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Relation to Extraction Operations From these phase relations it is possible t o predict various factors involved in extraction processes. In the transfer of the solute from the diluent t o the solvent phase, the desire is to have a minimum amount of diluent dissolved in the solvent phase and of solvent dissolved in the diluent phase. The approach to this condition is indicated by the tendency of the branches of the mutual solubility curve to rise with increase of solute concentration closely to the 0 per cent solvent and the 0 per cent diluent sides of the diagram. The plait point indicates the limiting concentration of solute in diluent that can be successfully handled in an extraction system; in fact, it is impracticable to work near the plait point. The minimum amount of solvent to give complete extraction of solute from diluent can be determined from the distribution curve. Thus, for a perfect unit of infinite number of extraction stages, handling a solution of solute in diluent of strength M per cent which, from the distribution curve, is in equilibrium with a solution of solute in solvent of strength N per cent, the minimum solvent-diluent ratio is N / M . This assumes that the amounts of solvent and of diluent in the opposite phases may be neglected. The effect of the extraction, whether it he to reduce the concentration of solute in the diluent to a minimum or to concentrate the solute in the solvent to a maximum, can be determined from these equilibrium data. Usually both aspects are important. An example is the extraction of acetic acid from water. The effect desired is first to strip, as completely
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as possible, acid from water (which is later wasted) ; secondly, to increase the ratio of acid to water by transferring the acid, containing as small an amount of water as possible, to the solvent phase. The acid concentration in each of the two phases is given by the distribution curve and the compositions by the solubility diagram. The relation of the ratio of acid/(acid water) in the solvent phase to that in the mater phase shows the concentrating effectof the extraction. These plots for several systems are shown in Figures 11, 12, and 13. The use of these data will be shown in subsequent papers; and their application in industrial systems has been covered in many patents. Data are also being determined for various other systems.
+
Literature Cited Bachman, I., IKD. ENG.CHEW,ANAL.ED.,12, 38 (1940). Bancroft, A., P h y s . Rev., 3, 31 (1895-6). ISD. ENG. Branckner, A. V., Hunter, T . B.. and Nash, A. W., CHEX.,ANAL.ED., 12, 35 (1940). Evans, T. W., IND. ENG.CHEM.,28, 860-4 (1936). International Critical Tables, Vol. 111, p. 405, Table 6 (1928). Lloyd, B. A, Thompson, S. O., and Ferguson, J. B., Can. J . Research, 15B, 98-102 (1938). Othmer, M., Chem. 62 M e t . Eng., 43, 326 (1936). Rosiu, J., “Reagent Chemicala and Standards”, New York, D. Van Nostrand Co., 1937. Seidell, A . , “Solubility of Inorganic and Organic Compounds”, New York, D. Van Nostrand Co., 1928. Taylor, H. S., Treatise on Physical Chemistry, 2nd ed., Vol. 11, New York, D. Van Nostrand Co., 1931. PR~SENTED before the Division of Industrial and Engineering Chemistry a t the 101s Meeting of the American Chemical Society, St. Louis, Mo.
Concentrated Nitric Acid by Fractionation of the Mixture Nitric Acid-Water-Sulfuric Acid J
W, E. SILLICK Eastman Kodak Company, Rochester, N. Y.
T
HE three-component system water-nitric acid-sulfuric acid has been of great importance t o the chemical industry for many years. It is not only the basis for most nitration processes, but it is also the system commonly employed in the preparation of concentrated nitric acid from dilute acids. I n this latter role the system was first employed in connection with the recovery of spent nitrating acids and then extended to the problem of concentrating the dilute tower acids obtained in the old sodium nitrate-sulfuric acid processes. In more recent years the system has had wide application in the concentration of the dilute nitric acid produced by the various synthetic processes-in particular, the ammonia oxidation process. A review of the literature covering the use of the system water-nitric acid-sulfuric acid in the preparation of concentrated nitric acid reveals a great number of patents describing equipment and process modifications for applying the prin-
Algebraic and graphical methods are presented for analyzing the operation of a fractionating column when applied to the preparation of concentrated nitric acid from the three-component system nitric acid-water-sulfuric acid.
ciples of distillation and evaporation. As to information of a more basic theoretical nature, the works of Berl and Samtleben (1) and of Carpenter and Babor (2) are outstanding. These works, conducted a t about the same time, were concerned primarily with the determination of boiling points and vapor compositions corresponding to the various possible combinations of the three components water, nitric acid, and sulfuric acid. As a result, dependable data are available covering these properties. The literature is lacking,
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I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
however, in any description of a method for applying these ~~. data to the prediction of operation in a fractionating column. The author presents such methods as he has found to be useful in solving these problems.
