Sugar beet: Encyclopedia II - Sugar beet - Processing

Sugar beet - Processing

Sugar beet - Reception

After harvesting the beet are hauled to the factory. Delivery in the UK is by haulier or, for local farmers, by tractor and trailer. Railways and boats were once used, but no longer.

Each load entering is weighed, and sampled before tipping onto the reception area, typically a "flat pad" of concrete, where it is moved into large heaps. The beet sample is checked for

• soil tare - the amount of non beet delivered
• crown tare - the amount of low sugar beet delivered
• sugar content ("pol") - amount of sucrose in the crop
• nitrogen content - for recommending future fertilizer use to the farmer.

From these the actual sugar content of the load is calculated and the grower's payment determined.

The beet is moved from the heaps into a central channel or gulley where it is washed towards the processing plant.

Sugar beet - Diffusion

Image:BeetFactory.jpg

After reception at the processing plant the beet roots are washed, mechanically sliced into thin strips called cossettes, and passed to a machine called a diffuser to extract their sugar content into a water solution.

Diffusers are long (many metres) vessels in which the beet slices go in one direction while hot water goes in the opposite direction. The movement may either be by a rotating screw or the whole unit rotates and the water and cossettes move through internal chambers. There are three common designs of diffuser, the horizontal rotating 'RT' (from "Raffinere Tirlemontoise", the manufacturer), inclined screw 'DDS' (De Danske Sukkerfabrik)), or vertical screw "Tower". A less common design uses a moving belt of cossettes and water is pumped onto the top of the belt and pours through. In all cases the flow rates of cossettes and water are in the ratio one to two. Typically cossettes take about 90 minutes to pass through the diffuser, the water only 45 minutes. These are all countercurrent exchange methods that extract more sugar from the cossettes using less water than if they merely sat in a hot water tank. The liquid exiting the diffuser is called raw juice. The colour of raw juice varies from black to a dark red depending on the amount of oxidation which is itself dependant on diffuser design.

The used cossettes, or pulp, exits the diffuser at about 95% moisture but low sucrose content. Using screw presses, the wet pulp is then pressed down to 75% moisture. This recovers additional sucrose in the liquid pressed out of the pulp, and reduces the energy needed to dry the pulp. The pressed pulp is dried and sold as animal feed, while the liquid pressed out of the pulp is combined with the raw juice or more often introduced into the diffuser at the appropiate point in the countercurrent process.

During diffusion there is a degree of breakdown of the sucrose into invert sugars and these can undergo further breakdown into acids. These breakdown products are not only losses of sucrose but also have knock-on effects reducing the final output of processed sugar from the factory. To limit (thermophilic) bacterial action the feed water may be dosed with formaldehyde and control of the feed water pH is also practised. There have been attempts at operating diffusion under alkaline conditions but the process has proven problematic - the improved sucrose extraction in the diffuser offset by processing problems in the next stages.

Sugar beet - Carbonatation

The raw juice contains many impurities that must be removed before crystallisation. This is accomplished via carbonatation. First, the juice is mixed with hot milk of lime (a suspension of calcium hydroxide in water). This treatment precipitates a number of impurities, including multivalent anions such as sulfate, phosphate, citrate, and oxalate, which precipitate as their calcium salts, and large organic molecules such as proteins, saponins, and pectins, which aggregate in the presence of multivalent cations. In addition, the alkaline conditions convert the simple sugars, glucose and fructose, along with the amino acid glutamine, to chemically stable carboxylic acids. Left untreated, these sugars and amines would eventually frustrate crystallization of the sucrose.

Next, carbon dioxide is bubbled through the alkaline sugar solution, precipitating the lime as calcium carbonate (chalk). The chalk particles entrap some impurities and adsorb others. A recycling process builds up the size of chalk particles, and a natural flocculation occurs where the heavy particles settle out in tanks (clarifiers). A final addition of more carbon dioxide precipitates more calcium from solution; this is filtered off, leaving a cleaner golden light brown sugar solution called thin juice.

Sugar beet - Evaporation

The thin juice, is concentrated via multiple-effect evaporation to make a thick juice, roughly 60% sucrose by weight, and similar in appearance to pancake syrup. Thick juice can be stored in tanks for later processing reducing load on the crystallization plant.

Sugar beet - Crystallization

The thick juice is fed to the crystallisers, with recycled sugar dissolved into it, this is called "mother liquor". This is concentrated further by boiling under vacuum in large vessels and seeded with fine sugar crystals. These crystals grow as sugar in the syrup forms around them. The resulting sugar crystal and syrup mix is called a massecuite (French. 'cooked mass'). The massecuite is passed to a centrifuge where the liquid is removed from the sugar crystals. Remaining syrup is rinsed off and the crystals dried in a granulator using hot air. The remaining syrup called is fed to another crystalliser from which a second batch of sugar is produced. This sugar ("raw") is of lower quality and is re-dissolved into the mother liquor. The syrup from the raw is also sent to a crystalliser and the very low quality sugar crystal that forms is also redissolved. The syrup separated is molasses; still containing sugar but with too many impurities to be economically processed.

There are variations on the above system, with different recycling and crystallisation paths.

Adapted from the Wikipedia article "Processing", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki