JPS5834450B2 - Formaldehyde purification method - Google Patents

Description

[Detailed description of the invention] This invention relates to a method for purifying formaldehyde.

Hemi-formal is produced from an aqueous formaldehyde solution and polyethylene glycol, water in this hemi-formal is removed by evaporation, and then the hemi-formal is thermally decomposed to obtain formaldehyde and decomposed hemi-formal, which is then decomposed with formaldehyde in a wet wall column. A method for purifying formaldehyde by bringing it into countercurrent contact with miformal is known (Japanese Patent Publication No. 19288-1988).

According to this method, in order to obtain formaldehyde with a water content of about 100 ppm, it is necessary to maintain the temperature of the wetted wall tower at a low temperature of -20 to 0°C using a refrigerant.

Furthermore, in order to further reduce the water content of formaldehyde, it is necessary to lower the temperature further, but when the temperature of the wet wall tower is lowered, formaldehyde gas is absorbed by decomposed hemiformal, and the acquisition rate of formaldehyde decreases. It is fraught with the problem of doing so.

This invention provides a method for purifying formaldehyde that is an improvement on the above method, and by introducing dehydrated decomposed hemiformal into a wet wall column, the water content can be reduced without maintaining the temperature of the wet wall column at a low temperature. This is based on the knowledge that formaldehyde of 1100 pp or less can be obtained in good yield.

That is, this invention comprises: a first step of reacting an aqueous formaldehyde solution with an alcohol having a boiling point of 150° C. or higher to remove hemiformal; a second step of removing water in the hemiformal obtained in the first step by evaporation; A third step in which the hemiformal obtained in the step is thermally decomposed into formaldehyde and decomposed into formaldehyde, and the formaldehyde obtained in the third step is introduced into the lower part of the wetted wall column, and the formaldehyde obtained in the third step is introduced into the upper part. In a formaldehyde purification method comprising a fourth step of introducing decomposed hemiformal, extracting purified formaldehyde from the top, and extracting decomposed hemiformal from the bottom and returning it to the first step as an alcohol source, the heat of the hemiformal in the third step is A gas-liquid contact tower is used as a decomposition device, the bottom temperature is maintained above the decomposition temperature of hemi-formal, the upper and center temperatures are maintained below the decomposition temperature of hemi-formal, hemi-formal is introduced into the upper part of the gas-liquid contact tower, and the hemi-formal is Formaldehyde gas generated and rising from the bottom of the column due to thermal decomposition of This is the lfR production method for formaldehyde, which is characterized by extracting miformal from the bottom through decomposition.

With reference to the drawings showing one embodiment of the invention,
This invention will be explained in detail.

In the first step, a formaldehyde aqueous solution and the decomposed hemiformal returned from the fourth step as an alcohol source are introduced into the hemiformal production tank 1 through conduits 11 and 12, respectively, and the hemiformal is removed by insects.

Furthermore, two parts of the decomposed hemiformal obtained in the third step described below can also be introduced into the hemiformal production tank 1 through conduits 18, 13 and 12 as an alcohol source.

As the alcohol, an alcohol having a boiling point of 150"C or more is used, and specific examples thereof include cyclohexanol,
Examples include trimethylolpropane, 2-ethylhexanol, nonyl alcohol, tacyl alcohol, polyethylene glycol, polypropylene glycol, and the like.

The molecular weight of polyethylene glycol and polypropylene glycol is preferably 200 to 400.

Second Step The hemiformal obtained in the first step is introduced into the evaporator 2 through the conduit 14, the water in the hemiformal is evaporated and extracted from the conduit 15, and the hemiformal is extracted from the conduit 16.

It is preferable to remove as much moisture as possible from the hemiformal, but even when a high-performance Smith type thin film evaporator is used as the evaporator 2, the hemiformal extracted from the evaporator 2 usually contains 0.5 to 1% by weight. of water.

When the water content of hemiformal is high, such as when 50% formalin is used as a raw material, multiple evaporators 2 are provided, and the water distilled from the first evaporator 2-a with a low formaldehyde content is discarded. It is also possible to supply the bottom liquid from the evaporator 2-a to the next evaporator 2-b, and return the water distilled therefrom having a high formaldehyde content to the first step.

Third Step The hemiformal obtained in the second step is introduced into the upper part of the gas-liquid contact tower 3 through the conduit 16, and the hemiformal is thermally decomposed to obtain formaldehyde and decomposed hemiformal.

