Mhlume Factory
Mhlume Factory

Processes

 

 

The Sugar Production Process

The sugar production process begins with accepting sugar cane from growers and weighing it at weigh bridges to determine each grower’s quantity of cane for payment purposes. The cane is delivered in trucks and tractors by road.   After off loading the cane is then prepared by cutting it into pieces in rotating primary and secondary knives. The cut cane is then shredded in rotating hammers into a fine pulp like substance to expose the juice containing cells. The juice is then extracted either through four roller mills that squeeze out the juice or through a diffuser which washes the juice from the fibre cells.   The extracted juice is then purified by adding lime to it in a large tank called a clarifier in which clear juice is extracted at the top and settled mud is removed from the bottom. The mud is filtered to remove any remaining juice and the filter cake is used as manure in cane growing. The clear juice is evaporated to remove the water, resulting in syrup.   The syrup is further boiled in pans and fine sugar crystals added to form a nucleus.  Full size crystals are then obtained, surrounded by mother liquor. The mother liquor, called molasses, is separated from the crystals in rotating screens through centrifugal forces and the sugar crystals are then dried in a drier using heat from steam.   The final sugar is then bagged and ready for sales.

In the refining process the sugar crystals before the drying stage are melted and passed through carbonation and sulphitation processes to remove the colour. After these processes the boiling in pans and separation of the mother liquor as above is then repeated and the final white sugar is dried and bagged for onward sales.

Cane Receipt and Preparation

Both the Mhlume and Simunye mills receive sugar cane by road and both mills receive whole stick cane and off-load it by Hilo Off-loading Systems.  

The cane is first weighed to determine the quantity of cane brought in by the grower, off-loaded and then passed through either two sets of knives (Simunye mill) or a single set of knives (Mhlume mill) before being finally shredded in a shredder to a finely chopped fibre, ready to extract the juice. The cane preparation process is designed to expose the sweet sucrose bearing cells to enable extraction

 

Juice Extraction           

Milling

The milling process consists of three grooved rollers, between which the prepared cane is injected and squeezed to remove the sweet sucrose from the cells. The remaining fibre, after forcing out the juice, still contains some sucrose and would be a waste to discard it after only one mill.

Typically, six mills are set in tandem and the fibre is passed through all of them, after adding water at each stage (called imbibition), to remove as much of the sucrose as possible. It has been researched that a seventh mill becomes uneconomical to remove the small portion of remaining sucrose in the fibre.

The fibre discharged from the mills is called Bagasse and is used as fuel in boilers to produce steam and electricity for the sugar production process.   Other mills use the Bagasse for cattle feed or paper production etc.

Diffusion

The diffusion process consists of a large and long enclosed tank, with perforated screens at the bottom below which are juice collection chambers. The prepared cane is slowly dragged over the screens whilst imbibition water is added at stages to “wash” the sweet sucrose from the cells. The wet fibre leaving the diffuser is then de-watered, traditionally in two de-watering mills similar to the milling process. The dry fibre (Bagasse) from the drying mills is then treated the same way as in the milling process.

Juice Purification

The extracted juice from both the milling and diffusion processes, called mixed juice, contains water, sucrose and other impurities such as mud, fibrous material etc.   This mixed juice is heated up and milk of lime is added in a large tank called a clarifier to neutralize the acids which then form a precipitate that settles at the bottom of the clarifier.   To assist the clarification process flocculent is added to the mixed juice.   At the top of the clarifier clear juice is collected and sent to a clear juice tank, ready for the next process.

 The precipitate at the bottom of the clarifier, now called mud, is then combined with fine Bagasse and sent to a mud filtration station where, using sprayed water, almost all the remaining sucrose is recovered and joined with the clear juice from the clarifier.   The filtered cake is then recovered and transported to the fields for use as manure in the cane growing process.

 

Crystallisation

Evaporation

The clear juice is pumped through a series of vessels called evaporators to evaporate all the water. The evaporators are so arranged that as the juice moves from one vessel to the other the vacuum increases to enable water evaporation at lower temperatures.  

The evaporated water from the first two vessels is sent to the boilers as boiler feed water. The rest of the evaporated water, which may contain some traces of sugar, now called sweet water is used further in the sugar production process.

The now concentrated sucrose, called syrup, is now ready to form the sugar crystals in the next process.

