INTRODUCTION

India is the largest consumer of sugar in the World and Indian sugar industry is the second largest agro-based industry located in the rural India. With 453 operating sugar mills in different parts of the country, Indian sugar industry has been a focal point for socio-economic development in the rural areas. About 50 million sugarcane formers and a large number of agricultural laborers are involved in sugar cane cultivation and ancillary activities, constituting 7.5% of the rural population. Besides the industry provides employment to about 2 million skilled/semi skilled workers and others mostly from the rural areas. The industry not only generates power for its own requirements but surplus power for export to grid based on by product bagasse. It also produces ethanol, an eco friendly and renewable energy for blending with petrol.

International trade is of strategic importance to India as it can help maintain stability in the domestic market, despite the cyclic ability in production. If there is a sugar surplus the either due to excess production or due to greater economic attractiveness of cane for ethanol in the future, export could be used if the surplus cannot be managed in the domestic market. Acceptability as a credible exporter will provide the Indian sector an alternative set of markets for diverting surplus production. Similarly in case of deficit, raw sugar imports could help bridge the supply gap.

The sugar industry in India is part of food & Beverage series reports. The market will be boosted by the rapidly growing food and beverage industry with increasing production of confectionaries resulting in increased demand for sugar.

PROCESS DESCRIPTION

The cane is transported from field to factory in trucks or tractor drawn trailers nearly 70-80%, the remaining being brought to factories by bullock carts from the adjoining areas mostly with in a radius of 10-15 km. While the bullock carts are unloaded in to the carriers manually by tilting, the truck or trailers, each carrying 5-10 tons cane are mechanically unloaded, The vehicles are fitted with usually chain slings equally spaced on the floor of the truck. The sling ends are joined after loading the truck or trailer, thus forming bundles of cane, to be lifted by overhead crane fitted with bars. At the carrier the bundle is lifted, conveyed to the feeder table and sling ends released with the sling left hanging at the end of operation when the bar is lifted.

CANE PREPARATION

The objective of cane preparation is to cut cane in to short pieces for feeding the mills as also to rupture the cells, without extracting juice. The preparatory devices commonly employed and installed before the milling tandem, are classified into three types- i) Knives which cut the cane to pieces. ii) Shredders which shred cut cane into long fine pieces. iii) Fibrizor System

-Cane preparation by knives essentially consists in disintegrating the hard rind, and nodes and breaking the cane to short pieces which in effect increases the mill capacity and efficiency of juice extraction.

Shredder is essentially a hammer mill adapted to the function of sugar cane pulverizing and swing hammer type shredder of different designs. The shredder preparation results in long slivers, with a high degree of cell rupture, which is uniform and thorough.

Fibrizor, the ingenious preparatory devices, combines the function of cutter knives and shredder. The rotor consists of a heavy duty special steel shaft on which are mounted a series of sturdy hubs. The special knives have cutting edges and two hammers projecting in opposite direction which has hammers. The cutting knives are secured to the hubs and serve the dual function of cutting and shredding cane which has passed through the leveler set.

MILLING

The cane sugar factories across the world either use the conventional three roller mills with trash plates for extraction of juice. or using cane or bagasse diffusers for the purpose.

The conventional mills have lately been installed with heavy duty pressure feeders to achieve higher capacities.

The conventional three roller mills with or without pressure feeders suffer from various drawbacks including high power consumption.

Re-absorption and heavy wear and tear.

The cane and bagasse diffusers therefore have been the choice of many sugar mills.

The diffusers also suffer from a number of drawbacks like high retention time, sugar inversion, high steam consumption and flooding.

Control the performance of mills by

By laying brix curves of ideal vs actual.

Sampling of juice until one hour through out the roller of feed, discharge separately and composite.

The observed degree of brixes of various mill juice are plotted against equal intervals on the graph.

Ideal Brix corresponding to the ratio of added water, fiber of cane.

Crushing must be uniform, uniform imbibitions.

Sample should be taken in all mills of the same cane.

Multiplication factor =1+S+S 2 +S 3 +-----S n-1

1+S+S2+S3+-----Sn

Where S=W/S

(w=Added water%cane,S=Fiber%cane)

DIFFUSERS

In the great majority of cane sugar factories throughout the world, extraction of sugar from cane is effected by means of mills.

This was the process originally employed; it has been progressively improved but never displaced.

The only exception was Egypt, where batch diffusion, by battery of diffusion vessels, as used in the beet industry, was employed for along time and was discontinued only towards 1960.

However continuous diffusion, originating before the Second World War, expanded rapidly afterwards with the rebuilding of European factories which had been destroyed.

From 1950 onwards several manufacturers strove to adapt to cane the type of continuous diffuser which they had designed and put into operation with beet.

Diffusion is the phenomenon of osmosis.

By which two solutions of different concentrations located side by side or for example, separated by a membrane, exchange by osmosis a cross the membrane. If it is a case of two solutions of the same substance, the exchange takes place until the concentration is the same on each side of the membrane.

This assumes that the membrane is permeable in relation to the substance dissolved.

If there are two or more substances in solution, the membrane may be permeable to the solvent and to certain substances, and impermeable to others, it is then described as semi-permeable.

This the case to certain extent with the cell walls of the sugar-beet, when certain organic constituents are removed from them by heat, they allow sucrose to pass more readily than certain salts.

This is why, with sugar- beet, the diffusion juice has a purity higher than that of the normal beet juice.

In the sugar factory, diffusion is therefore the phenomenon by which the cells of the beet or the cane, immersed in water or solution of lower concentration than the juice which they contain, give up to that water or to that solution a part or all of the sugar forming the excess of concentration of their juice.

Types of Diffusers:

Normally diffusers are two types.

