JPH0834757A - Purification of acrylic acid - Google Patents

Abstract

PURPOSE:To provide an inexpensive and efficient method for purifying acrylic acid in production of acrylic acid. CONSTITUTION:This purifying method comprises subjecting propylene and (or) acrolein to catalytic vapor phase oxidation using a molecular oxygen-containing gas, cooling the produced gas and (or) absorbing the produced gas into water to afford a crude acrylic acid aqueous solution, subjecting the aqueous solution or a crude acrylic acid aqueous solution obtained by previously removing aldehydes in the aqueous solution by stripping to azeotropic distillation under reflux together with a nonhydrophilic azeotropic agent capable of forming an azeotropic mixture with acetic acid and recovering the purified acrylic acid from the tower bottom of azeotropic distillation tower used. In this method, an azeotropic agent is additionally fed from at least one place which is located more downstream than feed source of the reflux flow of azeotropic distillation tower in addition to the azeotropic agent as a reflux flow and the azeotropic distillation is carried out and acrylic acid substantially containing no acetic acid is recovered from the tower bottom.

Description

Detailed Description of the Invention

[Background of the Invention]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for purifying acrylic acid obtained by catalytic gas phase oxidation of propylene and / or acrolein with a gas containing molecular oxygen.

The reaction product obtained by catalytic gas phase oxidation of propylene and / or acrolein with a molecular oxygen-containing gas, preferably in the presence of steam, is usually acetic acid in addition to the desired product, acrylic acid. , By-products such as formic acid, acetaldehyde, formaldehyde, carbon dioxide, or carbon monoxide, and unreacted propylene and acrolein. The present invention, from its reaction product,
It is intended to provide a method for efficiently separating acetic acid, which is difficult to separate from acrylic acid among side reaction products, to purify acrylic acid with high purity. More specifically, the present invention absorbs the reaction product with water to prepare an aqueous solution of acrylic acid, which is introduced into an azeotropic distillation column for purification,
Provide a method of purifying acrylic acid by separating by-products by effective azeotrope by additionally supplying an azeotropic agent at one or more points below the reflux supply stage in addition to the reflux of the azeotropic distillation column To do.

[0003]

BACKGROUND OF THE INVENTION In the past, the separation of acetic acid from the product acrylic acid in the acrylic acid refining process has generally been
It has been considered that the physical properties of both compounds are very difficult, and that a general separation method requires a great deal of cost. However, in recent years, the importance of reducing the concentration of acetic acid has been increasing as the quality of acrylic acid products. For example, as a general-purpose application of acrylic acid, it is possible to polymerize it to obtain a highly water-absorbent resin, but since acetic acid in acrylic acid does not polymerize, it remains in the product resin as an acetic acid simple substance and absorbs water. It is easy to cause problems such as deterioration of odor and odor, and there is a great demand for reduction of acetic acid from raw material acrylic acid.

Also in the ester produced by reacting acrylic acid with various alcohols, the acetic acid in the starting acrylic acid reacts with the alcohol to form an acetic ester. The acetic acid ester cannot be easily separated from the acrylic acid ester and is mixed in the product acrylic acid ester.

Even when a so-called emulsion is produced by polymerizing acrylic acid, the acetic acid ester does not polymerize, and thus causes various troubles as a "residual monomer". Since the acetic acid content in general acrylic acid and the allowable acetic acid ester content in the product acrylic acid ester are each 0.05% by weight or less, the allowable acetic acid content in the raw material acrylic acid is required to be 0.03% by weight or less. To be done.

From such a point, several methods have been considered as a method for purifying acrylic acid, particularly a method for removing acetic acid as an impurity. For example, Japanese Patent Publication No. 63-10
Japanese Patent No. 691 describes a method in which an aqueous solution of acrylic acid is subjected to azeotropic distillation together with an azeotropic agent that forms an azeotropic mixture with acetic acid to separate and purify acetic acid from acrylic acid (FIG. 2).
Indicates the flow sheet). However, even though this method provided a reasonable solution, since it is merely an azeotropic distillation for purification, it is difficult to practically completely separate acetic acid with an azeotropic distillation column alone. Is understood to be. As a result, since acetic acid is extracted together with acrylic acid from the bottom of the column, the resulting acrylic acid requires a further treatment for removing acetic acid, which complicates the process and increases the cost. It is understood that there is a problem.

