US2382107A - Polyform conversion of hydrocarbons - Google Patents

Description

Aug. 14, 1945. A. H. scHUTTE POLYFQRM CONVERS ION- OF YHYDROCARBONS Filed May 1. 194s Patented Aug. 14, 1945 POLYFORM CONVERSION AOF HYDRO- f CARBONS August Henry Schutte, Hastings on Hudson, N. Y., assignor to The Lummus Company, New York,

N. Y., a corporation lof Delaware Application May 1, 194s, serial N0.'485,28s

(ol. 19e-9) 3 Claims.

This invention relates to improvements in the conversion of hydrocarbons for the production of lower boiling materials, particularly those in the gasoline range; and it specifically relates to an improved method for obtaining a higher crack Der pass in the combined conversion of normally liquid hydrocarbon cracking stocks and normally gaseous hydrocarbons.

It has been established that the simultaneous conversion of the normally gaseous Ca and C4 hydrocarbons and of a normally liquid cracking stock permits an in-crease inthe cracking temperature over that permissible in conventional thermal cracking so that an average crack per pass of approximately 22% is obtained as compared to the usual average of 16%. With this higher crack per pass, which is produced by the use of the higher operating temperatures, the total amount of oil circulation to the cracking heater is less than that in conventional thermal cracking for a given amount of charge'stock. At the same time, the total amount of oil vapors to be handled in the tower fractionating system is correspondingly reduced. The decrease in the total amount of oil circulation to the heater, however, is partially or wholly offset by an increase in the total amount of heater throughput because of the introduction of the Cs and C4 hydrocarbons along with the cracking charge stock. The total amount of heater charge, which comprises the oil and the gas, is, therefore, approximately the same as the total amount of heater charge in conventional thermal cracking; but, because of the higher cracking temperature permissible, the gasoline octane number is considerably higher than that obtained Aby conventional thermal methods. The loss in gasoline yield, which would normally be expected when a higher crack per pass and a higher octane number are realized, is made up for by polymerization of Ca and C4 hydrocarbons.

In thermal cracking operations `as well as in the combined gas-liquid cracking or polyforming operations, the formation of coke is' a limiting factor with respect to the temperature at which the cracking heater may be operated for any commercially suitable period. The time temperature relation must be so adjusted as to produce an efficient conversion of the charge stock just short of coke formation. The higher permissible operating temperature obtained in the polyforming operation, whereby a higher yield per pass of vgasoline is produced, resultsv from the fact that the C3 and C4 hydrocarbons introduced with thel charge oil into the heaterl lower the pseudocritical temperature of the material in the heater tubes or coil and permit the formation of more and heavier tarry and polymeric coke-forming materials before such materials are deposited as a liquid phase in the tubes and coking difliculty sets in. Because of thiscoke formation, the gas-oil cracking mixture is subjected to polyforming conditions only until the pseudocritical temperature of the coil material approaches the temperatureof opera-- tion and there is danger of separation of a liquid phase, at which point the coil material Vis quenched to arrest thel cracking reaction and the tendency to form coke.

The principal object of my invention is to provide an improved process for'the simultaneous conversion of normally gaseous and normally liquid hydrocarbons whereby the crack per pass may be materially increased without any substantial shortening of the operating period. Another object of my invention is to conduct this improved process so that the total amount of heater charge, the total amount of material processed in the tower system, and the total amount of heat to be recovered by heat exchange are materially reduced.

I A further object of my invention is to increase the overall octane number of the gasoline obtained in the polyforming operation.

