US2998466A - Process and apparatus for treatment of hydrocarbons - Google Patents

Description

Aug. 29, 1961 F. F. A. BRACONIER ET AL 2,998,466

PROCESS AND APPARATUS FOR TREATMENT OF HYDROCARBONS Filed May 18, 1959 NVE A NTOR$ Frederic .Bmcomer BY Jean J-llERija mmmf ,l', I

A T01 EYS Uted States Patent 2,998,466 PROCESS AND APPARATUS FOR TREATMENT OF HYDROCARBONS Frederic F. A. Braconier, Plainevaux, and Jean J. L. E. Riga, Liege, Belgium, assignors to Societe Beige de LAzote et des Produits Chimiques du Marly, Liege, Belgium Filed May 18,1959, Ser. No. 813,811 Claims priority, application Austria Aug. 7, 1953 11 Claims. (Cl. 260-679) This invention relates to a process for pyrolysis of hydrocarbon gases and to apparatus therefor.

Unsaturated hydrocarbons, particularly acetylene, are now required in a very pure state for synthesis of organic chemicals and especially for production of vinyl compounds for manufacture of synthetic resins. Pyrolysis of hydrocarbons has long been known to produce such unsaturated products, but the art has sought continually to improve purity and yield. The pyrolysis reaction is conveniently carried out by partial combustion of gaseous or vaporized hydrocarbons. Heat released by the exothermal burning of a portion of the hydrocarbon is used to raise the remainder of the starting material to an elevated temperature at which conversion to less-saturated hydrocarbon products occurs. This flame pyrolysis is brought about by introducing a mixture of hydrocarbon fuel and oxygen into a reaction chamber. After partial combustion, the gases are quenched to terminate the pyrolysis reaction while a substantial yield of the desired pyrolysis product remains, i.e., before it is decomposed by further pyrolysis. As the reaction is very fast, this necessitates a total reaction time measured in fractions of seconds, and any lack of homogeneity in the feed, or lack of uniformity of reaction condtions, will impair the value of the product.

Any part of the reactants which is over-reacted or not fully reacted not only reduces yield and efiiciency, but makes impurity in the product which seriously impairs its value as an intermediate for use in synthesis.

It is advantageous to preheat both pyrolysis reagents (hydrocarbon and oxygen) to the highest possible temperature before reaction. Such preheating reduces the number of calories which have to be supplied by partial combustion of the feed hydrocarbon and thereby reduces consumption of purified oxygen as Well as hydrocarbon.

Such preheating of the combustible mixture is limited by its tendency to ignite spontaneously. This tendency is dependent both on the oxygen/hydrocarbon ratio and on the temperature. We have shown that, if one is to utilize fully the advantages of preheating, homogeneity of composition and temperature in the feed mixture are of great importance.

Our prior copending application, Serial No. 726,248, describes a process and apparatus in which oxygen and the hydrocarbon to be pyrolyzed are mixed first in an elongated and relatively narrow Venturi throat into which the oxygen is injected and then in a mixing chamber of expanding cross section, which leads to a distributor screen. The homogeneous mixture passes through the many parallel passages of the distributor into the combustion chamber with velocity greater than the flame propagation velocity of the mixture.

The mixing chamber in which are preheated oxygen and hydrocarbon, not yet quite thoroughly diffused together, is the place where pre-ignition would occur and imposes the limit on preheating. If the reagent gases are not homogeneously mixed on leaving the outlet of the mixing zone, pre-ignition by local concentrations of excess oxygen may occur. In such cases, for a given preheating temperature and a given flow time through the apparatus, there is a critical concentration of excess oxygen which will cause preignition. We have now found that, if the gases are completely mixed at once, in the injection zone, so that a mixture over-rich in oxygen does not persist for a time long enough to effect ignition, it is possible to preheat the gases to a higher temperature and thus gain a higher et'ticiency.

Thus, it is an object of this invention further to improve the mixing of gaseous reagents used in such pyrolysis. Another object of the invention is to preheat the reagents to higher temperatures and to reduce the risk of spontaneous ignition in the mixing apparatus. Still another object of the invention is to promote the most efiicient operation of a furnace used for partial combustion pyrolysis and to improve the purity of the acetylene product.

A feature of the present invention is the progressive injection of oxygen into a hydrocarbon stream in several successive stages with mixing between, whereby an improved mixing of hydrocarbon and oxygen gases is achieved.

