JP7591359B2 - Method for producing easily polymerizable compound - Google Patents

Description

The present invention relates to a method for producing an easily polymerizable compound, including a step of purifying an easily polymerizable compound such as (meth)acrylic acid or a (meth)acrylic acid ester by distillation.

Conventionally, a method for purifying easily polymerizable compounds such as (meth)acrylic acid and (meth)acrylic acid esters using a vaporization separation tower such as a distillation tower is known. The vaporization separation tower may be provided with a circulation path for extracting at least a portion of the bottom liquid, heating it in a reboiler, and returning it to the vaporization separation tower. Easily polymerizable substances are prone to polymerization, and polymerization may occur during the production process, forcing the equipment to be shut down. Various measures to prevent polymerization are being investigated.

For example, Patent Document 1 discloses a method for distilling an easily polymerizable compound or a liquid containing an easily polymerizable compound, in which a reboiler having a predetermined size is used to distill an easily polymerizable compound or a liquid containing an easily polymerizable compound using a distillation tower equipped with a reboiler, and the liquid level in the distillation tower is maintained within a predetermined range. Patent Document 2 discloses a method for producing a (meth)acrylic ester, which includes a step of introducing a process liquid stream containing a (meth)acrylic ester into a distillation tower and distilling the stream, in which the distillation tower is a reactive distillation tower equipped with a tower bottom withdrawal pump and a tower bottom circulation line, the tower bottom withdrawal pump discharge liquid is branched to withdraw the process liquid, the withdrawn liquid and an oxygen-containing gas are mixed in a static mixer to form a gas-liquid mixed phase flow, and the gas-liquid mixed phase flow is supplied to the tower bottom circulation line or the tower bottom liquid of the reactive distillation tower.

JP 2000-239229 A JP 2014-162783 A

When refining a liquid containing easily polymerizable compounds using a vaporization separation tower, it is necessary to suppress the polymerization of the easily polymerizable compounds not only in the vaporization separation tower but also in the circulation path. In the heating section installed in the circulation path, the extracted liquid, which is at least a portion of the bottom liquid of the vaporization separation tower, is heated, making it easier for the polymerization reaction of the easily polymerizable compounds to proceed, so measures to prevent polymerization are particularly important.

The present invention has been made in consideration of the above circumstances, and its purpose is to provide a method for producing an easily polymerizable compound, which includes a step of purifying an easily polymerizable compound using a vaporization separation tower provided with a circulation path equipped with a heating section, and which is capable of suppressing polymerization of the easily polymerizable compound in the heating section.

The present invention includes the following inventions.
[1] A method for producing an easily polymerizable compound, comprising a step of introducing an easily polymerizable compound-containing liquid into a vaporization separation column selected from a distillation column and a stripping column and purifying the liquid under reduced pressure,
The vaporization separation tower is provided with a circulation path for returning at least a portion of the bottom liquid extracted from the tower to the vaporization separation tower,
The circulation path is provided with a supply port for supplying an oxygen-containing gas and a reboiler equipped with a heating unit in this order from the upstream side,
The supply port is disposed below the inlet of the heating unit by a height difference of 0.5 m or more,
A method for producing an easily polymerizable compound, comprising: supplying an oxygen-containing gas to the extracted liquid through the supply port.
[2] The method for producing an easily polymerizable compound according to [1], wherein the pressure of the withdrawn liquid in the vicinity of the supply port is 30 kPa or more higher than the gas pressure at the top surface of the column bottom liquid.
[3] The method for producing an easily polymerizable compound according to [1] or [2], wherein an average residence time of the extracted liquid from the supply port of the circulation path to the heating part is 2 seconds or more.
[4] The method for producing an easily polymerizable compound according to any one of [1] to [3], wherein the circulation path has a bent portion between the supply port and the heating part.
[5] The method for producing an easily polymerizable compound according to any one of [1] to [4], wherein the supply port is provided in a horizontal portion of the circulation path extending in a substantially horizontal direction.
[6] The method for producing an easily polymerizable compound according to [5], wherein the average residence time in the horizontal portion of the column bottom liquid flowing through the circulation path is 0.5 seconds or more.
[7] The method for producing an easily polymerizable compound according to any one of [1] to [6], wherein a widening section is provided at a connection section between the circulation path and the heating section on the upstream side of the heating section.
[8] The method for producing an easily polymerizable compound according to any one of [1] to [7], wherein the reboiler equipped with the heating unit is composed of a plate heat exchanger, a shell-and-tube heat exchanger, a double-tube heat exchanger, a coil heat exchanger, or a spiral heat exchanger.
[9] The method for producing an easily polymerizable compound according to any one of [1] to [8], wherein the oxygen-containing gas is supplied in the form of microbubbles.
[10] The method for producing an easily polymerizable compound according to any one of [1] to [9], wherein the easily polymerizable compound is (meth)acrylic acid or a (meth)acrylic acid ester.

The method for producing an easily polymerizable compound of the present invention includes a step of purifying an easily polymerizable compound-containing liquid under reduced pressure using a vaporization separation tower, and an oxygen-containing gas supply port is provided upstream of the reboiler in the effluent circulation path of the vaporization separation tower and at a position 0.5 m or more below the inlet of the heating section of the reboiler. This allows the oxygen-containing gas to be supplied to the effluent in a higher pressure state, making it possible to dissolve the supplied oxygen quickly in the effluent or to dissolve more oxygen in the effluent. As a result, the polymerization prevention effect in the circulation path is enhanced, and the polymerization of the easily polymerizable compound can be suppressed, particularly in the heating section of the reboiler where polymerization reactions are likely to occur.

1 illustrates an example of a system surrounding a vaporization separation tower used in the method for producing an easily polymerizable compound of the present invention. 2 illustrates another example of a system surrounding a vaporization separation tower used in the method for producing an easily polymerizable compound of the present invention.

The method for producing an easily polymerizable compound of the present invention includes a step of introducing an easily polymerizable compound-containing liquid into a vaporization separation tower selected from a distillation tower and a stripping tower and purifying the liquid under reduced pressure. By introducing an easily polymerizable compound-containing liquid into a vaporization separation tower and purifying the liquid, at least a portion of components having a lower boiling point and/or a higher boiling point than the easily polymerizable compound are removed from the easily polymerizable compound-containing liquid, and a purified easily polymerizable compound can be obtained. Hereinafter, the step of introducing an easily polymerizable compound-containing liquid into a vaporization separation tower and purifying the liquid is referred to as the "vaporization separation step."

The easily polymerizable compound is not particularly limited as long as it is a compound that easily polymerizes when heated in a vaporization separation tower, and examples thereof include (meth)acrylic acid and its esters, maleic anhydride and its esters, acrylonitrile, styrene, divinylbenzene, vinyltoluene, diethylene glycol monovinyl ether, etc. Among these, a representative easily polymerizable compound is (meth)acrylic acid and its esters. Examples of (meth)acrylic acid esters include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, trimethylolpropane di(meth)acrylate, trimethylolpropane tri(meth)acrylate, 2-(2-vinyloxyethoxy)ethyl (meth)acrylate, methoxyethyl (meth)acrylate, benzyl (meth)acrylate, and glycidyl (meth)acrylate. Preferred examples of the easily polymerizable compound include (meth)acrylic acid, butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxyethyl (meth)acrylate, nonyl (meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, trimethylolpropane di(meth)acrylate, trimethylolpropane tri(meth)acrylate, 2-(2-vinyloxyethoxy)ethyl (meth)acrylate, benzyl (meth)acrylate, and glycidyl (meth)acrylate.

The easily polymerizable compound-containing liquid introduced into the vaporization separation column of the distillation column or the stripping column is not particularly limited as long as it is obtained in the manufacturing process of the easily polymerizable compound and contains the easily polymerizable compound. For example, when the easily polymerizable compound is (meth)acrylic acid, the easily polymerizable compound-containing liquid may be a (meth)acrylic acid-containing liquid obtained in a collection process or a condensation process prior to the vaporization separation process (specifically, a (meth)acrylic acid-containing liquid obtained by recovering a (meth)acrylic acid-containing gas as a liquid), a (meth)acrylic acid-containing liquid obtained in an arbitrary purification process prior to the vaporization separation process, or a purification residue obtained in an arbitrary purification process subsequent to the vaporization separation process. Examples of purification means that can be used in these arbitrary purification processes include crystallization, distillation (fractional distillation), stripping, extraction, etc., and a combination of a plurality of these may be used.

