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Knowing how formulation moves through a tablet process is invaluable in optimizing the process for the greatest tablet quality and production. Residence time refers to the total time a particle or batch of material spends in a particular process or piece of equipment.  In tablet compression, this refers to the time formulation spends in a hopper or a feeder. A good place to start is looking at the flow rate of the formulation since residence time is proportional to the flow rate at steady-state operation.  To calculate the formulation’s flow rate through a tablet press, you will need: bulk density, tablet weight, press output (tablets per minute), and yield percentage (%).

Mass flow rate = (Press Output * Tablet weight) / (Product Yield)
Bulk Density = 0.55 g/mm^3 Press Output = 1400 tablets per minute
Tablet Weight = 500mg Product Yield = 98%
Mass flow rate = (1400 tablets per minute * 0.5g/tablet) / (0.98) = 714.3 g/ minute
Residence time for a particle on average within a hopper is equal to the hopper’s capacity divided by the volumetric flow rate.  Feeders can be more complex to calculate since recirculation extends residence time depending on the flow rate of formulation re-entering the feeder.

Consistent residence time ensures uniform tablet compaction and mechanical strength.  Achieving consistency can mean 2 things.  First, it is important to work toward the prevention of fluctuations in overall powder flow (flooding and starving).  Second, individual particles must move as similarly as possible so that fewer particles are subjected to extended residence times.

Minimizing residence time often is beneficial in reducing segregation and overblending of lubricant.  For hoppers, ensuring mass flow or minimizing the effects of rat-holing is essential.  For feeders, minimizing the detrimental effects of residence time relies on selecting the right paddles, minimizing your feeder speed, and minimizing the amount of recirculation.  Contact Natoli Engineering for assistance in optimizing press configuration, setup, and operation. Understanding the powder flow for your product is fundamental to achieving the lowest residence times.  Powder flow characterization is defined per USP<1174> & USP<616> and includes bulk density, tapped density, compressibility index, Hausner ratio, Angle of Repose,.  Advanced testing with an FT4 Powder Rheometer can evaluate the effects of consolidation, aeration, flow rate, moisture, electrostatic charge, and storage time.

Please contact Natoli Scientific for these services. 

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