JPS6217976B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for purifying 1,3-butadiene (hereinafter abbreviated as butadiene) or 2-methyl-1,3-butadiene (hereinafter abbreviated as isoprene), and more specifically, relates to a method for purifying 1,3-butadiene (hereinafter abbreviated as butadiene) or 2-methyl-1,3-butadiene (hereinafter abbreviated as isoprene). The present invention relates to a method for efficiently purifying butadiene or isoprene by recovering thermal energy when selectively discharging acetylenes and/or arenes having the same carbon number as butadiene or isoprene from a column. Extractive distillation is conventionally known as a method for separating and recovering butadiene or isoprene from a C ₄ or C ₅ hydrocarbon mixture obtained by thermal decomposition or dehydrogenation of naphtha. In particular, a two-stage extractive distillation method is widely known as a method for efficiently separating and recovering butadiene or isoprene of a purity suitable for solution polymerization, which has recently required strict removal of acetylenes. _C4 or with butadiene or isoprene
When a C ₅ hydrocarbon mixture is distilled under a suitable selective solvent, paraffins and olefins, which are relatively insoluble in the solvent, are discharged as an overhead stream of the first extractive distillation column and are more easily soluble in the solvent. A hydrocarbon mixture consisting of butadiene or isoprene, acetylenes and arenes is taken off as the bottom stream of the first extractive distillation column together with a selective solvent.
In addition, when _C5 fraction is used as a raw material in this process, in addition to acetylenes and allenes, diolefins such as cyclopentadiene and piperylene, and sulfur compounds such as dimethyl sulfide (hereinafter acetylenes, allenes, and others are collectively referred to as Acetylenes, etc.) are also taken out as a bottom stream. The two-stage extractive distillation method is roughly divided into two types. One is that the first and second extractive distillation processes are independent, each extractive distillation process has an independent solvent stripping column, and the bottom stream of the first extractive distillation column is sent to the first solvent stripping column. This is a method in which the hydrocarbon mixture is supplied, separated into a hydrocarbon mixture and an extraction solvent, and the hydrocarbon mixture obtained from the top of the column is supplied to the second extractive distillation step. This example is
Hydrocarbon Processing Vol.45No.11P151〜154
discloses a process using dimethylformamide as a selective solvent. The other type is one in which the first and second extractive distillation columns are connected by a common solvent stripping column, and the bottom stream of the first extractive distillation column is sent directly to the second extractive distillation column without passing through the first solvent stripping column. In this method, the water is fed to an extractive distillation column. An example of this is Hydrocarbon Processing Vol.47No.
11P135-138 disclose a process using N-methylpyrrolidone as a selective solvent. Generally, in the second extractive distillation process, acetylenes, etc., which significantly inhibit the activity of catalysts such as solution polymerization, are substantially not contained, by taking advantage of the property that acetylenes, etc. have a higher affinity for solvents than butadiene or isoprene. , high purity butadiene or isoprene is obtained as an overhead stream of the second extractive distillation column, while acetylenes etc. are discharged as a side stream.
Butadiene or isoprene taken off as an overhead stream of the second extractive distillation column is fed to a final fractional distillation step to remove trace impurities, producing high purity butadiene or isoprene. Furthermore, as a method for removing acetylenes, etc. in the second extractive distillation step, a two-stage extractive distillation process using N-methylpyrrolidone as a selective solvent is disclosed in Ind.Eng.Chem.Vol.62No.4P43-48. . According to this method, solvents containing acetylenes, etc. are taken out as a side vapor stream from the middle stage of the solvent stripping tower and supplied to the solvent separation tower, and the solvent is taken out from the middle stage of the solvent stripping tower and supplied to the solvent separation tower. Water is supplied nearby and the solvent is recovered by washing with water. The solvent dissolved in water is extracted as a bottom stream of the solvent separation column and returned to the solvent stripping column, while the hydrocarbon mixture containing acetylenes and the like is discharged as an overhead stream of the solvent separation column. Furthermore, in Japanese Patent Publication No. 47-2702, the solvent is separated in a stripping column (corresponding to the second extractive distillation step in the present invention), and the side having a higher concentration of acetylenes, etc. than the concentration of acetylenes, etc. in the mixture to be extracted and