US2220795A - Method for cracking hydrocarbons - Google Patents

Description

Nov. 5, 1940. .H. V. SMITH METHOD FOR CRACKING HYDROCARBONS Filed Nov. 20, 1954 2 Sheets-Sheet l h if/a,

Glam/M43:

Nov. 5, 1940.

H. V. SMITH METHOD FOR CRACKING HYDROGARBONS Fild Nov. 20, 1934 2 Sheets-Sheet 2 hugs m w 0 Gum/M 3 S Patented Nov. 5, 1940 PATENT or is METHOD FOR CRACKING HYDROGARBONS Howard V. Smith, El Dorado, Kans, assignor to Skelly Oil Company, El Dorado, Kans.

Application November 20, 1934, Serial No. 753,945

operation of this character in which a dirty stock such as crude oil, topped crude, fuel oil or analogous substances may be cracked to produce a high yield of gasoline without the formation of excessive quantities of carbon.

A further object of my invention is to provide a process wherein the heavy hydrocarbons such as are found in topped crude, fuel oil or the like may be initially cracked to form lighter hydrocarbons of the character of gas oil and distillate and in which these lighter hydrocarbons initially formed are additionally cracked in, the vapor phase to produce a high yield of gasoline,

Another object of my invention resides in my improved method whereby cracking is conducted in two distinct steps as before stated, and the vapors generated in the first step are separated from the unvaporized oil under high pressure, said vapors being thereafter additionally cracked unpossible the additional cracking of the vapors separated by further heating.

Still another object of my invention is to provide a process for cracking oil in two steps, as before stated, wherein the vapors generated in the first step and additionally cracked in the second step are thereafter remixed with the unvaporized oil. remaining after the first step. v

A further object of my invention is to provide a process for cracking oil in two steps as indicated, in which the oil cracked in the first step is separated into two portions, one portion comprising vapor only which is subjected to further cracking as the second step, the other portion comprising a mixture of vapor and liquid which are remixed with the additionally cracked vapors in said second step.

A still further object of my invention is to pro- I vide a process for cracking oil in two steps in which all of the unvaporized oil and a portion of the vapors resulting from the initial cracking step by-pass the second cracking step, thereafter remixing with the product of the second step, the recombined cracked products being subjected to tar separation and fractional condensation in order that the insufiiciently cracked hydrocarbons may be returned to the first step for further cracking.

Another object of my invention is to provide a process for separating under high pressure, vapors generated in an initial step of a cracking process, subjecting vapors only resulting from said initial cracking step to additional cracking under high pressure, recombining at a lower pressure the additionally cracked vapors with the materials remaining after the initial cracking step and not additionally cracked, subjecting the recombined cracked products to tar separation and fractional condensation, and reintroducing the insufiiciently cracked products to the initial cracking step.

Still another object of my invention is to provide a process of this character for cracking oil continuously for the "formation of distillate and coke wherein a portion of the oil initially heated in the first step is additionally heated by remixing with vapors which are produced in the first step 7 but which have been additionally heated in the .second'step, thereby producing a temperature in the recombined products which will cause coking of the residue therein.

Another object of my invention is to provide a process of the character described in the preceding paragraph in which the coke chambers are externally heated to aid in the production of coke of low volatile content.

Finally, it is my desire to provide a process in which any type of charging stock may be subjected to cracking in the vapor phase and at high pressure in order to obtain a motor fuel'of the most desirable character for use in high compression motors. v

With the above and other objects in view which will appear as the description proceeds, my invention consists in the novel features herein set forth, illustrated in the accompanying drawings and more particularly pointed out in the appended claim.

Referring to the drawings in which numerals of like character designate similar parts throughout the several views, a

Fig. 1 is a diagrammatic view of one form of apparatus suitable for carrying out my invention.

Fig. 2 shows a modified form of apparatus.

Fig. 3 is an enlarged detail vertical sectional view of my improved vapor separator for separating the products of the initial cracking step.

Fig. 4 is a horizontal section of said vapor separator, and

Fig. 5 is a view in side elevation of a coking chamber which may be substituted for the residuum separator shown in Fig. 1.

