JPH0812612A - Separation of acetaldehyde from methyl iodide - Google Patents

Abstract

PURPOSE:To efficiently separate acetaldehyde from methyl iodide by distilling a mixture containing acetaldehyde and methyl iodide under specific condition. CONSTITUTION:A liquid mixture containing acetaldehyde and methyl iodide is distilled at a column top temperature of >=55 deg.C, a reflux tank temperature of >=25 deg.C and a pressure of >=1kg/cm<2> or distilled in the presence of an alcohol at a column top temperature of <55 deg.C and a reflux tank temperature of <25 deg.C to efficiently separate the acetaldehyde and the methyl iodide while controlling the formation and precipitation of paraldehyde and metaldehyde which are condensation products of acetaldehyde. The alcohol is introduced at a zone below the charging stage of the mixed liquid in the case of performing the distillation in the presence of alcohol. This process is effective for separating acetaldehyde from a process liquid in the case of carbonylating methanol and/or methyl acetate in a reaction catalyst containing a group 8 metal catalyst and methyl iodide and recycling the process liquid to the reaction system after removing the group 8 metal-catalyst and acetic acid and/or acetic anhydride from the reaction mixture.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for efficiently separating acetaldehyde and methyl iodide from a mixed solution containing acetaldehyde and methyl iodide.

[0002]

There are various methods for separating acetaldehyde and methyl iodide from a mixed solution containing acetaldehyde and methyl iodide, but in many cases, they are complicated and very difficult. Is accompanied by. The reason is that acetaldehyde and methyl iodide have similar boiling points and cannot be practically separated from each other. In order to solve this problem, Japanese Patent Publication No. 3-51696 and Japanese Patent Publication No. 2-39490 use azeotrope of a hydrocarbon having a boiling point of 25 to 55 ° C. under normal pressure and acetaldehyde, and It is separated from methyl iodide, and the azeotrope is disclosed in which the acetaldehyde is washed out with water and the hydrocarbon is fed again to the azeotropic distillation.

However, when the azeotrope is treated with water, not only is it necessary to replenish the hydrocarbons that are extracted into the aqueous phase and thus are wasted, and also high pressure conditions for handling low boiling components. Or, since low temperature cooling water is required, there is a problem that the cost is high in terms of equipment and operation.

Further, in a method for producing acetic acid and / or acetic anhydride by continuously reacting methanol and / or methyl acetate with carbon monoxide in the presence of a Group 8 metal catalyst and methyl iodide, a reactor is used. It is known that the methyl iodide-rich liquid that is recycled to the system contains carbonyl impurities such as acetaldehyde, butyraldehyde, crotonaldehyde, 2-ethylcrotonaldehyde, and the like. As a method for separating the carbonyl impurities and methyl iodide, a methyl iodide recycle stream to a carbonyl reactor is reacted with an amino compound that reacts with carbonyl to form a water-soluble nitrogen-containing derivative to give an organic methyl iodide. A method has been disclosed in which the phase is separated from the aqueous derivative phase and the methyl iodide phase is distilled to remove carbonyl impurities (JP-A-4-266843).

However, the concentration of carbonyl impurities contained in the organic stream recycled to the carbonylation reactor is high,
The amount is not sufficient, and there is a new problem of removing nitrogen-containing compounds.

Therefore, an object of the present invention is to efficiently separate acetaldehyde and methyl iodide.

Further, a second object of the present invention is to easily remove acetaldehyde contained in the process liquid recycled to the carbonylation reactor easily, particularly in the continuous production process of acetic acid and / or acetic anhydride. And efficiently recycle methyl iodide to the reactor.

[0008]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies to achieve the above-mentioned object, and as a result, when distilling a mixed solution containing acetaldehyde and methyl iodide, the overhead temperature, reflux tank temperature, and By controlling the pressure, or in the presence of alcohol, by controlling the overhead temperature, reflux tank temperature, it is possible to control the formation and precipitation of paraaldehyde and methaaldehyde, which are condensates of acetaldehyde, The present invention has been completed by discovering that acetaldehyde and methyl iodide can be efficiently separated.

