JPH0343259B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention uses sulfuric acid as a catalyst to convert unsaturated carboxylic acids (hereinafter simply referred to as "carboxylic acids") into lower alcohols (hereinafter simply referred to as "alcohols").
Unsaturated carboxylic acid ester (hereinafter simply referred to as "ester") produced by esterification reaction with
The present invention relates to a method for recovering carboxylic acid from waste sulfuric acid generated in the production of sulfuric acid.

It is well known that sulfuric acid is used as a catalyst in the esterification reaction of carboxylic acids. Furthermore, since the esterification reaction is an equilibrium reaction, a method in which the reaction rate of carboxylic acid is improved by adding more than an equivalent amount of alcohol is also well known, and this can be said to be an industrially effective esterification method.

Therefore, when esterifying a carboxylic acid using sulfuric acid as a catalyst and under conditions of excess alcohol, the resulting reaction solution contains unreacted substances in addition to the reaction products ester and water. Contains alcohol, carboxylic acid, sulfuric acid, etc. As a method for separating ester and carboxylic acid from such a reaction solution, a method of extraction using a hydrophobic organic solvent as an extractant can be considered. And JP-A-53
As in the example of esterification of methacrylic acid described in Publication No. 21114, an industrially advantageous method that allows esterification and separation to be performed simultaneously by supplying an organic solvent as an extractant to an esterification reactor. There is a way.

According to studies by the present inventors, separation of the ester from the esterification reaction solution can be carried out relatively easily by an extraction method using a hydrophobic organic solvent as an extractant. However, since carboxylic acids have relatively high compatibility with water and sulfuric acid, they cannot be completely extracted by extraction with hydrophobic organic solvents, and a considerable amount of carboxylic acids may remain in the waste sulfuric acid. become. On the other hand, since alcohol has higher compatibility with water and sulfuric acid than carboxylic acid, most of it remains in the waste sulfuric acid.
Therefore, disposing of this waste sulfuric acid as it is not only causes pollution problems, but is also undesirable from the standpoint of effective resource utilization. Therefore, useful components in the waste sulfuric acid should be recovered as much as possible and distilled as necessary. After concentration, it is preferable to recycle and reuse. Useful components here include alcohol, ester, carboxylic acid, etc. Among these, alcohol can be easily separated from waste sulfuric acid by distillation. It is also possible to separate esters from waste sulfuric acid by distillation, but since esters generally form an azeotrope with alcohol, they must be extracted with a hydrophobic organic solvent before distilling the alcohol. It is economically advantageous to collect the waste. However, since carboxylic acids have a high boiling point and high polymerizability, it is difficult to separate them by distillation.On the other hand, simple extraction using a hydrophobic organic solvent does not allow sufficient separation and recovery for the reasons mentioned above. Can not do it.

In addition, waste sulfuric acid is subjected to a sulfuric acid concentration process for reuse, and since its boiling point is actually higher than that of water, it is distributed to the bottom of the distillation column for sulfuric acid concentration, but as described above, carbon dioxide is present in this waste sulfuric acid. If acid remains,
Due to its high polymerizability, it has the drawback of causing polymerization loss and clogging of the equipment by polymerization products at the bottom of the distillation column for sulfuric acid concentration. Therefore, it has been strongly desired to recover as much carboxylic acid from the waste sulfuric acid as possible before supplying the waste sulfuric acid to the sulfuric acid concentration process.

In addition, in the recovery of carboxylic acid by extraction as described above, it is possible to increase the amount of hydrophobic organic solvent supplied to the extraction process to improve the overall extraction ability and thereby increase the recovery rate of carboxylic acid. Although it is possible, in this case, the energy cost required to separate the hydrophobic organic solvent and the carboxylic acid increases unnecessarily, and the capacity of the extraction equipment needs to be increased, which increases the equipment cost. Carboxylic acid recovery costs will increase significantly.

The present inventors have conducted extensive studies on the recovery of carboxylic acids from waste sulfuric acid from the above viewpoint, and have found that extraction of esters from waste sulfuric acid using hydrophobic organic solvents is much easier than extraction of carboxylic acids. The inventors have discovered that if the ester in the waste sulfuric acid is removed, the carboxylic acid easily reacts with excess alcohol and the ester is produced relatively quickly, and the present invention has been completed based on these findings.

