CA1220757A - Separation of c.sub.4-hydrocarbon mixture by extractive distillation - Google Patents
Separation of c.sub.4-hydrocarbon mixture by extractive distillationInfo
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- CA1220757A CA1220757A CA000470880A CA470880A CA1220757A CA 1220757 A CA1220757 A CA 1220757A CA 000470880 A CA000470880 A CA 000470880A CA 470880 A CA470880 A CA 470880A CA 1220757 A CA1220757 A CA 1220757A
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- solvent
- hydrocarbons
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- extractive distillation
- distillation zone
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- C—CHEMISTRY; METALLURGY
- C07—ORGANIC CHEMISTRY
- C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
- C07C7/00—Purification; Separation; Use of additives
- C07C7/04—Purification; Separation; Use of additives by distillation
- C07C7/05—Purification; Separation; Use of additives by distillation with the aid of auxiliary compounds
- C07C7/08—Purification; Separation; Use of additives by distillation with the aid of auxiliary compounds by extractive distillation
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- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Engineering & Computer Science (AREA)
- Analytical Chemistry (AREA)
- Oil, Petroleum & Natural Gas (AREA)
- Water Supply & Treatment (AREA)
- Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
- Production Of Liquid Hydrocarbon Mixture For Refining Petroleum (AREA)
- Vaporization, Distillation, Condensation, Sublimation, And Cold Traps (AREA)
Abstract
Abstract of the Disclosure: A C4-hydrocarbon mixture which contains hydrocarbons which are more readily soluble in the selective solvent and those which are more sparingly soluble in the said solvent is separated with the aid of a selective solvent by a process in which the C4-hydrocarbon mixture is separated, in an extractive distillation zone, into a top product (distillate), which contains the more sparingly soluble hydrocarbons, and an extract containing the more readily soluble hydrocarbons and the selective solvent, the extract taken off as the bottom product is fed to a solvent recovery zone, in which the extract is separated into a product containing the hydrocarbons, and the selec-tive solvent which has been partially or completely freed from the hydrocarbons, and the concentration, based on the C4-hydrocarbon/selective solvent mixture formed in the extractive distillation zone, of the selective sol-vent in the said zone falls below 75% by weight on one or more trays or, in the case of packed extractive dis-tillation columns, at one or more points within the packing.
Description
Separation of a C4-hydrocarbon mixture by extractive distillation .
The present invention relates to a process for the separation, by extractive distillation with the aid of a selective solvent, of a c4-hYdrocarbon mixture which con-tains hydrocarbons which are more readily soluble in the selective solvent, and those which are more sparingly soluble in the said solvent.
Extractive distillation is a conventionaL method for separating mixtures which cannot readily be separated by conventional fractional distillation, for example where the components to be separated form an azeotrope or have small differences in the relative volatilities. In extrac-tive distillation, a relatively sparingly volatile solvent is introduced into the distillation column in an amount such that the differences in the relative volatilities of the components to be separated are increased, and - separation by distillation therefore becomes possible.
Typical examples of the use of extractive distillation can be found in, for example, C.S. Robinson et al., Elements of Fractional Distillation, 4th edition, McGraw-Hill Book Company, Inc., New York (1959), page Z91.
The separation of a C4-hydrocarbon mixture by extractive distillation with the use of a selective solvent in order to recover buta-1,3-diene has been disclosed, for example in German Published Application DAS 1,568,902 and German Patent 1,163,795.
In the conventional processes, a relatively high concentration, based on the C4 hydrocarbon/selective solvent mixture formed in the extractive distillation zone, of the selective solvent is maintained in the extractive distillation zone in order to effect selec-tive separation of the C4-hydrocarbons. However, this relatively high concentration of the selective solve~nt necessitates the circulation of large amounts of sol-vent and therefore results in the consumption of large ~Z207S7
The present invention relates to a process for the separation, by extractive distillation with the aid of a selective solvent, of a c4-hYdrocarbon mixture which con-tains hydrocarbons which are more readily soluble in the selective solvent, and those which are more sparingly soluble in the said solvent.
Extractive distillation is a conventionaL method for separating mixtures which cannot readily be separated by conventional fractional distillation, for example where the components to be separated form an azeotrope or have small differences in the relative volatilities. In extrac-tive distillation, a relatively sparingly volatile solvent is introduced into the distillation column in an amount such that the differences in the relative volatilities of the components to be separated are increased, and - separation by distillation therefore becomes possible.
Typical examples of the use of extractive distillation can be found in, for example, C.S. Robinson et al., Elements of Fractional Distillation, 4th edition, McGraw-Hill Book Company, Inc., New York (1959), page Z91.
The separation of a C4-hydrocarbon mixture by extractive distillation with the use of a selective solvent in order to recover buta-1,3-diene has been disclosed, for example in German Published Application DAS 1,568,902 and German Patent 1,163,795.
