Making Sugar
- Agriculture
Sugar beets are typically grown within a 60 mile radius of a factory, but can be grown up to 100 miles away. Planting takes place in March and the beets are harvested in October of each year. Sugar beets take a lot of water and are an irrigated crop in the Western Sugar growing areas. The farmers harvest the beets and deliver them to pile grounds where beets are dry screened, sampled and weighed. Large machines called pilers stack the beets in piles as large as 18 feet tall by 180 feet wide by several hundred feet long. Each factory will have several pile grounds surrounding it where the beets are stored until needed by the factory. From the piles, large front-end loaders load 28 tons per truck to deliver beets to the factory. - Beet Handling
The sugar beets are dumped directly from trucks into a wet hopper, where jets of water help unload the beets. The beets are floated into the factory by means of water in a flume where they are elevated by a lifting device into an upper flume. Once in the upper flume, they pass through a rock catcher for removal of rocks, mud or sand and then through another section of the flume where trash, weeds and leaves are removed. The beets then flow into a beet washer where they are rubbed together to be cleaned. From the washer, they are elevated to the top of the factory where a final rinsing and de-watering takes place on a roller-spray table. The beets are then conveyed into a hopper where they are fed into the beet slicers. - Diffusion
The sugar beets are fed from the slicer hopper into the slicers where very sharp knives cut them into long noodle-like pieces called cossettes. Emerging from the slicers, the cossettes fall onto a conveyor belt to be weighed and fed into the diffusion system. Here the sugar is removed from the beets by hot water "washing" or diffusing the sugar from the beets. The beets are fed into the bottom of the diffuser and are moved upward through the diffuser where they emerge with 2% of the sugar left in them. The spent beets are called wet pulp and processed in presses and dryers to become livestock feed. Hot water is fed into the top of the diffuser and flows down through the beets continuously extracting the sugar from the beets and emerges from the diffuser as sugar water called "raw juice". The extraction process uses a process called osmosis where the sugar is passed through the porous membrane on the beet's cell wall while some of the non-sugars are retained by the cell. - Pulp Dryer
The wet pulp (90-92% water) from the diffuser is sent to pulp presses. The pulp presses squeeze the pulp to remove as much water as possible. This is then called pressed pulp (72-78% water) and is either sold as livestock feed or sent to the dryer for further processing. The water pressed out of the pulp is sent back to the diffuser to recover the sugar in it. The rotary drum pulp dryers are direct fired by oil, gas or coal and dry the pulp to 12% moisture. The dried pulp can be sold but is normally compacted into pellets for easier handling and stored in a bulk warehouse for future sales. Pressed pulp is sold for short-term use while pellets store for longer periods, but both are used for livestock feed. - Lime Kiln
The lime kiln supplies the burned lime and carbon dioxide for the purification system. Limestone (calcium carbonate) is heated to 2000 F in a vertical shaft lime kiln using hard coal or natural gas. The heating process releases carbon dioxide from the limestone reducing the calcium carbonate to calcium oxide. The calcium oxide (burned lime) is then mixed with sweetwater to form milk of lime in a lime slaker. The milk of lime is mixed with the juices in the purification system while the carbon dioxide is pumped directly to the purification system. - Purification & Filtration
Upon leaving the diffuser, the raw juice moves through various stages of purification and filtration to remove non-sugars. It is first heated to 85 C and then sent to the prelimer, where most of the non-sugars are flicked or precipitated by gradual pH elevation (progressive preliming). From preliming, the juice flows through the main limer where the rest of the lime is added on its way to the first carbonation station. Here carbon dioxide gas from the lime kiln is bubbled through the limed juice where it reacts with the lime to form calcium carbonate and adsorbs some of the non-sugars. This process is tightly controlled to give the juice the proper pH or alkalinity leaving the first carb station.
Now the carbonated juice flows to a clarifier where the precipitated calcium carbonate formed in first carbonation is settled out. This leaves a clear juice to be sent to heaters and on to the second carbonation system. The sludge (calcium carbonate precipitate) left behind in the bottom of the clarifier is removed and fed to vacuum drum filters where it is washed to recover the sugar left in it. The washed sludge has about 1% of the sugar from the beets in it and is sent to a lime pond. The washed water with the recovered sugar is called "sweetwater" and is sent to the lime house to be mixed with the burned lime to be recycled to the purification system.
