EP3022169A1 - Method for producing 1,3-butadien from n-butenes by means of an oxidative dehydrogenation - Google Patents

Abstract

The invention relates to a method for producing butadien from n-butenes, having the following steps: A) providing an n-butene-containing feed gas flow a; B) supplying the n-butene-containing feed gas flow a and an oxygen-containing gas to at least one oxidative dehydrogenation zone and carrying out an oxidative dehydrogenation of n-butenes into butadiene, a product gas flow b being obtained containing butadiene, non-converted n-butenes, steam, oxygen, low-boiling point hydrocarbons, high-boiling point auxiliary components, optionally carbon oxides, and optionally inert gases; Ca) cooling the product gas flow b by brining the product gas flow into contact with a coolant and condensing at least some of the high-boiling point auxiliary components; Cb) compressing the remaining product gas flow b in at least one compression stage, wherein at least one aqueous condensate flow c1 and a gas flow c2 containing butadiene, n-butenes, steam, oxygen, low-boiling point hydrocarbons, optionally carbon oxide, and optionally inert gases are obtained; Da) separating non-condensable and low-boiling point gas components comprising oxygen, low-boiling point hydrocarbons, optionally carbon oxide, and optionally inert gases as a gas flow d2 from the gas flow c2 by absorbing the C₄ hydrocarbons comprising butadiene and n-butenes in an absorption agent, an absorption agent flow loaded with C₄ hydrocarbons and the gas flow d2 being obtained, and Db) subsequently desorbing the C₄ hydrocarbons from the loaded absorption agent flow, a C₄ product gas flow d1 being obtained. The invention is characterized in that a methane-containing gas flow is additionally supplied at at least one point in the method section comprising the steps B), Ca), Cb), and Da) in such quantities that the formation of an explosive gas mixture in step Da) is prevented.

Description

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

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 may be used which as

Main component n-butenes (1-butene and / or 2-butene) and was obtained from the C ₄ fraction of a naphtha cracker 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 starting gas. Further, as the starting gas, n-butenes containing gas mixtures obtained by catalytic fluid cracking (FCC) can be used.

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

US 2012/0130137 A1, for example, describes such a process using catalysts which contain 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 feed or complete oxygen conversion in the oxydehydrogenation reactor (ODH reactor). In the US 2012/0130137 A1, for example, an oxygen content of 2.5 to 8 vol .-% im

Starting gas described. The problem of the formation of any explosive mixtures after the reaction step is discussed in paragraph [0017]. In particular, it is pointed out that in a so-called "rich" mode of operation above the upper explosion limit in the reaction part, the problem is that after absorption of a large part of the organic constituents in the workup, the gas composition crosses the explosive region in a transition from rich to lean gas mixture It is also pointed out that the risk of catalyst deactivation by coking exists in the reaction section during continuous lean operation, but US 2012/0130137 A1 does not provide a solution to this problem. [0106] mentions, incidentally, how the occurrence is more explosive Atmospheres in the absorption step can be avoided by, for example, diluting the gas stream with nitrogen prior to the absorption step. [0091] In the more detailed description of the absorption step in paragraphs [0132] ff, attention is given to the problem of formation

explosive gas mixtures not further discussed.

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 down. 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 high oxygen concentrations in addition to hydrocarbons such as butane, butene and butadiene or the organic absorbents used in the work-up part is fraught with risks. So can form explosive gas mixtures. If you only work with a small distance to the potentially explosive area, it is not always technically possible to prevent this area from entering due to fluctuations in the process parameters.

US 4504692 (Japan Synthetic Rubber Co. Ltd.) describes a process for the preparation of butadiene, in particular with the separation of isobutene. However, the problem of the formation or the avoidance of explosive gas mixtures is not mentioned.

EP 2 177 266 A2 describes a catalyst based on Bi / Mo / Fe oxides and an associated process for the oxidative dehydrogenation of butenes to butadiene. However, the problem of the formation or the avoidance of explosive gas mixtures is not mentioned.

