I N D U S T R I A L A N D ENGINEERING CHEMISTRY
1274
Vol. 22, No. 12
Effect of Time and Temperature on the Cracking of Oils' J. C. Geniesse and Raymond Reuter TABATLANTIC REPINING COMPANY, PHILADELPHIA, PA.
HE gradual increase in
T
Atmospheric-pressure vapor-phase cracking experi-
Experimental
ments were conducted on a representative Midcontithe use of high-comnent gas oil. Three sizes of apparatus were constructed pression p r e s s u r e The apparatus used over to enable the time of contact to be varied from 0.75 to motors in automobiles to obthe high-temperature range is 4800 seconds and the temperature from 430" to 700' C. illustrated in Figure 1. The tain more power from a given The yields of gasoline, gas, and coke and the properties c r a c k i n g c h a m b e r was a engine has necessitated the of these products indicate that the main effect of temPyrex tube 30 mm. outside development of fuels having perature is to increase the rate of reaction; that is, diameter by 100 cm. long, a greater tendency to resist it is possible to obtain very nearly the same results and was electrically heated detonation. Higher quality with either short-time high-temperature or long-time by means of three combusfuels of this type are being low-temperature experiments. An increase of 17" C. produced (1) from selected t ion- t y p e hinged furnaces halves the time necessary to produce the same results. crudes which are inherently each 30 cm. long. The tube Recycle experiments on a narrow fraction from a good, (2) by cracking, and was packed with broken unMidcontinent crude show the increasing refractoriness glazed porcelain in order to (3) by blending, such as the of the cracking stock. reduce temperature fluctuaaddition of benzene or tetrations. Temperature measure ethyl lead. Of these three methods the use of selected crudes and the use of blending ments were made by inserting alumel-chrome1 thermocouples agents are limited by availability, and are therefore not into 10-mm. indentations in the glass wall. The hot junction within the refiner's control. Cracking, on the other hand, was packed with a small amount of fine porcelain and then is strictly within the refiner's control and can be applied to backed with asbestos to prevent direct heat transfer from the heating elements to the junction. Experiments made with produce the required stocks. It has been recognized that, by raising the temperature thermocouples and thermometers inserted through the end of of cracking, fuels may be produced with less tendency to the tube showed that this method was satisfactory. The free detonate. The quality of a fuel obtained by high-pressure space in the heated zone was 285 cc. The feed arrangement or liquid-phase cracking is limited, however, owing to shown at the left consisted of a carburetor float chamber, the low strength of steel a t the higher temperatures. Vapor- a needle valve with sight feed, and a vaporizer to insure vapor phase or low-pressure cracking may be used where high feed to the cracking tube. The charging stock was fed to temperatures are required to produce the type of fuel neces- the float bowl continuously by means of low-pressure air. sary. With this thought in mind an investigation was under- The cracked products passed through a condenser kept a t 27'-C. and a scrubber to remove the Carburetor mist. The gas formed passed to drums, f/d clwmbet d i s p l a c i n g w a t e r which was drained I 1 through a pressure-controlled valve. An Feed under equalizing line between the regulator and pmss. the line to the drum kept the pressure at the proper value. By this means the deede %he total fluctuation in pressure due to displacing water with gas was never greater than 6 mm. of water. The success of the experiments depended very largely on close pressure control, since feeding a t a constant rate is impossible with variations in back pressure. For very short times of contact the 30-mm. tube was replaced by a 13-mm. Pyrex tube having a 55-cc. heated reaction space when packed with broken porcelain. Theapparatusshown in Figure 2 was used for low-temperature, long-time-ofcontact experiments. It consisted of a bank of ninety Pyrex tubes 25 mm. inFigure 1-High-Temperat ure Cracking Apparatus side diameter and 90 cm. long contaken for the purpose of studying the effect of time and nected in series. An annular space between asbestosboards temperature on the cracking reactions a t atmospheric pres- surrounding the tubes and an outside casing served as an sure. air channel. Resistance wires placed in the annular space furnished the heat. A motor and fan mounted a t the base 1 Received September 2, 1930. Presented before the Division of between the tubes and down the forced the heated air Petroleum Chemistry at the 80th Meeting of the American Chemical outer space. A Meker burner connected directly to the Society, Cincinnati, Ohio, September 8 to 12, 1930.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
December, 1930
inside furnished inert gases so as to maintain the oxygen content of the gas within the apparatus at a safe limit and prevent an explosion in case a tube cracked. The tubes were not packed. The reaction volume was 40 liters. The same feed and receiving equipment were used as with the smaller apparatus. feRi I
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The proper choice of a charging stock to be used in a study of this type involves a consideration of the amount of time available as well as the type of information wanted. The decomposition of pure hydrocarbons, such as n-hexadecane, and n-hexadecene undoubtedly offers a better opportunity to study the chemical reactions taking place than if a fraction of a given crude were taken. This is especially true if a great deal of analytical work is done on the products formed. I n order to determine the effect of the chemicalstructure of the charging stock on the reaction taking place, it would be necessary to prepare and crack large quantities of each of various types of hydrocarbons. In addition, if the information obtained on pure hydrocarbons is to be useful it would be necessary to determine the approximate chemical composition of the ordinary charge in order to predict the optimum cracking conditions. I n view of the tremendous amount of research work involved in such a program, i t was thought advisable to limit it to the study of a gas oil. Table I-Properties
