US2568967A

US2568967A - Cracking process

Info

Publication number: US2568967A
Application number: US662213A
Authority: US
Inventors: William C Offutt; Fogle Merald Clark
Original assignee: Gulf Research and Development Co
Current assignee: Gulf Research and Development Co
Priority date: 1946-04-15
Filing date: 1946-04-15
Publication date: 1951-09-25
Anticipated expiration: 1968-09-25

Description

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m00- aad/m Patented Sept. 25, 1951 CRACKING PROCESS William C. Oflutt and Merald Clark Fogle, Oakmont, Pa., assignors to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Application April 15, 1946, Serial No. 662,213

10 Claims.

This invention relates to a cracking process, and more particularly to. a process of cracking petroleum hydrocarbons to produce gasoline.

It is ordinary practice in the petroleum renery art to crack petroleum hydrocarbons such as heavy residual oils by the use of heat with or without a catalyst, to obtain gasoline. All of these processes, however,F yield in addition to gasoline substantial proportions of other products such as, for example, gases, gas oil, tar and coke. It is desirable to obtain the largest possible amount of good quality gasoline while at the same time producing the smallest possible amount of other products and particularly fuel oil.`

One of the methods which has been applied to increase gasoline yield has been the recycling of the gaseous products to the reaction zone and the present invention is directed to an improved process which may include this use of gaseous products. Some of these processes have utilized relatively light stock which was obtained by rst separating the heavy stock before sending it to the reaction zone.

It is an object of this invention tol crac-k heavier petroleum hydrocarbons to obtain a high yield of gasoline by the use of gaseous hydrocarbons.

It is a further object of this invention t crack a heavy residual oil to obtain a high yield of gasoline while producing only a small amount of by-products such as fuel oil, by utilizing gaseous hydrocarbons.

rlhese and other objects of this invention are achieved by the present process wherein a heavy petroleum hydrocarbon fraction is partially vaporized in a coil and then mixed in a reaction zone or chamber with a hot gas, preferably gaseous hydrocarbons. The gases are preheated in a coil separate from the heavy petroleum hydrocarbons before being passed into the reaction chamber at a temperature sufficiently high to add substantial heat energy to the already heated heavy petroleum hydrocarbons. The temperature and time of residence of the heavy petroleum hydrocarbons in the preheating coil should be sufficient to elect substantial vaporization and to crack some of the constituents. but not sufiicient to cause substantial coke deposition in the preheating coil. This temperature should be in the range 850 F. to 950 F; (at the coil outlet) and preferably about 900 F. depending on the stock treated. When the hot gases contact this material, preferably when both enter the top of the vertical reaction chamber, further vaporization occurs, thev vaporsv are subjected to further cracking and the unvaporized residual constituents pass quickly to the bottom of the reaction vessel without much opportunity to coke.

The temperature of the hydrocarbon gases should be substantially higher than that of the residual oil charged but not so high as to cause substantially instantaneous coking upon contact with a heavy oil. For this purpose the gases should be heated to a temperature in the range 1G00 F. to 1200" F. and preferably 1100 F. It will be understood that the temperature of the gas may be varied depending upon the amount of the gas compared to the amount of charge stock, residence time in the reactor, temperature of the charge, and other factors, but these should be adjusted to allow for the above preferred gas temperatures. Coking will not take place to any substantial extent at the reaction temperatures if the time of reaction is so controlled to prevent such coking. The invention contemplates a temperature high enough to cause coking if a soaking period were allowed, but this period of time should be shortened to avoid the coking. The temperature of the heated oil is not necessarily raised above that of the coil outlet temperature,

the hot gases rather serving to cancel out some of the cooling effects which come into action when the material is discharged into the reaction. chamber. The action of the added gases contributes a vaporizing elect.

The charge is a heavy petroleum oil containing residual constituents such as reduced crude and it is a feature of the invention that this can be handled directly without preliminary separation of residual constituents.

