INDUSTRIAL A S D EXGINEERING CHEMIBTRY
492
3
have been detected. It is not known whether or not cyclobutane, if present, would have responded to the test. Summary The analysis in Table V summarizes the results; some of the determinations are only roughly quantitative.
Vol. 19, No. 4
Acknowledgment The writers wish to express their indebtedness to A. C. Fieldner, R. L. Brown, J. D. Davis, and J. B. Shohan for valuable suggestions, to L. C. Karrick for carrying out the carbonizations that furnished the gas, and to R. B. Cooper for assistance in collecting the sample.
Separation of Individual Saturated and Unsaturated Hydrocarbons in Coal Gas b y Fractional Distillation' By F. E. Frey and W. P. Yant PITTSBURGH ExPsnrmBNT STATION, U.
s. BUREAUOF MINBS,PITTSBURGH, PA.
~~~
RACTIONAL distillaA method is described for determining methane, in the analysis of the composite fractions, which contain several ethylene, ethane, propylene, propane, butylene, and tion has proved to be butane in gas by supplementing t h e Orsat analysis an essential tool in the ~ ~ f t ~ ~ ~ t ~ analysis of gaseous hydroby fractional distillation. T r o p s c h and Dittrich,* who The method can be applied t o 50 to 2000 cc. or more give reference to earlier methcarbon mixtures because the ods.8 For a simple gas, such as of gas, according to its content of these hydrocarbons. chemical similarity of homoa natural gas containing only A n analysis requires 5 to 6 hours. logs makes their separate dethe Daraffin hvdrocarbons. the t e r m i n a t i o n by - chemical met6od woufd be especially means unsatisfactory. Especially in the case of gaseous hydro- suitable, as no fraction would contain more than two constituents and the measurement of a single physical property, such as recarbons is fractional distillation useful, because these hydro- fractive index by the interferometer, density, or thermal concarbons are limited in number, and chemically similar con- ductivity, would serve t o determine them both. As a rule the stituents vary widely in boiling point. The boiling point of the familiar combustion method of gas analysis has been used, but i t gaseous hydrocarbons depends chiefly upon the number of car- requires more time and loses in precision for the higher fractions. 2-The sample is distilled quantitatively into simple fractions, bon atoms per molecule and is affected little by differences each of which contains only compounds of a single number of in structure and unsaturation. For this reason a mixture carbon atoms per molecule. This fractionation is more difficult can be separated into fractions containing molecules of the and time-consuming than the first method, b u t the fractions are same number of carbon atoms, which can then be analyzed more easily analyzed. A procedure of this type is described by Burrell and Robertson.' by chemical means. A precise separation can be obtained by means of a fracWhen the first method is applied to coal gas, which contains tionating column which utilizes vapor-liquid contact. The unsaturated hydrocarbons as well as paraffins, it is necessary unavoidable hold-up of material in the column makes it neces- to analyze fractions containing such a mixture as propylene, sary to have a sample sufficiently large to make an error propane, butylene, and butane. It is difficult to determine from that source negligible. For the same reason, the separa- a constituent present in small proportion in such a mixture. tion of a very small fraction in a large sample is unsatisfactory. Further, if stopcock grease is used in the apparatus it absorbs The fractionation of samples too small to distil through a the hydrocarbons above propane to such a degree as to change column may be accomplished by simple evaporation a t con- the composition of a composite fraction. This error is untrolled low temperatures and correspondingly low pressures, important in the analysis of gas fractions that contain a single taking advantage of the difference in vapor pressure, and number of carbon atoms per molecule. hence in rate of evaporation, to achieve separation. With a final vapor pressure of less than 0.5 mm. the total vapor Apparatus and Fractionation Procedure hold-up may be made very small. As a single distillation of The apparatus and procedure used for fractionating are this kind does not give a sharp separation, it must be repeated several times. This method is also useful for com- with slight modification those described by Shepherd and pleting the separation of small fractions obtained by the Porter.6 For the sake of brevity, in the following comments on the procedure a familiarity with the reference given is column distillation. assumed. The sample is led into the fractionation apparatus Fractional Distillation Applied to Paraffins and Olefins through a small absorption bulb containing 2 cc. of 1 per in Gas cent sulfuric acid (this may be omitted if ammonia is absent), For the analytical determination of the individual paraffins one containing 4 cc. of 15 per cent potassium hydroxide to reand olefins in small samples of gas, fractional distillation has move carbon dioxide and hydrogen sulfide, and a small drying tube containing magnesium perchlorate trihydrate to been applied in two ways as follows: remove water. Phosphorus pentoxide has the disadvantage 1-The mixture of hydrocarbons condensed from the sample of removing olefins when moisture has entered it. No PzOb at a low temperature is distilled into composite fractions contube is incorporated in the apparatus. taining both 2 and 3, both 3 and 4, etc., carbon atoms per mol-
F
ecule. This may be accomplished in one distillation for each fraction, as i t is only necessary t o avoid a separation so imperfect The burden of the procedure lies
as to allow ternary fractions.
