EP0017891B1 - Process for producing combustion gases free of or containing little soot - Google Patents

Description

The invention relates to a process for the production of soot-free or low-soot combustion gases by mixing oxygen of a certain flow rate with liquid methanol in a certain weight ratio, igniting the mixture thus produced and incomplete combustion of the methanol in the flame formed in the presence of certain amounts of water vapor.

It is known from Ullmann's Encyclopedia of Industrial Chemistry, Volume 4, pages 649 ff. That fuel gases in the form of mixtures of air and gaseous or liquid hydrocarbons, e.g. B. burns methane, benzene, gasoline and the combustion gases thus obtained used as heating gases. These combustion gases practically always contain certain amounts of soot; in particular, soot forms when the hydrocarbons are incompletely burned (Ullmann, loc. cit., volume 14, pages 793, 794; Hoffmann, Inorganic Chemistry (Vieweg, Braunschweig, 16th edition), page 330). Soot is finely divided carbon that can still contain oxygen, hydrogen and to a lesser extent sulfur, nitrogen and by-products. The deposition of soot interferes with the heating effect, and an excess of air is therefore used so that the carbon in the starting hydrocarbons burns as simultaneously as possible with the hydrogen (Ullmann, loc. Cit., Volume 4, page 709). Depending on the completeness of the combustion, soot-containing combustion gases can contain up to 10 percent by weight of soot, based on the amount by weight of the hydrocarbon used. In addition to the reduced heating effect, the soot separation also brings with it other operational difficulties, e.g. B. increased cleaning costs of systems and pipe connections, blockage of valves or pipe constrictions and thus malfunctions, impairment of chemical reactions due to the presence of soot or reduction in activity or poisoning of catalysts.

DE-B-2224223 describes a process for the production of soot-free or low-soot combustion gases at 400 to 1300 ° C., wherein a starting mixture of methanol and oxygen in a ratio of 1 to 10 percent by weight oxygen, based on methanol, and a flow rate of 50 to 500 meters per second mixed with oxygen at a flow rate of 1 to 20 meters per second in a ratio of 10 to 80 percent by weight of additional oxygen, based on the starting mixture, the final mixture thus ignited and 85 to 99 percent by weight of the total amount in the flame formed Methanol burns. The combustion is carried out in a kiln which contains the feed lines of the starting mixture and the additional oxygen as well as corresponding ignition devices and the discharge of the combustion gases.

It has now been found that at 200 to 1600 ° C., soot-free or low-soot combustion gases can be obtained advantageously by igniting a mixture of oxygen and methanol and burning 85 to 99 percent by weight of the total amount of methanol in the flame formed, if methanol in liquid form with oxygen from a flow rate of 1 to 100 meters per second in a ratio of 50 to 150 weight percent oxygen, based on methanoi, mixes, the mixture ignites in a perforated combustion chamber, burns and with through the perforations of the combustion chamber in a ratio of 10 to 3000 weight percent water , based on methanol entering water vapor mixes.

In view of the prior art, the method according to the invention provides soot-free or low-soot combustion gases in a simple and economical way, generally without any detectable soot content. The combustion gases can also be used advantageously as heating gases in all cases in which soot deposition would interfere, e.g. B. for heating oxidic catalysts such as oxides of iron, molybdenum, cobalt, nickel, tungsten, aluminum, titanium, phosphorus and chromium. They can advantageously be used to heat catalysts in catalytic synthesis processes, for example silver catalysts, which catalyze the conversion of methanol with oxygen to formaldehyde. Such heating gases are also of particular interest in industries or areas where other starting materials for the production of combustion gases, such as benzene or gasoline, or nitrogen as heating gas are not available in sufficient quantities or are uneconomical. Nitrogen as heating gas must also be used for some syntheses, e.g. B. the above-mentioned formaldehyde production can be specially cleaned. Air as a heating gas is not suitable for syntheses in which air components could form explosive mixtures during the subsequent implementation, e.g. B. in reactions with alkanols. Luminous gas and town gas result in heating gases which, in addition to soot, also contain sulfur in particular and so, for. B. impair or poison numerous catalysts or promote corrosion of the systems. Indirect heating, on the other hand, is uneconomical, especially on an industrial scale, and it is not possible to achieve uniform heating to higher catalyst temperatures in this way. In the case of electrical heating, local heating in the middle of the catalyst bed and soot formation are frequently observed. In all these cases, the method according to the invention provides an economically advantageous and reliable heating gas in a simple manner. In the case of formaldehyde production, the catalyst is brought quickly and uniformly to the desired temperature and the usual heating-up time is reduced.