The temperature at the bottom of the gas-liquid contact tower 3 is maintained at a temperature higher than the decomposition temperature of hemiformal, for example, in the range of 140 to 170°C, and the temperature at the top and center is maintained at a temperature lower than the decomposition temperature of hemiformal, for example 115 to 170°C. It is necessary to maintain a temperature in the range of 125°C.

The hemiformal introduced into the gas-liquid contact tower 3 flows down inside the tower without being decomposed in the upper and central parts of the tower.

The hemiformal flowing down to the bottom of the gas-liquid contact tower 3 is thermally decomposed there to produce formaldehyde gas and decomposed hemiformal.

Formaldehyde gas generated at the bottom of the gas-liquid contact tower 3 rises inside the tower, comes into countercurrent contact with the hemiformal flowing down inside the tower, and entrains moisture in the hemiformal.

Therefore, the hemipolmar flowing down to the bottom of the tower contains almost no water.

Formaltehyde gas entrained with moisture in the hemi-formal is extracted from the top of the gas-liquid contact tower 30 through the conduit 17, and dehydrated decomposed hemi-formal is extracted from the bottom of the gas-liquid contact tower 3 through the conduit 18. .

The entire amount of the dehydrated decomposed hemiformal can be subjected to the fourth step, and two parts thereof can be returned to the hemiformal production tank 1 as an alcohol source through conduits 13 and 12.

In this step, it is desirable to adjust the water content in the dehydrated decomposed hemiformal to 1100 pp or less by changing the amount of hemiformal supplied, the decomposition rate of hemiformal, etc.

This adjustment can be easily made by one skilled in the art in accordance with the teachings of this invention.

Fourth step: The formaldehyde gas obtained in the third step is introduced into the lower part of the wetted wall tower 4 through the conduit 17, and the decomposed formaldehyde gas obtained in the third step is introduced into the upper part through the conduit 18, and both flow in countercurrent flow. contact to purify formaldehyde gas.

Purified formaldehyde is passed from the top of the wet wall tower 4 to a conduit 19.
The decomposed hemiformal that has absorbed impurities such as water, formic acid, and methanol in the formaldehyde gas introduced into the wet wall column 4 is extracted from the bottom through the conduit 12 and returned to the hemiformal production tank 1.

The water content in the purified formaldehyde depends on the trace amount of water in the dehydrated decomposed hemi-formal and the temperature of the wet wall tower 4, and the lower the water content in the dehydrated decomposed hemi-formal, the lower the temperature of the wet wall tower 4. The more the water content in purified formaldehyde decreases.

In the previously mentioned known method, the decomposed hemiformal introduced into the wet wall tower 4 is not sufficiently dehydrated, so unless the temperature of the wet wall tower 4 is maintained at -20 to 0°C with a refrigerant, polymerization will occur. It is not possible to obtain formaldehyde with a water content of 100 ppm or less, which is generally required for formaldehyde for commercial use.

On the other hand, the decomposed hemiformal introduced into the wet wall tower 4 in the fourth step of the present invention has been sufficiently dehydrated in the preceding third step, so the temperature of the wet wall tower 4 is 2.
Even at room temperature of 0 to 30℃, the moisture content is 110
Purified formaldehyde of 0 pp or less can be easily obtained.

Furthermore, since the temperature of the wet wall tower 4 can be set to room temperature, the yield of purified formaldehyde is increased compared to known methods.

Next, examples will be shown.

Example 1 A 50% by weight formaldehyde aqueous solution was supplied at a rate of 0.95 kg/hour and a polyethylene glycol (molecular weight: 300) having a formaldehyde content of 13.0% by weight was supplied at a rate of 5.6 kg/hour to a hemiformal production tank 1. Press down 105
Hemiformal was generated with stirring at .degree.

The formaldehyde content of the hemiformal produced is 1
It was 8.4% by weight.

The produced hemiformal was continuously extracted from the hemiformal production tank 1 and supplied to the Smith thin film evaporator 2-a, where the water in the hemiformal was evaporated at a temperature of 80°C and an absolute pressure of 6011L11LHg.

The water content in the can liquid was 3.0% by weight.

This bottom liquid was again supplied to the Smith type thin film evaporator 2-b, and moisture was removed at a temperature of 103° C. and an absolute pressure of 30 mmHg.

The water content of the hemiformal in the can liquid was 0.5% by weight.

6.0 kg of the obtained hemiformal was introduced into the upper part of the gas-liquid contact tower 3 at 7 o'clock.

As the gas-liquid contact tower 3, a reboiler was installed at the bottom.
A tower with an internal diameter of 4 CrIL and a height of 170 cm was used.

