Work-up of a 3-methyl-1-butene-containing mixture

Work-up of a 3-methyl-1-butene-containing mixture

The invention relates to a process for working up a mixture containing water, 3-methyl-1-butene and at least one other methylbutene.

Methylbutenes belong to the class of C 5 -olefins. There are 3 isomeric methylbutenes, namely 2-methylbut-1-ene, 2-methylbut-2-ene and 3-methylbut-1-ene. The terms 3-methylbut-1-ene and 3-methyl-1-butene are synonymous. Synonyms are formed analogously for the two other methylbutenes. If “another methylbutene” is spoken of here, what is meant is an isomer different from 3-methyl-1-butene, namely 2-methylbut-1-ene or 2-methylbut-2-ene.

Methylbutenes are valued starting materials in industry. 3-Methyl-1-butene and its double bond isomers are used, for example, for preparing isoprene.

3-Methyl-1-butene can be prepared by elimination of water (dehydration) from 3-methyl-1-butanol, with the latter being able to be produced by hydroformylation of isobutene and subsequent hydrogenation of the resulting 3-methylbutanal.

The dehydration of 3-methyl-1-butanol to 3-methyl-1-butene is disclosed in U.S. Pat. No. 4,234,752 and WO 2008/006633 A1.

The dehydration of 3-methyl-1-butanol to 3-methyl-1-butene forms a moist, i.e. water-containing, mixture which contains other methylbutenes in addition to the desired 3-methyl-1-butene.

However, only a highly pure 3-methyl-1-butene is suitable for use as comonomer. Water contents of above 50 ppm damage the catalysts used in the polymerization, for example metallocene catalysts or Ziegler-Natta catalysts, by hydrolysis. A content of the two other double bond isomers, 2-methyl-2-butene and 2-methyl-1-butene, is likewise undesirable since these olefins having internal double bonds are virtually impossible to copolymerize and can therefore accumulate in the reaction system. Should these nevertheless be incorporated to a small extent into the polymer chain, this could have a possibly undesirable influence on the reaction or the polymer properties.

Against this background, there is the technical problem of working up the moist, methylene-containing mixture originating from the dehydration so as to provide highly pure 3-methyl-1-butene.

It is therefore an object of the invention to provide a process for working up a mixture containing water, 3-methyl-1-butene and at least one other methylbutene, by means of which the mixture can be dried and freed of undesirable methylbutenes.

This object is achieved by a process comprising the following steps:

a) Provision of the mixture; b) Primary distillation of the mixture so as to give a gaseous primary overhead product containing methylbutene and water and a water-free primary bottom product containing 3-methyl-1-butene; c) Condensation of the gaseous primary overhead product so as to give a condensate comprising a liquid aqueous phase and a liquid organic phase; d) Separation of the condensate into liquid aqueous phase and liquid organic phase; e) Discharge of the liquid aqueous phase; f) Recirculation of the organic phase to the primary distillation; g) Secondary distillation of the water-free primary bottom product of the primary distillation so as to give a secondary overhead product comprising 3-methyl-1-butene and a secondary bottom product.

According to the invention, at least two distillation steps, which are here referred to as primary and secondary distillation to distinguish them verbally, take place. The prefixes “primary” and “secondary” bear no relationship to the technical embodiment of the distillation. In each distillation, at least one overhead stream and at least one bottom stream are formed. To be able to assign the two overhead streams and the two bottom streams which occur in the context of the invention unambiguously, use is made of the respective prefixes. Correspondingly, the prefixes “tertiary” and “quaternary” will be used later.

The process of the invention makes it possible to isolate a highly pure 3-methyl-1-butene having a water content of preferably less than 50 ppm by mass of water, preferably less than 30 ppm of water and particularly preferably less than 10 ppm of water, from a moist mixture of isomeric methylbutenes.

The present invention has the advantage that a pure virtually water-free 3-methyl-1-butene can be obtained from a moist mixture of methylbutenes regardless of the ratio of the isomers. The purity in respect of water and isomeric methylbutenes of the 3-methyl-1-butene which has been worked up according to the invention is sufficiently high for the 3-methyl-1-butene to be used as comonomer, in particular for the modification of polypropylene or polyethylene.

The invention further provides a process for modifying polypropylene or polyethylene using 3-methyl-1-butene as comonomer, in which the 3-methyl-1-butene has been produced according to the invention.

The primary distillation is advantageously an azeotropic distillation. In principle, the removal of water could also be carried out by means of alternative industrial drying processes such as permeation, pervaporation or adsorption (for example pressure swing adsorption on molecular sieves). However, compared to the alternative processes mentioned, the azeotropic distillation according to the invention has lower capital costs and operating costs.

