Method for producing synthesis gas

The present invention is an improved method for producing synthesis gas from gaseous raw hydrocarbons. The method includes combustion of a mixture of raw hydrocarbons and air at .varies.=0.5-0.8. The hydrocarbons are oxidized during the compression stroke of a piston in the cylinder of a modified internal combustion engine. The resulting products are expanded and cooled during the movement of the piston toward the bottom dead center. The products containing the synthesis gas are then expelled from the reaction volume when the piston moves toward the top dead center. A new portion of the working mixture is supplied when the piston moves back to the bottom dead center. The mixture of raw hydrocarbons with air is preheated to 200-450.degree. C. before being fed into the cylinders of the modified internal combustion engine. The mixture is compressed until self-ignition takes place. A temperature of 1300-2300.degree. C. is maintained over a time period of 10-2-10-3 s. The cycle is repeated with a frequency exceeding 350 min-1. The method may use as raw material methane, ethane, or other gaseous hydrocarbons mainly produced during the separation of the large fraction of light hydrocarbons from petroleum gas. The specific productivity of the method is about 2.5 times higher than known methods, making it possible to improve the ecological situation in regions of oil production and processing.

AREA OF THE INVENTION
 This invention relates to the technology of hydrocarbon processing and more
 particularly to the production of synthesis gas from gaseous hydrocarbon
 raw material.
 DESCRIPTION OF THE PRIOR ART
 The method for producing synthesis gas from hydrocarbon raw material, which
 involves mixing the raw mixture with an oxidizer, oxygen or
 oxygen-containing gas, or water vapor, feeding the mixture to the reaction
 zone at a temperature, which is 93.degree. C. no less than the
 self-ignition point of the mixture, at a rate of the turbulent flow
 exceeding the rate of flame flash-back, and conversion of the mixture in
 the presence of monolytic catalyst, is disclosed in RF Pat. 1,831,468, to
 Danster, M. And Kornchak, D. Method of producing synthesis gas from
 hydrocarbon raw, Byul. Izobret., 1993, no. 28, Int. Cl.: CO1B3/38.
 The above method needs creation of a special catalytic reactor and the
 usage of a selective catalyst.
 Other methods of incomplete oxidation of hydrocarbon raw material used, for
 example, to produce synthesis gas are known:
EQU CH.sub.4 +0.5O.sub.2 =CO+2H.sub.2
 The most similar to the present invention is the method for producing
 synthesis gas disclosed in (Kazarnovskii, Ya. S., Derevyanko, I. G.,
 Stezhinskii., A. I., and Kobozev, N. I., Explosive methane conversion.
 Trudy of State Research Institute of Nitrogen Industry, Moscow, 1957, vol.
 VIII, pp. 89-104). The said method comprises combustion of a gas mixture
 composed of hydrocarbon raw material and oxygen-enriched air at
 .alpha.=0.5-0.8 or air not enriched with oxygen at .alpha.=0.827-1.2,
 explosive partial oxidation of hydrocarbons in the cylinder of an internal
 combustion engine, expansion and cooling the products when the piston of
 the engine moves to the bottom dead center, output of the products
 containing the synthesis gas from the reactive volume when the piston
 moves to the top dead center, and input of a new portion of the working
 mixture when the piston moves to the bottom dead center. Enriched gas from
 coke production is usually used as the hydrocarbon raw material, which
 predominantly contains carbon oxide and methane and ethylene fractions.
 The mixture of said raw material and air is supplied into the cylinders of
 the internal combustion engine, and the explosive partial oxidation is
 preceded by the forced ignition of the mixture. The specific productivity
 of the process with respect to the hydrocarbon raw material is about 700
 kg/m.sup.3 h.
 The production of synthesis gas is combined with electric power production.
 The use of the enriched gas of coke production, the product of natural gas
 processing, rather than the natural gas itself makes the synthesis gas
 production tied to the coke production facilities.
 In addition, when this method uses air not enriched with oxygen at
 .alpha.=0.827-1.2, the content of CO.sub.2 is 1.5-2 times higher than that
 of CO, and the content of hydrogen does not satisfy the synthesis demands,
 while, at .alpha.&gt;1, hydrogen is absent at all. Thus, for not enriched air
 at .alpha.=0.827, the ratio H.sub.2 /CO is 0.76 and, in any example, does
 not reach the value of 2.0 generally accepted in methanol synthesis.
