(444e) Hopper Refill of Loss-in-Weight Feeding Equipment
In pharmaceutical powder handling, as well as other powder processing industries, the ability to consistently feed a powder continuously can be one of the most important parts of the overall processing. If a powder feeder cannot sufficiently feed a powder at a desired rate, then it will pass composition and flowrate variability issues to subsequent unit operations, such as mixing (Weinkotter 1995). As feeding is typically the first unit operation, this means that these variable flowrates are passed through to the entire process. Loss-in-Weight feeders have improved the ability to control feedrate and minimize flow variability due to density changes associated with the emptying of the feeding hopper (Hopkins 2006). To the various LIW feeders, there have been many additions and improvements to impove/reduce steady-state flow varibility, such as using devices on the discharge of the feeder (Kehlenbeck and Sommer 2003) or using vibratory hopper agitation (Tardos et al 1996). Under normal operation for a LIW feeder, gravimetric mode, the controller compares the observed gravimetric feedrate to the user entered setpoint. Depending on the deviation from setpoint, the controller may send a new signal to the feeder to increase or decrease the speed. This is beneficial to the performance of the feeder, as it allows the feeder to adjust for many known and unknown feedrate inconsistencies of running the feeder volumetrically or at constant speed. To keep the process continuous and uninterrupted required occasional refilling of the feeder hopper, but unfortunately during this refilling it is not possible to gravimetrically control the feed rate as it is not possible to monitor the loss-in-weight of the feed hopper. Because of this, feeders run in an alternative mode where the feeder is blind and non-reactive in controlling the actual feedrate which can cause deviations from setpoint. During refilling, it has become common practice to refill the feeder when it reaches the lowest level that the feeder manufacturer would recommend for operation, which is quite commonly around 20% and to refill it by dropping material into the hopper until it reaches 80%. All during this time and a short settling time (typically about 10-15 seconds), the feeder does not monitor and control the gravimetric feedrate, so deviation from setpoint is quite common due to the density of the material in the hopper being increased. Alternatively, feeders can be refilled more often with less material, but this leads to a longer ?blind? time as although each refill is shorter each refill also requires an additional settling time. This is often overlooked as an option, as normally a feeder is not monitored by an external ?catch? scale that will detect deviations, and this ?blind? time allows much material to pass without any control or knowledge of its actual feedrate. As this refill dilemma is a known issue to feeder manufacturers, they have come up with many methods to attempt to address it. Refill modes that have a variable screw speed during refill (Wilson and Loe 1985), redundant refill (Aalto and Bjorklund 2002) and/or feeder systems (Wilson and Bullivant 1986) that try to bypass the issues. All these methods or systems may work to reduce or eliminate the issue, but they may not completely eliminate or involve purchasing extra equipment. With a little testing and observing refills, it is possible to optimize refill scheduling. After doing which, this can be paired with other methods, to further improve feeder performance during refilling. This research focuses on observing the effects and issues during refill as well as the developing of a method for optimizing refill scheduling. By using a gain-in-weight ?catch? scale, which collects and weighs material as it is fed, deviations from the feed setpoint can be monitored during hopper refill. It has been observed that size of refill has a huge impact on feeder consistency and performance. Using gravimetric feedrate calculated from ?catch? scale data during refills at a few different hopper levels, it is possible to optimize the scheduling of loss-in-weight feeder refilling.
References Aalto, P. and Bjorklund, J. 2002. Loss-in-weight feeder control. U.S. Patent 6,446,836, filed May 19, 1999, and issued September 10, 2002. Hopkins M. 2006. LOSS in weight feeder systems. MEASUREMENT& CONTROL 39(8):237-240}. Kehlenbeck V and Sommer K. 2003. Possibilities to improve the short-term dosing constancy of volumetric feeders. Powder Technol 138(1):51-6. Tardos, G. and Lu, Q. 1996. Precision dosing of powders by vibratory and screw feeders: an experimental study. Advanced Powder Technology 7 (1): 51-58 Weinkotter, R. 1995. Continous Mixing of Fine Particles. Particle and Particle Systems Characterization 12(1): 46-53. Wilson, D. and Bullivant, K. 1986. Loss-in-weight gravimetric feeder. U.S. Patent 4,579,252, filed May 5, 1983, and issued April 1, 1986. Wilson, D. and Loe, J. 1985. Apparatus and method for improving the accuracy of a loss-in-weight feeding sytem. U.S. Patent 4,524,886, filed January 28, 1982, and issued June 25, 1985.