ACTIVATED CARBON IN SUGAR REFINING
E. W. HARRIS Darco Corporation, New York, N. Y.
An activated carbon found suitable for purifying one solution may not necessarily be used to best advantage for a different material. For instance, the activated carbon used in the refining of beet sugar sirup would not be employed for treating used dry cleaning solvent, though both materials may have color, odor, and other impurities that are objectionable and should be removed. For most efficient purification a carbon would be selected having the desired properties along with the peculiar afinity for the adsorption of those impurities found in the material to be treated. Lack of knowledge of the behavior of activated carbons retarded their use. Only in recent years have their properties been investigated extensively and sufficient knowledge has been accumulated to cause rapid increase in their use. While activated carbons are employed Courtesy, American Crystal Sugar Company successfully in many fields other than sugar FILTERPRESSES FOR REMOVING ACTIVATED CARBONFROM THE PROCESS refining, this paper is confined primarily to its usein the sugar industry. The properties of decolorizing carbons which are of greater interest to processors of beet, cane, and corn sirups are their CTIVATED carbons, as we know them today, are ability to remove color bodies, colloids, odors, tastes, and vastly different from the decolorizing charcoal emother impurities, and the ease with which the carbon may be ployed a t the beginning of the eighteenth century in filtered from the liquors. Europe. Zerban (17) in 1918 pointed out the early attempt to produce activated carbon, discussed its use a t that time, Color Removal and reported on results of laboratory and sugarhouse tests in Louisiana during the grinding season of 1917-18. In the past The refining value of an activated carbon is not necessarily two decades much progress has been made in the developdetermined by its ability to remove color but by its capacity ment and application of activated carhon for sugar refining: to adsorb colloids and other impurities, many of which are Carbons having high decolorizing power, with capacity to colorless. Sanders (11) showed the method for evaluating adsorb large quanti ties of colloids, high clarifying power, and color removal properties of activated carbon by using the good filterability, are the result of much study and research. Freundlich adsorption equation and the adsorption isotherm. Many grades of activated carbons have been developed, each Color removed by carbon from sugar solutions may be having properties for a specific purpose. measured by most of the commercial color comparators, as In the manufacture of commercial activated carbons the the Hess-Ives tint-photometers, the Lovibond instrument, or raw material may consist of almost any carbonizable subthe various photoelectric devices on the market. The Hessstance; examples are wood charcoal, lignite, bituminous coal, Ives instrument, together with a modification of the Meadeand black ash (a by-product of the paper industry). Harris units (6),have been used in much of our work. Two general methods of manufacture are in use. One is The presence of color in sugar liquors indicates the presence chemical activation. In this procedure a dehydrating chemiof impurities. Color may be due either to substances excal is heated with wood, with subsequent leaching or vaportracted with the sugar from beet or cane or to degradation ization of the chemical. The other is activation a t high products formed in the manufacturing operations-e. g., temperatures in the presence of a mild oxidant such as steam caramel. We may define the former as natural color and the or carbon dioxide. Both methods are employed in the United latter as process color. States. However, by far the greater part of the production is Either type is undesirable in the final product. Yamane by the latter method. (16)found that caramel is adsorbed by the sugar crystal. Activated carbons are specific in their adsorptive charExperiments have shown that decolorizing carbons will reacteristics; that is, they will adsorb and hold certain submove both types of coloring matter although caramel seems stances tenaciously while they show less affinity for other to be removed more easily. This is illustrated as follows: substances. Consequently it is necessary to select raw Color, materials and control methods of manufacture so that the Activated Carbon Original Sugar m i 1 Treatment, % sugar sirup (caramel ad&d) Difference resulting products are suitable for the purpose for which they 0.0 156 232 76 are intended. Noteworthy examples are the two general 0.5 145 207 68 1.5 127 167 26 classes-gas adsorbent carbons and the so-called decolorizing 8.0 105 129 24 carbons. 90 111 21 4.0 1057
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The data indicate that the caramel color is being more readily removed. With the 4 per cent treatment the colors of the two solutions are rapidly approaching each other.
