WO2016150940A1

WO2016150940A1 - Process for preparing 1,3-butadiene from n-butenes by oxidative dehydrogenation

Info

Publication number: WO2016150940A1
Application number: PCT/EP2016/056218
Authority: WO
Inventors: Jan Pablo JOSCH; Stephan DEUBLEIN; Regina Benfer; Hendrik Reyneke; Christine TOEGEL; Ulrike Wenning; Anton Wellenhofer; Heinz Boelt
Original assignee: Basf Se; Linde Ag
Priority date: 2015-03-26
Filing date: 2016-03-22
Publication date: 2016-09-29

Abstract

The invention relates to a process for preparing butadiene from n-butenes, comprising the steps of: A) providing a feedstock gas stream a containing n-butenes; B) feeding feedstock gas stream a containing n-butenes and a gas containing oxygen 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 by bringing same in contact with a coolant, and condensing at least some of the high-boiling secondary components; Cb) compressing the remaining product gas stream b in at least one compression 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) removing uncondensable and low-boiling gas constituents comprising oxygen, low-boiling hydrocarbons, any carbon oxides and any inert gases as a gas stream d2 from gas stream c2 by absorbing the C₄ hydrocarbons comprising butadiene and n-butenes in an absorbent, resulting in an absorbent stream laden with C₄ hydrocarbons and gas stream d2, and Db) then desorbing the C₄ hydrocarbons from the laden absorbent stream in a desorption column, resulting in a C₄ product gas stream d1. The disclosed method is characterized in that a lateral discharge stream containing methacrolein is withdrawn from the desorption column.

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 is an important basic chemical and is used for example for the production of synthetic rubbers (butadiene homopolymers, styrene-butadiene rubber or nitrile rubber) or for the production of thermoplastic terpolymers (acrylonitrile-butadiene-styrene copolymers). Butadiene is further converted to sulfolane, chloroprene and 1, 4-hexamethylenediamine (over 1, 4-dichlorobutene and adiponitrile). By dimerization of butadiene, vinylcyclohexene can also be produced, 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). As the starting gas mixture 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 starting gas. Furthermore, n-butenes containing gas mixtures can be used as the starting gas, which were obtained by catalytic fluid catalytic cracking (FCC).

Processes for the oxidative dehydrogenation of butenes to butadiene are known in principle. Such methods often include the following steps:

A) providing a n-butenes containing feed gas stream a;

B) feeding the n-butenes containing feed gas stream a and an oxygen-containing gas into 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 dende secondary components, optionally carbon oxides and optionally inert gases is obtained;

Ca) cooling of the product gas stream b by contacting with a coolant and

Condensation of at least a portion of the high boiling minor components;

Cb) compression of the remaining product gas stream b in at least one compression 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 is obtained;

Da) separation of non-condensable and low-boiling gas components comprising oxygen, low-boiling hydrocarbons, optionally carbon oxides and optionally inert gases as gas stream d2 from the gas stream c2 by absorption of C ₄ - hydrocarbons comprising butadiene and n-butenes in an absorbent, wherein a with C ₄ hydrocarbons laden absorbent stream and the gas stream d2 are obtained, and

Db) subsequent desorption of the C ₄ hydrocarbons from the loaded absorption medium stream in a desorption column, a C ₄ product gas stream d 1 being obtained.

US 2012/0130137 A1, for example, describes a process for the oxidative dehydrogenation of butenes to butadiene using catalysts comprising oxides of molybdenum, bismuth and usually other metals.

Paragraph [0122] 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 2012/0130137 A1 paragraph [0124] to [0126] it is recommended to cool the hot reactor exhaust gases directly by contact with a cooling liquid (quench tower) to initially 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 as few as possible high-boiling by-products are discharged from the reaction part into the cooling part (quench). A separation of isobutene or its reaction product methacrolein is not mentioned.

