ADSORPTION AS A MEANS OF SEPARATION JOHN W. HASSLER Wrest Virginia Pulp and Paper Cornpan). Tyrone, Penna.----
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This paper presents a brief survey of factors to be considered in the choice of solid adsorbents for the separation of materials from liquids. Industrially, adsorption methods are often used to remove impurities, especially if they are present a t relatively low concentrations. Traces of material which would interfere with chemical reaction or such physical processes as crystallization, filtration, and distillation may sometimes be removed by proper use of the correct adsorbent. When the adsorbed impurity has value, it may be economically practical to recover this value, especially since such recovery will usually yield the impurity in concentrated form. Separation by adsorption is possible because materials differ in the degree to which they are adsorbed by a given adsorbent. In a liquid mixture or solution some constituents will be selectively attracted to the surface of the adsorbent to the almost complete exclusion of others. The extent and character of separation are influenced by the
attraction between the solid adsorbent and the liquid, the adsorbent, and the dissolved impurity, as well as the attraction between the liquid and the dissolved impurity. The concentration and kind of dissolved materials, extent of the solid surface, temperature, time, and pH also influence the adsorption effect. Such important points as conditions favoring the use of adsorption, the Freundlich equation, the selection of a suitable adsorbent and the determination of proper dosage are discussed. Revivification and operating method, including countercurrent flow, percolation, and simple contact treatment, are considered. No attempt is made to give detailed methods of existing applications; this information can be obtained from manufacturers of adsorbents, and such instructions can rarely be applied directly to new applications. Rather, the purpose is to outline an approach that has been found useful in the solution of various adsorption problems.
HE nature of adsorption has been discussed in detail by many investigators (16, 19, 20, 28). In solutions the component having the greater surface tension is pulled from the surface into the bulk of the liquid and the other components remain in greater concentration in the surface film. Although such an adsorption layer is formed a t a liquid-air boundary, it is not extensive enough for practical separations. The presence of a porous solid will provide a large surface area and, in addition, will frequently supply a strong attraction for certain components. The total effect often alters the concentration of a solution and is the basis of the industrial use of adsorption for separation. Generally the adsorbed layer is oriented. Figure 1 shows a layer of sodium stearate adsorbed a t the interface between active carbon and water, with the soluble sodium end attracted to the liquid and the organic end drawn toward the carbon. This is a very simple form; frequently the composition and structure of the adsorbed layer is more complex. A strong attraction between the liquid phase and the adsorbable components will reduce the concentration in the adsorption layer. Again, while adsorption is usually considered in relation t o a solute, frequently the solvent is more strongly attracted t o the solid surface (wettability, B), and will displace an adsorbable solute. It follows that adsorption of a solute is favored by use of a solvent which does not readily wet the adsorbent and one in which the solute is only slightly soluble. Although in industry the choice of a solvent is usually determined by other factors, methods of reducing the solubility are sometimes available; e. g., the influence of pH on adsorption is often due to an effect on solubility. Abderhalden (1) and Wiegner (39) found that where the presence of a salt alters the solubility, the adsorption will also be affected. Theories regarding the nature of the bond between the surface of a solid and the components of a solution are dis-
cussed in works on colloids (7, 16, 64, 36). The attraction is very selective (4,18, 29, 34, 41), selectivity being a primary characteristic of adsorption. Indeed the commercial utility of adsorption depends upon its selective action.
T
Character of Adsorbable Solutes Much depends upon the chemical nature of the solute; thus free iodine is readily adsorbed whereas iodides are weakly adsorbed. The presence of certain groups in the molecule will influence adsorption ( I ? ) , but the exact effect depends also on the type of solvent and adsorbent (26); e. g., some surfaces attract only basic dyes, and others only acid dyes (IS, 31). Materials which lower the surface tension frequently are well adsorbed a t a solid surface, but this does not always follow (11).
