US2335551A - Combination catalytic and thermal cracking - Google Patents

Description

Nov. 30, 1943. R. F. 'rRow COMBINATION CATALYTIC ND THERMAL CRACKING Filed Dec. 23, 19'42 lll `\v mff/ J ....m4 w

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Patented Nov. 30, 1943 mc. AND THERMAL l i CKING Richard F. Trow, Lawrenceville, Ill., assigner to communion cem.

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The Texas Company. New York,

poration of Delaware N. Y., a cor- Application December 23, 1942, Serial No. 469,913

s claims. (ci. 19e-49) i This invention relates to a combination catalytic and thermal cracking process and is concerned particularly with the processing of crude petroleum stocks to obtain maximum yields of high anti-knock motor fuel.

In accordance with the invention straight-run stocks, such as gas oil, are combined with intermediate stocks from the thermal cracking stepl and the mixture subjected to catalytic cracking While crude oil residual stocks, such as topped or reduced crude, and the higher boiling products from the catalyticcracking step are subjected to thermal cracking. In the processing of the gas oil or intermediate stocks straight-run -gas oil and thermally cracked gas oil are subjected to single-pass catalytic cracking and the gas oil or higher boiling stock from the catalytic `cracking operation, is subjected to single-pass thermal cracking. The relationship of the thermal and catalytic gas oil cracking steps is such that the thermal cracking serves to produce a stock Well adapted for catalytic cracking and the catalytic cracking functions to produce a stock well adaptedfor thermal cracking. And by maintaining single-pass conditions lin the catalytic cracking and thermal cracking operations respectively it is possible in each cracking stage to carry on a maximum extent of cracking which is conducive to conversion into gasoline constituents of high anti-knock quality.

For the purpose of fully disclosing the invention reference is made to the accompanying drawing which is a ow diagram illustrative of the invention.

Crude petroleum is directed by a pump l through a heat exchanger il wherein the crude oil is heated to the desired distilling temperature and passes thence to a distilling and fractionating tower i2. The heat exchanger li may be supplied with hot products from the cracking system. In the tower i2 vapors separate from residue and the vapors are subjected to fractionation to segregate the fractions desired. In the specific embodiment illustrated intermediate constituents, such as gas oil fractions, collect as a reilux condensate in a tray i3, heavy naphtha fractions collect in tray i4, while uncondensedl. vapors pass through a condenser coil l5 thence to a receiving drum I6 wherein a gasoline distillate is coliected.

The thermal cracking portion of the system includes a heating coil l1 disposed in a furnace I3 and a heating coil IB'disposed in a furnace 2i). A transfer line 2| from the heating coil i1 and a transfer line 22 from the heating coil I9 and I9 are merged for cracking in a reaction chamber 2-3, the eilluent from which passes to a subsequent reaction chamber 24, the eflluent from which passes to a ilnal reaction chamber 25 wherein separation of vapors from liquid residue takes place. The several reaction chambers are insulated against heat loss so that each of the chambers may be maintained at a cracking tem perature. Liquid is prevented from accumulating in any of the reaction chambers, the mixed liquid and vaporous and gaseous constituents iiowing from chamber 23 through chamber 24 thence to chamber 25 where liquid consituents are separated from vapors and gases, the separated vapors passing through a vapor line 26 to a primary fractionating tower 2l and vapors uncondensed therein pass through a vapor line 28 to a secondary fractionating tower 29. Uncondensed' vapors pass from the tower 29 through a condenser 30 thence to a distillate receiver or gas separator 3l wherein the gasoline or naphtha distillate is collected.

The thermal cracking portion of the system l advantageously includes a heating coil 32 disposed in a furnace 33 and a heating coil 34 disposed in a furnace 35. These heating coils are intended particularly for the thermal cracki'ng or reforming of light stocks. such as naphtha, as is explained hereinafter,l The eiiiuent from these coils, instead of being directed to the primary reaction chamber 23, is advantageously passed to a subsequenty portion of the cracking zone or to the fractionating portion of the thermal cracking system. As illustrated a transfer line 36 from heating coil 32 and a transfer line 3l from heating coil 34 serve to direct the eiiiuent from these coils to the primary fractionator 2l.

