AUGUST, 1935
INDUSTRIAL AND ENGIKEERlNG CHEMISTRY
941
energy ~ o u l dbe less for the breaking of three bonds than that required for four. However, a dissociation different from the monomolecular ruptures of reactions 1 and 2 is necessary, and it is assumed in the suggested mechanism of the writers that the bimolecular reaction takes place upon activation sufficient to produce the dissociations of these two monomolecular reactions. I n other words, some of the residues of the monomolecular dissociations exist long enough to react as 3. In any case, the reactions that represent the behavior of propane on pyrolysis may be written:
composition. This condition does not agree with any of the data published or with the present results, as shown by comparison of the results of experiments 7 and 8 with those of experiments 1 and 2 . On the other hand, the ethane content is found to be proportional to the partial pressure of propane. Calculations based on the free energy equations for the hydrocarbons involved which are given by Parks arid Huffman (9) show that at 900" K. (627" C.)] for instance, the bimolecular reaction, 2C3H.3 --f C3H6 C2Hs CHa, is not only thermodynamically possible but probable as well because a t this temperature AF, 627" C. = -11,830 calories.
-4s has already been indicated, i t was suggested by Frey and Smith (4) t h a t these same equations might account for the results they obtained for the pyrolysis of propane a t approximately atmospheric pressure as s h o r n in Figure 2 . The present results a t different temperatures and pressures confirm this suggestion and show t h a t these equations can be explained by the mechanism of a hypothesis which most workers in the field have overlooked. Frey and Smith likewise thought that i t was improbable for ethylene and hydrogen to react to form ethane a t the concentrations existing under the cracking conditions and, in support of their view, used the von Wartenberg equation (14) to estimate the equilibrium ratios of these gases. Unless the temperature and the partial pressures of the gases are known quite accurately, this equation cannot be applied with any degree of precision. The hydrogenation reaction is known to proceed much faster than the dehydrogenation reaction and the tendency, therefore, would be in this direction. However, at 10x1- percentages of decomposition with correspondingly low partial pressures of ethylene and hydrogen, there should be lese ethane formed than with greater degrees of de-
(1) Burk, R . E., J . Phys. Chem., 3 5 , 2 4 4 6 (1931). 23, 1033 (2) Ebrey, G . 0 , and Engelder, C. J., IND.ENG.CHEM~, (1931). (3) Frey, F. E., Ibid.. 26, 198 (1934). (4) Frey, F. E., and Smith, D. F., Ibid., 20, 948 (1928). (5) Kassel, L. S.,Chem. Reu., 10, 11 (1932). (6) Lang, J. W., IND.ENG.CHEY.,rinal. Ed., 7, 150 (1935). (7) Kef, J. U., J . A m . Chem. SOC.,26, 1549 (1904). ( 8 ) Ibid., 30, 645 (1908). (9) Parks, G . S.,and Huffman, H. M., "Free Energies of Some Organic Compounds, A. C. S. Monograph 60, New York, Chemical Catalog Co., 1932. (10) Paul, R E., and Marek, L. F., IKD.ENG.CHEM.,2 6 , 4 5 4 (1934). (11) Pease, R. N., J. Am. Chem. Soc., 50, 1779 (1928). (12) Rice, F. O.,Ibid., 53, 1959 (1931); 5 5 , 3 0 3 5 , 4 3 4 5 (1933). (13) Schneider, V., and Frolich, P. K., IND.ENQ.CHEM.,23, 1405 (1931). (14) Wartenberg, H. von, 2.physik Chem., 6 1 , 3 6 6 (1908). (15) Witham, TV. C., dissertation, Columbia University, 1934.
+
+
Literature Cited
RECEIVED February 12, 1935. Presented before the Division of Gas and Fuel Chemistry a t the 88th Meeting of the American Chemical Society, Cleveland, Ohio, September 10 t o 14, 1934. This paper is taken from the dissertation of Joseph W. Lang, submitted in partial fulfillment of t h e requirements for the degree of doctor of philosophy in the Faculty of Pure Science, Columbia University.
Removal of Mercaptans from Naphtha by Caustic The scrubbing of naphtha to remove mercaptans by means of caustic soda solutions finds wide application in the petroleum industry. A method of applying the available data on mercaptans to the design and testing of countercurrent scrubbing systems has been developed. It is believed that this method will also be of general interest as a practical application of the mathematical analyses recently published on this subject. J. HAPPEL AND D. W. ROBERTSON Socony-Vacuum Oil Company, Incorporated,
*
New York, N. Y.
