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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Summary General information of the conditions under which 18-8 stainless steels corrode aids in establishing conditions to minimize failure in this and similar steels. Detailed study was made of the effect of temperature, concentration, and pH of aerated sodium chloride solutions, noting the nature of corrosion and corrosion rate for 24-hour periods. Corrosion of 18-8 increases sharply with temperature, going through a maximum a t approximately 90" C. for 4 and 10 per cent sodium chloride and above 90" C. for l per cent solution. Corrosion is by pitting. At the boiling point corrosion decreases t3 nearly zero in 24-hour tests owing to lack of dissolved oxygen. Increased salt concentration a t 90" C. increases corrosion of 18-8 from zero in distilled water to a maximum a t 4 per cent sodium chloride. Identical tests with mild steel show significant corrosion in distilled water and maximum corrosion in 2 per cent sodium chloride. At higher concentrations corrosion is less, dropping off more rapidly for mild steel. In 25 per cent solution the corrosion rates of 18-8 by pitting and mild steel by uniform solution approach each other. At 90" C. in 4 per cent sodium chloride the logarithm of corrosion weight loss is a linear function of pH in the range pH 12 to 8. Corrosion falls off rapidly below pH 5, goes through a minimum a t 2.9 to 4.5, and increases sharply at 2.9. Pitting is observed above p1-I 2.8, and uniform solution accompanied by hydrogen evolution occurs in more acid solutions. The number of pits per square decimeter follow the minimum and
Vol. 33, No. 7
maximum of corrosion plotted with pH. Maximum pit penetration occurs a t pH 6 to 7. The effects of pH in the alkaline region are explained on the basis of increasing effective cathode areas surrounding the pits with decrease in pH. The corrosion minimum is accounted for by partial breakdown of passivity. The sharp upturn of corrosion a t pH 2.9 is due to corrosion by hydrogen evolution. The results indicate that a t 90" C. corrosion of 18-8 in aerated sodium chloride solutions is less if the solutions are made acid to pH 3 or 4. Least corrosion occurs in the alkaline region of pH 12.
Acknowledgment To members of the Corrosion Committee a t the Massachusetts Institute of Technology, the authors express their gratitude for continued advice and support during this investigation.
Literature Cited (1) Bengough, G., Chemistry h I n d u s t r y , 1933, 195, 228; Borgmann, C . , IXD.ENQ.CHEM.,29, 816 (1937); Friend, J., and Brown, J., J . Iron Steel Inst. (London), 83, 128 (1911) : Evans, U., and Hoar, T., Proc. R o y . SOC.(London), A137, 343 (1932); Heyn, E., and Bauer, O., M i t t . kgl. Materialprufungsamt, GTOSS Lichterfelde W e s t , 26, 1 (1908). (2) Frese, F., IND. ENG.CHEM.,30, 83 (1938). (3) Jordan, D., Trans. Faraday Soo., 34, 1305 (1938). (4) Uhlig, H. H., Trans. Am. I n s t . M i n i n g M e t . Engrs., 140,387 (1940) (5) I b i d . , 140, 411 (1940). 16) Whitman, W., Russell, R.. and Altieri. V., ISD. ENG.C H E M .16, , 665 (1924).
MIXED SOLVENT EXTRACTION A. V. BRANCKER, T. G. HUNTER, AND A. W. NASH
PHASE EQUILIBRIUM
The University of Birmingham, England
ITHIN the last few years considerable development has taken place in the refining of oils by the use of mixed solvents of two or more components. Of the many interesting solvent combinations possible, the simplest would appear to be the case where the two solvents are completely miscible and, in addition, one of the solvents is partially miscible and the other wholly miscible with the oil. An even simpler example of such a double solvent-extraction process would be the separation of a mixture of two completely miscible liquids, A and B, by treatment with a solvent mixture of two mutually miscible liquids, C and D, where liquid C, the principal solvent, is partially miscible with A and completely miscible with B, while liquid D, the auxiliary solvent, is wholly miscible with A , B, and C. Such a system would be a four-component system existing as two liquid phases a t the operating temperature and pressure. In the belief that a study of such a system would be helpful in the proper understanding of double solvent refining processes, the equilibrium in the four-component system acetone-acetic acid-chloroform-water was studied a t 25" C.; a detailed investigation of the system was described elsewhere (I). At this temperature chloroform and acetone, corresponding to A and B above, are completely miscible. Water, corresponding t o principal solvent C, is partially mis-
cible with chloroform and completely miscible with acetone, while acetic acid, corresponding t o auxiliary solvent D, is wholly miscible with chloroform, acetone, and water. The study of this system was followed by the investigation of equilibrium, a t the same temperature, in the system lubricating oil-acetic acid-chloroform, where the oil and the principal solvent acetic acid are partially miscible, the oil and the auxiliary solvent chloroform are wholly miscible, and the two solvents are entirely miscible. These systems were chosen, not for their industrial importance, but rather for convenience of analysis in terms of the individual components.