KR 2013036467 A describes a process for the oxidative dehydrogenation of butene

Butadiene, in particular while avoiding fouling in the downstream area. The

Problem of education or the avoidance of explosive gas mixtures is not mentioned. No. 7,034,195 B2 describes suitable catalysts for the oxidative dehydrogenation of butenes to butadiene. The presence of low-boiling components such as methane in amounts of <5% by volume is also described. The problem of the formation or avoidance of explosive mixtures is, however, excluded.

No. 8,003,840 B2 also describes a catalyst based on Bi / Mo / Fe oxides and a process for the oxidative dehydrogenation of butenes to butadiene. The problem of the formation or avoidance of explosive mixtures is, however, excluded. A disadvantage of the methods of the prior art is thus the passage through a range of explosive gas mixtures in the workup of the oxygen-containing

Product gas mixture, if the composition of the oxygen-containing, C ₄ - hydrocarbons containing product gas mixture in the absorption of C ₄ - hydrocarbons from one (not explosive) "fat" to a (not

explosive) "lean" mixture changes.

The present invention overcomes the aforementioned drawbacks by feeding a separate, methane-containing gas stream into the process at one or more points so that a non-explosive, hydrocarbon-rich "product mixture is always used in the work-up of the oxygen-containing C ₄ -hydrocarbon-containing product gas mixture. In particular, the nitrogen used as diluent gas or contained in air is partially or completely replaced by methane.

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

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

B) feed of the n-butenes containing feed gas stream a and a

oxygen-containing gas in at least one oxidative dehydrogenation zone and oxidative

 Dehydrogenation of n-butenes to butadiene, whereby 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 the product gas stream b by contacting it with a coolant and condensing at least part of the high-boiling secondary components;

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

Compression stage, wherein 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, one with C4 Hydrocarbons laden absorbent stream and the gas stream d2 are obtained, and

Db) subsequent desorption of the C ₄ hydrocarbons from the loaded

Absorbent stream, wherein a C ₄ -Produktgasstrom d1 is obtained, characterized in that at least one point of the process section with the steps B), Ca), Cb) and Da) additionally a methane-containing gas stream is fed in such amounts that the formation an explosive gas mixture in step Da) is avoided.

Preferably, the steps E) and F) are subsequently carried out:

E) separation of the C ₄ product stream d1 by extractive distillation with a butadiene-selective solvent in a butadiene and the selective solvent-containing

Stream e1 and n-butenes containing stream e2;

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 a preferred variant, the methane-containing gas stream is fed in step B). In a further preferred variant, the methane-containing gas stream is fed in step Da). The methane-containing gas stream may also be fed between steps B) and Da). It can also be several methane-containing gas streams

be fed to different parts of the process.

By feeding one or more separate methane-containing gas streams at one or more points of the method section comprising steps B), Ca), Cb) and Da), ie by feeding into these steps or between these steps, it is ensured that in the separation step Da) always a sufficiently "fat", so

Hydrocarbon-rich gas mixture is present in order to avoid the formation of an explosive gas mixture during the absorption step safely. Preferably, a distance of at least 2 vol .-% oxygen is maintained to the explosive area. The explosive region is specific for the components present in the mixture and can be taken from databases or through experiments with the mixture different compositions are determined. These experimental methods are known to the person skilled in the art.

In general, sufficient methane is fed in that the methane content of the gas stream d2 separated off in step Da) is at least 15% by volume. Preferably, sufficient methane is fed in that the methane content of the gas stream d2 obtained in step Da) is at least 20% by volume.

In general, the methane-containing gas stream d2 separated off in step Da) is at least partially recycled to step B). The proportion of the recycled part in the total flow is based on the proportion of nitrogen, which is replaced by methane.

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. In a further embodiment, it is preferable to replace as much nitrogen as possible with methane and to return the stream d2 as completely as possible. It is then particularly preferred to use as pure oxygen as the oxygen-containing gas.

