US2735876A

US2735876A - Conversion of gaseoos- hydrocarbons

Info

Publication number: US2735876A
Authority: US
Publication date: 1956-02-21
Anticipated expiration: 1973-02-21

Description

Feb. 2l, 1956 H. v. Hess ETAL 2,735,876

CONVERSION oF GAsEoUs HYDRocARBoNs INT0 ETHYLENE AND ARoMATIcs Filed Aug. 20, 1952 `u Q 2z N l? @8g Qu l u 1 (t lu 0 u Pr N LY I 22 N f Q N l N w n L 5 N a w N Q Q N N lq 4 v Y Nq 3 r Q S M D LQ l) f` NSN Q Q* N Q 25 LA \`\\2 g L w Q 5m Q N N N t I N 1 x. 3 m i L u N l INVENTOR s HOWARD V. H555 /VOQMA A/D. CAI/P75@ BYgA ATTORNEY' United States Patent CONVERSIGN 0F GASEUS HYDROCARBONS INTO ETHYLENE AND AROMA'IICS Howard V. Hess, Glenham, and Norman D. Carter, Poughkeepsie, N. Y., assignors to The Texas Company, New York, N. Y., a corporation of Delaware Applicationy August'zl, 1952, Serial No.. 305,332

4 Claims. (Cl. 260-673) This invention relates to an improved process for pyrolyzing ethane and Cs hydrocarbonsA to .form a mixture of ethylene and a liquid aromatic fraction `containing substantial. quantities of benzene, toluene and .naphthalenes. More specifically, this invention discloses a process whereby a substantial quantity of aromatic hydrocarbons is produced as a byproduct of ethylene manufacture by theV pyrolysis of C3 hydrocarbons.

lt has been knownV in the past thataromatics can be producedA as a byproduct of ethylene production by pyrolysis of C3 hydrocarbons. However, previous proc,- esses for. concomitantly producing ethylene and aromatics from propane or propane-propylene mixtureshave, had the disadvantages thatincreased aromaticproduction has always been at the expense of, the ethylene content of the product mixture and that the aromatic byproducts have had a substantial diolen content. Moreover, in the known processes, conversion of' propane or propanepropylene mixtures has been held below 7'5 per cent per pass in order to minimize coke formation. As a consequence it has been necessary in conventional processes to resort to recycle of hydrocarbonsin, order to realize economic utilization ofthe charge hydrocarbon. Recycle requires a substantial Yinvestment in gas separation equipment such as oil adsorbers, and strippers to separate unconverted hydrocarbons from. the gaseous reaction product comprising ethylene, methane hydrogen etc. ln contrast to former pyrolysis methods for gaseous hydrocarbons, the process of' this invention etfects better lthan 97 per cent conversion of hydrocarbons per pass ,so that a once-through operation is realized,` and recycle iseliminated. in addition, the production of aromatics at the expense of ethylene is minimized with the consequence that good yields of both ethylene and aromatics are obtained. The aromatic product isyof high quality and. possesses a low dioleiin content; nitration grade benzene and toluene are simply prepared'from the aromatic product by acid treatment followed by fractional distillation.

