WO2012090690A1

WO2012090690A1 - Method for purifying acrylonitrile

Info

Publication number: WO2012090690A1
Application number: PCT/JP2011/078706
Authority: WO
Inventors: 和彦佐野
Original assignee: 旭化成ケミカルズ株式会社
Priority date: 2010-12-27
Filing date: 2011-12-12
Publication date: 2012-07-05
Also published as: TWI434821B; KR101528986B1; CN103261150B; CN103261150A; KR20130086643A; TW201231443A; JP5605921B2; JPWO2012090690A1

Abstract

Provided is a method that is for purifying acrylonitrile and that contains a step for distilling a solution containing acrylonitrile, hydrogen cyanate, and water using a distillation device having a distillation tower and an overhead gas condenser connected to the distillation tower. The method contains a step for fixedly maintaining the temperature of a temperature control stage positioned below the uppermost stage of the distillation tower and above the feed stage of the distillation tower.

Description

Purification method of acrylonitrile

The present invention relates to a method for purifying acrylonitrile by distilling a solution containing acrylonitrile, hydrogen cyanide and water.

In the process of producing acrylonitrile by reacting propylene and / or propane, ammonia and oxygen in the presence of a catalyst, first, the reaction product gas containing acrylonitrile, acetonitrile and hydrogen cyanide is cooled in a quenching tower and unreacted. The ammonia is neutralized and removed with sulfuric acid. Thereafter, the reaction product gas is sent to an absorption tower to absorb acrylonitrile, acetonitrile and hydrogen cyanide in water. Next, an aqueous solution containing acrylonitrile and the like obtained in the absorption tower is introduced into the recovery tower, and from this aqueous solution, a fraction containing acetonitrile and most of water by distillation operation, and a fraction containing most of acrylonitrile and hydrogen cyanide, To separate. After that, a fraction containing most of acrylonitrile and hydrogen cyanide is introduced into a dehydrating acid dehydration tower, hydrogen cyanide and water are separated, and then the bottom liquid is introduced into the product tower, and acrylonitrile is purified by distillation operation. Get a product that fits.
Patent Document 1 discloses a method of suppressing the polymerization of acrylonitrile and hydrogen cyanide by adding an acid and hydroquinone to a dehydride dehydration tower in the purification of acrylonitrile.

Japanese Unexamined Patent Publication No. 2007-39403

In the dehydrating acid dehydration tower, a solution containing acrylonitrile, hydrogen cyanide and water is distilled, a vapor containing hydrogen cyanide is distilled from the top of the tower, and a solution containing acrylonitrile is withdrawn from the bottom of the tower. The gas containing hydrogen cyanide distilled from the top of the column is cooled and fractionated by a condenser, and the hydrogen cyanide with less impurities that has not been condensed is used as a raw material for the hydrogen cyanide derivative. It is preferred to keep the acrylonitrile concentration low. For this reason, even if the operation for maintaining the top temperature at the target temperature is performed according to a general distillation method, the acrylonitrile concentration in the hydrogen cyanide gas distilled from the top is not stable, and the acrylonitrile concentration in the hydrogen cyanide gas is regulated. The phenomenon of rising above the value is often seen. When this phenomenon occurs, not only does the quality of the hydrogen cyanide derivative raw material become stable, but also the quality of the acrylonitrile product becomes unstable, and further, it becomes a factor in the polymerization of acrylonitrile and hydrogen cyanide in the dehydride dehydration tower.
In the past, increasing the yield of the product acrylonitrile has, of course, received much interest and consideration. On the other hand, in addition to improvements aimed at the direct effect of increasing yield, indirect improvements such as stabilization of product quality also have significant technical and economic benefits, but detailed studies have been made so far. The current situation is not.
In view of the above circumstances, the problem to be solved by the present invention is to provide a method for stabilizing product quality in an acrylonitrile manufacturing process.

The present inventor stabilizes the product quality by controlling the temperature of a specific stage of the distillation column in the process of distilling the solution containing acrylonitrile, hydrogen cyanide and water in the process of producing acrylonitrile. In addition, the present inventors have found that the process load can be reduced and completed the present invention.

That is, the present invention is as follows.
[1]
A method for purifying acrylonitrile, comprising a step of distilling a solution containing acrylonitrile, hydrogen cyanide and water using a distillation apparatus having a distillation column and a condenser for the top gas connected to the distillation column,
A method comprising a step of maintaining a constant temperature of a temperature control stage located above the feed stage of the distillation column and below the uppermost stage of the distillation column.
[2]
A regulating valve is provided in a pipe for supplying the refrigerant to the condenser and / or a pipe for discharging the refrigerant from the condenser, and a thermometer is provided in the temperature control stage,
Set the target temperature of the temperature control stage,
When the temperature of the temperature control stage is higher than the target temperature, the opening of the adjustment valve is adjusted to increase the supply amount of the refrigerant,
[1] The above [1], wherein when the temperature of the temperature control stage is lower than the target temperature, the temperature of the temperature control stage is kept constant by adjusting the opening of the regulating valve to reduce the supply amount of the refrigerant. the method of.
[3]
An upper limit value and a lower limit value of the temperature of the temperature control stage are set, and the supply amount of the refrigerant is adjusted by the adjustment valve so that the temperature of the temperature control stage changes between the lower limit value and the upper limit value. The method according to [2] above.
[4]
While giving a certain amount of heat from the reboiler to the distillation column, the heat removal amount of the condenser is increased or decreased,
In each heat removal amount, the temperature of each stage located above the feed stage of the distillation column and below the top stage of the distillation column;
Measure the acrylonitrile concentration and hydrogen cyanide concentration in each stage,
A stage located above the feed stage of the distillation column and below the top stage of the distillation tower, wherein the acrylonitrile concentration is lower than the hydrogen cyanide concentration, the lowest stage (bottom stage) is temperature controlled. Set to
The step of determining the target temperature of the temperature control stage from the temperature of each stage in each heat removal amount so that the concentration of acrylonitrile distilled from the top of the distillation column is minimized. The method for purifying acrylonitrile according to any one of [3].
[5]
While giving a certain amount of heat from the reboiler to the distillation column, the heat removal amount of the condenser is increased or decreased,
In each heat removal amount, the temperature of each stage located above the feed stage of the distillation column and below the top stage of the distillation column;
Measure the acrylonitrile concentration and hydrogen cyanide concentration in each stage,
A stage located above the feed stage of the distillation column and below the top stage of the distillation tower, wherein the acrylonitrile concentration is higher than the hydrogen cyanide concentration, and the uppermost stage (top stage) is temperature controlled. Set to
The step of determining the target temperature of the temperature control stage from the temperature of each stage in each heat removal amount so that the concentration of acrylonitrile distilled from the top of the distillation column is minimized. The method for purifying acrylonitrile according to any one of [3].
[6]
A distillation tower,
A thermometer provided in a temperature control stage located above the feed stage of the distillation column and below the uppermost stage of the distillation column;
A condenser connected to the distillation column;
A pipe for supplying a refrigerant and a pipe for discharging the refrigerant connected to the condenser;
A distillation apparatus comprising: a pipe for supplying the refrigerant and / or an adjustment valve for adjusting a supply amount of the refrigerant attached to the pipe for discharging the refrigerant,
The thermometer is connected to the regulating valve via a temperature controller,
The temperature of the temperature control stage is transmitted to the temperature controller by the thermometer,
When the temperature of the temperature control stage is higher than the target temperature by the temperature controller, the supply amount of the refrigerant is increased by adjusting the opening of the adjustment valve,
A distillation apparatus in which when the temperature of the temperature control stage is lower than a target temperature, the amount of refrigerant supplied is reduced by adjusting the opening of the regulating valve.