That is, the present invention easily and advantageously converts carboxylic acid in waste sulfuric acid generated in the production of ester by reacting a carboxylic acid having 3 or 4 carbon atoms with an alcohol using sulfuric acid as a catalyst with a high recovery rate. It provides a method that can be recovered, and its characteristics include the following steps.
The point is to perform steps (a)-(b)-(c) or steps (b)-(c). (a)
Step (b) of extracting and recovering carboxylic acids and esters contained in waste sulfuric acid with a hydrophobic organic solvent; Step (c) of producing esters by esterifying the carboxylic acid remaining in waste sulfuric acid with alcohol; Step (b) ) The reaction solution obtained in step ) is brought into contact with a hydrophobic organic solvent to extract and recover carboxylic acid and ester. from which most of the esters and carboxylic acids have been removed, for example, by extraction, or something similar in composition, and is a mixture consisting mainly of sulfuric acid, water, and alcohol, and containing small amounts of esters and carboxylic acids. Here, the carboxylic acid has 3 or 4 carbon atoms, and specific examples include acrylic acid and methacrylic acid.

The present invention includes the step (b) of reacting the carboxylic acid remaining in the waste sulfuric acid with an alcohol to esterify it, but the alcohol used in this step preferably has 1 to 1 carbon atoms.
Specific examples include methanol, ethanol, propanol, etc. and,
In the production of esters that produce waste sulfuric acid, which is the subject of the present invention, the esterification reaction is carried out under conditions with an excess of alcohol, and therefore the waste sulfuric acid used in step (b) contains a sufficiently large amount of alcohol. If so, it is not necessary to newly supply alcohol to carry out step (b). However, in the production of esters, if the esterification reaction is carried out under conditions where the alcohol is equivalent to or less than the carboxylic acid, or under conditions where the alcohol is slightly in excess of the carboxylic acid, when performing step (b), A separate supply of alcohol is necessary or very effective. Then, the waste sulfuric acid used in step (b) is
In order to improve the recovery rate of carboxylic acid, it is preferable that the alcohol be contained in a molar ratio of 10 or more to the remaining carboxylic acid.

The reaction temperature of the esterification reaction in the above step (b) is usually 30 to 100°C, preferably 40 to 70°C. If the reaction temperature is too low, the reaction rate will be low;
On the other hand, if it is too high, polymerization of esters and carboxylic acids and further corrosion of equipment by sulfuric acid may occur, leading to other problems. The reaction time of this esterification reaction is usually 0.5 hours or more, preferably 1 to 6 hours. However, if the reaction time is too long, the polymerization of esters and carboxylic acids will proceed.
There are problems such as the need to increase the size of the reactor. The form of the reactor is not particularly limited and may be a so-called tank reactor, but since strict temperature control or vigorous stirring is not required, it may be used, for example, in a tank or in the column used in step (a) or (c). It is also possible to substitute a part of it as a reactor.

In the first method of the present invention, an extraction and recovery step using a hydrophobic organic solvent as an extractant, that is, step (a) and step (c), are performed before and after the above step (b), and in the second method, Step (c) is performed after step (b). The hydrophobic organic solvent used here has a boiling point of 0 to 250℃
It is one type or a mixture of two or more types selected from aliphatic hydrocarbons, alicyclic hydrocarbons, and aromatic hydrocarbons, and specific examples thereof include butane, pentane, hexane, heptane, nonane, octane, and decane. ,
Cyclopentane, cyclohexane, methylcyclohexane, propylcyclohexane, benzene,
Examples include toluene, ethylbenzene, xylene, and cumene.

The amount of the above-mentioned hydrophobic organic solvent used is usually 0.05 to 5.0, preferably 0.05 to 5.0 in weight ratio to the liquid to be extracted.
It is between 0.3 and 1.0. If the amount of hydrophobic organic solvent used is too small, the recovery rate of carboxylic acid and ester will be low, and if it is too large, the cost for separating and recovering the hydrophobic organic solvent will increase. The operating temperature of the extractor is usually 15-100°C, preferably 30-70°C. If this temperature is set too low, the cost of cooling will increase, which is industrially disadvantageous.On the other hand, if it is set too high, the extraction efficiency will decrease, resulting in a decrease in the recovery rate of esters and carboxylic acids. descend.

