JP3128902B2 - Method for recovering N-vinylformamide - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for recovering N-vinylformamide. N-vinylformamide is known as a monomer material of a water-soluble polymer having various properties.

[0002]

2. Description of the Related Art As a method for producing N-vinylformamide, for example, a method in which an alcohol is eliminated from N- (α-alkoxyethyl) formamide (Japanese Patent Application Laid-Open No. Sho 50-7)
6015, JP-A-62-59248), a method of removing hydrogen cyanide from formylalanine nitrile (JP-A-61-134359), and a method of thermally decomposing ethylenebisformamide (US Pat. No. 4,578,515).

[0003] In any of the above methods, 90 to 7
It is thermally decomposed at a high temperature of 50 ° C. to obtain N-vinylformamide. N-vinylformamide is usually recovered from the pyrolysis mixture by distillation. However, a part of N-vinylformamide is decomposed or polymerized in the course of the distillation, thereby lowering the recovery rate. Also, the purity of the recovered N-vinylformamide decreases, and N-vinyl as a polymer raw material is reduced. There is a problem that the polymerizability of formamide is also adversely affected.

Therefore, the following methods have been proposed as means for suppressing the decomposition and polymerization of N-vinylformamide during distillation. A method for improving the stability of N-vinylformamide by adjusting the pH of the pyrolysis mixture to 6 to 8 before performing distillation (Japanese Patent Application Laid-Open No. Sho 62-19352). Prior to the distillation, the pyrolysis mixture is subjected to a contact treatment with a weakly acidic cation exchange resin to remove trace amounts of basic impurities which are considered to promote the decomposition of N-vinylformamide (Japanese Patent Application Laid-Open No. Sho 63). -190862). A method in which formamide is present in the pyrolysis mixture to carry out distillation (JP-A-62-190153).

[0005]

However, in the above method, salts of impurities generated by the neutralization accumulate at the bottom of the distillation column, and the stability of N-vinylformamide cannot be improved in continuous distillation for a long time. Not always enough. In addition, in the method (1), it takes time for the contact treatment with the ion exchange resin, and the combination with the continuous distillation step is inappropriate.

Further, as for the above method, it is rather common that the pyrolysis mixture 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. The specification of the above method also teaches a method of circulating a liquid by providing a thin film evaporator and a circulation pump at the bottom of a distillation column. In this method, the liquid at the bottom of the column can be preheated by a thin-film evaporator to improve the distillation efficiency of the column, but the liquid containing impurities stays in the bottom of the column for a long time. -Vinyl formamide is easily decomposed.

[0007]

Means for Solving the Problems In view of the above problems, the present inventors have conducted various studies on a method for efficiently recovering N-vinylformamide. As a result, N-vinylformamide obtained by a thermal decomposition method was obtained. A small amount of basic impurities by-produced during the thermal decomposition present in the mixed solution containing the mixed solution to promote the decomposition of N-vinylformamide; and
The basic impurities have a very low vapor pressure and can be separated from N-vinylformamide even in a thin-film evaporator having a low separation efficiency.
Vinylformamide was found to have significantly improved thermal stability and arrived at the present invention.

That is, according to the present invention, in recovering N-vinylformamide by distilling a mixed solution containing N-vinylformamide obtained by a pyrolysis method , the entire amount of the mixed solution is previously passed through a thin-film evaporator. Is recovered as an evaporate, and then the evaporate is precision distilled using a column to recover N-vinylformamide, and the remaining unevaporate is removed out of the system. Recovery method for vinyl formamide.

Hereinafter, the present invention will be described in detail. The mixed solution containing N-vinylformamide obtained by the pyrolysis method used in the present invention is not particularly limited as long as it is obtained by the above-described pyrolysis method. However, in view of a high yield of N-vinylformamide and no by-products of harmful substances such as hydrogen cyanide, the thermal decomposition method involves removing alcohol from N- (α-alkoxyethyl) -formamide. The method is preferred. Therefore, the method will be mainly described below.

