US3174877A - Decolorizing sugar by extracting coloring matter therefrom with a methanol solution of a hydrocarbon - Google Patents

Description

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DECOLOING SUGAR BY EXTRACTING COLGR- EJG MATTER THEREFRUM WITH A METHA- NOL SULUTION OF A HYDROCARBGN Byron B. Bohrer, Rosemont, Pa, assignor to Sun Oil Company, Philadelphia, Pa., a corporation of New Jersey No Drawing. Filed May 16, 1963, Ser. No. 281,055

4 Claims. (Cl. 127-64) This invention relates to decolorizing raw sugar. The invention involves contacting raw sugar with methanol, the latter containing a small amount of a hydrocarbon. Upon such contact, which results in a slurry of solid raw sugar in organic liquid, the film of syrup on the surface of the raw sugar crystals is substantially removed therefrom and decolorized raw sugar is subsequently separated from the slurry.

At several of the many purification steps involved in the manufacture of commercial sugar from sugar cane or beets, solid sugar is recovered which is characterized in that it is composed of a number of relatively light colored sugar crystals each of which is covered by a thin film of relatively dark colored sugar syrup. The presence of this dark colored film, which renders the sugar impure and in many cases unsalable, is due to the fact that in conventional sugar refining processes solid sugar is recovered by vacuum crystallization of an aqueous sugar solution followed by centrifugation to separate the solid sugar crystals from the remaining mother liquor. The sugar crystals which form are relatively light colored while the mother liquor becomes darker due to the increased concentration of coloring matter therein. The thin film of relatively dark colored sugar syrup mentioned above is mother liquor which is not removed in the centrifugation step. The actual amount and color of this residual film of sugar syrup on the crystals will depend mainly upon the color, viscosity, etc. of the aqueous sugar solution originally charged to the vacuum Thus in commercial White sugar, which is obtained by mixing 34 batches of sugar consecutively crystallized and centrifuged from an almost water-white, relatively low viscosity, very high purity sugar solution, the film of residual syrup is almost negligible, in both amount and color. On the other hand, in raw sugar, from which white sugar is ultimately obtained and which is a mixture of 3 batches of sugar consecutively crystallized from a dark, aqueous sugar solution obtained by treating the cane or beets with water followed by filtration, evaporation, etc., the residual film is relatively dark, i.e., it ranges from light tan to dark brown. Similarly remelt sugars, or recovery sugars as they are also conventionally known, are crystallized from affination syrup and other low grade Sugar solutions which are extremely viscous and almost black in color. Here also the residual film of sugar is dark. Soft sugar, or brown sugar as it is also conventionally known (especially to the housewife), is also crystallized from dark colored sugar solutions and here also the residual film of syrup on the crystal surface is dark. Raw sugar, remelt sugar, and brown sugar normally have a color by ASTM Dl500 gms. sugar in ml. water) of 1.0 or higher.

Since most of the sugar sold in this country is white sugar, a method of removing all or part of this aforesaid residual film of sugar, which contains a relatively large amount of color bodies, is highly desirable. It would be desirable even in the case of brown sugar for while there is presently a market for brown sugar, the supply of such sugar generally exceeds the demand except for 13 months of each year. Several methods of removing all or part of this film are known, one of which is to dissolve the sugar in water, treat the resulting solution ited States Patent with an adsorbent such as boneblack to adsorb the color bodies, and then recover solid sugar by crystallization-centrifugation. The recovered sugar crystals will still have a thin film of mother liquor on their surface, but it will not be as dark as initially. This method is presently used but requires large crystallizer capacity, large quantities of boneblack, and also consumes a large amount of steam in evaporating the water. Another method involves spraying the crystals with water in the centrifuging step after the sugar is first crystallized. The fact that such water washing does decolorize the sugar shows that the coloring matter is a surface film rather than being occluded uniformly within the crystal. The main disadvantage of this technique is that the water dissolves and carries away a large amount of sugar which then has to be crystallized again.

The present invention provides a new method of removing all or a substantial part of this residual film of syrup from raw sugar. According to the invention this film is extracted from the raw sugar crystal by a procedure which involves intimately mixing, i.e., contacting, the solid raw sugar crystals with a single phase, i.e., homogeneous mixture of methanol and a small amount of a hydrocarbon more fully defined hereinafter. Upon such contacting, all or most of the color bodies in the residual film dissolve in the liquid phase while most of the sugar remains undissolved. By subsequently separating the un dissolved sugar a decolorized raw sugar is obtained. The presence of the hydrocarbon in the methanol greatly increases the yield of Sugar, i.e., it reduces the amount of sugar which dissolves in the liquid phase, while having only a relatively small effect upon the amount of decolorization which occurs.

