Abstract:
A method is disclosed for converting methane to acetylene wherein a mixture of oxygen-containing gas and hydrocarbons is compressed and ignited to react the mixture to form a product gas containing acetylene, ethylene and C 2  + hydrocarbons, and the reacted mixture is rapidly expanded and cooled. The mixture contains a mole ratio of hydrocarbons to oxygen-containing gas ranging from about 0.75:1 to about 1.65:1.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the thermal conversion of methane to acetylene and particularly relates to compression, rapid heating and decompression, and short residence time reaction of methane to produce acetylene in the combustion space of a cyclically operated internal combustion engine. 
     2. Setting of the Invention 
     It is known that hydrocarbons, including methane, as well as higher aliphatic and aromatic hydrocarbons, will yield acetylene and ethylene when subjected to partial combustion or thermal cracking processes. In thermal cracking processes, the necessary heat for the reaction is supplied to the hydrocarbon charge by external or indirect heating. Thermal cracking has not been found to be commercially viable for methane pyrolysis. In partial combustion processes, necessary heat for the reaction is supplied by combusting part of the methane charge. The principal difficulty with the partial combustion system is coke formation within the reaction system. Both processes require high temperatures to form acetylene, together with rapid cooling of the reaction product to avoid decomposition of the acetylene formed. 
     DESCRIPTION OF THE PERTINENT ART 
     Pertinent art includes U.S. Pat. No. 1,939,018 (&#39;018), which discloses a method of combusting a fuel such as gasoline, natural gas and other hydrocarbons to form product gases such as aldehydes, alcohols and other oxygenated products in an internal combustion engine, including the steps of admitting the fuel and an insufficient amount of an oxidizing agent into the combustion chamber of the engine, igniting the mixture, causing combustion to occur in the chamber, removing the exhaust gases from the chamber and immediately cooling the exhaust gases, and recovering the product gases from the exhaust gases. U.S. Pat. No. 1,939,018 recognizes that the compression of the fuel/oxidizing agent mixture relates directly to the temperature of the mixture before and after combustion. In order to control temperature, a diluent gas such as nitrogen or water vapor is admitted with the fuel/oxidizing agent mixture to the combustion chamber. 
     In addition, to achieve greater product yield, a catalyst such as nitrogen dioxide, nitric acid or a finely divided metal oxide was disclosed as an addition to the fuel/oxidizing agent mixture prior to ignition. 
     The method of &#39;018 produces oxygenated products in relation to the amount of power produced. The method details also using two sets of chambers within the same engine for production of maximum power from the first set and a maximum product under a controlled lower temperature in the second set. 
     Also included in pertinent art is U.S. Pat. No. 2,748,179 (&#39;179), which discloses another method for conversion of low molecular weight hydrocarbons to olefins and aromatics, including the steps of preheating to 200° to 400° F. a mixture of hydrocarbons and an oxygen-containing gas having 0.5 to 2.8 moles of free oxygen per mole of hydrocarbon, admitting the mixture to an externally unheated reaction zone, compressing the mixture in the zone to about 1/10 to 1/20 of the original volume of the zone about 100 to 450 times a minute so that the temperature is increased substantially to a level conducive to a limited reaction between the hydrocarbons and the free oxygen, rapidly expanding the reaction products to reduce the temperature and withdrawing the reaction products from the zone. The fuel/oxygen-containing gas mixture may be diluted with an inert gas such as steam or nitrogen. The fuels disclosed as being useful in the method of &#39;179 include gaseous and liquid paraffinic hydrocarbons and liquid napthenic hydrocarbons. The method is disclosed as being particularly useful on normally low molecular weight liquid hydrocarbons such as n-pentane and n-hexane. 
     Pertinent art also includes an article entitled &#34;The Production of Synthesis Gas and Power in a Compression Ignition Engine&#34; authored by G. A. Karim and N. P. W. Moore. In this article, a method for producing a synthesis gas using a single-cylinder, compression-ignition, four-stroke engine of an open chamber type was disclosed. The natural gas and air were introduced into the chamber, compressed to a ratio of 14.2:1, and ignited by injection of a small quantity of liquid fuel such as diesel fuel near the top dead center (T.D.C.) position in the cycle of the engine piston. The liquid fuel used was usually less than 10 percent of the total full load quantity. The authors noted that two stages took place during the synthesis gas production. The first stage was the consumption of most of the oxygen following the ignition of the liquid fuel to produce carbon dioxide and water and the release of the energy which raises the temperature level of the mixture held in the compression chamber. When the higher temperature level was obtained throughout the combustion chamber, a second step included the reforming reactions of the feed gas which proceeded at a substantial rate and produced synthesis gas. During expansion, these reactions ceased as a certain reduced temperature level was attained. Rapid quenching rates were provided by high-speed engines resulting in better gas yields and the possible production of other compounds such as acetylene. However, in all testing, Karim and Moore found that acetylene was absent from the exhaust gas produced during the experiments. It was stated that the methane was converted to acetylene in the flame front very rapidly, but to an extent far short of equilibrium. The acetylene would subsequently react with methane and water to produce hydrogen and carbon monoxide. 
