How Sucrose Is Extracted from Sugarcane

Extraction The first stage of processing is the extraction of the cane juice. In many factories the cane is crushed in a series of large roller mills: similar to a mangle [wringer] which was used to squeeze the water out of clean washing a century ago. The sweet juice comes gushing out and the cane fiber is carried away for use in the boilers. In other factories a diffuser is used as is described for beet sugar manufacture.

Either way the juice is pretty dirty: the soil from the fields, some small fibers and the green extracts from the plant are all mixed in with the sugar. There are several important aspects to extraction which involve the energy balance of the factory, the efficiency of extraction and therefore ultimately the profitability of operations: •The manager needs to process the cane as soon as possible if sugar losses are to be avoided it needs to have a sufficient supply in storage for times when cutting and transport are stopped, whether deliberately or not.

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Typically, cane is processed within 24 hours of cutting; •Cane preparation is critical to good sugar extraction, particularly with diffusion extraction. This is achieved with rotating knives and sometimes hammer mills called “shredders”. However shredding requires extra energy and more equipment; •The extraction is actually conducted as a counter-current process using fresh hot water at one end being pumped in the opposite direction to the cane.

The more water that is used, the more sugar is extracted but the more dilute the mixed juice is and hence the more energy that is required to evaporate the juice; •The more accurately that the mills are set [adjusted], the drier is the residual fiber and hence the less sugar remaining in the fiber; •A typical mixed juice from extraction will contain perhaps 15% sugar and the residual fiber, called bagasse, will contain 1 to 2% sugar, about 50% moisture and some of the sand and grit from the field as “ash”.

A typical cane might contain 12 to 14% fiber which, at 50% moisture content gives about 25 to 30 tons of bagasse per 100 tons of cane or 10 tons of sugar. 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 extracted and the mud can be washed before discharge, producing a 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 vapours 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 vapour from the previous effect so has to be operated at a lower temperature and therefore lower pressure. Boiling

Physical chemistry assists with sugar purification during the crystallization 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 which are 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 vapour from the evaporator. ] Some modern pans are continuous flow devices but most are batch devices which go hrough 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 boiling. 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 boiling 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 boiling and the rest is re-melted. Storage

The final raw sugar forms a sticky brown mountain in the store and looks rather like the soft brown sugar found in domestic kitchens. It could be used like that but usually it gets dirty in storage and has a distinctive taste which most people don’t want. That is why it is refined when it gets to the country where it will be used. Additionally, because one cannot get all the sugar out of the juice, there is a sweet by-product made: molasses. This is usually turned into a cattle food or is sent to a distillery where alcohol is made. Power

So what happened to all that fiber from crushing the sugar cane? It is called “bagasse” in the industry. The factory needs electricity and steam to run, both of which are generated using this fiber. The bagasse is burnt in large furnaces where a lot of heat is given out which can be used in turn to boil water and make high pressure steam. The steam is then used to drive a turbine in order to make electricity and create low pressure steam for the sugar making process. This is the same process that makes most of our electricity but there are several important differences.

When a large power station produces electricity it burns a fossil fuel [once used, a fuel that cannot be replaced] which contaminates the atmosphere and the station has to dump a lot of low grade heat. All this contributes to global warming. In the cane sugar factory the bagasse fuel is renewable and the gases it produces, essentially CO2, are more than used up by the new cane growing. Add to that the factory use of low grade heat [a system called co-generation] and one can see that a well run cane sugar estate is environmentally friendly.

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