According to the invention the raw sugar is initially contacted with a single phase, i.e., homogeneous, liquid mixture of methanol and a hydrocarbon. The hydrocarbons suitable for the present purpose are aliphatic and cycloaliphatic hydrocarbons containing not more than one double bond. Thus if the hydrocarbon used is an olefin, it must be a monoolefin. The hydrocarbons suitable for the present purpose are additionally characterized in that they are soluble in, i.e., miscible with, methanol, at the temperature at which the contacting is carried out, to the extent of at least 0.25% by weight of methanol. The reason for this requirement is that in order for the hydrocarbon to improve the sugar yield in the decolorization process of the invention it must be dissolved in the methanol in the amount of at least 0.25%. Freferably the hydrocarbon is soluble in methanol to the extent of at least 1% by weight of methanol. Examples of hydrocarbons suitable for the present purpose are pentane, hexane, octane, methane, butane, cyclohexane, 2-methylbutane, methylcyciohexane, ethylene, 2-methyl-3-ethylbutane, pentene-l, etc. Suitable hydrocarbons as described above will be referred to hereinafter as the hydrocarbon. Preferred hydrocarbons for the present purpose are saturated aliphatics containing 51() carbon atoms.

The amount of hydrocarbon used should be at least 0.25% but not more than 30% by weight of the methanol. Some of the hydrocarbons may not be miscible with methanol to the extent of more than 30% at the temperature at which the contacting is carried out. For those that are, however, they should not be used in quantities greater than 30% by weight of the methanol. Furthermore, the amount used is considered to be, for the present purpose, only that portion of the hydrocarbon which dissolves in the methanol. Thus some hydrocarbons such as pentane are only partially miscible with methanol and if enough pentane is added to methanol a two phase system results, one phase being methanol rich and containing pentane dissolved therein, the other phase being pentane rich and containing methanol dissolved andhence Can-be used,it serves no useful purpose and hence is wasteful. In such a two phase system the extracting agent for the present purpose would be the methanol rich layer and to be a suitable extracting agent for the present purpose it would have to contain 0.25-

30.0% pentane.

It has also been found that the improvement in sugar v yield obtained by the addition of the hydrocarbon to the methanol is optimized at about 1-5% hydrocarbon. While further yield improvements can usually be achieved by-usinghigher amounts of hydrocarbon up to 30%, it

a will usually be found that the amount of decolorization which occurs at these higher levels of hydrocarbon is disproportionately reduced. Consequently the preferred amount of hydrocarbon is 1-5 by weight of the methanol.

The amount of methanol-hydrocarbon mixture used the sugar, the type of equipment in which the invention In any event, however, the amount of is practiced, etc. mixture should be suflicient to form a slurry of solid sugar in organic liquid; i.e., two distinct phases, a solid sugar phase and an organic liquid phase. Normally the amount of the methanol-hydrocarbon mixture will be at least one milliliter per gram of sugar.

The temperature at which the contacting is carried out can vary considerably. High temperatures are advantageous in that they result in more decolorization but are disadvantageous in that they result in more sugar being dissolved in the methanol-hydrocarbon extracting agent, i.e., a greater yield loss. Consequently, it is preferable to use only moderately elevated temperatures of approximately 35 -75 0, although higher temperatures such as 100 C. or lower temperatures such as C.

can .be used. Temperatures higher than 100 C. are generally undesirable because they result in excessive thermal decomposition of the sugar.

The pressure at which the contacting is carried out can also vary considerably but since the methanol and hydrocarbon are employed as a liquid mixture the pressure should be'suflicient at the temperature used to maintain the methanol and hydrocarbon in the liquid phase. Where the hydrocarbon is normally gaseous, such as butane, the use of superatmospheric pressure will almost always be required.

The time of contact will also vary depending upon the color of the raw sugar, the contacting temperature, the amount of extracting agent used, etc. but will usually be on the order of -10 minutes. Dissolution of the color bodies in the methanol-hydrocarbon phase occurs fairly ture can'be charged to a tank equipped with an agitator.

Withthe agitator running, the raw sugar is then charged to the tank. Stirring is continued until the color bodies are removed from the surface of the raw sugar.

- After the raw sugar has been contacted with the methanol-hydrocarbon mixture for the desired period of time the sugar is then separated from the organic liquid phase. Such separation can be effected by conventional means such as decantation, filtration, centrifugation, etc. The separated sugar will generally contain a small amount of residual methanol-hydrocarbon mixture. This can be removed in any convenient manner such as vacuum drying, etc.

The separated crystals are decolorized, i.e., they contain less color than the raw sugar starting material. The actual difference in color between the starting sugar and the separated crystals will depend upon variables already discussed. If the actual difference in color is not as large as desired, the separated crystals can be decolorized one r or more additional times by the method of the invention.

It will be found that not only is the separated sugar decolorized but it is also purified, i.e, the separated sugar contains a higher percentage of sucrose than the original raw sugar. It should be noted, however, that small reductions in the impurity content of the sugar often cause very large reductions in the color of the sugar.

The invention is limited to the use of methanol as the alcohol of the invention. When other alcohols such as ethanol are substituted for the methanol, essentially no decolorization occurs. The reason why ethanol is unsuitable is not known but such has nevertheless been found to be the case.

The invention is illustrated more specifically by the following examples. In Example I the extracting agent is methanol alone. Examples 11 and III show the benefits obtained by dissolving a hydrocarbon as described herein in the methanol, the amount of hydrocarbon being relatively large in Example 11 and relatively small in Example III.