The once-through ethane or Cs hydrocarbon pyrolysis process of this invention is a combination of cracking and soakingroperaticns, each of which is effected under prescribed critical conditions. Ethane can be used in the process of the invention, but since the charge is normally a, Cs hydrocarbon, the invention will hereafter be described' asinvolving Cs hydrocarbonv conversion. The C3 hydrocarbon, which may be propane, propylene or a mixture of the two, is introducedat an inlet temperature fr'Omatmospheric to 800 F. into a cracking zone at a pressure below 50 p. s. i. g. and' is raised to a temperature between 1350 and 1550 F. by indirect heat exchange in a periodV of 0.5 to 2 seconds, which is the residence time, calculated on the basis of'. charge gas, prescribed for the cracking zone. The residence time in the cracking section Varies within the prescribed limits with the composition of the C3 hydrocarbon charge mixture and with the amount of preheatV imparted to the Cshydro- Lcarbon mixture, that is the temperature between atmos- ICC pheric and 800 F. selected forl introduction of the gas mixture into the cracking zone;V Better than per cent conversion of the charge C3 hydrocarbon mixture is elfected in the cracking sectionwhich is, of necessity, constructed of a nickel-free chrome steel, and which possesses a critical amount of surface area per volume of freef space; in order to effect better than 90 per cent conver- Vsion in the cracking Zone, it is necessary that the amount of surface area per volume of free space be maintained above 20() square-feet per cubic foot. The etiiuent gas from the crackingzone which is at a temperature between 1350 and 1550 F., isV immediately passed into an adiabatic or isothermal soaker which is also constructed of nickel-free chrome steel., The gas is passed through the soaking section at a rate suchthat` a residence time of 10 to 40 seconds is realized'thereimand issues from the soaker at a temperature in the range of 1100 to 1350" F. in the soaking zone, additional conversion of C3 hydrocarbons takes place so that better than 97 per cent conversion of charge is effected in the combined cracking and soaking operations, In addition, aromatic production is raised toa level higher than l5 per cent of the charge hydrocarbon gas and the diene content of the aromatic fraction is substantially reduced by the soaking operation.

it is necessary thatl the cracking and soaking operations be effected within theprecise limits defined above inorder to obtain high yields, of both ethylene and aromatics. The production of high yields of both ethylene and aromatics and the prevention of excessive carbon deposition are also dependent on observance of the prescribed conditions in both; the cracking and soaking zones. The critical conditions inthe cracking zone are pressure, temperatures of charge introduction and of product withdrawal, residence time and amount of surface area per volume of free space in the cracking zone which is constructed of nickel-free chrome steel. The critical conditions in the soaking zone are the material of construction, the inlet and outlet temperatures, pressure and the residence time.

Ethane, propane, propylene ora mixture of these hydrocarbons can bev employed in the process of this invention for producing ethylene and a high-quality aromatic-rich fraction. Since pure ethane, propaneV and propylene are high cost products, they are not usually employed in commercial adaptations of thisV process. Excellent results have been obtained with pure propane and propylene in small-scale pilot units. rifhe most commonly used charge is a mixture comprising70 to 95 perV cent propane and 5 to 30 per cent propylene which is available as a relinery byproduct.

lnrorder to prevent coking in the cracking zone, it is necessary that pressures below 50p. s. i. g, be employed therein. The operational pressure should be as close as possible to atmospheric as is compatible with dow of the gases through the processequipment. Since the pressure limitation of 50 p. s. i. g. is also to avoid excessive pressure drop throughout the equipment, it is recommended that aV bundle of relatively short cracking tubes be used in the cracking section ratherthan a long cracking coil. The construction of the cracking section'out'of a bundle of short tubes minimizes pressure drop and, in addition, is helpful in obtaining the required surface to volume relationship.`

The C3 hydrocarbon mixture may be introduced into the cracking zone at temperatures inthe range of atmospheric to 800"V F., but it is desirable to impart as much preheat as possible into the C3 hydrocarbon mixture. Accordingly, the charge mixture is ordinarily introduced into thecraeking zone at a temperature between 500 and 800-nr F. Sutiicient'heat is applied to the cracking zone so' that the product mixture is raised by indirect heat exchange to a temperature between 1350 and 1550 F. and preferably between 1450 and 1550 F. in a period of 0.5 to 2 seconds. The necessity of supplying sufficient heat to the cracking zone to supply the necessary sensible heat and endothermic heat of reaction in the prescribed contact time is the basis for the requirement that the amount of surface area per volume in the cracking zone be maintained at a high level.

A residence time of 0.5 to 2 seconds based on the charge gas fed to the cracking zone is essential for effecting better than 90 per cent conversion of C3 hydrocarbon mixture in the cracking zone. As Would be expected, the residence time is dependent in part on the amount of preheat that has been imparted to the C3 hydrocarbon mixture prior to its introduction into the cracking zone, on the composition of the C3 hydrocarbon mixture and on the amount of surface area per volume of free space. Regardless of the variation in these conditions, the residence time must fall within the prescribed limits of 0.5 to

2 seconds. In the preferred modification wherein the C3 hydrocarbon mixture is preheated to a temperature between 500 and 800 F., contact times between 0.7 and 1.5 seconds are preferred in the cracking zone.

