JP6682850B2 - Method for distilling easily polymerizable compound - Google Patents

Description

  The present invention relates to a method for distilling a readily polymerizable compound.

  Monomers having a carbon double bond in the molecule and having high polymerizability, such as styrene, acrylonitrile, acrolein, acrylic acid and methacrylic acid, and their esters are raw materials for various polymer materials. Hereinafter, these monomers having high polymerizability are collectively referred to as easily polymerizable compounds.

  The easily polymerizable compound exhibits high polymerizability even in the distillation step, but the production of the polymer in the distillation step involves deterioration of product quality and operation stoppage of equipment, and therefore sufficient measures to prevent polymerization are indispensable. .

  The most common method of preventing polymerization is to add a polymerization inhibitor to the easily polymerizable compound. The added polymerization inhibitor traps a radical that causes a polymerization reaction, thereby inhibiting a chain polymerization reaction, or the added polymerization inhibitor and an easily polymerizable compound form a complex salt to form a carbon- The activity of the carbon double bond is reduced and the polymerization reaction is suppressed. Non-Patent Document 1 lists phenol compounds such as hydroquinone, soluble metal salts, phenothiazine, and molecular oxygen as polymerization inhibitors for acrylic acid.

  Most of the polymerization inhibitors have higher boiling points than the easily polymerizable compounds. Therefore, the volatilized easily polymerizable compound does not accompany the polymerization inhibitor, and accordingly, the condensation liquid contains almost no polymerization inhibitor. In particular, in the distillation column or the reactive distillation apparatus, volatilization and condensation of the easily polymerizable compound are always repeated, and the condensed liquid of the easily polymerizable compound continues to be produced, but the condensed liquid does not contain the polymerization inhibitor. Polymerization is suppressed by mixing with a liquid containing a polymerization inhibitor, such as a feed liquid or a reflux liquid.

  However, in the distillation column or the reactive distillation apparatus, there are places where the condensate containing no polymerization inhibitor and the liquid containing the polymerization inhibitor are not sufficiently brought into contact with each other. For example, the bottom surface of the support ring, the support beam, the side surface of the downcomer, etc. in the distillation column tray. Once the polymerized product is generated at the site, the polymerized product is continuously expanded and eventually clogged, making it impossible to continue the operation of the distillation apparatus.

As a method for preventing polymerization at the site, Patent Document 1 discloses a method of supplying a liquid containing a polymerization inhibitor to the site by using a spray nozzle or the like to keep the site clean.
Patent Document 2 discloses a method in which a liquid containing a polymerization inhibitor is sprayed onto the gas phase part of a reactive distillation apparatus to prevent the condensate attached to a part in the gas phase part from polymerizing. There is.

  Further, Patent Document 3 discloses a method for preventing polymerization by a condensate in a distillation column by using a compound having a corresponding vapor pressure as a polymerization inhibitor for distillation of an acrylic acid aqueous solution.

  Patent Document 4 discloses a method of preventing the blockage due to polymerization in the distillation column by simplifying the shape of the tray and eliminating the retention part of the liquid or gas.

JP, 2000-355570, A JP, 2003-292472, A Japanese Patent Laid-Open No. 47-17714 JP-A-2000-300903

Eizo Omori, "Acrylic acid and its polymer [I]", 3rd edition, Shokoido Co., Ltd., April 28, 1978, p. 46

  However, the conventional method for preventing polymerization in a distillation column or a reactive distillation apparatus still has some problems in its application site, particularly in the middle part of the distillation column or the reactive distillation apparatus.

  The use of a volatile polymerization inhibitor having a corresponding vapor pressure allows the condensed easily polymerizable compound to contain the polymerization inhibitor, but separates the polymerization inhibitor from the condensed easily polymerizable compound. It is not easy to remove, and the remaining polymerization inhibitor impairs the polymerizability, which is important as a raw material for polymer materials. Therefore, the cases where the easily polymerizable compound is applicable are limited and not sufficient. .

