JPS61125485A - Method for recovering dmf - Google Patents

Abstract

PURPOSE:To save the cost of equipment for recovering dimethylformamide (thereafter abbreviated as DMF) and to reduce the operating cost by combining a reverse-osmosis membrane method with the conventional distilling method, and making use of merits of each method. CONSTITUTION:An aq. soln. of DMF is concd., the concd. DMF is dehydrated, and the obtained crude DMF is purified. In this case, aq. DMF-contg. distillate discharged from the top of a distillation and concentrating tower is separated into the water A having extremely low concn. of DMF and an aq. DMF soln. B in a separator 4 by using a reverse-osmosis membrane consisting of a polyamide membrane, etc. The water A is generally sent to an artificial leather- manufacturing plant, and reused. The latter aq. DMF soln. B is returned to a concentrating tower 1. Consequently, the number of trays of the concentrating tower 1 can be decreased, the diameter of the tower can be reduced, and the reflux ratio can also be reduced as compared with the conventional purification using only a distilling method. Accordingly, the cost of equipment can be saved and the amt. of steam consumed in operation can be saved.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a method for recovering DMF from an aqueous solution of DMF (i.e., dimethylformamide) discharged from a manufacturing plant for, for example, polyurethane moldings or acrylic fibers, and in particular to a distillation method. The present invention relates to a method for recovering DMF that combines separation using the method and separation using the reverse transport membrane method.

Incidentally, in this specification, % and ppm all mean % by weight and ppI11 by weight.

BACKGROUND OF THE INVENTION DMF is a substance that exhibits extremely high dissolving power for polymeric compounds, and is therefore used as an excellent solvent in the production of polyurethane and the spinning of polyacrylonitrile.

By the way, for example, in the wet manufacturing method of synthetic leather,
A DMF solution of a resin composition containing polyurethane as a main component is applied to a leather base material, and this DMF is replaced with water to generate a porous structure. The DMF aqueous solution recovered from this replacement process is usually It contains approximately 60-98% water. However, dumping this DMF aqueous solution into waste water cannot be adopted because DMF is a fairly expensive chemical and there is a risk of causing pollution problems.

Prior Art and its Problems Distillation has conventionally been a common method for recovering DMF from an aqueous DMF solution. Normally, when recovering DMF by distilling an aqueous DMF solution, the operation is carried out in a short time and at a low temperature in order to reduce the decomposition of DMF.

That is, in the concentration step, most of the water is removed in a short time by low-temperature distillation under reduced pressure, and in the next dehydration step,
The water content in concentrated DMF is reduced to a predetermined value or less by low-temperature distillation under reduced pressure. Further, the crude DMF obtained in the dehydration step contains a small amount of formic acid produced by decomposition of DMF, and this formic acid may have a harmful effect when the DMF is reused. Therefore, it is necessary to remove this trace amount of formic acid by vacuum distillation in a purification step to recover DMF of a predetermined purity.

On the other hand, the distilled water discharged from the top of each column in the concentration step and dehydration step of such distillation methods is generally reused.
Even so, the surplus is being forced to be released. If this distilled water contains DMF and its decomposition products as impurities, not only will discharging the distilled water cause a pollution problem, but it will also cause odor to the product or change its color tone when the distilled water is reused. Distillate water also requires a fairly high degree of purity due to its negative effects such as spoilage. For this reason, the number of stages in the distillation column in the concentration step and dehydration step is significantly increased.

Furthermore, in order to increase the reflux ratio, the diameter of the distillation column is increased, which results in a larger distillation column and higher equipment costs, and energy consumption, especially steam consumption, increases operating costs. It was becoming a burden.

From this point of view, multiple effect methods have been adopted and various energy-saving devices have been installed, but all of these methods require high equipment costs, are complex to operate, and have little effect. .

Incidentally, in recent years, there has been significant research into permselective membranes and permeation methods, and they have become indispensable materials, particularly in medical devices and electronics.

However, since DMF generally exhibits a large dissolving power for synthetic 8-molecular compounds, it has been said that it is difficult to use the above-mentioned membrane for DMF recovery.

