US3499055A - Method for thermally decomposing saturated hydrocarbons to produce unsaturated hydrocarbons employing oxygen along with a fuel gas - Google Patents

Description

March 3, 1970 FREUND ETAL 3,499,055

METHOD FOR THERMALLY DECOMPOSING SATURATED HYDROCARBONS T0 PRODUCE UNSATURATED HYDROCARBONS EMPLOYING OXYGEN ALONG WITH A FUEL GAS Original Filed June 2, 1965 FIG. 2

ATTORNEY United States Patent US. Cl. 260-679 6 Claims ABSTRACT OF THE DISCLOSURE Saturated hydrocarbons, containing at most five carbon atoms, are thermally decomposed in a one-step flamereaction to produce unsaturated hydrocarbons. A gaseous mixture of the saturated hydrocarbons, fuel gases having a higher burning speed and a higher upper burning limit than the saturated hydrocarbons, and oxidizing gases is burned in a flame reactor to attain a reaction temperature of from 1,000-1,600 C. with at least 30 percent but preferably more than 80 percent of the heat quantity required for the reaction being provided by partial or total burning of the fuel gases. The decomposition reaction is immediately quenched when the unsaturated hydrocarbons have been formed.

CROSS REFERENCE TO RELATED APPLICATION BACKGROUND OF THE INVENTION It is known that unsaturated hydrocarbons of a mixture thereof may be prepared by thermal decomposition of saturated hydrocarbons or a mixture of the same. By cracking not only unsaturated hydrocarbons and saturated hydrocarbons of lower alkyl chains, but hydrogen, carbon, carbon monoxide, carbon dioxide, oxygen, oxygen-containing compounds and further oxidation products are also formed.

Cracking is carried out at a temperature of 800 to 1600 C. and for a very short period of time to 10* seconds), but it requires significant heat consumption. Therefore very great heat quantities are to be transferred to the raw material to be decomposed in a very short time and this comprises the greatest difiiculty of cracking.

Heat transfer is often carried out by mixing the hydrocarbon to be decomposed with oxygen or other oxidizing gases in such ratio that partial burning takes place. The heat quantity thus formed is directly transferred to the unburnt part of the raw material, thus providing the heat requirement of thermal decomposition. A well-known example of the above method is the partial oxidation of methane to give acetylene and synthesis gas.

According to the process, generally a mixture consisting of 62% of methane and 38% of oxygen, preheated to SOD-600 C., is partially burnt. The reaction takes 3,499,055 Patented Mar. 3, 1970 place at a temperature above 1400 C. and, on sudden quenching of the same, a mixture containing 8.2% by volume of acetylene, hydrogen, carbon monoxide, carbon dioxide and methane is obtained. The gas mixture used as the starting material must have a composition determined by the upper burning limit. If the oxygen content of the gas mixture is smaller than the above value, only unstable burning takes place, while a higher oxygen-content results in the deterioration of the conditions of equilibrium of acetylene formation. The mixture of methane and oxygen is to be preheated before reaction in order to improve the conditions of acetylene formation and to decrease oxygen consumption. The upper limit of preheating is determined by the ignition temperature of methane (645 C.). By partial oxidation of methane, when the gas-mixture is preheated to the temperature of 600 C., allowable from the point of View of safety, and oxygen gas having a purity above 99% is used, the specific oxygen consumption amounts to 5 kg. oxygen per kg. acetylene. The diminution of the purity of the oxygen gas causes a significant increase of the specific oxygen consumption.

