92
INDUSTRIAL AND ENGINEERING CHEMISTRY
so
100
--*----, --\
PER CENT DIMER IN
v)
u)
0 U
I-
e
0 W
'5 0
\,
THE LlOUlD PRODUCT+
a
w W 4
,
W
-
w
3 20
40
-
0 1 ' 1000
I
1050
I
I
I100 I150 TEMPERATURE,*F.
Run No. Feed type Temp., F. Temp., C. Contact time, sec. Partial pressure of hydrocarbon, mm. Conversion to gas, wt. % Av. mol. wt. of gas Dimer in condensed hydrocarbon, wt. % Cd hydrocarbons in gas, wt..% C4Hs in C4 cut, wt. %
a
40
Vol. 39, No. 1
DIMER
GAS
I
a
'
TABLE11. EFFECTOF RECYCLEON DEPOLYMERIZATION OF
AVERAGE MOL.WT.
OF
I
I
l2W
Figure 7. Effect of Temperature on Depolymerization of Dimeric Butadiene Contact time, 0.70 second; dimer partial pressure, 110 mm.; diluent, steam
c-33 c-34 c-35 Dimer (b.p., Condensed Condensed 129-131' C.) hydrocarbons hydrocarbon from C-33 from C-34 1078 1076 1078 582 581 682 ' 0.48 0.48 0.49 109 19.8 52.6
110 17.3 60.6
113 16.0 48.7
95
90
68
95 99
93 98
89
...
ture should be as close to the initial depolymerization (decomposition) temperatures as possible and still provide for a practical conversion per pass. I n practice this makes necessary the employment of temperatures somewhat higher than the initial decomposition temperature. LITERATURE CITED
gradient in the reaction tube was quite sensitive to changes in feed rate. The partial pressure of the hydrocarbon gases varies a$ the cracking takes place. The partial pressures r$ported are the arithmetic averages of the partial pressures a t the entrance and at the exit end of the reaction tube. I n general, low temperatures and relatively small conversions produced butadiene in the highest over-all yields, with a high percentage of dimer in the recycle product. The required tempera-
(1) Cuneo, J. F.,and Switzer, R. L., IND.ENQ.CHEM.,ANAL.ED.,15, 508 (1943). (2) Kistiakowsky, G. B., and Ransom, W. W., J . 'Chem. Phys., 7, 734 (1939). (3) Rice, F. O., and Murphy, M. T., J . Am. Chem. SOC.,66, 766 (1944). (4) Robey, R. F.. Wiese, H. K., and MorreI1, C. E., IND.ENQ. CHEM.,36,3 (1944). PREBENTED before the Division of Petroleum Chemistry at the 109th Meeting of the AMHIRICAN CHEMICAL SOCIETY, Atlantic City, N. J.
Formation of Static Electric Charges on
Agitating Petroleum ,Products with Air by C. M. KLAEKNER agitators, as demonstrated by the secured electric sparks require not Magnolia Petroleum Company, B ~ T~~~~ ~ following ~typical results ~ ~ with a Detroleum distillate of only the existence of high voltages 96" F. flash point: but also a certain current density and the presence of an explosive mixture. Formation of an Solvent Temperature, Relative Explosibility of Vapors explosive mixture in air agitation depends on the vapor pressure F. in Commercial Agitatorsn of the liquid. If the vapor pressure is low, the concentration of 88 0.62 92 0 85 the vapors in the air may be less than that corresponding to the 96 1.00 low explosibility limit, whereas if the vapor pressure is very high, Explosive limit = 1.00. the concentration of vapors may be above the high explosibility limit and, in either case, no explosion occurs. Explosive mixIt is believed, therefore, that the explosive temperature is withtures of hydrocarbons of known composition can be calculated in less than 1' F. of the flash point of the petroleum distillates. with a fair degree of accuracy (d), but these calculations are not The rate of air blowing should not have an appreciable effect applicable to commercial petroleum products which contain small on the explodbility limits, provided sufficient time is allowed to quantities of volatile components. For these reasons the flash reach the equilibrium conditions. In plant practice the depth of point remains the most reliable guide for such estimates. the liquid in agitators is sufficient to allow the attainment of Since the flash point determination is affected by the type of equilibrium conditions during the passage of air-bubbles. This procedure selected, experiments were made for determining the may be shown by the following typical data obtained by varying relation between the Pensky-Martens closed-cup flash point the air rate to a large commercial agitator of approximately 30(A.S.T.M. D93-36) and the explosibility of oil-air vapors obfoot diameter with a liquid depth of 20 feet: tained on agitating petroleum solvent and kerosenes of 92 ' to Agitation, Relative Explosibility of Vaqors 110O F . flash points with air a t various temperatures. The John-, Relative Degree Evolved from Commercial Agitator ' son-Williams explosibility indicator was used to measure the ex0.14 None plosibility of the oil-air vapors obtained during the experiments. 0.61 0.10 0 .64 0 . 2 6 The work showed a close correlation between the flash points and 0.59 0.60 0.W the temperature of explosibility of oil-air vapors in commerical 1.00
E
XPLOSIONS caused
(1
~
January 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
Air blowbg is widely used in treating kerosenes with sulfuric acid and alkalies. Although water-washing is desirable between applications of acid and sodium hydroxide solution, this is not always done in actual plant practice and was omitted in the present experiments. These experiments were conducted with a large cylindrical tank agitator of the type mentioned with approximately the same liquid depth. A distribution pipe which supplied the air for agitation was located a t the bottom of the agitator. Static charges developed in the agitator during treating were measured by means of an electrostatic voltmeter. No attempt was made to determine the current density; this was considered to be of secondary importance because of the necessity of first obtaining a spark in order to cause an explosion. This is determined by the voltage generated and the gradient between the charge and tank wall. The observed voltages varied widely with the conditions of the atmosphere, as might be expected. The following results, however, were obtained in dry weather with air of about 50% relative humidity. In determining voltages generated, measurements were made a t various intervals between the center of disturbance and the tank walls. The highest voltages were observed a t points between the tank malls and the center of greatest disturbance; these were the voltages that were recorded. The following observations were made with apparatus which required a finite operating current, the effect of which was not specified. The potential readings, therefore, are relative and do not indicate undisturbed values, which might be hundreds of times those indicated by experimental measurement. In the course of the experiments the following observations were made: S o charges were observed on blowing dry kerosene with air. This may possibly be ascribed t o the insufficient sensitivity of the instrument used but served as an indication that the current density was not great. During the acid-treating, voltages close to 1000 were detected. Apparently the acid globules surrounded with oil favored accumulation of fairly large quantities of electricity. After the acid sludge was settled and withdrawn, the sodium hydroxide solution was added in small steps. Upon addition of a ’ small amount of caustic the charges disappeared, notwithstanding the presence of certain quantities of water and vigorous air agitation. KO satisfactory explanation of this phenomenon was developed. Upon further additions of sodium hydroxide solution electric potentials well in excess of 1000 volts were observed. In some instances potentials as high as 2500-3000 volts n-ere recorded.
