Equilibria in a Chemical System Hydrogen Sulfide-Propylene-Isopropyl
Mercap tan-n- Propyl h!t ercaptan
FRANK T. BARRAND D. B. KEYES, University of Illinois, Urbana. 111.
T
HE synthesis of mercapside of the reaction e q u a t i o n The equilibria in the reaction of hydrogen sultans from olefins and hyshould be present in greater than fide and propylene to f o r m isopropyl mercaptan drogen sulfide has e q u i l i b r i u m amounts at the and n-propyl mercaptan are determined at B O " , recently attracted some attenstart. However this method is 275", and 300" C. The standard free-energy tion (10, 15, 16). The reactions not e v e n t h e o r e t i c a l l y foolchanges and lhe heat changes accompanying ihe involved in these syntheses, howproof, as will be shown later, and ever, have been more frequently frequently involves n e e d l e s E react ions are: studied through the decomposie x p e r i m e n t a1 and technical tion of the mercaptans ( 2 1 , I!?, difficulties. Some m e t h o d o 1' H2S C ~ H E= (CH3)zCHSH (1) 17). Previous work has all been guiding the e x t r a p o l a t i o n to H2S C,H, = CH3CH2CH2SH (2) of a qualitative nature, and no infinite time of reaction would equilibrium figures have been TEMP.A F o (1) AH (1) AF' (2) AH (2) obviously be helpful. "C. Calories * cited; but i t is of interest that, C o n s i d e r t h e c o u r s e of a 300 1900 - 11,600 2600 - 14,600 although Sabatier and hfailhe solid-catalyzed gaseous reaction 273 1180 - 14,600 1x30 - 14,600 ( I S ) have reported considerable of the type: 250 460 14,600 1100 - 14,600 sulfide to be associated in mercaptan reactions, more recent inA + B = C (1) T h e standard free-energy chunges m a y be vestigations tend to show that representpd as follows: This is a bimolecular reaction of m e r c a p t a n s are considerably more stable thermodynamically the type of t h e e q u i l i b r i u m AF" = -14,600 f 28.80T (1) under consideration. T h e than the corresponding sulfides AF' = -24,600 30.00T (2) ( 1 1 ) . Trenner and Taylor (19) work of Benton (1, 2, 3 ) on the have postulated an intermediate m e c h a n i s m of gaseous reacT h e possibility of selectiae catalysis in isocompound of the type (RSH)2 tions a t solid catalytic surfaces in the decomposition of both shows t h a t r e a c t i o n s of this meric mercap f a n formation is suggested. A mercaptans and sulfides, but i t type, although actually heteromethod for extrapolating dynamic equilibria to is assumed to be quite unstable. g e n e o u s , m a y be assumed to in-finife time qf reaction, thereby determining the I n previous work i t has proceed a t rates governed by true equilibrium, is dezieloped. generally been inferred that isohomogeneous reaction velocity mercantans are much more stable considerations mi t h o u t introthan normal mercaptans. Birch and Korris (4) have found ducing any serious error Csing this basic assumution. . , an isomercaptans preqent in large quantities in Persian crude oil equation representing the course of opposing simultaneous with no evidence of any normal isomers. Duffey (9) has as- reactions toward equilibrium may be developed. If a and I ) sumed that with some catalysts, a t least, isopropyl mercap- represent the original amounts of compounds A and B, and tan is synthesized from hydrogen sulfide and propylene with x is the amount of each converted to C in time t, one may the formation of practically none of the normal isomer. This write, as for homogeneous gas reactions, investigation was partially undertaken to determine more accurately the comparative thermodynamic stabilities of normal and isopropyl mercaptans.
