AU7945301A - Process for preparing highly pure raffinate II and methyl tert-butyl ether - Google Patents

Description

S&FRef: 567718

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name and Address of Applicant: Actual Inventor(s): Address for Service: Oxeno Olefinchemie GmbH Paul-Baumann-Strasse 1 D-45764Marl Germany Armin Rix, Gerda Grund, Wilfried Bueschken Spruson Ferguson St Martins Tower,Level 31 Market Street Sydney NSW 2000 (CCN 3710000177) Process for Preparing Highly Pure Raffinate II and Methyl Tert-butyl Ether Invention Title: The following statement is a full description of this invention, including the best method of performing it known to me/us:- *r-lW I 7 5845c J O.Z. 5713 1 Process for preparing highly pure raffinate II and methyl tert-butyl ether The invention relates to a process for preparing highly pure raffinate II (a

C

4 -hydrocarbon mixture) which has a low isobutene content and is particularly suitable for the preparation of pure 1-butene and methyl tertbutyl ether (MTBE).

Isobutene-free butene mixtures are suitable for preparing highly pure 1butene and/or for preparing butene oligomers having a low degree of branching. MTBE is a sought-after carburetor fuel component for increasing the octane number. For this purpose, it does no harm for other ethers such as methyl sec-butyl ether or oligomers of C 4 -olefins to be present in the MTBE. High-purity MTBE which is used as a solvent requires significantly tighter limits for the abovementioned secondary components.

MTBE and linear butenes are obtained from C 4 -olefin mixtures, for example the C 4 fraction from steam crackers or FCC units. These mixtures consist essentially of butadiene, the monoolefins isobutene, 1-butene and the two 2-butenes together with the saturated hydrocarbons isobutane and n-butane. Customary work-up methods used worldwide for such C 4 fractions comprise the following steps: firstly, the major part of the butadiene is removed. If butadiene can be readily marketed or there is a 25 use for it within the company, it is separated off, for example by extraction or extractive distillation. Otherwise, it is hydrogenated selectively to linear butenes so as to leave butadiene concentrations of from 1 to In both cases, a hydrocarbon mixture (corresponding to raffinate I or hydrogenated crack-C 4 comprising the saturated hydrocarbons (n-butane 30 and isobutane) together with the olefins (isobutene, 1-butene and 2butenes) remains. A possible way of removing the isobutene from this mixture is reaction with methanol to form MTBE. This leaves the saturated hydrocarbons, linear butenes and possibly a residual amount of isobutene.

The C 4 mixture obtained after removal of the butadiene and isobutene is referred to as raffinate II.

Depending on the further use of the two streams (MTBE and the olefin mixture, raffinate II), particular qualities of these streams are of special interest. If the isobutene from the C 4 fraction is utilized for producing O.Z. 5713 -2carburetor fuel components (be it as MTBE or oligomer), the purity of the MTBE is not subject to any critical requirements. Other ethers such as methyl sec-butyl ether and/or C 4 -olefin oligomers can be present in the

MTBE.

In addition to the linear olefins, relatively large amounts of isobutene can be present in raffinate II if this C 4 mixture is reacted, for example, over acid catalysts, to form mostly branched C 4 -oligomers, in particular C 8 and C 12 oligomers. After hydrogenation, this mixture gives a high-octane carburetor fuel component.

If the MTBE is to be used, for example, as a pure solvent or for preparing highly pure isobutene in a cleavage reaction, it is allowed to contain only small amounts of secondary components. The synthesis to form MTBE therefore has to be carried out very selectively.

If the raffinate II is to be used for preparing oligomers having low iso indices, i.e. a low degree of branching, the isobutene content has to be very low, preferably less than 1000 ppm by weight. Virtually isobutene-free raffinate II is necessary if pure 1-butene is to be obtained from this raffinate II. The isobutene concentration of the raffinate II should then not exceed 450 ppm by weight.