The gases used may be obtained from an outside source such as a hydrocarbon gas from a cracking unit, steam, or other inert gas. Preferably however, the gases used are a part of the entire gaseous product, having a boiling point below the gasoline fraction, recycled from the same cracking system in which they are used. Use of a part of such entire gaseous products avoids any necessity for fractionating the gases. Pressure conditions may be varied depending upon the degree of cracking desired, type of charge, temperature, etc., but it will usually be in the range of from to 2000 pounds per square inch and preferably 500 to 1000 pounds per square inch.

The amount of added gas may be varied greatly depending upon the amount available, the Various temperatures and pressures of operation, the capacity of the apparatus, the olenic content of the gas and other factors. The preferable 4quantity of gas, however, should be in the range 50 to 300 liquid volume per cent of the oil charge and preferably about '75 liquid volume per cent to about 100 liquid volume per cent of the oil charge.

The apparatus used for the process may be any of the conventional types and the reaction chamber is preferably an elongated vertical vessel.

Cracking processes in which only one coil is used to heat the charge are known as single coil systems. The invention is applicable to such systems and also to systems of the two coil type wherein heavy oil is heated in one coil and light oil, which may be recycled from within the process, is heated in a second coil at a higher temperature and the two discharged into a reaction chamber. In using a two coil system the light recycle oil may be heated to a temperature in the range 950 F. to l050 F. prior to entering the reaction chamber. In a preferred embodiment of the invention it is also desirable to s control the operations of cracking and tar separation so as to produce and separate out a heavy tar of, for example, 4 API gravity or heavier, in other words to a material not directly saleable as fuel oil. This tar may then be blended back with a portion of the light recycle oil of the process to form a saleable fuel oil. Operating under these conditions, a greater gasoline yield is 0btained.

Specific embodiments of the present invention are illustrated in the drawings wherein:

Figure 1 is a ow sheet of the process of the present invention as applied to a so-called single coil system;

Figure 2 is a flow sheet of the process of the present invention as applied to a so-called two coil system, and

Figure 3 is a flow diagram of a reactor alternative to the one shown in Figures l and 2.

Heretofore, in the operation of so-called Single coil cracking units on oils containing residual constituents, it has been necessary to employ relatively mild temperatures and relatively low conversions per pass, in order to avoid excessive coke formation. As a result, such operations have tended to result in relatively poor yields of gasoline. However, our invention makes it -possible to carry out the cracking of a wide variety of residual oils in apparatus of the single coil type under such conditions that higher yields of gasoline are obtained, but without causing excessive deposition of carbon or coke, especially in the cracking coil. By maintaining proper conditions in the coil, deposition of coke at that point in the system is prevented. Higher temperatures may be maintained in the reaction chamber by the addition of heat energy from added gases, but on account of the relatively short time of contact of the heavy oil in the reaction chamber, coking is reduced to a minimum and any coking as may occur is obviously of less consequence than would be the case if the same amount of coking were to take place in a pipe lcoil of restricted cross-sectional area. On the other hand, the high temperature and relatively long time of contact of the lighter oil vapors in the reaction chamber makes it possible to obtain a substantially higher degree of cracking, and higher degree of conversion per pass, than could otherwise be secured. As a result, higher overall degree of cracking is obtained, and the yield and quality of the gasoline product are enhanced.

With reference to Figure l it will be seen that the charge, which is reduced crude in this instance, is pumped by means of pump I0 through 4 conduits II to heating coil I2 where it is brought up to a temperature in the range 850 F. to 950 F. Gas recycled from the process is sent to heater I4 where it is brought up to a temperature in the range of 1000 F. to 1200 F. and passed to a reaction chamber I5 at a point at or near the top thereof and adjacent the point at which the hot oil is introduced. The cracking products are taken from the reaction chamber I5 by conduit IB to a separator I8 wherein the products are separated into two fractions, one relatively heavy and the other relatively light. The heavy fraction or bottoms is sent by conduit I9 and preheater 20 to tar stripper 2I. The tar stripper may operate in any known manner with or without the use of steam or an extracting material such as naphtha. This heavy residuum from the separator is thus separated into the three portions; reduced tar, heavy recycle stock, and an overhead which, together with the overhead from separator I8, is passed to the fractionator 23 from which light oil is removed as bottoms and returned to the oil heater, or mixed with reduced tar to produce fuel oil as will be hereinafter explained. The overhead from the fractionator 23 is passed by conduit 24 to a stabilizer 26 wherein the gaseous products are separated from the gasoline. The overhead from the stabilizer is sent to a gas condenser 2'I wherein the diiiicultly condensible gases may be removed and the condensed gas ordinarily rich in C3 and C4 hydrocarbons is returned to the gas heater I4.