 When the method is realized with oxygen-enriched air at .alpha.=0.5-0.8
 (the content of oxygen is 29 and 50%, respectively, for the values of
 .alpha. indicated above) the ratio H.sub.2 /CO does not satisfy the
 demands of catalytic synthesis (in some examples, this ratio is less than
 unity). At .alpha.=0.8, the contents of CO.sub.2 and CO are equal.
 DISCLOSURE OF THE INVENTION
 The object of this invention is to provide the method for producing
 synthesis gas useful in catalytic processing.
 This method allows production of synthesis gas in commercial slightly
 modified internal combustion engines. This invention utilizes compression
 self-ignition and external mixture preparation.
 The use of methane, ethane, and other gaseous hydrocarbons as raw
 materials, including the broad fraction of light hydrocarbons from the
 associated gases makes it possible to improve the ecological situation in
 the regions of oil production and processing. The specific productivity of
 this method is 2.5-3 times higher than in the method referred to above,
 and the volume ratios H.sub.2 /CO=1-2, depending on the process regime and
 composition of raw material. This is of particular importance, because the
 efficiency of synthesis gas production is known to affect significantly
 the economy of synthetic motor fuel production (Lykov, O. P., Chemistry
 and Technology of Fuels and Oils. 1996, no. 3, pp. 15-24).
 The object of the present invention is to provide an improved process,
 which includes combustion of the mixture of raw hydrocarbons with air and
 oxidation of hydrocarbons upon the compression stroke of the piston in the
 cylinders of the modified internal combustion engine, expansion and
 cooling of the products during the piston stroke to the bottom dead
 center, output of the products containing synthesis gas from the reaction
 volume upon the piston stroke to the top dead center, inlet of a new
 portion of the working mixture upon the piston stroke to the bottom dead
 center, wherein the mixture of raw hydrocarbons with air at
 .alpha.=0.5-0.8 preheated to 200-450.degree. C. is fed into the cylinders
 of the modified internal combustion engine, and the mixture is compressed
 until self-ignition takes place and a temperature of 1300-2300.degree. C.
 is maintained over a 10.sup.-2 -10.sup.-3 s period, and the cycle is
 repeated with a frequency exceeding 350 min.sup.-1.
 When the mixture of air and hydrocarbon raw material is preheated to the
 temperature lower than 200.degree. C., no self-ignition takes place in the
 cylinder of the modified internal combustion engine. The choice of the
 upper temperature limit for preheating the mixture (450.degree. C.) is
 based on safety considerations relating to the possible self-ignition of
 the mixture before it reaches the reaction volume.
 When the content of air in its mixture with hydrocarbon corresponds to
 .alpha.&lt;0.5, the intense carbon black formation takes place and, thus, the
 synthesis gas quality gets worse. At the content of air corresponding to
 .alpha.&gt;0.8, the share of CO.sub.2 in the exhaust gases becomes greater
 than that of CO. This also deteriorates the synthesis gas quality
 (Kazamovskii, Ya. S., Derevyanko, I. G., Stezhinskii., A. I., and Kobozev,
 N. I., Explosive methane conversion. Trudy of State Research Institute of
 Nitrogen Industry, Moscow, 1957, vol. VIII, pp. 89-104).
 The lower temperature limit (1300.degree. C.) is selected so as to ensure
 high conversion in the partial oxidation of the hydrocarbon raw material.
 The upper temperature limit (2300.degree. C.) is selected so as to
 eliminate the black carbon formation at the low values of .alpha. claimed
 and to provide survivability of the outlet valves.
 The cycle frequency should exceed 350 min.sup.-1 because no self-ignition
 takes place with slow compression.
 When the partial oxidation is accomplished at temperatures referred to
 above over a period &gt;10.sup.-2 s, the yield of the target product
 decreases.
 The partial oxidation accomplished at temperatures referred to above over a
 period &lt;10.sup.-3 s requires such an increase in the engine rotation speed
 that the inertial loads rise to the levels higher than allowable ones with
 respect to the strength considerations.
 The method is accomplished as follows:
 1. The raw hydrocarbon material is premixed with air to achieve
 .alpha.=0.5-0.8.