Another feature is the injection of oxygen in each of said stages by means of several opposing jets directed across and advantageously perpendicularly to, the direction of flow of the hydrocarbon gases.

A further feature of the invention is the passage of the hydrocarbon and oxygen mixture through an annular chamber from the narrow zone directly to a distributor, and through it into a reaction chamber.

A preferred type of mixing apparatus embodying our invention and for carrying out the process of our invention advantageously comprises a Venturi consisting. of a convergent annular entrance chamber for introducing hydrocarbon gases, an elongated, narrow, annular throat, which is the injection and mixing zone, including a plurality of small annular expansion chambers, opposite sides of which are provided with perforations for injection of oxygen transversely into the hydrocarbon, and a divergent annular exit chamber connecting said injection zone with a distributor. Within and surrounded by said convergent and divergent chambers and said mixing zone, is a hollow central core in the form of a double cone, one tip of which is situated at the center of the distributor. The outer wall of said divergent chamber, which diverges in the direction of gas flow, and that portion of the hollow core which converges in the direction of gas flow define between them an angle such that the gases are spread out evenly over the area of the distributor screen. We have found it advantageous to use an angle of approximately 14.

The process and apparatus described above produce a homogeneous mixture of the gaseous reagents at the outlet of the elongated, narrow, mixing zone, and thus provide a homogeneous gas stream in said divergent chamber leading to the distributor. Local excessive concentrations of oxygen within the gas mixture are thereby obviated, a condition particularly desirable for avoiding pre-ignition outside of the reaction chamber.

The nature of the present invention will be more clearly nndesrtood by reference to the accompanying drawings which are given for purposes of illustration and to enable others skilled in the art to fully understand the invention, so as to be able to modify it and adapt it for various conditions of use.

In the drawings,

FIGURE 1 is a front view, in section, of a preferred embodiment of an apparatus for the flame pyrolysis of hydrocarbons;

FIGURE 2 is an enlarged sectional view showing a portion of the mixing zone of the apparatus of FIGURE 1 in greater detail;

FIGURE 3 is a horizontal section through a portion of the mixing zone of the apparatus of FIGURE 1, taken at line 33 of FIGURE 1;

FIGURE 4 is a horizontal section taken approximately 3 along line AA of FIGURE 1, and is intended to show certain relationships of areas but not necessarily details of construction; and

FIGURE 5 is a vertical section, taken along line 5-5 of FIGURE 2, and showing a portion of a wall in the mixing zone of the apparatus of FIGURE 1.

In FIGURE 1, the apparatus shown comprises annular convergent chamber 11, narrow, elongated, annular mixing zone 12, annular divergent chamber 13, distributor 14, and a portion only of combustion chamber 15. Chambers 11 and 13 and zone 12 surround hollow, central bipyramidal core 16, one tip of which is situated at the center of distributor 14.

Hollow central core 16 is traversed throughout almost its entire length by conduit 17, which feeds to mixing zone 12 a portion of the oxygen necessary in the partial combustion reaction. Annular mixing zone 12 is surrounded by annular chamber 18 having conduit 19 entrant therein, through which conduit the remaining necessary oxygen is supplied. Upper portions of convergent chamber 11 are also surrounded by annular chamber 20 having coduits 21 through which the hydrocarbon to be treated is introduced. In order to assure uniform distribution, the hydrocarbon passes through openings 22 uniformly spread around the top of chamber 11. Walls 26 and 27 of core 16 and of divergent chamber 13 diverge at an angle of approximately 14 beyond the outlet from mixing zone 12.

As shown in FIGURE 2 in detail, the injection and mixing zone 12 is formed by a plurality of small annular chambers 23 sequentially arranged and connected with each other by narrow short annular passages 24. The walls of chambers 23 are provided with a plurality of openings or perforations 25, through which oxygen from conduits 17 and 19 is injected. The walls defining annular mixing zone 12 are advantageously slightly divergent in the direction of gas flow so that chambers 23 are of successively increasing volume. This volume increase maintains a substantially constant flow rate by providing for the progressive increase in the total volume of oxygen supplied to zone 12 through openings 25.

