JPH0723334B2 - Formaldehyde purification method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is a so-called hemiformal in which formaldehyde in an aqueous formaldehyde solution is once reacted with polyethylene glycol to produce hemiformal, and then hemiformal is thermally decomposed to purify formaldehyde. The present invention relates to a purification method, and more particularly, to a method capable of efficiently purifying polyethylene glycol that is circulated in the above-mentioned hemiformal purification method and stably purifying formaldehyde.

(Prior Art and Problems Thereof) A method for purifying formaldehyde in a formaldehyde aqueous solution using polyethylene glycol is disclosed in, for example, Japanese Patent Publication No. 34450/1983. This method
First step of reacting an aqueous formaldehyde solution with polyethylene glycol to produce hemiformal, first step
A second step of evaporating and removing water in hemiformal obtained in the step, a third step of thermally decomposing hemiformal obtained in the second step to obtain formaldehyde and decomposed hemiformal, formaldehyde obtained in the third step and decomposed hemiformal Is subjected to countercurrent contact to allow the trace impurities in formaldehyde to be absorbed by the decomposed hemiformal to purify the formaldehyde, and the decomposed hemiformal is circulated and fed to the first step in a fourth step of purifying formaldehyde.

When the above-mentioned formaldehyde purification method is continuously carried out for a long period of time, the sugar produced by the aldol condensation of formaldehyde, the polymer produced by the polymerization of formaldehyde, and the metal ion from the equipment are accumulated in the decomposed hemiformal, and the viscosity of the decomposed hemiformal is increased. Operation becomes unstable and the operation becomes unstable.
It will be recognized that the problem that the purity of the purified formaldehyde is lowered in the process occurs. For this reason, it is necessary to replenish new polyethylene glycol or replace the entire amount every certain period.

(Technical Means for Solving Problems) An object of the present invention is to provide a method capable of stably purifying formaldehyde by efficiently removing sugars, polymers and metal ions contained in decomposed hemiformal. To do.

According to the present invention, a first step of reacting polyethylene glycol and formaldehyde to produce hemiformal, a second step of removing water in hemiformal by evaporation, and a third step of thermally decomposing hemiformal to obtain formaldehyde and decomposed hemiformal In the step, the formaldehyde obtained in the third step and the decomposed hemiformal are brought into countercurrent contact to purify the formaldehyde, and the decomposed hemiformal is supplied to the first step and the fifth step described later, and the fourth step and the fourth step. Water and an organic solvent that is incompatible with water are added to the decomposed hemiformal, the impurities contained in the decomposed hemiformal are extracted with water, and then water is added to the upper layer from the fifth step and the fifth step of separating into two layers. After the addition, the decomposed hemiformal in the upper layer was extracted with water, and then separated into an aqueous layer and an organic solvent layer. Then, the organic solvent layer is circulated and supplied to the fifth step, the water in the aqueous layer is removed by evaporation to purify the decomposed hemiformal, and the evaporated water is circulated and supplied as the water for extracting the above-mentioned decomposed hemiformal. A method for purifying formaldehyde is provided.

The present invention will be described below with reference to the drawings illustrating one embodiment of the present invention.

The first to fourth steps of the present invention are the same as the method described in Japanese Patent Publication No. 58-34450, but each step will be described below.

First step In the hemiformal production tank 1, decomposed hemiformal, which is circulated and supplied as an aqueous formaldehyde solution and an alcohol source from the fourth step and the sixth step, is introduced from pipes 11, 42 and 64, respectively, to generate hemiformal. The molecular weight of polyethylene glycol constituting the decomposed hemiformal is preferably 200 to 400.

Second step The hemiformal obtained in the first step is introduced into the evaporator 2 through the tube 12 to evaporate the water content in the hemiformal and then the tube.
Withdraw from 22 and withdraw hemiformal from tube 23.

It is preferable to remove water in hemiformal as much as possible. For this purpose, a plurality of thin film evaporators 2 are provided as evaporators, and the water having a low formaldehyde content distilled from the first evaporator 2a is cut off from the pipe 22a and evaporated. The bottom liquid of the vessel 2a is supplied to the next evaporator 2b through a pipe 21, and water having a high formaldehyde content, which is distilled there, is first fed through a pipe 22b.
It can be returned to the process.

Third step The hemiformal obtained in the second step is introduced from the pipe 23 to the upper part of the gas-liquid contact pipe 3 to thermally decompose the hemiformal,
Obtain formaldehyde and decomposed hemiformal.