The secondary distillation is advantageously carried out as a fractional distillation. This improves the separation performance.

The mixture to be worked up can also contain low boilers in addition to the methylbutenes and the water.

For the present purposes, “low boilers” are a group of components of a liquid mixture whose respective boiling points are at lower temperatures compared to the boiling points of the other components of the mixture or whose vapour pressures are higher than the vapour pressures of the other components. The terms “low boilers” and “low-boiling components” are used synonymously. An example of a low boiler is isobutene, often a residual component from the preparation of 3-methyl-1-butanol which is in turn dehydrated to 3-methyl-1-butene.

If the mixture to be worked up comprises low boilers which get into the primary overhead product, it can be advisable to condense the gaseous primary overhead product only partially so that a gas containing the low boilers, which can then be discharged, is formed in addition to the condensate. In this way, undesirable low boilers can be separated off so that the purity of the target product is increased.

Despite the liquid phase separation, the aqueous phase can contain traces of organic impurities. To eliminate these, it is advisable to subject the liquid aqueous phase to a tertiary distillation before being discharged, with the tertiary overhead product formed here being able to be recirculated either to the condensation or to the primary distillation. The tertiary bottom product obtained here then consists virtually entirely of water and can be released into the environment.

The mixture to be worked up can also contain high boilers in addition to the methylbutenes, the water and any low boilers.

“High boilers” are, for the present purposes, a group of components of a liquid mixture whose respective boiling points are at higher temperatures compared to the boiling points of the other components of the mixture or whose vapour pressures are lower than the vapour pressures of the other components. The terms “high boilers” and “high-boiling components” are used synonymously. Examples of high boilers which may contaminate the mixture are 3-methyl-1-butanol which has not been reacted in the dehydration of 3-methyl-1-butanol to 3-methyl-1-butene and also di(3-methylbutyl)ether formed in the dehydration.

Should the mixture to be worked up contain high boilers, there are two possibilities for isolating the high boilers, which are selected depending on whether the high boilers only get into the primary bottom product or accumulate together with the other methylbutene in the secondary bottom product.

If the mixture contains high boilers going into the primary bottom product, it is advisable to take off not only the secondary overhead product and the secondary bottom product but also a side product from the secondary distillation, so that the secondary bottom product contains the high boilers and the side product contains the other methylbutene.

If high boilers present in the mixture even get into the secondary bottom product, it may be expected that the other methylbutene will also accumulate in the secondary bottom product. In such a situation, it is advisable to subject the secondary bottom product to a quaternary distillation so as to give a quaternary overhead product containing the other methylbutene and a quaternary bottom product containing the high boilers.

The invention is described by way of example below without the invention, whose scope is defined by the claims and the description, being restricted thereto.

In the process of the invention, mixtures containing 3-methyl-1-butene and its double bond isomers and also water can be worked up to give highly pure, dry 3-methyl-1-butene. The mixtures can contain small amounts of low boilers (materials having a boiling point lower than that of 3-methyl-1-butene) and small amounts of high boilers (materials having a boiling point higher than that of 3-methyl-1-butene, preferably a boiling point higher than that of 2-methyl-2-butene (38.6° C. at atmospheric pressure)).

The proportion of 3-methyl-1-butene in the mixture can be in the range from 5 to 98% by mass. The proportion by mass of 3-methyl-1-butene is preferably in the range from 80 to 95%. Such mixtures are obtained, for example, in the dehydration of 3-methyl-1-butanol.

Of course, other 3-methyl-1-butene-containing mixtures which have not been obtained by dissociation of 3-methyl-1-butanol can also be used in the process of the invention.

The mixture can be fed in liquid or gaseous form into the process of the invention. If the mixture is present in liquid form, it is advantageous to separate off undissolved water mechanically, for example by decantation. If the mixture is obtained at the same location by distillation, it is advantageous for energy reasons for it to be taken off in vapour form from the distillation and introduced without intermediate condensation into the process of the invention.

In the process of the invention, the separation of the water from the 3-methyl-1-butene-containing stream is preferably carried out in at least one column, preferably in precisely one distillation column (primary azeotropic distillation). Here, the water is removed by the overhead vapour obtained (primary overhead stream), which contains the major part of the water and also methylbutenes, being condensed completely and the water being separated off in liquid form by simple phase separation in a liquid-liquid separator (separation). The then largely water-free stream (organic phase) is returned to the primary distillation column. A stream having a content of at least less than 50 ppm of water, preferably less than 30 ppm, particularly preferably less than 10 ppm, is obtained as primary bottom product.

