TW200402408A - Preparation of unsaturated nitriles - Google Patents

Abstract

The invention relates to a process for preparing unsaturated nitriles from alkanes comprising the steps: (a) Feeding the alkane and oxygen-containing gas into a dehydrogenation zone and autothermally dehydrogenating the alkane to the corresponding alkene to obtain a product gas stream A which comprises the alkene, unconverted alkane, steam and possibly one or more further gas components selected from the group consisting of hydrogen, carbon oxides,' hydrocarbons having a lower boiling point than the alkane or the alkene (low-boilers), nitrogen and noble gases, (b) feeding the product gas stream A, ammonia and oxygen-containing gas into an oxidation zone and catalytically ammoxidizing the alkene to the corresponding unsaturated nitrile to obtain a product gas stream B which comprises the unsaturated nitrile, ammoxidation by-products, unconverted alkane and alkene, steam and possibly one or more further gas components selected from the group consisting of hydrogen, oxygen, carbon oxides, ammonia, low-boiling hydrocarbons, nitrogen and noble gases, (c) optionally removing ammonia from the product gas stream B to obtain an ammonia-depleted product gas stream C, (d) removing the unsaturated nitrile and ammoxidation by-products from the product gas stream B or C by absorption in an aqueous absorbent to obtain a gas stream D which comprises unconverted alkane and alkene and possibly one or more gas components such as from the group consisting of hydrogen, oxygen, carbon oxides, ammonia, low-boiling hydrocarbons, nitrogen and noble gases, and an aqueous stream which comprises the nitrile and the by-products, and recovery of the unsaturated nitrile from the aqueous stream, (e) separating the gas stream D into two substreams D' and D", optionally removing uncoverted alkane and alkene from the substream D' and recycling the substream D" and any unconverted alkane and alkene removed from the substream D' in the dehydrogenation zone.

Description

200402408 0) 玖. Description of the invention (The description of the invention should state: the technical field, prior art, content, embodiments, and drawings of the invention are briefly described) Technical Field The present invention relates to a method for preparing unsaturated nitriles from alkanes. It is known in the prior art that acrylonitrile and methacrylonitrile can be separately prepared from the corresponding alkane, propane and isobutylene in the presence of a suitable catalyst by using the known oxane ammoxidation reaction using an ammonia / oxygen mixture. The relevant alkane can be produced from the corresponding alkane in a previous dehydrogenation step. For example, the ammoxidation of acetonitrile produces propionitrile and the ammoxidation of isobutyridine produces methylpropionitrile. Generally, methyl-substituted olefins produce the corresponding α, yS-unsaturated nitrile when the methyl group is converted to a nitrile group. EP-A 0 193 3 10 describes a method for the preparation of propionitrile from propane, which comprises the catalytic dehydrogenation of propane to produce propylene, the ammoxidation of propylene to produce propionitrile, and the product gas stream from the ammoxidation is removed from the propionate The nitrile and the unconverted propane and propylene are recycled to the catalytic dehydrogenation zone. After removing the propionitrile from the product gas stream of ammonia oxidation, oxygen is used to selectively combust the hydrogen formed in the dehydrogenation process through an oxidation catalyst to produce water. The rest of the hydrogen-depleted gas stream contains unconverted Propane, Propane, Carbon Dioxide and Low Stagnation Smoke. After removing a secondary stream therefrom and recovering unconverted propane and propane, the gas stream is recycled to the dehydrogenation step. One disadvantage of this method is the heat transfer limitation associated with heating the reaction gas by external combustion. The object of the present invention is to provide an improved method for producing acrylonitrile from propane. 200402408

(2) Summary of the Invention We have discovered that a method for preparing unsaturated nitriles from a corresponding alkane includes the following steps: (a) feeding the hydrocarbon and oxygen-containing gas into a dehydrogenation zone and autothermally removing Hydrogen This hydrocarbon becomes the corresponding olefin to obtain product gas stream A, which contains the olefin, unconverted alkane, steam, and possibly one or more other gas components selected from hydrogen, carbon oxides, having a boiling point lower than that of the alkane or the olefin. Hydrocarbons (low boilers)

