Process for producing purified hydrogen cyanide

Abstract

Provided is a method which is for producing purified hydrogen cyanide, and by which hydrogen cyanide can be purified in a safe and stable manner while reducing the number of spots that require the use of materials having high corrosion resistance in equipment and members for purifying the hydrogen cyanide. This method for producing purified hydrogen cyanide comprises: a step (A) for obtaining crude mixed gas 3 by mixing crude hydrogen cyanide gas 1 with crude acrylonitrile gas 2 obtained through ammoxidation; a step (B) for obtaining deammoniation-treated gas 5 by mixing the crude mixed gas 3 with a sulfuric acid aqueous solution 4 to neutralize and remove unreacted ammonia contained in the crude mixed gas 3; a step (C) for obtaining a crude mixed liquid 7 by absorbing the deammoniation-treated gas 5 into water 6; a step (D) for obtaining an organic layer 9 containing hydrogen cyanide and acrylonitrile by separating an aqueous layer 8 from the crude mixed liquid 7; and a step (E) for obtaining purified hydrogen cyanide 11 by distilling the organic layer 9.

Description

Technical Field

[0001] The present invention relates to a method for producing purified hydrogen cyanide by purifying crude hydrogen cyanide containing impurities such as unreacted residual components of synthetic raw materials to obtain purified hydrogen cyanide.

Background Art

[0002] Hydrogen cyanide is widely used as a raw material for various compounds and pesticides. As an industrial production method of hydrogen cyanide, for example, ammoxidation of a mixture of methane, ammonia and air, so-called Andrussow method, is known. Further, a method of obtaining by-produced hydrogen cyanide when producing acrylonitrile by ammoxidation using propylene or propane as a raw material, so-called Sohio method, is also used.

[0003] Hydrogen cyanide synthesized by ammoxidation of methane as described above is crude hydrogen cyanide that may contain impurities such as unreacted residual components of synthetic raw materials such as methane, ammonia, and oxygen. In order to obtain high-purity hydrogen cyanide, the crude hydrogen cyanide is purified.

[0004] Hydrogen cyanide easily polymerizes in the presence of an alkali, and the polymerization is further accelerated by heat generation, and there is a risk of causing blockage of lines in the purification process and even fire and explosion due to a runaway reaction. Therefore, crude hydrogen cyanide is purified in a state where the risk of polymerization is reduced by pH adjustment or the like.

[0005] For example, Patent Document 1 describes that as a method for simultaneously producing acrylonitrile and hydrogen cyanide, the acrylonitrile production process and the hydrogen cyanide production process are operated in parallel, and the product flows from both processes are combined and recovered and purified, so that polymerization of relatively high-concentration hydrogen cyanide can be substantially prevented.

[0006] The schematic flow of the method described in the above Patent Document 1 is shown in FIG. 2. As shown in FIG. 2, in the method described in the above Patent Document 1, the acrylonitrile product 22 from the acrylonitrile synthesis reactor is neutralized with an ammonia spray of an aqueous sulfuric acid solution 23 in the quench column 100, and then in the absorption column 200, it is mixed with the hydrogen cyanide product 21 and water 25 from the hydrogen cyanide synthesis reactor and the pH is adjusted by the addition of an acid 26 to form a mixed product. After non-absorbed compounds are separated and removed as off-gas 24 in the absorption column 200, the acid 26 is added to the transferred recovery column 300 to adjust the pH. Then, the mixed product transferred to the decanter 400 is adjusted in pH with the acid 26, after which the aqueous layer is separated and refluxed to the recovery column 300. The organic layer is supplied to the head column 500, adjusted in pH with the acid 26, and then separated to obtain separated crude hydrogen cyanide 31 and separated crude acrylonitrile 32. The separated crude hydrogen cyanide 31 is further distilled to obtain purified hydrogen cyanide.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] By the way, a mixture whose pH is adjusted by mixing hydrogen cyanide and sulfuric acid or the like is likely to corrode carbon steel, which is a material generally used for plant pipes and containers. Therefore, from the viewpoint of safe operation of the purification process, it is necessary to frequently replace the equipment and members in the process of contacting the mixture, or alternatively, to use a material with high corrosion resistance such as stainless steel. In Patent Document 1 mentioned above, when recovering and purifying hydrogen cyanide by the method described in the document, in order to protect the equipment that comes into contact with hydrogen cyanide at a higher concentration than in the production of acrylonitrile by the ordinary Sohio method, it is described that the recovery and purification equipment should use a constituent material such as stainless steel, which has higher corrosion resistance than carbon steel, which is a material generally used for plant piping, containers, etc.