  Molecular oxygen contained in air or the like is an effective polymerization inhibitor because it is easy to separate from easily polymerizable compounds and has an excellent radical scavenging ability, but it is necessary to increase the concentration of dissolved oxygen in the solution. It is necessary to pressurize the system, the oxygen concentration in the gas phase increases, and an explosive composition is easily formed by mixing with a gas containing an easily polymerizable compound, and molecular oxygen that is a non-condensable gas Supply to the system greatly increases the load on the vacuum device such as a vacuum pump and a steam ejector, and since peroxide is generated as a by-product in parallel with the trapping of radicals, it also has a factor that accelerates polymerization It has some problems and its effects and application range are limited, so it cannot be said to be sufficient.

  There is a method to reduce the part where polymerization is caused by the condensate by using a perforated tray without a weir without a downcomer.However, since accessory parts such as a support ring and a support beam still exist, the part where the polymerization occurs It is not a complete solution, and it is difficult to suppress the polymerization that occurs in inserts other than the tray.

  For example, in the method of spraying the polymerization inhibitor-containing liquid with a spraying device such as a spray nozzle, it is necessary to separately secure the polymerization inhibitor-containing liquid. When a spraying device is installed near the top of the distillation column or near the feed part, a part of the reflux liquid or the feed liquid can be used as the polymerization inhibitor-containing liquid, and the spraying device is installed near the bottom part of the column. In this case, a part of the liquid extracted from the bottom of the column can be used as the polymerization inhibitor-containing liquid. However, for example, when a spraying device is installed in the middle part of the distillation column other than these, it is difficult to find a usable liquid in the vicinity thereof, and a part of the reflux liquid or the bottom liquid is a polymerization inhibitor-containing liquid. However, even if it is used as the above, a part of the light-boiling substances and high-boiling substances separated in the above-mentioned distillation column will be distilled again, which is inefficient in terms of distillation purification. As a countermeasure, it is conceivable to reduce the amount of spray liquid, but a decrease in the amount of spray liquid reduces the cleaning effect on the part where polymerization is desired to be suppressed. By providing a liquid holding part such as a chimney tray in the middle part of the distillation column and extracting the liquid from the liquid holding part and using the extracted liquid as the polymerization inhibitor-containing liquid, the distillation efficiency is lowered by mixing different liquid compositions. Also, it is possible to avoid a decrease in the cleaning effect due to a decrease in the amount of the supplied liquid, but since the chimney tray itself has a structure in which polymerization due to the condensate is likely to occur, there is a problem that a blocking site is newly created by the polymerization.

The present invention has been made in order to solve the above problems, in the distillation of the easily polymerizable compound in the distillation column, without impairing the distillation efficiency, especially to suppress the blockage due to the polymerization in the middle part for a long time. To aim.
Incidentally, the distillation of the easily polymerizable compound in the present invention by a distillation column means that the withdrawn portion of the distilled easily polymerizable compound may be the tower top part, the column bottom part, or the middle stage part of the column. I do not care.

  The present inventor initially used a spray nozzle having a wide spray range as a means for supplying the polymerization inhibitor-containing liquid to the distillation column, whereby the spray liquid spreads to accessory parts in the distillation column even with a small amount of liquid, We considered that it would solve the problem, but unlike the expectation, when the spray destination of the nozzle is directed upward, the nozzle will be blocked for a short period of time and spraying will not be possible, and the nozzle will be blocked. It was found that the result significantly deteriorated due to the decrease in the spray amount, irrespective of the concentration of the polymerization inhibitor in the supply liquid.

Furthermore, even if the spray destination of the nozzle is directed downward, according to the observation inside the distillation column during the distillation operation, the fine droplets ejected from the spray nozzle show that the easily polymerizable compound in the distillation column fluctuates. Grasping the phenomenon such that the accessory gas in the distillation column may not be sprayed depending on the flow of the contained gas, and that even if sprayed, the polymer adhering to the target object does not peel off, etc. As a result of investigation, it was found that the above-mentioned problems can be solved by entraining a droplet containing a polymerization inhibitor with a gas flow containing a readily polymerizable compound rising in the distillation column.
That is, the present invention is as follows.