Colloid Removal I n addition to being able to remove coloring matter from solution, decolorizing carbon is capable of removing many colorless substances which are surface active or are colloidal in nature. The removal of these substances is often of greater importance than getting rid of coloring matter since they interfere with manufacturing operations and their removal results in improved operation of the factory and a better product. The presence of surface-active materials is indicated by lowering of the surface tension of the sugar solutions. Methods of determining the presence of colloidal matter were described by Paine and Badollet (8) and by Newkirk. The former authors use the so-called dye test. Newkirk employed a method of fractional precipitation (unpublished) to separate and determine the various types of colloidal matter present in sugar solutions. It has been found that most activated carbons designed for sugar refining have a more pronounced tendency to adsorb colloids than color bodies. Tests made on a large number of beet sugar sirups of high and 10~3-purity showed the carbon to remove a higher percentage of colloids as compared to color; colloid removal was in the range of 12 to 20 per cent greater than color removal, depending on the material treated. This difference has been found t o hold over a range of color removal up to about 82 per cent of original color. Color and colloid determinations on corn and cane sugar sirups indicate that the same refining condition may be expected, but further study of these materials should be made. Color and dye test measurements made on materials before and after a singlestage carbon treatment are shown in Table I. I
/
I
14
I
I5
BRIX
I
16
/I
I
I I BEET SUGAR THIN JUICE
PI I
I 17
I
THINJUICE FIGURE 1. VISCOSITYCURVESFOR BEET SUGAR
Activated carbons are selective in their power to adsorb impurities from solutions and will generally remove the most objectionable materials. Those impurities found in sugar sirups, known as molasses-forming compounds, are among the groups removed. This is of great importance because of the effect on crystallization. Such impurities not only retard the rate of evaporation and speed of crystallization and impair the quality and character of the crystal, but also reduce the yield of crystallizable material. Newkirk (7) found that such impurities have a marked influence upon crystallization;
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Brewster and Raines ( 1 ) showed the detrimental effect of impurities in sugar factory performance aside from those which are responsible for color. Lindfors (6) in his work on surface tension found that mineral salts have little effect upon surface tension and, on the other hand, that colloidal silica, gums, and organic acids depress the surface tension. These colloidal substances and organic materials also tend t o increase viscosity of sugar sirups and, as pointed out, adversely affect evaporation and operation of the strike pan. Paine, Badollet, and Keane (9), in their study of colloids in cane and beet sugar, called attention to the effect of colloids in increasing viscosity and thereby diminishing the rate of filtration, boiling, and crystallization. Colloidal impurities in a sugar sirup causing increased viscosities are adsorbed by activated carbons. It has been found that, in regular factory operation when treating beet sugar thin juice with 0.5 per cent carbon on dry substance, there is an average reduction in viscosity of 5 per cent. I n treating beet sugar thick juice with the same quantity of carbon, the reduction averages 10 per cent. TABLE I. COLORAND DYETESTMEASUREMEYTS Treatment Mead-Harris Color Colloids by Dye Carbo; Unitsa Test ?on Dry % re70 reSubstance) Before After moved Before After moved
Material Beet sugar Thin juice 0.2 126 Thin juice 0.5 126 118 Standard liquor 0.2 Standard liquor 0.5 118 174 Raw 0.2 Raw 0.5 174 Cane sugar 327 Raw 0.2 0.5 327 Raw Washed raw 0.2 98 0.5 98 Washed raw a By Hess-Ives tint-photometer.
97 52 88 46
127 76
261 205 64
18
23.0 58,7 25.4 61.0 27.0 56.3
142 142 79 79 204 204
83 31 44 14 121 61
41.5 78.2 44.3 82.3 40.7 70.1
20.2 37.3 34.7 81.6
288 288 127 127
194 44 54 6
32.6 84.7 57.5 95.3
Typical viscosity-Brix curves for beet sugar thin juice are shown in Figure 1. These data were obtained during test work a t a beet sugar factory during the campaign of 1940. The viscosities were determined in an Ostwald viscometer a t 25" C. This decrease in viscosity after carbon treatment results jn increased rate of evaporation, higher filtration rates, and improved workability of the fillmass. The impurities enter the process along with the raw material, and in production they may form more complex substances due to heat and oxidation. They increase viscosity, adversely affect surface tension, and have a direct influence on quality of finished product. The adsorption of such impurities by activated carbon from sugar sirups is a direct means of reducing the complex products formed during evaporation and crystallization, and will result in increased process efficiency and a finished product of high purity. We have found that the impurities should be removed as near the beginning of the process as is economically possible. It is usual practice in corn sirup, beet sugar, and corn sugar refining that low-gravity materials to be evaporated are first treated with partially spent activated carbon and then the heavy sirup is treated with new carbon. A large percentage of the impurities is therefore adsorbed near the beginning of the process, and the virgin carbon removes further impurities from the heavy liquor as well as any that may be formed during the process of concentration.
lMethod of Application and Handling The successful and economical use of act~ivatedcarbon in sugar refining is dependent on its method of application and handling in the plant. It is not sufficient merely to select the
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for a 15-minute period to ensure complete dispersion of the carbon throughout the liquor. Thorough agitation is important since it is necessary for each particle of the carbon to be brought into contact with the solution. Activated carbon is added as a slurry to the material to be treated since this gives better dispersion than the addition of dry carbon to the sirup. A dry feed machine is employed in the preparation of the carbon slurry, or the carbon may be used in a batch system. Sugar solutions or water is used for making up the slurry.
