US3019271A - Process and apparatus for treatment of hydrocarbons - Google Patents

Description

Jan. 30, 1962 F. F. A. BRACONIER ETAL 3, ,27

PROCESS AND APPARATUS FOR TREATMENT OF HYDROCARBONS Filed May 18, 1959 1F? STEAM OXYGEN Z2 FUEL T i 2 Sheets-sheaf. 1

NAPHTHA .2 r Z 23 2 g4 T INVENTORS Frederw 1J1. Bracanber Jan. 30, 1962 F. F. A. BRACONIER ETAL 3,019,271

PROCESS AND APPARATUS FOR TREATMENT OF HYDROCARBONS Filed May 18. 1959 2 Sheets-Sheet 2 T1 31 FUEL [9 Z0 OXYGEN TEMPER ATU R E INVENTORS';

ls LENGTH OF COMBUSTION CHAMBER m m m BYJeanJL E"Ri a Frederic Z'T-H.Br0wonzer United States Patent PROCESS AND APPARATUS FOR TREATMENT OF HYDROCARBONS Frederic F. A. Braconier, Plainevaux, and Jean J. L. E. Riga, Liege, Belgium, assignors to Societe Belge de lAzote et des Produits Chimiques du Marly, Liege, Belgium Filed May 18, 1959, Ser. No. 813,772 Claims priority, application Belgium Sept. 8, 1958 8 Claims. (Cl. 260-679) This invention relates to a process for the thermal treatment of hydrocarbons and to apparatus therefor.

Unsaturated hydrocarbons, particularly acetylene, can be produced in a very pure state by pyrolysis of gaseous hydrocarbons having fewer unsaturations. Advantageously, the pyrolysis uses hot gases, preferably free of oxygen, originating from a flame produced by the combustion of a liquid or gaseous fuel with a combustion supporting gas such as oxygen.

The furnaces used in the process advantageously essentially comprise:

(1) A combustion chamber in which the fuel and combustion supporting gas, fed separately into the chamber, are mixed and form a flame at a burner outlet of the chamber;

(2) A pyrolysis chamber connected directly with the combustion chamber, at the junction of which the hydrocarbon to be pyrolyzed is injected into the hot gases from the combustion chamber; and

(3) A device for quenching the pyrolysis gases.

The principle of the method lies in very rapidly heating the hydrocarbon to be pyrolyzed to a high temperature in the substantial absence of oxygen. Consequently, a high concentration of heat is to be attained in the combustion chamber under conditions as adiabatic as possible; and the combustion gases, into which the hydrocarbon to be pyrolyzed is injected, are to be at very high temperature and to contain substantially no free oxygen.

In copending application, Serial No. 739,605, filed June 3, 1958, are described a process and apparatus for concentrating the energy of combustion flames in a chamber Which is of small volume and which is substantially adiabatic in its operation. To achieve this, oxygen and fuel were separately introduced at high speed and at substantially equal rate of flow, through perforations of small diameter distributed around concentric rings. For each perforation introducing a stream of oxygen, there was a corresponding stream of fuel emanating from another perforation in the same plane as the first, advantageously so positioned that the streams of oxygen and fuel impinge with an angle of 90 between them. Several elementary burners Were thereby realized, each of which efficiently mixed the reaction gases. The very short flames of the individual burners taken together produced a substantially continuous ring of short flame extending parallel to the axis of the combustion chamber.

By concentrating the energy of the flames in a chamber of small volume which is substantially adiabatic in operation, heat losses were reduced. The internal wall of the chamber, however, was subjected to intense radiation. According to the present invention, this wall is protected by a screen of steam used as a radiation shield between the wall and the flames. Two envelopes or jackets of steam having opposing directions are disposed symmetrically on either side of the ring of flames and meet so as to surround the ring completely. This is achieved by injection of the steam through two concentric slits which eject the steam in two directions advantageously forming between them an angle greater than 50 but less than 120,

most advantageously between 70 and The openings for introducing fuel and a combustion supporting gas lie between these steam injection slits.