That is, according to the present invention, a mixed solution containing acetaldehyde and methyl iodide is distilled at a column top temperature of 55 ° C. or higher, a reflux tank temperature of 25 ° C. or higher, and a pressure of 1 kg / cm ² or higher, or in the presence of alcohol. And the top temperature of 55
A method for efficiently separating acetaldehyde and methyl iodide by distilling at a temperature of less than 0 ° C and a reflux tank temperature of less than 25 ° C is provided.

An acetic acid production process will be described as an example of a reaction for providing a mixed solution containing acetaldehyde and methyl iodide which is treated according to the present invention.

In the presence of a Group 8 metal catalyst, methyl iodide,
Methanol is reacted with carbon monoxide to separate the reaction solution into a volatile phase containing acetic acid, methyl acetate and methyl iodide and a low volatile phase containing a Group 8 metal catalyst, and the volatile phase is distilled to obtain acetic acid. Is a process for the production of acetic acid, in which a product containing and an overhead containing methyl acetate and methyl iodide is obtained and the overhead is recycled to the reactor.

Examples of the Group 8 metal catalyst used in the above process include a rhodium catalyst, a palladium catalyst, a molybdenum catalyst and a nickel catalyst. Also, cobalt,
A compound containing one or more compounds selected from the group consisting of iridium, platinum, osmium and ruthenium can also be used as the catalyst. As the catalyst, only one kind may be used, or two or more kinds may be used. Among the above catalysts, a rhodium catalyst is more preferably used. The rhodium catalyst usually exists as a rhodium complex in the reaction solution. Therefore, the rhodium catalyst may be used in any form as long as it changes into a complex that dissolves in the reaction solution under the reaction conditions. Specifically, RhI ₃ , [Rh (CO) ₂ I ₂ ] ^-
Rhodium iodine complex and rhodium carbonyl complex are effectively used. The amount used is the concentration in the reaction solution,
200 to 1000 ppm, preferably 300 to 600 p
pm.

The iodide salt may be added for stabilizing the rhodium catalyst and suppressing side reactions, especially under a low water content.
This iodide salt may be any salt as long as it produces iodine ions in the reaction solution. Examples include alkali metal iodide salts such as LiI, NaI, KI, RbI, CsI, BeI ₂ , MgI ₂ , Ca.
Alkaline earth metal iodide salts such as I ₂ , BI ₃ and AlI
_And aluminum group metal iodide salts such as ₃ . In addition to the metal iodide salt, an organic iodide salt may be used, for example,
Quaternary phosphonium iodide salt (tributylphosphine,
Methyl iodide adduct such as triphenylphosphine or hydrogen iodide adduct), Quaternary ammonium iodide salt (tertiary amine, pyridines, imidazoles, imides etc. methyl iodide adduct or iodide) Hydrogenated products and the like). Particularly, an alkali metal iodide salt such as LiI is preferable. The iodide salt is used in an amount of 0.07 to 2.5 mol / liter, preferably 0.25 to 1.5 mol / liter as iodide ion in the reaction solution. .

In this process, methyl iodide is used as a catalyst promoter and is present in the reaction solution in an amount of 5 to 20% by weight, preferably 12 to 16% by weight.

The water concentration in the reaction solution is 15% by weight or less, preferably 10% by weight or less, more preferably 1 to 5%.
% By weight.

Further, since the reaction of this process is a continuous reaction, 0.1 to 30% by weight, preferably 0.5 to 5% by weight, of methyl acetate produced by the reaction of the raw material methanol with acetic acid is produced.
Acetic acid, which is present and remains in the reaction solution, is the product and the reaction solvent.