On the other hand, in JP-A-5-246941, acrylic acid is separated from acetic acid and purified by adding an azeotropic agent that forms an azeotropic mixture with acetic acid to an acrylic acid aqueous solution to carry out azeotropic distillation. The method of doing is described (Fig. 3
Indicates the flow sheet). However, even though this method also provided some solution, since the azeotropic agent shown here uses a hydrophilic substance such as diethyl ketone, an azeotropic agent distilled from the top, Even if the mixture of water and acetic acid is left undisturbed, a considerable amount of the azeotropic agent remains in the aqueous phase, and a dedicated azeotropic agent recovery tower for recovering the azeotropic agent from the aqueous phase is required. It is understood that there is. Further, it is understood that this known invention has one characteristic in using a mixture of a hydrophilic solvent such as diethyl ketone and a hydrophobic solvent such as toluene as an azeotropic agent. Changes over time during operation,
It is understood that it is not suitable for long-term continuous operation as an actual plant, considering the composition of the azeotropic liquid and the replenishment method during operation.

Other methods are known, but the conventional methods have any of the above problems.

[0009]

[Means for Solving the Problems]

[Summary of the Invention] In purifying an aqueous crude acrylic acid solution by azeotropic distillation under reflux, a non-hydrophilic azeotropic agent is used in addition to reflux, and at least one portion below the stage is further added. It is based on the discovery that high-purity acrylic acid can be obtained at low cost by supplying it.

<Summary> That is, the method of purifying acrylic acid according to the present invention is characterized in that propylene and / or acrolein is subjected to catalytic gas phase oxidation with a gas containing molecular oxygen to cool and / or absorb water into water. A crude acrylic acid aqueous solution obtained by the above, or a crude acrylic acid aqueous solution obtained by previously removing the aldehydes in the solution by stripping,
Is subjected to azeotropic distillation under reflux with a non-hydrophilic azeotropic agent that forms an azeotrope with acetic acid, in the method of recovering purified acrylic acid from the bottom of the azeotropic distillation column used, In addition to the azeotropic agent, the azeotropic agent is additionally supplied from at least one position downstream from the reflux source to carry out this azeotropic distillation, and acrylic acid substantially free of acetic acid is recovered from the column bottom. It is characterized by:

<Effect> According to the method of the present invention, a crude acrylic acid aqueous solution can be purified to a high purity at a low cost.

[Detailed Description of the Invention] <Purification Target> The crude acrylic acid aqueous solution to be purified by the present invention is propylene and / or
A crude acrylic acid aqueous solution obtained by cooling and / or absorbing water with a product gas obtained by catalytic gas phase oxidation of acrolein with a gas containing molecular oxygen, or aldehydes in the liquid were previously removed by stripping. A crude acrylic acid aqueous solution. Since these aqueous solutions of acrylic acid contain acetic acid as an impurity, the purification method of the present invention works effectively.

Catalytic gas phase oxidations of propylene and / or acrolein are well known. Since the present invention is not concerned with the catalytic gas phase oxidation itself, the details of catalytic gas phase oxidation of propylene and / or acrolein may be incorporated by reference. Such documents include, for example, Hydrocarbon Processing 56 No. 11,123.
(1977) and 58 No. 11, 124 (1979).

The catalytic gas phase oxidation of propylene and / or acrolein is often carried out in the presence of water vapor, and the gaseous reaction product of this gas phase reaction carried out at elevated temperature converts acrylic acid from it. In treating the gaseous reaction product for recovery, it is common to cool the gaseous reaction product, or to treat the gaseous reaction product with water in order to absorb acrylic acid, that is, scrubbing. It is in the form of a crude acrylic acid aqueous solution targeted by the present invention.

These crude aqueous solutions of acrylic acid can be purified by the method of the present invention at any concentration, but especially 20 to 85% by weight of acrylic acid and 0.5 to 6.5 of acetic acid.
It is preferable that the content of water is 10 to 75% by weight.

