopy and in-line NIRS to monitor the dehydration of MT in a lab-scale fluid bed dryer. A PLS model was generated to quantify the solid-state transformation using Raman spectra while water removal was monitored using NIRS. This proved to be an effective combination because physical and chemical information were monitored and extracted by way of NIRS and Raman spectroscopy, respectively. Such combinations of process analyzers offer enhanced capabilities to improve process understanding and control. 

For best operation, the feed rate to rotating equipment should be closely controlled and uniform in quantity ana quality. Because sohds temperatures are difficult to measure and changes slowly detected, most rotating-equipment operations are controlled by indirect means. Inlet and exit gas temperatures are measured and controlled on direct-heat units such as direct dryers and kilns, steam temperature and pressure and exit-gas temperature and humidity are controlled on steam-tube units, and direct shell temperature measurements are taken on indirect calciners. Product temperature measurements are taken for secondaiy control purposes only in most instances. 

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