[0010] N- (α-alkoxyethyl) formamide is usually obtained by an etherification reaction of N- (α-hydroxyethyl) formamide with an alcohol.
In this case, as the alcohol, a monovalent aliphatic lower alcohol such as methanol, ethanol, or propanol is generally used. In the obtained N-α-alkoxyethylformamide, there is no problem even if the raw material components remain, and it is usually subjected to the next thermal decomposition step as it is without purification.

Various conditions are selected for the method of thermal decomposition according to the type of alcohol used in the above-mentioned etherification reaction. For example, under a reduced pressure of 50 to 200 mmHg,
Examples include a method in which pyrolysis is performed in the gas phase at 500 ° C. to condense and recover product vapor, and a method in which the product is heated to 90 to 200 ° C. under reduced pressure of 1 to 20 mmHg to perform pyrolysis in the liquid phase while distilling off the product. Is done.

The mixed solution containing N-vinylformamide obtained by the above-mentioned thermal decomposition method contains N-vinylformamide and alcohol as a by-product as main components. In the present invention, the liquid may be directly passed through a thin-film evaporator, but depending on the boiling point of the by-produced alcohol, a low-boiling alcohol is usually distilled off under reduced pressure at 70 ° C. or lower before applying to the thin-film evaporator. Is also good. The liquid after the alcohol is separated is also included in the "mixture containing N-vinylformamide obtained by the pyrolysis method" used in the present invention.

[0013] In some cases, the stability of N-vinylformamide can be further improved by adding an acid to the above mixture and adjusting the pH of the mixture to near neutrality. Specifically, a part of the mixed solution may be sampled, and an acid may be added to the mixed solution in such an amount that the pH becomes 6 to 8 when the sampled solution is diluted with three times the amount of water. A feature of the present invention is that most of the above-mentioned mixed solution is recovered as an evaporating component, and then the evaporating component is precision distilled in a column to recover N-vinylformamide, and the remaining unevaporated component is removed out of the system. On the point.

Although the thin film evaporator has a separation efficiency similar to that of flash evaporation, it cannot separate impurities such as formamide in the mixed solution, but removes high boiling impurities including a trace amount of basic impurities in the mixed solution. And is suitable for evaporating non-thermally stable objects such as N-vinylformamide in the mixture. The structure of the thin film evaporator is the same as that of a commercially available device and does not need to have a special structure, and examples thereof include a rising thin film type, a plate type falling film type, and a tube type falling film type.

The operating conditions for thin film evaporation are usually 1-2
Under a reduced pressure of 0 Torr, preferably 2 to 10 Torr, the vapor temperature is usually 70 to 150 ° C., preferably 80 to 130 ° C.
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, most of the liquid is recovered as an evaporated component. "Most of the liquid" does not specifically set a boundary value,
This means that usually at least 90%, preferably 95%, of the mixture passed through the thin film evaporator is recovered. 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.

Next, the above-mentioned evaporation is used as it is, or
By introducing the condensed liquid to the tower and performing precision distillation,
High purity N-vinylformamide is recovered. The operating conditions of the distillation column are appropriately selected according to the requirements for the purity of the N-vinylformamide to be recovered.
20 Torr, preferably 2 to 10 Torr, and the overhead temperature is usually 6
The conditions are 0 to 90 ° C, preferably 70 to 85 ° C, and usually 5 to 50, preferably 10 to 30 as the number of theoretical plates in the tower. The structure in the column is the same as that of a general distillation column, and examples thereof include a packed type, a tray type and the like.

[0017]

EXAMPLES Next, the present invention will be described more specifically with reference to examples, but the present invention is not limited to the following examples unless it exceeds the gist. Note that “%” represents “% by weight” unless otherwise specified.