Chloroform-Acetone--4cetic Acid-Water System at 25" C. Equilibrium data in four-component systems may be r e p resented graphically in two ways, on the basis of a regular tetrahedron or a right equilateral triangular prism. The usual tetrahedron method of repreTETRAHEDRON. sentation is shown in Figure l a for this quaternary system. Curve E A H B G is the ternary equilibrium curve in the system acetone-chloroform-water, and curve ECWDG is the ternary equilibrium curve in the system chloroform-acetic acid-water. The frustum outlined by these two curves and the sloping surface HTV joining them represents equilibrium
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1941
in the four-component system. Any mixture whose composition can be represented by a point within or on the surface of the frustum exists as two liquid phases. Any mixture whose composition can be represented by a point outside the frustum exists as a single liquid phase. Equilibrium between coexisting liquid phases in the ternary system acetonechloroform-water is represented by the usual ternary tie lines; AB is such a tieline. Similarly, equilibrium in the ternary system acetic acida chloroform-water is represented by ternary tie lines such as CD. Equilibrium between coexisting quaternary liquid phases is represented by quaternary tie lines, such as X Y . Figure 1 shows tie line X Y lies on line KP which is the line of intersection of the two planes S R M and TFN. Plane S R M is a plane passing through tie line AB in the ternary system acetone-chloroform-water and through the apex M representing the remaining component-in this case, acetic acid. Plane TFN passes through tie line CD in the ternary system acetic acid-chloroform-water and also through the apex N representing the remaining component-in this instance, acetone. All quaternary tie lines in plane SRM will have their terminal points lying on curve AXYB, which is the curve of intersection of plane S R M with the frustum. Figure l b is an orthogonal projection of the tetrahedron onto the acetone-chloroform-acetic acid side as base, and i t contains the projections of two quaternary equilibrium isotherms, in each of which the ratio of acetic acid to water has a constant value. It shows that the profile HW of the frustum is a straight line, and that the frustum can be formed by joining the two ternary equilibrium curves by sloping straight lines which lie in a plane perpendicular to the base of the tetrahedron. If the position of any point within the tetrahedron is defhed by weight percentages w, x, y, z of the four components acetone, water, chloroform, and acetic acid, respectively, ACETONE
a
FIQURE 2
881
RNARY ISOTHERM
A%
ACETIC ACID
I9OTHERM
ACETONE
6
FIGURE 1 then the position of the projection of such a point-for example, onto the base triangle chloroform-water-acetic acid can be defined by the distances of the projected point from the three sides of the base triangle, which distances are weight percentages of water, chloroform, and acetic acid. If these distances are x', y', and zf, respectively, then the relation between w, 2, y, z and d,y', z' can, from geometrical considerations, be shown to be: 2'
=x
+ w/3;
y' = y
+ w/3;
2'
=z
+ w/3
These relations enable the required projection to be located. Equilibrium in this system may be summarized briefly as follows : The two ternary equilibrium curves acetonechloroform-water and acetic acid-chloroform-water define the heterogeneous region on two sides of the tetrahedron. The frustum outlined by these curves and by the sloping surface joining them defines the complete heterogeneous region. A plane passing through a tie line in one of the two-phase ternary systems and the opposite apex of the tetrahedron will intersect a second plane passing through a tie line of the other two-phase ternary system and its opposite apex to give a quaternary tie line, the two terminal points of which lie on the surface of the frustum. RIGHT EQUILATERAL TRIANGULAR PRISM.One of the two usual methods of usine a rieht equilateral triangular p&m ?or ScEToNE representing equilibrium data for four-component systems is shown in Figure 2. This method was employed by Defize (3) and by Buchel and Saal (2) for representing two-liquid-phase, oil-double solvent systems, and by Hunter, K k X I Nash and Ba Thi (6) for representing oil-wax-double solvent systems. I n the latter case, however, the systems investigated were all of the solid-liquid type. A In Figure 2a the two eauilibrium I ACETIC curves %WG and E'Hb' in the ACID ternary systems chloroform-acetic acid-water and chloroform-acetone-water define the heterogeneous region on the two bases of the prism. The frustum E'Gfb HWGE defines the complete heterogeneous region.