The bottom temperature of the gas-liquid contact tube 3 is maintained at a temperature equal to or higher than the decomposition temperature of hemiformal, for example, a temperature of 140 to 170 ° C., and the temperatures of the upper portion and the central portion are lower than the decomposition temperature of hemiformal,
For example, it is desirable to maintain the temperature at 115 to 125 ° C.

The hemiformal introduced into the gas-liquid contact tube 3 flows down in the column 3 without being decomposed in the upper part and the central part of the column 3. Hemiformal flowing down to the bottom of the gas-liquid contact tube 3 is
There it is pyrolyzed to formaldehyde gas and decomposed hemiformal. The formaldehyde gas generated at the bottom of the gas-liquid contact tower 3 rises in the tower 3, makes countercurrent contact with the hemiformal flowing down in the tower 3, and carries water in the hemiformal. Therefore, hemiformal flowing down to the bottom of the tower 3 contains almost no water.

Formaldehyde gas accompanied by water in hemiformal is withdrawn from the top of the gas-liquid contact tower 3 through a pipe 31,
The dehydrated decomposed hemiformal is withdrawn from the bottom of the gas-liquid contact tower 3 through a pipe 32 and supplied to the next fourth step. In addition, a part of the decomposed hemiformal can be circulated and fed to the first step to react with formaldehyde.

In the thermal decomposition of hemiformal, it is preferable to use the above-mentioned gas-liquid contact tower in order to reduce the water content in the decomposed hemiformal as much as possible, but a simple thermal decomposition tank can also be used.

Fourth Step A countercurrent contactor, for example, the formaldehyde gas obtained in the third step is supplied to the lower part of the wetting wall tower 4 through a pipe 31, and decomposed hemiformal is introduced from a upper part through a pipe 32 to bring them into countercurrent contact, Purify formaldehyde gas. Impurities in the formaldehyde gas introduced into the wet wall tower 4, such as water, formic acid, and methanol, are absorbed by the decomposed hemiformal flowing down the wet wall tower 4. The purified formaldehyde gas is withdrawn as a product from the top of the wet wall tower 4 through a pipe 41. Decomposed hemiformal that has absorbed impurities is the wet wall tower 4
Is taken out from the bottom of the tank and partly circulated to the hemiformal production tank 1 of the first step through a pipe 42. The remaining decomposed hemiformal is subjected to the next fifth step via the tubes 42 and 43.

This decomposed hemiformal contains, in addition to the above-mentioned impurities, sugar produced by the aldol condensation of formaldehyde,
It contains polymers produced by the polymerization of formaldehyde, and metal ions from the equipment. Water, formic acid, methanol, etc. contained in decomposed hemiformal are second
Although water in hemiformal in the process is removed by evaporation, it is not discharged to the outside of the system and accumulated in the system, but the above sugars, polymers, metal ions, etc. are accumulated in decomposed hemiformal, and the viscosity of decomposed hemiformal is increased. To increase instability to make the operation unstable, or to reduce the purity of the purified formaldehyde in the third and fourth steps.

Fifth Step The fifth step and the subsequent sixth step form an essential part of the present invention.

Decomposed hemiformal, water and an organic solvent incompatible with water are supplied to the extraction tank 5 through pipes 43, 51 and 52, respectively.

As the organic solvent incompatible with water, aromatic hydrocarbons such as benzene, toluene and xylene are preferably used. The usage rate of this organic solvent is 10 to 20 per 1 part by weight of water.
It is preferably 0 parts by weight, particularly preferably 50 to 100 parts by weight.

The total usage ratio of water and organic solvent is preferably 1 to 10 parts by weight, and more preferably 3 to 5 parts by weight per 1 part by weight of decomposed hemiformal.

By contacting decomposed hemiformal, water and an organic solvent in extraction tank 5, sugar and metal ions contained in decomposed hemiformal are extracted into water.

Then, the extraction mixture is supplied to the separation tank 6 through the pipe 53. By standing the extraction mixture in the separation tank 6, the extraction mixture is separated into two layers. The upper layer consists mainly of organic solvent and decomposed hemiformal. The lower layer is water extracted sugar and metal ions, decomposed hemiformal and organic solvent,
And a polymer formed by the polymerization of formaldehyde.

