US2529790A - Thermal and catalytic cracking of hydrocarbons - Google Patents

Description

Nov. 14, 1950 P. M. wADDlLL THERMAL AND CATALYTIC CRACKING oF HYnRocARBoNs Filed Dec. 50, 1947 A 7' TORNEVS Patented Nov. 14, 1950 THERMAL AND CATALYTIC CRACKING F HYDROCARBON S Paul M. Waddill, Bartlesville, Okla., assignor to Phillips Yetroleum Company, a corporation of Delaware Application December 30, 1947, Serial No. 794,555

3 Claims.

This invention relates to the art of hydrocarbon conversion and to the production of gasoline from hydrocarbons of a relatively wide boiling range. In a particular embodiment, it relates to the production of gasoline from such hydrocarbons by a novel process comprising a combinationof thermal and catalytic cracking. In one of its specific aspects, it involves the conversion of wide boiling range hydrocarbons by a novel process involving thermal cracking in combination with low temperature and high temperature catalytic cracking steps.

Many cracking methods are known and a wide range of cracking conditions has been disclosed. It has been desirable to improve the quality and yield of gasoline produced from various hydrocarbon stocks and it has been found that relatively specic conditions are usually best suited to different and particular feed stocks,'that is, a particular feed stock of a relatively narrow boiling range responds most advantageously to a particular set of conditions of temperature and pressure of a fairly narrow range and with certain particular catalytic agents. It has also been found that two or more of the particular feed stocks may be combined in conversion processes such as cracking, and slightly different conditions of processing applied thereto to prepare satisfactory gasoline. It has also been proposed to individually treat hydrocarbon stocks of different boiling range to cracking conditions particularly suited thereto and to combine the effluents in a common separation with further treatment of individual fractions thereof. Thus, it has been proposed to thermally crack a relatively heavy hydrocarbon stock and to thermally reform a lighter hydrocarbon stock and to combine the effluents in a common separating zone, thereafter separating the combined eiiluent into at least three fractions of different boiling range which are individually further treated. These processes have been attended by various disadvantages such as the necessity for extra equipment and handling in the former case and the diiculties of control and the limitation of conditions for the maximum recovery of desirable products in the latter type of process.

By the present invention, a maximum yield of gasoline is obtained with a minimum yield'of fuel oil by cracking heavier hydrocarbon stocks such as topped crude, and is favored by a condition of high dilution in the cracking Zone, lSuch dilution may be furnished by a suitable amount of an inert gas, such as steam or of lighter hydrocarbons, such as the xed hydrocarbon. gases Cil and/or vaporized hydrocarbon fractions of thej naphtha or gas oil boiling range. In employing hydrocarbons as the diluent, certain advantages are realized since at least a portion of the diluent may be converted into gasoline. However, recovery of the maximum gasoline yield from a heavy hydrocarbon stock containing gas oil as the diluent is normally limited inasmuch as conditions of temperature, pressure, contact time, etc., for recovering the maximum yield of gasoline from the heavier stocks -without excessive destruction and coke formation are usually less severe than the conditions necessary to produce gasoline from gas oil and hence, when the usual methods and conditions 4for conversion of mixtures of hydrocarbon of different boiling range or. of wide boiling range are employed, the gas oil and lighter fractions are converted only in an amount such that the gasoline potentially available from these fractions Ais not produced. By the present invention, it is. possible to realize maximum yields from each of the fractions andI the hydrocarbon feed stock while realizing the advantages of having the lighter hydrocarbons admixed with the heavier oil as a diluent therefor during the cracking of the heavy oil. Needed heavy oil dilution can be further augmented by the addition of increased quantities of virgin gas oil, recycle cracked gasoline and other diluent injections.

