Etiket Arşivleri: Centrifugation

Cream Seperation ( Prof. S K Dash )

CREAM SEPARATION

(Cream separation, Centrifugation, Tubular bowl centrifuge, Disc bowl centrifuge, Domestic cream separator)
The cream separation is a very important unit operation in dairy industry. The cream or fat is taken out of milk for standardisation purpose. Besides, cream is also required to prepare some value added products as ghee, butter, etc. Cream is usually separated from milk by the centrifugation process.

Centrifugation

Sometimes gravity separation may be too slow because of the closeness of densities of the particle and the fluid, or because of the association forces holding components together, as in emulsions like whole milk. In that case centrifugation helps in separation of the components on the basis of differences in their densities.
• The centrifuge increases the forces on particles many fold.
• Thus the particles that do not settle readily or at all in gravity settlers can often be separated from fluids by centrifugal force.
• The relative settling velocities of small particles are not changed, but the disturbing effects of Brownian motion and free convection currents are overcome.
The equipment using this principle of separation is known as a centrifuge. The
centrifuges are used for:
• separation of immiscible liquids,
• clarification of liquids by removal of small amounts of solids, and
• for removal of solids from liquids
Centrifuges are also used for centrifugal filtration, where the centrifugal force is used (not the pressure difference) to separate the solids through a filter medium. The major applications of the centrifuges are shown in Fig. 8.1.

Centrifugation

Centrifugation

Learning Outcomes

General Idea

Feed added to spinning bowl

Sedimentation of particles occurs in centrifugal field

Flow is upwards at a particular rate which determines residence time in device

Separation happens if sedimentation velocity is high enough for particle to reach side of bowl within residence time

Large particles have higher settling velocities than small particles

Both large and small are still particles, have small Reynolds no.s (<1) and obey Stokes’ Law

The Centrifuge

Separation of milk into skimmed milk and cream is done with a centrifuge

Centrifugal Motion

Compared to gravity

mg

Acceleration constant

In direction of earth

Equilibrium velocity reached

Terminal velocity given by:

mrω2

Acceleration increases with r

Acceleration increases with ω

Away from axis of rotation

Equilibrium velocity never reached

Instantaneous velocity:

Sigma Factor

The capacity of a centrifuge is defined by Σ

Q is the throughput (m3/s) at which all particles with a terminal velocity ≥ uT (m/s) are retained

Σ has units of m2 and is equivalent to the cross sectional area of a thickener with the same capacity

Activity – Determine Σ

The contents of a fermenter are discharged to a centrifuge

Volume of material is 100 m3

Centrifugation time is 5 hrs

Particle size is 3 mm – all particles of this size are separated

Density of solid phase 1090 kg/m3

Cell free liquid density 1025 kg/m3

Cell free liquid viscosity 0.005 Pa.s

Calculate the capacity factor, Σ

Bowl Centrifuge

Sigma Factor – Bowl Centrifuge

For the bowl centrifuge:

ω is the angular velocity (rad/s)

R is the outer radius of the bowl (m)

rc is the radius of the clarified discharge weir (m)

H is the height of the bowl (m)

r is the inner radius of the liquid in the bowl (m)

g is the acceleration due to gravity (m/s2)

A long thin bowl gives good separation

The Disc Stack Centrifuge

Benefit of Discs

Disc Stack Centrifuge Capacity

For the disc stack centrifuge:

ω is the angular velocity (rad/s)

n is the number of discs

R is the outer radius of the discs (m)

r is the inner radius of the discs (m)

θ is the angle between disc and vertical (rad)

g is the acceleration due to gravity (m/s2)

Activity

What is the capacity of the following disc stack centrifuge?

No. of discs = 400

Space between discs is 0.01 m

Inner radius = 0.05 m

Outer radius = 0.20 m

Half vertical angle = 45°

Rotational speed = 4500 rpm

What effect does an increase in diameter have on the capacity?

What diameter is needed for the previous example?

Decanter Centrifuge


Laboratory‎ > Centrifugation

Centrifugation

Cream Separation, Churning & Homogenization

Purpose of the experiment:

The aims are:

• to separate cream from milk and to obtain milk fat by application of churning process.

• to show how homogenization works

• to produce butter and pure fat using churning.

CLARIFICATION AND CREAM SEPARATION

The following topics will be covered in this section:

• Centrifugation

• Separation

• Clarification

• Standardization