GB2146636A

GB2146636A - Process for producing acrylic acid

Info

Publication number: GB2146636A
Application number: GB08420153A
Authority: GB
Inventors: Masao Baba; Hitoshi Takahashi; Haruo Tahara; Atsushi Okubo
Original assignee: Nippon Shokubai Co Ltd
Current assignee: Nippon Shokubai Co Ltd
Priority date: 1983-08-11
Filing date: 1984-08-08
Publication date: 1985-04-24
Also published as: KR850001720A; GB8420153D0; KR900006901B1; GB2146636B; FR2554809A1; FR2554809B1; MX161562A; DE3429391A1; DE3429391C2

Abstract

Acrylic acid is produced by catalytically oxidizing propylene and/or acrolein in the vapor phase with a gas containing molecular oxygen, the reaction product gas is rapidly cooled, the condensate is collected and the uncondensed gas is contacted with water. The resulting aqueous solution of acrylic acid composed mainly of acrylic acid, acetic acid and water is fed into a distillation column having at least 10 theoretical trays and distilled in the presence of an azeotropic agent. From the top of the column, is distilled an azeotropic composition consisting substantially only of the azeotropic agent and water and the separated water is then recycled. A mixture consisting mainly of acrylic acid and acetic acid and being free from water and the azeotropic agent is obtained from the bottom of the column.

Description

SPECIFICATION Process for producing acrylic acid This invention relates to a process for producing acrylic acid from propylene and/or acrolein. More specifically, this invention provides a process for producing acrylic acid, which comprises catalytically oxidizing propylene and/or acrolein in the vapor phase with a gas containing molecular oxygen, rapidly cooling the resulting reaction product gas and collecting it as a condensate, collecting the uncollected substances by causing them to be absorbed by absorbing water, distilling the resulting aqueous solution of acrylic acid composed mainly of acrylic acid, acetic acid and water in the presence of an azeotropic agent to distill off water substantially as an azeotrope, separating water from the azeotrope, recycling the separated water as the absorbing water, and in the meantime, obtaining a mixture composed mainly of acrylic acid and acetic acid as distillation bottoms, separating acrylic acid from the distillation bottoms, and purifying it.

A conventional process for producing acrylic acid comprises catalytically oxidizing propylene and/or acrolein in the vapor phase, treating the resulting aqueous solution of acrylic acid with an organic solvent to extract acrylic acid and purifying the acrylic acid. In other words, an extracting operation is generally practiced to separate water from the aqueous solution (see, for example, U.S. Patent No. 3,968,153). The waste liquor discharged in the extracting step, however, contains the unrecovered acrylic acid, by-product organic acids (such as acetic acid, propionic acid and maleic acid) and aldehydes (such as acrolein and acetaldehyde) and polymers or condensates of these compounds (such as tarry substances and high-boiling substances such as a dimer of acrylic acid).Discarding of the waste liquor, therefore, requires prior teatment for rendering it nontoxic by an active sludge method or a concentration burning method, and a great deal of expenditure must go to facilities and utilities required for this treatment. When this waste liquor is directly used in the process as an absorbing liquor for collecting acrylic acid, the amount of acids in the process increases because of the acids present in the waste liquor, and the efficiency of collecting acrylic acid in the acrylic acid collecting device is decreased. If the exhaust gas from this device is recycled as an inert gas source for the starting gas, the amount of acids in the starting gas increases and may cause accelerated degradation of the catalytic activity in the oxidation reaction (see, for example, U.S. Patent No. 4,147,885).

With an advance in recent years in the technology of performing the above oxidation reaction and of collecting the desired product, the aqueous solution of acrylic acid obtained can contain acrylic acid in a concentration of as high as 50 to 80% by weight.

According to this invention, there is provided a process for producing acrylic acid, in which by subjecting such a high concentration aqueous solution of acrylic acid to an azeotropic distillation operation under suitable conditions instead of subjecting it to an extracting operation as in the prior art as an operation of separating water, the distillation of acrylic acid and acetic acid from the top of the distillation column can be limited only to a trace.

Thus, according to this invention there is provided a process for producing acrylic acid which comprises catalytically oxidizing propylene and/or acrolein in the vapor phase with a gas containing molecular oxygen, wherein the reaction product gas obtained by the oxidation reaction is rapidly cooled to collect it as a condensate, the uncollected substances are collected by causing them to be absorbed by absorbing water, the resulting aqueous solution of acrylic acid composed mainly of acrylic acid, acetic acid and water is fed into a distillation column having at least 10 theoretical trays in its recovery section and distilled in the presence of an azeotropic agent, an azeotropic composition consisting substantially of the azeotropic agent and water and being free from acrylic acid and acetic acid is distilled off from the top of the column, water is separated from the distillate and recycled as the absorbing water to the collecting device, and in the meantime, a mixture composed mainly of acrylic acid and acetic acid and free from water and the azeotropic agent is obtained from the bottom of the column.