In comparison to the method described in DE-B-2224223, the method according to the invention is simpler and more economical; Mixing, regulating and control devices are saved since, according to the known method, a starting mixture of methanol and oxygen must first be prepared, which is then only mixed with additional oxygen. The method according to the invention comparatively uses only one burner, while the known method requires two burners, namely a main burner and a secondary burner for the ignition. For this reason, a simpler and more reliable ignition and combustion of the starting mixture is achieved comparatively in the method according to the invention. Despite the simpler ignition and combustion, smaller amounts of air are used to burn the methanol; surprisingly, a practically oxygen-free, soot-free combustion gas without by-products is obtained. The combustion chamber according to the invention is cooled with the aid of the steam flowing in for mixing with the combustion gases; local overheating, which can destroy the burner, is avoided. Since the temperature of the combustion chamber wall is therefore comparatively lower, the design of the combustion chamber jacket is cheaper and simpler. All these advantageous results of the method according to the invention are surprising with regard to the prior art. The incomplete combustion of methanol with air would have been expected to cool the combustion chamber walls and scour the combustion mixture and cause soot to form. The temperature of the heating gas can be adjusted within a wide range by adding water vapor. When generating the same hourly amounts of heating gas, the combustion chamber according to the invention is smaller and lighter than the known combustion chambers. For this reason, in many syntheses, in particular in formaldehyde synthesis, it can be arranged directly on or above the reactor, if appropriate using gas distributors. A much smaller volume has to be filled with heating gas in front of the catalyst layer. The heat losses are smaller and the heating of the catalyst is more energy efficient than in the known processes. On the other hand, the rooms that are in the event of a malfunction, e.g. B. the sudden failure of the methanol supply, can fill with explosive mixture, only very small. The procedure according to the invention is more reliable, especially since the steam supplied also reduces the explosion range of methanol with air.

Instead of oxygen, oxygen-containing mixtures, advantageously air, are also advantageously used. Mixers and advantageously nozzles come into consideration as mixing devices, e.g. B. injector mixers, mixing chambers or mixing sections with injectors, jet mixers, swirl chamber nozzles, eccentric nozzles, bundle nozzles, centrifugal pressure nozzles, slot nozzles, flat jet nozzles, hollow nozzles, spiral nozzles. The components methanol and oxygen or air, which form the starting mixture, are expediently introduced into the combustion chamber via corresponding nozzles. It is advantageous to add the methanol into the supplied air shortly before or at the entrance to the combustion chamber via a nozzle (input mixing nozzle). Methanol is added in liquid form; the inlet mixing nozzle advantageously forms a fine, droplet-shaped distribution of the liquid methanol in the oxygen or in the supplied air.

The starting mixture has a ratio of 50 to 150, in particular 100 to 140 percent by weight of oxygen, based on methanol, and generally a flow rate of 1 to 100, in particular 8 to 45 meters per second. For the finished mixture, pressures from 1 to 6, in particular from 1.1 to 2.5, bar are expediently considered. The flow rate of oxygen, advantageously air, is 1 to 100, in particular 8 to 45 meters per second. With separate supply, the components are supplied under such a pressure, e.g. B. in the oxygen or air expediently from 1 to 6, in particular from 1.01 to 2.5 bar, that the aforementioned weight ratios and flow rates of the starting mixture at the burner inlet. In general, the components are added at room temperature. Starting materials suitable for the process are pure methanol and technical grade methanol. Crude methanol, which can optionally be purified by treatment with oxidizing agents and / or alkalis by the processes described in DE-B-1 277 834 and DE-C-1 235 881, can also be used.

The inlet mixing nozzle or the feed of the methanol into the oxygen (air) stream is advantageously within a range of 2 centimeters from the entrance of the burner chamber.