This arrangement has numerous advantages, particularly with respect to the yield and the economy of the pyrolysis process. The steam jackets act as a particularly efficient radiation screen which limits thermal losses. In addition, they surround the flame ring and define a combustion Zone of small volume, thus strongly concentrating the energy of combustion. After the rapid initial combustion reaction of the fuel and oxygen in their substantially pure states, the steam becomes rapidly and intimately mixed with the combustion gases at high temperature. Because of the reaction equilibria in the presence of steam, free oxygen is substantially completely removed when the combustion reaction is completed. Consequently, if the fuel and combustion supporting gas are used in stoichiometric proportions, a homogeneous mixture of steam and combustion gases, substantially free of oxygen and unburned fuel gas, is produced in a very short time.

It has previously been proposed in this art that a secondary gas such as steam be introduced into combustion gases, containing hydrogen but free of oxygen, before injecting the hydrocarbon to be pyrolyzed. The presence of the secondary gas is intended to cause recombination of a major part of any oxygen and oxygen radicals formed by dissociation of components of the combustion gas during cracking (ionization) in the combustion chamber.

In the present invention, however, the combustion zone defined by the steam jackets is of such small volume that only the initial combustion phase can occur therein before mixing of the gases with steam occurs. During this initial combustion phase, which is of very short duration because of the use of very pure fuel and oxygen, the temperature attained can produce only an insignificant number of free radicals. After this initial phase, the reacting components are immediately and homogeneously mixed with steam, and the presence of this steam in the final phase leads to a completion of the reaction with substantially complete removal of free oxygen. When the fuel and oxygen are used in stoichiometric proportions, the combustion gases contain neither free oxygen nor unburned fuel in substantial amounts. This facilitates subsequent steps for separating and concentrating the unsaturated hydrocarbons formed by the pyrolysis and reduces to a minimum oxidation of the hydrocarbon to be pyrolyzed.

It has also been observed that the introduction of steam mixed with fuel gas and/or oxygen does not otter the same advantages as a separate injection of steam in the form of a ring surrounding the flame ring. To carry out the combustion reaction in a zone of very small volumethat is, to carry out the reaction at very high rate-substantially pure components must be used in at least the first phase of combustion. The dilution of the reagents with steam reduces the reaction rate, resulting in an increase in the volume of the combustion zone and, in consequence, in a less intense concentration of heat.

The nature and advantages of the invention will be more clearly understood by reference to the accompanying drawings, in which a preferred embodiment of a furnace for the pyrolysis of hydrocarbons is given by way of example.

In the drawings, FIGURE 1 is a front view, in section, of a circular furnace which can advantageously be used for producing unsaturated hydrocarbons by injection of the hydrocarbon to be pyrolyzed into hot combustion gases;

FIGURE 2 is a plan sectional view of the furnace at 2-2 of FIGURE 1;

FIGURE 3 is a front view, in section, of a portion of .3 the furnace of FIGURE 1, showing in detail a configuration for the injection of steam, fuel and oxygen; and

FIG. 4 is a plot of combustion temperature as a function of the length of the combustion chamber, both for processes known to the art and for a furnace such as is shown in FIG. 1.

In FIGURE 1, the embodiment shown comprises distributor manifold 11, combustion chamber 12, and pyrolysis chamber 13. Conduits 14 and 15, respectively feed fuel and a combustion supporting gas such as oxygen to distributor 11. Conduit 16 feeds a pyrolyzable hydrocarbon such as naphtha to pyrolysis chamber 13.

A device such as sprayer ring 17 for the injection of cold water is provided for quenching the pyrolysis gases.

That side of distributor 11 which faces combustion chamber 12 comprises circular notch 18, of trapezoidal cross section, the axis of which is also the longitudinal axis of the pyrolysis furnace. A perforated ring 19 connected with conduit 14 and a second concentric perforated ring 20 connected with conduit 15 feed fuel gas and oxygen, respectively, to chamber 12. Rings 19 and abut the inclined side walls of notch 18 and are inclined perpendicularly to said walls. The side walls form an angle of 45 with the longitudinal axis of the furnace. The perforations in rings 19 and 20 are symmetrically distributed around the circumference of the rings and are advantageously of a suitable diameter to give a high nozzle vel'ocity, of 100 to 200 m./ second, for example, to escaping fluids. The momenta of fluids leaving corresponding perforations' in rings 19 and 20 per unit time are preferably substantially equal.