[1] A method of distilling a readily polymerizable compound while spraying a polymerization inhibitor-containing liquid as a spray liquid with a spray device through a distillation tower,
The average droplet diameter of the sprayed spray liquid was calculated by calculating the superficial velocity of the easily polymerizable compound-containing gas rising in the distillation column as the falling velocity of the spray liquid in the distillation column under no wind. Is less than 1.5 times the droplet diameter of
A method for distilling a readily polymerizable compound, wherein the center of the spray direction is substantially vertically downward.

[2] The method for distilling a readily polymerizable compound according to [1], wherein the spraying device is a spray nozzle.
[3] The method for distilling a readily polymerizable compound according to [1] or [2], wherein the spray angle is 90 ° or more.
[4] The easy polymerization according to any one of [1] to [3], wherein the concentration of the polymerization inhibitor in the polymerization inhibitor-containing liquid is 5 times or more the average concentration of the polymerization inhibitor in the distillation column at the spray location. Method for distilling organic compounds.

  According to the present invention, in the distillation of the easily polymerizable compound by the distillation column, it is possible to suppress the blockage due to the polymerization particularly in the middle part of the distillation column without impairing the distillation efficiency.

1 is a diagram of a distillation apparatus including a distillation column according to the present invention. It is a figure in a distillation column containing a spraying device in the present invention.

Hereinafter, the method of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the following description, and various modifications may be made within the scope of the gist of the present invention. You can
Moreover, although the aspect which carries out azeotropic dehydration distillation of acrylic acid aqueous solution as an easily polymerizable compound is demonstrated below, this invention is applicable not only to acrylic acid but to distillation of other easily polymerizable compounds.

A diagram of a distillation apparatus including a distillation column according to the present invention is shown in FIG.
Reference numeral 1 is a distillation column having a plurality of trays 9 (dual flow trays) made of non-weir perforated plates having no downcomer. Aqueous acrylic acid solution is supplied from the supply line 2 to the middle part of the distillation column, and a solvent for azeotropic distillation (hereinafter sometimes referred to as "azeotropic solvent") is supplied from the reflux line 3 to the uppermost part of the distillation column. Water and an azeotropic solvent distilled from the top of the distillation column are condensed in a condenser 4 and then separated into a water layer and a solvent layer in a reflux tank 5 having a liquid-liquid separation tank, and the solvent in the solvent layer is reflux line. It is recycled to the distillation column 1 through 3 and the water layer is discharged to the outside of the system. The gas phase portion of the reflux tank 5 is connected to a vacuum device (not shown) by a decompression line 6, and the inside of the reflux tank 5 and the distillation column main body 1 is kept under reduced pressure by the operation of the vacuum device. An acrylic acid solution containing an azeotropic solvent and containing substantially no water is obtained from the bottom of the distillation column 1 (extraction line 7), and a part of the solution is branched and sent to the next purification step (not shown). At the same time, the rest is sent to the reboiler 8 to be heated and then recycled to the lower part of the distillation column 1. The feed line 2 and the reflux line 3 each have a feed line for a liquid containing a polymerization inhibitor, and a feed line for diluting air at the bottom of the distillation column (neither is shown). And dilution air are continuously supplied.

FIG. 2 shows a view of the inside of the distillation column including the spraying device of the present invention.
11 is a side wall surface of the distillation column 1, 12 is a support ring, 13 is a support beam, and 14 is a perforated plate, all of which are fixed in the distillation column by bolts, nuts, or welding. In FIG. 1, the support ring, the support beam, and the perforated plate are collectively referred to as a shelf plate. Reference numeral 21 denotes a nozzle for supplying a spray liquid, which also has a plurality of branch pipes, and the nozzle 21 and the branch pipe have a plurality of spray nozzles 22 at regular intervals. The spray nozzle 22 is provided with a supply line (not shown) so that a part of the liquid recovered through the reflux line 3 and the withdrawal line 7 from the tower bottom can be supplied as a polymerization inhibitor-containing liquid.