After mixing methanol with oxygen, the mixture thus formed is known in a known manner, for. B. ignited by electrical sparks of 5000 to 20 000 volts. The ignition temperature of the final mixture is between 400 and 500 ° C. After ignition, a soot-free or soot-free flame is formed and generally a combustion temperature of 600 to 1600 ° C, preferably of 650 to 1500 ° C (measured at the end of the flame cone). The heat of combustion is expediently 1400 to 3200 kJ / N _M ³ . It is advantageous to adjust the flame cone by regulating the oxygen so that the distance (Cone height) from the cone tip at the entrance to the burner chamber to the end of the cone is 10 to 230 centimeters. The diameter of the cone base or the maximum diameter of the flame cone is advantageously 10 to 60 centimeters. The more additional oxygen is added, the smaller this cone height and the hotter the flame; at a flow rate of 15 m / sec and 140 weight percent oxygen, based on the starting mixture, the temperature at the end of the flame cone is 1600 ° C., at a flow rate of 5 m / sec and 80 weight percent oxygen 600 ° C. The length of the combustion chamber is advantageously 1.2 to 2 times the flame cone height and the diameter or, in the case of a rectangular combustion chamber, a broad side of the combustion chamber is 1.5 to 2 times the flame cone diameter. The combustion is usually carried out continuously. A total pressure of 1 to 6 bar, preferably of 1.01 to 2.5 bar, is expediently set in the combustion chamber. In a preferred embodiment of the method, 1 to 30 grams of methanol are burned per second under the aforementioned preferred conditions of combustion and flame cone design.

The gas mixture obtained by the ignition and combustion generally contains 0.01 to 0.3 percent by weight of formaldehyde, 0.01 to 1 percent by weight of methanol, 10 to 16 percent by weight of water (without water optionally contained in the starting mixture), 8 to 19 percent by weight of carbon dioxide, 0 to 5 weight percent carbon monoxide and 58.7 to 66 weight percent nitrogen. If air is used as the oxidizing agent or crude methanol as the starting material, as is of particular interest on a large industrial scale, the corresponding proportions of noble gases, such as argon, or impurities, such as e.g. B. Dimethyl ether, ammonia, monomethylamine.

The combustion chamber contains the ignition device at the head and is expediently or advantageously tubular. The walls (tube wall) are perforated along the entire wall length below the ignition device or, advantageously, only from 6 centimeters below the ignition device. Preferred are from 2 to 10,000, in particular from 5 to 1000 perforations, diameter of the perforations from 2 to 20 millimeters, wall length of the combustion chamber from 0.2 to 4.6, in particular 0.6 to 1.5 meters, combustion chamber diameter or wall sides the rectangular, equilateral combustion chamber 0.1 to 1.5, in particular 0.2 to 0.8 meters. Immediately after leaving the flame, the gas mixture is mixed with water vapor, which enters through the perforations at a flow rate of 5 to 50, in particular 10 to 25, meters per second. A mixing ratio of 100 to 3000, in particular 1000 to 2000 percent by weight of water, based on methanol, is preferred. The perforations can be distributed anywhere on the combustion chamber walls (pipe wall), expediently in a uniform distribution or in a ring. The final mixture of gas and water vapor are the combustion gases; They expediently come out at the end of the combustion chamber opposite the ignition without pressure or under pressure, expediently 1.1 to 2.5 bar, expediently at a flow rate of 3 to 20, in particular 5 to 10 meters per second, and can be carried out without further process steps can be used as heating gas.

In a preferred embodiment, the combustion chamber is installed in front of a reactor or in particular placed on a vertical reactor. The combustion gases according to the invention are advantageously used for heating silver catalysts which catalyze the conversion of methanol with oxygen to formaldehyde. The silver catalyst does not become poisoned. You can with the combustion gases, the catalyst to the reaction temperature, for. B. 500 to 800 ° C, or to a lower temperature, for. B. from 300 to 500 ° C, heat and then connect the oxidation of methanol to formaldehyde. You can also the catalyst with the combustion gases on z. B. heat the aforementioned lower catalyst temperature and then carry out the oxidation of the methanol, the temperature, which is optionally controlled by suitable composition and / or flow rate of the starting mixture of the reaction, then to the z. B. increases above reaction temperature. You can also simultaneously heat the catalyst and carry out the reaction by a mixture of the combustion gases together with the starting mixture of formaldehyde synthesis, for. B. methanol, oxygen or air and appropriately water vapor, passes over or through the catalyst; in this way, the combustion gases and the heat of reaction of the methanol oxidation heat the silver catalyst. This procedure can also be carried out in one step up to the reaction temperature or in two steps, at a lower starting temperature and then at the reaction temperature. When the catalyst is heated, the flow rate is advantageously 50 to 1000 kg of combustion gases per square meter of catalyst bed cross section and hour; if they are mixed with the starting mixture of the reaction, a ratio of 0.1 to 0.5 parts by weight of oxygen in the starting mixture of the methanol oxidation to one part by weight of combustion gas is expediently chosen. It is advantageous to use a gas distributor in front of the catalyst layer, expediently in the reactor hood of the vertical reactor carrying the catalyst layer, which distributes the combustion gases and / or the starting mixture of formaldehyde synthesis evenly over the surface distributed the catalyst layer.

