376
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE111. COMPARISON OF AVAILABLE VALUESOF THERMAL CONDUCTIVITY Liquid Year Observer Methylene chloride 1933 Kardos (6) Chloroform 1885 Weber ( 8 ) Carbon tetrachloride 1911 Goldschmidt (4) Tetrachloroethylene 1890 Stankevi6 (7) p-Trichloroethane (1,2-dichloroethsne) i s g o StankeviC (7) a Using present authors’ temperature coefficient.
K Z Oc. O Bates Temp., Hamarh, KT C. Kw’ c . ~ Palmer o.ooo3s o 0.00036 o.00038 0.00033 12 0.00032 0.00031 0.00026 0 0.00024 0.00038 0.00030 22 0.00030 0.00039 o ,00038 20 0.00038 0.00033
Vol. 33, No. 3
temperature differences, and were checked by values calculated from tangents to the respective temperature gradient curves. The physical properties and specificationsof the chlorinated hydrocarbons are given in Table 11. Table 111 presents a comparison of the authors’ with those Of Others in ture. Very few recent data are available in the literature on the thermal conductivity of CMOrinated hydrocarbons.
Acknowledgment Table I presents the experimentally determined values of the thermal conductivity and temperature coefficients of thermal conductivity of the twelve chlorinated hydrocarbons. Since these determinations were made on 1-inch layers as contrasted to previously reported determinations (1, 8, 3) on 2-inch layers, and because of the general characteristics of the chlorinated hydrocarbons (toxicity, action on neoprene, selective solvent action, etc.), the precision of these determinations is not so high as that of the determinations in previous papers. However, with the exception of methylene chloride and dichloroethylene “Di-48”, the precision of the present determinations of the thermal conductivity is 2 per cent. Since the thermal conductivities of methylene chloride and dichloroethylene “Di-48” were obtained by the application of conduction corrections already mentioned, it is felt that these values are good to about 3 per cent. The values marked are considered most reliable since they were calculated for the inch thickness and corresponding
The authors wish to express their appreciation to Laurens H. Seelye and Ward C. Priest of The St. Lawrence University for their cooperation in the general research program. Acknowledgment is also made to E. I. du Pont de Nemours & Company, Inc., for whom the research was performed, for their perniission to publish the data on the thermal conductivity of the chlorinated hydrocarbons.
Literature Cited (1) Bates, IND.ENG.CHEM.,25,431 (1933). (2) Ibid., 28, 494 (1936). (3) Bates, Hazzard, and Palmer, IND. ENG.CHEM., Anal. Ed., 10,314 (1938). (4) Goldschmidt, Phgsik. Z.,12, 417 (1911). (5) Kardos, A.,Forsch. Gebiete Ingenieurw., 5, 14 (1934). (6) R. and H. Chemicals Dept., du P o n t Co., “Chlorinated Hydrocarbons”, Wilmington, Del., 1935. (7) StankeviE, International Critical Tables, Vol. V, p. 228, New York, McGraw-Hill Book Co., 1929. (8) Weber, H. F., Ibid., Vol. V, p. 228.
Rate of Cracking of Paraffin Wax L. B. BRAGGI Massachusetts Institute of Technology, Cambridge, Mass. TREMENDOUS amount of work has been done in studying the cracking of petroleum, petroleum fractions, and some of the simpler pure hydrocarbons found in petroleum. Most of this work has been carried on primarily to determine the quantity and characteristics of the products resulting from the cracking process. The published results of these studies occasionally contain reference to the rate of reaction, generally as the number of degrees rise in temperature which is necessary to double the cracking rate. It is only during the last ten years, however, that published results of cracking studies have given values of the reactionrate constant, although the constant may be calculated from some of the data previously reported. When this study was commenced, nothing had been published on the value of the reaction-rate constant for the cracking of paraffin hydrocarbons higher than butane. Data on the cracking of such small molecules are of comparatively little value in connection with the prediction of the rate of decomposition in commercial cracking units. The purpose of this work was t o obtain data on the value of the reactionrate constant for molecules of a size comparable to those encountered in commercial cracking. It would have been desirable t o limit the investigation to pure compounds, but they were not obtainable in sufficient
A
1
Present address, Foster Wheeler Corporation, New York, N. Y.
quantity except a t prohibitive cost. Ordinary petroleum cuts, on the other hand, contain such a wide variety of compounds, with respect to both size and structure, that their use would have been unsatisfactory. As a compromise, a commercial wax of 122’ F. (50’ C.) melting point was used in the belief that this material was more nearly representative of a single family of hydrocarbons than any other as readily available. Another important reason for using a paraffi wax was the fact that its use permitted an accurate determination of the amount of undecomposed material. I n other studies of the cracking of hydrocarbons of relatively large molecular weight, distillation had been almost universally relied upon to separate decomposed and undecomposed matter. When utilizing such a method of separation, any high-boiling compounds formed would be reported as undecomposed matter. By using wax it was possible to separate and remove the lowmolecular-weight decomposed hydrocarbons by distillation and the high-molecular-weight hydrocarbons formed by decomposition and subsequent polymerization, by solvent extraction.
