Process for preparing linear butenes from methanol

Abstract

The invention relates to a method for producing linear butenes from methanol. The problem addressed is that of specifying such a method in which the methanol used is converted, to the largest possible extent, into butenes. The problem is solved by combining a methanol-to-propylene process with a metathesis reaction by means of which the propene obtained from the methanol is converted into linear butenes.

Claims

1. Process for preparing linear butenes from methanol, comprising: a) reacting methanol in a first reaction stage to give a first reaction mixture containing dimethyl ether, water, and optionally unreacted methanol; b) reacting dimethyl ether in a second reaction stage to give a second reaction mixture containing propene and further hydrocarbons having two, four, and five carbon atoms, where the second reaction stage is at least partly supplied with the first reaction mixture; c) working-up the second reaction mixture to give a propene-rich fraction and at least one low-propene fraction, where the low-propene fraction is at least partly recirculated to the second reaction stage; d) reacting propene in a third reaction stage to give a third reaction mixture containing ethene and linear butenes selected from the group consisting of 1-butene, cis-2-butene, trans-2-butene, where the third reaction stage is supplied at least partly with or from the propene-rich fraction; e) working-up the third reaction mixture to give a target fraction rich in linear butenes and an ethene-rich fraction, wherein the propene-rich fraction contains propane, wherein the reaction in the third reaction stage occurs in the presence of propane, and wherein a propane-rich fraction is isolated during the course of working-up the third reaction mixture.

2. Process according to claim 1, wherein the ethene-rich fraction is at least partly recirculated to the second reaction stage.

3. Process according to claim 1, further comprising: f) converting ethene into a fourth reaction mixture comprising linear butenes selected from the group consisting of 1-butene, cis-2-butene, trans-2-butene in a fourth reaction stage, where the fourth reaction stage is supplied from the ethene-rich fraction.

4. Process according to claim 1, wherein a fraction rich in hydrocarbons having two carbon atoms, a fraction rich in hydrocarbons having four carbon atoms, and a fraction rich in hydrocarbons having five carbon atoms are also isolated during the course of working-up the second reaction mixture, where the fraction rich in hydrocarbons having two carbon atoms and the fraction rich in hydrocarbons having five carbon atoms are at least partly recirculated to the second reaction stage.

5. Process according to claim 1, wherein a high boiler fraction containing hydrocarbons having more than five carbon atoms is also isolated during the course of working-up the second reaction mixture.

6. Process according to claim 1, wherein an aqueous fraction is also isolated during the course of of working-up the second reaction mixture.

7. Process according to claim 1, wherein, prior to the reacting: preparing a synthesis gas containing carbon monoxide and hydrogen from a water-containing or water-free carbon source, and optionally with addition of water or water vapour; and in a fifth reaction stage, catalytically converting the synthesis gas into methanol for the reacting a).

8. Process according to claim 7, wherein the carbon source is a fossil carbon source, a renewable carbon source, or a mixture thereof, and wherein the carbon source is selected from the group consisting of: hard coal, brown coal, petroleum fractions, peat, natural gas, oil sand, shale gas, wood, biogas, biomass, domestic waste, manure, and sewage sludge.

9. Process according to claim 1, wherein the reaction in the first reaction stage occurs in the presence of a solid silica-alumina catalyst.

10. Process according to claim 1, wherein the reaction in the second reaction stage occurs in the presence of a zeolite catalyst.

11. Process according to claim 1, wherein the reaction in the third reaction stage occurs in the presence of a tungsten and/or molybdenum catalyst.

12. Process according to claim 11, wherein propene which has not reacted in the third reaction stage is separated off from the third reaction mixture and recirculated to the third reaction stage.

13. Process according to claim 3, wherein the reaction in the fourth reaction stage occurs in the presence of a catalytic system composed of trialkylaluminium and alkyl titanate in ethers.

Description

(1) Various embodiments of the invention will now be illustrated with the aid of flow diagrams. For the purposes of improved clarity and comprehensibility, the flow diagrams have been reduced to the essentials. In particular, conveying devices and devices for altering pressure and temperature are not drawn in. The figures show:

(2) FIG. 1: flow diagram of a conventional MTO process with recirculation of the C.sub.4-hydrocarbons (prior art);

(3) FIG. 2: flow diagram of a conventional MTO process with taking-off of the C.sub.4-hydrocarbons (prior art);

(4) FIG. 3: flow diagram of a first embodiment according to the invention with a propane-propene column installed upstream of the metathesis;

(5) FIG. 4: flow diagram of a second embodiment according to the invention with a propane-propene column installed downstream of the metathesis;

(6) FIG. 5: flow diagram of a third embodiment according to the invention with a propane-propene column installed downstream of the metathesis and only a single C.sub.4-C.sub.5 column.

