In majority of the units all over India Double Sulphonation process is used for Manufacturing of Sugar.
The basic process operations are as below,
Cane harvesting (Mechanical / Manual) and Transportation.
Cane preparation.
Milling-Juice extraction from cane
Clarification of cane juice (Sulphonation /Carbonation process)
Evaporation (Removal of water from clarified juice)
Vacuum pan Boiling (Crystallisation of sugar)
Centrifugalling (Separation of sugar crystals from Molasses)
Drying and grading of sugar.
Bagging and warehousing.
A) CANE HARVESTING AND TRANSPORTATION
The object of sugarcane harvesting is to produce sugarcane stalks of high quality. The quality of cane is reduced by physically damaging the cane, large amount of trash and delay in cane transport. The removal of tops is of prime importance in any harvesting operation. Hand stripping of cane reduces trash content to less than 5%. In mechanical harvesting, a machine cut the cane at the base using revolving knives and removes the tops with another set of revolving blades. Harvesting by mechanical means needs the removal of soil and other extraneous matter. This results in an enormously large quantity of water usage. Therefore, manual harvesting is desirable as it eliminates much of the trash and soil from sugarcane. The trash removed from cane serves as cattle feed and dry trash is used as domestic fuel.
B) CANE PREPARATION
Cane is prepared before milling:-
1. By revolving hubs, called kicker, which breaks the cane binders and makes the cane level uniform in the cane carrier.
2. By revolving blades, called knives, that cut cane stalks into chips but extracts no juice.
3. By revolving hammers, called shredders/ fibrizers, that shred the cane into long and thin pieces and opens the juice holding cells of cane but extracts no juice.
All the cane preparatory devices are located on the cane carrier, which takes the cane to the milling tandem.
C) MILLING
Extraction of juice from sugarcane is carried out in conventional mills. At the milling station, the prepared cane is passed through roller mills where juice extraction takes place. The classical combination of 3 rollers arranged in triangular form is the standard milling unit. Most of the factories use four to five sets of 3 roller mills depending on the crushing capacity of the plant. Each mill unit is commonly driven by separate drive unit either by steam engine, D.C electric motor, steam turbines or hydraulic drive. In order to enhance juice extraction, hot water is sprayed over the bagasse blanket as it emerges from each mill. The quantity of Imbibition process usually involves the use of recycled hot water.
The products of juice extraction process are mixed juice and bagasse. A major portion of bagasse (the single largest solid waste produced in a cane sugar factory) is sent to boilers for use as fuel for steam generation and the excess is either sent to open storage or baled for future use. The mixed juice is sent to clarification station after coarse straining and removal of fine bagasse particles.
D) CLARIFICATION OF CANE JUICE
Juice clarification in cane sugar factories broadly follows one of the following processes:
1. Chemical treatment for raw sugar.
2. Sulphitation
3. Carbonation
Chemical treatment using plant extracts is adopted in raw sugar manufacturing. However, Sulphitation is the major clarification process adopted in cane sugar factories in India.
At the clarification point, juice is subjected to thermal and chemical treatments. The incoming juice from milling station is weighed. After weighing the mixed juice, Phosphoric acid is added to it to increase the P2O5 content of raw juice to 350 ppm. It is then heated to 70oC temperature in juice heaters and then passed into juice sulphitation vessel. Milk of lime Ca (OH)2 prepared in a lime slacker by slaking lime stone with hot water, is added in the tank and the pH is raised from 5.5 to 9.00. Then SO2 derived from a sulphur burner is bubbled through the juice till the pH is lowered to 7.00 – 7.10. This procedure provides maximum flocculation to remove impurities.
The limed and sulphited juice is subsequently heated to 105 oC. The above treatment results in the formation of insoluble lime salts. Heating the juice slightly above the boiling point (i.e. above 103 oC) coagulates albumins and some of fats, waxes and gums present in cane juice and precipitates the suspended solids as well as fine particles.
After heating the juice to 105 oC, it is passed into a clarifier where polyelectrolytic flocculates are added to aid flocculation of impurities. The precipitated solids are separated by sedimentation. The products of clarification are clarified juice and settled impurities in the form of mud. The mud from the clarifier is filtered in rotary vacuum filters. The products of mud filtration are filtrate and filter cake. Filtrate is returned to the mixed juice tank and filter cake (Pressmud) is used for composting or applied in fields as soil conditioner. The clarified juice (also known as clear juice) from the clarifier is heated in a juice heater to 110 oC and then sent to Evaporators for concentration.
E) EVAPORATION
In the evaporator, the preheated clear juice with 15% solids concentration is evaporated in multiple effect evaporators to around 60% solids. A quadraple or quintiple evaporator system consisting of 4 or 5 bodies is commonly used in sugar factories. An additional pre-evaporator called semi kestners or vapour cells are also used in some factories.
