Dairy Engineering

Module 3. Separation equipment

Lesson 10
GRAVITY SETTING, SEDIMENTATION, PRINCIPLES OF CENTRIFUGAL SEPARATION

10.1 Introduction

Separation of components of immiscible liquids by use of density difference is a well established method. In the dairy industry, the application of centrifugal force is done particularly in separating fat from rest of the milk. It is also being used in separating bacteria from milk.

Different components of milk have the following specific gravities:

Fat: 0.93

Skim milk: 1.036

Whole milk: 1.028 to 1.032 depending on composition

10.2 Stoke's Law

The rapidity of separation depends on various factors, and can be estimated by use of Stoke’s law for gravity separation modified to include the centrifugal force, instead of gravitational force.

Origins of Centrifugal cream separation are from the fundamentals of particle separation by gravitational forces, due to density difference between various components. If the density of the particle is higher than that of the surrounding medium it sinks and if it is lower it rises. The particle remains in suspension if densities are equal.

Considering the particle in the figure 10.1, the particle is acted upon by two forces, viz. gravitational force and viscous force. The gravitational force pulls the particle down, and the viscous force is resisting the movement of the particle. The particle will come down, if its density is greater than the fluid, and it will move up, if its density is less than that of the fluid surrounding it. The expression for these forces is given below.

Fig. 10.1 Viscous force

Fig. 10.2 Cream separation

10.3 Energy of Rotating Body

Calculation of the approximate power required to rotate the cream separator can be done by assuming that the stack of discs along with bowl as a hollow cylinder with uniform density throughout. The weight of the bowl, W can be given as

Fig. 10.3 Energy of rotating body

10.4 Capacity of Cntrifugal Separator

The capacity of the centrifugal separator depends on the number of factors again, viz., the number and size of discs, speed of rotation, the viscosity of the fluid etc. An expression for the throughput of centrifugal cream separator is arrived at as shown below

Fig. 10.4 Centrifugal separator

Fig. 10.5 Drag force due to volumetric flow

‘C’ is added to take care of deviation due to

1. Velocity profile is not laminor flow

2. Decrease in cross sectional area due to back flow of lighter phase

3. Non- ideal distribution over the feed inlet circumference

4. Geometric in accuracies in the stacks of discs.

The position of neutral zone or the place where the milk enters the gaps between adjacent discs will influence the ease in separation of the denser and lighter phases of the fluid. In applications like Clarifier, where the component to be separated is the denser particles of dust, extraneous matter, the neutral zone should be towards periphery. In the case of cream separation, the component that is to be separated is the fat, which is of lighter phase, the zone has to be towards centre of the disc, to reduce the distance of travel for the separation.

Fig. 10.6 Drag force due to volumetric flow Q (m3/sec)

The pressure exerted by the lighter and heavier fluids, when in equilibrium, on the inner wall of bowl is same

The above is possible, only when the level of the lighter and heaver liquid, are different. (about 3 to 10 mm) by fitting overflow lips of different sizes

The distance RS of the rotating separated milk ring is greater than RC, the rotating of cream.