Apparatus and Procedure The wax was cracked under carefully controlled pressure and temperature conditions in a small flow type of apparatus (Figure 1). The apparatus was constructed so as to permit rapid heating
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1941
WATER-COOLED
311
COIL-
&VENTS
4 T
MANOMETER
-,
u GAS TANK
i WATER
S T E A M HEATERS
IN
WATER
OF APPARATUS FIGURE1. FLOWDIAGRAM
Commercial paraffin wax of 122' F. (50' C.) melting point was cracked in a small flow type of apparatus at temperatures of 788' to 900' F. (420' to 482' C.) and pressures of 490 to 1870 pounds per square inch gage. The reaction-rate constants were corrected to give the initial cracking rate and may be represented by the Arrhenius equation, In k
(-64,386)/RT
+ 16.713
Pressure does not seem to influence the rate , of reaction; therefore the reaction may be assumed to be unimolecular.
of the feed, during which essentially no cracking took place, followed by reaction at substantially constant temperature and pressure. Two reheating coils were used, both immersed in electrically heated gad baths. The first contained about 21 feet of No. 10 gage, a/S-inch outside diameter, Shelby steel tubing havin about '/,winch inside diameter. The second coil consisted o f about 7 feet of the same tubing. During most of the work the heaters in the second lead bath were used at their full ca acity, and control of the exit wax temperature was obtained Ey regulation of the heat input to the first lead bath. This method of heating minimized skin cracking durin the heatin period. After preheating, the feed entered t f e reaction chamber (Figures 2 and 3) which consisted of approximately 62 inches of ll/a-inch, double extra heavy, hydraulic pipe, A , with flanged connections B and C. The temperature of the wax enterin the reaction chamber was obtained by a thermocouple eened into a small hack saw cut in the Shelby tubing just aheaiof the inlet reaction chamber flange. The temperatures of the inlet and outlet flanges and the reaction chamber were obtained with thermocouples I J, and K . The tees, L, welded to the reaction chamber were fiiled with Pwdered Sil-0-Gel, M , to ensure a reading of the reaction chamer temperature not appreciably influenced by the temperature of lead bath E. The whole reaction chamber unit, including the flanges and release valve D,was covered with a 2-inch layer of asbestos insulation, N , and a 2-inch layer of magnesia pipe cover%teoieaction chamber was heated by electrical heaters # incased in cast-iron shoes F , and the flanges were heated by electrical heaters H .
In making a run the apparatus was first brought up to the desired temperature while gas oil was being umped through it. When the temperatures were nearly stabiEzed, the feed was changed t o wax. An amount of wax equal to about seven times the volume of the reaction chamber was then passed throu h the apparatus t o sweep out the gas oil. A t the same time tge rate of throughput of the wax was adjusted to that desired, and the pressure and the temperatures were stabilized at the desired points. The temperatures were maintained as nearly constant as ossible, and the pressure was re ulated by the release valve. %e throughput was determined f ~ y adding the weight of the bottoms, the overhead, and the gas.
Analysis of Products The bottoms contained cracked material that did not go into the overhead and the unreacted wax. As much as possible of this cracked material was removed by steam distillation of the bottoms, the distillation being continued until the still temperature reached 482" F. (250" C.). The remainder of the cracked material was separated from the wax by extraction with ethylene dichloride. The extracted material, the distillate, and the overhead were combined and distilled to yield a gasoline boiling between 90" and 400" F. (32" and 204" C.). Unknown amounts of wax were carried into the steam distillate and into the extract of the residue from the steam distillation. These amounts of wax were determined by a cloud-point crystallization method, t o be described. Knowing the quantities of unreacted wax present in the distillate and residue, the amounts of cracked material were obtained by difference, and were added to the overhead and gas t o give the total cracked material. The residue from the steam distillation was extracted three times with approximately equal volumes of ethylene dichloride to remove the remainder of the cracked material from the wax. The mixtures of wax and ethylene dichloride were warmed, if necessary, until there was only one phase present and then cooled and filtered a t progressively lower temperatures through a fine wire cloth in a suction filter until 50" F. (10"C.) was reached. It was necessary to cool and filter a t several progressively lower temperatures because the mixtures of wax and extract would otherwise become solid. The filtrates from the three extractions were combined, and the solvent was distilled off over boiling water; the final stages of the distillation were under a slight vacuum, with air bubbling through the liquid, in order to remove the last traces of solvent. The distillation was continued until no solvent could be detected by odor. The wax a t e r e d out was treated in the same manner to drive off the solvent.