Sugar Boiling

The syrup is concentrated further in vessels known as pans, also under vacuum, and all the remaining water is removed. Seed crystals are then added and act as a nuclei that grow into the larger sugar crystals. The crystal mixture and the surrounding mother liquor is called massecute. Typically the pan boiling process is done in three stages, with each stage separating the mother liquor and further growing crystals until very little sucrose remains in the final mother liquor, now called final molasses.

Separation of Crystals from Molasses

The massecuite from the pans is discharged into tanks with stirrers called crystallisers to grow the crystals further.   The crystals from the massecuite from the crystallisers are then separated from the mother liquor, called molasses, in centrifugals where a rotating perforated basket, with fine screens, spins at high speed and spins out the liquid molasses. The raw sugar crystals remain in the basket.

Final molasses from the third pan boiling process is then collected and used either as cattle feed, feedstock for ethanol production or other uses downstream

Sugar Drying 

The raw sugar is then passed through a drier to remove all moisture that could cause the sugar to cake when bagged or stored in bulk. The drying process consists of a large rotating drum through which  hot steam is passed counter-flow to the movement of the sugar. The dried sugar is then ready for dispatch, bagging or for the sugar refining process.

Sugar Refining               

The main purpose of the sugar refining process is to remove impurities that remain in the raw sugar crystals – called colour removal.  

The process involves re-melting the raw sugar crystals and passing the melt through a de-colourisation process.   Typically the carbonatation process is used where lime and carbon-dioxide (CO2) are added to the melt to form calcium carbonate precipitate. The mixture is then filtered and passed through sulphur-dioxide (SO2) for further colour removal. Again, this mixture is filtered and the fine clear liquor crystallized in vacuum pans.   The same crystals/ mother liquor separation as in raw sugar production is applied and the white sugar crystals dried and ready for dispatch as the final product.

 

The Ethanol Production Process

The ethanol production process begins with the receipt of molasses from the sugar manufacturing factories which is weighed and stored in storage tanks.   The molasses is then pumped into fermentation tanks, into which yeast, water and other nutrients are added.  

This mixture goes through a fermentation process that produces carbon-dioxide (CO2) and ethanol in a mixture called beer.  Some of The CO2 is recovered to produce liquid CO2. The beer is fed to a distillation process which produces the low and high grade ethanol called feints and potable ethanol respectively.  Some of the ethanol is passed through a dehydration process to remove as much water as possible to produce anhydrous grade ethanol at 99.9% absolute alcohol by volume.  

The distillation process also produces a byproduct called vinasse which is passed through an evaporation process to remove as much of the water as possible to produce Concentrated Molasses Stillage (CMS). The stillage is then blended with other chemicals to produce fertilizer that is used in the cane growing process.

Molasses as feedstock

In the production of ethanol various feedstock can be used in different production processes.   Such feedstock material includes molasses, corn, sweet sorghum, cassava etc. At RSSC the feedstock for the distillery is cane molasses.   Molasses produced from both Mhlume and Simunye mills is transported and stored in tanks and dams that feed the distillery.

Fermentation

Fermentation is the process by which the sugar in the molasses, in the form of sucrose, glucose and fructose, is converted to ethanol by the use of yeast.   The molasses is diluted with water and yeast and diamonium phosphate are added in the mixture.   Fermentation then takes place for 40+ hours to produce 7-9%+ beer.

Distillation

This is the final stage in the ethanol production process in which the ethanol is purified by removing all impurities and concentrating the product. The impurities removed results in two products called feints and fusel oil.   This alcohol, at 96.4% absolute alcohol by volume, is the final product in the ethanol production process and the product is then used downstream to make a wide range of other products.

Evaporation

After the distillation process a byproduct called vinasse is produced. The vinasse is then passed through an evaporation process to remove as much water as possible. The resulting product is called concentrated molasses stillage (CMS) which is then applied to the cane growing fields as a fertilizer.

Anhydrous Ethanol

Ethanol, when highly concentrated to 99.9%+, can be blended with petrol and used in motor vehicles. The ethanol from the distillery is passed through an anhydrous plant that removes most of the remaining water to concentrate the ethanol to 99.9%+, now called anhydrous ethanol. This product is then preserved in nitrogen covered to flow with the subsequent word tank to prevent the ingress of water, and finally blended with petrol, at various ratios, to produce bio-fuel.