The typical reaction temperature for the carbonylation in this process is about 150-250 ° C. and about 180-2.
A temperature range of 20 ° C. is preferred. The carbon monoxide partial pressure in the reactor can vary over a wide range, but is typically about 2-30 atm, preferably 4-15 atm. The total reactor pressure is about 15 due to the partial pressure of the by-products and the vapor pressure of the liquid contained.
Within the range of -40 atmospheres.

The process for producing acetic acid will be described below with reference to the drawings.

FIG. 1 is a flow chart showing a reaction-acetic acid recovery system used for carbonylation of methanol to acetic acid.

Reaction from methanol to acetic acid shown in FIG.
The acetic acid recovery system comprises a carbonylation reactor 10, a flasher 12 and a methyl iodide-acetic acid splitter column 14. In the carbonylation reactor 10, the reaction liquid contents are usually automatically maintained at a constant level. Fresh methanol and sufficient water are continuously introduced into the reactor as needed to maintain a measurable water concentration in the reaction medium. Alternative distillation systems can also be used as long as they include means for recovering crude acetic acid and means for recycling the catalyst solution, methyl iodide and methyl acetate to the reactor.

In the preferred process, carbon monoxide is continuously introduced into the carbonylation reactor 10 just below the stirrer used to stir the contents. The gaseous feedstock is dispersed throughout the reaction solution by this means. A gaseous purge stream is discharged from the reactor to prevent the accumulation of gaseous byproducts and maintain a set carbon monoxide partial pressure at a constant total reactor pressure. The reactor temperature is automatically controlled and the carbon monoxide feed is introduced at a reaction rate sufficient to maintain the desired total reactor pressure. The liquid product is withdrawn from the carbonylation reactor 10 at a rate sufficient to maintain a constant level and is introduced into the flasher 12 at a midpoint between its top and its bottom via line 11.

In the flasher 12, the catalyst solution is withdrawn as a bottom stream 13 (mainly acetic acid containing the catalyst and iodide salt together with small amounts of methyl acetate, methyl iodide and water) and returned to the carbonylation reactor 10. . The overhead 15 of the flasher 12 contains primarily the product acetic acid along with methyl iodide, methyl acetate and water.

Methyl iodide-acetic acid splitter column 1
The product acetic acid taken off by line 17 from the side near the bottom of 4, which can also be taken off as a bottoms stream, is further purified by the person skilled in the art in an obvious manner. The overhead 20 from the methyl iodide-acetic acid splitter column 14, which contains mainly water and acetic acid in addition to methyl iodide and methyl acetate, is recycled to the carbonylation reactor 10 via line 21. Overhead 20 typically condenses into two liquid phases when condensed, if sufficient water is present. The lower phase 30 consists mainly of methyl iodide plus some methyl acetate and acetic acid, and the upper phase 32 consists mainly of water and acetic acid plus some methyl acetate.

Methyl iodide-acetic acid splitter column 1
4 from lower phase 30, upper phase 32 or total overhead 20 if not separated, or streams thereof, together with other recycle products including methyl iodide, methyl acetate, water and impurities. The recycle stream 21 can be formed.

Methyl iodide-acetic acid splitter column 1
4 lower phase 30, upper phase 32, or total overhead 2
0 or the recycle stream 21 is introduced into the distillation column 40 and subjected to the treatment of the present invention. For the distillation column, separation, etc., any suitable device known in the art can be used. Further, the number of stages of the distillation column may be any number, but if it cannot be carried out by one distillation column due to facility cost or the like, the treatment of the present invention may be carried out by using two or more distillation columns. I don't care.

The case of carrying out using two distillation columns will be described below with reference to FIG.

Methyl iodide-acetic acid splitter column 1
4 lower phase 30, upper phase 32, or total overhead 2
0 or recycle stream 21 is introduced into distillation column 40 and the methyl iodide recycle stream is recycled from the bottom of the column via line 46 to the reactor. A distillate 44 is obtained from the top of the column.

