GB1594158A - Purification of methyl tert-butyl ether - Google Patents

Description

(54) PURIFICATION OF METHYL TERT.-BUTYL ETHER (71) We, BASF AKTIENGESELLSCHAFT, a German Joint Stock Company of 6700 Ludwig shafen, Federal Republic of Germany, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following Statement: The present invention relates to a new process for the purification of methyl tert.-butyl ether (MTB).

The manufacture of MTB by reacting methanol with isobutene, using acid catalysts, in the liquid phase at from 20 to 150 C and under a correspondingly elevated pressure (from about 2.5 to 50 bars) has been disclosed, for example in U.S. Patent 2 480 940 and in German Patent 1 224 294. To achieve industrially satisfactory conversions of isobutene, an excess of methanol is used, but this has the disadvantage that the excess must be separated from the MTB. Even if less than the equivalent amount of methanol is used, some methanol passes into the MTB. Since methanol and MTB form an azeotrope, the methanol is extracted with water, a procedure which entails expensive apparatus and high energy consumption and in turn requires that the methanolwater mixture should be worked up by distillation (cf., for example, Oil and Gas Journal, June 1975, pages 50-53). The methanol produced, which is recycled to the reaction, contains substantial amounts of MTB, which shift the reaction equilibrium in the adverse direction. Furthermore, the MTB obtained by the above process still contains about 1.5% by weight of water which has to be removed.

Since MTB, whether used as a solvent or as an antiknock compound in gasoline, must be virtually methanol-free (the presence of methanol in motor fuels would require special modifications of the fuel system), the present invention seeks to obtain MTB in a pure form from its mixtures with methanol by a more economical method than that provided by the prior art.

We have found that methyl tert.-butyl ether (MTB) may be obtained from its mixtures with methanol if the methanol is removed azeotropically from these mixtures by means of a hydrocarbon of 4 carbon atoms under a pressure of from 1 to 25 bars.

Since it is advantageous to employ from 1 to 3 moles of methanol per mole of isobutene when synthesizing MTB, the MTB/methanol mixtures resulting from the synthesis generally contain from 30 to 420 g of methanol per kilogram. Depending on the nature of the hydrocarbon of 4 carbon atoms and the pressure prevailing during distillation, from 10 to 120 g of entraining agent are required for the complete azeotropic removal of 1 g of methanol. Whilst the various hydrocarbons of 4 carbon atoms, and their mixtures, differ little in their effectiveness, the pressure exerts a substantial influence on the composition of the azeotrope.

The list below shows the proportions of methanol in the azeotrope with but-l-ene under various pressures.

bar % by weight of methanol in azeotrope 1 0.8 2 1.7 5 2.6 10 4.4 15 6.1 20 7.6 25 9.1 It is true that the proportion of methanol in the azeotrope increases with increasing pressure, but the technical problems also increase.

Furthermore, at the temperatures of azeotrope formation which correspond to the higher pressure, there is a danger of decomposition of the MTB. On the other hand, too low a pressure has the disdavantage that substantial amounts of the hydrocarbons of 4 carbon atoms are required to remove the methanol.

Accordingly, the optimum conditions for industrial purposes depend on the individual circumstances.

From a purely chemical point of view and from the point of view of process technology, isobutene is the most suitable hydrocarbon for the azeotropic distillation, since it can be recycled with the methanol to the etherification stage. However, in most cases it is more advantageous to carry out the etherification with C4-cuts instead of pure isobutene, since isobutene reacts selectively to give MTB, and the remaining hydrocarbons of the C4 cuts then serve for the azeotropic distillation. If the amount present does not suffice, the hydrocarbons are advantageously partially recycled to the azeotropic distillation after they have been separated from the methanol.

