Mixing and Homonization

Contents of this session

• Introduction to mixing

• Importance of mixing

• Applications of mixing

• Types of mixtures

• Mechanisms of mixing

• Factors affecting mixing

• Criteria for solid mixing

• Classification of equipments for solid mixing

• Ribbon Blender

• Sigma blade mixer

• Double cone and V cone blenders

• Planetary mixer

• Mixing of liquids with mechanisms

• Liquid mixing devices

• Flow pattern in impeller mixers

• Propellers

• Turbines

• Vortex formation and remedies

• Homogenization

• Factors influencing selection of homogenizers

• Silverson emulsifier

• Colloid mill

• Ultrasonic emulsifier

Learning objectives

At the end of this PDF lecture student will be able to:

• Define mixing with examples

• Enumerate the importance of mixing

• State the applications of mixing

• Describe different types of mixtures

• List the different mechanisms of mixing

• List out the criteria for solid mixing

• Classify equipment’s for solid mixing

• Describe ribbon blenders and sigma blade blenders with applications

• Explain tumblers with and without mixing blades

• Describe the working of planetary mixer

• Enumerate various mechanisms of liquid mixing

• State the differences between various liquid mixing devices

• Explain the flow patterns of liquid mixing

• Discuss the pharmaceutical role of propellers and turbines

• Define homogenization

• State the factors influencing selection of emulsifiers

• Describe Silverson emulsifier

• Explain the working of colloid mill and ultrasonic emulsifier

Summary

• Mixing is a process resulting in a randomization of dissimilar particles within a system

• Two terms used interchangeably in the pharmaceutical industry are mixing and blending

• Mixing is an essential to make simple physical mixture, induce physical change, and affect dispersion and promotion of reaction

• Mixing is highly necessary in the preparation of several dosage forms like tablet, capsules, dry syrup and insufflations

• Three types of mixtures are positive, negative and neutral mixtures

• Various factors influencing mixing are particle size, particle shape, particle attraction, material density and proportion of materials

• Important selection criteria for solid mixing are sufficient shearing force, prevention of damage to particles, absence of centrifugal effect and three directional movement of particles

• Solid mixing devices are classified on the basis of flow properties and scale of mixing

• Ribbon mixer works on shear and Low speeds are used for dry blending and fast for wet granulation

• Sigma blade mixer most commonly used to handle semi-solids of plastic consistency and also in the wet granulation process, pill masses and ointments

• Tumblers with Baffles are useful for both wet and dry mixing and wide range of Shearing force can be applied with agitator for intimate mixing of very fine as well as coarse powders

• Planetary mixer utilizes the shear between moving blade and stationary wall and low speeds are used for dry blending and fast for wet granulation

• Different mechanisms of mixing in liquids are bulk transport, turbulent mixing, laminar mixing and molecular diffusion

• Liquid mixing devices based on pitch and scale are classified into 3 types are propellers, turbines and paddles

• Three Flow patterns in liquid mixing are tangential (rotational) flow, radial flow and axial or longitudinal flow

• Propellers with blades are used when high mixing capacity is required with viscosity being the main restriction

• Turbines consists of a circular disc to which a number of short blades are attached to achieve greater shearing forces than propellers though the pumping rate is less which is suitable for emulsification and high viscosity solutions

• Vortex is a strong circulatory flow pattern near the impeller shaft reducing mixing intensity by reducing velocity of the impeller relative to the surrounding fluid

• Vortex can be prevented by placing the propeller in any one of the position that can avoid symmetry, using baffled containers, mounting two or more impellers on the same shaft and noncylindrical tanks

• Homogenization is an intensive mixing technique of mutually insoluble phases to obtain a suspension or emulsion

• Pharmaceutical homogenizers are used to blend liquids by reducing the size of particles of one material and dissolves it evenly throughout the other material

• The factors influencing the selection of emulsifiers are quantity of emulsion to be prepared, flow properties of liquids, temperature maintenance and desired rate of cooling

• Intense shearing forces and turbulence are used in Silverson emulsifier for rapid breakdown of the dispersed liquid into smaller globules

• Colloidal mill, a hydraulic shear producing device works on the principle of rotorstator and size reduction is affected due to shearing, when the material is passed between the narrow gap of milling surfaces of rotor and stator

• Ultrasonic vibrations are generated in ultrasonic homogenizer resulting in great forces for emulsification with a mixing capacity of 20 - 500 liters per minute

• Ultrasonic homogenizer is suitable for thermolabile substances since heat is not generated during mixing

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