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 JP201 1 -001341 A is therefore a occasional washing out of deposits from the heat exchangers with organic or aqueous solvents. As the solvent, for example, aromatic hydrocarbons such as toluene or xylene or an alkaline aqueous solvent such as the aqueous solution of sodium hydroxide are described. In order to avoid too frequent stopping of the method for cleaning the heat exchanger, JP 201 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). A separation of isobutene or its reaction product methacrolein is not mentioned. JP 201 1 -132218 limits the iso-butene content in the feed since it is known that isobutene forms oxy genates. The separation of the oxygenates is not described.

JP 2012240963 describes a process for butadiene production, in which the gas stream containing C ₄ -hydrocarbons is brought into contact with an absorbent b in an absorption stage b 'in order to absorb the C ₄ components.

JP 2010-090083 limits the amount of aldehydes and also discloses the formation of methacrolein in Table 1, but makes no proposals for its separation. Iso-butene is present in almost all C ₄ hydrocarbon streams that may be used for the ODH process. In particular, C ₄ hydrocarbon streams from FC crackers contain iso-butene in amounts of up to 15% by volume. The isoButene entering the ODH reactor is converted into methacrolein by about 50%, depending on the catalyst used and the reaction conditions. This accumulates in the recycle stream of the absorption / desorption part of the C ₄ -hydrocarbon separation and can cause side reactions such as oligomerizations and polymerizations, deposits on the column internals and in particular of evaporators and condensers as well as a deterioration of the separation performance. The object of the invention is to provide an improved process for the preparation of butadiene by oxidative Deyhdrierung of n-butenes and subsequent workup of the C ₄ -Kohlenwasser- substances and by-products containing product gas stream, which helps to overcome the disadvantages described above. The object is achieved by a process for the preparation of butadiene from n-butenes with the following steps:

A) providing a n-butenes containing feed gas stream a; B) feeding the n-butenes containing feed gas stream a and an oxygen-containing gas into 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 dende secondary components, optionally carbon oxides and optionally inert gases is obtained;

Ca) cooling the product gas stream b by contacting it with a coolant and condensing at least part of the high-boiling secondary components;

Db) subsequent desorption of the C ₄ hydrocarbons from the loaded absorption medium stream in a desorption column, a C ₄ product gas stream d 1 being obtained, characterized in that the desorption column is taken off a side draw stream containing methacrolein. Surprisingly, it was found that methacrolein on the lower stages of the

Enriched desorption column to a relatively high concentration. By means of a gaseous or liquid side draw on one of the separation stages, the concentration of methacrolein in the regenerated absorbent returned to the absorption column can be significantly reduced. It has also been found that a liquid side draw-off stream is advantageous in order to minimize butadiene loss. The side draw stream can be thermally recovered or, if it contains a sufficiently high concentration of butadiene, also worked up and the butadiene recycled.

The methacrolein-containing side draw-off stream can be taken off in liquid form or in gaseous form. In general, it is taken from the desorption column in the lower third, based on the number of theoretical plates. In general, the side draw stream in addition to methacrolein still contains water, C ₄ hydrocarbons and the absorbent. From the gaseous side draw stream, butadiene can be washed out with absorbent in a downstream washing column and returned to the desorption column.

Preferably, steps E) and F) are carried out: E) separation of the C4 product stream d1 by extractive distillation with a solvent which is selective for butadiene 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 selective solvent-containing material stream f2 in a substantially consisting of the selective solvent stream gl and a butadiene-containing stream g2.

In general, in the cooling stage Ca), an aqueous coolant or an organic solvent is used.

Preferably, in the cooling stage Ca), an organic solvent is used. These generally have a much higher solvent power for the high-boiling by-products, which can lead to deposits and blockages in the parts of the system downstream of the ODH reactor, than water or alkaline-aqueous solutions. Preferred organic solvents used as coolants are aromatic hydrocarbons, for example toluene, o-xylene, m-xylene, p-xylene, diethylbenzenes, triethylbenzenes, diisopropylbenzenes, triisopropylbenzenes and mesitylene or mixtures thereof. Particularly preferred is mesitylene. The following embodiments are preferred or particularly preferred variants of the method according to the invention.

