JPH10175932A - Purification of n-vinylcarb0xylic acid amide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress polymerization in a rectifying tower in purification of crude N-vinylcarboxylic acid amide using the rectifying tower and prevent accumulation of impurity in the tower bottom of the rectifying tower and continuously obtain high-purity N-vinylcarboxylic acid amide for a long period in high yield. SOLUTION: In continuously purifying crude N-vinylcarboxylic acid amide, (1) crude N vinylcarboxylic acid amide is fed to a rectifying tower and (2) a purified N-vinylcarboxylic acid amide is distilled away from the tower top and a tower bottom solution containing N-vinylcarboxylic acid is pulled out from the tower bottom and (3) the tower bottom solution is circulated to a thin-film evaporator and unevaporated content is removed to outside of the system and evaporation content is fed to the rectifying tower.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for purifying N-vinylcarboxylic acid amide by distillation.

[0002]

2. Description of the Related Art N-vinyl carboxylic acid amides have the general formula

[0003]

Embedded image CH ₂ ＣＨCHNHCOR (wherein, R represents a hydrogen atom or an alkyl group)

[0004] N-vinylamide or N-
It is a compound having high polymerizability, also called vinylacylamide, and specific examples include N-vinylformamide and N-vinylacetamide. The compound is polymerized alone or copolymerized with another vinyl compound,
Polymers having various properties can be obtained. Especially,
N-vinylcarboxylic acid amide is useful as a raw material of a water-soluble polymer such as polyvinylamine obtained by hydrolyzing this polymer and converting the amide group to a primary amino group, or as an intermediate of an organic chemical. It is known that there is.

As a method for producing this N-vinylcarboxylic acid amide, a method of removing an alcohol from N- (α-alkoxyethyl) carboxylic acid amide (US Pat. No. 3,914,304), formylalanine Method for desorbing hydrogen cyanide from nitrile
134359) and a method of thermally decomposing ethylene bisformamide or ethylidene bisacetamide (U.S. Pat. Nos. 4,018,826 and 4,490,
557 and 4,578,515) and the like. Each of these methods is performed under reduced pressure at 90 to 7
The N-vinyl carboxylic acid amide precursor is thermally decomposed at a high temperature of about 50 ° C. to obtain N-vinyl carboxylic acid amide. The obtained N-vinylcarboxylic acid amide is usually recovered from the pyrolysis reaction solution by distillation. However, the thermal stability of N-vinylcarboxylic acid amide is not sufficient, and a small amount of basic impurities by-produced in the thermal decomposition reaction is present in the thermal decomposition reaction solution to decompose N-vinylcarboxylic acid amide. Promote. For this reason, N-
There is a problem that the recovery rate of the vinyl carboxylic acid amide becomes low, or the purity of the recovered purified N-vinyl carboxylic acid amide is not sufficient, which adversely affects the polymerizability as a polymer raw material.

Accordingly, the following method has been proposed as a means for suppressing the decomposition of N-vinylformamide among the N-vinylcarboxamides, particularly during distillation.
(1) a method in which a pyrolysis reaction solution is subjected to a contact treatment with a weakly acidic cation exchange resin before distillation to remove trace amounts of basic impurities contained therein (JP-A-63-190862);
(2) A method in which a pyrolysis reaction solution is passed through a thin film evaporator to recover most of the mixed solution as an evaporating component, and then the evaporating component is precision distilled using a distillation column (Japanese Patent Laid-Open No. 5-155829). ) Distillation by allowing a large amount of formamide to be present in crude N-vinylformamide
-99374).

[0007]

However, (1)
In the method (1), operations such as regeneration of the ion exchange resin are complicated. Further, in the method (2), since the separation of the basic impurities in the thin film evaporator is not complete, the basic impurities partially mixed in the evaporate accumulate in the bottom of the distillation column, and the continuous distillation for a long time is performed. However, the improvement in stability of N-vinylformamide is not always sufficient. Furthermore, in the method (3), it is rather general that the pyrolysis reaction solution originally contains a considerable amount of formamide, and the stabilizing effect is not sufficient. Further, if formamide is added to the pyrolysis mixture and distillation is performed, the time and labor required for separating N-vinylformamide and formamide increase accordingly.