The distillation process according to the invention can be inserted particularly advantageously into the overall process for the synthesis of MTB as follows: A C4 -cut of the conventional industrial composition, from which the particularly valuable butadiene has as a rule already been removed, and which contains from 35 to 60% by weight of isobutene, is passed, in a conventional manner, over a strongly acid cation exchanger together with up to 100 mole% excess of methanol, at from 40 to 900C and from 5 to 20 bars. When the isobutene has been virtually completely converted, the mixture is passed into a pressure distillation column in which the methanol is removed azeotropic.

ally together with the remaining hydrocarbons of 4 carbon atoms at from 1 to 25 bars, preferably from 4 to 20 bars. In this column, the temperatures, for the general pressure range of from 1 to 25 bars, are from -7 to 11 50C at the top and from 55 to 2000C at the bottom, whilst for the preferred pressure range of from 4 to 20 bars they are from 35 to I030C at the top and from 105 to 1850C at the bottom. In general, the process parameters are selected so that the excess amount of methanol corresponds exactly to the amount of hydrocarbons for azeotropic distillation. If this is not possible, the amount of hydrocarbon corresponding to the shortfall must be added, and this is advantageously achieved by recycling the hydrocarbons of 4 carbon atoms which have been freed from methanol. The pure MTB obtained at the bottom of the column may be taken off as liquid or vapor. The latter method requires more energy but has the advantage that the less volatile substances are substantially left in the boiler.

Since a single-stage etherification, aiming at virtually complete conversion of isobutene, has the disadvantage of a realtively long reaction time, the overall economics of the process can in many cases be improved by dividing the etherification and distillation over a plurality of stages, in particular two stages. In this embodiment, in which the first stage is advantageously carried out with a slight excess of methanol or even with a less than equivalent amount of methanol, the isobutene is only converted to the extent of about 80-85% in the first etherification stage. The azeotrope of hydrocarbons of 4 carbon atoms and methanol, containing isobutene, which is obtained as the top product during subsequent distillation is mixed with fresh methanol and fed to the second etherification stage, where the isobutene reacts virtually completely. The MTB formed in the 2nd reaction stage is freed from hydrocarbons of 4 carbon atoms and from methanol by azeotropic distillation in a further distillation column, and is combined with the bottom product from the 1st column.

The methanol/hydrocarbon azeotropes are advantageously separated by extraction with water; because of the volatility of the hydrocarbons, the extraction is also carried out under pressure. The aqueous methanolic phase thus obtained is separated into its components by distillation. The water is recycled to the extraction stage and the methanol to the etherification stage.

If pure isobutene is used, i.e. if the latter also serves to form the azeotrope during distillation, the isobutene/methanol azeotrope obtained as the top product is advantageously recycled to the reaction. If C4-cuts are used, distillation and extraction gives a substantially isobutene-free mixture of the residual hydrocarbons, which are used for other syntheses, e.g. the manufacture of plastics.

The MTB may be obtained directly in from 99.0 to 99.9% purity and accordingly requires no further purification before use as a solvent or as an anti-knock compound in gasolines. The overall process for the manufacture of MTB is so advantageous that it can even be used to isolate isobutene from C4 -cuts, by re-decomposing the MTB into methanol and isobutene in the conventional manner over an acid catalyst at an elevated temperature.

EXAMPLE Per hour, 0.5 kg of a C4 -cut composed (in per cent by weight) of 50% of isobutene, 23% of but-l-ene, 9% of trans but-2-ene, 6% of cis but-2-ene and 12% of n- and iso-butane, was passed, together with 0.14 kg of methanol (molar ratio of methanol : isobutene = 0.97 1) at 70"C and 12 bars pressure through an 8 m long tubular reactor, the residence time being 14 minutes. In this reactor, the isobutene was selectively converted, by means of a strongly acid cation exchanger, to methyl tert.-butyl ether (MTB), the conversion of isobutene being 92%.

The mixture leaving the reactor, which contained 56.3% of MTB, 1.3% of methanol, 3.2% of isobutene, 39.0% of other hydrocarbons of 4 carbon atoms and 0.2% of by-products, was rectified in a distillation column with 16 theoretical plates under 4.5 bars at from 380C (top temperature) to 1100C (bottom temperature), using a reflux ratio of 1.4. The MTB taken off as vapor just above the bottom was 99.4% pure.

The top product (0.28 kglhour) contained 2.7% of methanol, 7.2% of isobutene, 89.3% of other hydrocarbons of 4 carbon atoms and 0.8% of MTB.

