CHISHTIA SUGAR MILLS LIMITED

Growing the Cane

Sugar cane is a sub-tropical and tropical crop that prefers lots of sun and lots of water - provided that its roots are not waterlogged. It typically takes about 12 months to reach maturity although the time varies widely around the world from as short as six months in Louisiana to 24 months in some places. Where it differs from many crops is that it re-grows from the roots so the plant lasts many cycles [or 'ratoons', a word derived from the Spanish to sprout] before it is worn out.

The More Detail button will take you to the web site of Kew Gardens in a separate window. You should return here when you close that down again.

Cane grows very tall in good growing regions - certainly up to 3 metres/10 feet tall - and still has some green leaves when ripe although most leaves have dried off by then. Where possible the cane is fired before harvesting to remove the dead leaf material and some of the waxy coating. The fire burns at quite high temperatures but is over very quickly so that the cane and its sugar content are not harmed.

In some areas burning is not permitted because of the nuisance value to local communities of the smoke and carbon specs that are released. However there is no environmental impact, the CO2 released being a very small proportion of the CO2 fixed with photosynthesis during growth and the improved sugar extraction meaning that less cane needs to be grown on fewer acres to satisfy the world's sugar demand.

Harvesting is done either by hand or by machine. Hand cut cane -- cane cutting is a hard and dirty job but can employ lots of people in areas where jobs are scarce -- is cut at about ground level, the top green leaves are cropped off and then the stalk is bundled whole. Once a complete bundle has been assembled it is removed from the field with a light cart and may then be transferred to a larger vehicle for transport to the mill.

Most machine-cut cane is chopped into short lengths but is otherwise handled in a similar way as hand cut cane. Machines can only be used where land conditions are suitable and the topography is relatively flat. In addition the capital cost of machines and the loss of jobs caused makes this solution unsuitable for many sugar estates.

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 fibre 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 fibres and the green extracts from the plant are all mixed in with the sugar.

How Cane Sugar is Made - Extraction

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 yet needs to have a sufficient supply in storage for times when cutting and transport are stopped, whether deliberately or not. 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 fibre and hence the less sugar remaining in the fibre;

A typical mixed juice from extraction will contain perhaps 15% sugar and the residual fibre, 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% fibre 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 factory can clean up the juice quite easily with slaked lime (a relative of chalk) which settles out a lot of the dirt so that it can be sent back to the fields. Once this is done, the juice is thickened up into a syrup by boiling off the water using steam in a process called evaporation. Sometimes the syrup is cleaned up again but more often it just goes on to the crystal-making step without any more cleaning. The evaporation is undertaken in order to improve the energy efficiency of the factory.

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 crystallisation 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

The syrup is placed into a very large pan for boiling, the last stage. In the pan even more water is boiled off until conditions are right for sugar crystals to grow. You may have done something like this at school but probably not with sugar because it is difficult to get the crystals to grow well. In the factory the workers usually have to throw in some sugar dust to initiate crystal formation. Once the crystals have grown the resulting mixture of crystals and mother liquor is spun in centrifuges to separate the two, rather like washing is spin dried. The crystals are then given a final dry with hot air before being stored ready for despatch.

How Cane Sugar is Made - Boiling

Physical chemistry assists with sugar purification during the crystallisation 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 crystallisation is repeated several times. However non-sugars inhibit the crystallisation. 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 crystallisation 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 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 crystalliser where it is cooled down and the crystals continue to grow. This also releases the pan for a new boiling. From the crystalliser 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 crystalliser 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 crystallise. The sugar is usually used as seed for B boilings 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 fibre 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 fibre.

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.

How Cane Sugar is Made - The Energy Aspects

The steam is raised in bagasse fired boilers which usually have a secondary fuel to accommodate imbalances in bagasse supply and steam or power demand. The factory designer attempts to balance the site such that bagasse is neither left over nor insufficient: any secondary fuel costs money and a large surplus of bagasse may cost money to dispose. Balancing is done by selecting the right mix of turbine and electric drives for major equipment and selecting the pressure of the steam to give the efficiency required. In many cases this does not recognise the full energy value of the bagasse and is therefore wasteful in an overall sense. Today, more and more factories are considering power export as another by-product of sugar production. To do this they are improving the efficiency of their thermodynamic cycles and converting equipment drives to optimise power output.

Factories are frequently in very undeveloped places and have no connection to an external power supply. This requires special techniques to start the factory and means that any breakdown in the power house impacts on the entire neighbourhood. Wives soon tell their husbands what happened to dinner when their spouses lost power!

Do you want to know more about bagasse fired boilers? Try the web domain of Thermal Energy Systems.( http://www.tesboilers.com)