JPS6152126B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a process for separating and refining butadiene or isoprene to high purity from a C ₄ or C ₅ hydrocarbon mixture obtained by a method such as naphtha cracking or dehydrogenation using an extractive distillation method. More specifically, the first stage is an extractive distillation process to separate and remove paraffinic and olefinic hydrocarbons, and the second stage is a process for separating and removing acetylenes, cyclopentadiene, dimethyl sulfide, etc., which are relatively heavier than butadiene or isoprene. It relates to a two-stage extractive distillation process with an extractive distillation step for removal. Extractive distillation is widely known as a method for separating and recovering high-purity butadiene or isoprene from a C ₄ or C ₅ hydrocarbon mixture containing butadiene or isoprene in high yield, and is often practiced industrially. As the selective solvent, polar substances such as acetonitrile, dimethylformamide, N-methylpyrrolidone, furfural, acetone, and dimethylacetamide are used. A two-stage extractive distillation process in which paraffins and/or olefins are removed as an overhead distillate in the first stage, and target butadiene or isoprene is separated and recovered as an overhead distillate in the second stage is also widely known and industrially There are also practical examples. This two-stage extractive distillation process can be broadly classified into two types. One is that the first and second extractive distillation processes are independent, and each extractive distillation process has an independent solvent separation column. For example, Hydrocarbon Processing
Vol.47, No.11, p135-138 describes a process using N-methylpyrrolidone as a selective solvent.
Vol. 46, No. 5, pages 166-168 discloses a process using dimethylacetamide as a selective solvent. In this process, butanes and butenes, which have a lower affinity for solvents than butadiene, are distilled out from the top of the column as light fractions in the first extractive distillation column, and the bottom fraction is sent to the first solvent stripping column. A hydrocarbon fraction (sometimes containing solvent) is distilled from the top of the first solvent stripping column, and is sent to the second extractive distillation column. On the other hand, a solvent substantially free of hydrocarbons is obtained from the bottom of the first solvent stripping column, and is recycled to the first extractive distillation column. Butadiene containing virtually no acetylene fraction is obtained from the top of the second extractive distillation column, and a mixed solution of acetylenes and solvent, which have a stronger affinity for solvents than butadiene, is obtained from the bottom of the column. Sent to the solvent stripping tower. A fraction mainly composed of acetylenes is obtained as the top or side stream of the second solvent stripping column, while a solvent substantially free of hydrocarbons is obtained from the bottom of the column, which is sent to the second extractive distillation column. It is circulated. The other type is one in which the first and second extractive distillation columns are connected by a common solvent stripping column, for example.
Hydrocarbon Processing Vol.47, No.11, p135
~138, a process using N-methylpyrrolidone as a selective solvent is also described in Chem.Techn., 28.Jg.
Heft8 (Aug. 1976) p463-466 discloses a process using dimethylformamide as a selective solvent, and JP-A-47-34207 discloses a process using acetonitrile as a selective solvent. In these latter processes, the selective solvent supplied to the first extractive distillation column and the second extractive distillation column is obtained from the bottom of a common solvent stripping column and is therefore of the same quality, whereas in the former process, the selective solvent is generally are not homogeneous. In order to obtain an overhead fraction (butadiene or isoprene) with a sufficiently low concentration of acetylenes in the second extractive distillation column, it is necessary to keep the acetylene fraction in the solvent circulated to the column sufficiently low. ,
Less stringent constraints are placed on the acetylene concentration in the solvent recycled to the first extractive distillation column. On the other hand, when separating solvents and hydrocarbons in a solvent stripping column, it is relatively difficult to separate acetylenes because they have a stronger affinity for selective solvents than diolefins; however, in the latter process, they are recycled to the second extractive distillation column. The concentration of acetylenes is kept sufficiently low not only in the solvent used but also in the solvent recycled to the first extractive distillation column. Moreover, the solvent that is circulated to the first extractive distillation column is the raw material that is supplied to the extractive distillation process.