According to another important group of cracking processes, the calorific and decomposing reactions are carrier out in two steps. These processes are characterized by burning the almost stoichiometric mixture of a se lected fuel gas and oxidizing agent, using preferably a mixture consisting of a fuel gas having a high hydrogencontent and of oxygen. The fuel gases thus formed, having a temperature of 2700-2900 C., are cooled to 2400" C. in order to save the structural material; the hydrocarbons to be decomposed are mixed into the heat carrier gas and the reaction is quenched at the appropriate moment in the usual way. As cracking reactions possess a high reaction velocity, the hydrocarbons are to be admixed with the heat carrier gases in an extremely effective way. At the place of admixture both the heat carrier gases and the hydrocarbons are to be circulated with very high flow velocity, generally amounting to the value of the speed of sound and complicated constructions are required. In order to obtain appropriate safety by heating and quick burning, the fuel gas and the gas to be oxidized are also to be admixed with a velocity equivalent to the speed of sound. The above process has considerable disadvantages: the flue gases of high temperature cause several difficulties, the admixture requires high energy consumption, the augmentation of the dimensions result in a more or more non-homogenous admixture the above factors decrease the economic efficiency of the process to a significant extent. Moreover, complicated reactors are required to perform the above method.

SUMMARY OF THE INVENTION This invention relates to an improved process for the production of unsaturated hydrocarbons from saturated hydrocarbons containing :at most five carbon atoms by means of thermal decomposition by flame reaction.

The present invention provides a process for the thermal decomposition of saturated hydrocarbons containing at most five carbon atoms by means of a one-step flamereaction, the heat quantity necessary for thermal decomposition being procured by using fuel gases having a higher burning speed and an upper burning limit than the hydrocarbons to be decomposed. The essential feature of the present invention resides in feeding a mixture of the hydrocarbons to be decomposed, fuel gases and the oxidizing gases into the burner of the flame reactor, in such ratio that, on burning the gas-mixture, a reaction temperature of 1000-1600 C. is achieved; at least 30%, but preferably more than 80% of the heat quantity required for the reaction are provided by the partial or total burning of the fuel gases and the decomposition reaction is immediately quenched in the usual Way, when the desired unsaturated hydrocarbons have been formed.

Burning becomes generally more stable than the usual partial oxidations, under the effect of the fuel gas added to the hydrocarbons to be decomposed. At the higher decomposing temperature, due to the higher burning velocity of the fuel gases, cracking of the saturated hydrocarbons soon starts. However, it is completed only after the fuel gas used for heat output has been burnt. Heat output is procured in the first place by fuel gases and burning only takes place to a greater extent to the detriment of hydrocarbons, of oxygen remains in the mixture after the fuel gases having been burnt. Thus the conditions of cracking become more uniform than by partial oxidation. On the one part, the yield of unsaturated hydrocarbons becomes higher and, on the other, specific oxygen consumption and soot formation decrease.

Acetylene, ethylene or a mixture of acetylene and ethylene are formed as a principal product of the reaction. As a by-product generally carbon dioxide, carbon monoxide, hydrogen and methane are formed. The concentration of hydrogen and carbon monoxide formed usually surpasses the amount of fuel gases, required for heat output, so that the by-product gases obtained by cracking cover the fuel-gas consumption of the process according to the present invention.

The nature and the ratio of the unsaturated hydrocarbons formed by the decomposition of saturated hydrocarbons, particularly that of acetylene and ethylene, are adjusted by the temperature of thermal decomposition, the latter being regulated by the ratio of the saturated hydrocarbons, fuel gases and oxidizing gas, depending on the quality thereof.

It the reaction is carried out for a longer period of time than required for acetylene formation and somewhat more oxygen is used, the saturated hydrocarbons decompose completely and synthesis gas free of soot is obtained.

The process of the present invention may be carried out at atmospheric pressure and at smaller or higher pressures too, which may even expand to the value of 20 atmospheres. Thus the reaction conditions and the quality of the raw material may be altered to a significant extent. The reaction conditions are determined by the quality and ratio of the raw-material, fuel gas and oxidizing gas. The ratio of acetylene and ethylene depends on the raw material, temperature, and duration of the reaction. The reaction may be carried out also when higher or smaller amounts of fuel gas are admixed with the hydrocarbon to be decomposed, than required for heat output.