LISBON
A. P. I.
charging stock is the fact that plant experience with lowtemperature cracking units has shown that all paraffin-base oils have very nearly the same cracking characteristics. High-temperature cracking should show less difference b e tween them. The particular gas oil used was taken from Lisbon crude. Its properties are given in Table I. Preparatory to making an experiment the apparatus was heated to approximately the correct temperature, when gas oil was fed a t a rate depending on the time of contact desired. The rheostats were then adjusted so as to obtain the correct temperature under operating conditions. The correct operating conditions having been obtained the volume of charging stock in the feed graduate and of gas in the drum were noted, the receiver was emptied, and the time recorded. At the conclusion of the experiment the same readings were taken and the distillate and gas were set aside for analysis. Experiments were conducted a t temperatures varying from 430" to 700" C. The time of contact varied from 0.75 to 4800 seconds, or 1 hour and 20 minutes. The quantity of oil put through the apparatus varied from 3000 to 300 cc., depending on the time of contact and the information wanted. The data obtained are given in Table 11. Fractionations of the synthetic crudes were conducted in a column similar to that described by Peters (2). Since a batch distillation was used, a large portion of the lighter hydrocarbons, such as butane and pentane, escaped from the receiving flask in the form of a vapor. This made it impractical to produce a gasoline having the proper volatility. During the first part of the program the column fractionation was continued until the vapor temperature was 225' C., thus giving a distillate having approximately 10 per cent evaporated a t 80' C., 35 to 50 per cent a t 140' C., and an end point of approximately 220" C. Later a decision was made to stop the column distillation a t a point that would give a distillate having 54 per cent at 140"C., with the thought that such a naphtha would be of more value. The distillates formed in this way had, on distillation, approximately 10 per cent evaporated a t 80" C., 54 per cent at 140' C., and an end point varying from 180" to 225' C. The chemical and physical tests reported later in the paper were I
of Charging Stock GASOIL
Gravity,
1275
37.9
c.
SEMIXOLB
SPECIAL CUT 37.7
c.
247
Ani1 Iodine number: Addition Substitution Sulfur, yo
A Midcontinent para&-base crude was used as the source of the gas oil because of the large quantity available to the industry. An additional point in favor of this type of
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OF co,vT/cr stcs Figure 3
obtained on the two types of naphthas described above. Since these arbitrary naphthas are not strictly gasoline, they cannot be applied to the study of gasoline yields with changes of temperature and time. For this purpose i t was therefore necessary to set up a hypothetical gasoline of definite and practical specifications. It was decided that the yield should be the maximum volume of gasoline produced having 10 per cent evaporated a t 60" C., not less than 54 per cent recovered
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1276
at 140" C., and not less than 90 per cent at 200" C. Considerable work was done studying the relation between column distillations and A. S. T. M. distillation of the naphtha and also relating 10 per cent evaporated on A. S. 7'. M. distillation with the light hydrocarbon composition of the gasoline, and it was found possible to estimate accurately the yield of gasoline with the above specifications. The times of contact were calculated on the assumption that the gas laws held. The velocity of the entering vapor was obtained by assuming a molecular weight of 215 for the gas oil. The velocity of the gases leaving the cracking zone was obtained from the quantity of gas, gasoline, and uncracked stock. For a short-time-of-contact experiment an average of these two values gave the information necessary for calculating the time of contact. Similar outlet velocity calculations made for various experiments gave values for the velocity of the gases a t points along the tube for the long-time experiments. The times of contact obtained from such calculations are given in Table 11. Discussion
The data obtained at 600" and 640" C. in the small apparatus have been plotted on ordinary coordinate paper in Figure 3 to show the type of curves obtained. A study of the gas and gasoline yields indicated that increasing the temperature by 17"C. resulted in approximately the same yield of products in one-half of the time. It should therefore be possible to simplify consideration of the data by combining time and temperature into a single function. If 480' C. is taken as a standard condition, the time-temperature index of cracking will therefore be: 2
'+x
time in minutes
where T is the temperature in C. The values for 1 minute contact are given in Table 111. I n Figures 4 to 7 the abscissa scale indicates the time-temperature index of cracking. Semi-logarithmic paper has been chosen so as to keep percentage approximately constant over the whole range. The lower curves in Figure 4 give the yield of gasoline by volume. As time or time-temperature index is increased it increases rapidly to a maximum and then decreases slowly. This decrease is due to a falling off of cracking stock and to increased decomposition of the gasoline already formed. The maximum point in each curve was obtained a t about the same index for each temperature. The deviations are not more than would be expected with errors in temperature measurements of 3" or 4" C. The maximum yields decrease with increasing temperature. Possibly the decomposition of gasoline has a greater temperature coefficient than the formation of gasoline.