The liquid residue separated in the reaction chamber 25 passes through a line 38 and pressure-reducing valve 39 to a coking drum 40. In practice a plurality of coking drums are employed so as to enable the removal of the coke without interrupting the continuity of the complete process. The coking drums are insulated against heat loss and a suilicient portion of the vapors from reaction chamber 25 is included with the liquid residue passed to the coking drum to accomplish autogenous coking therein. The

evolved vapors from the coking drum pass Y ythrough a line 4I to a fractionating tower 42. This tower is formed with a primary dephlegmating section 42A which receives the vapors from the coking drum and a secondary section 42B separated by a trapout tray 43. In some casesan additional fractionating -section 42C is provided separated from the section 42B by a trapout tray 44. Uncondensed vapors pass from the tower 42 to a condenser 45 thence to a distillate receiver or gas separator 46.

The catalytic cracking portion of the system includes a heating coil 41 disposed in a furnace 48 and a catalyst chamber 49 which receives the eiliuent from the heating coil and wherein the heated oil is contacted with a catalyst. The catalyst consists advantageously of a synthetic silica-alumina type of catalyst. Various acidtreated and metal-substituted clays such as the Super-Filtrols andacid-treated and metal-substituted natural or artificial zeolites, such as the artificial zeolite known as Doucil may be employed. Various metals such as uranium, molybdenum,` manganese, lead, zinc, zirconium, nickel and the like, may be substituted in the clays or zeolites. Likewise, the combination of certain acid-treated active clays of the character of .iltrol,v together with added proportions of alumina or silica or both may be employed. Alumina alone may be used under certain conditions. The synthetic alumina catalysts can be improved by the addition of other constituents such as zirconium oxide or molybdenum oxide. The oil may be contacted with the catalyst in various ways. Thus the oil may be vaporized inthe heating coil and the vapors passed through the reaction chamber provided with a stationary bed of the catalyst through which the vapors pass;` or a moving mass of granular catalyst may be maintained in the reaction chamber; or nely divided catalytic material may be admixed with the oil .stream prior to introduction to the reaction chamber; or the catalyst may be introduced in a finely divided, commlnuted or powdered form suspended in a Agaseous l,or vaporous medium whereby it is carried along with the vapors through the reaction chamber.

The products of the catalytic cracking pass to a separating and fractionating chamber 50 wherein the products are subjected to a primary separation and dephlegmation. Uncondensed vapors pass to a secondary fractionating tower 5i wherein a further fractionation occurs to separate reflux condensate from uncondensed vapors. coil 52 thence to a receiving drum or gas separator 53 wherein the gasoline or naphtha -distillate is collected.

in practicing the invention a crude oil charging stock is heated to a desired distilling temperature and separated into vapors and residue in the fractionator I2. `'Ihe crude oil residue is withdrawn from the tower through a. line 54 and directed by a pump 55 through aline 56 to the dephlegmating tower 50 to dephlegmate the vapors therein received from the catalytic cracking operation. The resultant mixture of residual constituents and reflux condensate comprising the higher boiling products from the catalytic cracking operation and unvaporized constituents of the crude residue is withdrawn from the tower 50 by al pump 51 and directed through a line 58 to the heating coil` I9 wherein the oil is subjected to cracking temperature ,under superatmospheric pressure. The vapors from the tower 50 are subjected to further fractionation in the sec- The latter vapors pass to a condenser ondary tower` 5| to separate intermediate constituents of the nature of gas oil and kerosene from the lower boiling motor fuel distillate; The intermediate condensate is withdrawn from the tower 5I by a' pump 59 and directed through a line GII to the heating coil I1 wherein the oil is subjected to cracking temperature under superatmospheric pressure. The condensate stock is subjected to higher cracking temperatures or to higher rates of cracking per pass in the coil I1 than are applied to the residual or higher boiling Y stock being subjected to cracking in the coll I9. The eluent from the two heating coils is com# bined for further thermal cracking in the reaction chambers 23, 24 and 25 and the products separated into vapors and residue in the latter reaction chamber. The separated vapors pass to the fractionators 21- and 29 wherein the vapors l are fractionated under superatmospheric pressure. The residue, together with a sufficient portion of the vapors to accomplish coking, is passed to the coking drum 40 maintained under reduced pressure wherein conversion to coke is accomplished by contained heat. The higher boiling reux condensate which is separated in the presline 68 to the tower 42 or to the section 42B thereof to reiiux the vapors therein. In this way intermediate constituents of the nature of gas oil or kerosene comprising straight-run constituents and constituents from the thermal cracking and coking operations are combined in the redux condensate collecting in tray 43. This condensate is withdrawn through a line 69 and is directed by a pump 10 through the line 1I to the 4heating coil 41 and constitutes the charge to the catalytic cracking step.

