Pharmaceutical mixing
Main purpose of mixing
⇨ To control heat and mass transfer
⇨ To secure uniformity of composition
⇨ To improve single phase and multiple phase systems.
⇨ To improve dissolution
Classification of mixing
1. Positive mixing
2. Negative mixing
3. Neutral mixing
Types of Liquid mixing
⇨ Turbulent mixing
⇨ Laminar mixing
Types of solid mixing
⇨ Convective mixing
⇨ Shear mixing
⇨ Diffusive mixing
Types of Semi-solid mixers
⇨ Sigma blade mixer
⇨ Planetary mixer
⇨ Mulling mixer
Types solid mixers
⇨ Agitator mixer
⇨ Ribbon mixer/Blender
⇨ Nauta mixer
⇨ Fluidized air mixer
Types of Rapid mixer granulators
⇨ Lodige mixer
⇨ Diosna mixer-granulator
⇨ Gral mixer-granulator
⇨ Continuous mixer
⇨ Barrel type continuous mixer
⇨ Zig-zag continuous mixer
Segregation
Dissimilar powder grains almost invariable promotes segregation.
Overblending is associated with physical degradation of material properties.
Three distinct types of segregation/Demixing
⇨ Radial segregation
⇨ Axial segregation
⇨ Competitive pattern segregation
Other types of segregation
⇨ Percolation segregation ( Due to smaller particle size)
⇨ Trajectory segregation (Due to larger particle size)
⇨ Elutriation segregation/ Fluidization segregation ( Due to Particle sedimentation)
Radial segregation is associated with migration of fine grains towards the center surrounded by larger grains.
Factors affecting segregation
⇨ Particle size and size distribution
⇨ Particle density
⇨ Particle shape
⇨ Particle charge
Segregation problem due to particle size can be retified by
⇨ Selection of particular sized fraction
⇨ Milling
⇨ Granulation of powder mix
Reason for Segregation & Mixing
⇨ For getting desirable size, shape, hardness and density
Classification of Impellers
⇨ Propellers
⇨ Turbines
⇨ Paddles (About 50 rpm speed)
Fick's fiirst law of Diffusion
dm/dt = - DA× dc/dx
dm/dt⇨ The rate of transport of mass
A⇨ Interface of Area
D⇨ Diffusion coefficient
dc/dx⇨ Concentration gradient
Vortexing
A vortex develops at the center of the vessel when liquids are mixed by a centrally mounted vertical-shaft impeller.
When a vortex is formed-
⇨ Air is drawn into the impeller and is dispersed into the liquid, which may lead to foaming, especially if surfactants are present.
⇨ The entrapped air also causes oxidation of the substances in some case and reduces the mixing intensity by reducing velocity of the impeller.
Vortex may be avoided by
⇨ Changing arrangement of the impeller.
⇨ Changing the tank geometry
⇨ Using a push-pull propeller
⇨ Using baffles
⇨ Using diffuser ring
Problems of Excessive mixing time
⇨ May fracture fragile excipients and ruin their compressibility
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