Although it depends on the composition of the C ₄ or C ₅ fraction, it is usually considerably larger than the amount recycled to the second extractive distillation column. Therefore, although the latter process has the advantage of simplifying the process by sharing a solvent stripping column, it is wasteful in separating the solvent and hydrocarbons, and in this respect, the former process This process would be superior. As described above, both processes have drawbacks, but as a method that eliminates the drawbacks of both, Japanese Patent Application Laid-Open No. 54-138508
A new process has been disclosed. This process belongs to a two-stage extractive distillation process that uses a shared solvent stripping column, but a portion of the mixture of acetylene-rich hydrocarbons and selective solvent obtained from the bottom of the second extractive distillation column is The solvent is circulated as it is to the first extractive distillation column, and a portion is sent to the solvent stripping column to diffuse the acetylenes, thereby ensuring that the solvent is circulated to the second extractive distillation column with a sufficiently low concentration of acetylenes. By doing this, the former process is simplified, and at the same time, only the solvent that is circulated to the second extractive distillation column (in the example of JP-A-138508, approximately 30% of the total) is treated in the solvent stripping column. In this respect, it can be said that the drawbacks of the latter process are overcome. However, it has become clear that this process also has major drawbacks. That is, in this process, separation of hydrocarbons and solvent in the solvent stripping tower still requires a large amount of energy and equipment. For example, JP-A-54-
138508 has been shown to operate with a reflux ratio of 140 to 1300, but even though the hydrocarbon concentration in the stream fed to the solvent stripping column is low, the energy consumption in this column is significant. Become. The present inventors have made extensive studies to overcome the problems of the prior art as described above, and as a result, the function of the solvent stripping tower is to combine hydrocarbon fractions rich in acetylenes and hydrocarbon fractions containing substantially acetylenes. It has been found that a more rational process can be achieved by dividing the separation into solvent fractions at the raw material supply stage and operating them separately. That is, as a result of a detailed analysis of the separation of the acetylene-rich hydrocarbon stream and the solvent in the solvent stripping column, the present inventors found that the reflux amount necessary for separation in this column is controlled by the raw material supply stage. The surprising fact has been found that the concentration section, which is the bottom recovery section and above the raw material supply stage, requires a much smaller reflux amount. Therefore, after considering various ways to utilize this fact to streamline the process, we decided to divide the function of the solvent stripping tower by the raw material supply stage, and collect all of the vapor flow from the top of the recovery section or the portion required by the concentration section. By returning the remainder to the second extractive distillation column and operating the concentration section at a much lower reflux rate, the energy required to separate acetylenes can be reduced to an almost negligible level in the overall process. made possible.
Furthermore, by connecting the concentrating section to the recovery section through a part of the recovery section of the second extractive distillation column, the degree of freedom not only in process design but also in plant operation increases, resulting in a more rational process. At the same time, we also discovered what could be done. The present invention is a two-stage extractive distillation process that essentially consists of a first extractive distillation process using a selective solvent, a _second extractive distillation process, and a solvent dispersion process using a selective solvent to extract butadiene from the _C4 fraction and isoprene from the C5 fraction. In a separation and purification method using a method, a part of the solvent containing a small amount of acetylenic hydrocarbons obtained in the second extractive distillation step is recycled to the first extractive distillation step, and the remainder is sent to the solvent diffusion step. To obtain a solvent that essentially does not contain acetylenes and to circulate the solvent to the second extractive distillation process, the function of the solvent stripping tower is divided at the raw material supply stage, and the steam from the top of the solvent stripping tower recovery section is The entire stream or the remainder except for the part required by the concentrating section is returned to the second extractive distillation process. The objective is to provide a process that reduces volume and at the same time saves equipment costs. Selective solvents that can be used in the method of the present invention include:
Acetonitrile, dimethylformamide, N-
Examples include methylpyrrolidone, furfural, acetone, dimethylacetamide, or aqueous mixtures of these with a small amount of water added, but those with relatively low boiling points such as acetonitrile and acetone are preferred, and those based on acetonitrile and water are even more suitable. preferable. Examples of acetonitrile and water-based solvents include alcohols such as allyl alcohol, t-butanol, as disclosed in JP-A-52-33601, and alcohols such as t-butanol, as disclosed in JP-A-53-111,001. Examples include those to which glycols such as ethylene glycol monomethyl ether are added. The present invention will be explained in detail below with reference to the drawings. For the sake of clarity, the diagram shows pumps that are not particularly necessary for explanation.