In one embodiment, therefore, methane is used instead of nitrogen as a diluent gas. It is preferred, then as little nitrogen as possible in the process

feed. The nitrogen content of the gas stream d2 obtained in step Da) is then preferably at most 10% by volume. Preferably, a gas stream which is as oxygen-rich as possible is then fed in step B) of the process. Preferably, an oxygen-containing gas having an oxygen content of at least 90 vol .-% is fed. Particularly preferred is technically pure oxygen is fed with a content of at least 95% oxygen.

The COx content in the gas stream d2 can be reduced by means of a wash before the gas stream d2 is returned to stage B). Such a wash is particularly preferred when all of the nitrogen is substituted by methane and most of the gas stream is d2

is returned. Preferably, the laundry is done in amine-powered scrubbers to remove CO2. 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.

Generally, in the cooling step 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 solubility for the high-boiling ones

 By-products, which can lead to deposits and blockages in the downstream of the ODH reactor plant, as water or alkaline aqueous solutions.

Preferred organic solvents used as coolants are aromatic ones

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, at least a part of the

Solvent after passing through the second stage Ca2) as a cooling agent of the first stage Ca1) supplied.

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 a part of this cooling agent is after

Passing through the at least one cooling stage Cbb) supplied 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 one

high-boiling absorbent, wherein a loaded with C4 hydrocarbons absorbent stream and the gas stream d2 are obtained Da2) Removal of oxygen from the C4 hydrocarbons laden

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

Db) desorption of the C ₄ hydrocarbons from the loaded absorbent stream to obtain a C ₄ product gas stream d1 consisting essentially of C ₄ hydrocarbons and comprising less than 100 ppm oxygen. Preferably, the high-boiling absorbent used in step Da) is an aromatic hydrocarbon solvent, more preferably that used in step Ca)

aromatic hydrocarbon solvents, in particular mesitylene. It is also possible to use diethylbenzenes, trietylbenzenes, diisopropylbenzenes and triisopropylbenzenes. Embodiments of the process according to the invention are shown in FIGS. 1 and 2 and are described in detail below.

As feed gas stream pure n-butenes (1-butene and / or cis- / trans-2-butene), but also containing butene gas mixtures can be used. 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 constituent and obtained from the C ₄ fraction of naphtha cracking by separating butadiene and isobutene , In addition, gas mixtures may also be used as starting gas comprising pure 1-butene, cis-2-butene, trans-2-butene or mixtures thereof obtained by dimerization of ethylene. Furthermore, as

Starting gas n-butenes containing gas mixtures, which were obtained by catalytic fluid cracking (FCC).

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 is a

Gas mixture containing addition butadiene 1-butene, 2-butene and unreacted n-butane secondary constituents. Usual minor components are hydrogen, water vapor,

Nitrogen, CO and CO2, methane, ethane, ethene, propane and propene. The composition of the gaseous mixture leaving the first dehydrogenation zone can vary widely depending on the mode of 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 n-butenes-containing feed gas stream and an oxygen-containing gas are fed into at least one dehydrogenation zone (the ODH reactor A) and those in the

Gas mixture containing butenes in the presence of an oxydehydrogenation catalyst oxidatively dehydrogenated to butadiene.

The methane content of the gas mixture reacted in step B) is generally at least 10% by volume, preferably at least 20% by volume, when a methane-containing gas stream is fed to step B). It is generally not more than 90% by volume.

A methane-containing gas stream can be fed in as methane fresh gas stream and / or methane-containing recycle stream d2 in step B).

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 Co4,5Ni2 _{3, 5} Po, 5KO, lox + Si0 _2), 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 Co4.5Ni2.5Wo, 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 (Moi2BiNi ₀ , 5Cr ₃ Po, 5Mg7, ₅ Ko, iOx + Si0 ₂ ) and DE-A 24 40 329

(Moi2BiCo4.5Ni ₂ , 5Cr ₃ Po, ₅ Ko, iOx).

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 with oxygen or an oxygen-containing gas and optionally additional inert gas, methane or

Water vapor are mixed. The resulting oxygen-containing gas mixture is then fed to the oxydehydrogenation.