Steam is supplied to boil juice in the first body, the vapour generated from it is used to boil juice in succeeding bodies. The vapour from the final body is condensed by direct contact with cooling water in a barometric condenser or multi-jet condenser. Condenser that maintains stepwise pressure and temperature drop across the bodies of the multiple effect evaporator.
The steam condensate from the calendria of the first evaporation body and the vapour condensate from the calendria of second body are usually free from entrained sugar and are normally recycled for use as boiler feed water. The condensate from third, fourth and fifth bodies are recycled for use as process hot water. The concentrated juice leaving the last body of the evaporator is called syrup and is subjected to a second sulphitation. SO2 gas produced from the sulphur burner is bubbled through the syrup in the syrup sulphitation vessel and the pH of 5 is attained. The colouring matter present in the syrup is bleached by sulphitation. Sulphured syrup is then sent to pan station for pan boiling and crystallisation.
F) VACUUM PAN BOILING
The main crystallisation of sugar takes place during pan boiling in vacuum pans of the boiling house. The function of the vacuum pan is to produce and develop satisfactory sugar crystals from the syrup. Vacuum pans are single effect evaporators and a barometric or multijet condenser supplied with cooling water is used to condense vapour from the pans by maintaining sufficient vacuum. Sulphited syrup is concentrated in pans by evaporation of water under vacuum using vapours from the first effect of the evaporator or exhaust steam as heating medium. Crystallisation takes place when syrup is saturated with sugar. At this point, seed grain is added to serve as nuclei for the sugar crystals, and more syrup is added as water evaporates. The growth of the crystals continues till the pan is full. The crystals and mother liquor form a dense mass known as massecuite. The contents of the pan is then discharged into crystallisers. Massecuite boiling is classified as A, B and C according to the purity of the massecuite. Mother liquor obtained from A massecuite curing will serve as feed material for B massecuite boiling and the mother liquor obtained from B massecuite curing serves as feed material for C massecuite boiling. The molasses from C massecuite curing has residual sugar and its recovery is uneconomical. Hence, it is stored in steel tanks after weighment and sold to distilleries and cattle feed manufacturers. Due to the presence of impurities in the syrup, the sugar present in it couldn’t be crystallised in single stage. To recover maximum sugar from syrup, three massecuite boiling system is adopted. A and B massecuites are higher grade massecuites and C massecuite is the low grade massecuite.
Condensers attached to evaporators and pans are the major units using about 90% of the cooling water required by the plant.
G) CRYSTALLISERS
The crystallisation of sucrose from the mother liquor in low grade massecuite can’t be carried to sufficient completion in the vacuum pan alone due to high viscosity and rapidly diminishing crystallisation rates. Therefore, the massecuite is discharged into a crystalliser where crystallisation in motion takes place until mother liquor (molasses) is adequately exhausted. This involves a cooling operation in which it cools from pan temperature to near ambient temperature. The progressively lowered temperature reduces solubility of sucrose and forces crystallisation to continue.
Crystallisers are cylindrical or U shaped vessels with low speed stirrers. Some factories currently use continuous vertical crystallisers. Forced cooling with cold water is employed in these units for cooling C massecuite. The cooled massecuite from the crystallisers is reheated before feeding to centrifugals for separation of sugar crystals. A and B massecuites are cooled in air cooled crystallisers and then sent to centrifugals without reheating.
H) CENTRIFUGALLING
The massecuite from the crystallisers is drawn into the centrifugal machines to separate out the sugar crystals from the mother liquor (Molasses). The cylindrical basket centrifuge with perforated sides permit the molassses to drain out. Washing the massecuite, with hot water or steam or both, is a common practice. The mother liquor molasses is returned to the next vacuum pan for reboiling.
In three massecuite boiling scheme, the first boiling of raw syrup yield A massecuite which is centrifuged to produce A sugar and A heavy and A light molasses. A light molasses is recycled to A boiling. A heavy molasses is sent to B massecuite boiling and B massecuite is produced. B massecuite is cooled in crystallisers and cured in B centrifugals which yields B sugar and B heavy molasses. The B heavy molasses is sent to C pans for reboiling and C massecuite is produced. C massecuite is cooled for about 36 hours in air and water cooled crystallisers and then cured in another centrifugal to yield C fore sugar and final molasses. Final molasses is weighed and stored in steel tanks. C fore sugar is magmised with water and then cured in another centrifugal to yield C seed and C light molasses. C seed is completely melted and sent to A massecuite boiling. C light molasses is recycled for C massecuite boiling. B seed sent for A massecuite boiling and excess B seed is melted and consumed in A massecuite boiling. Final molasses have a heavy, viscous constitution and contains approximately 30% sugar, 20% reducing sugars and the remaining ash and water. In addition to 3 massecuite boiling system, 4 massecuite-boiling schemes are also adopted in some sugar factories depending upon the purity of cane juice and the final molasses purity desired.