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Vol. 33, No. 3
To determine the amount of wax in the steam distillate and in the extract, samples of the wax used for feed to the cracking apparatus were steam-distilled and extracted in the same manner as the bottoms. The solubilities of the wax distillates and wax extracts in ethylene dichloride were determined and are shown in Figure 4; the data of Poole and collaborators (IO) on the solubility of commercial wax (melting a t 50" C.) are also included. This curve could not be properly used in determining the wax content of the distillates and extracts because the processes of steam distillation and extraction both cause a refining of the wax by removing a portion of the lower boiling constituents. Hence it was necessary t o employ solubility curves of wax samples which had been subjected t o the same refining processes as the uncracked wax in the bottoms. By using sufficiently dilute solutions of distillate and extract in ethylene dichloride (1 part by weight with 10 to 40 parts of solvent), the effect of the oil present on the wax solubility mas negligible, as shown by Poole and collaborators (IO). The crystallization temperatures of dilute solutions of the distillates and extracts in ethylene dichloride were determined by slowly chilling the clear solutions until they became cloudy. Then by using the proper solubility curve, the amounts of wax present in the distillates and extracts were calculated.
Calculation of Reaction-Rate Constants The temperature and pressure data were both averaged on a time basis. The temperature data might have been more correctly averaged by calculating the temperature corresponding t o the average value of e-' I R T . But the average value of T obtained this way does not differ from that obtained as an arithmetical time average by more than 0.9" F. (0.5' C.) for even the worst case, run 1. The average temperatures a t the three points in the reaction chamber were averaged arithmetically t o determine the average reaction temperature, the maximum deviation of individual temperatures from the average being 14.4" F. (8" C.). The molecular weight of the gas was calculated from the gas density balance data and the barometer reading, with the assumption that the perfect gas laws held over the small range of pressures used. The weight of the gas collected was calculated from its volume, molecular weight, pressure, and temperature. The quantity of distillate from steam stripping
%I
FIGURE 3. CROSSSECTION OF REACTION CHAMBER
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1941
TABLEI. TESTDATAAND CALCULATED VALUESOR
RunNo. 1 2 3 4 5 6 7 8 9 10 11 12
Gas, Lb. 0.28 0.00 0.00 0.06 0.08 0.24 0.15 0.09 0.10 0.14 0.07 0.06
Gasoline, Lb. 1.09
...
0.18 0.21 0.56 0.99 0.83 0.53 0.55 0.90 0.64 0.64
Total
Cracked,
Through-
Lb.
%!