(7) A flow diagram of a conventional MTP process is depicted in FIG. 1. A stream of provided methanol (1) is, after heating and vaporization, fed into a first reaction stage (V1) for conversion of the methanol into dimethyl ether (DME). In a second reaction stage (V2), DME is completely or partly converted into olefins. For this purpose, the first reaction output (2), which contains at least DME, methanol and water, is, after further heating to the reaction temperature, fed into the second reaction stage (V2) for conversion of the DME and methanol into olefins. The streams (3), (24) and (34) are also recirculated to the second reaction stage (V2). In the second reaction stage (V2), the methanol still present, the DME and recycled components are converted into hydrocarbons. Water may also be involved in the reaction. The second reactor output (10), which contains at least DME, methanol, water and C.sub.1-C.sub.6-hydrocarbons, is, after cooling, fed to a process step (V3) in which the second reaction output (10) is quenched and water and an organic phase (13) containing unreacted DME, methanol and possibly residual water are separated off by distillation and phase separation. DME, water and methanol are recirculated as an organic fraction (3) to the second reaction stage (V2) and excess water is discharged as aqueous fraction (14). The remaining hydrocarbon mixture (21) is, after compression, separated by distillation in a low boiler column (V4) into an ethene-rich low boiler fraction (23) containing predominantly C.sub.1- and C.sub.2-hydrocarbons and a higher-boiling fraction (26) containing C.sub.3-hydrocarbons and higher hydrocarbons. The low boiler fraction (23) is partly recirculated to the second reaction stage (V2). In order to avoid undesirable accumulation of by-products in the process, a purge stream (25) is discharged. The higher-boiling fraction (26) is separated by distillation in a C.sub.3 column (V5) into a fraction rich in C.sub.3-hydrocarbons (28) and a stream (27) which contains C.sub.4-hydrocarbons and higher hydrocarbons. The C.sub.3-rich fraction (28) is separated by distillation in a propane/propene column (V6) into a propene-rich fraction (32) containing virtually pure propene and a propane-rich fraction (31) which contains predominantly propane. The stream (27) is separated by distillation in a high boiler column (V7) into a relatively low-boiling fraction (30) containing predominantly C.sub.4- and C.sub.5-hydrocarbons and a high boiler fraction (29) containing C.sub.5+-hydrocarbons, i.e. C.sub.6-hydrocarbons and higher hydrocarbons. The relatively low-boiling fraction (30) is partly recirculated to the second reaction stage (V2). In order to avoid undesirable accumulation of by-products in the process, a purge stream (35) is discharged.

(8) A flow diagram of a variant of a conventional MTP process is shown in FIG. 2. In this variant, the C.sub.4-hydrocarbons are not recycled to the second reaction stage (V2) but instead are separated from the C.sub.5-hydrocarbons (7) in a C.sub.4 column (V8) and isolated as C.sub.4-rich fraction (6). This contains, due to the process, a mixture and saturated and unsaturated C.sub.4 isomers. The C.sub.4 yield leaves something to be desired since the process is optimized for the production of C.sub.3-olefins, namely propene (32).

(9) A flow diagram of a first embodiment according to the invention of a plant in which the process of the invention can be carried out is shown in FIG. 3. In so far as this embodiment corresponds to the above-described MTP process, reference is made to the detailed description of FIGS. 1 and 2.

(10) In this first embodiment according to the invention, a propene-rich fraction (32) together with the stream (38) are converted completely or partly into butenes and ethene and relatively small proportions of pentenes by metathesis in a third reaction step (V9). A third reaction mixture (34) from the metathesis (V9) is separated by distillation in a C.sub.3 column (V10) into a stream (35) containing predominantly propene, propane and ethene and a stream (40) containing predominantly butenes and pentenes. In a C.sub.2 column (V11), an ethene-rich fraction (36) containing predominantly ethene is separated off from stream (35). This gives a fraction (37) containing predominantly propene and propane. This is, after a purge stream (39) has been separated off in order to avoid accumulation of propane to undesirable concentrations, recirculated as propene-rich fraction to the metathesis (V9). The ethene-rich fraction (36) can, after a purge stream (36a) has been separated off, be partly recirculated to the second reaction step (V2).