<Azeotropic distillation><< Outline >> The azeotropic distillation used for purifying the crude acrylic acid as described above in the present invention, except that the azeotropic agent is supplied from a plurality of specific points, It is essentially the same as conventional ones, in particular those carrying out reflux.

That is, the azeotropic distillation of the present invention has a basic structure in which acetic acid as an impurity is first distilled from the top of the column and purified acrylic acid is recovered from the bottom of the column. Therefore, the azeotropic distillation column used can be said to be an azeotropic separation column for separating acetic acid from crude acrylic acid.

Since the azeotropic agent used in the present invention is preferably an azeotropic agent with water in addition to acetic acid (details described later), the overhead distillate of azeotropic distillation is substantially acetic acid, water and azeotrope. It typically consists of effervescent agents. Therefore, when such an azeotropic agent is used, purified acrylic acid as a column bottom liquid can be obtained without containing water and / or an azeotropic agent.

In the azeotropic distillation of the present invention carried out under reflux, the overhead distillate consisting essentially of these three components may be refluxed as it is, but the preferred embodiment is substantially only the azeotropic agent. Is to be refluxed. That is, since the azeotropic agent used in the present invention is non-hydrophilic, when the overhead distillate is left to stand, it is stratified into a phase consisting essentially of the azeotropic agent and a phase consisting essentially of acetic acid and water. So it is also possible to separate this entrainer phase and bring it to reflux, which is also a preferred embodiment of the invention.

One of the features of the present invention is the multi-stage feed of the azeotropic agent as described above, one of which is supplied as a reflux to the top of the column and the other of which is at least a stage below the stage where the reflux is supplied. Additional feed is provided in one, ie over one or several stages. It is suitable to carry out the additional supply of the azeotropic agent in one or more stages, preferably one stage (thus, two stages together with the supply as reflux), and the additional supply stage is near the crude acrylic acid aqueous solution supply stage, that is, the same stage. Or below, is preferred. The additional feed azeotropic agent may be recovered as the overhead distillate or may be fresh.

The amount ratio of the azeotropic agent as reflux and the total amount of the additionally supplied azeotropic agent is 1: 5 to 30: 1, preferably 1: 1 to 1.
It is usually 15: 1.

When the azeotropic agent is azeotropic with water, the total amount of the azeotropic agent to be supplied to the azeotropic distillation column is generally the same as that of acetic acid in the crude acrylic acid aqueous solution which is a raw material to be supplied to the azeotropic distillation column. The total amount of the azeotropic composition with the azeotropic agent and the amount of the azeotropic composition with water and the azeotropic agent is not less than the sum, preferably from 2.0 to 10 of the azeotropic composition with acetic acid
It is more than the sum total of double the amount and 1.0 to 3.0 times the azeotropic composition with water.

As a matter of course, the azeotropic agent as the reflux and the additional supply azeotropic agent can be re-supplied after treatment such as washing with water outside the azeotropic tower system.

<< Azeotropic Distillation Flow Sheet / Apparatus >> The above-described preferred embodiment of the method for purifying crude acrylic acid by azeotropic distillation under reflux according to the present invention is shown in a flow sheet as shown in FIG.

In the case of the flow sheet shown in FIG.
The crude acrylic acid aqueous solution which is supplied to No. 1 and flows out from the tower bottom via line 2 is supplied to the azeotropic distillation tower 12. In the azeotropic distillation column 12, in addition to the crude acrylic acid aqueous solution from the line 2, an azeotropic agent is used as a reflux from the line 4 and a stage below the reflux supply stage from the line 5, for example, a raw material supply stage in FIG. , And are supplied as divided azeotropic agents respectively. By dividing and supplying the azeotropic agent in this manner, the proportion of the azeotropic agent in the gas-liquid in the azeotropic distillation column, especially in the raw material supply stage and below, becomes large, and the azeotropic agent is azeotropically distilled into acetic acid. The effect is improved.