The objects of this invention will become apparent from the following detailed description.

Production of acrylic acid by the catalytic vapor-phase oxidation of propylene and/or acrolein using s multi-component catalyst based on molybdenum is being practiced industrially. In recent years, the techniques of performing the oxidation reaction and of collecting acrylic acid have made a striking advance, and the concentration of acrylic acid in the resulting aqueous solution can be increased to 50 to 80% by weight. This means that separation of water directly from the aqueous solution by distillation is more convenient than the conventional operation of separating acrylic acid and acetic acid from water by extraction of the aqueous acrylic acid solution with an organic solvent.If water alone can be efficiently separated from the aqueous acrylic acid solution and acrylic acid and acetic acid can be obtained substantially as a mixture, this mixture can be advantageously applied also to the conventional purification process sfterward. The present invention has been developed in order to achieve this purpose.

The azeotropic agent used in this invention is preferably a compound having a great ability to carry water.

Those which have a high boiling point in the range of 110 to 130"C at atmospheric pressure can be recommended. Specific examples include n-butyl acetate, isobutyl acetate, sec-butyl acetate and methyl isobutyl ketone.

These azeotropic agents have boiling points close to that of acetic acid. If they are simply subjected to distillation, acetic acid is also distilled off together with water. Specifically, the distillate is composed of the azeotropic agent, water and acetic acid, and a complex separating operation again becomes necessary.

The present inventors however found that distillation of acrylic acid can be inhibited by determining the amount of reflux so that the vapor at the top of the distillation column consists only of the azeotropic agent and water at the operating pressure, and the amount of acetic acid distilled can be limited to only a trace by setting the number of theoretical trays in the recovery section of the distillation column at 10 or more. It has thus been found that distillation bottoms composed mainly of acetic acid and acrylic acid and being free from the azeotropic agent and water can be taken out.

The present inventors found that in the above distillation operation, a tray on which the concentrating of acetic acid reaches a peak is separated from a tray on which the concentrating of water reaches a peak.

Presumably, this serves as an effective means of inhibiting the distillation of acetic acid. When the amount of acetic acid in the feed solution increases, there will be more trays on which acetic acid is concentrated increase and they become close to the tray on which water is concentrated, thus causing some distillation of acetic acid. Accordingly, as the concentration of acetic acid increases, the number of theoretical trays in the recovery section should be increased to about 15. This is because the concentration of acetic acid in the aqueous solution of acrylic acid changes to 2 to 8% by weight.

The process of this invention has led to the estabiishment of a distillation method which does not distill not only acrylic acid but also acetic acid using an azeotropic agent having a great ability to carry water in the distillation of an aqueous solution of acrylic acid containing acetic acid. In the prior art, the distilled water is required to be subjected to a treatment for rendering it pollution-free, for example by waste wafter treatment or an operation of separating and recoverying acetic acid. In contrast, in the process of this invention, the distillate, after recovering the azeotropic agent, is re-used as the absorbing water for collection of acrylic acid. Thus, the present invention has led to the accomplishment of a very effective pollution-free closed system process.

One embodiment of the present invention is further described below with reference to Figure 1.

A starting gas containing propylene, air, nitrogen, carbon dioxide gas and water vapor is passed through a first reactor filled with a molybdenum-base multi-component catalyst to oxidize propylene mainly to acrolein, and the reaction product gas is subsequently passed through a second reactor filled with a molybdenum-vanadium base multi-component catalyst to oxidise acrolein mainly to acrylic acid (not shown in the drawing). The reaction product gas contains acrolein, acetaldehyde, acetic acid, maleic acid, water vapor, carbon dioxide, carbon monoxide, nitrogen and hydrocarbons such as propane and propylene in addition to the desired acrylic acid.

The reaction product gas is introduced into an acrylic acid collecting device 101 through a line 1. In the collecting device 101, the gas is rapidly cooled and condensed. Most of the acrylic acid and acetic acid become an aqueous solution. The uncondensed gas is absorbed by cooled absorbing water containing a polymerization inhibitor and fed from a line 2 into the upper portion of the collecting device 101. Thus, the reaction product gas is taken out in the form of an aqueous solution of acrylic acid from the bottom of the collecting device 101 through a line 3.

The exhaust gas is discharged from the top of the acrylic acid collecting device 101 through a line 4. The exhaust gas contains non-condensabie carbon dioxide, carbon monoxide, propane, propylene and nitrogen and a small amount of steam but scarcely any acid. The exhaust gas can therefore be partly reused as a component of the reaction starting gas, and the remainder may be rendered pollution-free by a catalytic combustion device (not shown) or the iike and used as an inert gas in another part of the process.