The extraction device used here may be any commonly used extraction device, but it is preferably one that allows the hydrophobic organic solvent and waste sulfuric acid to come into contact with each other efficiently in countercurrent flow.
For example, a packed column filled with a Raschig ring or the like, a tray column type extraction column, etc. can be suitably used.

In the first method of the present invention, a total of two extraction steps are performed before and after the esterification step (step (b)), but the hydrophobic organic solvent used as the extractant does not necessarily need to be supplied separately for each step. Instead, the extract obtained in one extraction process can be supplied as an extractant for the other extraction process. This is possible because the amount of esters and carboxylic acids in the extracted liquid is small relative to the amount of hydrophobic organic solvent, and by doing this, the overall amount of hydrophobic organic solvent used can be reduced. The energy cost for hydrophobic organic solvent recovery can be reduced.

In the first method of the present invention, by performing step (a) before step (b), carboxylic acid can be recovered with high efficiency in step (b) and subsequent step (c). However, if the ester content in the waste sulfuric acid generated during ester production is very small, for example, 0.5% by weight or less, the process
A second method can be applied in which the extraction step before (b), ie step (a), is omitted. That is, this step (a)
In the esterification of step (b),
The main purpose is to shift the chemical equilibrium of the system to a direction that promotes esterification, and therefore, if the ester content of the waste sulfuric acid is low and the carboxylic acid in the waste sulfuric acid is in a state where it can be esterified. Step (b) can be carried out immediately, and the content of carboxylic acid in the finally obtained waste sulfuric acid can be made extremely small.

In addition, it may be effective in some cases to improve the recovery rate of carboxylic acid by repeating the esterification in step (b) and the extraction in step (c) after performing the recovery treatment according to the present invention. Yes, but the process
Since the esterification reaction in (b) occurs relatively quickly and the extraction rate of ester in step (c) is high, generally steps (b) and c can be performed only once to obtain a sufficiently high recovery rate of carboxyl. Acid can be recovered.

As described above, the main feature of the present invention is to convert the carboxylic acid in waste sulfuric acid, which is difficult to recover only by extraction with a hydrophobic organic solvent, into an easily recoverable ester and recover it. Since the method of the present invention is carried out incidentally to the ester production process, it is not necessary to recover the carboxylic acid as it is, and recovering the carboxylic acid in the form of an ester ultimately increases the yield of the ester. It will also improve. Moreover, by recovering carboxylic acid as an easily recoverable ester, the amount of hydrophobic organic solvent used as an extractant can be far reduced compared to recovering the carboxylic acid form to obtain the same recovery rate. Therefore, there is an advantage that the energy cost required for separating and recovering the hydrophobic organic solvent can be significantly reduced.

Next, preferred embodiments of the present invention will be described using the drawings.

A process of producing waste sulfuric acid to which the present invention is applied;
That is, the process in which an ester is synthesized by esterifying a carboxylic acid and an alcohol under a sulfuric acid catalyst, and most of the ester and carboxylic acid contained in the reaction system are separated to produce waste sulfuric acid is as follows:
Although not particularly limited, an example is JP-A-53-21114.
The following explanation assumes that ester production is carried out in accordance with the method described in the publication.

In FIG. 1, P indicates an ester production system,
This system consists of three complete mixing tank reactors 5, 6,
7 are connected in series through separators 19, 20, and 21 connected to these reactors 5, 6, and 7, respectively, and the first stage reactor 7 is supplied with a hydrophobic organic solvent and carboxyl as a reaction medium from the tube 1. Acid is supplied, and a sulfuric acid aqueous solution as a catalyst and excess alcohol are supplied through pipe 2 to reactor 5 at the final stage, and in each reactor 5, 6, and 7, the supplied reactants are brought into contact in countercurrent and reacted. death,
The obtained reaction liquid is separated in separators 19, 20, and 21 into a phase containing sulfuric acid, water, and alcohol as main components, and a phase containing carboxylic acid, ester, and hydrophobic organic solvent as main components. Separator 19 tube 3
From this, a hydrophobic organic solvent, a crude ester mainly composed of ester, is obtained, which is passed through the tube 4 of the first stage separator 21.
Waste sulfuric acid can be obtained from In the present invention, this waste sulfuric acid is treated as follows to recover carboxylic acid.

