FR2554809A1 - PROCESS FOR THE PREPARATION OF ACRYLIC ACID - Google Patents

Abstract

PROCESS FOR PREPARING ACRYLIC ACID BY CATALYTIC OXIDATION OF PROPYLENE OR ACROLEIN IN THE VAPOR PHASE BY MEANS OF A GAS CONTAINING MOLECULAR OXYGEN. THE GASEOUS PRODUCT PROVIDED BY THIS OXIDATION IS QUICKLY COOLED AND COLLECTED IN THE APPARATUS 101, THE SUBSTANCE NOT COLLECTED IN THIS WAY BEING ABSORBED BY THE WATER. THE AQUEOUS SOLUTION THUS OBTAINED, WHICH CONSTITUTES ESSENTIALLY ACRYLIC ACID, ACETIC ACID AND WATER, IS INTRODUCED IN A DISTILLATION COLUMN 102 WHERE IT IS DISTILLED IN THE PRESENCE OF AN AZEOTROPISM AGENT. FROM THE TOP OF THE COLUMN IS ESCAPED BY DISTILLATION AN AZEOTROPIC MIXTURE WHICH IS ESSENTIALLY CONSTITUTE OF AZEOTROPISM AND WATER AND WHICH IS FREE OF ACRYLIC ACID AND ACETIC ACID. THE WATER IS SEPARATED FROM THE DISTILLATE AND IS RETURNED AS ABSORPTION WATER IN THE APPARATUS 101. AT THE SAME TIME IS REMOVED FROM THE BOTTOM OF COLUMN 102, A MIXTURE ESSENTIALLY CONSISTING OF ACRYLIC ACID AND ACETIC ACID.

Description

S54809

  The present invention relates to a process for the preparation of acrylic acid from propylene and / or acrolein. More specifically, it relates to a process for the preparation of acrylic acid in which the propylene and / or the acrolein in the vapor phase are catalytically oxidized with a gas containing molecular oxygen, and the gaseous reaction product formed is rapidly cooled. and it is collected in the form of a condensate, the substances which have not been collected by absorbing them with water of absorption, the aqueous solution of acrylic acid obtained, which is essentially consisting of acrylic acid, acetic acid and water, in the presence of an azeotropic agent such that the water is distilled off essentially under the

  form of an azeotropic mixture, the water is separated from this mixture

  Azeotropic geometry, the separated water is recycled as absorbent water.

  tion and, having at the same time obtained the residue (distillate tails)

  a mixture consisting essentially of acrylic acid

  and acetic acid, the acrylic acid is separated from this

  distillation residue and purified.

  In a conventional process for the preparation of acrylic acid, propylene is catalytically oxidized and / or

  acrolein in the vapor phase is treated with an organic solvent.

  The aqueous solution of acrylic acid thus obtained is extracted to extract the acrylic acid and the latter is purified. In other words, an extraction operation is generally used to remove water from the aqueous solution (see, for example, US Patent 3,968,153). However, the spent liquor from the extraction step contains unrecovered acrylic acid as well as by-products which are organic acids (such as acetic acid, propionic acid and maleic acid), aldehydes (such as aroline and acetaldehyde) and polymers or condensation products of these compounds (especially

  tarry substances and substances with a high boiling point

  For example, a dimer of acrylic acid). Before rejecting the spent liquor, it is therefore necessary to make it lose its toxicity by a method using activated sludge or a combustion concentration method, treatment that requires high costs for the facilities and services it requires. When this spent liquor is directly used in the process as the absorption liquor for the acrylic acid collected the amount of acid in the process increases as there is in the spent liquor, and the yield of acrylic acid collected in the acrylic acid collector device is increased. If the spent gas leaving this device is recycled as an inert gas source for the starting gas the amount of acid in the starting gas increases and can accelerate the degradation of the catalytic activity during the oxidation reaction (see example the patent

US. 4,147,885).

  As a result of the technical progress made

  in recent years in the oxidation reaction mention-

  above and in the collection of the desired product, the aqueous solution of acrylic acid obtained may contain acrylic acid in a high concentration, including

  for example between 50 and 80% by weight.

