Symposium on Hydrocarbon Decomposition Presented before the Division of Petroleum Chemistry a t the 86th Meeting of t h e American Chemical Society, Chicago, Ill., September 10 t o 15, 1933.
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Fundamental Variables in Mixed-Phase Cracking HAROLDSYDNOR, Standard Oil Company of New Jersey, Elizabeth, N. J
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HE term "liquid-phase Perhaps this factor will become The fundamental variables entering into crackcracking" c a m e i n t o clearer from the following brief ing operations and the effect of changes of these general usage in connecdescription of the cracking coil variables on the yields and characteristics of tion with the early cracking decycle : Most modern cracking the products are described. There are three velopments and served to disprocesses recycle to some extent ; fundamental variables controlling cracking operatinguish the p r o c e s s e s u s i n g that is, they return a portion or moderate temperatures of 750" all of the cycle gas oil produced time, temperature, tions in general-namely, to 850" F. and substantial presto the s y s t e m as f e e d s t o c k and pressure. The cracking coil cycle is desure, from the atmospheric or along with the incoming straightscribed in detail, and the effect of the amount of very lo w-pr e s s u r e processes run gas oil. I n such a system recycling upon product yields is discussed. which employed temperatures of the total feed rate (straight-run A series of experiments is cited to illustrate the 1000" F. and above and were p l u s c y c l e oils) is maintained designated by the general term c o n s t a n t , and the amount of dual and progressive nature of the reactions. "vapor phase.'' straight-run gas oil added per The results of another series of experiments The designation was probu n i t of t i m e m u s t b e t h e carried out to study the cffect of changing one ably accurate as applied to the equivalent of the gasoline, fuel variable at a time are discussed; it is shown, oil, cycle gas oil, gas, and coke e a r l y b a t c h pressure-still dethat both temperature and pressure, as well as velopment, since cracking in the removed from t h e s y s t e m . liquid phase was n e c e s s a r y in Cycle gas oils may be removed the extent of the conversion per pass, in$uence most c a s e s b e f o r e substantial from t h e s y s t e m i n s e v e r a l the yields and characteristics of the products vaporization and consequent reways. They may be removed secured. moval of products from the reacdirectly as cuts from the fraction zone could occur. It is untionating e q u i p m e n t . T h e y likely that under the temperatures and pressures used in pres- may be removed by decreasing the specific gravity of the fuel sure-still operation substantial cracking of the vaporized frac- oil produced, in which cases they are designated as fuel oil tions occurred before these fractions were removed from the unless subjected to a redistillation process and recovered as reaction zone. With the increasing demand for gasolines of overhead products. I n most lowpressure (atmospheric up higher octane rating, the so-called liquid-phase processes began to, say, 350 pounds) cracking equipment, they are taken out to increase temperatures but in most cases maintained sub- to some extent along with the gasoline composing the product stantial pressure, ranging from 200 to 1000 pounds in the re- designated as distillate. After removal of the gasoline, the action chambers. There were three reasons for maintaining remaining cycle gas oil may be returned to the cracking pressure-namely, the suppression of gas and coke formation, process. Modern high-pressure (750- to 1000-pound) crackthe better quality of the distillate obtained as regards ease of ing equipment is built with fractionating equipment that mill finishing, and the higher capacities secured. The increase permit all of the cycle gas oil to be retained within the system of pressure and temperatures, and the change from pressure until completely converted to 400" F. end-point specification stills to continuous-cracking coil systems has resulted in gasoline, fixed gases, and fuel oil of the gravity desired down substantial cracking of vaporized fractions in the liquid-phase to about 5" A. P. I. (1.0366 specific gravity). Except as processes, especially where the feed stocks are low-boiling and otherwise noted, the data to which reference is made herein the pressure employed is high. The term "mixed-phase are based upon producing only 400" end-point gasoline, 12' cracking" has therefore been selected to apply to experiments 8.P. I. (0.9861 specific gravity) fuel oil, gas, and coke. covering a range of temperature from 850" to 900" F. (reacTUBEAND TANKUNIT tion chamber temperature) and pressures from 200 to 1500 pounds. The adaptation of this cycle to plant practice is shown The author believes that the differences in results secured in Figure 1. The fresh charging stock, normally gas oil, with various cracking processes are primarily caused by is charged through the spiral coils, A , and introduced to the differences in the relationships of certain fundamental bubble tower, B , where it comes into contact with cracked variables. These