The concentration of the easily polymerizable compound in the easily polymerizable compound-containing liquid introduced into the vaporization separation tower in the vaporization separation step is not particularly limited, but in order to more significantly exert the effects of the present invention, the concentration of the easily polymerizable compound in the easily polymerizable compound-containing liquid is, for example, preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more. The upper limit of the easily polymerizable compound concentration in the easily polymerizable compound-containing liquid is not particularly limited, but is, for example, 97% by mass or less.

As a distillation tower or stripping tower used as a vaporization separation tower, a plate tower (for example, a perforated plate tower or a bubble cap tower) with shelves (sieve trays) inside the tower or a packed tower with packing inside the tower is preferably used. A vaporization separation tower has a top, a middle, and a bottom. When a vaporization separation tower is divided in the height direction, the range in the height direction where the shelves or packing are provided is called the middle, the area closer to the top of the tower than that is called the top, and the area closer to the bottom of the tower than that is called the bottom.

The position at which the easily polymerizable compound-containing liquid is introduced into the vaporization separation tower and the position at which the purified easily polymerizable compound is extracted from the vaporization separation tower can be appropriately set depending on the composition of the easily polymerizable compound-containing liquid and the type of vaporization separation tower, i.e., whether the vaporization separation tower is a distillation tower or a stripping tower.

When the vaporization separation tower is a distillation tower, it is preferable to introduce the easily polymerizable compound-containing liquid into the distillation tower from the center of the tower. The extraction position of the purified easily polymerizable compound extracted from the distillation tower may be appropriately set according to the composition of the easily polymerizable compound-containing liquid introduced into the distillation tower. For example, when the (meth)acrylic acid-containing liquid is introduced into the distillation tower for purification, it is preferable to extract the purified (meth)acrylic acid from the distillation tower as follows. When the distillation tower is one that distills the (meth)acrylic acid-containing liquid obtained in the previous collection step or condensation step, since the (meth)acrylic acid-containing liquid contains many components with a lower boiling point than (meth)acrylic acid, it is preferable to extract the purified (meth)acrylic acid from the center and/or bottom of the distillation tower. In this case, the extraction position of the purified (meth)acrylic acid is preferably on the bottom side of the tower from the introduction position of the (meth)acrylic acid-containing liquid. On the other hand, when the (meth)acrylic acid-containing liquid contains a large amount of components with a higher boiling point than (meth)acrylic acid, such as Michael adducts and maleic acid, the purified (meth)acrylic acid is preferably withdrawn from the top of the column relative to the introduction position of the (meth)acrylic acid-containing liquid, and more preferably withdrawn from the top of the column. In this case, the purified (meth)acrylic acid is preferably withdrawn as vapor.

When the purified (meth)acrylic acid is extracted from the middle of the tower, it may be extracted as a liquid or as vapor. In order to extract the purified (meth)acrylic acid as a liquid from the middle of the tower, for example, in a plate tower, an extraction port may be provided at the position of the plate, or a chimney may be provided at the plate. By extracting it as a liquid, the purified (meth)acrylic acid can be supplied to the next process without additional operations such as condensation. When the purified (meth)acrylic acid is extracted as vapor, it becomes easier to obtain purified (meth)acrylic acid with a relatively low impurity content. In the distillation tower, the reflux liquid flows down from the top to the bottom of the tower, and a portion of the reflux liquid remains accumulated on the plate. However, by providing an extraction port at a position sufficiently above the plate, it becomes easier to suppress the extraction of the reflux liquid and preferentially extract the purified (meth)acrylic acid in the form of vapor.

When the purified (meth)acrylic acid is extracted from the bottom of the column, a line for extracting the purified (meth)acrylic acid may be connected to the effluent circulation line described below, and the purified (meth)acrylic acid may be extracted from the line, or a line for extracting the purified (meth)acrylic acid may be connected directly to the bottom of the distillation column, separate from the effluent circulation line. In this case, it is preferable to extract the purified (meth)acrylic acid as a liquid.

When the vaporization separation tower is a stripper tower, it is preferable to introduce the easily polymerizable compound-containing liquid into the stripper tower from the center or top of the stripper tower and extract the purified easily polymerizable compound from the bottom of the tower. In the stripper tower, the stripping gas is basically supplied from the bottom of the tower, and the low boiling point components contained in the easily polymerizable compound-containing liquid are vaporized and separated. It is preferable to extract the purified easily polymerizable compound as a liquid from the stripper tower. In this case, a pipe for extracting the purified easily polymerizable compound-containing liquid may be provided in the extract liquid circulation path described later, or a pipe for extracting the purified easily polymerizable compound-containing liquid may be directly connected to the bottom of the distillation tower, separate from the extract liquid circulation path.

The vaporization separation tower is provided with an extract circulation path that returns at least a portion of the bottom liquid accumulated at the bottom of the tower to the vaporization separation tower. The extract circulation path is provided with a reboiler equipped with a heating unit. The extract circulation path is a means for increasing the temperature in the vaporization separation tower and facilitating temperature control by heating the extract liquid extracted from the bottom of the tower in the reboiler and returning it to the vaporization separation tower, thereby increasing the separation efficiency in the vaporization separation tower. Therefore, to the extent that these can be achieved, there are no particular limitations on the connection type between the vaporization separation tower and the extract circulation path and the structure of the extract circulation path. However, in this invention, a liquid level gauge or the like that cannot achieve the above is not referred to as an extract circulation path.

The effluent circulation path may be one for one vaporization separation tower, or multiple for one vaporization separation tower, or one for multiple vaporization separation towers. When one effluent circulation path is provided for multiple vaporization separation towers, the effluents from the multiple vaporization separation towers can be collected and heated and then returned to each vaporization separation tower. From the viewpoint of increasing the separation efficiency in the vaporization separation tower and enabling more precise control, it is preferable to provide multiple effluent circulation paths for one vaporization separation tower. However, if the required accuracy can be obtained, it is simple and preferable to provide one effluent circulation path for one vaporization separation tower.

The circulation path is preferably set up so that the bottom liquid extracted from the bottom of the tower is returned to the bottom of the tower. This makes it easier to adjust the temperature inside the vaporization separation tower. The position of the bottom liquid circulation path for returning the liquid to the bottom of the tower may be higher than, lower than, or at the same height as the position at which the bottom liquid is extracted from the bottom of the tower. In the following, the extracted liquid flowing through the circulation path may be referred to as the "circulating liquid".

As the reboiler, it is preferable to use a heat exchanger with a heat transfer surface, which makes it easier to control the temperature of the circulating liquid. In this case, the heat transfer surface becomes the heating part of the reboiler. As the heat exchanger, any known heat exchanger can be used, and examples of such heat exchangers include a plate type heat exchanger in which one plate is arranged, or multiple plates are stacked at intervals, and the heat medium presence part and the circulating liquid presence part are arranged alternately through the plates; a multi-tube (shell-and-tube) heat exchanger in which multiple tubes are arranged in a container and heat exchange is performed inside and outside the tube; a double-tube heat exchanger in which an inner tube is arranged inside an outer tube and heat exchange is performed inside and outside the inner tube; a coil type heat exchanger in which one tube is arranged in a coil shape in a container and heat exchange is performed inside and outside the tube; and a spiral type heat exchanger in which two heat transfer plates are spirally wound around a central tube whose cross section is divided into two, forming two spiral flow paths. The cross-sectional shape of the tubes used in the multi-tube heat exchanger, double-tube heat exchanger, coil heat exchanger, and spiral heat exchanger is not particularly limited. In terms of heat transfer efficiency and ease of maintenance, plate heat exchangers and multi-tube heat exchangers in which a circulating liquid flows inside the tubes are preferred, with the latter being particularly preferred.

The heating section of the reboiler is preferably installed so that the circulating liquid flows vertically from below to above. As described later, in the present invention, since the oxygen-containing gas supply port is provided at a position lower than the inlet of the heating section of the reboiler, in order to prevent the oxygen-containing gas supplied to the circulating liquid from remaining in the heating section or from flowing unevenly in the heating section, the heating section is preferably installed so that the circulating liquid flows vertically from below to above. From the same viewpoint, when a heat exchanger is used as a reboiler, it is preferable to use a plate heat exchanger or a multi-tube heat exchanger. Note that, when a heat exchanger is used as a reboiler, the inlet of the heating section refers to the part located most upstream on the heat transfer surface of the heat exchanger. For example, in the case of a multi-tube heat exchanger in which the circulating liquid flows in the tubes, the upstream tube plate surface corresponds to the inlet of the heating section.