[0009] However, frequent replacement of plant equipment and components is not preferable because it leads to an increase in equipment costs, an increase in the burden of replacement work, and further a decrease in the production efficiency of purified hydrogen cyanide. On the other hand, materials with high corrosion resistance such as stainless steel are expensive. Therefore, when used as the constituent material for many parts of the recovery and purification equipment, or in a purification equipment for hydrogen cyanide alone, the equipment cost for obtaining purified hydrogen cyanide increases. Therefore, there is a need for a method that can obtain high-quality purified hydrogen cyanide safely and stably at low cost.

[0010] The present invention has been made to solve the above-mentioned problems, and in the purification equipment and components of hydrogen cyanide, it aims to provide a method for producing purified hydrogen cyanide that reduces the locations where materials with high corrosion resistance are required and can purify hydrogen cyanide safely and stably.

Means for Solving the Problems

[0011] The present invention is based on the finding that in the production of purified hydrogen cyanide, by incorporating the purification process of crude hydrogen cyanide into a predetermined step in the purification process of acrylonitrile, the locations where a material with high corrosion resistance rather than ordinary carbon steel must be used as the material of the purification equipment and its components can be reduced.

[0012] That is, the present invention provides the following [1] to [8]. [1] A process for producing purified hydrogen cyanide, comprising: step (A) of mixing crude hydrogen cyanide gas with crude acrylonitrile gas obtained by ammoxidation to obtain a crude mixed gas; step (B) of mixing the crude mixed gas with an aqueous sulfuric acid solution to neutralize and remove unreacted ammonia contained in the crude mixed gas to obtain a deammoniated gas; step (C) of absorbing the deammoniated gas in water to obtain a crude mixed liquid; step (D) of separating an aqueous layer from the crude mixed liquid to obtain an organic layer containing hydrogen cyanide and acrylonitrile; and step (E) of distilling the organic layer to obtain purified hydrogen cyanide. [2] The process for producing purified hydrogen cyanide according to [1] above, wherein the crude mixed gas contains 2.5 to 9.0 moles of acrylonitrile per mole of hydrogen cyanide. [3] The process for producing purified hydrogen cyanide according to [1] or [2] above, wherein the concentration of hydrogen cyanide in the crude mixed liquid is 0.1 to 20.0% by mass. [4] The process for producing purified hydrogen cyanide according to any one of [1] to [3] above, wherein in step (B), the crude mixed gas and the aqueous sulfuric acid solution are mixed by flowing the crude mixed gas into the aqueous sulfuric acid solution. [5] The process for producing purified hydrogen cyanide according to any one of [1] to [4] above, wherein step (C) is carried out in equipment made of carbon steel. [6] The process for producing purified hydrogen cyanide according to any one of [1] to [5] above, wherein step (D) is carried out in equipment made of carbon steel. [7] The process for producing purified hydrogen cyanide according to any one of [1] to [6] above, wherein the crude hydrogen cyanide gas is obtained by ammoxidation of a synthetic raw material containing methanol. [8] The process for producing purified hydrogen cyanide according to any one of [1] to [7] above, wherein the crude acrylonitrile gas is obtained by ammoxidation of a synthetic raw material containing propylene. [Effect of the Invention]