The stage Ca) is 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 solvent 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 in step Ca) as a cooling agent. 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 the stage Ca).

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). Preferably, step D) comprises the steps Da1), Da2) and Db):

Da1) absorption of the C4 hydrocarbons comprising butadiene and n-butenes in a high-boiling absorbent, wherein a loaded with C4 hydrocarbons absorbent stream and the gas stream d2 are obtained, Da2) removal of oxygen from the loaded with C4 hydrocarbons absorbent stream from step Da) by stripping with a non-condensable gas stream, and

Db) desorption of the C4 hydrocarbons from the loaded absorbent stream to obtain a C4 product gas stream d1 consisting essentially of C4 hydrocarbons and comprising less than 100 ppm oxygen. Preferably, the high-boiling absorbent used in step Da) is an aromatic hydrocarbon solvent, most preferably the aromatic hydrocarbon solvent used in step Ca) is mesitylene. Diethylbenzenes, trietylbenzenes, diisopropylbenzenes and triisopropylbenzenes can also be used. In one embodiment of the invention, the gas stream d2 contained in step Da) is recycled to at least 80%, preferably at least 90%, in step B). This can be useful if only a small purge of electricity has to be removed from the gas stream d2. Embodiments of the method according to the invention are illustrated in FIGS. 1 and 2 and will be described in detail below.

The feed gas stream used is n-butenes (1-butene and / or cis- / trans-2-butene) and isobutene-containing gas mixtures. Such a gas mixture can be obtained, for example, by non-oxidative dehydrogenation of n-butane. It is also possible to use a fraction containing n-butenes (1-butene and cis- / trans-2-butene) as the main component and obtained from the C4 fraction of naphtha cracking by separating butadiene and isobutene. 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 starting gas. Further, as the starting gas, n-butenes containing gas mixtures obtained by catalytic fluid cracking (FCC) can be used.

In one embodiment of the process according to the invention, the starting gas mixture containing n-butenes is obtained by non-oxidative dehydrogenation of n-butane. By the

Coupling a non-oxidative catalytic dehydrogenation with the oxidative dehydrogenation of the formed n-butenes, a high yield of butadiene, based on n-butane, 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 unreacted 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 hydrogenation 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 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 feed gas stream containing n-butenes and an oxygen-containing gas are fed into at least one dehydrogenation zone (the ODH reactor A) and the butenes contained in the gas mixture are oxidatively dehydrogenated to form butadiene in the presence of an oxydehydrogenation catalyst. In one embodiment, it is preferable to use an oxygen-containing gas containing more than 10% by volume, preferably more than 15% by volume and more preferably more than 20% by volume of molecular oxygen. In one embodiment, air is used as the oxygen-containing gas. The upper limit of the content of molecular oxygen in the oxygen-containing gas is then 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 in the oxygen-containing gas 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 in the oxygen-containing gas, it is generally 10% by volume or less, preferably 1% by volume or less.

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 further 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 Co4,5Ni2 _3, ₅ Po, 5KO, lox + S1O2), DE-A 25 30 959 ₍₃ Moi2BiFe Co4,5Ni2,5Cro, 5KO, iO _x,

Moi _3, 7 ₅ ₃ BiFe Co4,5Ni2,5Geo, 5KO, 80 _x, Moi2BiFe ₃ Co4,5Ni _2, 5Mno, 5KO, _x and OK

Moi2BiFe Co4 _3, ₅ Ni _2, 5Lao, 5KO, lox), U.S. 3,91 1, 039 ₍₃ Moi2BiFe Co4, ₅ Ni2,5Sno, ₅ Ko, lox), DE-A 25 30 959 and DE-A 24 47 825 (Moi2BiFe ₃ Co4, ₅ Ni2.5Wo, ₅ Ko, iOx).

Suitable multimetal oxides and their preparation are further described in US 4,423,281 (Moi2BiNi ₈ Pbo, ₅ Cr ₃ Ko, 20x and Moi2BibNi ₇ Al ₃ Cro, 5Ko, 50x), US 4,336,409 (Moi2BiNi ₆ Cd2Cr ₃ Po, ₅ Ox), DE-A 26 00 128

+ Si0 ₂ ) and DE-A 24 40 329

(Moi2BiCo4, ₅ Ni2,5Cr ₃ Po, 5KO, 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). X ¹ is preferably Si and / or Mn and X ² is preferably K, Na and / or Cs, particularly preferably X ² = K. Particular preference is given to a largely Cr (VI) -free catalyst.

For carrying out the oxidative dehydrogenation with high total 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 starting material gas can be mixed with oxygen or an oxygen-containing gas and optionally additional inert gas, methane or steam. The resulting oxygen-containing gas mixture is then fed to the oxydehydrogenation.

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 come z. As melting of salts or salt mixtures 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 the hearth furnace, in the fixed-bed tubular reactor or tube bundle reactor or in the 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 number of reaction tubes accommodated in the tube bundle reactor is generally at least 1000, or 3000, or 5000, preferably at least 10,000. Frequently, the number of reaction tubes accommodated in the tube bundle reactor is 15,000 to 30,000 or 40,000 or 50 000. The length of the reaction tubes normally extends 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, which is set up in the ODH reactor A, can consist of a single layer or of 2 or more layers. These layers may be pure catalyst or diluted with a material that does not react with the source gas or components of the product gas of the reaction. Furthermore, the catalyst layers may consist of solid material and / or supported shell catalysts.

The product gas stream 2 leaving the oxidative dehydrogenation contains, in addition to butadiene, generally 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 2 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. In this case, if two or more intended heat exchangers are arranged in parallel, thus allowing distributed cooling of the recovered product gas in the heat exchangers, the amount of high-boiling by-products deposited in the heat exchangers decreases and so 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 by other heat exchangers after a certain period of operation. In this method, the cooling can be continued, a portion of the heat of reaction recovered and in parallel, the deposited in one of the heat exchangers high-boiling by-products can be removed. As a coolant mentioned above, a solvent can be used as long as it is capable of dissolving the high-boiling by-products. Examples are aromatic hydrocarbon solvents such as toluene, xylenes, diethylbenzenes, trietylbenzenes, diisopropylbenzenes and triisopropylbenzenes. Particularly preferred is mesitylene. It is also possible to use aqueous solvents. These can be made both acidic and alkaline, such as an 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 2 by cooling and compression. Cooling is by contacting with a coolant. This stage is also referred to below as quench. This quench can consist of only one stage or of several stages (for example B, C in FIG. 1). The product gas stream 2 is thus brought into direct contact with the organic cooling medium 3b and 9b and thereby cooled. Suitable cooling media are aqueous coolants or organic solvents, preferably aromatic hydrocarbons, more preferably toluene, o-xylene, m-xylene, p-xylene or mesitylene, or mixtures thereof. Diethylbenzene, trietylbenzene, diisopropylbenzene and triisopropylbenzene can also be used.

Preference is given to a two-stage quench (comprising stages B and C according to FIG. 1, ie stage Ca) comprising two cooling stages Ca1) and Ca2)), in which the product gas stream 2 is brought into contact with the organic solvent. In general, the product gas 2, depending on the presence and temperature level of a heat exchanger before the quench B, a temperature of 100 to 440 ° C. The product gas is in the 1. Quenching stage B brought into contact with the cooling medium of organic solvent. 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 so that clogging is minimized by deposits in the area of the coolant inlet.

In general, the product gas 2 in the first quenching stage B 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 3b 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 B 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). If 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 B is designed as a cooling tower. The cooling medium 3b 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 3 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 4 with secondary components increases over time, a portion of the loaded cooling medium can be withdrawn from the circulation as Purgestrom 3a and the circulating amount to be kept constant by adding unladen cooling medium 6. 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.

The cooled and possibly depleted in secondary components product gas stream 4 can now be a second quenching C are supplied. In this he can now be brought into contact again with a cooling medium 9b.

In general, the product gas is cooled to 5 to 100 ° C, preferably 15 to 85 ° C and even more preferably 30 to 70 ° C, until the gas outlet of the second quenching stage C is reached. The coolant can be supplied in countercurrent to the product gas. In this case, the temperature of the coolant medium 9b 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 C 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 step is preferred designed as a cooling tower. The cooling medium 9b used in the cooling tower is frequently used in a circulating manner. The recycle stream of the cooling medium 9b 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 0.0001 to 1 l / g and particularly preferably from 0.002 to 0.2 l / g.

Depending on the temperature, pressure, coolant and water content of the product gas 4, it may come in the second quenching stage C for the condensation of water. In this case, an additional aqueous phase 8 may form, which may additionally contain water-soluble secondary components. This can then be subtracted in the phase separator D. The temperature of the cooling medium 9 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 9 with secondary components increases over time, a portion of the loaded cooling medium can be withdrawn as Purgestrom 9a from the circulation, and the circulating amount by the addition of unloaded cooling medium 10 are kept constant.

In order to achieve the best possible contact of product gas and cooling medium, internals in the second quenching stage C may be present. Such internals include, for example, bell, centrifugal and / or sieve trays, structured packing columns, e.g. B. Sheet metal packings with a specific surface area of 100 to 1000 m ² / m ³ as Mellapak® 250 Y, and packed columns.

The solvent circulations of the two quench stages can be both separated from each other and also connected to each other. For example, the current 9a can be supplied to the current 3b 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, wherein the solvent of the second stage Ca2) loaded with secondary components is passed into the first stage Ca1). The solvent taken from the second stage Ca2) contains less secondary components than the solvent 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, for example, further gas scrubbing.

There is obtained a gas stream 5, the n-butane, 1-butene, 2-butenes, butadiene, optionally oxygen, hydrogen, water vapor, in small quantities of methane, ethane, ethene, propane and propene, isobutane, carbon oxides, inert gases and Contains parts of the solvent used in the quench. Furthermore, 5 traces of high-boiling components can remain in this gas stream, which were not quantitatively separated in the quench. 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) by contacting with an organic solvent as the cooling agent.

The product gas stream 5 from the solvent quench is compressed in at least one compression stage E and subsequently cooled further in the cooling apparatus F, with at least one condensate stream 14 being formed. There remains a gas stream 12 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 5 can be carried out 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 is followed by a cooling step, in which the gas stream is cooled to a temperature in the range of 15 to 60 ° C. 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 and the solvent used in the quench. 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 current I 1 produced by compression of current 5 and / or to remove further secondary components from the current I 1, the condensed quench can be removed.

Cooled solvent in a heat exchanger and recycled as a refrigerant in the apparatus F. Since the loading of this cooling medium 13b with secondary components increases over time, a portion of the loaded cooling medium can be withdrawn from the circulation (13a) and the circulation rate of the cooling medium can be kept constant by adding unladen solvent (15).

The solvent 15 added as a cooling medium may be an aqueous coolant or an organic solvent. Preference is given to aromatic hydrocarbons, particular preference is given to toluene, o-xylene, m-xylene, p-xylene, diethylbenzene, triethylbenzene, diisopropylpolybenzene, triisopropylbenzene, mesitylene or mixtures thereof. Especially preferred is mesylene.

The condensate stream 13a may be recycled to the recycle stream 3b and / or 9b of the quench. As a result, the C4 components absorbed in the condensate stream 13a can be brought back 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: one outlet pressure) per compressor stage are between 1, 5 and 3.0 depending on the design. The cooling of the compressed gas takes place in flushed with coolant heat exchangers or organic quench, which can be performed, for example, as a tube bundle, spiral or plate heat exchanger. Suitable coolants may be aqueous or the above-mentioned organic solvents. As a coolant in the heat exchangers while cooling water or heat transfer oils are used. 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), optionally water vapor, optionally carbon oxides and optionally inert gases and optionally traces of secondary components containing gas stream 12 is as Output current fed to the further treatment.

In a step D) are non-condensable and low-boiling gas constituents, oxygen, low-boiling hydrocarbons (methane, ethane, ethene, propane, propene), carbon oxides and inert gases in an absorption column G as gas stream 16 from the process gas stream 12 by absorption of C4 hydrocarbons in a high-boiling absorbent (21 b and / or 26) and subsequent desorption of the C4 hydrocarbons separated. Preferably, step D), as shown in FIG. 1, comprises the steps Da1), Da2) and Db):

Da1) absorption of the C4 hydrocarbons comprising butadiene and n-butenes in a high-boiling absorbent (21 b and / or 26), wherein a loaded with C4 hydrocarbons absorbent stream and the gas stream 16 are obtained

Da2) removal of oxygen from the C4 hydrocarbons-laden absorbent stream from step Da1) by stripping with a non-condensable gas stream 18, wherein a C4 hydrocarbons laden absorbent stream 17 is obtained, and

Db) Desorption of the C4 hydrocarbons from the loaded absorbent stream, whereby a C4 product gas stream 27 is obtained, which consists essentially of C4 hydrocarbons. For this purpose, in the absorption stage G, the gas stream 12 is brought into contact with an absorbent and the C4 hydrocarbons are absorbed in the absorbent, whereby an adsorbent loaded with C4 hydrocarbons and an exhaust gas 16 containing the other gas constituents are obtained. In a desorption stage H, the C4 hydrocarbons are released from the high-boiling absorbent again.

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 z. B. tray columns with bell, centrifugal and / or sieve plates, columns with structured packings, eg. B. Sheet metal packings with a specific surface area of 100 to 1000 m ² / m ³ 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.

In one embodiment, an absorption column in the lower region of the butadiene, n-butenes and the low-boiling and non-condensable gas components containing gas stream 12 is supplied. In the upper part of the absorption column, the high-boiling absorbent (21 b and / or 26) is abandoned.

Inert absorbent used in the absorption stage are generally high-boiling non-polar solvents in which the C4-hydrocarbon mixture to be separated has a significantly higher solubility than the other gas constituents to be separated off. Suitable absorbents are comparatively nonpolar organic solvents, for example aliphatic Ce to Cis alkanes, or aromatic hydrocarbons, such as the paraffin distillation from the paraffin distillation, toluene or ethers with bulky groups, or mixtures of these solvents, these being a polar solvent such as 1, 2-dimethylphthalate may be added. Suitable absorbents are also esters of benzoic acid and phthalic acid with straight-chain C 1 to C 1 alkanols, as well as so-called heat transfer oils, such as biphenyl and diphenyl ether, their chlorinated derivatives and triaryl alkenes. A suitable absorbent is a mixture of biphenyl and diphenyl ether, preferably in the azeotropic composition, for example, the commercially available Diphyl ^®. Frequently, this solvent mixture contains dimethyl phthalate in an amount of 0.1 to 25 wt .-%.

In a preferred embodiment in the absorption stage Da1) the same solvent as in the cooling stage Ca) is used.

Preferred absorbents are solvents which have a solubility for organic peroxides of at least 1000 ppm (mg active oxygen / kg solvent). Preference is given to aromatic hydrocarbons, particularly preferably toluene, o-xylene, p-xylene and mesitylene, or mixtures thereof. Diethylbenzene, triethylbenzene, diisopropylbenzene and triisopropylbenzene can also be used.

At the top of the absorption column G, a stream 16 is withdrawn, which is essentially oxygen, low-boiling hydrocarbons (methane, ethane, ethene, propane, propene), optionally C4-hydrocarbons (butane, butenes, butadiene), optionally inert gases, optionally carbon oxides and optionally still contains water vapor. This stream can be partially fed to the ODH reactor. Thus, for example, the inlet flow of the ODH reactor can be adjusted to the desired C4 hydrocarbon content. At the bottom of the absorption column 18 residues of oxygen dissolved in the absorbent are discharged in a further column by rinsing with a gas. The remaining oxygen content should be so small that the stream 27 leaving the desorption column and containing butane, butene and butadiene only contains a maximum of 100 ppm of oxygen.