According to the present invention, a high-quality product can be stably obtained over a long period of time in the acrylonitrile manufacturing process.

It is the schematic which shows an example of an acrylonitrile manufacturing process notionally. It is a schematic diagram which shows notionally an example of a dehydration acid dehydration tower and equipment connected to it. It is a schematic diagram which shows notionally another example of a dehydration acid dehydration tower and the equipment connected to it.

Hereinafter, modes for carrying out the present invention (hereinafter referred to as the present embodiment) will be described in detail. In addition, this invention is not limited to this embodiment, It can implement in various deformation | transformation within the range of the summary.
Hereinafter, this embodiment will be described in detail with reference to the drawings as necessary. In the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. Further, the positional relationship such as up, down, left and right is based on the positional relationship shown in the drawings unless otherwise specified. The dimensional ratios of the devices and members are not limited to the illustrated ratios.

The purification method of acrylonitrile of this embodiment is:
A method for purifying acrylonitrile, comprising a step of distilling a solution containing acrylonitrile, hydrogen cyanide and water using a distillation apparatus having a distillation column and a condenser for the top gas connected to the distillation column,
The method includes a step of maintaining a constant temperature in a temperature control stage located above the feed stage of the distillation column and below the uppermost stage of the distillation column.

FIG. 1 is a schematic diagram schematically showing an example of an acrylonitrile production process, and FIG. 2 is a schematic diagram conceptually showing an example of a dehydrating acid dehydration tower and equipment connected thereto. In the following description, the “distillation tower” in the present embodiment will be described as a “debleaching acid dehydration tower”. However, the “distillation tower” is not limited to the “debleaching acid dehydration tower”, and may be any tower capable of performing distillation. Are all included in the range of the “distillation tower” of the present embodiment.

In the acrylonitrile production process, first, gaseous propylene and / or propane from line 2, ammonia from line 3, and oxygen (usually using air) from line 4, fluidized bed reactors packed with fluidized bed catalyst, respectively. 1 and propylene and / or propane are subjected to an ammoxidation reaction. The resulting reaction product gas is withdrawn from line 5 and introduced into quenching tower 6. In the quenching tower 6, the reaction product gas and water are brought into countercurrent contact, the reaction product gas is cooled, and the high-boiling substance and the fluidized bed catalyst contained in a trace amount in the gas are removed. In addition, unreacted ammonia is neutralized and removed with sulfuric acid. These high-boiling substances, catalysts and ammonium sulfate are extracted out of the process system from a line 7 at the bottom of the quenching tower 6.
A gas taken out from the upper part of the quenching tower 6 is introduced into the absorption tower 9 through a line 8. The water extracted from the recovery tower 12 is supplied to the top of the absorption tower 9 from the line 14 as absorption water, and acrylonitrile, acetonitrile and hydrogen cyanide in the reaction product gas are absorbed by water. Unabsorbed propylene, propane, oxygen, nitrogen, carbon dioxide, carbon monoxide, etc., and trace amounts of organic substances are extracted from the line 11 at the top of the absorption tower. The liquid at the bottom of the absorption tower 9 is supplied from the line 10 to the recovery tower 12.
Extracted water is introduced from the line 15 to the top of the recovery tower 12, and acetonitrile is extracted and separated by extractive distillation. Acetonitrile is extracted from the line 16 to the outside of the process system. Most of the water is extracted from the line 13 to the outside of the process system. From the top of the recovery tower, acrylonitrile, hydrogen cyanide and water are distilled off by a line 17 and condensed by a condenser not shown, and then separated into two layers of an organic layer and an aqueous layer by a decanter not shown. An organic layer containing acrylonitrile, hydrogen cyanide and a small amount of water is supplied to the dehydride dehydration tower 18. The water layer is recycled to the previous step as a recovery tower feed liquid (from line 10) or extracted water (from line 15).

Vapor containing hydrogen cyanide is distilled from the top of the dehydrating acid dehydration tower 18 through the line 19 and sent to the condenser 20 to be cooled and contracted. The condensed hydrogen cyanide liquid is refluxed to the top of the column by a line 22, and crude hydrogen cyanide gas with little impurities that has not been condensed is drawn out of the system from the line 21. The crude hydrogen cyanide gas is purified by a distillation column (not shown) as necessary and used as a raw material for the hydrogen cyanide derivative. The condenser 20 is preferably a vertical type, and acetic acid is sprayed on the upper tube sheet to suppress hydrogen cyanide polymerization. As the refrigerant 20a used in the condenser 20, water or an aqueous methanol solution having a supply temperature of 0 to 35 ° C., preferably 3 to 30 ° C. is used.

The liquid in the tower is extracted from the chimney tray D in the middle stage of the dehydrating acid dehydration tower 18 by the line 23, cooled by the refrigerant 23a by the side cut cooler 23b, supplied to the decanter 23d by the line 23c, and the organic layer and the water layer by the decanter 23d. Separate into two layers. In the present embodiment, the “middle stage” indicates a portion below the tower top and above the tower bottom, and in the case of a multistage distillation tower, indicates one stage between the tower bottom and the tower top. For example, in the case of a distillation column having 50 to 65 plates, the line 23 is preferably set to 20 to 30 plates from the bottom of the column from the viewpoint of efficiently separating water from the crude acrylonitrile. As the refrigerant 23a, the same refrigerant as the refrigerant 20a can be used. The amount of heat removed by the side cut cooler 23b is adjusted with reference to a thermometer (not shown) for measuring the temperature of the liquid installed in the decanter 23d. The liquid temperature in the decanter is preferably controlled to be constant in the range of 20 to 40 ° C. The aqueous layer in the decanter is recycled to a pre-process such as the recovery tower 12 through the line 23f. The organic layer in the decanter is returned to the lower stage by the line 23e from the stage from which the liquid in the tower is extracted. This organic layer may be preheated back.

The heat required for distillation is supplied from the reboiler 24a through the line 24c. As the heat medium 24b, steam or high-temperature process water taken out from the tower bottom (lines 14 and 15) and / or the tower bottom (line 13) of the recovery tower 12 is used.

The amount of heat given to the distillation column by the reboiler 24a is preferably 180 × 10 ³ to 260 × 10 ³ kcal / h / t-acrylonitrile, and is preferably 190 × 10 ³ from the viewpoint of efficiently separating and recovering acrylonitrile in the deblue acid dehydration column 18. ˜230 × 10 ³ kcal / h / t-acrylonitrile is more preferred. Here, the mass of acrylonitrile is the mass (t) of acrylonitrile obtained as a product from the product tower, and the above-mentioned numerical value represents the calorie per unit mass of acrylonitrile. Can do.