variables are thought to be temperature, gasoline and cycle stock vapors passing up through the bubble pressure, and time of reaction. There is a fourth factor tower. The cycle stock is condensed and returned to the which is not a fundamental variable in the sense that the accumulator, C, along with the incoming charge stock. From other three are, but which plays a large part in the results the bottom of accumulator C the fresh and cycle mixture secured. It is the extent to which the reactions are allowed known as the total feed is picked up by the hot feed-pumps, to proceed before the tarry bodies and gasoline fractions D, which force it under pressure through the tubes of the are separated from the recycle gas oil. This factor will be heating furnace, E , and then to the reaction chamber, F , referred to hereafter as the conversion per pass. It is the where the bulk of the cracking takes place. The desired product of the gasoline yield times the percentage fresh feed pressure is maintained on the cracking zone by means of i n total feed: the release valve, G. From the reaction chamber the oil flows to the separator, H , which is maintained under approxiConversion/pass = % gasoline X yofresh feed 181
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mately 110 pounds pressure. I n the separator the fuel oil fractions are removed through the cooler, H I , to storage. The gravity of the fuel oil removed is controlled by controlling the temperature of the separator. This is accomplished by circulating fuel oil through a cooling worm and returning it to the bottom of the separator. From the separator the vapors, which have been freed of fuel oil fractions, pass through the high-pressure exchangers, I , which are usually three in number, where heat is exchanged with the total feed on its way to the furnace. The partially cooled vapors enter bubble tower B , where a portion of the cycle gas oil is fractionated out, as described above, and then pass to the secondary bubble tower, J, from \vhich a distillate side stream may be taken if desired. The overhead vapors pass through condensers to the gas disengaging drum, K , where the gas is separated from the l i q u i d under approximately 100 pounds pressure. T h e l i q u i d is then pumped under pressure to a btabilizer. The temperature a t the top of the secondary bubble tower is controlled in qome cases by pumping back cooled distillate or by means of a n a u x i l i a r y c o o l i n g circuit. W h i l e this description is representative of only one particular cracking syst'em, t h e author believes that the fundamental factors to be discussed i n t h i s paper a r e equally applicable to other systems operating within the r a n g e s of t e m p e r a t u r e and pressure that have been studied. Aside from differences in temperature and pressure, the several cracking 5ystems differ primarily in the relative proportions of the heating and cracking zones and in the extent to which auxiliary fractionating equipment is employed to effect complete separation of the gasoline from the cycle gas oils. Another variation is the direction of flow through the soaking drums. With the increase of temperature it has been found advantageous in many installations to have the oil flow downward through the soaking drum in order to prevent the fuel oil fractions from being trapped out with subsequent cracking and formation of coke. Arrangements for holding levels in the soaking drums and for withdrawal of fractions in order to prevent the liquid from being completely cracked to coke have also been employed. There has been a tendency in recent years to increase the ratio of the cracking taking place in the soaking drum to that occurring in the coil proper. For the handling of fuel crudes i t is frequently the practice to "break" the crude viscosity in one furnace and recycle the resulting cycle gas oil through another furnace and soaking drum in order to obtain the most desirable operating conditions on each type of charge stock. Some of the systems separate the fuel oil under higher pressures and subsequently flash this fuel oil to recover the cycle gas oil. Various means have been used to secure clean feed stocks to the heating zones. One of the best illustrations is the de Florez clean recirculation system. There has been an increasing appreciation of the fact that coke formation in cracking coil tubes is very largely the result of the inclusion of small quantities of asphaltic material in the charge stock to the heating zone.
GEXERAL S A T U R E OF
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Before proceeding to the discussion of the individual effects resulting from a change of each of the fundamental variables, we shall consider for a moment the general nature of the reactions with which we are dealing. Because of the complexity of these reactions and the lack of adequate understanding of their chemistry, it is necessary to fall back upon certain physical characteristics of the charge stocks and end products of the series of reactions designated by the general term "cracking." The most useful of these physical characteristics are the boiling range, the gravity and the aniline point-i. e., the temperature a t which equal quantities of oil and aniline are miscible.