The tubes comprising the cracking zone must be constructed of steel which is free from carbon formers such as nickel and vanadium in order to prevent excessive carbon formation during the higher than 90 per cent C3 conversion effected therein. Chrome steel comprising 27 per cent chromium and 73 per cent steel, is a preferred material for the cracking tubes.

The transmission of sufficient heat to effect better than 90 per cent conversion of C3 hydrocarbons per pass in the cracking zone requires a large amount of heat exchange surface. It has been discovered that a minimum amount of 200 square feet per cubic foot of free space is necessary in the cracking tubes in order to effect better than 90 per cent conversion of C3 hydrocarbons per pass at the conditions prescribed in this invention. While the minimum amount of surface area per volume above 200 square feet per cubic foot must be maintained in the cracking zone, it is recommended that the cracking zone be so constructed that the amount of surface area per volume of free space be in the neighborhood of 225 to 350 square feet per cubic foot.

There are two general means of constructing the cracking zone so that the necessary surface to volume ratio is realized. The cracking zone can be constructed of a plurality of small diameter tubes which are, of necessity, of limited length because the pressure drop allowable in the cracking section is limited to a maximum of 50 p. s. i. g. The other alternative involves using larger diameter tubes which are packed with material of the same type employed in constructing the cracking tubes; for example, large diameter tubes can be packed with small diameter tubes or chrome steel wool. The necessary packing to give the required surface to volume ratio can also be obtained by inserting a honeycomb framework within larger diameter tubes.

The gas issuing from the cracking zone is immediately introduced into the adiabatic soaker. The adiabatic soaker contains a large volume of free space in comparison with surface area and is essentially a large insulated drum wherein the effluent from the cracking section cools adiabatically. The inlet temperature to the soaking section is the outlet temperature from the cracking zone and falls in the range of 1350 to 1550 F., and preferably in the upper half of this range. After the product gas has been in the soaker for a period of -40 seconds it issues from the soaking section at an outlet temperature in the range of 1100 to 1350 F. A residence time of 12 to 25 seconds and an outlet temperature of 1200 F. have been found to be the preferred conditions for withdrawing an eiuent from the soaking section. The prescribed residence times in the soaking section are also based on the gas charged to the cracking section.

During its passage through the soaking section, the conversion 0f C3 hydrocarbons is raised to higher than 97 per cent. There is also effected a substantial increase in the liquid aromatic product which is primarily realized from additional C3 conversion rather than from ethylene utilization. The third important result achieved during the soaking operation is that the diene content of the liquid aromatic product is substantially destroyed so that a high quality liquid aromatic fraction is obtained by the prescribed cracking and soaking operations of this invention.

As a result of passage through the cracking and soaking zones under the above-described conditions, better than 97 per cent conversion of the C3 hydrocarbon mixture is effected. The yield of ethylene, basis the weight of propane converted, is between 30 and 35 per cent. The yield of high-quality aromatic-rich fraction obtained by the process of the invention is between 23 and 30 per cent; the aromatic fraction comprises condensed liquid fraction and liquids recovered from the gas stream by absorption.

The high-quality of the aromatic product is indicated by the bromine number and diene number of the product prior to further refining such as acid treatment. The bromine numbers of the untreated aromatic fraction are in the neighborhood of 25 to 40, and the diene numbers vary from 5 to 15. The aromatic fraction comprises approximately 5 per cent non-aromatic fraction lighter than benzene, 45 per cent benzene, 10 per cent toluene, 10 per cent naphthalene and 30 per cent higher boiling aromatics. The benzene fraction obtained on fractional distillation of the aromatic product generally contains less than 0.6 weight per cent diolens and can be converted to nitration grade benzene by acid treatment.