The stripping out of the oxygen in step Db) 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 not or only weakly in the absorbent stream 21 b and / or 26 absorbable gases such as methane, through the loaded absorption solution. With stripped C4 hydrocarbons are washed in the upper part of the column G 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, this direct coupling can take place. Suitable Stippkolonnen are z. B. tray columns with bell, centrifugal and / or sieve tray, columns with structured packings, eg. B. Sheet metal packings with a specific surface area of 100 to 1000 m ² / m ³ 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 absorbent regenerated in the desorption stage is withdrawn as stream 20 together with the condensed water from the desorption column H. This biphasic mixture can be cooled in a heat exchanger and separated as stream 21 in a decanter I into an aqueous 21 a and an absorbent stream 21 b. The absorbent stream 21 b will again be supplied to the absorber column G, while the aqueous stream 21 a is vaporized in an evaporator and thus the stream 23 is generated. Additionally or alternatively, additional water (stream 24) can be evaporated in the evaporator. Further, only a portion of the stream 21 a can be evaporated and the unvaporized portion can be removed as stream 22.

Low boilers in the process gas stream such as ethane or propane and high-boiling components such as benzaldehyde, maleic anhydride and phthalic anhydride can accumulate in the absorption medium cycle stream. In order to limit the enrichment, a purge stream 25 can be withdrawn.

The C4 product gas stream 27 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.

Part of the condensed head effluent of the desorption column containing mainly C4 hydrocarbons is recycled as stream 30 to the column top to increase the separation efficiency of the column. The desorption step can be carried out in any suitable desorption column known to the person skilled in the art. The desorption can be effected by lowering the pressure and / or heating the desorption stage. The desorption stage may be heated by feeding a hot medium - such as steam - or by generating its own vapor by bottom evaporation.

Suitable desorption columns are, for example, tray columns with bells, centrifugal and / or sieve trays, 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. According to the invention, as shown in FIG. 1, the desorption column H is taken from a side draw stream 31 containing methacrolein in order to prevent accumulation of acrolein in the absorbent cycle. The side draw stream 31 can be both liquid and gaseous, preferably it is gaseous.

The desorption column preferably has from 5 to 30, particularly preferably from 10 to 20, theoretical plates. The side draw stream 31 is preferably removed in the lower third of the desorption column. The liquid side draw stream 31 generally contains from 0.1 to 2% by weight of methacrolein. In addition, it contains 5 to 15 wt .-% water, 0 to 3 wt .-% C ₄ -hydrocarbons and 70 to 90 wt .-% of the absorbent.

The gaseous side draw stream 31 generally contains from 1 to 10% by weight of methacrolein. In addition, it contains 30 to 60 wt .-% water, 0 to 6 wt .-% C ₄ hydrocarbons and 30 to 60 wt .-% of the absorbent.

In the preferred embodiment shown in FIG. 2, the gaseous side draw stream 31 is scrubbed in a scrubbing column H ^' with a stream 33 of the absorbent to obtain an absorbent stream 34 laden with C ₄ hydrocarbons which is recycled to the desorption column H. A stream 32 is withdrawn overhead and mainly contains the secondary components to be separated, such as metacrolein and small amounts of absorbent and water as stream 31.

The liquid (stream 28) or gaseous (stream 29) C ₄ product streams leaving the condenser are then converted into a butadiene and the selective solvent-containing material stream 31 and a butane by extractive distillation in step E) using a solvent selective for butadiene and n-butenes containing stream 32 separated.

