Issue link: https://www.e-digitaleditions.com/i/1000772
30 July 2018 Tablets & Capsules pure mixing applications, plug flow tends to be undesirable because accurately feeding bulk solid materials can be challenging. Feeders often directly precede blenders in the process train, and all feeders introduce a certain degree of variability. Feeders must also be periodically refilled, which can introduce sizable perturbations (also called noise) into the ingredient blend [5]. This noise will pass unfiltered through a blender operating in complete plug flow, which can affect the quality of the final product. For example, Figure 2a shows the concentration profile for one ingredient in a blend over time, after each unit operation in a tableting process train. The process train includes the ingredient feeders, a mill, a blender, and a tablet press, in that order. The ingredient concentration in the feed stream is indicated by the dark blue line, and the application's maximum acceptable ingredient concentration is indicated by the horizontal purple line. The green line indicates a perturbation (a 0.25-gram pulse) in the ingredient concentration introduced into the feed stream that passed unfiltered through the mill. As indicated by the red line, the mixing that occurred in the blender dampened the effects of the perturbation enough that the ingredient concentration in the tablets (indicated by the turquoise line) remained below the maximum acceptable value. Note that this example's RTD standard deviation value (a measure of the blender's noise-filtering ability) is 12 seconds. If a 1-gram pulse (instead of a 0.25-gram pulse) of the same ingredient is introduced into the feed stream, as shown in Figure 2b, a blender operating at the same RTD and standard deviation value won't be able to dampen the effects of the perturbation enough to keep the ingredient concentration in the tablets below the maximum acceptable value. However, if the RTD is widened to a standard deviation value of 24.9 seconds, as shown in Figure 2c, the blender can successfully dampen the larger ingredient pulse. Widening the RTD means that the blender is operating closer to continuous stirred tank flow, which allows the blender to filter the noise in the ingredient feed and maintain the quality of the final product. time by dividing the mass holdup by the mass flowrate. However, some particles experience faster convection in the forward axial direction, while other particles experience an unusual amount of backward pushing or occupy a "dead" volume within the blender for some time. This causes some particles to exit the blender very quickly, while other particles spend an unusually long time in the cylinder. As a result, a representative group of particles passing through a blender operating at steady state will have a range of residence times, called a residence time distribution (RTD). There are two idealizations of residence time behavior: plug flow and continuous stirred-tank flow. In plug flow, all particles move at the same speed along the cylinder's axis, so particles entering the blender together exit the blender together and only cross-sectional mixing occurs. Continuous stirred-tank flow, on the other hand, assumes that particles entering the blender inlet are instantly and completely mixed into the rest of the material in the blender, so some particles exit instantaneously, while other particles take a very long time to exit. In reality, all blenders operate somewhere between these two idealized states. As will be discussed shortly, it's possible to tune a blender's RTD by changing certain operating and design parameters. The question is: what is a desirable RTD? For Figure 3 Blender inlet and outlet pro le of a tracer material Tracer pulse at blender inlet Tracer measured at blender outlet Monitoring outlet Tracer concentration Time Achieved steady-state operation •Metolose SR •Hypromellose Phthalate NF/EP/JP •Hypromellose Acetate Succinate NF/JP •Low-substituted HPC NF/EP/JP •Directly Compressible Chewing Gum