From the preceding description it will be seen that the charging stock for the catalytic cracking step consists of a mixture` of straight-run stock and thermally cracked stock of intermediate boiling range and that the charge to the thermal cracking step consists of the higher boiling or residual component of the crude oil and the higher boiling as well as intermediate boiling products of the catalytic cracking step. Furthermore, it will be seen that `there is no recycling within either the thermal or catalytic cracking stage. Inv other words, the charge for the catalytic cracking operation is passed once through the catalytic cracking zone and the higher boiling or insuiiciently converted constituents are directed to the thermal cracking zone and likewise the charge for the thermal cracking zone is passed once through the thermal'cracking zone and intermediate bciling'range products recovered from the thermal cracking are directed to the catalytic cracking zone. v a In the processing of crudes containing gasoline of relatively low anti-knock quality it may be desirable to subject such gasoline to reforming. For this purpose a pump 12 withdraws a heavy naphtha fraction from the tray I4 through a. line 13 and directs it through a line 14 to the ing stock is heated to a distillate temperature of heating coil 32. In case it is desired to subject the entire gasoline fraction to reforming the distillate from receiver I6 may be conducted through a line thence through line 13, pump f 12 and line 14 to the heating coil 32. When reforming only the heavy' gasoline or naphtha fraction, the light fraction may be`withdrawn through a line 1B.

The gasoline product obtained in the flashcoking operation will ordinarily be of lower antlknock quality than that of the gasoline obtained by either the thermal cracking or catalytic cracking. For the purpose of reforming this gasoline, which ordinarily needv be applied only to the higher boiling constituents thereof, heavy naphtha condensate which collects in the tray 44 of the tower 42 is withdrawn through a line 11 and directed by a pump 18 through a line 19 to the heating coil 34 wherein it is subjected to reforming. For the purpose of reforming gasoline constituents from the thermal cracking operation a trapout tray 80 Amay be provided in the pressure fractionator 29 in which a heavy naphtha fraction may be collected. This fraction may be passed through a line 8l and pressure-reducing valve 82 to the fractionating section 42C. Due to the relatively high pressure in the tower 29 the fraction Withdrawn from tray 80 will contain f light fractions which are vaporized in the low pressure fractionator 42C andare consequently recovered in the distillate collected in the re.- ceiver 46. In this manner a maximum gasoline yield is obtained when improving the volatility and octane number.

In an alternative method of operation the cracked residue separated in the reaction chamber instead of being subjected to coking is passed through a branch-line 83 and vcombined with the separated vapors passed to the fractionator 21. The resultant residue and reflux condensate collected in tower 21 is then passed through pressure-reducing valve 62 and line 6l to thel dephlegmating section 42A wherein it is subjected to flash distillation. The flashed residue is withdrawn through a line 84. When the coking drum 40 is used the dephlegmate produced in the dephlegmator 42A and withdrawn through line 84 consists essentially of high boiling constituents produced in the coking operation. This product is of a highly aromatic character and is well adapted for fuel oil blending. g

In practicing the invention the catalytic cracking operation is conducted under relatively low pressures such as from about 100 p. s. i. down to atmospheric pressure, while the thermal cracking operation in the reaction chambers is conducted under higher pressures preferably over 200 p. s. i. The pressure in the ractionator ci the thermal cracking system is maintained rela'-1 tively high. It is advantageous to maintain a suiiciently high pressure in the reaction cham bers as to have pressures of 400 p. s. i. or higher in the fractionator 29 and gas separator 3l, parn ticularly when it is desired to recover normally gaseous constituents for recycling in the system.

