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