The process of the invention is diagrammatically illustrated in the accompanying drawing.

Propane is introduced through a pipe 1 into a preheater 2 wherein it is raised to a temperature of about 750 F. A C3 hydrocarbon mixture at a temperature of 750 F. is then introduced through a pipe 3 into a manifold 6 which is located in a furnace 4 and which communicates with the tube bundle 7 comprising the cracking zone. The cracking zone possesses a ratio of surface area to volume of free space above 200.

During passage through the cracking section in a period of about 1.3 seconds, more than per cent of the C3 hydrocarbon mixture is converted to product. The product from the cracking section issues at a temperature of about 1500 F. into a header 10 and immediately passes through the conduit 11 into a soaker 12 wherein the product from the cracking section is cooled adiabatically to a temperature of about 1200 F.

During passage through the soaker 12, the C3 hydrocarbon conversion is raised above 98 per cent and the yield of liquid aromatic fraction is raised from about 5 to 25 per cent. Increased aromatic production is obtained primarily from increased C3 hydrocarbon conversion. The soaker 12 may contain some packing which is not shown on the drawing.

The product gas from the cracking section is cooled to a temperature of about 1200 F. during its 20 second residence time therein. In addition to effecting increased conversion of C3 hydrocarbon mixture and a manifold increase in yield of liquid aromatic fraction, the passage of the product from the cracking section through the soaker substantially destroys all the acetylene in the product from the cracker and converts the dienes present in the cracker product to aromatics. As a consequence, the aromatic product obtained by the process of this invention is a high-quality product substantially free of acetylenes and containing a very low content of dienes.

There issues from the soaker 12 at a temperature of about 1200 F. a product containing a high concentration of ethylene and a high-quality aromatic fraction. The gaseous portion of the product comprises mainly ethylene,

:hydrogen and .methane .Further dreatmentnfitheaproduct is by conventional procedures for :separating :liquid l.product from 4gaseous zproduct. vA :representative `protcedure :for :separating the .ethylene Agas :from #the liquid aromatic fraction is shown in the drawing. l

lhereluenhfrompthe shaker 12:.s 4:introduced rthrough .a pipe :16 dntoathe heat exchanger :17 `whereiniit Ais `cooled Jand is hen passed .through 1a 4pipe '18 into .a :gas-liquid :separator ,20. In xthe separator 20, the condensed aro- .matic fraction isseparated from ethe :gaseous fraction :of tthelproduct. .Further .treatmentofthe .condensed aromatic -fraction which 4.is removed from :the =separator-20 .through Ava :pipe 12.2 twill die `described thereafter.

The gaseous portion of the product is withdrawnlfrom the separator 20 through a pipe 25 and is introduced into an=absorber 26 svherein .it is subjected to countercurrent contactwithan absorber oil such as mineral seal oil which ,is introduced into the absorber 26 through a `pipe 27. "Contact 'of the gaseous 'fraction of the product removes heavier hydrocarbons :from :the product gas.

The effluent-which Ais removed from the absorber '26 through a pipe 51) .comprises approximately 37.2 weight per 'cent ethylene, 48.8 -weight vper centvrnethane, l9.0 iper cent ethane, 2.3 weight per cent hydrogen `and 42.5 `weight per cent C3 hydrocarbons. lSince this product gas comprises such a low concentration of propylene, it may be used directly in a hydrochlorinator to prepare ethylene chlorohydrin, an intermediate in the preparation of ethylene oxide and ethylene glycol. If a pure ethylene stream is desired, the effluent from the absorber 26 is passed to a conventional separation system for recovering ethylene from lighter gases. Separation of the pure ethylene fraction from the product gas obtained through the pipe 30 is not shown, since separation does not constitute any part of the process of this invention. It is noteworthy that the Ca hydrocarbon content of the product gas is so low that the necessity of recycle to the cracking section is eliminated.