Thanks to the gas distributor in the upper part of the reactor hood, the silver catalyst is heated evenly across the catalyst cross-section. The known damage to the contact, especially when heating, e.g. B. formation of inactive areas, soot formation, sintering due to overheating and impairment of the activity of the catalyst are avoided. By combining the combustion chamber according to the invention with a gas distributor, the yield of formaldehyde is higher than in the known methods of catalyst heating. It is surprising that the combustion gases do not have any negative effects on the catalyst activity due to condensation of the steam, even at the beginning of the heating period. As a gas distributor it is advisable to use pipe sections that contain a narrower pipe section (inner material) on the inside; In this way, the interior of the pipe section (interior of the outer jacket) consists of two parts, the interior of the inner jacket (tube core) and the interior (ring rim), which is enclosed by the outer wall of the inner jacket and the inner wall of the outer jacket. A hollow tube core and a ring rim, which is provided with baffle walls, are expedient. The lines of contact of the baffle walls with the outer wall of the inner shell and the inner wall of the outer shell expediently form parallel curves or mostly straight lines with angles between the line of contact and the direction of flow of the incoming combustion gases. Advantageously, the wall angle is 10 to 60 °, the number of baffle walls 4 to 100 in the ring rim, the diameter (clear width) of the tube core 0.5 to 1000 centimeters, the clear width of the ring rim (= distance between outer jacket and inner jacket) 1 to 200 Centimeter. In this way, the outer regions of the gas stream contain a swirl, the turbulence of which causes the combustion gases to be distributed uniformly over the entire surface of the catalyst layer. The baffles are suitably arranged similar to the blades of axial turbines.

The oxidation can be carried out in the preferred combination of the process according to the invention with the methanol oxidation on the silver catalyst by the known processes, e.g. B. perform the process described in Ullmanns Encyklopadie der Technische Chemie, Volume 7, pages 659 ff. For the production of formaldehyde. Starting materials suitable for the oxidation are pure methanol, industrial methanol or advantageously their mixtures with water; the concentration of the aqueous mixtures can suitably vary between 60 and 95 percent by weight, preferably between 70 and 90 percent by weight, of methanol. Crude methanol, which can optionally be purified by treatment with oxidizing agents and / or alkalis by the processes described in DE-B-1 277 834 and DE-C-1 235 881, can also be used. As a rule, the methanol is fed to the reaction space in vapor form and, if appropriate, in a mixture with inert gas. Nitrogen, for example, can be used as the inert gas for the oxidation. Both pure oxygen and free oxygen-containing gases, in particular air, can be used as the oxidizing agent. Oxygen and methanol are expediently in a molar ratio of 0.15 to 0.6, in particular 0.15 to 0.5 mol, of oxygen per mol of methanol or methanol and air are expediently in a molar ratio of 1 mol of methanol to 1.4 to 2.9 Mole of air applied. The total volume of water vapor is preferably 0.8 to 1.9 parts by volume per volume of methanol vapor. Any silver catalysts are suitable as catalysts, e.g. B. the described in German Ausleschrift 1 231 229 and Ullmanns Encylklopadie der Technische Chemie, Volume 7, pages 659 ff. Preferably two-layer or multilayer silver catalysts, e.g. B. the catalysts listed in DE-B-1 294 360, in DE-A-1 903 197 or in DE-B-2 322 757. With regard to the preparation of the catalyst and implementation of the corresponding reaction with these catalysts, reference is made to the publications mentioned. The oxidation is otherwise carried out in a known manner by z. B. a gas mixture of methanol vapor, water vapor, air, optionally inert gas and exhaust gas; in the aforementioned quantities at temperatures of about 550 to 750 ° C, in particular 600 to 700 ° C, passes through the silver catalyst. The process is generally carried out continuously at pressures between 0.5 and 2 bar, preferably between 0.8 and 1.8 bar. It is expedient that, for the catalyst zone reaction gases leaving within a short time, for example in less than _^1/10 second cool down. B. to temperatures of 350 ° C. The cooled gas mixture is then expediently fed to an absorption unit in which the formaldehyde is washed out of the gas mixture with water, advantageously in countercurrent.