[0008]

In view of the above-mentioned problems, the present inventors have conducted various studies on a method for efficiently recovering N-vinylcarboxylic acid amides such as N-vinylformamide, and as a result, purified from the top of the tower. In a distillation method using a rectification column for obtaining N-vinylcarboxylic acid amide, at least a part of the bottom liquid containing N-vinylcarboxylic acid amide is taken out of the column and supplied to a thin film evaporator. If the method of removing the non-evaporated components out of the system and returning the evaporated components to the rectification column and distilling again is adopted, there is no accumulation of basic impurities in the bottom liquid of the rectification column and high purity N -It has been found that vinylformamide is recovered by distillation in high yield, and the present invention has been achieved.

That is, the gist of the present invention is to provide a method for continuously purifying crude N-vinylcarboxylic acid amide, which comprises the steps of:
Vinyl carboxylic acid amide is supplied to the rectification column, (2) purified N-vinyl carboxylic acid amide is distilled off from the top of the column, and a bottom liquid containing N-vinyl carboxylic acid amide is withdrawn from the bottom of the column. A method for purifying N-vinylcarboxylic acid amide, comprising circulating the bottom liquid in a thin-film evaporator, removing unvaporized components out of the system, and supplying the evaporated components to the rectification column.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
As the N-vinyl carboxylic acid amide targeted in the present invention, N-vinyl formamide and N-vinyl acetamide are the most common, but N-vinyl formamide is particularly suitable as the target substance of the present invention. The method for producing N-vinylcarboxylic acid amide is not particularly limited as long as it is the method exemplified above, but the yield of N-vinylcarboxylic acid amide is high and there is no by-product of harmful substances such as hydrogen cyanide. In the above, a method of thermally decomposing N- (α-alkoxyethyl) -carboxylic acid amide to remove the alcohol is particularly preferable.

The above reaction solution containing N-vinylcarboxylic acid amide may be directly used as a crude N-vinylcarboxylic acid amide in the purification of the present invention. Since the mixture is a mixture containing a low boiling point component or a high boiling point component, usually, at least a part, preferably the entire amount, of the crude N-vinylcarboxylic acid amide reaction solution is removed in advance to some extent of the light boiling component and the high boiling component. A method in which crude N-vinylcarboxylic acid amide is supplied to a rectification column is desirable. As a method of evaporating light boiling components,
A method of distilling off the low boiling components in the mixture under reduced pressure can be adopted. For example, a reaction solution containing N-vinylformamide obtained by a thermal decomposition method in which alcohol is eliminated from N- (α-alkoxyethyl) -formamide contains alcohol and formamide by-produced in addition to N-vinylformamide. Since it is a mixture containing methanol, ethanol, etc., the low boiling alcohol can be effectively removed from the thermal decomposition reaction solution by distillation under reduced pressure.

In addition to removing the light-boiling components, it is also preferable to use a thin-film evaporator to remove some of the high-boiling components in the reaction mixture. In this case, crude N-vinylcarboxylic acid amide is recovered as an evaporating component. The thin film evaporator is suitable for evaporating and separating an object having low thermal stability, such as N-vinylcarboxylic acid amide, from the mixture. The structure is the same as that of a commercially available device, and it is not necessary to have a special structure. The rotating film type, plate type falling film type, tube type falling film type, wiper type thin film type, centrifugal type thin film type etc. Although a type is exemplified, a device of a type that does not easily cause problems such as blockage with respect to a polymer contained in advance or generated during a unit operation is desirable, and a rotary type thin film type and a wiper type thin film type A type in which a thin film is mechanically formed such as a centrifugal thin film type is particularly preferable. The operating conditions for thin film evaporation are usually 0.1
Under a reduced pressure of from 3 to 3 KPa, preferably from 0.3 to 2 KPa, the vapor temperature is usually from 70 to 150 ° C., preferably from 80 to
At 130 ° C., the average residence time of the liquid is usually 30 seconds to 10 minutes, preferably 1 to 5 minutes. By evaporating the thin film under such conditions, usually 80% or more, preferably 85% or more of the liquid containing N-vinylcarboxylic acid amide is recovered as an evaporating component. On the other hand, the remaining unevaporated components, that is, the remaining high-boiling components are removed from the system without being circulated to the thin film evaporator.