Most of the heat exchangers, containers, etc. are omitted, and only the main parts are shown. Also, depending on the type of raw material, a pre-treatment step and/or a post-treatment step may be necessary before or after the steps shown below, but these are omitted as they are unrelated to the purpose of the present invention. be. In addition, in each figure below, if thermal coupling is performed between the first extractive distillation column, the second extractive distillation column, and the solvent stripping column,
Since the division of each process is not necessarily clear,
It is indicated by the following symbol in the figure. Symbol Process E First extractive distillation process E Second extractive distillation process S Solvent dispersion process Figure 1 is an example in which the recovery section and concentration section of the solvent dispersion process are directly connected. Feedstock hydrocarbons are fed through a conduit 1 to a first extractive distillation column 2, where they are extracted and distilled in the presence of a selective solvent fed through a conduit 3, and from the top of the column a distillate consisting mainly of paraffinic and olefinic hydrocarbons is produced. minute is obtained via conduit 4,
A stream consisting of hydrocarbons and solvent, consisting mainly of diolefins and acetylenes, is obtained from the bottom of the column via line 5, which stream is then sent to a second extractive distillation column 6. Extractive distillation is carried out in the second extractive distillation column 6 in the presence of a selective solvent supplied from a conduit 7 provided above the supply stage of the conduit 5, and from the top of the column a diolefin containing essentially no acetylenes is produced. can get. In this process, as a raw material
When a C ₅ fraction is used, in addition to acetylenes, sulfur compounds such as cyclopentadiene and dimethyl sulfide can also be separated and removed from isoprene. On the other hand, from the bottom of the column 6, a stream of solvent containing a small amount of acetylenic hydrocarbons (including the aforementioned cyclopentadiene, dimethyl sulfide, etc. when the raw material is _C5 ) is obtained from the conduit 9, and some of it is passed through the conduit. 3 to the first extractive distillation column 2, and the remainder is supplied to the solvent stripping column 12 via conduit 10. A vapor stream is extracted from the raw material supply stage of the solvent stripping column 12 (it may be near the raw material supply stage, but preferably the raw material supply stage) through the conduit 11 and is supplied to the bottom of the column 6, thereby controlling the operation of the column 6. All or part of the heat required for this is covered. From the top of the solvent stripping column 12, a fraction rich in acetylenic hydrocarbons (in the case of _C5 , it also includes the aforementioned cyclopentadiene, dimethyl sulfide, etc.) is distilled out via a conduit 13, and from the bottom of the column, essentially An acetylene-free solvent stream is obtained which is recycled via line 7 to the second extractive distillation column 6. In the case of acetonitrile-based solvents, column 12 is 10
It has about 30 stages, and a reflux ratio of about 2 to 20 at the top of the column is sufficient. In addition, in FIG. 1, a conduit 14 indicated by a broken line shows the case where the first extractive distillation step and the second extractive distillation step are thermally coupled, and the conduit 14 is located near where the conduit 5 opens into the column 6. A vapor stream is fed to the bottom of column 2 from stages (preferably identical stages). This vapor stream supplies all or part of the heat required for the operation of column 2, in which case the reboiler of column 2 is not required. Figure 2 shows a case in which the function of the solvent stripping column in Figure 1 is divided into a concentration section and a recovery section with the raw material supply stage as the border, and is indirectly inherited through the recovery section of the second extractive distillation process. . In Figure 2, the first extractive distillation process is exactly the same as in Figure 1 (in the case of Figure 1-15, 2-1
6, 3-17, 4-18, 5-19 correspond). Extractive distillation is carried out in the second extractive distillation column 20 in the presence of a selective solvent supplied through a conduit 21 opened above the opening of the conduit 19.