It is preferable to use a fuel gas having low oxygenconsumption and providing flue gases, for the reaction mixture, which exhibit a favorable effect on the equilibrium conditions of the decomposition reaction. Thus specific oxygen-consumption, being characteristic of the economy of the process, may be improved to a considerable extent. The requirements according to the present invention, with respect to the fuel gases, may be determined from the burning characteristics of the fuel gases; such value of some saturated hydrocarbons, fuel gases and pure oxygen are summarized in the following table.

It has been found that it is preferable by the process of the present invention to preheat the hydrocarbons, fuel gases and oxidizing gas to a temperature of 400 to 600 C. This may be carried out by preheating the components separately, before the mixing of the same or by preheating the mixture of the hydrocarbons and fuel gases and admixing the same with the preheated oxidizing gas. One may also proceed by preheating the previously prepared mixture of the components.

Product v Z p 1 p11 C H4 420 59. 2 4, 275 O. 803 3, 440 CzHe 50. 5 4, 390 0. 817 3, 580 C H 365 55. 0 4, 460 0. 822 3, 665 Gillie. 480 49. 0 4, 550 0. 825 3, 750 C7H- x 28. 8 4, 360 0. 820 3, 580 C O. 93. 3 6, 040 O. 785 4, 750 He 1, 93. 9 5, 0. 805 4, 140 (EH-O9]. 4,280 0.846 3, 625 C/+0.5 Oz/ 2, 530 0.673 1,700

1 The data relates to gasoline.

vs cin./sec., maximum burning velocity.

ZU Percent by volume, upper burning limit.

p keel/normal cubic metre of oxygen, specific heat output related to 0X13, lli ilaliout dimension, etliciency of furnace technique at a reaction temperature of 1,200 0.

p1; kcaL/normal cubic metre of oxygen, product of multiplication, characteristic of the economy of heat output.

According to the data of the above table, hydrogen meets all the requirements with respect to the fuel gases. Moreover, according to experimental data, the steam formed by burning exhibits a favorable effect from the point of view of the decomposition reaction, as it inhibits unfavorable side-reactions, particularly the precipitation of carbon. Carbon monoxide also complies with the requirements, but it may only be used as a fuel gas when admixed with hydrogen, due to the slow burning of carbon monoxide. The admixture of car-bon monoxide with the hydrocarbon to be decomposed is useful, in the first place, because it decreases the carbon monoxide formation in the decomposition reaction, the latter being very uneconomical according to the last column (p17) of the table.

It is to be noted that the known procedures, by whiEh only small amounts of 0.51.0% by volume of other gases or vapors, such as hydrogen, carbon monoxide, carbon dioxide, steam, formaldehyde, methanol, are added to the methane raw material, are only directed to promote more or less acetylene formation, but such small amounts of the above gases are completely unsuitable to achieve the effect of a fuel gas, used according to the present invention.

By appropriate addition of the oxidizing gas it is some times preferable to carry out the reaction at the upper burning limit, corresponding to the composition of the reaction mixture, under homogenous burning conditions. The concentration of the oxidizing gas in the starting mixture is to be regulated on the basis of the oxygen concentration of the product gas, so that the oxygen content of the residue should amount to a value between 0.2% and 0.5% by volume.

One may also proceed according to the present invention by using low-rate gases of low nitrogen and carbon dioxide content, comprising, apart from hydrogen and carbon monoxide, at least 25% of saturated hydrocarbons containing at most 5 carbon atoms, as a mixture of hydrocarbons and fuel gases.

For an understanding of the principles of the invention, reference is made to the following description of typical embodiments thereof as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a longitudinal sectional view of apparatus for performing the method of the invention; and

FIG. 2 is a section on line IIII of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the invention is carried out in the apparatus shown in the drawings, which forms the subject matter of co'pending application, Ser. No. 460,718 of which the present application is a division. Apparatus for carrying out flame reaction processes generally consist of three parts, as illustrated in the drawings. These parts comprise a diffuser 1 distributing the combustible mixture, a combustion chamber 3 and a section having a restricted free cross section and disposed intermediate parts 1 and 2 and separating the same from each other.