  Although the polymerization inhibitor-containing liquid is sprayed from the spray nozzle 22, the center of the spray direction is substantially vertically downward. The substantially vertical downward direction is less than ± 45 ° when the vertical downward direction is 0 °. By making the center of the spraying direction substantially vertically downward, the sprayed polymerization inhibitor-containing spray liquid is polymerized by being dispersed in a wide range together with the easily polymerizable compound-containing gas rising in the distillation column. There is a possibility that it will spread to easy places. Further, it is preferable that the spray angle is 90 ° or more. By setting the angle of spraying to 90 ° or more, it becomes easier to accompany with the above-mentioned rising easily polymerizable compound gas.

  During the distillation operation of the distillation column 1, the average droplet diameter of the spray liquid which is the polymerization inhibitor-containing liquid sprayed from the spray nozzle 22 is determined by the superficial velocity of the easily polymerizable compound gas rising in the distillation column. It is 1.5 times or less, and preferably 1.0 times or less, of the droplet diameter of the spray liquid calculated as the falling velocity of the spray liquid in the tower under no wind. By setting the average droplet diameter of the sprayed spray liquid within the above range, immediately after spraying, the spray liquid once diffused downward pushes up to the flow of the easily polymerizable compound-containing gas rising in the distillation column. As a result, it becomes possible to go up with the gas and spread to the places where polymerization is likely to occur in the distillation column.

The range of the average droplet diameter of the spray liquid will be described in detail.
First, the falling velocity of the spray liquid in the distillation column in a windless state means that the distillation column is filled with a gas containing a polymerizable compound and is under the conditions of temperature and pressure for distillation. It is the final velocity at which the sprayed droplets of the polymerization inhibitor-containing liquid reach the free-falling state of the gas in a static state with the gas stationary.

The droplet diameter of the spray liquid and the falling velocity of the spray liquid in the distillation column under no wind can be expressed by the following equations (1) to (4) according to the Stokes-Allen-Newton resistance law.
For example, when the Reynolds number (Re) represented by the following formula (1) is less than 2, the falling velocity of the spray liquid in a windless state is calculated according to the following formula (2). When the Reynolds number (Re) is 2 or more and 500 or less, the falling velocity of the spray liquid in a windless state is calculated according to the following formula (3), and when the Reynolds number (Re) exceeds 500, Is the drop velocity of the spray liquid under no wind condition is calculated according to the following equation (4).

In the above formulas (1) to (4), v is substituted with the superficial velocity of the easily polymerizable compound-containing gas that rises in the distillation column during the distillation operation of the easily polymerizable compound, and is back-calculated to obtain the droplet diameter. It can be calculated. The average droplet diameter of the sprayed spray liquid is 1.5 times or less, preferably 1.0 times or less, the droplet diameter calculated by the back calculation.
The superficial velocity of the easily polymerizable compound-containing gas rising in the distillation column means that the distillation column is regarded as an empty column, and the volume flow rate (m ³ / s) of the easily polymerizable compound gas is the cross-sectional area (m It is the value divided by ² ).

Since the droplet diameters of the spray liquid sprayed from the nozzles are not completely the same and have a certain range, they include droplets smaller than the value represented as the average droplet diameter. The smaller the ratio, the more rapidly the ratio decreases.
A larger average droplet diameter does not ensure a stable droplet rise. On the other hand, if the droplet size is too small, almost all of the droplets start to rise, but the smaller diameter droplets start to rise immediately after being sprayed from the spray nozzle, reducing horizontal diffusion, The coverage is narrowed. From the above, it is preferable that the droplet diameter of the spray liquid is at least 0.2 times or more the average droplet diameter. For the same reason, the spray angle is preferably 150 ° or less. The distillation method of the present invention is capable of preventing polymerization blockage for a long time by spraying a small amount of a polymerization inhibitor-containing liquid, and a shower nozzle for continuously supplying a liquid columnar polymerization inhibitor-containing liquid. Compared with the above, since the amount of spray liquid is small, there is almost no self-cleaning action of the spray nozzle by the spray liquid, and therefore the spray direction is approximately vertical downward so as not to be exposed to the falling liquid in the distillation column containing the polymer. There is a need to. It is not necessary to be completely below the vertical direction, but from the viewpoint of preventing the polymer from adhering to the tip of the nozzle, the vertical direction is 0 °, and preferably less than ± 45 °. The increase in the amount of the spray liquid causes a reduction in the distillation purification efficiency of the easily polymerizable compound in the distillation column, and it is necessary to enhance the required equipment capacity for spraying. Assuming that the moving distance from the spray nozzle having the spray angle θ to the time when the droplet sprayed downward turns to 0.3 m, its coverage range is about π × {0.3 × {t
an (θ / 2)} ² (m ² ), and the amount of the liquid supplied to the coverage range is preferably less than 100 L / m ² h. Further, the amount of the liquid to be supplied is preferably 10 L / m ² h or more in order to stably continue spraying in a range where the droplet diameter does not become too fine.

