Liquid mixing Flashcards
What is the definition of mixing?
operation in which 2 or more components are treated so that each particle lies as nearly as possible in contact with a particle of each of the other component. (seen in ideal mixing which is hard to achieve)
What are the objectives of mixing? (there are 4)
1) to produce a simple physical mixture (mix 2 solids or 2 liquids)
2) to produce a physical change (E.g. Soluble salt (solid) changed to liquid form if can mix the salt in a solvent)
3) to promote or control chemical reaction (Product obtained by chemical rxn by reactant/compound A and B)
4) to produce a dispersion (e.g. suspensions, emulsions)
degree of mixing needed depends on the objective of the mixing, as well as the type of mixture to be produced
- Mix 2 solids VS 2 miscible liquids
- prepare a solution VS an emulsion
Which requires a higher degree of mixing?
Solids and emulsion require higher degree of mixing
What are positive mixtures? What are some examples?
Irreversible mixing take place by diffusion, without the expenditure of work provided that time is unlimited (no time limit)
doesn’t require high order of mixing
E.g. Mixtures of gas/miscible liquids
What are negative mixtures and examples?
Consist of components which will separate unless work is continually expended on them
Require high degree of mixing
e.g. suspensions
What are neutral mixtures and examples?
Components that don’t mix or segregate spontaneously.
Intermediate mixing
E.g. Mixtures of powder; ointments; suppositories containing drugs
What are the 2 requirements in mixing operation?
1) Localised mixing (shear) - applies shear to the liquid
2) General movement (flow) - takes all parts of the material through the shearing zone to produce a uniform product
What are the 3 velocity components in the movement of the liquid at any point in the mixing vessel?
Longitudinal
Radial
Tangential
What if there is excessive radial movement?
Materials taken to vessel wall
If e.g. Liquid with solid particles; solid will move to wall the solid will sediment to the bottom of the vessel
If there is excessive radial movement in suspension; solid particles at bottom and mixing not effectively conducted
What if there is inadequate longitudinal movement? What is the term called?
Stratification
One layer of liquid above the other layer of immiscible liquid (layering effect)
If mixing suspension: solid particles in layer separate from liquid medium
What if there is dominant tangential movement?
Vortexing
Liquid will swirl around the impeller shaft; liquid separated by shaft by cone of air; if cone of air reaches the tip of blade –> introduce air - bubbles ; if any component prone to oxidation; bubbles air - degradation of product
What are the factors affecting flow patten in vessel?
- Form of impeller
- Position of impeller
- Container shape
- Presence of baffles
- Liquid properties
What are the 3 types of impeller and its properties (movement)?
Propeller mixer: strong longitudinal component
Turbine mixer: Strong radial component, weak tangential component
Paddle mixer: strong radial component, strong tangential component
What are the 3 positions of impeller?
Offset (not in centre)
Angled
Side-entering
What are 4 types of container shape?
- Double cone
- Slant cone
- V-shape
- Cylindrical
What does presence of baffles do?
Vessel can have more than 1-2 baffles; baffles reduce vortexing
In cylindrical vessels + baffles
BUT Installation of baffles might have dead spots; mixing cannot be effectively carried out
The solid and liquid will not mix properly if it’s at dead spots
Is more viscous or less viscous more difficult to mix liquids?
More difficult to mix liquids of higher viscosity
What if the apparent viscosity of liquids is low?
Speed of rotation is high
D/d 20
(smaller impeller is used)
D= vessel diameter d = impeller diameter
What if the apparent viscosity of liquids is high?
Speed of rotation is low
D/d ~1
D= vessel diameter d = impeller diameter
The size of impeller is as wide as the vessel diameter
Typically rotated at low speed
To mix high viscosity liquids large impeller with low speed
What are the 4 types of liquid mixers?
Shaker mixers
Propeller mixers
Turbine mixers
Paddle mixers
What are the properties of shaker mixers?
- Vessel containing the liquid is agitated in an oscillary or rotatory movement
- Mixing efficiency is highly variable
- Affected by properties of liquids and the constructional characters of the mixer
- limited use in practice
What are the properties of propeller mixers?
- Accentuates longitudinal movement but imparts little shear
- suitable for mixing liquids of LOW VISCOSITY and SUSPENSIONS
- D/d 20 and high speed up to 8000 rev/min
- Unsuitable for liquids of high viscosity (>1000 cps) and for emulsification
There are inclined blades as well
What are the properties of turbine mixers?
- Impeller has circular disc to which are attached a number of short and vertical blades which may be straight or curved
- May be fitted with a diffuser ring to increase the shear forces
Smaller the holes; greater shear force exerted on liquid as liquid move out from impeller to diffuser ring to other parts in vessel
- Produces STRONG RADIAL MOVEMENT and shear forces but little tangential movement
- turbine impeller rotated at lower speed than propeller mixer
- Suitable for:
~ Emulsification
~ Mixing liquids of HIGH VISCOSITY (up to 100000cps)
~ Mixing liquids that may stratify with a propeller mixer
(Despite strong longitudinal component; may not be efficient to mix the liquids - stratification; possible to separate into layers; thus turbine may be used) - Fitted with a diffuser ring, it is less suitable for suspending heavy solids
What are the properties of paddle mixers?
- flat blades attached to a vertical shaft; The blades will lie flat on surface; no inclined (propeller is inclined and turbine blades are vertical but curved or straight)
- low speed of the order of 100 rev/min usually employed
- If high speed used, baffles needed to avoid swirling and vortexing; Strong tangential movement will promote vortexing; to minimise that in paddle mixers; baffles are usually employed
What are the 3 variations of paddle mixers for mixing liquids of different viscosities?
- Plain paddles (mixing liquids at low viscosity)
- Paddles with blades that fit closely to the vessel wall (mixing liquids of higher viscosity)
- Smaller paddle that rotates on its own axis while travelling in a circular path round the vessel - aka planetary motion mixer (mixing highly viscous liquids); Require less energy; circular path - better mixing of liquid - more efficient mixing using less energy
How to overcome vortexing and aeration in liquid mixing? (There are 4 approaches)
1) Mounting impeller deep in liquid; Dont have the blade too close to the surface of the liquid
2) Avoiding symmetrical positioning of impeller (offset/ off-centre)
3) Employing a push-pull propeller; They are inclined differently - Note the different flow patterns
4) Employing baffles on vessel wall