Example 1 (Production of N- (α-methoxyethyl) formamide (hereinafter referred to as “ether compound”)) Formamide and a 0.5-liter glass reactor equipped with a stirrer and a temperature controller were added. N- (α-hydroxyethyl) formamide (hereinafter referred to as “hydroxy form”) crystals obtained by reacting acetaldehyde with a potassium carbonate catalyst (purity 73%, formamide content 0.5%,
30 g of K ₂ CO _{3 (} 0.5% K ₂ CO ₃ ) were charged, and 31.5 g of methanol (4 mole times based on the hydroxy form) and 0.25 g of sulfuric acid (neutralized amount of K ₂ CO ₃ + based on the hydroxy form) 0.5 mol%), and the mixture is stirred at 25 to 40 ° C.
The etherification reaction was performed at a temperature of until the remaining amount of the hydroxy compound in the reaction system became 8% (reaction time: 3 hours).
The pH was adjusted to 7 by adding 20% caustic soda to neutralize the catalyst. Further, the liquid was distilled under a reduced pressure of 3 mmHg after removing light boiling components under a reduced pressure of 50 mmHg.

(Preparation of N-vinylformamide) The ether compound obtained above was treated with a heater having a diameter of 10 mm and a length of 5 mm.
At a rate of 2 g / min at a reduced pressure of 100 mmHg while maintaining the internal temperature at 400 ° C., the gas discharged was immediately condensed into a 00 mm stainless steel tube at a rate of 2 g / min. N-vinylformamide 6
(7%, methanol 31%).

(Distillation of Pyrolysis Mixture Containing N-Vinylformamide) The above distillate was further subjected to a pressure of 50 Torr and 50 ° C.
Under the conditions described above, a mixture obtained by removing light-boiling components such as methanol was used as a pyrolysis mixture for distillation (81% N-vinylformamide, 13% formamide, 3% ether, 3% high boiling impurities). Using a glass falling thin film evaporator (heat transfer surface diameter: 50 mm, height: 200 mm), the pyrolysis mixture was used at a pressure of 3 Torr, a vapor temperature of 125 ° C., a feed rate of the mixture of 500 g / hr, and an average residence time of the mixture of 5 minutes. Under the conditions of
Evaporation was performed. As a result, 97% of the mixture was evaporated. The composition of the evaporating component is 85% N-vinylformamide,
Formamide was 12% and ether was 3%, and the recovery of N-vinylformamide was 99.8%. on the other hand,
Unevaporated components were removed from the system without circulation to the thin film evaporator.

Next, the condensed liquid of the above-mentioned evaporation is passed through a column (diameter 30).
mm, height 900 mm, filled with Dickson packing), pressure 5 Torr, tower temperature 80 ° C., number of theoretical plates 10 stages, reflux ratio 10, tower bottom residence time 1 hour, condensate feed amount 28
Precision distillation was performed under the condition of 0 g / hr (feed from the middle stage of the column). As a result, a liquid having a composition of 95.4% N-vinylformamide and 4.6% formamide was obtained at 160 g / h from the top of the tower. On the other hand, N-vinylformamide 71.1% and formamide 21.9 from the bottom of the column at 120 g / hr.
%, N-methoxyethylformamide having a composition of 7%. Further, N-vinylformamide was not substantially decomposed, and no decomposed product was accumulated.

Comparative Example 1 The same pyrolysis mixture as in Example 1 was directly subjected to precision distillation in a column under the same conditions as in Example 1, and 143 g from the top of the column
/ H: N-vinylformamide 95%, formamide 5
% Of the liquid was obtained. On the other hand, 137 g / h from the bottom
-55% vinylformamide, 21% formamide, N
-A liquid having a composition of 6% methoxyethylformamide and 18% high boiling component was obtained. In this distillation, the decomposition rate of N-vinylformamide was 7.5%.

[0023]

According to the present invention, N-vinylformamide can be efficiently distilled and recovered without decomposing it.

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C07C 231/24 C07C 233/03 CA (STN)

Claims (1)

(57) [Claims] 1. In recovering N-vinylformamide by distilling a mixed solution containing N-vinylformamide obtained by a pyrolysis method , the entire amount of the mixed solution is previously mixed with a thin film evaporator. Most of the liquid is recovered as an evaporate, and then the evaporate is subjected to precision distillation using a column to recover N-vinylformamide, and the remaining unevaporate is removed outside the system. -A method for recovering vinylformamide.