882
INDUSTRIAL AND ENGINEERING CHEMISTRY
Buchel and Saal (9)presented an example of right equilateral triangular prism where the quaternary tie lines appear to be situated on planes parallel to the base of the prism. Tie line X Y in plane I J K of Figure 2a is an example of a quaternary tie line so situated. Such a position for a quaternary tie line means that, in the case under considerat,ion, the ratio of acetone to acetic acid in the two coexisting phases X and Y is constant. I n the present system, however, this is not the case.
WATER
FIGURE 3
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of its orthogonal projection p on, for example, the lower base chloroform, water, acetic acid, can be defined by the distances of the projected point p from the three sides of the triangle; these distances are the weight percentages of water, chloroform, and acetic acid-say x’, y’, and z’, respectively The relations between w, 2, y, z and x’,y’, z’ are given by 2’ = 2; y‘ = y;
2’ =
w
+z
which enable the projection to be located. The effect of temperature on a ternary system is normally represented by using the lateral edges perpendicular to the bases of a right equilateral triangular prism as axis. SimiIarly the effect of acetone addition on the system acetic acid-chloroform-water can be represented by plotting the percentages of acetone along the lateral edges perpendicular to the triangular base of the prism as in Figure 3. The heterogeneous region is defined by the frustum EHWG. X Y is a quaternary tie line formed by the intersection of the triangular plane S R M , containing tie line AB in the ternary system acetone-chloroform-water, with the plane T F N O containing tie line CD in the ternary system acetic acidchloroform-water. These two planes intersect a t K P ; and the quaternary tie line lies on KP, its terminal points, X and Y , being given by the intersection of K P and AXYB, the curve of intersection of plane S R M and the frustum. If the position of any point within the prism is defined by weight percentages w, x,y, and z of acetone, water, chloroform, and acetic acid, respectively, then the position of its orthogonal projection on, for example, the base plane chloroform-water-acetic acid can be defined as before by the distances of the projected point from the three sides of the triangle, which are x’, y’, and x’, the weight percentages of water, chloroform, and acetic acid, respectively. The relation between 20, x,y, x and x’, y’, z’ enabling the required position of the projection t o be located will be:
I n Figure 2b X and Y represent the two quaternary phases jfound to be in equilibrium; X is situated in a different plane from Y since the ratio of acetone to acetic acid in these two phases is not the same. Line RS contains a tie line A B in the ternary system acetone-chloroform-water, and TF contains a tie line CD in the ternary system aoetic‘acid-chloroformx‘ = (A) 100 - w 100; y’ = 100; z’ = 100 water. Also RS, TF, AB,-and CD are the same as the lines similarly lettered in Figure la. Since X Y in Figure l a falls Equilibrium data in four-component systems represented on the line of intersection K P of planes S R M and TFN, so graphically on the basis of a regular tetrahedron have been XY in Figure 2b falls on the intersection KP of planes SRM found less confusing and, on the whole, more convenient to and TFN as shown. handle than similar data represented on a right equilateral Quaternary tie line X Y can lie in a plane, such as I J K in triangular prism; and this tetrahedron method of representaFigure 2a, which is parallel