In the vaporization separation step, it is desirable to suppress the polymerization of easily polymerizable substances. Therefore, it is preferable that the liquid containing easily polymerizable compounds introduced into the vaporization separation tower contains a polymerization inhibitor, or that a polymerization inhibitor is added to the vaporization separation tower. In particular, since the temperature is high at the bottom of the vaporization separation tower and polymerization reactions tend to proceed easily there, it is preferable that the bottom liquid contains a polymerization inhibitor.

As the polymerization inhibitor, a conventionally known polymerization inhibitor can be used, for example, quinones such as hydroquinone and methoquinone (p-methoxyphenol); phenothiazines such as phenothiazine, bis-(α-methylbenzyl)phenothiazine, 3,7-dioctylphenothiazine, and bis-(α-dimethylbenzyl)phenothiazine; N-oxyl compounds such as 2,2,6,6-tetramethylpiperidinooxyl, 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl, and 4,4',4"-tris-(2,2,6,6-tetramethylpiperidinooxyl)phosphite; dialkyldithiocarbamate; Examples of the polymerization inhibitor include copper salt compounds such as copper carbamate, copper acetate, copper naphthenate, copper acrylate, copper sulfate, copper nitrate, and copper chloride; manganese salt compounds such as manganese dialkyldithiocarbamate, manganese diphenyldithiocarbamate, manganese formate, manganese acetate, manganese octanoate, and manganese naphthenate; and nitroso compounds such as N-nitrosophenylhydroxylamine and its salts, p-nitrosophenol, and N-nitrosodiphenylamine and its salts. These polymerization inhibitors may be used alone or in combination of two or more. Of these, it is preferable to use at least quinones as the polymerization inhibitor.

The concentration of the polymerization inhibitor in the bottom liquid may be set as appropriate, but in order to ensure that the polymerization inhibitor effect in the vaporization separation tower is properly exerted, it is preferably, for example, 1 ppm by mass or more, more preferably 10 ppm by mass or more, even more preferably 100 ppm by mass or more, and particularly preferably 1000 ppm by mass or more. There is no particular upper limit to the concentration of the polymerization inhibitor in the bottom liquid, but in order to reduce the amount of polymerization inhibitor added, it is preferably 50,000 ppm by mass or less, more preferably 10,000 ppm by mass or less, even more preferably 5,000 ppm by mass or less, and particularly preferably 2,000 ppm by mass or less.

In the vaporization separation tower, it is preferable to carry out vaporization separation at the lowest possible temperature in order to suppress the polymerization of easily polymerizable compounds. Therefore, in the present invention, the pressure inside the vaporization separation tower is set lower than atmospheric pressure. In order to ensure that separation and purification are carried out appropriately in the vaporization separation tower, it is preferable to control the pressure inside the vaporization separation tower so that the pressure at the bottom of the tower > the pressure in the middle of the tower > the pressure at the top of the tower.

In a vaporization separation tower, the temperature tends to be higher toward the bottom of the tower, and in the reboiler of the effluent circulation path in particular, the effluent is heated, which makes it easier for the polymerization reaction of easily polymerizable compounds to proceed, so measures to prevent the polymerization of easily polymerizable compounds are important in the reboiler of the effluent circulation path. If the polymerization reaction of easily polymerizable compounds proceeds in the heating section of the reboiler, the polymers will adhere and accumulate on the heating section, which can cause blockage of the flow path in the heating section.

Therefore, a supply port for supplying oxygen-containing gas is provided upstream of the reboiler in the circulation path, and the oxygen-containing gas is supplied from the supply port to the extracted liquid (circulating liquid) flowing in the circulation path. This enhances the polymerization prevention effect in the circulation path, and makes it possible to suppress polymerization of easily polymerizable compounds in the heating section of the reboiler, where polymerization reactions are particularly likely to occur. As the oxygen-containing gas, pure oxygen gas, air, a gas with an arbitrary oxygen concentration obtained by mixing nitrogen, oxygen, and air, process waste gas, etc. can be used.

When supplying oxygen-containing gas to the circulating liquid as described above to suppress polymerization in the reboiler heating section, it is desirable that as much of the oxygen as possible dissolves in the circulating liquid before the oxygen-containing gas is supplied to the circulating liquid and reaches the reboiler heating section. To achieve this, it is desirable to supply the oxygen-containing gas to the circulating liquid under as high a pressure as possible. On the other hand, from the viewpoint of suppressing polymerization of easily polymerizable compounds in the vaporization separation tower, it is desirable to be able to set the temperature required for vaporization separation low, and for this reason, the vaporization separation tower refines easily polymerizable compounds under reduced pressure. However, in this case, the bottom liquid of the vaporization separation tower contains very little oxygen, which has a polymerization-inhibiting effect, and polymerization is more likely to occur in the reboiler heating section. In other words, the gas pressure on the top surface of the bottom liquid of the vaporization separation tower is less than normal pressure, so the saturated concentration of gas dissolved in the bottom liquid is low.

In this invention, therefore, oxygen-containing gas is supplied at a location of high liquid pressure upstream of the reboiler where the oxygen saturation concentration in the circulating liquid is high. Specifically, the supply port for the oxygen-containing gas is provided in a circulation path located at a height difference of 0.5 m or more below the inlet of the heating section of the reboiler, thereby dissolving more oxygen into the circulating liquid. This will be explained with reference to the drawings.

1 and 2 show schematic diagrams of the system around the vaporization separation tower. The vaporization separation tower 1 is provided with a circulation path 2 for extracting at least a portion of the tower bottom liquid 3 and returning it to the vaporization separation tower 1. The circulation path 2 is provided with a reboiler 4 equipped with a heating section 5, and a supply port 7 for supplying oxygen-containing gas upstream and below the heating section 5. In the vaporization separation tower 1, the gas pressure on the top surface of the tower bottom liquid is set to less than normal pressure, which makes it possible to lower the vaporization temperature of the liquid containing the easily polymerizable compound and suppress polymerization of the easily polymerizable compound in the vaporization separation tower 1. On the other hand, by providing the oxygen-containing gas supply port 7 at a position 0.5 m or more below the inlet of the heating section 5 of the reboiler 4, the oxygen-containing gas can be supplied to the circulation liquid under higher pressure. In the drawings, the difference in height between the inlet of the heating section 5 of the reboiler 4 and the oxygen-containing gas supply port 7 is indicated by an arrow h. By providing the oxygen-containing gas supply port 7 at such a position, it is possible to dissolve the supplied oxygen into the circulating liquid quickly or to dissolve a larger amount of oxygen into the circulating liquid. In this case, the oxygen-containing gas supply port 7 is subjected to a liquid head pressure difference of 5 kPa or more between the oxygen-containing gas supply port 7 and the inlet of the heating section 5 of the reboiler 4 in addition to the liquid head pressure at the inlet of the heating section 5 of the reboiler 4, so that the oxygen-containing gas can be supplied under higher pressure. In addition, sufficient time can be secured for the supplied oxygen to dissolve into the circulating liquid between the supply port 7 and the heating section 5 of the reboiler 4. By supplying the oxygen-containing gas to the circulating liquid in this way, the polymerization reaction of the easily polymerizable compound can be suppressed in the heating section 5, and the supplied oxygen dissolving in the circulating liquid can also be suppressed from being unevenly distributed in the heating section 5.

The pressure at the bottom of the vaporization separation tower 1, i.e., the gas pressure (absolute pressure) at the top of the bottom liquid, may be set appropriately based on the temperature dependency of the polymerization reaction of the easily polymerizable compound or a composition containing it in the vaporization separation tower and the temperature-vapor pressure correlation, and is usually, for example, preferably 50 kPa or less, more preferably 40 kPa or less, and even more preferably 30 kPa or less. By setting the gas pressure at the top of the bottom liquid to 50 kPa or less, the temperature required for vaporization of the easily polymerizable compound can be lowered. The lower limit of the gas pressure at the top of the bottom liquid may be set appropriately based on the cost of reducing the pressure and the durability of the vaporization separation tower, and is usually, for example, preferably 5 kPa or more, more preferably 8 kPa or more, and even more preferably 10 kPa or more.

