CRYSTALLISATION & SEEDING OF SUGAR

CRYSTALLISATION

Natural or artificial process of formation of Solid crystals precipitating from a solution or a melt. ; Crystallization is therefore an aspect of precipitation, obtained through a variation of the solubility conditions of the solute in the solvent, as compared to precipitation due to chemical reaction.

It consists of 2 major steps namely: Nucleation & Crystal Growth.

Nucleation

Nucleation is a step wherein the solute molecules dispersed in the solvent start to gather into clusters.

These stable clusters constitute the nuclei.

After attaining the critical size ( Nuclei) , the atoms arrange in a defined and periodic manner that defines the crystal structure.

Nucleation can be of two types: Primary – Homogeneous & Heterogeneous Secondary

Primary Nucleation Secondary Nucleation/Seeding

Homogeneous• Due to random movement

through the medium, few particles stick together to form a crystal.

Heterogeneous Assisted by suspended

particles of foreign substance such as rod immersed in a solution. These objects catalyse the process of nucleation.

Occurs when seed crystals are added.

Requires lower super saturation than primary nucleation.

Due to impact with agitator or vessel wall. Nuclei are created by striking a crystal.

Embryos are created & swept away that would have incorporated the existing crystal.

Types of Nucleation

Crystal Growth

Is the subsequent growth of the Nuclei that succeeds in achieving the critical cluster size.

Crystal growth ( Shape and Size) depends upon the conditions of super saturation .

Once the super saturation is exhausted , the solid – liquid system reaches equilibrium and the crystallisation is complete.

Sugar Formation (Vacuum Pan Processes)

Initially, a sub saturated standard liquor is pumped in the vacuum pan.

To heat the juice & evaporate the water, vacuum pans are equipped with CALANDRIA. Here, steam flows at a pressure greater than the atm. Pressure and as the steam condenses, it releases heat to the massecuite (mixture of raw sugar and syrup).

BAROMETRIC CONDENSER helps in maintaining the partial vacuum (0.2-0.3 bar) and 65-70°C temperature, as it absorbs the steam and condenses it by means of cooling.

Working of a Vacuum Pan

LOADING- Standard liquor is made to enter the vacuum pan followed by supplying steam which increases the temp.

CONCENTRATION- Evaporation of water leads to an increase in the concentration of the juice from 0.6-0.7 to super saturation level of 1.2-1.3.

SEEDING- Seeds of sucrose crystals dissolved in alcoholic solution are added to the vacuum pan. The sucrose mass generally added is around 10gm and the size of the crystals is about 5μm.

STABILIZATION- Super saturation conditions are maintained till

the crystals attain their final size 0.5μm and a brix level of 90.4.

Working of a Vacuum Pan

WAITING- Water is introduced in the vacuum pan where it is evaporated by heat from Calandria.

DISCHARGE- The massecuite is left to fall or discharged. This is facilitated by ‘vacuum breaking’ i.e. pressure in the chamber differs from the atm. Pressure by less than 10%. Massecuite is discharged until it occupies less than 6% of the volume of pan.

WASHING- steam is passed through the cleaning valve at 90 C which dissolves the remains of the massecuite stuck to the walls.

Centrifugation

The centrifuge is filled with the massecuite and centrifuged at 150rpm.

This presses the massecuite against the wall.Centrifuge is accelerated to a max. speed of 1100rpm.Sugar crystals and the syrup is separated and is made

to pass through centrifugal screens.Sugar is washed with water. Remains of the syrup are

eliminated by injecting hot water.Speed of rotation is decreased to 100rpm, sugar is then

discharged and dried.

Sugar- formation

Sugar crystals separated in A centrifugals constitute “ white sugar”.

The residual mother liquor obtained known as A molasses is again crystallised in vacuum pan B and passed through sugar centrifuges to yield B strike and B molasses.

The molasses obtained is converted into alcohol.