Broadly speaking, the invention comprises thermally cracking a heavier hydrocarbon oil of relatively Wide boiling range, such as a hydrocarbon mixture containing naphtha and gas oil, or a topped crude, in combination with relatively lighter hydrocarbons, separating in the presence of heavy reflux oil the thermally cracked product into an unvaporized residue comprising fuel oil and into a lighter vaporized fraction comprising gases, gasoline, and intermediate boiling hydrocarbons heavier than gasoline, further heating the vaporized fraction in the presence of added superheated steam or low endpoint straight run hydrocarbon vapors, catalytically cracking said heated vapor fraction at relatively low temperatures, fractionating the low temperature catalytically cracked effluent into normal gases, a gasoline fraction, a heavy fraction which may be recycled to the thermal cracking zone and an intermediate fraction, at least a portion of which may be passed to a relatively high temperature catalytic cracking zone from which the cracked efliuent is returned for fractionation -with the effluent from the low temperature catalytic cracking zone, and any remainder of which' can be recycled as the reux oil to said first separation. A lighter portion of the unvaporized residue from the separation following the thermal cracking may be recovered in a flash separation and returned to the thermal cracking with the charge stock as a part or all of the lighter hydrocarbons present therein. The heavier portion of the ash separation may be vacuum distilled to separate from the residual fuel oil any entrained gas oil which may then be passed to fractionation with the cracked eiiluents and thereby recycled to the process.

In operating according to the present invention, the maximum conversionand recovery of gasoline are obtained both from the heavy hydrocarbon oils and from the naphthas and gas oils concurrently employed therewith as diluent, and with the most effective recovery and recycle of all fractions potentially convertible to gasoline, while operatingV with the desired minimum production of. fuel. oil. By employing a flashV separation on the unvaporized fuel oil, material is recoveredand returned to the process which is usually lost by the removal of unvaporizedfractions.. Furthermore, all of the vaporizedy hydrocarbons following. the initial heating and thermal cracking stepare subjected to subsequent treatment in the process, Thus, unconverted material and lighter products are progressively subjected to increasingly more severe l.

conversion conditions while having always present during any specific conversion step lighter hydrocarbon fractions which are relatively refractory under the operating conditions of that conversion step and which serve as diluent to the hydrocarbon undergoingI conversion in that step. However, although somewhat refractory and usefull asa diluent in any particular step, a portion of these lighter hydrocarbons undergoes transformations to desirable products and/ or to products which are more readily converted to desirable products in subsequent steps of the process. For example, during the thermal. crackingstep diluent lighter hydrocarbons such as gas oilv and/or' naphtha. are thermally reformed while in admixture with heavy hydrocarbon oils which are being thermally cracked. During the subsequent catalytic cracking of. the thermally cracked products, diluent gas oil and naphtha fractions are catalytically cracked andl reformed extensively. In addition, thermally cracked gases and gasoline from the thermal cracking step are also present during the catalytic crackingsteps for conversion and further improvement of their properties. of operation and separation4 whereby hydrocarbon fractions present or produced in the process are given the most effective treatment for the production of a maximum yield'of valuable gasoline stock.

An unusually flexible process is formulated by operating according to the present invention. The process is eojlally applicable to feed stocks of wide or narrow boiling range and to stocks containing intermediate or extremely heavy hydrocarbons. The conditions of conversion may be easily varied, as more-completely described below, to be most effective for converting the feed stock being treated at each particular step of the process. Additional diluents may be added along the course of the process or the major portion introduced at the first. During successive conversion stages, relatively light products are formed which comprise at least a part of the diluent particularly desirable in subsequent steps. Fewer fifactionating steps and separation of the mate- Thus, there is a unity r rial into a smaller number of readily recovered fractions whose end-points may be less closely taken than usual may be realized by the practice of the present process.

A better understanding of the invention may be had in connection with the attached drawing in which a mixed hydrocarbon feed of relatively wide boiling range, such as a topped crude, is introduced through a line I0 into an absorber II where it is admixed with an overhead fraction from a flash tower I2 entering the absorber II through a line I3. Additional diluent hydrocarbons or steam may be introduced through either or both lines 13a and I4a as required or desired. The mixture of heavier feed stock and light overhead ispassed by a line I4 through a heater I5 in which the mixture is substantially vaporized and mildly cracked. The eiiluent from the heater is passed to a separator I9 for removal of any unvaporized material. If desirable, the absorber may be by-passed by all or any portion of the feed through a line I9 and passed directly to line i4. Oil from a subsequent fractionation is introduced into the top of separator IS by a line Il as reflux, and unvaporized hydrocarbons are withdrawn from the bottom of separator I5 through a line I3. Liquid material in line I8 is passed to flash tower I2 wherein entrained light materials are taken overhead to the absorber as described and' heavier material comprising fuel oil is withdrawn through a line 2U to a vacuum still 2| in which further separation is made of the fuel oil. Residual fuel oil is removed by a line 22 and any gas oil which may be present is recovered and passed by a line 23 to subsequent.