2, a stream of diolefin essentially free of acetylenes is obtained. A solvent vapor stream containing a small amount of acetylenes is withdrawn via conduit 23 from a stage below the opening of conduit 19 of column 20 and fed to the bottom of distillation column 24. From the top of the column 24 an acetylene-enriched stream is withdrawn via line 25, while from the bottom of the column a solvent stream with a reduced acetylenic hydrocarbon content is obtained via line 26, which is transferred to column 30. It is returned to the stage at or near the opening of conduit 23. From the bottom of the column 20, a solvent containing a small amount of acetylenic hydrocarbons flows into the conduit 27.
A portion of the distillate is recycled to the first extractive distillation column 16 via conduit 17, and the remainder is supplied to the top stage of distillation column 29 via conduit 28. The vapor stream obtained from the top of column 29 is transferred via conduit 30 to second extractive distillation column 2.
It is supplied to the bottom of the column 20, and covers all or part of the heat required for the operation of the column 20. tower 24
and tower 29 each have about 5 to 10 stages,
Further, a reflux ratio at the top of the column 24 of about 2 to 20 is sufficient. The difference between the processes in FIG. 2 and FIG. , the concentration of acetylenes in the solvent recycled to the first extractive distillation step can be changed independently from the second extractive distillation step. Acetylene contained in the solvent is partially distilled out from the top of the first extractive distillation column, so if there is a restriction on the acetylene concentration in this stream, the method shown in Figure 2 is more effective than the method shown in Figure 1. can also be said to be preferable. In addition, the conduit 31 indicated by a broken line in FIG. 2 shows the case where the first extractive distillation process and the second extractive distillation process are thermally coupled, and the conduit 31 is connected to a stage near where the conduit 19 opens to the column 20. A vapor stream (preferably the same stage) is fed to the bottom of column 16. This vapor stream supplies all or a portion of the heat required for the operation of column 16, and in the case of full supply, column 16 does not require a reboiler. In the explanation of Figures 1 and 2 above, as the circulating solvent gradually accumulates components that are inconvenient for continued operation, such as polymers, during use, some of the circulating solvent is intermittently or continuously drawn out of the system. After removing and purifying these undesirable components, it is returned to the system. The amount to be extracted varies depending on the conditions, but is usually about 0.1 to 5% of the amount of circulating solvent. Next, the present invention will be explained in more detail with reference to Examples. Example 1 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 from 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 15 to the 50th stage from the bottom of a distillation column 16 having 116 trays. The operating conditions for columns 16, 20, 24 and 29 were as follows.

[Table] Raw material is supplied to the column 24 in the form of a vapor stream from the 20th stage from the bottom of the column 20 through a conduit 23.
The bottom fraction of the column was returned to the 20th column from the bottom of the column 20 via conduit 26. Columns 16, 20 and 24 have overhead condensers and column 1
Reboilers are provided at the bottoms of columns 6, 20 and 29. The flow rates in the main conduits were:

[Table] Rufid

[Table] Water 〓

Claims (1)

[Claims] 1. Butadiene or isoprene is extracted from the _C4 or _C5 fraction containing 1,3-butadiene (hereinafter abbreviated as butadiene) or 2-methyl-1,3-butadiene (hereinafter abbreviated as isoprene) using a selective solvent. Two-stage extractive distillation essentially consists of a first extractive distillation step for the purpose of separating hydrocarbons and/or olefins, a second extractive distillation step for the purpose of separating acetylene hydrocarbons, and a solvent dispersion step. In the separation and purification method using a method, a part of the solvent containing a small amount of acetylenic hydrocarbons obtained in the second extractive distillation step is circulated to the first extractive distillation step, and the remainder is sent to the solvent dispersion step to remove the solvent. All of the vapor stream from the top stage of the recovery section of the stripping column or the remainder excluding the part required by the concentration section is returned to the second extractive distillation step, and the concentration section is directly or indirectly passed through the second extractive distillation column. A method for purifying butadiene or isoprene, characterized in that the method is followed by a recovery section.