JP4467312B2 - Method for producing acrylonitrile - Google Patents

Description

  The present invention relates to a method for producing acrylonitrile, and more particularly to a method for producing acrylonitrile by an oxidation reaction by mixing hydrocarbon, ammonia and an oxygen-containing gas.

  In recent years, acrylonitrile has been produced on a large scale by an oxidation reaction of propylene and ammonia in the gas phase. As a production method thereof, an oxygen-containing gas such as propylene, ammonia and air is supplied to a reactor filled with a catalyst. A typical method is to produce acrylonitrile by an oxidation reaction at a high temperature.

  In this method, the product gas containing acrylonitrile distilled out from the reactor contains unreacted ammonia, oxygen, nitrogen and propylene in addition to acrylic acid, acetic acid, hydrocyanic acid, acetonitrile, acrolein and the like.

  If this product gas is left as it is, side reactions such as the formation of β-aminopropionitrile and the polymerization of hydrocyanic acid occur, so in order to prevent such side reactions, it is washed with an aqueous sulfuric acid solution and cooled to leave it unreacted. The ammonia is neutralized and removed as an aqueous ammonium sulfate solution.

  The oxidation reaction gas from which unreacted ammonia has been removed, after cooling, absorbs or dissolves products such as acrylonitrile, hydrocyanic acid, acetonitrile, and acrolein in water in an absorption tower and separates them from the oxidation reaction gas.

  From the aqueous solution in which acrylonitrile or the like is dissolved, the produced acrylonitrile is recovered by appropriately combining means such as stripping or distillation in a recovery tower.

  At this time, acrylonitrile is recovered as crude acrylonitrile and further sent to a purification system, where light-boiling components containing hydrocyanic acid are separated, dehydrated, and high-boiling components are separated, and finally product acrylonitrile is obtained.

  In the reactor in such a continuous production process of acrylonitrile, the concentration of supplied oxygen is set to a predetermined concentration (specifically, 0.1 to 1.0 vol%, preferably with respect to hydrocarbons such as propylene supplied). 0.1 to 0.4 vol%, acrylonitrile according to the characteristics of the catalyst by adjusting to a molar ratio of air / propylene of 8.0 to 11.0, preferably 8.6 to 9.5) Production efficiency can be maintained as high as possible (Patent Document 1).

  In addition, regarding the technology for controlling the oxygen supplied to the reactor, a sampling line (collection path) is provided between the quenching towers connected downstream of the reactor, gas is collected from the sampling line, and monoxide is oxidized in the gas. Connect a gas chromatography that measures the amount of each component of carbon, carbon dioxide and unreacted olefin (hydrocarbon), calculate the amount of change in the yield or concentration of these components, and these measured values and the amount of oxygen supplied The amount of oxygen supplied to the reactor in the continuous production state is changed so that the pattern of the correlation curve indicating the relationship between the two is identical (Patent Document 2).

JP-A-8-206643 (see paragraph [0014]) JP-A-9-208550 (refer to claim 6)

  However, according to the conventional acrylonitrile manufacturing method described above, the measuring line such as the sampling line (collecting channel) or gas chromatography provided at the outlet of the reactor or downstream thereof is clogged by the high boiling point impurities in the reactor. Therefore, there is a problem that it is difficult to accurately measure the components of the product gas over a long period of time.

  Such clogging of the sampling line and measuring equipment can be resolved by repair work such as cleaning or replacement near the line, but if the repair work interval is increased, the flow rate gradually decreases, The problem of fluctuations in measured values is unavoidable, and stable and accurate measurement cannot be performed.

  Accordingly, an object of the present invention is to solve the above-mentioned problems, continuously measure the oxygen concentration of the product gas at the outlet of the reactor or downstream thereof, and accurately measure it stably for a long time, and supply it to the reactor. It is to accurately control the oxygen concentration in the raw material gas to an optimum concentration so that the production efficiency of acrylonitrile can be maintained as high as possible.

  In order to solve the above problems, in the present invention, a raw material gas containing hydrocarbon, ammonia and oxygen is supplied to a reactor, and an acrylonitrile component in a product gas obtained by a gas phase oxidation reaction in the reactor is recovered. In the method for producing acrylonitrile, a product gas sample collection path is provided on the downstream side of the reactor, and the product gas sample is brought into contact with water through the collection path, and then water components are separated and separated. The product gas sample is dehumidified, then the oxygen concentration of the product gas sample is measured, and the raw material gas composition is adjusted based on the oxygen concentration so that the amount of acrylonitrile component in the product gas is increased. This is a method for producing acrylonitrile.

  According to the acrylonitrile manufacturing method of the present invention configured as described above, the product gas sample collected when the product gas sample is brought into contact with water in the product gas sample collection path and then the water-soluble component is separated thereafter. From there, water-soluble hydrocyanic acid, acrylonitrile and the like are removed by being dissolved in water, and further, water vapor in the generated gas and water-soluble components dissolved therein are also removed by dehumidification.

  Therefore, the dehumidified product gas contains almost no condensable component, and the product gas flow rate does not decrease because the liquid component does not condense during the oxygen concentration measurement, and the product gas sample maintained at a constant flow rate. Thus, the oxygen concentration can be measured stably and accurately.

  In this way, when the concentration of oxygen to be supplied can be accurately controlled to the optimum concentration based on the oxygen concentration of the product gas sample, the source gas composition, especially the oxygen concentration can be adjusted appropriately at all times. Thus, the production efficiency of acrylonitrile can be maintained as high as possible.

  When the step of mixing the product gas with water vapor is adopted as the step of passing the product gas in contact with water, the contact efficiency is improved, and the water-soluble components in the product gas can be efficiently separated in a short time.

  In addition, if the dehumidification is a dehumidification by the condensation step of the water vapor in the product gas or the contact step with the desiccant or both steps, the water-soluble component in the product gas can be efficiently separated, which is a more preferable method as a production method .

  As described above, according to the present invention, in the method for producing acrylonitrile, a product gas collection path is provided on the downstream side of the reactor, and the product gas is brought into contact with water through the collection path to remove water components. Since the oxygen concentration was measured after dehumidifying the product gas and the composition of the raw material gas supplied to the reactor was adjusted based on this oxygen concentration, the oxygen concentration of the product gas at the outlet of the reactor or downstream thereof Can be accurately measured continuously for a long period of time, and the concentration of supplied oxygen can be quickly adjusted to an optimum concentration, which has the advantage that the production efficiency of acrylonitrile can be increased as much as possible.

  Acrylonitrile in the present invention supplies a raw material gas containing hydrocarbon, ammonia and an oxygen-containing gas to a reactor, causes an oxidation reaction using an ammoxidation catalyst in this reactor, and from the gas generated at that time The acrylonitrile component is recovered.

  As the reactor used in the present invention, either a fluidized bed reactor or a fixed bed reactor may be used, but it is preferable to use a fluidized bed reactor for the reason that the reaction heat can be easily removed.

  The hydrocarbon used in the present invention is a gaseous hydrocarbon that is a component of the raw material gas. For example, an olefin such as propylene, propane, or a gas mixture thereof can be used.

  The oxygen-containing gas may contain a predetermined concentration of oxygen, and the remaining component may be composed of a mixed gas composed of an inert gas. Usually, preferable results are obtained using air.

  As the catalyst used in the gas phase oxidation reaction for oxidizing ammonia, for example, a composite metal oxide catalyst composed of elements such as phosphorus, molybdenum, bismuth, iron, antimony, and the like can be used.

  As shown in FIG. 1, in the acrylonitrile production method of the embodiment, propylene, ammonia and air are supplied to a fluidized bed reactor 1 filled with a catalyst and reacted at a high temperature to produce acrylonitrile.

  The reaction product gas containing acrylonitrile flowing out from the fluidized bed reactor 1 contains by-products such as ammonia, acrylic acid, acetic acid, hydrocyanic acid, acetonitrile, and acrolein. Acrylonitrile is recovered by the following recovery process.

  First, the reaction product gas flowing out from the fluidized bed reactor 1 is supplied to the quenching tower 2 and washed and cooled with sulfuric acid water, and unreacted ammonia is neutralized and absorbed as an aqueous ammonium sulfate solution. It is removed from the bottom.

  Next, the reaction product gas from which unreacted ammonia has been removed is supplied to the absorption tower 3, and by-products such as acrylonitrile, acrylic acid, acetic acid, hydrocyanic acid, acetonitrile, and acrolein are absorbed and dissolved in water, and then from the bottom of the tower. Extract and separate from reaction product gas. The gas not absorbed is supplied to the waste gas incinerator and incinerated.

The aqueous solution in which the extracted acrylonitrile and the like are dissolved is supplied to the recovery tower 4, and water vapor having an azeotropic composition amount of gaseous acrylonitrile and hydrocyanic acid and acrylonitrile as a main component by means of stripping or distillation (in the figure) In the following purification system, hydrocyanic acid, water and the like are removed and rectified to obtain product acrylonitrile.
In the figure, reference numeral 13 indicates a dehydration tower, 14 indicates a drying tower, and 15 indicates a purification tower.

  On the other hand, a part of the waste water extracted from the bottom of the recovery tower 4 is circulated to the absorption tower 3 and a part is treated as waste water. Further, impurities such as acetonitrile (indicated by “ATN” in the figure) are separated from the middle stage of the recovery tower 4.

  As shown in FIG. 1, the product gas sample collection channel 5 may be provided on the downstream side of the reactor, for example, between the quenching tower 2 and the absorption tower 3 or upstream of the quenching tower 2 although not shown. It can also be provided near the outlet of the reactor 1.

  As shown in FIG. 2, in the collection channel 5, the generated gas sample is brought into contact with water using a gas scrubber 6, and then water components are separated from the lower part of the gas scrubber 6. A product gas sample is collected from the gas.

  Next, the product gas sample with increased humidity is introduced into the gas cooler 8 through the pressure compensator 7 and cooled, and dehumidification treatment is performed to remove the condensed water, followed by solid or liquid desiccant such as silica gel. When passing through the filled dryer 9, it is brought into contact with a desiccant to obtain a sufficiently dried product gas sample.

  The product gas sample thus dried is introduced into an oxygen concentration meter (oxygen concentration automatic measuring device) 10 to automatically measure the oxygen concentration, and the amount of acrylonitrile component in the product gas increases based on this oxygen concentration. Thus, the raw material gas composition is adjusted to an appropriate oxygen / hydrocarbon molar ratio.

  Acrylonitrile was produced by the production process shown in FIGS. That is, 7.8 kg / hr of propylene, 3.5 kg / hr of ammonia, and 54 kg / hr of air were supplied to the fluidized bed reactor 1 at each flow rate, and the pressure was 90 kPa and the temperature was 460 ° C. in the presence of 84 kg of an Fe—Sb catalyst. The ammonia oxidation reaction gas that flowed out of the reactor 1 was brought into contact with the sulfuric acid aqueous solution in the quenching tower 2 to remove unreacted ammonia.

  As shown in FIG. 2, a part of the reaction gas (quenching tower outlet gas) from which unreacted ammonia has been removed is introduced into the collecting channel 5 and mixed with steam, and is washed with water in the gas scrubber 6, thereby being washed with water. The reaction gas was cooled and dehydrated with a gas cooler 8 while stabilizing the flow rate with a pressure compensator 7 and passed through a dryer 9 filled with 1 kg of silica gel for dehumidification (water vapor pressure of 15 mmHg or less, preferably 10 mmHg). Then, after passing through the filter 11 and the rotor meter 12, when the oxygen concentration was continuously measured with the oxygen concentration meter 10, the oxygen concentration was in the range of 0.1 to 1.0 vol%.

  When the oxygen concentration is outside this range, the air / propylene molar ratio of the raw material gas is adjusted within the range of 8.0 to 11.0, and the value of the oxygen concentration is maintained within a predetermined range. I did it.

Although the manufacturing method in Example 1 was continuously performed for three months, the silica gel in the dryer 9 installed in the collection channel 5 in which the oximeter was installed was exchanged once a week. Other than the above, there was no need for maintenance, and the oxygen concentration meter 10 could be continuously and accurately measured without any problem causing clogging. As a result, the oxygen concentration in the raw material gas supplied to the reactor could be accurately controlled to an optimum concentration, and the acrylonitrile production efficiency could be maintained as high as possible.

Comparative Example 1

  In Example 1, except that the gas cooler (vortex cooler) and the dryer were not installed in the collection channel 5 in which the oxygen concentration meter 10 was installed, that is, the collected product gas sample was not dehumidified. Similarly, acrylonitrile was produced.

  In this way, water droplets and the like adhere to the filter 11 and the rotor meter 12 of the collection channel 5 shown in FIG. 3, and the flow rate gradually decreases. Finally, the gas flow rate to the oximeter 10 becomes 0 on the scale. In order to avoid this, the line cleaning operation and the drying operation were required twice per day.

  As a result, it was recognized that the oxygen concentration in the raw material gas supplied to the reactor could not always be controlled to an optimum concentration, and that the production efficiency of acrylonitrile was therefore lowered.

The flowchart which shows the acrylonitrile manufacturing process of embodiment Schematic which shows the connection state of the collection path of the product gas sample in the manufacturing process of Example 1, and an oximeter Schematic which shows the connection state of the collection path of the product gas sample in the manufacturing process of the comparative example 1, and an oximeter

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fluidized bed reactor 2 Quenching tower 3 Absorption tower 4 Recovery tower 5 Collection path 6 Gas scrubber 7 Pressure compensator 8 Gas cooler 9 Dryer 10 Oxygen meter 11 Filter 12 Rotor meter 13 Dehydration tower 14 Drying tower 15 Purification Tower

Claims (3)

In a method for producing acrylonitrile, a raw material gas containing hydrocarbon, ammonia and oxygen is supplied to a reactor, and an acrylonitrile component in a product gas by a gas phase oxidation reaction in the reactor is recovered.
A sample collection path is provided downstream of the reactor and upstream of the quenching tower or between the quenching tower and the absorption tower, and a part of the product gas is introduced , and the product gas sample is brought into contact with water through the collection path. Then, the water component is separated and the separated product gas sample is dehumidified, and then the oxygen concentration of the product gas sample is measured. Based on this oxygen concentration, the amount of acrylonitrile component in the product gas increases. The raw material gas composition is adjusted to a method for producing acrylonitrile.   The method for producing acrylonitrile according to claim 1, wherein the step of bringing the product gas into contact with water is a step of mixing the product gas with water vapor. The method for producing acrylonitrile according to claim 1 or 2, wherein the dehumidification is a condensation step of water vapor in the product gas, a contact step with a desiccant or both steps.

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