« ForrigeFortsett »
water flows through the column, and this serves, both by rapid condensation of the steam and by the barometric pull of its column of liquid, to maintain a high degree of vacuum.
A charge of sirup is first drawn into the vacuum pan; this sirup as it leaves the evaporators has a specific gravity of about 1.25 (or about 50 per cent solids) and is boiled down in the vacuum pan to a specific gravity of 1.50 or about 90 per cent solids. The ebullition in the vacuum pan is violent, and unless the sugar boiler is careful some of the sirup may be carried over with the vapor into the condenser. This is called entrainment and is a source of frequent losses in sugar manufacture. In all modern sugar factories the chemist makes constant examination of the condensation water, so that any loss due to this cause may be promptly detected and stopped.
The handling of the vacuum pan requires more skill than any other operation of the sugar house; care must be taken to avoid entrainment and care must be taken to build up crystals of uniform grain or size. The usual practice is to boil down the first charge of sirup to what is called “string proof,” i.e. to the point when a few drops of sirup withdrawn from the pan will draw out between the fingers in fine strings or threads. When this point is reached, a large charge of fresh cold sirup is drawn into the pan, the sudden cooling of the supersaturated contents starting the formation of innumerable fine crystals. These first crystals constitute the foundation so to speak of all the sugars obtained in a given boiling or strike of the pan.' The boiler aims to build up these crystals without forming new ones; he aims from now on to avoid supersaturation and to avoid sudden chilling through drawing in too much sirup at one time. He controls his process by drawing out samples every few minutes and examining these upon glass against a light; if he sees fine new crystals appearing among the old ones, he reduces the vacuum a little, thus raising the temperature and dissolving this false grain, as the fine crystals are called. By skillful manipulation, which comes only with long practice and experience, the sugar boiler is able to build up his crystals to any desired size. The usual practice is a crystal about the size of ordinary granulated sugar; in certain localities, however, a large crystal is favored; as, for example, in Peru, where the sugar is boiled slowly and for a long time, thus building up a very large grain. The
attachment for withdrawing samples of sirup from the vacuum pan is called the “proof stick.”
When the vacuum pan is filled with a thick magma of sugar crystals, of about the consistency of mortar, the steam is shut off, air is admitted, the bottom of the pan opened, and the entire contents dumped into a mixer, which keeps the mass in slow movement by means of revolving arms. This mixer is situated
over a row of centrifugal machines; the mass of crystals (sometimes called masse cuite from the French, or Füllmass from the German) is drawn off gradually in successive charges into the centrifugals. The inner walls of the latter are lined with fine brass meshing, and as the drums are rotated the masse cuite is whirled against the meshing, which retains the sugar but allows the molasses to pass through. After spinning for a few minutes until as much of the molasses is removed as possible, the revolving mass of sugar may be sprayed with a fine spray of water or a jet of steam in order to remove more of the film of molasses which remains adhering to the crystals; the amount of spraying depends upon the whiteness of sugar desired. In Louisiana a very pure, white sugar is made by spraying with several sprinklings of water; such sugar is over 99 per cent pure sucrose, the remainder being mostly moisture. In Cuba and Porto Rico they aim to make a 96 per cent sugar. In Hawaii and Java a sugar testing about 97 per cent is desired. Spraying will, of course, dissolve some of the sugar, so that the process is one which must be carefully controlled.
When the molasses has been removed as completely as possible, the centrifugals are stopped and the sugar emptied through the bottom of the drum into a conveyor, by which it is carried to the bagging department, where it is prepared for shipment. The raw sugar from the centrifugal contains considerable moisture, and in some countries the sugar is dried in revolving drums before being bagged. This drying is advantageous for two reasons: first, the excess moisture is removed, thus saving the cost of transporting water; and, second, the sugar is sterilized and protected against the attacks of ferments and bacteria. The drying of raw sugar is not practiced in Cuba, Porto Rico, or Louisiana, but is carried out in Java and the Hawaiian Islands, where the sugar has to be shipped long distances for refining. The storage of undried raw sugar for long periods of time is a risky operation, as many speculators in sugar have found to their cost.
F16. 42. — View in a sugar factory showing multiple evaporators in the background (American Photo Co., Havana).
The sugar which is made from the pure juice of the cane is called “first sugar” and the molasses drained from this sugar is called “first molasses.” The latter still contains a large amount of sucrose, and various processes are used to recover as much of this as will crystallize. The first molasses is sometimes boiled down again in the vacuum pan and a second crop of sugar crystals obtained; this is the second sugar and the molasses obtained from this the second molasses. The second molasses may be boiled over again and a third sugar obtained, the molasses from which is the third or final molasses. Of course, as the sugar is removed the impurities become more and more concentrated in the molasses, until finally a thick, stringy mass is obtained which will no longer crystallize. Such a molasses may still contain, however, 30 per cent sucrose; there is also present about 30 per cent invert sugar, 8 to 10 per cent of ash, and 8 to 10 per cent of gums, organic acids, amino compounds, etc.
The tendency of modern methods in cane sugar manufacture is against the repeated boiling of molasses, and the aim is to get as much sugar as possible in one operation. Many processes have been devised to attain this end. One method is to take the molasses from the first strike of sugar, draw this into the vacuum pan with the sirup for the succeeding strike, and boil the two down together. The masse cuite from this mixture is then run while still hot into large tanks, called crystallizers (Fig. 41), where it is kept in slow motion by means of revolving arms. As the mass cools and thickens more molasses is drawn to keep the proper degree of fluidity. When no more sugar will crystallize, as determined by analysis of samples, the contents of the crystallizer are spun out in centrifugals and the molasses withdrawn from the factory.
Several of the features above described are shown in Fig. 42. In the foreground at the left are the large wheels of the cane mill; at the right is the conveyor which carries away the bagasse. In the background a multiple-effect evaporator may be seen