PROCESS FOR THE PREPARATION OF METHANOL

Abstract: The invention relates to a process for the preparation of methanol by converting a lower hydrocarbon or mixture of lower hydrocarbons to a first gas mixture containing CO and H₂ and converting this first gas mixture to methanol at elevated temperature and increased pressure in the presence of a suitable catalyst. In order to obtain low energy requirements this process is characterized in that said lower hydrocarbon or mixture of lower hydrocarbons is converted in a first reactor to said first gas mixture consisitng substantially of CO, CO₂ and H₂, by partial oxidation thereof with oxygen and optionally water, at a reactor outlet temperature of between 800 and 1500°C and a pressure of between 40 and 150 bar, that said first gas mixture is supplied to a second reactor where it is converted to a second gas mixture containing methanol at a tempera­ ture of between 240 and 320°C and a pressure of between 40 and 150 bar in the presence of a suitable catalyst, that a third gas mixture consisting of the non converted gaseous components is separated from said second gas mixture by cooling, that from this third gas mixture prior to recirculation to the reactor carbon dioxide is removed in an amount that the molal ratio H₂(2CO + 3CO₂) in the combined said first gas mixture and the depleted third gas mixture is at least 0.90, which combined mixture is to be supplied to said second reactor, that the crude methanol, obtained by said cooling of said second gas mixture is expanded to a lower pressure, and purified and that the energy released during said partial oxidation and the conversion of said combined gas mixture to said methanol containing third gas mixture is at least in part used for the generation of steam.

Description: The invention relates to a process for the preparation of methanol by converting a lower hydrocarbon or mixture of lower hydrocarbons to a first gas mixture consisting substantially of CO, CO₂ and H₂, by partial oxidation thereof with oxygen and optionally water, at a reactor outlet temperature of between 800 and 1500 °C and a pressure of between 40 and 150 bar, supplying said first gas mixture to a second reactor where it is converted to a second gas mixture containing methanol at a temperature of between 240 and 320 °C and a pressure of between 40 and 150 bar in the presence of a suitable catalyst, separating a third gas mixture consisting of the non converted gaseous components from said second gas mixture by cooling, controlling the molar ratio H₂/(2CO + 3CO₂) in the combined said first gas mixture and the depleted third gas mixture, which combined mixture is to be supplied to said second reactor, expanding the crude methanol, obtained by said cooling of said second gas mixture to a lower pressure, and purifying said methanol, in which process the energy released during said partial oxidation and the conversion of said combined gas mixture to said methanol containing second gas mixture is at least in part used for the generation of steam. Such a process is described in US-A-3920717. In this process synthesis gas is produced by partial oxidation of a hydrocarbonaceous feed in a non catalytic synthesis gas generator. The molar ratio H₂/CO is subsequently adjusted by non catalytic thermal direct water-gas shift, whereafter the gas stream is cooled and purified preferably with a portion of the methanol product. It is further known to convert lower hydrocarbons such as methane ethane and propane, for example natural gas, to a synthesis gas by passing the hydrocarbons with steam over a heated catalyst (for example based on nickel), at elevated temperature and increased pressure. In this process a gas mixture is formed which contains more hydrogen than the theoretical stoichiometric requirement for the preparation of methanol. A disadvantage of this process is the relatively high energy consumption. Moreover, after the preparation of methanol, unused hydrogen remains, which can in most cases be used only as a fuel gas for heating purposes. The object of the invention is to provide a process for the preparation of methanol from lower hydrocarbons which has a low energy consumption and which does not require a higher investment than in the known processes. The invention is characterized in that CO₂ is removed from the said third gasmixture in an amount that the molar ratio H₂/(2CO + 3CO₂) in the said combined gas mixture is at least 0.90. Preferably the amount of said steam being generated is substantially equivalent to the energy demand of an oxygen plant which supplies the oxygen needed for the said partial oxidation. In summary, essential