In the hitherto known apparatus the free cross-section of section 2 has already been restricted to such an extent that the mixture passes through it with a higher speed than the burning velocity thereof, so that the back-fire in diffuser 1 could be hindered. The entire cross section of section 2 was generally filled with a one-pieced or split fire proof ceramic block, on the whole cross section of which regularly distributed slits of circular section were formed in the direction of flow, as a restricted cross section. Combustion chamber 3 has such a cross section that therein the flowing velocity of the mixture is smaller than the burning velocity thereof, and this enables the formation of a stable flame in the combustion chamber. In order to provide stable flame and to hinder back-fire in the diffuser, the ratio of the free cross section of section 2 and that of combustion chamber 3 was adjusted preferably to a value between 1:4 and 1:12, while the diameter of the borings in section 2 were preferably 7 to mm. By the above expedients, at the junction of section 2, having a restricted free cross section, and of combustion chamber 3, so great a dead area is formed, that coke deposits occur in the dead area as a result of back flows, and such deposits must often be removed during processing. In a known apparatus scraping was used every two hours. The scraping of coke disturbs, on one parts, normal processing and, on the other, it causes the early waste of the fireproof material on section 2.

It is further known that flame-reaction processes may be carried out in apparatus in which the restricted cross section 2 is formed of metal and cooled with water, in order to protect the same from detrimental heat quantities radiated from the combustion chamber. Known apparatus is a complicated hollow construction, and moreover, a large dead area is formed at the junction of sec tions 2 and 3. Consequently, known apparatus processes significant disadvantages. A further type of known apparatus includes borings of greater and smaller resistance used to form a guard-flame. Such apparatus, however, is absolutely unsuitable for performing the method of the present invention.

In the apparatus used in performing the method of the present invention, the restricted cross section of section 2 is formed by slots 5, bordered by parallel metal-plates 4, having preferably a gauge of 1.5 to 3 mm. The size of slots 5 is determined by spaced spacer ribs 6 formed on the plates and illustrated in FIG. 2. The ratio of the cross section of slots 5 and that of plates 4 varies between the values of 1:1 and 1:2, thus the ratio of the free cross section of section 2 and of the cross section of combustion chamber 3 is considerably smaller than the corresponding ratio in known apparatus of the same type. The apparatus shown in the drawings provides the required safety against back-fire in the diffuser and it also produces a stable flame, even with the above ratio, as slots 5 are of such size, that, by loadings used, the Reynolds number amounts to a value entering into the laminar range among the slots and into the turbulent range in combustion chamber 3. This is also advantageous, as the burning velocity of the mixture is significantly smaller in the laminar range than in the turbulent range. Thus, the ratio of the flow velocity and burning velocity will be greater among the slots, so that the cross section ratio used in the present invention procures sufficient safety against back-fire in the diffuser.

Safety against back-fire in the diffuser depends naturally also on the height of plates 4. The higher the safety intended to be achieved and the greater the pressure-oscillation which may occur in the system, the higher are the plates to be inserted between the diffuser and the combustion chamber.

The dense distribution of slots 5 having identical cross sections on the cross section of section 2 provides regular burning and thus preferable reaction conditions too. Dead cross sections are significantly smaller. Consequently, the dead flow area is decreased so that there is no possibility for the formation of coke-deposit.

Thermal loading of the plates 4 is relatively small, as the ratio of the surface radiated by the combustion chamber related to that cooled by the mixture circulating among the plates is very preferable (it varies between the values of 1:50 and 1:100). Accordingly, section 2'may be safely prepared from a metal resistant to a temperature not higher than C. above the temperature of preheating, and no water-cooling is required.