Subsequent distillation of this mixture under pressure, in a column with 20 theoretical plates, under 4.5 bars at from 380C (top temperature) to 1 100C (bottom temperature), using a reflux ratio of 0.3, gave a top product which in addition to the other hydrocarbons of 4 carbons atoms, and small amounts of impurities, contained 2.4% of methanol and 1.0% of isobutene.

This mixture was extracted in the conventional manner in a pulsating packed column at 25"C under 3 bars. The organic raffinate phase was taken from the system as one of the products, whilst the water and methanol, after conventional separation by distillation, were recycled.

Overall, the MTB was obtained in 99% yield based on isobutene employed, and was 99.4% pure.

WHAT WE CLAIM IS:- 1. A process for obatining pure methyl tert.-butyl ether (MTB) from a mixture thereof with methanol, wherein the methanol is removed azeotropically from the mixture by means of a hydrocarbon of 4 carbon atoms, under a pressure of from 1 to 25 bars.

2. A process as claimed in Claim 1, wherein the starting mixture has been obtained by reacting isobutene and methanol over a strongly acid cation exchanger to give MTB with incomplete conversion of the methanol.

3. A process as claimed in Claim 2, wherein the isobutene has been employed together with one or more other hydrocarbons of 4 carbon atoms in the manufacture of the starting mixture.

4. A process as claimed in Claim 3, wherein the residual C4 hydrocarbon(s) in the starting mixture serve for the azeotropic distillation without addition of other C4 hydrocarbon material.

5. A process as claimed in any of Claims 1 to 4, wherein the distillation pressure is from 4 to 20 bars.

6. A process for the manufacture of pure MTB from isobutene by reaction with methanol over a strongly acid cation exchanger, with incomplete conversion of the methanol, followed by a process as claimed in Claim 1, 3, 4 or 5, wherein the reaction is carried out in two stages with the azeotropic distillation being performed after each stage and unconverted isobutene from the first stage being reacted in the second stage after admixture with fresh methanol.

7. A process as claimed in Claim 1 or 6 carried out substantially as described in the foregoing Example.

8. Methyl tert.-butyl ether having a purity of from 99.0 to 99.9% when obtained by a process as claimed in any of Claims 1 to 7.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. plates, under 4.5 bars at from 380C (top temperature) to 1 100C (bottom temperature), using a reflux ratio of 0.3, gave a top product which in addition to the other hydrocarbons of 4 carbons atoms, and small amounts of impurities, contained 2.4% of methanol and 1.0% of isobutene. This mixture was extracted in the conventional manner in a pulsating packed column at 25"C under 3 bars. The organic raffinate phase was taken from the system as one of the products, whilst the water and methanol, after conventional separation by distillation, were recycled. Overall, the MTB was obtained in 99% yield based on isobutene employed, and was 99.4% pure. WHAT WE CLAIM IS:-

1. A process for obatining pure methyl tert.-butyl ether (MTB) from a mixture thereof with methanol, wherein the methanol is removed azeotropically from the mixture by means of a hydrocarbon of 4 carbon atoms, under a pressure of from 1 to 25 bars.

2. A process as claimed in Claim 1, wherein the starting mixture has been obtained by reacting isobutene and methanol over a strongly acid cation exchanger to give MTB with incomplete conversion of the methanol.

3. A process as claimed in Claim 2, wherein the isobutene has been employed together with one or more other hydrocarbons of 4 carbon atoms in the manufacture of the starting mixture.

4. A process as claimed in Claim 3, wherein the residual C4 hydrocarbon(s) in the starting mixture serve for the azeotropic distillation without addition of other C4 hydrocarbon material.

5. A process as claimed in any of Claims 1 to 4, wherein the distillation pressure is from 4 to 20 bars.

6. A process for the manufacture of pure MTB from isobutene by reaction with methanol over a strongly acid cation exchanger, with incomplete conversion of the methanol, followed by a process as claimed in Claim 1, 3, 4 or 5, wherein the reaction is carried out in two stages with the azeotropic distillation being performed after each stage and unconverted isobutene from the first stage being reacted in the second stage after admixture with fresh methanol.

7. A process as claimed in Claim 1 or 6 carried out substantially as described in the foregoing Example.

8. Methyl tert.-butyl ether having a purity of from 99.0 to 99.9% when obtained by a process as claimed in any of Claims 1 to 7.