Sugar Refinery Process Selection
Refined sugar manufactured in two ways:
Standalone Sugar Refinery
Raw sugar is purchased from external sources and only the refining step is carried out.
The raw sugar is first manufactured and is then, continuously, taken to the second step of refining.
The conversion of an existing direct plantation white sugar mill to produce refined sugar will make it a back-end refinery. The advantages are:
· The raw sugar quality can be maintained as per requirement of the refinery.
· The returns and run-off from refinery can be sent back to raw house for further processing, giving better yield.
· Minimal additional requirements of steam and power can generally be met from existing utilities.
· No additional cost on storage/handling/transport and inventory of raw sugar
· Flexibility to produce various grades of sugar.
Steps for Removing Colour and Impurities in Sugar Refinery
1. The process relies on the use of CO2 and the lime for forming Calcium Carbonate precipitate. The CO2 is obtained from the boiler flue gas and the lime is added to the melt prior to the carbonation. Multiple stages are used to bring the final pH to 8.4.
2. The flue gas has to be scrubbed and the SO2 removed by passing through water prior to its use for carbonation.
3. A substantial quality of water gets used in the milk of lime and this lowers the brix of the melt subsequently requiring higher steam quantity for evaporation. For certain melts, the carbonation proceeds better at lower brix hence evaporators are required to bring back the brix to higher levels.
4. The carbonated melt is filtered through process filters using filter aids for pre-coating.
1. This process is based on the creation of calcium phosphate floc that captures the suspended impurities and the colouring matter precipitates formed by the use of special colour precipitants.
2. The conglomerate of these flocs, precipitates, suspended impurities is floated with the aid of air and removed as scum.
3. The clarified melt is filtered through a multi-bed filter before it goes to the next stage of decolourisation.
Ion Exchange Resin
1. Batch Ion exchange : Click Here
2. Continuous Ion Exchange : m/s Puritech designed the continious ion exchange process described by them in their attached paper
Click on link below to download the paper :
Granular Activated Carbon (GAC)
Granular Carbon is a similar particle to resin but it has a diameter of around 1mm and is not perfectly spherical in shape. It can also be considered as a sponge with the sugar solution going into the pores of the particles to be decolourised. In the case of carbon, the decolourising site is activated carbon and this is created by kilning the granules in a furnace at around 950oC, with very little oxygen present. So granular carbon is held in a vessel and the sugar solution is passed through it to decolourise it. Then the carbon has to be desweetened and removed from the vessel in order to revivify it. This is performed in the furnace.
In small installations, the plant will consist of several granular carbon vessels and a system to transfer the carbon into and out of the vessel to the furnace.
In larger installations, as would be economical for a 1,000 tonne per day Refinery, a single Carbon Column is used and a small proportion of the carbon is removed as a pulse and then it is topped up with revivified carbon. This makes for better utilisation of the carbon and minimises the plant cost.
Granular carbon is also good at removing odours such as from Amino Acids and so provides further clean-up capability. This is of limited use where the carbon is before crystallisation because the boiling in the pan tends to remove these impurities in the vapour. It would be useful in liquid sugar applications where the sugar is not crystallised.
Powdered Activated Carbon (PAC)
Powdered Activated Carbon works in the same way as Granular Activated Carbon with the active carbon sites absorbing the colour in the sugar solution. The small size of the powdered carbon particles (30 - 40 microns) means the colour is absorbed more rapidly than Granular Carbon and so a small amount is gently mixed in the sugar solution. The carbon is mixed with a diatomaceous earth filter aid and filtered in a pressure leaf filter.
The waste filter aid and carbon is then dumped since it cannot be reused. The greater the quantity of powdered carbon used, the greater the amount of decolourisation. An acceptable level of 50% colour removal can usually be obtained with a carbon addition of 0.1% of the sugar solids. This can be improved by increasing the quantity of carbon up to 0.5% on solids to give a colour removal of up to 80%.
The pressure leaf filters are expensive items but because of the low dosage of carbon, the requirement is less than the full filtration duty that would be required in Carbonatation. The area requirement is half way between a full and a polish filtration duty. The powdered carbon is dirty and dusty if handled wrongly.