Crude acrylonitrile is extracted from the bottom of the dehydration acid dehydration tower 18 through the line 24 and sent to the product tower 25. A part of the column bottom liquid extracted by the line 24 is supplied to the reboiler 24a.

The product column 25 is a plate distillation column operated under a pressure lower than atmospheric pressure. The distillate vapor from the product column 25 is withdrawn through a line 26 and sent to a condenser 30 for condensation. The condensed liquid is refluxed to the product column 25 through the line 31, and a part of the liquid is extracted through the line 29. The column bottom liquid containing the high boiling point substance is extracted from the line 28. In the process shown in FIG. 1, acrylonitrile is obtained as a product from line 27.

In the acrylonitrile manufacturing process, the amount of acrylonitrile produced may be increased or decreased due to production plans, even during normal operation. In this case, the amount of the solution fed to the dehydrating acid dehydration tower 18 is increased or decreased, and it becomes necessary to adjust the operating conditions of the distillation apparatus. In the present embodiment, the “distillation apparatus” is a concept including ancillary equipment of a distillation column such as a reboiler and a condenser. A part of the solution is extracted from the middle stage of the distillation column, and the middle stage extracted liquid is cooled. In the case of oil-water separation, a cooler and / or oil-water separator is also included in the distillation apparatus.

The dehydrocyanic acid dehydration tower 18 is a tray distillation tower operated under normal pressure, and the number of shelves is preferably 50 to 65. The shelves to be used include types such as a sheave tray and a dual flow tray, but are not limited thereto.

The feed liquid to the dehydrating acid dehydration tower is supplied to the feed stage A from the line 17. The position of the feed stage A is the upper part of the chimney tray D, preferably the upper part of the 10 to 25 stages of the chimney tray D. When the feed liquid is supplied, the vapor rises in the column, and the vapor containing hydrogen cyanide is distilled from the line 19 from the top of the column. The distillate vapor is sent to the condenser 20 and cooled to be condensed. The condensed hydrogen cyanide liquid is refluxed to the uppermost stage C of the tower through a line 22, and crude hydrogen cyanide gas with little impurities, which has not been condensed, is drawn out of the system from the line 21. Distillation purification is performed by bringing the reflux liquid flowing down the column into contact with the vapor rising in the column.

In the method of the present embodiment, the temperature of the stage B located above the feed stage A and below the uppermost stage C of the tower is kept constant. Here, “above feed stage A” does not include feed stage A itself, and “below uppermost stage C” does not include uppermost stage C itself. In the present embodiment, “maintaining the temperature constant” means maintaining the set target temperature, and when setting an upper limit value and a lower limit value, which will be described later, a temperature not lower than the upper limit value and not higher than the lower limit value. It also includes maintaining the range. It also includes the amplitude due to hunting of instrument measurements. Here, “stage B” does not mean all stages located above the feed stage A and below the top stage of the tower, but refers to a specific stage where a thermometer 22b selected from the stages in between is installed. , Called “temperature control stage”. More preferably, the temperature of a specific stage B located between the upper three stages from the feed stage A to the lower three stages from the uppermost stage of the tower is kept constant.

The target temperature is preferably set at a specific temperature. However, in practice, even when the temperature of the temperature control stage deviates from the target temperature, the distillation at the target temperature is acceptable in terms of distillation separation. There are upper and lower temperature limits. In the present embodiment, the values are referred to as an upper limit value and a lower limit value, respectively. In general, the upper limit value and the lower limit value are preferably set such that upper limit value = target temperature × 1.05 and lower limit value = target temperature × 0.95. For example, when the upper limit value is the target temperature + 2 ° C. and the lower limit value is the target temperature −2 ° C., the temperature of the temperature control stage is maintained within the target temperature ± 2 ° C.

The thermometer 22b is connected to a flow rate control valve 20b of the refrigerant 20a provided in a pipe for discharging the refrigerant via the temperature controller 22a, and the temperature of the temperature control stage B is controlled by the thermometer 22b. 22a, when the temperature of the temperature control stage B is higher than the target temperature, the supply amount of the refrigerant is increased by adjusting the opening of the regulating valve 20b, and the temperature of the temperature control stage B is higher than the target temperature. When it is low, the supply amount of the refrigerant is decreased by adjusting the opening degree of the regulating valve 20b. There are two modes of “adjusting the opening degree” of the adjusting valve: a mode in which the opening degree is increased, that is, the valve is opened, and a mode in which the opening degree is decreased, that is, the valve is closed. When the adjustment valve is provided in the discharge pipe as in the example shown in FIG. 2, the refrigerant 20 a is discharged by opening the adjustment valve 20 b, and the refrigerant having a lower temperature than the discharged refrigerant flows into the condenser 20. As a result, the cooling effect of the condenser 20 is enhanced. On the contrary, by closing the regulating valve 20b, the discharge of the refrigerant 20a is suppressed, and the refrigerant having a lower temperature than the discharged refrigerant is prevented from flowing into the condenser 20, so that the cooling effect of the condenser 20 is reduced. . Thus, the temperature of the reflux liquid returned to the tower from the condenser 20 is changed by changing the supply amount of the refrigerant 20a by the control valve 20b, and the temperature of the temperature control stage B is kept constant.
When setting the upper limit value and the lower limit value of the temperature of the temperature control stage, the supply amount of the refrigerant can be adjusted by the adjusting valve so that the temperature of the temperature control stage changes between the lower limit value and the upper limit value. .

The target temperature of the temperature control stage B is preferably 40 to 55 ° C. from the viewpoint of lowering the acrylonitrile concentration in the distillate vapor to increase the hydrogen cyanide purity and from the viewpoint of lowering the hydrogen cyanide concentration in the bottom liquid and increasing the acrylonitrile purity. More preferred is ˜50 ° C. When the temperature of the temperature control stage B is higher than the target temperature, the concentration of acrylonitrile in the distillate vapor increases, leading to loss of acrylonitrile, and the purity of distillate hydrogen cyanide is lowered, which adversely affects the quality of the hydrogen cyanide derivative. On the other hand, when the temperature of the temperature control stage B is lower than the target temperature, the concentration of hydrogen cyanide in the column bottom liquid rises and cannot be sufficiently removed by the downstream product column, and the acrylonitrile product may become an off-spec product.

In this embodiment, the “target temperature” is an optimum temperature derived from an acrylonitrile distillation experiment in a laboratory and / or an experiment on temperature dependence of distillation separation performance using a commercial scale distillation apparatus. For example, the relationship between each temperature distribution from the top of the distillation column to the bottom of the column (hereinafter referred to as “temperature profile”), the concentration of the key substance at the top and the concentration of the key substance at the bottom is examined. Here, the key substance is a substance that serves as a guideline for carrying out distillation separation, and generally refers to a trace amount of impurities. If a large amount of the substance is mixed, it is not preferable for purification. It is preferable that a specification of the key substance concentration is determined, and this is used as a separation specification and used for operation management of the distillation column.

FIG. 3 is a schematic diagram showing another example of the dehydration acid dehydration tower 18 and equipment connected thereto. Since the flow rate adjustment valve 20b 'connecting the supply pipe and the discharge pipe for the refrigerant 20a of the condenser is substantially the same as the example shown in FIG. 2, only the differences will be described. When the control valve 20b 'is opened, a part of the refrigerant 20a flows from the supply pipe to the discharge pipe without passing through the condenser, so that the supply amount of the refrigerant 20a is reduced by opening the control valve 20b'. The thermometer 22b is connected to the flow

rate control valves

20b and 20b ′ via the temperature controller 22a, the temperature of the temperature control stage B is transmitted to the temperature controller 22a, and the temperature of the temperature control stage B is the target temperature. If higher, the regulating valve 20b is opened and / or the regulating valve 20b 'is closed to increase the supply amount of the refrigerant 20a. When the temperature of the temperature control stage B is lower than the target temperature, the regulating valve 20b is closed and / or the regulating valve 20b ′ is opened to reduce the supply amount of the refrigerant 20a, and the temperature of the temperature control stage B is kept constant. To maintain.

In the example shown in FIG. 3, both the flow

rate control valves

20b and 20b ′ are operated by a command from the temperature controller 22a. However, as long as the function of “maintaining the temperature of the temperature control stage constant” is achieved. Both of them need not be opened / closed by the temperature controller 22a, only the flow rate control valve 20b may be opened / closed by the temperature controller 22a, and the flow rate control valve 20b ′ may be manually operated. When the control valve 20b ′ is manual, the opening degree of the control valve 20b ′ is kept constant, and the temperature of the temperature control stage B is kept constant by operating the control valve 20b as in the example shown in FIG. .

In a commercial scale acrylonitrile distillation apparatus, it is preferable to use acrylonitrile as the key material at the top of the column and hydrogen cyanide and water as the key material at the bottom. By maintaining the acrylonitrile in the hydrogen cyanide gas distilled from the top of the column at a low concentration, it is possible to prevent a decrease in the mass of acrylonitrile obtained as a product. Hydrogen cyanide is also one of commercially available products and is used in various hydrogen cyanide derivatives. By keeping the concentration of acrylonitrile in hydrogen cyanide low, for example, methacrylic acid obtained by the acetone cyanohydrin (ACH) method is used. Undesirable coloring of methyl acid (MMA) can be prevented. Even if acrylonitrile is distilled off from the top of the column, it is possible to increase the purity of hydrogen cyanide by further separation by distillation or the like. Processing equipment is also an essential requirement. Therefore, considering the use of hydrogen cyanide, it is preferable to keep the concentration of acrylonitrile in the hydrogen cyanide distilled from the top of the column low. The acrylonitrile concentration in the hydrogen cyanide distilled from the top of the column is preferably 1000 ppm or less, more preferably 700 ppm or less, and even more preferably 500 ppm or less.

When a large amount of hydrogen cyanide is mixed in acrylonitrile extracted from the bottom of the tower, it causes coloring of acrylic fibers and ABS resin obtained using the acrylonitrile. Moreover, when a lot of water is mixed, the purity of the product acrylonitrile falls. The hydrogen cyanide concentration in acrylonitrile extracted from the column bottom is preferably 100 ppm or less, more preferably 70 ppm or less, and further preferably 50 ppm or less.

Measure the change in the concentration of key substances at the top and bottom of the tower by changing the temperature profile in the tower by increasing / decreasing the heating amount of the reboiler and / or the heat removal amount of the condenser. From the measurement results, the position of the preferred temperature control stage and its target temperature for forming an in-column temperature profile exhibiting good distillation separation performance are determined.

Hereinafter, an example of a method for determining the position of the temperature control stage and the target temperature will be described.
First, the reboiler heating amount and the condenser heat removal amount are made constant, and the concentration (mass%) of the key substance is examined in each stage (concentration profile). In addition, the temperature of each stage is also measured (temperature profile). Next, without changing the heating amount of the reboiler, only the heat removal amount of the condenser is changed, and the concentration profile and the temperature profile are measured. When comparing the temperature profiles in each column when the heat removal amount of the condenser is different, even if the temperatures of the feed stages and the temperatures of the uppermost stages are the same in each case, The temperature of each stage located below the uppermost stage may be different, and at this time, a difference occurs in the key substance concentration at the top and bottom of each case. That is, even if only the temperature of the feed stage and / or the uppermost stage is monitored, the concentration of the key substance at the top and the bottom of the tower cannot be controlled. The present inventor has discovered that the temperature change in the stage located above the feed stage and below the top stage affects the key substance concentration.
In the concentration profile, the temperature at the top and the bottom of the column is reversed, specifically, the temperature of the lowest level (the lowest level) among the levels where the acrylonitrile concentration is lower than the hydrogen cyanide concentration, and / or Of the stages where the acrylonitrile concentration is higher than the hydrogen cyanide concentration, the uppermost stage (the uppermost stage) was found to show a strong correlation with the concentration of the key substance at the top and / or bottom of the tower.
Since acrylonitrile is highly concentrated at the bottom of the column and hydrogen cyanide is highly concentrated at the top of the column, the concentration of both is reversed at a certain stage. The temperature of this stage affects the concentration of the key substance at the top and / or bottom of the tower. Therefore, it is preferable to refer to the concentration profile and determine the stage where the key substance concentration is reversed as the position of the temperature control stage. The target temperature can be set from the temperature of the temperature control stage in the preferred temperature profile. In general, in a preferable temperature profile, the temperature of the temperature control stage shows a rapid change, and the inflection point of the temperature profile often corresponds to the temperature control stage.

That is, as a preferable aspect in the method of the present embodiment,
While giving a certain amount of heat from the reboiler to the distillation column, the heat removal amount of the condenser is increased or decreased,
In each heat removal amount, the temperature of each stage located above the feed stage of the distillation column and below the top stage of the distillation column;
Measure the acrylonitrile concentration and hydrogen cyanide concentration in each stage,
A stage located above the feed stage of the distillation column and below the top stage of the distillation column, wherein the acrylonitrile concentration is lower than the hydrogen cyanide concentration, the lowest stage (bottom stage) and / or the stage Among the stages where the acrylonitrile concentration is higher than the hydrogen cyanide concentration, the uppermost stage (the uppermost stage) is set as the temperature control stage,
Determining the target temperature of the temperature control stage so that the concentration of acrylonitrile distilled from the top of the distillation column is minimized from the temperature of each stage in each heat removal amount.

From the viewpoint of purifying acrylonitrile so as to satisfy the separation specifications of the key substance, it can be said that referring to the concentration profile and the temperature profile, the position of the temperature control stage and the target temperature are a preferable aspect.

At the start of operation of the distillation tower, the reboiler heating amount and the condenser heat removal amount are repeated in parallel, but in the final adjustment stage, the reboiler heating amount and the condenser heat removal amount are referred to as If the two heat quantity variables are increased or decreased at a time, it becomes difficult to operate the distillation column stably. Therefore, from the viewpoint of determining the position of the temperature control stage and the target temperature while stably operating the distillation column, the reboiler has a constant heating amount in the range of 180 × 10 ³ to 260 × 10 ³ kcal / h / t-acrylonitrile. It is preferable to control so that the heat removal amount of the condenser is increased or decreased and the temperature of the temperature control stage of the distillation column becomes the target temperature. By doing so, there is a tendency that good separation performance of the distillation column can be drawn out early, and the amount of off-spec products that need re-purification can be suppressed. In addition, the product acquisition time can be advanced.

In the acrylonitrile manufacturing process, the amount of acrylonitrile produced may be increased or decreased due to production plans, even during normal operation. In this case, the amount of the solution fed to the dehydrating acid dehydration tower 18 is increased or decreased. The amount of product produced according to the change in the mass of the feed liquid and the amount of heat added to the reboiler from the above-described reboiler calorific value (hereinafter referred to as “reboiler heating amount”) are adjusted and changed. When the reboiler heating amount is increased or decreased, the amount of steam inside the distillation column changes. For example, when the reboiler heating amount is increased, acrylonitrile may be cooked in the upper part of the tower and the proportion of distilling in the crude hydrogen cyanide may be increased. On the other hand, when the reboiler heating amount is decreased, hydrogen cyanide may fall to the lower part of the tower, and the ratio existing in the bottom extract may rise. All of these adversely affect the purity of the product (acrylonitrile, hydrogen cyanide derivative). In order to prevent these, it is useful to appropriately adjust the distillation tower temperature in accordance with the amount of increase / decrease in the reboiler heating amount.

Although it does not specifically limit as an apparatus for performing the purification method of this embodiment, For example, it can carry out using the following distillation apparatuses.
A distillation tower,
A thermometer provided in a temperature control stage located above the feed stage of the distillation column and below the uppermost stage of the distillation column;
A condenser connected to the distillation column;
A pipe for supplying a refrigerant and a pipe for discharging the refrigerant connected to the condenser;
A distillation apparatus comprising: a pipe for supplying the refrigerant and / or an adjustment valve for adjusting a supply amount of the refrigerant attached to the pipe for discharging the refrigerant,
The thermometer is connected to the regulating valve via a temperature controller,
The temperature of the temperature control stage is transmitted to the temperature controller by the thermometer,
When the temperature of the temperature control stage exceeds the target temperature by the temperature controller, the supply amount of the refrigerant is increased by adjusting the opening of the adjustment valve, and the temperature of the temperature control stage is less than the target temperature. In this case, the amount of refrigerant supplied is reduced by adjusting the opening of the adjusting valve.

Hereinafter, the present embodiment will be described in more detail with reference to examples. However, the present embodiment is not limited to the examples described below. In addition, the acrylonitrile manufacturing process in an Example is the same as that of what was shown in FIG. Further, the dehydrating acid dehydration tower in the examples is the same as that shown in FIG.
Analysis of acrylonitrile was performed by gas chromatography using the following apparatus and conditions.
In the gas chromatography, Shimadzu GC-17A was used as an apparatus, and TC-FFAP 60 m × 0.32 film thickness 0.25 μm was used as a column. The detector used was FID and the carrier gas used helium.
The column temperature conditions were as follows.
Initial temperature: 50 ° C
Temperature rising rate: 5 ° C / min Final temperature 1: 180 ° C 15 minutes HOLD
Temperature increase rate: 10 ° C / min Final temperature 2: 230 ° C 10 minutes HOLD
Final temperature 3: 50 ° C, 5 minutes HOLD

Hydrogen cyanide and water were analyzed by silver nitrate titration method and Karl Fischer method, respectively.

The following were used as a flow meter and a thermometer.
Flow meter: YKOGAWA differential pressure flow meter (orifice type) Differential Pressure Transmitter DP hard EJX
Thermometer: Resistance Thermometer, Temperature Trans made by OKAZAKI

[Example 1]
Propylene, ammonia and air were supplied to a vertical cylindrical fluidized bed reactor 1 having an inner diameter of 8 m and a length of 20 m, and propylene ammoxidation reaction was carried out as follows. The fluidized bed reactor 1 had a raw material gas dispersion pipe, a dispersion plate, a heat removal pipe, and a cyclone inside. The dehydrating acid dehydration tower 18 comprises 55 sheave trays, has a feed stage A at the 37th stage from the bottom of the tower, a chimney tray D at the 24th stage, and a line 23 for extracting a side cut flow at the 24th stage. After the side cut cooler 23b and the decanter 23d, the 23rd stage had a line 23e for returning the organic layer in the decanter.
As the fluidized bed catalyst, a molybdenum-bismuth-iron-based supported catalyst having a particle size of 10 to 100 μm and an average particle size of 55 μm was used and packed so as to have a stationary bed height of 2.7 m. Air was 56000Nm ³ / h supplied from the air distribution plate, propylene 6200Nm ³ / h and ammonia from a raw material gas dispersion tube was 6600Nm ³ / h feed. The reaction temperature was controlled with a heat removal tube so as to be 440 ° C. The pressure was 0.70 kg / cm ² G.
The reaction product gas was introduced into the quenching tower 6 and brought into countercurrent contact with water, and unreacted ammonia was neutralized and removed with sulfuric acid. The gas flowing out of the quenching tower 6 was introduced into the absorption tower 9 from the line 8. Absorbed water was introduced from the line 14 at the top of the absorption tower 9 and brought into countercurrent contact with the gas, so that acrylonitrile, acetonitrile and hydrogen cyanide in the gas were absorbed into water. The amount of absorbed water was adjusted so that the acrylonitrile concentration in the gas discharged from the top of the absorption tower was 100 volppm. The gas that was not absorbed was taken out from the absorption tower top line 11 and incinerated.
The absorption tower bottom liquid was preheated to 80 ° C. and supplied to the recovery tower 12. Acetonitrile and most of the water were separated in the recovery tower 12, and acrylonitrile, hydrogen cyanide and water were distilled from the tower top line 17. The distillate vapor is condensed, an organic layer and an aqueous layer are formed by a recovery tower decanter (not shown), the aqueous layer is recycled to the supply line 10 of the recovery tower 12, and the organic layer is supplied to the dehydride dehydration tower 18. .
The mass and temperature of the feed liquid to the dehydrating acid dehydration tower 18 were measured by a flow meter and a thermometer (not shown) installed in the line 17. The measured values were 13595 kg / h and 35.0 ° C., respectively.
Crude hydrogen cyanide gas was extracted from the top line 19 of the dehydrating acid dehydration tower 18 and sent to the condenser 20, where it was cooled and fractionated. The refrigerant 20a used in the condenser 20 was 6 ° C. water. The condensed hydrogen cyanide liquid was refluxed to the top of the column, and hydrogen cyanide gas with little impurities that was not condensed was extracted from the line 21 to the outside of the system.
The liquid in the tower was extracted from the 24th stage of the dehydration acid dehydration tower 18 and cooled by the side cut cooler 23b. The refrigerant 23a used for the side cut cooler 23b was 25 ° C. water. The heat removal amount Q3 of the side cut cooler was adjusted by the flow rate of the refrigerant 23a so that the liquid temperature of the decanter 23d was 30 ° C. The side stream extracted from the tower was separated into two layers of an organic layer and an aqueous layer by a decanter 23d, and the aqueous layer was extracted through a line 23f and recycled to the supply liquid of the recovery tower 12. The organic layer was returned to the 23rd stage of the tower by line 23e.
110 ° C. process water extracted from the lower part of the recovery tower 12 was used as a heat source for the reboiler 24a. The amount of heat Q1 applied was 200 × 10 ³ kcal / h / t-acrylonitrile, and the mass of acrylonitrile obtained as a product in the product tower 25 was 11.5 t per hour, so 2300 × 10 ³ kcal / h Thus, the flow rate of the process water 24b leading to the reboiler 24a was adjusted.
Crude acrylonitrile was extracted from the tower bottom line 24 and sent to the product tower 25. The bottom extract liquid was measured for mass by a flow meter (not shown) installed in the line 24, and the measured value was 11,585 kg / h. The temperature of the liquid extracted from the bottom of the tower was 86 ° C., which was the same as the liquid temperature at the bottom of the dehydration acid dehydration tower 18.
Here, the heating amount of the reboiler 24a and the heat removal amount of the condenser 20 were made constant, and the acrylonitrile concentration and the hydrogen cyanide concentration in each stage above the feed stage were measured. As a result, of the stages where the acrylonitrile concentration was lower than the hydrogen cyanide concentration, The lower stage was 51 stages from the tower bottom.
Next, as a result of adjusting the heat removal amount of the condenser so that the acrylonitrile concentration in the hydrogen cyanide distilled from the top of the column is 300 ppm while the heating amount of the reboiler 24a is kept constant, the temperature of the stage is 48 ° C. there were.
Here, the stage is the temperature control stage B, the thermometer installed in the stage is the thermometer 22b, the target temperature of the stage is 48 ° C., and the heat removal amount of the condenser is set so that the temperature of the stage is 48 ° C. It was adjusted.
The above operation was continued for about 6 months when the acrylonitrile production amount was 11.5 ± 0.2 t / h. During this time, the temperature of the temperature control stage was 48 ± 0.4 ° C.
The dehydrating acid dehydration tower can be operated stably. During this time, the concentration of acrylonitrile in hydrogen cyanide distilled from the top of the dehydrating acid dehydration tower is 300 ± 20 ppm, and the concentration of hydrogen cyanide in acrylonitrile extracted from the bottom of the tower is 40 ± 10 ppm. Met. During this time, the concentration of hydrogen cyanide in the acrylonitrile product was 5 ppm or less, and a high-quality acrylonitrile product was stably obtained. Moreover, the purity of the crude hydrogen cyanide was stable, and there was no problem with the quality of the hydrogen cyanide derivative.

[Example 2]
Acrylonitrile was produced using the same equipment and method as in Example 1 except that the production amount of acrylonitrile was increased to 12.7 t / h by changing the production plan.
The reboiler heat was increased to 2540 × 10 ³ kcal / h. The flow rate control valve 20b of the refrigerant 20a was controlled through the temperature controller 22a so that the temperature of the temperature control stage B of the dehydration acid dehydration tower 18 was 48 ° C. Each temperature in the dehydrating acid dehydration tower 18 and the temperature of the decanter 23d were substantially the same as those in Example 1.
The above operation was continued for about 3 months when the acrylonitrile production amount was 12.7 ± 0.2 t / h. During this time, the temperature of the temperature control stage was 48 ± 0.4 ° C. The dehydrating acid dehydration tower 18 can be operated stably. During this time, the concentration of acrylonitrile in hydrogen cyanide distilled from the top of the dehydrating acid dehydration tower is 300 ± 20 ppm, and the concentration of hydrogen cyanide in acrylonitrile extracted from the bottom of the tower is 40 ±. It was 10 ppm. During this time, the concentration of hydrogen cyanide in the acrylonitrile product was 5 ppm or less, and a high-quality acrylonitrile product was stably obtained. Moreover, the purity of the crude hydrogen cyanide was stable, and there was no problem with the quality of the hydrogen cyanide derivative.

[Example 3]
Production of acrylonitrile under the same conditions as in Example 1, with the heating amount of the reboiler 24a and the heat removal amount of the condenser 20 made constant, and the acrylonitrile concentration and hydrogen cyanide concentration in each stage above the feed stage were measured. Among the stages having a hydrogen cyanide concentration higher than that of the hydrogen cyanide, the lowest stage was 50 stages counted from the tower bottom.
Next, as a result of adjusting the heat removal amount of the condenser so that the acrylonitrile concentration in the hydrogen cyanide distilled from the top of the tower is 300 ppm while keeping the heating amount of the reboiler 24a constant, the temperature of the stage is 51 ° C. there were.
Here, assuming that the stage is the temperature control stage B, the thermometer installed in the stage is the thermometer 22b, and the target temperature of the stage is 48 ° C. It was adjusted.
The above operation was continued for about 6 months when the acrylonitrile production amount was 11.5 ± 0.2 t / h. During this time, the temperature of the temperature control stage was 51 ± 0.4 ° C. The dehydrating acid dehydration tower can be operated stably. During this time, the concentration of acrylonitrile in hydrogen cyanide distilled from the top of the dehydrating acid dehydration tower is 300 ± 20 ppm, and the concentration of hydrogen cyanide in acrylonitrile extracted from the bottom of the tower is 40 ± 10 ppm. Met. During this period, the hydrogen cyanide concentration in the acrylonitrile product was 5 ppm or less, and high-quality acrylonitrile products could be obtained stably. Moreover, the purity of the crude hydrogen cyanide was stable, and there was no problem with the quality of the hydrogen cyanide derivative.

[Example 4]
Propane, ammonia and air were supplied to the same fluidized bed reactor 1 as in Example 1, and propane ammoxidation reaction was performed as follows.
The fluidized bed catalyst was a molybdenum-vanadium-based supported catalyst having a particle size of 10 to 100 μm and an average particle size of 55 μm, and packed so that the stationary bed height was 2.2 m. Air was 64500Nm ³ / h supplied from the air distribution plate, propane 4300Nm ³ / h and ammonia from a raw material gas dispersion tube was 4300Nm ³ / h feed. The reaction temperature was controlled with a heat removal tube so as to be 440 ° C. The pressure was 0.75 kg / cm ² G.
The reaction product gas was introduced into the quenching tower 6 and brought into countercurrent contact with water. Further, unreacted ammonia was neutralized and removed with sulfuric acid.
The gas taken out from the quenching tower 6 was introduced into the absorption tower 9 from the line 8. Absorbed water was introduced from the top line 14 and brought into countercurrent contact with the gas to absorb acrylonitrile, acetonitrile and hydrogen cyanide in the gas into the water. Unabsorbed gas was taken out from the absorption tower top line 11 and incinerated. The amount of absorbed water was adjusted so that the acrylonitrile concentration in the gas taken out from the top of the absorption tower was 100 volppm.
The absorption tower bottom liquid was preheated and supplied to the recovery tower 12. Acetonitrile and most of the water were separated in the recovery tower, and acrylonitrile, hydrogen cyanide and water were distilled from the top line 17. The distillate vapor was condensed to form an organic layer and an aqueous layer, the aqueous layer was recycled to the supply line 10 of the recovery tower, and the organic layer was supplied to the dehydrating acid dehydration tower 18.
The mass and temperature of the feed liquid to the dehydrating acid dehydration tower 18 were measured by a flow meter and a thermometer (not shown) installed in the line 17. The measured values were 6219 kg / h and 35.0 ° C., respectively.
Crude hydrogen cyanide gas was extracted from the top line 19 of the dehydrating acid dehydration tower 18 and sent to the condenser 20, where it was cooled and fractionated. The refrigerant 20a used in the condenser 20 was 6 ° C. water. The condensed hydrogen cyanide liquid was refluxed to the top of the column, and hydrogen cyanide gas with little impurities that was not condensed was extracted from the line 21 to the outside of the system.
The liquid in the tower was extracted from the 24th stage of the dehydration acid dehydration tower 18 and cooled by the side cut cooler 23b. The refrigerant 23a used for the side cut cooler 23b was 25 ° C. water. The heat removal amount Q3 of the side cut cooler was adjusted by the flow rate of the refrigerant 23a so that the liquid temperature of the decanter 23d was 30 ° C. The side stream extracted from the tower was separated into two layers of an organic layer and an aqueous layer by a decanter 23d, and the aqueous layer was extracted through a line 23f and recycled to the supply liquid of the recovery tower 12. The organic layer was returned to the 23rd stage of the tower by line 23e.
110 ° C. process water extracted from the lower part of the recovery tower 12 was used as a heat source for the reboiler 24a. The amount of heat Q1 applied was 250 × 10 ³ kcal / h / t-acrylonitrile, and the mass of acrylonitrile obtained as a product in the product tower 25 was 5.22 t per hour, so 1305 × 10 ³ kcal / h Then, the flow rate of the process water 24b leading to the reboiler 24a was adjusted.
Crude acrylonitrile was extracted from the tower bottom line 24 and supplied to the product tower 25. The bottom extract was measured for mass by a flow meter (not shown) installed in the line 24, and the measured value was 5312 kg / h. The temperature of the liquid extracted from the bottom of the tower was 86 ° C., which was the same as the liquid temperature at the bottom of the dehydration acid dehydration tower 18.
Here, the heating amount of the reboiler 24a and the heat removal amount of the condenser 20 were made constant, and the acrylonitrile concentration and the hydrogen cyanide concentration in each stage above the feed stage were measured. As a result, of the stages where the acrylonitrile concentration was lower than the hydrogen cyanide concentration, The lower stage was 51 stages from the tower bottom.
Next, as a result of adjusting the heat removal amount of the condenser so that the acrylonitrile concentration in the hydrogen cyanide distilled from the top of the column is 300 ppm while the heating amount of the reboiler 24a is kept constant, the temperature of the stage is 48 ° C. there were.
Here, the stage is the temperature control stage B, the thermometer installed in the stage is the thermometer 22b, the target temperature of the stage is 48 ° C., and the heat removal amount of the condenser is set so that the temperature of the stage is 48 ° C. It was adjusted.
The above operation was continued for about 4 months when the acrylonitrile production amount was 5.22 ± 0.17 t / h. During this time, the temperature of the temperature control stage B was 48 ± 0.4 ° C. The dehydrating acid dehydration tower can be operated stably. During this time, the concentration of acrylonitrile in hydrogen cyanide distilled from the top of the dehydrating acid dehydration tower is 300 ± 10 ppm, and the concentration of hydrogen cyanide in acrylonitrile extracted from the bottom of the tower is 40 ± 10 ppm. It was the following. During this time, the concentration of hydrogen cyanide in the acrylonitrile product was 5 ppm or less, and a high-quality acrylonitrile product was stably obtained. Moreover, the purity of the crude hydrogen cyanide was stable, and there was no problem with the quality of the hydrogen cyanide derivative.

[Example 5]
Production of acrylonitrile under the same conditions as in Example 4, with the heating amount of the reboiler 24a and the heat removal amount of the condenser 20 made constant, and the acrylonitrile concentration and hydrogen cyanide concentration in each stage above the feed stage were measured. Among the stages having a hydrogen cyanide concentration higher than that of the hydrogen cyanide, the lowest stage was 50 stages counted from the tower bottom.
Next, as a result of adjusting the heat removal amount of the condenser so that the acrylonitrile concentration in the hydrogen cyanide distilled from the top of the tower is 300 ppm while keeping the heating amount of the reboiler 24a constant, the temperature of the stage is 51 ° C. there were.
Here, the stage is the temperature control stage B, the thermometer installed in the stage is the thermometer 22b, the target temperature of the stage is 48 ° C., and the heat removal amount of the condenser is set so that the temperature of the stage becomes 51 ° C. It was adjusted.
The above operation was continued for about 4 months when the acrylonitrile production amount was 5.22 ± 0.17 t / h. During this time, the temperature of the temperature control stage B was 51 ± 0.4 ° C. The dehydrating acid dehydration tower can be operated stably. During this time, the concentration of acrylonitrile in hydrogen cyanide distilled from the top of the dehydrating acid dehydration tower is 300 ± 20 ppm, and the concentration of hydrogen cyanide in acrylonitrile extracted from the bottom of the tower is 40 ± 10 ppm. Met. During this time, the concentration of hydrogen cyanide in the acrylonitrile product was 5 ppm or less, and a high-quality acrylonitrile product was stably obtained. Moreover, the purity of the crude hydrogen cyanide was stable, and there was no problem with the quality of the hydrogen cyanide derivative.

[Comparative Example 1]
The ammoxidation reaction of propylene was carried out using the same equipment and method as in Example 1 except that the uppermost stage of the dehydration acid dehydration tower was a temperature control stage and the temperature of the stage was 30 ° C. Inter-acrylonitrile was produced. During this time, the temperature in the temperature control stage was not changed at 30 ° C., but one month after the start of production, the concentration of acrylonitrile in hydrogen cyanide distilled from the top of the dehydrating acid dehydration tower increased to 1000 ppm. It was judged that the heat removal amount Q2 of the condenser was insufficient, and when Q2 was increased by increasing the flow rate of the refrigerant leading to the condenser, the temperature of the uppermost stage of the dehydrating acid dehydration tower was not changed at 30 ° C. The acrylonitrile concentration in the hydrogen cyanide distilled from the top of the column was reduced to 300 ppm.
Two months after the start of production, the concentration of hydrogen cyanide in acrylonitrile obtained as a product increased to 20 ppm and became an off-spec product. At this time, the concentration of hydrogen cyanide in the bottom liquid of the dehydride dehydration tower was 120 wtppm. When it was judged that the amount of heat removal Q2 in the condenser was excessive and Q2 was reduced by reducing the flow rate of the refrigerant leading to the condenser, the hydrogen cyanide concentration in the acrylonitrile obtained as a product was reduced to 5 ppm and became an on-spec product. Moreover, the ratio of acrylonitrile distilled in the hydrogen cyanide distilled from the tower top rose to 600 ppm, and the quality of the hydrogen cyanide derivative was lowered. During this time, the temperature of the uppermost stage of the dehydrating acid dehydration tower was 30 ° C. and did not change.

[Comparative Example 2]
The propane ammoxidation reaction was carried out using the same equipment and method as in Example 4 except that the uppermost stage of the dehydration acid dehydration tower was a temperature control stage and the temperature of the stage was 30 ° C. Inter-acrylonitrile was produced. During this time, the temperature in the temperature control stage was not changed at 30 ° C., but two weeks after the start of production, the concentration of acrylonitrile in hydrogen cyanide distilled from the top of the dehydrating acid dehydration tower increased to 1000 ppm. It was judged that the heat removal amount Q2 of the condenser was insufficient, and when Q2 was increased by increasing the flow rate of the refrigerant leading to the condenser, the temperature of the uppermost stage of the dehydrating acid dehydration tower was not changed at 30 ° C. The acrylonitrile concentration in the hydrogen cyanide distilled from the top of the column was reduced to 300 ppm.
Four weeks after the start of production, the concentration of hydrogen cyanide in acrylonitrile obtained as a product increased to 20 ppm and became an off-spec product. At this time, the concentration of hydrogen cyanide in the bottom liquid of the dehydride dehydration tower was 120 wtppm. When it was judged that the amount of heat removal Q2 in the condenser was excessive and Q2 was reduced by reducing the flow rate of the refrigerant leading to the condenser, the hydrogen cyanide concentration in the acrylonitrile obtained as a product was reduced to 5 ppm and became an on-spec product. Moreover, the ratio of acrylonitrile distilled in the hydrogen cyanide distilled from the tower top rose to 600 ppm, and the quality of the hydrogen cyanide derivative was lowered. During this time, the temperature of the uppermost stage of the dehydrating acid dehydration tower was 30 ° C. and did not change.

This application is based on a Japanese patent application (Japanese Patent Application No. 2010-290914) filed with the Japan Patent Office on December 27, 2010, the contents of which are incorporated herein by reference.

The method of the present invention has industrial applicability in a process for producing acrylonitrile in which propylene and / or propane, ammonia and oxygen are reacted in the presence of a catalyst.

DESCRIPTION OF SYMBOLS 1 Fluidized bed reactor 2 Propylene and / or propane supply pipe 3 Ammonia supply pipe 4 Air (oxygen) supply pipe 6

Quenching tower

5, 7, 8 line 9

Absorption tower

10, 11 line 12

Recovery tower

13, 14, 15, 16, 17 line 18 debrocyanic acid dehydration tower 19 line 20 deblueening acid dehydration tower condenser 20 a refrigerant supplied to deblue acid dehydration tower condenser 20 b refrigerant flow control valve 20 b ′ condenser supplied to deblue acid dehydration tower condenser

Flow control valve

21, 22 line 22a temperature controller 22b temperature detector (thermometer) connecting refrigerant supply pipe and discharge pipe
23, 23c, 23e, 23f Line 23a Refrigerant to be supplied to the decut acid dehydration tower side cut cooler 23b Deblue acid dehydration tower side cut cooler 23d Deblue acid

dehydration tower decanter

24, 24c Line 24a Deblue acid dehydration tower reboiler 24b Deblue acid dehydration tower reboiler 25

Heat column

26, 27, 28, 29 Line 30 Product tower condenser 31 Line A Feed stage B Temperature control stage C Top stage D Chimney tray

Claims

A method for purifying acrylonitrile, comprising a step of distilling a solution containing acrylonitrile, hydrogen cyanide and water using a distillation apparatus having a distillation column and a condenser for the top gas connected to the distillation column,
A method comprising a step of maintaining a constant temperature of a temperature control stage located above the feed stage of the distillation column and below the uppermost stage of the distillation column.
A regulating valve is provided in a pipe for supplying the refrigerant to the condenser and / or a pipe for discharging the refrigerant from the condenser, and a thermometer is provided in the temperature control stage,
Set the target temperature of the temperature control stage,
When the temperature of the temperature control stage is higher than the target temperature, the opening of the adjustment valve is adjusted to increase the supply amount of the refrigerant,
2. The temperature of the temperature control stage is kept constant by adjusting the opening of the regulating valve to reduce the supply amount of the refrigerant when the temperature of the temperature control stage is lower than the target temperature. Method.
An upper limit value and a lower limit value of the temperature of the temperature control stage are set, and the supply amount of the refrigerant is adjusted by the adjustment valve so that the temperature of the temperature control stage changes between the lower limit value and the upper limit value. The method according to claim 2.
While giving a certain amount of heat from the reboiler to the distillation column, the heat removal amount of the condenser is increased or decreased,
In each heat removal amount, the temperature of each stage located above the feed stage of the distillation column and below the top stage of the distillation column;
Measure the acrylonitrile concentration and hydrogen cyanide concentration in each stage,
A stage located above the feed stage of the distillation column and below the top stage of the distillation tower, wherein the acrylonitrile concentration is lower than the hydrogen cyanide concentration, the lowest stage (bottom stage) is temperature controlled. Set to
The step of determining a target temperature of the temperature control stage from the temperature of each stage in each heat removal amount so as to minimize the concentration of acrylonitrile distilled from the top of the distillation column. The method for purifying acrylonitrile according to any one of the above.
While giving a certain amount of heat from the reboiler to the distillation column, the heat removal amount of the condenser is increased or decreased,
In each heat removal amount, the temperature of each stage located above the feed stage of the distillation column and below the top stage of the distillation column;
Measure the acrylonitrile concentration and hydrogen cyanide concentration in each stage,
A stage located above the feed stage of the distillation column and below the top stage of the distillation tower, wherein the acrylonitrile concentration is higher than the hydrogen cyanide concentration, and the uppermost stage (top stage) is temperature controlled. Set to
The step of determining a target temperature of the temperature control stage from the temperature of each stage in each heat removal amount so as to minimize the concentration of acrylonitrile distilled from the top of the distillation column. The method for purifying acrylonitrile according to any one of the above.
A distillation tower,
A thermometer provided in a temperature control stage located above the feed stage of the distillation column and below the uppermost stage of the distillation column;
A condenser connected to the distillation column;
A pipe for supplying a refrigerant and a pipe for discharging the refrigerant connected to the condenser;
A distillation apparatus comprising: a pipe for supplying the refrigerant and / or an adjustment valve for adjusting a supply amount of the refrigerant attached to the pipe for discharging the refrigerant,
The thermometer is connected to the regulating valve via a temperature controller,
The temperature of the temperature control stage is transmitted to the temperature controller by the thermometer,
When the temperature of the temperature control stage is higher than the target temperature by the temperature controller, the supply amount of the refrigerant is increased by adjusting the opening of the adjustment valve,
A distillation apparatus in which when the temperature of the temperature control stage is lower than a target temperature, the amount of refrigerant supplied is reduced by adjusting the opening of the regulating valve.