Symbols
~
Algebraic Analysis
All quantities are measured in weight and weight percentages, though mole units may be employed if desired: 0 = weight of liquid overflow = weight of feed P = weight of product V = weight of vapor B = weight of bottom effluent ' R = weight of reflux N = per cent " 0 8 S = per cent HzS04 W = per cent H20 n = plate identification numbered from the top down
F
In the following discussion, consideration is given only to a column operating with all three Subscripts are used for complete identification; thus components N V , + 1 refers to the percentage of nitric acid in the vapor present on every 1. rising from plate n plate. OtherOne symbol denotes a derived quantity: wise the system would reduce to L, = 0, - B e i t h e r of t h e simple two-comIn Figure 1 we represent a fractionating column where the ponent systems feed is introduced on the top plate. As described above, water-nitric acid we may arbitrarily choose F, P , and R, and by means of a heat or water-sulfuric and material balance determine the composition on plate acid. It is as1; or we may choose F , P , and the plate composition and, sumed that the by means of a heat and material balance, determine R . Such FIGURE 1. DIAGRAMMATIC FLOWOF conditions are represented in Figure 2 by points F , P , and I. MATERIALS IN FRACTIONATING COLUMN system nitric acid-sulI represents the liquid composition on the top plate. furic acid rarely To determine the conditions on the second plate, IT, we exists, This discussion also does not consider the presence set up the following material balance equation: of oxides of nitrogen which in practice are always present to some extent in the mixtures of low water content. A further limitation may be made by assuming that the vapor pressure of the sulfuric acid is negligible. This assumption holds This equation is based on the assumption that the sulfuric fairly well for temperatures up t o 200" C. a t atmospheric acid is always in the liquid state and therefore flows a t a conpressure. stant known rate from plate to plate. Thus we have a maMaking these limitations and then applying the phase rule, terial balance equation in nitric acid in which the only unwe find that in order to define the system it is necessary to known is V,, + 1. By applying this equation and calculating state three of the variables. The pressure may be assumed V , + 1, we fix one of the two variables necessary to define constant, which leaves two variables to be determined. These may be (a) the percentage of two of the components in the liquid state, ( b ) the percentage of one of the components in the liquid state and the percentage of one of the components in the vapor state, (c) the percentage of one of the components in the liquid state and the temperature, ( d ) the percentage of one of the components in the vapor state and the temperature. This may be represented graphically if we consider a triangular three-component chart on which have been interposed isotemperature lines representing boiling points and isocomposition lines representing the percentage of the components in the vapor ( I ) . Thus, the intersection of any two of these lines represents a point and defines a system. We can arbitrarily fix certain independent variables of column operation which will define or fix all other conditions in the column. For example, we can fix the rate and composition of the feed, the yield and composition of the product, and either the liquid composition on the top plate or the rate of reflux. I n practice it simplifies calculations to fix the top plate composition at some point which will be in equilibrium with a vapor of the product composition. Inspection of Figure 1 shows that fixing these particular variables makes it possible to employ material balance equations to interrelate all auantities exceDt the reflux ratio. This ;due must be -determined by means of a FIGURE 2. ILLUSTRATION FOR ALGEBRAICANALYSISSHOWING ISOVAPORheat balance. COMPOSITION CURVEIS
+
c