Light recycle oil from fractionator 23 may be returned by conduit 29 to oil heater I 2, or a portion of it may be used to blend with heavy tar to make a saleable fuel oil. The cracking by the present process produces a large amount of gasoline and this is increased by so controlling distillation to produce a quantity of heavy tar (4 API gravity or heavier) rather than a heavy oil which might be used directly as a fuel oil. The tar produced is utilized by mixing therewith a. small proportion of the light recycle oil to obtain a fuel oil. This is accomplished by tapping conduit 29 by means of a conduit 30 and removing a portion of the light oil to admixture with the reduced tar coming from the tar stripper by conduit 3l. Conduit 32 provides a means for returning light recycle oil to the reactor as a quench. Most of the light recycle however is subsequently passed to the reactor and the ultimate yield of gasoline is higher than if a saleable fuel oil were separated out instead of a low gravity tar.

Alternatively, but not shown herein, it may be desirable to use a gas absorber in the place of the gas condenser. In that instance, an absorbing material, such as naphtha or gas oil, may be used to absorb the gases and the absorbed material may be passed to another vessel in which the absorbed gases are then stripped from the naphtha and sent to the gas heater. The naphtha or gas oil may be recycled continually between the gas absorber and the gas stripper.

With reference to Figure 2 there is shown a flow sheet of the process of the present invention as applied to a so-called two coil system. A charge of reduced crude or other heavy oil is sent to heater 40 where it is heated up to a temperature in the range 850 to 950 F., then passed to reaction chamber 4I. A light recycle oil which is recycled from the process is heated in heating coil 43 and subsequently charged to the reaction chamber together With the heavy oil. There is above.

. Figures l. or 2.

.further provided a gasheater 44 forv heating the gas recycled in the system and this gas is heated to a temperature in the range 1000o F. to 1200 F.,

`and isalso introduced into the reaction chamber at a point adjacent. the introduction of the light and heavy oil. The reaction products from reaction chamber d Ik are passed into separator 46 lby means of conduit l'l-wherein they are separated into light and heavy ends. The heavy material passes by conduit 48 lthrough a tar preheater 49 to a tar stripper 50.v This tar stripper may operate in any of the conventional manners with or without the use of steam, but essentially the heavy ends from separator 46 are divided into ythree portions, one a reduced tar product, an-

other a heavy recycle oil, which is passed by conduit 52 to the heavy oil heater, and an overhead fraction. The lighter overhead material from ,thev tar stripper ispassed to a fractionator 55 as isV also the overhead from separator 46, which passes to the fractionator 55 by conduit 55. The

fractionator separates the materials into a light and a heavy product, the light product passing Aby conduit 58 to a stabilizer 59 and the heavy product, which is a light recycle oil, is passed by conduits 60 and '6I to the light oil heater 43. A portion of the light oil, however, .may be bled 01T vby conduit B2 for admixture with the reduced ytar from conduit (i3Y to form fuel oil When operating under conditions such that reduced tar is formed instead of a heavy oil which would be saleable as fuel oil, as explained in Figure l Conduit Si! provides a means for returning light recycle oil to the reactor to act as a quench. In the stabilizer 59- the gasoline product is taken olf as bottoms and the overhead gases passed to a gas condenser 55 by means of conduit 56 wherein the condensible gases are condensed and' returned by conduit Bl to the gas heater 44 and the non-condensible gases, are passed oif by conduit 58.

While the two-coil cracking system may not be as flexible as the single-coil system, it does have the advantage that the light oil may be heated to a higher temperature than the heavy oil without coking and thus more gasoline is obtained.

Figure 3 shows an alternative method of operating the reaction chamber from that shown in In this case the preheated oil and gas inlets are at a point in the chamber substantially below the topY thereof and means are provided for venting the vapors at the top of the reaction'chamber. Some of the vapors may pass 4 out With the liquid at the bottom of the chamber as shown in Figures l and 2.