The reaction temperature of the oxydehydrogenation is generally by a

Heat exchange agent, which is located around the reaction tubes, controlled. As such liquid heat exchange agents come z. B. melting 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 reaction

Heat exchange agent and it forms a so-called hotspot. The location and height of the hot spot 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 become. The difference between hotspot temperature and the temperature of the

Heat exchange agent is generally between 1 -150 ° C, preferably between 10-100 ° C and more preferably between 20-80 ° C. The temperature at the end of the

Catalyst bed is generally between 0-100 ° C, preferably between 0.1 -50 ° C, more preferably between 1 -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 in fixed bed tubular reactors or

Fixed bed tube bundle reactors performed. 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 im

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 extends in

Normally 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 consist of pure catalyst or be diluted with a material that is not compatible with the

 Starting gas or components from the product gas of the reaction reacts. 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 also 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 to a

Temperature of 150-400 ° C, preferably 160-300 ° C, more preferably 170-250 ° C. brought. 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 with this system the temperature of the

Product gas can be kept at the desired level. As an example of one

Heat exchanger can be spiral heat exchangers, plate heat exchangers,

 Double tube heat exchangers, multi-tube heat exchangers, boiler spiral heat exchangers, shell and 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 after a certain period of operation of other heat exchangers. 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. B.

Toluene, xylenes, diethylbenzenes, triethylbenzenes, 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 are 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 ones

Hydrocarbons, particularly preferably toluene, o-xylene, m-xylene, p-xylene or mesitylene, or mixtures thereof. It is also possible to use diethylbenzene, trietylbenzene,

Diisopropylbenzene and triisopropylbenzene. Preference is given to a two-stage quench (comprising stages B and C according to FIG. 1), ie stage Ca) comprises 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 has 2, depending on the presence and temperature level of a

Heat exchanger before quench B, a temperature of 100-440 ° 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 by deposits in the area of the

Coolant inlet is minimized.

In general, the product gas 2 in the first quenching stage B is cooled to 5-180 ° C, preferably to 30-130 ° C and even more preferably to 60-1 10 ° C. The temperature of the

Coolant medium 3b at the inlet may generally be 25-200 ° C, preferably 40-120 ° C, most preferably 50-90 ° C. The pressure in the first quenching step 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, it can easily be used to polymerize high-boiling

By-products and deposits of solids by high-boiling

By-products caused in this process section come. 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 stream of the cooling medium in liters per hour, based on the mass flow of butadiene in grams per hour, can generally be 0.0001 -5 l / g, preferably 0.001-1 l / g and particularly preferably 0.002-0.2 l / g be.

The temperature of the cooling medium 3 in the bottom can generally be 27-210 ° C., preferably 45-130 ° C., particularly preferably 55-95 ° 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 15-85 ° C and even more preferably 30-70 ° C, to the gas outlet of the second quenching stage C. 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 5-100 ° C, preferably 15-85 ° C, particularly preferably 30-70 ° C. The pressure in the second quenching stage C is not particularly limited, but is generally 0.01 -4 bar (ü), preferably 0.1 -2 bar (g) and more preferably 0.2-1 bar (g). The second quenching stage 7 is preferably 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 0.0001 -5 l / g, preferably 0.3001-1 l / g and more preferably 0.002-0.2 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 20-210.degree. C., preferably 35-120.degree. C., particularly preferably 45-85.degree. Since the loading of the cooling medium 9 with

As components increase over time, a portion of the loaded cooling medium can be withdrawn from the circulation as Purgestrom 9a, 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 m2 / m3 such 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 second stage Ca2) removed

Solvent 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 by further structural measures, such as

For example, further gas washes are removed from the product gas.

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, iso-butane, carbon oxides, inert gases and parts of the quench used

Contains solvent. 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 at high-boiling

 Secondary components is depleted, 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 a cooling agent. The product gas stream 5 from the solvent quench is in at least one

 Compression stage E compressed and subsequently cooled further in the cooling apparatus F, wherein at least one condensate stream 14 is 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 can

Product gas stream still contains 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 solvent can be cooled in a heat exchanger and recycled as a coolant into 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 13b, which is added as a cooling medium, may be an aqueous

Coolant or an organic solvent. Preferred are aromatic

Hydrocarbons, particularly preferred are toluene, o-xylene, m-xylene, p-xylene, diethylbenzene, triethylbenzene, diisopropylbenzene, triisopropylbenzene, mesitylene or mixtures thereof.

