JPH03176439A - Recovering of isobutane and methacrolein - Google Patents

Abstract

PURPOSE:To simultaneously recycle both of isobutane and methacrolein by separating methacrylic acid by means of cooling the condensed vapor-phase catalytically oxidized gas of isobutane, bringing a gas containing unreacted isobutane and generated methacrolein into contact with an organic solvent and distilling the resultant absorbed solution. CONSTITUTION:Steam and supplementary isobutane are supplied from a line 6. Air, a large portion of isobutane and methacrolein are recycled from a distilling tower 5 through a line 15, then methacrylic acid and methacrolein are generated in a reactor 1. Methacrylic acid and high boiling matter are extracted from a line 8 in a quenching tower 2 and gas is introduced into an absorbing tower 3, then brought into contact with an organic solvent (e.g. n- paraffin) supplied from a line 13, thus absorbed and absorbed solution is pre- heated in a heat exchanger 4, then introduced into the distilling tower 5. Distillation is performed in the tower and isobutane and methacrolein are recovered, then recycled from the line 15. Organic solvent is recycled from lines 12 and 13.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention provides a method for producing methacrylic acid and/or methacrolein in one step by catalytically oxidizing isobutane in a gas phase.
The present invention relates to an economical method with excellent operational stability for recovering unreacted isobutane and produced methacrolein from the reaction gas.

(Prior art) In the past, saturated hydrocarbons such as isobutane were considered to be inert gases, but in recent years, isobutane has been processed at high temperatures (300°C) in the presence of a catalyst without passing through isobutylene.
A method of directly producing methacrolein or methacrylic acid through catalytic oxidation at temperatures of ~500°C has been studied and is attracting attention. For example, British Patent No. 1340891, JP-A-55-62041, JP-A-62-132
This method is disclosed in Japanese Patent Application Laid-open No. 832, Japanese Patent Laid-Open No. 145249/1983, and the like. These methods use specific catalysts to directly convert isobutane, which has poor reactivity, into methacrolein or methacrylic acid, but in all of these methods, the one-pass conversion rate of isobutane is extremely low at 10% or less. Therefore, we took advantage of the fact that the conversion rate did not change even if the concentration of isobutane was increased, and reacted with a high concentration of isobutane.

By increasing the product concentration in the reaction gas, the productivity of the catalyst has been improved to a practical level. However, since most of the isobutane remains unreacted, unreacted isobutane must be recovered at a low cost, especially for industrial implementation of water droplets. On the other hand, since methacrolein can be converted into methacrylic acid by further oxidation, it is necessary to recover the produced methacrolein at low cost and recycle it to the reactor.

By the way, isobutane is a type of liquefied gas.

Although it is possible to easily compress and liquefy it and separate it from nitrogen gas, if the amount of nitrogen gas, which is a non-condensable gas component, is large, the loss of isobutane gas entrained therein will be large. At this time, if an attempt is made to reduce the loss of isobutane to an industrially practicable range, the power required for compression and liquefaction or the cost of cooling become extremely large, making it impractical. Moreover, depending on the oxygen concentration in the reaction gas, there is a risk that the gas composition will temporarily become explosive during the process in which the isobutane gas concentration decreases due to compression and liquefaction of isobutane, which poses a problem in industrial implementation. Therefore, the method of recovering unreacted isobutane and produced methacrolein by compression liquefaction has many practical problems from the viewpoint of economy and safety, and cannot be said to be a preferable method.

Various methods for recovering methacrolein have been proposed so far, such as Japanese Patent Publication No. 48-23409;
Japanese Patent Publication No. 55-100334, Japanese Patent Publication No. 60-416
54, Japanese Patent Publication No. 60-59891, and the like. All of these are isobutyne or tertiary-butanol (hereinafter referred to as isobutynes).

This method uses a solvent to absorb and recover the methacrolein produced during the gas phase catalytic oxidation of (2), and does not attempt to recover unreacted isobutins as well. From isobutins,
Catalytic oxidation to methacrylic acid is a two-stage reaction, and unreacted isobutyne is not absorbed in a solvent, but rather recycled to the first stage reactor together with accompanying gas, and methacrolein is extracted from the absorbed liquid by methods such as distillation or stripping. It was necessary to separate and recover isobutyne and methacrolein in order to recover them and recycle them to the second stage reactor.