A group consisting of 5 nitrogen and rare gases' (b) Feed the product gas stream A, ammonia and oxygen-containing gas into an oxidation zone and catalytically ammoxidize the feminine hydrocarbon to the corresponding unsaturated nitrile to obtain the product gas stream B, which Contains the unsaturated nitrile, by-products of ammoxidation, unconverted alkanes and olefins, steam and possibly one or more other gas components selected from the group consisting of hydrogen, oxygen, end oxides, ammonia, low-carbon hydrocarbons, nitrogen and rare gases Group, (c) selectively removing ammonia from the product gas stream B to obtain an ammonia-depleted product gas stream C,

(d) removing the unsaturated nitrile and ammoxidation byproducts from the product gas stream B or C in an aqueous absorbent by an absorption method to obtain a gas stream D, which contains unconverted alkanes and alkenes and possibly one or more gas components selected from A group consisting of hydrogen, oxygen, carbon oxides, ammonia, low boiling hydrocarbons, nitrogen and rare gases, and an aqueous stream containing the nitrile and the by-products, and recovering the unsaturated moonlight from the aqueous stream, (e) Separate the gas stream D into two secondary streams Df and Dff, selectively remove unconverted alkanes and olefins from the secondary stream D 'and recycle the secondary stream Dn and any unconverted alkanes removed from the secondary stream IT and Tritene hydrocarbons reached the dehydrogenation zone and reached 200,402,408. (3) Description of the invention Achievement: Lian_mine full ore lion mixed for this purpose. Embodiment In step (a) of the first method, the alkane and oxygen-containing gas are fed into the dehydrogenation zone and the system hydrocarbon is dehydrogenated by autothermal catalytic dehydrogenation to the corresponding feminine hydrocarbon. In the self-heating method, 'the hydrogen formed in the dehydrogenation by selective combustion of hydrocarbons in the reactor system by the presence of oxygen and the selective combustion of hydrocarbons directly generate the heat required for the reaction. Oxygen may be fed into the dehydrogenation zone as a co-feed and / or present in the recirculated gas stream D, for example, 'may be fed into the ammoxidation zone only with oxygen and present in the (recirculation) ) The (excess) oxygen in gas stream 0 functions as a difficult oxygen source for the autothermal catalytic dehydrogenation in such a way that the method according to the invention is carried out. If necessary ', an additional hydrogen-containing co-feed can be incorporated. The starting material for burning tobacco using the method according to the present invention is usually C3-C14-alkane, and preferably, propane and isobutane. The latter can be obtained from, for example, LPG (liquefied petroleum gas) or LNG (liquefied natural gas). In addition to dehydrogenation, it also produces cracked products of the alkanes. Depending on the dehydrogenation method, carbon oxides (c0'c02) 'water and nitrogen may also be present in the alkane dehydrogenation product gas mixture. In addition, unconverted alkane is present in the product gas mixture. In principle, all reactor types and methods known in the art can be used for the dehydrogenation of autothermally burned hydrocarbons. The " Catalytica® Studies Division, Oxidative Dehydrogenation and Alternative Dehydrogenation Processes 11 (Study Number 4192 OD, 1993, 430 Ferguson Drive, Mountain View, CA, 94043-5272 USA) are used for dehydrogenation according to the present invention 200402408 (4) The comparison of methods includes a sexual description.