Distributor manifold 11 is also provided with a central steam chamber 21 connected with steam conduit 22. Steam from chamber 21 is introduced into combustion chamber 12 through annular slit 23, inclined at an angle of from about 35 to 50 to the longitudinal furnace axis. A- second annular slit 24, also inclined at an angle from about 35 to 50 with the furnace axis injects another steam jet so that two jackets or envelopes of steam are present. The two steam jets meet, advantageously including an angle of about 70 to 100 between them. The steam jets meet approximately on the same line, parallel to the furnace axis, as the meeting of the jets of fuel and oxygen. The relative width of slits 23 and 24 is such that substantially equal amounts of steam pass through both in unit time.

FIGURE 2 and FIGURE 3 show in detail the configuration ofrings 19 and 20 and slits 23 and 24.

In operation, hydrogen, or a hydrogen rich fuel gas, and oxygen are introduced into combustion chamber 12 through conduit 14 and ring 19 and through conduit 15 andring 20, respectively. The gaseous reagents, which may be preheated, meet at an angle of approximately 90 on issuing from rings 19 and 20 with high linear velocity and at substantially equal flow rates. This results in an efficient and rapid local mixing, with the formation of a ring of short flames extending generally in a direction parallel to the axis of the combustion chamber 12.

Steam fed through conduit 22 passes through chamber 21 of" distributor 11, protecting the lattter against overheating, and is then injected into combustion chamber 12 through slit 23. Additional steam passes into chamber 12 from slit 24. The steam jets meet at an angle between about 70 to 100" and surround the ring of flames completely, outlining a combustion zone within which only a primary combustion phase occurs. The steam jets form a thermal screen protecting the walls of combustion chamber- 12 against radiant heat.

The fuel and oxygen react in the combustion zone, which is of small volume, outlined by the steam jackets. As only the first combustion phase occurs therein, a minimum of free radicals are formed. At the completion of the first combustion phase, the steam jackets and combustion gases meet rapidly and with a homogeneous distribution as regards both heat and composition. After this initial phase, the products of the combustion are mixed With steam. The final combustion phase, or completion of the reaction, then occurs, during which phase substantially all free oxygen is consumed and the fuel is completely burned.

At the outlet of chamber 12, the mixture of steam and combustion gases passes into pyrolysis chamber 13. The hydrocarbon to be pyrolyzed is injected through conduit 16 and apertures 25. It is decomposed at high temperatures to yield principally acetylene and ethylene. The pyrolysis gases are quenched by transverse injection of cold water through sprayer ring 17.

A particularly advantageous embodiment of a furnace of the kind shown in FIGURE 1 is described in Example 1 below.

Example A furnace of the type shown in FIGURE 1 has been. used for the simultaneous production of acetylene and ethylene at rates as high as 1922 kilograms/day and 4282 kilograms/day, respectively.

Such a furnace is advantageously constructed with a combustion chamber 12 defined by a distributor manifold 11 conveniently of steel and side walls conveniently of refractory brick. The wall of pyrolysis chamber 13 is conveniently made of steel. The walls of both chambers are advantageously cooled externally by circulation of cold water (not shown in FIGURE .1). Combustion chamber 12 has an internal diameter of 140 mm. and a height of 168 mm. Distributor 11 comprises notch 18, the sides of which are each inclined at' 45 with the vertical. Perforated ring 20 comprises 24 apertures of 7 mm. diameter distributed symmetrically on a circle having a diameter of 104 mm. Ring 19 comprises 24 holes of 4.5 mm. diameter on a circle having a diameter of 66 mm. Annular slit 23 in distributor 11 has a diameter of 52 mm. (measured on the lower face of the distributor), a width of 5.5 mm., and is inclined at an angle ofv 37 from the vertical. Slit 24 has a diameter of 116 mm.,. a width of 3 mm., and is inclined at 37 with the vertical. axis of the furnace.

A coke oven gas of the following. composition was fed through conduit 14 and ring- 19 into chamber 12- at a throughput of 260 Nm /hour:

Percent by volume- Oxygen of 93.5 percent purity was introducedinto chamber 12 through conduit 15 and ring 20 at a throughput of 250 Nm /hour. On entering the combustion chamber, the gaseous reactants interpenetrated at an angle of and ignited very rapidly to form a flame ring extending. in a direction parallel to the axis of chamber 12.

The flame ring was completely surrounded by a screen of steam obtained by injecting steam through slits 23 and 24. The total throughput of steam, at 600 C., was 500 kilograms/hour, under a pressure (before passing through a preheater not shown in FIGURE 1) of 2 kg./crn. The steam throughputs and velocities were substantially the same at each slit, and the steam jackets met at an angle of 74 and on a vertical line beneath the meetingpoint of the combustion reagents.