fractionation. Excess vapors or gases which are introduced through line I3 into the absorber or present in the original feed and which are unabsorbed by the feed stock may be removed. from the absorber through a line 25, if desirable. Gases, gasoline, and intermediate hydrocarbon fractions are removedV overhead as vapor from separator I5 and passed through a heater 21 in a line 28 to a low temperature catalytic cracking system 29. SteamY or low end-point straight run hydrocarbon vapors may be introduced by a line 39 and passed with the vapor fraction through the heater 2 into the catalytic cracking system Z9 under conditions described hereinafter.

The catalytically cracked hydrocarbon eiuent which emerges from the catalyst zone 29 through a line 32 is quenched with cooler oil or with water from a line 33 before passing through a heat exchanger 34 and a line 35 into a fractionating system 36 from which a lighter fraction comprising gasoline and lighter compounds is taken overhead through a cooler 31 in a line 3B to an accumulator 39. Fixed gases are removed from the accumulator through a line 40 and the gasoline which separates from any condensed water is recovered through a line 4 I. Condensed water, if present, is withdrawn from the accumulator through a line 42 and reflux to the fractionator may be recycled through a line 43. An intermediate side stream is withdrawn from fractionator 3E through line I1, a portion of which side stream is recycled as reux to the hot oil separator I9 and the remainder of which is passed through a line 44 to storage or through a line 45 and a heater 46 to a high temperature catalytic cracking system 41 in which the hydrocarbon vapors are further cracked as described below and returned through a cooler 4B and through lines 49 and 35 to the fractionator 3G for separation with the eluent from the catalyst zone 29. Superheated ste-am or low end-point straight run hydrocarbon vapors may be introduced by av line" fractionator 36 through a line 5I and is passed.

into a stripper 52. Steam for the stripping may be introduced into the stripper through a line 53 and a lighter hydrocarbon fraction removed overhead through a condenser 55 in line 54 and into an accumulator 56 from which condensed water is removed through a line 5l. Liquid hydrocarbon is withdrawn from the accumulator through a line 58 to furnish all or a portion of the quenching oil in line 33. If water is employed as the quenching medium and is present in the heavy residue in line 5 l it may be removed in the stripper and returned directly to the quench line through lines 59 and 58 after condensation in condenser 55, by-passing,T the accumulator. Any water recovered from the overhead of the fractionator may be returned to the quench line by means not shown, if desired. The heavy residue from the stripping zone may be recycled to thermal cracking in heater l5, if desired. Various valves, pumps and other conventional equipment essential to the operation of the process will be apparent to those skilled in the art and have been omitted from the drawing and discussion for sake of clarity.

In the discussion of the invention broadly, diluent vapors have been described as steam or other inert gases, or as low end-point hydrocarbon vapors which particularly refers to low octane naphthas or light gas oils as well as other hydrocarbons which are normally liquid but are readily volatile. It is not suggested that the value of each of these alternative diluents is equal since it has been found that best effects in the resulting products are realized when low endpoint hydrocarbon vapors are employed. The preferred manner of operation is then to employ low end-point hydrocarbon vapors for diluent in an amount such that deeper conversion of the heavier fraction may be effected with less coke formation. Such diluent may be heated, if desired, to temperatures above the conversion temperature in the particular step to which it is added and thereby furnish at least a part of the required heat to promote the conversion. In addition to any inherent production of hydrocarbons boiling in the gasoline range by the partial conversion of such diluents, it has further been found that, even when conditions may be too mild to promote conversion of the diluent, the presence of low end-point hydrocarbon vapors, such as low octane number straight run'naphthas, does promote the production from gas oil of gasoline having a higher octane value for blending with other gasoline stocks in the preparation of a finished gasoline than the product obtained when using other diluents, such as steam. The presence of such diluents permits a more complete cracking of the heavier stock with less coke formation and promotes the vaporization of more of the heavier material in the feed stocks.