In the practice of the present invention the unvaporized oil in the coil, as well as in the reactor, is subjected to vis-breaking, andthe coking of this reduced amount of residuum is avoided by removing it rapidly from the bottom of the reactor. Thus, no accumulation of liquid should be permitted vin the bottom of the reactor. The charge emergesfrom the coil to the reactor as a mixture of vapors with more or lessV subdivided drops or very small slugs of unvaporized liquid. This liquid because of its density will drop or be projected quite rapidly to the bottom of the reactor where it will pass. out at once. The vapor on the other hand will not pass through the reactor as rapidly and will mix with the hot gas injected from the gas heating coil. Thus the time of residence oi vapors is greater than it would be in a coil since normal vapors in a coil travel more rapidly than unvaporized liquid. Thus in .the present process the vapor is subjected to a longer time of treatment than-.it would be. in a coil.

The; present process eiects a vis-breaking; of the residuum in order to reduce its amount as far as possible. The intention in the process is to make asy little and as heavy a tar as possible, since it may subsequently be converted to marketable fuel oil by blending with some of the light recycle oil. In thisr manner the largest possible Iamount of gasoline is produced. The process thus is adequatelyadapted to deal directly with heavy petroleum distillates to obtain the maximum amount of gasoline with the minimum amount of fuel oil.

VThus. We havev described a new and useful process for cracking heavy petroleum hydrocarbons to obtain a high yield of gasoline. z

What we claim is: l. AA process for cracking a heavy petrol'eu hydrocarbon fraction containing residual constituents to obtain gasoline Without preliminary separation of such residual constituents which comprises heating said heavy fraction at a temperature and for a time suhcient to cause partial vaporization but insufficient to cause coking, heating a gas to a temperature substantially higher than the temperature of heating of said heavy fraction but not so high as to cause substantiallyV instantaneous coking upon contact with said heated heavy fraction, charging both vaporized and unvaporized portions of said heavy fraction together with said heated gas to a reaction zone containing no catalyst and further vaporizing said unvaporized portion and subject'- ing it to vis-breaking, thereby producing minimal amounts of unvaporized residuum, passing the `unvaporized portions of said heavy fraction rapidly through said reaction Zone Without substantial coking, and maintaining the vaporized portionsof said heavy fraction in said reaction zonev for a time sufficient to cause substantial cracking of said vaporized portions.

2. A process for cracking a heavy petroleum hydrocarbon fraction containing residual constituents to obtain gasoline without preliminary separation of such residual constituents which comprises heating said heavy fraction at a temperature in the range 850 to 950 F. and for'a time suiiicient to cause partial vaporization but insuicient to cause coking, heating a normally gaseous hydrocarbon mixture to a temperature substantially higher than the temperature of heating ofv said heavy fraction but not so high as to cause substantially instantaneous coking upon contact with said heated heavy fraction, charging both vaporized and unvaporized portions of said heavy fraction together with said heated gas to a reaction zone containing no catalystV and further vaporizing said unvaporized portion and subjecting it to vis-breaking, thereby producing minimal amounts of unvaporized residuum, passing the unvaporized portions of saidheavy fraction rapidly through said reaction zone without-substantial coking, and maintaining the vaporized portions of said heavy fraction in said reaction zone for a time suflicient to cause substantial cracking of said vaporized portions.

3. The process of claim 2, wherein the temperature of heating of the normally gaseous hydrocarbon mixture is inthe range 1000 to 1200ov F.

4. A process for cracking a heavy petroleum hydrocarbon fraction containing residual constituents to. obtain gasoline without preliminary separation of such residual constituents which comprises heating said heavy fraction at a temperature in the range 850 to 950 F. and for a. time su'icient to cause partial vaporization but insuilicient to cause coking, heating a normally gaseous hydrocarbon mixture in an amount between fty per cent and three hundred per cent by liquid volume of said petroleum hydrocarbon to a temperature in the range 1000 to 1200 F., charging both vaporized and unvaporized portions of said heavy fraction together with said heated gas to a reaction zone containing no catalyst and further vaporizing said unvaporized portion and subjecting it to vis-breaking, thereby producing minimal amounts of unvaporized residuum, passing the unvaporized portions of said -heavy fraction rapidly through said reaction zone Without substantial coking and maintaining the vaporized portions of said heavy fraction in said reaction zone for a time suflicient to cause substantial cracking of said vaporized portions.