However, when isobutane is subjected to gas phase catalytic oxidation, both unreacted isobutane and methacrolein produced must be recycled to the same reactor. No recovery method was proposed.

(Problems to be Solved by the Invention) Therefore, an object of the present invention is to produce methacrylic acid and/or methacrolein in one step by catalytically oxidizing isobutane in the gas phase, from the gas phase catalytic oxidation reaction gas. It is an object of the present invention to provide an economical recovery method for recovering reacted isobutane and produced methacrolein, which is also excellent in operational stability.

(Means for Solving the Problems) In order to solve the above 111 problems, the present inventors have conducted extensive research and found that isobutane and methacrolein are simultaneously absorbed and separated by bringing a certain organic solvent into contact with the reaction gas. death,
It has been discovered that this objective can be achieved by recovering isobutane and methacrolein from the obtained absorption liquid by distillation, and the present invention has been completed.

That is, 1. When recovering unreacted isobutane and generated methacrolein from a reaction gas obtained by gas-phase catalytic oxidation of isobutane, the reaction gas is brought into contact with an organic solvent and isobutane and methacrolein are simultaneously absorbed and separated. This is a method for recovering isobutane and methacrolein, which is characterized in that the obtained absorption liquid is subjected to a distillation treatment, and the obtained isobutane and methacrolein are recycled to a reactor after being separated from the organic solvent.

The gist of the present invention is to cool the reaction gas obtained by the gas phase catalytic oxidation, and to contact the gas containing unreacted isobutane and produced methacrolein after condensing and separating methacrylic acid with an organic solvent, The purpose is to simultaneously absorb and separate methacrolein and recover isobutane and methacrolein from the resulting absorption liquid by distillation. Therefore, compared to the case where isobutane and methacrolein are recovered separately, the process is shortened and one type of absorption solvent can be used. Furthermore, by manipulating the distillation conditions, the amount of organic solvent in the recovered isobutane and methacrolein can be easily reduced.

Furthermore, fluctuations in the amount of isobutane and methacrolein fed to the reactor have a considerable effect on the reaction results, but in the stripping method, fluctuations in the concentration of isobutane and methacrolein in the absorption liquid, air supply amount, stripping tower temperature, and pressure Therefore, the amounts of isobutane and methacrolein supplied to the reactor are likely to fluctuate. On the other hand, with the distillation method, it is possible to temporarily store the recovered isobutane and methacrolein, and the supply amount of isobutane and methacrolein in the form of gas or liquid can be controlled with precision, making it a method with excellent operational stability. It is.

Therefore, according to the method of the present invention: (1) Since isobutane and methacrolein are recovered at the same time, the process is shortened and one type of absorption solvent can be used. (2) Even when the one-pass conversion rate of isobutane is as low as about 10%, it is possible to provide an extremely economical and practical method for recovering isobutane and methacrolein. (3) The recovered isobutane and methacrolein can be supplied to the reactor with high precision, making it a process with excellent operational stability.

Hereinafter, the present invention will be roughly explained in detail.

When isobutane is catalytically oxidized in the vapor phase with a molecular oxygen-containing gas (mostly air) in the presence of water vapor, in addition to the target products methacrylic acid and methacrolein, acetic acid, carbon oxide, carbon dioxide, unreacted isobutane, and unreacted are produced. A high temperature reaction gas containing oxygen, nitrogen, etc. is obtained. This reaction gas may be brought into contact with the organic solvent as it is, but the reaction gas may be cooled or brought into contact with water to be separated into a gas phase and a liquid phase by an appropriate method, as is usually done. . It is preferable that the methacrylic acid, high-boiling products, and water vapor produced in the reaction be lowered in advance from the gas phase by this cooling step, and then brought into contact with the organic solvent. It is more preferable that the reaction gas having a boiling point higher than that of the organic solvent used in the present invention is substantially completely removed in the cooling step. The gas phase thus obtained contains unreacted isobutane, methacrolein, acetone, nitrogen, oxygen, carbon oxides, carbon dioxide and water vapor, etc. This gas phase is passed to an absorption step and brought into contact with an organic solvent. As a result, a gaseous body from which unreacted isobutane and produced methacrolein have been almost completely separated is obtained from the top of the column, and an absorption liquid which has absorbed isobutane, methacrolein, and some oxidized by-products from the bottom of the column is obtained from the column bottom. I can do it.