+ +
-
+
I
THEORETICAL CONSIDERATIOKS The equilibrium of a gaseous reaction may be determined by allowing the component materials to react, either with or without a catalyst, until the equilibrium concentrations of all the compounds involved are attained. This method frequently takes too long for actual laboratory work and involves some ambiguity if a catalyst is present, so that a dynamic rather than a static method is usually used. The dynamic method involves the measuring of the progress of the reaction a t various times of reaotion and extrapolating this data to infinite time. The method of varying the time of reaction is usually the changing of the rate of progress of the reactants through a given amount of catalyst. When this is done, any equilibrium shifting or product-absorbing effect the catalyst may have is automatically eliminated, provided a state of dynamic equilibrium is reached before each measurement is taken. It has been assumed that for most conclusive results the equilibrium should be approached from both sides; that is, the substances on the one side and then those on the other
if the starting materials are pure A and B and no C, where k , is the reaction velocity constant when the equation is read from left to right and k2 from right t o left. Solve this differential equation and make use of two sets of known limiting conditions: (1) When t = 0, x = 0; and ( 2 ) since a t equilibrium the rate of the reaction to the right must equal that tc. the left, when t = a ,d x / d t = 0, and x = xs, where x s represents the equilibrium conversion to compound C. Then, 1-- XXe In ___ ab
kit = ab - x e 2
To simplify matters, consider n = 6, a situation which can easily be arranged experimentally. Then,
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The equation now applies to a given amount of starting material and to be more general should be applicable to percentage data. Therefore, putting z = x / a , and z. = x d a , so that z becomes the fractional conversion, and substituting a single constant, K , for the product, akl, and the factor for the change to common logarithms, gives : (3)
T o apply this equation, any one point of the plot of t against z, and an approximation of zs only are necessary. It would be possible to write an equation which would define the curve
Vol. 26, No. 10
Czt = log 2%ze - z
The difference between Equations 3A and 5A is simply the term zz.*, which is very small compared with z a t low conversions. It appears therefore that, for the purpose of assisting in the extrapolation to equilibrium, the unimolecular equation is accurate for both types of reaction. The fact that both equations give so nearly the same results indicates that all reactions will approach equilibrium in the same general fashion, and the influence of mechanism of reaction on the rate is minimized in the extrapolation.
EXPERIMENTAL PROCEDURE APPARATUS.The determination of the hydrogen sulfidepropylene equilibria was carried out by means of a dynamic method:
-------A-
JOO'C. M
, ' C
The apparatus consisted of gas holders for the pro ylene and hydrogen sulfide, flowmeters for the measurement o r the dried gases, a catalyst chamber, and the sampling system. The catalyst chamber, having a volume of about 200 cc., was constructed so that the furnace kept the catalyst at an even temperature throughout, and those small variations in temperature which did occur were not concentrated in any one position. The regulator held the catalyst temperature constant to within *lo C. The sampling system consisted of a fritted glass bubbler for absorption of the gases in solvent naphtha and a spiral condenser in which the gases were liquefied by means of carbon dioxide snow in acetone.
"
0
0
YIELDSOF MERCAPTANS AT FIGURE 1. TOTAL 300 275 AND 250 C . O,
O,
from two points not including the one a t equilibrium, but it would involve so much more arithmetic manipulation that the trial-and-error method for determining zeseems preferable. T o use the equation, a probable value of Z. is substituted, together with one point experimentally determined, and K is calculated. The curve drawn from this equation is compared with the experimentally determined points. When the closest fit is obtained, Ze, the equilibrium concentration, is found. Any units of t may be used, the units being absorbed in K. Thus, when the reaction is caused by gases being conducted through a catalyst, one may write:
CATALYSTAND MATERIALS. The catalyst, which was kindly supplied by J. C. Elgin of Princeton University, was prepared essentially by a method detailed in patents of the I. G. Farbenindustrie A,-G. (IQ), involving precipitation from a nickel nitrate solution by means of sodium carbonate solution. Kieselguhr was used as the support material, however, instead of silica gel as directed by the patents.
t = l/v
where v
=
velocity of gas
When a volume change takes place in the reaction, this equality does not hold exactly, but for conversions under 30 per cent the discrepancy is very small and well within the limits of experimental error. The curve representing the approach to equilibrium is very much the same for both unimolecular and bimolecular reactions. Using the same sort of reasoning for a unimolecular reaction of the type, A = C
(4)
gives the equation, (5)
where the letters have analogous significance. That this equation varies little from the other for small values of z. becomes apparent when Equations 3 and 5 are rewritten:
0'
I
2.0
I
I
I
4.0 60 40 Gas kk&, cc. perm/&
/QO
FIGURE2. TOTAL YIELDSOF MERCAPTANS AT 225" AND 200" c.
