A
DSORPTION, while known for a long time, is difficult to define properly and to differentiate from absorption. I t is an established fact that an evacuated solid will take up part of a gas or vapor in contact with it. At constant volume, the pressure in the system will fall; a t constant pressure, the volume will shrink. If the molecules from the gas phase enter the inside of the solid, the phenomenon is designated absorption; if, however, the molecules remain attached to the surface of the solid, that occurrence is called adsorption. This definition of adsorption was first given by H. Kayser a t the suggestion of E. du Bois-Reymond in 1881.2 Prior to that time both phenomena were named absorption. Quite frequently the two effects take place together, and the term sorption, introduced by J. W. McBain in 1909, is used to describe the joint r e s ~ l t . The ~ solid phase is known as the adsorbent, and the gas or solute molecule attached to the surface of the solid is named the adsorbate. This precise demarcation is often difficult or impossible to apply experimentally, but no better one has been advanced. The history of adsorption is as old as the greater part of chemistry itself. C. W. Scheele, in 1773, and the Abbe F. Fontana, in 1777, investigated the uptake of gases by charcoal; and T. Lowitz, in 1785, discovered that charcoal took the coloring matter out of certain
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' Industrial
Fellow, Multiple Industrial Fellowship of the Co., Mellon Institute, Pittsburgh, Pennsyl-
Texas Gulf Sulphur
vania. BRUNA~ER S.,, "The Adsorption of Gases and Vapors," Princeton University Press, 1943, p. 3. a B R U N A ~S., R ,ibid.
solution^.^ The industrial use of adsorption followed rather quickly. In 1794 an English sugar refinery employed charcoal to remove color, one of the 6rst times that adsorption was applied as a chemical engineering unit operation. In 1812 bone char was introduced in the retining of cane sugar, and the bulk of our cane sugar is still refined by percolation through bone black. Today adsorption is used in many industrial processes. unfortunately in empirical ways in many cases. THEORETICAL CONSIDERATIONS
According to Brunauer,6 if a gas molecule remains on the sulfaceof a solid, there may occur a weakinteraction between solid and gas, like a condensation, or a strong interaction, similar to a chemical reaction. The former is called physical adsorption; the latter, chemical adsorption or chemisorption. Adsorption from solution may become even more complicated. Much of the study of adsorption has been devoted to equilibrium conditions. One of the most popular, although empirical, equations is the one attributed to Freundlich.Vt is x / m = ac1/*. Langmuir7 evolved an equation based on theoretical grounds that has been very valuable. Gibbs8 in 1874 proposed an isotherm based on thermodynamic reasoning, but it has been of little use because of the lack of knowledge about the - MCBAIN,I. W., "The Sorption of Gases and Vapors by Solids," George Routledge and Sons, Ltd., London, 1932, p. 1. = 0Ob.cit.. o.4. MCBAIN,J. w., op. it., P.5. LANDMUIR, I., I.Am. Chm. Soc., 38,2267 (1916). GIBES,J. W., "The Collected Works of J. Williard Gibbs," Longmans, Green and Co., New York, 1928, p. 232.