The manner in which a narrowboiling gas oil fraction breaks down as the cracking reaction is allowed to proceed to different extents of conversion is illustrated by Figures 2 and 3. These summarize the results of a series of experiments in which a partially cracked gas oil fraction obtained as recycle stock from a Burton-Clark pressure still operation and boiling between 440" and 675" F. was used as the base stock for a series of laborat,ory experiments. The oil was heated in a coil immersed in a lead bath and then passed through soaking drums of different sizes at various feed rates, the liquid products being collected in one cut, measured, and subsequently distilled to determine gasoline content. The gas was separated from the liquid and metered at atmospheric pressure. All results are for once-through experiments in which there was no return of the cycle gas oil to the system. For the most part the temperatures used range from 895' to 950' F., the pressures from 200 to 3jO pounds, and the time of reaction from 30 seconds to 30 minutes. This work was carried out before the part played by stabilization in accurately determining cracked gasoline yields was appreciated fully and as a consequence the data were not as accurate as those which will be discussed subsequently. They serve, however, to illustrate the general manner in which a narrow-boiling fraction is decomposed into gasoline and gas on the one hand and fuel and coke on the other. Figure 2 was prepared from the distillations of the total liquid products collected from the once-through experiments mentioned above. With increase of reaction time at any one temperature the percentage boiling below 460" F. in the liquid product was found to increase. The selection of the percentage boiling below 460" F. was purely arbitrary
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but serves as a basis of reference for illustrating the effect of increasing conversion to fractions boiling below a given point upon the percentages of the total liquid products boiling within the other intervals of temperature shown on the chart.
Vol. 26, No. 2
will give results that reflect accurately the effect of making such a change in the plant, the effects that are noted are usually the result of a change of both pressure and conversion per pass. An accurate knowledge of the effect of changing pressure and temperature when holding conversion per pass constant appears to be of more than academic interest. EFFECT OF CHANGISG EACHVARIABLE SEPARATELY
FIGURE 2. EFFECT OF CONVERSION UPON BOILING LIQUIDPRODUCT OF A OSCERANGEOF TOTAL THROUGH CRACKING OPERATION
The relationships shown were found to hold throughout the range of temperature and pressure covered by this work and to depend only upon the extent of the conversion. Figure 2 shows that, when increasing the extent of the conversion per pass, as measured in this case by the percentage of the liquid products boiling below 460" F., there is a gradual increase in the formation of the lighter fractions at the expense of those of the immediately higher boiling range. It will also be observed from Figure 3 that with increasing formation of gasoline, as measured by the percentage boiling below 374' in the liquid product, there is an increase in the specific gravity of the final 15 per cent of the liquid. It is evident, therefore, that we are dealing with two types of reactions-namely, the decomposition of intermediate fractions to gasoline and gas on the one hand and polymerization to fuel oil and coke on the other. By this we do not mean to infer that gas is necessarily formed solely as a result of the decomposition of light ends or that coke is formed entirely from the breakdown of fuel oil. It does appear however that, whatever the exact mechanism of the cracking reaction, light products and gas are formed a t the expense of intermediate fractions as the conversion per pass is increased. Furthermore, increasing polymerization to fuel oil and coke appears to take place as a result of increasing conversion. I n considering these facts, it should be borne in mind that these experiments were conducted entirely on a once-through basis and dealt with a fairly narrow range of pressures. They are cited here primarily to illustrate the general nature of the cracking reactions. It seldom happens in plant practice that only one of the fundamental variables mentioned above is changed a t a time. Engineering limitations make it impractical to study in the plant the effect of a change of one variable a t a time through a sufficiently wide range for the data to be significant. The general relationships are complicated by the fact that a change of pressure alone results in a change of time factor as well as pressure per se. It can be shown that pressure, as such, influences the relative yields of gasoline and fuel products by adjusting the feed rate or volume of the reaction zone so as to obtain the same conversion per pass at different pressures. While it is true that a change of pressure without a compensating change of feed rate or reaction zone volume