The material was then filtered from solution, partially dried, and pressed into tablets. These tablets were introduced into the catalyst chamber and were activated merely by passing a mixture of propylepe and hydrogen sulfide through the chamber a t a temperature of 300' to 350' C. The catalyst was then undoubtedly a form of nickel sulfide. The hydrogen sulfide was taken from a tank of the commercial product after 25 to 30 per cent of the original gas in the tank had been removed. The hydrogen sulfide thus obtained was about 99.5 per cent pure. The propylene was of the commercial grade and was 98 per cent unsaturated. The amount of ethylene present in the unsaturated portion was not great enough to modify the results seriously.
October, 1934
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EXPERIMENTAL AND ANALYTICAL PROCEDURE. In making mercaptan sulfur given above are of the same order as this runs the furnace was allowed t o remain at the desired tem era- difference, thus strengthening the postulation of the secondary ture for at least 12 hours and the mixture and velocity o f the hydrogen sulfide and propylene were kept constant until dynamic reaction. Figure 3 shows a corresponding plot of the ratio of equilibrium had been reached before any sample was taken. isopropyl to n-propyl mercaptan in the equilibrium mixture. Then the product was absorbed in solvent naphtha by means The extrapolations are carried out in a manner similar to that of the fritted glass bubbler, flowmeter readings being taken meanwhile t o determine the constancy and amount of the flow for total mercaptan conversion, since the relation between the of each gas. When a large enough sample had been collected, two mercaptans can be expressed as an equilibrium of the type it was analyzed for mercaptan by titration with silver nitrate A = B. The extrapolation of the data a t 300" C. was carsolution, according to the method of Borgstrom and Reid (5), ried out in such a fashion as to resemble the curves a t 250" the hydrogen sulfide being taken out by means of acid cadmium and 275", with allowance made for increased rate of reaction. chloride solution (23). The percentage total conversion was calculated from the amount of gas put in and the amount of Since the true equilibria a t all three temperatures are so mercaptan obtained. KO evidence of reactions involving forma- nearly the same, a ratio of 65 t o 35 of isopropyl mercaptan to tion of any compounds condensing at room temperature was n-propyl mercaptan was used a t all temperatures in the found, indicating that there were practically no unsuspected calculations. side reactions. The fraction of each mercaptan present was determined by making the run as above, but condensing the product gases in TABLEI. TOTAL YIELDS OF MERCAPTANS carbon dioxide snow-acetone mixture, and fractionating the TOTAL TOTAL yield in a microdistilling column (8). Av. VBLOCITY,MERCAPTAN Av. VELOCITYMERCAPTAN TEMP. HIS, CgHs CONVERSIONTEMP. HzS, CsHe CONVERSION The determination of the nonmercaptan sulfur in the product was made by the A. S. T. M . lamp method (7), after the hydroC. Cc./min. % C. Cc./min. 70 20.7 1.90 13.9 200 1.75 275 gen sulfide had been removed by cadmium chloride and the 21.6 2.00 13.1 2.10 mercaptans by silver nitrate. 15.2 12.5 3.70 4.25 IDENTIFICATION OF MERCAPTANS. The identification of the two isomeric propyl mercaptans was accomplished by the preparation of the derivatives with 2,4-dinitrochlorobeneene, as described by Bost (6). The samples separated in the microfractionating column were used for this purpose. The derivative from the isopropyl mercaptan fraction melted after one recrystallization a t 93" to 93.5" C.; Bost gives 94.5" C. That from the n-propyl fraction melted a t 80" to 81" C. with no recrystallization; the literature gives 81" C. as the true melting point. It was noted that the derivative of the n-propyl mercaptan precipitated much more readily than that of the iso- compound. Bost also indicates that the derivatives of the normal mercaptans in general are prepared more readily than those of the iso- series. RESULTS. The total conversions of hydrogen sulfide and propylene to mercaptans a t various temperatures and rates of flow are given in Table I, and the ratios of iso- to n-propyl mercaptan in the mixtures are given in Table 11. The determination of the nonmercaptan sulfur was very inaccurate, but the method showed a conversion to organic sulfur compounds other than mercaptans of about 4.6per cent a t 275" C. and an average velocity of 2.0 cc. of each gas per minute, and of 3.2 per cent a t 300" C. and 1.95 cc. per minute. These conversions are calculated on the assumption that the secondary reaction is of the type,
2RSH = H,S
+ RnS
which will remove one-half of the sulfur from organic environment. Owing to the small amounts of the product of this secondary reaction, no identification could be made. The reaction above, therefore, is purely hypothetical. The data on the total yield of mercaptans are shown graphically in Figures 1 and 2 . I n Figure 1 both the actual yields and the equilibrium yields, estimated by the process of extrapolation, are plotted against the rate of flow of the input gases. Figure 1 gives the curves for the data a t 300°, 275O, and 250" C., for which the extrapolation was done, as shown, with some precision. Figure 2 shows the graphs of the data a t 225" and 200" C., a t which temperatures the rates of conversion were comparatively so low that the method of extrapolation could not be applied accurately; the true equilibrium conversions a t these temperatures, therefore, are only estimates. From these graphs the figures for total conversion to mercaptan are taken. The effect of the secondary reaction is apparent in the difference between the curves for actual conversion and those for extrapolation. The figures for non-
225
250
8.15 10.0
11.0 10.0
2.00 4.15 5.90 9.60
20.7 16.7 9.9 8.4
3.05 5.00 7.05 9.75
20.8 26.2 26.0 21.7
TABLE11. TEMP.