Since the boiling point difference between isobutene and 1-butene is only 0 C, economical separation of the two components by distillation is not 25 possible. In this case, isobutene has to be reacted virtually completely in the MTBE synthesis.

The highest demands are placed on the MTBE synthesis if solvent-quality MTBE is to be produced and the raffinate II is at the same time to be used 30 for 1-butene production. Here, both a very high isobutene conversion and a very high MTBE selectivity are necessary.

The preparation of MTBE from isobutene-containing C 4 -hydrocarbon mixtures such as raffinate I or hydrogenated crack-C 4 by reaction with methanol is frequently carried out industrially using acid ion exchange resins (sulfonic acid groups) as heterogeneous catalysts. The reaction is carried out in one or more reactors connected in series, with the catalyst preferably being present as a fixed bed. This gives a product in which O.Z. 5713 3 methanol, isobutene and MTBE are in equilibrium. The equilibrium conversion is established in each reactor as a function of the reaction conditions (temperature, methanol excess, etc.). This means that under the reaction conditions customarily set in industrial processes, about 96% of the isobutene used is reacted. This mixture can subsequently be fractionally distilled to give a bottom fraction comprising MTBE and a top fraction comprising C 4 -hydrocarbons and methanol. After removal of the methanol present as an azeotrope, the raffinate II produced in this way is not suitable for producing pure 1-butene because of its high residual isobutene content.

To obtain virtually complete isobutene conversion, reactive distillation columns are used in industry. These are columns which contain both separation trays (or mesh packing) and catalysts on separation trays or integrated into other internals or mesh packing. In such columns, the reaction of the residual isobutene with methanol to form MTBE and the separation of the products by distillation occur simultaneously. The feed olefin mixture, for example raffinate I or selectively hydrogenated crack-C 4 can also be fed into such a column. These columns are particularly useful for the abovementioned equilibrium mixture in order to achieve very high conversions. Products obtained are an azeotrope comprising methanol S and C 4 -hydrocarbons, which in the case of 1-butene production has to be virtually free of isobutene, at the top and MTBE at the bottom.

US 4 504 687 describes a process for preparing MTBE and a lowisobutene C 4 stream. Here, the reaction of a C 4 stream comprising both isobutene and linear butenes with methanol is carried out in a reactive distillation column in which reaction and distillation are, due to structural measures, carried out at different pressures. The division of the column in 30 terms of pressure into a distillation section and a reaction section is structurally complicated. No information is given on the purity of the products prepared in US 4 504 687. A large reflux ratio of 0.5 20 1 is *-disclosed for the reactive distillation column.

ooo In US 5 120 403, the same reaction is carried out in a reactive distillation column in which the catalyst is flooded. Although the reaction to form MTBE can proceed more readily in a liquid phase, the distillation is made more difficult, as a result of which the separation of the components to produce highly pure products is not ensured.

O.Z. 5713 4 EP 0 885 866 Al discloses a process in 6 embodiments for preparing MTBE and a low-isobutene C4 stream by reacting a C 4 -hydrocarbon stream comprising isobutene and n-butenes with methanol. The feature common to all embodiments is that at least one prereactor, a reactive distillation column and an after-reactor are connected in series.

In all three abovementioned publications, neither the quality of the MTBE prepared nor the isobutene content of the remaining C4 stream is disclosed.

US 5 368 691 describes the reaction of a C 4 -hydrocarbon mixture comprising isobutene and linear butenes with methanol to form MTBE and a C4 stream comprising the linear butenes in a reactive distillation column.

Here, MTBE is obtained as bottom product in a purity of greater than 98%, which does not meet the requirements for the preparation of solventquality MTBE. The example describes a top product having a residual isobutene content of This isobutene content is far too high for further processing to produce pure 1-butene. The reflux ratio of the column is stated to be from 0.5 1 to 5 1.

