Fluid mixing Flashcards
what is mixing
Mixing is defined as an operation in which two 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.
The objectives of mixing may be broadly classified as follows:
1)To produce a simple physical mixture
(2 or more miscible fluid / solid; diluting potent drug; need high degree of mixing)
2)To produce a physical change (eg salt mixing no need high degree of mixing)
3)To promote or control chemical reaction
(eg acid and base; if sensitive to ph change then need good mixing if not nvm)
4)To produce a dispersion (suspension / emulsion)
The degree of mixing needed depends on the objective of the mixing, as well as the type of mixture to be produced
Mixtures may be divided into three types that differ fundamentally in their behaviour
Positive mixtures (low mixing)
these are formed from materials where irreversible mixing would take place by diffusion, without the expenditure of work provided that time is unlimited.
(tend to be miscible)
Negative mixtures (high mixing)
These consist of components which will separate unless work is continually expended on them.
(suspension and emulsion)
Neutral mixtures (intermediate mixing) These consist of components that do not mix or segregate spontaneously. eg powder and pastes
The degree of mixing needed depends on the objective of the mixing, as well as the type of mixture to be produced
The degree of mixing needed depends on the
objective of the mixing, as well as the type of mixture to be produced
The fluid mixing operation requirements:
(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
Liquid mixing is usually performed with
a rotational device (mixing element) in a vessel
The mixing element has a suitable shape to produce shear forces and appropriate flow pattern
The movement of the liquid at any point in the mixing vessel has _____ components
1) longitudinal (parallel to shaft)
2) radial (perpendicular to shaft)
3) tangential (tangent to circle of rotation)
Assuming shaft is vertical
excessive radial movement cause
material to be taken to vessel wall.
solid particles will sediment under the influence of gravity and rotate at the bottom and wont achieve good mixing.
Assuming shaft is vertical
inadequate longitudinal movement cause
stratification
eg 2 immiscible liquid will remain as 2 phase and rotate within its components layer.
Assuming shaft is vertical
dominant tangential movement cause
vortexing
not effective mixing
cone of air the separate the impeller from the liquid.
vortexing is form because
a vortex form when the centrifugal force imparted to the liquid by the propeller blades causes it to back up round the sides of the vessels and form a depression around the shaft.
as the speed of rotation is increased, air may be sucked into the fluid due to the formation of a vortex; this can cause frothing and possible oxidation
factor affecting flow pattern
Form of impeller Position of impeller Container shape Presence of baffles Liquid properties
components of propeller mixer
strong longitudinal component
components of turbine mixer
strong radial component,
weak tangential component
components of paddle mixer
strong radial component
strong tangential component
position of impeller
offset
angled
side-entering
container shape
cylinder or symmetrical container tends to form vortex
which can be overcome by baffles
presence of baffles
create dead spot
(liquid near here dont mix well with other)
guide movement of fluid and prevent vortex so mix well
create turbulence by diverting the rotating fluid from its circular path to the centre of the vessel where the vortex would otherwise form.
liquid properties
Mixing parameters depend on the viscosity of the liquid
Apparent Viscosity = LOW
Speed of rotation = HIGH
D/d = 20
Apparent Viscosity = HIGH
Speed of rotation = LOW
D/d = ~ 1
D= vessel diameter; d=impeller diameter
types of fluid mixer
Shaker mixers
Propeller mixers
Turbine mixers
Paddle mixers
shaker mixer
The vessel containing the liquid is agitated in an oscillary or rotatory movement
Mixing efficiency is highly variable
It is affected by the properties of the liquids and the constructional characters of the mixer
Such liquid mixers have limited use in practice
propeller mixer
The impeller usually resembles the ordinary marine propeller
It accentuates the 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
stratification occur if mixing high viscosity fluid
Turbine mixer
The impeller has a circular disc to which are attached a number of short and vertical blades which may be straight or curved.
It may be fitted with a diffuser ring to increase the shear forces
The turbine mixer produces strong radial movement and shear forces but little tangential movement
The turbine impeller is usually rotated at a lower speed than a propeller impeller
It is suitable for:
Emulsification
Mixing liquids of high viscosity (up to 100000 cps)
Mixing liquids that may stratify with a propeller mixer Fitted with a diffuser ring, it is less suitable for suspending heavy solids
principle of turbine mixer
The impeller has four flat blades surrounded by perforated inner and outer diffuser rings. The rotating impeller draws the liquid into the mixer ‘head’ and forces the liquid through the perforations with considerable radial velocity, sufficient to overcome the viscous drag of the bulk of the fluid. One drawback is the absence of an axial component, but a different head with the perforations pointing upwards can be fitted if this is desired. As the liquid is forced through the small orifices of the diffuser rings at high velocity, large shear forces are produced. When mixing immiscible liquids, if the orifices are sufficiently small and velocity sufficiently high, the shear forces produced enable the generation of droplets of the dispersed phase which are small enough to produce stable dispersions (water-in-oil or oil-in-water). Turbine mixers of this type (homogenizers) are therefore often fitted to vessels used for the large-scale production of emulsions and creams. Turbine-type mixes will not cope with liquids of very high viscosity since the material will not be drawn into the mixer head. These liquids are best treated as semi-solids and handled in the same equipment as used for such materials
paddle mixer
They use an impeller which consists of flat blades attached to a vertical shaft.
Low speed of the order of 100 rev/min is usually employed.
If high speed is used, baffles are needed to avoid swirling and vortexing.
paddle mixer accentuates tangential movement so mixing is often accompanied by vortexing which is made worse when rotating at high speed.
speed of impeller
fastest propeller turbine paddle slowest
types of paddle mixer
A variety of paddle mixers is available for mixing liquids of different viscosities
Plain paddles
For mixing liquids of low viscosity
Paddles with blades that fit closely to the vessel wall For mixing liquids of higher viscosity
Smaller paddle that rotates on its own axis while travelling in a circular path round the vessel
Known as planetary motion mixer
For mixing highly viscous liquids
Vortexing and aeration are common problems encountered in liquid mixing
how to overcome
a) Mounting impeller deep in the liquid
b) Avoiding symmetrical positioning of impeller c)Employing a push-pull propeller
d) Employing baffles on vessel wall
planetary motion mixer principle
small clearance between the vessel and the paddle in order to ensure sufficient shear.
scrapping down of the bowl is usually necessary several times during a run to mix the contents well, since some materials are forced to the top of the bowl.
The problems that arise during the mixing of semi-solids (ointment and pastes) stem from the face that, unlike liquids, semi-solid will not flow easily.
Material that finds its way to a dead spot will remain there.
therefore mixer must have rotating elements with narrow clearances between themselves and the mixing vessel wall and they must produce a high degree of shear mixing since diffusion mixing cannot occur.