     Further pertinent art includes U.S. Pat. No. 2,823,243 (&#39;243), which discloses a method of pyrolysis of hydrocarbons to produce an acetylene-containing product. The method includes the steps of burning hydrogen and oxygen to form a combustion gas having a temperature of between 4500° and 5300° F.; introducing a stream of gas (hydrogen or steam) to reduce the temperature of the combustion gas to not higher than 4200° F.; preheating a methane-containing gas to 1800° to 2400° F.; introducing the heated gas into the combustion gas to produce a gas admixture having a temperature between 2400° and 3500° F.; increasing the linear velocity of the resulting mixture to 200 to 500 feet per second; decreasing the velocity; maintaining the mixture in reaction for a time between 0.001 and 0.05 seconds; quenching the mixture to below pyrolysis temperature; and recovering the pyrolysis products. 
     The principal object of the present invention is to provide the process of production of acetylene and ethylene from methane. A more specific object of the present invention is to maximize the acetylene yields by minimizing feed hydrocarbon losses to carbon dioxide and coke. A further object is to maximize the overall yield of acetylene from a methane feedstock while still producing usable power. A still additional object of the present invention is to provide a convenient small-scale method of converting a methane-containing natural gas to power and acetylene which can be further converted into a liquid form for transport. 
     SUMMARY OF THE INVENTION 
     The present invention covers a process for converting low molecular weight normally gaseous hydrocarbons into valuable hydrocarbons containing acetylene and include the steps of introducing the hydrocarbons and an oxygen-containing gas into a reaction zone, the mole ratio of the hydrocarbons and oxygen ranging from about 0.75:1 and about 1.65:1, compressing the gaseous mixture in the zone to about 1/5 to about 1/13 of the original volume of the reaction zone, igniting the mixture causing combustion of a portion of the hydrocarbons so that the temperature of the mixture is increased substantially to a level conducive to a limited reaction of the mixture under compression to produce an acetylene-containing reaction product, rapidly therafter expanding the reaction product to reduce the temperature of the reaction product, removing the reaction product from the reaction zone and cooling the removed reaction product. These steps are repeated between about 500 and about 2000 times per minute. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The FIGURE is a schematic representation of an apparatus used in the process of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The process of the present invention may best be illustrated in accordance with the FIGURE, which shows one embodiment of the present invention. For purposes of the explanation of this embodiment, methane is employed, although it will be understood that other hydrocarbons such as natural gas may be and are used. The term &#34;methane&#34; is used in the description of the embodiment, but should not be read to limit the type of hydrocarbons useful in this process. The hydrocarbons used in the process of the present invention may be any gaseous, low molecular weight hydrocarbon which is combustible such as city gas, natural gas, ethane, propane, butane, pentane, and other gaseous petroleum products. The feedstock useful in the present invention also includes hydrocarbons which can be vaporized at conditions under which the process is conducted. The hydrocarbons may include additional hydrogen. 
     The methane is pumped from storage tank 10 through line 12 to metering device 14 and thence through line 16. The methane is introduced into mixing zone 20 where oxygen-containing gas is introduced via line 11 from oxygen source 13. The mixture flows via line 21 to heat exchanger 18 for preheating. A predetermined amount of mixture is metered into lines 23a, b, c, d, and e, and then into the reaction zones 22a, b, c, d, and e. Reaction zones 22 are a battery of internal combustion cylinders as part of a multicylinder internal combustion engine 24. Pistons 26 of engine 24 are externally actuated to alternately compress and expand the methane and the reaction products. 
     Pistons 26 move within reaction zones 22 to compress the mixture held in reaction zones 22. Near the top of the compression stroke (T.D.C.), a spark is provided by means 34 to ignite the mixture. This ignition causes combustion of the mixture of oxygen-containing gas and methane, which in turn raises the temperature of both the combustion gases and the remaining combusted portion 38 of the compressed methane. Portion 38 of the compressed methane reacts to form desired products, including acetylene. Following the combustion and the reaction, pistons 26 move within reaction zones 22 to expand the combustion gases and the reaction gases. The gaseous mixture of combustion and reaction is partially cooled during the expansion stroke of pistons 26. 