From the viewpoint of supplying oxygen-containing gas to the circulating liquid under higher pressure, the position of the supply port 7 is preferably at least 1.0 m below the inlet of the heating section 5 of the reboiler 4, and more preferably at least 1.5 m below. There is no particular upper limit to this value, but taking into account the practical installation mode of the vaporization separation tower 1 and the reboiler 4, the height difference is preferably within 8.0 m, more preferably within 6.0 m, and even more preferably within 4.0 m.

The pressure of the circulating liquid (extracted liquid) at the supply port 7 is preferably 30 kPa or more higher than the gas pressure at the top surface of the bottom liquid of the vaporization separation tower 1, more preferably 40 kPa or more higher, and even more preferably 50 kPa or more higher. This makes it possible to more effectively suppress polymerization of the easily polymerizable compound in both the vaporization separation tower 1 and the reboiler 4. There is no particular limit to the upper limit of the pressure difference between the pressure of the circulating liquid at the supply port 7 and the gas pressure at the top surface of the bottom liquid of the vaporization separation tower 1, but taking into account the practical installation mode of the vaporization separation tower 1 and the reboiler 4, the pressure difference is preferably 100 kPa or less, more preferably 90 kPa or less, and even more preferably 80 kPa or less. The pressure of the circulating liquid at the supply port 7 means the pressure in a state that is not affected by the oxygen-containing gas supplied from the supply port 7, and can be determined, for example, by measuring the pressure of the circulating liquid at the supply port 7 in a state where no oxygen-containing gas is supplied from the supply port 7. Alternatively, the pressure of the circulating fluid at the supply port 7 can be obtained by measuring the pressure of the circulating fluid at a position away from the supply port 7 and correcting the obtained pressure value with the pressure loss from the supply port 7 to the measurement position calculated based on the piping shape of the circulation path 2, the flow rate of the circulating fluid, etc.

Only one supply port 7 may be provided in the circulation path 2, or multiple supply ports 7 may be provided. When multiple supply ports 7 are provided, it is preferable that the pressure difference between the pressure of the circulating liquid at at least one supply port and the gas pressure at the top surface of the liquid at the bottom of the tower is in the above range. In addition, it is preferable that the amount of oxygen-containing gas supplied from a supply port located at a position where the above pressure difference is achieved is 50% or more of the total amount of oxygen-containing gas supplied from all supply ports, more preferably 70% or more, and even more preferably 80% or more. It is particularly preferable that the pressure difference between the pressure of the circulating liquid at all supply ports and the gas pressure at the top surface of the liquid at the bottom of the tower is in the above range.

The flow rate of the circulating liquid in the circulation path 2 may be set arbitrarily, but a high Reynolds number in the circulation path 2 causes the circulating liquid to be in a turbulent state, which increases the dissolution rate of the oxygen-containing gas, so it is preferable for the Reynolds number to be 2300 or more. The Reynolds number of the circulating liquid is more preferably 5000 or more, even more preferably 10000 or more, even more preferably 50000 or more, and particularly preferably 100000 or more.

Methods for flowing the extracted liquid in a predetermined direction in the circulation path include a self-circulation method using the thermosiphon principle and a forced circulation method using a liquid feed pump. In the latter method, if the supply port 7 is provided downstream (discharge side) of the liquid feed pump, the oxygen-containing gas can be supplied to the circulating liquid at a higher pressure by the discharge pressure of the liquid feed pump. On the other hand, if the supply port 7 is provided upstream (suction side) of the liquid feed pump, the dissolution of oxygen can be promoted by stirring in the pump. Note that if an excessive amount of oxygen-containing gas is supplied to the suction side of the liquid feed pump, cavitation may occur and the amount of liquid fed may become unstable, so in this case, it is preferable to set the supply amount of oxygen-containing gas appropriately. The supply port 7 may be provided both upstream and downstream of the liquid feed pump. Note that the suction side of the liquid feed pump is connected to the bottom of the vaporization separation tower 1, and the liquid at the bottom of the tower is extracted by this liquid feed pump, heated in the reboiler 4, and then returned to the bottom of the tower.

The average residence time of the circulating liquid (extracted liquid) from the supply port 7 of the circulation path 2 to the inlet of the heating section 5 of the reboiler 4 is preferably 2 seconds or more, more preferably 3 seconds or more, and even more preferably 4 seconds or more. This makes it easier for the supplied oxygen to be sufficiently dissolved in the circulating liquid from the time when the oxygen-containing gas is supplied to the circulating liquid until the time when the oxygen-containing gas reaches the heating section 5 of the reboiler 4. The upper limit of the average residence time is not particularly limited, but since there is no particular need to install the circulation path 2 excessively long, it may be, for example, 60 seconds or less, 30 seconds or less, or 15 seconds or less. The average residence time of the circulating liquid from the supply port of the oxygen-containing gas in the circulation path to the inlet of the heating section of the reboiler can be obtained by dividing the volume (m ³ ) of the circulation path from the supply port of the oxygen-containing gas in the circulation path to the inlet of the heating section of the reboiler by the flow rate (m ³ /sec) of the circulating liquid flowing through the circulation path. The average residence time can be appropriately adjusted by adjusting the position of the supply port or changing the diameter of the circulation path.

The supply port 7 may be provided at a portion where the circulation path 2 extends upward (e.g., vertically upward or diagonally upward) as shown in FIG. 1, or at a horizontal portion where the circulation path 2 extends horizontally as shown in FIG. 2. Alternatively, the supply port 7 may be provided at a bent portion of the circulation path 2. When multiple supply ports 7 are provided, the average residence time from the supply ports 7 of the circulation path 2 to the inlet of the heating section 5 of the reboiler 4 is set based on the supply port that provides 50% or more of the total amount of oxygen supplied.

The circulation path 2 is preferably provided with a bend between the supply port 7 and the heating section 5 of the reboiler 4. By providing a bend in the circulation path 2, the oxygen-containing gas is well mixed with the circulation liquid when the circulation liquid passes through the bend, and oxygen is easily dissolved in the circulation liquid. The bend may be, for example, a portion that changes a horizontal flow to a vertical flow, or a portion that changes a horizontal flow in one direction to a horizontal flow in the other direction. In FIG. 2, a bend 8 that changes a horizontal flow to a vertical flow is provided between the supply port 7 and the heating section 5 of the reboiler 4.

As the bent section, so-called elbow pipes or bend pipes can be used. The bending angle of the bent section is preferably 40° or more, more preferably 55° or more, even more preferably 70° or more, and preferably 125° or less, more preferably 110° or less, and even more preferably 95° or less. The bending angle of the bent section refers to the angle difference between the tube axis extending direction on the upstream side of the bent section and the tube axis extending direction on the downstream side, and the bending angle is 0° in a straight pipe section.

From the viewpoint of increasing the mixing efficiency of the oxygen-containing gas and the circulating liquid, it is preferable that the widening section 6 is provided at the connection between the circulation path 2 upstream of the heating section 5 of the reboiler 4 and the heating section 5. In the widening section 6, the cross-sectional area of the flow path through which the circulating liquid flows is formed so as to widen from the upstream side to the downstream side, so that the circulating liquid passes through the widening section 6 to form a turbulent flow, the oxygen-containing gas is well mixed with the circulating liquid, and oxygen is easily dissolved in the circulating liquid. In addition, by slowing down the linear velocity of the circulating liquid in the widening section 6, it is possible to obtain an effect that the residence time from the supply port 7 to the inlet of the heating section 5 can be increased, and the amount of oxygen dissolved in the circulating liquid can be increased. Furthermore, even if a part of the oxygen-containing gas supplied to the circulating liquid passes through the widening section 6 in a gaseous state, the oxygen-containing gas is mixed with the circulating liquid in the widening section 6, so that the uneven distribution of the oxygen-containing gas in the heating section 5 can be suppressed. The widening section may be provided independently, and a heat exchanger having a widening section may be used as a reboiler.

In the widening section 6, the cross-sectional area of the flow path through which the circulating liquid flows is preferably expanded by 1.2 times or more, more preferably 1.5 times or more, even more preferably 2 times or more, and preferably 50 times or less, more preferably 30 times or less, and even more preferably 20 times or less.

