US2319926A - Cracking of light petroleum stocks - Google Patents

Description

W. 'r. HANCOCK- Filed NOV. 27, 1959 CRACKING OF LIGHT PETROLEUM STOCKS May 25, 1943.

Patented May 25, 1943 UNITED STATES PATENT OFFICE CRAOKING F LIGHT PETROLEUM STOCKS Application November 27, 1939, Serial No. 306,345

2 Claims.

This invention relates to the rening of petroleum hydrocarbons, and has for its primary object to provide a relatively simple system where-A by it is possible to obtain in essentially a single rening sequence or operation, abnormally high yields of cracked gasoline of high anti-knock lvalue, and possessed of water white stability without the necessity of treatment other than that given the hydrocarbons in this same refining operation. More particularly, the present invention has to do with the cracking and rening of relatively low boiling charging stocks, such as those coming within the kerosene distillate and gas oil range, which upon liquid phase cracking, pro duce comparatively little amounts of free carbon and tarry bodies. In a copending application, Ser. No. 306,346, entitled System for cracking and polymerizing hydrocarbons, led on even date herewith, I describe a similar system and process that is more particularly adapted to the rening of heavier cracking stocks such as fuel oil and crudes that require elimination from an early stage in the system of carbon and heavy hydrocarbons formed in the cracking stage.

The principal novelty and importance in the present method is the use, in conjunction with the cracking system, of divided siliceous or adsorptive material to accomplish three main objectives: (l) cracking or dissociation of the charging stock to produce maximum percentages of actual or potential gasoline constituents, (2) conversion by polymerizing of low boiling cracked hydrocarbons into gasoline fractions with minimum uncondensible or fixed gases remaining, and (3) simultaneous removal from the distillate of gum and color forming compounds so that the product gasoline will have stable water white color. The present system is further characterized with respect to its operation, in that it requires the maintenance of considerably lower cracking temperatures and pressures than the usual plants designed to produce cracked gasoline from lower boiling charging stocks of the type herein contemplated.

In accordance with the invention, the charging stock is subjected to cracking in liquid phase or mixed liquid and vapor phase, and the cracked hydrocarbons are discharged directly from the cracking zone into what may be termed a reaction or polymerizing zone containing preferably a stationary, vertically extending body of siliceous material adapted to catalyze the reactions involved in the conversion and reformation of the cracked hydrocarbons. Although broadly speaking, divided siliceous materials of any suitable character and composition may be employed, Y

I prefer to use adsorptive materials or clays such as bauxite, fullers earth, Death Valley or Muroc clays, and the like, which if desired, may have been prepared or activated, as by treatment with an electrolyte or any of the various methods known to those familiar with the art. Mixed liquid and vapor hydrocarbons are maintained in the reaction zone under conditions such that the mixture is subjected to intimate contact and agitation within the body of adsorptive material, to the end that equilibrium conditions between the liquid and vapor phases may be reached or approached in the presence of the adsorptive material, with resultant maximum conversion of heavier than gasoline fractions, and maximum reformation, by polymerization, of low boiling -fractions lighter than gasoline. By thus intimately contacting and washing the adsorptive material with the hydrocarbon mixture, it is possible to so effectively remove color-producing and gum-forming compounds from the vapors as to obviate necessity for further treatment of the vapors or product condensate to stabilize the color or remove gums.

To assure proper intimacy of contact between the mixed liquid and vapor hydrocarbonswith the adsorptive material in the reaction zone, a body of the liquid hydrocarbons may be maintained within the lower portion of the clay and the cracked hydrocarbon stream from the cracking zone introduced directly into this liquid body. By this method the vapors are caused to pass upwardly through the clay and carry with them the liquid hydrocarbons, all in a manner such that the latter remain continuously in a state of surging and washing agitation within the clay. Such intimate and repeated washing of the clay with the liquid and vapor hydrocarbon mixture provides an effective condition and medium for maximum polymerization and removal of gumforming compounds fromthe vapor stream. The unvaporized polymer-containing residue may be removed from the system and taken to storage or disposal, or a portion of the residue may be subjected to recracking and the resulting cracked hydrocarbons returned to the reaction zone. Where the latter method is employed, that portion of the unvaporized residue to again be subjected to cracking preferably is withdrawn from the reaction zone at an intermediate location in the liquid body, in order that the more heavy carbon and polymer contaminated oil may be withdrawn from the bottom of the liquid body.

The vapors leaving the reaction zone may be specifically described, such condensate may be recirculated to the cracking zone in which the charging stock is undergoing cracking, and a mixture of the cracked charging stock and recirculated condensate discharged into the reaction zone. As will appear, cracking of both charging stock and recirculated condensate may, though not necessarily, occur inthe same cracking coil in view of the basic similaritycof vthe two oils.