     Power is generated by the expansion of the hot gases contained in reaction zones 22 against pistons 26, which in turn drive piston rods 52 downward during various stages of the overall process. The movement of piston rods 52 in turn drives shaft 54. Shaft 54 is connected to a power collection means 56 for generation of useful mechanical power or conversion to electrical power. 
     The gases are withdrawn via outlets 40 as pistons 26 reach full expansion and during a portion of the exhaust stroke. From outlets 40, the hot product gases flow to heat exchanger 18 through lines 42 to be cooled by incoming methane mixture stream 21. The cooled gases from heat exchanger 18 are moved to condenser 44 via line 45 to be cooled further to a desired temperature. The cooled gases are moved via line 46 to separation tower 48. The cooled gases may be separated and purified in separation tower 48 by any of the various known techniques for recovery of individual products such as by distillation, absorption, gravity separation and the like. Products, including acetylene, as well as recoverable olefins such as ethylene, are recovered via line 51 in product tank 52. Unused methane can be recycled via line 50 to methane source 10. 
     The oxygen-containing gas may be air, air enriched with oxygen, oxygen, mixtures of them or compounds containing one or more of them which support combustion; that is, compounds that will react with and oxidize the hydrocarbon fuel under conditions of combustion in an internal combustion engine. Preferably, the oxygen-containing gas is either air or oxygen. More preferably, it is oxygen. Still more preferably, the oxygen has a purity greater than 99.5 percent. 
     An important feature of the present invention is the rapid formation of a heat source within the reaction zone without the combustion of the entire hydrocarbon fuel held within the reaction zone. The heat generated by combustion raises the temperature of the mixture to a temperature at which methane is converted to form a desirable reaction product containing acetylene and C 2  + hydrocarbons. 
     The reaction within the engine is sensitive to temperature, time, compression ratio, expansion ratio, degree of mixing of the oxygen-containing gas with the hydrocarbon fuel, engine design, ratio of hydrocarbon to oxygen, and ratio of combusted hydrocarbons to total compressed hydrocarbons. Relatively long contact time due to low revolutions per minute (rpm) of the internal combustin engine produces carbon dioxide and water, rather than the desired product gas. A high preheat temperature (for instance, about 400° F.) causes prereaction of the mixture, which has the effect of reducing the amount of desired acetylene and C 2  + hydrocarbons in the reaction product. 
     The mole ratio of methane to oxygen should range from about 0.75:1 to about 1.65:1. Preferably, the mole ratio of methane to oxygen is between about 1.2:1 to about 1.3:1. More preferably, the ratio is about 1.29:1. 
     The number of compressions per minute may vary from about 500 to about 2000 times per minutes. Preferably, the compressions occur about 800 to about 1400 times per minute. More preferably, the compressions occur about 1000 to about 1200 times per minute. 
     The timing of ignition is set such that shaft 54 is continuously turned at a smooth rate. Preferably, the timing is set to be between about 20° before T.D.C. and about 30° after T.D.C. More preferably, the timing is about T.D.C. to about 20° after T.D.C. 
     The compression ratio ranges from about 5:1 to about 13:1, and preferably from about 6:1 to about 8:1. The more preferred compression ratio is about 10:1. The temperature obtained at T.D.C. should range from about 2700 to about2900° F. Preferably, the temperature is from about 2700° to about 2800° F. The temperature obtained at T.D.C. may be controlled by compressing the methane to a higher ratio, preheating the methane before compression, or a combination thereof. 
     The expansion rate should be as rapid as possible and need not be identical to the compression rate. The methane may be injected under a vacuum or after partial completion of the piston cycle. Preferably, however, the reaction product gases are expanded into a large volume outside the reaction zone to increase the expansion rate. Preferably, the expansion ratio should be from about 10:1 to about 20:1. More preferably, the expansion ratio is about 13:1 to about 18:1. 
     If a lubricant is used with the piston and cylinder, it is preferable that the lubricant be convertible to a usable product. 
     It is preferable to cool the exhaust gases immediately after reaction or at least as they are discharged from outlets 40 of the engine for purposes of preventing the destruction of unstable compounds. It is more preferable to introduce to reaction zones 22 cooling quench gas during the expansion step of pistons 26 to quickly lower the temperature of the product gases. The cooling quench gas may be water, steam, water vapor, methane, ethane, propane, butane, or other fluid which can be readily separated from the product gases in separation tower 48. Alternatively, or in combination with cooling, the reaction gas is contacted with a catalyst upon exiting outlets 40 or following cooling to convert more methane to the desired end products. 