The distillate 44 of the distillation column 40 is introduced into the distillation column 60, subjected to the treatment of the present invention, and the methyl iodide recycle stream from which most of the acetaldehyde has been removed is sent to the upper part of the distillation column 40 via a line 66. Is recycled. Alternatively, if a methyl iodide-rich liquid from which most of the acetaldehyde has been removed is obtained overhead, the overhead distillate is recycled to the distillation column 40.

Generally, the total overhead 20 process liquid from the methyl iodide-acetic acid splitter column 14 is:
Methyl iodide 5 to 90% by weight, acetaldehyde 0.0
5 to 50% by weight, methyl acetate 0 to 15% by weight, acetic acid 0 to
It contains 80% by weight, 0.1-40% by weight of water and other carbonyl impurities.

Since the process liquid containing acetaldehyde as described above contains useful components such as methyl iodide and methyl acetate, it is circulated to the carbonylation reactor 10 and reused. Therefore, it is preferable that after the acetaldehyde is separated and removed from these process liquids as much as possible, it is circulated to the reactor.

When the acetaldehyde is not sufficiently removed, acetaldehyde is accumulated in the process liquid to promote the aldol condensation of acetaldehyde, and reducing substances such as crotonaldehyde and 2-ethylcrotonaldehyde and hexyl iodide. The by-product rate of alkyl iodide is increased, and a product acetic acid containing a large amount of these carbonyl impurities is obtained.

Separation of acetaldehyde and methyl iodide is not only difficult because the boiling points of acetaldehyde and methyl iodide are close to each other. Distillation and concentration in a non-aqueous system such as methyl iodide requires paraaldehyde, Not only does it interfere with the concentration of acetaldehyde by producing methaaldehyde, but also the methaaldehyde precipitates in the process, making safe operation impossible.

Paraaldehyde is a trimer of acetaldehyde and is a liquid having a boiling point of 124 ° C. and a melting point of 10 ° C.
Paraaldehyde is generally 0 to -1 from acetaldehyde.
It is likely to occur at a low temperature of 0 ° C, and the production limit temperature is 55 ° C.
Is. In the laboratory, it was confirmed to occur at 20 ° C.

Methaldehyde is a tetra-hexamer of acetaldehyde and is a white needle crystal with a melting point of 140-246 ° C. Methaldehyde is produced at a lower temperature than paraaldehyde, and is generally -10 to -from acetaldehyde.
It occurs at about 40 ° C. In the laboratory, it was confirmed that it occurs even at about 5 ° C. If the temperature is -40 ° C or lower, polymer polymerization occurs. Also, for paraaldehyde and methaldehyde,
It was confirmed that there are stereoisomers and the melting points and solubilities in solvents differ.

As shown here, the formation of paraaldehyde and methaldehyde is affected by temperature. That is,
By controlling the operating pressure and operating temperature in the distillation column, it became possible to separate and remove acetaldehyde.

That is, the temperature at the top of the distillation column is 55 ° C.
As described above, the distillation at the reflux tank temperature of 25 ° C. or higher and the pressure of 1 Kg / cm ² can suppress the formation of paraaldehyde and methaaldehyde, and improve the separation efficiency of methyl iodide and acetaldehyde. I found it. It is also effective to suppress the generation of paraaldehyde and methaaldehyde by shortening the residence time from the top condenser through the reflux tank to return to the distillation column.

In the distillation column, the column top temperature is lower than 55 ° C.,
By distilling at a reflux tank temperature of less than 25 ° C., acetaldehyde is converted to paraaldehyde and methaaldehyde at the top of the tower and has a high boiling point, so that it moves to the bottom side of the tower. In the form, it was found to be removed from the bottoms. However, since methaaldehyde is a solid that has low solubility especially in methyl iodide, it precipitates, clogging not only the porous plate and packing of the distillation column but also each nozzle, pipe, valve, etc. Become. The inventors have found that metaaldehyde dissolves in alcohols such as methanol, ethanol, propanol. That is, it became possible to prevent clogging by distilling in the presence of alcohol.