5. A process for cracking a heavy petroleum hydrocarbon fraction containing residual constituents to obtain gasoline without preliminary separation of such residual constituents which comprises heating said heavy fraction at a temperature of about 900 F. and for a time suillcient to cause partial vaporization but insufficient to cause coking, heating a gas at a temperature of about l100 F. in an amount between seventyve per cent and one hundred per cent by liquid volume of said heavy petroleum hydrocarbon fraction, charging both vaporized and unvaporized portions of said heavy fraction together with said heated gas to a reaction zone containing no catalyst and further vaporizing said unvaporized portion and subjecting it to vis-breaking, thereby producing minimal amounts of unvaporized residuum, passing the unvaporized portions of said heavy fraction rapidly through said reaction zone without substantial coking and maintaining the vaporized portions of said heavy fraction in said reaction zone for a time suicient to cause substantial cracking of said vaporized portions and under'a pressure in the range of from 100 to 2000 pounds per square inch.

6. The process of claim 5, wherein the pressure is in the range 500 to 1000 pounds per square inch.

7. A process for cracking a heavy petroleum hydrocarbon fraction containing residual constituents to obtain gasoline without preliminary separation of such residual constituents which comprises heating said heavy fraction at a temperature and for a time suilicient to cause partial vaporization but insufcient to cause coking, heating a gas theretofore obtained in the process to a temperature substantially higher than the temperature of heating of said heavy fraction but not so high as to cause substantially instantaneous coking upon contact with said heated heavy fraction, charging both vaporiezd and unvaporized portions of said heavy fraction together with said heated gas to a reaction zone containing no catalyst and further vaporizing said unvaporized portion and subjecting it to visbreaking, thereby producing minimal amounts of unvaporized residuum, passing the unvaporized portions of said heavy fraction rapidly through said reaction zone without substantial coking and maintaining the vaporized portions of said heavy fraction in said reaction zone for a time sumcient to cause a substantial cracking of said vaporized portions, separating the reaction products to obtain tar, light and heavy recycle oils, gasoline and gaseous hydrocarbons, recycling said recycle oils for heating with said heavy petroleum hydrocarbons, and recycling a portion of said gaseous hydrocarbons.

8. A process for cracking a heavy petroleum hydrocarbon fraction containing residual constituents to obtain gasoline without preliminary separation of such residual constituents which comprises: heating said heavy fraction at a temperature in the range 850 to 950 F. and for a time suiiicient to cause partial vaporization but insufcient to cause coking; heating a light recycle oil theretofore obtained in the process to a temperature in the range 950 to 1050 F.; heating a normally gaseous hydrocarbon mixture theretofore obtained in the process to a temperature in the range 1000 to 1200o F.; charging both vaporized and unvaporized portions of said heavy fraction together with said heated gas and light recycle oil to a reaction zone containing no catalyst and further vaporizing said unvaporized portion and subjecting it to vis-breaking, thereby producing minimal amounts of unvaporized residuum; passing the unvaporized portions of the charge rapidly through said reaction zone Without substantial coking; maintaining the vaporized portions of the charge in said reaction zone for a time suilicient to cause substantial cracking of said vaporized portions; separating the reaction products from said reaction zone into gasoline, light recycle 011, heavy recycle oil, tar and normally gaseous hydrocarbons; recycling the light recycle oil to the light recycle oil heating step; recycling the heavy recycle oil for heating with the heavy petroleum hydrocarbon fraction; and recycling a portion of the normally gaseous hydrocarbons to the gaseous hydrocarbon heating step.