Under such pressures the gases separated in re cer/ing drum 3i 'consist predominantly oi hydrogen,'methane and C2 hydrocarbons and the gasoline product collected in the receiver 3l may be debutanized to obtain a normally gaseous fraction comprising CsCl hydrocarbons which may be recycled to any of the cracking or reforming coils for gas reversion with the normally liquid hydrocarbons undergoing cracking or reforming.

In an example of invention a crude oil charg= 670 F. and fractionated in the tower, I2 to obtain a gas oil fraction of 800 F. end point, heavy naphtha of 450 F. end point andlight naphtha of 300 F. end point. The straight-run gas oil is refluxed in the fractionator 42B, while the reduced crude is refluxed in the fractionator 50.

A gas oil fraction consisting of straight-run constituents and constituents produced in thermal cracking 'and coking is withdrawn from tray 43 and after heating in the coil 41 to 1000 F. is.

vboiling constituents of the catalytic cracking are withdrawn from tower 50 at a temperature of '750 F. and subjected to cracking in the coil I9 at a temperature of 930 F. The vapors from the tower 50 are subjected to further fractionation in the tower 5I to separate higher boiling reflux condensate from the catalytic, gasoline. The reflux condensate is withdrawn from the tower at a temperature of 580 F. and is subjected to cracking in the coil. I1 at a temperature of 930 F. The effluent from coils I1 and I9 is combined. and subjected to further thermal cracking in the reaction chambers under a pressure of 500 p. s. i. with a temperature of 900 F.

in the final reaction chamber 25. The bottoms from reaction chamber 25, together with a minor .portion Aof the vapors sufllcient to support autogenous coking, are expanded into 'the coking .maintained at 450 p. s. i. and the bottoms are withdrawn at a temperature of 850 F. and refluxed in the lower pressure dephlegmator 42A. The tower 29 is maintained at a pressure of 430 p. s. i. and reflux condensate is withdrawn therefrom at a temperature of '180 F. and reiluxed in the dephlegmator 42A. Heavy condensate constituting a synthetic fuel oil is withdrawn from dephlegmator 42A at av temperature of 800 F. A heavy naphtha fraction consisting predomi nantly of gasoline fractions produced in the coking is Withdrawn from tray 44 and, after being` subjected to thermal reforming in the coil 34 at a temperature of 1000 F. and under a pressure of 1200 p. s. i. is' directed to ractionating tower 2. The light coke still gasoline is collected in the receiver d6 and the thermally cracked gasof line is collected in receiver 3l Inanother example of the invention the process is conducted in a manner similar to that of the preceding example except that the coking operation is omitted and the residue from the reaction chamber 25 is combined with the reflux condensate formed in the fractionator 21 and flashed in the dephlegmator 42A from which a fuel oil product is withdrawn through the line 84.

The coire still gasoline4 collected in receiver' 46 may, with advantage, be reuxed on the pressure tower 2Q so that the total gasoline product from the thermal cracking and hashing or coking operation may be collected in the receiver 3 l. This procedure is advantageous when it is desired to subsequently debutanize the gasoline product so as to obtain C304 fractions for recycling to any of the cracking or reforming coils.

While I have described a particular embodiment of my invention 'for purposes of illustration, it should be understood that various moditlcations and adaptations thereof which will be obvious to one skilled in the art, may be made within the spirit of the invention as set forth in the appended claims. I claim:

