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Nigeria Sugar Manufacturing By Milling Process
Pemas Engineers Ltd provides services in cane sugar industry as professional engineers and technical adviser in cane sugar manufacturing by milling process specializing in project planning,feasibility studies,technical reports, factory operations,optimization of steam and power requirements,and balancing of plants and equipment for better efficiency. Basically, sugar manufacturing by milling process goes through the process of sucrose extraction, clarification, evaporation, sugar boiling and sugar drying and bagging using various plant and equipment and chemicals along the production line. The boiler plant is required for steam production for power plant, mill and knife turbines and steam exhaust for process use. The sugar crystals produced can be in the form of raw sugar or refined sugar depending on the extent to which colour in the grains formed have been removed. In this regard, some factories stopped their sugar process at what is known as “Brown Sugar” stage without going further to the next stage referred to as “Refined White Sugar”. Sugar is sucrose extracted from cane or from Beet. Cane and Beet are different but sugar can be obtained from them using different technologies. Sucrose from cane is extracted by milling or diffusion process while sucrose from Beet is essentially extracted by diffusion process. Sugar production from Beet is more expensive than sugar production from cane reason being that while a cane sugar factory can work conveniently for about 5 – 6 months, a Beet factory can only work for about 3 months because Beet is grown from seed sown in the spring. This means a cane factory could be only half of a Beet factory in size. From 0.680kg of beet, it is possible to get 0.113kg of sugar. In this write up, we are concerned with cane sugar manufacture starting from the raw material, which is cane. The basic agronomic practices involve the following a. Land Development/Preparation This process is capital intensive since heavy agricultural machinery are needed for various operations. Among these operations are tree felling, clearing, filling and levelling, land surveying, construction of drainages and canals. b. Seed Planting This is done manually or mechanically depending on the farm size, geological nature of the soil and availability of equipment and labour. c. Fertiliser Application Various types of fertilisers are available for use depending on the type of soil. These include NPK and Urea. Other source of manure like the filtered mud could also be applied for improved yield. d. Weeding This is the process of eliminating competitive weeds e.g. grasses. This could be done manually or mechanically depending on age of sugarcane. Chemical weeding is also common in most cases. e. Irrigation This is the method by which water is applied at a predetermined cycle. This is done either by surface or overhead systems. This is a very important aspect of cane growing; hence, a permanent source of water must be available for irrigation during the dry period of the year. f. Harvesting Canes are harvested at maturity. Maturity in cane is determined by the purity at the harvesting period, which must be above 80%. Harvesting could be done manually or mechanically depending on the nature of the cane and furrow type. However, manual harvesting is common due to availability of cheap labour during the dry season. THE MANUFACTURING PROCESS STAGE 1: Extraction Of Sugar a. The weighbridge b. The derricks c. Conveying and preparation d. The mills The first operation is weighing the cane. The cane weighbridge has a capacity of 30 tonnes. The cane is brought to the factory in cane carts by tractors. The unloading derricks system is capable of handling 120 TCH. The feed tables, which are also suitable for 120 TCH, transport the cane onto the 1st feeder carrier, which runs under a leveller knife set. The leveller knife is driven by a 205KW steam turbine. The clearance between the carrier and the knife is small to allow the cane to be cut to pieces. The cane on the first feeder carrier is passed to the auxiliary carrier which runs under a heavy-duty knife set driven by a 205KW turbine. The knives further cut the cane into further smaller pieces. The cane leaving the auxiliary carrier goes on to the main carrier and enters the heavy-duty shredder for fine preparation before entering the mill. In the mill house, the cut cane will pass successfully through four mills, each incorporating three rollers. These rollers will be disposed in such a way that there will be two squeezing of the cane per mills – the first between the top and feed roller, and the second between the top and back roller. The top rollers will be free to move vertically to a limited degree in order to provide for any uneven thickness of the cane blanket, and to ensure that a steady pressure is at all times maintained. This pressure will approach a total of 450 tonnes on each top roller, and will be imposed by means of hydraulic pressure cylinders operating on the bearings. In order to provide for the increased throughput of cane up to 4000 tonnes per day, which will be required at a later period, provision is made in the layout of the mill house for the installation when required of two additional mills. The mills are driven by steam turbines each developing up to 410KW. The normal speed of the turbines will be 5700 rpm, which will be reduced to the mill speed of 4.3 rpm through heavy multiple gearing. In the first instance, - with the crushing requirement at 2000 tonnes cane per day – the mill was driven in pairs through gearing by the turbines. This gearing was designed, however, such that, when the throughput of cane is increased and more power per mill becomes necessary, a re-arrangement of the gear wheels and the addition of further turbines could be effected to provide for one turbine to drive each mill. The stopping of the turbines – either singly or all together – and individual adjustment for speed is effected from a central control platform located in a position from which one attendant will have a free access to the plant in operation. The juice squeezed out of the cane by the mills is passed through mechanically operated strainers and is then pumped across to the process house where, before being treated, it will be weighed automatically for purposes of process control. STAGE 2: Production of Brown Crystal Sugar. This stage comprises the following successive operations:- a. The clarification of the juice by the action of chemicals and heat, and the subsequent precipitation of the impurities, which are thereby separated from the liquor. The precipitate – normally called “mud” – is subsequently