Available reactor types are fixed-bed tube reactors or tube-bundle reactors. In these reactors, the dehydrogenation catalyst and optionally a special oxidation catalyst are arranged as a fixed bed in a reaction tube or a bundle of reaction tubes. The inside diameter of conventional reaction tubes is from about 10 to 15 cm. A typical dehydrogenated tube bundle reactor contains from about 300 to 1000 reaction tubes. The internal temperature in this reaction tube is customarily in the range from 300 to 1200 ° C, preferably in the range from 600 to 1000 ° C. Conventional operating pressures range from 0.5 to 8 bar, usually from 1 to 2 bar when using a small amount of steam dilution (similar to the Linde method for propane dehydrogenation), or from 3 to 8 bar when diluting a high amount of steam (similar Phillips Petroleum Co. for steam-activated recombination methods (STAR method) for propylene or sinter, see US 4,902,849, US 4,996,387 and US 5,389,342). Typical gas hourly space velocity (GHSV) is from 500 to 2000 per hour, based on the alkane to be dehydrogenated. The shape of the catalyst can be, for example, spherical or cylindrical (hollow or solid).

Catalytic alkane dehydrogenation can also be performed in a fluidized bed with heterogeneous catalysis as described in Chem. Eng. Sci. 1992 b, 47 (9-11) 2313. Two parallel fluidized bed operations are preferred, one of which is usually a regeneration process. The operating pressure is typically from 1 to 2 bar, and the dehydrogenation temperature is usually from 550 to 60 ° C. The heat required for dehydrogenation is introduced into the reaction system by preheating the dehydrogenation catalyst to the dehydrogenation temperature. When mixed with an oxygen-containing co-feed, the preheater may not be needed and by burning hydrogen and / or hydrocarbons in the presence of oxygen to directly generate the heat required in the reactor system. If required You can mix another hydrogen-containing co-feed. -10- 200402408

(5)

Self-heating alkane dehydrogenation can be performed in a disc reactor. This contains one or more consecutive catalyst beds. The number of catalyst beds may be from 1 to 20, preferably from 1 to 6, more preferably from 1 to 4 and even more preferably from 1 to 3. The reaction gas preferably flows radially or axially through the catalyst bed. Usually, such a disk reactor is operated using a fixed catalyst bed. In the simplest case, the fixed catalyst bed is arranged axially in a shaft furnace reactor or in the annular gap of the concentric cylindrical grid. One shaft furnace reactor corresponds to one tray. Performing this dehydrogenation in a single shaft furnace reactor corresponds to a desirable system. In another preferred system, the dehydrogenation is carried out in a disc reactor with three catalyst beds. This includes mixing an oxygen-containing gas with the alkane dehydrogenation reaction gas mixture in at least one reaction zone and burning hydrogen and / or hydrocarbons contained in the reaction gas mixture, which is directly in the reaction gas mixture Heat is generated for at least a portion of the dehydrogenation required in the at least one reaction zone. Optionally, the reaction gas mixture can be subjected to intermediate heating in the path from one catalyst bed to the next one catalyst bed, for example, by borrowing heat exchanger ribs heated by hot gas or by heating it by hot combustion gas. tube. Generally, the amount of oxygen-containing gas added to the reaction gas mixture is selected in such a way that the heat required for the dehydrogenation of the alkane is the hydrogen present in the reaction gas mixture and / or the Combustion of hydrocarbons in the reaction gas mixture and / or