[0013] According to the method for producing purified hydrogen cyanide of the present invention, in the hydrogen cyanide purification equipment and members, the equipment cost can be suppressed by reducing the locations where materials with high corrosion resistance are required, and moreover, hydrogen cyanide can be purified safely and stably. As a result, it becomes possible to improve the production efficiency of purified hydrogen cyanide.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0015] Hereinafter, the method for producing purified hydrogen cyanide of the present invention will be described with reference to the drawings. FIG. 1 shows the schematic flow of the method for producing purified hydrogen cyanide of the present invention. The method for producing purified hydrogen cyanide of the present invention includes a step (A) of mixing crude hydrogen cyanide gas 1 with crude acrylonitrile gas 2 obtained by ammoxidation to obtain a crude mixed gas 3, a step (B) of mixing the crude mixed gas 3 with an aqueous sulfuric acid solution 4 to neutralize and remove unreacted ammonia contained in the crude mixed gas 3 to obtain a deammoniated gas 5, a step (C) of absorbing the deammoniated gas 5 in water 6 to obtain a crude mixed liquid 7, a step (D) of separating an aqueous layer 8 from the crude mixed liquid 7 to obtain an organic layer 9 containing hydrogen cyanide and acrylonitrile, and a step (E) of distilling the organic layer 9 to obtain purified hydrogen cyanide 11. That is, the method for producing purified hydrogen cyanide of the present invention includes steps (A) to (E) in order as shown in FIG. 1. Through such steps, by purifying crude hydrogen cyanide, in the hydrogen cyanide purification equipment and members, the locations where materials with high corrosion resistance are required can be reduced, so that the equipment cost can be suppressed. In addition, hydrogen cyanide can be purified safely and stably. The following describes steps (A) to (E) in order.

[0016] [Step (A)] Step (A) is a mixing step of mixing crude hydrogen cyanide gas 1 with crude acrylonitrile gas 2 obtained by ammoxidation to obtain a crude mixed gas 3. As described above, since hydrogen cyanide has a risk of polymerization in the presence of an alkali, it is necessary to maintain it at neutral to acidic with an acid such as sulfuric acid. However, a mixed state of high-concentration hydrogen cyanide and an acid such as sulfuric acid easily corrodes carbon steel. From such a viewpoint, in the present invention, before mixing with an aqueous sulfuric acid solution for suppressing the polymerization of hydrogen cyanide, the crude hydrogen cyanide gas 1 and the crude acrylonitrile gas 2 are mixed to form a crude mixed gas 3.

[0017] [Crude hydrogen cyanide gas] The supply source of the crude hydrogen cyanide gas 1 is not particularly limited, and may be a product gas containing hydrogen cyanide produced by a known hydrogen cyanide synthesis method. As an industrial production method of hydrogen cyanide, in addition to the Andrussow method described above, there are also a so-called BMA method (also called the Degussa method) using methane as a raw material, a method by ammoxidation of methanol, and the like.

[0018] The Andrussow method is a method of reacting a mixture of methane, ammonia and air (oxygen) usually in the presence of a catalyst such as a rhodium-platinum catalyst at a high temperature of about 800 to 1000 °C. This reaction is the ammoxidation of methane and proceeds as shown in the following reaction formula (1). CH4 + NH3 + 3 / 2O2 → HCN + 3H2O (1)

[0019] The BMA method is a method of reacting in a bundle-shaped alumina tube whose inner surface is coated with a platinum catalyst at a high temperature of about 1300 °C in the absence of air (oxygen), and this reaction proceeds as shown in the following reaction formula (2). CH4 + NH3 → HCN + 3H2 (2)

[0020] In addition, in the method of ammoxidation of methanol, the reaction proceeds as shown in the following reaction formula (3). In this method, a mixture of methanol, ammonia and air (oxygen) can be reacted at about 300 to 600 °C, which is a lower temperature than those in the above reaction formulas (1) and (2), in the presence of a catalyst such as a composite oxide catalyst of molybdenum, bismuth and other metals. CH3OH + NH3 + O2 → HCN + 3H2O (3)

[0021] Among these methods for producing hydrogen cyanide, from the viewpoints of energy efficiency and the like, the production method by ammoxidation of methanol with a relatively low reaction temperature is preferred. Therefore, the crude hydrogen cyanide gas 1 is preferably obtained by ammoxidation of a synthesis raw material containing methanol. The pressure during the reaction is preferably normal pressure to 1 MPaG, more preferably normal pressure to 0.5 MPaG, and even more preferably normal pressure to 0.2 MPa. The residence time of the product in the synthesis reactor is preferably 0.1 to 60 seconds, more preferably 0.1 to 50 seconds, and even more preferably 0.1 to 30 seconds.

[0022] The crude hydrogen cyanide gas 1 is a gas containing impurities such as unreacted residual components of the synthesis raw material, contaminants during production, and by-products in addition to hydrogen cyanide. The crude hydrogen cyanide gas 1 may be directly supplied with the product gas discharged from the hydrogen cyanide synthesis reactor. When the temperature of the product gas is high, it is preferably cooled to a temperature at which the mixing operation with the crude acrylonitrile gas 2 can be safely performed, and then subjected to the mixing step. The temperature is preferably 250 °C or lower, more preferably 100 to 230 °C, and even more preferably 150 to 200 °C from the viewpoints of safety and energy efficiency.