Particularly preferred is mesitylene. The condensate stream 13a may be recycled to the recycle stream 3b and / or 9b of the quench. As a result, the C ₄ components absorbed in the condensate stream 13 a 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:

Inlet 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 come

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, iso-butane), optionally water vapor, if appropriate

Carbon oxides and optionally inert gases and optionally traces of

Complementary gas stream containing 12 is the output current of the other

Treatment supplied. According to the invention, methane can be additionally added to the gas stream 12 to ensure that in the column G always a not

explosive, hydrocarbon-rich "rich" gas mixture is present.

In a step D), non-condensable and low-boiling gas components comprising 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 C ₄ Hydrocarbons in a high-boiling absorbent (20b and / or 23) and subsequent desorption of C ₄ hydrocarbons separated. Preferably, step D), as shown in FIG. 1, comprises the steps Da1), Da2) and Db):

Da1) absorption of C ₄ hydrocarbons comprising butadiene and n-butenes in one

high-boiling absorbent (21 b and / or 26), wherein a loaded with C ₄ - hydrocarbons absorbent stream and the gas stream 16 are obtained,

Da2) Removal of oxygen from the charged with C ₄ hydrocarbons

Absorbent stream from step Da) by stripping with a non-condensable gas stream 18, wherein a loaded with C ₄ hydrocarbons 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 contacted with an inert absorbent and the C4 hydrocarbons are absorbed in the inert absorbent to obtain an absorbent laden with C4 hydrocarbons and an exhaust gas 16 containing the remaining gas constituents. In a desorption stage, the C4 hydrocarbons are released from the high-boiling absorbent again.

It is essential to the invention that during the absorption stage G and at each point of the

Absorption column is always a non-explosive, hydrocarbon-rich rich gas mixture. In general, the oxygen content of the gas mixture during the absorption stage G is in the range of 3 to 10% by volume. The hydrocarbon content (C4 hydrocarbons + methane + low-boiling secondary constituents + vapor

Absorbent) is generally in the range of 15 to 97% by volume.

The absorption step may be in any suitable manner known to those skilled in the art

Absorption column can be performed. 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 cocurrent, countercurrent or cross flow. Preferably, the absorption is carried out in countercurrent. Suitable absorption columns are z. B. tray columns with bell, centrifugal and / or sieve tray, columns with structured packings, eg. B. sheet metal packaging with a specific

Surface of 100 to 1000 m ² / m ³ as Mellapak® 250 Y, and packed columns. However, there are also trickle and spray towers, graphite block absorbers, surface absorbers such as thick film and thin-layer absorbers and rotary columns, dishwashers, cross-flow scrubbers and rotary scrubbers into consideration.

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 non-polar organic solvents, for example aliphatic C 1 to C 6 alkanes, or aromatic hydrocarbons, such as the paraffin distillation, toluene or ethers with bulky groups, or mixtures of these solvents, these being a polar solvent such as 1, 2 Dimethyl phthalate may be added. Suitable absorbents are also esters of benzoic acid and phthalic acid with straight-chain d-Cs-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) becomes the same

Solvent as used in the cooling stage Ca).

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

Material flow 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. Further, according to the invention, a large part of the methane can be recycled via this recycling. Then the methane used is not completely burned but is at least partially reused.

The methane content of stream 16 is generally at least 15% by volume, preferably at least 20% by volume. In general, the methane content is at most 95% by volume. The stream 16 is divided and fed as stream 16b into the reactor A. The remaining partial flow 16a can be used thermally or materially, for example in a synthesis gas plant.

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 contains only 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 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 Absorbing solution done. 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 achieved by a piping of the

Stripper column as well as a direct assembly of the stripping column below the

Absorber column carried out. 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 bottom, columns with structured packing, 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.