Furtherobjects and advantages of my invention will be apparent from the following description thereof, taken in connection with the attached drawing, which shows a flow sheet of a combination cracking system embodying the features of my invention. l i

Asv the polyform process is now carried out, a suitable cracking stock in admixture with a sufficient quantity of hydrocarbon gases,7 primarily those having 3 to 4 carbon atoms per moleculey is passed through a conversion zone under conditions more drastic than those to which the cracking stock alone could be subjected withoutcoking. As already pointed out, these gases serve to lower the pseudocritical temperature of the mixture so that a higher conversion temperature can be employed before tar deposition and accompanying coke formation take place. The hydrocarbons are subjected. to the conversion conditions until the incipient coking stage or the coking threshold is reached', at which point they are cooled by the `addition of a quenching oil which serves to arrestV the conversion reactions. The cooled hydrocarbons are then fractionated. under reduced pressure for the separation of the particular cracked fractions desired, Because of the coke-forming tendency of theA heavy polymeric and tar-forming constit- By means of my improved process, the initial lcharging stock may be subjected to two or more conversion stages maintained at progressively increasing temperatures without the operating limitation imposed by the formation of coke in the conversion zone as the temperature is increased.

According to my invention, a suitable cracking stock, ranging in type from naphtha to reduced crude, is commingled with normally gaseous hydrocarbons having from 3 to 4 carbonatoms` per molecule, andthe mix-ture is then passed through a rst conversion stage wherein it is subjected to conversion conditions until the point of incipient coking or the coking threshold is reached. The partly converted hydrocarbons are then withdrawn from the conversion stage and cooled, preferably by the `addition of a suitable quenching oil, in order to arrest the conversion reactions and to separate out the heavy polymeric and tar-forming constituents in a liquid phase.` Duringv such step, the conditions are so adjusted that substantially none or, at the most, only a very small percentage of the light, nontarry constituents is transferred into the liquid phase.` The remaining vaporous constituents, to which additional normally gaseous C3 and C4 hydrocarbons may iirst be added, are then passed through a second conversion stage wherein they are subjected to a higher conversion temperature than that maintained in the first stage.

'Again in the second stage the conversion is carried out until the coking threshold is reached, at which point the reacting hydrocarbons are withdrawn and are cooled to separate out additional tarryy constituents as a liquid phase. The remaining yapors may then be subjected to treatment in further conversion stages, the number of which is limited only by the pressure on the hydrocarbons entering the initial conversion stage and the pressure drop through each conversion stage in the system. The various products of conversion are then fractionated into normally gaseous constituents, gasoline, a fuel oil residuum, and an intermediate cycle cracking stock. The C3 and C4 gases, from which those hydrocarbons having 2 or less carbon atoms per molecule have vbeen iirst removed, are preferably recycled for ing stock may be combined with the` cracking charge stock.

and a particular cracking Although any desired number of conversion is particularly adapted for use in conjunction with a combination unit wherein all the cracking stocks are simultaneously but independently treated.

The increased yield of gasoline per pass obtained by my improved process requires a smaller total amount of heater charge than that inthe customary polyform operation ior a given production of gasoline. As a result of the increased gasoline yield, the recycle streams of cracking stock and hydrocarbon gases are materially reduced in amount. The advantages of such operation are that the cracking heater duties are decreased, the necessary fractionating equipment is smaller, and the required heat exchange surface for heat recovery is less.

In one embodiment of my process, a heavy gas oil in admixture With C3 and C4 hydrocarbons is introduced into a heater and subjected to cracking conditions, as in the case of polyforming, up

.to the coking threshold, i. e., up to the point Where the pseudocritical temperature of the coil materialapproaches the operating temperature and there is danger of separation of a liquid phase. When this point is reached, the coil material, instead of being quenched and introduced into a tower system under reduced pressure, is removed from the heater and cooled only suiiiciently at full pressure to separate out a liquid phase containing the heavy tar and polymers and as small an amount of lighter constituents as possible.. The separated liquid phase is withdrawn to the tower system for fractionation into fuel oil, cycle gas oil, gasoline, and gas. The vapors recovered from the initial separation constitute a clean .cracking-stock and are subjected to further pyrolysis with further addition of Cs and C4 hydrocarbons, if desired, but without intermediate fractionation. In this fashion a 25 A. P. I. gas

, oil, which had a 50%'point of 700 F. and could be cracked to give only about a 16% crack perpass by conventional methods or about a 22% crack per pass by the polyform process, can be processed to yield up to 35% gasoline per pass.