If low-boiling components are present in the 3-methyl-1-butene-containing mixture, these can likewise be separated from the methylbutenes in the primary distillation column by not completely condensing the overhead vapour (primary overhead product) but condensing it only partially. The liquid stream obtained in this way (condensate), which is largely free of low boilers, can be passed to the liquid-liquid separation, while the smaller (volume-wise) gas stream containing the predominant part of the low boilers is taken off as offgas stream.

A distillation column which is preferably used for separating the water from the 3-methyl-1-butene-containing mixture into low and high boilers preferably has from 5 to 50 theoretical plates, preferably from 10 to 38 theoretical plates and particularly preferably from 18 to 32 theoretical plates. The feed to the column is preferably introduced between plates 1 and 16 (from the top), particularly preferably between plates 1 and 8 . The ratio of amount of feed introduced to amount of vapour is, as a function of the number of theoretical plates achieved, the water concentration of the feed to the column and the required purities of the bottom product, preferably less than 5, preferably less than 1. The operating pressure of the column can preferably be set in the range from 0.1 to 2.0 MPa (absolute), preferably in the range from 0.2 to 1.0 MPa (absolute), particularly preferably in the range from 0.25 to 0.5 MPa (absolute).

To separate the heavy, aqueous phase from the condensate, it is possible to use generally known technologies for liquid-liquid separation, e.g. gravity separators with or without internals or centrifugal separators such as separators or centrifuges. To separate off the heavy, aqueous phase, preference is given, in the process of the invention, to using gravity separators, preferably gravity separators configured as horizontal vessels with internals.

The liquid-liquid separation enables the water to be separated from the organic components down to its physical solubility limit. The light organic phase is recirculated to the upper part of the distillation column, with the recirculation being carried out either directly or after mixing with the feed stream to the primary column.

Despite the liquid phase separation, the aqueous phase can contain traces of organic impurities. To eliminate these, the liquid aqueous phase can be subjected to a tertiary distillation before being discharged, with the tertiary overhead product formed here being able to be recirculated either to the condensation or to the primary distillation. The tertiary bottom product obtained here then consists virtually entirely of water and can be released into the environment.

The bottom stream from the primary column, on the other hand, contains virtually no water; the water content is preferably less than 50 ppm, preferably less than 30 ppm, particularly preferably less than 10 ppm. The primary bottom stream may be a saleable product. However, according to the invention, the primary bottom product is subjected to a secondary distillation in which the 3-methyl-1-butene present is worked up to higher purities.

The largely water-free primary bottom stream is separated in a secondary fractional distillation into a secondary overhead stream which has a purity of at least 98% by mass of 3-methyl-1-butene and a secondary bottom stream which comprises predominantly 2-methyl-2-butene and 2-methyl-1-butene and possibly other components which have a boiling point higher than that of 3-methyl-1-butene and are present in traces. In the process of the invention, the isomers are preferably separated off in at least one column, preferably precisely one distillation column (secondary distillation). A distillation column which is preferably used for separating off the isomers and any high-boiling components present from the 3-methyl-1-butene preferably has from 30 to 120 theoretical plates, preferably from 40 to 80 theoretical plates and particularly preferably from 55 to 70 theoretical plates. The feed to the column is preferably introduced in the middle part of the column. The reflux ratio is, depending on the number of theoretical plates realized, the composition of the feed to the column and the required purities of the overhead product and bottom product, preferably in the range from 0.5 to 10, preferably from 1.1 to 4.5. The operating pressure of the column can preferably be set in the range from 0.1 to 2.0 MPa (absolute), preferably in the range from 0.2 to 1.0 MPa (absolute), particularly preferably in the range from 0.25 to 0.5 MPa (absolute).

The overhead stream from the secondary column (secondary overhead product) preferably contains more than 98% of 3-methyl-1-butene, more preferably more than 99.0%, particularly preferably more than 99.5%.

The secondary overhead product thus has a purity which enables it to be used directly as monomer or comonomer for preparing polymers or copolymers. It can also be used as starting compound for the preparation of epoxides, ketones, aldehydes, alcohols and carboxylic acids. Furthermore, it can be used as alkylating agent or as a component in ene reactions.

The secondary bottom product contains at least one of the two undesired isomers, namely 2-methyl-2-butene and/or 2-methyl-1-butene, and possibly high boilers. The secondary bottom product can, if desired, be fractionated by quaternary distillation in a further column. The high boilers leave the quaternary distillation as quaternary bottom product, and the other (undesired) methylbutenes leave it as quaternary overhead product.

If the product taken off in the lower part of the secondary column contains high boilers having a boiling point higher than that of the highest-boiling 3-methylbutene isomer present, 2-methyl-1-butene and/or 2-methyl-1-butene or a mixture thereof can be taken off as side product a few plates above the bottom. The high boilers are then discharged with the secondary bottom product. If the high-boiling components are 3-methyl-1-butanol and/or di(3-methylbutyl)ether, the secondary bottom product obtained in this way can be passed to the dehydration in order to dissociate 3-methyl-1-butanol and/or di(3-methylbutyl)ether into the desired target product 3-methyl-1-butene.

