The Amazing Sugarcane

                                       Sugarcane Taxonomy

Sugarcane is a genus of 6 to 37 species (depending on taxonomic interpretation) of tall grasses (family Poaceae, Order Cyperales), native to warm-temperate to tropical regions of the Old World. Sugarcane have stout, jointed, fibrous stalks that are rich in sugar and measure 2 to 6 meters tall. All of the sugarcane species interbreed, and today’s major commercial cultivars are complex hybrids.

Sugarcane is responsible for around 70% of the raw table sugar production worldwide, with the remaining production coming from sugar beet (in temperate countries).  Sugarcane stores energy as the non-reducing disaccharide sucrose, which accumulates in large amounts in the vacuoles of parenchyma cells of stem tissues (up to 23% w/v). This organic compound is especially used for producing table sugar via simple crystallization of sucrose from stem juice and further refining and clarification. Ethanol and spirits such as rum are produced via yeast fermentation and subsequent distillation of the stem juice.  Chopped sugarcane stalks are also widely used as cattle feed, especially during dry season when pastures are unavailable for grazing. 

Sugarcane is considered a first-generation biofuel crop. In sugarcane-producing countries, most energy converted to ethanol biofuel is derived from the sucrose extracted from squeezing the sugarcane stems and fermenting the juice. However, only a third of the plant’s potential energy is extracted using this technology, with the remaining being stored in less readily available compounds, such as the cellulose deposited in plant cell walls.

                                                Sugarcane Harvest

Hand cutting is the most common harvesting method throughout the world but some locations have used mechanical harvesters for several years. After cutting, the cane is loaded by hand, mechanical grab loaders, or continuous loaders. Cane is transported to the mills using trailers, trucks, railcars, or barges, depending upon the relative location of the cane fields and the processing plants. Shortly after the cane is cut, it starts to deteriorate rapidly. Therefore, unlike sugar beets, sugarcane cannot be stored for later processing without excessive deterioration of the sucrose content.

Sugar Production

The cane is received at the mill and prepared for extraction of the juice. At the mill, the cane is mechanically unloaded, placed in a large pile, and, prior to milling, the cane is cleaned. The milling process occurs in two steps: breaking the hard structure of the cane and grinding the cane. Breaking the cane uses revolving knives, shredders, crushers, or a combination of these processes. For the grinding, or milling, of the crushed cane, multiple sets of three-roller mills are most commonly used although some mills consist of four, five, or six rollers in multiple sets. Conveyors transport the crushed cane from one mill to the next. 

Imbibition is the process in which water or juice is applied to the crushed cane to enhance the extraction of the juice at the next mill. In imbibition, water or juice from other processing areas is introduced into the last mill and transferred from mill to mill towards the first two mills while the crushed cane travels from the first to the last mill. The crushed cane exiting the last mill is called bagasse. The juice from the mills is strained to remove large particles and then clarified. 

Evaporation is performed in two stages: 1) initially in an evaporator station to concentrate the juice and then 2) in vacuum pans to crystallize the sugar. The clarified juice is passed through heat exchangers to preheat the juice and then to the evaporator stations. Evaporator stations consist of a series of evaporators, termed multiple-effect evaporators; typically a series of five evaporators. Steam from large boilers is used to heat the first evaporator, and the steam from the water evaporated in the first evaporator is used to heat the second evaporator. This heat transfer process continues through the five evaporators and as the temperature decreases (due to heat loss) from evaporator to evaporator, the pressure inside each evaporator also decreases which allows the juice to boil at the lower temperatures in the subsequent evaporator. Some steam is released from the first three evaporators, and this steam is used in various process heaters in the plant. The evaporator station in cane sugar manufacture typically produces a syrup with about 65 percent solids and 35 percent water. Following evaporation, the syrup is clarified by adding lime, phosphoric acid, and a polymer flocculent, aerated, and filtered in the clarifier. From the clarifier, the syrup goes to the vacuum pans for crystallization.

Crystallization of the sugar starts in the vacuum pans, whose function is to produce sugar crystals from the syrup. In the pan boiling process, the syrup is evaporated until it reaches the supersaturation stage. At this point, the crystallization process is initiated by “seeding” or “shocking” the solution. When the volume of the mixture of liquor and crystals, known as massecuite, reaches the capacity of the pan, the evaporation is allowed to proceed until the final massecuite is formed. At this point, the contents of the vacuum pans (called “strike”) are discharged to the crystallizer, whose function is to maximize the sugar crystal removal from the massecuite. Some mills seed the vacuum pans with isopropyl alcohol and ground sugar (or other similar seeding agent) rather than with crystals from the process. From the crystallizer, the massecuite (A massecuite) is transferred to high-speed centrifugal machines (centrifugals), in which the mother liquor (termed “molasses”) is centrifuged to the outer shell and the crystals remain in the inner centrifugal basket. The crystals are washed with water and the wash water centrifuged from the crystals.