One application of this principle to a combination cracking unit is shown in the accompanying drawing. In this case a fresh heavy gas oil charge stock is introduced by pump IIJ through line II into the gas oil coil or heater I2. This charge may be augmented by a cycle gas oil introduced through line I3. C3 and C4y hydrocarbons from line I5 are added to the gas oil charge stock to provide the desired polyiorm charge mixture. The quantity of these gaseous hydrocarbons is so controlled as to give the proper gas-oil ratio for the desired pseudocritical temperature of the mixture owing through the heater and usually amounts to 50 to 75% by volume on the oil. The conversion in heater I2 is carried on to the greatest extent and for the longest time permitted by coke formation at the particular operating temperature and pressure employed, Which may range from about 950 to 990 F. and from about 600 to 1500 pounds per square inch, respectively. The partially converted material xis then discharged from the heater and quenched suiiiciently by admixture with a suitable quench oil introduced through line I6 to condense the undesiredv heavy tarry constituents present therein. 'I'he quenched mixture is passed to separator I'I, from which the separated `liquid is withdrawn through line I8 under control of liquid level control I9 and valve 20. The remaining vapors are removed fromseparator I'I through line 2| for further treatment as hereinafter described.

The amount of quench oil used is 'so regulated as to condense only the heavy tarry, coke-forming constituents present in the eiiiuent mixture laaeaio'r vadmixed with the chargeto coil |2 'by reason of the partial pressure effect exerted by these gasecus hydrocarbons. f

A further quantity of C3 andCi hydrocarbons may be added through line 22 to the separated vapors before introduction thereof into the second coil or heater 23 if such addition is desirable. The conversion is then continued in heater 23,

' until the coking threshold is again approached,

at a temperature ranging from about 980 to 1000o F. and a pressure ranging from about 500 tol-100 pounds per square inch. *Quenchi oil from line 25 is then admixed with the eiliuentfrom heater 23 in the same manner as described above, and the cooled mixture is introduced into separator 26. The separated liquidl tarry material is drawn off through line 21 under control of valve 28 and liquid level control 29, and the separated vapors are withdrawn through line 3l);

The separated vapors may again be subjected to further conversion in coil 3| u nder appropriate temperature and pressure conditions either alone or in admixture withl further C3 and C4 hydrocarbons from line 32. As before, the .con-

'version is carried out until the cking threshold means of pump 39 into the` vapors and gasesv in line 30, and the mixture isy passed to cracking coil 3|. In such case, suiiicientCa and C4 hydrocarbons are added through line 32 to lower the pseudocritical temperature of the reduced crude to the desired degree; and the conversion in coil 3| is then conducted under lower pressure and temperature, for, example, at a pressure ranging from about 300 to 500 pounds per square inch and a temperature ranging from about 850 to 950 F., to effect a viscosity-breaking of or a wax destruction in the reduced crude. The resulting eluent from coil 3| is then suitably quenched and introduced into tower 36 for the desired separation.

Alternatively, the reduced crude introduced through line 31 may be separately treated in heater 40 either alone or in admixture with Ca andk C4A hydrocarbons from line 42 ranging in amount from about 50 to 100% by volume on the oil. The conditions in heater 40 are such that a viscosity-breaking of the reduced crude is primarily eifected. The coil eilluent is admixed with quench oil from line 34a, and the resulting lline 65:

yquenched stream is passed'to bubble tower' through line 35 in admixture 'with the conversion products from heater 3|.` 1 c Y l In bubbletower 36, the cracked products 'are separated into an overhead consisting of unconverted C3 and `Crhydrocarbons -and .-gasoline, which is'removed through= line 44,f and a fuel oil bottoms consisting of the .heaviest cracked material, which is withdrawn through line `45. Anintermediate fraction comprising a cycle'gas oil is collected on tray '46 and is removedthr'oug'h line 46a by means of pump 41 for admixture with thel incoming gas oil charge' in -line The'l`iq uid separated out in separators |1 and 26 isalso passed through line 48to the lower'part of tower 36 for separation into the respective fractions;`