1. Batch type –before 1960

2. Continuous type-after1960

The batch diffusion system operated in Egypt for over 50 years.

The first successful continuous diffusers installed in 1960.

The continuous diffusers operated as counter-current solid/liquid system

The continuous diffusers are categorized into two types. 1. Cane diffusers, 2.Bagasse diffusers.

Cane diffusers having different types.

True counter current diffusers(Ex: DDS, Saturn)

Moving bed diffusers (Ex:BMA,Desmit,Silver ring ,Tanget, Halet)

Other types are –F&S/van Hengel, Rotocel.

Moving bed diffusers are now used to the exclusive of others in sugar mills.

De-smith diffusers of -TS type are used for baggas.

CLARIFICATION

The clarification station constitutes an important section of the entire factory in view of the effects of its functioning on the subsequent sugar crystallization. In the context of Indian condition where in direct consumption sugar is produced from cane juice and the high standards of sugar have to be maintained, any inadequacies in operation of clarification process would be very costly for the factory. The major reagents employed at this stage are heat, lime, so2 and phosphoric acid in sulphitation & defecation (only heat &lime). Clarification influences the colour and quality of marketable sugar as well as the loss of sugar in crystallization process.

The object of juice clarification is to convert dark green, muddy juice, turbid & acidic juice into clear & transparent juice of pH 7.0 with out disturbing the reducing sugars.The main aim of cane juice clarification is

1) Elimination of suspended impurities and collides.

2) Removal of maximum amount of non sugars .

3) Not to destroy the sucrose in any manner .

4) Not to destroy or decompose the reducing sugars.

5) To remove as much colour of the juice as possible .

6) Clear and transperant juice production.

Composition of mixed juice:

water - 75 to 88 %

sucrose - 10 to 21 %

R.s. - 0.4 to 1 %

Organic matter than sugar - 0.5 to 1 %

Inorganic compounds - 0.2 to0.6 %

Nitrogenous compounds - 0.5 to 1 %

Brix - 12 to 18 %

Purity - 80 to 87 %

Ash - 0.4 to 0.7 %

Cao - 500 to 800 ppm

P2o5 - 60 to 300 ppm

There are three different types of clarification process.

1) Defecation process

2) Sulphitation process

3) Carbonation process.

1) Defacation process : This process is the oldest & cheapest method of clarification. The raw sugar is manufactured from cane juice the clarification process used of all production of raw sugar is called defacation process. In this process basically two agents are used , 1) heat 2)lime.

The importance of this process is that the phosphate level in the juice should be maintained at 300ppm in order to form sufficient tri calcium phosphate. As ppt occludes the collides ,suspended impurities .

Characteristics of defecation process can be high lighted as follows.

1) changing the acidic nature of the M.J. to slightly alkaline juice by adding proper amount of milk of lime.

2) heating of the juice above its boiling pt .

3) settling of lime and heated juice.

4) decantation of supernent juice from the clarifier which has much anfarior than that of sulphited juice.

Different Process of Deffication:

1) Cold liming

2) Hot liming

3) Fractional liming

4) Fractional liming and double heating.

cold liming :

Raw juice

| -----Mol addition (2.5-7.0% 0n juice)

Raw juice pH 7.8-8.2

|

R.Jc heating to 102-1030c

|

Clarifier /settler

2) Hot liming :

Raw juice

|

R.Jc heated to 102-1030c

|

limed with MOL to 7.4-7.8pH -----clarifier

3) Fractional Liming:

Raw juice|

1st stage liming of R.Jc|

Limed juice (pH-6.2-6.4)|

2nd stage liming of R.Jc|

Llimed juice (pH 7.2-7.4)|

Heated to 102-1030c|

settler

4) Fractional Liming and Double heating

Raw juice|

1st stage liming of R.Jc|

Limed Jc (pH 6.2-6.4)|

Limed juice heated upto 90- 930c|

2nd stage liming of R.Jc|

Limed Jc ( pH 7.2-7.4)|

Heated to 102-1030c|

Clarifier

Calcium Phosphate PrecipitationThe reaction between lime and phosphoric acid and lime & original

phosphate in the juice can be written as follows .1) ca++ + Hpo4

-- -------- caHpo4 |2) 3ca++ + 2po4 -- ------- ca3(po4) |

The rates of these reactions are increased by boiling. When the first reaction approaches completion its rate slows down and second reaction exceeds it .The mechanism of lime defecation :

1) By adding MOL to R.Jc the free organic acids will form calcium organates . 2) On continuous addition , lime unites with phosphoric acid present in juice and form tri calcium

phosphate .3) A number of substances originally present in raw juice in colloidal form precipitates.4) In ,addition part of the proteins flocculate when the tempis raised . The floc and tha precipitated

calcium phosphate, absorb other particles such as clay , cane wax, bagacillo ,bacteria, probably some polysaccharides including starch etc.

5) The following non sugars are ppted .1) Albumin -- both soluble and in soluble forms2) coloring substances – anthocyanin (a small % )3) Nitrogenous compounds – about 50-60 % of the total .4) pectins –a small %

6) When lime is added some of it under goes reaction and some removes in solution in the form of soluble cao, as indicated by noel deer on defecated juices.

Effects of defacation process :1) There is a small purity rise of 0.5-1.0 unit due to removal of above non sugars and collids in

lesser proportion .2) There is possibility of higherdestruction of reducing sugars upto 3to5%.3) The cao content rise from rawjuice to clarified juice is in the range of 250-300 ppm 4) This process is not helping for color removal.5) The waxes & gums etc are removed with calcium in filter mud.6) Results in less scale formation as compared to sulphitation

Sulphitation Process

sulphitation process is the 3rd developed clarification process after carbonation and defecation . In this process the purification of juice is arranged with mol and so2gas .The complete name of this process is sulphitation – defacation or sulpho – defecation process.This process is widely used for the production of plantation white sugar .