1 Received January 15. 1927. Published with approval of the Dircctor. U. S. Bureau of Mines.
d",
Brennstof-Chem., 6, 169 (1925). Also see Shepherd and Porter, THISJOURNAL, 15, 1143 (1923), Bibliography; Wollers, Sfah2 Eisen, 42, 1455 (1922). 4 Chem. E n g . , 20, 223 (1914). 6 LOC. cil. 16, 1143 (1923). f
a
;
April, 1927
INDUSTRIAL AND ENGINEERING CHEMISTRY
The aluminum block has been found satisfactory for maintaining distilling temperatures higher than that of liquid air. A cylinder of aluminum 5 inches long and 2l/4 inches in diameter, in which are drilled holes to serve as well for the distilling tube, thermometer, and liquid air, rests in a cylindrical vacuum flask. After cooling to the desired temperature by introducing liquid air portionwise into its well the temperature of the block rises about 10" C. an hour. A little non-flammable cryostat liquid6 is placed in the thermometer and distilling tube wells to hasten the temperature equalization. Mixture No. 40, given in the paper cited,6 is used to cover the required temperature range. It has the following percentage composition: chloroform, 18.1; ethyl chloride, 8.0; ethyl bromide, 41.3; dichloroethylene, 12.7; and trichloroethylene, 19.9. The oxygen, nitrogen, hydrogen, carbon monoxide, and methane are first removed as described by Shepherd and Porter in a single fraction. For the higher hydrocarbons the procedure is somewhat different from that described by these writers. The ethane-ethylene is first distilled off from the residue from the methane separation at a beginning temperature of -1143" C. for 20 minutes, at the end of which time the temperature will have risen to about -139" C. Twice in the course of the distillation the distilling tube is cut off from the receiver, the temperature bath removed, and the hand applied to the bottom of the tube for a moment to agitate the distilling liquid. The distillate is then redistilled a t -145" C. until the vapor pressure, measured with the receiver cut off, falls to 0.5 mm., and the distillation continued up to 5 minutes longer. That distillate is redistilled a t -150" C. just until the vapor pressure drops to 025 mm. This distillate is the final fraction. The same procedure at temperatures of -114" to - l l O o , -115", and -120' C. is used for separating the propanepropylene fraction, and at -85" to -82", -87", and -92' C . for the butane-butylene. This procedure is suitable for samples in which the amount of each constituent above methane is from 10 to 60 cc. The temperature and duration ,of distilling operations, as well as their number, must be varied for gases differing much in composition from those analyzed in this work. When the fraction to be distilled is associated with much of the less volatile material, the first distillation is made a t a temperature somewhat higher than that given, but seldom more than 10" C. higher. When the volume of the fraction is much less than 10 cc., a temperature as much as 5" C . lower is used for the final distillation. Because of smaller boiling point differences, one more distillation is advisable for the butane-butylene fraction. The residue, which contains pentane, amylene, and higher hydrocarbons, is distilled into a small, calibrated, rat-tailshaped receiver attached to the tubing leading into the first ,distilling tube. The receiver is cut off from the train by a T-cock through which air is let into it, and the volume of liquid is measured after the temperature has risen. This receiver joins the train by a ground-glass joint to permit its removal for cleaning. For more accurate analysis when more gas is available a fractionating apparatus of the Leslie-Geniesse' type modified for low temperatures has been incorporated in the ShepherdPorter apparatus. The fractionating column is of Pyrex glass, 9 111111. in diameter and 50 cm. long, and surrounded b y a vacuum jacket. It is packed with 5-mm. brass rings. Controlled reflux is provided by withdrawing heat a t a controlled rate from the uppermost 10 cm. of the column by liquid air through an interspace 1 111111. thick filled with hydrogen. 'The rate of heat withdrawal is controlled by varying the de-
' Kanolt, Bur. Standards, Sci. Pager 6'20 (1926). 1
Leslie, "Motor Fuels." p. 555 (1923).