In the conventional processes, a relatively high concentration, based on the C4 hydrocarbon/selective solvent mixture formed in the extractive distillation zone, of the selective solvent is maintained in the extractive distillation zone in order to effect selec-tive separation of the C4-hydrocarbons. However, this relatively high concentration of the selective solve~nt necessitates the circulation of large amounts of sol-vent and therefore results in the consumption of large ~Z207S7
- 2 - O.Z.0050/36883 amounts of valuable energy.
The present invention is intended to improve the pro-cedure and cost-efficiency of the conventional processes.
It is an object of the present invention to pro-vide a process for the separation of a c4-hYdrocarbon mixture with the aid of a selective solvent, wherein both the complexity of the apparatus and hence the capi-tal costs, and the amount of selective solvent circu-lated and hence the energy consumption, can be reduced.
We have found that these and other objects and advantages are achieved, in accordance with the inven-tion, by a process for the separation, with the aid of a selective solvent, of a C4-hydrocarbon mixture which contains hydrocarbons which are more readily soluble in the selective solvent and those which are more sparingly soluble in the said solvent, wherein the C4-hydrocarbon mixture is separated, in an extractive distillation zone, into a top product (distillate), which contains the more sparingly soluble hydrocarbons, and an extract which contains the more readily soluble hydrocarbons and the selective solvent, the extract taken off as the bottom product is fed to a solvent recovery zone, in which the extract is separated into a product which con-tains the hydrocarbons, and the selective solvent which has been partially or completely freed from the hydro-carbons, and the concentration, based on the C4-hydro-carbon/selective solvent mixture formed in the extractive distillation zone, of the selective solvent in the said zone falls below 75% by weight on one or more trays or, in the case of a packed extractive distillation column, at one or more points within the packing.
In the novel process, the amount of selective solvent circulated can be reduced while maintaining the same separation efficiency as in the conventional pro-cesses operating with higher solvent concentration.This is surprising since the selectivity which is essen-tial for the separation decreases when the solvent ~;~Z0757 ~ 3 ~ O.Z.0050J36883 concentration in the extractive distillation zone is reduced. Reducing the amount of solvent circulated per-mits smaller apparatuses to be used, so that the capital costs are correspondingly reduced. Furthermore, in the case of the separation of butane and butene, the heat losses which result when the recycled selective solvent is recooled to the temperature at which the solvent enters the extractive distillation can be kept small if the heat remaining in the recycled solvent after heat exchange with laden solvent is used profitably for the separation in a distillation zone.
The process of the present application is generally applicable to the separation of C4-hydrocarbon mixtures which contain various unsaturated compounds.
In such systems, the more highly saturated compound is the comPonent which is the more sparingly so~uble in the selective solvent, and the less saturated compound is the component which is more readily soluble in the said solvent. In the case of isomers, for example an acetylenic - 20 compound and a diolefin, the acetylenic compound is more readily soluble than the diolefin.
For e%ample, the process according to the inven-tion is advantageously used for the separation of a buta-1,3-diene-containing C4-hydrocarbon mixture.
Such C4-hydrocarbon mixtures are obtained as, for example, C4-fractions in the preparation of ethy-lene and/or propylene by thermal cleavage of a petroleum fraction, for example liquefied petroleum gas ~LPG), naphtha, gas oil or the like. C5-fractions of this type are also obtained in the catalytic dehydrogenation of n-butane and/or n-butene. The C4 fractions contain, as a rule, butanes, n-butene, isobutene, buta-1,3-diene, vinylacetylene, ethylacetylene and buta-1,2-diene, with or without small amounts of C5-hydrocarbons, the buta-1,3-diene content being in general from 10 to 80, pre-ferably from 20 to 70, in particular from 30 to ~0, X by weight, while the total content of vinylacetylenes, S~22~757 - 4 - O.Z.OQ50/36883 ethylacetylene and buta-1,2-diene in the C4 fractions generally does not exceed 5% by weight. The novel extractive distilLation of these C4 fractions generally gives the saturated and monoolefinically unsaturated C4-hydrocarbons, such as butanes, n-butene and isobutene, as the toP product of the extractive distillation zone, and buta-1,3-diene together with other hydrocarbons which are more readily soluble in the selective solvent, such as vinylacetylene, ethylacetylene and bu~a-1,2-diene, as the product of the solvent recovery solvent,This butadiene obtained as a product of the solvent recovery zone is as a rule subjected to further purifi-cation operations in order to obtain very pure butadiene.
Other suitable buta-1,3-diene-containing C4-hydrocarbon mixtures which are advantageously separated using the process according to the invention are crude butadienes which in generaL contain not less than 90, preferably not less than 95, and in particular not less than 98, X by weight of buta-1,3-diene together ~ith, as impurities, higher acetyLenes, such as vinylacetyLene and ethylacetylene, and higher allenes, such as buta-1,~-diene.