524- kg./hour of naphtha were injected into the mixture of combustion gases and steam. The. temperatureof the naphtha was about 580 C. at the inlet of the,

pyrolysis furnace. The naphtha had the following characteristics:

Naphthenic hydrocarbons 10.5 percent by weight.

As mentioned earlier, the steam jackets preferably meet to include an angle between 70 and 100. The selection of the angle value and of the distance between the injection slits depends mainly on the nature of the fuel used in producing the hot combustion gases. For each fuel, the combustion zone outlined by the steam jackets advantageously has a volume such that the first combstion phase proceeds normally and such that the temperature attained in this phase is in the same range as the temperature reached on completion of combustion after mixture with the steam of the screen has occurred.

This condition is shown in FIGURE 4 of the accompanying drawings. In FIGURE 4, temperature is plotted in arbitrary units on the ordinate, and the length of a combustion chamber such as that shown as 12 in FIGURE 1 is plotted on the absicssa in millimeters. According to the process of this invention (curve 26), the temperature within the chamber increases rapidly during the primary combustion phase (portion 0A). Then, after the mixing in of steam with the combustion gases, the temperature remains substantially constant, the cooling effect of the steam (curve 27) being compensated by the exothermicity of the reaction completing the combustion (curve 28). A combustion chamber 168 mm. long (as in Example 1) is thus sufficient to give a mixture of steam and combustion gases which is both thermally and compositionally homogeneous.

By way of contrast, curve 30 shows the temperature pattern in a combustion chamber when a fuel gas and combustion supporting gas containing steam admixed prior to any combustion are used. The combustion, since carried out with impure reagents, proceeds more slowly. A long combustion chamber is necessary, resulting in significant heat losses.

Curve 29 is descriptive of a process in which steam is introduced after complete combustion. The temperature increases steeply during the combustion (portion OB of curve 29), and then decreases due to the introduction of relatively colder steam. In this case, a long combustion chamber, at least 300 mm. long, must be provided to get a thermally and compositionally homogeneous mixture of combustion gases and steam. In comparison with the process of the present invention, the introduction of steam after complete combustion results in heat losses due to:

(l) The higher temperature in the combustion chamber (i.e., 08 greater than OA); and

(2) The greater volume required for the combustion chamber and the resultant larger exchange surface of the walls thereof.

In addition, because of the elevated temperature, there is significant formation of free radicals during the combustion.

Comparative measurements have been made with the steam screen of the present invention in the form of two jackets surrounding the flame ring and including an angle of about 74, and with a steam screen simply directed along the side walls of combustion chamber 12. Under the same conditions of nature and throughput of reagents and ethylene/ acetylene ratio in the pyrolysis gas, the measurements show that with the steam screen of the present invention:

(1) The length of the combustion chamber can be substantially reduced (e.g., from 300 mm. to 168 mm), resulting in a reduction or the heat losses by over 50 percent; and

(2) The oxidation of the hydrocarbon to be pyrolyzed by oxygen and free radicals in the combustion gases can be reduced from 9 percent to 4.5 percent.

The process of the present invention may also be used when several flame rings are formed, each of which is then surrounded by two steam jackets. Such a configuration is particularly suitable for the large-scale production of unsaturated hydrocarbons.

Although specific embodiments have been shown and described, it is to be understood that they are illustrative and are not to be construed as limiting on the scope and spirit of the invention.

What is claimed is:

1. In a process for the preparation of unsaturated hydrocarbons by pyrolysis of hydrocarbons with hot combustion gases comprising forming a ring of flames in a combustion chamber by the combustion of a fluid fuel and a comburent gas introduced into said chamber through perforations distributed on a plurality of concentric rings, injecting the hydrocarbon to be pyrolyzed into the hot combustion gases originating from said ring of flames, to obtain pyrolysis products, and quenching the pyrolysis products, the steps comprising surrounding said ring of flames with a plurality of steam screens formed by pairs of opposing steam jets meeting to define a combustion Zone of small volume wherein said combustion is first partially effected, mixing said steam and said hot combustion gases rapidly and homogeneously prior to completion of said combustion whereby said combustion is completed in the presence of steam, and thereafter injecting the hydrocarbon to be pyrolyzed into the hot gases originating from the completion of the combustion reaction in the presence of steam.