493
gree of vacuum in the interspace by a mercury leveling bulb. The distillate may be condensed and weighed, or it may be passed into a gasholder or into the Shepherd-Porter train. The column in this form has not been used to fractionate below -50°C. Analysis of the Fractions
The first fraction obtained, consisting of oxygen, nitrogen, hydrogen, carbon monoxide, and methane, is analyzed on the Orsat apparatus.8 Analysis of the other fractions is carried out on a simplified Orsat apparatus built into fractionation apparatus, which carries only the potassium hydroxide and slow-combustion pipets and a mercury-filled absorption vessel similar to the one used in the Bone-Wheeler gas analysis apparatuss for treating the gas with small volumes of absorbents. The olefin content of each fraction is determined by sulfuric acid absorption. Fuming sulfuric acid containing silver and nickel sulfates was used for the absorption of ethylene. A 0.6 per cent solution of silver sulfate in concentrated sulfuric acid (sp. gr. 1.84) saturated with nickel sulfate, recommended by Tropsch and Dittrich, was used for the absorption of propylene. The same reagent diluted with one-tenth its volume of water was used for butylene. The water prevents carbon formation but does not interfere with the absorption of the less reactive butylenes. One cubic centimeter of fresh reagent was used for each determination. Combustion analyses have been made on the fractions in many cases both before and after olefin removal, and the separation by distillation was shown to be quantitative within the limit of error of the combustion analysis. For this type of analysis, then, combustion data on hydrocarbons above methane may be eliminated; the determination of olefin by absorption suffices. This compensates for the large time-consumption of the fractionation. The method as described does not furnish a separate determination of any higher unsaturated hydrocarbons. The acetylenes and isomeric butylenes are determined separately if desired in the respective fractions in which they occur. Analysis of Synthetic Mixtures
Two synthetic mixtures were prepared from measured portions of pure hydrocarbons. The "liquid hydrocarbons" contained about 20 per cent of amylene; the remainder was mostly hexylene. The mixtures were analyzed by the method described, using three distillations to separate each fraction except the butane-butylene in mixture B, for which four were used. Mixture B is a somewhat more difficult one to analyze than mixture A . A complete analysis requires 5 or 6 hours. Analyses of Synthetic Mixtures CONSTITUENT
.
.
MIXTURE A Present
Found
P e r cent
Per cent
5.25 20.17 49.1 2.66 6.60 4.67 4.66 3.35 1.84 1.70
5.2 19.96 49.3 2.82 6.51 4.62 4.82 3.27 1.80 1.7
* Fieldner, Jones, and Holbrook, Bur. 9
Gnce and Payman, Fuel.
MIXTURE B Present
Found
Per cent Per cent 0.0 0.0 0.0 0 0 0.0 0.0 5.68 5.6 34.5 34.9 4.30 4.65 12.6 12.5 26.1 25.6 9.8 9.80 7.00 7.0
Mines, Tech. Paper 3'20 (1925).
S, 236 (1924).
Successful utilization of the airplane for the spraying of alfalfa fields suffering from the alfalfa weevil is reported to the Department of War from Utah, where an army plane was recently used to spray poison on three test fields.
494
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 19, No. 4
Kinetics of Catalyzed Gas Reactions in Flow Systems1’* By Arthur F. Benton UNIVERSITY OF V I R G I:IA, ~
UNIVERSITY,
VA.
OR purely scientific purposes the velocity of gas re- mediate compounds with the catalyst-processes which actions has been studied principally by “static” meth- are equivalent if the compound formation is purely superods, in which the reaction is allowed to proceed in a ficial. The quantity of gas adsorbed by a solid is a function closed system and the extent of conversion is determined of the nature of the materials, and of the temperature and at intervals by analysis or by the change in some property pressure. Although the actual amount taken up cannot of the mixture, such as the pressure. On the other hand, a t present be predicted theoretically, it is known that this technical gas reactions are usually carried out by flow meth- ordinarily decreases with rising temperature, and increases ods. Although the analysis and interpretation of results with increasing pressure, rapidly at first and then more obtained by the first method have been highly developed,a slowly, finally becoming nearly independent of pressure. the treatment of flow exOn the assumption-that the periments has received little adsorbed film does not exattention, particularly in the ceed one molecule in thickThis paper embodies an attempt to set u p equations ness, Langmuir5derived the case of reactions catalyzed expressing the yield i n catalyzed gas reactions carried by solids. following equation for the out by flow processes in terms of the quantity of catav a r i a t i o n of adsorption However, in a recent lyst, the rate of passage, the total pressure, and the with pressure: paper,4 in which were recomposition of the gas mixtures involved. ported the results of %ow Following a consideration of the mechanism of such s = s- afi measurements on the comreactions, based o n the adsorptions of the various 1 ap bination of hydrogen and gases,suitable approximations are introduced and where s is the area of the oxygen in contact with a equations are derived for a number of special cases. surface covered by gas at silver catalyst, a theory of These are shown t o represent satisfactorily the availthe pressure p , S is the total t h e m e c h a n i s m was deable data in the literature, which include the combinasurface, and a is a constant veloped, based on measuretion of hydrogen and oxygen with a silver catalyst, a t a given temperature. Alments of the adsorption by the synthesis of ammonia with molybdenum and t h o u g h t h i s