Reaction of so2 gas :

combustion of sulphur in the presence of air generates so2 gas.s+ o2 ----so2+2217 k.cal

So2 is higher soluble in water ,45 volumes of so2 are soluble in one volume of water .In the presence of moisture air, so2 oxidised to form so3 during the process of combustion of sulphur , this so3 when reacts with water sulphuric acid is formed.

so3 +H20 -----H2so4

The so2 gas combines with water to form sulphuric acid .H2o + so2 ----H2so3

This sulphurous acid is di basic and forms both normal and acid salts .H2so3 ----- H+ + Hso3 – -

H2so3 ----- 2H+ + so3 -- Equipments required for Sulphitation process:

1) Air dryer2) Air compresser 3) Sulphur burner : 1) Batch type

2) Continuos type a) cont. type with horizontal burning chamber.

b) film type sulphur burner.4) Juice sulphitation vessel.

Brief Outline of Sulphitation Process:

The juice which is free of bagacillo and weighed in the weighed tank of pH 5.2-5.3 is heated in the juice heater at 700c .this juice is subjected for continuous liming and sulphitatiion process to get the sufficient removal of non sugars and to have neutral pH of 7.0.This reactions are carried out inspected type of vessels called as juice sulphitation tank .This tank consist of cylindrical portion and conical bottom portion .it is also provided with stirrer having 16to18 rpm to have intimate mixiing of juice,lime &so2 gas . The capacity of the juice sulphitation tank sholud such that retention time of min 8minutes to be achieved for satisfactory ppt of calcium sulphite .The lime & sulphited juice is over flowed and same to the sulphited juice tank . From sulphured juice is transfer to the juice heaters to heat upto 1030c and then it sent to the clarifier via flash tank.Methods :

1) Pre sulphitation followed by liming & sulphitation 2) Pre liming follwed by simultaneous liming & sulphitation 3) Simultaneous liming & sulphitation 4) Shock liming & sulphitation

Fundamental Reactions taking place in sulphitation process:Phosphate ppt :

ca++ + Hpo4 ------ caHpo4

3ca ++ + 2po4 -- -------ca3(po4)2

Precipitation of calcium sulphite : ca(oH)2 ----- ca++ + 2oH – Dissociation of calcium hydroxide. so2 + H2o ----- H2 so3 Formation of sulphurous acid H2so3 ------ 2H + + so3 - - Dissociation of sulphurous acid ca ++ + so3 - - ------ caso3 precipitation of calcium sulphite 2H + + 2oH - ------- 2H2o Formation of un dissociated water molecule

Ca(oH)2 + H2 so3 ------ caso3 + 2H2oCalcium bi sulphite formation :

H2so3 ----- H+ + Hso3 –

caso3 ------ ca ++ + so3 - -

so3 - - + H + ------ Hso3 –

ca ++ + 2Hso3 - ------- ca( Hso3 )2

caso3 + H2 so3 ------- ca( Hso3 )2 calcium bi sulphite

EVAPORATION

The clear juice contains about 83 to 85% water, the remaining portion being represented by the sugar and impurities known as non sugar components. The object of evaporator station is to reduce the water content of clear juice to a level where the sugars are still in the dissolved state ie., the solution is unsaturated with respect to sucrose. The limit of evaporation corresponds with concentration of juice to syrup of 650 Bx and usually the syrup proceeding from the evaporator stage has a brix of 600 – 650 Bx. Thus out of the water to be evaporated for sugar crystallization nearly 80 – 85% is removed at evaporator stage.Multiple Effect EvaporationNorbert Rillieux, born in New Orleans in 1806, invented multiple effect evaporators in 1844 which is now used all over the globe. Rillieux’s Principles:Three important principles are involved in his invention.1. It states that in a multiple effect evaporator, each pound of steam supplied to the first body will evaporate one pound of water in each body in series. With ‘N’ effects one pound of steam evaporates ‘n’ pound of water.2. It states that if a weight of vapor ‘W’ is bled from effect ‘M’ of ‘N’ effects and used in place of steam for a given duty, the saving in steam is equal to, (M/N)*WEg. If bleeding is done from the first body of the quadruple set, the saving is 1/4 th the weight of the vapor used.3. In any apparatus in which steam or vapor is condensed, it is necessary to withdraw continuously the non-condensable gases which are unavoidable left in the heating surface compartment.TYPES OF EVAPORATORS

Special Multiple effects

1)Kestner evaporator - Designed by panl kestner based on principle of 'climbing film'. When a liquid is heated in a very long vertical tube the vapour bubble form at the base of the tube increase in size as they rise & their diameter rapidly attains that of the tube. Hence in the upper part of tubes, there is chain of bubbles, then a film of juice & entrained by friction of upward current of vapour.

a)Length of Tubes - generally 7 mt in length classen shown that maximum heat transmission occurs with tube length 4.9 mts.

b)Diameter of Tubes - Varies from 27 to 38mm tube length is 42 mm ID/45 mm OD.

Kestner Evaporation

Juice Level :- The kestner principle by realization of climbing film permits decrease of hydrostatic Head which would be caused by greater length of tubes. The bubbles in the tubes when occupies full diameter of the tube, break the continuity of liquid column in such a way that ordinary hydrostatic law is not applicable. The climbing film produced in kestner is due to friction of vapour with film of juice. The optimum juice level + in kestner is 20% length of tubes instead of 30 to 35 % in ordinary evaporators.