Example I 400 g. of a Puerto Rican raw sugar having a color by ASTM D-1500 (10 g. sugar in 25 ml. Water) of 2.0 and polarizing 98.5% sucrose is charged to a separatory funnel at room temperature (25 C.). The funnel is then charged with 1000 ml. of methanol at room temperature. The mixture of raw sugar and methanol is shaken vigorously for several minutes after which the methanol is decanted. A second 1000 ml. portion of methanol is then added to the flask, the mixture is shaken vigorously, and the methanol is decanted. A third 1000 ml. portion of methanol is then added to the flask, the mixture is shaken vigorously, and the methanol is decanted. Thus the sugar is thrice extracted with 1000 ml. of methanol. The solid sugar remaining in the funnel after the third decantation of methanol is dried in a vacuum oven. The dried product weighs 376 g., has a color of 0.75 and polarizes 99.4% sucrose. The yield loss thus is 20.3 g. sucrose or 5.2%. The color of water by ASTM D-1500 is zero.

Example 11 The procedure of Example I is repeated except that instead of extracting the sugar with 1000 ml. portions of methanol it is thrice extracted with 1000 ml. portions of a mixture of methanol and pentane having a pentane content of 19.6% by weight of the methmol. The dried sugar product Weighs 387 g., has a color of 1.0 and polarizes 99.3%. The yield loss thus is 9.7 g. or 2.5%. By comparing the results of Examples 1 and II it can be calculated that the yield loss when methanol alone is used is 108% higher than when the methanol-pentane mixture is used while the color improvement when methanol alone is used is only 25% higher than when the methanolpentane mixture is used.

Example III The procedure is the same as in Example 11 except that the pentane content of the methanol-pentane is 3% by weight of the methanol. The dried sugar product Weighs 386 g., has a color of 0.85, and polarizes 99.3% sucrose. The yield loss is thus 11 g. or 2.8%. By comparing the results of Examples '1 and III it can be calculated that the yield loss when methanol alone is used is 86 0 higher than when the methanol-pentane mixture (3% pentane) is used while the color improvement when methanol alone is used is only 9% higher than when the methanol-pentane mixture is used. On the other hand, by comparing Examples II and III it can be calculated that the use of the higher amount of pentane results in an 11% improvement in yield but also results in a sugar color 18% higher.

The invention claimed is:

1. Method of decolorizing raw sugar which comprises contacting solid raw sugar at a temperature of 0l00 C. with a single phase liquid mixture consisting essentially of methanol and a hydrocarbon to form a slurry of solid raw sugar in said organic liquid mixture, whereby coloring matter in said raw sugar is extracted from said raw sugar into said organic liquid mixture, and separating decolorized raw sugar from said slurry, wherein (1) said hydrocarbon contains a maximum of one double bond, is se lected from the group consisting of aliphatic hydrocarbons and cycloaliphatic hydrocarbons, and is miscible with methanol to an extent of at least 0.25% by weight of methanol at the temperature of said contacting, and (2) the amount of said hydrocarbon in said single phase, liquid mixture is 0.25-30.0% by weight of said methanol.

2. Method according to claim 1 wherein said hydrocarbon is miscible with methanol to an extent of at least 1% and wherein the amount of said hydrocarbon is 1-5 3. Method according to claim 1 wherein said hydrocarbon is a saturated aliphatic hydrocarbon containing 5-10 carbon atoms.

4. Method according to claim 3 wherein said hydrocarbon is miscible with methanol to an extent of at least 1% and wherein the amount of said hydrocarbon is 1 5% References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Spencer-Meade Cane Sugar Handbook, 1945, Eighth Edition, John Wiley & Sons, New York, pp. 205-215 and 335-337 relied on.

MORRIS O. WOLK, Primary Examiner.

Claims (1)

1. METHOD OF DECOLORIZING RAW SUGAR WHICH COMPRISES CONTACTING SOLID RAW SUGAR AT A TEMPERATURE OF 0*-100*C. WITH A SINGLE PHASE LIQUID MIXTURE CONSISTING ESSENTIALLY OF METHANOL AND A HYDROCARBON TO FORM A SLURRY OF SOLID RAW SUGAR IN SAID ORGANIC LIQUID MIXTURE, WHEREBY COLORING MATTER IN SAID RAW SUGAR IS EXTRACTED FROM SAID RAW SUGAR INTO SAID ORGANIC LIQUID MIXTURE, AND SEAPRATING DECOLORIZED RAW SUGAR FROM SAID SLURRY, WHEREIN (1) SAID HYDROCARBON CONTAINS A MAXIMUM OF ONE DOUBLE BOND, IS SELECTED FROMTHE GROUP CONSISTING OF ALIPHATIC HYDROCARBONS AND CYCLOALIPHATIC HYDROCARBONS, AND IS MISCIBLE WITH METHANOL TO AN EXTENT OF AT LEAST 0.25 BY WEIGHT OF EMTHANOL AT THE TEMPERATURE OF SAID CONTACTING, AND (2) THE AMOUNT OF SAID HYDROCARBON IN SAID SINGLE PHASE, LIQUID MIXTURE IS 0.25-30.0% BY WEIGHT OF SAID METHANOL.