During the normal operation of the furnace, preheated hydrocarbon gases to be pyrolized are fed through conduit 21 into chamber 20 and then into chamber 11 through openings 22. As the gases approach mixing zone 12, their rate of flow increases because of the decreasing cross section of convergent chamber 11.

The other gaseous reagent, advantageously oxygen, is first preheated, and then introduced in two parts of substantially equal volumes through conduits 17 and 19, respectively. The oxygen fed through conduit 17 passes through central core 16 and is then injected into chambers 23 through openings 25 in the wall of the core 16. The oxygen introduced through conduit 19 passes through chamber 18 and is likewise injected into said chamber 23 through the openings 25 in the outer wall. As shown in FIGURES 1 and 2, this injection is advantageously perpendicular to the direction of flow of the hydrocarbon through mixing zone 12, and it is believed that a swirling of the gases is produced in each chamber 23. The mixing within each chamber, multiplied by the effect of passage through several of chambers 23, with successive expansion and constriction of the stream, effects a rapid and complete mixing of the gases, so that a homogeneous blending of hydrocarbon and oxygen is obtained at the outlet of mixing zone 12 and throughout the divergent increasing gas volume. This also avoids excessive pressure losses as the gases pass between difiereut parts of the system.

A particularly advantageous arrangement of openings 25 is shown in FIGURES 3 and 5 of the drawings, which figures are sectional views respectively taken at 33 of FIGURE 1 and 55 of FIGURE 2. Although a large number of openings 25 is advantageously used to inject oxygen into the hydrocarbon, for purposes of clarity in FIGURE 3 only sixteen have been shown as entrant on mixing zone 12 from each of core 16 and chamber 18. Advantageously, openings from core 16 and chamber 18 are respectively opposed in pairs. As shown in FIGURE 5, openings 25 are also advantageously staggered so that the openings in any of the chambers 23 are displaced along the circumference of the chamber so as not to lie in the same vertical line with other openings in chambers above or below the chamber in question.

Since the gaseous reagents have been thoroughly mixed on reaching the outlet of mixing zone 12, chamber 13 serves only to connect the outlet of mixing zone 12 with distributor 14 and as the expansion zone for the Venturi which is constituted by the converging annular chamber 11, the annular throat 12 and the diverging chamber 13. To obtain uniform distribution of the gaseous mixture across the distributor 14, it is advantageous to choose the dimensions of the apparatus such that the area of a circular central portion of the distributor, defined by projecting central core 16 on the plane of the distributor, equals the area of a remaining annular portion of the distributor surface. As shown in FIGURE 4, a circular central portion 28 has an area equal to that of annular portion 29.

The top angle of chamber 13, between walls 26 and 27, as shown, is 14. This, as mentioned earlier, gives an advantageous smooth spreading flow to assure uniform distribution. An angle significantly smaller than this value necessitates an unwanted elongation of chamber 13. An angle significantly larger than 14, which results in a decrease in the length (ie.e., height) of chamber '13, may be conducive to lack of uniformity in distribution of the gases or may tend to retain gases in some zones for a time substantially longer than the average time of passage through the chamber.

The apparatus and process described above produce an intimate mixture of hot gaseous reagents in a very short time period. Numerous tests of samples taken in several locations around the circular outlet of the annular zone 12 have shown the uniformity of oxygen content of the mixtures.

In addition, the mixture flows through chamber 13 in a very short period, due to the selection of the approximate 14 value of the angle mentioned, and further due to the small volume of chamber 13, the height of which is advantageously a little less than twice the greatest diameter of the chamber.

Under circumstances such as these, it is possible to preheat the gaseous reagent to a higher temperature depending on the length of time the gases remain in mixing zone 12 and chamber 13. This is particularly advantageous to industry, since by increasing the preheating temperature by 50 C. approximately 0.2 ton of pure oxygen and 200 to 250 cubic meters of methane may be saved for each ton of acetylene produced by pyrolysis.

It may be helpful for understanding the device shown in the accompanying figures to give the dimensions of an actual embodiment which has shown particular usefulness.