I n order t o secure basic data concerning the effect of changing each of the vatiables singly, experimental work was carried out in the laboratory. The equipment used for these experiments has been described in detail.' As stated above, we are dealing with three variables (temperature, pressure, and time) and a fourth factor (the extent to which the reactions are allowed to proceed before the gasoline and tar products are separated from the cycle gas oil, which is then returned to the system). Obviously the return of the cycle gas oil results in an increase in the time factor for the fractions returned, but the results that are obtained differ from those that would be obtained if the gasoline and tarry bodies were allowed to remain in the system until complete decomposition of the cycle gas oil had occurred. For this reason it is necessary to consider the conversion per pass as a separate factor. Since we are in a sense dealing with four variables and since the conversion per pass is a product of the other three, it follows that if we fix any two of these variables and alter a third, we of necessity change the fourth. Temperature and pressure are readily measurable, and the conversion p e r p a s s can be d e t e r m i n e d from the yields and i n s p e c t i o n s of t h e p r o d u c t s obtained. 20 The accurate ineasurement of the time of reaction in a con- IS tinuous system is ext r eme 1y d i f f j c u l t IO owing to the lack of a d e q u a t e data concerning the physical state of the oil in the system. H o w e v e r , o 1 1 II 1 SPEbIF1 dGR jV I Td 1 1 1 we c a n establish .M .e .w .96 .pa 1.0 ~
curves of conversion FIGURE 3. RELATION OF GRAVITY OF FINAL15 PER CENT OF DISTILLATE per pass ""* (LIOUIDPRODUCTS) TO PERCENTAGE gasoline Yield u n d e r OF CONVERSION TO FRACTIONS BOILING BELOW 374' F. conditions of constant temperature and pressure by varying the time factor, although the exact time factor corresponding t o a given conversion per pass will not be known accurately. In this manner it is possible to study the effect of changes of temperature and pressure a t constant conversion per pass, and it is these relationships that are of the greatest practical significance in commercial cracking operations. I n order to aroid cracking during the heating period, i t was necessary to heat the oil to the desired temperature as rapidly as possible and pass it through a reaction chamber held under constant temperature and pressure. While i t was impossible to eliminate cracking in the heating zone entirely, samples taken from time to time indicated that the extent of cracking in this region was comparatively smalI in most cases. Since this work was carried out in the laboratory where the heat losses are large in proportion t o the total heat content of the oil, it was necessary to heat the soaking drum as well as the coil. For this reason the relationship between the coil outlet temperature and the 1
Sydnor and Patterson, IND ENQ.CHEY.,22, 1237 (1930).
I N D U S T R I A L A N D E N G I N E E R I N G C H E iM I S T R Y
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750 pounds pressure and 18 per cent conversion per pass with those that would be secured at 200 pounds pressure and 10 to 12 per cent conversion per pass. Further examination of Table I reveals the fact that increase of temperature a t constant conversion per pass and pressure results in a decrease in the ultimate yield of gasoline and that the decrease in yield at 750 pounds is caused primarily by increase of gas formation. It will also be observed that this increase of gas formation is accompanied by an improvement in the octane number of the gasoline produced. At 200 pounds the higher gasoline yield a t 860" F. and 7 . 5 per cent conversion per pass appears to be associated with the lower fuel oil yield. The higher yield of coke indicates the possibility that the additional gasoline was formed, in part at least, from the coking of fuel oil. It also appears that the decrease in fuel oil formation with increase of conversion per pass a t 900" F. and 200 pounds pressure is the result of the fuel oil breaking down to form gasoline, gas, and coke. TABLE I. ULTIMATEYIELDSSECURED B Y CRACKITG A 33.7' A. P. I. MIDCONTIXENT GASOIL The author has attempted to correct the table to a coke:A11 gasoline yields, octane numbers, and volatilities calrulated to a 100 free basis in order to illustrate this point. per cent butane recovery basis) The octane numbers of the gasoline obtained in the experiPressure, lb./sq. in. ,--200---750 Temperature (soaker), F. 860 900 860 900 ments referred to above are not as consistent as was hoped Conversion per pass 7.5 7.520.0 7.520.0 7.520.0 for. This is probably due primarily to the difficulty of corGaso!ine (400' F. end point), % by vol. 6 9 . 3 6 7 . 0 62.1 6 3 . 0 5 9 . 5 61.5 57.9 recting the octane numbers to represent the same extent Fuel oil (12 . . . of butane recovery under different conditions of operation. vol. 24.0 2 7 . 1 23.4 30.0 32.4 29.6 32.0 11.2 1 1 . 4 18.0 11.4 11.4 1 2 . 6 1 2 . 6 Gas, % by weight I n general, i t seems that these data indicate only a slight Coke, % by weight 2.0 0.2 2.0 0 0.8 0 0.8 100.0 100.0 100.0 100.0 100.0 100.0 100.0 Recovery, % b y weight effect of conversion per pass upon octane number a t constant Characteristics of gasoline: temperature and pressure. Apparently there is very little Octane NO.^ 63.5 68.8 68.8 . . . . . . 67.0 69.6 effect of pressure at constant temperature. Temperature % ' dietd. + loss a t 140' F. 1 5 . 1 1 8 . 1 20.0 15.1 17.5 18.1 20.0 appears to be the controlling factor in so far as octane number Y I E L D S C O R R E C T E D T O COLE-FREE BABISb is concerned. Gasoline (400' F. end 67.4 6 6 . 