As shown in FIG. 2, the oxygen-containing gas supply port 7 is preferably provided in a horizontal portion of the circulation path 2 that extends in a substantially horizontal direction. For example, if the supply port 7 is provided in a portion of the circulation path 2 that extends in a vertical direction, the oxygen-containing gas may rise regardless of the flow direction of the circulating liquid at that point, which may result in poor flow of the circulating liquid or a short contact time between the oxygen-containing liquid gas and the circulating liquid. However, by providing the supply port 7 in a horizontal portion of the circulation path 2, the oxygen-containing gas and the circulating liquid can be kept coexisting under high pressure for a longer period of time, making it possible to more efficiently dissolve oxygen in the circulating liquid. In this case, the average residence time of the circulating liquid in the horizontal portion is preferably 0.5 seconds or more, and more preferably 1 second or more.

The horizontal section of the circulation path 2 may be provided in a portion between the supply port 7 and the inlet of the heating section 5 that does not include the supply port 7, and in this case, the above-mentioned effect of the horizontal section can also be obtained. A plurality of horizontal sections may be provided between the supply port 7 and the inlet of the heating section 5. Therefore, it is preferable that the total average residence time of the circulating liquid in all horizontal sections between the supply port 7 and the inlet of the heating section 5 is 0.5 seconds or more, and more preferably 1 second or more.

It is preferable that the circulation path 2 does not have any portion extending downward (including diagonally downward) from the upstream side to the downstream side between the supply port 7 and the heating section 5 of the reboiler 4. This allows the oxygen-containing gas to move favorably toward the reboiler 4 side along with the flow of the circulating liquid, and also makes it difficult for the oxygen-containing gas to remain in a part of the circulation path 2 and obstruct the flow of the circulating liquid. When a horizontal section is provided in the circulation path 2, it is preferable that a bent section 8 is provided downstream of the horizontal section, and the circulation path 2 is configured to extend upward (for example, vertically upward or diagonally upward) downstream of the bent section.

The amount of oxygen-containing gas supplied to the circulating liquid may be set appropriately according to the dissolved oxygen concentration of the circulating liquid at the inlet of the heating section 5 of the reboiler 4. It is preferable that the dissolved oxygen concentration of the circulating liquid at the inlet of the heating section 5 of the reboiler 4 is, for example, 5 ppm by mass or more.

For example, a nozzle can be used as the supply port 7 for supplying the oxygen-containing gas. For example, the nozzle can be a ring-shaped, cylindrical, or any other shaped nozzle with one or more holes with a diameter of 0.1 mm to 10 mm. A sintered filter can also be used to increase the bubble surface area of the oxygen-containing gas supplied to the circulating liquid.

To dissolve oxygen in the circulating liquid more quickly, the oxygen-containing gas may be supplied to the circulating liquid as microbubbles. Since microbubbles have a smaller bubble diameter than normal bubbles, when the same amount of oxygen-containing gas is supplied, the total surface area of the microbubbles is larger, making it easier for the oxygen to dissolve in the circulating liquid more quickly.

The microbubbles may include so-called nanobubbles, and those with a bubble diameter of 0.1 μm to 100 μm at the time of generation are preferably used. A known device may be used as the microbubble generator. Examples of methods for generating microbubbles include a gas-liquid two-phase flow swirling method (e.g., swirling flow method, static mixer method, mechanical shear (rotation) method) in which a vortex or shear flow is generated by a water flow or mechanical stirring to break down the bubbles, a pressurized dissolution method in which a gas is dissolved in the liquid to be treated under pressure and then reduced in pressure to re-vaporize the gas dissolved in the liquid to be treated as microbubbles, a Venturi method in which a gas-liquid fluid is passed through a tube having a throat section and compressed and reduced in pressure when passing through the throat section to break down the bubbles, a cavitation nozzle method in which the liquid static pressure is locally lowered to vaporize part of the liquid to generate vapor bubbles, and the pressure is restored in a short time to break down the vapor bubbles, and a filter method in which a gas is passed through a filter having a nanoporous section in the liquid to break down the bubbles, and any of these methods may be used.

In the present invention, the easily polymerizable compound-containing liquid introduced into the vaporization separation step can be the easily polymerizable compound-containing liquid obtained in the previous step. For example, when the easily polymerizable compound is (meth)acrylic acid, the (meth)acrylic acid-containing liquid obtained in the collection step, condensation step, or the like can be used. Below, a method for obtaining the (meth)acrylic acid-containing liquid to be introduced into the vaporization separation tower will be described using the (meth)acrylic acid production process as an example.

When the easily polymerizable compound is (meth)acrylic acid, the method for producing an easily polymerizable compound of the present invention (in this case, the method for producing (meth)acrylic acid) preferably includes a step of subjecting a raw material for producing (meth)acrylic acid to a catalytic gas-phase oxidation reaction to obtain a (meth)acrylic acid-containing gas, and a step of contacting the (meth)acrylic acid-containing gas with an absorbent and/or cooling and condensing the gas to obtain a (meth)acrylic acid-containing liquid.

The raw material for producing (meth)acrylic acid to be subjected to the catalytic gas phase oxidation reaction can be any raw material that produces (meth)acrylic acid through a reaction, and examples of such raw materials include propane, propylene, (meth)acrolein, and isobutylene. Acrylic acid can be obtained, for example, by oxidizing propane, propylene, or acrolein in one stage, or by oxidizing propane or propylene via acrolein in two stages. Acrolein is not limited to a raw material obtained by oxidizing propane or propylene, and may be a raw material obtained by dehydrating glycerin. Methacrylic acid can be obtained, for example, by oxidizing isobutylene or methacrolein in one stage, or by oxidizing isobutylene via methacrolein in two stages.

Conventionally known catalysts can be used as the catalyst for catalytic gas phase oxidation. For example, when propylene is used as the raw material for acrylic acid production, it is preferable to use a composite oxide catalyst containing molybdenum and bismuth (molybdenum-bismuth catalyst) as the catalyst. When propane or acrolein is used as the raw material for acrylic acid production, it is preferable to use a composite oxide catalyst containing molybdenum and vanadium (molybdenum-vanadium catalyst) as the catalyst.

As the reactor for carrying out the catalytic gas phase oxidation reaction, a fixed bed reactor, a fluidized bed reactor, a moving bed reactor, etc. can be used. Among them, it is preferable to use a multi-tube fixed bed reactor because of its excellent reaction efficiency. When producing (meth)acrylic acid by subjecting the (meth)acrylic acid production raw material to a two-stage oxidation reaction, a reactor for carrying out the first stage oxidation reaction and a reactor for carrying out the second stage oxidation reaction can be combined, or a single reactor can be divided into an area for carrying out the first stage oxidation reaction and an area for carrying out the second stage oxidation reaction, thereby producing (meth)acrylic acid from the (meth)acrylic acid production raw material. In the latter case, for example, in a fixed bed reactor, the inlet side (the introduction side of the (meth)acrylic acid production raw material) of the reaction tube of the fixed bed reactor can be filled with a catalyst for carrying out the first stage oxidation reaction, and the outlet side can be filled with a catalyst for carrying out the second stage oxidation reaction.

The reaction for producing a (meth)acrylic acid-containing gas from a (meth)acrylic acid production raw material may be carried out under known reaction conditions. For example, when propylene is oxidized in two stages to produce acrylic acid, a propylene-containing gas may be introduced into a reactor together with molecular oxygen, and the first stage oxidation reaction may be carried out under conditions of, for example, a reaction temperature of 250°C to 450°C, a reaction pressure of 0 MPaG to 0.5 MPaG, and a space velocity of 300 h ^-1 to 5000 h ^-1 , and then the second stage oxidation reaction may be carried out under conditions of a reaction temperature of 250°C to 380°C, a reaction pressure of 0 MPaG to 0.5 MPaG, and a space velocity of 300 h ^-1 to 5000 h ^-1 .

The (meth)acrylic acid-containing gas obtained by subjecting the raw material for producing (meth)acrylic acid to a catalytic gas phase oxidation reaction is brought into contact with an absorbent and/or cooled and condensed to obtain a (meth)acrylic acid-containing liquid. In the former case, the (meth)acrylic acid-containing gas is introduced into an absorption tower and brought into contact with the absorbent, whereby the (meth)acrylic acid is absorbed into the absorbent to obtain a (meth)acrylic acid-containing liquid (absorption step). In the latter case, the (meth)acrylic acid-containing gas is introduced into a condensation tower and cooled to condense the (meth)acrylic acid to obtain a (meth)acrylic acid-containing liquid (condensation step).