Satisfactory conversion conditions in each respective zone of the process will of course depend somewhat upon the type and refractoriness of the feed stock. In the initial, or thermal, cracking zone the conditions are relatively mild since the heaviest and most easily cracked hydrocarbons are present. There may be little more than sub- 6. stantial vaporization in this initial heating zone. However, it is a preferred condition, especially with feed stocks containing relatively heavy hydrocarbons, that a mild thermal cracking of the heavier hydrocarbons be effected therein. The feed stock is heated in the vaporization zone to a temperature between about 800 and 950 F. under superatmospheric pressures up to about 300 pounds per square inch and, under these conditions, cracking of heavier hydrocarbons occurs, as well as vaporization of these and lighter fractions. After the separation of the unvaporized material, the hydrocarbon vapors may be further diluted and are heated to a temperature between about 800 and 1100 F. while under a pressure between atmospheric and pounds per square inch before being introduced into the low temperature catalytic cracking zone. Following fractionation of the elliuent from this zone, at least a portion of a heavy residue from the fractionation is diluted and adjusted to the desired conversion conditions in a second, or high temperature catalytic cracking zone where the diluted heavy hydrocarbon residue is catalytically treated at a temperature between 1000 and 1200 F. and a pressure between about 50 and 250 pounds per square inch to obtain further production of the desired gasoline.

The contact catalysts mentioned above are those having activity in promoting the cracking of hydrocarbons and are particularly those rugged minerals or materials comprising bauxite, brucite, various clay-type minerals and active aluminum silicates. These natural catalysts may be used after activation by various means such as acid treating and/or may be promoted with minor amounts of active metals or metal salts or oxides. Also useful are natural materials comprising zirconia, titania, and synthetic preparations comprising zirconia, titania, magnesia, alumina, and various silica-alumina combinations. These latter-*materials may be promoted with minor quantities of metal oxides, particularly those of chromium, nickel, and zinc.

The same catalyst or different catalysts may be employed in the two catalytic conversion zones. Since the hydrocarbon charge to the second or high temperature catalytic cracking zone is usually more refractory than that charged to the low temperature catalytic cracking zone, conditions peculiarly effective for the maximum conversion of each type of feed to these respective zones may be utilized. Thus, higher temperatures and more drastic conditions may be required in the second catalytic zone if the catalyst in each zone is of relatively the same activity, or the conversion conditions may be more nearly the same when the activity of the catalyst in the second zone is greater. The most desirable conditions for each step for a particular feed stock may be readily ascertained by mere routine test. It may also be desirable to use fresh or highly active catalyst in the second catalyst zone until it is partially deactivated and then to transfer the partially deactivated catalyst to the first catalyst zone where less active or less drastic conversion conditions are required.

After periods of service in the process, these catalysts are gradually deactivated through accumulation of tarry deposits and carbonaceous residues. When their activity has declined to a degree which renders the conversion unsatisfactory, they are quickly and completely restored to substantially their original activity by conventional reactivation at controlled temperatures in an oxidizing atmosphere. For this reactivation,` itis usually preferred to pass oxygen-con:- ta-ining gases such as mixtures of air with steam, nitrogen, carbon dioxide, or inert combustion gas through the catalyst to burn off the materials responsible for deactivation without thereby producing combustion temperatures harmful to the catalyst. By providing a plurality of catalyst chambers, it is possible to operate continuously, with one or more chambers on stream while the spent catalyst is being reactivated.