From the top line 3 of the azeotropic distillation column 12,
The overhead distillate, that is, the condensate, which is a mixture of substantially acetic acid, an azeotropic agent, and water, uses a non-hydrophilic azeotropic agent having low solubility in water, and therefore, when it is left standing, it becomes an aqueous phase. Although it is separated into two phases, an azeotropic agent phase and an aqueous phase, which substantially consists of all the water and acetic acid in the acrylic acid aqueous solution from the acrylic acid collection tower 11, is discharged from the system through a line 8. It
A part of it is circulated to the acrylic acid collection tower 11 through the line 7 and used as an absorbing liquid for collecting acrylic acid.

On the other hand, a part of the azeotropic agent phase is refluxed in line 4 as the reflux of the azeotropic distillation column 12, and the rest is line 5 in the stage below the reflux stage of the azeotropic distillation column 12, FIG.
It is circulated to the raw material supply stage, which is the stage supplied in line 2.

The azeotropic distillation column used in the present invention is not particularly limited, but preferably 15 to 60 plates, more preferably 3 plates.
It is usual to use the one having 5 to 50 steps.

As the operating conditions of the azeotropic column, the column top pressure is preferably set so that the column top temperature is 35 ° C. or higher and the column bottom temperature is 100 ° C. or lower. In addition, it goes without saying that a gas containing a polymerization inhibitor and molecular oxygen may be supplied during the operation.

<Azeotropic agent> Since the azeotropic distillation in the present invention has the basic purpose of distilling acetic acid as an azeotropic mixture, the azeotropic agent to be used is an azeotropic agent with acetic acid. Although it must be present, it is preferably azeotropic with water so that water present in significant amounts in the crude acrylic acid can also be distilled off. Further, since the purified acrylic acid is recovered as the bottom liquid of the column, the azeotropic agent to be used should be one that does not substantially azeotrope with acrylic acid. In particular, those having a solubility in water of 5% by weight or less are preferable. In particular,
Toluene, heptane, methylcyclohexane, cyclohexane, ethylbenzene, isobutyl ether, cyclohexene, cyclohexanediene, butyl ethyl ketone, n-propyl propionate, isobutyl acetate and the like can be given. It is preferable to use one that is substantially insoluble in water, such as toluene, cyclohexane, or methylcyclohexane.

[0031]

EXAMPLES Experimental Example 1 The azeotropic distillation column used was adjusted to have a top pressure of 110 mmHg. The crude acrylic acid aqueous solution as a raw material contains 38.
A material containing 5% by weight, 3.2% by weight of acetic acid, and some aldehydes, carboxylic acids and the like was used. This aqueous solution is a condensed product gas obtained by the catalytic gas phase oxidation reaction of propylene.

The above raw materials were supplied to the azeotropic distillation column at 14.6 kg / hour, and azeotropic distillation was performed using toluene as an azeotropic agent. The entire amount of the toluene phase in the condensate from the top of the column was circulated to the azeotropic distillation column. The circulation point and the circulation amount of the azeotropic agent were 51.4 kg / hour for reflux and 5.7 kg / hour for the divided azeotropic agent supply to the raw material supply stage, ie, 57.1 kg / hour. During operation, the tower top temperature was about 40 ° C and the tower bottom temperature was about 100 ° C. The component of the bottom liquid at this time was 99.0% by weight of acrylic acid.
As described above, acetic acid was 0.03% by weight or less, and toluene was a trace amount, which was sufficiently usable as a raw material acid for acrylic ester. The toluene concentration in the aqueous phase after leaving the condensate from the top of the column to stand was 0.03% by weight.

Experimental Example 2 14.6 kg of a raw material crude acrylic acid aqueous solution to the azeotropic distillation column
The azeotropic distillation was carried out using toluene as an azeotropic agent. The circulation point and circulation amount of the azeotropic agent are 5 for the reflux.
1.4kg / hr, 7.
A total of 58.5 kg / hour of 1 kg / hour was supplied. As a result, the components of the bottom liquid were 99.0% by weight or more of acrylic acid, 0.01% by weight or less of acetic acid, and a trace amount of toluene. In this case as well, acetic acid was largely separated and removed from the raw material acrylic acid aqueous solution. I was able to. After allowing the condensate from the top of the column to stand, the concentration of toluene in the aqueous phase was 0.01% by weight.