HE presence of mercaptans in light petroleum distillates is undesirable because of their disagreeable odor and corrosive nature. Gasoline] solvents, and kerosene are acceptable m-hen w e e t according to the doctor test. I n order to meet this condition, the mercaptan sulfur content of a distillate must not exceed about 0.001 77-eight per cent of mercaptan wlfur (6). The use of caustic soda is an effective means of accomplishing reduction in the mercaptan content of naphthas. Because of their weakly acidic nature, mercaptans can be only partially removed by direct caustic washing, and such washing must usually be followed by a sweetening process to remove final traces. I t is the purpose of this paper to present data and methods of calculation in typical refinery applications.
QT
Chemistry of Caustic Scrubbing
.
The tn-0-phase system of naphtha and aqueous caustic solution usually will contain mercaptan in three forms: free (HSR) dissolved in the oil and in the caustic layer (NaSR), and (SR-) ions (mainly the latter).
942
INDUSTRIAL AND ENGINEERING CHEMISTRY
The equilibrium equation becomes
K =
A(sR-) A (HSR) A(oH-)
where ~ ( s R - ) , A(HsR),and A(oH-) represent the activities of mercaptide ion, mercaptan, and hydroxyl ion, respectively. Since the solution is very dilute with respect to mercaptan content, A(sR-) may be assumed to be proportional to the concentration of mercaptide dissolved in the caustic layer, and A(HSR,to the mercaptan dissolved in the oil layer. Therefore, for a given hydroxyl-ion concentration, the ratio of the ion concentrations of mercaptan in aqueous and oil layers (i. e., the distribution coefficient) should be a constant, as demonstrated by Meyer (7). hfeyer (7), Birch and Korris ( I ) , and Borgstrom and Reid (3) have studied the effects of chemical composition of mercaptan, strength of caustic, composition of naphtha, and temperature of extraction. The following generalizations can be drawn : (1) Ratio of distribution coefficients for two different mercaptans under identical Conditions is independent of those conditions. Roughly, a given normal mercaptan will have a distribution coefficient four times as great as the normal mercaptan containing one additional carbon atom per molecule up to amyl mercaptan, above which the ratio tends to become constant. (2) Secondary mercaptans are more difficult to remove than the corresponding normal mercaptans. (3) By assuming activity coefficients of 0.95, 0.70, and 0.50 for solutions containing 0.0834, 0.417, and 0.834 pounds per gallon of sodium hydroxide, respectively, a reasonably accurate correlation of coefficients obtained at different concentrations is obtained. With solutions more dilute than 0.0834 pound per gallon, an activity coefficient of 1.0 is used. Borgstrom (3) notes that the removal of mercaptans by a given amount of caustic increases with the concentration up to 2 molal, and then begins to drop off. (4) The effect of the character of the naphtha is slight. (5) Data indicate that a 20’ F. drop in temperature will increase the distribution coefficient by approximately 50 per cent, a factor hitherto inadequately considered.
VOL. 27, NO. 8
mercaptans in the propyl and butyl mercaptan fractions. The mercaptans containing one and two carbon atoms per molecule are probably methyl and ethyl compounds, respectively. Beyond this point, some unknown mixture of mercaptans \vi11 be obtained, increasing in complexity with the number of carbon atoms per molecule. It seems reasonable to suppose that the boiling range of mercaptans having a given number of carbon atoms will fall fairly close together, as in the case of the hydrocarbon isomers. Examination of boiling point data on known mercaptans ( 8 ) indicates that, if cuts are taken in a true boiling-point still, the boiling range of mercaptan molecules having a given number of carbon atoms will be practically the same as the boiling range of the corresponding hydrocarbon group having three additional carbon atoms. If a given distillate is split into a number of cuts, each cut may be analyzed for its mercaptan content conveniently and rapidly by the method of Bond ( 2 ) by titrating the unknown sample with a standard kerosene solution of copper soap and precipitating cuprous mercaptides; the end point is reached when a drop of the copper soap colors the solution blue.
