Sugar

Chemical Treatment of the Juice.

As the dissolved non sugars are colloidal (see COLLOIDS), the juice must undergo some form of chemical treatment before it can be filtered satisfactorily. The natural acidity of the juice must also be neutralized, because sucrose decomposes on heating acid juices, the sucrose combining with water and breaking up into two simpler sugars, glucose and fructose, as shown in the equation:— This is termed inversion of sucrose, and the resulting mixture of glucose and fructose is termed invert sugar.

The object of chemical treatment is fourfold:—(i) To neu tralize the natural acidity (as just explained) ; (2) to remove suspended impurities, thus rendering the juice transparent; (3) to precipitate dissolved non-sugars, thus increasing the purity of the juice; and (4) to decolourize the juice in the manufacture of white sugar.

The chemical agents used are :—quicklime (Ca0), usually added in the form of lime-cream ; sulphur dioxide gas (SO2) ; carbon dioxide gas (CO2) ; and, occasionally, phosphoric acid The effect of these chemical agents is completed by heating the juice to boiling point in an apparatus called a juice heater, the juice flowing continuously through a number of brass tubes heated externally by steam. Certain impurities, previously dissolved in the juice, are thus rendered insoluble and form a dark coloured precipitate which can be separated in various ways: (a) by causing it to rise to the surface (with the escaping air bubbles), forming a scum which is removed by skimming ladles; (b) by allowing it to subside (after separation of the air) and removing it as a sediment; (c) by filtration through cloth or other filtering media; (d) by centrifugal action.

In the manufacture of raw sugar, about two gals. of lime-cream ( Is° Beaume) are added per i,000 gals. of juice ; the mixture is heated to boiling point and discharged into settling tanks (sub siders or clarifiers), each holding i,000 gal. or more, and filled in rotation. After subsidence for about an hour, the clear juice is drawn off into one gutter, and the sediments into another. The sediments are usually resubsided in additional tanks, more clear juice being then drawn off. The final sediments are pumped through filter-presses (see article FILTRATION), yielding clear filtered juice and cakes of solid matter. The clear juices flowing from the first subsiders, resubsiders and filter-presses, are mixed and pass to the multiple-effect evaporator (to be described later).

In the manufacture of white sugar, a more thorough purification of the juice is essential, and two methods are used, (a) acid sulphitation and (b) carbonation. As the latter method is mainly used for treating beet juice it will be described under that heading. In the former, a, from 4 to 5 gal. of lime-cream are added per I,000 gal. of juice, and sufficient sulphur dioxide gas to precipi tate this lime, with the formation of a granular precipitate of calcium sulphite as shown in the equation The gas may be added either before or after liming, the final result being the same. The juice is then heated and subsided.

Evaporation of Water.

A large quantity of water must be evaporated from the purified juice before the dissolved sucrose can begin to crystallize. To avoid destruction of sucrose at high temperatures the juice is boiled in closed vessels under a vacuum, thus lowering the boiling point. Further, in order to economize fuel, multiple effect evaporators are used.

Fig. 4 is a sectional view of a double-effect evaporator. The lower part of each vessel contains numerous vertical brass tubes, their open ends passing through perforations in two horizontal plates. The space between these plates forms a steam-drum, with steam-inlet. The shaded space below the drum and inside the tubes is filled with the boiling juice. The large space above the drum is filled with steam generated by the boiling juice. Steam entering the drum of No. I condenses on the exterior of the tubes, transmitting its latent heat to the juice inside the tubes and causing it to boil. The steam thus generated passes into the steam drum of No. 2 and there acts in the same manner. The steam generated here by the boiling juice passes to a condenser and air pump, which maintain a vacuum of about 25 inches in No. 2. This vacuum fixes the boiling point of the juice here, also the tempera ture at which steam (leaving No. I) can condense on the tubes in No. 2, also the pressure of that steam and the boiling point of the juice in No. 1. Consequently, the high vacuum and low boiling point in No. 2 produce a lower vacuum and higher boiling point in No. I. Multiple effect evaporation is applicable to any number of connected vessels.