Method for the Purification of (Meth) Acrylic Acid

Abstract

The present invention relates to a process for production of (meth)acrylic acid, whereby first a crude (meth)acrylic acid is produced and this crude (meth)acrylic acid is then continuously purified, whereby the continuous purification of the crude (meth)acrylic acid comprises the following process steps: a) in a composition comprising (meth)acrylic acid and impurities, precipitating the impurities in crystalline form from the composition; and b) separating the crystalline impurities precipitated from the composition. The invention also relates to the (meth)acrylic acid obtainable by this process, a device for production of (meth)acrylic acid, the use of the device or of a purification device for production of (meth)acrylic acid, (meth)acrylic acid, water-absorbing polymers, the use of water-absorbing polymers as well as fibers, sheets, foams and composites.

Description

The present invention is more closely illustrated by means of non-limiting figures.

FIG. 1 shows the basis solution of the process according to the invention, in which crystallized impurities are separated continuously from a composition stored in a container.

FIG. 2A shows the process course of a process according to the invention, in which, in a one-step purification process, impurities are separated from melted (meth)acrylic acid, which were obtained after separation from a crystal suspension.

FIG. 2B shows the process course depicted in FIG. 2A, whereby, however, the separation of the impurities does not occur directly within the product cycle, but by means of a separate cycle which is connected with the product cycle.

FIG. 2C shows the process course of a process according to the invention, in which in a one-step purification process impurities are separated from the crystal suspension, which was obtained in the crystallizer and which is, by means of a product cycle, at least partially melted and conducted back into the crystallizer.

FIG. 2D shows the process course of a process according to the invention, in which, in a one-step purification process, impurities are separated from the mother liquor, which was obtained on separation of the crystallized (meth)acrylic acid in the wash column and which is at least partially conducted back into the crystallizer.

FIG. 3 shows at which positions in a two-step purification process a separation according to the invention of impurities can occur.

According to the basis solution of the process according to the invention depicted in FIG. 1, a composition comprising (meth)acrylic acid and impurities, in particular maleic acid anhydride, and stored in a stock container 1, is stored for so long until a sufficient hydrolysis of the maleic acid anhydride has occurred. In the case of a continuous crystallization process for purification of (meth)acrylic acid, the stock container 1 corresponds to a suspension generator, to a wash column or to a supply line or discharge line for a crystal suspension, for crystallized (meth)acrylic acid separated from the crystal suspension, or for mother liquor, depending on from which composition the crystallized impurities, in particular the crystallized maleic acid and/or fumaric acid, are separated.

By means of a product cycle 2, the composition is removed continuously or discontinuously from the stock container 1, whereby the product cycle is operated by means of a product cycle pump 3. From the product cycle 2, by means of a further product cycle 4, the composition is conducted through the separating device, in which crystallized impurities, in particular crystallized maleic acid and/or fumaric acid are separated by means of discharge 8. The composition freed from the crystallized impurities is than conducted back into the product cycle 2 and, via this, then into stock container 1. Besides the procedure depicted in FIG. 1, it s also possible to conduct the composition conducted in product cycle 2 directly (and not by means of the separate product cycle 4), through the separating device 5.

FIG. 2A shows the process course of a process according to the invention, in which, in a one-step purification process, impurities are separated from the at least partially melted (meth)acrylic acid, which was obtained after the separation in the wash column 10.

The composition stored in stock container 1 and comprising (meth)acrylic acid and impurities is conducted to a crystal suspension generator 8. The formed crystal suspension is then conducted by means of a feed 9 into a separating device (e.g. a wash column) 10. In the separating device 10, the (meth)acrylic acid crystals are separated from the mother liquor. In addition, a part of the (meth)acrylic acid crystals are transferred again into a crystal suspension and this crystal suspension conducted in a product cycle 11 driven by means of a product cycle pump 12, in which product cycle 11 the (meth)acrylic acid crystals are melted by means of a heat exchanger 13 and, for increase of the purity of the (meth)acrylic acid, at least partially conducted back as wash liquid to the counter-flow wash in the wash column 10. Thus, the wash column 10 serves to separate solid and liquid as well as to carry out a displacement washing, whereby the displacement washing is carried out without loss of wash fluid. The other part of the (meth)acrylic acid crystals leaves the system and flows into the product container 14 (if the purity of the (meth)acrylic acid obtained by means of the one-step purification process shown in diagram 2A is not sufficient, the (meth)acrylic acid crystals can be conducted to a further purification step). The at least partially melted (meth)acrylic acid crystals are conducted through the separation device for separation of impurities 5. According to FIG. 2B, the at least partially melted (meth)acrylic acid crystals can also be conducted to the separating device by means of a separate product cycle. The separated impurities are removed via discharge line 6 and the composition freed from the impurities is conducted back into the separating device 10. In one embodiment of this one-step purification process, at least a part of the mother liquor separated in wash column 10 is conducted back via feed 15 into the suspension generator 8, the other part of the mother liquor is conducted as effluent into the mother liquor container 16.