Hydrocarbons, halogenated hydrocarbons, ketones, ethers, etc. can be used as organic solvents for absorption, but the selection of the absorption solvent is extremely important, considering economic efficiency, safety, and organic solvent content. It must be decided taking into account stability and other factors. In particular, ketones and ethers tend to generate peroxides, which can cause radical polymerization.
If a substance that is extremely easily polymerized, such as methacrolein, remains in the reaction gas, it will not be a preferred organic solvent. Preferred are hydrocarbons, especially aliphatic hydrocarbons, aliphatic cyclic hydrocarbons, and/or aromatic hydrocarbons having 6 to 20 carbon atoms, more preferably 7 to 18 carbon atoms. aliphatic hydrocarbons, aliphatic cyclic hydrocarbons and/or aromatic hydrocarbons.

When using an organic solvent having 5 or less carbon atoms, the relative volatility of methacrolein and the organic solvent is low, and when isobutane and methacrolein are recovered by distillation.

It requires a large number of distillation column plates and a large amount of thermal energy. In addition, when isobutane and methacrolein are absorbed by the organic solvent, economic efficiency may deteriorate due to the loss of the organic solvent, and feeding the organic solvent to the reactor may have an adverse effect on the reaction. On the other hand, if the number of carbon atoms exceeds 20, the viscosity becomes high and the absorption efficiency becomes extremely poor at the operating temperature of a conventional absorption tower, so that it cannot be used. Suitable organic solvents include, for example, hexane, heptane, octane, isooctane, dimethylcyclohexane, trimethylhebutane, decane, dodecane, tridecane, tetradecane, pentadecane, hexadecane, heptadecane, octadecane, eicosane, cyclohexane, benzene, toluene, xylene, Ethylbenzene, butylbenzene, cymene, durene, methylnaphthalene, ethylnaphthalene, propylnaphthalene,
Examples include, but are not limited to, dimethylnaphthalene and diphenyl. Each of these can be used alone or in the form of a mixture. Further, the organic solvent in which high-boiling components such as oxidation reaction by-products have accumulated can be purified by continuously or intermittently extracting it and distilling it.

The method of contacting the reaction gas containing isobutane and methacrolein with the above organic solvent may be a countercurrent method or a cocurrent method, and may also be a two-stage or multistage method. Contact is more preferred. The absorption tower can be of any type, such as a packed tower, perforated plate tower, bubble bell tower, or spray tower.

The amount of organic solvent supplied varies depending on the absorption tower operating temperature, operating pressure, gas amount, type of organic solvent, etc., so it is difficult to define it unambiguously, but methacrolein is small compared to isobutane. Generally, it is sufficient to focus on isobutane in the reaction gas and set the amount of organic solvent at 0.5% per mole of unreacted isobutane.
It is preferable to supply 100 moles, preferably 1 to 620 moles. By supplying an appropriate amount of organic solvent in this manner, it is possible to recover substantially the entire amount of unreacted isobutane and produced methacrolein.

The operating temperature of the absorption tower can be controlled by installing a cooler inside or outside the absorption tower, preferably from -20°C to 100°C, more preferably from 20°C to 50°C.
You can choose from a range of 'C. The operating pressure should not be set solely from the optimization conditions of the absorption tower alone, but in order to improve the absorption efficiency of organic solvents, it should be operated under pressure and the operating pressure should be within the range of about 1 to 10 kg/cm2G. It is preferable to choose.

In addition, when operating the absorption tower under pressure, it is also possible to operate by increasing the operating pressure of all equipment such as the reactor before the absorption tower by the amount that increases the operating pressure of the absorption tower.

Isobutane and methacrolein absorption liquid.