Types of Commercial Adsorbents Most solid materials have some adsorptive power, but few possess this property to a marked degree. Among those important commercially (SO) may be mentioned fuller’s earth, activated clay, bauxite, silica gel, zeolites, bone char, and activated carbons. From a tonnage viewpoint, fuller’s earth leads all others; it and activated clays are used in petroleum and edible oil industries. Bone char is used in sugar refining and silica gel for dehydration. Activated carbons are utilized in such diversified fields as the purification of organic and inorganic chemicals, of edible oils, sugars, and other food products, of alcoholic liquors, and of municipal water supplies. Adsorbents of the same kind differ widely in quality. Gurvitch (18) showed that fuller’s earth from different sources will vary both in chemical composition and in selective adsorptive power. Although all activated carbons have a similar ultimate chemical analysis, these carbons 640
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SEPARATION OPERATIONS
cannot be considered chemical identities in the same sense as sodium chloride or acetic acid. An activated carbon produced by one method may be very effective for the adsorption of iodine whereas a carbon prepared by a different method may be chiefly effective in decolorizing sugar; still a third might be called “just a reasonably good all-around carbon”. To the extent that chemical or residual valence forces are accepted as a factor in adsorption, the activated carbon surface may be visualized as a mosaic or checkerboard of various active patches ( 1 7 ) . As stated by Bartell (4), “the exact properties of a given charcoal will depend both on the type of activation and on the relative proportion of the surface which has the desired activation”.
64 1
of temperature is not uniform. Substances having an a p pfeciable vapor pressure are generally better adsorbed as the temperature is lowered, whereas the adsorption of many other materials (e. g., colors, gums, etc.) may be favored by elevating the temperature. In some cases there is a definite
Influence of Surface Area and Particle Size Many of the more powerful adsorbents have numerous capillaries and pores which provide a large internal surface: a kilogram of gas-mask carbon is reported to possess a total surface of one million square meters (29). Although the extent of the surface is of great importance, there are practical limits to the extent to which the external surface can be increased by reducing the particle size. An adsorbent of extremely small particle size will be difficult t o separate from the liquid with which it may be mixed. Where very small dosages of adsorbents are used (e. g., a few parts per million), as in the removal of tastes from water supplies (SS), an extremely small particle size is essential to furnish sufficient dispersion to provide the probability of making a contact with most of the taste-bearing molecules (6).
FIGURE 1. ORIEKTED ADSORPTION OF SOAPMOLECULES AT A LIQUID-SOLID INTERFACE
optimum temperature (32). Where higher temperatures are favorable, it may be due to chemosorption, denaturing, or, in some cases, to an increase in the ratio of the vapor pressure For well-dispersed adsorbents, adsorption equilibrium is of the solvent to that of the solute. In view of our limited generally reached rapidly. Often 10 to 20 minutes (11) are knowledge of the effective forces a t the solid-liquid interface, ample for practical purposes. Hon-ever, cases occur where it is reasonable t o suppose that still other factors are involved. much longer periods of time are necessary. Also, some adViscous liquids are generally treated at elevated temperatures sorbents require more time than others to reach equilibrium. because of greater ease in handling. -4s is true of so many factors in adsorption, the influence The adsorbability of bodies often varies with changes in pH. Often a low pH is favorable, although in some cases the reverse is true. If the relationship of pH to adsorption is plotted, frequently the curve will show maximum and minimum points of adsorption (14). The important influence of p H is i n d i c a t e d b y t h e known value of pH ccntiol in the dyeing of textiles acd paper, n-hich is basically an adsorption phenomena ( 3 , 16). Some adsorbents may contain sufficient acid or alkali t o alter the pH of the liquid being treated. This may be an advantage or disadvantage according to the specific problem a t hand. The effect of the pH on color bodies of an indicator type must be considered t o avoid confusing a possible color change with adsorption. Curiously, the matter Courtesy, Penick & Ford, Ltd. of whether the adsorbent PRESSESUSED IS APPLYINGACTIVECARBONT O REMOVECOLORS,COLLOIDS,A N D OTHER has an acid, alkaline, or SOLUBLE IMPERITIES FROM GLUCOSE neutral character may in-
Influence of Time, Temperature, and pH
INDUSTRIAL AND ENGINEERING CHEMISTRY
642
fluence adsorption from nonaqueous liquids-. oils (23).