2-Methyl-1-butene or 2-methyl-2-butene or a mixture thereof, possibly together with high boilers, can be utilized, for example, for preparing isoprene.

The distillation columns used in the process of the invention can be provided with internals, e.g. trays, rotating internals, random beds and/or ordered packing.

In the case of column trays, it is possible to use, for example, the following types:

Trays having holes or slits in the base plate. Trays having necks or chimneys which are covered by bells, caps or hoods. Trays having holes in the base plate which are covered by moveable valves. Trays having special constructions.

In columns having rotating internals, the runback can, for example, be sprayed by means of rotating funnels or spread as film over a heated tubular wall by means of a rotor.

In the process of the invention, it is possible, as stated above, to use columns which have random beds of various packing elements. The packing elements can comprise virtually all materials, in particular, for example, steel, stainless steel, copper, carbon, stoneware, porcelain, glass or plastics, and can have a wide variety of shapes, in particular the shape of spheres, rings having smooth or profiled surfaces, rings having internal webs or openings through the wall, wire mesh rings, saddle bodies and spirals.

Packings having a regular/ordered geometry can comprise, for example, metal sheets or woven meshes. Examples of such packings are Sulzer woven mesh packings BX made of metal or plastic, Sulzer laminar packings Mellapak made of metal sheet, high-performance packings from Sulzer such as Mella-pakPlus, structured packings from Sulzer (Optiflow), Montz (BSH) and Kühni (Rombopak).

The present invention will now be illustrated with the aid of the figures. The figures show:

FIG. 1 : Plant for carrying out a first embodiment of the process of the invention without tertiary distillation;

FIG. 2 : Plant for carrying out a second embodiment of the process of the invention with tertiary distillation;

FIG. 3 : A detail of a plant for carrying out a third embodiment of the process of the invention with side product in secondary distillation.

A block diagram of an embodiment of a plant in which the process of the invention can be carried out is shown in FIG. 1 . The 3-methyl-1-butene-containing mixture ( 1 ) is fed into a column K 1 for the purpose of primary distillation. The primary overhead product ( 2 ) from the column K 1 , which contains the major part of the water and also methylbutenes, is condensed completely (condenser not shown) and fed as condensate to the liquid-liquid separator D 1 . In the separator D 1 , the water is separated off down to its physical solubility limit and discharged as heavy aqueous phase ( 4 ). The then largely water-free stream of the organic phase ( 3 ) is returned to the distillation column K 1 .

The largely water-free primary bottom product ( 6 ) from the column K 1 is separated in the secondary distillation column K 2 into a secondary overhead product ( 7 ) which has a purity of at least 98% by mass of 3-methyl-1-butene and a secondary bottom product ( 8 ) which comprises predominantly 2-methyl-2-butene and/or 2-methyl-1-butene and possibly other higher-boiling components.

Low boilers can optionally be taken off as gas ( 5 ). In this case, the primary overhead stream ( 2 ) is only partially condensed.

A second variant of the process of the invention is shown in FIG. 2 . The variant in FIG. 2 differs from the variant in FIG. 1 in that the water phase ( 4 ) is additionally separated in the column K 3 into a tertiary overhead product ( 9 ) which comprises mainly water/3-methylbutene azeotropes and possibly additional water and a water ( 4 a ) contaminated with a small amount of organic materials (tertiary distillation). The tertiary overhead product ( 9 ) is condensed in the condenser of the column K 3 (not shown) and introduced into the separator D 1 or fed into the condenser of the column K 1 (not shown in FIG. 2 ).

FIG. 3 shows, as third variant, a particular embodiment of the column K 2 , in which the largely water-free primary bottom stream ( 6 ) from the column K 1 is separated into a secondary overhead product ( 7 ) which has a purity of at least 98% by mass of 3-methyl-1-butene and a side product ( 8 a ) which comprises predominantly 2-methyl-2-butene and/or 2-methyl-1-butene and a secondary bottom product ( 10 ) which comprises predominantly other higher-boiling components.

The following examples illustrate the invention without restricting its scope which is defined by the description and the claims.

The following illustrative calculations were carried out using the steady-state simulation program ASPEN Plus (Version 2006.1 from Aspen Tech). To produce transparent, reproducible data, only generally available materials data were used. The property method “UNIFAC-DMD” (see J. Gmehling, J. Li, and M. Schiller, Ind. Eng. Chem. Res. 32 (1993), pp. 178-193) was used as materials data model. A person skilled in the art can therefore readily understand the calculations.