1. In a combination thermal and catalytic cracking process the method that comprises fractionating ,crude petroleum to obtain a residual fraction and a condensate fraction, subjecting oil to catalytic cracking vin a catalytic cracking zone, directing the resultantproducts of the catalytic cracking to a dephlegmating zone, dephlegmating the vapors therein with said residual fraction, passing unvaporized constituents from said dephlegmating zone through a'heating zone wherein the oilis subjected to cracking temperature, subjecting the dephlegmated vapors from said dephlegmating zone to further fractionation to separate higher boiling fractions from lower boiling fractions, passing said higher boiling fractions through a heating zone wherein the oil is subjected tocracking temperature, combining the eliluent from said heating zones for thermal cracking in a reaction zone maintained under superatmospheric pressure and wherein `separation, of vapors from liquid residue takes place, fractionating the separated vapors under superatmospheric pressure lto separateI reflux condensate from lighter fractions, directing said liquid residue into a lower pressure ilashing zone wherein vapors separate from residue, passing the separated vapors to a separate dephlegmating zone, directing reflux condensate, obtained in the fractionation of the vapors from the thermal cracking operation, and condensate, obtained from the crude oil fractionation, to the latter dephlegmating zone to dephlegmate the flashed vapors therein'and subjecting the com- 4mingled constituents to fractionation to obtain a fraction containing flashed constituents as well as constituents from the crude oil fractionation and thermal cracking, and directing said fraction to the aforesaid catalytic cracking zone.

2. In a combination -thermal and catalytic cracking process the method that comprises fractionating crude petroleum to obtain a residual fraction and a condensate fraction, subjecting oil to catalytic cracking in a catalytic cracking zone, directing the resultant products of the catalytic cracking into a dephlegmating zone, de-

' phlegmating the vapors therein with said residual fraction, passing unvaporized constituents from said dephlegmating zone through a heating zone wherein the oil is subjected to cracking temperature, subjecting the dephlegmated vapors from said dephlegmating zone to further fractionationA to separate higher boiling fractions from lower boiling fractions, passing said higher boiling fractions through a heating zone wherein the oil is subjected to cracking temperature, combining the eiiiuent from said-heating zones for thermal cracking in a reaction -zone maintained under superatmospheric pressure and wherein separation of the vapors from liquid residue takesplace, fractionating the separated vapors under superatmospheric pressure to separate reflux condensate from lighter fractions, directing said liquid residue to a coking zone wherein it is converted to coke, passing evolved vapors from the coking zone to a separate dephlegmating zone, introducing reflux condensate, obtained in fractionating the vapors from the thermal cracking operation, and condensate, obtained in thecrude petroleum fractionation. to the latter dephlegmating zoneto dephlegmate the vapors therein and subjecting the commingled constituents to fractionation to produce a fraction containing constituents from the cokl ing operation and from the crude petroleum fractionation and thermal cracking and directing said fraction to the catalytic cracking zone.

3. In a combination thermal and catalytic cracking process the method thatV comprises fractionating crude petroleum to obtain a gas oil fraction and a residual oil fraction, subjecting oil to catalytic cracking in a catalytic cracking zone, directing the resultant productsof the catalytic cracking to a dephlegmating zone, introducing said residual fraction to said dephlegmating zone to dephlegmate the vapors therein, withdrawing a residual fraction from said dephlegmating zone and passing said fraction through a heating zone wherein it is subjected to cracking temperature, subjecting lvapors uncondensed in said dephlegmating zone to further fractionation to separate a gas oil fraction from lighter con-- stituents, passing said gas oil fraction through a heating zone wherein it is subjected to cracking temperature, combining the eiiiuent from said heating zones for thermal cracking in a reaction zone maintained under superatmospheric pressure and wherein separation of vapors from liquid residue takes place, fractionating the separated vapors to obtain a gas oil fraction, flash-` distilling said liquid residue to obtain a gas oil fraction, combining gas oil constituents obtained in the fractionation of the crude petroleum with gas oil constituents from the thermal cracking and coking operations and directing the resultant composite gas oil fraction to said catalytic cracking zone.