Returning to the treatment of the absorber oil, there is withdrawn from the bottom portion of the absorber 26 through a pipe 35 absorber oil saturated with the fraction comprising C4. hydrocarbons and benzene removed from the product gas. The saturated absorber oil is introduced into a heat exchanger 36 wherein it is raised to a temperature suicient to free the absorbed C4 plus fraction. The heated absorber oil is introduced through a pipe 37 into a stripper 38 wherein the C4 plus hydrocarbon fraction is removed as an overhead through a pipe 39. The lean absorber oil is removed from the bottom of the stripper 38 through a pipe 42, is cooled in a heat exchanger 43 and is recycled through a pipe 44 to the absorber 26.

The C4 plus hydrocarbon fraction from the stripper 38 is cooled in a heat exchanger 46 and is then combined through a pipe 47 with the condensed aromatic fraction which was obtained from the separator 20.

The condensed aromatic fraction which is separated from the gaseous portion of the product in the separator 20 is introduced into a stabilizer 50 wherein absorbed gases are separated from the condensed aromatics. The gas fraction stripped from the condensed aromatic is removed from the upper portion of the stabilizer 50 through a pipe 51 and is normally combined with the product gas in the pipe 25 prior to the latters introduction into the absorber 26 for the removal of C4 plus hydrocarbon fraction.

If the gas removed from the condensed aromatic fraction of the stabilizer S does not contain C4 plus hydrocarbons, it is possible to bypass the absorber 26 and recombine the gas from the stabilizer 50 directly with the product gas from the absorber 26. The alternate line 52 provides for direct combination of the gas from the stabilizer 50 with the product gas from the absorber 26.

The condensed aromatic fraction free of absorbed .gases ,is withdrawn V'from .the bottom portion of Vthe stabilizer 5,0 ,through `a pipe 55 :for combination -with `the C4 plus hydrocarbon fraction.

The following example :illustrates the :process 'of Ythis invention for ;ob.taining :high yields :of ethylene .and of Ahigh quality aromatic fraction by pyrolyzing propane Iin a ,combination `of :cracking and soaking operations at prescribed conditions:

'Propane `.vvas l(passed :atroom :temperature to a 'cracking 4coil where there was .281.6 -.sq. ft. of fsurface `per cubic rfoot of free volume. .The cracking coil -outletftemperature was l500 .F. and the -contact `time in the cracker -was -l seconds. Iheeluentfrom'the'cracker*contained .9.-4per 1centsof propane and .l-1..7 j.per :cent-of propylene.

craoker-efuent was passed immediately :to a soaker where Ait was `heldfor 34.4 seconds. The A-produots were vremoved `from the soaker sat 1200 .F. Yields of .27.9 weight per -cent .ethylene .and .24.:73 weightjper .cent aromatic rliquids were obtained. No :propane -and .only 1.9 weight per cent propylene appeared in the -outlet from the -soaker.

lhejproduct obtained in .this Yrun had the composition shown in Table kI wherein .the fyields are shown yon `the basis-,of weight per centtcharge.

Table I Weight per cent Hydrogen 1.7 Methane 36.6 Ethylene 27.9 Ethane 7.0 Propylene 1.9 Total liquids including C4 plus hydrocarbon fraction and condensed liquids 24.73

In Table II there is shown the approximate composition on a volume per cent basis of the total liquid product.

Table 1I Volume per cent Butadiene 5 Pentadiene 1 Cyclopentadiene 12 Benzene prerun 4 Benzene 38 Toluene 8 Naphthalene 8 Heavy aromatics 24 The benzene fraction obtained by straight distillation of the liquid product had a bromine number of about 1.0, a diene number of about 0.7 and a specii'ic gravity of 0.8840; after acid treatment, the benzene fraction had a speciiic gravity of 0.8847, a R. I. at 68 F. of 1.5009 and an acid wash color between 0 and l. Acid treatment of the toluene fraction gave a product having a specific gravity of 0.8723, a R. I. at 68 F. of 1.4981 and an acid Wash color between 1 and 2. The high quality of the aromatic products of the process of this invention is well demonstrated by these properties.

