JPH0425257B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for separating a mixed liquid of methyl acetate and methanol, for example, a mixed liquid of methyl acetate and methanol that is produced as a by-product in the saponification process of polyvinyl acetate in the manufacturing process of polyvinyl alcohol. be.

Usually, the above-mentioned methyl acetate/methanol mixture, which is a by-product in the saponification process of polyvinyl acetate, is separated and the methyl acetate is sold as is, or it is hydrolyzed into acetic acid and methanol, and acetic acid is converted into vinyl acetate monomer. Since methanol is reused as a polymerization solvent for vinyl acetate and as a saponification solvent for polyvinyl acetate, the quality of separation and recovery of the methyl acetate/methanol mixture determines the viability of the polyvinyl alcohol manufacturing industry. It is no exaggeration. Although various methods can be considered for such separation operations, a method of hydroextractive distillation of the mixture has been put into practical use. That is, a mixture of methyl acetate and methanol is charged from the middle of the distillation column, and water is introduced from the top of the column in an amount of 4 to 5 times the mole of methyl acetate while distillation is carried out, thereby transferring methyl acetate from the top of the column to the bottom of the column. A methanol/water mixture is obtained from the

However, as we enter an era of resource and energy conservation, the reality is that the distillation method, which requires a large amount of expensive thermal energy, must be reconsidered, and countermeasures are required.

Therefore, the inventor of the present invention attempted to solve this problem and conducted various studies on vacuum distillation based on the premise that if distillation is carried out under reduced pressure, the temperature required for the heating source can be lowered by lowering the boiling point. It has been found that efficient separation cannot be achieved simply by reducing the distillation conditions under normal pressure to reduced pressure.

However, as a result of further studies on the vacuum distillation method, the present inventor found that when a mixture of methyl acetate and methanol is hydroextractively distilled to separate the two, (1) a distillation column with the equivalent of 20 to 60 theoretical plates is required. (2) The amount of water added should be 0.5 to 3 times the mole of methyl acetate, (3) The distillation should be carried out under reduced pressure of 200 to 600 torr, and the distillation should be carried out under conditions that satisfy the above three requirements. The present inventors have discovered the novel fact that, when carried out, even low-temperature waste liquid, which conventionally had to be discarded, can be used as a heat source for the repoiler, making it possible to carry out the separation operation in an extremely economically advantageous manner. Ivy.

As mentioned above, in order to carry out distillation at a reduced pressure of 200 to 600 torr, the present invention uses a distillation column with 20 to 60 theoretical plates, and also adjusts the amount of water added relative to methyl acetate.
It is necessary to specify 0.5 to 3 times the mole.
Through this operation, methyl acetate is distilled out from the top of the column, and a methanol/water mixture is distilled out from the bottom of the column, resulting in separation. The methyl acetate concentration is over 95%, resulting in extremely high purity products.

The vapor-liquid equilibrium relationship in a methyl acetate/methanol/water mixture system is that as the vacuum level of the system decreases, the specific volatility of methyl acetate increases, making separation easier, but on the other hand, the condensation temperature of the vapor at the top of the distillation column decreases. It must be above 200 Torr to make it difficult to condense with normal cooling water. Also, if it exceeds 600 torr, a practical reduction in the boiling point cannot be expected, so 200 to 600
It must be carried out under a vacuum of 350 to 450 Torr, preferably 350 to 450 Torr for practical purposes.

Further, in the above pressure range, the theoretical plate number of the distillation column must be 20 to 60 plates. Theoretically, the greater the number of stages, the greater the separation effect, but in practice it must be within the above range in consideration of the reflux ratio and pressure loss.

Furthermore, in such distillation under reduced pressure, the amount of water added must be 0.5 to 3 times the molar amount of methyl acetate. Outside this range, the purity of methyl acetate decreases and becomes impractical. This amount of water added is extremely small considering that conventional distillation methods require 4 to 5 times as much water, which is a major feature of the present invention. The small amount of water added means that less thermal energy is required to remove water from the methanol/water mixture distilled from the bottom of the column.