I) DRYING OF SUGAR
The purged A sugar from A centrifugal machine is discharged into an open vibrating sugar hopper to enable natural drying or hot air is passed through the sugar to aid drying. After drying, sugar is cooled by blowing cold ambient air through it. After drying, sugar is sent for sieving in the vibrating sugar grader. Lumps, Rori, twin grains and powder sugar are segregated, melted and recycled for A massecuite boiling. The white crystal sugar confirming to Indian Sugar Standards (ISS) is elevated to sugar storage bins.
J) BAGGING AND WARE HOUSING
The dried final sugar product is weighed and bagged in 100 Kgs. jute gunny bags. These bags are taken to sugar godawons where they are stored prior to sale.
A) Floor washings and boiler blowdown:-
A considerable amount of waste water results from floor washings which contain oil and grease and spillover of juice etc. The BOD concentration of this waste may be in the range of 1000 to 1500 mg/l. The boiler blowdown may not contain any organic pollutants, however, the inorganic contents are high.
B) Soda and Acid wash waters:-
It is general practice to clean heat exchangers and evaporators with caustic soda and hydrochloric acid in order to remove scale on the surface of tubings. Normally the caustic soda washings are stored and reused for next cleaning operation. However, most of the sugar factories discharge this waste stream into drains. If recycling is followed, not only the problem of disposal of waste would be solved but a considerable saving of chemicals can be achieved. After the equipment is boiled with caustic soda and rinsed with fresh water, the equipment is treated with dilute hydrochloric acid along with an inhibitor. This wastewater is discharged into drains, as the recovery of dilute HCL may not prove to be economical.
The washings of soda and acid contribute considerable amount of organic and inorganic pollution load and would lead to shock load on wastewater treatment plants. These washings should be stored in the holding tank and discharged in a gradual manner into waste treatment plant or allowed to dry in evaporation pits and the sludge can be disposed off in low lying areas.
C) Excess condensate turbine overflow:-
The condensate does not normally contain any pollutants and is used as boiler feed water, washing operations, imbibition water etc. It may so happen sometimes that it may get contaminated with juice due to entrainment in which case it is let out into wastewater gutter. The treatment requirement in this case is nominal and can be let out directly as irrigation water after cooling or can be stored in a pond to percolate into ground which would help to recharge ground water table.
D) Condenser cooling water:-
In some industries mini cooling towers are used to cool waters and recycle back for process. Condenser cooling water is recirculated unless it gets contaminated with juice due to defective entrainment separators, faulty operation beyond the design rate capacity of evaporators etc. If it gets contaminated the water would go into drain invariably. The best way is to avoid entrainments and if it is not possible to recharge into groundwater after neutralisation.
E) Sulphur and lime house:-
The washing of sulphur and lime house may contribute a inorganic solids concentration. The sulphur burner and compressor cooling waters can be recycled after passing through spray pond / cooling tower.
F) Vacuum filter washing:-
The filter is washed occasionally and this would contribute considerable amount of suspended solids. This waste contains both organic and inorganic pollutants. The waste streams may be diverted to holding tank.
Waste water streams.
The characteristics of industrial streams as well as combined effluents are given below
Sr. No | Type | Oil & Grease | pH | COD | BOD | TS | TDS | SS | Chloride |
1 | Mill house | 40-80 | 6.0-7.7 | 1500-2000 | 800-900 | 600 -800 | 300 -400 | 300-400 | 60-80 |
2 | Boiler | 20-40 | 9.0-10.0 | 300-500 | 200-300 | 3000-3500 | 2800-3400 | 100-200 | 80-160 |
3 | Floor washing | 100-400 | 5.0-6.0 | 4000-6000 | 2000-3000 | 3000-4000 | 2500-3400 | 500-600 | 40-60 |
4 | Pan section | 20-40 | 4.0-4.5 | 800-1000 | 200-300 | 1000-1200 | 800 -900 | 200-300 | 100-150 |
5 | Boiling House | 10-20 | 4.0-4.5 | 800-2000 | 450-850 | 1000-1900 | 900-1700 | 100-200 | 30-50 |
6 | Filter washing | Nil | 4.5-7.0 | 10,000-15,000 | 4,000-6,000 | 15,000-20,000 | 12,000-15,000 | 3,000-5,000 | 200-300 |
7 | Cleaning waste | 100-200 | 4.0-5.0 | 4,000-6,000 | 2,000-3,000 | 7,000-8,000 | 6,500-7,000 | 500-1,000 | 150-200 |
8 | Excess condensate | Nil | 6.0-7.0 | 100 -300 | 50-100 | 500 -800 | 400 –600 | 100-200 | 30-50 |
9 | Spray pond overflow | Nil | 7.0-7.5 | 100-300 | 50-100 | 600-1000 | 500 -800 | 100-200 | 30-50 |
10 | Combined | 100-150 | 5.0-5.5 | 2000-3000 | 800-1200 | 2500-3000 | 1000-2000 | 300-400 | 80-100 |
Note : All values except pH are in mg/l.