to the two base planes of the tion is recommended in preference to the prism method. prism only when certain conditions are satisfied. The required conditions are that the tie lines in the two ternary Oil-Acetic Acid-Chloroform System at 25’ C. systems represented on the two bases of the prism should be parallel. It can be proved geometricaIIy that if the two lines The double-solvent oil system investigated was composed S R and TF of Figure 2b, which lie on the two ternary tie of Anglo-Iranian second-cooled blue oil (viscosity-gravity lines AB and CD, are parallel, then the line of intersection constant 0.8553), acetic acid, and chloroform, in which acetic K P of the two planes SRM and TFN is parallel to the two acid was the principal solvent and chloroform the secondary base planes of the prism. That is, the quaternary tie line or auxiliary solvent. This system was studied in two secXY which forms part of this line of intersection K P will lie tions-oil-acetic acid a t 25’ C. and then oil-acetic acidin a plane, like I K J of Figure 2a, which is parallel to the two chloroform a t 25” C. base planes. OIL-SINGLESOLVENT.The oil-acetic acid data are represented by means of an equilateral triangle (4,6) where a It would appear from this that only in the very special physical property is used to characterize the oil. The physicase in which the component ( N in Figure 2b) of the treated cal property employed in this case was viscosity-gravity liquid which is completely miscible with both solvents and the auxiliary solvent (iM in Figure 2b) have identical solvent constant (V. C. G.). All tie line data were obtained by direct analysis of two phases in equilibrium; that is, the percentage properties (with respect to the other two components), will acetic acid in each phase was determined by actual analysis the quaternary tie lines lie in planes parallel to the base of the prism. (1) and the V. G. C. of each phase was measured after removal of solvent. The data obtained are given in Table I. One advantage obtained by employing this method of repOIL-DOUBLESOLVENT.The method of determining the resentation is the ease in constructing an orthogonal projection necessary data with two solvents is illustrated in Figure 4. on either of the triangular bases. If the position of any point This figure also shows the method of representation by a P (Figure 2a) within the prism is defined by the weight perregular tetrahedron. The physical property (V. G. C. in this centages w, 5,y, and z of the four components acetone, water, case) is represented by one edge of the tetrahedron, and the chloroform, and acetic acid, respectively, then the position
ew) (eW)
(
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1941
ACET\C ACID
a
FIGURE 4
DATAFOR OIL (V. G. C . 0.8553) TABLE I . EQUILIBRIUM ACETICACIDAT 25" C. -ComplexAcetic acid, wt. yo 29.68 45.83 58.89 63.36 78.50 90.01
Oil, wt. 7 0 70.32
54.17
41.11 36.64 21.50 9.99
-Upper Acetic acid, wt. 7% 11.65 10.67 10.07 9,62 9.99 9.85
Phase---Lower V. G. C. of Acetic solvent-free acid oil wt. .i, 0.8529 0.8486
0.8455 0.8418 0.8364 0.8299
89.32 90.61 92.00 93.03 95.98 98.00
AND
PhaseV. G. C. of solvent-free oil 0.9231 0.9365 0.9408 0.9532
0.9623 0.9687
FOR OIL-ACETICACID-CHLOROFORM AT TABLE 11. ISOTHERMS 25' C.
-Weight Oil (V. G. C.
Per Cent-
0.8553)
Acetic acid
90.0
10.0
44.2 22.7
21.6 30.5 49.4
75.5 58.4
9.5
13.5
57.5
Chloroform 0.0
11.0 25.3 27.9 33.0 20.0
--Weight Oil (V. G. C.
Per Cent-
0.8260)
Acetic acid
Chloroform
90.0 72.0 50.7
10.0 10.0 15.5 29.5
33.8 35.5
35.0 24.0
11.4
45.4 70.8
0.0
18.0
30.6 17.8
883.