A further process for preparing MTBE and a low-butene C4 stream using a reactive distillation column is known from US 4 475 005. Here, the column is operated at a reflux ratio of 1. The isobutene content of the distillate is 25 4830 ppm by weight and is thus significantly too high for further use for producing pure 1-butene.

Since the known processes are not fully satisfactory in respect of the isobutene content of the top product or of the raffinate II produced therefrom and/or the quality of the MTBE obtained and/or capital cost and/or energy consumption, it is an object of the invention to develop a process which produces a raffinate II which is suitable for cost-effective production of 1-butene and at the same time gives MTBE in solvent quality.

It has surprisingly been found that the acid-catalyzed reaction of methanol and a C 4 -olefin mixture in a two-stage plant with a reactive distillation column as second stage makes it possible to obtain an overall isobutene conversion of above 99.9% and a virtually isobutene-free raffinate II and at O.Z. 5713 5 the same time gives an MTBE which contains virtually no impurities if specific reaction conditions in terms of reflux ratio, temperature and pressure are adhered to in the reactive distillation column.

The present invention provides a process for preparing methyl tert-butyl ether (MTBE) and a virtually isobutene-free C 4 -hydrocarbon mixture by reaction of an isobutene-containing C 4 -hydrocarbon stream with methanol over an acid catalyst, wherein, in a first stage in one or more reactor(s), isobutene is reacted with methanol to equilibrium formation of MTBE and, in a second stage in a reactive distillation column, the remaining isobutene present in the mixture is reacted over an acid ion exchange resin, where the reactive distillation column is operated in a pressure range from 3 to bar abs. and in a temperature range in the reaction zone from 55 0 C to 0 C at a reflux ratio of less than 1.

Owing to the high conversion, the product obtained at the top contains less than 450 ppm by weight of isobutene and is therefore very suitable for the production of pure 1-butene. The MTBE produced has a purity which enables it to be used as solvent.

The reflux ratio is defined as the ratio of the runback (reflux) stream in the column to the distillate stream taken off.

This finding that a better isobutene conversion is obtained at low reflux 25 ratios and temperatures in the catalyst packing than at higher reflux ratios is surprising, since the opposite is described in the literature.

For example, it is shown in the following publications that, similar to the case of a distillation, the reaction rate of the reaction of the isobutenecontaining C 4 streams with methanol to form MTBE in a reactive distillation column rises with increasing reflux ratio (Lawrence A. Smith, D.Hearn, Catalytic Destillation, Proc. Intersoc. Energy Convers. Conf. (1984) 1 9 th (Vol p 998-1002; Miguel A. Isla, Horazio A. Irazoqui, Modeling, Analysis, and Simulation of a Methyl tert-Butyl Ether Reactive Destillation Column, Ind. Eng. Chem. Res. 1966, 35, 2696-2708; Hoshang Subawalla, James R. Fair, Design Guideline for Solid-Catalyzed Distillation Systems, Ind. Eng. Chem. Res. 1999,38, 3696 3709, or in "Rate-Based Modeling of Reactive Destillation Systems", V. Pinjala and T. L. Marker et al., Topical Conference on Separations Technologies ATChE, 1-6, 11. 1992).

O.Z. 5713 -6- Setting a low reflux ratio was therefore contrary to the teachings of the prior art.

The process of the invention has a series of advantages. The process makes it possible to obtain a distillate containing less than 450 ppm by weight of isobutene based on the C 4 -hydrocarbons and/or containing less than 0.5 ppm by weight of MTBE (except for methanol) and is thus suitable for producing pure 1-butene containing less than 1000 ppm by weight of isobutene. The MTBE is of such high quality that it can be utilized both as precursor for the preparation of high-purity isobutene and as solvent.

The lowering of the reflux ratio also leads to a significant steam saving, as a result of which the process of the invention has a low energy requirement.