In one embodiment of the method according to the invention stripping is carried out in step Db) with a methane-containing gas stream. In particular, this gas stream (stripping gas) contains> 90% by volume of methane.

The laden with C4 hydrocarbons absorbent stream 17 can in a

Heat exchangers are heated and then passed as stream 19 in a desorption column H. In a process variant, the desorption step Db) by relaxing and stripping the loaded absorbent by a steam flow 23rd

carried out.

The absorbent 20 regenerated in the desorption stage can be cooled in a heat exchanger. The cooled stream 21 contains water in addition to the absorbent, which is separated in the phase separator I.

In the process gas stream low boilers such as ethane or propane and high-boiling components such as benzaldehyde, maleic anhydride and

Phthalic anhydride can accumulate in the absorbent cycle stream. To limit the enrichment, a purge stream 25 can be deducted. This can be used alone or combined with the streams 3a and / or 9a and / or 13a in a distillation column T (FIG. 2) according to the prior art in low boilers 36, regenerated absorbent 26 and / or regenerated cooling medium 10 and / or 15 (FIG and 2) and high boilers 37 are separated. Preferably, it will be fed directly to the streams 15 and / or 10 and / or 6 to backwash the C ₄ hydrocarbons still dissolved in the streams 25 and / or 3a and / or 9a and / or 13a into the process gas stream. When electricity is 25 in the

Distillation column T is separated, the streams 36 and 37, for example, burned and thus be used energetically. Of course, the streams 13a and / or 9a and / or 3a can also be purified without the stream 25 in a distillation column T and returned to the process. The circulation amount of the absorbent stream 21 b can be kept constant by adding unladen solvent 26. The absorbent stream 21 b can be returned to the absorption stage G. Since the loading of the water flow 21 a with secondary components increases over time, this can be partially evaporated and the circulation rate of the water flow by adding unladen water 24 are kept constant.

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, 0 to 50% by volume of 2-butenes and 0 to 10% by volume of methane, the total amount being 100% by volume. Furthermore, small amounts of iso-butane may be included.

A portion of the condensed, mainly C4 hydrocarbons headspace of the desorption column is recycled as stream 25 in the column head to increase the separation efficiency of the column.

The liquid (stream 30) or gaseous (stream 29) C4 product streams leaving the condenser are then removed by extractive distillation in step E) with a butadiene-selective solvent into a butadiene and the material flow 31 containing the selective solvent and a butane and n-butane. Buteneene 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 C4 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

 Separation stages, 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 of 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 , in particular 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 withdrawn 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 and, if appropriate, methane can be supplied in whole or in part or else not in 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 slower to butadiene 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 falls an extractant and

optionally water-containing stream, wherein the composition of the

Extractant and water-containing material stream of the composition corresponds to how 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 so deducted

Extracted extraction solution in a desorption zone, wherein the butadiene is desorbed again from the extraction solution and backwashed. The desorption zone can be designed, for example, in the form of a wash column which contains from 2 to 30, preferably from 5 to 20, theoretical stages and, if appropriate, 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 of 100 to 300 ° C, in particular in the range of 150 to 200 ° C and a

Head temperature in the range of 0 to 70 ° C, in particular carried out 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 34 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 The example describes the normal operating state with 6 vol.% Oxygen at the end of the ODH reactor without the addition of methane.

It is shown in FIG. 3 that, during these process conditions, during the C4 absorption the explosive area is just touched. The numbers represent the composition of the currents 2, 4, 5, 12 and 16 in the diagram. In the absorber column, only a distance of 0.8% by volume of oxygen to the LOC (lowest oxygen concentration, the lowest oxygen concentration at which an explosion can still occur) can be maintained. This can be formed in fluctuations in the process, for example, caused by faulty or failing valves or flow meter, explosive gas mixtures and not respond quickly enough. An explosion can therefore not be completely ruled out, and thus the relevant oxygen-carrying apparatus must be designed shockproof. Table 1: Data Simulation Comparative Example

