Spurred by the pharmaceutical industry's demand for continuous, low-rate powder feeders, where an hour's worth of material might be as small as a teaspoon of sugar and accuracy is measured in milligrams, the development of precision microfeeding technology today opens up new avenues of operation and efficiency improvements for processors in a variety of industries. To realise its full promise, however, microfeeding requires that especially rigorous attention is paid to every aspect of feeder design and application.
Accurately and reliably controlling the flow of material at normal process rates can be challenging on its own. Add the need to control the flow of some minor ingredients in the process in micro-regions as low as 20 g/h. Further compound the problem by requiring a level of precision that permits only a scant few percent sample-to-sample variation. This is the challenge of microfeeding. The development of microfeeding technology emerged primarily as a result of the recent decade's shift towards Process Automation Technology (PAT) as sanctioned by the FDA. This initiative has hastened the application of continuous processing techniques in the pharmaceutical industry. Moving to a continuous process necessitates the elimination of manual feeding of minor ingredients and creates a real need for microfeeding technology.
Unlike continuous, higher-rate feeding, microfeeding tests the limits of measurement and control and requires scrupulous attention be paid to every aspect of design and execution. As will be seen throughout this discussion, a single reality governs the challenge of continuous microfeeding: in microfeeding, every detail matters. For the purposes described here, microfeeding can be defined as controlling the flow of a powdered or other small particlesize material at a range of feed rates from 2000 g/h down to 20 g/h or lower. At 20 g/h, this translates to 0.33 g/min or just 5.5 mg/s. Expressing this limit in terms of a second-to-second flow rate is intended to highlight the need to first consider the question of performance timescale, the possible or practical to obtain physical secondto- second samples of a discharge stream, at some point – as sample duration and weight diminish –, the ability to confidently resolve these sample weights becomes compromised, depriving the feeder control system of the weight measurement integrity it needs to make useful control corrections.