Types of Graining Techniques in sugar crystallization process | Pan Boiling

Methods of graining techniques in sugar industry pan boiling

Graining is defined as a process of establishment of fine sugar nuclei or grain in graining material( sugar material like syrup or molasses) for further development during the crystallization process in the sugar industry. (pan boiling operation). Graining techniques play very important role in the exhaustion and centrifuging of the massecuite.

Generally, the graining techniques can be classified into three methods.

• Waiting method.
• Shock seeding method.
• True seeding method.

Waiting method :

This is an old graining technique in crystallization process. The following steps are involved in this waiting method.

1. Take the graining material (syrup or molasses) and concentrate up to reach labile zone of supersaturation. (As per the characteristics of the labile zone fine grain nuclei appear suddenly without any presence of crystal).
2. As per the characteristic of the labile zone, crystal (grain) formed spontaneously.
3. Sufficient grain nuclei are established, and further crystal germination is restricted either by reducing the brix (concentration of material) by application of movement water. Now the concentration of material brings back the super saturation in metastable zone.
4. The concentration of the martial was kept in metastable zone by application of moment water for hardening of the grain.

This method is entirely dependent on the skill of the person. As the crystal appears suddenly, it is very difficult to control number of grains and uniformity of grains. For low purity graining material (below 65), the viscosity increases very high in labile zone of super saturation almost decreasing in the circulation of material in the pan. So in this case it is very difficult to form uniform grain. The coefficient of crystal variation is always above 30 %. Due to its drawbacks waiting method not followed in sugar industry crystallization process(pan boiling process).

Shock seeding method:

The shock seeding technique of graining, results in some improvement over the waiting method.  The following steps involve in this method.

1. Concentrate graining material up to the intermediate zone of supersaturation. The length of string generally kept 8 to 15 cm, but mostly depends on purity of material. (As per characteristics of intermediate zone, the fine grain nuclei start appearing due to the presence of crystal).   
2.  After material concentration reaches to intermediate zone, Introducing of crystal in the form of Powder sugar or Icing sugar or fondant sugar. ( Icing sugar more uniform when compare to powder and fondant sugars).
3. In the presence of introducing crystal starts formation of new crystal.
4. As soon as sufficient number of crystals appears, further crystal growth is stopped by reducing the brix% ( decreasing the material concentration) by the using of moment water (condensate water).
5. If the crystal crops is high, wash with hot water to remove fine crystal.
6. After the getting required grain, the concentration of the material is reduced to the metastable zone by introducing condensate water, and the grain is further hardened. 

Similar to waiting method, it is very difficult to produce the required grain uniformity in this method also. The coefficient of crystal variation is always above 25%. To control the number of crystals in grain is highly skilled job and needs experienced person.

While following the both Waiting & Shock seeding methods,  slight error of judgment may create serious in exhaustion of massecuite and also creates problem in purging of massecuite at centrifugal section.

True seeding method:

The true seeding method is the best technique for graining ( crystallization) in the pan-boiling process of sugar industry. The following steps take place in the method

1. Take the appropriate footing (graining) material and make it crystal free.
2. Concentrate grain-free material up to border of meta stable and intermediate zone. ( As per characteristics of metastable zone, no new crystals formed.  Only present crystals grow will in size) .
3. Now introducing the predetermined quantity of slurry into the concentrated footing material. Take care of no air is to be passed in graining martial. And also control maximum vapour because its speed change in material concentration.
4. Wait some tome till grain starts appearing (grain visibility size 25 to 30 microns). The grain appearing time varies according to the purity of the material.
5. Once the crystal appears reduced the brix% (concentration) of the material slightly for the resisting further chances of new crystal formation.
6. Now the concentration of material maintained super saturation in meta stable zone to hardening the grain.

In this method when graining point kept in only meta stable zone, then no new crystals formed.  Only present crystals grow in size.  For this case uniformity of sugar crystals variation is remain even below 20%. Quantity and crystal size of slurry sugar play a very important role in this true seeding method.

The main advantages of true seeding method is less chance to form false grain and Conglomerates(joint grains). Due to this advantage better exhaustion takes place in final massecuite.

The successful development and standardization of this system is credited to E.G. Gillet, the Technical Director of the California and Hawaiian Sugar Refining Company.

Helpful Points in true seeding methods :

1. The meta stable zone is generally tested by the thumb rule of thread(string) length. The string length depend upon martial purity, viscosity, and temperature. Generally, its having for syrup 4 to 5cm, For AH molasses  6 to 8cm and CL molasses it’s having 10 to 12cm.
2. The grain appearing time varying according to purity, concentration, and temperature of the material. Generally, 5 minutes time taken for appearing grain in B-grain material, and for C-grain material takes 8 to 10 minutes.
3. The size of the crystal in slurry size is in the range of 3 to 5 microns. 6 to 8 grams of slurry sugar are required per tonne of B-massecuite. 8 to 12 grams of slurry sugar are required per tonne of C-massecuite. 

The establishment of grain of right size and sufficient numbers of grains is purely depend upon the art of skilled pan men’s rather than technology.  But now a day with different pan control instruments like conductivity or brix sensors are being shifted to technology than art.

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