METHOD FOR OBTAINING PURE 1,3-BUTADIENE

Abstract

Process for isolating pure 1,3-butadiene from a crude C.sub.4 fraction by extractive distillation using a selective solvent, wherein (a) the crude C.sub.4 fraction is introduced into a predistillation column, a first low boiler fraction comprising C.sub.3-hydrocarbons is taken off as overhead stream, a gaseous C.sub.4 fraction is taken off as side stream and a first high boiler fraction is taken off as bottom stream, (b) the gaseous C.sub.4 fraction is brought into contact with a selective solvent in at least one extraction column, giving an overhead fraction comprising butanes and butenes and a bottom fraction comprising 1,3-butadiene and selective solvent, (c) crude 1,3-butadiene is desorbed from the bottom fraction in at least one stripping column, with a stripped selective solvent being obtained and the stripped selective solvent being recirculated to the extraction column, and (d) at least part of the crude 1-3-butadiene is fed to a pure distillation column and a second high boiler fraction is separated off and a gaseous purge stream is taken off. Gaseous purge streams from the columns which are necessary in order to keep the concentration of molecular oxygen below a predetermined concentration limit are consolidated with output streams which are in any case provided for discharging other components in the process. The recirculation of the second high boiler fraction to a lower section of the predistillation column creates a further degree of freedom in operation of the pure distillation column.

Claims

1-13. (canceled)

14. A process for isolating pure 1,3-butadiene from a crude C.sub.4 fraction by extractive distillation using a selective solvent, the process comprising: a) introducing a liquid crude C.sub.4 fraction into an inflow region of a predistillation column, which is divided in a middle section by a dividing wall aligned essentially in a longitudinal direction of the predistillation column into the inflow region and a side offtake region, taking off a first low boiler fraction comprising C.sub.3-hydrocarbons as an overhead stream, taking off a gaseous C.sub.4 fraction as a side stream from the side offtake region and taking off a first high boiler fraction as a bottom stream, b) bringing the gaseous C.sub.4 fraction into contact with a selective solvent in at least one extraction column, to obtain an overhead fraction comprising butanes and butenes and a bottom fraction comprising 1,3-butadiene and the selective solvent, c) desorbing crude 1,3-butadiene from the bottom fraction in at least one stripping column, with a stripped selective solvent being obtained and recirculating the stripped selective solvent to the at least one extraction column, and d) feeding at least part of the crude 1-3-butadiene to a pure distillation column and separating off a second high boiler fraction and taking off a gaseous purge stream and recirculating the second high boiler fraction to a lower section of the predistillation column.

15. The process of claim 14, wherein the gaseous purge stream from the pure distillation column is conveyed together with vapor from the predistillation column through an overhead condenser of the predistillation column.

16. The process of claim 14, wherein the crude 1,3-butadiene is brought into contact with the stripped selective solvent in an after-scrubbing zone.

17. The process of claim 15, wherein the desorbed crude 1,3-butadiene is partially condensed, a condensed part of the crude 1,3-butadiene is conveyed as runback into the at least one stripping column and/or into an after-scrubbing zone and the other part of the crude 1,3-butadiene is fed in gaseous form to the pure distillation column.

18. The process of claim 14, wherein the gaseous C.sub.4 fraction is brought into contact with the selective solvent in an extraction column and in an upper section of an extraction and prestripping column and the crude 1,3-butadiene is desorbed from the bottom fraction in a lower section of the extraction and prestripping column and a stripping column.

19. The process of claim 18, wherein an after-scrubbing zone is an upper section of the extraction and prestripping column separated off by a dividing wall running essentially in a longitudinal direction of the extraction and prestripping column.

20. The process of claim 15, wherein a gas comprising C.sub.4-acetylenes is also desorbed from the bottom fraction.

21. The process of claim 20, wherein the gas comprising C.sub.4-acetylenes is taken off as a side offtake stream from the at least one stripping column.

22. The process of claim 20, wherein the gas comprising C.sub.4-acetylenes is diluted with uncondensed constituents of vapor from the at least one extraction column.

23. The process of. claim 14, wherein the crude C.sub.4 fraction comprises from 15 to 85% by weight of 1,3-butadiene, from 4 to 13% by weight of butanes, from 24 to 64% by weight of butenes, from 0.2 to 0.5% by weight of C.sub.4-acetylenes, from 0.01 to 2.0% by weight of C.sub.3-hydrocarbons and from 0.01 to 0.5% by weight of C.sub.5+-hydrocarbons.

24. The process of claim 14, wherein the selective solvent comprises at least 80% by weight of N-methylpyrrolidone.

Description

[0056] The invention will be illustrated in detail by the accompanying drawing and the following examples.

[0057] FIG. 1 schematically shows a preferred plant for carrying out the process of the invention.

