EP3274319A1 - Process for preparing 1,3-butadiene from n-butenes by oxidative dehydrogenation - Google Patents

Abstract

The invention relates to a process for preparing butadiene from n-butenes, comprising the steps of: A) providing a feedstock gas stream a1 containing n-butenes; B) feeding feedstock gas stream a1 containing n-butenes, an oxygenous gas and an oxygenous cycle gas stream a2 into at least one oxidative dehydrogenation zone and oxidatively dehydrogenating n-butenes to butadiene, resulting in a product gas stream b containing butadiene, unconverted n-butenes, steam, oxygen, low-boiling hydrocarbons, and high-boiling secondary components, with or without carbon oxides and with or without inert gases; Ca) cooling product gas stream b and optionally removing at least some high-boiling secondary components and steam, resulting in a product gas stream b'; Cb) compressing and cooling product gas stream b' in at least one compression and cooling stage, resulting in at least one aqueous condensate stream c1 and one gas stream c2 containing butadiene, n-butenes, steam, oxygen and low-boiling hydrocarbons, with or without carbon oxides and with or without inert gases; Da) absorbing the C₄ hydrocarbons comprising butadiene and n-butenes in an aromatic hydrocarbon solvent as an absorbent stream A1 in an absorption column K1 and removing uncondensable and low-boiling gas constituents comprising steam, oxygen, low-boiling hydrocarbons, any carbon oxides, aromatic hydrocarbon solvent and any inert gases as a gas stream d2 from gas stream c2, resulting in an absorbent stream A1' laden with C₄ hydrocarbons and a gas stream d2, and then desorbing the C₄ hydrocarbons from the laden absorbent stream A1', resulting in a C₄ product gas stream d1; Db) recycling at least part of gas stream d2 as a cycle gas stream a2 into the oxidative dehydrogenation zone. The disclosed method is characterized in that the content of aromatic hydrocarbon solvent in cycle gas stream a2 is limited to less than 1% by volume by bringing gas stream d2 discharged in removal stage Da) into contact with an at least partially circulating liquid absorbent stream A2 for aromatic hydrocarbon solvent A1 in another column K2 and limiting the water content in the liquid absorbent stream A2 to no more than 80% by weight.

Description

 Process for the preparation of 1, 3-butadiene from n-butenes by oxidative dehydrogenation

description

The invention relates to a process for the preparation of 1, 3-butadiene from n-butenes by oxidative dehydrogenation (ODH).

Butadiene (1,3-butadiene) is an important basic chemical and is used, for example, for the preparation of synthetic rubbers (butadiene homopolymers, styrene-butadiene rubber or nitrile rubber) or for the preparation of thermoplastic terpolymers (acrylonitrile-butadiene-styrene -Copolymers) used. Butadiene is further converted to sulfolane, chloroprene and 1, 4-hexamethylenediamine (over 1, 4-dichlorobutene and adiponitrile). By dimerization of butadiene it is also possible to produce vinylcyclohexene, which can be dehydrogenated to styrene.

Butadiene can be prepared by thermal cracking (steam cracking) of saturated hydrocarbons, usually starting from naphtha as the raw material. Steam cracking of naphtha produces a hydrocarbon mixture of methane, ethane, ethene, acetylene, propane, propene, propyne, allenes, butanes, butenes, butadiene, butynes, methylalls, Cs and higher hydrocarbons.

Butadiene can also be obtained by oxidative dehydrogenation of n-butenes (1-butene and / or 2-butene) in the presence of molecular oxygen. As a feed gas stream for the oxidative dehydrogenation (oxydehydrogenation, ODH) of n-butenes to butadiene, any n-butenes containing mixture can be used. For example, a fraction containing n-butenes (1-butene and / or 2-butene) as a main component and obtained from the C ₄ fraction of a naphtha cracker by separating butadiene and isobutene can be used. Furthermore, gas mixtures which comprise 1-butene, cis-2-butene, trans-2-butene or mixtures thereof and which have been obtained by dimerization of ethylene can also be used as the feed gas stream. Furthermore, n-butenes containing gas mixtures obtained by catalytic fluid cracking (FCC) can be used as the feed gas stream. In addition to n-butenes and molecular oxygen, the reaction gas mixture usually contains inert components. Inert components here mean that they are converted under the reaction conditions of the ODH to less than 90%. Inert components are, for example, water vapor and nitrogen, but also, for example, alkanes, such as methane. The molar ratio of the inert component to molecular oxygen is usually higher than that in air, especially to prevent the risk of explosions. This can be done, for example, by using air as the oxygen-containing gas and using molecular Nitrogen is diluted. However, the provision of large quantities of concentrated nitrogen is expensive and economically unfavorable. Furthermore, this can be done by using a molecular oxygen-depleted air (lean air) as the oxygen-containing gas. Furthermore, this can be done by diluting air with lean air.

Processes for the oxidative dehydrogenation of butenes to butadiene are known in principle.

For example, US 2012 / 0130137A1 describes such a process using catalysts containing oxides of molybdenum, bismuth and, as a rule, other metals. For the sustained activity of such catalysts for the oxidative dehydrogenation, a critical minimum oxygen partial pressure in the gas atmosphere is required in order to avoid too extensive a reduction and thus a loss of performance of the catalysts. For this reason, it is generally also not possible to work with a stoichiometric oxygen input or complete oxygen conversion in the oxydehydrogenation reactor (ODH reactor). In the US 2012/0130137, for example, an oxygen content of 2.5 to 8 vol .-% is described in the starting gas. The N2 / 02 ratio in the reaction gas mixture is adjusted to the desired value by diluting air as the oxygen-containing gas with nitrogen gas.

The need for an excess of oxygen for such catalyst systems is well known and manifests itself in the process conditions when using such catalysts. Representing the recent work of Jung et al. (Catalan Surv. Asia 2009, 13, 78-93, DOI 10.1007 / s10563-009-9069-5 and Applied Catalysis A: General 2007, 317, 244-249, DOI 10.1016 / j.apcata.2006.10.021) ,

However, the presence of oxygen in addition to butadiene according to the ODH reactor stage in the work-up part of such operated under excess oxygen process is fraught with risks. Especially in the liquid phase, the formation and accumulation of organic peroxides is to be checked. These risks have been discussed, for example, by D.S. Alexander (Industrial and Engineering Chemistry 1959, 51, 733-738).

JP 201 1 -006381 A of Mitsubishi addresses the risk of peroxide formation in the working up part of a process for the preparation of conjugated alkadienes. The solution describes the addition of polymerization inhibitors to the absorption solutions for the process gases and the adjustment of a maximum peroxide content of 100 ppm by weight by heating the absorption solutions. However, no information is given on the prevention or control of peroxides in upstream process steps. In particular, the cooling step of the ODH reactor discharge with a water quench can be seen critically. Formed organic peroxides are hardly soluble in water so that they can deposit and accumulate in solid or liquid form in the apparatus instead of being discharged with the aqueous purge stream of the quencher. At the same time, the temperature of the water quench is not so high that it can be assumed that the peroxides formed are sufficiently high and steady. In the catalytic oxidative dehydrogenation, high-boiling minor components such as maleic anhydride, phthalic anhydride, benzaldehyde, benzoic acid, ethylbenzene, styrene, fluorenone, anthraquinone and others can be formed. Such deposits can lead to blockages and a pressure drop increase in the reactor or behind the reactor in the field of workup and thus interfere with a regulated operation. Deposits of said high-boiling secondary components may also impair the function of heat exchangers or damage moving apparatus such as compressors. Water-volatile compounds such as fluorenone can penetrate through a quench apparatus operated with water and precipitate in the gas discharge lines behind it. Thus, in principle there is also the risk that solid deposits in downstream equipment parts, such as compressors, get there and cause damage.

US 2012 / 0130137A1 also refers to the problem of high-boiling by-products. In particular, phthalic anhydride, anthraquinone and fluorenone are mentioned, which are typically present in concentrations of 0.001 to 0.10 vol .-% in the product gas. In US 2012 / 0130137A1 paragraph [0124] - [0126] it is recommended to cool the hot reactor exhaust gases directly by contact with a cooling liquid (quench tower) usually first 5 to 100 ° C. As cooling liquids are called water or aqueous alkali solutions. The problem of blockages in the quench by high boilers from the product gas or by polymerization products of high-boiling by-products from the product gas is expressly mentioned, which is why it is advantageous that high-boiling by-products are carried as little as possible from the reaction part into the cooling part (quench). In KR 2013-0036467 and KR 2013-0036468 it is also recommended to cool the hot reactor exhaust gases directly by contact with a coolant. Water-soluble organic coolants are used as the coolant to better cool the minor components. In JP 201 1 -001341 A, a two-stage cooling process is described for a process for the oxidative dehydrogenation of alkenes to give conjugated alkadienes. The product discharge gas of the oxidative dehydrogenation is first adjusted to a temperature between 300 and 221 ° C and then further cooled to a temperature between 99 and 21 ° C. In paragraphs et seq. It is described that for adjusting the temperature between 300 and 221 ° C preferably heat exchangers are used, but also a part of the high boilers from the product gas could fail in these heat exchangers. In JP 201 1 -001341 A, therefore, an occasional washing out of deposits from the heat exchangers with organic or aqueous solvents is described. Examples of suitable solvents are aromatic hydrocarbons such as toluene or xylene or an alkaline aqueous solvent such as, for example, the aqueous solution of sodium hydroxide. In order to avoid too frequent stopping of the method for cleaning the heat exchanger, JP201 1 - 001341 A describes a structure with two heat exchangers arranged in parallel, each of which is operated or purged alternately (so-called A / B mode of operation). JP 2010-90083 A describes a process for the oxidative dehydrogenation of n-butenes to butadiene, in which the product gas of the oxidative dehydrogenation is cooled and dehydrated. Butadiene and unreacted butenes and butane are then absorbed in a solvent from the feed gas stream containing C 4 -hydrocarbons. The residual gas not absorbed by the solvent is then disposed of by incineration. When a solvent such as low boiling point toluene is used as the absorbent, it is recovered from the residual gas stream by absorption in a high boiling point solvent such as decane to avoid solvent losses.

The residual gas which is not absorbed by the solvent and which is largely freed from the C4 hydrocarbons can also be recycled as recycle gas into the oxydehydrogenation.

JP 2012-072086 A describes that, as the oxygen-containing gas, a gas in which the hydrocarbons such as butadiene, n-butene, n-butane and isobutane have been separated from the product gas mixture can be recycled to the oxydehydrogenation. The document makes no statements as to how such a recycle gas stream is recovered and what impurities are contained therein. JP 2012-240963 describes a process for Butadienherstellung, wherein the C ₄ -Kohlen- hydrogen-containing product gas stream from the dehydrogenation in a first absorption stage with a first adsorbent for C ₄ hydrocarbons is brought into contact. The gas stream freed from C ₄ hydrocarbons is then contacted with a second liquid absorbent in a second absorption stage to reduce the content of vaporous first absorbent in the gas stream. The second absorbent has a higher boiling point than the first absorbent. The first absorbent is for example toluene, the second absorbent another hydrocarbon with a higher boiling point. The disadvantage is that for regeneration, the first and second absorbent must be separated from each other. The object of the present invention is to provide a method which remedies the abovementioned disadvantages of known methods. In particular, a method should be provided in which deposits are avoided by high-boiling organic secondary constituents in the ODH downstream equipment. Furthermore, a method is to be provided in which the possible accumulation of organic peroxides is avoided. Furthermore, it is an object of the invention to avoid high loads of wastewater with organic compounds in dissolved, emulsified or suspended form and to reduce the accumulation of polluted with organic compounds wastewater. These objects are to be achieved without there being any significant impairment of the catalyst activity by traces of organic solvents in the recycle gas returned to the ODH.

The object is achieved by a process for the preparation of butadiene from n-butenes with the following steps:

Providing a feed gas stream a1 containing n-butenes, Feed of the n-butenes containing feed gas stream a1, an oxygen-containing gas and an oxygen-containing cycle gas stream a2 in at least one oxidative dehydrogenation and oxidative dehydrogenation of n-butenes to butadiene, wherein a product gas stream b containing butadiene, unreacted n-butenes, water vapor, oxygen , low-boiling hydrocarbons, high-boiling secondary components, optionally carbon oxides and optionally inert gases is obtained,

Cooling of the product gas stream b and, if appropriate, at least partial separation of high-boiling secondary components and of water vapor, whereby a product gas stream b 'is obtained,

Compression and cooling of the product gas stream b 'in at least one compression and cooling stage, at least one aqueous condensate stream c1 and a gas stream c2 containing butadiene, n-butenes, water vapor, oxygen, low-boiling hydrocarbons, optionally carbon oxides and optionally inert gases,

Absorption of the C4 hydrocarbons comprising butadiene and n-butenes in an aromatic hydrocarbon solvent as absorbent stream A1 in an absorption column K1 and separation of non-condensable and low-boiling gas components comprising water vapor, oxygen, low-boiling hydrocarbons, optionally carbon oxides, aromatic hydrocarbon solvent and optionally inert gases as gas stream d2 from the gas stream c2, wherein a C ₄ - loaded hydrocarbons absorbent stream aT and the gas stream are obtained d2, and subsequent desorption of the C ₄ hydrocarbons from the laden absorbent stream aT, wherein a C ₄ product gas stream d1, at least partial recirculation of the gas stream d2 as circulating gas stream a2 into the oxidative dehydrogenation zone, characterized in that the content of aromatic hydrocarbon solvent in the circulating gas stream a2 is limited to less than 1% by volume, by the gas stream d2 leaving the separation stage Da) is brought into contact in a further column K2 with an at least partially recirculated liquid absorbent stream A2 for the aromatic hydrocarbon solvent A1, and the water content of the liquid absorbent stream A2 in the further column K2 is maximally 80% by weight. -% is limited.

The determination of the volume fraction of the aromatic hydrocarbon solvent and the other gas components is carried out by gas chromatography. The calibration of the aromatic hydrocarbon solvent, for example of mesitylene, takes place here by means of an external standard. For a gasifiable solvent, such as m-xylene, with, for example, mesitylene in a certain molar ratio in a solvent, such as Example acetone, solved. Assuming that both substances and the solvent behave as ideal gases, the molar fraction is converted into volume fractions.

The gas sample with a known volume fraction of the gasifiable solvent is fed to the GC via a test loop. The defined volume sample loop is operated at constant pressure and temperature, whereupon an external factor can be determined from the areas of the reference substance and, for example, mesitylene. This can then be set in relation to the mesitylene. The other components are similarly calibrated singly or in mixtures. All components are treated like ideal gases. This also applies to the analysis of gas flows in the ODH process.

It has been found that increased levels of aromatic hydrocarbon solvents in the reaction mixture of oxydehydrogenation affect catalyst activity. The amount of aromatic hydrocarbon solvent in the reaction gas mixture depends on the proportion of the aromatic hydrocarbon solvent in the recycle gas and the proportion of the recycle gas in the reaction gas mixture. By providing a further column K2, the content of aromatic hydrocarbon solvent in the circulating gas stream a2 is limited to less than 1% by volume by the absorbent A1 contained in the gas stream d2) in vapor form or in the form of fine droplets in an absorbent stream A2 is absorbed. In this case, A2 may contain an absorption agent different from A1 or else the appropriate absorption medium. Also, the current A2 may have a lower temperature than the gas flow d2. If the absorbent contained in stream A2 and the absorbent contained in stream A1 are the same, the temperature of stream A2 is even below the temperature of stream d2 to reduce the content of aromatic hydrocarbon in stream d2.

The fact that the water content of this further absorbent stream A2 in the column K2 is limited to a maximum of 80, preferably to a maximum of 50 wt .-%, it is avoided that the solubility of the absorbent A2 for organic peroxides (primarily butadiene peroxide) is reduced so far that a separate phase of organic peroxides can form. The formation of a peroxide phase is safety-critical and should be avoided at all costs.

The water content of the further absorbent stream A2 is limited to a maximum of 80% by weight, preferably to a maximum of 50% by weight.

The selection of the coolant in the cooling stage Ca) is not subject to any restrictions. However, in the cooling step Ca), an organic solvent is preferably used. These generally have a much higher solubility for the high-boiling by-products found in the downstream of the ODH reactor parts to deposits and blockages can lead to as water or alkaline-aqueous solutions. Preferred organic solvents used as cooling agents are aromatic hydrocarbons, particular preference is given to toluene, o-xylene, m-xylene, p-xylene, mesitylene, all possible constitutional isomers of mono-, di- and triethylbenzene and all possible constitutional isomers of mono-, di-, and triisopropylbenzene or mixtures thereof. Preference is given to aromatic hydrocarbons having a boiling point at 1013.25 hPa of more than 120 ° C. or mixtures thereof. Especially preferred is mesitylene.

The absorbent used in the separation step Da) is an aromatic hydrocarbon solvent. Preference is given to toluene, o-xylene, m-xylene, p-xylene, mesitylene, all possible constitutional isomers of mono-, di- and triethylbenzene and all possible constitutional isomers of mono-, di- and triisopropylbenzene or mixtures thereof. Preference is given to aromatic hydrocarbons having a boiling point at 1013.25 hPa of more than 120.degree. Particularly preferred is mesitylene. More specifically, in the separation step Da), the same aromatic hydrocarbon solvent is used as in the preceding cooling step Ca) when an organic solvent is used in the cooling step Ca).

By absorption of the C4 hydrocarbons comprising butadiene and n-butenes from the gas stream c2 in the aromatic hydrocarbon solvent A1, non-condensable and low-boiling gas constituents comprising water vapor, oxygen, low-boiling hydrocarbons, optionally carbon oxides and optionally inert gases are obtained as gas stream d2. At least part of this gas stream d2 is recycled as circulating gas stream a2 into the oxidative dehydrogenation (step B)). According to the invention, the content of aromatic hydrocarbon solvent in the cycle gas stream a2 is less than 1% by volume.