In the drawings, referring particularly to Fig. 1, numeral l designates a pipe line leading from a suitable source of supply and in which is mounted a pump 2 of any suitable type, said pipe line I merging into two branch lines '3 and 4 provided with valves 5 and B, respectively, whereby the oil pumped through line I may be directed into either of said branch lines as hereinafter described. The branch line 3 is connected into a pipe line 1 which terminates in communication with the inlet of a heating coil 8 located in the combustion chamber of a suitable furnace 9 adapted to receive its heat by any conventional heating means such as gas burners or the like.

When cracking a heavy stock such as topped crude or residues, the coil 8 is preferably arranged so as to raise the oil to the desired temperature in a short time in order that the oil may reach a high temperature without an excessive amount of cracking taking place in the coil. However, for certain types of charging stocks which may be more severely cracked without carbon formation, for example, gas oil or distillate, the coil 8 is preferably arranged to raise the oil rapidly to a cracking temperature with thereafter a slower rise or no rise for an extended flow, in order to allow time for greater conversion to take place.

The outlet of the coil 8 is provided with a controlling valve I!) through which it is connected into a pipe line II which discharges at a point adjacent the upper end of a primary vapor separator l2, the function of which is to remove the unvaporized oil from the vapors without permitting any material amount of cracking of the liquid during the process of removal.

For operating at low pressures, this vapor separator [2 may be of any conventional type, for example, a vertically placed tank with or without bafiles in the upper portion. However, for operating at high pressures, I prefer to employ the type of separator shown in detail in Figs. 3 and 4. This structure consists in a relatively small receptacle built with heavy walls to withstand high pressure, and is provided with a tangential inlet [3 (Fig. 4) whereby the stream entering the separator near the top is directed tangentially and thus caused to assume a substantially horizontal rotation in its downward travel. This causes the liquid to be thrown against the shell of the separator by centrifugal force and run down the wall of the separator, the vapors escaping through a series of apertures 14 arranged in a centrally disposed pipe I5 which forms the vapor outlet for the separator. The bottom of the separator is preferably of substantially funnel shape and is provided with a liquid discharge opening which is connected into a pipe line l6 and controlled by a valve IT.

The vapor discharge pipe 15 is connected into a pipe line l8 which terminates in the inlet end of a second heating coil I9, the latter being disposed in the combustion chamber of. a furnace 29 adapted to receive its heat from any suitable source as in the case of furnace 9. It will be noted that the coils 8 and I9 may be advantageously placed within a suitable single furnace if desired, the same being shown in the drawings as arranged in two separate furnaces merely in the interest of simplicity.

The discharge end of the second coil I9 is provided with a control valve 2| andis connected into a pipe line 22 'which terminates in communication with the lower portion of a reaction chamber 23, the latter preferably comprising a vertical chamber of relatively large capacity and constructed to withstand high temperatures and pressures and providing a convenient means of obtaining a greater degree of cracking of the vapors from coil l9. This reaction chamber 23 is preferably equipped with a drain line 25 and a valve 24 which are used to remove water and condensate which may collect in the chamber While it is being heated up at the start of a run. A vapor line 26 leads from the upper extremity of the reaction chamber 23 to the lower portion of a secondary vapor separator or residuum separator 28, a valve 21 being provided in the vapor line to control the flow therethrough. V

The residuum separator 28 may be of any conventional type such for example as a vertically placed tank having a vapor outlet at its upper extremity leading into a vapor line 29 and a residuum draw-off in its lower extremity leading into line 30 which in turn is connected into a cooling coil 3| and line 32 which leads to storage, a valve 33 being provided in the latter line to control the flow therethrough. The line 16 from the lower extremity of the primary vapor separator I2 leads into the residuum separator 28 preferably at a point above the connection of line 26 conducting vapors to the residuum separator 28.