Vol. 22, No. 12
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
December, 1930
m b
10
m
mm
mow N
Nm
m - w3
0
C
1277
The upper curves in Figure 4 show the ratio of gas to gasoline. Increasing the amount of cracking results in a decided increase in gas production over that of the gasoline. The effect of temperature is relatively small except at a very high time-temperature index. Gas analyses were made in a modified Bureau of Mines apparatus for complete analysis of gas. The olefins, with the exception of ethylene, were absorbed in 87 per cent by weight sulfuric acid solution. Ethylene was then absorbed in bromine water. Hydrogen was burned over copper oxide a t 300" C. The density of the gas was obtained by the effusion method. Gas yield percentages by weight are plotted in the lower part of Figure 5 . The weight of gas increases rapidly to a moderate value and then increases more slowly owing to the refractoriness of the remaining liquid. The olefinic content of the gas is given in the upper part of the same figure. These two sets of curves show that although the weight of gas is not affected by the temperature, the unsaturated content increases appreciably. The latter may indicate that the temperature coefficient of polymerization of unsaturated hydrocarbons is less than that for their formation. The ethylene c9ntent of the gas parallels the olefins, showing no irregularity in its decomposition of formation. Iodine values were obtained by the method described by Johansen (1). The iodine addition numbers for the naphthas have been plotted in the lower part of Figure 6. This property indicates that temperature has no effect other than to reduce the time of contact necessary for a given amount of reaction. The unsaturation increases rapidly to a maximum and then decreases slowly, suggesting polymerization. The aniline critical temperature of solution was determined as the maximum temperature a t which the two liquids are not miscible in any proportion. The data have been plotted in the upper part of Figure 6. As with the unsaturation content, these data indicate that the effect of temperature is the same as time. Polymerization is accompanied by the formation of naphthenes and possibly aromatics which lower the critical temperature of solution with aniline. I n the upper part of Figure 7 have been plotted the antiknock values expressed as octane numbers, or the per cent by volume of 2, 2, 4-trimethyl pentane in a blend with n-heptane which duplicates the sample under test. The antiknock values, expressed as benzene equivalents, are listed in Table 11. Although the octane numbers are not as regular as some of the properties cited, there is no general trend due to temperature.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Vol. 22, No. 12
tion reactions. As the lighter hydrocarbons polymerke they pass out of the vapor phase, form droplets which adhere to any surface, and continue to polymerize and split oil hydrogen and methane, ultimately forming coke. I n the open tube a larger proportion of the droplets are carried forward to the condenser. e z Table 111-Time-Temperature
Indices for 1 Minute Contact
INDEX
TEMPERATURE
430 460 480 500 550
INDEX
TEMPERATURS
0.135 0.44 1.00 2.28 17.4
580 800 625 840 700
58 132 363 690 7940
Table IV-Recycling Data Temperature, 600' C. Time of contact, 0.083 minute Time-temperature index, 11 Oil feed, 10 cc. per minute ORIGI- 1ST RE- 2ND RE- 3RD RE- 4TH RE- 5TH RENAL CYCLE CYCLE CYCLE CYCLE CYCLE
Yields:
vol.
by vol. Coke, % by at. Gasoline:" Specific gravity Benzene equivalent A n i l i n e critical temp. soh. Gas: Densitv Hydrogen % by vol. Ethylene,'% by vol. Total olefins, % b y vol. a
Figure 4
Coke formation was determined by aspirating oxygen through the heated reaction zone a t the completion of the cracking experiment and determining the carbon dioxide. The data have been plotted in the lower part of Figure 7. It may be seen that more coke was formed in the packed tubes, although the effect of increasing the temperature in each case was simply to reduce the time of contact necessary to produce a given amount of coke. Undoubtedly, coke is the end product of a series of polymerization and decomposiI
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8
0
,
I
16.5 18.0 18.0 11.1 13.9 11.9 2.5 2.9 4.3 0.054 0.097 0.12
10.7 7.9 5.5 0.19
0.7870 0.7831 0.7967 0.8017 46 47 48 25.5
18.0 1.00
1.01 8.0 24 52
... .... ..