8 60.2 63.0 5 8 . 8 6 1 . 5 57.2 point), yo by vol. The data covering the volatility of the gasolines are someFuel oil (12' A. P. I.), Yo what more satisfactory than those dealing with octane numbv vol. 2 8 . 1 27.5 27.5 3 0 . 0 3 4 . 0 29.6 3 3 . 6 G a s , % by weight 10.2 1 1 . 3 1 6 . 9 1 1 . 4 11.0 12.6 12.2 ber, and appear to show conclusively that there is a distinct a Cooperative Fuel Research Steering Committee motor. increase in the percentage of light ends formed with increasb I t waa assumed t h a t 12' A. P. I. fuel oil cracks t o the following yields: ing conversion per pass. Also, a t constant conversion per gasoline, 45 per cent by volume; gas, 22 per cent by weight; coke, 42 per c e n t by weight. pass there is a distinct increase in the formation of light ends with increase of temperature. There appears to be With increasing conversion per pass there is a decrease in little effect of pressure a t constant conversion per pass over t h e ultimate yield of gasoline obtainable at a given tempera- the range of pressures studied in these experiments. These t u r e and pressure. At high pressure (750 pounds per square data are particularly important in view of the rapid rate of inch) this decrease of gasoline yield is a result of an increase increase in the formation of light ends in refining operations in the formation of fuel oil-i. e., increased polymerizationin recent years. This increase is obviously the result to a while at low pressure it appears to take place as a result of considerable extent of the demand for gasoline of higher octane increased gas formation even though the gas has been cor- number. In order to secure this higher octane number, it rected to a butane-free basis. It is quite possible that the has been necessary to increase the temperature of the comgas yield at high pressure may increase slightly with increasing mercial cracking operations, and this in turn has resulted in an conversion, but i t was not detectable within the limits of increase in the conversion per pass in most cases. The net accuracy of the data over the range of conversion shown. result has been a decided increase in the formation of the The figures given in Table I were taken from curves based lower boiling fractions a t the expense of a reduction in the upon a number of determinations. The gas yields are probably ultimate yield of gasoline that can be obtained. The price accurate to within one per cent on the charge stock. of obtaining gasoline of high octane rating by cracking Also, distinctly higher yields of gasoline are obtained a t is a decided reduction in the yield of gasoline obtainable 200 pounds as compared with 750 pounds per square inch for from a barrel of crude. operations a t constant temperature whether the temperature The foregoing discussion has dealt entirely with the beemployed is 860' or 900" F. This result appears to be en- havior of one gas oil of known gravity and aniline point when tirely consistent with plant practice. The use of high pres- cracked under various operating conditions. However, it sure in commercial operations results in increased fresh feed has been found that the results obtained by cracking a capacity for a unit having a given total pumping and heating fairly wide range of straight-run and cycle gas oils under capacity, but this increased capacity is obtained a t the ex- constant operating conditions can be related to the aniline pense of increased polymerization to fuel oil. I n plant point and gravity of the gas oils. A very good relationship practice the gasoline yield obtained a t 200 pounds exceeds has also been found to exist between the gravity, aniline t h e yield a t 750 pounds by a greater amount than is indicated point, and boiling range for both straight run and cycle gas by a comparison of the effect of pressure a t constant con- oils. From these interrelationships it is possible to predict version per pass, since a lower conversion is obtained in plant with reasonable accuracy the behavior of a fairly wide range practice when operating a t the lower pressure. For the feed of distilled fractions when cracked over the range of temperastocks studied in these experiments, a better idea of the ture and pressure that has been studied to date. practical significance of using reduced pressure would be obtained by comparing the gasoline yields corresponding to RECEIVEDSeptember 7, 1933.
soaking drum temperature used in these experiments is of no significance as regards the relation which exists between these two temperatures in the plant. I n some instances the coil outlet was held a t or below the soaking drum temperature to minimize the cracking in the coil proper, whereas in plant practice the coil outlet temperature usually exceeds the average soaking drum temperature by about 40' F. for the normal operation of tube and tank units. Therefore, only t h e soaking drum temperature has been shown, and only this temperature has been used as the basis for comparison of the results obtained. The effects of changes of conversion per pass, temperature, and pressure are illustrated by Table I. It should be borne in mind that these data represent complete decomposition of the cycle gas oil into gasoline, fuel oil, coke, and gas in all cases.
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