Centrifugation

Question 1

What are the 5 applications of centrifugation?

Answer 1

1) Separation of immiscible liquids
2) Sedimentation - centrifugal rate of separation
3) Centrifugal clarification
4) Desludging (large particles to remove)
5) Centrifugal filtration

Question 2

Describe the formula for separation of immiscible liquids

Answer 2

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

Question 3

Rotational speeds can also be expressed as rev/min (formula)

Answer 3

ω = 2 pi N / 60
or
ω = pi N / 30
Where N = rotational speed in revs/min 
ω = radians /sec

Question 4

Hence what is the total equation?

Answer 4

F = mr (2 piN/60)^2 = 0.011 m rN^2

Question 5

What is the gravitational force acting on the particle?

Answer 5

Fg = mg

(Gravitational acceleration g has been replaced by 0.011 rN^2

Question 6

What does centrifugal force depend on? (4)

Answer 6

1) Radius of rotation
2) Speed of rotation
3) Mass of body
4) Density of material

Question 7

If the size of particles and speed of rotation are fixed then…..

Answer 7

Separation is a function of mass or density

Question 8

Describe the centrifuge diagram - circle - centre to outside

Answer 8

Air space, Liquid B (light), Neutral zone, Liquid A (dense), Bowl wall

Question 9

If the liquid rotates at bowl speed i.e. no slippage, negligible friction then:

Answer 9

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

Question 10

What is the equation for radius of neutral zone

Answer 10

Ri^2 = pARA^2 - pBRB^2 / pA - pB

Question 11

Separation of milk and cream describe

Answer 11

Milk is fed continuously in he machine which is rotated about its vertical axis

Question 12

The separation between the 2 different densities is called the

Answer 12

interface

Question 13

Why would you change the outlet radius?

Answer 13

To move the neutral zone to vary the amount of fat ie skimmed, semi-skimmed milk

Question 14

The radius of the neutral zone depends on ……

Answer 14

the radii of the 2 phase outlets and liquid densities

Question 15

For example, 2 liquids of known densities - if RB is fixed, then as RA is increased….

Answer 15

so Ri increases - therefore Ri can be controlled and therefore Zone A and Zone B sizes can be controlled

Question 16

When the neutral zone is nearer the central axis the

Answer 16

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

Question 17

What is the maximum velocity reached?

Answer 17

Terminal velocity

Question 18

Centrifugal rate of separation - velocity of particles falling in a fluid - depends on….

Answer 18

Size, density and shape of particles and properties of the fluid

Question 19

Stokes law - when a particle falls steadily through a fluid there are two principal forces acting on it

Answer 19

1) Drag force resisting motion (from frictional action of the fluid.
2) External accelerating force

Question 20

Stokes Law formula

Answer 20

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

Question 21

Stokes law only applies:

Answer 21

in streamline flow and to spherical particles

Question 22

Applications of stokes law:

Answer 22

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

Question 23

Describe centrifugal clarification:

Answer 23

• 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

Question 24

Describe centrifugal filtration:

Answer 24

• 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

Question 25

Difference between pressure filtration and centrifugal filtration

Answer 25

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

Question 26

Disadvantages of sedimentation by gravity

Answer 26

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

Question 27

Efficiency of cyclone depends on:

Answer 27

• 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