Such crude butadienes are obtained, for example~ by extractive distillation, eg. with the novel process, of C4 fractions resulting from the thermal cleavage of petroleum fractions or from the catalytic dehydrogenation of n-butane and/or n-butene. In the separation of such a crude butadiene by the novel process, bu~a-1,3-diene, being a hydrocarbon which is more sparingly soluble in the selective solvent, is obtained as the top product of the extractive distillation zone, and the higher acetylenes and some or all of the higher allenes, being hydrocarbons ~hich are more readily soluble in the selective solvent, are obtained as a product of the solvent recovery zone.
Other C4-hydrocarbon mixtures which are suitable as starting C4-hydrocarbon mixtures for the novel pro-cess are, for example, mixtures containing butanes, n-butenes and isobutene, as obtained, for example, as a ~220757 - 5 - O.Z.0050/36833 disti llate (raffinate) from a butadiene extraction plant, mixtures containing butanes and n-butenes, as obtained, for example, after separation of isobutene from the raffinate described above, and mixtures con-taining butanes and but-2-ene, as obtained, for example, from plants for the dimerization of n-butenes. The separation of the C4-hydrocarbons into the distillate, containing the more sparingly soluble hydrocarbons, and a product containing the more readily soluble hydrocar-bons can be used not only to separate buta-1,3-diene from the butenes and from the acetylenes, as described above, but also, for example, to separate butane from the but-1-enes, from the but-Z-enes or frôm buta-1,3-diene.
Examples of suitable selective solvents for the novel process are butyrolactone, nitriles, such as acetonitrile, propionitrile or methoxypropionitrile, ketones, such as acetone, furfurol, N-alkyl-substituted _ lower aliphatic acid amides, such as dimethylformamide, diethylformamide, dimethylacetamide, diethylacetamide or N-formyLmorpholine, and N-alkyl-substituted cyclic acid amides (lactams), such as N-alkylpyrrolidones, in par-ticular N-methylpyrrolidone. In general, N-alkyl-substituted lower aliphatic acid amides or N-alkyl-substituted cyclic acid amides are used. Particularlyadvanta~eously used solvents are dimethylformamide and, in particular, N-methylpyrrolidone.
However, the selective solvent used can also be a mixture of these solvents with one another, for example N-methylpyrrolidone with acetonitrile, a mixture of these solvents with cosolvents, such as water and/or a tert.-butyl ether, eg. methyl tert.-butyl ether, ethyl tert.-butyl ether, propyl tert.-butyl ether or n- or isobutyl tert.-butyl ether.
An essential feature of the novel process is that the concentration, based on the C4-hydrocarbon/
selective solvent mixture formed in the extractive dis-tillation zone, of the selective solvent in the said ~2207S7 - 6 - O.Z.aO50/36883 zone, ie. in the zone between the solvent feed point and the solvent recovery zone falls below 75% by weight on one or more trays in the case of tray-containing extrac-- tive distillation zones or, in the case of packed extrac-tive distillation zones, at one or more points within the packing. In general, the concentration of the selective solvent is from 30 to 75, preferably from 35 to 65, in particular from 40 to 60, ~ by weight. The çoncentration of the said solvent in the extractive dis-tillation zone is established, for example, by varyingthe amount of hydrocarbon reflux at the top of the said zone, by varying the pressure or the solvent feed tem-perature or, where the solvent contains water, also by changing the water content of the solvent.
The distillate obtained at the top of the extrac-tive distillation zone is advantageously taken off at the top or as a sidestream. To remove small amounts of selective solvent present in the distillate, the latter _ is advantageously washed with water, or fed counter-current to a liquid hydrocarbon reflux in a distillation zone, the solvent being retained as a result.
The absolute pressure in the extractive dis-tillation zone and the water-wash zone or the distilla-tion zone is in general from 1.5 to 9, preferably from 2 to 8, in particular from 3 to 7, bar.
The temperature in the extractive distillation zone is dependent on the number of trays or the height of the packing, and on the pressure. In general, it is from 2û to 80C, preferably from 40 to 70C, at the point at which the solvent concentration is lowest.
The pressure in the solvent recovery zone can be lower than that in the extractive distillation zone. In this procedure, the pressure in the solvent recovery zone is in general from 1.2 to 3 bar. However, it may also be advantageous, particularly in the separation of a C4-hydrocarbon mixture containing butanes and butenes, if the process is carried out in such a way that the top - 7 - O.Z.0050/36883 of the extractive distillation zone is connected to a distillation zone, and the pressure in the solvent recovery zone is not less than that in the extractive distillation zone and the distillation zone, the pressure in these zones being no lower than that required for the condensation temperature of the top product of the dis-tillation zone to be 30C or h;gher. The separation of the C4-hydrocarbon mixture containing butanes and butenes is preferably carried out using the distillation zone in addition to the extractive distillation zone. It may furthermore be advantageous if only some of the selective solvent freed from the hydrocarbons is removed from the solvent recovery zone and then recycled to the extrac-tive distillation zone.