2. A process as in claim 1 wherein said steam jets set to include an angle between about 50 and 120.

3. A process as in claim 1 wherein said steam jets meet to include an angle between about 70 to 100.

4. A process as in claim 1 wherein said steam jackets meet on a line parallel to the longitudinal axis of the combustion chamber and substantially directly below said ring of flames.

5. In a process for the preparation of unsaturated hydrocarbons by pyrolysis of hydrocarbons with hot combustion gases, the steps comprising mixing steam with hot gases formed by partial combustion of a fluid fuel and a comburent gas prior to complete combustion thereof, and injecting the hydrocarbons to be pyrolyzed into the hot gases after completion of the combustion reaction in the presence of steam.

6. In a process for the preparation of unsaturated hydrocarbons by pyrolysis of hydrocarbons with hot combustion gases, the steps comprising cooling the reagents in an exothermal combustion reaction by mixing steam with said reagents after combustion has begun but before it is completed, whereby a constant temperature gaseous mixture of steam and combustion gases is produced, and thereafter injecting the hydrocarbons to be pyrolyzed into said constant temperature gaseous mixture.

7. A furnace for the preparation of unsaturated hydrocarbons by pyrolysis of more saturated hydrocarbons with hot combustion gases, which apparatus comprises concentric annular conduits for feeding, respectively, fuel gases, comburent gases, and steam, a distributor for said gases, a combustion chamber and a pyrolysis chamber, said combustion chamber having a small volume, said distributor provided, on its side facing the combustion chamber, with an annular groove concentric with said combustion chamber and with the annular conduits for feeding the gaseous reagents and the steam, the sides of said groove diverging at an angle not greater than and having small perforations connecting said groove with the annular conduits for the fuel and comburent gases, the perforations connected to the conduit for introducing one of the gases being paired with corresponding perforations for introducing the other reactive gas, said groove also having in the sides thereof annular slits connecting said 8. A furnace as in claim 7 wherein said steam jets groove with the annular conduits for the steam, said anmeet to include an angle between about 50 and 120. nular slits being disposed pairwise on either side of each set of paired perforations for fuel and comburent gases References Cited in the file of this patent and opposed to define an acute angle therebetween, where- 5 UNITED STATES PATENTS by to inject paired steam jets which meet to define a re- 2 706 210 Harris A pr. 12, 1955 actlon zone of small volume surroundlng said paired per 2,912,475 Krause et a1. Nov. 1959 forations.

Claims (1)

1. IN A PROCESS FOR THE PREPARATION OF UNSATURATED HYDROCARBONS BY PYROLYSIS OF HYDROCARBONS WITH HOT COMBUSTION GASES COMPRISING FORMING A RING OF FLAMES IN A COMBUSTION CHAMBER BY THE COMBUSTION OF A FLUID FUEL AND A COMBRURENT GAS INTRODUCED INTO SAID CHAMBER THROUGH PERFORATIONS DISTRIBUTED ON A PLURALITY OF CONCENTRIC RINGS, INJECTING THE HYDROCARBON TO BE PYROLYZED INTO THE HOT COMBUSTION GASES ORIGINATING FROM SAID RING OF FLAMES, TO OBTAIN PYROLYSIS PRODUCTS, AND QUENCHING THE PYROLYSIS PRODUCTS, THE STEPS COMPRISING SURROUNDING SAID RING OF FLAMES WITH A PLURALITY OF STEAM SCREENS FORMED BY PAIRS OF OPPOSING STEAM JETS MEETING TO DEFINE A COMBUSTION ZONE OF SMALL VOLUME WHEREIN SAID COMBUSTION IS FIRST PARTIALLY EFFECTED, MIXING SAID STEAM AND SAID HOT COMBUSTION GASES RAPIDLY AND HOMOGENEOUSLY PRIOR TO COMPLETION OF SAID COMBUSTION WHEREBY SAID COMBUSTION IS COMPLETED IN THE PRESENCE OF STEAM, AND THEREAFTER INJECTING THE HYDROCARBON TO BE PYROLYZED INTO THE HOT GASES ORIGINATING FROM THE COMPLETION OF THE COMBUSTION REACTION IN THE PRESENCE OF STEAM.

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