FIGS. 2C and 2D show the one-step purification process corresponding to that of FIG. 2A, whereby, however, not the at least partially melted (meth)acrylic acid crystals which were obtained after the separation in the wash column 10, but rather the crystal suspension (FIG. 2D) obtained in the suspension generator 8 or the mother liquor of the separating device 5 conducted back into the suspension generator 8 are supplied.

FIG. 3 shows a two-step purification process, in which the mother liquor separated in wash column 8 is at least partially conducted into a further suspension generator 17. The crystal suspension obtained in suspension generator 17 is conducted into a further wash column 18, in which the (meth)acrylic acid crystals are separated from the mother liquor. The (meth)acrylic acid crystals separated in this second wash column 18 are, in crystalline form or after they have been at least partially melted by means of a heat exchanger, conducted via feed line 20 to the first suspension generator. It is also exemplary in this two-step purification process that the crude (meth)acrylic acid is conducted out of the stock container 1 into the head of the second wash column, in order, in this way, to conduct the product crystals scraped off in the second process step as crystal suspension into the suspension generator of process step A. This variant has the energetic advantage of being able to dispense with a melting in the second step and not having to freeze again the crystals now present in the first step.

In FIG. 3, it is illustrated at which positions a separation of crystallized impurities, in particular of crystallized maleic acid and/or fumaric acid is possible by means of separating device 22.

The invention is now more closely illustrated by means of an example:

EXAMPLE

113.5 g of a bottom product from a column with the composition given in table 1 was placed with 1.6 g water in a Erlenmeyer flask. The mixture was allowed to stand over a time period of 42 hours at room temperature and then filtered using a vacuum filter. 108.5 g filtrate with the composition likewise given in table 1 were obtained. The solid remaining on the filter was then centrifuged at 4,000 rpm for 5 minutes, whereby 3.6 g of a solid was obtained. This solid was dissolved in 129.0 g highly pure acrylic acid, for the purpose of analysis. The composition of the solid is likewise given in table 1 (the components comprised in the compositions were analyzed by means of respectively gas chromatography and Karl-Fischer-Titration; the relative amounts of the components in the bottom product, in the filtrate and in the solid do not add up to 100 wt % exactly, because of the rounding up and rounding down of the analysis data obtained for these compositions.

TABLE 1
Components (all	Bottom product	Filtrate (based	Solid (based
entries in wt. %,	(based on the total	on the total	on the
unless otherwise	amount of the	amount of the	total amount
given)	bottom product)	filtrate)	of the solid
Water	0.018	0.979	1.22
MEHQ1)	0.0044	0.0037	0.51
Hydroquinone	0.113	0.125	<800	ppm
Acryl acid	88.6	89.0	48.43
Acetic acid	0.055	0.049	0.59
Propionic acid	0.036	0.034	0.48
Dimeric acrylic acid	2.759	2.959	4.79
MA/MS2)	8.131	6.215	44.16
Furfural	0.0327	0.036	<40	ppm
Benzaldehyde	0.075	0.08	<40	ppm
Acrolein	0.001	0.0019	<40	ppm
Protoanemonine	0.039	0.0398	<40	ppm
Remainder	0.06	0.440	<800	ppm
1)Methyl hydroquinone
2)Maleic acid anhydride/maleic acid

Claims

1. A process for production of (meth)acrylic acid, whereby first a crude (meth)acrylic acid is produced and this crude (meth)acrylic acid is then continuously purified, wherein the continuous purification of the crude (meth)acrylic acid comprises the following process steps: a) in a composition comprising (meth)acrylic acid and impurities, precipitating the impurities from the composition in crystalline form;b) separating the crystalline impurities precipitated from the composition.

2. The process according to claim 1, wherein the impurity is maleic acid anhydride, maleic acid, fumaric acid or a mixture of these compounds.