  In the process-object of the invention, which is a process for the preparation of acrylic acid, an aqueous solution of acrylic acid of such high concentration is subjected to azeotropic distillation under conditions which

  rather than subject it to an operation of ex-

  pulling as was done in the prior art to remove water; the quantities of acrylic acid and acetic acid that come out

  from the top of the distillation column.

  The present invention thus provides a process for the preparation of acrylic acid by catalytic oxidation of propylene and / or acrolein in the vapor phase by means of a gas containing molecular oxygen, the process according to which: the gaseous reaction product supplied by the oxidation reaction is cooled rapidly to form a condensate; the substances which have not been collected are collected by absorbing them in the form of a condensate; absorption water, the aqueous solution of acrylic acid thus obtained, which essentially consists of acrylic acid, acetic acid and water, is introduced into a distillation column comprising at least ten theoretical plates in its recovery section, and distilled in the presence of an azeotropic agent, - at the top of the column is distilled off azeotropic mixture which consists essentially of the azeotropic agent and water and which cunt holds neither acrylic acid nor acetic acid, - the water is separated from the distillate and recycled as absorption water in the collecting device, and - at the same time a mixture is obtained at the bottom of the column which essentially consists of of acrylic acid and acetic acid and which is devoid of water and the azeotropic agent.

  The following detailed description will make a good

  take what are the objects of the present invention.

  The preparation of acrylic acid by catalytic vapor phase oxidation of propylene and / or acrolein

  in the presence of a complex catalyst (multi-component

  molybdenum-based) is applied industrially.

  In recent years the execution techniques

  of the oxidation reaction and the collection of acrylic acid

  that remarkable progress has been made, and the acrylic acid concentration of the aqueous solution obtained can be increased to a value of between 50 and 80% by weight. This means that the direct removal of water from the aqueous solution by distillation is better than the usual method of separating acrylic acid and acetic acid from water by extraction of the aqueous solution. acrylic acid with an organic solvent. If water alone can be effectively removed from the aqueous acrylic acid solution and acrylic acid and acetic acid can be obtained essentially in the form of a mixture, this mixture can advantageously also be subjected to the usual purification method. The present invention has been developed in order to achieve this goal. The azeotropic agent used in the present invention is preferably a compound having a high ability to convey water. It is recommended to use those having a high boiling point, between 110 and C under atmospheric pressure. As examples of such

  compounds which may be mentioned are n-butyl acetate,

  butyl, sec-butyl acetate and methyl isobutyl ketone.

  These azeotropic agents have boiling points

  very close to that of acetic acid. If they are simply distilled, acetic acid also distils with water. More precisely, the distillate is

  consisting of the agent of azeotropism, water and acetic acid

  that we are again faced with the need to

  kill a complete separation operation.

  The present inventors, however, have found that the distillation of acetic acid can be inhibited by controlling the amount of reflux such that the vapor at the top of the distillation column is formed.

  that azeotropic agent and water at the operating pressure

  the amount of acetic acid escaping by distillation by fixing the number of theoretical plates in the recovery section of the distillation column has a value at least equal to 10. They have thus found that the distillation residue can be removed

  that is, the product of the bottom of the column, which is essential

  consisting of acetic acid and acrylic acid and

  is devoid of the azeotropic agent and water.

25548O

  The present inventors have found that in the distillation operation mentioned above, a plateau on which the concentration of acetic acid reaches a maximum is separated by a plateau on which the concentration of water reaches a maximum of maximum. We can see there a way

  effective in preventing the distillation of acetic acid.

  When the quantity of acetic acid in the solution introduced increases the number of plates on which the acid

  acetic acid is concentrated increases and is closer to the

  water on which the water is concentrated, resulting in some distillation of acetic acid. Therefore, as the concentration of acetic acid increases, the number of theoretical plates in the recovery section should be increased to about 15, because the acetic acid concentration of the aqueous solution of acrylic acid undergoes a modification

from 2 to 8% by weight.