That is, the waste sulfuric acid from the pipe 4 is cooled by a heat exchanger 8 if necessary, and then is supplied to the upper part of the extraction tower 9, and the hydrophobic organic solvent is supplied to the lower part of the tower through the pipe 11. Most of the esters and some of the carboxylic acids in the waste sulfuric acid are extracted by the extractant, which is the main component. The extract from the top of the extraction tower 9 is discharged through a pipe 10 and sent to a separation step (not shown) for each component. From the pipe 12 at the bottom of the extraction tower 9, waste sulfuric acid from which most of the esters and some of the carboxylic acids have been removed is extracted, and after being heated with a heat exchanger or the like if necessary, it is transferred to the esterification reactor 13. Supplied. Most of the esters in the waste sulfuric acid supplied here have been removed in the extraction tower 9, so that the chemical equilibrium has shifted in a direction that promotes esterification. Most of the carboxylic acids in the waste sulfuric acid
Ester is produced by reaction with excess alcohol.

Here, when the content of alcohol supplied to the reactor 13 is low, it is preferable to supply alcohol separately from the pipe 22 and adjust the amount of alcohol to a molar ratio of 10 or more with respect to the carboxylic acid. .

The reaction liquid from the reactor 13 is extracted through a pipe 14, cooled by a heat exchanger 15 as necessary, and then supplied to an extraction tower 16, where it is supplied to the extraction tower 16 through a pipe 17. Most of the esters and some of the carboxylic acids are extracted by a hydrophobic organic solvent. The extract containing a hydrophobic organic solvent as a main component from the extraction column 16 is supplied to the extraction column 9 as an extractant through a pipe 11. Waste sulfuric acid containing almost no esters and carboxylic acids is obtained from the pipe 18 at the bottom of the extraction tower 16, and is normally sent to a sulfuric acid concentration step.

In the above, the hydrophobic organic solvents supplied to tube 1 and tube 17 may be of different types, but are preferably the same. Moreover, when the reaction temperature in the reactors 5, 6, 7 or the reactor 13 is low, the heat exchanger 8 or the heat exchanger 15 may not be necessary.

Although an independent reactor 13 is used in the example shown in FIG. 1, a device in which the reactor 13 is integrated with the bottom of the extraction column 9 as shown in FIG. As shown in FIG. 3, an apparatus may be used in which the extraction columns 9 and 16 are integrated and the central portion of the column is enlarged to have the function of the reactor 13. Furthermore, various changes can be made to the apparatus, such as increasing the volume of the extraction towers 9 and 16 to lengthen the residence time to allow the reaction to proceed while extraction is being carried out, and these changes may be made without departing from the spirit of the present invention. All are included in the present invention.

Example 1 Methyl methacrylate was produced using methacrylic acid and methanol using an apparatus configured according to the flow sheet shown in FIG. That is, in the ester production system P, methacrylic acid and n-hexane containing 500 ppm of hydroquinone as a polymerization inhibitor are added to tube 1 at flow rates of 299 g/hr and 146 g/hr, respectively, and methanol is added to tube 2. and 70% concentration sulfuric acid aqueous solution, respectively.
The esterification reaction was carried out in reactors 5, 6, and 7 at a reaction temperature of 65° C. and a residence time of 6 hours.

As a result, from tube 3, normal hexane 28.4
A crude ester having a composition of 64.5% by weight of methyl methacrylate and 6.1% by weight of methanol and further containing small amounts of methacrylic acid, sulfuric acid, water, etc.
g/hr, and from pipe 4, waste consisting of 0.3% by weight of n-hexane, 2.5% by weight of methyl methacrylate, 1.0% by weight of methacrylic acid, 23.3% by weight of methanol, 40.7% by weight of sulfuric acid, and 32.2% by weight of water. Sulfuric acid was obtained at a flow rate of 403 g/hr.