The methyl acetate/methanol mixture to be subjected to distillation is usually a mixed solvent produced as a by-product from the saponification process of polyvinyl acetate, and has a composition of 35 to 55% by weight of methyl acetate and 65 to 45% by weight of methanol. Of course,
The present invention is not limited to the above-mentioned by-product mixed solvent, and even if it is a mixed solvent produced as a by-product from any manufacturing process or a waste liquid after use in various applications, in essence, as long as it is a mixture of methyl acetate and methanol. Both are applicable.

The distillation column used in carrying out the method of the present invention may be a distillation column similar to that used in atmospheric distillation, as long as it is equipped with a pressure reducing device. Plate column type, packed column type, spray type, falling film type,
It may be of any structure, such as a stirred liquid film type, but
It is desirable to have a small pressure loss, and usually a packed column type or a plated column type is practical.

Next, the flow of the method of the present invention will be explained based on the drawings, but of course the method of the present invention is not limited thereto.

In the figure, a mixed solution of methyl acetate and methanol is supplied from 1 to a vacuum distillation column 3. In step 3, water is introduced into the column outlet from point 2 to eliminate azeotropy between methyl acetate and methanol, and then heated in reboiler 5 to obtain highly concentrated methyl acetate from the top of the column. The reflux liquid is returned to the top of the column through reflux tube 4, and the methyl acetate fraction is taken out from 7. The fraction is sent to the next step as required, where it is hydrolyzed and converted into acetic acid and methanol. On the other hand, from the bottom of the column, a mixture of methanol and water is supplied to a distillation separation column through a passage 6, where a mixture of methanol and water is obtained, and methanol is recovered from the top 8 of the column, and water is recovered from the bottom 9 of the column.

The method of the present invention will be explained in more detail by way of examples.

Example 1 Saponification mother liquor of polyvinyl acetate (methyl acetate 45
%, a mixture of 55% methanol) is added every hour to the 15th plate (from the top of the column) of a packed pressure distillation column with 35 theoretical plates.
Water was supplied at a rate of 100 kg/hour, while water was supplied at a rate of 17 kg/hour from the top of the column to the second stage. Reflux ratio 0.7,
Distillation was carried out by adjusting the system pressure to 400 Torr.

Methyl acetate (96% purity) is delivered from the top of the column at 47% per hour.
Kg, methanol/water mixture (mixture ratio) from the bottom of the column
55/15) at a rate of 70 kg/hour.

At this time, the tower top temperature was 40°C, the tower bottom temperature was 56°C, and the amount of steam (waste organic solvent vapor 70 to 80°C) used was 16.5 kg/hour.
It was hot.

Control Example 1 The method of Example 1 was carried out at 760 torr. (However, the amount of water added was changed to 45 kg/hour) As a result, the tower top temperature was 54°C, the tower bottom temperature was 81°C, and the amount of steam (steam over 100°C) used was 17.9 kg/hour.

Example 2 The same experiment as Example 1 was conducted except that the amount of water added was changed to 15 kg/hour and distillation was performed at 350 torr. The amount of steam used was 16.0 kg/hour.

Example 3 The same experiment as Example 1 was conducted except that the number of plates in the distillation column was changed to 30 theoretical plates and the amount of water added was changed to 23 kg. Steam consumption was 17.1 kg/hour.

[Brief explanation of drawings]

The drawing is a flow sheet showing one embodiment of the invention. 1...Mixed liquid supply pipe, 2...Water supply pipe, 3...
Vacuum distillation column, 4... Reflux tube, 5... Reboiler,
6... Methanol/water supply pipe, 7... Methyl acetate distillation pipe, 8... Methanol distillation pipe, 9... Water distillation pipe.

Claims (1)

[Claims] 1. When a mixture of methyl acetate and methanol is subjected to hydroextractive distillation to separate the two, (1) a distillation column with a number of theoretical plates equivalent to 20 to 60 is used; (2) the amount of water added; 0.5 to 3 times the molar amount of methyl acetate to methyl acetate; and (3) a reduced pressure of 200 to 600 torr. Separation method.