distilled is A process is described in which acetylenes, etc. are removed by discharging the stream from the stripping column and butadiene or isoprene is recovered from the top of the stripping column. In this example, the side stream containing acetylenes etc. is first cooled through a "condenser" and then introduced into a separation column or combined with raffinate into a rotating disk column type washing column, where it is introduced into a separation column or Water is supplied to the washing tower and the solvent is recovered by washing with water. Furthermore, Japanese Patent Application Laid-Open No. 138508/1983 describes a method for removing acetylenes and the like from a solvent stripping tower. It has been shown that this method requires a very high reflux ratio of 140 to 1300 in order to separate hydrocarbons containing acetylenes and the like from the solvent in a solvent stripping tower and recover the solvent. The present inventors studied the above-mentioned conventional technology from the standpoint of effective energy utilization, and as a result, the following became clear. That is, since the concentration of acetylenes having the same number of carbon atoms as butadiene or isoprene contained in the raw material C ₄ or C ₅ fraction is usually much lower than that of butadiene or isoprene, it is difficult to separate the acetylenes, etc. The majority of the stream fed to the solvent stripping tower is solvent. Taking these special characteristics into consideration, we conducted a detailed analysis of the separation of acetylenes, etc. and solvent in a solvent stripping tower, and found that the reflux amount required for separation in this tower is controlled by the reflux amount below the raw material supply stage. The surprising fact was discovered that in the concentration section, which is the recovery section and is above the raw material supply stage, it is possible to separate the acetylenes, etc. and the solvent even with a much smaller reflux amount. In other words, in the conventional technology, much more reflux was applied in the concentration section of the solvent stripping column than the reflux amount required for separation, and a large amount of heat was provided at the top of the column to generate the reflux. This means that the water is not effectively utilized because it is discarded from the condenser to the outside of the system, or it is transformed into an ineffective form by water supplied from the outside to the top of the tower. Therefore, the present inventors investigated various ways to make use of the facts found as a result of detailed analysis of the column to rationalize the process, and as a result, the present inventors extracted the side vapor flow from a stage near the raw material supply stage of the solvent stripping tower. The side vapor stream is condensed by passing it to a heat recovery process, and then returned to the adjacent stage from which the side vapor stream of the solvent stripping column was extracted, thereby achieving the objective in the first extractive distillation process or the second extractive distillation process. The present invention was achieved by discovering that it is possible to save energy in the process with almost no effect on the separation of components. That is, the present invention provides a method for recovering high-purity butadiene or isoprene from a _C4 or _C5 hydrocarbon mixture by a two-stage extractive distillation process in the presence of a selective solvent. In the process of separating acetylenes, etc. having the same number of carbon atoms as butadiene or isoprene contained in a small amount in the liquid from a solvent stripping tower, a selection including acetylenes, etc. extracted as a vapor phase from a stage near the raw material supply stage of the stripping tower. The method is characterized in that the side stream consisting of the solvent is supplied to the heat recovery step and then returned to a stage adjacent to the withdrawal stage of the side stream of the solvent stripping column.
The present invention provides a method for efficiently separating and purifying butadiene or isoprene from a C ₄ or C ₅ hydrocarbon mixture. The amount of side vapor stream withdrawn from the solvent stripping column according to the present invention is exclusive of that required by the concentration section of the column. The stage for extracting the side vapor flow may be a stage in the vicinity of the raw material supply stage, but preferably it is extracted from the raw material supply stage. On the other hand, the return stage for the condensate after heat recovery may be a stage close to the steam flow withdrawal stage, but it is preferably the same stage as the side steam flow withdrawal stage. In the present invention, selective solvents that can be used include acetonitrile, dimethylformamide, N