The parts listed in the following examples are parts by weight.

example 1

The ignition and combustion is carried out in the following combustion chamber (see drawing): The combustion chamber (1) contains an outer jacket (2), an air inlet (3), a steam inlet (4), an ignition device (5) and a partially perforated inner jacket ( 6). The methanol supply is in the air supply via a pressure atomizing nozzle (7). The combustion gases emerge from the outlet (8) of the combustion chamber. The height between chamber entrance (9) and chamber exit (8) is 70 centimeters, the diameter of the tubular chamber 50 centimeters, the inner jacket (6) is perforated from 40 centimeters below the ignition to 10 centimeters above the chamber exit (8) with 24 perforations with a perforation diameter of 17 millimeters. The ignition (5) is 0.5 centimeters above the chamber entrance (9), the nozzle outlet of the methanol supply (7) is 0.7 centimeters above the chamber entrance (9).

Hourly, 30 parts of liquid methanol (flow rate 40 meters per second; temperature 20 ° C) through the nozzle (7) into the air flow (flow rate 15 meters per second; temperature 20 ° C; pressure 1.1 bar) of the air supply (3) dusted and mixed with 200 parts of air per hour. The starting mixture thus formed enters the combustion chamber at the combustion chamber entrance (9) (flow rate 15 meters per second; pressure 1.1 bar) and is ignited at the ignition (5) by means of an electrical spark (10,000 volts). After ignition of the final mixture, a flame of 50 cm cone height (measured from the entrance to the combustion chamber) and a cone diameter of 40 cm and a combustion temperature of 1500 ° C (measured at the end of the flame cone) is formed. The heat of combustion is 3000 kJ / N _M ^3. After the combustion, the gas mixture mixes with 300 parts of water vapor per hour (flow rate 20 meters per second; temperature 120 ° C), which occurs through the perforations. The combustion gases in the chamber (temperature 700 ° C; pressure 1.1 bar) emerge from the chamber at the chamber outlet (8) and then have a flow rate of 6 meters per second, a temperature of 650 ° C and a pressure of 1, 1 bar. 530 parts of combustion gases at 650 ° C. are formed per hour, which contain 0.02 percent by weight formaldehyde, 62.6 percent by weight water, 7.8 percent by weight carbon dioxide, 0.5 percent by weight carbon monoxide, 28.67 percent by weight nitrogen, 0.4 percent by weight argon and 0 , 01 percent by weight of impurities, discharged at the combustion chamber outlet (8) and immediately used as heating gas. No soot can be detected in the combustion gases even after 1000 hours of operation.

Example 2 (use)

The combustion chamber is connected via a pipe (10) to the feed (11) of a methanol / air / water vapor mixture of the formaldehyde synthesis.

The combustion is carried out analogously to Example 1 and the combustion gases in a composition analogous to Example 1 via feeds (10) and (11) and a gas distributor (12), which is attached to the head of the reactor (13) containing the silver catalyst, via a two-layer catalyst in Reactor (13) passed, wherein the lower layer of the catalyst is 25 millimeters thick and 90 percent by weight of crystals with a grain size of 1 to 2.5 millimeters and the upper layer has a thickness of 1.5 millimeters and crystals with grain sizes of 0 , 2 to 0.75 millimeters. The throughput is 150 kilograms of combustion gases per square meter of catalyst bed cross section and hour. The catalyst is heated to 330 ° C. within 15 minutes. An hourly mixture of 300 parts of methanol and 200 parts of water in a mixture with 402 parts by volume of air is evaporated in the reactor and passed over the catalyst at 1.3 bar, the temperature rising to 650 ° C. within 10 minutes. Now the supply of the combustion gases is stopped. The reaction mixture emerges at the outlet (14) of the reactor and is then rapidly cooled to 350 ° C. and then, after further cooling, absorbed in water in an absorption tower. 251 parts of formaldehyde (calculated as 100%) per hour are obtained in the form of a 40% by weight aqueous solution with a methanol content of 0.7% by weight. This corresponds to a yield of 89.3 percent of theory. The yield of end product and the methanol content of the formaldehyde solution obtained remain constant for 17 weeks.

If the heating is carried out in a known manner using electrical heating instead of the combustion gases and then the reaction is carried out in the aforementioned manner, the same yield of formaldehyde is obtained which drops to 86 percent of theory within 12 weeks.

Claims (1)

1. A process for the preparation of combustion gases which are completely or substantially free from soot and are at from 200° to 1600°C, by igniting a mixture of oxygen and methanol and combusting from 85 to 99 per cent by weight of the total amount of methanol in the resulting flame, wherein methanol, in the liquid form, is mixed with oxygen at a flow velocity of from I to 100 meters per second in a proportion of from 50 to 150 per cent by weight of oxygen, based on methanol, the mixture is ignited, and combusted, in a perforated combustion chamber and steam entering, through the perforations of the combustion chamber, in a proportion of from 10 to 3000 per cent by weight of water, based on methanol, is admixed.

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