GB2159153A - Process for the production of oxygenated hydrocarbons - Google Patents

Abstract

Oxygenated hydrocarbons, for example methanol, are produced from a gaseous hydrocarbon feedstock mixture comprising methane and/or ethane by feeding the hydrocarbon to the combustion chamber of a burner having a pulsating mode of operation wherein a series of explosion or pressure waves is produced by repeated ignition of a gaseous oxidant/fuel mixture, the hydrocarbon being fed to the chamber at a point close to the source of ignition.

Description

SPECIFICATION Process for the production of oxygenated hydrocarbons The present invention relates to a process for the production of oxygenated hydrocarbons by the oxidation of a gaseous hydrocarbon feedstock comprising methane and/or ethane.

Oxygenated hydrocarbons, for example methanol and ethanol, are valuable industrial products useful as solvents, as chemical intermediates and as internal combustion engine fuel supplements. Methanol is produced from methane, for example, on a commercial scale in two steps. In a first step, methane is steam reformed to synthesis gas (carbon monoxide/hydrogen), which is a highly endothermic reaction, and in a subsequent second step the synthesis gas is catalytically converted into methanol by a route which is highly exothermic. Taken together the two steps are energetically unfavourable. Ethanol is commonly produced on a commercial scale either by biological fermentation of carbonaceous materials or by catalytic hydration of ethylene, the ethylene being derived from petroleum sources.

With the vast available resources of natural gas principally comprising methane and ethane in many parts of the world it would be highly desirable to convert methane and/or ethane in a single step into oxygenated hydrocarbons and in particular into methanol and/or ethanol.

We have now found that this can be achieved by oxidising the hydrocarbon in a burner having a pulsating mode of operation.

Accordingly, the present invention provides a process for the production of oxygenated hydrocarbons from a gaseous hydrocarbon mixture comprising methane and/or ethane which process comprises feeding the hydrocarbon to the combustion chamber of a burner having a pulsating mode of operation wherein a series of explosion or pressure waves is produced by repeated ignition of a gaseous oxidant/ fuel mixture, the hydrocarbon being fed to the combustion chamber at a point close to the source of ignition.

The gaseous hydrocarbon feed may be substantially pure methane or ethane or mixtures of hydrocarbon containing substantial proportions of methane and/or ethane, such as those obtained from natural hydrocarbon gas reservoirs.

The oxidant may suitably be any source of molecular oxygen, for example air or a mixture of air and oxygen. Alternatively, pure oxygen may be used as the oxidant.

The fuel may suitably be any combustible gas.

Suitable combustible gases include hydrogen, methane, ethane, propane and butane, or mixtures thereof. The fuel gas may be identical to the reactant gas. Alternatively, the fuel may be liquid, provided it is fed in the form of fine droplets.

The ratio of fuel to oxidant must be within the flammable, preferably within the detonation limits, of such mistures. The fuel and oxidant may suitably be fed separately or combined and fed as a mixture to the combustion chamber. Supplementary oxidant may be fed in whole or in part with the hydrocarbon reactant or separate from the hydrocarbon reactant.

A gaseous diluent may be employed if so desired.

For example, it is particularly preferred to feed a gaseous diluent when using hydrogen as the fuel gas and molecular oxygen as the oxidant. The gaseous diluent may suitably be any inert gas, for example nitrogen or carbon dioxide.

Some, or all of the feeds may be heated if desired.

The process may suitably be carried out at atmospheric pressure, though subatmospheric and superatmospheric pressures may be employed if so desired.

The process is preferably operated in a continuous manner with reference to the hydrocarbon reactant, fuel and oxidant feeds. It is further preferred to separate from the product, and recycle, a stream comprising unreacted gaseous hydrocarbon, oxidant, fuel and if present, gaseous diluent.

The process is carried out in the combustion chamber of a burner having a pulsating mode of operation. Suitably the burner comprises a combustion chamber housing (a) pulsed ignition means, and (b) a gaseous oxidant/fuel inlet system arranged to mix the fuel and oxidant in one zone of the combustion chamber, the chamber being further provided with a hydrocarbon reactant inlet port adjacent to the pulsed ignition means.

Cylindrical combustion chambers are particularly suitable, especially those having an axial length at least 20 times, preferably at least 40 times their average diameter. The combustion chamber is preferably adapted to promote gas mixing by, for example, roughening the internal walls or the provision of internal baffles or acoustic pulsators or a combination of any of these. The combustion chamber preferably incorporates a terminal quench or cooling zone which suitably communicates with means for separating and removing liquid products, such as a catch pot.

The combustion chamber may suitably be fabricated in a ceramic material, metal or glass.

The gaseous oxidant/fuel inlet system preferably has a low resistance to gaseous flow. The inlet system may be arranged to admitthe oxidant and fuel directly into the combustion chamber, thereby forming a mixing zone at one end of a cylindrical combustion chamber. Alternatively, the oxidant and fuel may first pass into one or more small antechambers where mixing occurs before the oxidant and fuel pass into the combustion chamber.

The pulsed ignition means may be, for example, a spark plug or plugs connecting with an electrical circuit adapted to feed an electrical pulse to the plug or plugs. The pulse rate may suitably be in the range from about 100 to 0.5 pulses per second, though higher and lower pulse rates may be employed if so desired. However, the pulse rate should not be so high as to produce a continuous flame.

The hydrocarbon inlet port may be constructed in any fashion and at any angle with reference to the direction of flow of the oxidant/fuel gases. Within the constraint that the inlet must be close to the source of ignition, its precise position is preferably variable, since this will depend for optinum conversions to oxygenated hydrocarbons on the reaction conditions (such as ignition rate,fuel/oxidantfeed rate, pressure etc) employed.