- Polar substances such as methylpyrrolidine, dimethylacetamide, furfural, acetone, etc., or an aqueous mixture of these substances with 0 to 20% by weight of water. The heat recovery process can be carried out anywhere as long as it is lower than the temperature of the side stream taken out from the solvent stripping column and the temperature difference can be 5°C or more.A specific example is the bottom of the first extractive distillation column. or a reboiler or side reboiler in a stage near it (see Figure 4) A stage for feeding butadiene- or isoprene-rich hydrocarbons supplied from the first extractive distillation step to the second extractive distillation step or a stage near it. Side reboiler (see Figure 5) Feedstock preheater for the raw _C4 or _C5 hydrocarbon mixture Preheater or reboiler in the pretreatment process before the extractive distillation process Distilled as the overhead stream of the second extractive distillation column Preheaters or reboilers used in purification processes to remove impurities from crude butadiene or crude isoprene; preheaters for low-pressure steam generators; and preheaters or reboilers used in selective solvent purification processes. Among these, it is desirable to minimize contamination due to leakage troubles in heat exchangers, etc., and to have a relatively high heat retention capacity compared to the thermal energy of the side flow, and to have little impact on operation when heat recovery is performed. Particularly preferred. The present invention will be explained in detail below with reference to the drawings. In order to simplify the drawing, most of the pumps, heat exchangers, containers, etc. that are not particularly necessary for the explanation are omitted, and only the main parts are shown. Depending on the type of raw material, a pre-treatment step and/or a post-treatment step may be provided before or after the steps shown below, but these are omitted because they are irrelevant to the gist of the present invention. Furthermore, since the divisions between the first and second extractive distillation columns and the first and second solvent stripping columns may not always be clear in the diagram, the following symbols are used in the diagram. Symbol Process E 1st extractive distillation column E 2nd 〃 S 1st solvent stripping column S 2nd 〃 S Solvent stripping column (shared with E and E) Figure 1 shows the first extractive distillation process and the second extractive distillation process, respectively. This is an example of having an independent solvent stripping tower. The raw material hydrocarbons are supplied to the first extractive distillation column E, 2 through conduit 1, and are subjected to extractive distillation in the presence of a selective solvent supplied through conduit 3, and from the top of the column are mainly composed of paraffinic and olefinic hydrocarbons. The fraction is discharged through conduit 4, and from the bottom of the tower, a flow consisting of hydrocarbons and solvents mainly composed of diolefins and acetylenes passes through conduit 5 to the next first solvent stripping tower S, 6. Sent. A substantially hydrocarbon-free solvent is obtained from the bottom of the first solvent stripping column, and this stream is recycled via line 3 to the first extractive distillation column. On the other hand, a stream of hydrocarbons (which may contain a small amount of solvent) consisting of diolefins, acetylenes, etc. is obtained from the top of the first solvent stripping column. This stream is fed via conduit 7 to a second extractive distillation column E, 8;
Extractive distillation is carried out in the presence of a selective solvent supplied via conduit 9, and diolefins from which acetylenes having the same number of carbon atoms as butadiene or isoprene are substantially removed are obtained from the top of the column via conduit 10. On the other hand, a stream mainly consisting of acetylenes and the like and a solvent is discharged from the bottom of the column via a conduit 11, and this stream is sent to the next second solvent stripping column S, 12.
The diolefins obtained via conduit 10 are sent to a final purification step (not shown), if necessary, to be purified into high-purity butadiene or isoprene.
Acetylenes, etc. are discharged from the top of the second solvent stripping column via conduit 13, while a substantially hydrocarbon-free solvent stream is obtained from the bottom of the column, which is fed to the second solvent stripping column via conduit 9. It is recycled to the extractive distillation column 8.
According to the present invention, the side stream extracted from the raw material supply stage of the second solvent stripping tower 12 via the conduit 14 gives heat to the object to be heated in the heat exchanger 15, condenses, and becomes a liquid stream, which is passed through the conduit 16. It is returned to the raw material supply stage of the stripping tower. When using acetonitrile as the solvent, column 1
2 usually has 10 to 30 stages, and a reflux ratio at the top of the column of 10 to 20 is usually sufficient. FIG. 2 shows an example of a process in which the first extractive distillation column and the second extractive distillation column share a solvent stripping column. This process is the first step in Figure 1.
The solvent stripping column is omitted, and the first extractive distillation column E, 2