The extractive distillation can be carried out in a column. In the case of a large number of trays, eg.
more than 1~0 trays, it may be advantageous to carry out the extractive distillation in more than one column, in general in two columns. Where two columns are used, it is advantageous if the absorption stage above the point at which the C4-hydrocarbon mixture is fed into the extractive distillation zone is located in the first column, and the concentration stage below the feed point of the hydrocarbon mixture is located in the second column, ie. the feed point of the hydrocarbon mixture is at the top of the second column or, preferably, at the bottom of the first column. It is preferable if there is no compression stage between the absorption stage and the concentration stage, but instead pressure conditions as arise automatically in the extractive distillation zone in the absence of compression and/or pressure-reduction stages in the said zone are maintained in this zone, so that the pressure at the bottom of the extrac-tive distillation zone corresponds to that at the top of the said zone, taking into account the usual pressure loss in the columns. As a rule, the pressure difference between the top and the bottom of the extractive ~220757 .
- 8 - 0.~.0050/36883 distillation zone is from 0.1 to 3, preferably from 0.2 to 2, bar.
The extract taken off as a bottom product from the extractive distillation zone is fed to a solvènt recovery zone, in which the extract is separated into a hydrocarbon-containing product and the selective sol-vent which is partially or completely freed from the hydrocarbons. The solvent recovery zone can be operated as, for example, a devolatilizer or a solvent stripper, or a combination of these.
In general, the solvent recovery zone is supplied with heat, for example via a reboiler. The selective solvent which has been partially or completely freed from the hydrocarbons and is obtained as the bottom pro-duct of this zone is ~dvantageously recycled to the extractive distillation zone.
The Example which follows illustrates the inven-tion.
EXAMPLE
- An extractive distillation (cf. Figure) was operated with N-methylpyrrolidone as the selective sol-vent, this being fed in an amount of 3 kg/h via line 10 to the extractive distillation zone 2 equipped with trays. The C4-hydrocarbon mixture employed, which was fed in an amount of 1 kg/h via line 1 to the extractive distillation zone 2, was derived from a butene dimeriza-tion plant and had the following composition:
C3-hydrocarbons 0.76% by weight butane 47.68% by weight 3û isobutane 11.57% by weight isobutene 0.83% by weight but-1-ene 1.18X by weight cis-but-2-ene 10.54% by weight and trans-but-2-ene 27.44% by weight.
In the distillation zone 4, traces of solvent which were present in the distillate obtained at the top of the extractive distillation zone were retained. At ~2Z07S7 - 9 - O.Z.~050/36883 the same time, the distillation zone served to effect further distillative separation of the distillate obtained at the top of the extractive distillation zone, with the result that the concentration of cis but-2-ene in the distillate was reduced and this compound was retained. The concentration of the solvent on the trays of the extractive distillation was adjusted by varying the amount of hydrocarbon reflux. For this purpose, some of the raffinate taken off via line 5 and condensed was recycled via line 6. Butane raffinate was removed via line 7, while a stream containing butenes was taken off via line 8. The selective solvent removed at the bottom of the solvent recovery zone 3 was subjected to heat exchange to adjust the temperature and then re-cycled via lines 9 and 10 to the top of the extractivedistillation zone.
At a hydrocarbon reflux of 1 kg/h, the solvent concentration fell below 51X by weight on several trays.
The stream taken off via line 8 contained 75X by weight of butenes and 25% by weight of butanes and therefore had a composition corresponding to that of a starting mixture for the n-butene dimerization plant. The top product removed via line 7 contained 75% by weight of butanes. If the hydrocarbon reflux was increased to just 2 kg/h, the solvent concentration fell below 47% by weight on several trays, and at the same total butene content of the butene stream 8, ie. 75X by weight, the butane content of the top product obtained via line 7 increased to as much as 82X by weight.
Surprisingly, it was not necessary to reduce the pressure of the solvent recovery zone 3, at 4.75 bar, with respect to the pressure of the extractive distilla-tion zone 2 and the distillation zone 4 in order to obtain sufficiently pure products. Hence, compression of the hydrocarbons stripped off in the solvent recovery zone was not necessary. It was 'possible to condense the top product with cooling water, a coolant being unnecessary.
COMPARATIVE EXAMPLE
When the experiment was carried out as described above, with the exception that a minimum solvent conc-en-tration of 75.5% by weight was established in theextractive distillation zone by adjusting the hydro-carbon reflux to 0.18 kg/h and increasing the tempera-- ture of the selective solvent, the top product contained only 65% by weight of butane, ie. in the top product, about 3/4 of the butenes present in the C4-hydrocarbon mixture used were lost, so that only about 1/4 of the butenes were obtained ;n the butene product; conse-quently, th;s procedure ;s of no further interest from - an economic point of view.
The present invention is intended to improve the pro-cedure and cost-efficiency of the conventional processes.