 Power number 2 4 5 12 16

Temperature ° C 380 88 53 40 35

Pressure bar 1.5 1.4 1.35 10.042 10

Flow rate kmol / h 4739 5044 4751 4213 3823

PROPANE mole.% 0.00 0.00 0.00 0.00 0.00

PROPOS Mol. -% 0.00 0.00 0.00 0.00 0.00

BUTAN Mol. -% 1.38 1.32 1.41 1.55 0.02 L-BUTANE mole. -% 0.00 0.00 0.00 0.00 0.00 L-BUTENES Mol. -% 0 , 12 0.12 0.13 0.14 0.00

1-BUT mol. -% 0.00 0.00 0.00 0.00 0.00

C-2 BUTEN mol.% 0.54 0.52 0.56 0.61 0.01

T-2 BUTEN mol.% 0.76 0.72 0.77 0.85 0.01

1.3-BUTADIENE Mol. -% 6,32 6,01 6,44 7,10 0,04

H2O mol.% 11.73 11.07 10.58 0.73 0.52

ETHEN Mol. -% 0.02 0.02 0.02 0.02 0.02 L-PENTAN Mol. -% 0.00 0.00 0.00 0.00 0.00

Mesitylene Mol. -% 0.03 5.92 1.20 0.06 0.06

CO2 mol.% 1.54 1.45 1.54 1.73 1.59

H2 mol.% 0.01 0.01 0.01 0.01 0.01

N2 mol.% 70.73 66.46 70.56 79.55 89.30

02 mol.% 6.00 5.64 5.99 6.75 7.43

CO mole.% 0.79 0.74 0.79 0.89 0.98

example 1

The example describes the use of methane in the gas stream in reactor, quench, compressor and absorber column, which ensures that the gas composition is always at least 2 vol .-% of the explosive limit. Part of the methane can be recycled from the exhaust stream of the absorber column in the reactor. Due to the distance of 2 Vol .-% to the explosion limit can be technically realized that fluctuations in the process, caused by faulty or failing valves or flow meters, leaving the operating state detected quickly enough and thus can be reacted quickly enough to enter the explosion area to prevent. Possible technical solutions are switching off the supply of oxygen when limit value is reached, intertizing, uncoupling the apparatus and switching the gas flow to a torch.

From the ODH reactor A, a process gas 2 having a temperature of 380 ° C., a pressure of 1.3 bar and the composition shown in Table 2 is provided. This gas stream is cooled in Quenchteil B with a mesitylene circulation stream at a temperature of 35 ° C and to a temperature of 90 ° C. Here are a number of

Secondary components dissolved out of the gas stream and the composition of

Process gas flow changes to the concentrations shown for stream 4. The

Mass ratio of the circulation stream 3b to the process gas 2 and the purge current 3a is 1: 1 to 0.067.

The gas stream 4 is in the second quenching stage C with another mesitylene circulation stream 9b which enters the quench at 35 ° C at the head end, further cooled to 45 ° C. The resulting gas stream 5 has the composition shown in Table 2, while the

Mass ratio between stream 4 and 9b 1 to 2.38 and a purge stream 6 is withdrawn with a share of 1% from the stream 9 and passed into the first quench.

The gas stream 5 is drawn in a 4-stage compressor as shown in Figure 1

Intercoolers compressed to 10 bar absolute. The purge streams 13a, 13a ', 13a "and 13a'" are all fed to the stream 10.

The resulting gas flow 12 "'(output flow of the heat exchanger F'" behind the 4th

Compression stage) has a temperature of 40 ° C and that shown in Table 2

Composition. This stream is in the absorber column G by passing in countercurrent and entering the column with 10 bar absolute and 35 ° C at the top of the column

Absorbent stream 21 b separated into a gas stream 16 and a mainly charged with C ₄ - hydrocarbons absorbent stream. The loaded one

Absorbent stream is passed through a stream 18 consisting of nitrogen with a

Freeing the temperature of 35 ° C from the oxygen insofar that the gas stream leaving the desorber 27 only 20 ppm oxygen. The mass ratio between Current 12 "'and 21b is 1 to 4.6 and the mass ratio between 21b and 18 is 1 to 0.003.

