DE2613229A1 - PROCESS FOR THE RECOVERY OF SOLVENTS IN THE MANUFACTURE OF SYNTHESIS FIBERS - Google Patents

Description

Bayer Aktiengesellschaft 2 e 13 2 2 9

• Ζ · · Central area

Patents, trademarks and licenses

5090 Leverkusen, Bayerwerk Dn / Wes

* b. Harz 1976

Process for the recovery of solvents in the manufacture of synthetic fibers

The invention relates to the regeneration of dimethylacylamides which are used as solvents in the production of threads or fibers made of acrylonitrile polymers or polyurethanes can be used and thereby with water and other substances become contaminated.

Dimethylformamide (DMF) and dimethylacetamide are known (DMA) excellent solvents for acrylonitrile polymers, with the help of which the manufacture of acrylonitrile fibers succeeds with excellent quality both after the dry and after the wet spinning process. These solvents are also used in the production of molded parts and fibers from polyurethanes. The energies to be expended for the regeneration of the solvent that is used in the manufacturing process with water and possibly with others Substances contaminated are significant. Therefore, several methods have been used for solvent recovery been tried. Some processes are based on extraction with an extractant that does not mix with water and subsequent separation, using, for example, methylene chloride or acrylonitrile as the extractant will.

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However, these processes are only suitable if the mixture to be worked up contains less than 15 % solvent, since the distribution coefficients of the solvents between water and the extractant are relatively small.

Rectification is suitable for other mixtures with solvent contents above 15 %. Normal rectification has the disadvantage that it has to be carried out under vacuum, the pressure in the column being below 500 Torr so that the dimethylacylamides, which are unstable to thermal stress and present in the mixtures, do not decompose. Furthermore, it has the disadvantage that in the rectification all water, which is known to have a high heat of evaporation, has to be evaporated and the use of the high heat of evaporation of the water in the top product can usually not be sensibly carried out, since this heat at a low temperature level of below 75 ° C occurs.

The present invention was therefore based on the object of making the process for recovering the spinning solvent more economical to design.

It has now been found that this can be achieved by mixing the resulting mixtures of solvent and water and other impurities such as the corresponding acids formic acid and acetic acid, their salts not directly rectified, but the mixtures with high water contents are preconcentrated and, during preconcentration, the heat of evaporation of the water is used in that the The mixture is concentrated, the subsequent columns being heated with the top product of the preceding columns.

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The invention therefore relates to a process for the regeneration of dimethylacylamides from aqueous solutions which arise in the production of threads from acrylonitrile polymers or polyurethanes, which is characterized in that the solvent-water mixtures first of all a preconcentration system consisting of at least two successive distillation columns are fed, using the overhead vapor products of each column, the mixtures up to a solvent content of at most 60 wt.% concentrated as the heating medium for the subsequent column and then distilled in another column from which the solvent is removed in vapor form.

The method according to the invention is described in more detail as follows: In the case of high water contents in the mixtures, the boiling temperatures of the same are lower than in the case of low water contents. Therefore, according to the invention, mixtures with high water contents can be distilled at higher pressures without that excessive thermal stress and thus decomposition takes place. This decomposition must be stopped, on the one hand, the decomposition products in the solvent lead to a yellowish or brownish discoloration of the polyacrylonitrile or polyurethanes and since on the other hand their removal is cumbersome.

Since at solvent contents below about 60 wt.%, The possibility of the distillation at higher pressures is given as about 300 Torr with a low tendency to decompose the solvent, a pre-concentration to about 60 wt.% Can be performed at higher pressures in a plurality of distillation columns. The heat of vaporization of the top product of the first column,

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those at the highest pressure, preferably from 1500 to 5600 Torr, is operated, is used to heat the second column, which operates at a lower pressure, preferably from 760 to 2800 Torr, operated, exploited. In the first column, the expulsion of the water makes the mixture lighter concentrated volatile components and passed into the second column. There the mixture is further concentrated and optionally passed into a further column which is operated at an even lower pressure, preferably from 100 to 2100 Torr, is operated as the second column and which is heated with the top product of the second column. This process can be repeated with further columns. It makes sense that the mixtures do not have a solvent content Concentrate from 60 #, as this is the number of preconcentration columns becomes too large and the last preconcentration column at too low a pressure or the first Preconcentration column would have to be operated at too high a pressure.

According to the invention, the bottom product from the last preconcentration column is fed to a further distillation column and rectified there preferably in such a way that steam and the more volatile constituents are overhead in the side stream the solvent is gaseous and in the sump the less volatile components are liquid and solid in a suspension subtracted from.