g., edible
Adsorption Calculations From a purely scientific view, adsorption deals with the formation and composition of the interfacial region. Although any change in concentration of the liquid phase is incidental, in fact, adsorption from a liquid can occur without a concentration change; general interest centers in those cases which do involve concentration change. Most adsorption equations include a function involving such changes. Of these, the so-called (69)Freundlich equation (11) is most generally used by industry: x / m = kC1jn where x = amount of material adsorbed m = mass of adsorbent
(1)
C = residual concentration at equilibrium k, 1,fn = constants
The equation is empirical, and efforts to give it a theoretical basis have not proved entirely satisfactory. As Proctor (35) stated, it is “a mathematical expression which will closely represent any chemical or physical phenomenon which proceeds a t a diminishing rate”. The equation holds rather well a t low concentrations and, as a rule, covers the range involved in most industrial applications. It is conveniently used in graphical form, where the logarithm of x / m is plotted against the logarithm of C. If the results of an experiment follow the equation, this plotting will give a straight line. The results of an adsorption of color by an activated carbon are given in Table I and plotted in Figure 2.
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result is that adsorption is more applicable where impurities are present in small concentrations. The trend is illustrated in Table 11. TABLE11. IKFLUENCE OF ORIGINAL CONCENTRATION AS SHOWN BY DECOLORIZATION OF BLACKSTRAP MOLASSES SOLUTIONS Concn. of Blackstrap Molasses -Grams 2.0 1.5
Activated Carbon Doeage per IO0 ml.-
Color Removed
% 70
0.6
so
0.6 0.6
1.0
86
Any commercial evaluation of adsorbents must indude a factor for relative cost to produce the same change in concentration; in view of “selectivity”, it is apparent that the choice of an adsorbent should be based on tests with the product to be treated (34). Seldom is it possible to duplicate plant operating conditions in laboratory tests, but fortunately experience has shown that the plant operation generally will require
[
‘-O
50
d
[
60 70 80 9 0 COLOR REMOVED
FIGURE 3. CARBON DOSAGE us.
REMOVAL OF COLOR FROM A CRUDE LACTICACID(FROM TABLEI) 20
10
30
less absorbent than the laboratory tests indicate. In some cases the amount saved may be considerable. De Witt (10) states that in flotation (an adsorption phenomena) it has similarly been found that large-scale operations require a smaller quantity of reagents than laboratory data indicate.
50
C
FIGURE 2. LOGARITHMIC DIAGRAM OF ADSORPTIONISOTHERM (FROM TABLE I)
Countercurrent Application So far a single-stage contact treatment has been considered. A countercurrent treatment (37) involving two or more stages
TABLEI. DECOLORIZATION OF CRUDE LACTIC ACID“ Units of Color Carbon Dosage, m
Adsorbed,
Residual,
Adsorbed per g. carbon,
X
C
z/m
x/m to
51 70
49 30
170
3.5 4.7 6.5 8.2
Ratio of
C
Gram/lSO 4.