4. In a combination thermal and catalytic cracking process the method that comprises fractionating crude petroleum to obtain a residual fraction and a condensate fraction, subjecting oil to catalytic cracking in a'catalytic cracking zone, directing the resultant products of the catalytic cracking into a dephlegmating zone, dephlegmating the vapors therein with said residual fraction, passing unvaporized constituents from said dephlegmating zone through a heating zone wherein the oil is subjected to cracking temperature, subjecting the dephlegmated vapors from said dephlegmatingzone to further fractionation to separate higher boiling fractions from lower boiling fractions, passing said higher boiling fractions through a heating zone wherein `the oil is subjected to cracking temperature, combining the eiiluent from said heating zones for thermal crackingA in a reaction zone maintained under superatmospheric pressure and wherein separation of the vapors from liquid residue takes place, fractionating the separated vapors under superatmospheric pressure to separate reflux condensate from lighter fractions, directing said liquid residue to a coking zone wherein it is converted to coke, passing'evolved vapors from the coking zone to a separate dephlegmating zone, introducing reflux condensate, obtained in fractionating the vapors from the -thermal cracking operation, and condensate, obtainedin the crude petroleum fractionation, to the latter dephlegmating zone to dephlegmate the `vapors therein and subjecting Athe commingled constituents to fractionation to separate a naphtha. fraction from higher boiling reflux condensate containing constituents from the coking operation and from the crude petroleum fractionation and thermal cracking, directing the latter higher boiling reflux condensate to the catalytic cracking zone, passing said naphtha fraction through a reforming zone wherein the v alytic cracking zone, directing the resultant products of the catalytic cracking to a dephlegmating zone, dephlegmating the vapors therein with said residual fraction, passing unvaporized constituents from said dephlegmating zone through a heating zone wherein oil is subjected to cracking temperature, subjecting the dephlegmated vapors from said dephlegmating zone to further fractionation to separate higher 'boiling fractions from lower boiling fractions,

passing said higher boiling fractions through a heating zone wherein the oil is subjected to cracking temperature, combining the eiiiuent from said heating zones for thermal cracking in a reaction zone maintained under superatmospheric pressure and wherein separation of vapors from liquid residue takes place, fractionating the separated vapors under superatmospheric pressure to separate reflux condensate from lighter fractions, directing said liquid residue into a lower pressure flashing zone wherein vapors separate from residue, passing the separated vapors toa separate dephlegmating zone, directing reflux condensate, obtained in the fractionation of the vapors from the thermal cracking operation, and

condensate, obtained from the crude oil \frac' tionation, to the latter dephlegmating zone to dephlegmate the ashed vapors therein and subjecting the commingled constituents to fractionationto obtain a fraction containing flashed constituents as well as constituents from the Y.

crude oil fractionation and thermal cracking. di-

recting said fraction to the aforesaid catalytic cracking zone, passing the naphtha fraction from the crude petroleum through a thermal reforming zone wherein the naphtha is subjected to a reforming temperature to raise the anti-knock quality thereof and fractionating the resultant reformed products with vapors from the aforesaid thermal cracking.

6. In a combination thermal and catalytic cracking process the method that comprises fractionating crude petroleum to obtain a residual fraction, a gas oil fraction and a naphtha fraction, subjecting oil to catalytic cracking in a catalytic cracking zone, directing the resultant products of the catalytic cracking into a dephlegmating zone, dephlegmating the vapors therein with said residual fraction, passing unvaporized constituents from said dephlegmating zone through a heating zone wherein the oil is subjected to cracking temperature, subjecting the dephlegmated vapors from said dephlegmating zone to further fractionations to separate higher boiling fractions fromy lower boiling fractions, passing said higher boiling fractions through a heating zone wherein ,the oil is subjected to cracking temperature, combining the,

eiiiu'ent from said heatingv zones for thermal cracking in a reaction zone maintained under superatmospheric pressure and wherein separation of the vapors from liquid residue takes place, fractionating the separated vapors under superatmospheric pressure to separate reflux conden-A sate from lighter fractions, directing said liquid residue to a coking zone wherein it is converted to coke, passing evolved vapors from the coking zone to a separate dephlegmating zone, introducing reflux condensate, obtained in fractionating the vapors from the thermal cracking operation, and condensate, obtained in the crude petroleum fractionation, to the latter dephlegmating zone to dephlegmate the vaporsv therein and subjecting the commingled constituents to fractionation to separate a naphtha fraction from higher boiling reflux condensate containing constituents from the coking operation and from the crude petroleum fractionation and thermal cracking, directing the latter higher boiling re-` flux condensate to the catalytic cracking zone, passing the aforesaid naphtha fractions'through a reforming zone wherein the naphtha is subjected to a reforming temperature to raise the anti-knock qualityI thereof and fractionating resultant products of the reforming with vapors from the aforesaid thermal cracking.

RICHARDF. TROW.

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