The alcohol used in the present invention may be any alcohol such as alicyclic alcohols such as methanol, ethanol and propanol, aromatic alcohols such as phenol and benzyl alcohol, and polyhydric alcohols such as ethylene glycol. However, methanol is preferably used also as a raw material.

Further, as a result of detailed examination of the solubility of methaaldehyde, the solubility was increased in the order of methyl iodide << acetaldehyde = methanol <mixed solution of methyl iodide and methanol, and a mixed solution of methyl iodide and methanol was obtained. It was found that there is an optimum point of solubility in. In the distillation column bottoms composition in the continuous production process of acetic acid, the recrystallization temperature is 18 ° C. if the methyl iodide / methanol weight ratio is 3/1, 12 ° C. if it is 5/4, and 6 if it is 3/4. It was confirmed that the temperature was -9 ° C or lower if the temperature was 1/2 ° C. The preferred methyl iodide / methanol weight ratio is 5/4 to 1/2, although it depends on the heat retention state.

The alcohol charging position may be any charging stage which allows separation without loss of alcohol from the top of the column, but is preferably a mixing liquid charging stage containing acetaldehyde and methyl iodide to be treated according to the present invention. Below.

Further, the production and decomposition of paraaldehyde and methaaldehyde are affected not only by temperature and time but also by coexisting acid strength.

[0042]

The present invention will be described in more detail based on the following examples, but the invention is not intended to be limited by these examples. Parts in the examples are by weight unless otherwise stated.

Example 1 During the operation of the acetic acid production test apparatus shown in FIGS. 1 and 2, the total number of the lower phase liquid 30 of the separation tank after condensing the overhead 20 of the methyl iodide-acetic acid splitter column 14 was 80. It was introduced at the 70th stage from the top of the distillation column 40 of the stage (Sieve Tray) and distilled under the conditions of a reflux ratio of 270, a column top temperature of 54 ° C and a column bottom temperature of 82 ° C. With the charged amount as 100 parts, 0.033 parts from the top of the tower and 99.67 parts from the bottom of the tower were withdrawn. For convenience of equipment, the second distillation column 60 is charged with the overhead distillate of the distillation column 40, conditions under which paraaldehyde and methaldehyde are not produced, column top temperature 56 ° C, reflux tank temperature 32 ° C, column top pressure. Distilled at 2.5 kg / cm ² G. The distillation column 60 was a packed column with 8 theoretical plates, and the total amount of the distillate at the top of the distillation column 40 was charged in the 4th plate from the top. The reflux ratio was 40 and the bottom temperature was 74 ° C. With the amount charged to the distillation column 60 as 100 parts, 38.5 parts of the acetaldehyde concentrate from the top of the column (acetaldehyde concentration 88.1 w
The liquid rich in methyl iodide (82.8 wt% of methyl iodide) was separated from the bottom of the column as 61.
Five parts were withdrawn and recycled to the distillation column 40. The concentration of hexyl iodide in the product acetic acid was 28 ppb. Table 1 shows the composition of the reaction liquid, Table 2 shows the composition of the liquid charged to the distillation column 40, and the composition of the liquid extracted at the top of the distillation column 40. Table 3 shows the composition of the distillation column 6.
The composition of the liquid charged to 0, the distillate of the distillation column 60, and the bottom liquid are shown respectively.

[0044]

[Table 1]

[Table 2]

[Table 3] Comparative Example 1 Overhead 20 from a methyl iodide-acetic acid splitter column was not distilled and was recycled to the reactor as it was. As a result, acetaldehyde was not separated and removed, and the concentration of hexyl iodide in the product acetic acid was 100 ppb.
Met. The wet product stream taken out from the vicinity of the bottom of the methyl iodide-acetic acid splitter column was dried by distillation, and the concentration of propionic acid in the dried product liquid was 620 ppm.