The lower layer discharged from the pipe 54 may be entirely discarded, and after the contained polymer is removed by filtration,
It is also possible to cut off the amount commensurate with sugars and metal ions produced in all the steps of purification, and circulate and supply the residue to the extracted sugar 5.

The upper layer is supplied from the pipe 55 to the next step.

Sixth step The upper layer of the fifth step and water are supplied to the extraction tank 7 through pipes 55 and 61, respectively.

The proportion of water used is preferably 1 to 10% by weight based on the mixture of the organic solvent and decomposed hemiformal in the upper layer.

In the extraction tank 7, the decomposed hemiformal contained in the upper layer of the fifth step is extracted with water.

The extraction mixture is fed to the separation tank 8 through the pipe 62. By standing the extraction mixture in the separation tank 8, the extraction mixture is separated into an upper layer mainly composed of an organic solvent and a lower layer composed of decomposed hemiformal and water.

The organic solvent in the upper layer is circulated and supplied to the extraction tank 5 in the fifth step through the pipe 52.

The mixture of decomposed hemiformal and water in the lower tank is supplied to a dehydration tower, for example, a reduced pressure evaporator 9 through a pipe 63 to evaporate and remove water, and the decomposed hemiformal from which water has been removed passes through a pipe 64 to a first position. It is supplied to the hemiformal production tank 1 of the process. The removed water is circulated and supplied to the extraction tank 7 in this step through the pipe 61.

(Example) An example is shown below.

Example 1 Formaldehyde was purified using the apparatus shown in FIG.

In the extraction tank 5 of the fifth step, the decomposed hemiformal from the fourth step was fed at 50 kg / hr through the pipe 43, and the water and the benzene containing a small amount of hemiformal from the sixth step were fed through the pipe 52.
Each is circulated and supplied at 2500 kg / hr and 300 kg / hr, and makeup water is further introduced from the pipe 51 at 2.2 kg / hr and mixed at 40 ° C.,
The impurities in the decomposed hemiformal were extracted with water. After this, the extraction mixture was separated into two layers fed to the separation tank 6.

Circulating water (80 kg / hr) containing benzene and formaldehyde containing the upper layer of hemiformal was supplied to the extraction tank 7 of the sixth step through the pipes 55 and 61 and mixed to extract hemiformal with water. The extraction mixture was supplied to the separation tank 8 and separated into two layers. Tube the mixture of lower hemiformal and water
It was fed to the vacuum evaporator 9 via 63 and continuously dehydrated to obtain purified hemiformal at 50 kg / hr. The upper layer of benzene containing a small amount of hemi-formal and the evaporated water are respectively collected by the pipe 52.
And was circulated to the fifth step via the pipe 61.

Impurity contents in the decomposed hemiformal (decomposed HF) from the fourth step and the hemiformal purified in the sixth step (purified HF) supplied to the fifth step are shown below.

Decomposed HF Purified HF polymer (%) 0.5 0 Metal content Fe 25.9 0.09 (ppm) Ca 2.3 0.09 Ni 3.9 0.09 Na 6.0 0.09 Sugar (brown) Yes No

[Brief description of drawings]

FIG. 1 is a schematic view showing an embodiment of the present invention. 1 ... Hemiformal production tank, 2 ... Evaporator, 3 ... Gas-liquid contact tower, 4 ... Wetting wall tower, 5 ... Extraction tank, 6 ... Separation tank, 7 ... Extraction tank, 8 ... Separation Tank

Claims (1)

[Claims] 1. A first step of reacting polyethylene glycol with formaldehyde to produce hemiformal, a second step of removing water in hemiformal by evaporation, and a third step of thermally decomposing hemiformal to obtain formaldehyde and decomposed hemiformal. The formaldehyde obtained in the third step and the decomposed hemiformal are brought into countercurrent contact to purify the formaldehyde, and the decomposed hemiformal is supplied to the first step and the fifth step described below, and the decomposition supplied from the fourth step. Water and an organic solvent incompatible with water are added to hemiformal, impurities contained in the decomposed hemiformal are extracted with water, and then water is added to the upper layer from the fifth step and the fifth step to separate into two layers. Then, decomposed hemiformal in the upper layer was extracted with water, and then separated into an aqueous layer and an organic solvent layer. The organic solvent layer is circulated and supplied to the fifth step, the water in the water layer is removed by evaporation to purify the decomposed hemiformal, and the evaporated water is circulated and supplied as the water for extracting the above-mentioned decomposed hemiformal. Purification method.