In Figure 4 it is shown that by the higher generated by the methane stream

Total volume fraction of organic compounds in the gas stream in the absorber column a greater distance to the explosive range can be maintained. The numbers represent the composition of the currents 2, 4, 5, 12 and 16 in the diagram.

Table 2: Data Simulation Example 1

 Power number 2 4 5 12 16

Temperature ° C 380 88 45 40 35.37

 Pressure bar 1, 5 1, 4 1, 45 10.042 10

 Flow rate kmol / h 4740 5045 1001 4212 3772

 concentrations

 PROPANE mole.% 0.00 0.00 0.00 0.00 0.00

 PROPOS Mol. -% 0.00 0.00 0.00 0.00 0.00

 BUTAN Mol. -% 1, 38 1, 32 1, 41 1, 55 0.02

 l-BUTAN Mol. -% 0.00 0.00 0.00 0.00 0.00

 1-BUTEN mole.% 0.12 0.12 0.13 0.14 0.00

 1-BUT mol. -% 0.00 0.00 0.00 0.00 0.00

 C-2 BUTEN mol.% 0.54 0.52 0.56 0.61 0.01

 T-2 BUTEN mol.% 0.76 0.72 0.77 0.85 0.01

 1, 3-BUTADIENE Mol. -% 6,32 6,01 6,44 7,1 1 0,04

 H2O mol.% 1 1, 73 1 1, 07 10.57 0.72 0.51

 ETHEN Mol. -% 0.02 0.02 0.02 0.02 0.02

 l-PENTAN mol.% 0.00 0.00 0.00 0.00 0.00

 Mesitylene Mol. -% 0.03 5.92 1, 20 0.06 0.06

 CO2 Mol. -% 1, 53 1, 43 1, 52 1, 71 1, 70

 H2 mol.% 0.01 0.01 0.01 0.01 0.01

 N2 mol.% 53.05 49.85 52.92 59.68 67.47

 O2 mol.% 6.00 5.64 5.99 6.75 7.54

 CH4 mol.% 17.80 16.73 17.76 20.00 21, 73

 CO mol.% 0.69 0.65 0.69 0.78 0.87

Claims

claims

1 . A process for the preparation of butadiene from n-butenes comprising the steps of: A) providing a feed gas stream containing n-butenes 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 minor 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;

Cb) compression of the remaining product gas stream b in at least one compression stage, wherein 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 the C4 hydrocarbons comprising butadiene and n-butenes in an absorbent, wherein one charged with C4 hydrocarbons

 Absorbent stream and the gas stream d2 are obtained, and Db) subsequent desorption of the C4 hydrocarbons from the loaded

Absorbent stream, wherein a C4 product gas stream d1 is obtained, characterized in that at least one point of the process section with the steps B), Ca), Cb) and Da) additionally a methane-containing gas stream is fed in such amounts that the formation of a explosive

 Gas mixture in step Da) is avoided.

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

Stream f2 in a substantially consisting of the selective solvent stream g1 and a butadiene-containing stream g2.

A method according to claim 1 or 2, characterized in that the methane-containing gas stream in step B) is fed.

A method according to claim 1 or 2, characterized in that the methane-containing gas stream in step Da) is fed.

Method according to one of claims 1 to 4, characterized in that the methane content of the gas stream d2 obtained in step Da) is at least 10 vol .-%.

Method according to one of claims 1 to 5, characterized in that the methane content of the gas stream d2 obtained in step Da) is at least 20% by volume.

Method according to one of claims 1 to 6, characterized in that the nitrogen content of the gas stream d2 obtained in step Da) is at most 10% by volume.

Method according to one of claims 1 to 7, 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 8, 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 9, 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.

A method according to claim 10, characterized in that in step Da2) is stripped with a methane-containing gas stream.

Method according to one of claims 1 to 1 1, characterized in that the absorbent used in step Da) is an aromatic

Hydrocarbon solvent is.