The absorption tower is not particularly limited as long as it can bring the (meth)acrylic acid-containing gas and the absorbent into contact with each other in the absorption tower. For example, the (meth)acrylic acid-containing gas is introduced into the absorption tower from the lower part of the absorption tower, and the absorbent is introduced into the absorption tower from the upper part of the absorption tower. As the (meth)acrylic acid-containing gas rises in the absorption tower, it comes into countercurrent contact with the absorbent, and the (meth)acrylic acid is absorbed into the absorbent and recovered as a (meth)acrylic acid-containing liquid. As the absorption tower, for example, a plate tower with shelves (sieve trays) provided in the tower, a packed tower with a filling material packed in the tower, a wet-wall tower in which the absorbent is supplied to the tower inner wall surface, a spray tower in which the absorbent is sprayed into the tower space, etc. can be used.

The absorbent is not particularly limited as long as it can absorb and dissolve (meth)acrylic acid, but examples of the absorbent that can be used include diphenyl ether, diphenyl, a mixture of diphenyl ether and diphenyl, water, and (meth)acrylic acid-containing water (for example, an aqueous solution containing (meth)acrylic acid produced in the (meth)acrylic acid production process). Of these, it is preferable to use water or (meth)acrylic acid-containing water as the absorbent, and it is more preferable to use (meth)acrylic acid-containing water or water that contains 50% by mass or more of water (more preferably 70% by mass or more, and even more preferably 80% by mass or more).

The temperature and supply amount of the absorbent may be appropriately set so that the (meth)acrylic acid contained in the (meth)acrylic acid-containing gas is sufficiently absorbed by the absorbent. The temperature of the absorbent is preferably 0°C or higher, more preferably 5°C or higher, and more preferably 35°C or lower, more preferably 30°C or lower, from the viewpoint of increasing the efficiency of collection of (meth)acrylic acid. The supply amount of the absorbent is preferably 2L/m3 or ^{more, more preferably 3L/m3 or more} , more preferably 5L/m3 or more, and more preferably 15L/m3 ^{or less} , more preferably 12L/m3 or less, and even more preferably 10L/m3 or ^less , in terms of the liquid-gas ratio (L/G) represented by the ratio of the supply amount (L) of the absorbent to the absorption tower to the supply ^amount ( ^G) of the (meth)acrylic acid- ^containing gas to the absorption tower.

The (meth)acrylic acid absorbed in the absorbent is extracted from the absorption tower as a (meth)acrylic acid-containing liquid. The (meth)acrylic acid-containing liquid may be extracted, for example, from a position in the absorption tower below the supply position of the (meth)acrylic acid-containing gas (for example, the bottom of the absorption tower).

The absorption tower preferably has a circulation path that returns a portion of the (meth)acrylic acid-containing liquid discharged from the absorption tower to a position above the (meth)acrylic acid-containing gas supply position and the (meth)acrylic acid-containing liquid discharge position of the absorption tower and below the absorbent supply position. By circulating a portion of the (meth)acrylic acid-containing liquid extracted from the absorption tower back to the absorption tower through the circulation path, the (meth)acrylic acid concentration of the (meth)acrylic acid-containing liquid can be increased. The circulation path is preferably provided with a heat exchanger for cooling the (meth)acrylic acid-containing liquid passing through the circulation path.

On the other hand, when the (meth)acrylic acid-containing liquid is obtained by condensing the (meth)acrylic acid-containing gas, it is preferable to use a condensation tower, and for example, a heat exchanger equipped with a heat transfer surface can be used as the condensation tower. By cooling the (meth)acrylic acid-containing gas through the heat transfer surface, (meth)acrylic acid can be condensed from the (meth)acrylic acid-containing gas, and as a result, a (meth)acrylic acid-containing liquid is obtained. As the heat exchanger, a conventionally known heat exchanger may be used, and for example, a plate-type heat exchanger, a multi-tube type (shell-and-tube type) heat exchanger, a double-tube type heat exchanger, a coil type heat exchanger, a spiral type heat exchanger, etc. can be adopted. By connecting a plurality of heat exchangers in series, the (meth)acrylic acid-containing gas can be cooled in multiple stages, and the (meth)acrylic acid-containing liquid can be recovered by fractional condensation.

The condensation tower may be one in which the (meth)acrylic acid-containing gas is brought into contact with the condensed liquid to obtain the (meth)acrylic acid-containing liquid from the (meth)acrylic acid-containing gas. In this case, it is preferable to provide shelves (sieve trays) in the condensation tower or fill the condensation tower with a packing material to increase the contact efficiency between the (meth)acrylic acid-containing gas and the condensed liquid. By using such a condensation tower, the (meth)acrylic acid-containing gas is, for example, introduced into the condensation tower from the lower part and is fractionally condensed while moving from the lower part to the upper part of the condensation tower. At this time, for example, the (meth)acrylic acid-containing liquid is extracted from the middle part of the condensation tower, a substance having a higher boiling point than (meth)acrylic acid is extracted from the lower part of the condensation tower, and a substance having a lower boiling point than (meth)acrylic acid is extracted from the upper part of the condensation tower. A part of the condensed liquid may be extracted from each stage of the condensation tower and cooled by a heat exchanger or the like, and then returned to the same stage of the condensation tower. In addition, the condensed liquid returned to the condensation tower generally does not contain any liquid medium other than the condensed liquid generated in the condensation tower.

When obtaining a (meth)acrylic acid-containing liquid from a (meth)acrylic acid-containing gas, a polymerization inhibitor may be added to the collection tower or condensation tower to suppress polymerization of (meth)acrylic acid. As the polymerization inhibitor, the polymerization inhibitors described above can be used.

In the present invention, the (meth)acrylic acid-containing liquid thus obtained can be subjected to the vaporization separation step described above. In this case, a distillation tower can be used as the vaporization separation tower, since it is easier to obtain purified (meth)acrylic acid with a higher purity. In this case, it is preferable that the distillation tower functions as a light boiling separation tower. The light boiling separation tower is equipped with a liquid circulating path at the bottom of the tower.

In the light boiling separation tower, purified (meth)acrylic acid is obtained by removing at least a portion of the low boiling fraction from the (meth)acrylic acid-containing liquid. The low boiling fraction refers to a fraction with a lower boiling point than the purified (meth)acrylic acid, that is, a fraction withdrawn from the top side of the light boiling separation tower from the purified (meth)acrylic acid. The low boiling fraction includes compounds with a boiling point lower than that of (meth)acrylic acid, such as water and acetic acid.

In the light boiling separation tower, the (meth)acrylic acid-containing liquid is preferably introduced into the light boiling separation tower from the middle of the tower. For example, the (meth)acrylic acid-containing liquid is preferably introduced from a position in the range of 5% to 75% of the total theoretical number of stages from the top to the bottom of the light boiling separation tower, with the range being more preferably 10% or more, even more preferably 15% or more, and even more preferably 70% or less, and even more preferably 65% or less.

In the light boiling separation tower, the low boiling fraction is extracted from the top side (preferably the top of the tower) of the (meth)acrylic acid-containing liquid. The low boiling fraction is preferably extracted from a position in the range of 0% to less than 5% of the total theoretical number of stages from the top of the light boiling separation tower to the bottom of the tower, starting from the top side of the tower. The low boiling fraction distilled from the light boiling separation tower is preferably returned to the previous collection tower or condensation tower. There are no particular limitations on the amount of return to the collection tower, but for example, it is preferable to return to a range of 20% to 90% of the total length of the collection tower when the bottom of the collection tower is the starting point (0%) and the top of the tower is the end point (100%), and it is even more preferable to return to a range of 30% to 80%.

On the other hand, the purified (meth)acrylic acid is withdrawn from the outlet at the center and/or bottom of the light boiling separation tower. The outlet for purified (meth)acrylic acid is preferably located on the bottom side of the tower from the supply position of the (meth)acrylic acid-containing liquid, and is preferably provided, for example, in a range of 20% to 100% of the total theoretical number of stages from the top of the light boiling separation tower to the bottom of the tower, more preferably 25% or more, and even more preferably 35% or more. The outlet for purified (meth)acrylic acid is preferably provided in the center of the tower, and in this case, the bottom liquid containing a large amount of high boiling point components is withdrawn from the bottom of the tower, making it easier to increase the (meth)acrylic acid concentration in the purified (meth)acrylic acid withdrawn from the center of the tower. In this case, the outlet for the purified (meth)acrylic acid is preferably provided in a range of 90% or less of the total theoretical number of stages from the top to the bottom of the light boiling separation tower, starting from the top side, more preferably 80% or less, and even more preferably 70% or less.