Vapour Separator - located between two vessels and slightly towards rear. The climbing film in the kestner evaporator actually exists only on the condition that juice is boiling juice most be heated to boiling temperature when it arrives to the vessel. Hence it is necessary that certain portion of the heating surface should be serve as a heater. If follows that kestner will not function on satisfactorily unless it receives juice very close to its boiling point. This is the point in which the kestner is inferior to the ordinary evaporators, which are very well suited for heating the entering juice as well as evaporating it. A kestner will be installed as a first effect on condition that it is fed with the juice at a temperature within 3 oC of the boiling point of juice in the vessel.

Application of kestner - The juice form a climbing film only when it is light while syrup climbs badly due to thick & viscous nature. Hence kestner is applied as first effect in evaporator set.

Advantages & Disadvantages

Advantages : 1) takes less space

2) juice passes rapidly in about 30 sees hence less inversion &

coloration when working at high temperature & high pressure.

3) Scale formation is much slow.

Disadvantages

1) Kesnter requires high head room.

2) If it is provided with independent separate requires more space.

3) Requires hot juice & supplementary heater.

Evaporation under pressure - Steam economy of the evaporator set depend on the bleeding vapours from the vessels in the set. A consideration has been given to raising the lower limit of the temperature range of a multiple effect in such a way that vapour from the last body would be at a temperature sufficient to ploy its full part in the principle heating application in the factory.

Author (Hugot) propose that lengths of the tube should decrease & their diameters increase from 1st to last vessel in keeping with the increase in brix.

Falling film Evaporators

These are class of evaporator vessels in which the juice travels from top to bottom as in general evaporator juice travels from bottom to top i.e. climbing film evaporation when the juice fed at the top of tubes, it descends in the tube entraining with vapours and collects in lower chamber where vapour separates & from which it is evacuated to the following vessel or condenser.

Advantages

1)They have a good heat transfer since there is no boiling point elevation due to hydrostatic pressure.

2)There is no expenditure of energy to produce upward movement of juice.

3)The evaporators are designed so that juice is in contact with heating surface in a thin layer over length of heating surface, hence the vapours being unable to leave from the upper portion is entrained with the juice & fills interior of the tube hence there are no vapour bubbles to obstruct the upward flow of juice as in climbing film evaporator.

The main problem with descending film evaporator is the distribution of juice on the upper tube plate. It is necessary that all tubes from this tube plate should receive required volume of juice so that the juice distributed only in a thin layer around the tube and that no section of the tube remains dry at any moment.

With this object, the total area of the tube plate is divided into enough sections to assure optimum wetting of the tubes.

m = Q/nxπxd

Q = wt of juice in kg/hr

n = No. of tubes

d = dia of tube

m = optimum wetting in kg/h/m

The coefficient of heat transfer is superior to that of climbing film evaporators hence falling film evaporator can work under an appreciably lower temperature drop.

On account of very brief period of juice contact, these evaporators may be operated with steam at 135oC or 2.1 kg/cm2 pressure, which is a principal advantage.

CRYSTALLISATIONAfter sulphitation of syrup to ph 5.0 to 5.4 the syrup is subject to vacuum pan boiling process. The pan boiling essentially consists of the removal of water by evaporation in single effect vessel known as vacuum pan and crystallizing out sugar by increasing the concentration. The vessel in which process is completed is called as vacuum pan. The function of vacuum pan is to produce and develop sugar crystals of desired size from syrup or the molasses known as mother liquour.

Object of Pan BoilingThe object of pan boiling is to obtain maximum amount of the sugar from syrup

through crystallization by concentration under vacuum in vessels known as vacuum pans.

Duty of Pan

a. Concentration of feedb. Nucleationc. Finishing the strike with tight Massecuite i.e. mixure of crystals suspended in the almost exhausted mother liquor.

The basic material used in the pan boiling is syrup of 550 to 650 Bx. and purity of 82 to 85. The entire process of crystallization on pan floor is conducted in three or four stages, the mother liquor from the final stage being discarded as final molasses. The feed material of the pans at second and later stages of boiling is of about 700 Bx. For recovering maximum sugar in crystalline form, high concentration of the massecuites under vacuum is essential consistent with the requirements of discharging the massecuite from the pans. The brixes of first grade massecuites i.e. 'A' strikes from which while sugar is produced are 92-94 while the final massecuites, whose mother liquor when separated is the final molasses are concentrated to 1000 to 1020 Bx. Concentrating beyond the certain

limits at any stage can cause unwanted secondary grain formation or conglomerates besides affecting the fluidity of the massecuites.

Why vacuum is Required

Vacuum is mainly required to lower the boiling point of the massecuite as at high temperature, sucrose is lost due to inversion and the colour of sugar increases due to caramelisation and such other reasons.

Further when the boiling point is considerably reduced by maintaining vacuum the temperature of the boiling massecuite being low, exhaust steam or vapour at lower temperature can be used as a medium of heat.

Principles involved in Pan Boiling

1. Solubility

Pure sucrose dissolved in water. This dissolution is called solubility. Solubility increases with increase in temperature. For Example 1 kg of water dissolves 2.334 kg sugar, at 40 0C, but at 80 0C 1 kg water dissolves 3.703 kg of sugar.

2. Saturated Solution

When a solution contains the total/maximum quantity of sugar/sucrose, which it can dissolve, it is said to be saturated solution. When this state of the solution is reached it cannot further accept a smallest amount of sucrose in it.

3. Solubility Coefficient

The solubility of sucrose varies with the impurities present in the solution. In case of Beet juice, impurities increase the solubility. The opposite hold with the cane products. In this case the solubility of sucrose decreases with decreasing purities.

The effect varies with the nature of impurities in each case. With cane, it is mainly reducing sugar which cause the decrease in the solubility of sucrose.