[0058] A liquid crude C.sub.4 fraction, stream 1, is introduced into the predistillation column K1. The predistillation column K1 is divided, in a middle section, by a dividing wall 2 aligned essentially in the longitudinal direction of the predistillation column K1 into an inflow region 3 and a side offtake region 4. The inflow region 3 and the side offtake region 4 each extend in the vertical direction from the upper to the lower end of the dividing wall 2. A gaseous C.sub.4 fraction 7 is taken off from the side offtake region 4. The vapor from the predistillation column K1 is passed through the overhead condenser 5. The condensate formed therein is fed back into the predistillation column K1. The uncondensed part of the vapor forms the overhead stream 6 which is taken off as a low boiler fraction from the predistillation column K1. In addition, a first high boiler fraction is taken off as bottom stream 9 from the predistillation column K1. The bottom of the predistillation column K1 is heated by means of the vaporizer 8.

[0059] The gaseous C.sub.4 fraction 7 is introduced into a lower section of an extraction column K2 and is brought into contact therein with a stripped selective solvent 10 which is fed into an upper section of the extraction column K2. Condensation of the vapor from the extraction column K2 in the condenser 11 gives an overhead fraction 12 (known as raffinate I) comprising butanes and butenes and also an uncondensed part 13 of the vapor. The stream 13 serves as the diluent gas necessary for safety reasons for the acetylenes 32. At the same time, the stream 13 is the purge stream for the extraction column K2 in order to control the content of molecular oxygen in the gas space of the extraction column K2 and keep it below the detection limit. In addition, a bottom fraction 14 which consists essentially of selective solvent in which 1,3-butadiene and also methylacetylene, vinylacetylene, ethylacetylene and C.sub.5+-hydrocarbons are dissolved is obtained.

[0060] The bottom fraction 14 is fed into an upper section 15 of an extraction and prestripping column K3. A substantial part of the crude 1,3-butadiene is desorbed in the lower section 18 of the extraction and prestripping column K3. The crude 1,3-butadiene is brought into contact with stripped selective solvent 23 in an after-scrubbing zone in order to separate off C.sub.4-acetylenes. The after-scrubbing zone is an upper section 16 of the extraction and prestripping column K3 which is separated off by a dividing wall running essentially in the longitudinal direction of the column. The crude 1,3-butadiene 19 discharged at the top of the extraction and prestripping column K3 from the scrubbing section located immediately above the after-scrubbing zone is partially condensed in the condenser 20. The condensed part 21 of the crude 1,3-butadiene is conveyed as runback into the scrubbing section via which the runback flows into the after-scrubbing zone. The other part 22 of the crude 1,3-butadiene is fed in gaseous form to the pure distillation column K5. The bottom of the extraction and prestripping column K3 is heated by means of the vaporizer 25.

[0061] A gas 17 comprising butanes and butenes which is discharged from section 15 at the top of the extraction and prestripping column K3 is conveyed back into a lower section of the extraction column K2. Thermodynamically, the section 15 of the extraction and prestripping column K3 and the extraction column K2 together correspond to a single extraction column, which for reasons of the column height has been divided in two vertically.

[0062] Predegassed solvent from the bottom of the extraction and prestripping column K3 is conveyed further as stream 26 into a stripping column K4. In the stripping column K4, further crude 1,3-butadiene is desorbed and this is conveyed via conduit 27, the direct cooler K6, the compression 31 and conduit 24 into the lower section of the extraction and prestripping column K3. Cooling medium is introduced and discharged via the conduits 29 and 30. Intrinsic condensate, which consists essentially of components of the selective solvent, e.g. water and NMP, serves as cooling medium. The bottom of the stripping column K4 is heated by means of the vaporizer 37.

[0063] A gas 28 comprising C.sub.4-acetylenes is taken off as side offtake stream from the stripping column K4. In the acetylene scrubber K7, the gas 28 comprising C.sub.4-acetylenes is scrubbed with water introduced via conduit 36. The stream 32 obtained at the top of the acetylene scrubber K7 is diluted with the purge stream 13 from the extraction column K2. Condensed constituents are condensed in the condenser 33 and can partly be introduced as runback 35 into the acetylene scrubber K7; the remainder is essentially process wastewater. Uncondensed constituents are discharged as stream 34 (diluted acetylene stream).

[0064] In the pure distillation column K5, a second high boiler fraction 43 is separated off from the crude 1,3-butadiene 22. The vapor is introduced into the condenser 39. The uncondensed part of the vapor forms the purge stream 38. The pure 1,3-butadiene is obtained as condensate in the condenser 39 and is taken off as stream 40; a substream is introduced as runback into the pure distillation column K5. The bottom of the pure distillation column K5 is heated by means of the vaporizer 42.

[0065] The second high boiler fraction 43 is recirculated into a lower section of the predistillation column K1. The deaeration stream 38 is conveyed via the blower 41 to the vapor of the predistillation column K1.

EXAMPLE 1

[0066] The process of the invention was simulated on the basis of a plant as shown in FIG. 1. The BASF in-house software Chemasim was used for the simulation calculation; comparable results would be obtained using commercially available software such as Aspen Plus (manufacturer: AspenTech, Burlington/Mass., USA) or PRO II (Fullerton, USA). The set of parameters was based on comprehensive equilibrium measurements, studies on laboratory columns and operating data from various plants. The target specification for the pure 1,3-butadiene was: at least 99.5% of 1,3-butadiene, not more than 20 ppm of 1,2-butadiene, not more than 20 ppm of acetylenes.

