Chapter 5 Sugar Manufacturing

Chapter – 5 Sugar Manufacturing Why we get two different colours of sugar? 1 Introduction • Have produced from two main commercially known raw materials Sugarcane  sugar beat • Manufacturing steps: 1. Extraction of juice 2. purification of juice 3. Concentration or evaporation 4. Crystallisation 5. Separation of crystals 6. Refining of crystals 7. Recovery of sucrose from molasses 2 1. Extraction of cane juice • Washed and cut into small lengths with a sharp rapidly moving knives (500600 rpm) • The crusher converts the cane into chips , then by passing the chips into a series of mills the juice is extracted. • All the spent bagasse is collected in the storage which is then either burned for fuel or used for the manufacture of paper, hard board and insulating materials. 3 2. Juice purification/clarification The principal aims in purification are: • To remove suspended impurities in juice, to bring down color contents, to obtain juice free from turbidity and to achieve purity by reducing the overall non sugar content in juice. • The juice obtained in extraction process is brownish green opaque liquid contain 10-20% sugar. • Impurities: Colloidal carbohydrates, water, organic matter, colouring matter, reducing sugars, gums, mineral salts, fine particles of bagasse and small quantities of vegetable proteins. 4 Defection • The cane juice is acidic (pH 5.1 to 5.7) which leads to inversion of sucrose to reducing non-crystallisable sugars. So, it should be immediately made alkaline because on standing the juice the pectic substances present will undergo fermentation into pectic acids. Those pectic acids catalyse the change of cane sugar into non-crystallisable sugar. • Defection is the preparation of raw liquor for filtration and clarification by removing solid impurities. • Neutralized by adding adequate amount of milk of lime till the pH reaches 7 to 7.3 5 Cont’d….. • The treatment precipitates pectic substances as insoluble calcium pectates, coagulates the vegetable proteins, gums etc and converts sucrose into soluble calcium sucrosate. The limed juice passed through a tubular heater and heated till boiling. • The hot limed juice pumped into settling tank, where it separates into three layers (a thick precipitated scum at the surface upper layer, clear juice middle layer and heavy precipitates or mud at bottom layer). • Removed mechanically by passing through a filter press. 6 Double Sulphitation Process Cane harvesting crushers cane preparation weighing crushing and milling 10 and 20 juices 𝑆𝑂2 Setting in a settling tank Sulphited juice heated to 103 oC Mills Weighted juice heated to 75 oC 𝑚𝑖𝑙𝑘 𝑜𝑓 𝑙𝑖𝑚𝑒 1st sulphited juice pH=6.9 – 7.1 Filtration in rotary filters evaporation 𝑆𝑂2 Muddy waste are used for fertilizer and dry filtrates are added to weighed juice Unsulphured syrup subject to 2nd sulphitation) Sulphured syrup (pH=5.5) Massecuite (crystallised sugar + molasses) evaporation Crystallization Cont’d Massecuite (crystallised sugar + mother liquor) Wet sugar Centrifuging Mother liquor (molasses A) Boiling and centrifuging Drying Cooling Sieving or grading Bagging, weighing and storage Low grade sugar used as external seed in crystallization (not bagged) Mother liquor (molasses B) Low grade sugar used as external seed in crystallisation (not bagged) Final Molasses 8 Double Carbonation:Double Sulphitation Process Cane harvesting weighing cane preparation crushers crushing and milling Mills Mixed juice o 𝐶𝑂2 Weighted juice heated to 55-60 C 𝑚𝑖𝑙𝑘 𝑜𝑓 𝑙𝑖𝑚𝑒 is limed and carbonated Filtration and Setting in settling tank Filtered juice undergo 2nd 𝐶𝑂2 carbonation 𝑺𝑶𝟐 1st 2nd carbonated juice (pH= 8.5) Unsulphured syrup treated with SO2 for 2nd stage (pH=5.5) carbonated juice 1st carbonated juice pH=10.5 – 11 10% of the juice is precipitated as impurities 2nd carbonated juice heated to 75-80 0C and filtered Clear juice undergo evaporation (75% complete) Mud as a waste 2nd filtered juice 𝑺𝑶𝟐 1st sulphured syrup (pH=6.9-7.1) The same as in the previous method 3. concentration/evaporation • Concentrated into a thick syrup under reduced pressure in multi-effect vacuum evaporators. 4. Crystallisation • after a further concentration of the syrupy liquid to remove most of the water left with a single effect evaporator (vacuum pan), it is allowed to cool slowly. • the process involves three stages: clustering of crystals, nucleation and growth. 10 Cont’d… • the crystallisation usually carried out by adding sugar crystals externally into the syrup to induce (fasten) the crystallisation process which saves fuel, the time and disposes off low grade sugar. 5. Separation of crystals • Crystals of sugar are separated from molasses (mother liquor) by whirling in centrifugal machine (1200-1450 rpm) • Last traces of molasses still sticking with the sugar crystal are removed by spraying cold water and whirling again. 6. Drying: by dropping them in a long pipe through which hot air or super heated steam is passed. 11 7. Refining • Sugar crystals obtained are dissolved in hot water and then boiled with animal charcoal, coconut charcoal, polymeric media-synthetic ion exchange resin or adsorbent resin or bone char for decolourization of the solution, then filtered. • The filtered solution is concentrated under pressure in multi-effect evaporator. 8. Recovery of sugar from Molasses • The molasses obtained from mother liquor still has considerable amount of sucrose. The mother liquor is boiled, cooled slowly when some fresh crystals separates out. • The crystals obtained are diluted with water and treated with strontium hydroxide. Distrontium sucrosate will precipitate out and filtered through a filter press. 12 Cont’d… • Distrontium sucrosate ppt will decomposed into sucrose and strontium carbonate when a strong current of CO2 is passed through in it. Such sugar recovered from molasses in this manner is low grade and mainly used for manufacture of alcohol. Sugar quality analysis • • • • Visual colour comparison Sieve analysis pH Polarimetry measurement 13 Refining process of sugar manufacture 14 15