It is an object of the present invention to pro-vide a process for the separation of a c4-hYdrocarbon mixture with the aid of a selective solvent, wherein both the complexity of the apparatus and hence the capi-tal costs, and the amount of selective solvent circu-lated and hence the energy consumption, can be reduced.
We have found that these and other objects and advantages are achieved, in accordance with the inven-tion, by a process for the separation, with the aid of a selective solvent, of a C4-hydrocarbon mixture which contains hydrocarbons which are more readily soluble in the selective solvent and those which are more sparingly soluble in the said solvent, wherein the C4-hydrocarbon mixture is separated, in an extractive distillation zone, into a top product (distillate), which contains the more sparingly soluble hydrocarbons, and an extract which contains the more readily soluble hydrocarbons and the selective solvent, the extract taken off as the bottom product is fed to a solvent recovery zone, in which the extract is separated into a product which con-tains the hydrocarbons, and the selective solvent which has been partially or completely freed from the hydro-carbons, and the concentration, based on the C4-hydro-carbon/selective solvent mixture formed in the extractive distillation zone, of the selective solvent in the said zone falls below 75% by weight on one or more trays or, in the case of a packed extractive distillation column, at one or more points within the packing.
In the novel process, the amount of selective solvent circulated can be reduced while maintaining the same separation efficiency as in the conventional pro-cesses operating with higher solvent concentration.This is surprising since the selectivity which is essen-tial for the separation decreases when the solvent ~;~Z0757 ~ 3 ~ O.Z.0050J36883 concentration in the extractive distillation zone is reduced. Reducing the amount of solvent circulated per-mits smaller apparatuses to be used, so that the capital costs are correspondingly reduced. Furthermore, in the case of the separation of butane and butene, the heat losses which result when the recycled selective solvent is recooled to the temperature at which the solvent enters the extractive distillation can be kept small if the heat remaining in the recycled solvent after heat exchange with laden solvent is used profitably for the separation in a distillation zone.
The process of the present application is generally applicable to the separation of C4-hydrocarbon mixtures which contain various unsaturated compounds.
In such systems, the more highly saturated compound is the comPonent which is the more sparingly so~uble in the selective solvent, and the less saturated compound is the component which is more readily soluble in the said solvent. In the case of isomers, for example an acetylenic - 20 compound and a diolefin, the acetylenic compound is more readily soluble than the diolefin.
For e%ample, the process according to the inven-tion is advantageously used for the separation of a buta-1,3-diene-containing C4-hydrocarbon mixture.
Such C4-hydrocarbon mixtures are obtained as, for example, C4-fractions in the preparation of ethy-lene and/or propylene by thermal cleavage of a petroleum fraction, for example liquefied petroleum gas ~LPG), naphtha, gas oil or the like. C5-fractions of this type are also obtained in the catalytic dehydrogenation of n-butane and/or n-butene. The C4 fractions contain, as a rule, butanes, n-butene, isobutene, buta-1,3-diene, vinylacetylene, ethylacetylene and buta-1,2-diene, with or without small amounts of C5-hydrocarbons, the buta-1,3-diene content being in general from 10 to 80, pre-ferably from 20 to 70, in particular from 30 to ~0, X by weight, while the total content of vinylacetylenes, S~22~757 - 4 - O.Z.OQ50/36883 ethylacetylene and buta-1,2-diene in the C4 fractions generally does not exceed 5% by weight. The novel extractive distilLation of these C4 fractions generally gives the saturated and monoolefinically unsaturated C4-hydrocarbons, such as butanes, n-butene and isobutene, as the toP product of the extractive distillation zone, and buta-1,3-diene together with other hydrocarbons which are more readily soluble in the selective solvent, such as vinylacetylene, ethylacetylene and bu~a-1,2-diene, as the product of the solvent recovery solvent,This butadiene obtained as a product of the solvent recovery zone is as a rule subjected to further purifi-cation operations in order to obtain very pure butadiene.
Other suitable buta-1,3-diene-containing C4-hydrocarbon mixtures which are advantageously separated using the process according to the invention are crude butadienes which in generaL contain not less than 90, preferably not less than 95, and in particular not less than 98, X by weight of buta-1,3-diene together ~ith, as impurities, higher acetyLenes, such as vinylacetyLene and ethylacetylene, and higher allenes, such as buta-1,~-diene.
Such crude butadienes are obtained, for example~ by extractive distillation, eg. with the novel process, of C4 fractions resulting from the thermal cleavage of petroleum fractions or from the catalytic dehydrogenation of n-butane and/or n-butene. In the separation of such a crude butadiene by the novel process, bu~a-1,3-diene, being a hydrocarbon which is more sparingly soluble in the selective solvent, is obtained as the top product of the extractive distillation zone, and the higher acetylenes and some or all of the higher allenes, being hydrocarbons ~hich are more readily soluble in the selective solvent, are obtained as a product of the solvent recovery zone.