According to the invention, the content of aromatic hydrocarbon solvent A1 in the circulating gas stream a2 is limited to less than 1% by volume in that the gas stream d2 leaving the separation stage Da) is brought into contact in a further column K2 with a liquid absorbent A2 for the aromatic hydrocarbon solvent A1 , The absorbent A2 used in this further column K2 must be miscible with the aromatic hydrocarbon solvent A1 from the absorption column K1 of the separation stage Da) and may optionally also be the same solvent. If the absorption medium A2 used in the further column K2 is the same solvent as the absorption medium A1 used in the column K1, the pressure in this further column K2 is higher than in the absorption column K1 of the separation stage Da), or but the absorbent stream A2 fed to this further column K2 is colder than the gas stream d2 entering this column, whereby the aromatic hydrocarbon solvent A1 contained in the gas stream d2 is at least partially separated from the gas stream d2. Furthermore, according to the invention, the water content of the absorption medium A2 in the further column K2 is limited to a maximum of 80% by weight, preferably at most 50% by weight. This can be done by (i) continuously withdrawing a portion of the aqueous absorbent A2 from the further column K2 and replacing it with fresh, not or not so strongly hydrous absorbent A2; or

 (ii) the water-containing absorbent is separated in a phase separator into an absorbent phase and a water phase, the water phase is separated off and the absorbent phase is applied again to the further column K2; the phase separator may be a separate phase separator or integrated into the column bottom of the further column K2; or

 (Iii) a part of the water-containing absorbent is fed into the absorption column K1.

In one embodiment of the invention, the absorption column K1 used in the absorption step Da) or the further column K2 used after the absorption step Da) has one or more devices, for example a demister or droplet separator, which entrain the liquid components from the absorption column K1 or K1 reduce the further column K2 into the gas stream d2. Suitable are all devices that reduce the proportion of liquid components in the gas stream d2. Generally, demisters or mist eliminators are understood to mean devices for separating the finest liquid droplets from gases, vapors or mists, generally aerosols. In columns, the liquid entrainment can be reduced by demisters or droplet separators. Demisters or droplet separators may, for example, consist of wire mesh packings, lamella separators or fillings of fillers with a large internal surface area. As a rule, steels, chromium-nickel steels, aluminum, copper, nickel, polypropylene, polytetrafluoroethylene and the like are used as materials. The degree of separation decreases with decreasing droplet diameters. Demisters can be counted among the coalescence separators. Demisters are described inter alia in applications US 3,890,123 and US 4,141,706 and the references cited therein. The demister or mist eliminator can be located either within the absorption column or the absorption columns, or downstream of it.

The content of the circulating gas stream a2 of aromatic hydrocarbon solvent is preferably less than 0.5% by volume, more preferably less than 0.2% by volume, in particular less than 0.1% by volume.

The process according to the invention preferably also comprises the following further process steps: E) separation of the C4 product stream d1 by extractive distillation with a butadiene-selective solvent into a butadiene and the stream e1 containing the selective solvent and a stream e2 containing n-butenes;

F) distillation of the butadiene and the material stream e2 containing the selective solvent into a stream f1 containing the selective solvent and a stream f2 containing butadiene.

The following embodiments are preferred or particularly preferred variants of the method according to the invention: The stage Ca), at least one cooling stage can be connected upstream, in which the product gas stream b is cooled by indirect cooling in a heat exchanger. The stage Ca) can be carried out in several stages in stages Ca1) to Can), preferably in two stages in two stages Ca1) and Ca2). In this case, particularly preferably at least part of the coolant is supplied after passing through the second stage Ca2) as cooling agent of the first stage Ca1). The stage Cb) generally comprises at least one compression stage Cba) and at least one cooling stage Cbb). Preferably, in the at least one cooling stage Cbb) the compressed in the compression stage Cba) gas is brought into contact with a cooling agent. More preferably, the cooling agent of the cooling step Cbb) contains the same organic solvent used as the cooling agent in step Ca) when an organic solvent is used in the cooling step Ca). In a particularly preferred variant, at least part of this cooling agent is fed after passing through the at least one cooling stage Cbb) as cooling agent of stage Ca). Alternatively, the cooling stage Cbb) may consist of heat exchangers. Preferably, the stage Cb) comprises a plurality of compression stages Cba1) to Cban) and cooling stages Cbb1) to Cbbn), for example four compression stages Cba1) to Cba4) and four cooling stages Cbb1) to Cbb4).

Step Da) preferably comprises the steps Daa) to Dac):

Daa) absorption of the C4 hydrocarbons comprising butadiene and n-butenes in an aromatic hydrocarbon solvent as absorption medium, wherein a C ₄ -Kohlen- hydrogens laden absorbent stream and the gas stream are obtained d2, Dab) removing oxygen from the C ₄ hydrocarbons loaded absorbent stream from step Daa) by stripping with a non-condensable gas stream, and

Dac) desorption of the C ₄ hydrocarbons from the loaded absorbent stream, whereby a C ₄ product gas stream d1 is obtained, which consists essentially of C ₄ hydrocarbons and less than 100 ppm oxygen.

Embodiments of the method according to the invention are described in detail below.

As feed gas stream a1 it is possible to use pure n-butenes (1-butene and / or cis-2-butene and / or trans-2-butene), but also gas mixtures containing butenes. Such a gas mixture can be obtained, for example, by non-oxidative dehydrogenation of n-butane. It it is also possible to use a fraction which contains n-butenes as the main constituent and was obtained from the C ₄ fraction of naphtha cracking by removal of butadiene and isobutene. In addition, gas mixtures comprising pure 1-butene, cis-2-butene, trans-2-butene or mixtures thereof, obtained by dimerization of ethylene, can also be used as the feed gas stream. Further, as feed gas stream n-butenes containing gas mixtures can be used, which were obtained by fluid catalytic cracking (FCC).

In one embodiment of the process according to the invention, the feed gas stream containing n-butenes is obtained by non-oxidative dehydrogenation of n-butane. By coupling a non-oxidative catalytic dehydrogenation with the oxidative dehydrogenation of the n-butenes formed, a high yield of butadiene, based on n-butane used, can be obtained. In the non-oxidative catalytic n-butane dehydrogenation, a gas mixture is obtained which, in addition to butadiene 1-butene, 2-butene and unconverted n-butane, contains minor constituents. Common secondary constituents are hydrogen, water vapor, nitrogen, CO and CO2, methane, ethane, ethene, propane and propene. The composition of the gas mixture leaving the first dehydration zone can vary greatly depending on the mode of operation of the dehydrogenation. Thus, when carrying out the dehydrogenation with the introduction of oxygen and additional hydrogen, the product gas mixture has a comparatively high content of water vapor and carbon oxides. When operating without oxygen feed, the product gas mixture of the non-oxidative dehydrogenation has a comparatively high content of hydrogen.

In step B), the reaction gas mixture containing the n-butenes containing feed gas stream a1, an oxygen-containing gas, an oxygen-containing cycle gas stream a2 and optionally other components in at least one dehydrogenation zone (the ODH reactor) is fed and the butenes contained in the gas mixture in the presence an oxydehydrogenation catalyst oxidatively dehydrogenated to butadiene. Catalysts suitable for oxydehydrogenation are generally based on a Mo-Bi-O-containing multimetal oxide system, which generally additionally contains iron. In general, the catalyst system contains other additional components, such as potassium, cesium, magnesium, zirconium, chromium, nickel, cobalt, cadmium, tin, lead, germanium, lanthanum, manganese, tungsten, phosphorus, cerium, aluminum or silicon. Iron-containing ferrites have also been proposed as catalysts.

In a preferred embodiment, the multimetal oxide contains cobalt and / or nickel. In a further preferred embodiment, the multimetal oxide contains chromium. In a further preferred embodiment, the multimetal oxide contains manganese.

Examples of Mo-Bi-Fe-O-containing multimetal oxides are Mo-Bi-Fe-Cr-O or Mo-Bi-Fe-Zr-O-containing multimetal oxides. Preferred systems are described, for example, in US 4,547,615 (Moi2BiFeo, iNi ₈ ZrCr ₃ Ko, 20x and Moi2BiFeo, iNi ₈ AICr ₃ Ko, 20x), US 4,424,141

(Moi2BiFe ₃ Co _4, 5Ni2, ₅ Po, 5KO, lox + S1O2), DE-A 25 30 959 (Moi2BiFe ₃ Co _4, 5Ni2,5Cro, 5KO, iO _x, Moi3,75BiFe3Co4,5Ni _2, 5Geo, 5KO, 80 _x, Moi2BiFe3Co4,5Ni2,5Mn _0, 5KO, _x and OK

Moi2BiFe ₃ Co4,5Ni2,5Lao, 5K _0, lox), U.S. 3,91 1, 039 ₍₃ Moi2BiFe Co4,5Ni2,5Sno, ₅ Ko, lox), DE-A 25 30 959 and DE-A 24 47 825 ( Moi2BiFe3Co4,5Ni2,5Wo, ₅ Ko, iOx). Suitable multimetal oxides and their preparation are further described in US 4,423,281 (Moi2BiNi ₈ Pbo, 5Cr ₃ Ko, 20x and Moi2BibNi ₇ Al ₃ Cro, 5Ko, 50x), US 4,336,409 (Moi2BiNi ₆ Cd2Cr ₃ Po, 50x), DE-A 26 00 128 (Moi2BiNio, 5Cr ₃ Po, ₅ Mg ₇ , 5Ko, iOx + Si0 ₂ ) and DE-A 24 40 329

(Moi2BiCo4, ₅ Ni2,5Cr Po _{3, 5} Ko, lox). Particularly preferred catalytically active, molybdenum and at least one further metal-containing multimetal oxides have the general formula (Ia):

Moi2BiaFebCOcNidCr _e X ¹ fX ² gOy (la), with

X ¹ = Si, Mn and / or Al,

X ² = Li, Na, K, Cs and / or Rb,

 0.2 <a <1,

0.5 <b <10,

 0 <c <10,

 0 <d <10,

 2 <c + d <10

 0 <e <2,

0 <f <10

 0 <g <0.5

y = a number determined on the assumption of charge neutrality by the valence and frequency of the elements other than oxygen in (1a). Preference is given to catalysts whose catalytically active oxide composition of the two metals Co and Ni has only Co (d = 0). Preferred is X ¹ Si and / or Mn and X ² is preferably K, Na and / or Cs, more preferably X ² = K.

The molecular oxygen-containing gas generally contains more than 10% by volume, preferably more than 15% by volume, and more preferably more than 20% by volume of molecular oxygen. It is preferably air. The upper limit of the content of molecular oxygen is generally 50% by volume or less, preferably 30% by volume or less, and more preferably 25% by volume or less. In addition, any inert gases may be contained in the molecular oxygen-containing gas. Possible inert gases include nitrogen, argon, neon, helium, CO, CO2 and water. The amount of inert gases for nitrogen is generally 90% by volume or less, preferably 85% by volume or less, and more preferably 80% by volume or less. In the case of components other than nitrogen, it is generally 10% by volume or less, preferably 1% by volume or less. For carrying out the oxidative dehydrogenation at full conversion of n-butenes, preference is given to a gas mixture which has a molar oxygen: n-butenes ratio of at least 0.5. Preference is given to operating at an oxygen: n-butenes ratio of 0.55 to 10. To set this value, the input gas stream with oxygen or at least one oxygen-containing gas, such as air, and optionally additional inert gas or steam can be mixed. The resulting oxygen-containing gas mixture is then fed to the oxydehydrogenation. Furthermore, inert gases such as nitrogen and also water (as water vapor) may be contained together in the reaction gas mixture. Nitrogen can be used to adjust the oxygen concentration and prevent the formation of an explosive gas mixture, the same applies to water vapor. Steam also serves to control the coking of the catalyst and to dissipate the heat of reaction.

The reaction temperature of the oxydehydrogenation is generally controlled by a heat exchange medium located around the reaction tubes. As such liquid heat exchange agents, e.g. Melting of salts such as potassium nitrate, potassium nitrite, sodium nitrite and / or sodium nitrate and melting of metals such as sodium, mercury and alloys of various metals into consideration. But ionic liquids or heat transfer oils are used. The temperature of the heat exchange medium is between 220 to 490 ° C and preferably between 300 to 450 ° C and more preferably between 350 and 420 ° C. Due to the exothermicity of the reactions taking place, the temperature in certain sections of the interior of the reactor during the reaction may be higher than that of the heat exchange medium and a so-called hotspot is formed. The location and height of the hotspot is determined by the reaction conditions, but it may also be regulated by the dilution ratio of the catalyst layer or the flow rate of mixed gas. The difference between hotspot temperature and the temperature of the heat exchange medium is generally between 1 to 150 ° C, preferably between 10 to 100 ° C and more preferably between 20 to 80 ° C. The temperature at the end of the catalyst bed is generally between 0 to 100 ° C, preferably between 0.1 to 50 ° C, more preferably between 1 to 25 ° C above the temperature of the heat exchange medium.

The oxydehydrogenation can be carried out in all fixed-bed reactors known from the prior art, such as, for example, in a hearth furnace, in a fixed-bed or shell-and-tube reactor or in a plate heat exchanger reactor. A tube bundle reactor is preferred. Preferably, the oxidative dehydrogenation is carried out in fixed bed tubular reactors or fixed bed bundle bundle reactors. The reaction tubes are (as well as the other elements of the tube bundle reactor) usually made of steel. The wall thickness of the reaction tubes is typically 1 to 3 mm. Their inner diameter is usually (uniformly) at 10 to 50 mm or 15 to 40 mm, often 20 to 30 mm. The submerged in the tube bundle reactor The number of reaction tubes usually amounts to at least 1000, or 3000, or 5000, preferably to at least 10,000. Frequently, the number of reactor tubes accommodated in the tube bundle reactor is 15,000 to 30,000 or up to 40,000 or up to 50,000. The length The reaction tubes usually extend to a few meters, typical is a reaction tube length in the range of 1 to 8 m, often 2 to 7 m, often 2.5 to 6 m.

Furthermore, the catalyst layer configured in the ODH reactor may consist of a single layer or of two or more layers. These layers may be pure catalyst or diluted with a material that does not react with the feed gas stream or components from the product gas of the reaction. Furthermore, the catalyst layers can consist of solid material or supported shell catalysts.

In addition to butadiene, the product gas stream leaving the oxidative dehydrogenation generally contains unreacted 1-butene and 2-butene, oxygen and water vapor. As secondary components it furthermore generally contains carbon monoxide, carbon dioxide, inert gases (mainly nitrogen), low-boiling hydrocarbons such as methane, ethane, ethene, propane and propene, butane and isobutane, optionally hydrogen and optionally oxygen-containing hydrocarbons, so-called oxygenates. Oxygenates may be, for example, formaldehyde, furan, acetic acid, maleic anhydride, formic acid, methacrolein, methacrylic acid, crotonaldehyde, crotonic acid, propionic acid, acrylic acid, methyl vinyl ketone, styrene, benzaldehyde, benzoic acid, phthalic anhydride, fluorenone, anthraquinone and butyraldehyde.

The product gas stream at the reactor exit is characterized by a temperature near the temperature at the end of the catalyst bed. The product gas stream is then brought to a temperature of 150 to 400 ° C, preferably 160 to 300 ° C, more preferably 170 to 250 ° C. It is possible to isolate the line through which the product gas stream flows to maintain the temperature in the desired range, or to use a heat exchanger. This heat exchanger system is arbitrary as long as the temperature of the product gas can be maintained at the desired level with this system. As an example of a heat exchanger, there can be mentioned spiral heat exchangers, plate heat exchangers, double tube heat exchangers, multi-tube heat exchangers, boiler spiral heat exchangers, shell-shell heat exchangers, liquid-liquid contact heat exchangers, air heat exchangers, direct contact heat exchangers and finned tube heat exchangers. Because, while the temperature of the product gas is adjusted to the desired temperature, a portion of the high-boiling by-products contained in the product gas may precipitate, therefore, the heat exchanger system should preferably have two or more heat exchangers. If two or more intended heat exchangers are arranged in parallel, and thus a distributed cooling of the product gas obtained in the heat exchangers is made possible, the amount of high-boiling by-products that are deposited in the heat exchangers, and thus their service life can be extended. As an alternative to the above-mentioned method, the two or more intended heat exchangers may be arranged in parallel. The product gas is supplied to one or more, but not all, heat exchangers, which are replaced after a certain period of operation of other heat exchangers. In this method, the cooling can be continued, recovered a portion of the heat of reaction and parallel to the in one of the heat exchangers deposited high-boiling by-products are removed. As an organic solvent mentioned above, a solvent may be used so long as it is capable of dissolving the high-boiling by-products. Examples are aromatic hydrocarbon solvents such as toluene and xylenes, as well as alkaline aqueous solvents such as the aqueous solution of sodium hydroxide.

Subsequently, a large part of the high-boiling secondary components and the water is separated from the product gas stream by cooling and compression. This stage is also referred to below as quench. This quench can consist of only one stage or of several stages. The cooling can be effected by contacting with a coolant, preferably an organic solvent. The cooling medium used are organic solvents, preferably aromatic hydrocarbons, particularly preferably toluene, o-xylene, m-xylene, p-xylene, mesitylene, all possible constitutional isomers of mono-, di- and triethylbenzene and all possible constitutional isomers of mono-, di- and triisopropylbenzene or mixtures thereof. Also preferred are aromatic hydrocarbons having a boiling point at 1013.25 hPa of above 120 ° C or mixtures thereof.

Preferred is a two-step quench, i. H. the stage Ca) comprises two cooling stages Ca1) and Ca2), in which the product gas stream b is brought into contact with the organic solvent.

In general, the product gas, depending on the presence and temperature level of a heat exchanger before the quench, a temperature of 100 to 440 ° C. The product gas is brought into contact with the cooling medium in the 1st quench stage. In this case, the cooling medium can be introduced through a nozzle in order to achieve the most efficient possible mixing with the product gas. For the same purpose, internals, such as, for example, additional nozzles, can be introduced into the quenching stage and pass through the product gas and the cooling medium together. The coolant inlet into the quench is designed to minimize clogging due to deposits in the area of the coolant inlet.

In general, the product gas in the first quenching stage is cooled to 5 to 180 ° C, preferably to 30 to 130 ° C and even more preferably to 60 to 1 10 ° C. The temperature of the coolant medium at the inlet may generally be 25 to 200 ° C, preferably 40 to 120 ° C, particularly preferably 50 to 90 ° C. The pressure in the first quenching stage is not particularly limited, but is generally 0.01 to 4 bar (g), preferably 0.1 to 2 bar (g) and more preferably 0.2 to 1 bar (g). When larger amounts of high-boiling by-products are present in the product gas, polymerization of high-boiling by-products and deposits of solids caused by high-boiling by-products in this process section can easily occur. In general, the quenching stage is designed as a cooling tower. The cooling medium used in the cooling tower is often used in a circulating manner. The recycle flow of the cooling medium in liters per hour, based on the mass flow of butadiene in grams per hour, can generally 0.0001 to 5 l / g, preferably 0.001 to 1 l / g and more preferably 0.002 to 0.2 l / g be. The temperature of the cooling medium in the bottom can generally be 27 to 210 ° C, preferably 45 to 130 ° C, particularly preferably 55 to 95 ° C. Since the loading of the cooling medium with secondary components increases over time, a portion of the loaded cooling medium can be withdrawn from the circulation as a purge stream and the circulation volume can be kept constant by adding unzula- denem cooling medium. The ratio of effluent amount and added amount depends on the vapor load of the product gas and the product gas temperature at the end of the first quench stage.

Depending on the temperature, pressure and water content of the product gas, condensation of water may occur in the first quenching stage. In this case, an additional aqueous phase may form, which may additionally contain water-soluble secondary components. This can then be subtracted in the bottom of the quenching stage. Preference is given to an operation in which no aqueous phase is formed in the first quench stage. The cooled and possibly depleted in secondary components product gas stream can now be fed to a second quenching stage. In this he can now be brought into contact again with a cooling medium.

The choice of the coolant is not particularly limited. Preferred cooling media are organic solvents, preferably aromatic hydrocarbons, particularly preferably toluene, o-xylene, m-xylene, p-xylene, mesitylene, all possible constitutional isomers of mono-, di- and triethylbenzene and all possible constitutional isomers of mono-, di-, and triisopropylbenzene or mixtures thereof. Also preferred are aromatic hydrocarbons having a boiling point at 1013.25 hPa of above 120 ° C or mixtures thereof.

Generally, the product gas is cooled to 5 to 100 ° C, preferably 15 to 85 ° C and even more preferably 30 to 70 ° C, to the gas exit of the second quench stage. The coolant can be supplied in countercurrent to the product gas. In this case, the temperature of the coolant medium at the coolant inlet may be 5 to 100 ° C, preferably 15 to 85 ° C, particularly preferably 30 to 70 ° C. The pressure in the second quenching stage is not particularly limited, but is generally 0.01 to 4 bar (g), preferably 0.1 to 2 bar (g), and more preferably 0.2 to 1 bar (g). The second quenching stage is preferably designed as a cooling tower. The cooling medium used in the cooling tower is often used in a circulating manner. The circulation stream of the cooling medium in liters per hour, based on the mass flow of butadiene in grams per hour, can generally be from 0.0001 to 5 l / g, preferably from 0.3001 to 1 l / g and particularly preferably from 0.002 to 0, 2 l / g.

Depending on the temperature, pressure and water content of the product gas, condensation of water may occur in the second quench stage. In this case, an additional aqueous phase may form, which may additionally contain water-soluble secondary components. This can then be subtracted in the bottom of the quenching stage. The temperature of the cooling medium in the bottom can generally be from 20 to 210 ° C., preferably from 35 to 120 ° C., particularly preferably from 45 to 85 ° C. Since the loading of the cooling medium with secondary components increases over time, part of the loaded cooling medium can be used as purge stream are withdrawn from circulation, and the circulating amount to be kept constant by adding unladen cooling medium.

In order to achieve the best possible contact of product gas and cooling medium, built-in components can be present in the second quenching stage. Such internals include, for example, bell, centrifugal and / or sieve trays, columns with structured packings, eg sheet metal packings having a specific surface area of 100 to 1000 m ² / m ³ such as Mellapak® 250 Y, and packed columns. The coolant circulations of the two quench stages can be both separated from each other and connected to each other. For example, the power can be supplied to the power or replace it. The desired temperature of the circulating streams can be adjusted by means of suitable heat exchangers. In a preferred embodiment of the invention, therefore, the cooling stage Ca) is carried out in two stages, with the second-stage coolant Ca 2) loaded with secondary components being led into the first stage Ca1). The coolant removed from the second stage Ca2) contains fewer secondary components than the coolant removed from the first stage Ca1). In order to minimize the entrainment of liquid components from the quench into the exhaust pipe, suitable structural measures, such as the installation of a demister, can be taken. Furthermore, high-boiling substances, which are not separated from the product gas in the quench, can be removed from the product gas by further structural measures, such as further gas scrubbing.

It is obtained a gas stream of n-butane, 1-butene, 2-butenes, butadiene, oxygen, hydrogen, water vapor, in small quantities of methane, ethane, ethene, propane and propene, isobutane, carbon oxides, inert gases and parts of contains coolant used in the quench. Furthermore, traces of high-boiling components can remain in this gas stream, which were not quantitatively separated in the quench. Such high boiling components include, for example, methyl vinyl ketone, methyl ethyl ketone, crotonaldehyde, acrylic acid, propionic acid, maleic anhydride, ethylbenzene, styrene, furanone, benzoic acid, benzaldehyde, fluorenone and anthraquinone. Furthermore, this gas stream may contain formaldehyde, methacrolein and / or furan.

Subsequently, the gas stream b 'from the cooling step Ca), which is depleted in high-boiling secondary components, is cooled in step Cb) in at least one compression stage Cba) and preferably in at least one cooling stage Cbb). The product gas stream from the quench is compressed in at least one compression stage and subsequently further cooled in the cooling apparatus, wherein at least one condensate stream containing water is formed. If a coolant other than water is used in the quench, the coolant used in the quench can continue to condense and optionally form a separate phase. There remains a gas stream containing butadiene, 1 - Butene, 2-butenes, oxygen, water vapor, optionally low-boiling hydrocarbons such as methane, ethane, ethene, propane and propene, butane and isobutane, optionally carbon oxides and optionally inert gases. Furthermore, this product gas stream may still contain traces of high-boiling components.

The compression and cooling of the gas stream can take place in one or more stages (n-stage). Generally, a total pressure is compressed in the range of 1.0 to 4.0 bar (absolute) to a pressure in the range of 3.5 to 20 bar (absolute). After each compression stage, a cooling stage follows, in which the gas stream is cooled to a temperature in the range of 15 to 60 ° C. The cooling is preferably carried out by contacting with an organic solvent as a cooling agent. Alternatively, heat exchangers can also be used. The condensate stream can therefore also comprise a plurality of streams in the case of multistage compression. The condensate stream consists to a large extent of water (aqueous phase) and optionally the refrigerant used in the quench (organic phase). Both streams (aqueous and organic phase) may also contain minor components such as low boilers, C4 hydrocarbons, oxygenates and carbon oxides.

In order to cool the stream and / or to remove further secondary components from the stream, the condensed quench coolant can be cooled in a heat exchanger and recycled as coolant into the apparatus. Since the loading of this cooling medium with minor components increases over time, a portion of the loaded cooling medium can be withdrawn from circulation and the circulating amount of the cooling medium can be kept constant by adding uncharged coolant. The coolant, which is added as a cooling medium, thus also preferably consists of the aromatic hydrocarbon solvent used as quench coolant.

The condensate stream can be returned to the recycle stream of the quench. As a result, the C4 components absorbed in the condensate stream can be reintroduced into the gas stream and thus the yield can be increased. Suitable compressors are, for example

Turbo, rotary piston and reciprocating compressors. The compressors can be driven, for example, with an electric motor, an expander or a gas or steam turbine. Typical compression ratios (outlet pressure: inlet pressure) per compressor stage are between 1, 5 and 3.0, depending on the design. The cooling of the compressed gas is carried out with organic solvent purged heat exchangers or organic quench, which may be designed, for example, as a tube bundle, spiral or plate heat exchanger. As coolant, cooling water or heat transfer oils are used in the heat exchangers. In addition, air cooling is preferably used using blowers. The butadiene, n-butenes, oxygen, low-boiling hydrocarbons (methane, ethane, ethene, propane, propene, n-butane, isobutane), water vapor, optionally carbon oxides and optionally inert gases and optionally traces of minor components containing gas stream c2 is used as the output stream the further processing supplied. In a step Da) are non-condensable and low-boiling gas components comprising water vapor, oxygen, low-boiling hydrocarbons (methane, ethane, ethene, propane, propene), carbon oxides and inert gases in an absorption column K1 from the process gas stream c2 by absorption of C4 hydrocarbons separated in a hydrocarbon aromatic solvent as a high-boiling absorbent A1 and subsequent desorption of the C4 hydrocarbons. Preferably, the step Da) comprises the steps Daa) to Dac):

Daa) absorption of the C4 hydrocarbons comprising butadiene and n-butenes in an aromatic hydrocarbon solvent as absorbent A1, one with C ₄ -

Hydrocarbons laden absorbent stream AT and the gas stream d2 are obtained

Dab) removal of oxygen from the laden with C ₄ hydrocarbons absorbent stream A1 'from step Daa) by stripping with a non-condensable

 Gas stream, and

Dac) desorption of the C ₄ hydrocarbons from the loaded absorbent stream A1 ', whereby a C ₄ product gas stream d1 is obtained, which consists essentially of C ₄ -carbon hydrogens.