(11) In a C.sub.5 column (V12), the butenes are separated off from higher-boiling components, predominantly pentenes formed in the metathesis. The C.sub.5-rich fraction (41) can be completely or partly recirculated to process step (V2). The butene-rich fraction (50) contains predominantly linear butenes (1-butene and 2-butenes) and together with stream (6) represents the product of the process of the invention. To increase the yield of C.sub.4-hydrocarbons further, partial recirculation of stream (36) can be omitted. Instead, this stream can optionally be fed to an ethylene dimerization. The ethylene dimerization takes place in a fourth reaction stage which is not shown in the figure. By-products from the dimerization can optionally also be recirculated to the second reaction stage (V2).

(12) A flow diagram of a second embodiment according to the invention of a plant in which the process of the invention can be carried out is shown in FIG. 4. In this variant, a partial removal of propane (in process step (V6) in FIG. 3) is omitted. Instead, a propane-containing, propene-rich fraction (28) is fed directly to the metathesis (V9) and the latter is thus carried out in the presence of propane. In process step (V13), a propane-rich fraction (39a) is then separated off from the recycle stream (37) by distillation. This variant has the advantage over the variant in FIG. 3 that the outlay for separating off the propane (39a) from stream (37) is lower than from stream (28).

(13) A flow diagram of a third embodiment according to the invention of a plant in which the process of the invention can be carried out is shown in FIG. 5. In this variant, the process steps (V8) and (V12) in the embodiment shown in FIGS. 3 and 4 are omitted. Hence, the C.sub.5-hydrocarbons in streams (30) and (40) can be separated off from the target product, viz. the C.sub.4-hydrocarbons, in only one process step (V14). This variant has the advantage over the variants shown in FIGS. 3 and 4 that the outlay in terms of apparatus for purification of the C.sub.4-hydrocarbons is lower.

LIST OF REFERENCE SYMBOLS

(14) V1: First reaction stage for conversion of methanol into DME (Process step b): DME synthesis) V2: Second reaction stage for conversion of DME into olefins (Process step c): MTP reactor) V3: Quench, isolation of water and recirculation of water, methanol and DME V4: Low boiler column (Isolation of C.sub.2-hydrocarbons and lower-boiling components) V5: C.sub.3 column (Isolation of C.sub.3-hydrocarbons) V6: Propane-propene column V7: High boiler column (Isolation of C.sub.5+-hydrocarbons and higher-boiling components) V8: C.sub.4 column V9: Third reaction stage for conversion of propene into olefins (Process step e): metathesis reaction) V10: C.sub.3 column (Isolation of C.sub.3-hydrocarbons and lower-boiling components) V11: C.sub.2 column V12: C.sub.5 column V13: Propane-propene separation V14: C.sub.4-C.sub.5 separation 1: Methanol 2: First reaction mixture from V1, mixture of, inter alia, methanol, DME, water 3: Recycle stream into V2, mixture of, inter alia, methanol, DME, water 6: C.sub.4-rich fraction containing, inter alia, 1-butene, 2-butenes, n-butane 7: C.sub.5-hydrocarbons 7a: C.sub.5-hydrocarbons 8: C.sub.5-hydrocarbons 10: Second reaction mixture from V2, mixture of, inter alia, methanol, DME, water, hydrocarbons 14: Water discharge (quench) 21: Hydrocarbon mixture, including C.sub.1-C.sub.6-hydrocarbons 23: Low boiler fraction containing, inter alia, methane, ethene, ethane 24: Hydrocarbon mixture, including methane, ethene, ethane 25: Purge stream composed of low boilers, including methane, ethene, ethane 26: Relatively high-boiling fraction containing, inter alia, C.sub.3-C.sub.6-hydrocarbons 27: Hydrocarbon mixture, including C.sub.4-C.sub.6-hydrocarbons 28: C.sub.3-hydrocarbons (propene, propane) 29: High boiler fraction, C.sub.5+-hydrocarbons and higher-boiling components 30: Hydrocarbon mixture, including C.sub.4-C.sub.5-hydrocarbons 31: Propane-rich fraction 32: Propene-rich fraction 34: Third reaction mixture from V9, including ethene, propane, propene, butenes, pentenes 35: Purge composed of ethene, propane, propene 36: Ethene-rich fraction 36a: Purge stream composed of ethene 43: Ethene 37: Propane, propene 38: Propane, propene 39: Propane, propene 39a: Propane-rich fraction 40: Butenes, pentenes 41: C.sub.5-rich fraction containing, inter alia, pentenes 41a: Purge stream composed of pentenes 42: Pentenes 45: C.sub.5-hydrocarbons 46: C.sub.5-hydrocarbons 47: C.sub.5-hydrocarbons 50: Butene-rich fraction (target fraction) 51: Butenes