steps in the process are the partial oxidation of the hydrocarbon to a gas mixture containing CO, CO₂, H₂ and H₂O, the depletion of CO₂ from the gas mixture from the synthesis reactor, which is to be recycled, in an amount that the desired ratio of the components of the reactor feed is established, and the preparation of methanol therefrom in a reactor, whereby the heat of reaction, therfrom as well as the heat of reaction generated during the partial oxidation is converted to medium pressure or high-pressure steam. Advantages of the process according to the invention are that the energy consumption per tonne of methanol produced is low, that the only by-product is carbon dioxide, that no steam reformer is required, that a separate water gas shift reactor is dispensed with, and that it is possible to prepare a mixture of methanol and other alcohols which is pre-eminently suitable for application as engine fuel, mixed with for example petrol. As process feedstock, methane, ethane or propane or a mixture thereof can be used. If required, the hydrocarbons can be desulphurised before being introduced into the process or at any stage in the process prior to the methanol synthesis. Advantageously the gas mixture has a pressure of between 40 and 150 bar, more in particular between 70 and 90 bar. The partial oxidation can be effected by feeding the hydrocarbon with oxygen and water vapour to a reactor containing a suitable catalyst. This catalytic partial oxidation can be carried out at a reactor outlet temperature of between 800 and 1200 °C and a pressure of between 20 and 100 bar, usually 40-65 bar. The ratio of water molecules to carbon atoms can be between 1.0 and 2.5. In this case, the gas stream obtained has to be cooled in order to substantially remove the water present in it. More preferably, an uncatalysed partial oxidation is used. This can be carried out at a pressure of between 40 and 150 bar and, a reactor outlet temperature of between 1100 and 1500 °C. In such case the amount of water in the feed can be low. The gas mixture formed will then contain only a small amount of water, i.e. water supplied to the process and water formed during the oxidation, which need not be removed. The fresh feed to the synthesis reactor or combination of reactors contains a relatively small amount of water. As a result of the shift reaction which occurs in the reactor besides the methanol synthesis, hydrogen and carbon dioxide are formed. The mixture of non-converted, gaseous components leaving the reactor is relatively rich in carbon dioxide. Thus it is easy to remove a portion of the carbon dioxide by washing. The gas mixture to be recycled might be recovered by cooling and expanding the reactor effluent, in which process crude methanol would be separated out as a liquid. Preferably, however, the effluent is cooled to, for example, between 10 and 40 °C and the gases are separated out at the pressure prevailing in the said second reactor. The gas mixture thus obtained can then be washed at roughly the synthesis-pressure. The washing step can be effected with any one of the known media for washing out carbon dioxide. Preferably, crude methanol is used as the washing medium, at a temperature of between -35 and +40 °C, preferably between 15 and 35 °C. The carbon-dioxide-containing methanol can be expanded to about atmospheric pressure, in which process carbon dioxide evolves. Of the separated-out gas from the synthesis reactor, a portion can be discharged as waste and a portion might be added to the hydrocarbon feed. Also, a portion of the gas might be recycled direct to the synthesis, without a washing step. By washing it is achieved that the molar ratio H₂/(2CO + 3 CO₂) in the reactor becomes at least 0.90 and preferably at least 0.95. When the ratio is lower, the synthesis becomes less efficient. For the preparation of pure methanol, a ratio of about 1, especially between 1.0 and 1.02, is desired. By choosing a slightly lower ratio, more higher alcohols are formed as by-products. A mixture of methanol with such higher alcohols is particularly suitable for application as engine fuel, mixed with petrol. The methanol synthesis takes place in a reactor which is operated at a temperature of between 240 and 320 °C and a pressure of between 40 and 100 bar, in the presence of a suitable catalyst. Preferably, the pressure is between 70 and 90 bar and the temperature between 250 and 270 °C. The reactor design should enable the removal of the heat of reaction with generation of steam. Suitable reactors and reaction conditions are known and have been described in the literature. The known catalysts, e.g. catalysts based on