For this purpose, in the absorption stage, the gas stream c2 is brought into contact with the absorbent A1 and the C ₄ hydrocarbons are absorbed in the absorbent A1, whereby an absorbent A1 'laden with C ₄ hydrocarbons and a gas stream d 2 containing the other gas constituents are obtained which is at least partially recycled as a recycle gas stream in the oxidative dehydrogenation. In a desorption stage, the C ₄ -hydrocarbons are released from the loaded absorbent A1 'again.

As absorption medium A1, organic solvents, preferably aromatic hydrocarbons, particularly preferably toluene, o-xylene, m-xylene, p-xylene, mesitylene, all possible constitutional isomers of mono-, di- and triethylbenzene and all possible constitutional isomers of mono-, Di- and triisopropylbenzene or mixtures thereof. Also preferred are aromatic hydrocarbons having a boiling point at 1013.25 hPa of above 120 ° C or mixtures thereof. More specifically, in the separation step Da), the same aromatic hydrocarbon solvent is used as in the preceding cooling step Ca) when an organic solvent is used in the cooling step Ca). Preferred absorbents are solvents which have a solubility for organic peroxides of at least 1000 ppm (mg active oxygen / kg solvent). In a preferred embodiment, A1 mesitylene is used as the absorption medium.

The absorption stage can be carried out in any suitable absorption column known to the person skilled in the art. Absorption can be accomplished by simply passing the product gas stream through the absorbent. But it can also be done in columns or in rotational absorbers. It can be used in DC, countercurrent or crosscurrent gear- be processed. Preferably, the absorption is carried out in countercurrent. Suitable absorption columns are, for example, tray columns with bell, centrifugal and / or sieve trays, columns with structured packings, for example sheet metal packings with a specific surface area of 100 to 1000 m ² / m ³ such as Mellapak® 250 Y, and packed columns. Rice and spray towers, graphite block absorbers, surface absorbers such as thick-film and thin-layer absorbers, as well as rotary columns, rags, cross-flow scrubbers and rotary scrubbers are also suitable.

Preferably, the absorption column K1 is a tray column with bells, centrifugal and / or sieve bottom or a column with structured packing or a packed column, more preferably a column with structured packings. This generally has 10 to 40 theoretical plates. The absorption column K1 is generally operated at a pressure of 5 to 15 bar, preferably 8 to 12 bar. The temperature of the absorbent A1 added to the column K1 is generally from 5 to 50 ° C, preferably from 20 to 40 ° C.

In one embodiment, the absorption column K1 is supplied in the lower region of the butadiene, n-butenes and the gas stream c2 containing the low-boiling and non-condensable gas constituents. In the upper part of the absorption column, the absorbent is applied.

At the top of the absorption column K1 a gas stream d2 is withdrawn, which is essentially water vapor, oxygen, low-boiling hydrocarbons (methane, ethane, ethene, propane, propene), the aromatic hydrocarbon solvent, optionally C4-hydrocarbons (butane, butenes, butadiene) , optionally inert gases and optionally containing carbon oxides. This stream is brought according to the invention in the further column K2 with a liquid absorbent A2 for the aromatic hydrocarbon solvent in contact and then at least partially supplied as a circulating gas stream a2 the ODH reactor. Thus, for example, the inlet flow of the ODH reactor can be adjusted to the desired C4 hydrocarbon content. In general, optionally after separation of a Purgegasstroms, at least 30 vol .-%, preferably at least 50 vol .-% of the gas stream d2 recycled as circulating gas stream a2 in the oxidative dehydrogenation. The purge gas stream may be subjected to thermal or catalytic afterburning. In particular, it can be thermally utilized in a power plant.

In general, the recycle stream is 10 to 70% by volume, preferably 30 to 60% by volume, based on the sum of all material streams fed into the oxidative dehydrogenation B).

According to the invention, the content of aromatic hydrocarbon solvent in the circulating gas stream a2 is limited to less than 1% by volume by passing the gas stream d2 leaving the separation stage Da) in contact with a liquid absorbent A2 for the aromatic hydrocarbon solvent in a further column K2 , wherein the water content of the absorbent A2 in the further column K2 is limited to a maximum of 50 wt .-%. This can be done by (i) continuously withdrawing a portion of the aqueous absorbent A2 from the further column K2 and replacing it with fresh, not or not so strongly hydrous absorbent A2; or

(ii) the water-containing absorbent is separated in a phase separator into an absorbent phase and a water phase, the water phase is separated off and the absorbent phase is applied again to the further column; the phase separator may be a separate phase separator or integrated into the column bottom of the further column; or

(Iii) a part of the water-containing absorbent stream A2 from the column K2 is fed into the absorption column K1.

Suitable absorbents are organic solvents, preferably aromatic hydrocarbons, particularly preferably toluene, o-xylene, m-xylene, p-xylene, mesitylene, all possible constitution isomers of mono-, di- and triethylbenzene and all possible constitution isomers of mono-, di- and Triisopropylbenzene or mixtures thereof. Preferred absorbents are solvents which have a solubility for organic peroxides of at least 1000 ppm (mg active oxygen / kg solvent). In a particularly preferred embodiment, mesitylene is used as absorption medium A2. In particular, the same aromatic hydrocarbon solvent is used in the further column K2 as the absorbent A2, which is also used in the absorption column K1 as the absorbent A1.

As a further absorption column K2, for example, tray columns with bells, centrifugal and / or sieve plates, columns with structured packings, for example sheet metal packings having a specific surface area of 100 to 1000 m ² / m ^3, such as Mellapak® 250 Y, and packed columns are suitable. These generally have 1 to 15 theoretical plates. The further column K2 is generally operated at a pressure of 5 to 15 bar, preferably 8 to 12 bar. The temperature of the absorbent A2 added to the column K2 is generally from 0 to 30 ° C, preferably 5 to 15 ° C.

When column K1 and column K2 are operated at the same pressure, the temperature of the absorbent A2 charged to the column K2 is generally 1 to 50, preferably 20 to 30 ° C lower than the temperature of the absorbent A1 applied to the column K1.

In a first variant (i), part of the water-containing absorption medium A2 from the further column K2 is continuously withdrawn and replaced by fresh, not or not so strongly hydrous absorbent A2. In general, the part of the water-containing absorbent stream A2 which is withdrawn and is not fed back to the column K2 is 0.1 to 10% of the total stream of the absorbent A2. The withdrawn stream is optionally passed to column K1 or solvent regeneration. In a second variant (ii), the water-containing absorbent is separated in a phase separator into an absorbent phase and a water phase, the water phase is separated off and the absorbent phase is again applied to the further column K2. The phase separator may be a separate phase separator or integrated into the column bottom of the further column K2.

In a third variant (iii), part of the water-containing absorption medium from the column K2 is fed into the absorption column K1. If there are two separate columns, a part of the bottom effluent from the column K2 is fed into the column K1. However, it can also run over an overflow part of the stream from column K2 in the column K1, while the rest of the stream is withdrawn and added back to the top of the column K2, wherein the column K2 and the column K1 column sections of a single combined column are. In one embodiment, this combined column contains a chimney tray between the column sections K1 and K2. In general, the part of the aqueous absorption medium A2, which is led into the absorption column K1 and is not applied again to the column K2, amounts to 0.1 to 10% of the total flow of the absorbent stream A2.

At the bottom of the absorption column K1 residues in the absorbent dissolved oxygen can be discharged in a further column K3 by flushing with a gas. The remaining oxygen content is preferably so small that the stream d1 leaving the desorption column and containing butane, butenes and butadiene only contains a maximum of 100 ppm of oxygen.