  The spray liquid pushed up by the flow of the easily polymerizable compound-containing gas rising in the distillation column accompanies the point at which the easily polymerizable compound-containing gas condenses, so that the polymerization inhibitor contained in the spray liquid is It is considered that it effectively acts to prevent polymerization due to condensation of the easily polymerizable compound-containing gas. However, since the amount of the spray liquid is small compared to the amount of gas, it is preferably 5 times or more, more preferably 10 times or more, of the concentration of the polymerization inhibitor having a sufficient concentration, specifically, the average polymerization inhibitor concentration in the distillation column. preferable.

  Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples.

[Reference example]
In FIG. 1, obtained by collecting the acrylic acid-containing gas obtained by the catalytic gas phase oxidation reaction using propylene as a raw material with water using an azeotropic dehydration distillation apparatus for an acrylic acid aqueous solution without a spraying device. The resulting aqueous solution of acrylic acid was subjected to continuous dehydration distillation. The acrylic acid aqueous solution contained 35% by weight of water, 1.5% by weight of acetic acid and other impurities, and toluene was used as an azeotropic solvent. The pressure in the reflux tank was maintained at 14 kPa by a vacuum pump, and the reflux liquid amount was 8 times the weight of the distilled water amount. Phenothiazine as a polymerization inhibitor was added to the reflux liquid in an amount of 300 ppm by weight. The heat load of the reboiler was adjusted so that the tower bottom temperature was 83 ° C., and the tower bottom liquid was withdrawn so that the tower bottom liquid level became constant. The column bottom pressure increased by 3 kPa by continuous operation for 6 months of dehydration distillation. In the open inspection of the distillation column after the operation was stopped, a large amount of acrylic acid polymer was confirmed on the first to third trays from the bottom of the column, the back surface thereof, the support beam, and the support ring.

[Example]
Using an azeotropic dehydration distillation apparatus having a spraying apparatus as shown in FIG. 2, continuous dehydration distillation was carried out in the same manner as in Reference Example 1. The spray device was installed on the lower side of the 1st to 3rd trays counting from the bottom of the tower, and a part of the bottom liquid of the tower was branched to remove the solid matter with a strainer, and then supplied to the spray device. The spray nozzles were arranged at intervals of about 1 m, and most of them were installed such that the center of the spray direction was upward, some were lateral, and some were vertically downward. The spray angle of the spray nozzle was 120 °, the droplet diameter (catalog value) was 0.35 mm, and the spray amount was 80 L / m ² on average. The bottom liquid used as the polymerization inhibitor-containing liquid had a polymerization inhibitor concentration about 6 times that of the first to third trays. Plural observation windows for observation and daylight were provided on the side of the distillation column. The observation window had a double glass structure having a wiper for removing dirt on the side facing the inside of the tower, and was completely covered with a heat insulating material or the like except during observation.

The column bottom pressure increased by 3 kPa in the dehydration distillation operation for 5 months. In the open inspection of the distillation column after the operation was stopped, it was confirmed that most of the spray nozzles installed with the center of the spray direction facing upward were blocked. During the operation period, all the polymers formed on the support beam observed through the observation window were in the process of growing, and no peeling was confirmed.
The effect of alleviating the blockage due to polymerization due to the installation of the spraying device was not confirmed, and it was considered that the cleaning effect would not be obtained when the amount of liquid supplied was small, but there were some locations such as some peepholes where the spray nozzle was not aimed. Therefore, it was confirmed that the amount of polymer was less than that in other places. Initially, it was considered that there was an error for each operation, but similar results were obtained even when similar operations were repeated, so the study was continued.
According to the observation through the viewing window, droplets exist in the distillation column during the dehydration distillation operation regardless of the presence or absence of the spray nozzle, and due to the periodic fluctuation of the gas flow, much of the distillation column gets wet with the droplets. However, it was confirmed that even the support beam and the support ring, which caused blockage due to polymerization, were wet with the droplets. The observation window was similar, and it was confirmed that droplets gradually adhered during the observation.

No significant difference in droplet adhesion could be confirmed both in the case where the peephole was fogged due to the polymerization and in the case where it was not. The droplet diameter during the observation was relatively large, exceeding several millimeters, and was scattered sparsely on the glass surface. However, by observing with a strong light source, a minute liquid with a diameter of less than 1 mm was observed. It was possible to confirm the adhesion of the droplets, and it was confirmed that the more the minute droplets adhered, the less the fog on the viewing window. In the peep window that did not fog, the spray liquid from the spray nozzle installed above the peep window was blown up by the rising gas flow in the tower, and did not reach the peep window at all. However, there was a spray nozzle installed vertically downward in the spray direction center below the viewing window, and in a viewing window with cloudiness, the spray nozzle below the viewing window with clouding was blocked. Therefore, it was concluded that the minute droplets ejected from the spray nozzle ascended together with the gas in the tower and reached the observation window to exhibit the polymerization inhibiting effect. The gas density during the operation of the distillation column was about 0.7 kg / m ³ in consideration of the vapor phase association of acrylic acid, and the linear velocity for moving up the openings of the dual flow tray was 10.5 m / sec. The droplet diameter that can be raised by the gas was calculated to be 2.6 mm from the formula for calculating the falling velocity of the spray liquid under no wind. It is considered that the droplets having a relatively large diameter, which are sparsely confirmed on all the observation windows, originate from this. It is presumed that the droplets also have a polymerization inhibitory effect, but it is not possible to evenly wet the inner members of the tower except for a part thereof, and the number is small particularly in the region where the gas velocity is slow, and a sufficient polymerization inhibitory effect is obtained. I don't think I can get it. Similarly, the droplet diameter was calculated to be 0.33 mm from the gas superficial velocity of 2.1 m / sec in the distillation column. That is, the average droplet diameter from the spray nozzle installed below was 0.35 mm, which was less than or equal to 1.5 times 0.33 mm, and it was confirmed by calculation that it rose with the gas rising in the column. .

1 Distillation Tower 2 Supply Line 3 Reflux Line 4 Condenser 5 Reflux Tank 6 Decompression Line 7 Extraction Line 8 Reboiler 9 Shelf 11 Side Wall 12 of Distillation Tower 12 Support Ring 13 Support Beam 14 Perforated Plate 21 Nozzle 22 Spray Nozzle

Claims (4)

By a distillation column, while spraying a polymerization inhibitor-containing liquid as a spray liquid by a spraying device, a method of distilling a readily polymerizable compound,
The average droplet diameter of the sprayed spray liquid was calculated by calculating the superficial velocity of the easily polymerizable compound-containing gas rising in the distillation column as the falling velocity of the spray liquid in the distillation column under no wind. Is less than 1.5 times the droplet diameter of
A method for distilling a readily polymerizable compound, wherein the center of the spray direction is substantially vertically downward.   The method for distilling a readily polymerizable compound according to claim 1, wherein the spraying device is a spray nozzle.   The distillation method of the easily polymerizable compound according to claim 1 or 2, wherein the spray angle is 90 ° or more.   4. The easily polymerizable compound according to claim 1, wherein the concentration of the polymerization inhibitor in the polymerization inhibitor-containing liquid is 5 times or more the average concentration of the polymerization inhibitor in the distillation column at the spraying location. Distillation method.

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