The aqueous acrylic acid solution taken out from the line 3, after, as required, light-boiling substances such as acrolein and acetaldehyde have been recovered, is fed into an azeotropic dehydrating column 102. The azeotropic dehydrating column 102 has at least 15, preferably 20 to 30, theoretical trays, and the number of theoretical trays in the recovery section should be adjusted to at least 10. An azeotropic agent is fed into the top of the dehydrating column 102 through a line 5 so that the vapor distilled from the top of the column through a line 6 becomes nearly a water-azeotropic agent azeotrope.

From the bottom of the dehydrator 102 a distillation bottoms composed of acrylic acid, acetic acid and other high-boiling substances and being free from water and the azeotropic agent is taken out through a line 7. The distillation bottoms is then subjected to an acetic acid separating step and a step of rectifying acrylic acid to obtain a final product (not shown).

The liquid which has been distilled from the top of the dehydrator 102 is separated into an azeotropic agent layer and an aqueous layer in a separating tank 104. The aqueous layer is sent to an azeotropic agent recovering column 103 through a line 8, in which the azeotropic agent and a part of water are distilled. The distillate is sent through a line 9 to the separating tank 104 and recovered. On the other hand, water not containing the azeotropic agent is taken out from the bottom of the column 103 through a line 10, and is recycled to the acrylic acid collecting device for re-use. Thus, the occurrence of waste water can be substantially avoided. A process substantially free from the occurrence of waste water can be performed by recycling a part of the exhaust gas from the acrylic acid collecting device as a part of the starting gas for the first-stage reaction after adding a required inert gas (nitrogen, steam, etc.) thereto, discharging the remainder out of the system, and adjusting the amount of steam contained in the discharged waste gas at least to that of steam generated by the catalytic vapor-phase oxidation reaction.

The distillation bottoms composed substantially of acrylic acid and acetic acid is subsequently subjected to an operation of separating acetic acid and then to an operation of rectifying acrylic acid to obtain acrylic acid as a final product. This operation can be carried out industrially and stably for an extended period of time.

Example 1 Propylene was subjected to a two-stage oxidation reaction using a molybdenum-base multi-component catalyst and a molybdenum-vanadium base multi-component catalyst. The amount of the reaction product gas was 23 Nm3/hr and consisted of 5.9% by volume of acrylic acid, 0.2% by volume of acetic acid, 7.2% by volume of water and the remainder being mostly nitrogen with traces of acrolein, propylene, propane and acetaldehyde.

The reaction product gas was introduced into an acrylic acid collecting device 101 and cooled and collected as a condensate. The collecting device was a stainless steel column having an inside diameter of 200 nm and equipped with 50 bubble cap trays at its upper portion and a multitubular cooling section at its lower portion and designed to feed water containing hydroquinone as a polymerization inhibitor from the topmost part of the column. The reaction product was fed at 200"C, and cooled to 70"C at the cooling section.

Water at 600C was fed at a rate of 2.4 kg/hr to the absorbing section, thereby maintaining the ratio of collection of acrylic acid at more than 99%. The temperature of the exhaust gas leaving the collecting device 101 was adjusted to 60"C.

The amount of the aqueous solution of acrylic acid so collected was 7.66 kg/hr, and consisted of 66.0% by weight of acrylic acid, 2.2% by weight of acetic acid, 31.2% by weight of water and 6.0% by weight of impurities.

The aqueous solution of acrylic acid was fed to the 23rd tray from the top of an azeotropic dehydrating column 102 having 48 perforated trays without a downcomer (100 mm) (19 theoretical trays). Methyl isobutyl ketone (MIBK} was used as the azeotropic agent, and fed from the topmost portion of the column so that the overhead fraction at a temperature of 46"C and a column top pressure of 110 mmHg (absolute pressure) consisted of 21% by weight of water and 79% by weight of MIBK.

From the top of the column, hydroquinone was fed as a polymerization inhibitor in an amount of 0.1% by weight of the acrylic acid fed. From the bottom of the column, air was fed at a rate of 100 liters/hour. When the distillation became nearly steady, the temperature distribution of the components in the column was measured, and the results are shown in Table 1 below.

The distillation bottoms was obtained at a rate of 5.3 kg/hr, and consisted of 96.9% by weight of acrylic acid, 3.1% by weight of acetic acid and traces of MIBK and water.

The distillate from the top of the column was cooled and separated into an aqueous layer and an azeotropic agent layer in the separating tank 104. The aqueous layer was obtained at a rate of 2.4 kg/hr, and traces of acetic acid and acrylic acid were contained in the aqueous layer and MIBK was contained in an amount of 1.8% by weight. The aqueous layer was distilled under atmospheric pressure in an azeotropic agent recovering column 103 having a diameter of 50 mm and filled with Raschig rings to a height of 3 meters. From the top of the column, MIBK and water were distilled at a rate of 0.1 kg/hr. The resulting water containing MIBK was recycled to the separating tank 104. In the meantime, water free from MIBK was obtained at a rate of 2.3 kg/hr as the distillation bottoms.This water was fed to the collecting device 101 together with 0.1 kg/hr of water containing a fresh supply of the polymerization inhibitor.