Example 2 The top withdrawal liquid of the distillation column 40 in Example 1 was used as the distillation column 60.
To produce paraaldehyde and methaaldehyde, column top temperature 28.7 ° C, reflux tank temperature -1
Distillation was performed at 0 ° C. The other conditions in the distillation column 60 are the reflux ratio of 15 and the total number of stages is an older show of 20 stages.
Tower bottom temperature 64.6 ° C, tower top pressure 1.033 kg / cm ²
Met. Further, 100 parts of a methanol solution was introduced at the 17th stage from the top of the distillation column 60. Distillation tower 40 overhead liquid 100
Part into the distillation column 60, withdrawing 74 parts from the top of the column,
Recycled to the top of distillation column 40. The remaining 26 parts were separated and removed as bottoms. Further, by charging methanol, it was possible to prevent the bottom nozzle of the distillation column from being blocked and to extract the bottom liquid. The concentration of hexyl iodide in the product acetic acid was 40 ppb. Table 4 shows the composition of the liquid charged to the distillation column 60, the distillate of the distillation column 60, and the bottoms composition.

[0046]

[Table 4] Comparative Example 2 (conditions in which methanol is not charged) The same operation as in Example 2 was carried out except that no methanol solution was charged, and as a result, the nozzle at the bottom of the distillation column was clogged due to the precipitation of metaaldehyde crystals, and the operation could not be performed. .

[0047]

According to the present invention, acetaldehyde and methyl iodide can be efficiently separated. Furthermore, in the continuous production process of acetic acid and / or acetic anhydride, acetaldehyde contained in the process liquid recycled to the carbonylation reactor can be sufficiently removed, and methyl iodide can be efficiently fed to the reactor. You can recycle.

[Brief description of drawings]

FIG. 1 shows a flow chart of a reaction-acetic acid recovery system used for carbonylation of methanol to acetic acid.

FIG. 2 shows an example of a flow chart of a distillation system for separating acetaldehyde and methyl iodide.

[Explanation of symbols]

 10 Carbonylation reactor 12 Flasher 14 Methyl iodide-acetic acid splitter column 30 Separation tank Lower phase 40,60 Distillation column

Continuation of front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display area C07C 51/54

Claims (5)

[Claims] 1. A mixed liquid containing acetaldehyde and methyl iodide is a tower top temperature of 55 ° C. or higher and a reflux tank temperature of 25 ° C.
Above, a method for separating acetaldehyde and methyl iodide, which comprises distilling at a pressure of 1 Kg / cm ² or more. 2. A mixture liquid containing acetaldehyde and methyl iodide, the overhead temperature is lower than 55 ° C., the reflux tank temperature is 25 ° C.
A method for separating acetaldehyde and methyl iodide, characterized in that the distillation is carried out in the presence of less than and alcohol. 3. The method for separating acetaldehyde and methyl iodide according to claim 2, wherein alcohol is introduced into the lower part of the charging stage of the mixed solution containing acetaldehyde and methyl iodide. 4. The method for separating acetaldehyde and methyl iodide according to claim 2, wherein the alcohol is methanol. 5. A mixed solution containing acetaldehyde and methyl iodide contains methanol and / or methyl acetate,
Carbonylating in a reaction medium containing a group metal catalyst and methyl iodide, the product of the carbonylation being a volatile phase containing the product and unreacted methanol and / or methyl acetate and methyl iodide and the Group 8 It is separated into a low-volatile phase containing a metal catalyst, and the volatile phase is further distilled to obtain a product and an overhead containing unreacted methanol and / or methyl acetate and methyl iodide. Methanol and / or recycle to the carbonylation reactor
The method for separating acetaldehyde and methyl iodide according to any one of claims 1 to 4, which is the overhead in the method for carbonylating methyl acetate.