US1938406A - Process of cracking mineral oil - Google Patents

Description

Dec. 5, 1933. H. THOMAS PROCESS OF CRACKING MINERAL OIL Filed July 5, 1928 ./m. E 5 WM M Q Mdm Patented Dec. 5, 1.9733

UNITED STATES PATENT OFFECEl Henry Thomas, Ridley Park, Pa assigner to Sun Oil Company, Philadelphia, Pa., a corporation of New Jersey Application July 5, 1928. Serial No. 299,288

2 Claims.

In the art of treating boiling and high gravity hydrocarbons oils for the purposeoi decomposing them to convert them into lower boiling and lower gravity hydrocarbon oils, which art is generally known as cracking, the most common procedure is to subject the oil to be cracked to high temperature and a substantial pressure above atmospheric. The art is a ver old one. In recent years, its inost useful industrial application is in the production c1" gasoline from such higher boiling and higher gravity petroleum oils as gas oil, fuel oils, and kerosene distillates. The cracking operation may be conducted in batch or it may be carried on in a continuous manner.

In continuous processes for cracking higher boiling petroleum constituents to gasoline, it is old and well known to convey the cil to be de* composed, which may be initially preheated, through a long tube cr tubes in immediate contact with heating gases and therein to raise the oil to a cracking temperature and either co-; plete the cracking therein or perform a part i the cracking therein. In the latter case, the partly cracked oil may be conveyed to a container or reaction chamber of relatively large crosssection, wherein the pressure is maintained, and wherein the temperature of the oil falls, although not below an effective cracking temperature. En this chamber the cracking is completed and the oil will be relatively quiescent to allow deposition of the coke which is formed under the high temperature conditions. The oil may be allowed to escape through a loaded valve into a vaporizer, wherein there is maintained a pressure but little above atmospheric. After passing the valve, most of the oil immediately vaporizes. The vapors are fractionated and condensed. Ii all the cracking be eiiected in the tubes, the formation and accumulation of coke in the tubes tends to insulate the oil from the heating ga es and clog the tubes, necessitating their irequent cleaning. lf only part or the oil is cracked in the tubes and if the partly cracked oil conveyed from the tubes to a large reaction chamber, in which the cracking is completed, as explained above, the trouble is merely ameliorated, in that some of the coke 'that v ould otherwise form or deposit in the tubes forms or deposits in the large 4reaction chamber. Frequent cleaning oi the reaction chamber is therefore necessary. This is entirely practicable, but the drop in temperature that occurs in the reaction chamber slowsdown the reaction to a very great degree; it being possible to appreciate the extent of such slowing down if it be borne in inind that with every increase of temperature of about 13 degrees F., the speed of reaction doubles.

The obje-c of the present invention is to pro .3 vide a process wherein the cracking operation is eiected in a long tube or series of tubes, under such conditions as to minimise the formation, and accumulation of coke.

The process comprises establishing a flowing stream of oil through a tube, or a series of tubes that in ject constitute one long tube, in so regulating the tcny-erature of the heating Inediura as to raise the temperature o1" the oil through a cracking range to about the maximum crack im' ing temperature which it is desired to establish while it is traveling through a part (preferably a minor part) of the length or" the tube, in holdthe oil, while it is flowing through the remaining part (preferably the major part) of the g5 length or the tube, at about said maximum desirable crack" r temperature, and in owing the oil through .ie tube, and particularly through that part thereof within which the oil is at a comparatively high cracking temperature, at a rate of speed which, though very high, is limited as hereinafter described` The process also comprises the maintenance of the more advanced (preferably major) part or the oil stream a niaxiinum cre king temperature very close, 35 b t below, its critical temperature, in which phase all, er at least the predominating part, of the cracking occurs. The process also comprises the application ci heat to a tube which is inserted in a single furnace and in which the oil circulates laterally in progressively higher planes se that, although the saine furnace gases transmit heat to the entire stream of oil, the furnace gases are progressively cooled. as they iiow upward and the heat received by the oil as it advances progressively diminishes, so that just enough heat is received by the oil to balance the heat lost by the endotherinic reaction of cracking.

Other advantageous features of the process will be developed in the following description.

While the process is not dependent for its execution on any particular construction and arrangement of apparatus, the drawing represents the lay-out of a plant in which the process is adapted to be carried out in a particularly practicable and advantageous way.

Figure l is an elevation, in diagram, of a complete cracking plant.