It is usually necessary to cool or quench the hot cracked products in the separator 28 in order to prevent cracking of the heavy residuum with the resultant carbon formation and in order to obtain a residuum sufiiciently fluid to be marketable as a fuel oil. This may be done by providing a line 34 controlled by valve 35 leading into the residuum line 16 from a suitable source of cooler oil, or a line 36 provided with valve 31 leading into the vapor line 26 anterior to its connection with the residuum separator 28. This may also be accomplished by introducing a quantity of cooled residuum or other oil directly into the separator 28 through line 38 controlled by valve 39 and connected into the upper end of the separator 28. If desired, this quenching may be effected by a combination of these methods.

The vapor line 29 leading from the upper portion of the separator 28 is provided with a control valve 49 and terminates in communication with the lower portion of a fractionating tower or dephlegmator 4| which is preferably of the bubble type, a line 42 leading from the upper extremity of said tower to a suitable condenser 43, the latter being connected by line 44 and valve 45 to a gas separator 46. This gas separator is provided at its upper extremity with a line 41 and valve 48 leading to a vapor recovery plant of other suitable point of disposition, not

shown, its'lower extremity being provided with a distillate discharge line 49 having therein a control valve 50, whereby the distillate is conyed to stora e.

The lower extremity of the fractionatingtower t] is arranged in communidation with a pipe line i having valve 52 therein, said line 5I joining the charging line 1 as clearly shown in the drawings, a suitable pump 53 being interposed in the line ill for the purpose hereinafter set forth.

In carrying out the operation of my improved process in an apparatus of the type shown in Fig. 1 and hereinbefore described, the fresh charge, which may be any type of crude oil, crude oil distillate, crude oil residue or even residue from other cracking operations, is pumped by pump 2 from any suitable supply tank, not shown, through line I with or without preheating, as desired. The pump 2 may either discharge through line 4 into the side of the fractionating tower GI or it may discharge directly into the inlet of coil 8 through lines 3 and I, the valves 5 and B being regulated to control the desired direction of flow. In some instances, it may be desirable to split the stream of fresh charge, introducing a portion thereof intoboth the fractionating tower M and the coil 8.

In starting the run, however, it is preferable to close valve 6 and open valve 5 so that pump 2 'will force the charging stock through line 3 into line I and through coil 8. After the apparatus is on-stream, the valve 5 may be closed and the valve 6 opened so that the charge is diverted into the fractionating column 4|.

The pump 53 forces the oil flowing from the fractionating tower throughline 5|, through line 'I into the inlet of the cracking coil 8, the conditions of temperature and pressure under which the oil is cracked in this coil being determined by such factors as the type of charge and the length of the coil relative to the throughput. It is desirable to obtain as much cracking in the coil 8 as possible without the formation of carbon. For example, a gas oil may be cracked to the extent that the stream leaving the coil 8 would contain as much as 30% of gasoline, whereas a heavy fuel oil charge might be limited to not over 5% gasoline. In general, topped crude, which is the preferred type of charging stock, will be cracked in the coil 8 to the extent of producing between 5% and ofgasoline in the stream at the outlet of coil 8. I have found that the pressures carried on the coil 8 may runfrom approximately 100 pounds per square inch to in excess of 1000 pounds per square inch, and that the temperatures may range from 750 F. to 1000 F. Very satisfactory resultsmay be obtained with an outlet temperature and pressure of 880 F. to 950 F. and 400 to 500 pounds per square inch, respectively.

The initially cracked products leave the coil 8 through line II which, by means of valve III, may be throttled to cause a reduction of pressure and enter the primary vapor separator I2 which may be operated at pressures of from 50 to 500 pounds per square inch and at a temperature of from 800 F. to 925 F. In this separator the vapors produced as a result of the initial heating and cracking in coil 8, are separated from the unvaporized oil, the greater portion of such vapors passing out of the separator I2 through the line IS controlled by valve Him, and entering the vapor heating coil I9 which forms the second step of my process. The unvaporized oil passes out of the separator I2 through line I8 and enters the residuum separator 28, the rate of withdrawal of such unvaporized oil being regulated by valve II.