     The matter of generating power efficiently is typically secondary to production of a maximum amount of desired end products; that is, acetylene. Conditions may be adjusted with use--either a different natural gas or a different oxidizing agent such as air or pure oxygen or air enriched with oxygen. The maximum amount of power is not always obtained simultaneously with the maximum yield of incompletely oxidized desired end products. 
     Alternatively, a number of the cylinders may be fed with power producing fuel oxygen ratios so as to drive the shaft and the other cylinders which would be directed to maximum methane conversion. 
     EXAMPLE 
     A series of tests using a spark-ignited, single-cylinder, four-cycle, internal combustion engine to demonstrate the effect of enging speed, compression ratio, ignition spark timing, and methane-to-oxygen ratio on the conversion of methane to C 2  + hydrocarbons was conducted. Predetermined amounts of methane and oxygen were introduced into the engine as a homogeneous mixture through a modified intake manifold. The engine speed was set at various rpm&#39;s between 500 and 1400. The compression ratio is set at various ratios from 7:1 to 14:1. The ignition spark timing is set to occur at between 30° before T.D.C. and 20° after T.D.C. The methane-to-oxygen ratio is varied from 0.5:1 to 2:1. The exhaust gas was analyzed with a gas chromatograph. Results are presented in Table 1 below. 
     
                       TABLE 1______________________________________              IGNITION   RATIO OF COMPRESSION  SPARK      METHANE   C.sub.2RPM   RATIO        TIMING     TO OXYGEN (%)______________________________________ 500  10:1         30°                     BTDC    1:1     0.0 500  12:1         30°                     BTDC    1:1     0.0 500   8:1                TDC    1.3:1    0.01000  10:1         30°                     BTDC  1.21:1    0.31000  10:1                TDC   1.32:1    0.61000  12:1         30°                     BTDC  1.28:1    0.51000  12:1                TDC   1.31:1    0.91000  12:1         30°                     BTDC  0.50:1    0.01000  12:1         20°                     ATDC  0.50:1     0.011000  12:1         20°                     ATDC  1.30:1    --1200  10:1         30°                     BTDC  1.32:1    0.81200  10:1                TDC   1.29:1    2.01200  12:1         30°                     BTDC  1.27:1    0.41200  12:1                TDC   1.20:1    1.11400   7:1         30°                     BTDC   0.5:1    0.01400   7:1                TDC    0.5:1    0.0______________________________________ 
    
     The maximum conversion to C 2  + hydrocarbons from methane occurred when the engine was operated at 1200 rpm with a compression ratio of 10:1, spark timing at T.D.C., and a methane-to-oxygen ratio of 1.29:1. Under these conditions, approximately 2 percent of the methane on a carbon basis was converted to C 2  + hydrocarbons; 65 percent of C 2  + hydrocarbons being acetylene. The exhaust gas temperature reached a maximum value of 1400° F. and the engine produced a low level of positive crankshaft power. Increasing the methane-to-oxygen ratio above 1.3:1 caused an abrupt loss of combustion and accompanying methane conversion and power. The conversion of methane to C 2  + hydrocarbons occurred to a lesser degree when the engine was operated at 1000 rpm with a compression ratio of 10:1 and a spark timing at T.D.C. As with the previous conditions, the maximum conversion occurred when the methane-to-oxygen ratio was at a value slightly less than the rich limit of combustion. Engine operation and other parameter settings outside of those indicated resulted in essentially no conversion to C 2  + hydrocarbons. 
     A C 2  + hydrocarbon concentration in the product gas with the ideal parameters of carbon monoxide and hydrogen remains high regardless of the engine speed, compression ratio or spark timing. At a 12:1 compression ratio at 1200 rpm, the conversion of methane to C 2  + hydrocarbons drops to 0.4 percent with an exhaust gas temperature of 1300° F. With the spark timing set at 30° before T.D.C., the predominant C 2  + hydrocarbon was ethylene, as opposed to at T.D.C. where the predominant C 2  + hydrocarbon was acetylene. At spark timings after T.D.C., combustion decreased rapidly with the exhaust gas becoming a mixture of carbon dioxide, water, carbon monoxide, unburned methane, and oxygen. The concentration of C 2  + hydrocarbons was very low. 
     The versatility of the present invention is well demonstrated in that desired compounds are produced which can be transported or converted to highly valued hydrocarbons and that mechanical and/or electrical power is also produced.