Figure 2 is a diagram of a modication oi a part ci such plant.

A suitable charging stock, preferably gas oil iiows from a charging tank through line a. (in which interposed a booster pump b) which extends through the gasoline condenser e and biings the charging stock in heat exchange relation with gasoline vapors coming from the gasoline vapor purifier wherein, by heat exchange, the charging stock is preliminarily heated and the gasoline vapors cooled and partially condensed. The charging stock from line a thence ows into a tank c, which serves as a surge tank and from which the oil may be fed through any number oi` cracking units. Only one cracking unit is shown. The process is, oi course, applicable to one r more cracking units.

From tank c the oil iiows through line d, wherein a high pressure pump p is interposed, which may maintain any desired pressure in the cracking unit, say from 600 to 15(20 pounds per square inch. In line d is also interposed a heat exchanger c, wherein the charging stock is brought into heat exchange relation with the cracked oil flowing from the cracking unit. Herein the temperature of the charging stock may be raised to (say) l00" to 500F., although the extent to which the temperature ci the charging stock is raised before entering the tubular cracker is not an essential feature of the process.

From heat exchanger e the charging stock flows into a furnace f and enters the lower bank of tubes g, after traversing which laterally and upwardly the stock flows laterally and upwardly through an upper bank of tubes h and thence through line i (in which is interposed the above described heat exchanger e) to a vaporizing and ractionating tower t.

The two banks of tubes may comprise different numbers and arrangements of tubes, which may be of variable individual sizes and total length; but in order that those skilled in the art may have full information regarding one practicable construction and arrangement, the following description of lan operative unit is given.

The lower bank comprises 126 tubes each of about three inches internal diameter and about 32 feet long, adjacent tubes being connected at the end by curved couplings in such manner that the sto la will flow through the tubes in seri s. The upper bank or banks comprise 252 tubes of the same length and diameter and similarly connected in series, the two banks of tubes being also connected in series. Thus there is provided a continuous conduit of small crosssectional area and over 12000 feet, or nearly two and one-half miles, in length, about onethird of which is formed by the lower bank of tubes and about two-thirds of which is formed by the upper bank or banks of tubes.

The lower tube bank g is subjected to the highest degree of heat and in this bank the temperature of the stock is raised from that at which it leaves the heat exchanger e (say 400 to 500 F.) to about the maximum cracking temperature which it is desired to establish. i the gas oil has a critical temperature of 364 F., it is heated in the lower tube bank to nearly, but not quite, this critical temperature, say 860 F. kThe stock in the upper tube bank or banks is subjected to a lower and variable degree of heat which decreases progressively upward; the heat beine so adjusted as to hold the oil flowing through the upper bank at substantially an even maximum cracking temperature.

The furnace may be of any suitable construction. in the one shown, Dutch ovens, containing burners lc, are positioned in the lower forward part ci the furnace, fresh air is admitted at the burners, and thc hot gases flow upward through the spaces around the tube banks g and h. The cooled gases now out the top through tube m and thence down the stack n, part of them going to the main draft stack o and the remainder being forced through flue i' into the furnace, where they mix with, and are reheated by, the hot gases or combustion. A more ldetailed de4 ption or" a desirable arrangement of p and passages for air and gases is `contained in an application filed by Henry IThomas une 14, 1927, Serial No. 198,703, while Va desirable construction of furnace, including efiicient means for supporting the tube banks, is set forth in an application by Henry Thomas July 6, 192'?, Serial No. 203,402. The present process, however, is not dependent on such details of construction, a description ci which herein being therefore unnecessary.

The method of maintaining the oil at or nearly at even temperature while it is traveling through the upper tube bank will be understood by the following description of a typical operation.