9. A process for cracking a heavy petroleum hydrocarbon fraction containing residual constituents to obtain gasoline without preliminary separation of such residual constituents which comprises heating said heavy fraction at a temperature and for a time suiicient to cause partial vaporization but insufficient to cause coking, heating a gas to a temperature substantially higher than the temperature of heating of said heavy fraction but not so high as to cause substantially instantaneous coking upon contact with said heated heavy fraction, charging both vaporized and unvaporized portions of said heavy fraction together with said heated gas to a reaction zone containing no catalyst and further vaporizing said unvaporized portion and subjecting it to vis-breaking, thereby producing minimal amounts of unvaporized residuum, passing the unvaporized portions of said heavy fraction rapidly through said reaction zone without substantial coking, and maintaining the vaporized portions of said heavy fraction in said reaction zone for a time suilicient to cause substantial cracking of said vaporized portions, the unvaporized portions of said heavy fraction and the products of cracking of the vaporized portions of said heavy fraction being withdrawn carbon mixture substantially higher than the temperature of heating of said heavy fraction but not so high as to cause substantially instantaneous coking upon contact with said heated heavy fraction, charging both vaporized and unvaporized portions of said heavy fraction together with said heated gas to a reaction zone containing no catayst and further vaporizing said unvaporized portion and subjecting it to vis-breaking, thereby producing minimal amounts of un- 10 vaporized residuum, passing the unvaporized portions of said heavy fraction rapidly through said reaction zone without substantial coking, maintaining the vaporlzed portions of said heavy fraction in said reaction zone for a time suicient to cause substantial cracking of said vaporized portions, subjecting the total effluent from said reaction zone to separation to obtain a reduced tar product having an A. P. I. gravity .heavier than 4, fractionating the rest of the reaction products to obtain gasoline, gas, heavy recycle oil and a light recycle oil,- and combining 10 a portion of said light recycle oil with said tar to produce fuel oil.

WILLIAM C. OFFUTT. M. CLARK FOGLE.

y REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,995,005 Morrell et al Mar. 19, 1935 2,067,782 Nelson Jan. 12, 1937 2,132,129 Roberts Oct. 4, 1938 2,139,672 Crittenden Dec. 13, 1938 2,251,571 Howard Aug. 5, 1941 2,294,565 Lemen Sept. 1, 1942 2,326,186 Watson Aug. 10, 1943 2,332,562 Egloff Oct. 26, 1943 2,335,551 Trow NOV. 30, 1943 2,406,312 Barron Aug. 27, 1946 2,470,680 Beuther 1 May 17, 1949

Claims

1. A PROCESS FOR CRACKING A HEAVY PETROLEUM HYDROCARBON FRACTION CONTAINING RESIDUAL CONSTITUENTS TO OBTAIN GASOLINE WITHOUT PRELIMINARY SEPARATION OF SUCH RESIDUAL CONSTITUENTS WHICH COMPRISES HEATING SAID HEAVY FRACTION AT A TEMPERATURE AND FOR A TIME SUFFICEINT TO CAUSE PARTIAL VAPORIZATION BUT INSUFFICIENT TO CAUSE BOKING, HEATING A GAS TO A TEMPERATURE OF HEATING OF SAID HIGHER THAN THE TEMPERATURE OF HEATING OF SAID HEAVY FRACTION BUT SO HIGH AS TO CAUSE SUBSTANTIALLY INSTANTANEOUS COKING UPON CONTACT WITH SAID HEATED HEAVY FRACTION, CHARGING BOTH VAPORIZED AND UNVAPORIZED PORTIONS OF SAID HEAVY FRACTION TOGETHER WITH SAID HEATED GAS TO A REACTION ZONE CONTAINING NO CATALYST AND FURTHER VAPORIZING SAID UNVAPORIZED PORTION AND SUBJECTING IT TO VIS-BREAKING, THEREBY PRODUCING MINIMAL AMOUNTS OF UNVAPORIZED RESIDUUM, PASSING THE UNVAPORIZED PORTIONS OF SAID HEAVY FRACTION RAPIDLY THROUGH SAID REACTION ZONE WITHOUT SUBSTANTIAL COKING, AND MAINTAINING THE VAPORIZED PORTIONS OF SAID HEAVY FRACTION IN SAID REACTION ZONE FOR A TIME SUFFICIENT TO CAUSE SUBSTANTIAL CRACKING OF SAID VAPORIZED PORTIONS.