[0023] <Crude acrylonitrile gas> The crude acrylonitrile gas 2 is obtained by ammoxidation. As a method for producing acrylonitrile by ammoxidation, known methods can be applied. Among these methods, the above-described Sohio method is common. Usually, a mixture of propylene, ammonia, and air is reacted at a temperature of about 350 to 500°C in the presence of a catalyst such as a composite oxide catalyst of molybdenum, bismuth, and other metals. From the viewpoint of production efficiency and the like, a production method by ammoxidation of propylene is preferable as a method for producing acrylonitrile. Therefore, it is preferable that the crude acrylonitrile gas 2 is obtained by ammoxidation of a synthetic raw material containing propylene.

[0024] In addition, the synthetic raw material containing propylene in the production of acrylonitrile may contain methanol. Thereby, it is possible to increase the content of hydrogen cyanide in the crude acrylonitrile gas 2. However, the ammoxidation of a synthetic raw material containing methanol together with propylene places a large load on the acrylonitrile synthesis reactor and has a high energy cost, and the catalyst may also be easily deactivated. Therefore, when increasing the amount of hydrogen cyanide in the crude mixed gas 3, rather than increasing the amount of methanol supplied as a synthetic raw material together with propylene to the acrylonitrile synthesis reactor, as described above, it is preferable to increase the amount by adjusting the supply amount of the product gas discharged from the hydrogen cyanide synthesis reactor separately.

[0025] The crude acrylonitrile gas 2 is a gas containing impurities such as unreacted residual components of the synthetic raw material, contaminants during production, and by-products such as acetonitrile and hydrogen cyanide in addition to acrylonitrile. The crude acrylonitrile gas 2 may be directly supplied with the product gas discharged from the acrylonitrile synthesis reactor. When the temperature of the product gas is high, it is preferable to cool it to a temperature at which the mixing operation with the crude hydrogen cyanide gas 1 can be safely performed and then subject it to the mixing step. The temperature is preferably 250°C or lower, more preferably 100 to 240°C, and even more preferably 150 to 230°C from the viewpoints of safety and energy efficiency. Since the crude acrylonitrile gas 2 may contain tar and heavy components due to modified products and polymers of acrylonitrile as impurities, it is preferable to wash it with water to separate and remove these tar and heavy components. The washing can be carried out, for example, in an absorption tower into which water is flowing. The crude acrylonitrile gas 2 washed with water is preferably cooled to 100°C or lower, more preferably 95°C or lower, and even more preferably 85°C or lower.

[0026] As described above, from the viewpoint of the safety of the mixing operation and the like, both the crude hydrogen cyanide gas 1 and the crude acrylonitrile gas 2 are preferably cooled to a temperature lower than the temperature of the product gas in the synthesis reactor and then mixed to obtain the crude mixed gas 3.

[0027] <Crude mixed gas> The crude mixed gas 3 preferably contains 2.5 to 9.0 moles of acrylonitrile per mole of hydrogen cyanide, more preferably 2.6 to 6.0 moles, and even more preferably 2.8 to 5.0 moles. If there are 2.5 moles or more of acrylonitrile per mole of hydrogen cyanide, the hydrogen cyanide is in a state of being sufficiently diluted by the acrylonitrile, and even when sulfuric acid is mixed to suppress the polymerization of hydrogen cyanide, the corrosion of carbon steel is likely to be suppressed. Also, from the viewpoint of the production efficiency of purified hydrogen cyanide, the upper limit of the amount of acrylonitrile per mole of hydrogen cyanide is preferably 9.0 moles or less.

[0028] In addition, since it is difficult to determine by actual analysis and measurement in the crude mixed gas 3 the molar amount of acrylonitrile per mole of hydrogen cyanide in the crude mixed gas 3 of the present invention, that is, the molar ratio of acrylonitrile to hydrogen cyanide, it is a calculated value based on the charged amount of the synthesis raw materials and the assumed yield in the production of each of acrylonitrile and hydrogen cyanide.