Comparative Example 1 Under the experimental conditions of Experimental Example 1, the toluene circulation to the azeotropic distillation column was 57.1 kg / hour only in the reflux supply stage (the total amount of azeotropic agent circulation to the two locations in Experimental Example 1). The azeotropic distillation was performed only by changing to. The components of the bottom liquid were acrylic acid of 99.0% by weight, acetic acid of 0.1% by weight, and trace amounts of toluene. As it was, only acrylic acid of a level that could not be used as a raw material acid for acrylic ester I couldn't get it. After that, the toluene circulation rate was increased and the change in the purity in the bottom liquid was observed. Before reaching the purity of Experimental Example 1, the circulation rate was set to 124 kg / hour, and the azeotropic distillation column was used. The interior fell into flooding and it became impossible to operate.

[0035]

According to the present invention, acetic acid by-produced during the production of acrylic acid can be efficiently removed at a low cost to obtain high-purity acrylic acid. I have just done it.

[Brief description of drawings]

FIG. 1 is a flow sheet showing a method for purifying acrylic acid according to the present invention.

FIG. 2 is a flow sheet showing a method for purifying acrylic acid using the method described in JP-B-63-10691.

FIG. 3 is a flow sheet showing a method for purifying acrylic acid using the method described in JP-A-5-246941.

[Explanation of symbols]

 1 line (propylene oxidation reaction gas) 2 lines (crude acrylic acid aqueous solution) 3 lines (azeotropic distillation column overhead) 4 lines (azeotropic distillation column reflux) 5 lines (splitting azeotropic agent) 6 lines (purified acrylic acid) 7 lines (recycled wastewater for absorbing acrylic acid) 8 lines (wastewater) 9 lines (recycled acrylic acid) 10 lines (crude acrylic acid before deacetic acid) 11 acrylic acid collection column 12 azeotropic distillation column 13 stationary tank 14 acetic acid separation Tower 15 Azeotropic agent recovery tower

Claims (6)

[Claims] 1. A crude acrylic acid aqueous solution obtained by cooling and / or absorbing water with a product gas obtained by catalytically vapor-phase oxidizing propylene and / or acrolein with a gas containing molecular oxygen, or in the liquid. The crude acrylic acid aqueous solution in which the aldehydes have been removed by stripping in advance is subjected to azeotropic distillation under reflux with a non-hydrophilic azeotropic agent that forms an azeotropic mixture with acetic acid, and the azeotropic distillation column bottom used In the method for recovering more purified acrylic acid, in addition to the azeotropic agent as the reflux, the azeotropic agent is additionally supplied from at least one position downstream of the source of the reflux of the azeotropic distillation column. A method for purifying acrylic acid, characterized by performing distillation to recover acrylic acid substantially free of acetic acid from the bottom of the column. 2. The method for purifying acrylic acid according to claim 1, wherein the acrylic acid from the bottom of the column does not contain water and / or an azeotropic agent. 3. The azeotropic distillation column used has a number of stages of 15 to 60, and the ratio of the amount of the azeotropic agent supplied as reflux and the total amount of the azeotropic agent additionally supplied is 1: 5 to 30. The method for purifying acrylic acid according to any one of claims 1 to 2, which is in a range of 1 to 1. 4. The method for purifying acrylic acid according to claim 1, wherein the azeotropic agent used satisfies all of the following conditions. (A) In addition to acetic acid, it forms an azeotropic mixture with water, (b) the solubility in water is 5% by weight or less, and (c) an azeotropic mixture with acrylic acid. That does not form. 5. The azeotropic distillation column distillate is a mixture consisting essentially of acetic acid, water and an azeotroping agent, and the mixture is allowed to stand to form a phase substantially consisting of the azeotropic agent. 5. Purification of acrylic acid according to claim 4, wherein the acrylic acid is separated into a phase composed of acetic acid and water, the former is supplied to an azeotropic distillation column as reflux and the latter is used as water for absorbing a catalytic gas phase oxidation product gas. Law. 6. A crude acrylic acid aqueous solution to be subjected to azeotropic distillation comprises 20 to 85% by weight of acrylic acid and 0.5 to 50% of acetic acid.
The method for purifying acrylic acid according to any one of claims 1 to 5, which contains 6.5% by weight and water in the range of 10 to 75% by weight.