The present invention relates to the field of sugar refining, and provides a system and method which improves the efficiency of the refining process.
Raw sugar is obtained by extracting the juice from sugar cane, and processing the juice to produce sugar crystals. The raw sugar is light brown in color, due to the presence of color bodies in the crystals. The color of the crystals is determined by the content of organic chemicals in the sugar. A primary object of the refining process is to convert the raw, brown sugar into white sugar.
A major component of the sugar refining process is known as carbonation. In the carbonation step, carbon dioxide is added to raw sugar which has been dissolved to form a clarified liquor. The carbon dioxide reacts with calcium in the sugar to form calcium carbonate (CaCO.sub.3). The calcium carbonate precipitates out of the sugar, and takes with it a large proportion of the color bodies. In fact, in a single carbonation step, more than 60% of the coloring matter may be removed. The precipitate can then be removed by filtration. The carbonation step may be repeated, or it may be followed by additional refining steps, such as treatment with activated carbon. These further steps can remove most or all of the remaining color bodies.
The carbonation step may be enhanced by adding lime (CaO) to the reactor which contains the raw sugar. The lime provides more calcium than that which is naturally found in raw sugar or sugar cane. The lime thereby enhances the production of calcium carbonate by providing more calcium atoms to react with the applied carbon dioxide, according to the reaction EQU CaO+C0.sub.2.fwdarw.CaCO.sub.3.
Because carbonation is the major step in removing coloring matter from raw sugar, it is important to maintain a reliable source of carbon dioxide in a sugar refinery. The reliability of the source of carbon dioxide is a major determinant of the productivity of a sugar refining plant. If the supply of carbon dioxide is curtailed, the entire operation of the plant is correspondingly limited.
Various methods of providing carbon dioxide have been used in the sugar refining industry. A typical approach is to derive carbon dioxide from the effluent of the exhaust of a boiler. A sugar refinery includes a boiler which provides steam which heats the contents of the reactor, thereby increasing the rate of the sugar-refining reactions. The boiler exhaust is itself a source of carbon dioxide. It has therefore been known to recover the boiler exhaust, to purify it (such as by use of a gas scrubber), and to use the purified stream in the above-described sugar-refining reactions. If the fuel for the boiler is natural gas, which is the usual fuel in such applications, the boiler exhaust will contain about 6-9% carbon dioxide, by volume. If some other fuel is used (such as coke, propane, heavy oil, or fuel oil), the percentage of carbon dioxide could be outside of the above range.
The major disadvantage of using the boiler exhaust as a source of carbon dioxide is that if a problem with the boiler develops, it may be necessary to reduce the boiler output. In the latter case, the supply of carbon dioxide is thereby reduced, thus affecting the operation of the entire plant. Similarly, if the purity of the fuel decreases, less carbon would be available for combustion, and the amount of carbon dioxide produced would be correspondingly reduced. The latter occurrence is quite possible where the boiler is fueled by natural gas, because the purity of a natural gas stream may vary continuously over time.
The risk of an interruption in the carbon dioxide supply, due to a problem with the boiler, or due to fluctuations in the purity of the fuel, can be offset by providing a backup source of carbon dioxide on the premises. But providing such backup, which could require storage of large tanks of compressed gas or liquefied gas, is inconvenient and expensive. For this reason, the usual approach is to reduce production when there is an interruption in the source of carbon dioxide.
Another solution, proposed in an article entitled "The use of pure CO.sub.2 in the sugar industry", Sakharnaya Promyshlennost (1973), is to treat chemically the flue gases from the boiler so as to produce pure carbon dioxide for use in the refining process. The above-cited article suggests treating the flue gas with monoethanolamine, which absorbs carbon dioxide, and later desorbs it, thereby providing pure C0.sub.2 for use in carbonation. While the latter system works, it is expensive, as it requires the additional steps of handling chemicals. Moreover, most existing installations do not have the capability of processing sugar rapidly enough to justify the use of pure carbon dioxide. Thus, for most sugar refineries, the use of pure carbon dioxide would be unduly expensive relative to the benefit conferred.
The present invention provides a system and method which substantially increases the efficiency of a sugar refining plant. The present invention requires no special chemicals, and can be conveniently used with existing refining plants to increase their productivity.