A further characteristic of the apparatus resides in the fact that the restricted cross section of section 2 is formed from two different channel-systems, through which the mixture flows with different velocities. One channel-system, through which the majority of the mixture flows, consists of slots 5 having a relatively large cross section. Flow resistance is here smaller and thus high flow velocity may be achieved, which safely hinders the back fire into the diffuser. On the other hand in grooves 7 formed in the spacer ribs and having narrower cross sections, flow-resistance is higher and consequently, flow velocity is smaller. By appropriate dimensioning of the grooves 7, it may be achieved that the flow velocity of the mixture amounts to a value below the breaking down speed of the flame, but it approaches the burning velocity only to such an extent that there is no back-fire in the diffuser through the narrow grooves; the latter being also hindered by the cooling effect of the metal walls. Thus the fact that the mixture flows from grooves 7 into the combustion chamber with a velocity possibly smaller than the burning velocity but certainly smaller than the breaking down speed of the flame, ensures the continuity of burning, as the mixture flowing out from grooves 7 provides permanent ignition flames for the mixture, flowing out with greater velocity from slots 5, which are in the immediate vicinity of grooves 7. The ignition flames are regularly distributed over the whole cross section and ensure the stability of burning, while stable burning enables a considerable variation of the loading of the burner, as loading may be increased until the velocity of the mixture, flowing out from grooves 7, remains below the breaking down speed of the flame.

The method of the present invention will now be described by reference to examples, the method being performed with essentially the apparatus shown in the drawings.

Examples 1, 2 and 3 relate to different embodiments of the method, While Examples 4 and 5 relate to certain specific constructions of the apparatus in connection with the well-known partial oxidation of methane.

Example 1 60 normal cubic metres per hour of a gas-mixture at atmospheric pressure, preheated to 400 C., having the following composition are fed into the burner of a flame reactor:

As gas-mixture to be decomposed Percent by volume On partial burning of the above mixture by the oxygen content of same, a reaction temperature of about 1450" C. was obtained. The reaction took place during 3.10 seconds, whereupon the reaction product was quickly cooled with water to a temperature below 100 C., 65

normal cubic metres per hour of a dry product gas of the following composition were obtained:

Percent by volume Acetylene 8.2 Carbon monoxide 17.2 Hydrogen 61.2 Methane 4.0 Oxygen 0.3 Nitrogen 4.9 Carbon dioxide 4.2

Thus the use of hydrogen as the fuel gas results in the specific oxygen consumptioncharacteristic of the economy of the process-is 3.74 g. oxygen per kg. acetylene, while that of the known partial oxidation method with preheating to 600 C. was significantly higher, kg. oxygen per kg. acetylene. The value of 3.75 kg. oxygen per kg. oxygen per kg. acetylene achieved by the present example may be further decreased if the starting gas mixture contains no nitrogen contamination.

It is to be noted the above favorable result was obtained by using preheating to only 400 C., which means further savings in the structural material of preheater and in the heat-requirement for preheating.

EXAMPLE 2 60 normal cubic metres of a gas mixture at atmospheric pressure and preheated to 400 C., having the following composition are fed into the burner of flame reactor:

Percent by volume Methane (as gas to be decomposed) 30.5 As fuel gas:

Carbon monoxide 15.3 Hydrogen 32.0 Oxygen 22.0

The above gas-mixture was partially burnt by the oxygen content thereof under the reaction conditions described in Example 1. 52.5 normal cubic metres per hour of a dry gas mixture of the following composition were obtained:

Nitrogen (as contamination of the oxygen) Percent by volume Acetylene 9.3 Carbon monoxide 26.1 Hydrogen 56.5 Methane 3.5 Oxygen 0.25 Nitrogen 0.25 Carbon dioxide 4.1

According to the present example, if an approximately 1:2 mixture of carbon monoxide and hydrogen is used as a fuel gas, and oxygen having a purity of 99% is used as the oxidizing gas, specific oxygen consumption decreased to the value of 3.3 kg. oxygen per kg. acetylene.