Laboratory Work on Plant Cuts
Experimental work was done on light, stabilized, cracked naphtha from Luling crude, to design a caustic scrubbing system for this material. A given naphtha was split up into cuts in a true boiling-point still, using a freezing mixture to obtain reflux, before and after shaking a given volume of sample with half its volume of 0.417 pound per gallon of caustic solution in a separatory funnel, and the cuts were analyzed for mercaptan sulfur. Table I gives a summary of the data, the analyses being expressed as pounds of mercaptan sulfur per gallon. If concentrations are expressed per unit volume, the distribution coefficient is independent of the units. Since for the data reported in Table I the caustic had half the volume of the naphtha, the coefficient is twice the drop in naphtha concentration divided by the final naphtha concentration. A comparison of the values of mixed propyl, butyl, and Birch and Sorris found that the mercaptans present in the amyl mercaptans with values reported for pure mercaptans by overhead distillates from Persian crude contained only isoBirch and h’orris and by Meyer, as well as the present writers’ data, indicates that they are largely iso- compounds. ANALYSISAND DISTRIBUTION COEFFICIENTS FOR TABLEI. MERCAPTAN LIGHTCUT FROM LULINQCRUDE Cut No.
End Point F. Fractional Analysis of Original 65 Methyl 1 Ethyl 2 125 Propyl 3 180 Butyl 4 235 Amyl 5 280 Hexyl+ Residue . Mercaptan
..
Mercaptan Mercaptan Mercaptan DistriS in Each S per Gal. S as % of bution Total Off Cut Naphtha Total 9 Coe5cient yo by vol. Lb./gal. Lb. Naphtha (0.01301 Lb./Gal. Mercaptan Sulfur before Distn.) 0-23.0 0,02494 0.00574 46.2 ... 23.C-40.0 0,02027 0,00345 27.7 . . 40.0-55.0 0.01084 0,00155 12.5 .. . 55.0-75.5 0,00617 0.00126 10.2 ... 75.5-85.5 0,00284 0.00028 2.3 . .. 85.5-99.5 0.00092 0.00013 1.1 ...
.
-
-
0.01241
100.0
Fractional Analysis of Naphtha after Soda Wash (0.001167 Lb./Gal. Mercaptan Sulfur before Distn.) Methyl Ethyl Propyl Butyl Amyl
Hexyl
1 2 3 4
65 125 180 235
Residue
...
5
+
0-23.0 23.0-39.5 39.5-55.0
55.0-75,5
75.5-85.5 85.5-99.5
280
0,00000 0.00051 0.00158 0.00244 0.00187 0.00092
0.0 7.3 21.4 43.7 16.3 11.3
0.000000 0,000084
0.000245 0,000600
0.000187 0.000129
- 0.001145
Temperature of extraction, Initial *
2!:50%
O
F. 60 Gravity of naphtha, A. S. T. M . distillation, O F.:
83 94 106 121 140
60% 70%
%!2 Final
213a
80.0 10.7
3.0
1.0 0
100.0 O
A. P. I.
75.1
184 210
240
302 338 Recovery 93 % a B y adding methyl mercaptan in excess of that originally present, the same technic gave the coe5cient shown, 213. 160
Plant Operation For plant operation, countercurrent washing is desirable. The computation technic of Evans ( 6 ) ,analogous to that used in the design of distill a t i o n a n d absorption equipment (Lewis and Cope, 4 ) is a p p l i c a b l e . If a large excess of caustic were used so that the caustic concentration did not change, the distribution coefficient would be constant throughout the operation. Calling x the concentration of any given mercaptan in the caustic and y the corresponding concentration in the naphtha, the equilibrium relationship is a straight line through the origin. I n fact, however, caustic Concentration falls because of conversion to mercaptide. Assume a suitable percentage utilization-i. e., conversion of caustic in the operation as a whole. This fixes once for all the value of the distribution coefficient in the final stage of contact. If OA in Figure 1 represents the equilibrium of a given component for fresh caustic and OB the corresponding line for spent caustic, the actual equilibrium line for each s t a g e of c o n t a c t , o t h e r than the final one, will be r e p r e s e n t e d b y s o m e l i n e t h r o u g h
INDUSTRIAL AND ENGINEERING CHEMISTRY
AUGUST, 1935
the origin of slope intermediate between these two. The exact position of each line must be determined by trial and error. The operating line for any given component is fixed and is a straight line, the slope of which is equal to the ratio of the volume of caustic to the volume of naphtha passing through t h e system per unit time. To locate its position, a trial-anderror method is used. The terminal conditions are thus fixed and it is possible to proceed with a stepwise calculation of the number of countercurrent stages required to effect the desired caustic utilization. A suitable efficiency, equal to the percentage approach to equilibrium of the mercaptan removal for any given stage, may be used to advantage. The following plant test illustrates the applicability of the method. EXAMPLE 1. Twelve thousand gallons of naphtha were washed with 5300 gallons of caustic containing 0.03 pound per gallon of sodium hydroxide a t 40" F. in a two-stage countercurrent system. I n the first stage raw naphtha was bubbled through partially spent caustic; in the second stage the washed naphtha was intimately mixed in a centrifugal pump with fresh caustic and allowed to settle. It is desired to determine the efficiency of extraction of each of the stages. Naphtha analyses are as follows: Raw Naphtha
Mercaptan Sulfur Lb./gal. 0.0077
Methyl mercaptan Ethyl mercaptan Propyl mercaptans Butyl mercaptans .4myI mercaptans Hexyl mercaptans
0.0027
0,0014 0.0005 0.0002
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943
s /GAL. CAUSTIC - ~ X * A X I S J DIAGRAM FOR MERC.4PTAN REMOVAL
1-85, MERCAPTAN
FIGURE1.