  The process of the present invention has led to the development of a distillation method which does not remove acrylic acid or acetic acid and which uses azeotropic gent having a great ability to convey water. during the distillation of an aqueous solution of acrylic acid containing acetic acid. In the prior art the water from the distillation must be subjected to a treatment rendering it non-polluting, for example to a treatment of waste water or to an operation of separation and recovery of the acid.

  acetic. In contrast, in the process of the present invention

  tion, the distillate, after recovery of the azo

  tropism, is reused as an absorption water for the col-

  lect of acrylic acid. Thus the present invention has led to the development of a closed system process,

very efficient and non-polluting.

  One embodiment of the present invention is

  described below with reference to the accompanying drawing.

  In order to oxidize the propylene and thus to convert it mainly to acrolein, a starting gas containing propylene, air, nitrogen, carbon dioxide and water vapor is passed through a first reactor packed with a molybdenum-based multicomponent catalyst, and the gaseous reaction product is then passed through a second reactor packed with a molybdenum-based multi-component catalyst to oxidize acrolein primarily to acid acrylic (operations not shown in the drawing). The gaseous reaction product

  contains acrolein, acetaldehyde, acetic acid

  that maleic acid, water vapor, carbon dioxide,

  carbon monoxide, carbon monoxide, nitrogen and hydro-

  carbides such as propane and propylene, in addition to

the desired acrylic acid.

  The gaseous reaction product is introduced, via a line 1, into an acid collecting device

  acrylic 101, where it is rapidly cooled and condensed.

  Most of the acrylic acid and acetic acid gives an aqueous solution. The uncondensed gas is absorbed by cooled absorption water which contains a polymerization inhibitor and which is introduced via line 2 into the upper part of the device.

  In this way, the gaseous reaction product is removed.

  re in the form of an aqueous solution of acrylic acid,

  at the bottom of the collecting device 101, by a pipe 3.

  The exhaust gas is exhausted by a pipe

  4, from the top of the acrylic acid collector device 101.

  It contains non-condensable carbon dioxide, carbon monoxide, propane, propylene, and nitrogen, as well as a small amount of steam but barely contains acid. It can therefore be reused in part as constituent of the starting reaction gas, the remainder can be made non-polluting by means of a catalytic combustion apparatus (not shown) or of another apparatus of the same kind, and be used as gas inert in another -7-

part of the process.

  The aqueous solution of acrylic acid withdrawn from line 3, after being removed, as necessary, the low-boiling compounds, such as acrolein and acetaldehyde, is introduced into a Azeotropic dehydration column. It has at least 15 theoretical plateaus, preferably 20 to 30, and

  the number of theoretical plateaus in the recovery section

  The azeotropic agent is introduced through a line 5 at the top of the dewatering column 102 so that the vapor which escapes from the top of the column by a pipe 6, become almost an azeotropic mixture

  of water and the agent of azeotropism.

  From the bottom of the dehydration column 102 on

  by a line 7, a distillate residue

  which consists of acrylic acid

  that, acetic acid and other compounds with high points

  boiling. and which is devoid of water and the azotic agent

  tropism. This distillation residue (bottoms product) is then subjected to a step of separating acetic acid at a step of rectification of the acrylic acid so as to obtain a final product (operations).

not represented).

  The liquid which has escaped by distillation from the top of the dehydration column 102 is separated, in

  a separating container 104, in a layer of an azeotropic agent

  and an aqueous layer. The latter is sent, via line 8, to a recovery column of the azeotropic agent 103 where the azeotropic agent and a part of

  water are distilled. The distillate is sent by a con -.-

  9, in a separation vessel 104 and is recovered.

  On the other hand, the water which does not contain azeotropic agent is removed from the bottom of the column 103 by a pipe 10 and

  is returned for reuse in the device

  acrylic acid reader. The formation of wastewater is therefore largely avoided. The process can be carried out substantially without the formation of wastewater by recycling a portion of the exhaust gas exiting the acrylic acid collecting device as part of the starting gas for the reaction of the first stage, after having added thereto an inert gas if necessary (nitrogen, steam, etc.), evacuating the remainder of the gas from the system by adjusting the amount of vapor contained in the exhaust gas exhausted to a value at least equal to that of the vapor generated by the catalytic vapor phase oxidation reaction.