In a typical example of the process, 3340 b./d. (barrels per day) of topped crude plus 650 b./d. of recycle oil is charged to a thermal cracking zone and cracked at 900 F. and 100 p. s. i. The effluent is separated into a heavier bottom from which the above recycle and about 650 b./d. of fuel oil is obtained and into a vaporized overhead to which 673 b./d. of straight run naphtha is added. The mixed stream is further heated to 1050U F. and passed to a low temperature catalytic chamber and catalytically cracked between 1050-1000 F. at 85 p. s. i. in the presence of bauxite. rfhe effluent from the catalyst zone is passed to a fractionator and fractionated together with the eiuent from a subsequent high temperature catalyst Zone into a normally gaseous fraction, a gasoline fraction, an intermediate fraction, and a heavy oil fraction. The intermediate fraction is withdrawn as a side stream at the rate of about 2200 b./d. and admixed with 560 b./d. of straight run naphtha. The mixture is heated to 1100 F. and cracked in the presence of bauxite in the subsequent catalyst zone at 110D-1050D F. and 85 p. s. i. The eilluent from this cracking is returned to the fractionator together with the effluent from the low temperature catalytic cracking zone and 2548 b./d. of stabilized 400 F. end-point gasoline is recovered overhead from the fractionation.

I claim:

1. A process for the conversion of hydrocarbons which comprises thermally cracking a hydrocarbon oil, separating the non-vaporized residue from the thermally cracked oil, flashing a lighter gas oil portion from said residue and admixing said portion in liquid phase with the hydrocarbon oil before subjecting it to said thermal cracking, catalytically cracking all of the non-residual portion of the thermally cracked hydrocarbons in a first catalyst zone, recovering an intermediate hydrocarbon fraction from the catalytically cracked hydrocarbons directly passing said fraction to another catalyst zone and therein cracking said portion under more severe conditions of temperature than those in first mentioned zone.

2. The process of claim 1 wherein said thermal cracking of said hydrocarbon oil is accomplished in the presence of an added diluent consisting of a vaporized fraction of the gas oil boiling range.

3. The process of claim l wherein said thermal cracking of said hydrocarbon oil is accomplished in the presence of an added diluent consisting of a Vaporized fraction of the gas oil boiling range and wherein said catalytic cracking in said first and second catalyst zones is accomplished in the presence of added diluent naphtha.

PAUL M. WADDILL.

REFERENCES CTED The following references are of record in the ile of this patent:

UNITED STATES PATENTS Number Name Date 2,270,071 McGrew Jan. 13, 1942 2,284,493 Noll et a1. May 26, 1942 2,287,940 McGreW June 30, 1942 2,297,773 Kanhofer Oct. 6, 1942 2,298,434 Thomas Oct. 13, 1942 2,339,995 Kanhofer Jan. 25, 1944 2,358,149 Cooke Sept. l2, 1944 2,403,486 Barron July 9, 1946 Certificate of Correction Patent No. 2,529,790 November 14, 1950 PAUL M. WADDILL It is hereby certed that error appears in the printed specification of jf the above numbered patent requiring correction as follows:

Column 8, line 12, for the Word portion read fmetz'on;

and that the said Letters Patent should be read as lcorrected above, so that the same may conform to the record of the case in the Patent OIice.

Signed and sealed this 5th day of June, A. D'. 1951.

[SEAL] THOMAS F. MURPHY,

Assistant 'ommssz'oner of Patents.

Claims (1)

1. A PROCESS FOR THE CONVERSION OF HYDROCARBONS WHICH COMPRISES THERMALLY CRACKING A HYDROCARBON OIL, SEPARATING THE NON-VAPORIZED RESIDUE FROM THE THERMALLY CRACKED OIL, FLASHING A LIGHTER GAS OIL PORTION FROM SAID RESIDUE AND ADMIXING SAID PORTION IN LIQUID PHASE WITH TH HYDROCARBON OIL BEFORE SUBJECTING IT TO SAID THERMAL CRACKING, CATALYTICALLY CRACKING ALL OF THE NON-RESIDUAL PORTION OF THE THERMALLY CRACKED HYDROCARBONS IN A FIRST CATALYST ZONE, RECOVERING AN INTERMEDIATE HYDROCARBON FRACTION FROM THE CATALYTICALLY CRACKED HYDROCARBONS DIRECTLY PASSING SAID FRACTION TO ANOTHER CAYALYST ZONE AND THEREIN CRACKING SAID PORTION UNDER MORE SEVERE CONDITIONS OF TEMPERATURE THAN THOSE IN FIRST MENTIONED ZONE.

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