3. The process according to claim 2, wherein the separation of the maleic acid anhydride from the composition occurs in that in process step a) maleic acid anhydride is hydrolyzed, to a proportion of at least about 50 mol %, based on the total amount of the maleic acid anhydride comprised in the composition before the start of the hydrolysis, with formation of maleic acid and/or fumaric acid, wherein maleic acid and/or fumaric acid precipitate in crystalline form from the composition.

4. The process according to claim 3, wherein the hydrolysis of the maleic acid anhydride in process step a) occurs over a time period in a range from about 1 to about 250 hours at a temperature in a range from about −70 to about 130° C.

5. The process according to claim 1, wherein the composition is the (meth)acrylic acid crystals in at least partially melted form or the mother liquor, which are obtained in step B or in step D of a process which comprises the following process steps: A) crystallization of (meth)acrylic acid from an impure crude (meth)acrylic acid stream from a process for production of (meth)acrylic acid with formation of a crystal suspension by means of a first suspension generator;B) separation of the (meth)acrylic acid crystals from the crystal suspension by means of a first separating device, whereby a mother liquor is retained; as well as optionallyC) renewed crystallization of (meth)acrylic acid from the first mother liquor obtained from step B) with formation of a second crystal suspension by means of a second suspension generator;D) separation of the (meth)acrylic acid crystals obtained in step C) from the second crystal suspension by means of a further separating device, whereby a second mother liquor is retained.

6. The process according to claim 1, wherein the composition is the crystal suspension in at least partially melted form which is obtained in process step A or step C of a process which comprises the following process steps: A) crystallization of (meth)acrylic acid from an impure crude (meth)acrylic acid stream from a process for production of (meth)acrylic acid with formation of a crystal suspension by means of a first suspension generator;B) separation of the (meth)acrylic acid crystals from the crystal suspension by means of a first separating device, whereby a mother liquor is retained; as well as optionallyC) renewed crystallization of (meth)acrylic acid from the first mother liquor obtained from step B) with formation of a second crystal suspension by means of a second suspension generator;D) separation of the (meth)acrylic acid crystals obtained in step C) from the second crystal suspension by means of a further separating device, whereby a second mother liquor is retained.

7. A device for production of acrylic acid comprising, as components connected with each other in fluid-conducting fashion, an acrylic acid reactor, a quench tower, a distillation device and a purification device, which comprises a separating device for separation of impurities, which comprises the features (ε1) to (ε5): (ε1) the device unit comprises a crystallization region, a separating region, a separating device, at least two guides and optionally a melter;(ε2) the crystallization region comprises at least one inlet ε2E—1 for an impure (meth)acrylic acid stream and an outlet ε2A—1 for a crystal suspension, wherein the outlet ε2A—1 is connected by means of a first guide with an inlet ε3E—1 of the separating region ε3;(ε3) the separating region comprises at least one inlet ε3E—1 and at least one outlet ε3A—1, whereby the inlet ε3E—1 of the separating region ε3 is connected by means of the first guide with the outlet ε2A—1 of the crystallization region and the outlet ε3A—1 is connected with a second guide for the mother liquor separated in the separating region;(ε4) the melter comprises an inlet ε4E and an outlet ε4A, whereby the inlet ε4E of the melter is connected by means of a third guide with a further outlet ε3A—2 of the separating region for the removal of separated (meth)acrylic acid crystals and the outlet ε4A is connected by means of a fourth guide with a further inlet ε3E—2 of the separating region or whereby the inlet ε4E of the melter is connected by means of a fifth guide with a further outlet ε3A—2 of the crystallization region for the removal of a crystal suspension and the outlet ε4A is connected by means of a sixth guide with a further inlet ε2E—2 of the crystallization region;(ε5) the second guide, the fourth guide or the sixth guide is conducted through the separating device.

8. The device according to claim 7, wherein the separating device (ε5) is a filter.

9. Use of a device according to claim 7 for the production of (meth)acrylic acid which has a purity of more than about 90 wt. %, such as more than about 95 wt. % and such as more than about 99.5 wt. %, respectively based on the (meth)acrylic acid with impurities.

10. (Meth)acrylic acid obtainable by a process according to claim 1.

11. (Meth)acrylic acid, wherein the (meth)acrylic acid has a maleic acid anhydride content of less than about 50 ppm.

12. Water-absorbing polymers obtainable by polymerization of acrylic acid according to claim 11.

13. Use of water-absorbing polymers according to claim 12 in fibers, sheets, foams and composites.

14. Fibers, sheets, foams and composites comprising water-absorbing polymers according to claim 12.