In the light-boiling separation tower, distillation may be performed under conditions that allow separation of the low-boiling fraction containing the absorbent and acetic acid from the (meth)acrylic acid, and it is preferable to appropriately adjust the temperature and pressure in the light-boiling separation tower so that such distillation is possible. For example, the tower top pressure (absolute pressure) is preferably 2 kPa or more, more preferably 3 kPa or more, and also preferably 50 kPa or less, more preferably 40 kPa or less, and even more preferably 30 kPa or less. The tower top temperature is preferably 30°C or more, more preferably 40°C or more, and also preferably 70°C or less, and more preferably 60°C or less. The tower bottom temperature is preferably 70°C or more, more preferably 80°C or more, and also preferably 120°C or less, and more preferably 110°C or less. By performing distillation under such conditions, it becomes easy to obtain purified (meth)acrylic acid with reduced absorbent and acetic acid concentrations (for example, water concentration and acetic acid concentration are both 1 mass% or less).

In the distillation of the (meth)acrylic acid-containing liquid, an azeotropic solvent may or may not be used. When the (meth)acrylic acid concentration of the (meth)acrylic acid-containing liquid is 80% by mass or more, it is preferable not to use an azeotropic solvent, since it is easy to separate the low boiling fraction without using an azeotropic solvent.

The (meth)acrylic acid concentration of the purified (meth)acrylic acid obtained from the light boiling separation tower is not particularly limited as long as it is higher than that of the (meth)acrylic acid-containing liquid, but is preferably 80% by mass or more, more preferably 85% by mass or more, even more preferably 90% by mass or more, and particularly preferably 92% by mass or more. The upper limit of the (meth)acrylic acid concentration of the purified (meth)acrylic acid is not particularly limited, but is usually less than 99.5% by mass.

In the production method of the present invention, the bottom liquid obtained from the light boiling separation tower may be subjected to the vaporization separation step described above. In this case, it is preferable to use a distillation tower that functions as a high boiling separation tower as the vaporization separation tower. The bottom liquid of the light boiling separation tower contains a considerable concentration of (meth)acrylic acid and also contains a large amount of components with a higher boiling point than (meth)acrylic acid, such as Michael adducts and maleic acid. Therefore, by introducing the bottom liquid of the light boiling separation tower into the high boiling separation tower and distilling it, a (meth)acrylic acid-containing distillate with reduced high boiling point components can be obtained. In this case, the bottom liquid of the light boiling separation tower becomes a (meth)acrylic acid-containing liquid, and the (meth)acrylic acid-containing distillate becomes purified (meth)acrylic acid. It is preferable to return the (meth)acrylic acid-containing distillate to the light boiling separation tower, which can increase the (meth)acrylic acid yield of the entire process.

At the bottom of the high boiling separation tower, a bottom liquid is generated in which high boiling components such as Michael adducts are concentrated. This bottom liquid is preferably withdrawn from the bottom of the high boiling separation tower and introduced into a (meth)acrylic acid dimer decomposition unit. Since the Michael adducts contained in the bottom liquid of the high boiling separation tower can be converted back into (meth)acrylic acid by thermal decomposition, it is preferable to introduce the bottom liquid into a (meth)acrylic acid dimer decomposition unit for thermal decomposition, and return the resulting decomposition liquid to the high boiling separation tower.

The (meth)acrylic acid dimer decomposition device is composed of a thermal decomposition tank, and the bottom liquid of the high boiling separation tower is heated in the thermal decomposition tank at a temperature of 120°C to 220°C (preferably 140°C to 200°C) to decompose the Michael adducts contained in the bottom liquid. In order to promote the decomposition of the Michael adducts, it is preferable to add a decomposition catalyst such as an alkali metal salt, an alkaline earth metal salt, or an N-oxyl compound described in JP 2003-89672 A to the thermal decomposition tank. In particular, when an N-oxyl compound is used as a polymerization inhibitor in the light boiling separation tower or the collection tower or condensation tower upstream of the light boiling separation tower, it is preferable because it can also act as a decomposition catalyst for the Michael adducts.

In the high boiling separation tower, at least a portion of the bottom liquid in which high boiling components are concentrated can be extracted and heated in a reboiler provided in the extraction liquid circulation path, and returned to the high boiling separation tower. When the Michael adducts contained in the bottom liquid are decomposed in a (meth)acrylic acid dimer decomposition device, the decomposition liquid may be returned to the high boiling separation tower via a reboiler, or the extracted bottom liquid heated in the reboiler may be introduced into the (meth)acrylic acid dimer decomposition device (thermal decomposition tank).

The purified easily polymerizable compound obtained in the vaporization separation process may be subjected to another purification process, or may be handled as a product without further purification, or may be used as a raw material for the production of other compounds. In the former case, it is preferable to employ crystallization in the purification process, since it is easy to obtain a highly pure easily polymerizable compound while suppressing the polymerization of the easily polymerizable compound.

The crystallization may be performed in a batch or continuous manner. As a crystallizer used in the crystallization operation, for example, a crystallizer having a heat transfer surface and capable of crystallizing and/or melting the easily polymerizable compound by heat exchange on the heat transfer surface can be used. By supplying a cold heat transfer medium to one side of the heat transfer surface and supplying the purified easily polymerizable compound obtained in the vaporization separation process to the other side, the liquid containing the purified easily polymerizable compound is cooled by heat exchange via the heat transfer surface, and the easily polymerizable compound is crystallized. In the batch crystallization operation, the crystallized easily polymerizable compound is heated by heat exchange via the heat transfer surface by supplying a hot heat transfer medium to one side of the heat transfer surface, to obtain a molten liquid containing the easily polymerizable compound. The easily polymerizable compound may be melted by heating the same heat transfer surface used in the crystallization, or the crystallized easily polymerizable compound may be recovered and the recovered crystals of the easily polymerizable compound may be heated via a heat transfer surface other than that used in the crystallization.

In the batch crystallization operation, the purified easily polymerizable compound obtained in the vaporization separation step is cooled to crystallize the easily polymerizable compound, and the crystallized easily polymerizable compound is melted to obtain a molten liquid containing the easily polymerizable compound. In this case, in order to increase the purity of the obtained molten liquid of the easily polymerizable compound, it is preferable to first partially melt (sweat) the crystals of the easily polymerizable compound, wash out the impurities present between the crystals and on the crystal surface, and then melt the remaining crystals of the easily polymerizable compound to obtain a molten liquid of the easily polymerizable compound. It is preferable that the uncrystallized residue when the easily polymerizable compound is crystallized and the sweated liquid obtained by partially melting the crystals of the easily polymerizable compound are discharged from the crystallizer as crystallization residue. As a crystallization apparatus used in the batch crystallization operation, for example, a layer crystallization apparatus (dynamic crystallization apparatus) manufactured by Sulzer Chemtech, a static crystallization apparatus manufactured by BEFS PROKEM, etc. can be used.

Continuous crystallization operations can be carried out, for example, by continuously supplying the purified easily polymerizable compound obtained in the vaporization separation process to a crystallizer, cooling it to crystallize the easily polymerizable compound, continuously discharging a slurry consisting of the crystals of the easily polymerizable compound and the mother liquor from the crystallizer, and further supplying the slurry to a washing column to continuously separate the crystals of the easily polymerizable compound from the mother liquor while washing them. Examples of crystallization apparatuses used in continuous crystallization operations include crystallization apparatuses in which a crystallization section, a solid-liquid separation section, and a crystal purification section are integrated (e.g., a BMC (Backmixing Column Crystallizer) type crystallizer), crystallization apparatuses (e.g., a CDC (Cooling Disk Crystallizer) crystallizer manufactured by GOUDA Co., Ltd. or a DC (Drum Crystallizer) crystallizer manufactured by GEA Co., Ltd.), solid-liquid separation section (e.g., a belt filter, a centrifuge) and a crystal purification section (e.g., a KCP (Kureha Crystal Purifier) purification apparatus manufactured by Kureha Engineering Co., Ltd. or a WC (Wash Column) purification apparatus manufactured by GEA Co., Ltd.) are combined.