The liquor (A molasses) from the first centrifugal is returned to a vacuum pan and reboiled to yield a second massecuite (B massecuite), that in turn yields a second batch of crystals. The B massecuite is transferred to the crystallizer and then to the centrifugal, and the raw sugar is separated from the molasses. This raw sugar is combined with the first crop of crystals. The molasses from the second boiling (B molasses) is of much lower purity than the first molasses. It is reboiled to form a low grade massecuite (C massecuite), which goes to a crystallizer and then to a centrifugal. This low-grade cane sugar is mingled with syrup and is sometimes used in the vacuum pans as a “seeding” solution. The final molasses from the third stage (blackstrap molasses) is a heavy, viscous material used primarily as a supplement in cattle feed. The cane sugar from the combined A and B massecuites is dried in fluidized bed or spouted bed driers and cooled. After cooling, the cane sugar is transferred to packing bins and then sent to bulk storage. Cane sugar is then generally bulk loaded to trucks, railcars, or barges. 

Cane sugar is refined either at the same location where it was produced as part of an integrated facility or at separate raw sugar refineries. The initial step in cane sugar refining is washing the sugar, called affination, with warm, almost saturated syrup to loosen the molasses film. This is followed by separation of the crystals from the syrup in a centrifugal and washing of the separated crystals with hot water or a high purity sweetwater. If the refinery is part of the cane sugar production facility, the cane sugar may be washed more heavily in previous steps and the affination step omitted.

The washed raw sugar is sent to a premelter and then to a melter, where it is mixed with high-purity sweetwaters from other refinery steps and is steam heated. The resultant syrup is passed through a screen to remove any particulate in the syrup and sent to the clarification step. The syrup from the crystal washing, called affination syrup, is transferred to a remelt processing station or reused in the raw sugar washing step. 

In the remelt station, the syrup volume is reduced to form the massecuite, and the sugar crystals are separated from the syrup. The separated liquor is blackstrap molasses. The sugar crystals are sent to a melter and then to the clarification step. Two clarification methods are commonly used: 1) pressure filtration and 2) chemical treatment; chemical clarification is the preferred method. Two chemical methods are also commonly used: 1) phosphatation and 2) carbonation; both processes require the addition of lime. The phosphatation uses phosphoric acid, lime (as lime sucrate to increase solubility), and polyacrylamide flocculent to produce a calcium phosphate floc. Air flotation is usually used to separate the floc from the liquor and the floc skimmed from the liquor surface. Carbonation consists of adding lime to the raw melter liquid and then bubbling carbon dioxide (CO2) through the liquor to produce a calcium carbonate precipitate. The source of CO2 is boiler flue gas, which contains about 12% CO2 by volume. The clarifier systems yield either presscakes, muds, or scums which are treated to remove entrapped sugar, and then sent to disposal.

The next step is decolorization, which removes soluble impurities by adsorption. The two most common adsorbents are granular activated carbon and bone char, manufactured from degreased cattle bones.  Powdered carbon and synthetic resins are less commonly used. Bone char or activated carbon are used in either fixed or moving bed systems. Spent adsorbent is removed from the bed, regenerated, and stored for reuse.

Some refineries then send the decolorized sugar liquor to heaters, followed by multiple-effect evaporators, and then to the vacuum pans.  The sugar liquor from the evaporators is transferred to the vacuum pans to further reduce the liquor volume and form the massecuite. In refined sugar production, the most common boiling system is the four-strike system. When the liquor in the pans has reached the desired level of supersaturation, the liquor is “seeded” to initiate formation of sugar crystals. At this point, the strike is discharged to a mixer and then to the centrifugal. In the centrifugal, the white sugar is retained in the inner basket and the liquor centrifuged to the outer shell. The sugar liquor is returned to a vacuum pan for further volume reduction and white or brown sugar production. The white sugar is then washed in the centrifugal; the separated wash water (containing liquor and dissolved sugar) is returned to the vacuum pans. The moist sugar from the centrifugals contains about 1% water by weight.

To produce refined granulated sugar, white sugar is transported by conveyors and bucket elevators to the sugar dryers. As you can see, it requires a lot of specialized equipment and many skilled workers to produce sugar and molasses, but the results of their hard work literally make life sweeter around the world!

References:

  1. Food And Agricultural Industry (June 1997)
  2. Sugar And Sweetener Yearbook, U. S. Department Of Agriculture, Economic Research Service, Washington, DC, June 1995.