Let it be assumed that the oil entering the lower tube bank t a temperature of 5000 and that the furnace gases have a temperature, when they reach theloi. er part of the lower tube bank of (sr y) 1350O and that these gases, in passing through the lower tube bank, are reduced in temperature to (say) i000O F. and heat the oil to a temperature of 8580 F. As the oil news through the upper bank or 'tubes it continually absorbs heat from the gases but at a constantly decreasing rate, so that the gases leave the upper tube bank at a temperature of (say) S900 to 9200 F. This action would tend, or" course, to continually raise, at a decreasing rate, the temperature ci the oil above its desired maximum were it not for certain conditions tending to reduce the temperature or" the oil. These conditions may be enumerated as follows, First, the cracking reaction is endothermic. Second, sensible heat is being converted into latent heat. Third, at te temperature specined, there is a continuing conversion o oil into lighter condensible o1 accompanied by reduction in pressure, ich in turn, without added heat, causes addit nal vaporization, with a resultant loss of heat by expansion. The problem, therefore, is to just balance the loss of heat arising from the change ci condition of the oil by absorption or" heat :from the furnace With a furnace o the character speciiied it is possible to maintain this balance. If it is found that the temperatur` el the oil in the upper tube bank is increasing, a smaller volume of gas is diverted from .ck 7i through nue r into the furnace, thereby decreasing the volume or heating gases and decreasing the absorption of heat by the oil. Ii it is found that the temperature ci the oil in upper tube bank is decreasing, a larger volume of gas reci" ulated ytain maximum temperature, because increase of temperature of the furnace gases tends Yto intensify the conditions which produce cooling of the oil, which in turn tends to accelerate cooling of the furnace gases; while decrease in temperature of the furnace gases tends to retard that change in the condition of the oil which tends to cool it, which in turn tends to retard the cooling of the furnace gases. This limited automatic regulation, together with a careful regulation of the volume of recirculated gases, makes it quite possible to hold the oil at a temperature, just below the critical temperature, which will not Vary more than one or two degrees from the temperature desired.

Regulation of the temperature of the oil in the upper tuber bank or banks may be, of course, effected by other expediente.

In explanation of the statement, hereinbefore made, that evolution of oil into lighter condensible vapors is accompanied by reduction in pressure, which in turn causes additional vaporization, it should be said that, in the normal operation of my process in the described cracking coils, the slide valve s maintains a pressure of 650 pounds to the square inch at the outlet and that the pump p operates at about 1200 pounds to the square inch at the inlet. This pressure differential is due entirely to friction through the tubes. The larger the proportion of liquid that is converted into vapor, the greater the velocity of flow through the tubes, the greater the increase in frictional resistance, and the greater the pressure drop. If, now, the oil should be heated above its critical temperature, resulting in its complete conversion into a true gas, the friction drop, due to greatly increased velocity, would be tremendously increased, with the result that for a constant pressure on the pump the throughput would be greatly diminished. It is clear that this gas, flowing with such greatly increased Velocity through a tube of the length herein described, would not be subjected to a cracking temperature for a sufficient length of time to secure an economically sufficient yield. This difliculty may be overcome, however, by dividing the tube h into a plurality of tubes arranged in parallel with each other and in series with tube g, or by increasing the diameter of tube h especially at its outlet end, or by both expediente, as hereinafter described.

In the described process, one of the most remarkable features is the almost complete absence of coke formation, In ordinary cracking processes, the percentage of coke formed to the percentage of charging stock varies from about .Gol to .084. In my process this percentage is reduced to about .600015 or less than one-sixtieth the usual proportion. Moreover the coke does not tend to bake hard on the inner wall of the cracking tube. Such coke as form after a protracted period of cracking is of a comparatively soft and non-adherent character and may be washed out without resorting to the use of scrapers or other mechanical cleaners, the use of which is highly objectionable, since they cannot scrape out the coke without also scraping off the metal of the inner tube wall and their operation is laborious and time-consuming. Finally, in View of the extreme slowness with which the coke accumulates, it is possible to operate the process continuously for a much longer period than is possible in the usual cracking processes,

The factors that may minimize coke formation and accumulation'are the relativelyv low. heat of the furnace gas (the desirable temperature in the hottest zone is between 1200 and 1300 F. and should not exceed 1400o FJ, the extremely rapid flow of the oil, the fact that the critical temperature of the oil is not exceeded, and the stationary temperature of the oil throughout the more advanced and major part of the length of the tube. It is believed that such coke as forms at all is formed in the lower coils, due to the hotter temperature of the furnace gases and consequent local overheating.

While the rapid fiow of oil tends to forcibly wash away any coke that may form, the effect of the rapid flow seems to be to more evenly distribute it .through the tube and not .to carryv itl into the vaporizer, there being little evidence of the presence of coke in the heavy oil precipitated therein.

The oil that leaves the bank h of cracking tubes is reduced in temperature in the heat exchanger e. ducing valve s into the tower t, the great superatmospherie pressure to which it has been continuously subjected after it has passed beyond the high pressure pump is removed and the greater part ofthe liquid oil, as it enters the vapor space of the vaporizer, bursts into a vapor, the remaining liquid oil (about five or ten per cent. of the total) being thrown down and escaping through line u to a suitable receptacle.

