Spray modeling and measurements for pharmaceutical tablet coating applications
Abstract
A method of spray modeling and measurements for pharmaceutical pan tablet coating applications is proposed. It consists of (1) spatially resolved spray characterization and (2) computational predictions of subsequent spray evolution using CFD. Both (1) and (2) are in line with PAT guidelines that emphasize process understanding and analytical process design. (1) utilizes a dual-mode Phase Doppler Anemometry (PDA) system. Data are obtained at multiple points covering the entire spray cross-sectional area. Measurements are validated by ensuring that PDA-measured mass flowrate matches measured mass flowrate and by comparing PDA-calculated absorptances to the optical patternator measured ones. Experimental data show that drop size, velocity, and volume flux vary spatially throughout a spray cross-sectional area. Spatial variation in size and velocity do not significantly change with variations in spraying parameters. Drop size decreases with an increase in atomizing/shaping air pressure, a decrease in liquid supply rate, and a decrease in formulation concentration (viscosity). Drop velocities increase with an increase in atomizing air pressure, a decrease in liquid supply rate, a decrease in formulation concentration, and a decrease in gun-to-target distance. In contrast to size and velocity distribution, spatial variation in volume flux is sensitive to changes in operating conditions. For elliptical sprays, an increase in shaping air pressure and an increase in liquid viscosity lead to the development of a dumbbell-shaped pattern. Increasing atomizing air pressure narrows the spray whereas increasing liquid supply rate produces a more elliptical spray. On the contrary, volume flux distributions for round sprays are significantly less sensitive to changes in either operating conditions or liquid viscosity. (2) utilizes a commercially-available CFD package to predict how sprays evolve under drum coater conditions. Results show that drum rotation induces gas swirling that transports small drops away from the spray zone. It also lengthens the spray. Drying air promotes spray evaporation, therefore reducing local drop number density. Furthermore, since drying air enters from the top of the drum, it pushes the spray downward, leading to spray patterns that resemble the letter ‘C’. Drying air flow also helps reduce the effects of drum rotation on spray evolution by providing droplets with additional axial momentum. The additional momentum in turn increases droplet velocities. Computed drop sizes and drop velocities generally agree well with measured data, but ∼46% difference is observed when comparing measured and computed volume flux magnitude. Measured volume flux data, however, are likely less accurate due to the systematic limitations of the standard (Fiber) PDA system. Results from (1) provide a comprehensive understanding on how changes in atomizer operational parameters influence spray characteristics and patterns, and on how drum coater parameters (drying air flow and drum rotational speed) influence the spray evolution process. Results from (2) provide a means to model spray evolution in the drum coater and to reinforce process understanding, particularly for the influence of drum rotation on drying air flow, and for the influence of process variables on spray pattern.
Degree
Ph.D.
Advisors
Sojka, Purdue University.
Subject Area
Chemical engineering|Mechanical engineering|Pharmacy sciences
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