hydrocarbons in the form of coke. Generally, based on the total amount of the alkane, the total amount of oxygen added is from 0.001 to 0.5 mole / mole, preferably from 0.005 to 0.2 mole / mole, more preferably from 0.05 to 0.2 mole / mole. Oxygen can be added as pure oxygen or as oxygen-containing compound in the mixture with inert gas -11-200402408. It is desirable to use the recirculated oxygen-containing stream D "recirculated in step (e) as the oxygen-containing gas, which contains residual oxygen from the ammoxidation and can be optionally fed with pure oxygen. It is less suitable to feed air as a co- -Feeding. The inert gas and the gas produced by self-combustion generally provide additional dilution and therefore support the heterogeneous catalyzed dehydrogenation. The hydrogen produced by combustion to generate heat is the hydrogen formed by catalyzing the dehydrogenation of alkanes and and Is any additional hydrogen added to the reaction gas mixture. The amount of hydrogen should preferably be such that the H2 / 02 mole ratio φ in the reaction gas mixture immediately after the oxygen is fed is from 2 to 10 moles / mole In a multi-stage reactor, this applies to each intermediate oxygen feed point and any intermediate hydrogen feed point.-Catalytically burns the hydrogen. The commonly used dehydrogenation catalyst also catalyzes hydrocarbons and hydrogen and oxygen Combustion is based on the principle that no other special oxidation catalyst is required. In one system, one or more oxidation catalysts operate in the presence of hydrocarbons to selectively catalyze the combustion of hydrogen and oxygen. Therefore, the hydrocarbons and oxygen Burning In order to produce CO and CO 2 only to a minor extent, it has a significant positive impact on the selectivity for the formation of fluorene. The de-φ hydrogenation catalyst and the oxidation catalyst should preferably exist in different reaction zones. When is When the reaction is performed in more than one step, the oxidation catalyst may be present in only one, in more than one, or in all reaction zones. It is desirable to configure the catalyst to selectively catalyze the oxidation of hydrogen in the position where it is The reactor has a higher oxygen partial pressure than other spaces, especially near the feed point of the oxygen-containing gas. The oxygen-containing gas and / or hydrogen can be fed into one or more spaces of the reactor -12- ⑺ 1 ^^ 200402408 In a system of the method according to the invention, an intermediate feed of oxygen-containing gas and hydrogen takes place before each disk of a disk reactor. Before the method according to the invention In another system, oxygen-containing gas and hydrogen are fed before each disk except the first disk. In one system, a layer of a special oxidation catalyst exists downstream of each feed point, and continues One layer of this dehydrogenation catalyst. In another system, no special oxidation catalyst exists. The dehydrogenation temperature is usually from 400 to 1100 ° C. The pressure in the final catalyst bed of the disc reactor is usually from 0.2 Up to 5 bar, preferably from 1 to 3 · bar. GHSV, based on the alkane to be dehydrogenated, is usually from 500 to 2000 per hour, and at high-load to load operation even reaches 100,000 per hour, preferably From 4,000 to 16,000 per hour. A desirable catalyst that selectively catalyzes the combustion of hydrogen and contains oxides or phosphates selected from the group consisting of germanium, tin, tungsten, sulphur, antimony, indium, or oxides or phosphates Group. Another desirable catalyst for catalyzing the combustion of hydrogen comprises a precious metal of transition group VIII or I of the periodic table.