The stripping out of the oxygen in step Dab) can be carried out in any suitable column known to the person skilled in the art. The stripping can be carried out by simply passing non-condensable gases, preferably inert gases such as nitrogen, through the loaded absorption solution. With stripped C4 hydrocarbons are washed in the upper part of the absorption column back into the absorption solution by the gas stream is passed back into this absorption column. This can be done both by a piping of the stripping column and a direct assembly of the stripping column below the absorber column. Since the pressure in the stripping column part and the absorption column part is the same according to the invention, this direct coupling can take place. Suitable spiked columns are e.g. Tray columns with

Bell, centrifugal and / or sieve bottom, columns with structured packings, eg sheet metal packings with a specific surface area of 100 to 1000 m ² / m ³ such as Mellapak® 250 Y, and packed columns. But there are also trickle and spray towers and rotary columns, dishwashers, cross-flow scrubbers and rotary scrubbers into consideration. Suitable gases are for example nitrogen or methane. The loaded with C4 hydrocarbons absorbent stream A1 'contains water. This can be separated in a decanter as a stream from the absorbent A1 ', so that a stream is obtained which contains only the redeemed water in the absorbent. The laden with C4 hydrocarbons, largely freed from water absorbent stream AT can be heated in a heat exchanger and then passed into a desorption. In a process variant, the desorption step De) is carried out by relaxation and / or heating of the loaded absorbent. A preferred process variant is the use of a reboiler in the bottom of the desorption column.

The absorbent A1 regenerated in the desorption stage can be cooled in a heat exchanger and returned to the absorption stage. Low boilers in the process gas stream, such as ethane or propane, as well as high-boiling components, such as benzaldehyde, maleic anhydride and phthalic anhydride, can accumulate in the circulation stream. To limit the enrichment, a purge can be deducted. This can be separated in a distillation column according to the prior art in low boilers, regenerated absorbent and high boiler. The C4 product gas stream d1 consisting essentially of n-butane, n-butenes and butadiene generally contains from 20 to 80% by volume of butadiene, from 0 to 80% by volume of n-butane, from 0 to 10% by volume 1 - Butene, and 0 to 50% by volume of 2-butenes, the total amount being 100% by volume. Furthermore, small amounts of iso-butane may be included. A portion of the condensed head effluent of the desorption column, mainly containing C 4 hydrocarbons, is returned to the top of the column to increase the separation efficiency of the column.

The liquid or gaseous C4 product streams leaving the condenser can subsequently be separated by extractive distillation in step E) with a butadiene-selective solvent into a butadiene and the material stream containing the selective solvent and a stream containing n-butenes.

The extractive distillation may, for example, as described in "petroleum and coal - natural gas - petrochemistry", Volume 34 (8), pages 343 to 346 or "Ullmann's Encyclopedia of Industrial Chemistry", Volume 9, 4th edition 1975, pages 1 to 18, be performed. For this purpose, the C4 product gas stream with an extractant, preferably an N-methylpyrrolidone

(NMP) / water mixture, contacted in an extraction zone. The extraction zone is generally designed in the form of a wash column which contains trays, fillers or packages as internals. This generally has 30 to 70 theoretical plates, so that a sufficiently good release effect is achieved. Preferably, the wash column has a backwash zone in the column head. This backwash zone is used to recover the extractant contained in the gas phase by means of a liquid hydrocarbon reflux, to which the top fraction is condensed beforehand. The mass ratio extractant to C4 product gas stream in the feed of the extraction zone is generally from 10: 1 to 20: 1. The extractive distillation is preferably carried out at a bottom temperature in the range from 100 to 250 ° C., in particular at a temperature in the range from 110 to 210 ° C., a top temperature in the range from 10 to 100 ° C., in particular in the range from 20 to 70 ° C. ° C and one Pressure in the range of 1 to 15 bar, in particular operated in the range of 3 to 8 bar. The extractive distillation column preferably has from 5 to 70 theoretical plates.

Suitable extractants are butyrolactone, nitriles such as acetonitrile, propionitrile, methoxypropionitrile, ketones such as acetone, furfural, N-alkyl-substituted lower aliphatic acid amides such as dimethylformamide, diethylformamide, dimethylacetamide, diethylacetamide, N-formylmorpholine, N-alkyl-substituted cyclic acid amides (lactams) such as N Alkylpyrrolidones, especially N-methylpyrrolidone (NMP). In general, alkyl-substituted lower aliphatic acid amides or N-alkyl substituted cyclic acid amides are used. Particularly advantageous are dimethylformamide, acetonitrile, furfural and in particular NMP.

However, mixtures of these extractants with each other, e.g. NMP and acetonitrile, mixtures of these extractants with cosolvents and / or tert-butyl ether, e.g. Methyl tert-butyl ether, ethyl tert-butyl ether, propyl tert-butyl ether, n- or iso-butyl tert-butyl ether can be used. Particularly suitable is NMP, preferably in aqueous solution, preferably with 0 to 20 wt .-% water, particularly preferably with 7 to 10 wt .-% water, in particular with 8.3 wt .-% water.

The overhead product stream of the extractive distillation column contains essentially butane and butenes and in small amounts of butadiene and is taken off in gaseous or liquid form. In general, the stream consisting essentially of n-butane and 2-butene contains up to 100% by volume of n-butane, 0 to 50% by volume of 2-butene and 0 to 3% by volume of further constituents, such as isobutane, isobutene , Propane, propene and Cs ⁺ hydrocarbons. The stream consisting essentially of n-butane and 2-butene can be supplied wholly or partly to the C ₄ feed of the ODH reactor. Since the butene isomers of this recycle stream consist essentially of 2-butenes, and 2-butenes are generally more slowly dehydrogenated to butadiene than 1-butene, this recycle stream can be catalytically isomerized prior to being fed to the ODH reactor. As a result, the isomer distribution can be adjusted in accordance with the isomer distribution present in the thermodynamic equilibrium.

In a step F), the stream comprising butadiene and the selective solvent is fractionated by distillation into a stream consisting essentially of the selective solvent and a stream comprising butadiene.

The stream obtained at the bottom of the extractive distillation column generally contains the extractant, water, butadiene and, in minor proportions, butenes and butane and is fed to a distillation column. In this can be obtained overhead or as a side take butadiene. At the bottom of the distillation column, an extractant and optionally water-containing material flow is obtained, wherein the composition of the extractant and water-containing material stream corresponds to the composition as it is added to the extraction. The extractant and water-containing stream is preferably returned to the extractive distillation. If the butadiene is recovered via a side draw, the extraction solution thus withdrawn is transferred to a desorption zone, wherein the butadiene is desorbed again from the extraction solution and washed back. The desorption zone can be embodied, for example, in the form of a wash column which has 2 to 30, preferably 5 to 20 theoretical stages and optionally a backwashing zone with, for example, 4 theoretical stages. This backwash zone is used to recover the extractant contained in the gas phase by means of a liquid hydrocarbon reflux, to which the top fraction is condensed beforehand. As internals packings, trays or packing are provided. The distillation is preferably carried out at a bottom temperature in the range from 100 to 300 ° C., in particular in the range from 150 to 200 ° C. and a top temperature in the range from 0 to 70 ° C., in particular in the range from 10 to 50 ° C. The pressure in the distillation column is preferably in the range of 1 to 10 bar. In general, a reduced pressure and / or elevated temperature prevails in the desorption zone relative to the extraction zone.

The product stream obtained at the top of the column generally contains 90 to 100% by volume of butadiene, 0 to 10% by volume of 2-butene and 0 to 10% by volume of n-butane and isobutane. For further purification of the butadiene, a further distillation according to the prior art can be carried out.

example

A variant of the method according to the invention is shown in FIG. The process gas mixture leaving the compressor enters the latter at 64 ° C. and the composition shown in Table 1 as stream 1 at the 30th stage of the absorption column 22 comprising 60 stages. The top pressure of the column is 10 bar absolute. The column contains bubble trays. In the absorber column, the process gas stream flows in countercurrent to the unloaded absorbent stream 10 fed from above, consisting mainly of mesitylene saturated with water. The absorbent preferably absorbs the C ₄ - hydrocarbons and small amounts of non-condensable gases. The ratio between the mass of the absorbent stream 10 and the process gas stream 1 is 2.2: 1. The non-condensable gases leave the absorption column mainly as stream 3 above the top of the column at a temperature of 35 ° C and the composition shown in Table 1. In order to further reduce the mesitylene concentration in the exhaust gas stream 3, this is passed into a further absorber column 25 and further cooled in contact with the stream 17. The resulting exhaust gas flow 20 then has only 80 mol. Ppm of mesitylene. According to the invention it is important that condensed water is removed from the circuit 17. This is possible via the withdrawal 18 or the recirculation 21 in the absorber column 22. It has been found to be particularly advantageous that in the circuit 17, the same absorbent as in the circulation of the absorber column 22 is used and a part of the consisting of absorbent and water circuit 17th is conducted as stream 21 in the absorber column 22. By this take-off stream 21 and the fresh absorbent, which is supplied to the circuit 17 as stream 19, a maximum water concentration of 42.3 mol .-% can be maintained. In order to the circuit 17 has a sufficient amount of organic absorbent with a high peroxide solution behavior.

Nitrogen flow 2 desorbs oxygen from the absorbent stream loaded with C 4 hydrocarbons. The largely deoxygenated, with C ₄ -Kohlen- hydrogens laden absorbent stream 4 is heated in the heat exchanger 28 and passed as stream 7 in the desorber 26. There, the C ₄ -hydrocarbons are removed from the absorbent stream by the stripping steam stream 6 and leave the column 26 as stream 13. This is partially condensed in the condenser 29, the gas stream 15 remaining. Part of the condensate is recycled as stream 16 to the desorber column 26, while stream 14 is the C ₄ product stream.