3.41 0.73 1.08 1.48 1.60 3.07 2.63 1.60 2.09 3.08 2.29 1.95
8.64 3.30 5.22 6.38 6.35 6.22 5.9s 5.77 6.00 6.04 5.85 8.13
5
Total Cracked 0.395 0.221 0.208 0.232 0.252 0.494 0.440 0.277 0.349 0.510 0.392 0.240
Time of Run, Min. 60.3 66.0 84.0 44.0 22.2 44.0 22.0 12.8 12.0 12.2 12.0 8.7
the bottoms was corrected for dissolved wax, using the solubility curves determined for the wax distillates (Figure 4). The amount of extract collected was corrected for dissolved wax in the same manner. The gasoline cut obtained was corrected for the samples removed to determine the amounts of oil in the distillate and extract. The melting point of the paraffin wax used was determined from a cooling curve to be 122" E'. (50" C.). According to the data of Buchler and Graves (3) on the melting point of petroleum waxes, the wax had a molecular weight of 338.8. The data of Hildebrand and Wachter (8) on normal paraffins indicate that the wax contained an average of 23.7 atoms of carbon per molecule and had a molecular weight of 333.8. The data of Garner, Van Bibber, and King (6) indicate that the wax contained an average of 24.0 carbon atoms per molecule and an average molecular weight of 338.4. From an average of these three values, the wax was taken to have an average molecular weight of 337 and an average of 23.9 carbon atoms per molecule. From the extended Cox chart of Calingaert and Davis (4), the hypothetical paraffin C ~ H 4 9 . 8would have a critical temperature of 529" C. (802" K.) and a critical pressure of 9120 mm. of mercury (12.0 atmospheres). Cope, Lewis, and Weber (6) pointed out that, for normal paraffins, RT,/P,V, is approximately constant a t 3.85 and also that the critical density remains essentially constant a t 0.234 gram per cc. Hence V , = (82.07 X 802 / 12.0 X 3.85) = 1425 cc. per gram mole or V , = 337/0.234 = 1440 cc. per gram mole. Taking the average of these two values, V , is 1432 cc. per gram mole. Values of reduced temperature, reduced pressure, and reduced volume for isopentane were calculated from the data of Young (11). These values are presented graphically in Figure 5 so plotted that the data may be extrapolated with a minimum of error. As indicated by Brown, Souders, and Smith (2) for the higher hydrocarbons, the value of V R is independent of the molecular weight when close to the critical temperature and a t pressures well above the critical. Hence, the data obtained by Young (11) for isopentane, as plotted on a reduced basis, may be used to obtain the value of V Rfor the paraffin wax with a probable error of not over about 10 per cent, which would be in the direction of too small a value of V B . Knowing V Band V,, the value of V as cc. per gram mole may be readily calculated as V = V , X VB. The volume of the reaction chamber and flanges, as calculated from its physical measurements and then corrected for expansion due to temperature, was 926 cc. The time of reaction was calculated from the volume of the reaction chamber and the volume of the wax passing through the chamber per unit of time under reaction chamber conditions of pressure and temperature. To use this method of calculation, the assumptions were made that there was only one phase present throughout the entire length of the chamber and that the small amounts of gaseous hydrocarbons formed would not
REACTION-RATE CONSTANT
THE
Av. Temp.
F. 858 788 790 817 836 864 867 862 856 900 873 874
O
379
c.
459 420 421 436 446 462 464 461 458 482 467 468
Av. Pressure Lb./Sq. Ih. Gage 1463 995 1034 1046 1020 1015 1004 490 987 990 1870 969
-
eaea.
kses.
473 1310 1060 451 229 449 233 132 127 128 139 68
0.00106 0.000190 0.000219 0.000585 0.00127
k/(l 5) 0,0018 0.00027 0.00024 0.0017 0.00076
0,00152 0.00249 0.00246 0.00337 0,00557 0.00357 0.00402
0.0030 0.0044 0.0034 0.0052 0.011 0.0059 0.0053
appreciably change its specific volume. Reaction-rate constants were then determined by means of the equation,
where e = time of reaction, sec.; i. e., length of time in the reaction chamber x = fraction of wax decomposed 15
l4 I3
12 I1
10
8
5
x2
E
9k 6 :
a
7 0 0
6 0 Y r
5 a x 4 s
3b YI
2 2 1 0
0 6
8 10 12 14 16 18 20 2 2 24 TEMPERATURE IN DEGREES CENTIGRADE
26
FIGURE4. SOLUBILITIES OF WAX DISTILLATES, WAX EXTRACTS,AND COMMERCIAL WAX I N ETHYLENE DICHLORIDE
A study of the data of various investigators has revealed the fact that cracking reaction-rate constants may apparently be corrected empirically for the reverse reaction and for the fact that in a mixture the larger molecules tend to crack first; this may be done with satisfactory accuracy by multiplying the calculated value of k by u/(u - z), where a is the amount of the material being studied in the feed and (a - z) is the amount remaining after pyrolysis. The accuracy of this method is particularly evident in the work of Geniesse and Reuter (7). These experimenters obtained data on the cracking of gas oil to widely varying extents a t a given temperature level for several different temperature levels. When the calculated values of the reaction rate constant are corrected as above, they apparently differ by no more than the experimental error. In addition to being consistent, this method of correcting has the added advantage over graphical extra-
380
Vol. 33, No. 3
INDUSTRIAL AND ENGINEERING CHEMISTRY
of 10 to 15 per cent. If corrected, such an error would improve the data slightly since these points, particularly that for run 10, a t the highest temperature lie below the best straight line through the data as plotted in Figure 6. As previously noted, there was essentially no cracking during the heating of the feed. The temperature of the lead in the first lead bath was always lower than that of the feed leaving the second lead bath. The temperature of the lead in the second lead bath exceeded the temperature of the heated feed by a maximum of 260" C., but the time of heating in this lead bath was always less than 2 per cent of the time of reaction, and the reaction temperature was reached only a t the end of the heating coil. It is obvious that there could have been no cracking during the heating period that was of importance in comparison to the cracking within the reaction chamber.