2 is fed directly to the second extractive distillation column E, 26, and the substantially hydrocarbon-free solvent obtained from the bottom of the common solvent stripping column S, 30 via conduit 32 is fed directly to the second extractive distillation column E, 26. The flow is divided into two parts, and one part goes to conduit 2.
3 to the first extractive distillation column 22, and the rest to the conduit 27
It is circulated to the second extractive distillation column 26 via the . The first method is to extract the side vapor stream from the raw material supply stage of the solvent stripping tower 30, send it to the heat recovery process, and then return the liquid stream.
It is similar to the figure. FIG. 3 shows another example in which the first extractive distillation column and the second extractive distillation column share a solvent stripping column.
In the figure, feedstock hydrocarbons are supplied to a first column E (first extractive distillation column) 42 through a conduit 41. Extractive distillation is carried out in the presence of a selective solvent supplied via conduit 43, and a fraction consisting mainly of paraffins and olefinic hydrocarbons is discharged from the top via conduit 44, and from the bottom via conduit 45. As a result, a stream consisting of hydrocarbons and solvents mainly composed of diolefins and acetylenes is obtained, and this stream is divided into the 3rd column E/E/S (functionally, the 1st and 1st columns in Figure 1). 46 (consisting of a part of the recovery section of the two extractive distillation columns and a recovery section of the solvent stripping column). The vapor stream obtained from the top of the third column via conduit 47 is divided into two parts, one part via conduit 48 to the bottom of the first column, and the remainder via conduit 49 to the second column E (second column).
(extractive distillation column) 51. On the other hand, the selective solvent necessary for the operation of the second column is supplied to the upper part of the column through a conduit 52, and as a result, diolefins that do not contain acetylenes, etc., which have the same number of carbon atoms as butadiene or isoprene, are passed through the conduit from the top of the column. 53, and from the bottom of the column a stream consisting of hydrocarbons and solvent relatively rich in acetylenes etc. is obtained via conduit 54, which, after combining with the stream in conduit 45, passes through conduit 50. It is then supplied to the top of the third column. A substantially hydrocarbon-free solvent is obtained from the bottom of the third column via line 55, and this stream is divided into two
A portion is recycled to the first column 42 via conduit 43, and the remainder is recycled to column 51 via conduit 52. From the middle stage of the third column (this stage becomes the raw material supply stage for the solvent stripping tower), a side vapor stream consisting of a small amount of hydrocarbons rich in acetylenes and a large amount of solvent is passed through conduit 56. Some parts were extracted and placed in the 4th Tower S.
Direct conduit 57 to ensure the amount of steam necessary for operation of 61 (consisting of the concentration section of the solvent stripping tower)
The remaining heat is supplied to the bottom of the fourth column 61 via a conduit 58 to a heat exchanger 59 for heat recovery.
The fluid to be heated is heated and condensed to form a liquid stream, which is supplied to the bottom of the fourth column 61 via the conduit 60. From the top of the fourth column, hydrocarbons rich in acetylenes, etc. are discharged out of the system via conduit 62, while from the bottom of the column, solvent with a reduced hydrocarbon concentration than in the vapor stream in conduit 56 is discharged. A stream is obtained via conduit 63 which is returned to the same stage from which it is withdrawn in conduit 56 of the third column. In FIG. 3, the recovery section S of the third column 46 and the fourth
Together with the tower 61, they function as one solvent stripping tower. In Figures 1 to 3, the flow of hydrocarbons is indicated by conduits 1, 21, 41 Raw material conduits 4, 24, 44 Paraffins and olefins conduits 10, 28, 53 Diolefins conduits 13, 31, 62 Acetylenes, etc. are doing. The effect of the present invention is that by providing a heat recovery process for the side vapor stream extracted from the solvent stripping tower, the amount of heat can be used effectively, and the amount of heat discarded from the top of the solvent stripping tower to the outside of the system can be minimized. It is possible to save energy in the entire process. Next, the present invention will be explained in more detail with reference to Examples. Example 1 Using the same distillation apparatus as described in FIG.
Acetonitrile-water mixed solvent (weight ratio: 92:
8) was used to separate and purify butadiene from the _C4 fraction. The feed C ₄ fraction was fed as a vapor stream to column 42 via conduit 41 (FIG. 3, hereinafter the same). The operating conditions of columns 51, 46 and 61 were as follows.