c.
250
300
5.80 7.95 9.95
16.6 14.6 13.9
1.70 1.95 3.80 7.95 8.00 9.90 10.0
5.2 8.3 7.4 9.7 11.7 10.2 10.9
FRACTIONATION ANALYSISOF MERCAPTANS Av. VELOCITY, H&, C3Hs Cc./min. 6.55 10.05
ISOPROPYL MERCAPTAN
n-PRoPYL
MERCAPTAN
%
%
68 75
32 25
275
6.10 9.30
68 72
32 28
300
6.10 9.80
57" 66
43Q 34
The sample for fractionation here wa8 contaminated with water, which had escaped removal b y the drying tubes, in a distinctly separate phase' the fraction of n-propyl mercaptan was probably considerably less than thd figure given.
These results are a t some variance with the work of Duffey (9) who, using a catalyst of nickel sulfide on pumice, obtained
results which gave a much smaller equilibrium concentration of total mercaptan, with no separation of the two being made. Runs were therefore made using Duffey's catalyst; the yield of mercaptan was not great enough to make any fractionation analysis possible, but the attempted preparation of the 2,4-dinitrophenyl derivative gave a minute amount of a substance which, without any recrystallization or purification, melted a t 90" C. At no time was any compound isolated which melted near 81" C., the melting point of the normal mercaptan derivative. Since the latter was found to be the more easily prepared of the two, the derivative actually prepared appears to be that of the isopropyl mercaptan (melting point, 94" C.) and the yield seems to consist for the most part of the isopropyl compound. The conversions obtained with Duffey's catalyst. calculated on the basis of a yield of a single mercaptan, therefore, indicated conversion such as to give an equilibrium constant of 0.144 a t 300" C.; Duffey gives 0.16, approaching the equilibrium from both sides.
DISCUSSIOK OF RESULTS The results give the folloxTing conversion for the equilibria in the system hydrogen sulfide-propylene-isopropyl mercaptan-n-propyl mercaptan:
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TDMP.
TOTAL MERCAPTAN
CONVERSION
c.
I~OPROPYL n-PRoPYL MERCAPTAN MERCAPTAN
%
300 275 250 225" 200a
.
12 19 29 40 50
%
%
7.8 12.3 18.8 26.0 32.5
4.2 6.7 10.2 14.0 17.5
Vol. 26, No. 10
out previously, is applicable to all cases of the determination of equilibria by dynamic methods whether the equilibrium is approached from both sides or not, is that the catalyst used in his measurements catalyzed the secondary yield-reducing reaction to a proportionately greater extent that the catalyst
Figures a t this temperature are estimated values.
These conversions are calculated on the basis of pure materials, since the small amounts of impurities in the hydrogen sulfide and propylene were well within experimental error. The figures above lead to the following values of the equilibrium constant, K , and of AF" for the reactions:
+ +
HzS C3H6 = (CH3)zCHSH HzS CsHs = C H I C H ~ C H ~ S H (CH3)zCHSH = CHaCHZCH9SH TEMPERATURE K (1) 'C. 300 275 250 225a 200" a
O K .