filtered to enhance the recovery of juice, and hence of sugar. b. The evaporation of the large quantity of water, which is present in the juice, followed by, further concentrated to produce sugar crystals. c. The separation of the crystals from their mother liquor by the application of centrifugal force. The first operation is the clarification of the juice expressed by the milling plant, which has a dark opaque yellow/green colour due to occluded dirt and other impurities, which in addition to sugar, are extracted from the cane during milling. For clarification a small quantity of Sulphur dioxide gas is first bubbled through the juice, after which a controlled amount of milk of lime is added. The chemically treated juice is then pumped through heaters to bring the temperature to around 215oF, after which it is passed to the clarifier. In this the settlement of impurities takes place, and the resultant clear juice is decanted off from above in to continuous rotary vacuum filters, which separate the remaining juice, which is returned to process. The second operation in the process house is to remove the greater part of the water contained in the juice. This is done in a multiple effect evaporator, which utilizes the exhaust steam from the various turbines as the heating medium. The juice passes successively through four vessels and the vapour provided by the evaporation from each vessel is transferred in turn to the next. By this means the juice is concentrated to a thick syrup at a relatively low boiling temperature in the fourth vessel. The vapour from this vessel passes to a condenser, which maintains the vacuum. The clarified juice enters the evaporator containing 85% of water by weight, and emerges as a thick syrup containing about 35%. Further operation in the process house is to continue to evaporate the water from the syrup such that, when the liquor becomes saturated, crystals of sugar will form and can be grown to the size required under carefully controlled conditions of boiling. This operation is performed in vacuum pans, which are large vessels, similar to those of the evaporator, but which operate individually on an intermittent batch system. This boiling operation produces a composite mass of sugar crystals embedded in heavy molasses, which is termed massecuite. On discharge from the vacuum pans the massecuite gravitators into crystallizers located below, in which it is kept in motion by stirrers until in a condition for passing to the centrifugals. The third operation in the process house is the separation of the crystals from the molasses, this being effected by spinning the massecuite in centrifugals, which are in the form of an open perforated drum attached on a vertical spindle driven by an electric motor. When the basket rotates the sugar crystals are retained within the screen of the basket, and the molasses passes through the fine perforations. The centrifugals operate at speeds of 1000 rpm and 1500 rpm, creating in the massecuite forces up to 1500 times that of gravity. At the conclusion of stage two a brown crystal sugar results. This is the “raw sugar” of commerce, and contains 97% to 98% chemically pure sucrose. The brown colour is due to an envelope of mother liquor with which each crystal is covered, together with a proportion of impurities, which are occluded with the crystal structure. STAGE 3: Production of White Sugar. The removal of the molasses and the other impurities from the brown sugar produced in stage 2 will be effected by re-melting the sugar and re-crystallising this operation being done in the white sugar section of the process house. The brown crystal sugar is dissolved in water under controlled conditions to produce a liquor of desired density at a particular temperature. Thereafter this raw melt liquor is subjected to chemical treatment by the metered addition of a solution of a phosphoric acid followed by a controlled amount of milk of lime. The liquor is then aerated and fed to a clarifier in which the precipitated impurities float to the surface to form a scum and the clarified liquor is drawn off from below. Both the liquor and the scum are filtered, after which the liquor is passed for boiling in the white sugar vacuum pans, which are operated in the same manner as those used earlier in the production of brown sugar. The separation of the white sugar crystals from the massecuite is carried out in the white sugar centrifugals. The product from these machines is thereafter passed through a sugar drier to reduce the moisture content to less than 0.05%. From the driers the sugar is conveyed out of the process house in 50kg bags to the sugar store. Steam Generation and distribution. The crushed cane – called bagasse – from which the juice has been extracted by the mills, will be conveyed to the boiler house and will be burned directly as fuel in specially designed furnaces. The generation of steam for the total power and heating requirements of the factory will normally be derived from this fuel source. Steam will be generated in the boiler at a pressure of 250 p.s.i.g, and will be sued directly for driving the steam turbines operating the mills and electric generators. The steam will be exhausted from these turbines at a pressure of 15 p.s.i.g, and will be distributed around the factory as the heating medium for juice heaters, evaporators and vacuum pans. A very useful factory by-product is molasses, which the company sells to Nigerian Yeast and Manufacturing Company Plc as raw material for their products. Molasses is usually 4% in cane. In Nigeria, only Nigerian Sugar Company, Bacita and Savannah Sugar Company, Numan produce refined white sugar. Lafiagi produces raw sugar otherwise known as “Mill White Sugar” while Sunti Sugar Company is expected to produce also “Mill white Sugar” when the factory is ready. In terms of plant and equipment of production, the process of sucrose extraction, clarification, evaporation, boiling, crystallisation and drying are the same. The differences being in different manufacturers designs and economics of operation. The plant and equipment reviews and operational performances can be related to the types of plant and equipment installed at respective sugar companies comparing them to where similar plants and equipment are installed elsewhere in the world and their operational performances. The capacity of the mills (factory) in terms of tonnes cane crushed per hour determines the capacities and input requirements of other plants and equipment in the factory. It is therefore necessary from the onset to assess the mill capacities as installed for the factories while from the operational point of view the cane milling efficiency is equally assessed. |
PEMAS ENGINEERS,NIGERIA