The tower vt is shown as comprising a lower vaporizing section (which receives the oil from line i) and an upper section in which partial condensation of the ascending vapors occurs, due to the cooling action of the gasoline which is pumped into the top of the tower, as hereinafter explained. These two sections of the tower may comprise, if desired, separate towers,

as inthe patent issued to Henry Thomas Aprilat the top and, through line w, flow to a puri-l iier and decolorizer which may be a so-called clay tower comprising a bed of decolorizing material, such as fullers earth, charcoal, Death Valley earth, bauxite, a product resulting from the reaction of a soluble silicate with ammonium aluminum sulfate, or other suitable filtration material. Such high boiling gasoline as may condense in the tower :c and flow through the ltering material prevents accumulation of coloring bodies in the gasoline and the clogging of the lter bed. This gasoline condensate mayA flow out with the vapors through line y into the gasoline condenser z, or the gasoline condensate maybe led into either the vaporizing section or the refluxing section of the reflux towerft, as set forth in the last named Thomas patent. It is led, preferably, through a reflux condensate tank 12, a cooler 13, and a line 14, to the pipe s.y

In the upper part of the condenser e, the gasoline vapor exchanges `heat with the cold charg- In passing through the pressure re ing stock iniiowing through line a and then drops into a lower Water-cooled section, where its condensation is completed. The gasoline thence flows into a separating tank l0, wherein permanent gases are removed, and thence flows through line 1l to storage. To control the vapor temperature from the tower t, gasoline may be pumped from separator 10 through line 15 (in which is a temperature control valve 16) into the top of the tower. This gasoline is revaporized in the tower, thus extracting heat from the ascending vapors and condensing components heavier than gasoline. The control valve 16 on line 15 is a thermostatically operated valve with the sensitive element in the vapor line w and is so arranged that if the temperature in this vapor line exceeds the desired temperature, the valve opens and allows more gasoline to be pumped into the top of the tower, thus cooling the vapors to the desired point. If the temperature on the vapor line drops below the desired point, the valve shuts against the pump and, byv thus decreasing the amount of gasoline pumped into the tower, permits the temperature or" the outgoing vapors to rise. A description of this valve is not necessary, as it is a well known device.

The speed with which the oil travels through the tube banks g and iL will depend on the diameter of the tubes, their total length, the character of the charging stock, the pressure to which the oil is subjected, the temperature of the oil, whether it is. largely in liquid phase or whollyV in gas phase, and on other factors. With the typical structural conditions hereinbefore enumerated, the s eed of travel will average probably five feet per second, based on cold oil; the speed accelerating with increase of temperature and increase of formation of permanent gas and condensible vapor. The average speed of travel, based on the specified operating temperature, should not be less than ten feet per second, and may be much higher.

The temperature to which the oil is desirably raised and at which it is held during the major part of the cracking operation is a feature of great importance. This ideal temperature cannot be expressed in terms of an exact range of degrees because of the variation in the critical temperatures of different charging stocks. Bearing in mind, however, that the speed of reaction doubles with each increase of 18 F., it will be understood that 18 F. below theV critical temperature is about the minimum temperature at which the process can be efficiently conducted, and even at this temperature the yield of gasoline is too slow to be economically satisfactory and an uneconcmical re-run is imperative. By maintaining the cracking temperature at 9 F. below the critical temperature, the yield is increased approximately 5() per cent. above the yield at 18 below the critical temperature. is, however, quite practicable to maintain the oil at a temperature of 4 or 5 F. below the critical temperature, thereby increasing the yield about 75 per cent. above the yield at 18 below 'the critical temperature. An even closer approximation to the critical temperature, within about 2 F. thereof, is possible with careful temperature regulation.

With heavy gas oils having a relatively high critical temperature, the Vcracking temperature may be maintained so high as to give an extremely high yield of gasoline.