 2. The prepared mixture is heated to a temperature of 200-450.degree. C.
 3. The preheated mixture is drawn into the cylinder of the modified
 internal combustion engine type during the motion of the piston to the
 bottom dead center.
 4. The partial oxidation of hydrocarbons is accomplished by compression of
 the mixture in the cylinder by means of the piston stroke to the upper
 dead center until self-ignition of the mixture takes place and a
 temperature of 1200-2300.degree. C. is maintained over the period of
 10.sup.-2 -10.sup.-3 s.
 5. The products are cooled by expansion during the piston stroke to the
 bottom dead center.
 6. The process products containing synthesis gas are removed out of the
 cylinder during the piston stroke to the upper dead center.
 7. The cycle is repeated at a frequency exceeding 350 min.sup.-1.
 8. The kinetic energy of the engine motion is used to produce energy in the
 generator connected to the engine shaft.
 BRIEF DESCRIPTION OF DRAWING
 The method for producing synthesis gas is more fully described with
 reference to the accompanying drawing, which shows the scheme of the
 setup.
 The setup consists of the chemical compression reactor based on the
 modified internal combustion engine. It includes cylinder 1, representing
 the closed reaction volume, wherein piston 2 is disposed, the intake valve
 3 arranged in the zone of the top dead center of the cylinder 1 and
 intended to deliver the mixture of the oxidant and hydrocarbon raw
 material and connected by a piping with reactor 4, where the said mixture
 is preheated, connected with mixer 5 of the said components of the raw
 material, and the outlet valve 6 disposed in the zone of the top dead
 center of the cylinder 1 and intended to remove the products. Piston 2 of
 cylinder 1 is connected with crank drive. Crankshaft of drive 7 is
 connected either with the electric motor 8, or with a drive of another
 type, depending on the extent of autonomy of the setup and conditions of
 its operation. Generator 9 is installed on the same shaft of the internal
 combustion engine.
 The setup is equipped with the system for preparing the working mixture
 from the hydrocarbon raw material and air, which includes the dosing
 apparatus and measuring devices. The reactor 4, where the working mixture
 is preheated, includes the heater or the recuperative exchanger-preheater,
 where the products are directed from the engine cylinder 1.
 The operation of the setup and realization of the method proceed as
 follows.
 The hydrocarbon raw material and air are fed at the ratios referred to
 above into mixer 5. The mixture is fed into reactor 4 and the mixture is
 preheated to the temperatures referred to above. The preheated mixture is
 supplied through valve 3 into cylinder 1, where upon the movement of
 piston 2 to the top dead center 8, the mixture is compressed until
 self-ignition takes place and a temperature of 1400-2300.degree. C. is
 maintained over a time period of 10.sup.-2 10.sup.-3 s, during which
 combustion and thermal decomposition of the working mixture are
 accomplished.
 When piston 2 moves in cylinder 1 downwards the bottom dead center, the
 products are expanded, cooled, and quenched, the thermal energy of the
 products being converted to the mechanical energy of the motion mechanism
 utilized by means of generator 9. During the subsequent stroke of piston 2
 to the top dead center, the products are removed from cylinder 1 through
 the outlet valve 6. A new portion of the working mixture is fed into
 cylinder 1 through intake valve 3 when piston 2 moves to the bottom dead
 center. The reciprocal motion of piston 2 in the cylinder 1 is performed
 at a frequency not less than 350 min.sup.-1.

Examples of the present invention are presented in the Table. The method is
 accomplished by means of a setup which includes the modified
 Ch8,5/11(1R2-6) two-cylinder diesel engine with an effective volume of
 1.24 l processing the hydrocarbon raw material.
 As is seen from the Table, the H.sub.2 /CO volume ratio lies in the claimed
 range (1-2). It is quite suitable for further catalytic production of
 fuel, methanol, or dimethyl ether. The conversion of the natural gas is
 close to 100%. In addition, the specific productivity of the process with
 respect to the raw material amounts to 1400-2000 kg/m.sup.3 h. This is
 2.5-3 times higher as compared to the prototype.
 Examples 6-10 illustrate possible technological variations within the
 framework of this method. Thus, the pressure at the outlet, temperature of
 preheating, and the phase of the mixture ignition can be controlled if
 some amount of process gases are left in a cylinder (including water
 vapors).
 TABLE