SUGAR MANUFACTURING PROCESS

SUGARCANE

Sugar Cane

The sugarcane(Saccharum spp.) is a thick, tall, perennial grass that flourishes in tropical or subtropical regions. It requires a frost-free climate. Sugar synthesized in the leaves is used as a source of energy for growth or is sent to the stalks for storage. It is the sweet sap in the stalks that is the source of sugar as we know it.

The typical composition of Sugarcane is as follows:

• 15% dissolved matter (13% sucrose; 2% are other sugars-mainly glucose and fructose)

• 15% fibre (insoluble), and

• 70% water. 

For every 100 tons cane crushed, 30 tons of fibrous residues (bagasse), about 12 tons sugar and 4 tons molasses, around 4 tons of press mud and, molasses gives 2200-2500 litres of alcohol.

sugar cane planting and harvesting

PLANTING AND HARVESTING

Sugarcane requires an average temperature of 75 degrees Fahrenheit (23.9 degrees Celsius) and uniform rainfall of about 80 inches (203 centimeters) per year. Therefore, it is grown in tropical or subtropical areas.Sugarcane takes about seven months to mature in a tropical area and about 12-22 months in a subtropical area. At this time, fields of sugarcane are tested for sucrose, and the most mature fields are harvested first.Harvesting cane is done primarily by hand or machine. The harvested cane stalks are loaded mechanically into trucks or railroad cars and taken to mills for processing into raw sugar.

CANE PREPARATION AND PROCESSING

Cane Preparation and Processing

After the cane arrives at the mill yards, it is mechanically unloaded and excessive soil and rocks are removed. The cane is cleaned by flooding the carrier with warm water (in the case of sparse rock and trash clutter) or by spreading the cane on agitating conveyors that pass through strong jets of water and combing drums (to remove larger amounts of rocks, trash, and leaves, etc.). At this point, the cane is clean and ready to be milled. 

Juice can be removed from cane either by repeated crushing and washing (milling) or by washing alone, with a final squeezing simply to dry the spent fibre (diffusion). Better sucrose extraction can be obtained by crushing finely shredded cane rather than intact stalks and “Preparation” refers to that step in which cane supply is put onto a conveyer chain and passing it through one or two sets of cane knives and then through shredder Cane is finely shredded before juice is extracted either by milling or diffusion.

JUICE EXTRACTION

sugar cane crusher

A basic cane mill consists of three grooved rollers. Prepared cane is squeezed between the rollers, thus forcing the juice out of the fibre. The basic work of a mill is the separation of juice from fibre. Fibre, however, has the natural property of always retaining approximately its own weight of juice regardless of the pressure applied to it. To displace retained juice, water is poured onto the cane fibre before crushing, this is called imbibition. The residual or bagasse left in the last set of roller will be used for fuel to power the boilers to produce steam and generate electricity for use in subsequent sugar production process. 

A single milling unit would give an unacceptably low extraction. Typically, five mills are set in tandem and cane is passed in series from Mill 1 to Mill 5.

JUICE CLARIFICATION

Sugar Cane Juice Clarifier

Juice from a milling tandem contains a large amount of cane fibre, dirt and unwarranted particles that falls out with the juice between the rollers of the mills. To make it lucid and clear juice the juice is poured over a wire-mesh screen, or cascaded over an inclined wedge-wire screen. And then the juice is heated and lime is added to neutralise the natural acidity. It is then placed in a large settling tank called a Clarifier which produces a clear juice that is light in colour and free of suspended matter. To improve the precipitate formation, flocculent is added. 

The settled precipitate, referred to as mud, is pumped out of the trays of the clarifier and sent to the filtration station where the juice it contains will be recovered.

EVAPORATION

Sugar Cane Juice Evaporator

The clarified juice is then put through multiple evaporators to extract up to 60 – 65% of water from it, leaving the juice becoming concentrated or syrup with 60 – 65 brix. Multiple effect evaporation is the system where juice is boiled in series in several vessels, with steam fed to vessel 1 only. Vapour from vessel 1 boils the juice in vessel 2, vapour from 2 boils the juice in 3, and so on until vapour from the final vessel goes to waste.

 

SUGAR BOILING

Sugar boiling is conducted under “vacuum pan” process in order to lower operating temperatures, but some small scale manufacturer of sugar “open pan” processes. The syrup produced by the evaporators is concentrated further in pans. As the concentration rises the dissolved sugar crystallises and the work of the pans is to grow sugar crystals (from the sucrose in syrup) in several steps to maximise the amount of sucrose recovered in raw sugar.

CRYSTALLIZATION

Sugar Cane Juice Crystallizers

When the massecuite (raw sugar crystals mixed with molasses) is discharged from the pans it is retained in stirred tanks called crystallisers, where the sugar crystals continue to grow through cooling rather than boiling. The crystallisers are U-shaped vessels where massecuite is slowly rotated and allowed to cool for up to 48 hours. This technique is often referred to as “crystallisation in motion”. Rotation promotes even cooling of the massecuit, which helps to achieve uniform crystal growth. Uniform crystal growth can also be encouraged by placing granulated massecuit from a previous batch into the crystalliser with the fresh massecuit.  This process is known as “seeding”. 

After this process, the massecuit is now a suspension of sugar crystals in molasses. It is transferred to a centrifuge for separation.

CENTRIFUGATION

Sugar Centrifugal Machine

Massecuite leaving the crystallisers has now to be separated into crystals and molasses. The more efficient this separation, the more sucrose will be recovered as sugar and the less sucrose will be lost in molasses. A centrifugal is a machine that separates crystals from molasses. Centrifugation involves spinning massecuite in a perforated basket; centrifugal force acts on the molasses, forcing it through the perforations while the sugar remains on the basket wall. Water and steam may then be sprayed onto the crystals to wash off the remaining molasses.

SUGAR DRYING

Sugar Hopper Dryer

Sugar leaving the centrifugals has excess moisture which has an extremely detrimental effect on the keeping quality of the raw sugar and drying is therefore important. In a drier, the moisture is driven off from the surface of the liquor layer covering the crystal by passing heated air around the sugar crystals. The product from the process described so far is a raw sugar (Brown sugar) that can be used as is, or sent to a refinery to be converted to a white (refined) sugar.

BYPRODUCTS

Sugar Cane Baggasse

Bagasse is the final residual of the cane after it has underwent crushing/milling process. Nowadays, bagasse is used as fuel in generating electricity and steam the sugar production process. Furthermore, bagasse can be used as raw material for producing paper, insulating board, and hardboard, as well as furfural, a chemical intermediate for the synthesis of furan and tetrahydrofuran. 

The end product derived from sugar refining is blackstrap molasses. It is used in cattle feed as well as in the production of industrial alcohol, yeast, organic chemicals, and rum. 

Sugar Cane Molasses

QUALITY CONTROL

Mill sanitation is an important factor in quality control measures. Bacteriologists have shown that a small amount of sour bagasse can infect the whole stream of warm juice flowing over it. Modern mills have self-cleaning troughs with a slope designed in such a way that bagasse does not hold up but flows out with the juice stream. Strict measures are taken for insect and pest controls. 

Because cane spoils relatively quickly, great steps have been taken to automate the methods of transportation and get the cane to the mills as quickly as possible. Maintaining the high quality of the end-product means storing brown and yellow refined sugars (which contain two percent to five percent moisture) in a cool and relatively moist atmosphere, so that they continue to retain their moisture and do not become hard.