The dehydrogenation catalyst used usually has a support and an active ingredient. The support is composed of a refractory oxide or a mixed oxide. The dehydrogenation catalyst preferably contains as a carrier a metal oxide selected from the group consisting of zirconium dioxide, zinc oxide, aluminum oxide, dioxide dioxide, dioxin, magnesium oxide, lanthanum oxide, bromide oxide, and mixtures thereof. The mixture may be a physical mixture or a chemically mixed phase such as a magnesium alumina or a zinc alumina mixed structure. Desirable carriers include hammer dioxide and / or silica, and particularly preferred is a mixture of zirconium dioxide and crushed dioxide.

The active ingredient of the dehydrogenation catalyst usually contains transition group VIII of the periodic table -13-200402408

One or more elements of rhenium, preferably platinum and / or palladium, and more preferably platinum. The dehydrogenation catalyst may and may contain one or more elements of the main group I and / or II of the periodic table, preferably potassium and / or cesium. The dehydrogenation catalyst may include one or more elements of transition group III of the periodic table, including lanthanoids and actinides, preferably lanthanum and / or samarium. Finally, the dehydrogenation catalyst may have one or more elements of the main group III and / or IV of the periodic table, preferably one or more elements selected from the group consisting of boron, gallium, silicon, germanium, tin and lead, more preferably tin. In a preferred system, the dehydrogenation catalyst comprises at least one element of transition group VIII of the periodic table, at least one element of main group I and / or II, at least one element of main group III and / or IV, and transition group III includes at least one element of the lanthanide and the-element. For example, disclosed in WO 99/46039, US 4,788,37 and EP-A 705,136, WO 99/29420, US 5,220,091, US 5,430,220, US 5,877,369, EP 0 117 146, DE-A 199 37 106, DE-A All catalysts from 199 37 105 and DE-A 199 37 107 can be used in the process according to the invention. Particularly preferred catalysts for use in the above-described variants of autothermal alkane dehydrogenation include catalysts according to Examples 1, 2, 3 and 4 of DE-A 199 37 107. It is preferable to perform the dehydrogenation of the alkane in the presence of steam. The added steamer serves as the heat medium and the chlorination of organic deposition on the catalyst. It suppresses the carbonization of the catalyst and increases the operating period of the catalyst. The organic deposits are converted to complete monoxide and complete dioxide. The dehydrogenation of alkanes can also be performed by the pipeline gas method described in the pending German patent application P 102 1 1 275.4. The dehydrogenation catalyst can be regenerated by a known method. For example, you can add > " -14- 200402408 (9) steam to the reaction gas mixture or, from time to time, send an oxygen-containing gas through the catalyst bed and burn off the deposited char at an elevated temperature. The dehydrogenation catalyst can then be reduced in a hydrogen-containing atmosphere. The alkane dehydrogenation produces a gaseous mixture that contains a second component in addition to the alkane and unconverted alkane. Common second components include hydrogen, water, nitrogen, carbon oxides (CO and co2) and low boiling hydrocarbons such as methane, ethane and acetamidine. For example, when autothermal dehydrogenation is performed while oxygen and additional hydrogen are fed in, the product gas mixture has a higher content of water and interfering oxides. For example, in the case of propane dehydrogenation, the product gas mixture leaving the dehydrogenation reactor contains at least the components propane, propane, and steam and, in general, additional carbon oxides and molecular hydrogen. Generally, it will contain, in addition, nitrogen and rare gases, and low boiling hydrocarbons methane, ethane and acetamidine. Dehydrogenation of isobutane can also obtain propane, propylene, propyne, and propane, as cracking products. Usually, the dehydrogenation product gas mixture will be at a pressure of 0.3 to 10 bar and usually a temperature of from 400 to 1200 ° C, and in an advantageous case from 450 to 800 ° C. In method step (b), the autothermal dehydrogenation product gas stream A, ammonia and oxygen-containing gas are fed into the oxidation zone and the fluorene is ammonium oxidized to the corresponding unsaturated nitrile. The product gas stream A is fed (without removing individual gas components first) into the reaction zone. The catalytic dehydrogenation product gas stream A fed to the ammoxidation does not require cooling and reheating, and selective decompression or boosting. The self-heating dehydrogenation method generates the heat of reaction in the dehydrogenation reactor and heats the reaction gas mixture, which is a problem that external heating is not required and there are no heat transfer restrictions associated with external heating. In addition, the hydrogen present in the product gas stream A lengthens the ammoxidation to 200402408