In the process of the invention, the reaction of isobutene with methanol to form MTBE is carried out in two stages (see Fig. The first stage comprises the reaction of isobutene in the C 4 mixture with methanol in one or more reactors until thermodynamic equilibrium between MTBE, methanol and isobutene has been established. This is generally at an isobutene conversion of from 94 to 96%. The reactors of the first stage can be conventional fixed-bed reactors containing the same catalysts as are described below for the second stage. The reactors are usually operated at 30 110°C and 5 50 bar abs.

,.2 Compositions of the reaction mixtures obtained in this way are described in the examples. In general, these mixtures contain less than 1% by weight o of isobutene which is reacted very selectively to form MTBE in the subsequent second stage, viz. the reactive distillation column.

The catalyst is present in the enrichment section of this reactive distillation column, and separation trays or distillation packing are/is present below and above the catalyst packing. The catalyst is either integrated into a packing unit, for example KataMax® (EP 0 428 265), KataPak® (EP 0 396 650) or MultiPak® (utility model No. 298 7 007.3), or polymerized onto shaped bodies (US 5 244 929).

The zone above the catalyst packing comprises from 5 to 20 theoretical plates, in particular from 10 to 15 theoretical plates. The catalyst zone can O.Z. 5713 -7be estimated as having a distillation efficiency of from 1 to 5 theoretical plates per meter of packing height. The separation zone below the catalyst comprises from 12 to 36 theoretical plates, in particular from 20 to theoretical plates.

The actual catalyst used in the two stages of the process is a solid which is soluble neither in the feed mixture nor in the product mixture and has acid centers on its surface. The catalyst must not release any acidic substances into the product mixture under reaction conditions, because this would lead to yield losses.

The activity of the catalysts must be such that, under reaction conditions, they effect the addition of methanol onto isobutene but do not bring about addition onto linear butenes to a significant extent. Furthermore, they must not catalyze the oligomerization of olefins and the formation of dimethyl ether to any significant extent.

A group of acid catalysts which can be used in the process of the invention are solid ion exchange resins containing sulfonic acid groups. Suitable ion exchange resins are, for example, ones prepared by sulfonation of phenol/aldehyde condensates or cooligomers of aromatic vinyl compounds. Examples of aromatic vinyl compounds for preparing the cooligomers are: styrene, vinyltoluene, vinylnaphthalene, vinylethylbenzene, methylstyrene, vinylchlorobenzene, vinylxylene and 25 divinylbenzene. Particular preference is given to using the cooligomers formed by reaction of styrene with divinylbenzene as precursors for the preparation of ion exchange resins containing sulfonic acid groups. The resins produced can be in gel form, macroporous or in sponge form.

Strong acid resins of the styrene-divinyl type are sold, inter alia, under the following trade names: Duolite C20, Duolite C26, Amberlyst Amberlyst A35, Amberlite IR-120, Amberlite 200, Dowex 50, Lewatit SPC 118, Lewatit SPC 108, K2611, K2621, OC 1501.

The properties of these resins, in particular specific surface area, porosity, stability, swelling or shrinkage and ion exchange capacity, can be varied by means of the production process.

In the process of the invention, the ion exchange resins can be used in their H form. Preference is given to using macroporous resins, for example O.Z. 5713 -8- Lewatit SCP 118, Lewatit SCP 108, Amberlyst A15 or Amberlyst K2621. The pore volume is from 0.3 to 0.9 ml/g, in particular from 0.5 to 0.9 ml/g. The particle size of the resin is from 0.3 mm to 1.5 mm, in particular from 0.5 mm to 1.0 mm. The particle size distribution selected can be relatively narrow or relatively broad. Thus, for example, ion exchange resins having a very uniform particle size (monodisperse resins) can be used. The capacity of the ion exchanger is, based on the product as supplied, 0.7 2.0 mol/l, in particular 1.1-2.0 mol/l.

Feedstocks which can be used for the process of the invention are C 4 hydrocarbon mixtures comprising both isobutene and linear butenes, but no acetylene derivatives and less than 8000 ppm by weight of butadiene.