mixtures lie within the heterogeneous region and form two liquid phases. Themixture represented by point N1, for ex-. ample, separates in-. to the two coexisting liquid phases X and Y ; these points,, when joined,.giv.e a, quaternary tie line. The particular acetic acid-oil mixture, N,. which was used had the composition 63.4 per cent by weight of ACID acetic acid and 36.6 per cent by weight of b stock oil of V. G. C. 0.8553. At 25" C. this mixture separated into the two coexistingliquid phases as given by the fourth tie line in Table I. To this mixture chloroform was added so that the weight percentages of chloroform in the five total mixtures actually used were 5.0, 9.7,15.0, 20.0, and 25.0 per cent. These mixtures a t 25" C. were all two-liquid-phase systems and were separated into two layers which were analyzed for acetic acid and chloroform content (1). The V. G. C. of the solvent-free oil present in each layer was also determined (Table 111). The representation of the results is shown in Figure 5a which is a schematic rather than a quantitative figure. The heterogeneous region of the oil-double solvent system is defined by the frustum; that of the oil-acetic acid system is defined by curve CABD. When oil of V. G. C., represented by point T,is mixed with acetic acid in the proportion of solvent to oil to give the mixture whose composition is represented by point N , then this mixture separates into the two liquid phases A and B . AB is a tie line in the oil-acetic acid system. The removal of acetic acid from the two liquids, whose compositions are represented by A and B, results in two solvent-free oils having V. G. C.'s of R1and El. When oil of V. G. C. represented by point T is treated with a mixture of acetic acid and chloroform of composition represented by point MI, a complex whose composition is represented by point N 1results. This complex lies within the heterogeneous region and therefore separates into two coexisting liquid phases of compositions X and Y . XY is a tie line in the double solvent system. To ascertain the V. G. C. of the oil in these two coexisting liquid phases, it is convenient to consider the removal of solvent in two stages. First, if all the chloroform is abstracted from the complexes represented by X and Y , there result the two chloroform-free complexeswhose compositions are represented by H and F in Figure 5b. Secondly, if all the acetic acid is removed from complexes H and F,two solvent-free oils having V. G. C.'s of Rz and EZare obtained. The double-solvent-system tie lines, such as X Y , lie
two opposite apexes M and C represent the two solvents, acetic acid and chloroform. By titrating oil S, of known V. G. C., with known mixtures of chloroform and acetic acid M , M1, and M 2 , in Figure 4a, the points a, ul, and & are oband Mz with oil S, tained. By titrating mixtures M, MI, points b, bl, and bz are obtained. M is pure acetic acid. I n this way the isothermal curve aalazbzblb is defined. Oils of various V. G. C., such as S1or 8 2 , prepared from stock oil S by batch extraction with acetic acid, were used to get similar isotherms in other planes, S I M C or SSMC, and hence determine the outline of the equilibrium frustum. The data obtained are shown in Table 11. To determine the position of the double-solvent tie lines the following procedure was adopted: The stock of V. G. C. S was treated with acetic acid until a mixture of known composition N (Figure 4b) was obTABLE111. EQUILIBRIUM DATAFOR v. G. c. 0.8553 OIL-ACETIC ACID-CHLOROFORM AT 25' c.. tained; this mixture was, in c ComplexU er PhaseLower Phase turn, treated with chloroform Acetio ChloroAcetic g l o r o - V. G. C. of Acetic ChloroV. G. C. ofin varying amounts to obtain acid, form, Oil, acid, form, solvent-free mid, form, solvent-freea series of double solvent mixwt. % wt. % wt. % wt. % wt. % oil wt. 7 0 wt. yo 011 60.82 5.00 34.18 11.6 10.22 0.8420 90.41 1.98 0.9393 tures, such as N1 and Nz, the 57.80 9.70 14.49 18.93 32.50 0,8424 87.50 3.62 0.9210 54.43 15.00 30.57 15.79 26.58 0.8431 81.00 7.75 0.8969 known compositions of which 51.22 20.00 28.78 18.22 32.43 0.8440 73.40 12.83 0.8828 were represented by points 48.00 25.00 27.00 20.00 36.03 0.8447 67.00 17.31 0.8742 situated on line N C . These
--
INDUSTRIAL AND ENGINEERING CHEMISTRY
884
G
LOROFORM
ACID
a
FIGURE 5