The components that can be removed by the above-mentioned thin film evaporator are crude N from which light-boiling components and the like have been removed at room temperature (25 ° C.).
-It can be quantified as an acetone-insoluble component generated as a precipitate when a 5-fold amount of acetone is added to vinylcarboxylic acid amide. The component is identified as a high-boiling component mainly consisting of a polymer of N-vinylcarboxylic acid amide by analysis such as IR spectrum, NMR spectrum and liquid chromatography. The polymer is crude N
-In the production of vinyl carboxylic acid amides, for example, N
It is a by-product in the thermal decomposition step of a precursor such as-(α-alkoxyethyl) carboxylic acid amide. Also, N-
Because of the high reactivity of vinylcarboxylic acid amide itself,
It is produced and increased spontaneously even during transportation and storage of the purified N-vinylcarboxylic acid amide. Such an acetone-insoluble component is dissolved in the crude N-vinyl carboxylic acid in a normal state. When such an acetone-insoluble component is contained in the purified raw material, polymerization is promoted using the component as a core, and even in the presence of a stabilizer, the growth of the polymer is inevitable. In a distillation column, insolubles tend to precipitate where the packing is not uniformly wetted by the liquid. It is presumed that once the insoluble matter precipitates, the polymer grows with the nucleus as a nucleus, and the insoluble and infusible polymer eventually accumulates. Such a phenomenon is remarkable in the raw material supply port where the acetone-insoluble component is always supplied. Therefore, the acetone-insoluble component in the crude N-vinylformamide is always removed as much as possible during continuous distillation purification. The content of the high molecular weight component in the crude N-vinylformamide is usually 400 ppm or less, preferably 200 ppm or less, particularly preferably 50 ppm.
The following is assumed.

Further, N-vinyl carboxylic acid amide is very sensitive to heat change, so that even if the acetone-insoluble matter can be removed once, the crude N-vinyl carboxylic acid amide recovered by the thin film evaporator can be used. The acetone-insoluble matter is regenerated, including when the vapor components are cooled and condensed, or when the crude N-vinyl carboxylic acid amide is supplied immediately in the rectification column and not purified, but temporarily stored or stored. Special attention is required because it is easy to do. In general, when evaporating a thin film of an organic substance, it is often the case that the evaporated portion is cooled and once recovered as a condensate.However, in the case of N-vinylcarboxylic acid amide, when the time of gas-liquid contact during condensation becomes longer, Care must be taken because acetone-insoluble matter is easily regenerated. In addition, when this condensate is supplied to a rectification column, when rectifying general organic matter, a method of preheating and supplying the condensed liquid to the rectification column in order to reduce the heat load in the rectification column as much as possible. Although generally desirable, N-vinylcarboxylic acid amide is not preferable because heating too much will regenerate the acetone-insoluble component remarkably. Even when heating, the temperature is usually kept at 50 ° C or lower, preferably 40 ° C or lower. Should. Further, in order to avoid the regeneration of the acetone-insoluble component and supply it to the rectification column, crude N-vinyl carboxylic acid amide is supplied to a thin film evaporator to remove the non-evaporated components, while N-vinyl carboxylic acid amide is A method of supplying the evaporating component to the rectification column without condensing it is preferable. At this time, usually 90% or more of the evaporated portion is supplied without condensation, and preferably substantially the entire amount is supplied to the rectification column by steam. For this purpose, it is necessary to keep the line from the thin film evaporator to the rectification tower kept warm.