It is then fed to a distillation column to separate isobutane and methacrolein from the solvent. At that time, a condenser is installed at the top of the distillation column to apply reflux to improve the separation of components, and isobutane and methacrolein can be recovered as a liquid. If organic solvents are used, installing a partial condenser and recycling most of the isobutane and methacrolein as gas to the reactor is an energetically advantageous method. The presence of organic solvent components in the isobutane and methacrolein recycled to the reactor leads to organic solvent losses. In addition, since it may have an adverse effect on one reaction, the amount of solvent in the gas and/or liquid obtained from the top of the column should be 1 volume or less, more preferably 100 volume p.
Keep it below pm.

On the other hand, the organic solvent obtained from the bottom of the column, from which almost all the isobutane and most of the methacrolein have been separated, is used again as an absorption solvent in the absorption column.

At this time, if the operating temperature at the bottom of the distillation column is low, isobutane and methacrolein will not be cut properly.

When this solvent is recycled to the absorption tower, losses of isobutane and methacrolein increase at the top of the absorption tower, and the absorption efficiency decreases.

For this reason, the isobutane concentration in the solvent extracted from the bottom of the column is 1% by weight or less, more preferably 100% by weight or less, and the methacrolein concentration is 1% by weight or less, more preferably 0.1% by weight or less. Set the bottom temperature to .

The distillation column can also be operated under pressure, in which case the absorption column and possibly the reactor can also be operated simultaneously under pressure, and the entire process can also be operated under pressure throughout. can.

As an example of a preferred embodiment of the present invention, a flow sheet of a methacrylic acid manufacturing process is shown in FIG. 1, and will be described below accordingly.

Most of the isobutane and methacrolein, which are reaction raw materials, are recycled to the reactor 1 from the top of the distillation column 5 through a line 15. Line 6 supplies air, steam and make-up isobutane, which is oxidized in reactor 1 primarily to methacrylic acid and methacrolein. The generated reaction gas containing methacrylic acid passes through line 7 and enters quenching bath 2, where methacrylic acid and high-boiling substances are condensed and extracted through line 8. On the other hand, the unreacted isobutane and methacrolein-containing gas is led to the absorption tower 3 through line 9, where it is brought into contact with the organic solvent supplied from line 13, and an isobutane and methacrolein absorption liquid is obtained from line 10. It will be done. The exhaust gas then passes through line 11 and is treated in a gas incinerator before being discharged into the atmosphere. After the isobutane and methacrolein absorption liquid is preheated in the heat exchanger 4, it passes through the line 14 and enters the distillation column 5, where the isobutane and methacrolein in the absorption liquid are recovered by distillation. The recovered isobutane and methacrolein pass through line 15 to reactor 1.
will be recycled. The organic solvent extracted from the line 12 is cooled in the heat exchanger 4 and then passed through the line 3 and recycled to the absorption tower 3.

Next, the present invention will be explained more clearly with reference to Examples.

(Example) Example 1 After introducing the reaction gas generated in the gas phase catalytic oxidation reaction of isobutane to a quenching bath and removing methacrylic acid and high boiling point substances,
The following gas composition was obtained.

That is, 36.5 mol 2 of isobutane, 0.4 mol 2 of methacrolein, 7.5 mol % of oxygen, and 55.6 mol 2 of nitrogen, carbon oxide, carbon dioxide, acetone, and steam. This gas was supplied at a flow rate of 8.1Nl/min to a tube with an inner diameter of 1.5B and a length of 150cm (Dixo
n Packing (diameter 3 mm, length 4I). ) from the bottom of the absorption tower, and n-paraffin was fed from the top at a rate of 4.5 kg/hr. This n-paraffin was a mixture of decane, undecane, and dodecane, and the contents were 22% by weight, 52% by weight, and 26% by weight, respectively. When a refrigerant at -12°C was placed in the outer tube of the absorption tower and operated at normal pressure, almost the entire amount of methacrolein and 99.5% of unreacted isobutane could be recovered. The suction 181 liquid thus obtained was packed into a tube with an inner diameter of 3B and a length of 100cm (Dixon Packin as a filling material).
g (diameter 3 m+a, length 4 m+a) was filled. ) was supplied to the middle stage of the distillation column at a rate of 4.9 kg/hr, and 6 kg/cm2G of steam was supplied to the outer tube at the bottom of the distillation column, and the distillation column was operated at normal pressure. As a result, 92% of the methacrolein in the absorption liquid was
and 99.9% of isobutane could be recovered. Isobutane and methacrolein discharged from the top of the distillation column were supplied to the reactor and subjected to a gas phase catalytic oxidation reaction together with newly supplied isobutane. Note that the n-paraffin accompanying isobutane and methacrolein is 1
00 weight ρρth or less.