equation is the catalyst of the gases inpromoted iron catalysts, and the contact sulfuric acid derived on the supposition volved, and an equation was reaction i n presence of platinum. that the adsorbing surface derived which was found to is uniform, a c o n d i t i o n represent the results with which is amarentlv not considerable accuracy. In the present communication it is proposed to treat the prob- fulfilled by typical active catalysts,B it may iiverthefess be lem from a more general standpoint and to apply the equa- considered as a satisfactory empirical approximation for the tion derived to such data as are available in the literature present purpose. The adsorption, when small, is thus proportional to the pressure, but when the surface becomes nearly for other reactions. covered, it approaches independence of the pressure. Notation Since reaction occurs only at the surface of the catalyst, The symbols to be employed most frequently are the fol- its rate will depend on the quantity of reactants adsorbed. If only one reactant is appreciably taken up, the reaction lowing: rate will be proportional to the surface area covered, and P = total pressure PI, pz. . . = partial pressures of the several gases 1, 2. . . . . . also proportional to the pressure of the second reactant, X I , XZ.. = mol. fractions = volume fractions of gases 1, 2. . ., since this strikes the surface a t a rate proportional to its pressure. In case the adsorptions of both reacting gases if these are regarded as perfect gases X = volume fraction of the reaction product desired are small, the rate should be proportional to the product V = rate of flow (liters per minute) referred to the of their partial pressures. If both gases are strongly adexit gases sorbed, the rate usually proves to be a more complex funcS = total surface area exposed by the catalyst tion of the pressures. Here the question of adsorption in Y = X V = yield of product desired (liters per minute) a = constant of adsorption equation mixtures of gases is involved, about which very little is k1, kz = velocity constants of forward and reverse reactions, known. respectively For the special case of two gases, only one of which is much adsorbed, we may write for the forward reaction rate Mechanism of Catalyzed Gas Reactions
F
+
As in homogeneous reactions, a theoretical equation for reaction rate must be based on a postulated reaction mechanism. In contact catalysis this may involve adsorption of the gases concerned or the formation of definite interReceived November 30, 1926. Contribution No. 35 from the Cobb Chemical Laboratory, University of Virginia. a For example, Hinshelwood. “The Kinetics of Chemical Change in Gaseous Systems.” Oxford University Press, 1926. 4 Benton and Elgin, J . A m . Chcm. Soc., 48, 3027 (1926). 1
2
For small adsorption the second term reduces to klaSplp2; for large adsorption it becomes klSp2. Many catalytic gas reactions of technical importance may be treated as examples of one or other of these special cases. The time t in this equation, when applied to flow measure‘ J . Am. Chrm. Soc., 40, 1370 (1918). Compare Taylor, Pros. Roy. SOC.(London), 108A, 105 (1925).
6
495
April, 1927
tiur of contact” (1 X volume) may be x (surface per unit V O ~ U U ~ C Jrhe , introduced, where t h e time of contact is the reciprocal of the space velocity ( V + volume).
version is very small, p l and p z are practically constant, independent of t. In this case integration shows that the partial pressure, p , of the product and, therefore, also its volume fraction, X, in the exit gases, is inversely proportional to the rate of passage, V , of the gas mixture. Hence the yield of product, X V , is independent of the rate of passage. This fact was clearly recognized by Unger7 in reporting on experiments performed in Haber’s laboratory on the synthesis of ammonia in which conversions of only a few per cent were obtained. Nevertheless, Unger’s data showed that the yields actually increased with increasing rate of passage, and he accounted for this on the assumption that the ammonia formed retards the reaction by becoming adsorbed on the catalyst. If the conditions are such that a reaction gives a large conversion, the yield will, of course, increase with increasing rate of flow even if the product is not adsorbed, but cases in which such adsorptions actually occur appear to be common, particularly in reactions involving synthesis. When the product is adsorbed, the reaction rate will evidently be proportional to the area of the surface which is free from this adsorption. This area is s-s=- S 1
+ up
In case of large adsorption, unity may be neglected in comparison with up, and the bare surface is inversely proportional to the pressure of the product. A Useful Approximation
From these considerations it would be possible to set up a general equation to represent all cases, but the result would be too cumbersome for practical purposes. In particular cases, however, simple equations may be obtained by introducing suitable approximations. The calculations are much simplified if the effective pressure of each gas is taken as the average of the pressure in the entering and exit gases, rather than the average obtained by integration. That this substitution is justifiable in treating the retarding effect of a reaction product may be shown as follows: Since the reaction rate is proportional to the area of surface free from the adsorbed product, we may write kS dp = dt 1 up
+
+
p l may be replaced by - c p ) , where p’l is the pressure of the reactant in the entering gases and c is a factor which represents the number of molecules of reactant which are used up in forming one molecule of product. Making this substitution and integrating, we obtain Expansion of the exponential term gives (ckt)* ( ~ k t ) ~ = 1 - ckt + - - ~f .. .. .. 2!
cp