Filtration of Activ'ated Carbons An activated carbon may have a very high refining value and a filtration rate so low that its use commercially is not
FIGURE 2.
J.
FILTERABILITY APPARATUS
FilKer paper
K. Filter cloth L. 1.5-inohiron companion flanges M. Screen grids N . Perforated plate
economical. Therefore, in the production of activated carbons the manufacturer must produce a well-balanced carbon having a filterability and refining value that will permit ease of operation and low-cost refining, a carbon that is readily wetted and can be held on the usual filter press cloth. There is considerable variation in the filterability of the different grades of activated carbons due, in many cases, to porosity, structure, and particle size. Carman (8), Underwood ( I C ) , Sperry (IS), Ruth, Montillon, and Montonna (IO), van Gilse, van Ginneken, and Waterman (4,and Walker, Lewis, McAdams, and Gilliland (I@, have published equations relating the variables in the filtration of insoluble substances. For filtration at constant pressure, most of the equations can be put in the form, T / V = aV
+6
where V = volume of filtrate collected in time, T a,b = constants
proper grade of carbon for the refining of a certain sirup. Lines for conveying the carbon-treated sirup, tanks, and pumps should be constructed of suitable material to give the best processing conditions. For example, sirups being treated in the pH range of 3.5-5.5 should not be processed in iron tanks. Wooden or iron tanks properly coated should be used; also proper coating of metals to prevent galvanic action when using heavy concentrations of activated carbon is desirable. Activated carbon should be employed so as to get the maximum refining value from the quantity used. Sanders ( l a ) described the countercurrent use of activated carbons and gave the method of calculation for the re-use of partially spent carbon. This method gives the greatest refining value a t the least cost. Frequently the carbon is employed in a two-step countercurrent procedure on a sirup, and then the partially spent carbon is used on a different sirup of lower purity.. I n such cases the treatment is not a true countercurrent operation, but there are times when certain advantages may be had in this scheme from a refining viewpoint. It is important when using the countercurrent system that the correct amount of carbon be used in each stage; otherwise the procedure cannot be kept balanced. A simplified graphical method of determining the dosage is available (3). The three-stage treatment of sirup finds considerable use where a high degree of refining is necessary. Four-stage countercurrent treatment and higher multiples are employed in refining sugar sirups only in special cases, since the carbon after three runs is generally fairly well spent and further use is not considered economical. The use of activated carbon in the sugar factory, whether it be for the refining of corn, beet, or cane sugar, consists essentially in the addition of carbon to the material to be refined, agitation, and subsequent filtration to separate the carbon from the liquid. Adsorption of the impurities by the carbon takes place rapidly, but in most cases agitation is continued
Constant pressure filtration is rarely used commercially, but laboratory filtration tests at constant pressure enable the filterability characteristics of the carbon to be easily measured. It can be shown that the term aV is equivalent to the depth of the cake. This permits a determination of the relative filterability of an activated carbon in a comparatively simple apparatus as a routine control test. The filtration test as carried out a t this laboratory for comparing the so-called filterabilities of activated carbon is applicable to the evaluation of the filtration rate of most insoluble substances. The apparatus is illustrated in Figure 2. The technique is described briefly as follows: The sample to be evaluated is mixed with sufficient distilled water to form a thick, pasty mass. The mixture is subjected to a vacuum of 28 inches of mercury in order to free it of air bubbles. It is then transferred to the filtering tube by means of a syringe. The excess water is drawn off by carefully applying suction below the filter medium (cloth and paper). It is important that no air be drawn into this preformed cake. Water is carefully placed above the cake, the filter tube is closed, and the water is forced through the bed of carbon by air at a constant pressure of 40 pounds per square inch. The volume of water collected from the filter tube is recorded a t ,constant time intervals. The temperature of the water is recorded. Normally a volume of 250 ml. is collected. The filter tube is now opened. The depth of the cake is measured in the filter tube by means of a micrometer. The test is repeated for three depths of carbon cakes for a given sample of carbon. The T/V value for each determination is calculated by dividing the recorded time in seconds by the volume of filtrate in milliliters measured. This value is adjusted to 20" C. by introducing a viscosity-temperature correction. The T/V values (corrected to 20" C.) of the above tests are plotted against the corresponding cake depths in centimeters.