The cracking and polymerizing stages of the process are maintained under suitable pressure, say in the neighborhood of 100 lbs. per sq. in.,

which however is considerably less than the pressures ordinarily employed and required for the cracking of light stocks ofthe type herein con-y templated. This feature is of vimportance iroin both plant construction and operation standpoints, in thatvit avoids the necessity for having to design and build equipment for high temperature and high pressure operation. Yet I am ablerto produce gasoline yields higher in quality and quantity Ythan those obtainable by the usual systems, including those designed for cracking athigher temperatures and pressures. In actual plant operation, the present system has produced yields of unusually high octane gasoline'running in the neighborhood 'of around 90% of the charging stock. The ability to obtain such yields at the relatively low temperatures and pressures employed, appears to be due largely to the conditions obtaining in the reaction zone which, as observed above, tend to procal and physical properties of the clay. Under proper temperature and pressure conditions, the silicates present in the clay have, by reason of their chemical constituencies, a catalytic eiect in inducing cracking. On the other hand, polymerization results at least largely from the known physical characteristics of the clay, i. e., its porous and extended surface form giving the clay adsorptive properties and quality of a polymerizingcatalyst. Y y

The invention will be understood to better advantage from the following detailed description of a typical and illustrative system for cracking and refining relatively light charging stocks in accordance with the general method as outlined in the foregoing. In the description, reference is had to the accompanying drawing illustrating the process diagrammatically and in flow sheet form. K

Referring to the drawing, a kerosene distillate or gas oil charging stock, having for example a boiling range between 417 F. and '750 F., is discharged by pump I from a suitable supply source such as tankII, through line I2 to the cracking still I3. The latter is shown typically and conventionally to comprise a preheating coil I4 from which the charging stock ows to the crackin-gcoil I5 in which the oil is heated to a proper cracking temperature, ordinarily under 900 F., and which for a charging stock of the particular boiling range named above, preferably is in the neighborhood of 800 F. The cracked hydrocarbons are discharged from still I3 through line I6 into reaction zone Il Within a vertically extending shell I8 containing the divided siliceous material or adsorptive clay I9.

As illustrated, the hydrocarbon stream from the cracking still is discharged into an intermediate location within the clay ybody I9 so that the vapors tend to ilow upwardly through the clay with the unvaporized hydrocarbons remaining within the lower portion of the clay. Provi- "sion preferably is made for maintaining the liquid hydrocarbons at a minimum level L-L' above the entry of the cracked hydrocarbons at 20, so that in rising within the chamber, the vapors tend to agitate and displace the liquid upwardly within the body of clay and in this manner maintain an intimate mixture of both liquid and vapors in thorough and extended contact with the clay. The liquid level may be suitably maintained as by a valve 2| in the outlet line 22, operated by a connection 23 with a liquid level responsive control diagrammatically indicated at 24.

As previously stated, the clay chamber has a dual function in that it serves both to increase the yield of recoverable gasoline by the catalytic effect of the clay on the cracking reaction and polymerization of low boiling fractions, and also by such polymerization to remove from the vapors gum-forming compounds heavier than gasoline. The heavy polymers and residue or carbon present in the oil settle to the bottom of the reaction chamber where they may be intermittently or continuously drawn on, in accordance with their rate of formation, through line 25 under control of a suitable valve 26. If desired, line 25 may be employed as the sole drawoiT line to storage of liquid hydrocarbons from the reaction zone, in which event this unvapor-4 ized residue and the contained polymers are removed from the system. Alternatively, a portion of the.condensate may be drawn oi through line 22 from a higher location in the liquid body (leaving the heavier lpolymers to be discharged at the bottom through line 25), and sent to storage via line 21, or all or a portion of the oil in line 22 may be recirculated through lines 28 and 29 to the cracking coil I5.

The clay treated vapors leaving the reaction zone through line 30 are subjected to partial condensation and fractionation in a dephlegmator or fractionating column 3|, wherein the heavier vapor fractions having boiling temperatures extending through the kerosene distillate range are condensed. The vapors pass from the fractionating column through line 32 to a condenser 33 from which the product gasoline condensate is taken through line 34 to receiver 35. Fixed and uncondensible vapors are discharged from the system through line 36. Reflux condensate formed in fractionating column 3| is withdrawn through line 31 and again subjected to cracking, as by returning the condensate through line 29 to be admixed with the charging stock undergoing cracking in coil I5. The cracked condensate thence is discharged together with the cracked charging stock stream through line I6 into the reaction zone. The cracking still and reaction zone may be maintained under proper pressure by a suitably located valve, as for example valve 38 in the vapor line 32 beyond the fractionating column. As previously mentioned, the pressure on the cracking still and reaction zone ordinarily need not exceed around 100 lbs. per sq. in., although if desired, higher pressure may be employed. In cases where it is desired to subject the vapors to fractionation under less pressure, the required back pressure on the cracking still and clay chamber may be maintained by valve 40 in line 30 and fractionating column 3| then operated under suitable reduced pressure.