Examples of industrial mixtures which may comprise both isobutene and linear butenes are light gasoline fractions from refineries, C 4 fractions from FCC units or steam crackers, mixtures from Fischer-Tropsch syntheses, mixtures from dehydrogenation of butanes, mixtures from skeletal isomerization of linear butenes, mixtures formed by metathesis of olefins or other industrial processes.

After removal of multiply unsaturated compounds, these mixtures can be used in the process of the invention. For example, a suitable feed mixture S.. can be obtained from the C 4 fraction from a steam cracker by extraction of the butadiene or by selective hydrogenation of the butadiene to linear butenes. This mixture (raffinate I or selectively hydrogenated crack-C 4 25 comprises n-butane, isobutane, the three linear butenes and isobutene and is a preferred starting material for the process of the invention.

The hydrocarbon feed mixture can be fed together with methanol into the first stage of the process. Catalysts used are the same catalysts as those used in the reactive distillation column or catalysts similar to these. The first stage produces a mixture in which isobutene, methanol and MTBE are in equilibrium. A preferred embodiment of the process of the invention comprises producing an equilibrium mixture or a mixture close to equilibrium in the first stage and feeding it to the reactive distillation column (second stage).

The feed to the column of the second stage can contain more methanol than is required for complete reaction of the remaining isobutene.

O.Z. 5713 -9 However, the methanol excess should be limited so that, on the one hand, a sufficient amount of methanol for the azeotrope formed from methanol and C 4 -hydrocarbons is present but, on the other hand, not so much that methanol gets into the bottom product, so that an MTBE which meets specifications (methanol content less than 5000 ppm by weight) is obtained.

If the methanol content in the feed to the column is below the maximum permissible value, additional methanol may, if appropriate, be introduced into the feed mixture before it is fed into the column. Furthermore, methanol can be fed in at the top of the reactive distillation column via a separate facility.

The temperature of the feed to the column is, regardless of its composition, the reaction pressure in the column and the throughput, from 0 C to 80 0 C, preferably from 60 0 C to 75 0

C.

The mean temperature in the catalyst zone is, depending on the pressure in the column, preferably from 55 0 C to 70 0 C, particularly preferably from 58 0 C to 67 0

C.

The reactive distillation column is operated at pressures, measured at the top of the column, of 3 15 bar abs., preferably from 5 bar abs. to 9 bar abs., in particular from 7 25 bar abs. to 8.5 bar abs.

o The hydraulic loading in the catalyst packing of the column is preferably from 10% to 110%, more preferably from 20% to 70%, of its flooding point loading. For the purposes of the present invention, hydraulic loading of a distillation column is the uniform hydrodynamic loading of the column cross section by the rising stream of vapor and the downflowing stream of liquid. The upper loading limit is the maximum loading by vapor and downflowing liquid above which the separation efficiency drops as a result of entrainment or backing-up of the downflowing liquid by the rising stream of vapor. The lower loading limit is the minimum loading below which the separation efficiency decreases or breaks down as a result of irregular flow or empty running of the column, e.g. the trays. (Vauck/MOller, "Grundoperationen chemischer Verfahrenstechnik", p. 626, VEB Deutscher Verlag fOr Grundstoffindustrie.) O.Z. 5713 10 At the flooding point, the shear stresses transmitted from the gas to the liquid become so great that the entire liquid is entrained in the form of droplets in the gas and carried along with it or phase inversion occurs in the column Mackowiak, "Fluiddynamik von Kolonnen mit modernen Fullkorpern und Packungen fur Gas/FlCssigkeitssysteme", Otto Salle Verlag 1991).

In the process of the invention, the column is operated at reflux ratios of less than 1, in particular reflux ratios which are greater than 0.6 and less than 1, preferably in the range from 0.7 to 0.9.

At these reflux ratios, residual isobutene concentrations in the raffinate II of less than 450 ppm by weight, preferably less than 400 ppm by weight, very particularly preferably less than 300 ppm by weight (based on the C 4 mixture in the distillate), are obtained according to the invention. The optimum reflux ratio depends on the throughput, the composition of the feed to the column and the column pressure. However, it is always within the abovementioned ranges.