On the other hand, n-paraffin containing a small amount of methacrolein and a very small amount of isobutane was obtained from the bottom of the tower, and this liquid was used again as an absorption liquid in the absorption tower.

Example 2 Gas phase catalytic oxidation reaction of isobutane at 2.4 kg/cI11
The reaction was carried out under a pressure of 2 G, and the resulting reaction gas was introduced into a quenching bath in the same manner as in the example above to remove methacrylic acid and high-boiling substances, and the following gas composition was obtained.

That is, 37.7 mol% of isobutane, 0.4 mol% of methacrolein, 5.7 mol% of oxygen, and 56% of other nitrogen, carbon oxide, carbon dioxide, acetone, and water vapor. Hle%
Met. 8.2 This gas was transferred to the same absorption tower as in Example 1.
N1/min was fed, and xylene was supplied at 3.0 kg/hr. 2. Circulate cold water at 5°C through the jacket tube of the absorption tower.
When operated at 0 kg/cc+2G, almost the entire amount of methacrolein and 99.7% of unreacted isobutane could be recovered.

The absorption liquid thus obtained was sent to the same distillation column as in Example 1 at a rate of 3.4 kg/hr, and was transferred to the jacket tube at the bottom of the distillation column at a rate of 6.4 kg/hr.
kg/ca2G of steam is supplied, and the pressure at the top of the distillation column is increased to 1.
．． When operated at 4 kg/c'G, 90% of methacrolein and 99.8% of isobutane in the absorption liquid could be recovered. Isobutane and methacrolein discharged from the top of the distillation column were supplied to a reactor and subjected to reaction in the same manner as in Example 1. Note that the amount of xylene entrained in isobutane and methacrolein was 100 ppm by weight or less. On the other hand, xylene containing a small amount of methacrolein and a very small amount of isobutane was obtained from the bottom of the tower, and this liquid was used again as an absorption liquid in the absorption tower.

Example 3 A gas containing 34.3 mol 2 of isobutane and 0.4 mol
4.0k of dimethylcyclohexane from the top.
When it was fed and absorbed at a rate of g/hr, almost the entire amount of methacrolein and 99.8% of unreacted isobutane could be recovered. The absorption liquid thus obtained was transferred to a distillation column controlled at the same operating temperature and pressure as in Example 2.
．． When supplied at a rate of 4 kg/hr, 92% of methacrolein and 99.9% of isobutane in the absorption liquid could be recovered. Note that dimethylcyclohexane entrained in isobutane and methacrolein weighs 100 tons.
It was less than ppm.

(Effects of the Invention) According to the method of the present invention: (1) Since isobutane and methacrolein are recovered at the same time, the process is shortened and one type of absorption solvent can be used.

(2) One pass conversion rate of isobutane is 10
%, it is possible to provide an extremely economical and practical method for recovering isobutane and methacrolein.
(3) The recovered isobutane and methacrolein can be supplied to the reactor with high precision, making it a process with excellent operational stability.

[Brief explanation of drawings]

FIG. 1 is a flow sheet showing a preferred embodiment of the present invention. In the figure, 1 is a reactor, 2 is a recovery column, 3 is an absorption column, 4 is a heat exchanger, and 5 is a distillation column.

Claims (1)

[Claims] 1. When recovering unreacted isobutane and methacrolein produced from the reaction gas obtained by gas-phase catalytic oxidation of isobutane, isobutane and methacrolein are separated by contacting the reaction gas with an organic solvent. A method for recovering isobutane and methacrolein, which comprises simultaneously performing absorption separation, distilling the obtained absorption liquid, and recycling the obtained isobutane and methacrolein to a reactor after separation from an organic solvent. 2. The method according to claim 1, wherein an aliphatic hydrocarbon, an aliphatic cyclic hydrocarbon, and/or an aromatic hydrocarbon having 6 to 20 carbon atoms is used as the organic solvent.