(ίο) Operating time with catalyst. The catalytic ammoxidation is carried out in a known manner. This ammoxidation is usually carried out at a molar ratio of ammonia to olefins from 0.2: 1 to 2: 1 at temperatures from 375 to 550 ° C and pressures from 0.1 to 10 bar. Useful catalysts are known to those skilled in the art and are described, for example, in WO 95/05241, EP-A 0 573 713, US 5,258,543 and US 5,212,317. The ammoxidation can be carried out in a tubular reactor in which the catalyst is stored in flakes and surrounded by a cooling liquid to eliminate the heat of reaction. It is desirable to perform this ammoxidation in a fluidized bed reactor φ. The volume ratio of oxygen to olefin is usually from 1.6: 1 to 2.4: 1. The volume ratio of ammonia to fluorene is usually from 0.7: 1 to 1.2: 1. ~ The oxygen-containing gas fed into the oxidation zone can be pure oxygen, air, or oxygen-rich air. The preferred oxygen-containing gas is pure oxygen. In a preferred embodiment of the present invention, oxygen is fed into the oxidation zone in an excess amount chemically relative to the ammonia oxidation. A product gas stream B is obtained, which includes unsaturated nitriles, by-products of ammonia oxidation, unconverted hydrocarbons and women's hydrocarbons, steam with or without oxygen, hydrogen, carbon oxides, ammonia, low hysteresis hydrocarbons, nitrogen And / or olefinic gas. In a preferred embodiment, the product gas stream contains excess residual oxygen. Generally, it will also contain ammonia, low boiling hydrocarbons, and hydrogen dehydrogenated from alkanes. When the air is fed as the oxygen-containing gas, it will contain nitrogen and a rare gas. For example, the product gas stream B from the ammoxidation of propionate to propionitrile may include the byproducts of the ammoxidation, propionaldehyde, acetonitrile, and HCN, and the product gas stream B from the ammoxidation of isobutene to methacrylonitrile may -16- 200402408 (η) Methylpropionaldehyde, HCN, acetonitrile and propionitrile as by-products. Alternatively, ammonia can be removed from the product gas stream B in method step (c) to obtain a highly ammonia-depleted or ammonia-free product gas stream C. In a variant of the method, another ammonia removal is achieved by contacting the hot ammoxidation product gas stream B with an aqueous sulfuric acid solution in a grate cooling tower, and thus washing ammonia from the product gas stream B as ammonium sulfate ( c). This produces an aqueous ammonium sulfate solution which may include dissolved unsaturated nitriles and by-products that have ammonia oxidation effects. These materials can be removed from the aqueous ammonium sulphur acid ammonium acid solution in a downstream steam stripper and fed to further distillation refining. In the other variant, no other ammonia removal is done (c). However, ammonia is almost, if not completely, removed from the ammonia oxidation product gas stream in this subsequent absorption step (d) by absorption in an aqueous absorbent.

Alternatively, methanol can be fed to the top part of the fluidized bed reactor (from about 85 to 95% of the total length) of the ammoxidation, which reacts with ammonia to produce HCN, water, and carbon dioxide. The oxidation product gas mixture is removed from the ammonia. In method step (d), the unsaturated nitrile and any ammoxidation by-products are removed from the product gas stream B or C by absorption in an aqueous absorbent. For this purpose, the product gas stream B or C is contacted with the aqueous absorbent in a gas scrubber to produce an aqueous stream containing the unsaturated nitrile, any by-products of ammonia oxidation and any ammonia, and the unsaturated is subsequently recovered therefrom Nitrile, and an outlet gas stream D containing unconverted alkanes and alkenes and any hydrogen, oxygen, carbon oxides, ammonia, low boiling hydrocarbons such as methane, ethane and ethylene, nitrogen and / or noble gases. Generally, the outlet gas stream D will also contain oxygen, hydrogen, and carbon.

-17- 200402408

(12) Oxides and low boiling hydrocarbons. When the product gas stream B from which the unsaturated nitrile is washed out by the aqueous absorbent, because, for example, there is no ammonia removal (c) and still contains a considerable amount of ammonia, the ammonia will be dissolved in the aqueous absorbent accompanied by at least Partially with the carbon dioxide which is also present in the product gas stream B, ammonium carbonate is formed. The unsaturated nitrile was recovered from the aqueous stream obtained in the absorption stage by distillation. For example, in the case of acrylonitrile production from propylene calcination, the aqueous stream obtained from the absorption step can be separated in a first distillation column into a top stream containing crude propionitrile and a bottom stream containing acetonitrile, water, and high boilers. This crude acrylonitrile is obtained as an overhead stream, and in particular it is also possible to obtain HCN, which can be further purified by distillation. Pure acetonitrile can be recovered from the bottom stream by distillation. In the case of fluorenylpropionitrile, the refining method is similar. In method step (e), the gas stream D is divided into two secondary streams D, and D '', and unconverted alkanes and fluorenes are selectively removed from the secondary stream Df, and the secondary stream D, and the secondary stream Any unconverted alkanes and amidines removed by D 'are recycled to the dehydrogenation zone. Whether or not unconverted alkanes and fluorenes are removed from the secondary stream EΓ and recycled to the dehydrogenation zone depends on the size of the secondary stream Df. Generally, the D7D "ratio is from 10 to 1/1000, preferably from 1 to 1/100, and more preferably from 1/10 to 1/50. Usually only when the D '/ D,' ratio is greater than 1 / Only 10 is removed from the secondary stream D 'and paraffins and fluorenes are recycled and recycled. When the Df / D "ratio is less than 1/20, usually the hydrocarbons and fluorenes present in D' are not removed and recycled. Carbon monoxide and low-boiling hydrocarbons removed from the secondary stream IT to provide the formation of carbon oxides and low-boiling hydrocarbons, and the autothermal dehydrogenation and / or ammoxidation use air as -18- 200402408