Optionally, a top product comprising a C 4 -hydrocarbon mixture and methanol and having an isobutene content of less than 450 ppm by weight, preferably less than 400 ppm by weight, very particularly preferably less than 300 ppm by weight, and a bottom product comprising 25 MTBE and having a methyl sec-butyl ether (MSBE) content of less than 2500 ppm by weight can be obtained in the second stage of the process.

This top product can in turn be separated into a C 4 -hydrocarbon mixture and methanol, with the C 4 -hydrocarbon mixture containing less than 30 ppm by weight of MTBE and/or TBA.

The bottom product from the reactive distillation column preferably comprises MTBE and less than 2500 ppm by weight of methyl sec-butyl ether and less than 2500 ppm by weight of C 8 -hydrocarbons. Further purification of the MTBE is no longer necessary if it is to be used as a component of 4-stroke fuels.

The methanol can be separated off from the top product by, for example, extraction with water. Traces of butadiene can be removed from the O.Z. 5713 11 resulting raffinate II by selective hydrogenation (SHP). This mixture can be fractionally distilled to give 1-butene, isobutane and a mixture of 2-butenes and n-butane or to give 1-butene, 2-butene and n-butane.

The pure 1-butene produced in this way contains less than 1000 ppm by weight of isobutene and is a sought-after intermediate. It is used, for example, as comonomer in the production of polyethylene (LLDPE or HDPE) and of ethylene-propylene copolymers. It is also used as alkylating agent and is a starting material for the preparation of 2-butanol, butene oxide, valeraldehyde.

A further use of the virtually isobutene-free raffinate II produced according to the invention is the preparation of n-butene oligomers, in particular by the Octol process.

The hydrocarbons which remain after removal or reaction of the linear butenes in the raffinate II can, if desired, be worked up by hydrogenation (CSP) to give isobutane and n-butane.

20 The MTBE obtained as bottom product from the reactive distillation column can be used for various purposes. Since it contains only extremely small amounts of methyl sec-butyl ether (MSBE), it is suitable for the preparation of highly pure isobutene by redissociation, since virtually no linear butenes can be formed (by redissociation of methyl sec-butyl ether). The cleavage 25 of MTBE can be carried out as described in, for example, DE 100 943.2.

Owing to its low content of by-products (MSBE and C 8 -olefins), the MTBE obtained in this way can, after removal of the residual alcohols, be used as 30 solvent in analysis or in organic syntheses.

It is also possible for it to be used as a component of 4-stroke fuels.

A block diagram of a plant in which the process of the invention can be carried out is shown in figure 1.

A C 4 -hydrocarbon mixture (raffinate I or selectively hydrogenated crack-C 4 is reacted with methanol in the reactor in which an acid ion exchange resin is present to give an MTBE-containing reaction mixture (4) O.Z. 5713 12 which is fed into a reactive distillation column at a point below the catalyst packing As top product methanol and a C 4 stream containing less than 300 ppm by weight of isobutene is obtained. MTBE is taken off as bottom product The following examples illustrate the invention without restricting its scope, which is defined by the claims.

Example 1 (Comparative example) The reaction of the C 4 mixture having the composition indicated in Table 1 was carried out in a reactive distillation column (see fig. 1) which was provided with Amberlyst A15 in KATAMAX packing. The packing was located in the upper part of the column. Above the feed point there are separation trays, followed by three KATAMAX packings each having a liquid distributor, and finally more distillation trays. Below the feed point, there are appropriately dimensioned stripping sections in order to bring about the separation of MTBE from C 4 -hydrocarbons.

