Issue link: https://www.e-digitaleditions.com/i/1000772
Tablets & Capsules July 2018 29 Residence time distribution When material enters the blender during steady-state operation, most particles remain in the blender for a time close to a certain mean value, defined as the mean residence time. You can calculate the mean residence The most common type of continuous blender is the convective tubular blender, as shown in Figure 1. The convective tubular blender consists of a stationary horizontal mixing cylinder with a material inlet at one end and a material outlet at the other end. An overflow weir is typically mounted inside the cylinder before the material outlet to control the holdup of material within the blender. The cylinder contains a rotating shaft with impellers (such as blades, paddles, or ribbons) that mix the material by convection (moving the particles within the cylinder relative to each other). In operation, unmixed ingredients enter the blender's inlet and are lifted, tumbled, sheared, and conveyed by the rotating impellers to the blender's outlet, where the material exits the blender as a homogeneous stream. During steady-state operation, the mass flowrate into the blender equals the mass flowrate out of the blender, and the mass of material inside the blender (called the mass holdup) remains constant. Continuous mixing modes Material mixing within a tubular blender can be divided into two modes: cross-sectional (or radial) mixing and axial mixing. In Figure 1, for example, as two ingredients (Ingredient A and Ingredient B) are fed into the blender, we can consider them to be completely unmixed at the blender inlet, as shown in the cross section on the left in the figure. As the blender's impellers rotate, they lift and tumble the ingredients, mixing the material in the cross-sectional plane. At steady state, cross-sectional mixing can be largely time independent, which means that each cross section in the blender exhibits the same arrangement of ingredients over time. As the figure shows, however, subsequent cross sections exhibit different ingredient arrangements. This is because when the impellers lift and tumble the material in the cross-sectional plane, some particles are pushed forward (and to a lesser extent, backward) in the cylinder, resulting in axial mixing as the material is transported and mixed along the cylinder's axis. Figure 1 Cross-sectional mixing in a continuous tubular blender Feeder A Feeder B Material Inlet Impellers Material outlet Rotating shaft Weir Blended material Ingredient A Ingredient B Figure 2 a. Small (0.25-gram) pulse sufficiently dampened by blender at 12-second RTD standard deviation Ingredient concentration profiles in a tableting process train b. Large (1-gram) pulse not sufficiently dampened by blender at 12-second RTD standard deviation c. Large (1-gram) pulse sufficiently dampened by blender at 24.9-second RTD standard deviation 200 400 600 800 1,000 1,200 1,400 200 400 600 800 1,000 1,200 1,400 200 400 600 800 1,000 1,200 1,400 0.090 0.085 0.080 0.075 0.070 0.065 0.060 0.090 0.085 0.080 0.075 0.070 0.065 0.060 0.090 0.085 0.080 0.075 0.070 0.065 0.060 Concentration (percentage by mass) Concentration (percentage by mass) Concentration (percentage by mass) Time (seconds) Time (seconds) Time (seconds) Feed stream Mill exit Blender exit Tablets 0.25-gram pulse Blender MRT: 41.6 seconds RTD standard deviation: 12 seconds 1-gram pulse Blender MRT: 41.6 seconds RTD standard deviation: 12 seconds 1-gram pulse Blender MRT: 71.7 seconds RTD standard deviation: 24.9 seconds