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concentration instead of increasing it. When we approach the condition where S v , + 1 = Nv,, we approach a condition requiring an infinite number of plates in order to accomplish a concentration of nitric acid. Such a condition is represented in Figure 3, since X v n + I = 95 per cent nitric acid and Nv, (in equilibrium with 0,) = 95 per cent nitric acid. If we assume that B in Figure 3 is the desired bottom composition for a fractionating process, we can draw lines B , V , + 1 for h-vn A 1 = 95, 80, and 60 per cent nitric acid, etc., and we find that where these lines intersect their respective isovapor-composition curves, as a t 0, in Figure 3, we have the condition Nvn + = Y v ~ .Thus, if we draw a line through these points, we have a curve iepresenting the locus of the points where A'v, + = N v , for the particular value of B. This line is represented by curve I1 in Figure 3. It is apparent that, if any plate composition approaches this line, the number of plates required t o obtain the desired separation approaches infinity. This line therefore serves YO Has04 as a qualitativeguide to the approximate plate FIQURB 3. ILLUSTRATION FOR GRcPHIC 4L ANALYSIS SHOWINQ ISOV LPORcompositions R hich must be maintained in COMPOSITION CURVES the column in order for the column to perform its task efficiently. Heat balances have shown that these plate compositions must lie to the left of this limiting curve (Figure 3 ) . If we extend the liquid composition on the second plate. The second this reasoning to various values of B, allowing B to vary variable, composition, is fixed by trial and error in such a = 0, we get the family of curves shown along the line "03 way as to provide a heat balance. In this manner we dein Figure 3. Curve 111, for B = 68 per cent sulfuric acid, is termine the composition of the liquid on the second plate, 11, discontinuous; that is, it does not pass through point B. in Figure 2. By repeating this process in a stepwise manner, This indicates that me cannot by simple fractionation conit is possible to trace the system from the top plate to the tinuously separate 95 per cent nitric acid from a mixture of bottom. From such calculations we can judge the feasibility sulfuric acid-water-nitric acid and remove 68 per cent sulof the process. furic free of nitric acid a t the bottom of the column. In I n making calculations for a heat balance, it is necessary other words, 70 per cent is approximately the lowest strength in some cases to consider the sensible heat of the liquids insulfuric acid from which 95 per cent nitric acid can be stripped volved and the heat of separation, because for some mixtures completely by simple fractionation. If we extend this reasonthese quantities are of appreciable magnitude. Useful data ing, we arrive a t curve IV (Figure 3 ) , representing the minion specific heats of mixtures are available in the International mum possible concentration of nitric acid in the bottom efCritical Tables. Data on the heat of dilution of mixed acids fluent from a fractionating column operated to produce 95 have been prepared by Rhodes and Nelson (3'). per cent nitric acid vapor a t the top plate. By applying the above graphical methods, it is possible Graphical Method to examine a great number of combinations of feed and prodWith all of the variables which must be arbitrarily fixed, uct compositions as they might be applied to a fractionating it is evident that the use of the above algebraic method would column. From such a study it is possible to reduce greatly be slow and laborious if one desired to investigate any conthe amount of algebraic calculation necessary to solve a siderable range of operating conditions. A simple graphical specific problem. method has been found useful for preliminary examinations. Referring to Figure 1, we may write the equation, On=B
+ Vn+
Range o f Application
1
If we represent the composition of these quantities by points on a triangular three-component chart (Figure 3 ) , we find that V , + must be along the line H2S04 = 0 per cent; and with 100 per cent recovery of nitric acid a t the top of the still, = 0 per cent. Also, 0, must lie on the straight line between the points Vm + 1 and B , and be distanced between them inversely proportional to the amounts of the two components V , + and B. Having thus located these three points with relation to one another, we can read values directly for N v , + 1 and Nv,. In order for the fractionating column t o accomplish the desired result, it is necessary that N v , + 1 < Nv,. It also follows that, when Nvn + 1 > Nv,, the column is reducing the nitric acid
B must lie along the line HNOa
These methods of analysis have been found helpful in determining the usefulness of packed columns applied to fractionating mixtures of sulfuric acid-nitric acid-water as distinguished from packed columns used in the capacity of stripping with steam. They have also been helpful in establishing a theoretical basis on which to plan engineering tests.
Literature Cited (1) Berl, E., and Samtleben, O., Z.angew. Chem., 35, 206-11 (1922). (2) Carpenter, C. D., and Babor, J. A., Trans. Am. Inst. Chem. Engrs., 16, 111-47 (1924). (3) Rhodes, F. H., and Nelson, C . C . , IND.Exa. CHEM., 30, 648-50 (1938).