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 C ₄ - product gas stream with an extractant, preferably an N-methylpyrrolidone

(NMP) / water mixture, contacted in an extraction zone. The extraction zone is generally carried out in the form of a wash column which contains trays, fillers or packings as internals. This generally has 30 to 70 theoretical plates on, 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 a 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-butenes and, if appropriate, methane can be recycled completely or partially or else not into 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 dehydrogenated oxidatively to butadiene more slowly than 1-butene, this recycle stream can be catalytically isomerized prior to being fed to the ODH reactor. Thereby, the isomer distribution can be adjusted according to the isomer distribution present in the thermodynamic equilibrium. Furthermore, the stream can also be fed to a further work-up to separate butanes and butenes from one another and to return the butenes in whole or in part to the oxydehydrogenation. The stream can also go into maleic anhydride production. 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 extracted is transferred to a desorption zone, wherein the butadiene is desorbed again from the extraction solution and backwashed. 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 serves to recover the extractant contained in the gas phase by means of a liquid hydrocarbon reflux, for which purpose 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 of 100 to 300 ° C, in particular in the range of 150 to 200 ° C and a top temperature in the range of 0 to 70 ° C, in particular in the range of 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 38 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.

Examples

Comparative example

In the comparative example without side draw stream 31, methacrolein (MAC) accumulates up to 1.08% by weight in the coolant circulation.

The results of the simulation calculation are summarized in Table 1. Table 1

Continuation Table 1

example 1

By a gaseous side take-off of Figure 1 on 1. theoretical bottom of the desorption column is achieved a reduction of the MAC concentration to 0.5 wt .-% in the circulation solvent mesitylene. The results of the simulation calculation are summarized in Table 2.

Table 2

Continuation Table 2

Example 2

By means of a liquid side draw according to FIG. 1 on the first theoretical bottom of the desorption column, a reduction of the MAC concentration to 0.5% by weight in the circulation solvent mesitylene is achieved. The results of the simulation calculation are summarized in Table 3.

Table 3

Continuation Table 3

Claims

claims

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

A) providing a n-butenes containing feed gas stream a;

B) feeding the n-butenes containing feed gas stream a and an oxygen-containing gas into 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;

Da) separation of non-condensable and low-boiling gas constituents comprising oxygen, low-boiling hydrocarbons, optionally carbon oxides and optionally inert gases as gas stream d2 from the gas stream c2 by absorption of the C4 hydrocarbons comprising butadiene and n-butenes in an absorbent, wherein one with C4 Hydrocarbons laden absorbent stream and the

Gas stream d2 can be obtained, and

Db) subsequent desorption of the C4 hydrocarbons from the loaded absorbent stream in a desorption column, wherein a C4 product gas stream d1 is obtained, characterized in that the desorption column a methacrolein-containing side draw stream is removed.

2. The method of claim 1 with the additional steps:

E) separation of the C4 product gas stream d1 by extractive distillation with a solvent which is selective for butadiene 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 selective solvent-containing material stream f2 in a substantially consisting of the selective solvent stream gl and a butadiene-containing stream g2.

Process according to Claims 1 or 2, characterized in that the side draw-off stream is taken off in the lower third of the desorption column.

Method according to one of claims 1 to 3, characterized in that the desorption column, a liquid side draw stream is removed.

Method according to one of claims 1 to 3, characterized in that the desorption column, a gaseous side draw stream is removed.

A process according to claim 4 or 5, characterized in that the side draw stream is scrubbed in a scrubbing column with a stream of the absorbent to obtain an absorbent stream laden with C4 hydrocarbons which is recycled to the desorption column.

Method according to one of claims 1 to 6, characterized in that the oxygen-containing gas fed in step B) contains at least 90% by volume of oxygen.

Method according to one of claims 1 to 7, characterized in that in step Da) separated gas stream d2 is at least partially recycled in step B).

Method according to one of Claims 1 to 8, characterized in that the step Da) comprises the steps Da1), Da2) and Db):

Da1) absorption of the C4 hydrocarbons comprising butadiene and n-butenes in a high-boiling absorbent, wherein a loaded with C4 hydrocarbons absorbent stream and the gas stream d2 are obtained

Da2) removal of oxygen from the loaded with C4 hydrocarbons absorbent stream from step Da) by stripping with a non-condensable gas stream, and

Db) desorbing the C4 hydrocarbons from the loaded absorbent stream to yield a C4 product gas stream d1 comprising less than 100 ppm oxygen.

0. The method according to any one of claims 1 to 9, characterized in that in

Step Da) used absorbent is an aromatic hydrocarbon solvent.