It is essential that the operation of the vapor separator I2 be so conducted that nomaterial cracking of the liquid occurs therein, as cracking of this dirty oil would naturally produce carbon which would result in the clogging of the separator and the line. leading therefrom. Hence, in order to prevent this cracking, I have found that it is very desirable to withdraw a portion of the vapors from the separator I2 together with the liquid through'valve I I and line I6 leading to the residuum separator 28. This vapor tends to sweep out the liquid as rapidly as it falls to the bottom of the vapor separator I2, and thus prevents the liquid from remaining in the separator asuificient length of time to crack therein.

I have also found that in some instances, for example when charging a very light stock, it may be advantageous to introduce a cooler oil into the vapor'separator I2 through line I2a in order to reduce the vaporization in the separator. This may also be accomplished by introducing such cooler oil into the line II leading from the coil 8,

if desired.

The vapors leaving the separator l2 pass through'line I8 and valve l8a into the vapor heating coil I9 wherein they are heated to a higher cracking temperature preferably of from 900 F. to 1200 F. These additionally heated.

vapors pass out of the coil I9 through the line 22 and valve 2| and enter the reaction chamber 23 wherein a greater degree of cracking is obtained. Any suitable pressure less than that in the vapor separator I2 may be maintained in the reaction chamber 23. The cracked vapors leave the reaction chamber 23 through line 26 and valve 21 and are introduced into the lower portion of the residuum separator 28 where they are remixed with the liquid withdrawn from the primary separator I2. Here again the pressure in the residuum separator 28 may be'any amount less than that in the reaction chamber 23, for obvious reasons.

This remixing of the cracked vapors and the heavy liquid serves a'two-fold purpose. The liquid from the vapor separator I2 contains certain fractions which should be recycled for further cracking as well as residuum which should be removed, and the intimate mixing of the hot vapors from the reaction chamber 23 with this liquid serves to strip the lighter constituentsfrom the residuum so that they may be carried on through the remainder of the process and eventually recycled as hereinafter set forth. The additionally cracked vapors on the other hand, contain certain heavy cracked tars which should be removed and withdrawn with the residue. The intimate contact of the vapors with the liquid from the vapor separator serves to wash these tars out of the vapors very eiTectively. Thus, it will be seen that the remixing is very advantageous both to the additionally cracked vapors from coil I9 and to the liquid material from the bottomof the vapor separator l2. 1 The residuum is drawn off from the separator 28 through line'38 and the cooler 3 I, from whence it is run to storage through line 32. The vapors, composed of the desired product, fixed gases and any insufiiciently cracked hydrocarbons, pass out of the residuum separator 28 through line 28 and valve 40 into the lower portion of the fractionating tower 4| which is regulated so as to permit the desired product and permanent gas to pass overhead through line 42, condenser and line 44 into the gas separator 46. In most instances, it is necessary to cool the hot cracked products in the separator 28 in order to prevent cracking of the heavy residuum with the resultant carbon formation and in order to obtain a residuum sufficiently fluid to be marketable as fuel oil. As before stated, this may be done by introducing a cooler oil, such for example as some of the cooled residuum either directly into the separator 28 through line 38 and valve 39, or into the line 26 through line 36 and valve 31, or into the line l6 through line 34 and valve 35, or into a combination of these lines.

The insufficiently cracked vapors entering the fractionating tower 4| are collected as a reflux condensate in the bottom of the tower, from whence they are pumped by pump 53 and recirculated through coil 8 together with the fresh charging stock entering the tower 4| through line '4. The regulation of fractionating towers for obtaining these results is believed to be so well known to those skilled in the art as not to require a detail description herein.

The permanent gases separate from the cracked distillate in the gas separator 43 and pass out through line 41 and valve 48 to a vapor recovery plant or other suitable point of disposition, not shown, the distillate from said separator being discharged through line 49 and valve 50 to a suitable storage receptacle.

In the form of apparatus just described, the coil I9 is preferably arranged to effect a rapid heating of the vapors, the reaction chamber 23 permitting a continued and greater degree of cracking of such vapors. However, in some instances it may be desirable to eliminate the reaction chamber 23 and rely solely upon the coil IQ for the desired cracking.