The waste sulfuric acid thus obtained was cooled to 40°C with cooling water in the heat exchanger 8, and then heated to a tube with an internal diameter of 2.5 cm.
403 g /
hr flow rate, and an extract containing 97% by weight of normal hexane, methyl methacrylate, etc. obtained from the top of the extraction column 16 was supplied as an extractant to the bottom stage for extraction.

As a result, from the pipe 10 at the top of the extraction tower 9, 89.1% by weight of normal hexane and methyl methacrylate were extracted.
A liquid consisting of 8.5% by weight and small amounts of methacrylic acid, methanol, etc. was obtained at a flow rate of 137 g/hr, and from the tube 12 at the bottom of the extraction column 9, a liquid containing 0.6% by weight of methanol, sulfuric acid, and water as main components was obtained. of methacrylic acid and 0.05% by weight of methyl methacrylate was obtained at a flow rate of 391 g/hr. The liquid in the tube 12 was heated to 60°C using a heat exchanger (not shown) and then supplied to the reactor 13 for an average residence time of 2 hours.
Esterification of methanol acid was carried out at a reaction temperature of 60°C. As a result, 63% by weight of methacrylic acid in the liquid from tube 12 was esterified, and the concentration of methacrylic acid in the reaction liquid obtained from tube 14 of reactor 13 was 0.23% by weight, and the concentration of methyl methacrylate was 0.5.
% by weight, and the reaction liquid from this tube 14 was cooled to 40°C with water in a heat exchanger 15, and then transferred to an extraction column 9.
Extraction was carried out by supplying n-hexane to the uppermost stage of an extraction column 16 having the same structure and capacity as , and to the lowermost stage thereof from a pipe 17 at a flow rate of 121 g/hr.

As a result, an extract containing n-hexane as a main component was obtained from the pipe 11 at the top of the extraction tower 16, and this liquid was supplied to the extraction tower 9 as described above. A liquid containing 0.1% by weight of methanol acid and 0.06% by weight of methyl methacrylate and whose main components were methanol, sulfuric acid, and water was discharged from the pipe 18 at the bottom of the extraction column 16 at a flow rate of 387 g/hr. .

As a result of continuous operation for 48 hours as described above, the recovery rate of methacrylic acid was 90%, and 42% of the recovered methacrylic acid was recovered as methyl methacrylate.

Comparative Example 1 In the flow sheet shown in FIG. 1, the same procedure as in Example 1 was used except that the reactor 13 and heat exchanger 15 were removed and an apparatus in which the extraction tower 9 and the extraction tower 16 were connected in series was used. I performed the operation.

As a result, from the pipe 10 at the top of the extraction tower 9, 89.6% by weight of n-hexane and methyl methacrylate were extracted.
7.3% by weight, methacrylic acid 1.9% by weight, methanol
A liquid consisting of 1.2% by weight was obtained at a flow rate of 136 g/hr, and from the pipe 18 at the bottom of the extraction column 16, a liquid containing methanol, sulfuric acid, and water as main components, 0.02% by weight of methyl methacrylate, and 0.36% by weight of The liquid containing methacrylic acid was discharged at a flow rate of 388 g/hr, and methacrylic acid was eventually recovered with a recovery rate of 65%.
Although the number of plates in the extraction column and the amount of n-hexane as the extractant were the same as in Example 1, the methacrylic acid recovery rate was significantly lower than in Example 1 because no esterification reaction was performed. is clear.

Comparative Example 2 The same operation as in Comparative Example 1 was performed except that the flow rate of normal hexane supplied from the pipe 17 to the extraction column 16 was changed to 363 g/hr.

As a result, from the pipe 10 at the top of the extraction tower 9, 96.0% by weight of normal hexane and methyl methacrylate were extracted.
2.6% by weight, methacrylic acid 1.0% by weight, methanol
A liquid consisting of 0.4% by weight was obtained at a flow rate of 379 g/hr, while a liquid containing methanol, sulfuric acid, and water as main components, 0.02% by weight of methyl methacrylate, and 0.09% by weight was obtained from the pipe 18 at the bottom of the extraction column 16. A liquid containing methacrylic acid was obtained at a flow rate of 387 g/hr, and methacrylic acid could be recovered with a recovery rate of 91%.

According to this example, almost the same recovery rate as in Example 1 was obtained, but since no esterification reaction was performed, about three times as much normal hexane as in Example 1 was used. It is clear that a large amount of energy is required for recovery.