0.3
0.5
0.7
82 88
18 12
140
117
0.9 98 1.0 90 10 90 9.0 Colors measured in KCH colofimeter, baaed on 100 color units in original liquid: data arranged for Freundlich equatlon. a
The question is sometimes raised as to whether adsorption is more effective a t high or low residual concentrations. This would appear to depend on the angle of view. Table I shows that with decreasing residual concentrations there is a decrease in the amount of color held by each unit of adsorbent. This is illustrated clearly by plotting the percentage of color removed against carbon dosage (Figure 3). The first 0.5 per cent of carbon removes 70 per cent of the color; the next 0.5 per cent of carbon (1 per cent total) removes but 20 per cent additional. On the other hand, with decreasing residual concentrations, there is an increase in the ratio of material adsorbed to the amount left in solution (Table I). Since Equation 1 is independent of the original concentration, the
will require less adsorbent. Figure 1 indicates that a t 10 per cent residual color concentration (90 per cent removal) each unit of carbon holds 90 units of color, whereas a t 30 per cent residual color, each unit of the same carbon holds 140 units of color. For this specific case the spent carbon from the treatment for 90 per cent removal has a reserve power of 50 units to bring an additional quantity of the original solution to 30 per cent residual color and thus furnish a partially decolorized solution which requires less virgin carbon to produce the finished product of 10 per cent color. Frequently, a twostage system will reduce by 30 to 50 per cent the amount of adsorbent required. The slope of the Freundlich curve is an index to the benefit to be so derived, since a steeper slope indicates greater reserve power. Countercurrent treatment is most valuable where it is desired to obtain a small residual concentration.
Percola tion To a modified extent the percolation method of adsorption inherently embodies a continuous countercurrent principle, as only the last layer of adsorbent is in equilibrium with the finished product. An increasing quantity of the reserve power of each successively higher layer is utilized. This
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SEPARATION OPERATIONS
benefit is often offset by certain disadvantages of the percolation method, such as the necessity of using larger size partides, and channeling (37) may occur whicli eliminates part of the bed from effective contact with the liquid.
Factors Involved in Removal of Impurities Although adsorbents are frequently used in industry to remove impurities, they are not a cure-all. Their use is governfd by Certain considerations. Where the amount of impurity is small, the adsorbent cost may be considerably less than another type of separation. Again, some materials may contain a variety of impurities, removal of which would require separate chemical treatments. The proper adsorbent or mixture of adsorbents may provide
643
purification in one simple treatment. Adsorption is useful when small amonnts must be handled, and where the volume and ralue of the product may not warrant special or extensive chemical control and supervision. Moreover, an impurity either may not be removed by known chemical treatments, or chemical treatment may result in contamination. Adsorption may replace physical separation in some casese. g., where the impurity distills within tlie boiling range of the product. The use of adsorbents is so strongly associated with the idea of color and taste removal that other possible benefits may be overlooked. Thus, although adsorbents have long been used for bleaching edible oils, only recently has i t b e come known that certain adsorbents are beneficial in other ways-e. g., removal of incipient rancidity (22, 83)
Adsorption in Other Separations There are cases where adsorption is of value in causing a slightly reversible reaction to go to completion. An illustration is found in the neutralization of an edible oil in those cases where complete removal of the free fatty acids is desired. Although reaction with caustic is noticeably reversible, the addition of an adsorbent removes tlie soap as formed and results in complete neutralization and removal ($8). Adsorbents have been used t o remove certain impurities that act as negative catalysts for some chemicat reactions. The fact that a nogative catalyst for one reaction may be a stabilizer for another must he considered in choosing( - an adsorbent, ($3). Many liquids contain seemingly unimportant traces of impurities which, however, may be adsorbed at surfaces to an extent that will retard other separations. Frcundlich (18) and Clayton (6') have reviewed many oases where such bodies, when adsorbed on a crystal surface, retard o~ even prevent crystal growth. Again, these impurities may be ad-
* V r ~ w sIN A GOVERNMENT-OWNED SUGAR MSLLd'r ZACATEPEC, MEXICO, WXICH USES ACTIVATED CARBON
(Top) Auto filten which separate sugar liquors from the csrhon after decolori-
zation bas been completed; ( c a t e r ) vacuum filters which wash and dewater the spent activated carbon; (bottom) batteries of electric furnaces which revivify the spent carbon.