The purity of the crude N-vinylcarboxylic acid amide prepared as described above is usually at least 50% by weight, preferably 80 to 95% by weight. The crude N-vinyl carboxylic acid amide means N-vinyl carboxylic acid amide having a lower purity than after the purification according to the present invention, and has a lower boiling point component or In addition to liquids from which high boiling point components have been removed to some extent, even once purified, the purity of the N-
Vinyl carboxylic acid amides and the like are also included.

In the present invention, the purification of the crude N-vinylcarboxylic acid amide as described above is performed by continuous distillation using a rectification column. The structure of the rectification column used is the same as that of a general distillation column, and examples thereof include a packed type, a tray type and the like.
The supply port of the crude N-vinylcarboxylic acid amide solution in the rectification column is
In the middle stage of the tower, usually 1/5 to 4/5
It is installed at the position. There are no particular restrictions on the operating conditions for the distillation, but as conditions that are easy to carry out industrially, the theoretical number of columns in the column is usually 3 to 30, preferably 5 to 20, and the pressure at the top of the rectification column is usually 0. 1-3 KPa, preferably 0.3-
2 KPa, and the corresponding tower temperature range is usually 50 to
100, 70-85 ° C. In the present invention, the column bottom temperature can be generally maintained at 80 to 120 ° C., preferably 100 ° C. or lower, and the N-vinylcarboxylic acid amide is maintained at about 5 to 10 ° C. lower than the conventional distillation method. And a stable distillation operation is possible.

Then, usually 40 to 90%, preferably 50 to 80% of the N-vinylcarboxylic acid amide component of the crude N-vinylcarboxylic acid amide supplied to the rectification column is distilled off from the top. When the distillation rate of N-vinylcarboxylic acid amide in the rectification column is too low, when the rectification column is fed again after passing through the thin film evaporator, the amount of the bottom liquid of the rectification column becomes too large. In addition, the necessity of increasing the size of the thin film evaporator and the rectification column arises, which is not industrially advantageous. On the other hand, if the distillation rate of N-vinylcarboxylic acid amide in the rectification column is too high, the amount of the bottom liquid in the rectification column will be too small, and the residence time at the bottom will increase, and , The accumulation of basic impurities cannot be ignored, and the effect of the present invention cannot be sufficiently exhibited. The basic impurities here are ammonia, picoline,
It is a mixture of ethyl picoline and various other basic substances of unknown structure. In the present invention, the tower top has a broad meaning, and includes the tower itself and its vicinity. That is, a method is also possible in which a small amount of light-boiling impurities is removed from the top of the column, and a desired N-vinylcarboxylic acid amide-rich component is distilled off from the vicinity of the top of the column.

The purity of the N-vinylcarboxylic acid amide in the distillate can be appropriately adjusted according to the desired degree of purification, but is usually 97% by weight or more, preferably 98% by weight or more of high-purity N-vinylformamide. Can be obtained stably.
On the other hand, with respect to the composition of the bottom liquid of the rectification column,
-The distilling rate of vinyl carboxylic acid amide from the top is usually 9
Since the content of the N-vinylcarboxylic acid amide component is usually 50% by weight or more, preferably 60% by weight,
% By weight or more.

Next, in the present invention, the bottom liquid containing the above-mentioned N-vinylcarboxylic acid amide is continuously discharged outside the column. Then, the extracted liquid is supplied to a thin-film evaporator, and in the thin-film evaporator, an unevaporated component is removed from the system, and an evaporated component containing N-vinylcarboxylic acid amide is recovered. Supply port. The higher the proportion of the bottom liquid which is extracted outside the column and supplied to the thin film evaporator, the higher the more, the more preferably it is usually 50% or more, preferably 80% or more, particularly preferably 90% or more.