In addition, the fact that the DMF aqueous solution from the synthetic leather manufacturing process is reduced by 1,161 times using only the reverse osmosis membrane method means that various impurities such as textile waste from the leather base material, glue, and inorganic salts are contained in this DMF aqueous solution. There are many points to be industrialized. This invention was made in view of these circumstances, and by combining the conventional distillation method with the reverse osmosis membrane method and making use of the advantages of each,
DMF that can overcome the above problems and reduce the equipment cost of DMF recovery equipment as well as the operating cost.
The purpose is to provide a collection method for

Means for Solving the Problems The method for recovering DMF according to the present invention involves concentrating an aqueous DMF solution, dehydrating the concentrated DMF, and purifying the obtained DMF by removing a DMF-containing distillate from the top of a concentrating column. The water,
It is characterized by separating into water with a significantly low DMF concentration and a DMF aqueous solution with a high concentration by reverse permeation removal method.

The water, which has a significantly lower DMF of 1 degree, is typically sent to a synthetic leather manufacturing plant for reuse. Further, the highly concentrated DMF aqueous solution and the distilled water discharged from the top of the dehydration tower are each returned to the concentration tower and distilled again.

In this invention, as the reverse permeation membrane, a membrane having excellent durability, such as a polyamide membrane, is often used.

The DMFIf1 degree of the distillate is 10% or less, preferably 1% or less.

The above-mentioned water with a significantly low DMF of 111 degrees has a DMF concentration of 0.
The amount of water is 1% or less, preferably 0.01% or less, and more preferably 50 ppm or less.

The high concentration DMF aqueous solution preferably has a DMF1I degree of about 10% or more.

Effects of the Invention According to the glue and MF recovery method of this invention, the distilled water coming out from the top of the concentration column is separated into water with a significantly low DMF concentration of 1 degree and a highly concentrated DMF aqueous solution using a reverse filtration membrane. ,
By returning the latter aqueous solution to the concentrating column, the number of stages in the concentrating column can be reduced and the column diameter can be reduced, as well as the reflux ratio, compared to the conventional 1R production method using only distillation. . Therefore, not only can equipment costs be reduced, but also the amount of steam consumed during operation can be reduced, resulting in significant cost reductions in DMF recovery.

EXAMPLE Next, an example will be shown to illustrate the effects of this invention.

A DMF aqueous solution collected from a wet synthetic leather manufacturing plant and consisting of 89.9% DMFIO, 89.9% water, and 0.1% others was concentrated, dehydrated, and purified according to the process shown in the drawing, and DMF was recovered. The capacity of this recovery plant is 250 ONy/h.

First, the pressure is reduced (100t) in the concentration column (1).

The above DMF aqueous solution was concentrated in the usual manner under rr).

The concentration column (1) has 9 concentration stages (2) and 5 recovery stages (3
), and the column diameter is 1450 mm. The operation was carried out at a reflux ratio of 0.15, and DMFi was removed from the top of the concentration column.
Distilled water with an i degree of 1% was obtained. This distilled water was divided into 11 units by the road-passing membrane method @ (4) 1. :Through, DMFi 1 degree 501
)I) Separated into water with I11 or less and an aqueous solution with a DMFa content of 10%.

The former water is returned to the synthetic leather manufacturing plant and reused. The latter aqueous solution is returned to the concentration column (1) and distilled again. , the bottom liquid of the concentration column (1) has a DMFa degree of 90
% and supplied to the dehydration tower 5). In this dehydration tower (5), normal distillation is performed under reduced pressure like the concentration tower, and DMF? Distilled water with an 1E degree of 5% is distilled out. This distilled water is also returned to the concentration column (1) and distilled again. The crude DMF discharged from the bottom of the dehydration tower is sent to the purification tower (
6), purified DMF with a DMF purity of 99.99% or more was recovered from the top of the column, and the residue was taken out from the bottom of the column.

For the DMF recovery method using the conventional distillation method and the recovery method according to the present invention, the capacity of each plant was set to 2500.
As Kg/h, the configuration of the concentrating column and the steam quenching 1! ! We compared the amounts. The results are shown in Table 1.

(Margin below) Table 1

[Brief explanation of the drawing]

The drawing is a flow sheet showing a method for purifying recovered DMF according to the present invention. (1) Concentration column, (4) Separation device, (5)
... Dehydration tower, (6) ... Purification tower. that's all

Claims (3)

[Claims] (1) When concentrating the DMF aqueous solution, dehydrating the concentrated DMF, and purifying the obtained crude DMF, the DMF-containing distilled water coming out from the top of the concentrating column is treated with a reverse filtration membrane method to reduce the DMF concentration significantly. A method for recovering DMF, characterized by separating it into low concentration water and a high concentration DMF aqueous solution. (2) The method according to claim 1, wherein the highly concentrated DMF aqueous solution is returned to the concentration column. (3) The method according to claim 1, wherein the distilled water discharged from the top of the dehydration tower is returned to the concentration tower.