644
INDUSTRIAL AND ENGINEERING CHEMISTRY
sorbed only on certain faces of the crystal and thus result in a distorted crystal form. In filtrations such bodies may be adsorbed on the surface of the filter medium, and thus decrease the free space and reduce filtration rates. In distillations they may be adsorbed a t the liquid and vapor interface to produce a surface film which, if tenacious, will result in foaming. Again, these bodies may produce an emulsoid or an emulsion. Inasmuch as adsorbable materials cause these difficulties, they can be eliminated by a prior treatment of the liquid with a suitable adsorbent.
Adsorption for Recovery of Materials Sometimes an adsorbed impurity may have sufficient value to be recovered, and adsorption may be applied for the sole purpose of recovering some valuable material-. g., dyestuffs, alkaloids (36), precious metals (SO), biological products (25140). Utilization of adsorbents for the recovery of materials involves three steps: (a) establishment of conditions for the preferential adsorption of the component to be recovered; (b) separation of the adsorbent carrying recoverable materials from the liquid; (c) establishment of changed conditions such that the desired component is no longer adsorbed-i. e., desorption-which enables the product to be recovered in a concentrated and relatively pure form. Occasionally a material may be altered chemically when adsorbed on some surfaces; while this seldom happens, it is a possibility to be studied in a recovery application. The desorption methods include: (a) addition of some material more strongly adsorbed with resultant driving out of the desired component; (b) change of pH, where the absorbability varies with the pH; (c) steaming, where the adsorbed body has a relatively high vapor pressure; ( d ) extraction with a solvent in which the component is very soluble; ( e ) chemical reaction. Different successive desorption methods can furnish a further separation in addition to that provided by the original adsorption. Sometimes this can be accomplished by the use of different solvents. Jukes (97) recently prepared aqueous yeast extracts and found that an activated carbon (Nuchar C-115) adsorbed not only pantothenic acid but also certain color bodies. The Xuchar containing the adsorbed materials was treated with alcohol which desorbed the pantothenic acid but not the color bodies; the latter were then desorbed by ammonia. This suggests that, where several important biological materials are present in the same source, they may be individually isolated by properly selected combinations of adsorption and desorption. The use of a chemical reaction can best be illustrated by an operation (2) on the West Coast, which for a number of years produced iodine equal t o one third the total domestic consumption in the United States. The sump water from the oil wells contains iodides to the extent of about 70 t o 100 parts per million; oxidizing agents converted this to free iodine which was adsorbed by activated carbon. The carbon with the adsorbed layer of iodine was separated from the liquid and treated to convert the free iodine to sodium iodide, which was extracted with a limited quantity of water.
Revivification Desorption frequently restores much of the original adsorptive power and, in a sense, can be considered as revivification. Often desorption is employed for this purpose alone. Another method, “reburning”, is extensively used for revivifying granular adsorbents (SO), Reburning of powdered adsorbents presents dusting and other problems. Although commercial equipment is available for this purpose (9, 58), powdered adsorbents are not reburned except where the daily
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consumption is sufficient to warrant the necessary supervision and capital expense.
Separation of Adsorbent from Liquid Filter presses are generally employed although gravity settling is sometimes used. A method used in the treatment of dry-cleaning solvents is based on the fact that some adsorbents can be wet by different immiscible liquids. The dirty solvent is treated with activated carbon which removes color and odor, and also with a sodium hydroxide solution to saponify any fatty material. While the carbon is preferentially wet by the solvent, it is partially wet by the caustic soap solution. Thus it forms a link between the two phases and collects a t the interface where it can be separated.