Prevention of fire hazard involved in agitating light petroleum distillates may be secured either by eliminating the conditions leading towards formation of explosive mixtures, or by using devices favoring dissipation of electric charges within the agitators. The first can be accomplished by calculating the maximum temperature rise that would occur on treating. This can be done as follows: In treating straight-run products such as kerosene or petroleum solvents in commercial equipment with reasonable quantities of surfuric acid, the temperature rise is not over 0.5 O F. per pound of acid per barrel. It varies somewhat with the strength and quantity of acid used (3) and with the nature of charge stock used. Similar data can be established easily for other types of petroleum products. On neutralizing with caustic, the heat developed depends on the extent of sludge separation, but it seldom results in a tsmperature rise over 8 O F. This information, as well as the knowledge of the flash point of the stock treated, permits an easy estimation of the maximum safe temperature for the charge stock in the agitators. The problem of dissipating electric charges is more complex from the practical vieolTpoint. Maintenance of a highly humid atmosphere a t the top of the agitators for creating conditions favorable to dissipation of electric charges is difficult, particularly because of the tendency of the acid to pick up moisture. Another method involves the use of metal screens. However,
.
93
Formation of static electric charges in petroleum products flowing through pipe lines or subjected to air agitation is a well known phenomenon which has been investigated at various times because of its practical importance in preventing &e hazards. A minimum 300-volt potential may be considered as representingthe dangerous sparking limit (4) under the conditions that may at times be encountered in plant practice around petroleum refineries, although potentials well in excess of this may frequently be generated without causing sparking. Potentials of several thousand voIts are easily generated by friction produced by petroleum distillates flowing through pipe lines at a velcrcity of a few feet per second. The terminal velocity of air bubbles rising through light petroleum products depends on viscosity and gravity of the liquid medium but is of the same magnitude (5). The size of plant agitators does not afford means for a quick dissipation of electric charges by contact with metal walls; this facilitates accumulation of high voltages in excess of those that can be expected in pipe flow experiments.
this requires that the screen be kept close to the surface of the liquid, because otherwise new charges will be formed while the air bubbles rise from the screen to the surface. The distance a t which such screens are capable of protection depends also on their mesh size, as demonstrated by the following data obtained by immersing screens of various mesh sizes at different depths below the petroleum liquid level in the agitator: Screen Mesh, In.
Maximum Voltage, Approx. % ’ 2-in. screen ?-in screen Fin. screen immersion immersion immersion
Another method of achieving protection from sparking is the hanging of wires at close intervals between the top and bottom of the tank so that they penetrate the oil surface. In connection with these discussions it may be of interest to mention that observations are reported (1) claiming that gasoline, after passing over copper or brass filings, completely loses its ability to acquire electric charges when it flows over a metal surface; zinc powder, on the other hand, has temporary effect only, and magnesium and iron filings have no influence whatever. ACKNOWLEDGMENT
The writer would like to express his appreciation to J. W. Newton, W. W. Leach, and P. L. Smith for permission to publish this manuscript, and to V. A. Kalichevsky for valuable suggestions in carrying out the experimental work and in the preparation of this paper. He also acknowledges the helpful assistance of Morris Muskrat, s. s. Mackeown, ana G. M. Pearson, who reviewed the manuscript. LITERATURE CITED
Bruninghaus, L., Recherches inventions, 7,735-7 (1936). Jones, G . W., Bureau of Mines, Report of Investigations, 3787 (1944); Mack, E., Boord, C . E., and Barham, H. N., IND. ENG.CHEM.,15,963-5 (1923). (3) Kalichevsky, V. A,, and Stagner, B. A., “Chemical Refining of Petroleum”, 2nd ed., New York, Reinhold Pub. Corp., 82, 1942. (4) Mackeown, S. S., and Wouk, V., IND.ENG.CHEM.,34, 659-64
(1) (2)
(1942).
(5) Miyagi, O., Phil. Mug., 50, 112-40 (1925). before the Texas Regional Meeting of the AMERICAN CHEMICAL PRESENTED SOCIETY a t Austin, Texas, 1945.
