Issue link: https://www.e-digitaleditions.com/i/1163217
Tablets & Capsules September 2019 37 and fixing the problem begins with a new understanding of the blending unit operation. Total volume versus working volume A blender's total volume (or capacity) is the total amount of material the vessel can hold, typically expressed in liters. A blender's working volume is the amount of material the vessel is capable of mixing at once, also typically expressed in liters and sometimes given as a narrow range. When discussing working volume, it can be helpful to use the term "occupancy," which more accurately describes what we are trying to achieve. Every blender class has a target fill level—an ideal occupancy level— regardless of the material's weight and bulk density. For example, 5 kilograms of popcorn will occupy a different volume of space than 5 kilograms of wet sand in a blender, so the ideal occupancy level of popcorn for a given blender will have a different weight than the ideal occupancy level of wet sand for that same blender. Successful scale-up depends in part on understanding this concept of occupancy. Blender classes such as twin shell, double cone, cube, and bin, each have different tar- get fill levels. Even slight changes in a blender's fill level or rotation rate during operation can induce changes in the material's mixing pattern that may result in segrega- tion. The further away the blender's fill level is from the target, the more trouble you're likely to have. Regardless of a blender's size, when particle velocities in specific regions of the vessel are below a critical value, one mixing pattern appears, and when the particle veloci- Class Mixing mechanism Suitable materials Air currents Tumble (including drum, twin-shell, double-cone, IBC, octagonal) Diffusion and axial blending • Ratios depend on the manufacturer and model • Distributes materials along an ever- changing angle of repose surface • Materials that are not significantly cohesive or lumpy and that have enough cohesion to prevent sifting • Or use an intensifier bar Low Ribbon, Sigma Convection • More axial than diffusion • Inside and outside ribbons convey materials in opposite directions, forcing them to intermix • Sigma has kneading action • Moderately cohesive materials of similar particle densities without fluidizing tendencies Low to moderate High-shear Shear • Axial with little diffusion • Materials are thrown around, providing great mechanical particle interaction and shear • Large flow velocity and rapid mixing • Non-friable materials • No cohesive materials • No lubricant blending High Octagonal Diffusion • Distributes materials along an ever- changing angle of repose surface • No cohesive materials Low Table 1 Blenders commonly used in pharmaceutical and nutraceutical manufacturing Acid Resistant Vegetable Capsules ) u . u, ' . AN EXCELLENT CHOICE FOR DELIVERY OF ACID-SENSITIVE ACTIVE INGREDIENTS . <> . BioCaps Enterprises, Inc. www.biocaps.net • info@biocaps.net US Patent 9452141