The column was operated at a pressure at the top of 8.2 bar abs., temperatures in the packing units, in order from the feed tray upward, of 65.8 0 C, 65.4 0 C and 65.1 0 C, at a hydraulic loading of the catalyst packing of 36% and a reflux ratio of 1.02. When the reactive distillation column was operated in this way, an isobutene conversion in 25 the column of 93.4% was obtained not enough to achieve the necessary residual isobutene concentration in the raffinate (see analyses in Table 1 *o ooo O.Z. 5713 13 Table 1A: Composition column of the streams to/from the reactive distillation Feed Distillate Bottoms by by by weight) weight) weight)

C

4 -hydrocarbon 58.93 94.14 0.13 mixture* MTBE 37.02 0 98.00 MSBE 0.04 0 0.32 Methanol 3.68 5.86 0.57 TBA 0.29 0 0.60

C

8 0.04 0 0.27 Others 0 0 0.11 Table 1B: Distribution of C 4 -hydrocarbons in the mixture' in the feed and distillate to/from the column (in each case normalized to 100%) Feed Distillate by weight) by weight) Isobutane 4.79 6.00 n-butane 14.46 14.32 trans-2-butene 23.66 24.56 1-butene 44.92 43.335 Isobutene 0.68 0.048 cis-2-butene 10.99 11.40 1,3-butadiene 0.50 0.34 10 Example 2 according to the invention The reactive distillation column was operated under the same pressure (8.2 bar abs.), temperatures in the packing of 66.5 0 C, 66.2 0 C and 65.8 0

C,

the same hydraulic loading in the catalytic packing of 36% and a composition of the feed stream comparable to that in Example 1. However, the reflux ratio in the column was reduced to 0.89. Tables 2A and 2B show the composition of the streams and allow conversion and selectivity to be derived.

O.Z. 5713 14 Table 2A: Composition of the streams to/from the reactive distillation column Feed Distillate Bottoms by by by weight) weight) weight)

C

4 -hydrocarbon 61.471 93.88 0.17 mixture* MTBE 34.82 0 98.04 MSBE 0.04 0 0.31 Methanol 3.41 6.12 0.48 TBA 0.22 0 0.62 C8 0.039 0 0.23 Others 0 0 0.15 Table 2B: Distribution of C 4 -hydrocarbons in the mixture in the feed and distillate to/from the column Feed Distillate by weight) by weight) isobutane 5.426 5.798 n-butane 13.549 13.861 trans-2-butene 25.733 26.138 1-butene 42.383 42.004 isobutene 0.720 0.018 cis-2-butene 11.880 11.906 1,3-butadiene 0.309 0.275 Compared to Example 1, Example 2 shows the advantageous effect of reducing the reflux ratio from 1.02 to 0.89. In Example 2, the isobutene concentration of the distillate is 0.018%, based on the C 4 -hydrocarbons.

This distillate is, in contrast to the distillate from Example 1, suitable for producing 1-butene containing less than 1000 ppm by weight of isobutene.

O.Z. 5713 15 Example 3 (accordinq to the invention) The reactive distillation column was operated at a pressure of 7.4 bar abs., temperatures in the packing units of 62.2 0 C, 62.0 C and 61.6 0 C, at a hydraulic loading in the catalytic packing of 37% and a reflux ratio of 0.89.

Tables 3A and 3B show the composition of the streams to/from the column.

Table 3A: Composition column of the streams to/from the reactive distillation

S

Feed Distillate Bottoms by by by weight) weight) weight)

C

4 -hydrocarbon 61.77 95.4 0.11 mixture* MTBE 34.84 0 98.49 MSBE 0.04 0 0.21 Methanol 3.06 4.6 0.32 TBA 0.19 0 0.51

C

8 0.04 0 0.19 Others 0.06 0 0.17 Table 3B: Distribution of C 4 -hydrocarbons in the mixture in the feed and distillate to/from the column Feed Distillate by weight) by weight) Isobutane 5.271 5.576 n-butane 15.088 15.350 trans-2-butene 24.589 24.335 1-butene 42.185 43.107 Isobutene 0.709 0.018 cis-2-butene 11.925 11.393 1,3-butadiene 0.233 0.221 O.Z. 5713 16 Example 3 shows that, at the same reflux ratio as in Example 2, reducing the pressures in the catalyst packing units lowered the concentration of methyl sec-butyl ether (MSBE) in the bottom product without increasing the isobutene content of the distillate. A bottom product of this quality is suitable for producing MTBE of solvent quality.