Although the combined cracking-soaking operation of this invention for producing ethylene and aromatics has been described using C3 hydrocarbons as the charge material, it is reiterated that the process of the invention is applicable to the conversion of ethane into a product cornpxising ethylene and aromatics. As long as the cracking and soaking operations are effected at the prescribed critical conditions, a high yield of ethylene and aromatics are obtained by cracking ethane, propane, propylene or a mixture thereof.

Obviously many modifications and variations of the invention, as hereinbefore set forth, may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. A process for converting a gaseous hydrocarbon selected from the group consisting of ethane, propane, propylene and mixtures thereof to ethylene and a liquid aromatic fraction which comprises introducing said hydrocarbon at a temperature vfrom atmospheric to 800 F. into cracking tubes constructed of nickel-free chrome steel and having a relationship of surface area to volume of free space above 200 square feet per cubic foot, passing said hydrocarbon through said tubes at a pressure below 50 p. s. i. g. and at a rate such that a contact time of 0.5 to 2 seconds is obtained, raising said gaseous hydrocarbon to a temperature level of 1350 to 15.50 F. during passage through said cracking tubes, removing an eluent gas from said cracking tubes at a temperature between 1350 and 1550 F., passing said eflluent gas immediately through a soaker at a rate such that a residence time of 10 to 40 seconds is maintained and in said soaker allowing said eiuent to cool essentially adiabatically, whereby more than 97 per cent hydrocarbon conversion is effected and the yield and quality of the liquid aromatic fraction are substantially improved, issuing a product gas from said soaker at a temperature inthe range of 1100 to 1350 F. and separating said product gas into an ethylene-rich fraction and into a liquid aromatic fraction.

2. A process according to claim 1 in which the resi- 8 dence time of the cracker eiuent in the soaker is between 12 and 25 seconds.

3. A process according to claim 1 in which the gaseous hydrocarbon comprises 70 to 95 per cent propane and 5 to 3() per cent propylene.

4. The process according to claim 1 wherein gaseous hydrocarbon is introduced into the cracking tubes at a temperature between 50G-800 F.; the cracking tubes have between 225 and 350 square feet of surface area per cubic foot of free space therein; said gaseous hydrocarbon is present in the cracking tubes for a contact time between 0.7 and 0.5 seconds; and the eluent gas is removed from the cracking tubes at temperatures between 1450-1550 F.

References Cited in the le of this patent UNITED STATES PATENTS OTHER REFERENCES Schneider et al.: Industrial and Engineering Chemistry, vol. 23, No. 12, (Dec. 1931), pages 1405-1410 (6 pages).

Claims

1. A PROCESS FOR CONVERTING A GASEOUS HYDROCARBON SELECTED FROM THE GROUP CONSISTING OF ETHANE, PROPANE, PROPYLENE AND MIXTURES THEREOF TO ETHYLENE AND A LIQUID AROMATIC FRACTION WHICH COMPRISES INTRODUCING SAID HYDROCARBON AT A TEMPERATURE FROM ATMOSPHERIC TO 800* F. INTO CRACKING TUBES CONSTRUCTED OF NICKEL-FREE CHROME STEEL AND HAVING A RELATIONSHIP OF SURFACE AREA TO VOLUME OF FREE SPACE ABOVE 200 SQUARE FEET PER CUBIC FOOT, PASSING SAID HYDROCARBON THROUGH SAID TUBES AT A PRESSURE BELOW 50 P. S. I. G. AND AT A RATE SUCH THAT A CONTACT TIME OF 0.5 TO 2 SECONDS IS OBTAINED, RAISING SAID GASEOUS HYDROCARBON TO A TEMPERATURE LEVEL OF 1350 TO 1550* F. DURING PASSAGE THROUGH SAID CRACKING TUBES, REMOVING AN EFFLUENT GAS FROM SAID CRACKING TUBES AT A TEMPERATURE BETWEEN 1350 AND 1550*F., PASSING SAID EFFLUENT GAS IMMEDIATELY THROUGH A SOAKER AT A RATE AT SUCH THAT A RESIDENCE TIME OF 10