VThe overhead fromy bubble tower 36 is passed to stabilizer. 49, vfrom which stabilized high octane gasoline is removed through bottoms"line 50. The overhead from the stabilizen'consi'sting principally of C3 and C4' hydrocarbons as Awell as'y lighter hydrocarbons' and hydrogen, v"is vremoved through line 52 and is'partially condensed in condenser 53 to produce a Ca--Cr condensate which is separated out in accumulator 54.' f A portion lof this condensate'is returned as reflux to the stabilizer 49 through line55. The major portion of the liquefied C3 and C4 hydrocarbons is passed by pump 56 into line 51, which 'supplies the C3 and v'C41 hydrocarbons for the various heatersthrough" the branch lines |5, 22, 32, and 42.y The balance of this condensate may be passed throughline'58"intoth`ebottom Vof absorber 59, into which the uncondensedCs 'andC4 hydrocarbon-mixture from accumulator'54 isf'also introduced through line 60."l Y

`In'abso'rber 59, the gaseous hydrocarbons' are contacted with naphthaintroduced through line 62 to-absorb the :C3 and Cil-'components therefrom. Additional C3 and C4 stock may be introduced into the system. through line 63. The Xenriched naphtha is removed through' bottoms yline 64, and the stripped gases are discharged through additionalv naphtha, if desired,y introduced through vline-lili, is then preferably passedib'y means of pump 66a throughf'lthe heater`61, wherein the conditions are `such that a reforming 'of the naphtha rand a polymerizingxofthe gases into Ahigh octanegasoline take placent' The eflluent from coil 61 is passedthrough line 68 vto v bubble tower 36 for separation into-the desired fractions; "The C3 and Cl'hydrocarbons'present inthe naphtha charge may range in amount from -about to.150% byl volume on thenap'htha, and the pressure incoil 61 may'rang'efrom' about. l000 to 2000 pounds per squareinchl with `a e temperature ranging from about 1020 t to Thequench oil for the gas oil: coils may be derived fromseveral sources. AA portion'offgth'e cycle gas oil inline'46a may be passed through line 1|) intothe separate lines I6, 25, 34, and34a leading to the outlet ylines from the respective yheavy tarry polymers from the beaten` streaima This enriched naphtha together with` absorption methods.

4 l assale? :lower overall. heater chargeis f obtained. ySince the` amount of C3' Aand `Ct'hydrocarbons necessary vnormally the vcase in conventional apolyforming.

In spite ofthis feature, -the overall gas chargev to the heaters is considerably less than vthat in con` .-ventional combination polyforming. v

'Ihe 'above description relates'particularly to a combined process for simultaneously treating the reduced crude, the gas oil; and the naphtha re-` covered in the distillation of a crude-oil, which distillation can,l of course, be conducted in asingle primary still or .fractionatingv towerv (not shown). The naphtha recovered as .a side stream from such tower canzbefedltothe above-described systern through lines 62 and `66; Vthe lgas oil, alsoy recovered as a side stream, can ybe fed throughline I I; and the reduced crude removed from the bottom of such towercan be fed through 1ine31., ,l

But the described system is also applicableto the treatment of ythese stocks individually with only slight modiiications- :If .a gas oil or a re-` duced crude stock istreatedv alone, for example, it can be fed to heater I2 through linell as pref `viously described. A*In this case theheater 40 and the line 31 are not used, and no feed offnaphtha isrequired. Instead of feeding-naphthaintovthe absorber59 fortabsorption of the C3 and C4 hydrocanbons, aportion of the recycle gas oil,l which is recovered from plateMi yin bubble tower; 36, can be used. gas oil-can be passed -abyiline' 16 through a cooling uniti'Vl 'and then into the top of the absorber through line 62, and the enriched gas oil can be subjected to conversion either in a single-stage system or in a multi-stage system.

In one specific embodiment of this. invention wherein a 25 A. P. I. gas oil having a meanlboil- :ing point of 700 F. was introduced through line Il, the total furnace charge amounted to only 240% onthe fresh feed oil, Ilhis example demonstrates the important advantages gained by the present invention in comparison with the con--` ventional polyforming process, in which a total furnace `charge ofwapproximately 415% would have been required for the same oil. In my proc- -ess less heat is required, decreased tower diameterscan be employed, less 'overall heat transfer surface is required, and higher yields .of'gaso'line with a higher overall octane number are obtained.