398
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surface tension a t an interface of a solid and a liquid. Other equations, both theoretical and empirical, have been proposed by Williams and Henry, Magnus, Polyani, McGavack, Patrick, and many other^.^ Equilibrium is always of importance to the chemical engineer because it is a limiting condition in the design of equipment. Isotherms, however, indicate only a relation between the adsorbate concentration in the adsorbent and the adsorbate concentration in the fluid phase a t one temperature. Temperature, in many cases, has a profound effect on the nature of sorption. In the instance of gases much greater adsorption occurs a t low temperatures. The values of both a and n in the Freundlich equation diminish continuously with rise of temperature, n tending toward the value of unity, so that the sorption is more nearly proportional to the pressure. In sorption from solutions, however, the effect of temperature is often different and may be in the opposite direction. Where the amount adsorbed is expressed as a function of the temperature with the pressure or concentration kept constant, an adsorption isobar is obtained; where the pressure or concentration is defined as a function of the temperature with the amount adsorbed kept constant, an adsorption isostere results. The rate of adsorption, particularly in the case of physical adsorption, has been shown to be extremely
rapid, so speedy in fact that it often cannot be measured. B r ~ n a u e rhas ' ~ pointed out that all adsorption processes are spontaneous and result in a decrease of the free energy of the system. I t bas likewise been demonstrated that a decrease in entropy accompanies the adsorption process. From these two findings the thermodynamic equation AH = AF
indicates that the heat of an adsorption process is exothermic, as has been proved experimentally. Therefore, in the use of an adsorbent on a large scale, the removal of heat may be a problem for the chemical engineer. CALCULATIONS
There are several practical ways in which an adsorbent may be utilized and for some of these procedures the results to be expected are easily calculated. The simplest application of adsorption is a process involving a single batch treatment. The equipment consists of a mixing tank and a filter. The solution and adsorbent, in powdered form, are mixed for a definite length of time and filtered. This process is wasteful of adsorbent, but is often adopted because of -
BRUNAU~R. S., oP.6% pp. 84-139
+ TAS
'0
Op, c i l . , p. 5.
the low cost and the simplicity of the equipment. A typical calculation of thigtype is given in the treatise of Walker, Lewis, McAdams, and Gilliland." More solution, however, may be purified to the same extent by the use of an adsorbent in several equal portions. This type of calculation, representing a more efficient use of the adsorbent, is presented in Perry's Handbook.'2 Where the adsorbent is of a granular nature, the gas or solution is usually percolated through a bed of the material. Such an operation involves an unsteady state condition which renders calculations, on a theoretical basis, almost impossible a t the present extent of our knowledge. Nevertheless a few generalizations may be made. The limit of the adsorption is the equilibrium condition. The capacity of any industrial system is dependent on the rate of exchange of adsorbate from one phase to the other. The equilibrium conditions and the rate of transfer are affected by a number of variables, such as the nature of the adsorbent and the adsorbate, temperature, pressure or concentration, viscosity, and time of contact. In 1916 W. K. Lewisla proposed the following equation for adsorption in a tower:
This equation bas proved too d i c u l t to combine with the experimentally determined equilibrium isotherms for integration. In recent years work on the subject has been carried on in the Chemical Engineering Department of the University of Pittsburgh under the guidance of Dr. James Coull. There EngelIs has studied the adsorption of chlorofom vapor from air, developing a probability approach to the problem. Both Friend1%and Tesi:' have investigated the adsorption of phenol from solution. Tesi has applied Furnas' analysis of heat transfer from a gas stream to a bed of broken solids. WestL8has done research on desorption of oxalic and acetic acids by water from adsorbent beds. The correlation of these doctoral dissertations by another investigator promises to lead to some explanation of the problem. More efficient use of an adsorbent can be obtained by using more than one column. By connecting several columns in series and rotating the solution inlet and outlet from column to column as the exhausted adsorbent is removed from the line and a fresh percolator inserted, an approach to continuous countercurrent operation can be made. The most efficient use of the adsorbent could be obtained by passage of the adsorbent countercurrent to the fluid, but it is a relatively difficult matter to move solids about, so that true countercurrent operation is not encountered. The type of calculation required for continuous countercurrent adsorption is also illustrated in the text by Walker, Lewis, McAdams, and Gilliland.lg MANUFACTURE OF ADSORBENTS
where c = oncentration of adsorbate in fluid z = concentration of adsorbate in solid a = constant in Freundlich isotherm n = constant in Freundlich isotherm p = fluid per unit amount of adsorbent t = time k = constant
In this equation Lewis limited himself to a Freundlich type of equilibrium. Hitchcock and Robinson14 derived an equation of continuity for this process, as follows:
where u
= fluid velocity y = concentration of adsorbate in fluid e = concentration of adsorbate in solid x = bed height
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a = volume of fluid per unit length of bed 6 = volume of solid per unit Length of bed Il " = +imp
WALKER, LEWIS,MCADAMS, AND GIL.L.~.AND, t principle^ of Chemical Eneneerinx." 3d Ed.. McGraw-Hill Book Co.. Inc., New York, 1937, p. 508 ff. la PERRY,J. H., editor, "Chemical Engineers' Handbook," 2nd Ed., McGraw-Hill Book Co., Inc., New York, 1941, p. 1245. L~wrs.W. K.. "The orinci~lesof countercurrentextraction." I n d . Eng.Chem.,8,827 (i916). H~rcncocx,F. L., AND S. M. ROBINSON, "Differential Equations in Applied Chemistry," John Wiley and Sons, Inc., New York, 1923, pp. 81f.