To give an example of the proportions of the clay column and the quantity of clay that may be used to give satisfactory results, I have used a six foot diameter shell 40 ft. high containing a bed of clay approximately 30 ft. in depth. Normal heat losses from the shell, or such losses together with the cooling effect of the condensate into which the vapors are introduced, produce considerable condensation of higher boiling fractions of the vapors. The liquid level control was positioned to maintain a depth of about 8 to 9 ft. of unvaporized residue and condensate in the base of the column and the cracked hydrocarbon stream was introduced to this liquid body at around 4 to 5 ft. below the minimum level maintained by the liquid level control. The rate of through-put averaged around 500 barrels cracking stock per 24 hours.

In operation, best results both with respect to percentage conversion and polymerization, were obtained by maintaining, typically, a maximum temperature of 885 F. to 900 F. at the vapor outlet of the clay column. When operating at considerably higher vapor outlet temperatures, the percentage of xed and uncondensible gases increase to the extent of materially reducing the gasoline yield. On the other hand, it is found that so long as a maximum temperature of about 900 F. is maintained at the top of the clay column, it is possible to increase considerably the cracking temperature and still obtain an exceptionally high yield of condensible distillate, and also that by increasing the cracking temperatures, the anti-knock properties of the distillate may be benefited substantially, Thus, operating on a gas oil charging stock, it may be desirable, particularly after the clay has lost some of its initial activity and polymerizing capacity, to increase the cracking temperature of the oil and thereby maintain the anti-knock rating of the product gasoline at a high value. Under such conditions, the cracking temperature may be raised as high as around 1100 F. to 1l75 F., with or without an increase of pressure, While the temperature at the top of the clay column is maintained, as stated above, at a maximum of around 900 F. In this same connection, during a continuous operation using the same body of clay, it may be desirable to progressively increase the cracking temperature as the efliciency of the clay decreases. As previously explained, however, the conditions, and particularly the washing action of the condensate, within the clay tower, are conducive to materially prolonging the life and sustained efficiency of the clay, as has been determined in plant operations where it has been found possible to use the same quantity of clay over a long period of time.

I claim:

1. The method of rening petroleum oil included within substantially the kerosene distillate-gas oil range, that comprises passing a stream of the oil under super-atmospheric pressure through a cracking zone wherein the oil is heated to cracking temperature, passing the cracked hydrocarbons directly into a reaction zone containing a body of divided adsorptive material and within which the hydrocarbons undergo cracking and also polymerization, maintaining a body of the hydrocarbons in liquid phase within the lower portion of said body of adsorptive material, subjecting mixed liquid and vapor fractions of the hydrocarbons to intimate contact and agitation within an extended height of the material above said body of liquid hydrocarbons by introducing said hydrocarbons from the cracking zone directly into the body of liquid hydrocarbons so that the liquid fractions are upwardly displaced by the vapors to constantly wash the material in the presence of vapors undergoing conversion and constantly remove polymers being formed, removing the vapors from said reaction zone and condensing them to produce gasoline, and separately withdrawing from said body of liquid hydrocarbons, unvaporized residual oil and its contained polymers.

2. The method of refining petroleum oil included within substantially the kerosene distillate-gas oil range, that comprises passingastream of the oil under superatmospheric .pressure through a cracking zone wherein the oil is heated to cracking temperature, passing the cracked hydrocarbons directly into a reaction Zone containing a body of divided adsorptive material and within which the hydrocarbons undergo cracking and also polymerization, maintaining a body of the hydrocarbons in liquid phase within the lower portion of said body of adsorptive material, subjecting mixed liquid and vapor fractions of the hydrocarbons to intimate contact and agitation within an extended height of the material above said body of liquid hydrocarbons by introducing said hydrocarbons from the cracking zone directly into the body of liquid hydrocarbons so that the liquid fractions are upwardly displaced by the vapors to constantly wash the material in the presence of vapors undergoing conversion and constantly remove polymers being formed, removing the vapors from said reaction zone and condensing them to produce gasoline, separately withdrawing from said body of liquid hydrocarbons, unvaporized residual oil and its contained polymers, maintaining within the reaction zone a temperature at which the hydrocarbons undergo cracking in the presence of said material but sufficiently low that the temperature of the vapors leaving said body of material does not exceed about 900 F.

WILLIAM T. HANCOCK.

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