Centrifugation Flashcards
What are the 5 applications of centrifugation?
1) Separation of immiscible liquids
2) Sedimentation - centrifugal rate of separation
3) Centrifugal clarification
4) Desludging (large particles to remove)
5) Centrifugal filtration
Describe the formula for separation of immiscible liquids
The centrifugal force F acting on an object of mass m, rotating in a circular path of radius r at an angular velocity ω is:
F = m r ω ^2
Rotational speeds can also be expressed as rev/min (formula)
ω = 2 pi N / 60 or ω = pi N / 30 Where N = rotational speed in revs/min ω = radians /sec
Hence what is the total equation?
F = mr (2 piN/60)^2 = 0.011 m rN^2
What is the gravitational force acting on the particle?
Fg = mg
(Gravitational acceleration g has been replaced by 0.011 rN^2
What does centrifugal force depend on? (4)
1) Radius of rotation
2) Speed of rotation
3) Mass of body
4) Density of material
If the size of particles and speed of rotation are fixed then…..
Separation is a function of mass or density
Describe the centrifuge diagram - circle - centre to outside
Air space, Liquid B (light), Neutral zone, Liquid A (dense), Bowl wall
If the liquid rotates at bowl speed i.e. no slippage, negligible friction then:
the drop in pressure from centrifugal force between Ri and RA = the drop in pressure from centrifugal force between Ri and RB - if not the interface at radius will be unstable
What is the equation for radius of neutral zone
Ri^2 = pARA^2 - pBRB^2 / pA - pB
Separation of milk and cream describe
Milk is fed continuously in he machine which is rotated about its vertical axis
The separation between the 2 different densities is called the
interface
Why would you change the outlet radius?
To move the neutral zone to vary the amount of fat ie skimmed, semi-skimmed milk
The radius of the neutral zone depends on ……
the radii of the 2 phase outlets and liquid densities
For example, 2 liquids of known densities - if RB is fixed, then as RA is increased….
so Ri increases - therefore Ri can be controlled and therefore Zone A and Zone B sizes can be controlled
When the neutral zone is nearer the central axis the
lighter component is exposed only to a small centrifugal force compared to the heavy liquid - this principle is used in cream separation where the maximum yield is the aim
What is the maximum velocity reached?
Terminal velocity
Centrifugal rate of separation - velocity of particles falling in a fluid - depends on….
Size, density and shape of particles and properties of the fluid
Stokes law - when a particle falls steadily through a fluid there are two principal forces acting on it
1) Drag force resisting motion (from frictional action of the fluid.
2) External accelerating force
Stokes Law formula
Vm = D^2g (Pp -Pf) / 18u
Vm = terminal velocity D = diameter of particle Pp and Pf = the density of the particle and the denisty of the fluid g = acceleration due to gravity u = dynamic viscosity
Stokes law only applies:
in streamline flow and to spherical particles
Applications of stokes law:
1) Creaming in emulsions
2) Sedimentation eg. particles in fruit drinks
3) Rate of separation in a centrifuge (modified equation)
4) Certrifugal clarification
5) Velocity and time required for particle to travel a centain distance
6) Residence time of a particle in the bowl
7) Sedimentation of solids in a gas
Describe centrifugal clarification:
- Removal of small quantities of insoluble solids
- If a liquid containing dense particles is spun, the particles move towards the bowl wall and remain there
- Smaller particles remian in the liquid, and are carried out
- Controlled by the rate of feed i.e. dwell time in the bowl
Describe centrifugal filtration:
- Separation of solids from a liquid by filtration when the flow of liquid is induced by centrifugal means
- Centrifugal force is used to provide the driving force in some filters. These machines are really centrifuges fitted with a perforated bowl that may also have filter cloth on it. Liquid is fed into the interior of the bowl and under the centrifugal forces, it passes out through the filter material
Difference between pressure filtration and centrifugal filtration
a) Centrifugal force and filtering area increase with increase in radius
b) Centrifugal force acts on the filtrate passing through the cake and on the cake itself
Disadvantages of sedimentation by gravity
a) Large chamber areas required
b) Long contact time between particles and ehated air - product denaturation - therefore use centrifugal forces in a cyclone separator - removes particles > 10u diameter
Efficiency of cyclone depends on:
- Small diameter cyclone produces greater Fc (centrifugal force acting on the particle)
- Double helical flow, undisturbed
- Air should enter tangentially at the top
- Baffles or lids should be avoided at the outlet