The absorbent and water leave the desorber 26 as stream 8, are cooled in the heat exchanger 27, wherein the stream 9 is formed, which is divided in the phase separator 24 in an absorbent stream 10 - which is returned to the absorber column - and the aqueous stream 26. From the absorbent stream, a partial stream 5 can still be deducted for solvent regeneration. The aqueous stream - from which a partial stream 30 can still be withdrawn - is vaporized in the steam generator 26 to stream 6. It can still be given a fresh water stream 1 1 in the steam generator 26.

table

Electricity 12 13 14 15 16 17 18 19 20 21

Aggregate liquid gas liquid gas liquid liquid liquid liquid gas liquid

Temperature 35.0 48.5 25.0 25.0 25.0 5.0 1 1, 7 1, 0 5.0 1 1, 7

Pressure 10.0 5.5 5.3 5.4 5.4 10.0 10.0 10.0 10.0 10.0

Mass flow 0.02 2.47 1, 14 0.07 1, 25 15.00 0.02 0.05 5.74 0.05

 Concentrations [% by weight]

 Butane 0.0 15.6 15.8 10.6 15.8 0.2 0.2 0.0 0.2 0.2 iso-butane 0.0 0.0 0.0 0.0 0.0 0 , 0 0.0 0.0 0.0 0.0 iso-butene 0.0 0.1 0.1 0.1 0.1 0.0 0.0 0.0 0.0 0.0

1-Butene 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 cis-2-butene 0.0 6.8 6.9 3.6 6.9 0.0 0.0 0.0 0.0 0.0 trans-2- 0.0 8.5 8.6 4.9 8.6 0.0 0.0 0.0 0.0 0 , 0 butene 1, 3-butadiene 0.0 67.2 67.8 47.2 67.8 0.2 0.2 0.0 0.1 0.2

Water 0.0 0.7 0.7 0.3 0.7 10.0 10.0 0.0 0.1 10.0

TMB 100.0 0.0 0.0 0.0 0.0 89.3 89.3 100.0 0.0 89.3

C0 ₂ 0.0 0.1 0.0 1, 3 0.0 0.1 0.1 0.0 1, 9 0.1

N ₂ 0.0 1, 0 0.1 32.1 0.1 0.2 0.2 0.0 89.4 0.2

0 ₂ 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 8,0 0,0

CO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.0

LIST OF REFERENCE NUMBERS

1 process gas stream with O2, N2, C _4, CO, CO2, H2O

 2 stripping medium, non-condensable gas (e.g., nitrogen, methane or like gases)

3 exhaust gas stream with 0 ₂ , N ₂ , CO, C0 ₂

4 absorption solution loaded with C ₄ -hydrocarbons, cool

 5 fume cupboard unloaded absorption solution

 6 uncharged water, evaporated

7 with C ₄ hydrocarbons loaded absorption solution, heated

 8 uncharged absorption solution with H2O, heated

 9 uncharged absorption solution with H2O, cool

 10 uncharged absorption solution

 1 1 fresh water

 12 fresh absorbent

13 C ₄ stream, gaseous

14 condensed C ₄ stream to the extractive distillation

15 gaseous C ₄ stream for extractive distillation, still contains intergas

16 C ₄ stream, liquid reflux

 17 circulation around absorber column 25

 18 withdrawal from circulation 17

 19 fresh absorbent

 20 gas stream containing less absorbent from column 22 than stream 3

 21 liquid stream containing water and absorbent from column 25th

 22 absorber column with stripping column mounted directly below

 23 desorber column

 24 decanters for the separation of absorber liquid 10 and water 6

 25 absorber column to reduce absorbent concentration in the gas stream 3

26 steam generator

 27 heat exchangers

 28 heat exchangers

 29 capacitor

 30 water outlet

Claims

claims

1 . Process for the preparation of butadiene from n-butenes, comprising the steps of:

A) providing a feed gas stream a1 containing n-butenes,

B) feeding the n-butenes containing feed gas stream a1, an oxygen-containing gas and an oxygen-containing cycle gas stream a2 in at least one oxidative dehydrogenation and oxidative dehydrogenation of n-butenes to butadiene, wherein a product gas stream b containing butadiene, unreacted n-butenes, water vapor, oxygen , low-boiling hydrocarbons, high-boiling secondary components, optionally carbon oxides and optionally inert gases is obtained,

Ca) cooling of the product gas stream b and, if appropriate, at least partial separation of high-boiling secondary components and of water vapor, a product gas stream b 'being obtained,

Cb) compression and cooling of the product gas stream b 'in at least one compression and cooling stage, at least one aqueous condensate stream c1 and a gas stream c2 containing butadiene, n-butenes, water vapor, oxygen, low-boiling hydrocarbons, optionally carbon oxides and optionally inert gases,

Da) absorption of the C4 hydrocarbons comprising butadiene and n-butenes in an aromatic hydrocarbon solvent as the absorbent stream A1 in an absorption column K1 and separation of non-condensable and low-boiling gas constituents comprising water vapor, oxygen, low-boiling hydrocarbons, optionally carbon oxides, aromatic hydrocarbon solvent and optionally inert gases as the gas stream d2 from the gas stream c2, where a loaded with C4 hydrocarbons absorbent stream aT and the gas stream are obtained d2, and subsequent desorption of the C ₄ - hydrocarbons from the laden absorbent stream aT, wherein a C ₄ - product gas stream is obtained d1,

Db) at least partially recirculation of the gas stream d2 as a circulating gas stream a2 in the oxidative dehydrogenation, characterized in that the content of aromatic hydrocarbon solvent in the circulating gas stream a2 is limited to less than 1 vol .-% by the gas stream d2 leaving the separation stage Da) in one another column K2 is brought into contact with an at least partially circulated liquid absorbent stream A2 for the aromatic hydrocarbon solvent A1, and the water content of the liquid absorbent stream A2 in the column K2 is limited to a maximum of 80 wt .-%.

2. The method according to claim 1, characterized in that the water content of the sorbent stream A2 in the further column K2 is limited to a maximum of 80 wt .-% by a partial stream of the aqueous absorbent stream A2 from the further column K2 is withdrawn continuously and by fresh, not or not so strongly hydrous absorbent A2 is replaced.

3. The method according to claim 1 or 2, characterized in that the water content of the absorbent stream A2 in the further column K2 is limited to a maximum of 80 wt .-% by the water-containing absorbent A2 is separated in a phase separator in an absorbent phase and a water phase, the water phase separated and the absorbent phase is again applied to the further column K2.

4. The method according to claim 4, characterized in that the phase separator is integrated into the column bottom of the further column K2.

5. The method according to any one of claims 1 to 4, characterized in that the water content of the absorbent stream A2 in the further column K2 is limited to a maximum of 80% by weight, by a partial stream of the water-containing absorbent A2 from the column K2 in the absorption column K1 to be led.

6. The method according to any one of claims 1 to 5, characterized in that in

 Step Da) used as the absorbent A1 aromatic hydrocarbon solvent is selected from the group consisting of toluene, o-, m-, p-xylene, mesitylene, mono-, di- and triethylbenzene and mono-, di- and triisopropylbenzene and mixtures thereof.

A method according to claim 6, characterized in that the aromatic hydrocarbon solvent is mesitylene.

8. The method according to any one of claims 1 to 7, characterized in that as absorption medium A1 and A2, the same aromatic hydrocarbon solvent is used.

9. The method according to any one of claims 1 to 8, characterized in that the proportion of the circulating gas stream a2 10 to 70 vol .-%, based on the sum of all fed into the oxidative hydrogenation hydrogenation zone, is.

10. The method according to any one of claims 1 to 9, characterized in that the step Da) the steps Daa) to Dac) comprises: Daa) absorption of the C4 hydrocarbons comprising butadiene and n-butenes in the aromatic hydrocarbon solvent as an absorbent to obtain a C4 hydrocarbons-laden absorbent stream and the gas stream d2,

Dab) removal of oxygen from the C4 hydrocarbons laden

 Absorbent stream from step Daa) by stripping with a non-condensable gas stream, and

Desorption of the C4 hydrocarbons from the loaded absorbent stream, wherein a C4 product gas stream d1 is obtained, which consists essentially of C4 hydrocarbons.

Method according to one of claims 1 to 10, characterized by the additional steps:

Separation of the C4 product stream d1 by extractive distillation with a butadiene-selective solvent in a butadiene and the selective solvent-containing stream e1 and a n-butenes containing stream e2;

Distillation of the butadiene and the selective solvent-containing stream e2 in a material containing the selective solvent stream f1 and a butadiene-containing stream f2.

Method according to one of claims 1 to 1 1, characterized in that the two columns K1 and K2 are designed as column sections K1 and K2 a combined column.

A method according to claim 12, characterized in that the combined column contains a chimney tray between the column sections K1 and K2.

Method according to one of claims 1 to 13, characterized in that the water content of the liquid absorbent stream in the column K2 is limited to a maximum of 50 wt .-%.