The thin film evaporator to which the extracted bottom liquid is supplied is not particularly limited, and the thin film evaporator used for obtaining the crude N-vinyl carboxylic acid amide from which the high boiling point component in the above-mentioned pyrolysis reaction solution has been removed. The same types and operating conditions as the evaporator can be adopted. Although the liquid extracted from the bottom of the column can be returned to the above-mentioned thin film evaporator, this method is particularly preferable because one thin film evaporator is effectively used. In the thin-film evaporator, the non-evaporated portion is removed from the system, the evaporated portion containing N-vinylcarboxylic acid amide is recovered, and the evaporated portion is circulated as it is or the condensed liquid is circulated to the supply port of the rectification column. Let it. The above-mentioned method of the present invention operates continuously each of distillation by a rectification column and evaporation and separation by a thin film evaporator, and
It is composed by combining both.

In the thin-film evaporator, the unevaporated component is removed from the system, while the evaporated component is supplied to the supply port of the rectification column. Even if the yield of carboxylic acid amide is not so high, high-purity N-vinylcarboxylic acid amide can be recovered by distillation in high yield. Since the retention time at the bottom of the rectification column and the distillation rate of distillation are originally in inverse proportion, the method of simply distilling N-vinylcarboxylic acid amide requires the use of N at the bottom of the rectification column.
-To prevent the decomposition of the vinyl carboxylic acid amide,
It was necessary to extremely suppress the distillation rate of N-vinylcarboxylic acid amide. However, according to the present invention, the decomposition of N-vinylcarboxylic acid amide at the bottom of the rectification column is reduced, and high-purity N-vinylcarboxylic acid amide can be recovered in high yield.

In Japanese Patent Publication No. 6-99374, the presence of a large amount of formamide in crude N-vinylformamide allows the bottom liquid to be pumped through a thin film evaporator during distillation in a rectification column. There is a statement to recycle.
This is one of the methods for effectively heating the bottom liquid, which is often performed in ordinary distillation.The bottom liquid mainly containing formamide extracted is heated by a thin film evaporator, and the unevaporated portion is left as it is. While being circulated to the bottom of the column, the evaporating component due to heating is also circulated to the bottom of the column. That is, a thin film evaporator is used as a reboiler. Therefore, as in the present invention, a bottom liquid mainly containing N-vinyl carboxylic acid amide is withdrawn from the bottom of the column, and the extracted liquid is supplied to a thin film evaporator, and only the evaporated portion is recovered to obtain a crude N-vinyl carboxylic acid amide. Is different from the method of supplying to the rectification column.

In the above continuous distillation of the present invention, a stabilizer effective for preventing thermal polymerization of N-vinylcarboxylic acid amide may be used. In the above-mentioned purification system, the stabilizer is preferably added when N-vinylcarboxylic amide is supplied to the thin film evaporator and the rectification column. Examples of effective stabilizers include quinones, alkali-modified quinones,
Phenol compounds, aromatic amine compounds, thiourea compounds, etc., and the amount of addition is usually 50 to 1000.
0 ppm, preferably 100-5000 ppm.

Particularly preferred as the stabilizer are alkali-modified quinones.
-Benzoquinone, benzoquinones such as o-benzoquinone, naphthoquinones, and anthraquinones are exemplified.When preparing an alkali-modified quinone, distillation separation from N-vinylcarboxylic acid amide is easy. Quinones are usually dissolved in a solvent such as methanol, ethanol, water, toluene, benzene or the like, or N-vinyl carboxylic acid amide at a concentration of 5 to 150 g / l, and sodium hydroxide, potassium hydroxide, sodium carbonate and sodium bicarbonate are dissolved therein. And the like may be added and stirred at room temperature or under heating. A sufficient amount of alkali to be added is about 10 ^-4 to 10 ^-2 mol / l. Quinones generally act as polymerization inhibitors, but since the alkali-modified quinones are non-volatile, they can be easily separated from purified N-vinylcarboxylic acid amide through continuous distillation.

[0025]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the present invention is limited thereto. In the following examples, “%” is “weight%”, and “ppm” is “weight ppm”.
Is shown. In the examples, a method for analyzing an acetone-insoluble component in N-vinylformamide is as follows.