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The slope of this line is the “filterability” value for this sample of carbon. Since all carbon samples are evaluated on the same filter tube, the area term does not appear in this slope evaluation but the true filterability value for a sample should include a unit area-i. e., seconds per milliliter filtered, per centimeter depth of cake, per square centimeter of filtering area. Typical values of activated carbons are as follows: Carbon A
B
c
1.O sec./ml./cm. depth 0.4 0.2
It should be remembered that the lower the filterability value, the better the filtering characteristics. For example, in the above table, 0.2 second is required to filter 1 ml. of water through 1 cc. of carbon C a t 40 pounds per square inch (2.8 kg. per sq. em.) pressure. For carbon A, 1.0 second is required to filter 1 ml. of water under identical conditions. Thus water would pass through the carbon C cake five times as fast as through a carbon A cake of equal depth a t the same pressure. The removal of impurities by adsorption from sugar solutions, especially those that interfere with evaporation and crystallization, result not only in better plant performance, but a product of higher quality and greater economy. It is apparent that the increasing use of activated carbon in the sugar industry has resulted in continued development of properly balanced carbons having high refining value, good filterability, and low operation cost, along with flexibility.
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Acknowledgment Tests on caramel adsorption by activated carbons and viscosity determinations were made by George H. Scheffler, Director of Research of this company.
Literature Cited Brewster and Raines, J. IND. ENG. CHEM.,13,929 (1921). Carman, J . SOC.Chem. Ind., 53, 159-65T, 280-2T, 301-9T (1934); Trans. Inst. Chem. Engrs. (London), 16, 168-88 (1938). Darco Gorp., Handbook for Counter-Current Treatment with Activated Carbon, 1939. Gilse, van, Ginneken, van, and Waterman, J . SOC.Chem. Ind., 49,444T, 483T (1930); 50, 41T, 95T (1931). Lindfors, IND.ENG. CHEM.,16, 813 (1924). Meade and Harris, Ibid., 12,686 (1920). Newkirk, Ibid., 16, 1173 (1924). Paine and Badollet, Planter Sugar Mfr., 79,121 (1927). Paine, Badollet, and Keane, Intern. Sugar J . , 28, 23 (1926). Ruth, Montillon, and Montonna, IND.ENG.CHEW,25, 76, 153 (1933). Sanders, Chem. & M e t . Eng., 28, 541 (1923). Sanders, IND.ENG. CHEM.,20, 791 (1928). Sperry, Chem. & Met. Eng., 15, 198 (1916). Underwood, Proc. World Eng. Congr. Tokyo, 1929,31, 245, 264 (1931). Walker, Lewis, McAdams, and Gilliland, “Principles of Chemical Engineering”, p. 343, New York, McGraw-Hill Book Co., 1937. Yamane, Facts About Sugar, 35, No. 11, 32 (1940). Zerban, Louisiana Agr. Expt. Sta., Bull. 161 (1918). PRESENTED in a group of papers on Decolorizing Carbons and Analysis before t h e Division of Sugar Chemistry and Technology a t t h e 102nd Meeting of the A M ~ R I C ACHEMICAL N S o c ~ n r u Atlantic , City, N. J.
PARTICLE SIZE STUDIES Effect of Viscosity of Medium on Rate of Grinding in Pebble Mills H. E. SCHWEYER’ Columbia University, New York, N. Y. Particle size distributions in the subsieve ranges have been used to study the development of surface and the decrease in top size in pebble mill operations using fluid media of different viscosities. It was found under the conditions used that the development of surface takes place in two stages. In the initial period the rate of surface development is essentially constant and is a function of the viscosity of the medium. In the second period the rate decreases and is practically independent of the viscosity of the medium. The rate of decrease of top size was found to be rapid at the start but decreases with time of grinding and is a function of the viscosity of the fluid medium used.
RINDING operations as employed in metallurgy have been given considerable study in the reduction of minerals to the sizes of the order of 100 microns (1 micron = 0.001 mm.). However, in the production of subsieve sizes little attention has been given to the evaluation of grinding operations. This has been true because methods for determining size distribution (IS) of the product have not been generally applied. Such particle size determinations are
G 1
Present address, The Texas Company, Port Neches, Texas.
being used to a relatively large extent in the ceramic industries because of the influence of the small particles in the manufacture of ceramic materials. The purpose of the present study is to illustrate the application of particle size determinations in the subsieve ranges to such research and, more specifically, their application t o a pebble mill study made under certain operating conditions. Andreasen (6)showed the application of the pipet method in studies on the ball milling of flint, where the relative surface developed after different times of grinding for a particular charge was evaluated. More recently this author and others (1) used a pipet method for studies on iron oxide and barytes ground in different types of ball mills. A general discussion of the grinding of ceramic materials in ball and pebble mills was given by Metz (IO). Farrant (7) discussed all types of grinding mills in different applications. It is interesting that both these authors evaluate fineness of product by the percentage smaller than various sieve sizes with such values as “99 per cent through 300 mesh” being noted. Evaluations such as these are obviously inadequate since a finely ground material may have an infinite number of size distributions and still meet such a specification. Gross (9) discussed crushing and grinding with considerable emphasis on the efficiency of grinding operations; an extensive bibliography is appended.