The preconcentration of the mixtures with a low solvent content is particularly advantageous for the production of fibers by the dry spinning process. There, through condensation of the vapors from the shaft, mixtures are produced which have a solvent content of over 90 %

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and which of course are not pre-concentrated. However, there are further mixtures with solvent contents of about 10 to 30 % , which are advantageously concentrated according to the invention. Preferably, all solvent-water mixtures that are not formed by condensation of the vapors in the spinning shaft are combined and regenerated according to the invention.

The process for the recovery of dimethylformamide is preferably carried out. According to the process according to the invention, the steam consumption during the distillation of the solvent-water mixture can be reduced by at least 20%.

The process according to the invention is illustrated in the following examples explained:

example 1

In the Paserherstellung by the dry spinning process from a spinning solution of polyacrylonitrile and dimethylformamide a mixture with 97 fo DMP and 3% water drops on the shaft by condensing vapors. In addition, a further mixture with a solvent content of 16.2% DMF, 83.3 % water and 0.5% pesticides dissolved in the mixture is obtained from the aftertreatment. This latter mixture is concentrated to 18.3% in a first column at a bottom temperature of 133 * 8 ° C. and a pressure of 22oo Torr. The top product, which is drawn off in gaseous form at a pressure of 2100 Torr and a temperature of 131 ° C., contains 0.024 % DMP and is used to heat the further column. The bottom product from the first column, which has been concentrated to 18.3% , is passed into this column and concentrated further to 21.9%. In the bottom of the second column is at

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a pressure of 1500 Torr and a temperature of 121.4 ° C. At the top of the second column, a temperature of 118 ° C. prevails at a pressure of 400 torr. The gaseous top product of the second column contains 0.027 % DMF and is used to heat a third column. In the third column, the bottom product from the second column is concentrated to 25.8%. At the bottom of this column, 105 ° C. at 860 torr and 100 ° C. at 760 torr were found in the top. The gaseous top product of the third column has a DMP content of 0.08 % and is used to heat the fourth column. In this fourth column, the bottom product from the third column is pre-concentrated to a solvent content of 33 * 8%. In the fourth column, rectification is carried out in the bottom at 85 ° C. and \ oo Torr and in the top at 76 ° C. and 300 Torr. All preconcentration columns are distilled at a reflux ratio of 1. In order to drive 1 t of water out of the mixture in this way, 0.5 ^ t of steam with 5 ata are required. If this pre-concentrated mixture is combined with the condensate of the shaft vapors, a mixture with a solution concentration of 80 % results. This mixture is separated in a distillation column at a reflux ratio of 2. In the head, water with a DMF content of 0.02 % is drawn off in vapor form at a pressure of 216 Torr and a temperature of 68 ° C. In the lower third of the column, DMP is withdrawn in vapor form in the side stream. At a pressure of 336 Torr, the temperature in the sump is 126 ° C. In order to evaporate one ton of water, about 3.3 t Γ-ampf at 5 ata are required in this column. On average, this type of solvent recovery requires 1.73 t of steam per t of water to be expelled. The recovered solvent

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is of the same quality as that of a normal one
Distillation of recovered solvents.

Example 2 (comparison; recovery with a conventional
Distillation)

When all solvent-containing mixtures are combined that arise in fiber production using the dry spinning process from a spinning solution of polyacrylonitrile and DMF, a mixture with a DMF content of 6% is obtained.

This mixture is separated in a distillation column at a reflux ratio of 1. In the head of this column
water with a DMF content of 0.07 % is drawn off in vapor form at a pressure of 220 Torr and a temperature of 67 ° C. In the lower third of the column, DMF is withdrawn in vapor form in the side stream. There is a pressure in the sump
of 3 ^ o Torr a temperature of 125 ° C. By a ton

Evaporating water will produce 2, ^ 7 tons of steam at 5 ata
needed.

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Claims (1)

Claim 1.) A process for the regeneration of dimethylacylamides from aqueous solutions which arise in the production of threads from acrylonitrile polymers or polyurethanes, characterized in that the solvent-water mixtures are first fed to a preconcentration system consisting of at least two successive distillation columns, with use of the overhead vapor products as the heating medium for the subsequent column until a solvent content of at most 60 wt.% concentrated mixtures of each column and then distilled in another column from which the solvent is removed in vapor form. Le A 17 072 - 8 - 709840/0283