SUGAR BOILING AND CRYSTALLIZATION-IN-MOTION 403 



results. Until the saturation point is reached the water added does not 

 dissolve sugar or increase the purity of the molasses, but per orms solely 

 the function of reducing the supersaturation. At the same time crystalliza- 

 tion proceeds freely, and when all the operations have been properly performed 

 a free-spinning material from which is obtained exhausted molasses results. 

 The calculation of the quantity of water to add is best indicated by an 

 example. The table given early in this chapter indicates that a strike of 

 60 '5 polarization gravity purity and Brix 93-8° will give exhausted molasses. 

 Let such a strike on leaving the pan be of 97 • 2° Brix. Let x be the quantity 

 of water required to be added to reduce the material to a Brix of 93-8". 

 Then 0-972 = (i + a;) 0-938, whence x = 0-036. That is to saj'-, per 100 

 lbs. of massecuite there is to be added 3-6 lbs. of water or roughly 334 gallons 

 per 1,000 cu. ft. This quantity of water should be added gradually, and in 

 such a way as to ensure an equal distribut'on. The rational location for 

 introduction is at the bottom of the container, as the water will have a 

 tendency to rise through the denser massecuite. Failing this, a perforated 

 pipe ma}' be arranged running the whole length of the upper surface of the 

 crystallizer. The water when introduced should be at the same temperature 

 as the massecuite. 



It is not to be understood that the table under wliich this calculation 

 is made is generally applicable without change ; rather every factory should 

 determine for its own use what are the most appropriate concentrations at 

 which to strike and to dry these low-grade massecuites. On the other hand 

 the general law under which the table in question was constructed remains 

 valid and is applicable to any factory. 



The rate of cooling is of importance, since the rate at which sugar can 

 separate from solution on to the surface of crystals is limited. Hence, if 

 the rate of cooling be too great, a supersaturated solution is again formed. 

 When crystaUizers were first used, many were installed with jackets, into 

 which either steam or water could be admitted. For use in the tropics 

 this has been found unnecessary, and the natural rate of cooling as determined 

 by the outside temperature seems to be what is required for the deposit 

 of the sugar. Similarly, no advantage is gained if the rate of revolution is 

 increased beyond that necessary to give the maximum rate of deposit of 

 sugar from solution. The rate of revolution that experience has found 

 desirable is about one revolution in if minutes. 



The size of crystal is also of importance since the deposit of sugar is essen- 

 tiaUy a contact process between soUd and solid in solution. If n be the num- 

 ber of crystals in unit volume the surface area of the crystals is proportional 

 to ^y/n, and consequently the rate of desaccharification of the mother liquor 

 will vary as the cube root of the mmiber of crystals. Conversely, if d be the 

 diameter of the crystals, the total surface area is inversely proportional to d. 

 It follows then that as regards the rate of desaccharification a fine-grained 

 massecuite is superior to one of larger grain. Larger crystals will then imply 

 a longer period of cooling and more crystallizer volume unless compensated 

 for by an increase in the rate of revolution, whereby the surface of contact 

 between crystal and mother liquor is increased. This increase in speed 

 should be in proportion to the diameter of the crystal or inversely in propor- 

 tion to the cube root of the number of crystals. 



In operating crystallizers it is of importance to see that the blades of 

 the stirrers are submerged, as otherwise in their movement they will force 



