Liquid Mixing Flashcards
Definition of mixing
An operation in which 2/more components are treated so that each particle lies as nearly as possible in contact with a particle of each of the other component
Objective of mixing
1) Produce a simple physical mixture
2) Produce a physical change
3) Promote or control chemical reaction
4) Produce a dispersion
How to determine degree of mixing?
Depends on the objective of mixing, as well as type of mixture to be produced
Types of mixtures (& their behaviours)
1) Positive mixtures (Eg. Mixture of gases/miscible liquids)
- Formed frm materials where irreversible mixing would take place by diffusion, provided time is unlimited
2) Negative mixtures (Eg. Suspension)
- Consist of components which will separate unless work is continually expended on them
3) Neutral mixtures (Eg. Mixture of powders, ointments, suppositories)
- Consist of components that do not mix/segregate spontaneously
2 requirements of mixing operation?
1) Localised mixing (shear)
- Applies shear to the liquid
2) General movement (flow)
- Takes all part of the material through the shearing zone to produce a uniform mixture
How is liquid mixing performed?
- Performed with a rotational device (mixing element) in a vessel
- > Mixing element has a suitable shape to produce shear forces & appropriate flow pattern
- Satisfactory flow pattern depends on the balance of the 3 velocity components (longitudinal, radial, tangential)
Excessive radial movement: Materials taken to vessel wall
“ longitudinal movement: Stratification (layering effect)
“ tangential movement: Vortexing
Factors affecting flow pattern in the vessel
1) Form of impeller
Propeller mixer: Strong longitudinal component
Turbine mixer: Strong radial, weak tangential
Paddle: Strong radial & tangential
2) Position of impeller (Offset/Angled/Side-entering)
3) Container shape
(Cylindrical, Double cone, slant cone, V-Shape)
4) Presence of baffles (Vertical strips extending frm wall)
- Cons: Can result in some parts not effectively mixed
5) Liquid properties
- More difficult to mix liquids of higher viscosity
List types of liquid mixers
- Shaker mixers
- Propeller mixers
- Turbine mixers
- Paddle mixers
Shaker mixers (Use, advantages, disadvantages)
Cons:
- High variable mixing efficiency
- Affected by properties of the liquids & the constructional characters of the mixer
- Limited use in practice
Propeller mixers (Use, advantages, disadvantages)
- Accentuates the longitudinal movement but imparts little shear
- D/d = 20 & high speed up to 8000rev/min
Use: Liquids of low viscosity & suspensions
(Resembles marine propeller of ship)
-> 2-bladed/3-bladed/Flat-bladed/Torrance ‘Trifoil’
Cons:
- Not suitable for high viscosity liquids (>1000cps) & emulsification
- Shearing not strong
Turbine mixers (Use, advantages, disadvantages)
- Has circular disc attached to a no. of short & vertical blades
- May be fitted with a diffuser ring to increase shear forces
- Usually rotated @ lower speed than propeller impeller
Use (Strong radial & shear forces, weak tangential):
- Emulsification (Where shearing is impt)
- Mixing liquids of high viscosity (up to 100,000cps)
- Mixing liquids that may stratify with propeller mixer
Cons:
- Less suitable for suspending heavy solids when fitted with a diffuser ring
Paddle mixers (Use, advantages, disadvantages)
- Use an impeller consisting of flat blades attached to a shaft (Eg. plain/abciliary/anchor/anchor-gate paddle)
- Low speed usually employed (100rev/min)
- -> If high speed, need baffles to avoid swirling & vortexing
Variety of paddle mixers available:
1) Plain paddles
- Mixing liquids of LOW VISCOSITY
2) Paddles with blades fitted closely to vessel wall
- Mixing liquids of HIGH VISCOSITY
3) Planetary motion mixer (small paddle rotating on its own axis while travelling in a in a circular path round the vessel)
- Mixing HIGHLY VISCOUS liquids
- Eg. mixer for baking
Common problems in liquid mixing
Vortexing & aeration
How to overcome problems in liquid mixing?
1) Mounting impeller deep in liquid
2) Avoid symmetrical position of impeller
3) Employing a push-pull propeller (b/c diff flow patterns)
4) Employing baffles (Overcome vortexing)
Why is dilatant flow a prob in the production of liquid preparations?
Dilatant flow: Viscosity increases with increasing shear stress
- Liquid can freeze @ very high speed bc it exhibits high resistance to flow
- Exerts overload on motor