The solubility coefficient (s) is the ratio of the quantity of sucrose soluble in a given weight of water in an impure solution at a certain temperature, to the quantity soluble in pure water at the same temperature.

Solubility of sucrose % water in impure solution

(s) =

Solubility of sucrose % water in pure solution

According to Thieme in java, he states that the solubility lies in the neighborhood of 80% for typical temperature and purities.

4. Super saturation

When a saturated sucrose solution is cooled or when its water is evaporated in such a way that no sugar crystals are formed in the solution is said to be super saturated.

5. Super Saturation Coefficient

(S) super saturation coefficient is the ratio of weight of sucrose % water content in a super saturated solution to the weight of sucrose % water which would be present in a saturated solution having the same temperature and the same purity.

Sucrose % water in super saturated solution

(S) =

Sucrose % water in saturated solution

The value of this coefficient is equal to 1.0 or more than 1.0. The coefficient plays an important role in massecuite boiling. As the crystallization progresses the water content of the solution, decrease the Brix % of the material increases. In other wards the super saturation increases. Finally the super saturation state is reached and the coefficient increased from 1.0 to 1.5.

On the basis of super saturation coefficient, Weber distinguishes different zones in the super saturated phase. They are as under:

1. Unsaturated zone - Coefficient below 1.0

2. Meta stable zone - Coefficient between 1.0 to 1.2

3. Inter-mediate zone - Coefficient between 1.2 to 1.4

4. Labile zone - Coefficient above 1.4

1. Unsaturated Zone

In this state of boiling of sugar solution the super saturated coefficient is below

1.0. In this state no crystal formation takes place or the crystals already formed

get dissolved. Hence in Pan boiling this state is not desired at all.

2. Metastable Zone

The coefficient remains between 1.0 to 1.2. In this state of boiling existing

crystals grow in size but new crystals cannot be formed.

3. Intermediate Zone4.

The coefficient remains between 1.2 to 1.4. In this state of boiling new crystals

may form along with existing crystals. This is not a stable zone.

4. Labile Zone

The coefficient is more than 1.4. In this state of boiling existing crystals grow, and

new crystals form even in the absence of existing crystals.

All these states of boiling are based on super saturation coefficient for pure

sucrose solution. For impure sugar solution the values differ a little and depends

upon the purity of the solution. Coefficient 1.2 is an ideal state for pan boiling and

the boiling of massecuite in a pan should be conducted between 1.2 to 1.4

coefficient.

Mechanism of Boiling

The primary function of a pan, is to evaporate water in order to have

crystallisation of sugar. This fundamental process is taking place by way of heat

transfer from the heating media to the massecuite inside the pan. The most

important factors involved in this mechanism are-

1. Circulation of Massecuite

2. Effect of hydrostatic head3. Heat injury to massecuite / sugar crystals4.

1. Circulation of Massecuite

A sectional view of a single tube in the pan if viewed, the mechanism of the

boiling of the fluid and the massecuite flow within the pan can be studied. The

observations may be split into four zones as under.

MECHANISM OF BOILING

HIGH SUPER SATURATION AND

ZONE 4 LOWER TEMPERATURE

ZONE 3 SLAG FORMED

ZONE 2 BOILING COMMENSES

SUPPRESSED BOILING DUE

ZONE 1 TO HYDROSTATIC HEAD

The massecuite enters the base of a tube due to density difference which exists between the column of massecuite in and above the tube the massecuite in the return path. Observations are as under

Zone 1a. As the massecuite is in the lowest part of the tube, boiling is suppressed due to the hydrostatic head of the fluid.b. No change in the phase takes place.c. No contribution to the circulation as it contains no bubbles and thus a drag on the circulation fluid.d. Super saturation within the pan massecuite fails down due to higher temperature of it in this part of the tube.

Zone 2a. Bubbles being to appear at the wall and eventually become stable.b. Contribution slightly to the circulation as bubbles are formed.c. Super saturation begins to rise.

Zone 3a. Hydrostatic head/pressure of the liquid above the massecuite falls.b. Heat continuous to be added through the wall and massecuite rise in the tube.c. Size of bubbles increases due to(a) and (b).d. The bubbles ultimately increase and approach the size of the tube and slug flow results.e. This zone contributes most to circulation.f. Density of massecuite in this zone and the massecuite in the return path differs and the difference is the heighest.g. Super saturation rises further.

Zone 4 a. Hydrostatic head on the massecuite is lowest.b. Bubbles size expands much, so that, it occupies the size more than the diameter of the tube.c. Larger bubble size translates to a fast bubble rise velocity. Thus the inventory of bubbles in this zone is relatively small.d. This zone does not contributes to much circulation.e. Temperature of the massecuite drops as it looses vapour to equilibrate at the low static pressure.f. Super saturation rises rapidly.g. Ultimately massecuite leaves the top of the tube.

From the above mechanism of boiling in a single tube, the importance of the circulation of massecuite inside a batch type pan can be judged.

In modern sugar factories for crystallization only vacuum pans used. The vacuum pans are having number of advantageous over open pan boiling (Still in some khandasari units open pan boiling procedure being adopted). These advantages are listed below.1. Temperature of boiling massecuite is very low as compared to open pans. This avoids colour formation and caramalisation of sugar. This helps to produce a superior quality sugar.