Fig. 2 shows an embodiment of my invention modified in accordance with this operation. In this figure, it will be noted that the coil 19 is considerably longer than the coil l9 and is preferably arranged to provide a section for the rapid heating of th vapors to a point near the desired cracking temperature, followed by a section in which the temperature of the vapors is raised more slowly or in which the vapor temperature remains relatively constant. In this way the time factor necessary for the desired cracking may be obtained without the use of the reaction chamber 23, the discharge line 22' leading from the coil l9 directly into the residuum separator 28.

The operation of the apparatus shown in Fig. 2 is otherwise the same as that heretofore described.

In Fig. 5 I have shown diagrammatically an apparatus which may be used in place of the residuum separator 28 when it is desired to operate the process so as to continuously convert the residuum into coke with the resultant increased yield of gasoline from the charging stock. In this figure the coking chamber 28' is substituted for the residuum separator 28 and preferably consists of a heavily insulated horizontal tankwhich may, if desired, be set in a furnace 28" for additional heating. As a further alternative, this chamber 28' may be composed of refractory material and arranged to facilitate external heating. Regardless of the type of coking chamber employed, in actual practice it is advisable to use a plurality of chambers in order to provide a continuous process so that coke may be removed from onewhile another is being utilized for coking.

This coking chamber 28 may be readily installed in connection with the apparatus described with reference to Fig. l by simply connecting the pipe line Hi from the bottom of the vapor separator l2 and the'pipe line 26 from the reaction chamber 23 into the upper portion of the coking chamber 28, the volatile constituents of the mixture being removed from the coking chamber through line 29 leading to the fractionating tower 4|. Otherwise, the operation of the process is the same as that heretofore described. The hot vapors from the vapor cracking step will usually supply sufficient heat to the residuum from the vapor separator l2 so that it will be reduced to coke and vapors without the addition of external heat, but when a very dry coke is desired, the coke chambers may be additionally heated. The

process is particularly well adapted to the production of dry coke by this method because of the fact that the high heat content of the combined materials entering the coke chambers causes a very high degree of vaporization, and thus there is a relatively small amount of volatil material remaining in the residue which falls to the bottom of the chamber. In any coking operation in which heat is supplied externally, one of the principal difficulties encountered is to induce the necessary heat to penetrate the coke bed because the coke initially formed on the heating surfaces acts as an insulator against the transmission of heat. Hence the advantage of a process which requires the addition of only a small amount of external heat as in the present instance is apparent.

Obviously, the size of the various elements of the apparatus will be determined by a variety of factors such as the throughput capacity, the typeof charging stock, the degree of cracking desired in the various parts of the apparatus and the type of product desired. Without in any way limiting the scope of my invention to the particular sizes hereinafter referred to, the elements may be as follows for a fairly large commercial unit.

The coil 8 may be from 3000 to 4000 feet in length and composed of tubing approximately three inches inside diameter; the coil I9 may be from approximately 1000 to 2000 feet in length and composed of tubing about three and one half inches inside diameter. As before stated. both of these coils may be arranged in a single furnace, if desired. The primary vapor separator l2 may be about three feet in diameter and about eight feet in height; the reaction chamber 23 may be approximately five feet in diameter by forty feet in height; the residuum separator 28 may be about eight feet in diameter by forty feet in height; the fractionating tower 4| may be about eight feet in diameter by sixty feet in height, and the coking chambers 28' if employed instead of the residuum separator 28. may be of any convenient capacity to suit conditions.

The pipes connecting the various elements are all provided with valves which are used to regulate pressures, control rates and direction of flow and liquid levels, or to completely out off certain elements when the unit is shut down.

Among the advantages of operating a cracking process under high pressures are large capacity in an apparatus of any size, high heat economy, lower production of fixed gases, and a product more easily treated to produce a finished gasoline, all of which advantages will be readily recognized by those skilled in the art. Likewise, the advantages of cracking in the vapor phase which are, among others, the attainment of a high degree of conversion without carbon formation, and the production of a gasoline of high antiknock quality, are apparent. By my improved process and apparatus, I combine these advantages by conducting a vapor phase cracking operation under high pressure, and furthermore, I utilize as charging stock any oil which may be available, without incurring diificulty from carbon formation.