Example 1 In a preferred embodiment, the device shown in FIG- URE 1 has a total height of 1965 millimeters between the inlet end of conduit 17 and distributor 14. Conduit 17 has a diameter of mm. and is surrounded by hollow central core 16, the tip of which is situated at the center of distributor 14. Distributor 14 has a diameter of 430 mm. Annular convergent chamber 11 has a height of 945 mm. and converges to an annular slit having a width of 16 mm., an outer diameter on the core 26 of 136 mm. and inside diameter of the shell 23 of 168 mm. Mixing zone 12 has a total height of 193 mm. and comprises 8 annular chambers 23 of progressively increasing volume, the first of which has a height of 15 mm. and a width of 32 mm., and the last of which has a height of 15 mm. and a width of 46 mm. Said chambers 23 are interconnected by narrow annular passages 24 having a height of approximately mm. and a width varying from 16 mm., for that passage interconnecting convergent chamber 11 with the first of chambers 23, to a width of 24 mm. for that passage connecting the last of chambers 23 with divergent chamber 13. Mixing zone -12 is surrounded by chamber 18 through which is fed a portion of the oxygen consumed. Zone 12 itself surrounds central core 16 through which the remaining portion of the required oxygen is supplied. Chamber 48 and central core 16 are each connected with any one of each of the chamber 23 of zone 12 through seventy-two openings 25 having a diameter of 3 mm. and regularly staggered. In the illustrated embodiment there are eight expansion chambers 23, which are thus connected with chamber 18 and core 16 through a total of 1152 openings 25.

From zone 12 for contacting and mixing the reagents, divergent chamber 13 extends down with a height of 830 mm. and an interior angle of 14 between the top of walls 26 and 27. Said chamber 13 is used only for connecting the mixing zone 12 with the distributor 14, the latter having a diameter of 430 mm. and a height of 215 mm. Distributor 14 comprises a plurality of parallel passages leading to reaction chamber 15, only part of which is shown in FIGURE 1. The number of passages shown is reduced for clearer showing. The number and diameter are such that the gas passes through with a velocity greater than the flame propagation velocity of the gas.

The apparatus of the preferred embodiment is made of refractory chromium-molybdenum steel, preferably an A.I.S.I. type 321 steel, as specified in the Steel Products Manual, Number 24, of the American Iron and Steel Institute.

In the operation of this preferred embodiment, methane of 97 percent purity (containing 2 percent of carbon monoxide and 1 percent of nitrogen) is preheated to 650 C. and introduced under a pressure of 1.25 absolute atmospheres through conduit 21 into said mixing apparatus at a rate of 2150 cubic meters per hour (calculated at normal pressure and temperature). The methane passes through annular chamber 20 and then through openings 22 into convergent chamber 11. On passing into chamber 11, the gas has a linear speed of approximately 90 meters per second.

Oxygen of 97 percent purity (containing 3 percent of nitrogen) is preheated to 650 C. and introduced through conduit 17 under a pressure of 1.3 absolute atmospheres, at a rate of 550 cubic meters per hour. This oxygen, after passing through conduit 17, enters central core 16 surrounding said conduit, and is then injected through the 576 openings 25 in core 16 into the 8 expansion chambers 23 at a linear speed of approximately 140 meters per second. The remaining oxygen is fed through conduit 19 at an equal rate of 550 cubic meters per hour, passes through chamber 18 and is similarly injected into the 8 annular chambers 23 through another 576 openings 25 and with a linear speed of approximately 140 meters per second.

Under such conditions, the gaseous reagents are rapidly brought into contact with each other and are thoroughly mixed in passing through the 8 chambers 23. The resulting homogeneous mixture then passes through divergent chamber 13 and is uniformly distributed to distributor 6 14 in such a manner that the throughput of the mixture entering reaction chamber 15 is substantially the same in each of the parallel passages of distributor 14.

After entering reaction chamber 15, the gas is slowed down to the flame propagation velocity and is then ignited and the methane is thereby subjected to a partial combustion and pyrolysis. The flame is advantageously stabilized by supplying additional oxygen at a rate of approximately cubic meters per hour distributed in small pilot jets, at the bottom of the distributor 14.

The product gases obtained by this combustion contain approximately 7.8 percent of acethylene and are produced at a rate of approximately 3,800 cubic meters per hour of dry gas.

What is claimed is:

1. In a process for making acetylene of high purity by passing a stream of hydrocarbon in gaseous phase through a Venturi, feeding oxygen into the hydrocarbon stream at the constricted throat of the Venturi in amount adapted to provide by partial combustion heat needed for pyrolysis of the hydrocarbon and passing the mixture through the divergent chamber of the Venturi and parallel closely spaced passages of a distributor into a pyrolysis flame maintained beyond said distributor, the improvement which comprises injecting the oxygen in jets at successive locations along the throat and repeatedly constricting and expanding the stream in the zone of the Venturi throat.