2. Heating medium used is steam and could be controlled as per wish. In direct fire open pans this is not possible.

3. Time and energy saving.4. Various methods could be adopted in vacuum pan to improve exhaustion while in open pan it is not possible. In evaporator our aim is only to evaporate the water and concentrate juice upto desired brix. But in pan boiling is having various aims as follows.1. Evaporation of water as per need to increase super saturation. Separation of sucrose molecules from its associates impurities either by forming grain or by development of grain or by development of grain. Thus transfer of sucrose molecule from liquid phase to solid phase.2. Separation of irregular crystallization particles by centrifugation and get maximum sugar recovery by maximum exhaustion of mother liquor.In 1813, howard invented the first vacuum pans equipped with vapour pipe condenser, proof stic, sight glasses etc. This has a steam jacket around the lower portion.Now a day the most of the batch vacuum pans used in sugar industry are calandria pans. The advantageous of calandria pans over coil pans are1. Use of low pressure vapour in calandria pans2. Simple construction of heating surface3. Low graining volume could be kept.4. Very large heating surface could be arranged in comparatively small space.

Similar to conventional evaporators the calandria pan has tubular calandria with the difference that the tubes are of larger diameter and shorter length as also larger down take to facilitate circulation of high brix material of high brix material of circulation of the m/c of high brix material. Circulation of thick m/c is or almost importance in the design of calandria and this is brought about by providing large down-take for the heat transfer from the steam or vapour to the m/c is raises in the tubes and reaching the top of upper tube plates descends through the down take. The efficiency and speed of circulation of mass is an important characteristic of any calandria pan design.Advantages of low head pans are: 1. Low graining or footing volume, about 25-30% the strike volume. 2. Good natural circulation of m/c even when the strike level reaches the maximum limits, resulting in reduced time of boiling. 3. Higher H.s. to volume ratio can be maintain which adds to the speed of boiling.

DIFFERENT TYPES OF LOW HEAD CALANDRI PANS: In calandria pans the tubes are shorter and of large diameter than the

evaporator. In calandria pans m/c raises through the tubes of calandria. There must be an equal descending mass, corresponding to this ascending m/c. with liquid and mobile material in evaporator, this posses no problem. With thick and viscous m/c the manner

in which this circulation is planned and successfully effected forms the most important characteristic of the pan.They are several types of pans as follows: 1. Flat fixed calandria with central down take.2. Floating calandria 3. Inclined plate calandria 4. Pan with mechanical circulator

1. FLAT FIXED CALANDRIA WITH CENTRAL DOWN TAKE: The calandria consists of the top and bottom tube plates having brass

or steel tube plates. The tube plates are fixed to the shell. The steam or vapour heating medium remains outside the tube and the heated material flows through the inside of the tubes. The calandria is fixed to the bottom discharge cone of the body of the pan. The upper part of the pan above the calandria constructed with M.S. or C.I. has sufficient space for m/c and also for vapours. The bottom saucer joined to the calandria facilitates discharge of m/c from pan through a discharge valve provided in the saucer.

Generally the diameter of central down take is 40-50% to that of calandria. The tubes are made by brass. The diameter of the tube is 100mm and length of the tube varies from 700-1000mm. Thickness of the tube remains 2mm for all diameter of brass tubes.

The pitch of the tube i.e., distance between centers of two adjacent tube is generally 16mm higher than O.D. of tube. In the calandria pan of M.S. construction the down take is provided with deflector of fanned shape at an angle of 450 at the upper end of down take opening to prevent short circuiting of m/c flow. Usually one steam connection is provided and the condensate drained from the opposite side into a tank located on floor. The feed is connected to the bottom ne away from the discharge valve but in some design, the feed enter the pan in the downtake. The conical bottom of each pan is joined to the cutover line through an opening the valve for good circulation the height of boiling m/c should not exceed 1.5 mts. Circulation of m/c in the pan is of great importance. Circulation of the m/c is due to temperature and density difference between two streams (upward and downward).

The boiling material on being heated in the tubes has lesser viscosity and density raises upwards. Thus upward movement is aided by vapour bubbles which are formed in the heated portions of the m/c’s. As heated m/c goes upward the cooler m/c from the upper portion flows downward through central downtake in pans.

FLOATING CALANDRIA PAN: The floating calendria pan has been performed in the past by many designers due to:-

1. Very good operation performance2. Increase in the down take area

3. Efficient circulation.

Its operating performance generally has been very good. As the down take area was increased and thus an efficient circulation was established. The heating surface generally is less, due to the smaller calandria diameter, which cannot be offset by the heated calandria evolute. In some of the floating calandria pans center well analogues the evaporator vessel. It was provided with the annular space of two passages this might create further problem to the massecuite circulation.

To overcome this Hugot designed a floating calandria pan with only annual space down and heating surface at the center of the pan increase by adopting conical bottom plates and flat top tube plate. Thus the massecuite receives maximum heat input at center and consequent lifting force and descends on all sides along the cool outer wall of the calandria. This arrangement permits higher enlargement of the pan body above the calandria. The speed of the movement of the massecuite from ascending to the descending zone is lower than in a pan with central down take since the massecuite near the surface must move from the region about the ascending zone to that situated above the descending zone. With central down take the speed of the horizontal movement increase I the intermediate area separating these two zones since the massecuite has to flow radically in case of central well while with floating calandria this movement takes place in perfectly opposite style.

To avoids restriction to circulation entry of steam to pan s made through a pipe entering through save all and descending vertifically to center of the calandria. This arrangement found suitable for distribution of steam in the calandria and location of incondensable gas outlet.

One great disadvantage with floating calandria pans in that mechanical circulator cannot be installed in such pan.ADVANTAGES: 1. Better circulation due to uniform steam distribution.2. Low graining volume due to bottom cone type tube plate.3. Maximum heat input at center and consequent lifting force and descends on all sides along the cool outer wall of calandria.4. No adding and dead pockets.DIS ADVANTAGES: 1. Installation of circulator not possible.