16.5
9.5 8.9 9.3 0.17
8.5 5.4 0:28
..
..
16.0
0.98 9.4 22 50
0.96
... ... ...
0.97
..
19.7 17 37
....
225' C. endpoint naphtha.
One set of recycle experiments was conducted on a narrow fraction taken from Seminole crude. Identification data for the cut are given in the third column of Table I. Cracking conditions were kept a t 600" C. and 0.083 minute, which is equivalent to an index of 11. The modus operandi consisted of cracking the gas oil, fractionating the synthetic 50
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TL7'F- TEmPL'RATUPf
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Figure 6
i
crude, and then recracking the gas oil cut. The fractionation was conducted to a vapor temperature of 225" C. for the gasoline and 260' C. for the recycle stock. The residue was arbitrarily taken as tar. Tests could not be made on the tar owing to the small quantities available. Recycling was repeated until the quantity of gas oil remaining wa8 too small to run another experiment. The data, including yields on the volume of charge actually fed, are tabulated in Table IV and plotted in Figure 8. The yields of gas, gasoline, and tar indicate that recycling the cracking stock changes its chemical composition so that decomposition de-
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
December, 1930
1279
&Coke yields are higher in packed tubes, owing probably to the wetting of the surface with droplets of partially polymerized hydrocarbons. 6-Recycling runs show a considerable change in the chemical character of the charging stock due to its continued heat treatment. As recycling is continued gas and gasoline yields continually drop, whereas tar and coke yields rapidly increase. The hydrogen content of the gas increases due in part to the increased coking reaction taking place.
Figure 7
creases and polymerization increases. The increased hydrogen content of the gas is probably due to the increased tar and coke formation. Conclusion
1-For the cracking reaction at atmospheric pressure similar results are obtained by increases in temperature or in time; that is, it is possible to duplicate approximately short-time high-temperature results by long times a t low temperatures. A temperature increase of 17” C. approximately halves the time. It is possible to consider time and temperature together as a time-temperature index. 2-As time-temperature is increased the yield of gasoline increases rapidly to a maximum, which is slightly higher at the lower temperatures, and then decreases slowly. 3-The properties of the gasoline are the same for a given time-temperature index. Increasing time-temperature results in (a) an increase in the antiknock value of the fuel, (a) a rapid increase in unsaturation followed by a slow decrease, and ( c ) a rapid decrease in the critical temperature of solution with aniline. 4-Gas yields increase rapidly to a maximum of 60 to 70 per cent by weight. The ratio of gas to gasoline increases considerably with time-temperature and slightly with temperature for a given time-temperature index. The unsaturation content of the gas increases with temperature a t a given index only under severe cracking conditions.
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Figure 8
Acknowledgment
Acknowledgment is due J. L. Oberseider, who assisted in the preparation of the data. Literature Cited (1) Johansen, J. IND.ENQ. CHBM.,14, 288 (1922). (2) Peters, I b i d . , 18, 70 (1926).
Half World’s Silver Output Obtained as By-product Silver production occupies a unique position in the production of metals in that more than half of it is derived from ores valued chiefly for their metals other than silver. The price of silver is thus of less weight in inducing the bulk of silver production than t h a t of other metals associated with it. Only onethird of the silver produced is derived from ores having silver as a predominant factor. On the other hand, the bulk of most other common metals is extracted from ores deriving a large proportion of their value from a single metal, the price of which practically determines the possibility of production. The recent decline in the price of silver t o a record low level has provoked much inquiry as to the extent t o which silver is a by-product of other metals, the extent t o which its by-product nature will tend t o maintain its production in spite of price, and the extent to which other metal production may be affected by the price of silver. In response to the need for a n analysis of silver production data, the output of about 1000 mines producing 91 per cent of the world’s silver has been studied by the Bureau of Mines.
The salient features of this economic study indicate that-over one-half of the world’s production of silver is from ores that derive less than 40 per cent of their recoverable value rom silver. Almost one-third is from ores carrying not more than 20 per cent of their value in the form of silver. The most productive single class of ore-that which carries 10 to 20 per cent of its value as silveraccounts for one-fifth of the world’s silver production. One-third of the world’s silver production is from ores that derive over 70 per cent of their value from silver. Almost one-half of this silver is recovered from the single class of ores which depend on silver for 80 to 90 per cent of their value. Over one-half of the world’s silver production may be considered by-product silver, as that amount is derived from ores in which some metal other than silver provides the principal source of revenue. Over one-quarter of the world’s silver production may be considered derived from “straight silver” ores-that is, ores depending on silver for over 80 per cent of their recoverable value.