Other C4-hydrocarbon mixtures which are suitable as starting C4-hydrocarbon mixtures for the novel pro-cess are, for example, mixtures containing butanes, n-butenes and isobutene, as obtained, for example, as a ~220757 - 5 - O.Z.0050/36833 disti llate (raffinate) from a butadiene extraction plant, mixtures containing butanes and n-butenes, as obtained, for example, after separation of isobutene from the raffinate described above, and mixtures con-taining butanes and but-2-ene, as obtained, for example, from plants for the dimerization of n-butenes. The separation of the C4-hydrocarbons into the distillate, containing the more sparingly soluble hydrocarbons, and a product containing the more readily soluble hydrocar-bons can be used not only to separate buta-1,3-diene from the butenes and from the acetylenes, as described above, but also, for example, to separate butane from the but-1-enes, from the but-Z-enes or frôm buta-1,3-diene.
Examples of suitable selective solvents for the novel process are butyrolactone, nitriles, such as acetonitrile, propionitrile or methoxypropionitrile, ketones, such as acetone, furfurol, N-alkyl-substituted _ lower aliphatic acid amides, such as dimethylformamide, diethylformamide, dimethylacetamide, diethylacetamide or N-formyLmorpholine, and N-alkyl-substituted cyclic acid amides (lactams), such as N-alkylpyrrolidones, in par-ticular N-methylpyrrolidone. In general, N-alkyl-substituted lower aliphatic acid amides or N-alkyl-substituted cyclic acid amides are used. Particularlyadvanta~eously used solvents are dimethylformamide and, in particular, N-methylpyrrolidone.
However, the selective solvent used can also be a mixture of these solvents with one another, for example N-methylpyrrolidone with acetonitrile, a mixture of these solvents with cosolvents, such as water and/or a tert.-butyl ether, eg. methyl tert.-butyl ether, ethyl tert.-butyl ether, propyl tert.-butyl ether or n- or isobutyl tert.-butyl ether.
An essential feature of the novel process is that the concentration, based on the C4-hydrocarbon/
selective solvent mixture formed in the extractive dis-tillation zone, of the selective solvent in the said ~2207S7 - 6 - O.Z.aO50/36883 zone, ie. in the zone between the solvent feed point and the solvent recovery zone falls below 75% by weight on one or more trays in the case of tray-containing extrac-- tive distillation zones or, in the case of packed extrac-tive distillation zones, at one or more points within the packing. In general, the concentration of the selective solvent is from 30 to 75, preferably from 35 to 65, in particular from 40 to 60, ~ by weight. The çoncentration of the said solvent in the extractive dis-tillation zone is established, for example, by varyingthe amount of hydrocarbon reflux at the top of the said zone, by varying the pressure or the solvent feed tem-perature or, where the solvent contains water, also by changing the water content of the solvent.
The distillate obtained at the top of the extrac-tive distillation zone is advantageously taken off at the top or as a sidestream. To remove small amounts of selective solvent present in the distillate, the latter _ is advantageously washed with water, or fed counter-current to a liquid hydrocarbon reflux in a distillation zone, the solvent being retained as a result.
The absolute pressure in the extractive dis-tillation zone and the water-wash zone or the distilla-tion zone is in general from 1.5 to 9, preferably from 2 to 8, in particular from 3 to 7, bar.
The temperature in the extractive distillation zone is dependent on the number of trays or the height of the packing, and on the pressure. In general, it is from 2û to 80C, preferably from 40 to 70C, at the point at which the solvent concentration is lowest.
The pressure in the solvent recovery zone can be lower than that in the extractive distillation zone. In this procedure, the pressure in the solvent recovery zone is in general from 1.2 to 3 bar. However, it may also be advantageous, particularly in the separation of a C4-hydrocarbon mixture containing butanes and butenes, if the process is carried out in such a way that the top - 7 - O.Z.0050/36883 of the extractive distillation zone is connected to a distillation zone, and the pressure in the solvent recovery zone is not less than that in the extractive distillation zone and the distillation zone, the pressure in these zones being no lower than that required for the condensation temperature of the top product of the dis-tillation zone to be 30C or h;gher. The separation of the C4-hydrocarbon mixture containing butanes and butenes is preferably carried out using the distillation zone in addition to the extractive distillation zone. It may furthermore be advantageous if only some of the selective solvent freed from the hydrocarbons is removed from the solvent recovery zone and then recycled to the extrac-tive distillation zone.
The extractive distillation can be carried out in a column. In the case of a large number of trays, eg.
more than 1~0 trays, it may be advantageous to carry out the extractive distillation in more than one column, in general in two columns. Where two columns are used, it is advantageous if the absorption stage above the point at which the C4-hydrocarbon mixture is fed into the extractive distillation zone is located in the first column, and the concentration stage below the feed point of the hydrocarbon mixture is located in the second column, ie. the feed point of the hydrocarbon mixture is at the top of the second column or, preferably, at the bottom of the first column. It is preferable if there is no compression stage between the absorption stage and the concentration stage, but instead pressure conditions as arise automatically in the extractive distillation zone in the absence of compression and/or pressure-reduction stages in the said zone are maintained in this zone, so that the pressure at the bottom of the extrac-tive distillation zone corresponds to that at the top of the said zone, taking into account the usual pressure loss in the columns. As a rule, the pressure difference between the top and the bottom of the extractive ~220757 .