[0067] A crude C.sub.4 fraction comprising 1300 ppm of C.sub.3-hydrocarbons, 2.0% of n-butane, 0.6% of isobutane, 19.0% of n-butene, 28.3% of isobutene, 5.5% of trans-2-butene, 4.4% of cis-2-butene, 39.0% of 1,3-butadiene, 0.2% of 1,2-butadiene, 1200 ppm of 1-butyne, 4500 ppm of vinylacetylene and 3000 ppm in each case of C.sub.5-hydrocarbons was taken as starting point.

[0068] Table 1 summarizes the mass flows and compositions of relative streams. The designations of these streams in the table relate to the designations in FIG. 1.

TABLE-US-00002 TABLE 1 Stream Example 1 1 7 9 43 Mass flow 32 000 31 935 200 37 [kg/h] 1,2-Butadiene 0.15 0.07 12.13 40.00 [% by weight] Acetylenes 0.57 0.57 0.38 0.20 [% by weight] C.sub.5+ components 0.30 0.02 45.00 8.97 [% by weight] 1,3-Butadiene 3.63 35.94 [% by weight]

[0069] A loss of 1,3-butadiene of about 7 kg/h results from the single high boiler purge stream 9. The content of C.sub.5 components as the high boilers which are most difficult to separate off is reduced by about 94% in the predistillation column.

COMPARATIVE EXAMPLE 2

[0070] A process according to the prior art was simulated. The composition of the crude C.sub.4 fraction and the target specification for the pure 1,3-butadiene were the same as in example 1. The plant used as a basis had a preceding distillation column as per WO 2013/083536 A1 to which the liquid crude C.sub.4 fraction was fed instead of the predistillation column K1 provided with the dividing wall 2. Recirculation of the high boiler fraction from the pure distillation column into the predistillation column is not provided.

[0071] Table 2 summarizes the mass flows and compositions of relevant streams.

TABLE-US-00003 TABLE 2 Stream Comparative Crude Crude Bottoms Bottoms example 2 C.sub.4 liq..sup.1 C.sub.4 purif..sup.2 dist...sup.3 pure.sup.4 Mass flow 32 000 31 743 110 69 [kg/h] 1,2-Butadiene 0.15 0.14 40.00 40.00 [% by weight] Acetylenes 0.57 0.57 0.21 0.18 [% by weight] C.sub.5+ components 0.30 0.13 8.38 30.18 [% by weight] 1,3-Butadiene 29.88 14.88 [% by weight] .sup.1liquid crude C.sub.4 fraction (reference symbol 1 in FIG. 1 of WO 2013/083536 A1) .sup.2vaporous purified crude C.sub.4 fraction (reference symbol 4 in FIG. 1 of WO 2013/083536 A1) .sup.3bottom stream obtained from the upstream distillation column (reference symbol 3 in FIG. 1 of WO 2013/083536 A1) .sup.4bottom stream obtained in the pure distillation

[0072] In this process, two C.sub.5+-comprising high boiler streams which are discarded are obtained: the bottom stream from the upstream distillation column and the bottom stream obtained in the pure distillation. There is a loss of 1,3-butadiene of about 43 kg/h. In the upstream distillation column of comparative example 2, only a small proportion of the components was separated off (55%). A far higher proportion of the C.sub.5+ components was carried via the gaseous purified crude C.sub.4 fraction into the extraction column.

EXAMPLE 3

Comparison of the Purge Streams for Removal of Oxygen

[0073] Table 3 compares the mass flows of the conventional purge streams for removal of oxygen with the internal streams proposed according to the invention for the removal of oxygen. The table illustrates that replacement of purge streams by internal streams enables the mass flow thereof to be multiplied without loss of product of value. Total gases comprising from about 30 to 60 kg of 1,3-butadiene are typically discharged per hour via the purge streams customary according to the prior art. The streams 13, 22 and 38 each replace one purge stream in the plant of FIG. 1. Nevertheless, the loss of product of value occurring via these streams is lower since the streams are recirculated into the process. Increasing the mass flows from 10-20 kg/h to 360 kg/h, 18 376 kg/h and 360 kg/h, respectively, results in more oxygen being discharged from the columns. As a result, the oxygen content can be pushed to below the detection limit of conventional detectors and the undesirable polymerization tendency of 1,3-butadiene can consequently also be reduced.

TABLE-US-00004 TABLE 3 Process according to the invention Prior art (FIG. 1) Purge stream at Stream Top of the 10-20 kg/h 13 360 kg/h extraction column Top of the 10-20 kg/h 22 18 376 kg/h extraction and prestripping column Top of the 10-20 kg/h 38 200 kg/h pure distillation column Total 30-60 kg/h Total 18 936 kg/h Loss 30-60 kg/h Loss 0