  The bottoms of distillation, essentially

  of acrylic acid and acetic acid, are then subjected to an operation of separation of acetic acid, then to a rectification operation of acrylic acid

  affecting the final product, that is acrylic acid.

  This operation can be carried out industrially and

stay stable for a long time.

Example 1

  Propylene is subjected to two-step oxidation using a molybdenum-based multi-component catalyst and a molybdenum-vanadium multi-component catalyst. The gas generated by the reaction produced at a flow rate of 23 m3 (normal conditions) per hour and it contains 5.9% by volume of acrylic acid, 0.2% by volume of acetic acid, 7.2% volume of water, the remainder consisting essentially of nitrogen as well as

  traces of acrolein, propylene, propane and acetal-

  The gaseous reaction product is introduced into

  an acrylic acid collecting device 101-o il = est-

  cooled and collected in the form of a condensate. The collector device is a stainless steel column, 200 millimeters inside diameter, which is equipped with 50 bubble trays at its upper part and a

  multitubular cooling section at its lower

  Higher; said column is designed to be

  fed from the top with water containing hydro-

  quinone as a polymerization inhibitor. The reaction product is introduced at 2000C and is cooled to 70 ° C. in the cooling section. In the absorption section, water is introduced at 60 ° C. at a rate of 2.4 kg / h.

  and thus maintains the ratio of acidic acid

  more than 99%. The temperature of the exhaust gas

  which leaves the collector device 101 is set at 60 C.

  The amount of the aqueous solution of acrylic acid

  The amount collected is 7.66 kg / h. This solution consists of 66.0% by weight of acrylic acid, 2.2% by weight of acetic acid, 31.2% by weight of water

and 0.6% by weight of impurities.

  The aqueous solution of acrylic acid is introduced into an azeotropic dehydration column 102 at the twenty-third plateau from the top, which column comprises 48 perforated trays without a downcomer (100 mm) (19 theoretical trays). We introduce

  the azeotropic agent, which in this case is methyl-

  isobutyl-ketone (MIBC), at the top of the column such that the top fraction, which is at a temperature of 46eC and a column pressure of 146 mbar (absolute pressure), is constituted of 21% by weight of water and

79% by weight of MIBC.

  At the top of the column is introduced hydro-

  quinone as a polymerization inhibitor in a quantity

  0.1% by weight relative to the acrylic acid introduced.

  At the bottom of the column is introduced air at a rate of 100 liters per hour. When the distillation has reached its state of regime, the temperature distribution of the components in the column is determined: the results obtained are

  recorded in Table 1 (see below).

  The bottoms are obtained at a flow rate of 5.3 kg / h and consist of 96.9% by weight of acrylic acid, 3.1% by weight of acetic acid and traces of MIBC. and water. The distillate leaving the top of the column is cooled and separated, in the separator vessel

  104, in an aqueous layer and a layer of azo agent

  tropism. The aqueous layer, which is obtained at a flow rate of 2.4 kg / h, contains traces of acetic acid and acrylic acid and 1.8% by weight of MIBC. It is distilled under atmospheric pressure in a recovery column of the azeotropic agent 103 which has a diameter of 50 mm and which is packed with Raschig rings up to a height of 3 meters. The methyl isobutyl ketone and the water escape by distillation from the top of the column at a flow rate of 0.1 kg / h. MIBC-laden water is thus obtained, which is returned to the separator vessel 104. At the same time, MIBC-free water is obtained at a rate of 2.3 kg / h as distillation bottoms. This water is fed to the collecting device 101 with 0.1 kg / h of water containing a

  fresh intake of polymerization inhibitor.

  The tails of distillation of the azeotropic dehydration column,

  which contain acrylic acid, are introduced suc-

  in a separation column of acetic acid.

  than and in a rectification column of acrylic acid so that acrylic acid is obtained as

final product.