- 8 - 0.~.0050/36883 distillation zone is from 0.1 to 3, preferably from 0.2 to 2, bar.
The extract taken off as a bottom product from the extractive distillation zone is fed to a solvènt recovery zone, in which the extract is separated into a hydrocarbon-containing product and the selective sol-vent which is partially or completely freed from the hydrocarbons. The solvent recovery zone can be operated as, for example, a devolatilizer or a solvent stripper, or a combination of these.
In general, the solvent recovery zone is supplied with heat, for example via a reboiler. The selective solvent which has been partially or completely freed from the hydrocarbons and is obtained as the bottom pro-duct of this zone is ~dvantageously recycled to the extractive distillation zone.
The Example which follows illustrates the inven-tion.
EXAMPLE
- An extractive distillation (cf. Figure) was operated with N-methylpyrrolidone as the selective sol-vent, this being fed in an amount of 3 kg/h via line 10 to the extractive distillation zone 2 equipped with trays. The C4-hydrocarbon mixture employed, which was fed in an amount of 1 kg/h via line 1 to the extractive distillation zone 2, was derived from a butene dimeriza-tion plant and had the following composition:
C3-hydrocarbons 0.76% by weight butane 47.68% by weight 3û isobutane 11.57% by weight isobutene 0.83% by weight but-1-ene 1.18X by weight cis-but-2-ene 10.54% by weight and trans-but-2-ene 27.44% by weight.
In the distillation zone 4, traces of solvent which were present in the distillate obtained at the top of the extractive distillation zone were retained. At ~2Z07S7 - 9 - O.Z.~050/36883 the same time, the distillation zone served to effect further distillative separation of the distillate obtained at the top of the extractive distillation zone, with the result that the concentration of cis but-2-ene in the distillate was reduced and this compound was retained. The concentration of the solvent on the trays of the extractive distillation was adjusted by varying the amount of hydrocarbon reflux. For this purpose, some of the raffinate taken off via line 5 and condensed was recycled via line 6. Butane raffinate was removed via line 7, while a stream containing butenes was taken off via line 8. The selective solvent removed at the bottom of the solvent recovery zone 3 was subjected to heat exchange to adjust the temperature and then re-cycled via lines 9 and 10 to the top of the extractivedistillation zone.
At a hydrocarbon reflux of 1 kg/h, the solvent concentration fell below 51X by weight on several trays.
The stream taken off via line 8 contained 75X by weight of butenes and 25% by weight of butanes and therefore had a composition corresponding to that of a starting mixture for the n-butene dimerization plant. The top product removed via line 7 contained 75% by weight of butanes. If the hydrocarbon reflux was increased to just 2 kg/h, the solvent concentration fell below 47% by weight on several trays, and at the same total butene content of the butene stream 8, ie. 75X by weight, the butane content of the top product obtained via line 7 increased to as much as 82X by weight.
Surprisingly, it was not necessary to reduce the pressure of the solvent recovery zone 3, at 4.75 bar, with respect to the pressure of the extractive distilla-tion zone 2 and the distillation zone 4 in order to obtain sufficiently pure products. Hence, compression of the hydrocarbons stripped off in the solvent recovery zone was not necessary. It was 'possible to condense the top product with cooling water, a coolant being unnecessary.
COMPARATIVE EXAMPLE
When the experiment was carried out as described above, with the exception that a minimum solvent conc-en-tration of 75.5% by weight was established in theextractive distillation zone by adjusting the hydro-carbon reflux to 0.18 kg/h and increasing the tempera-- ture of the selective solvent, the top product contained only 65% by weight of butane, ie. in the top product, about 3/4 of the butenes present in the C4-hydrocarbon mixture used were lost, so that only about 1/4 of the butenes were obtained ;n the butene product; conse-quently, th;s procedure ;s of no further interest from - an economic point of view.
Claims (7)
1. A process for the separation, with the aid of a selective solvent, of a C4-hydrocarbon mixture which contains hydrocarbons which are more readily soluble in the selective solvent and those which are more sparingly soluble in the said solvent, wherein the C4-hydrocarbon mixture is separated, in an extractive distillation zone, into a top product (distillate), which contains the more sparingly soluble hydrocarbons, and an extract which con-tains the more readily soluble hydrocarbons and the selec-tive solvent, the extract taken off as the bottom product is fed to a solvent recovery zone, in which the extract is separated into a product, which contains the hydrocar-bons, and the selective solvent which has been partially or completely freed from the hydrocarbons, and the con-centration, based on the C4-hydrocarbon/selective sol-vent mixture formed in the extractive distillation zone, of the selective solvent in the said zone falls below 75% by weight on one or more trays or, in the case of a packed extractive distillation column, at one or more points within the packing.