The process is, however, but not desirably,

capable of being carried out at a cracking teniperature above the critical temperature of the oil, but both the yield and the throughput, in the described apparatus, are greatly reduced. This is due to the fact that, when the oil is above the critical temperature, it is in gas phase, and oil in gas phase has a much higher velocity than when in liquid phase or in mixed liquid and vapor phase and hence is in the cracking coil a much shorter time. Time, as well as temperature, is a factor in cracking; and while the cracking operation proceeds more rapidly when the oil is above, than when it is below, the critical temperature, it is subjected to the cracking temperature for a more than correspondingly shorter time. Hence, the ideal conditions for cracking, in the apparatus and under the pressure herein described, are the maintenance of the oil, by high pressure, in largely a liquid phase, at a steady temperature as near as possible, but just below, the critical temperature, for the major portion of the time during which it is subjected to the cracking operation.

` While the reduced throughput and yield, when operating at above the critical temperature, may be, to some extent, counterbalanced by increasing the length of the tube, this is not unobjectionable and may be impractcable, because of the increased power required to force the oil through the tube and the excessive pressure to which the oil is subjected until it is converted into a gas.' As hereinbefore stated, however, this objection may be largely or wholly overcome, in a more practicable manner, by providing a plurality of tubes h arranged in parallel with each other and in series with tube g', as shown diagrammatically in fig. 2; or the diameter of the tube h, or of a part thereof, preferably the part nearest the outlet end, may be substantially increased. Such a mode of operation is necessary in the cracking of a kerosene distillate or a light gas oil, wherein, although the critical temperature is relatively low, the economically practicable cracking temperature is very high and above its critical temperature; but in the case of heavier gas oil, whose critical temperature is lrelatively high but which may be economically cracked at a relatively low temperature, the maintenance of the oil during a major part of the cracking operation, at a temperature just below the critical temperature of the oil, is an independent speciiic inventive feature of great importance. i

The process must not be confused with a process wherein the oil is gradually raised in temperature to its maximum cracking temperature throughout substantially the entire tube. Such a process is extremely inefficient, since, as above stated, cracking is a factor of both temperature and time. 1f the oil is substantially below its maximum cracking temperature for most of the time, the yield of gasoline will be small, or more or less of the cracking must be conducted in a large reaction chamber.

While the process as described is one in which the heating element is fiunace gases, it is advantageous to use mercury vapor or some equivalent as a heating medium. Mercury vapor is advantageous, first, because, accurate temperature control is most readily securable, and, secondly, because thereby a relatively small temperature difference between the heating medium and thel oil may be maintained, thus materially reducing the tendency to coke formation.

When the process is carried out in the preferred manner, namely, by maintaining the oil, during its flow along the more advanced and major part of the oil stream, at a temperature very close to and just below its critical temperature, it is necessary, prior to introducing the gas oil or other hydrocarbon into the cracking tube, to determine its critical temperature` which, in the case of different gravity gas oils, varies considerably. It is obvious that my process, in its preferred embodiment, is especially valuable when applied to the cracking of heavy gas oils, because these may be heated to Very high cracking temperatures that are nevertheless below their critical temperatures and because at such cracking temperatures the yield of gasoline greatly exceeds that obtainable from heating lighter gas oils to the same cracking temperatures but above their critical temperatures.

Having now fully described my invention, what I claim and desire to protect by Letters Patent l. The process of cracking mineral oil which comprises determining the critical temperature of the oil to be subjected to the cracking operation, rapidly heating the oil, for a relatively short time, to a gradually increasing temperature ranging from the temperature at which itstarts to substantially crack to a temperature close to said initially determined critical temperature and below the critical temperature at which it would be converted, regardless of pressure, into a true gas, while maintaining it under a high superatmospheric pressure suiiicient to retain it largely in liquid phase, and performing the predominant part of the cracking operation by maintaining the oil during a relatively long time at a temperature not below the temperature to which it Was rapidly raised as specified but below the critical temperature at which it would be converted, regardless of pressure, into a true gas, while maintaining a high superatmospheric pressure suiiicient to retain it largely in the liquid phase.

2. The process o cracking mineral oil which comprises establishing a flowing stream lof the oil of great length and small thickness, heating the oil, in the course of its flow through a minor part of the length of said stream, to a gradually increasing temperature ranging from the temperature at which it starts to substantially crack to a temperature just below but close to the'critical temperature at which it would be converted into a true gas while maintaining a pressure sufficient to retain the oil largely in liquid phase, and maintaining the oi'l, during its flow along a more advanced and major part of the length of said stream, at a temperature not below the temperature to which it was raised in said minor part of the oil stream length and below the critical temperature at which it would be converted, regardless o1 pressure, into a true gas while maintaining a pressure sufficient to retain a substantial proportion of the oil in liquid phase.

HENRY THOMAS.

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