The burner may suitably be provided with means for heating or cooling specific zones thereof. An example of a suitable burner which may be modified by the provision of a hydrocarbon reactant inlet port and extension into a quench or cooling zone communicating with means for separating and removing liquid products is described and illustrated in the complete specification of our British Patent No.

1254453, which is incorporated herein by reference.

The invention will now be illustrated by reference to the following Examples. In all the Examples a pulsed flame activated burner was employed. The organisation and construction of such a burner is illustrated in the accompanying Figure 1. With reference to the Figure, 1 is a 154.5 cm long brass tube of 2.5 cm outer diameter and 1.1 cm internal diameter expanding at 2 into a 220 cm long tube 3 of 2.5 cm internal diameter, 4 is a liquid product catch pot of 15 cm internal diameter and 27.5 cm total height communicating with tube 3 and having a tap 5 for draining liquid product and an exit port 6 communicating with a silencer and vent (not shown), 7 is a spark plug connected to an electrical circuit (not shown) for generating pulsed sparks, 8 is a fuel and oxidant inlet port and 9 is a gaseous hydrocarbon reactant inlet port.A Gas Chromatography analysis port (not illustrated) is provided in the tube 3. 10 is a nitrogen purge gas inlet.

EXAMPLE 1 With reference to Figure 1, a stoichiometric mixture of hydrogen and air was fed through inlet port 8 of the burner 1 at a combined feed rate of 9.7 dm3 min-1 and was ignited at a rate of twice per second by a high tension spark generated by the spark plug 7. Methane of chemical purity was injected through the port 9, which was situated at a distance of 15 cm from the plug 7, at a rate such that the methane to oxygen ratio was 1:1. Liquid condensate was separated from the gaseous product stream in the catch pot 4 and removed through the tap 5. The liquid condensate was analysed by on-line gas chromatography.

The selectivity to methanol, the major component of oxygenated hydrocarbons, was 29% with a 6.1% methane conversion.

EXAMPLE2 The procedure of Example 1 was repeated except that the methane injection port 9 was located at a distance of 6 cm from the plug 7.

EXAMPLE3 The procedure of Example 1 was repeated except that the methane injection port 9 was located at a distance of 24 cm from the plug 7.

Comparison Test 1 The procedure of Example 1 was repeated except that the methane injection port 9 was located at a distance of 33 cm from the plug 7.

Comparison Test2 The procedure of Example 1 was repeated except that the methane injection port 9 was located art a distance of 67 cm from the plug 7.

Comparison Test3 The procedure of Example 1 was repeated except thatthe methane injection port9was located ata distance of 107.0 cm from the plug 7.

Comparison Test4 The procedure of Example 1 was repeated except that the methane injection port 9 was located at a distance of 142 cm from the plug 7.

Comparison Tests 1 to 4 are not examples according to the present invention and are included solely forthe purpose of comparison.

The results of Examples 2 and 3 and Comparison Tests 1 to 4 are illustrated graphically in Figure 2 as a plot of per cent selectivity to methanol versus methane injection distance from the ignition source.

Methane injection at a distance of 33 cm and greater from the ignition source results in zero selectivity to methanol.

EXAMPLE4 A stoichiometric mixture of hydrogen and air, with a combined feed rate of 9.7 dm3min-',was ignited twice per second by a high tension spark. Methane of chemical purity was injected adjacent to the spark (235 mm) in such a manner that the methane to oxygen ratio was 5:1. Liquid condensate was removed from the product stream, which was anaiy- sed by on-line gas chromatography.

The selectivity to methanol, the major component of oxygenated hydrocarbons, was 22% with 2.5% to methane conversion.

Claims (13)

1. A processforthe production of oxygenated hydrocarbons from a gaseous hydrocarbon mixture comprising methane and/or ethane which process comprises feeding the hydrocarbon to the combustion chamber of a burner having a pulsating mode of operatin wherein a series of explosion or pressure waves is produced by repeated ignition of a gaseous oxidant/fuel mixture, the hydrocarbon being fed to the combustion chamber at a point close to the source of ignition.

2. A process according to claim 1 wherein the gaseous hydrocarbon mixture is one obtained from a natural hydrocarbon gas reservoir.

3. A process according to either claim 1 or claim 2 wherein the oxidant is air.

4. A process according to any one of claims 1 to 3 wherein the fuel is either hydrogen, methane, ethane, propane or butane, or a mixture thereof.

5. A process according to any one of the preceding claims wherein there is fed a gaseous diluent.

6. A process according to any one of the preceding claims wherein the burner comprises a combustion chamber housing (a) pulsed ignition means, and (b) a gaseous oxidant/fuel inlet system arranged to mix the fuel and oxidant in one zone of the combustion chamber, the chamber being further provided with a hydrocarbon reactant inlet port adjacent to the pulsed ignition means.

7. A process according to claim 6 wherein the combustion chamber is cylindrical and has an axial length at least 40 times its average diameter.

8. A process according to either claim 6 or 7 wherein the combustion chamber is adapted to promote gas mixing by either roughening the internal walls or the provision of internal baffles or the provision of acoustic pulsators or a combination of any of these.

9. A process according to any one of claims 6 to 8 wherein the combustion chamber incorporates a terminal quench or cooling zone which communicates with means for separating and removing liquid products

10. A process according to any one of claims 6 to 9 wherein the pulsed ignition means is a spark plug or plugs connecting with an electrical circuit adapted to feed an electrical pulse thereto.

11. A process according to any one of the preceding claims wherein the gaseous hydrocarbon mixture comprises methane and the oxygenated hydrocarbon product comprises methanol,

12. A process according to claim 1 substantially as hereinbefore described with reference to Examples 1 to 4.

13. Oxygenated hydrocarbons whenever produced by a process as claimed in any one of claims 1 to 12.