on plane S R M , which is the plane passing through a tie line, such as AB, in the single-solvent system and the apex of the tetrahedron opposite to it. That is, the rule with regard to tie lines in the double-solvent system appears t o be similar to that in the four-component system. Since tie lines in curve CFD of Figure 5a were not ascertained, no information is available to show that a plane drawn through apex G and a tie line in curve CFD would intersect plane XRM in a tie line. It is, however, highly probable that this is so. We showed previously (4, 6) that the equilibrium data obtained by batch extraction of a given oil stock with a single solvent when plotted on triangular coordinates can be used to forecast results from countercurrent and multiple extractions of the stock with the solvent. I n the case of the oildouble solvent system studied, i t has not yet been proved that the batch extraction data, when used to represent equilibrium relations as shown above, can be applied to multiple or countercurrent extraction computations, but it is to be expected that this will be possible. However, as far as the present work has gone, the oil-double solvent data represent only the effect obtained by adding either acetic acid, chloroform, or any mixture of acetic acid and chloroform to the given stock oil. The two prism methods of representation, as illustrated in Figures 2 and 3, can be used to represent equilibrium in an oil-double solvent system. Buchel and Saal (a) used the prism method of representation illustrated in Figure 2 to represent equilibrium in an oil-sulfur dioxide-benzene system. They considered the oil to consist of two components, aromatics and nonaromatics, and the double-solvent system was treated as a four-component system consisting of aromatics, nonaromatics, sulfur dioxide, and benzene. The diagrams, however, appear to be entirely qualitative and are employed in a patent specification to discuss certain advantages of sulfur dioxide-benzene mixtures compared with sulfur dioxide as a solvent. Defize (3) investigated quantitatively the equilibrium in the system oil-sulfur dioxide-benzene; like Buchel and Saal, he used the prism method of representation illustrated in Figure 2 and also the device of considering the oil as a twocomponent system of aromatics and nonaromatics. Defize found that the sulfur dioxide-benzene ratio in coexisting liquid phases was not constant. The tie line data of Defize, however, do not agree with the previously determined miscibility data; that is, instead of lying on the boundary outlining the heterogeneous region, the terminal points of the tie lines in
Vol. 33, No. 7
most cases lie well inside it. Defize explains this as due to difficulty in completely separating coexisting layers. This separation difficulty is undoubtedly a major problem, especially with dark or 0viscous oils. In addition, the accuracy of the data used for defining the heterogeneous region obtained by turbidity experiments is beACETIC lieved t o be affected ACID by the same cause. b It has been found in the work described here that such turbidity experiments, unless carried out carefully, are a fruitful source of error; and i t is felt that, where failure to secure agreement between tie line and turbidity data exists, the general reliability of the data is open to doubt. I n consequence, the results of Defize unfortunately add little of value to the knowledge of oil-double solvent systems of the type considered here. Thompson (7) investigated equilibrium in the system oilsulfur dioxide-benzene and finds, like Defize, that the sulfur dioxide-benzene ratio in coexisting liquid phases is different; therefore in this system, double-solvent tie lines would not lie in planes parallel to the prism base if the method of representation illustrated in Figure 2 was used. A similar effect was found for the system oil-furfural-benzene. It is now well established that phase equilibrium data for three-component and single solvent-oil systems can be used to forecast solvent extraction results. Since it has been shown here and in a previous publication ( I ) that phase equilibrium data for four-component and double solvent-oil systems can be readily handled, the forecasting of extraction results for such cases may be placed on an equally sound basis.
Literature Cited (1) Brancker, Hunter, and Nash, J . Phys. Chem., 44, 683 (1940). (2) Buchel and Saal, U. S. Patent 1,945,516 (1934). (3) Define, “On the Edeleanu Process for Selective Extraction of Mineral Oils”, pp. 142-93, Amsterdam, D. B. Centen, 1938. (4) Hunter and Nash, IND. ENC.CHEM.,27, 836 (1935). (5) Hunter and Nash, J . Inst. Petroleum Tech., 22, 49 (1936). (6) Hunter, Nash, and Ba Thi, Ibid., 24, 453 (1938). (7) Thompson, in “The Science of Petroleum”, Vol. 111, p. 1856, Oxford Univ. Press, 1938.
Purification of Glycerol by CrystallizationCorrection In this article by H. B. Hass and J. A . Patterson, which appeared in May, 1941 (page 615), the following reference should have been included: ‘LLossesin Distillation of Crude and Refined Glycerol. Removal of Arsenic from Glycerol and Its Purification by Crystallization”, A. C. Langmuir [IND.ENG. CHEM.,24, 378-84 (1932)]. Langmuir proved what had been earlier stated by Kraut-that glycerol of the highest purity could be obtained by crystallization of impure samples procured from soap lyes. We are sorry that Langmuir’s article was overlooked. H. B. HAW
