sugar plant

8/3/2019 Sugar Plant

http://slidepdf.com/reader/full/sugar-plant 1/7

Sugar Plant

Sugarcane is a perennial herb belonging to the grass family. Native to tropical

and subtropical regions of the world, this tropical grass is 10-24-feet tall, bears

long, pointed leaves, and has several stalks. The segmented stalks have a bud at

each joint and as the plant matures, small flowers appear.

THE DIFFERENT TYPES

There are a bewildering number of sugars and syrups available in the shops while

other types are available for the industrial user. Some of the basic differences

are discussed below.

White sugar is essentially pure sucrose and there is no difference between that

derived from cane and that from beet. Different manufacturers produce crystals

of different sizes however and this leads to some apparent differences. Smaller

crystals dissolve more readily and might therefore appear to be sweeter because

none is left at the bottom of the cup and they seem sweeter on the tongue if

eaten alone. Similarly smaller crystals have more surfaces per spoonful and

appear whiter than larger crystals. [Having said that, some white sugars are lesswhite than others: it depends on how much processing the manufacturer

applies.]

There are several speciality white sugars:

caster sugar is just a very small crystal size white sugar

icing sugar is ground up white sugar, essentially sugar dust

sugar cubes are lumps of sugar crystals "glued" together with a sugar

syrup

preserving sugar is a special large crystal

Brown sugars come in many different styles but are essentially one of two types:

sticky browns and free-flowing browns. The sticky browns were originally the sort

of mixture that comes out of a cane sugar crystallising pan. The extreme of this,

still made in India today, is "juggeri" or "gur" which is essentially such a mixture

boiled until dry.

In modern refining practice both of these types are made by mixing a refined or

at least purified sugar with a suitable syrup. The colour of the sugar and the

syrup determines the colour of the final product and the ratio of syrup to sugar

plus any drying applied determines whether the product is sticky or free-flowing.

Syrups, of which there are again an enormous range, range from pure sucrose

solutions as sold to industrial users to heavily treated syrups incorporating

flavours and colours. Refiners or "Golden" syrup is a sugar solution which has

been carefully treated to invert some of the sucrose. Inversion is a chemical

process which breaks down the disaccharide sucrose to its constituent sugars:

glucose and fructose. This helps ensure that crystallisation does not occur during



storage. Treacle is a similar product made from molasses rather than a pure

sugar

STEPS INVOLVED IN MAKING SUGAR

PLANTING

Sugarcane cuttings are planted in fields by workers or mechanical planters. In

order for the cane to grow, the seeds must be planted in well-drained soil. Typical

cane soil is made of a mixture of silt, sand, clay particles and organic matter.

Canes are spaced at least 4-feet apart and lined in rows and covered with soil.

Fertilizers are applied from the time of planting up until the beginning of the

ripening period. Cane fields are also routinely weeded to provide for optimum

growth of the cane. Depending on the region where the crop is planted, cane

seasons last from 8-22 months. In the United States, sugarcane is grown in

Florida, Hawaii, Louisiana and Texas.

COLLECTING THE HARVEST

Mature canes are gathered by a combination of manual and mechanical

methods. Canes are cut at ground level, its leaves are removed and the top istrimmed off by cutting off the last mature joint. Cane is then placed into large

piles and picked up, tied, and transported to a sugar factory.

CLEANSING AND GRINDING

Stalks are thoroughly washed and cut when reaching the sugar mill. After the

cleaning process, a machine led by a series of rotating knives, shreds the cane

into pieces. This is known as "grinding." During grinding, hot water is sprayed on

to the sugarcane to dissolve any remaining hard sugar. The smaller pieces of

cane are then spread out on a conveyer belt.

JUICING

The shredded pieces of sugarcane travel on the conveyer belt through a series of heavy-duty rollers, which extract juice from the pulp. The pulp that remains or

"bagasse" is dried and used as fuel. The raw juice moves on through the mill to

be clarified.

CLARIFYING

Carbon dioxide and the milk of a lime are added to the liquid sugar mixture and it

is heated to the boiling point, as the process of clarifying begins. As the carbon

dioxide travels through the liquid it forms calcium carbonate, which attracts non-

sugar debris (fats, gums, and wax) from the juice, and pulls them away from the

sugar juice. The juice is then pushed through a series of filters to remove any

remaining impurities.