An industrial granular adsorbent, in order to be economically applicable in percolation, must be hard so that it can be handled readily without much loss. In addition, it should have good density and high adsorptive capacity, and be readily reactivated. A powdered adsorbent need only possess a good adsorptive capacity. Adsorbents have been made in a number of different ways. The most widely used and best known products are the carbonaceous adsorbents in which carbon is the active ingredient. Other industrial adsorbents include silica gel, activated alumina, fuller's earth, clays, and magnesia. With a few exceptions these noncarbonaceous adsorbents usually are manufactured by simple calcination of the raw material. Very fortunate indeed is the producer of carbonaceous adsorbents who can render his product active by simple heating in the ab-, sence of air. Bone char is about the only industrial carbonaceous adsorbent in wide use that is rendered -
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ENGEL, H. C., AND J. COULL,"Adsorption studies of vapors in carbon-packed towers." Trans. Am. Inst. Chem. Eng.,38, 917 (1942). l6 FRIEND. L. L., "The Flow Kinetics Involved in the Adsorption of Phenol by Activated Carbon," dactoral dissertation. University of Pittsburgh, 1942. Tesr, A. F., "Kinetic Studies of the Adsorption of Phenol by Activated Carbon in Packed Towers," doctoral dissertation, University of Pittsburgh, 1943. l8 WEST,J. R., "Desorption of Oxalic and Acetic Acids from Charcoal Beds," doctoral dissertation, University of Pittsburgh, 1944. W ' Op cit.. pp. 508ff.
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active during the calcination of the raw material. I t has been made in large vertical retorts through which the charge took 36 to 48 hours to pass. At least one manufacturer is now using continuous, horizontal, rotary retorts through which the charge travels in approximately an hour. Coconut shell charcoal, which is widely used for solvent recovery and for industrial and military gas masks, requires a more complicated method of production. The shells are first carbonized, the residue being inactive; but upon treating this residue with steam a t an elevated temperature the carbon becomes active as an adsorbent. Other methods have been followed successfully in the art. One involves a partial oxidation of the carbon with air instead of steam; another depends on the presence of a chemical, such as zinc chloride. INDUSTRIAL APPLICATIONS
Adsorbents are used technologically for purificational Crurirrr of R m r r Sugar Refinery FOR SUGAR PURIFIand recovery purposes and as catalysts. H a ~ s l e r ~ON ~ TOPOP THE BONECHARPERCOLATORS CATION lists a few of the applications of activated carbons. The petroleum industry has used hone black, silica gel, fulls's earth, and activated alumina as adsorbents. hut, properly understood and used, that it can be an Recent attempts to introduce activated alumina, pre- effectual and economical unit operation in many inpared by the calcination of selected bauxite, as a dustrial processes. granular adsorbent in sugar refining have not been sucACKNOWLEDGMENT cessful thus far owing, primarily, to inadequate hardness. The assistance of W. A. Hamor and E. P. Barrett of Adsorbents have been employed as catalysts in the production of sulfur from hydrogen sulfide, in the air Mellon Institute in the preparation of this paper is oxidation of nitrous and nitric oxides, and in the gratefully acknowledged. Thanks are given to the Attapulgus Clay Co. of Philadelphia, Pennsylvania, the chlorination of hydrocarbons. In closing this general article it is well to emphasize Carrier Corp. of Syracuse, New York, the Floridin Co. that adsorption is no cure-all for production problems, of Warren, Pennsylvania, and the Revere Sugar Re*%ASSLER. J. W., "Active Carbon, The Modern Purifier," finery of Charlestown, Massachusetts, for permission to use their respective photographs. Githens-Soh1Corp.. New York, 1941.