It is preferable to return the crystallization residue generated in the crystallization process to the preceding vaporization separation process, particularly to the light boiling separation tower. On the other hand, the high-purity easily polymerizable compound obtained in the crystallization process may be further purified by any purification means, but it is preferable to handle it as a product without further purification or use it as a raw material for manufacturing other compounds.

The present invention will be explained in more detail below by showing examples, but the scope of the present invention is not limited to these examples.

Example 1
An acrylic acid-containing liquid (crude acrylic acid) containing hydroquinone and hydroquinone monomethyl ether was supplied to the distillation tower, and distillation purification was performed. The distillation tower was provided with a circulation path for the bottom liquid, and the circulation path was provided with a reboiler having a vertical multitubular heat exchanger. The bottom liquid was heated by extracting a part of the bottom liquid from the distillation tower, heating it in the reboiler, and returning it to the distillation tower. The distillation tower was operated under the conditions of a bottom pressure of 23 kPa, a reboiler heating section inlet pressure of 62 kPa, and a bottom temperature of 95°C to 105°C, and the bottom liquid had a composition of 90% or more acrylic acid, 1300 ppm hydroquinone, and 250 ppm hydroquinone monomethyl ether. An oxygen nozzle was installed upstream of the reboiler in the circulation path, and oxygen gas was introduced into the circulation liquid flowing through the circulation path at 9 Nm ³ /h (gas volume / circulation liquid volume = 13%). The supply port of the oxygen nozzle was installed 1,850 mm below the inlet of the heat exchanger (heating section), the pressure of the circulating liquid at the supply port of the circulation path was 80 kPa, and the average residence time of the circulating liquid from the supply port of the circulation path to the inlet of the reboiler heating section was 5.1 seconds. After operating for about six months, the operation was stopped and the inside of the distillation column and the heat exchanger were inspected, but no polymers or the like were found.

(Comparative Example 1)
In Example 1, the supply port of the oxygen nozzle was installed 250 mm below the inlet of the heat exchanger, and operation was performed under conditions where the pressure of the circulating liquid at the supply port was 64 kPa. After operating for about six months, the operation was stopped and the insides of the distillation column and the heat exchanger were inspected, and it was found that 70% or more of the total number of heat exchanger tubes in the heat exchanger were clogged with polymers.

Example 2
An acrylic acid-containing liquid (crude acrylic acid) containing hydroquinone and hydroquinone monomethyl ether was supplied to the distillation tower, and distillation purification was performed. Distillation purification was performed. The distillation tower was provided with a circulation path for the bottom liquid, and the circulation path was provided with a reboiler having a vertical multitubular heat exchanger. The bottom liquid was heated by extracting a part of the bottom liquid of the distillation tower, heating it in the reboiler, and returning it to the distillation tower. The distillation tower was operated under the conditions of a bottom pressure of 15 kPa, a reboiler heating section inlet pressure of 54 kPa, and a bottom temperature of 80°C to 90°C, and the bottom liquid had a composition of 90% or more acrylic acid, 1600 ppm hydroquinone, and 250 ppm hydroquinone monomethyl ether. An oxygen nozzle was installed upstream of the reboiler in the circulation path, and oxygen gas was introduced into the circulation liquid flowing through the circulation path at 3 Nm ³ /h (gas volume / circulation liquid volume = 5%). The supply port of the oxygen nozzle was installed 1,750 mm below the inlet of the heat exchanger (heating section), the pressure of the circulating liquid at the supply port of the circulation path was 71 kPa, and the average residence time of the circulating liquid from the supply port of the circulation path to the inlet of the reboiler heating section was 5.2 seconds. After operating for about six months, the operation was stopped and the inside of the distillation column and the heat exchanger were inspected, but no polymerized products were found.

(Comparative Example 2)
In Example 2, the supply port of the oxygen nozzle was installed 250 mm below the inlet of the heat exchanger, and operation was performed under conditions where the pressure of the circulating liquid at the supply port was 57 kPa. After operating for about 6 months, the operation was stopped and the insides of the distillation column and the heat exchanger were inspected, and it was found that 50% or more of the total number of heat exchanger tubes in the heat exchanger were clogged with polymers.

Example 3
The acrylic acid-containing liquid (crude acrylic acid) containing hydroquinone was supplied to the distillation tower, and distillation purification was performed. Distillation purification was performed. The distillation tower was provided with a circulation path for the bottom liquid, and the circulation path was provided with a reboiler having a vertical multitubular heat exchanger. The bottom liquid was heated by extracting a part of the bottom liquid of the distillation tower, heating it in the reboiler, and returning it to the distillation tower. The distillation tower was operated under the conditions of a bottom pressure of 14 kPa, a reboiler heating section inlet pressure of 53 kPa, and a bottom temperature of 80°C to 90°C, and the bottom liquid had a composition of 90% or more acrylic acid and 1300 ppm hydroquinone. An oxygen nozzle was installed upstream of the reboiler in the circulation path, and oxygen gas was introduced into the circulation liquid flowing through the circulation path at 3 Nm ³ /h (gas volume / circulation liquid volume = 10%). The supply port of the oxygen nozzle was installed 1,800 mm below the inlet of the heat exchanger (heating section), the pressure of the circulating liquid at the supply port of the circulation path was 71 kPa, and the average residence time of the circulating liquid from the supply port of the circulation path to the inlet of the reboiler heating section was 5.3 seconds. After operating for about six months, the operation was stopped and the inside of the distillation column and the heat exchanger were inspected, but no polymers or the like were found.

(Comparative Example 3)
In Example 3, the supply port of the oxygen nozzle was installed 400 mm below the inlet of the heat exchanger, and operation was performed under conditions where the pressure of the circulating liquid at the supply port was 58 kPa. After operating for about six months, the operation was stopped and the insides of the distillation column and the heat exchanger were inspected, and it was found that 40% or more of the total number of heat exchanger tubes in the heat exchanger were clogged with polymers.

1: vaporization separation tower 2: circulation path 3: tower bottom liquid 4: reboiler 5: heating section 6: widening section 7: (oxygen-containing gas) supply port 8: bent section

Claims (9)

A method for producing an easily polymerizable compound, comprising a step of introducing an easily polymerizable compound-containing liquid into a vaporization separation column selected from a distillation column and a stripping column and purifying the liquid under reduced pressure,
the easily polymerizable compound is (meth)acrylic acid or a (meth)acrylic acid ester,
The easily polymerizable compound concentration in the easily polymerizable compound- containing liquid introduced into the vaporization separation tower is 50% by mass or more,
The vaporization separation tower is provided with a circulation path for returning at least a portion of the bottom liquid extracted from the tower to the vaporization separation tower,
The circulation path is provided with a supply port for supplying an oxygen-containing gas and a reboiler equipped with a heating unit in this order from the upstream side,
The heating section of the reboiler is installed so that the extracted liquid flowing through the circulation path flows vertically from below to above,
The supply port is disposed below the inlet of the heating unit by a height difference of 0.5 m or more, and the circulation path is provided on a horizontal portion extending in a substantially horizontal direction,
A method for producing an easily polymerizable compound, comprising: supplying an oxygen-containing gas to the extracted liquid through the supply port. The method for producing an easily polymerizable compound according to claim 1, wherein the pressure of the liquid discharged at the supply port is 30 kPa or more higher than the gas pressure at the top surface of the liquid at the bottom of the column. The method for producing an easily polymerizable compound according to claim 1 or 2, wherein the average residence time of the extracted liquid from the supply port of the circulation path to the inlet of the heating section is 2 seconds or more. The method for producing an easily polymerizable compound according to any one of claims 1 to 3, wherein the circulation path has a bent portion between the supply port and the heating section. The method for producing an easily polymerizable compound according to any one of claims 1 to 4 , wherein an average residence time of the extracted liquid in the horizontal portion is 0.5 seconds or more. The method for producing an easily polymerizable compound according to any one of claims 1 to 5 , wherein a widening section is provided at a connection portion between the circulation path and the heating section on an upstream side of the heating section. The method for producing an easily polymerizable compound according to any one of claims 1 to 6 , wherein the reboiler equipped with the heating section is composed of a plate heat exchanger, a shell-and-tube heat exchanger, a double-tube heat exchanger, a coil heat exchanger, or a spiral heat exchanger. The method for producing an easily polymerizable compound according to any one of claims 1 to 7 , wherein the oxygen-containing gas is supplied in the form of microbubbles. The method for producing an easily polymerizable compound according to any one of claims 1 to 8, wherein the distillation column does not use an azeotropic solvent.

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