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May
19

Monitoring Process Performance Online

Process efficiency measurements involve the measurement of process flows and utility or raw material flows. The energy efficiency depends upon the utility temperatures and heat transfer coefficient. The raw material efficiency depends upon feed concentrations and the desirable reaction rate to product and the undesirable reaction rate to a byproduct. For simply changing a stream’s temperature or pressure, the measurement of exit stream flow is sufficient. For changes in composition, the composition of exit stream composition is needed besides flow. The efficiency is proportional to the ratio of the exit stream flow to utility or raw material flow (e.g. kg product per kg fuel gas or kg product per kg reactant). For synchronization, the utility or raw material flow should have dynamic compensation by means of an adjustable deadtime and filter time. For heat exchangers, inline blenders, dryers, kilns, and plug flow reactors just a deadtime equal to the residence time is sufficient. For columns, a deadtime and filter time equal to 15% and 85% of the residence time, respectively, is a good first approximation to the interactive time constants in series corresponding to equivalent trays. For well mixed vessels with negligible injection and conversion dynamics, a simple filter time constant equal to the vessel residence time can be used. Change of state dynamics can be considered negligible for condensation, evaporation, and drying. The conversion dynamics in crystallization and reaction can be significant. The dynamic compensation can be adjusted manually by observing the response. If the raw material efficiency first decreases below its final value for an increase in raw material feed rate (inverse response), the deadtime or filter time should be increased. A notably sharp inverse response is indicative of insufficient deadtime.

For monitoring the performance of completed batches, the totals just need to be ratioed at the batch end point eliminating the need for time synchronization and dynamic compensation. For monitoring the progress of fed-batch operations some of the considerations for continuous operations apply.

In sustainable manufacturing energy efficiency is often referred to as energy productivity. Raw material efficiency is often cited as yield. Wireless measurements enable the prototyping and evaluation of these and other online process metrics.