0.01
2 0.4
0.5
0.7
0.6
0.8
O.g
,
1.0
Y
v.
B
FIGURE5 , RZDUCEDPRESSURE, VOLUME, A X U TExPERATURE OF ISOPENTASE
I 0,001
polation of the data to zero per cent cracking that a single isolated bit of datum may be corrected. The method tends to overcorrect data slightly for pure hydrocarbons, which of course do not involve the initial cracking of the larger, more easily cracked molecules, and to undercorrect data for mixtures such as gas oil and wax. The calculated values of k were corrected in this manner by dividing by (1 - x), since a = 1for the wax; the values obtained are shown in Table I.
l 13.2
l
l 13.4
l
l 13.6
I I I I I I I 1 l l o , o o o , 13.8
14.0
-i
14.2
14.4
14.8
x 101 K
RATECOXSTANTS FOR CRACKING FIGURE 6 . REACTION OF PARAFFIX WAX
Discussion
The corrected values of the reaction-rate constant are plotted on a logarithmic scale against the reciprocal of the absolute temperature as shown in Figure 6. The best straight line through the data deviates from the data by no more than the experimental error. Hence the effect of temperature on the reaction rate may be expressed by the ilrrhenius equation (1 1
In IC
=
-E/RT
+C
where constants E and C = 64,386 and 16.713, respectively R = gas constant = 1.985 Pressure does not seem to influence the rate of cracking over the range of pressures used, from 490 to 1870 pounds per square inch gage. Thercfore the reaction may be assumed to be unimolecular. I n calculating the time of cracking, the assumption was made that only one phase was present. The validity of this assumption was investigated by the methods of Lewis and Luke (Q), using their data, some additional unpublished data kindly furnished by Luke, and data of Brown, Souders, and Smith (2). It was found that two phases were present in the material leaving the reaction chamber. The amount of the charge present in the gas phase varied from about 5 mole per cent in the lowest temperature runs t o 48 a t the highest temperature. Thus, i t is evident that the times of cracking were not so great and the rates of reaction were greater than those calculated. However, there was only one phase in the major portion of the reaction chamber; and except perhaps, for runs 6, 7 , and 10 which were a t the higher temperatures and had over 40 per cent cracking, the error is negligible. I n these three runs there may have been an error
It was further assumed that cracking ceased when the charge left the reaction chamber. A few simple calculations will readily prove that, even neglecting the cooling due to expansion on passage through the pressure release valve, the products were so rapidly cooled in the bare copper tubing that any cracking after leaving the reaction chamber was entirely negligible as compared to that within the chamber, ,4 cknowled g ment
The author wishes to express his appreciation t o H. C. Hottel, J. T. Ward, T. A. Mangelsdorf, L. F. Marek, J. W. Poole, H. G. Mangelsdorf, and B. De Lorenzo for their advice and assistance during the investigation. He is also indebted to the Tidewater Oil Company for the wax and gas oil used in the study, which they kindly furnished. Literature Cited (1) Arrhenius, Z . physik. Chem., 4, 226 (1889). (2) Brown, Souders, and Smith, IXD. ENG. CHEM.,24, 513--18 (1932). (3) Buchler and Graves, I b i d . , 19, 718-24 (1927). (4) Calingaert and Davis, I b i d . , 17,1287 (1925). (5) Cope, Lewis, and Weber, I b i d . , 23, 887-92 (1931) (6) Garner, Van Bibber, and King, J. Chern. Soc.. 1931, 1533-41. (7) Geniesse and Reuter, IND.ENG.CHEM.,24, 219-22 (1932). (8) Hildebrand and Wachter, J . Am. Chem. Soc., 51, 2487-8 (1929). (9) Lewis and Luke, IKD. ENG.CHEM., 25, 728-7 (1933). (10) Poole and collaborators, I b i d . , 23, 170 (1931). (11) Young, Proc. P h y s . SOC.(London), 13, 602-57 (1894-5).
TAKENfrom 8. thesis submitted by the author in partial fulSllment of t h e requirpments for the degree of doctor of science in chemical engineering f r o m the 1Iasuachujetts Institute of Technology.