Claims (15)

1. A process for preparing methyl tert-butyl ether and a virtually isobutene free C4 hydrocarbon mixture by reaction of an isobutene containing C4 hydrocarbon stream with methanol over an acid catalyst, wherein, in a first stage in one or more reactor(s), isobutene is reacted with methanol to equilibrium formation of methyl tert-butyl ether and, in a second stage in a reactive distillation column, the remaining isobutene present in the mixture is reacted over an acid ion exchange resin, where the reactive distillation column is operated in a pressure range from 3 to abs and in a temperature range in the reaction zone from 550C to 750C at a reflux ratio of less than 1.

2. The process as claimed in claim 1, wherein the reactive distillation column is operated at a reflux ratio which is greater than 0.6 and less than 1.

3. The process as claimed in claim 1 or claim 2, wherein, in the second stage a top product comprising methanol and a C4 hydrocarbon mixture having an isobutene content of less than 450ppm by weight, based in the C4 hydrocarbon mixture, is obtained.

4. The process as claimed in claim 1 or claim 2, wherein the temperature in the reaction zone is from 550C to 700C. The process as claimed in any one of claims 1 to 3, wherein the pressure is from 7bar abs to 8.5bar abs.

6. The process as claimed in any one of claims 1 to 5, wherein the reactive distillation column is operated at a hydraulic loading of the catalytic packing of from 10% to 110%. 20 7. The process as claimed in any one of claims 1 to 6, wherein, in the second stage a top product comprising methanol and a C4 hydrocarbon mixture having an isobutene content of less than 450ppm by weight, based in the C4 hydrocarbon mixture, and a bottom product comprising methyl tert-butyl ether having a methyl sec-butyl ether content of less than 2500ppm by weight are obtained.

8. The process as claimed in claim 7, wherein the top product is separated into a C4 hydrocarbon mixture and methanol.

9. The process as claimed in claim 7 or claim 8, wherein the C4 hydrocarbon mixture contains less than 0.Sppm by weight of methyl tert-butyl ether. The process as claimed in claim 7 or claim 8, wherein the bottom product contains less than 2500ppm by weight of methyl sec-butyl ether and less than 2500ppm by weight of C8 hydrocarbons.

11. A process for preparing methyl tert-butyl ether and a virtually isobutene free C4 hydrocarbon mixture by reaction of an isobutene containing C4 hydrocarbon stream with methanol oooo• over an acid catalyst, said process being substantially as hereinbefore described with reference to any one of the examples, but excluding the comparative examples.

12. A virtually isobutene free C4 hydrocarbon mixture prepared by the process as claimed in any one of claims 1 to 11.

13. The use of the virtually isobutene free C4 hydrocarbon mixture as claimed in claim 12 for preparing n-butene oligomers.

14. Methyl tert-butyl ether prepared by the process as claimed in any one of claims 1 to 11. LibC567718speci The use of the methyl tert-butyl ether as claimed in claim 14 for preparing thoroughly pure isobutene.

16. The use of the methyl tert-butyl ether as claimed in claim 14 as a solvent.

17. A solvent containing the methyl tert-butyl ether as claimed in claim 14.

18. The use of the methyl tert-butyl ether as claimed in claim 14 as a component for 4-stroke fuels.

19. A 4-stroke fuel containing the methyl tert-butyl ether as claimed in claim 14. Dated 15 October 2001 OXENO OLEFINCHEMIE GMBH Patent Attorneys for the Applicant/Nominated Person SPRUSON FERGUSON .O° ooooo* i LibC567718speci