EXTR.4CTION
per gallon, intersects the assumed operating line a t the y axis, the assumed operating line is correctly placed when using fresh caustic solution containing no mercaptans. A similar calculation is made for each of the other mercaptans present in the system. The mercaptan concentrations in pounds per gallon for all components leaving the first and >secondstages are as follows:
O.OO03 ~
0.012s
Total Total after 1st stage Total after 2nd stage
0.0073 0.0032
It is possible to compute the free caustic concentration a t t h e end of each stage. Then, using the generalizations previously outlined, the distribut)ion coefficients are as follo~vs: 2 n d stage 1st stage
Methyl 41.9 25.2
Ethyl 1.5 7 9.4
Propyl 2.1 1.3
Butyl 0 59 0 35
Amyl 0.20 0.12
A separate calculation for the scrubbing of each mercaptan is then carried out as illustrated in detail for propyl mercaptan. I n Figure 1 the equilibrium relationships for propyl mercaptan are plotted as two straight lines. The analyses of the feeds (using subscript p to designate propyl mercaptan) are yop = 0.0014 pound per gallon in the naphtha and zap = 0.0 pound per gallon in the caustic. The slope of the operating line is 5,300/12,000 = 0.44. T o locate it, a trial-and-error method is employed and a position is assumed. The intersection of the operating line and the horizontal line 2~ = yop gives the concentration of mercaptan in the caustic leaving the system. A vertical line dropped from this intersection to the equilibrium line for the first stage would give the mercaptan content of the naphtha leaving t h e first stage, ylp, if .equilibrium were attained in this extraction. An efficiency of 60 per cent is assumed, and consequently the value of yIp = 0.0013 will be given by a point 60 per cent of the distance between the operating and equilibrium lines. The intersection of the horizontal line y = ylP with the operating line gives the mercaptan content of the caustic leaving the second stage of naphtha scrubbing. The same procedure as above is followed in obtaining the mercaptan removal from the naphtha in the second stage, except that in this case an efficiency of 80 per cent is assumed. Since the horizontal line representing .the concentration of mercaptan sulfur, y..= = 0.0007 pound
Mercaptan Methyl Ethyl Propyl Butyl .4myl Hexvl 4-
1st stage
2nd stage
-
_-
0.0074
0.0030
Since the sums of the mercaptan concentrations are practically the same as the data obtained in the actual operation, the assumed efficiencies of 60 and 80 per cent are correct. The agreement of the plant test with laboratory data is reasonable, and it is to be expected that simple bubbling of naphtha through caustic will be less efficient than mixing in a centrifugal pump.
Acknowledgment The authors are indebted to W. K. Lewis who reviewed the manuscript and offered helpful suggestions regarding the presentation.
Literature Cited Birch and Norris, J . Chem. SOC., 127, 898 (1925). Bond, 1x1).ENQ.CHEM.,Anal. Ed., 5 , 257 (1933). Borgstrom, Ibid., 22, 249 (1930). C o p e and Lewis, Ibid., 24, 498 (1932). Evans, Ibid., 26, 860 (1934). Lachman, Ibid., 23, 354 (1931). Meyer, P., J . Inst. Petroleum Tech., 17, 621 (1931). Wiezevich, T u r n e r , and Frolich, IND. ENO.CHEM.,2 5 , 2 9 5 (1933). RECEIVEDMarch 26, 1936. Presented before the Division of Industrial and Engineering Chemistry a t the 89th Meeting of the American Chemical Society, New York, N. Y.,April 22 t o 2 6 , 1935.