2. Steam entry pipe at the center of the pan due to this restriction of the circulation.

3. Surrounding steam pipe m/c temp may be increased and caramalisation may be possible.

INCLINED PLATE CALANDRIA PAN: This may be fixed or suspended and plates may both be inclined at same angle or lower plate may be placed to a steeper angle than the upper. French engineer generally adopt 100 for the top plate and 10-250 for the lower plate.The object of the angle of the upper plate is to facilitate the washings and removal of the m/c remaining on the plate. This type of calandria causes the loss of heating surface and undesirable increase in the graining volume. This type of pan used in beet sugar industry. However the negative cone of the upper tube plate is not justified, as the calandria with flat offer little or no disadvantage from on un desirable increase in the graining volume.The positive cone of the lower plate, permits a useful gain in heating surface and useful decrease in graining volume.

ADVANTAGES:1. Low graining volume compared to flat fixed calandria.2. To facilitate the washing and removal of the m/c remaining on the plate.

DIS ADVANTAGES:1. The negative cone of the upper tube plate causes loss of heating surface an unnecessarily graining volume of the pan get increases.2. Condensate with drawl pipe is provided at the center of the bottom of the calandria. Any small leakage not dentified easily.

PAN WITH MECHANICAL CIRCULATOR The permissible range of super saturation is narrow. If super

saturation is raised too high new crystal nuclei appear causing false grain on the other hand if the solution becomes locally unsaturated, crystals will commence to dissolve and the result will bear erosion of the crystals will commence to dissolve and the result will bear erosion of the crystal surface. At a given concentration the degree of super saturation falls with raise in temperature. So, local hotspots due to slow movement of massecuite cause local unsaturated zones. These differences in super saturation caused due to inadequate circulation of massecuite in a pan can be avoided if mechanical circulator is provided to improve circulation in the pan.

Weber introduced a new mechanical circulator design after conducting studies on the circulation characteristics of pans with natural circulation.

Generally a shaft is inserted in the pan from top. At the bottom of central down take a screw pump impeller equal to the diameter of calandria is attached. Generally six vanes

are attached to the propeller drum to guide the descending column of massecuite towards the tubes of calandria. The shaft is supported in the centre of central down take by bearing and coupled to an electric motor trough a reduction gear located on top of the pan dome. An indicator ammeter provided for the motor serves to indicate the current and load variations during boiling which assists in regulating feed to the pan. A funnel shaped deflector 450 Angle and 8” height mounted at the edge of the center well prevents short circuiting of massecuite at the top of the tube plate by guiding the rising hot massecuite towards pan wall and avoiding the mixing of part of rising hot massecuite with down ward current of cooler massecuite.

Weber designs his circulator for velocity of massecuite in the tubes of 45cm/sec, which allows only 3 sec for contact with heating surface reducing substantially the maximum temp. However the velocity given to the massecuite is far from attaining that value, it has been measured and found to vary from 20cm/sec at the beginning of strike to 5-10cm/sec at the tightening of C-massecuite. Never the less this marks considerable improvement over pan with natural circulation. The load on electric motor goes on increasing as the massecuite level rises in the pan, reaching maximum during tightening phase. Average power consumption is about 1.05 KW/MT3 of capacity of the pan while the maximum would be 75% higher than average. The normal installed power will have to be 25% above the maximum. Speed of the circulator should be determined to the duty required.

A and B – Grain Strike … 80 rev / min.B strike (96-970 Brix) … 60 rev / min.C strike (100-1020Brix)… 50 rev. / min.

1) Higher power consumption.

2) Higher maintenance cost 3) Extra investments required.

2. Mechanism of Crystallisation in Pans:Crystallisation in pan is maintained by supersaturation which is itself achieved through four consecutive operations.1.Heating of the massecuite at the heating surface with formation of some small unstable bubbles.2.Movement of massecuite upwards to the boiling level during which movement some mixing of hot and cold material takes place.3.Evaporation – This takes place in the upper layer about 12 inches deep with the formation of stable vapour bubbles & if ideal conditions prevail with release of all these bubbles through the massecuite surface. This results in the cooling of the massecuite & an increase in super saturation.

4. The cooled & super saturated material should then flow vertically down the down take and later re-inter the heating surface for the commencement of another cycle. With a good pan the major part of the crystallisation will be done in the last period i.e., after the evaporation & before the reheating in the tubes. This part has been referred to as ‘Useful Crystallisation Period’.5.Heating surface should correspond to crystallisation rate. Both surplus and deficiency should be avoided.6.High ratio of tube area to plate area will promote velocity of material ensuring proper mixing.7.All the bubbles should be disengaged from material otherwise with the increase of hydrostatic head they will collapse raise the temperature and decrease the super saturation.3.Down TakeFor optimum circulation the pressure drop through the downtake should be as low as possible since such friction loss serves no useful purpose. Pressure drop through the tubes may be considered a useful loss as it is associated with the transfer of heat at increased pressure drop heat transfer is improved.

1. Circulation ratio is considered or practical importance through it is empirical figures. In old design it was kept below 3.0 where as in new designs it is kept below 2.0.

2. Mechanical circular is favoured for low grade highly viscous massecuite to improve the circulation. However, improvement in the design of natural circulation pan & if resistance to flow is minimized a natural circulation pan can handle the heaviest massecuite.

3. Older pans had very small centre well e.g. 20% of inside diameter. Hence very poor circulation. (‘Weber’ recommended down take diameter 50% of ID of pan. Tromp recommended 40% in Hawai Generally 40% of ID of pan is the practice adopted. This old concept of maintaining down take area in % (Percentage) to the tube plate area has no relevance in the today’s pan designs. The correct parameter to apply shall be circulation ratio.4. As the down take zone is an useful crystallisation zone no heating should take place during the stay of massecuites in the down take-in order to maintain achieved supersaturation. Therefore, the down take shell should be jacketed so that massecuite doesn’t come in contact with calendria shell of downtake.