' -While I have described what I consider `to be the most advantageous modifications of my process, itwill be evidentztoithose skilled .in thisv art that various other modifications can be made in the specic procedures set out without departing from the scope of this invention. Thus, it is possible to recover the C3 and lC4' hydrocarbons in the rgases leaving the stabilizer-*by other than And, asA mentioned previcusly, itl is possible to employ three `or even more conversion stages, preferably operated at increasingly higher temperatures.` It is also possible to employ centrifugal type separators in separators l1 and 26 if desired. Other modiiica.-`

tions of 'my process which fall Within' the scope of the following claims will be immediatelyevij dent to those skilled in the art.

I claim:

,comprise mixing `a gasfoil with C3 and C4 hydrocarbonsjin'proportions'varyingfrom about 50 to 75% `based-on thevolume ofthe gas oil, passing the mixture. through a conversion zone under a pressure 4.of 600 to.1,500,lbs. per square inch and Aat atemperature of 950 to 990 F.. until the in-` cipientl coking, stageisreached,A quenching the .conversion products to separate out substantially only the heavy tar-forming constituents, mixing -additionalrCs and C4- hydrocarbons with there- ;maining vaporsand passing the resulting mixture l througha second .conversion zone under a pressurejof 500 to,,1,400 lbs. per square inch and at a ternpera ture of 980 to .1,000o F. until the incipient colsingr Ystage isi again reached, quenching the conversion products to separate out additional yheavy tar-forming constituents, combining the remaining :vapors with additional C3 and C4 hydrocarbonsand with a reduced crude, passing the lresultingmixture through a third-conversion zone undera pressure of V3.00 to 500 lbs. per square line land ray viscosity-breaking of the reduced -rpar. ssing theresulting `mixtureinto the next succrude, quenching the A conversion products, introyducingthequenched conversion products into a fractionating zone and therein separating out a heavy bottoms fraction, anintermediate cycle gas oil fraction and an overheadgasoline fraction containing C3 and Crhydrocarbons, separating the-Cs and `CiAhydrocarbons fromthe gasoline overhead, andrecycling such C3 and C4 hydrolcarbonsfor admixture with `the charges to the respective,,conversion zones. 2.,In..the process as. claimed .in claim 1, the

steps of :returning ai portion ,of the y intermediate cycle gas oil fraction, for Vadmixture with the -,1n1tial gas oil charge and utilizing the remainder of. thecycle'gas oilfraction in the respective quenching steps.

.v .3.. In the cracking of `a gasoil to obtain a gasolinev of. high loctanerating, the' process which' comprises ,passing the gas oilr in adinixture with normally gaseous 0 3 and C4v hydrocarbons through a .plurality y ofl conversion zones main- .,tained yunder superatmospheric pressure and at successively. I higher ytemperatures substantially higher than thosewhich could be employed without undesirable coke formation in the absence of the gaseous hydrocarbons, mixing the conversion products from each conversion zone with a quench oil in acseparating zone lin quantity sufcient to separate outv onlyr theA heavy tar-forming constituentsv as av liquid phase, introducing into the vapors remainingafterl each separation anadditionalquantity of .Csand C4 hydrocarbons and ceeding conversionzonewadmixing the vapors 'from the final separating zone with a reduced ycrude and with ,additional C3 and C4' hydrocarbons and'passingthe mixture through a nal conversion Zone at a superatrnospheric pressure and ata temperature lower than those employed in the previous conversion zones to effect further conversion of the vaporsinto gasoline, passing the products from the final conversion zone into a fractionating Zone, recovering from the fractionating Zone an overhead fraction containing gasoline and C3andf'C4 hydrocarbons; separating the .C3 andrC4 hydrocarbons from said overhead frac- Aton,V and recyclingthem to be admixedr with the hydrocarbon charges entering the successive conversion lZones.

AUGUST HENRY SCHUTIE.

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