573 548 523 498 473
0.189 0.339 0.639 1.155 1.950
!Fo (1) Calories 1900 1180 460
-140 -630
K (2) 0.102 0.184 0.347 0.621 1.050
& F a (2) Calories 2600 1850 1100 470 -50
CF" (3) Calories 700 670 640 610 580
Figures a t this temperature are estimated values
The variations of the standard free-energy changes with temperature are shown graphically in Figure 4. The value of the heat of reaction, AH, may be obtained from the figures above by plotting log K against 1/T (temperature in " K.). Over the range of temperature for which accurate determinations of equilibrium conversion were made, 250" to 300" C., the heat changes for ali three of the reactions above are found to be constant. For reactions 1 and 2, AH = -14,600 calories, and for reaction 3, AH = 0.
used in this work. This would decrease the fraction of mercaptain in the equilibrium mixture, even when approached from the mercaptan side. However, i t has been pointed out that the yield of isopropyl mercaptan from the kieselguhr catalyst a t 300" C. is nearly the same as the total yield from the pumice-base catalyst. Furthermore, results indicate that the effect of the secondary reaction is not as consistent as the primary, mercaptan-producing reaction. Since the results with the pumice-base catalyst are closely reproducible, it would seem that the differing conversions brought about by the two catalysts can be explained somewhat better by assuming a selective catalytic effect for the nickel sulfide deposited on pumice
Gar Worm,cc.per m i m OF ISOPROPYL TO FIGURE 3. RATIOS MERCAPTAN
LITERATURE CITED
n-PRoPYL
The free energy and heat changes for the above reactions may be expressed as equations: AF" AH AF" AH
= -14,600
= -14,600 = -14,600 = -14,600
+ 28.80T + 30.00T
FIGURE4. STANDARD FREE ENERGIESOF FORMATION OF MERCAPT4NS FROM HYDROGEN SULFIDE AND PROPYLEXE
(1)
(2)
These equations are developed from the data in the range 250" to 300" C. and are probably accurate over a somewhat larger range. The accuracy of the results compares favorably with that of other thermodynamic data and is somewhat higher than is usual for determinations of free-energy change. Although the possibility of unsuspected error is present, the work was carried out with a certain precision and can be expected to be correct to within *50 calories in the standard free-energy values. The conversions obtained are much greater than those of Duffey (9) a t corresponding temperatures; this is troublesome in view of the fact that he arrived at the equilibrium from both sides. A possible explanation, which, as pointed
(1) (2) (3) (4) (5) (6)
(7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)
Benton, IND.ENG.CHEM.,1 9 , 4 9 4 (1927). Benton, J . Am. Chem. SOC.,53, 2984 (1931). Benton and Elgin, Ibid., 48, 3027 (1926). Birch and Norris, J . Chem. SOC.,127, 898 (1925). Borgstrom and Reid, IND.ENQ.CHEW,Anal. Ed., 1 , 186 (1929). Bost, Turner, and Norton, J . Am. Chem. Soc., 54, 1985 (1932). Bur. Mines, Tech. Paper 323A, p. 81 (March 18, 1924). Cooper and Fasce, IND. ENQ.CHEM.,20, 420 (1928). Duffey, unpublished Ph.D. thesis, Univ. Ill., 1933. Duffey, Snow, and Keyes, IND. ENG.CHEM.,26,91(1934). Elgin, Ibid., 22, 1290 (1930). Faragher, Morrell, and Comay, Ibid., 20, 527 (1928). Faragher, Morrell, and Monroe, Ibid., 19, 1281 (1927). I. G . Farbenindustrie A.-G., British Patent 265,884 (July 25, 1925). Johansen, U. S. Patents 1,836,170, 1,836,171 (Dec. 15, 1931); Nisson and Mandelbaum, Ibid., 1,836,183 (Dee. 15, 1931). Mailhe and Renaudie; Compt. rend., 195, 391 (1932). Malisoff and M a r k , IND.ENG.CHEM.,23, 1114 (1931). Sabatier and Mailhe, Compt. rend., 150, 1217, 1569 (1910). Trenner and Taylor, J.Chem. Phys., 1, 77 (1933).
RBCEIVED July 16, 1934. Presented before the Division of Industrial and Engineering Chemistry a t the 88th hleeting of the American Chemical Society, Cleveland, Ohio, September 10 t o 14, 1934. This paper is a contribution from the Chemical Engineering Division of the Chemistry Department, University of Illinois.