[Table] A side vapor stream of a mixed solvent containing acetylenes and arenes is taken out from the 16th stage from the bottom of the column 46 through a conduit 56 and sent to a column 61 for reflux required to separate the hydrocarbon mixture and the solvent. removing a side vapor stream corresponding to the corresponding amount of heat and passing the remaining side vapor stream through a heat exchanger 59 where it is heat exchanged with a diolefin-enriched liquid mixed solvent stream removed from the bottom of column 42; After the heat was recovered, it was supplied in liquid form to the bottom of the column 61. Columns 42, 51 and 62 are equipped with overhead condensers, and columns 42 and 46 are equipped with reboilers at the bottom. The flow rates in the main conduits were:

【table】

【table】

[Table] Since acetylenes are concentrated near the top of column 61, the pressure was operated at 1.5 Kg/cm ² -G, which is lower than the other columns, as mentioned above, to avoid the risk of explosion due to self-decomposition. Although not shown in the figure, it was diluted with ruffinate, which is a byproduct at the top of column 42, to further improve safety. In terms of effective use of energy, comparing the energy lost by cooling water etc. in the condenser at the top of the tower 61 between the conventional method (without the heat recovery device 59 in FIG. 3) and the method of the present invention. It was as follows. Conventional technology (without heat recovery process) Qc = 2555 x 10 ³ Kcal/H Method of the present invention Qc = 1000 x 10 ³ Kcal/H Therefore, the method of the present invention is approximately 61% lower than the conventional technology. It has become clear that the savings are significant and the effect is significant. Example 2 A mixed solvent of acetonitrile-allyl alcohol-water (weight ratio: 75:15:
10) was used to separate and purify isoprene from the _C5 fraction. Although not shown in the figure, the raw material _C5 fraction obtained by naphtha cracking is heated in advance to dimerize most of the cyclopentadiene, and then the heavy fraction is separated in a preconcentration column, and a certain amount of isoprene is removed. In concentrated form, it was fed as a vapor stream via line 41 to the 54th stage from the bottom of column 42 having 120 trays. The operating conditions of columns 42, 51, 46 and 61 were as follows.

[Table] A side vapor stream of a mixed solvent containing acetylenes, etc. is taken out from the 20th stage from the bottom of the column 46 through a conduit 56, and the side vapor stream corresponding to the amount of heat corresponding to the reflux required in the column 61 is removed. , the remaining side vapor stream is passed through heat exchanger 59 where it is heat exchanged with the liquid mixed solvent stream taken from the third stage from the bottom of column 42 and, after heat recovery, is transferred in liquid form to the column of column 61. Supplied to the bottom. Columns 42, 51 and 62 are equipped with an overhead condenser, and columns 42 and 46 are equipped with reboilers at the bottom. The flow rates in the main conduits were as follows:

【table】

【table】

[Table] From the viewpoint of effective use of energy, the energy dissipated by the cooling water etc. in the condenser at the top of the column 61 was compared between the conventional prior art method and the method of the present invention, and the results were as follows. Conventional technology (without heat recovery process) Qc = 222000 (Kcal/H) Method of the present invention Qc = 61000 (Kcal/H) Therefore, the method of the present invention can save 73% compared to the conventional technology. It became clear that the effect was remarkable.

[Brief explanation of the drawing]

FIG. 1 is a flow sheet of one embodiment of the method of the present invention, and FIGS. 2, 3, 4, and 5 are flow sheets of another embodiment. 1, 21, 41... Raw material conduit, 4, 24, 44
... Conduit for paraffins and olefins, 10,
28,53...Diolefin conduit, 13,3
1,62...Acetylene conduit, 2,22,4
2,E...first extractive distillation column, 8,26,51,
E...Second extractive distillation column, 6, S...First solvent stripping column, 8, E...Second solvent stripping column, 30,6
1, S... Solvent stripping tower, 15, 34, 59... Heat exchanger, 3, 5, 7, 9, 11, 14, 16, 2
3, 25, 27, 29, 32, 33, 35, 4
3, 45, 47, 48, 49, 50, 52, 5
4, 55, 56, 57, 58, 60, 63... Conduit.

Claims (1)

[Claims] 1. Highly purified 1,3-butadiene or 2-methyl-1,3 from a C ₄ or C ₅ hydrocarbon mixture by a two-stage extractive distillation process in the presence of a selective solvent.
- When recovering butadiene, acetylenes and/or allenes having the same number of carbon atoms as 1,3-butadiene or 2-methyl-1,3-butadiene are contained in a small amount in the bottom liquid taken out from the extractive distillation column. In the step of separating similar hydrocarbons from the solvent stripping tower, the acetylenes and/or extracted as a vapor phase from a stage near the raw material supply stage of the solvent stripping tower.
Or C ₄ or C _{5 carbonization} characterized by supplying a side stream consisting of a selective solvent containing arenes to the heat recovery step and then returning it to a stage near the extraction stage of the side stream of the solvent stripping tower. Efficiently convert 1,3-butadiene or 2-methyl-1,3- from a hydrogen mixture
A method to separate and purify butadiene. 2. The method according to claim 1, wherein the heat recovery step is a reboiler or side reboiler at or near the bottom of the first extractive distillation column. 3 The heat recovery step is performed using 1,3-butadiene or 2-methyl-1,3- supplied from the first extractive distillation column.
2. The method according to claim 1, wherein the process is a side reboiler at or near the stage for feeding butadiene-rich hydrocarbons to the second extractive distillation column.