oxides of copper, zinc, chromium and/or aluminium can be used. The process according to the invention will be described with reference to the figure. The gaseous hydrocarbon, preferably natural gas, is through line 1 fed to the partial oxidation reactor 5, via compressor 2, preheater 3 and desulphurisation unit 4. From an oxygen plant 6, oxygen is sent to the reactor through line 7. Through line 39, finally, a small amount of steam is supplied, for example such an amount that the H₂O/C ratio is between 0.1 and 0.5. The gas which leaves the reactor through line 8 is cooled in the steam generation unit 9, in which process the water supplied, through line 10 is converted to steam which is carried off through line 11. Through line 12, incorporating the heat exchanger 13, the gas is led to the synthesis reactor 13. The heat released in the conversion to methanol is used for the generation of steam. To this end, through line 14 water is supplied which is carried off as steam through line 15. In heat exchanger 13, this steam is further heated and then discharged through line 16. The reaction mixture leaves the reactor through line 17 and is cooled to a temperature of about 30 °C in heat exchanger 18 and, possibly, in further heat exchangers omitted from the drawing. In gas/liquid separator 19, the reaction mixture is separated into a gas substan tially consisting of CO, H₂ and CO₂, with some methane and inert, which through line 20 is sent to the washer 22, and the crude methanol, which through line 24 is also sent to the washer. Through the branched line 21, a portion of the gas is recycled to the partial oxidation, and through line 23 a portion is discharged as waste, for example to a burner. The gas rid of a portion of the carbon dioxide is discharged from the washer through line 26, brought to synthesis pressure in compressor 27, if necessary, and recycled, through heat exchanger 18 and line 28. The crude methanol loaded with CO₂ is via turbine 30 expanded to about atmospheric pressure and is through line 31 led to the gas/liquid separator. The CO₂ is discharged from the separator through line 33. The crude methanol is discharged through line 34 and is in part supplied to the washer through pump 35 and line 25, another part being sent, through line 36, to the working-up section 37 from which the end product is discharged through line 38.

Claims: Process for the preparation of methanol by converting a lower hydrocarbon or mixture of lower hydrocarbons to a first gas mixture consisting substantially of CO, CO₂ and H₂, by partial oxidation thereof with oxygen and optionally water, at a reactor outlet temperature of between 800 and 1500 °C and a pressure of between 40 and 150 bar, supplying said first gas mixture to a second reactor where it is converted to a second gas mixture containing methanol, at a temperature of between 240 and 320 °C and a pressure of between 40 and 150 bar in the presence of a suitable catalyst, separating a third gas mixture consisting of the non converted gaseous components from said second gas mixture by cooling, controlling the molar ratio H₂/(2CO + 3CO₂) in the combined said first gas mixture and the depleted third gas mixture, which combined mixture is to be supplied to said second reactor, expanding the crude methanol, obtained by said cooling of said second gas mixture to a lower pressure, and purifing said methanol, in which process the energy released during said partial oxidation and the conversion of said combined gas mixture to said methanol containing second gas mixture is at least in part used for the generation of steam, characterized in that CO₂ is removed from the said third gasmixture in an amount that the molar ratio H₂/(2CO + 3CO₂) in the said combined gas mixture is at least 0.90. Process according to claim 1 wherein the amount of said steam being generated is substantially equivalent to the energy demand of an oxygen plant which supplies the oxygen needed for the said partial oxidation. Process according to claim 1, wherein the said partial oxidation is carried out as a non-catalytic partial oxidation at a pressure of between 25 and 150 bar and a reactor outlet temperature of between 1100 and 1500 °C. Process according to claim 1, characterized in that the said molar ratio H₂/(2CO + 3CO₂) in the feed to the said second reactor is between 1.0 and 1.02. Process according to claim 1, characterized in that the carbon dioxide is removed from said third gas mixture by washing with crude methanol.

Inventors: DE LATHOUDER, HANS CHRISTIAAN
Applicants: STAMICARBON B.V.
Application year: 1983
Main classification: C07C29
IPC codes: C07C29, C01B3, C07C27, C07C67, C07C31