2. The process defined in claim 1 in which the oxygen is injected through opposed jets on opposite sides of the expanded stream.

3. The process defined in claim 1 in which the stream is annular and the zones of expansion in the Venturi throat are toroidal and the jets are distributed on opposite sides thereof at uniform angular spacing.

4. The process as defined in claim 3 in which the jets on opposite sides of the toroidal expansion zones are opposed on a common angle but are distributed along lines skew to the direction of flow of the stream.

5. In a process for making acetylene of high purity by passing a stream of hydrocarbon in gaseous phase through a Venturi, feeding oxygen into the hydrocarbon stream at the throat of the Venturi in amount regulated to provide by partial combustion the heat needed for pyrolysis of the hydrocarbon to acetylene, and passing the mixture through the divergent end of the Venturi and narrow parallel passages of a distributor screen into a flame reaction therebeyond and quenching the reaction products when the pyrolysis has reached the desired stage, the improvement which comprises substantially completing the mixing of hydrocarbon and oxygen to a homogeneous gas by repeated constriction and expansion of the flowing stream in the constricted throat of the Venturi, and by passage therethrough at velocity sufliciently high to assure complete mixing, whereby local spots of high oxygen concentration are avoided in the divergent passage of the Venturi- 6. An apparatus for uniformly distributing a pyrolyzable gaseous mixture of hydrocarbon and oxygen into a pyrolysis reaction chamber which apparatus comprises an annular conduit for feeding a stream of said hydrocarbon, said conduit converging from a wide end thereof, at which said hydrocarbon is introduced thereinto, to a narrow end, a plurality of annular injection chambers open from the narrow end of said convergent conduit and successively open into one another through constricted annular ports, through which said hydrocarbon stream successively passes, said chambers having a plurality of perforations therein for injecting oxygen inwardly, and an annular mixing chamber open to a constricted annular port in the last of said injection chambers, said mixing chamber diverging from said port to a pyrolysis reaction chamber and comprising a divergent outer wall and a convergent inner wall.

7. An apparatus as in claim 6 wherein said perforations are distributed pairwise on opposing walls of said annular injection chambers.

8. An apparatus as in claim 7 wherein said perforations are distributed along lines skew to a perpendicular to parallel planes defined by said annular injection chambers.

9. An apparatus as in claim 6 wherein each of said plurality of annular expansion chambers successively increases in volume.

10. An apparatus as in claim 9 wherein the inner and outer walls of said divergent annular chamber define an angle of about 14.

11. An apparatus as in claim 10 wherein the height of said divergent annular chamber is less than twice its greatest width.

References Cited in the file of this patent UNITED STATES PATENTS Armor Mar. 7, 1916 Lundgaard May 25, 1920 Schurs Nov. 2, 1920 Porter Oct. 25, 1921 Schroeder Sept. 5, 1922 Rusch July 14, 1925 Blood Jan. 12, 1937 MacQueen Nov. 12, 1957 Heller Sept. 9, 1958

Claims (1)

1. IN A PROCESS FOR MAKING ACETYLENE OF HIGH PURITY BY PASSING A STREAM OF HYDROCARBON IN GASEOUS PHASE THROUGH A VENTURI, FEEDING OXYGEN INTO THE HYDROCARBON STREAM AT THE CONSTRICTED THROAT OF THE VENTURI IN AMOUNT ADAPTED TO PROVIDE BY COMBUSION HEAT NEEDED FOR PYROLYSIS OF THE HYDROCARBON AND PASSING THE MIXTURE THROUGH THE DIVERGENT CHAMBER OF THE VENTURI AND PARALLEL CLOSELY SPACED PASSAGES OF A DISTRIBUTOR INTO A PYROLYSIS FLAME MAINTAINED BEYOND SAID DISTRIBUTOR, THE IMPROVEMENT WHICH COMPRISES INJECTING THE OXYGEN IN JETS AT SUCCESSIVE LOCATIONS ALONG THE THROAT AND REPEATEDLY CONSTRICTING AND EXPANDING THE STREAM IN THE ZONE OF THE VENTURI THROAT.

Images