EXAMPLE 3 60 normal cubic metres per hour of a gas-mixture at atmospheric pressure, preheated to 400 C. and having the following composition were fed into the burner of a flame reactor:

Percent by volume Methane (as gas to be decomposed) 20.4 Carbon monoxide 20.4 Hydrogen 42.5 Oxygen 16.5

Nitrogen (as contamination of the oxygen) 0.2

of the introduced amount of the fuel gas (an approximately 1:2 mixture of carbon monoxide and hydrogen) at the expense of methane, resulted a further reduction of the specific oxygen consumption.

It is to be noted that the total amount of the introduced carbon monoxide and hydrogen gases is somewhat smaller than that of the carbon monoxide and hydrogen gases formed by the reaction, thus the carbon monoxide, hydrogen and methane components of the product-gases may be directly used as starting material of the reaction.

EXAMPLE 4 Acetylene 8.6 Carbon monoxide 26.2 Hydrogen 57.5 Carbon dioxide 3.7

Methane 3.5 Ethylene 0.5 Oxygen 0.2

EXAMPLE 5 A gas mixture consisting of 63% of methane and 37% of oxygen, preheated to 600 C. and having a pressure of 5 atm. was passed through the burner. Specific loading, related to the free cross-section of the burner, increased to 600 normal cubic metres of methane per square decimetre hour, due to the higher pressure. The height of the plates amounted to 75 mm. according to this example also. However, the highest temperature of the plates was only 650 C. on their side toward the combustion chamber. No coke-deposit was observed on the plates.

Data according to Examples 4 and 5 relate to experiments carried out with the apparatus illustrated in the drawings provided with spacer ribs 6 and grooves 7. If the spacer elements and grooves 7 are formed from plates, being supported against plates 4, being thinner than the latter and in some places recessed according to the breadth of the slot in the direction of the flow, or if they are formed from separately inserted tubes of small diameter, similar results are then obtained.

What is claimed is:

l. A single step process for thermal decomposition of saturated hydrocarbons containing at most 5 carbon atoms to produce unsaturated hydrocarbons selected from the group consisting of acetylene, ethylene and mixtures thereof, which comprises introducing into the combustion zone of a flame reactor a gaseous mixture composed of a hydrocarbon component containing saturated hydrocarbons having at most 5 carbon atoms, an oxygen containing gas and a fuel gas, said fuel gas having a higher burning velocity and a higher upper burning limit than said saturated hydrocarbon component, said fuel gas being present in said gaseous mixture in an amount sufficient to provide at least 30% of the heat necessary to carry out said thermal decomposition with the temperature maintained at from 1,000 C. to about 1,600 C., and burning said gaseous mixture in said flame reactor combustion zone at a temperature of from 1,000 C. to about 1,600 C. to cause the thermal decomposition of said saturated hydrocarbons producing a residue containing said unsaturated hydrocarbons.

2. The process according to claim 1, wherein the hydrocarbon component, the oxygen containing gas and the fuel gas are heated to a temperature of from about 400 C. to 600 C. prior to introducing them into said flame reactor.

3. The process according to claim 1, wherein the amount of oxygen gas in said gaseous mixture is suflicient to provide oxygen in said residue in an amount of from about 0.2% to about 0.5% by volume of said residue.

4. The process according to claim 1, wherein said residue contains in addition to said unsaturated hydrocarbons, hydrogen and carbon monoxide.

5. The process according to claim 1, wherein said fuel gas is composed of hydrogen and carbon monoxide and said oxygen containing gas is composed of oxygen.

6. The process according to claim 1, wherein said fuel gas and said saturated hydrocarbon component contain 15 at least 25% by weight of saturated hydrocarbons having at most five carbon atoms,

References Cited UNITED STATES PATENTS 10 DELBERT E. GANTZ, Primary Examiner J. M. NELSON, Assistant Examiner US. Cl. X.R.

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