Example 2 Methyl acrylate was produced from acrylic acid and methanol using exactly the same equipment as in Example 1. That is, in the ester production system P, hydroquinone is added to the tube 1 as a polymerization inhibitor.
Acrylic acid and normal petane containing 500 p.pm are supplied at flow rates of 251 g/hr and 302 g/hr, respectively, and methanol and 70% concentration are supplied to tube 2.
of sulfuric acid aqueous solution at 234g/hr and 230g/hr, respectively.
The esterification reaction was carried out in reactors 5, 6, and 7 at a reaction temperature of 60° C. and a residence time of 5 hours.

As a result, tube 3 contained 48.1% by weight of normal pentane, 44.5% by weight of methyl acrylate, and methanol.
With a composition of 6.1% by weight, small amounts of acrylic acid, sulfuric acid,
Crude ester containing water etc. was obtained at a flow rate of 625 g/hr, and from tube 4, 0.3% by weight of normal pentane,
Methyl acrylate 2.5% by weight, acrylic acid 1.0% by weight, methanol 22.6% by weight, sulfuric acid 40.8% by weight, water
Waste sulfuric acid consisting of 32.8% by weight was obtained at a flow rate of 392 g/hr.

The waste sulfuric acid thus obtained was cooled to 40°C with cooling water in the heat exchanger 8 and then supplied to the extraction tower 9, while normal pentane was introduced into the extraction tower 16 from the pipe 17.
Methyl acrylate and acrylic acid were recovered in the waste sulfuric acid by supplying it at a flow rate of 234 g/hr. Extraction tower 9
The liquid obtained from the tube 12 at the bottom of the column was heated to 50° C. by a heat exchanger (not shown) and supplied to the reactor 13. In the reactor 13, the acrylic acid that could not be recovered in the extraction column 9 is esterified at a reaction temperature of 50° C. for an average residence time of 2 hours, and the reaction liquid from the tube 14 at the bottom of the column is exchanged with water in a heat exchanger 15. After cooling to 40°C, it was supplied to an extraction column 16 to recover methyl acrylate and acrylic acid.

As a result, a liquid containing 92.6% by weight of normal pentane, 4.3% by weight of methyl acrylate, and small amounts of acrylic acid, methanol, etc. was obtained from the pipe 10 at the top of the extraction column 9 at a flow rate of 254 g/hr. On the other hand, from the pipe 18 at the bottom of the extraction column 16, a liquid containing 0.08% by weight of acrylic acid and 0.01% by weight of methyl acrylate, and whose main components are methanol, sulfuric acid, and water, is released.
Obtained at a flow rate of 372 g/hr. As above
As a result of continuous operation for 48 hours, the recovery rate of acrylic acid was
92%, and 27% of the recovered acrylic acid was recovered as methyl acrylate. Note that the reaction rate of acrylic acid in reactor 13 was 57%.

Comparative Example 3 In the flow sheet shown in FIG. 1, the reactor 13 and the heat exchanger 15 were removed, and an apparatus in which the extraction tower 9 and the extraction tower 16 were connected in series was used. I performed the operation.

As a result, from the pipe 10 at the top of the extraction column 9, 93.2% by weight of n-pentane and 3.9% methyl acrylate were extracted.
A liquid consisting of 1.1% by weight of acrylic acid, 1.1% by weight of acrylic acid, and methanol etc. was obtained at a flow rate of 252 g/hr, and from the pipe 18 at the bottom of the extraction column 16, a liquid consisting of methanol, sulfuric acid, and water as main components and 0.03% by weight of Methyl acrylate, a liquid containing 0.32% by weight of acrylic acid, is 374
g/hr, and was able to recover acrylic acid with a recovery rate of 70%, although the number of plates in the extraction column and the amount of n-pentane as the extractant were the same as in Example 2. First, since no esterification reaction was performed, it is clear that the recovery rate of acrylic acid was significantly lower than in Example 2.