TABLE 1

  (unit:% by weight) Compound Distillate 15th 27th 35th 43rd Tails Tray Tray Tray Tray Acid Trace 34.2 58.8 67.8 86.7 96.9 Acrylic Trace Acid 6.5 8.6 16.7 13, 0 3.1 acetic acid MIBC 1.8 14.3 0.2 0, 1 Trace Trace Water 98.2 40.8 24.0 9.5 0.3 Trace -11-

Example 2

  An aqueous solution of acrylic acid identical to that obtained in the collector device 101 of Example 1 is introduced into the twenty-first plate: from the top of the same column of azeotropic dehydration 102 which is Example 1 is used. As the azeotropic agent n-butyl acetate is introduced at the top of the column such that the top fraction, which is under a top pressure

  column pressure of 173 mbar (absolute pressure) and at a temperature of

  50 C, consisting of 28% water and 72%

  n-butyl acetate. The same distillery is then

  In Example 1, a mixture consisting of 3.0% by weight of acetic acid, 96.2% by weight of acrylic acid and traces of sodium acetate was obtained as distillation bottoms. n-butyl and water. The amount of acetic acid

  and acrylic acid in the distillate is 0.07% by weight.

  The aqueous layer of the distillate is distilled, of the same -

  as in Example 1, in the recovery column of the azeotropic agent 103. The n-butyl acetate is thus recovered and, at the same time, the water of the distillation bottoms is introduced into the collector device 101. The distillation bottoms of the azeotropic dehydration column 102, which contain acrylic acid and acetic acid, are successively sent to a column of

  separation of acetic acid and in a rectification column

  fication of acrylic acid: one thus obtains the acid

acrylic as the final product.

Example 3

  An aqueous solution of acrylic acid identical to that obtained in the collecting device 101

  example 1 is introduced in the twenty-third

  water from the top of the same dehydration column

  azeotropic 102 than that used in Example 1.

  As a zeotropic agent, isoacetate acetate is introduced

  butyl at the top of the column such that the top fraction consists of 19.2% by weight of water and 80.8% by weight of isobutyl acetate. The same distillation treatment is then carried out as in Example 1. As the distillation bottoms, a mixture consisting of 3.1% by weight of acetic acid, 96.1% by weight of acrylic acid and traces of isobutyl acetate and water. Layer

  the distillate is distilled in the same

  no recovery of the azeotropic agent 103 than that

  used in Example 1. This recovers

  isobutyl ester; the water of the distillation bottoms is introduced into the collecting device 101. The distillation bottoms of the azeotropic dehydration column 102, which contain acrylic acid and acetic acid, are introduced successively into a separation column of acetic acid and in a rectification column of acrylic acid so that one

  gets acrylic acid as the final product.

-13-

Claims (3)

  A process for the preparation of acrylic acid by catalytic oxidation of propylene and / or acrolein in the vapor phase by means of a gas containing molecular oxygen, the process according to which: - the gaseous reaction product is cooled rapidly supplied by the oxidation reaction so as to obtain it in the form of a condensate,   - substances that have not been collected are collected   bound by absorbing them with absorption water, the aqueous solution of acrylic acid thus obtained, which consists essentially of acrylic acid, acetic acid and water, is introduced into a distillation column. comprising at least ten theoretical plates in its recovery section, and is distilled in the presence of a azeotropic agent, - at the top of the column is distilled off an azeotropic mixture which consists essentially of the agent of azeotropism and water and which contains neither acrylic acid nor acetic acid, - the water is separated from the distillate and recycled as absorption water in the collecting device, and - at the same time we obtain, at the bottom of the column, a mixture which consists essentially of acrylic acid and acetic acid and which is devoid of water and the azeotropic agent. 2. Method according to claim 1 characterized in that the azeotropic agent has a boiling point of   at 130 ° C under atmospheric pressure.   3. Method according to one of claims 1 and 2,   characterized in that the aqueous solution of acrylic acid to be introduced into the distillation column contains: from 50 to 80% by weight of acrylic acid and from 2 to 8% by weight of acetic acid.