[0029] [Step (B)] Step (B) is a deammoniation treatment step in which the crude mixed gas 3 obtained in step (A) is mixed with an aqueous sulfuric acid solution 4 to neutralize and remove the unreacted ammonia contained in the crude mixed gas 3, thereby obtaining a deammoniated gas 5. Thus, by subjecting the crude mixed gas 3 to deammoniation treatment with the aqueous sulfuric acid solution 4, it is not necessary to perform a deammoniation treatment for removing unreacted ammonia, which is a raw material for synthesizing hydrogen cyanide, in subsequent steps.

[0030] The mixing of the crude mixed gas 3 and the aqueous sulfuric acid solution 4 is preferably carried out by introducing the crude mixed gas 3 into the aqueous sulfuric acid solution 4. For example, it can be carried out by introducing the crude mixed gas 3 into a sulfuric acid tank filled with the aqueous sulfuric acid solution 4 and mixing it with the aqueous sulfuric acid solution 4. By contacting the crude mixed gas 3 with a sufficient amount of the aqueous sulfuric acid solution 4 in such a manner, the unreacted ammonia of the synthetic raw material contained in the crude mixed gas 3 is neutralized with sulfuric acid, and the generated ammonium sulfate is removed as an aqueous solution. The concentration of sulfuric acid in the aqueous sulfuric acid solution 4 depends on the amount of unreacted ammonia contained in the crude mixed gas 3, but from the viewpoints of efficient removal of ammonia and operational safety, it is preferably 0.1 to 20.0% by mass, more preferably 1.0 to 10.0% by mass, and still more preferably 2.0 to 7.0% by mass. The temperature of the sulfuric acid tank is preferably 30 to 100°C, more preferably 50 to 95°C, and still more preferably 70 to 90°C from the viewpoint of efficiently performing the deammoniation treatment of the crude mixed gas 3.

[0031] [Step (C)] Step (C) is an absorption step in which the deammoniated gas 5 obtained in step (B) is absorbed in water 6 to obtain a crude mixed liquid 7. In step (C), the deammoniated gas 5 from which ammonia has been sufficiently removed is absorbed in water 6, and the gas that is not absorbed in water is separated and removed as off-gas.

[0032] The crude mixture 7 has had ammonia sufficiently removed, has a pH of 7.00 or less, preferably 4.00 to 6.50, more preferably 5.00 to 6.00, and the hydrogen cyanide is in a stable state where polymerization is suppressed. Therefore, in step (C), it is not necessary to add an acidic liquid such as sulfuric acid again to adjust the pH. From the viewpoints of efficient production of purified hydrogen cyanide and operational safety due to sufficient suppression of hydrogen cyanide polymerization, the concentration of hydrogen cyanide in the crude mixture 7 is preferably 0.1 to 20.0% by mass, more preferably 0.3 to 10.0% by mass, and even more preferably 0.5 to 5.0% by mass.

[0033] Also, from the viewpoints of sufficiently dissolving hydrogen cyanide in the crude mixture 7 and efficiently producing purified hydrogen cyanide, the crude mixture 7 is preferably cooled to 50°C or lower, more preferably 1 to 35°C, and even more preferably 5 to 25°C.

[0034] As described above, although the crude mixture 7 is adjusted to be neutral to acidic with an aqueous sulfuric acid solution, the hydrogen cyanide is diluted in a state mixed with acrylonitrile, and carbon steel is not easily corroded. Therefore, in step (C), it is not necessary to use equipment made of a material with high corrosion resistance but high cost, such as stainless steel like SUS304 or SUS316, and it can be carried out using carbon steel equipment. In this regard, the method of the present invention has the advantage of being able to suppress equipment costs.

[0035] The crude mixture 7 obtained by absorbing the deammoniation-treated gas 5 into water 6 contains, in addition to hydrogen cyanide and acrylonitrile, acetonitrile which is a by-product in the production of acrylonitrile. In order to separate and remove acetonitrile, it is also preferable to perform a stripping treatment. The recovered liquid containing hydrogen cyanide and acrylonitrile after separating and removing acetonitrile is returned, and this is also combined as the crude mixture 7 and subjected to the next step (D).