Literature Cited (1) Abderhalden, I. E., and Fodor, A , Kolloid-Z., 27, 49 (1920). (2) Anonymous, Chem. & M e t . Eng., 42, 605 (1935). (3) Bancroft, in Alexander’s “Colloid Chemistry”, Vol. 4, pp. 21933, New York, Chemical Catalog Co., 1932. (4) Bartell, F. E., and Lloyd, L. E.. J . Am. Chem. SOC.,60, 2120 (1938). Bartell, F. E., and Osterhoff, H. J., Colloid S y m p o s i u m Monograph, 5, 113 (1928). Braidech, Helbig, Smith, and Baylis, J . A n . Water W o r k s ASSOC., 30, 1299-1334 (1938). Buzagh, von, ”Colloid Systems”, pp. 170-215, London, Technical Press, 1937. Clayton, W., J . Oil Colour Chem. Assoc., 18, 412 (1935). Davis, R. G., U. S. Patent 1,945,479 (Jan. 30, 1934). De Witt, C. C., personal communication, 1939. Freundlich, H., in Alexander’s “Colloid Chemistry”, Vol. 1. pp. 575-83, New York, Chemical Catalog Co., 1926. Freundlich, H., “Colloid and Capillary Chemistry”, (tr. by Hatfield), pp. 122-38, 190, 233-9, London, Methuen & Co., 1926. Ibid.. pp. 206-19, 32841. Friedman, L., and Kuykendall, D. V., Paper Trade J . , 99, No. 12, 103 (1934). Georgievics, G., in Alexander’s “Colloid Chemistry”, Vol. 4, pp. 197-204, New York, Chemical Catalog Co., 1932. Gibbs, J. W., “Collected Works”, Vol. 1, New York, Longmans, Green 8: Co., 1928. Gortner, R. A,, “Colloid Chemistry”, pp. 108-18, New York, Cornell Univ. Press, 1937. Gurvitch and Moore, “Scientific Principles of Petroleum Technology’’, pp. 488,491, New York, D. Van Nostrand Co., 1934. Hardy, Proc. Roy. SOC.(London), 86,634 (1912) ; 88,303 (1913). Harkins, in Alexander’s “Colloid Chemistry”, Vol. 1, pp. 192264, esp. 221, New York, Chemical Catalog Co., 1926. Harris, J. P., and Welch, W. A., Oil & Soup, 14, 3 (1937). Harris, J. P., and Welch, W. A , , U. S. Patent 2,105,478 (Jan., 1938). Hassler, J. W.. y d Hagberg, R. A., Oil & Soap, 15, 115 (1938). Hauser, E. A., Colloid Phenomena”, pp. 114-44, New York, McGraw-Hill Book Co., 1939. Holmes, H., Cassidy, H., Manly, R., and Hartzler, E., J . A m . Chem. SOC.,57, 1990 (1935). Holmes, H., and McKelvey, J., J . P h y s . Chem., 32, 1522 (1928). Jukes, T . H., personal communication, 1939. Langmuir, J . Am. Chem. Soc., 39,1848 (1917). McBain, J. W., “Sorption of Gases and Vapors by Solids”, pp. 5, 13, 77, London, George Routledge & Sons, Ltd., 1932. Mantell, in Perry’s Chemical Engineers’ Handbook, pp. 10581104, New York, McGraw-Hill Book Co., 1934. Rheinboldt, H., and Wedekind, E., Kolloid-Chem. Beiheffe, 17, 115 (1923). Robertson, Munsberg, and Gudheim, Oil & Soap, 16,153 (1939). Sigworth, E. A., J. Am. Water W o r k s Assoc., 29, 688 (1937). ENQ.CHEM.,16, 498 (1924). Teeple, J.,E., and Mahler, P., IND. Thomas, Colloid Chemistry”, p. 280, New York, McGrawHill Book Co., 1934. Ibid., pp. 286-304. Walker, Lewis, and McAdams, “Principles Chemical Engineering”, 2nd ed., pp. 669, 718, New York, McGraw-Hill Book eo., 1927. Wickenden and Okell, U. S. Patent 1,854,387 (April, 1932). Wiegner, Magasnik, and Virtanen, Kolloid-Z., 28, 51 (1921). Williams, R. J., Truesdail, J. H., Weinstock, H. H.. Rohrmann, E., Lyman, C. M., McBurney, C. H., J . Am. Chem. Soc., 60, 2719 71938). Zechmeister, “Carotinoide”, pp. 94-101, Berlin, Julius Springer, 1934. ~~