(Method of analyzing acetone-insoluble components) Room temperature (2
Under a condition of 5 ° C.), 250 g of acetone was added to and mixed with 50 g of the sample N-vinylformamide, and the precipitate was collected by filtration with a 0.5 μm Teflon filter. This was dried under reduced pressure at 60 ° C. until the weight became constant, and the amount was measured to determine the content (unit: ppm) of the high molecular weight component (dry weight of insoluble matter / 50).
It was indicated by × 10 ⁶ .

Example 1 [Production of N- (α-methoxyethyl) formamide (hereinafter referred to as “ether compound”)] Formamide and acetaldehyde were placed in a 500-liter stainless steel reactor equipped with a stirrer and a temperature controller. N- (α-hydroxyethyl) formamide (hereinafter referred to as “hydroxy form”) crystal (purity 73) obtained by reacting in the presence of a potassium carbonate catalyst.
30 kg), 31.5 kg of methanol (4 mol times based on the hydroxy form) and 0.25 kg of sulfuric acid (potassium bicarbonate). Neutralization amount and 0.5 mol% amount to the hydroxy form).
The etherification reaction is performed for 3 hours under stirring at a temperature of 25 to 40 ° C. until the remaining amount of the hydroxy compound in the reaction system becomes 8% by weight, and then 20% by weight of sodium hydroxide is added thereto to neutralize the catalyst. This brought the pH to 7. Further, the liquid was evaporated under a reduced pressure of 6.7 KPa to a light boiling point.
The ether compound was recovered by distillation under reduced pressure of KPa.

[Production of N-vinylformamide] The ether compound obtained above was prepared by heating the ether compound to a heater equipped with a diameter of 40 mm and a length of 1 mm.
It is supplied at a rate of 0.2 kg / min to a 000 mm stainless steel tube under a reduced pressure of 13.3 KPa while maintaining the internal temperature at 400 ° C. On the other hand, the discharged gas is immediately condensed to be thermally decomposed. 23 kg of liquid (67% by weight of N-vinylformamide, 31% by weight of methanol) were recovered.

[Distillation of Crude N-Vinyl Formamide] The above distillate was further evaporated to remove light boiling components such as methanol under the conditions of a pressure of 6.7 KPa and a temperature of 50 ° C. -Vinyl formamide (88% by weight of N-vinyl formamide, 5% by weight of formamide, 5% by weight of ether compound, 2% by weight of high boiling impurities) was obtained. This crude N-vinylformamide contains one acetone-insoluble component.
It was contained at 500 ppm.

On the other hand, p-benzoquinone (parabenzoquinone) was dissolved in methanol to a concentration of 5% by weight,
Caustic soda was added to the solution to a concentration of 20 meq / kg, and a solution was stirred at 20 ° C. for 3 hours. When the liquid was analyzed by liquid chromatography, p-
Benzoquinone was not detected (detection lower limit: 50 ppm) and all had been converted to alkali-modified products.

Alkali-modified p-benzoquinone obtained above (equivalent to 3000 ppm as p-benzoquinone) as a distillation stabilizer in the crude N-vinylformamide
Was added. The above crude N-vinylformamide was applied to a glass falling film evaporator (heat transfer surface diameter: 50 mm, height: 200 mm).
mm), pressure 0.4 KPa, steam temperature 125 ° C.,
Evaporation was performed under the condition that the average residence time of the liquid was 5 minutes.

Unevaporated components were removed from the system without being circulated to the thin film evaporator. In addition, the condensate (50 ppm in acetone-insoluble component) of the evaporating portion is directly supplied to a rectification column (diameter 50 mm, height 900 mm, packed with Sulzer Lab Packing) without heating, and the pressure is 6.7 KPa, and the column top is heated. Precision distillation was performed under the conditions of a temperature of 70 ° C., a tower bottom temperature of 90 ° C., a number of theoretical plates of 15, a reflux ratio of 4, and a tower bottom residence time of 2.5 hours. At this time, the p-benzoquinone alkali-modified product was converted to p-benzoquinone at 2000 pp with respect to the condensate.
m. The recovery rate of N-vinylformamide from the top in the precision distillation was 70%.
The purified N-vinylformamide having a purity of 99% by weight was obtained from the top of the column. In addition, the bottom liquid is the crude N-
It was combined with vinylformamide and supplied to the glass falling thin film evaporator.

