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Ma.Adeliza F. Mortalla BSChE-4 OJT April 18, 2012

Liming

Hydrated lime is essential to the production of sugar from both sugar cane and sugar beets. It

is also used to purify sugar from other sources, such as maple or sorghum, although these are

produced in much smaller quantities.

Sugar cane and sugar beets are harvested and processed with water to form raw juice, which

has low pH and contains dissolved impurities.The juice from the mills, a dark green color, is acid and

turbid. The clarification (or defecation) process is designed to remove both soluble and insoluble

impurities (such as sand, soil, and ground rock) that have not been removed by preliminary screening.

The process employs lime and heat as the clarifying agents. Hydrated lime (milk of lime) is added to

the juice to raise the pH and to react with the impurities to form insoluble calcium organic

compounds that can be removed.Milk of lime (about one pound per ton of cane) neutralizes the

natural acidity of the juice, forming insoluble lime salts. Heating the lime juice to boiling coagulates

the albumin and some of the fats, waxes, and gums, and the precipitate formed entraps suspended

solids as well as the minute particles.

Excess lime is removed by carbonation or by the addition of phosphoric acid. This process

may be repeated several times depending on the purity of final product required.

Clarification

The cane juice, as delivered from the milling train, contains some soil and other undesirable

impurities. To remove these juices is heated and lime is added to settle the unwanted material. The

impurities settle out in the clarifier and then go to the rotary vacuum filters, which filter out any

remaining juice. The filter mud from the vacuum filters is rich in nutrients and is recycled back to canefields. Clear amber coloured clarified juice containing about 15% sugar is decanted from the clarifiers

to the evaporators.

Evaporation

The mixed juice from extraction is preheated

prior to liming so that the clarification is

optimal. The milk of lime, calcium hydroxide

or Ca(OH)2, is metered into the juice to hold

the required ratio and the limed juice enters a

gravitational settling tank: a clarifier. The juice

travels through the clarifier at a very low

superficial velocity so that the solids settle out

and clear juice exits.

The mud from the clarifier still contains

valuable sugar so it is filtered on rotary

vacuum filters where the residual juice is


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extracted and the mud can be washed before discharge, producing sweet water. The juice and the

sweet water are returned to process.

The clear juice has probably only 15% sugar content but saturated sugar liquor, required

before crystallization can occur, is close to 80% sugar content. Evaporation in a steam heated

multiple effect evaporator is the best way of approaching the saturated condition because low

pressure water vapors can be produced for heating duties elsewhere in the factory.

The evaporator sets the steam consumption of the factory and is designed to match the

energy balance of the entire site: the manager wants to avoid burning auxiliary fuel and equally

wants to avoid paying to dispose of surplus bagasse. The greater the number of effects, the less

steam is required to drive the first effect. Each subsequent effect is heated by the vapor from the

previous effect so has to be operated at a lower temperature and therefore lower pressure.

Pan Boiling

Physical chemistry assists with sugar purification during thecrystallization process because there is a natural tendency for the

sugar crystals to form as pure sucrose, rejecting the non-sugars.

Thus, when the sugar crystals are grown in the mother liquor they

tend to be pure and the mother liquor becomes more impure. Most

remaining non-sugar in the product is contained in the coating of

mother liquor left on the crystals

The mother liquor still contains valuable sugar of course so

the crystallization is repeated several times. However non-sugars

inhibit the crystallization. This is particularly true of other sugars such as glucose and fructose whichare the breakdown products of sucrose. Each subsequent step therefore becomes more difficult until

one reaches a point where it is no longer viable to continue.

The crystallization step itself - a "boiling" - takes place in a vacuum pan: a large closed kettle

with steam heated pipes. [In practice the heating is done with a low pressure water vapor from the

evaporator.] Some modern pans are continuous flow devices but most are batch devices which go

through a discrete cycle and are then emptied for a new boiling. A typical cycle might be 4 hours long.

The mixture of crystals and mother liquor from a boiling, called the "massecuite", is dropped into a

receiving tank called a crystallizer where it is cooled down and the crystals continue to grow. This also

releases the pan for a new boiling. From the crystallizer the massecuite is fed to the centrifuges.

In a raw sugar factory it is normal to conduct three boilings. The first or "A" boiling produces

the best sugar which is sent to store. The "B" boiling takes longer and the retention time in the

crystallizer is also longer if a reasonable crystal size is to be achieved. Some factories re-melt the B

sugar to provide part of the A boiling feedstock, others use the crystals as seed for the A boilings and

others mix the B sugar with the A sugar for sale. The "C" boiling takes proportionally longer than the

B boiling and considerably longer to crystallize. The sugar is usually used as seed for B boilings and the

rest is re-melted.


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Crystallization

Crystallization is the next step in the manufacture of sugar. Crystallization takes place in a

single-stage vacuum pan. The syrup is evaporated until saturated with sugar. As soon as the

saturation point has been exceeded, small grains of sugar are added to the pan, or "strike." These

small grains, called seed, serve as nuclei for the formation of sugar crystals. (Seed grain is formed by

adding 56 ounces [1,600 grams] of white sugar into the bowl of a slurry machine and mixing with 3.3parts of a liquid mixture: 70 percent methylated spirit and 30 percent glycerine. The machine runs at

200 RPM for 15 hours.) Additional syrup is added to the strike and evaporated so that the original

crystals that were formed are allowed to grow in size.

The growth of the crystals continues until the pan is full. When sucrose concentration reaches

the desired level, the dense mixture of syrup and sugar crystals, called massecuite, is discharged into

large containers known as crystallizers. Crystallization continues in the crystallizers as the massecuite

is slowly stirred and cooled.

Massecuite from the mixers is allowed to flow into centrifugals, where the thick syrup, or

molasses, is separated from the raw sugar by centrifugal force.

Centrifugation

The high-speed centrifugal action used to separate the massecuite into raw sugar crystals and

molasses is done in revolving machines called centrifugals. A centrifugal machine has a cylindrical

basket suspended on a spindle, with perforated sides lined with wire cloth, inside which are metal

sheets containing 400 to 600 perforations per square inch. The basket revolves at speeds from 1,000

to 1,800 RPM. The raw sugar is retained in the centrifuge basket because the perforated lining retains

the sugar crystals. The mother liquor, or molasses, passes through the lining (due to the centrifugal

force exerted). The final molasses (blackstrap molasses) containing sucrose, reducing sugars, organicnonsugars, ash, and water, is sent to large storage tanks.

Once the sugar is centrifuged, it is "cut down" and sent to a granulator for drying. In some

countries, sugarcane is processed in small factories without the use of centrifuges, and a dark-brown

product (non-centrifugal sugar) is produced.

References:

http://www.madehow.com/Volume-1/Sugar.html

http://www.tullysugar.com/production-process

http://www.sucrose.com/evaprate.html

http://www.graymont.com/applications_sugar.shtml
http://www.madehow.com/Volume-1/Sugar.htmlhttp://www.tullysugar.com/production-processhttp://www.sucrose.com/evaprate.htmlhttp://www.graymont.com/applications_sugar.shtmlhttp://www.graymont.com/applications_sugar.shtmlhttp://www.sucrose.com/evaprate.htmlhttp://www.tullysugar.com/production-processhttp://www.madehow.com/Volume-1/Sugar.html

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