In the second carbonation system, carbon dioxide gas is again bubbled through the juice reacting with the residual lime to form calcium carbonate precipitate. The amount of gas is controlled by pH to obtain the proper alkalinity and the carbonated juice is sent to the second carbonation filters where the calcium carbonate precipitate is removed. This precipitate is sent to the sludge tank and then to the drum filters for processing with the precipitate from first carbonation. The clear juice moves on to sulfitation where sulfur dioxide is added to the juice to remove some color forming materials that would carry through to the finished sugar and to adjust the pH to allow for easier boiling in the evaporators and vacuum pans in further processing. - Evaporation
The sulfitated juice is now called "thin juice" and is heated and fed into the evaporators. The evaporation station is 4 to 6 evaporator bodies (or effects) that have vertical tubes in them. Steam (or vapors) is fed to the outside of the tubes and the juice is on the inside of the tubes. The heat transfers from the steam to the juice. This evaporates some water from the juice helping to concentrate it while the steam is condensed. The water evaporated in one evaporator becomes the steam feeding the next evaporator while the juice travels from one effect to the next. The thin juice enters the evaporators at 13 to 15% solids and leaves the station at 60% solids and is called thick juice. The multiple effect station allows 2.5 to 2.8 pounds of water to be evaporated per every pound of steam fed to the first effect of the evaporator station. - Crystallization & Separation
The thick juice from the evaporator station is sent to the high melter where the high and low raw sugars are dissolved in it through vigorous agitation and heat. The melter juice is then heated and filtered and further concentrated through the "concentrator" to 70 to 75% solids. This is then called standard liquor and is fed to the white pan where white sugar is crystallized from solution. Crystallization takes place in a batch process when water is evaporated from the solution in the vacuum pan. Finely ground sugar is used to "seed" the pan and each seed crystal grows into a typically sized sugar crystal. More and more water is evaporated forcing more and more sugar to crystallize on each sugar crystal. The solution is concentrated to 92% solids and consists of sugar crystal surrounded by syrup of sugar-water and this is call white massecuite. The massecuite batch is dropped into a mixer that then feeds the white centrifugals.
The white centrifugals separate the sugar crystals from the syrup in the massecuite by spinning the sugar against a screen while the syrup spins through the screen. Hot water is used to wash the residual syrup from the crystal and the spinning helps to partially dry the sugar. The white sugar is then conveyed to the granulator for further drying and cooling. The finished sugar is then sifted for lumps and moved on to the bulk sugar bins for storage. The bulk storage "conditions" the sugar before it is loaded for bulk shipment or sent to the warehouse for packaging in various package sizes - from restaurant size packets to 2300 lb. Super Sacks.
The white centrifugal produce two syrups. The high wash is higher purity syrup that is recycled to the high melter. The green is the lower purity syrup from the white centrifugals and it still contains a considerable amount of sugar. It is sent to the high raw vacuum pan for processing. Here again, seed crystals and evaporation are used to remove sugar from the syrup solution and deposit it on the sugar crystal. The high raw massecuite (sugar crystal and sugar syrup mixture) is sent to a mixer and then to another set of centrifugals to separate the sugar crystal from the syrup. The sugar crystals made here are not a good enough quality to sell as white sugar (they look like raw sugar) so they are sent to the high melter to be re-dissolved and re-processed in the white pan. This pan, mixer and centrifugal system is called the high raw system.
The syrup leaving the high raw centrifugals is called machine syrup. It is an even lower purity than the high green syrup but still contains enough sugar that it needs for further processing. The machine syrup is sent to the low raw system that contains one additional piece of equipment as compared to the high raw system. The low raw vacuum pan is used to crystallize sugar out of solution but instead of feeding the low raw massecuite straight to the low raw mixer and centrifugals, the low raw massecuite is sent to a crystallizer where cold water is used to continue the crystallization process for an additional 48 hours. From here it is sent to a reheater and to the low raw centrifugals where sugar crystals are separated from the syrup called molasses. The sugar crystals are sent to the high melter for re-processing in the white pan. The molasses is about 50% sugar by weight and this is sent to the Scottsbluff Facility where about 80% of this sugar is recovered. The molasses is another place in the process where sugar is lost. About 10-15% of the sugar from the beets leaves the sugar factory in molasses. This is the biggest loss of sugar in a standard sugar factory. - Molasses Desugarization
The molasses from all of the plants is sent to the molasses desugarization plant. This plant processes 132,000 tons of molasses annually and removes about 80% of the sugar in the molasses. The molasses must be treated to remove some non-sugars that will cause processing problems in the separation (ion exclusion) process. These non-sugars consist of calcium, magnesium and suspended solids. The molasses is softened by replacing non-sugars with sodium and filtering the precipitates formed in the reaction. The pretreatment process consists of dilution, heating, soda ash addition (sodium source), reactors (softeners) and filtering. The filtering process uses plate and frame filters to squeeze the softened molasses through filter leaves while the precipitated non-sugars remain behind.
The softened molasses is sent to the separation (ion exclusion) process that consists of four separation trains of three columns each. The columns are loaded with a resin that attracts certain chemicals while letting other chemicals pass through quickly. This difference in travel time is used to separate the molasses into a sugar fraction, a betaine fraction and a residual molasses fraction. The fractions are then concentrated and the sugar fraction is stored for processing in the sugar end of the Scottsbluff factory into white sugar while the betaine and molasses fractions are sold as byproducts. - Steam & Power
Sugar factories use steam for heating the various process streams as well as a lot of electricity. Coal or natural gas boilers are used to generate all of the steam needs as well as some of the electrical needs of the factory by using steam turbo-generator systems. The remaining electrical needs are purchased from the local power company.