The distillation bottoms of the azeotropic dehydrating column containing acrylic acid and was successively fed into an acetic acid separating column and an acrylic acid rectifying column to obtain acrylic acid as a final product.

TABLE 1 unit: wit %) Distil- 15th 27th 35th 43rd Substance late tray tray tray tray Bottoms Acrylic Trace 34.2 58.8 67.8 86.7 96.9 acid Acetic Trace 6.5 8.6 16.7 13.0 3.1 acid MIBK 1.8 14.3 0.2 0.1 Trace Trace Water 98.2 40.8 24.0 9.5 0.3 Trace Example 2 The same aqueous solution of acrylic acid as obtained in the collecting device 101 in Example 1 was fed into the 21st tray from the top of the same azeotropic dehydrating column 102 as used in Example 1. n-Butyl acetate as an azeotropic agent was fed from the top of the column so that the overhead fraction at a column top pressure of 130 mmHg (absolute) and a temperature of 50"C consisted of 28% of water and 72% of n-butyl acetate. Subsequently, the same distillation as in Example 1 was carried out.A composition composed of 3.0% by weight of acetic acid, 96.2% by weight of acrylic acid and traces of n-butyl acetate and water was obtained as distillation bottoms. The amount of acetic acid and acrylic acid in the distillate was 0.07% by weight. The aqueous layer of the distillate was distilled in the azeotropic agent recovering column 103 in the same way as in Example 1. Thus, n-butyl acetate was recovered and at the same time, water of the distillation bottoms was fed into the collecting device 101. The distillation bottoms of the azeotropic dehydrating column 102 containing acrylic acid and acetic acid was successively fed into an acetic acid separating column and an acrylic acid rectifying column to obtain acrylic acid as a final product.

Example 3 The same aqueous solution of acrylic acid as obtained in the collecting device 101 of Example 1 was fed into the 23rd tray from the top of the same azeotropic dehydrating column 102 as used in Example 1. As an azeotropic agent, isobutyl acetate was fed into the column from its top so that the overhead fraction consisted of 19.2% by weight of water and 80.8% by weight of isobutyl acetate. The same distillation treatment as in Example 1 was then carried out. A composition consisting of 3.1% by weight of acetic acid, 96.1% by weight of acrylic acid and traces of isobutyl acetate and water was obtained as the distillation bottoms. The aqueous layer of the distillation was distilled in the same azeotropic agent recovering column 103 as used in Example 1. Thus, isobutyl acetate was recovered, and water of the distillation bottoms was fed into the collecting device 101. The distillation bottoms of the azeotropic dehydrating column 102 containing acrylic acid and acetic acid was fed successively to an acetic acid separating column and an acrylic acid rectifying column to obtain acrylic acid as a final product.

Claims

1. A process for producing acrylic acid, which process comprises (i) catalytically oxidizing propylene and/or acrolein in the vapor phase with a gas containing molecular oxygen, (ii) rapidly cooling the reaction product gas thus obtained and collecting the condensate which forms, (iii) contacting the uncondensed gas with water, (iv) feeding the aqueous solution of acrylic acid composed mainly of acrylic acid, acetic acid and water collected as a result of steps (ii) and (iii) into a distillation column having at least 10 theoretical trays in its recovery section and distilling the said solution in the presence of an azeotropic agent, (v) distilling off from the top of the said column an azeotropic composition consisting substantially only of the azeotropic agent and water and being free from acrylic acid and acetic acid, (vi) recovering water from the distillate and recycling the recovered water for use in step (iii), and (vii) obtaining a mixture composed mainly of acrylic acid and acetic acid and free from water and the azeotropic agent from the bottom of the column.

2. A process according to claim 1 wherein the azeotropic agent has a boiling point of 110 to 1300C at atmospheric pressure.

3. A process according to ciaim 1 or 2 wherein the aqueous solution of acrylic acid fed to the distillation column in step (iv) contains 50 to 80% by weight of acrylic acid and 2 to 8% by weight of acetic acid.

4. A process according to any one of the preceding claims which further comprises: (viii) separating the acetic acid from the mixture obtained in step (vii).

5. A process according to any one of the preceding claims which further comprises: (ix) recovering the acrylic acid from the mixture obtained in step (vii).

6. A process for producing acrylic acid, said process being substantially as hereinbefore described in Example 1 or 2 or 3.

7. A process for producing acrylic acid, said process being substantially as hereinbefore described with reference to the Figure of the accompanying drawing.