Example 3 Preceding esterification of methacrylic acid in waste sulfuric acid consisting of 5% by weight methacrylic acid, 21.5% by weight methanol, 40.2% by weight sulfuric acid, 33.2% by weight water and containing 0.1% by weight methyl methacrylate It was recovered in the same manner as in the previous example except that extraction was not performed. That is, the waste sulfuric acid was directly supplied to the reactor 13 at a flow rate of 390 g/hr, esterification of methacrylic acid was performed at a reaction temperature of 50° C. for an average residence time of 2 hours, and the resulting liquid was transferred to the extraction tower of Example 1. 1
The mixture was supplied to the top of an extraction column having the same structure and capacity as No. 6, and normal hexane was supplied to the bottom of the extraction column at a flow rate of 156 g/hr for extraction.

As a result, a crude ester consisting of 87.7% by weight of n-hexane, 8.3% by weight of methyl methacrylate, 2.5% by weight of methacrylic acid, and a small amount of methanol was obtained from the top of the extraction column at a flow rate of 178 g/hr. On the other hand, from the bottom of the extraction column, a liquid containing 0.05% by weight of methyl methacrylate and 0.7% by weight of methacrylic acid, and whose main components were methanol, sulfuric acid, and water, was obtained at a flow rate of 368 g/hr.

As a result of continuous operation for 48 hours as described above, the recovery rate of methacrylic acid was 87%. Note that the reaction rate of methacrylic acid in the reactor was 65%.

When the amount of carboxylic acid to be recovered is small, it was possible to recover the waste sulfuric acid at a high recovery rate by esterifying and extracting the waste sulfuric acid without an extraction step.

[Brief explanation of drawings]

FIGS. 1 to 3 are flow sheet diagrams of steps preferably used in carrying out the present invention. 5,6,7,13...reactor, 9,16...extraction column, 19,20,21...separator.

Claims (1)

[Claims] 1. Unsaturated carboxylic acid in waste sulfuric acid produced in the production of an unsaturated carboxylic acid ester by reacting an unsaturated carboxylic acid having 3 or 4 carbon atoms with a lower alcohol using sulfuric acid as a catalyst. A method for recovering an unsaturated carboxylic acid, the method comprising recovering the acid through the following steps (a)-(b)-(c). (a) A step of extracting and recovering unsaturated carboxylic acids and unsaturated carboxylic acid esters contained in waste sulfuric acid using a hydrophobic organic solvent. (b) Esterifying the unsaturated carboxylic acid remaining in the waste sulfuric acid with a lower alcohol. Step (c) of producing an unsaturated carboxylic acid ester by contacting the reaction solution obtained in step (b) with a hydrophobic organic solvent to extract and recover the unsaturated carboxylic acid and the unsaturated carboxylic acid ester. The method for recovering an unsaturated carboxylic acid according to claim 1, wherein the alcohol is an alcohol having 1 to 3 carbon atoms. 3. The hydrophobic organic solvent according to claim 1 or 2, wherein the hydrophobic organic solvent is at least one selected from aliphatic hydrocarbons, alicyclic hydrocarbons, and aromatic hydrocarbons having a boiling point of 0 to 250°C. Method for recovering saturated carboxylic acid. 4. The unsaturated carboxylic acid in the waste sulfuric acid produced in the production of unsaturated carboxylic acid esters, which is carried out by reacting an unsaturated carboxylic acid having 3 or 4 carbon atoms with a lower alcohol using sulfuric acid as a catalyst, is purified by the following process ( A method for recovering an unsaturated carboxylic acid, the method comprising recovering an unsaturated carboxylic acid according to b)-(c). (b) A step of esterifying the unsaturated carboxylic acid remaining in the waste sulfuric acid with a lower alcohol to produce an unsaturated carboxylic acid ester. (c) Contacting the reaction solution obtained in step (b) with a hydrophobic organic solvent. Step 5 of extracting and recovering unsaturated carboxylic acids and unsaturated carboxylic acid esters by extracting and recovering unsaturated carboxylic acids and unsaturated carboxylic esters. The method for recovering unsaturated carboxylic acids according to claim 4, wherein the lower alcohol is an alcohol having 1 to 3 carbon atoms. 6. The non-container according to claim 4 or 5, wherein the hydrophobic organic solvent is at least one selected from aliphatic hydrocarbons, alicyclic hydrocarbons, and aromatic hydrocarbons having a boiling point of 0 to 250°C. Method for recovering saturated carboxylic acid.