EVAPORATION

The clear juice which results from the clarifying process is put under a vacuum,

where the juice boils at a low temperature and begins to evaporate. It is heated

until it forms into a thick, brown syrup.

CRYSTALLIZATION

By evaporating what little water is left in the sugar syrup, crystallization takes

place. Inside a sterilized vacuum pan, pulverized sugar is fed into the pan as the



liquid evaporates, causing the formation of crystals. The remaining mixture is a

thick mass of large crystals, which is sent to a centrifuge to spin and dry the

crystals. The dried product is raw sugar, still inedible.

REFINERY

Raw sugar is transported to a Cane Sugar Refinery for the removal molasses,

minerals and other non-sugars, which still contaminate the sugar. This is known

as the purification process. Raw sugar is mixed with a solution of sugar and water

to loosen the molasses from the outside of the raw sugar crystals, producing a

thick matter known as "magma." Large machines then spin the magma, which

separate the molasses from the crystals. Crystals are promptly washed, dissolved

and filtered to remove impurities. The golden syrup which is produced is then

sent through filters to remove the color and water. What's left is a concentrated,

clear syrup, which is again fed into a vacuum pan.

SEPARATION AND PACKAGING

Once the final evaporation and drying process is done, screens separate the

different sized sugar crystals. Large and small crystals are packaged andshipped, labeled as white, refined, sugar.

DETAILS:TECHNOLOGY/PROCESS DESCRIPTION:

Ponni Sugars produces sugar from sugarcane .Ponni is the first sugar mill in the

country to use alternative fuel in its boilers and release its bagasse in entirety for

paper making. The production process is as shown below.

How Sugar is Refined

Raw sugar is made in tropical countries where sugar cane can be grown

profitably. It is then shipped in bulk to a refinery in the country where the sugar

is required. It now has to be finally cleaned up, purified and made ready for the

consumer.

It helps to think of refining as a series of steps from left to right where colour and

non-sugars are concentrating to the left and pure sugar is concentrating to the

right. However the raw sugar comes into the process to the left of centre, not at

one end. In the description that follows the flow of sugar is followed first and then

the remainder of the process is reviewed.



Affination

The first stage of processing the raw sugar is to soften and then remove the layer

of mother liquor surrounding the crystals with a process called "affination". The

raw sugar is mixed with a warm, concentrated syrup of slightly higher purity than

the syrup layer so that it will not dissolve the crystals. The resulting magma is

centrifuged to separate the crystals from the syrup thus removing the greater

part of the impurities from the input sugar and leaving the crystals ready for

dissolving before further treatment.

The liquor which results from dissolving the washed crystals still contains some

colour, fine particles, gums and resins and other non-sugars.

Carbonatation

The first stage of processing the liquor is aimed at removing the solids which

make the liquor turbid. Coincidentally some of the colour is removed too. One of

the two common processing techniques is known as carbonatation where small

clumps of chalk are grown in the juice. The clumps, as they form, collect a lot of

the non-sugars so that by filtering out the chalk one also takes out the non-

sugars. Once this is done, the sugar liquor is now ready for decolourisation. The

other technique, phosphatation, is chemically similar but uses phosphate rather

than carbonate formation.

Decolourisation

There are also two common methods of colour removal in refineries, both relying

on absorption techniques with the liquor being pumped through columns of

medium. One option open to the refiner is to use granular activated carbon [GAC]

which removes most colour but little else. The carbon is regenerated in a hot kiln

where the colour is burnt off from the carbon. The other option is to use an ion

exchange resin which removes less colour than GAC but also removes some of

the inorganics present. The resin is regenerated chemically which gives rise to

large quantities of unpleasant liquid effluents.

The clear, lightly coloured liquor is now ready for crystallisation except that it is a

little too dilute for optimum energy consumption in the refinery. It is therefore

evaporated prior to going to the crystallisation pan.



Boiling

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 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 packed and/or stored ready for

despatch.

Recovery The liquor left over from the preparation of white sugar and the washings from

the affination stage both contain sugar which it is economic to recover. They are

therefore sent to the recovery house which operates rather like a raw sugar

factory, aiming to make a sugar with a quality comparable to the washed raws

after the affination stage. As with the other sugar processes, one cannot get all

of the sugar out of the liquor and therefore there is a sweet by-product made:

refiners' molasses. This is usually turned into a cattle food or is sent to a distillery

where alcohol 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 mightcontain 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 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 washedbefore 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

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.

sugar plant

Documents