In the operation for recovering N-vinylformamide, substantially no decomposition of N-vinylformamide, no accumulation of decomposition products and basic impurities at the bottom of the column, and high purity (99% by weight) of N-vinylformamide Formamide yield 9
It was recovered at 3% (the base of the yield was crude N-vinylformamide). A flow sheet showing the above-mentioned steady-state relationship of the purification process is shown in FIG. In FIG. 1, "VFA" is N-
Vinylformamide, “FA” is formamide, “ether” is N- (α-methoxyethyl) formamide,
"Base" indicates a basic impurity. Each component was analyzed by liquid chromatography. The value obtained by neutralization titration with a 0.1 N aqueous sulfuric acid solution was defined as the total amount of basic impurities.

Comparative Example 1 The same crude N-vinylformamide solution as in Example 1 was circulated to the thin film evaporator using the same glass thin film evaporator and rectification column as in Example 1. Purification was performed in the same manner as in Example 1 except that the compound was removed from the system without using the same. At this time, in order to obtain N-vinylformamide having a purity of 99% by weight in a consistent yield of about 90%, the distillation rate of N-vinylformamide was increased to about 93%. The bottom residence time exceeds 10 hours, and the accumulation of decomposition products and basic impurities at the bottom of the column is large. Even if the temperature of the bottom rises from 90 ° C. to about 110 ° C., the temperature does not stop and the continuous operation is continued. Could not. And the decomposition rate of N-vinylformamide at the beginning of distillation was 5%. FIG. 2 shows the relationship of the above purification process in a steady state.

[0035]

According to the present invention, in the purification of crude N-vinylcarboxylic acid amide using a rectification column, polymerization in the rectification column is suppressed, and impurities at the bottom of the rectification column are reduced. Of N-vinylcarboxylic amide of high purity can be obtained continuously for a long period of time with a high yield.

[Brief description of the drawings]

FIG. 1 is a flow sheet showing a purification process of N-vinylformamide of Example 1.

FIG. 2 is a flow sheet showing a purification process of N-vinylformamide of Comparative Example 1.

Continued on the front page (72) Inventor Eiichiro Kozezawa 1-1 Kurosaki Castle Stone, Yawata-Nishi-ku, Kitakyushu-shi, Fukuoka Inside the Mitsubishi Chemical Corporation Kurosaki Office

Claims (7)

[Claims] 1. A process for continuously purifying a crude N-vinylcarboxylic acid amide, comprising: (1) supplying the crude N-vinylcarboxylic acid amide to a rectification column; and (2) purifying the purified N-vinylcarboxylic acid from the top. The amide is distilled off, and the bottom liquid containing N-vinylcarboxylic acid amide is withdrawn from the bottom of the column. (3) The bottom liquid is circulated to a thin film evaporator, and the unevaporated components are removed from the system. Is supplied to the rectification column. 2. At least a part of the crude N-vinyl carboxylic acid amide is supplied to a thin film evaporator in advance, the unevaporated component is removed from the system, and the evaporated component containing the N-vinyl carboxylic amide is supplied to a rectification column. 2. The method of claim 1, wherein the method comprises providing. 3. The method according to claim 1, wherein 40 to 90% by weight of the N-vinylcarboxylic acid amide component supplied to the rectification column is distilled off from the top of the column. 4. The method according to claim 1, wherein the N-vinylcarboxylic acid amide component accounts for 50% by weight or more of the bottom liquid. 5. The process according to claim 1, wherein the distillation is carried out at a pressure of 0.1 to 3 KPa at the top of the rectification column. 6. The method according to claim 1, wherein the bottom temperature of the rectification column is maintained at 100 ° C. or lower. 7. The method according to claim 1, wherein the N-vinylcarboxylic acid amide is N-vinylformamide.