(13) A storage tank for nitrogen and rare gases produced by oxygen-containing gas. This tank is also called a clear tank.

The condensable gas component can be cooled and condensed in the condenser from the secondary stream D 'and subsequently removed from the non-condensable gas component in a separator. This removes the non-condensable gas components (if present in the gas stream D or D ') that interfere with oxides, hydrogen, oxygen, nitrogen, ammonia and low boiling hydrocarbons such as methylbenzene, ethylbenzene and ethylene, and is discharged from the process. This coagulation can be followed by refining to achieve almost complete removal of low-boiling hydrocarbons from unconverted alkanes and fluorenes. When a large amount of steam is present in the gas stream D ', a water phase and a pseudo-organic phase consisting of the burned hydrocarbon / women hydrocarbon mixture are formed in the separator. The hydrocarbon / halocarbon mixture can be easily separated from the aqueous phase by phase separation. Unconverted alkanes and women's hydrocarbons can also be removed from the secondary stream D 'by a high boiling absorbent in an absorption / desorption cycle. In this way, almost all non-condensable or low boiling gas components (nitrogen, hydrogen, carbon oxides, oxygen, low boiling hydrocarbons) present in the secondary stream D · are discharged from the process line.

For this purpose, the unconverted alkanes and alkanes in an absorption step are selectively absorbed by a passive absorbent under elevated pressure to produce an absorbent loaded with the alkanes and alkenes and an outlet Qi contains this subsidiary component. In the desorption step, unconverted alkanes and hydrazones are released from the absorbent at a pressure lower than that in the absorption step. The passive absorbent used in this absorption stage is usually a high boiling non-polar solvent in which the alkane / fluorene mixture to be removed has a significantly higher solubility than the remaining components of the gas stream Df. Absorption can be achieved by simply sending the gas stream through the absorbent. Can also be used in towers or in rotary absorbers -19- 200402408

(14) Now. Can operate in co-current, counter-current, or cross-flow. Examples of usable absorption towers include tray towers with blister trays, centrifuge trays and / or sieve trays, towers with structured packing, for example, sheet metal packing with a special surface area from 100 to 1000 square meters per cubic meter. Such as Mellapak® 250 Y, and towers with random packing. It is also possible to use trickle and spray towers, graphite block absorbents, surface absorbers such as thick film and thin film absorbers and rotary towers, plate scrubbers, cross-spray scrubbers and rotary scrubbers.