Moreover, 33 trays were provided inside the gas-liquid contact tower 3.

The temperature of the can liquid was set at 150°C, and the temperature of the tray was set at 125°C.

Formaldehyde gas with a water content of 4.0% by weight is fed from the top of the gas-liquid contact tower 3 at a rate of 0.75 kg/hour, and decomposed hemiformal with a water content of 80 ppm is fed from the bottom at a rate of 5.3 kg/hour.
In time, I pulled them out.

The formaldehyde gas was introduced into the lower part of the wetted wall tower 4 set at 20°C, and the decomposed formaldehyde was introduced into the upper part of the wetted wall tower 4 to purify formaldehyde.

The decomposed hemiformal extracted from the bottom of the wet wall tower 4 was returned to the hemiformal production tank 1.

The water content of purified formaldehyde was 50 ppm.

Moreover, the acquisition rate of purified formaldehyde (based on the formaldehyde gas introduced into the wet wall tower 5) was 62.5% by weight.

Comparative Example 1 A hemi-formal decomposition tank with an internal volume of 81 was used in place of the gas-liquid contact tower, and hemi-formal was heated to 150°C to decompose, the formaldehyde gas generated was extracted from the top of the decomposition tank, and the decomposed hemi-formal (water-containing rate, 0.2% by weight
) was extracted from the lower part of the decomposition tank, and Example 1 was repeated except that each was introduced into the wet wall column 4.

The water content of purified formaldehyde was 400 ppm, and the yield was 62.5% by weight.

Example 2 In hemiformal production tank 1, 1.3 kg/hour of a 50% by weight formaldehyde aqueous solution and 5.9 kg of cyclohexanol with a formaldehyde content of 12.7% by weight were added.
The mixture was fed at 7 o'clock, and hemiformal was generated while stirring at 80° C. under normal pressure.

The produced hemiformal is continuously extracted from the hemiformal production tank 1 and supplied to the Smith type thin film evaporator 2-a at a temperature of 70° C. and an absolute pressure of io.

Water in the hemiformal was removed by evaporation using m*Hg.

The water content in the can liquid was 0.5% by weight.

The obtained hemiformal was supplied to the gas-liquid contact tower 3 used in Example 1, and the hemiformal was decomposed at a bottom liquid temperature of 150°C and a tray temperature of 115°C, and the water content was 3.2% by weight from the top. formaldehyde gas of 1.0
kg/h, and 5.4 kg/h of decomposed hemiformal with a moisture content of 50 ppm was extracted from the bottom.

The formaldehyde gas was purified at 20° C. using the wet wall column 4 in the same manner as in Example 1, purified formaldehyde was extracted from the top of the column, and mi-formal was extracted from the bottom of the column for decomposition and returned to the hemi-formal production tank 1.

The water content of purified formaldehyde was 50 ppm, and the yield was 50% by weight.

[Brief explanation of the drawing]

The drawing is a schematic diagram showing one embodiment of the invention. 1... Hemiformal production tank, 2...
Evaporator, 3... gas-liquid contact tower, 4... wet wall tower.

Claims (1)

[Claims] 1. A first step in which hemiformal is produced by reacting an aqueous formaldehyde solution with an alcohol having a boiling point of 150° C. or higher.
a second step of evaporating and removing water in the hemiformal obtained in the first step; a third step of thermally decomposing the hemiformal obtained in the second step to obtain formaldehyde and decomposed hemiformal; , The formaldehyde obtained in the third step is introduced into the lower part of the wet wall column, the decomposed hemiformal obtained in the third step is introduced into the upper part, the purified formaldehyde is extracted from the top, and the decomposed hemiformal is extracted from the bottom and alcohol is extracted. In a formaldehyde purification method comprising a fourth step of returning the hemiformal to the first step as a source, a gas-liquid contact tower is used as a thermal decomposition device for hemiformal in the third step, and the bottom temperature is maintained at a temperature higher than the decomposition temperature of the hemiformal, The temperature of the upper and central parts is maintained below the decomposition temperature of hemi-formal, and hemi-formal is introduced into the upper part of the gas-liquid contact tower, and formaldehyde gas generated and rising from the bottom of the tower due to thermal decomposition of hemi-formal and hemi-formal flowing down from the upper part of the tower are separated. A method for purifying formaldehyde, which comprises bringing moisture in hemiformal into formaldehyde gas through countercurrent contact, extracting water-containing formaldehyde gas from the top of the column, and extracting decomposed hemiformal from the bottom of the column.