500 pounds per square inch and the pressure on the reaction chamber 23 need be only enough lower than that in the vapor separator to force the vapors through the vapor heating coil I!) at high velocity. For example, the vapor separator I2 may be operated under a pressure of 300 pounds and at a temperature of from 875 F. to 900 F., and th reaction chamber 23 operated under a pressure of 200 pounds, said latter pressure being regulated by throttling the valve 2'! in line 26.

One of the principal advantages of my process lies in the fact that all of the charging stock is subjected to an initial cracking step, which, however, is stopped before the dirtier constituents of the charge have reached the stage of carbon formation, and that these dirtier constituents are then removed to prevent further cracking, while the relatively clean constituents comprising the vaporsare subjected to additional cracking in the coil Hi. In the initial cracking step in coil 8, many of the heavy, relatively non-volatile hydrocarbons are converted into more volatile hydrocarbons which are then cracked into the final desired product in the second cracking step in coil l9. It will be noted that if the oil were not subjected to the initial cracking step, these more volatile hydrocarbons would be lost in the residue.

Another special feature of my process resides in the remixing of the material withdrawn from the bottom of the vapor separator l2 with the vapors which have been additionally cracked in coil l9. As has been previously pointed out, this remixing serves a two-fold purpose, namely, the stripping of the lighter constituents from the residuum and the washing of the heavy cracked tars out of the vapors.

As an illustration of the results to be obtained from this process, I have operated a unit charging between 800 and 1000 barrels daily of 40 A. P. I. gravity Texas crude. The still yielded to of gasoline of approximately 60 A. P. I. gravity and having an octane number of 70; 20% to 25% of about 10 A. P. I. gravity fuel oil having less than 2% of sediment by centrifuge test; and about 10% of gas and loss. This still was equipped with the low pressure type of vapor separator.

The crude oil was fed into a bubble tower fractionator where it was stripped of its gasoline content by the hot vapors from the residuum separator and mixed with the reflux condensate. The

Thus the vapor sep-. arator l2 may be operated at pressures of 50 to the residuum separator.

said liquid.

mixture of topped crude and reflux condensate was picked up from the bottom of the bubble tower by a pump and discharged through the primary cracking coil where it was heated to a temperature of 850 F. to 900 F. under an outlet pressure to 100 pounds per square inch regulated by a valve in the outlet transfer line and was discharged into the vapor separator. The pressure in the vapor separator was about '75 to pounds per square inch and the temperature about 825 F. to 850 F, Vapors passed out of the top of the separator .into a vapor cracking coil wherein they were heated to a temperature of 1020 F. to 1060 F. and passed thence into a reaction chamber which was maintained at a pressure of about 35 pounds and a temperature of approximately 900 F. to 950 F. The vapors then passed from the reaction chamber through a valved transfer line into the residuum separator.

Mixed liquid and vapor passed out of the bottom of the vapor separator through a valve which was trottled to regulate the rate of the withdrawal and thence through a line leading into Residuum was withdrawn from the bottom of the residuum separator through a cooling coil and delivered to storage. Some of the cooled residuum was sprayed back into the residuum separator to regulate the temperature therein, which was maintained at from 650 F. to 725 F. The vapors passed out of the top of the residuum separator through a line leading into the fractionator. Gasoline vapors and gas were withdrawn from the top of the fractionator, passed through a condenser into a gas separator from which the gas discharged to a vapor recovery system and v the details of construction and operation with-- out departing from the spirit of the invention as set out in the following claim.

What I claim and desire to secure by Letters Patent is:

A process of treating hydrocarbon charging stock comprising initially cracking the entire charge in a stream while flowing through a zone of restricted cross section, introducing the same tangentially near the top of a separating zone whereby the vapors are .separated fromthe liquids by centrifugal force, avoiding further cracking of liquids by withdrawing the liquid from said separating zone at such a rate that substantially no accumulation of liquid occurs therein, additionally cracking said vapors, and re-mixing the additionally cracked vapors with HOWARD V, SMITH.

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