CENTRIFUGINGThe machine in which crystals in the massecuite are separated from the surrounding molasses or mother liquor by a centrifugal force is called centrifugal. It is the last operation of sugar manufacturing process.

In India the centrifugals came in operation in early 1930 with the water driven centrifugal and then onwards developments in drive, speed, size etc., took place to the sophisticated continuous and fully automatic centrifugals with higher capacities. Now a days some factories have even replaced their self discharging machines by high speed fully automatic batch of 1750Kg/charge capacities supplied by various suppliers viz K.C.P., Wallchandnagar, Krupp India Ltd.

The advantages of using high capacity machines are 1. Lower space requirement. 2. The large size basket, namely 1000 kg/charge and above have very high movement of inertia. Fully loaded 1000kg basket has an inertia of the order of 550 kg sq. m. and similarly the 1750 kg basket is in the order of 975 kg sq. m. 3. This makes the acceleration and deceleration times very long and there by affect the productive time considerably. 4.The large size basket depending upon diameter and height require spinning speed between 1100 to 1300 rpm. Higher speeds even with high content massecuite make the sugar layer hard in the basket.

Drives: centrifugals mainly based on drives, before 50 years belt driven centrifugals were used. In some early installation of electric motor driven machines, the shaft is kept in line with the centrifugal basket by means of friction clutch. But the system of friction clutch results in trouble of working of brake lining. In the case hydraulic couplings are found better. But these couplings involve high power consumption almost more than double of normal consumption.There are two types of direct drives, which can be used. 1. Modern AC drive 2. Modern DC driveAC drives: Now a days AC motors of the totally enclosed Fan cooled – IP55 (TEFC) type in connection with appropriate frequency converters are used to drive centrifugal machines.

Depending upon the size of centrifugal the motor powers ranges from 110-355 K.W. (preferably 8-pol) DC Drives: Advantages of using DC motors with Thyristor controls. 1. Choice of speed of centrifugal independent on frequency of AC supply.2. Precise control of speed.3. Lower specific power consumption 3-4 KWH/ton of Sugar in massecuite.4. Elimination of peak current demand.In Thyristor control ON/OFF signals are through PLC system, hence power consumption is somewhat on higher side.

Importance of Washing in Centrifugal Machines

Washing in centrifugal is designed to remove the high colour molasses layer adhering to crystal surface. Once this has been achieved further washing serves to shrink the crystal by dissolution without further reducing the colour.Water washing à It is theoretically given that wash water is to be applied when 75% of molasses has been eliminated. Steam washing à By applying steam to the sugar layer during spinning speed, the temperature & fluidity of the material is maintained. Quantity of water & steam to the sugar washing :

As per Noel Deer values are Water 1 kg / 10 kg of sugar or 10 % on weight of sugar.Steam 1 kg / 05 kg of sugar or 20 % on weight of sugar.But practically following figures can be considered.Water maximum 4 - 5% on sugar or 2 –2.5% on massecuite.Steam maximum 20 % on weight of sugar means 10 % on weight of massecuite, with 100 TCH = 30 tons massecuite.The steam required will be 3 tons / hour = 3% on cane.It is higher value. So we take 2% on cane & it is sufficient.

SUGAR CONVEYORSThe sugar leaving the centrifugals is hot at 60-700C and contains 0.5-1.5% moisture, and as such it cannot be bagged for sale. Hence drying of sugar is done.White sugar discharged from centrifugals is conveyed on grass hopper to sugar elevator chute. Sugar conveyors are of three designs1. Screw or scroll conveyors2. Grass – hopper conveyor3. Belt or slat conveyor

Sugar Dryers- Types of dryers:1. Rotary Drum Dryers2. Rotary Lauver Dryers3. fluidized Bed Dryers4. Rotary Tray Dryers The rotary tray dryer is no more used because -Poor contact between sugar and air- High damage to sugar crystal- Low drying efficiency- High energy damage- High retention timeThe most important points in selecting the dryers are:1. Drying and cooling to the desired level at lowest possible energy uses2. Capital requirement, operating cost (mainly energy requirement), ease of operation.

Sugar Elevators:It is essential to transport the dry sugar from hopper to top of sugar grader which is accomplished by a bucket elevator.Grader:The grading of sugar involving separation of different size grains is performed in the sugar house. The elevator delivers the sugar from hopper to the top of the grader through a wide chute. A rotating distributer spreads the incoming sugar over the entire width of the top coarse screen. The function of the grader is to classify sugar into different size grains and to separate small lumps as well as dust from the sugar to be marketed.

PACKING AND MARKING:

Packing- Crystal sugar shall be packed in clean, sound and new A-twill jute bags (see IS 1943). The bags may be lined with polyethylene film. The mouth of each bag shall be either machine stitched or rolled over and hand stitched. If hand stitched, the stitches shall be in two rows with at least 14 stitches in each row.

Marking -Each bag shall be suitably marked so as to give the following information:1. Name of the place where the producer of sugar carries on the business of manufacture of sugar by the vacuum pan.2. Grade of sugar at the time of packing in terms of Indian Sugar Standards in force at the time of manufacture, ensuring the quality of sugar at the time of delivery.3. Net mass of sugar in the bag.4. Year of packing (year being the period beginning on the Ist of October and ending on 30th day of September of the following year).5. In the case of sugar obtained from the re-processing of : (1) Damaged or defective or rori or brown sugar of any previous season, or (2) Sugar house products left in process at the end of any previous year and not already included in the production of that year, the marking on the bag shall indicate the year in which it was re-processed.