2. A process as claimed in claim 1, which is used for separating a buta-1,3-diene-containing C4-hydro-carbon mixture into a distillate, containing saturated and monoolefinically unsaturated C4-hydrocarbons, and a product containing buta-1,3-diene, with or without higher acetylenes and buta-1,2-diene, as hydrocarbons which are more readily soluble in the selective solvent.
3. A process as claimed in claim 1, which is used for separating a crude butadiene into buta-1,3-diene as a distillate, and a product containing higher acetylenes, with or without buta-1,2-diene, as hydrocarbons which are more readily soluble in the selective solvent.
4. A process as claimed in claim 1, which is used for separating a C4-hydrocarbon mixture containing butanes and butenes into a distillate containing butanes, and a product containing the butenes as hydrocarbons which are more readily soluble in the selective solvent.
5. A process as claimed in claim 4, wherein the top of the extractive distillation zone is connected to a distillation zone, and the pressure in the solvent recovery zone is not less than that of the extractive distillation zone and the distillation zone, the pressure in the two last mentioned zones being no lower than that required for the condensation temperature of the top product of the distillation zone to be 30°C or higher.
6. A process as claimed in claim 5, wherein the distillation zone is used in addition to the extractive distillation zone for the separation of the C4-hydro-carbon mixtures containing butanes and butenes.
7. A process as claimed in claim 6, wherein the selective solvent taken off from the solvent recovery zone and recycled is only partially freed from the hydrocarbons.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19833346695 DE3346695A1 (en) | 1983-12-23 | 1983-12-23 | METHOD FOR SEPARATING A C (DOWN ARROW) 4 (DOWN ARROW) HYDROCARBON MIXTURE BY EXTRACTIVE DISTILLATION |
| DEP3346695.5 | 1983-12-23 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1220757A true CA1220757A (en) | 1987-04-21 |
Family
ID=6217889
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000470880A Expired CA1220757A (en) | 1983-12-23 | 1984-12-21 | Separation of c.sub.4-hydrocarbon mixture by extractive distillation |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP0149145B1 (en) |
| JP (1) | JPH0641428B2 (en) |
| AT (1) | ATE32877T1 (en) |
| CA (1) | CA1220757A (en) |
| DE (2) | DE3346695A1 (en) |
Families Citing this family (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3823772A1 (en) * | 1988-07-14 | 1990-02-08 | Krupp Koppers Gmbh | METHOD FOR SEPARATING PARAFFINIC AND OLEFINIC C (ARROW DOWN) 4 (ARROW DOWN) HYDROCARBONS |
| JP2842091B2 (en) * | 1992-09-11 | 1998-12-24 | 日本ゼオン株式会社 | Method for obtaining high-purity 1-butene and 2-butene |
| JP4134391B2 (en) | 1998-04-07 | 2008-08-20 | 日本ゼオン株式会社 | Separation and purification apparatus and method for separation and purification of unsaturated hydrocarbons |
| US8680353B2 (en) | 2011-11-21 | 2014-03-25 | Basf Se | Process for preparing oligomers of butene |
| MY166872A (en) | 2011-11-21 | 2018-07-24 | Basf Se | Process for preparing oligomers of butene description |
| DE102017114902A1 (en) * | 2017-07-04 | 2019-01-10 | Thyssenkrupp Ag | IMPROVED METHOD AND APPARATUS FOR SEPARATING C4 CARBON HYDROCARBONS BY EXTRACTIVE DISTILLATION |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4128457A (en) * | 1977-03-09 | 1978-12-05 | Societa' Italiana Resine S.I.R. S.P.A. | Process for the separation of butadiene by plural stage extractive distillation |
| JPS5920232A (en) * | 1982-07-26 | 1984-02-01 | Japan Synthetic Rubber Co Ltd | Production of n-butene |
-
1983
- 1983-12-23 DE DE19833346695 patent/DE3346695A1/en not_active Withdrawn
-
1984
- 1984-12-12 EP EP84115280A patent/EP0149145B1/en not_active Expired
- 1984-12-12 AT AT84115280T patent/ATE32877T1/en not_active IP Right Cessation
- 1984-12-12 DE DE8484115280T patent/DE3469728D1/en not_active Expired
- 1984-12-21 JP JP59268667A patent/JPH0641428B2/en not_active Expired - Lifetime
- 1984-12-21 CA CA000470880A patent/CA1220757A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| EP0149145A3 (en) | 1985-10-16 |
| DE3346695A1 (en) | 1985-07-04 |
| EP0149145A2 (en) | 1985-07-24 |
| EP0149145B1 (en) | 1988-03-09 |
| ATE32877T1 (en) | 1988-03-15 |
| DE3469728D1 (en) | 1988-04-14 |
| JPS60156623A (en) | 1985-08-16 |
| JPH0641428B2 (en) | 1994-06-01 |
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