Useful absorbents include relatively non-polar organic solvents, such as aliphatic c8- to c18-fluorenes, or aromatic hydrocarbons such as middle oil fractions distilled from paraffin hydrocarbons, or ethers with large base-iL, or these solvents Mixtures, each of which may be added with a polar solvent such as 1,2-dimethylphthalate. Other available absorbents include benzoic acid and esters of phthalic acid and linear C ^ Cs-alkanols, such as n-butyl benzoate, methyl benzoate, ethyl benzoate, dimethyl phthalate, phthalic acid Diethyl esters, and those known as heat transfer media, such as biphenyls and diphenyl ethers, their chlorine derivatives and triaryl olefins. One useful absorbent is a mixture of biphenyl and diphenyl ether, preferably in an azeotropic composition, such as commercially available Diphyl®. This solvent mixture often contains dimethyl phthalate in an amount from 0.1 to 25% by weight. Usable absorbents also include Simian, Nonane, Decane, Rhenium-Burner, Twelve Burns, Thirteen Burners, Tetradecane, Fifteen Burners, Sixteen Burners, Seventeen Burners and Eighteen Burners and Self Refining The stream is obtained with a linear paraffin as its main component. Desorption is performed by heating the loaded absorbent and / or reducing it to a lower pressure. Alternatively, desorption can be performed by stripping or by depressurizing, heating, and stripping in a combination of one or more steps. -20- 200402408 (15) The absorbent regenerated in this desorption phase is recycled to this absorption phase. Finally, the gas stream Dn and any unconverted alkanes and olefins removed from D 'are recycled to the dehydrogenation zone (step (a)). The presence of hydrogen from the recirculation stream has a positive effect on the catalyst operating time of the dehydrogenation catalyst and also results in a higher selectivity to isocyanate and isobutylene in the autothermal dehydrogenation. Ammonia in the recirculated gas stream D is not detrimental to the dehydrogenation of the alkane and is oxidized to nitrogen or nitrogen oxides.

Claims (1)

200402408 Patent application scope 1. A method for preparing unsaturated nitrile from an alkane, comprising the following steps: (a) feeding the alkane and oxygen-containing gas into a dehydrogenation zone and autothermally dehydrogenating the alkane into Corresponding olefins to obtain product gas stream A, which contains the alkanes, unconverted alkanes, steam and possibly one or more other gaseous components selected from hydrogen, carbon oxides, having a lower boiling point than the alkanes or the alkanes A group of hydrocarbons (low boilers), nitrogen and noble gases, (b) feed the product gas stream A, ammonia and oxygen-containing gases into the oxidation zone and catalytically ammoxidize the arsine hydrocarbon to the corresponding unsaturated nitrile To obtain a product gas stream B, which contains the unsaturated nitrile, by-products of ammonia oxidation, unconverted alkanes and fluorenes, steam and possibly one or more other gaseous components selected from hydrogen, oxygen, carbon oxides, ammonia, low boiling A group consisting of hydrocarbons, nitrogen and rare gases, (c) selectively removing ammonia from the product gas stream B to obtain an ammonia-depleted product gas stream C, (d) absorbing from the product gas stream B by absorption in an aqueous absorbent Or C remove that unsaturated And ammoxidation by-products to obtain a gas stream D comprising unconverted alkanes and hydrazones and possibly one or more components selected from the group consisting of hydrogen, oxygen, carbon oxides, ammonia, low boiling hydrocarbons, nitrogen and rare gases And an aqueous stream containing the nitrile and by-products, and recovering the unsaturated nitrile from the aqueous stream, (e) separating the gas stream D into two sub-streams D 'and Dn, selectively from the 200402408 Stream EΓ removes unconverted alkanes and fluorenes and recycles the secondary stream D'f and any unconverted alkanes and olefins removed from the secondary stream D 'to the dehydrogenation zone. 2. The method according to item 1 of the scope of the patent application, wherein a high boiling absorbent is used to remove unconverted alkanes and women's hydrocarbons from the stream D in an absorption / desorption cycle and recycle them into the dehydrogenation zone. 3. The method according to item 1 or 2 of the scope of patent application, wherein in step (c), ammonia is removed by ammonium sulfate by washing with sulfuric acid. φ 4. The method according to any one of items 1 to 3 of the scope of the patent application, wherein oxygen is fed into the reaction zone (step (b)) in excess by stoichiometry relative to the ammonia oxidation. The recycle stream D " contains residual oxygen. 5. The method according to any one of claims 1 to 4, wherein the alkane is propane and the unsaturated nitrile is propionitrile. 6. The method according to any one of claims 1 to 4, wherein the alkane is isobutane and the unsaturated nitrile is methylpropionitrile. -2- *-: r * rv 200402408 Lu, (1), the designated representative of the case is: Figure (2), the representative symbols of the representative diagram are briefly explained: 柒, if there is a chemical formula in this case, please disclose the best Chemical formula showing features of the invention: