Manufacture of Sugar from Sugarcane | Chemical Technology - Chemical Engineering PDF Download

28.1 Introduction

 Sucrose is a disaccharide that occurs naturally in most  fruits and vegetables.

Sugar occurs in greatest quantities in sugarcane and sugar beets from which sugar is separated economically and commercially.

Chemical formula – c₁₂H₂₂O₁₁

Molecular weight – 342

Density = 1.58 kg/m3

Sucrose is soluble in water but slightly soluble in methyl alcohol and ethyl alcohol.

Process flow sheet: Illustrated in Figure.

Figure 28.1 Flow sheet of manufacture of sugar from sugarcane

Raw material: Sugar cane

28.2  Functional role of various processes (Figure 28.1)

(g) Cutter

• The cutter consists of knives on a cylindrical shaft which rotate at a velocity of 400 to 500 rpm.
• The knives cut the canes into small pieces.

(h) Crusher

• Canes are shredded here.
• It consists of two rollers rotating in opposite direction.

(i) Series of Pressure mills

• Crushed canes are passed through four pressure mills to extract juice.
• Each pressure mill is made up of cast iron rolls.
• Rolls are grooved and the   width decreases from first roll to the last.
• Make up water  added in the third and fourth mill is recycled back to the first two mills.
• About 85-90% of juice present in cane is extracted.
• Bagasses are produced as byproduct.

(j) Clarifier

• In general two methods of clarification are available for the manufacture of white sugar, namely, Sulfitation process and Carbonation process.
• The juice now comes to thickener.
• To precipitate the colloids, calcium phosphate(CaHPO4) is added followed by milk of lime.
• The milk of lime used has 9 to 10% strength and about 400mg CaO/litre alkalinity.
• Apart from maintaining pH about 7, SO2 gas also acts as a bleaching agent.
• Phosphoric acid or CO2 can also be substituted as acidifying agent depending upon the type of extracted juice.
• At the bottom of clarifier, mud (impurities) are settled and drained.
• Steam is used to slightly heat the juice.

(k) Rotary filter

• The underflow mud from the bottom of thickener is passed to a continuous rotary filter press to recover sugar solution.
• This sugar solution if it is clear, is passed to multieffect evaporator or otherwise recycled back to clarifier.
• The filter cake produced is used for fertilizer.

(l) Multieffect evaporator

• The clarified liquor overflows to the 3-4 forward feed multieffect evaporator.
• Here juice is concentrated from 80-85% H₂O to 40% H₂O to make juice ready for crystallization.
• A vacuum of 63cm is maintained in the last effect.

(m)Crystallizer

• The clarified concentrated sugar solution comes to crystallizer.
• The sugar solution is further boiled in vacuum pans at vapor temperature of 57˚C until fine cloud of crystals is seen.
• Crystallization is completed in vacuum pan unit.

(n) Centrifuge

• The mixture of crystals and syrup is called masscuite.
• The masscuite from crystallizer is centrifuged in basket type centrifuged to basket type to centrifuge remove mother liquor (molasses) which is a byproduct.
• The high grade sugar crystals are obtained here.
• The centrifuge speed is maintained at 800 to 1000 rpm.

28.3 Technical Questions

1. What is special about the sugar production process from sugarcane

Ans: 

Sugarcane is a biological resource for sugar.  If sugar is used for the production of alcohols which can be in turn used as fuels in modern cars, then the original source for obtaining these fuels is the agricultural industry.  In other words, a biological source for fuels is an interesting technological concept for energy requirements of the modern society.  On the other hand, over consumption of sugars for fuel requirements could enhance sugar demand for food usage and could spiral up the prices of the sugarcane.  Therefore, a careful policy needs to be adopted for the utilization of sugar towards various process routes and technologies. 

2. Why last effect evaporator is maintained at 63cm vacuum?
Ans: 

• To allow the flow of juice from first effect to other due to pressure drop.
• To increase temperature gradient for better heat transfer.

3. What is the technical difference between sulfitation and carbonation clarification process?

Ans: Sulfitation process is based on large quantities of sulfurous acid with proportional quantity of lime. Calcium sulfite formed prevents oxidation and darkening of juice. It also precipitates gums and albuminous matter thereby it helps in filtration.

In this process at about 65°C, SO₂ gas is bubbled to juice to slightly increase acidity. The juice is then heated to boiling point for 2 hours. After sulfitation process, PH becomes 7 and concentration becomes 12 to 13 Brix.

In carbonation process, 1 to 1.5% of lime by weight at 50 to 55°C is added to the juice. CO₂ gas is bubbled to remove alkalinity. The temperature is raised to the boiling point to remove excess carbonic acid.

4. What are the byproducts of sugar industries?

Ans: Bagasse, filtercake and molasses.

5. Discuss energy economy in the sugar manufacturing process?

Ans: 

• Bagasse can be used to generate steam. Steam can be used for electricity generation which can be used to run machinery in the plant. Additional power can be exported to household usage and thus save consumption of fuel.
• Bagasse is also used as raw material for paper industries as fertilizer and cattle feed.
• The final mother liquor; molasses is sent for the production of ethyl alcohol by fermentation process.
• Press mud is used as phosphatic fertilizer in farms.

6. What is the problem in storage of sugarcane?

Ans: There should be no delay in transporting freshly cut sugarcane to sugarmill because after 24 hours of cutting causes loss of sucrose by inversion to glucose and fructose (monosaccaharides).

C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆

Sugar                   glucose           fructose

The extent of inversion is measured by polarimeter. The non inverted sugar has +97° polarization and completely inverted sugar has  -20° polarization.

This inversion of sucrose is maintained by quick delivery of freshly cut sugarcane to sugar mills.

7. Why vacuum pan crystallizer is used?

Ans: With the use of vacuum pan crystallizer, the pressure is reduced so as to super saturate the sugar solution. After supersaturation the formation of crystals starts. When the volume of massecuite (sugar solution  16crystals) exceeds certain limit, then it is transferred to centrifuge for separation.

8. What are the environmental problems faced by sugar industries.

Ans: 

• Flue gas(fly ash) produced by combustion of bagasse.
• The untreated effluent from sugar mills have BOD of 1.7 to 6.6 g/lit, COD of 2.3  to 8g/lit, TSS up to 5g/lit and high ammonium content.
• Mill generates  lot of dust and odor.
• Sometimes pesticides are  also present in sugar cane juice.

28.4 Starch 

28.4.1 Introduction

Starch consists of a chain of D-glucopyranosyl units. It is used in the manufacture of textiles, paper, adhesives, insecticides, paints, soaps, explosives, and derivatives as dextrins, nitrostarch and corn sugar.

Chemical formula- C₆H₁₀O₅ Starch gets hydrolysed by acids, alkalis and enzymes giving dextrin, dextrose. Starch forms gels with water within several minutes at 60-80˚C.

The major source of starch is maize kernels.

Process flow sheet: Illustrated in Figure. 

28.4.2 Functional role of various processes 

(a) Air cleaner:-

• At first, maize kernel containing 60-65% starch is introduced in air cleaner.
• Air is passed through the bottom of cleaner.

(b) Steeped tank:-

• Air cleaned maize kernel is passed to steeped tank to make it soft.
• 50-55˚C water and 0.15-0.3% SO2 is added.
• Here SO2 acts as a bacteriostatic.
• Steeping is done to yield high production and quality of starch.
• The residence time is 40-50 hours.

(c) Grinding mill:-

• The soft grains are passed through coarse grinding mill to rupture the cells.
• The floating germ oil from the tank is removed.

(d) Buhrstone mill:-

• The kernel is then wet ground in buhrstone mill.
• This mill completely disrupts the cells of endosperm and release starch granules.

(e) Nylon screen:

• The wet ground kernel is then passed through nylon cloth.
• Water washes the starch through the screens.
• The fibre and hulls is left over on the nylon screen.

(f) Two stage centrifuge:

• The gluten is water insoluble protein.
• Gluten is a light fraction which is separated in two-stage centrifuge.

(g) Rotary filter: 

• Water starch mixture is then passed then passed to continuous rotary filter.
• Starch is separated as filter cake which is of yellow colored and contains high amounts of protein.
• Other products can also be made by hydrolysis of starch.

(h) Dryer: 

• Starch as filter cake is dried and powdered in dryer with the introduction of steam in dryer and produced as pearl starch.

28.4.3 Technical Questions 

1.How is degermination carried out?

Ans: To free germs the maize kernels are coarsely ground in mill with cane to avoid oil leakage from germs otherwise oil will get soaked up with starch granules, which reduce starch quality. Lighter germs are separated by hydrocyclones.

For complete degermination, grinding and degeneration steps are performed twice. Then the germs are washed, dried and sold for corn oil production.

2.What is the use of steep water?

Ans: Concentrated steep water is consumed in the growth of penicillin and streptomysin. Steep water softens the kernels and release solubles.

3.How is gluten removed?

Ans: Gluten is starch milk, which contains water insoluble proteins. It is mostly separated by two successive nozzle type continuous centrifugal separators. The separation occurs due to density difference between starch and protein.