[0036] [Step (D)] Step (D) is a liquid-liquid separation step of separating the aqueous layer 8 from the crude mixture 7 obtained in Step (C) to obtain an organic layer 9 containing hydrogen cyanide and acrylonitrile. In the organic layer 9 in Step (D), hydrogen cyanide is diluted with acrylonitrile, and the pH is maintained at 7.00 or less. It is not necessary to add an acid or the like for suppressing the polymerization of hydrogen cyanide. The pH of the organic layer 9 is preferably from 4.00 to 6.50, more preferably from 5.00 to 6.00.

[0037] Even in Step (D), as in Step (C), hydrogen cyanide is diluted in a state of being mixed with acrylonitrile, and since carbon steel is not easily corroded, it is not necessary to use equipment made of a highly corrosion-resistant material, and it can be carried out using carbon steel equipment. In this regard, the method of the present invention has the advantage of being able to suppress equipment costs.

[0038] The separated aqueous layer 8 contains a small amount of acrylonitrile, hydrogen cyanide, acetonitrile, etc. dissolved therein. The separated aqueous layer 8 can be recovered in order to improve the yield of purified hydrogen cyanide, and can also be circulated so as to be mixed with the crude mixture 7 together.

[0039] [Step (E)] Step (E) is a distillation step of distilling the organic layer 9 obtained in Step (D) to obtain purified hydrogen cyanide 11. In Step (E), the organic layer 9 is distilled to separate hydrogen cyanide contained in the organic layer 9 from acrylonitrile, thereby obtaining purified hydrogen cyanide 11.

[0040] In Step (E), since the distillation equipment for purifying hydrogen cyanide comes into contact with high-concentration hydrogen cyanide, in order to suppress the corrosion of the equipment and obtain high-purity purified hydrogen cyanide, it is preferably composed of a material having high corrosion resistance such as stainless steel. In addition, in order to suppress the polymerization of hydrogen cyanide during the distillation process and stabilize the organic layer 9, it is preferable to add an acid or the like when performing distillation. Examples of the additive for stabilizing hydrogen cyanide include glycolic acid, acetic acid, sulfurous acid gas, phosphoric acid, and the like. When adding an acid or the like, the pH of the organic layer 9 after addition is preferably 3.50 to 6.00, more preferably 4.00 to 5.50.

[0041] The fraction containing hydrogen cyanide recovered by separating and removing acrylonitrile can be further rectified to produce highly pure purified hydrogen cyanide 11. In addition, the hydrogen cyanide removal treatment liquid 12 containing acrylonitrile can be made into highly pure purified acrylonitrile, for example, by performing liquid-liquid separation treatment again, drying the obtained organic layer, and further rectifying it.

Examples

[0042] Examples of the present invention will be described below, but the present invention is not limited to these examples. [Example 1] Purified hydrogen cyanide was produced according to the flow shown in FIG. 1. The crude acrylonitrile gas 2 at 230°C obtained by ammoxidation of propylene was washed with water, cooled to 85°C, and then mixed with the crude hydrogen cyanide gas 1 at 200°C obtained by ammoxidation of methanol to obtain a crude mixed gas 3 (step (A)). The ratio of acrylonitrile to hydrogen cyanide contained in the crude mixed gas 3 was set to 2.83 moles of acrylonitrile per 1 mole of hydrogen cyanide. The crude mixed gas 3 was mixed with a sulfuric acid aqueous solution 4 having a sulfuric acid concentration of 5.0% by mass in a sulfuric acid tank (85°C) to obtain a deammoniation treatment gas 5 at 85°C (step (B)). Next, the deammoniation-treated gas 5 was absorbed in water to obtain a crude mixed solution 7 at about 20°C (step (C)). The crude mixed solution 7 was subjected to stripping treatment to separate and remove acetonitrile, and the recovered liquid containing hydrogen cyanide and acrylonitrile was returned and combined. The pH of the crude mixed solution 7 was 5.95. The pH was the arithmetic mean value of the values measured three times with a desktop pH meter ("F-71S", Horiba, Ltd., temperature correction (20°C)) (the same shall apply hereinafter). Next, the crude mixed solution 7 was separated into an organic layer 9 and an aqueous layer 8 by liquid-liquid separation (step (D)). The aqueous layer 8 was recovered and returned, and this was also combined with the crude mixed solution 7. The concentration of hydrogen cyanide contained in the crude mixed solution 7 was about 1.8% by mass. The pH of the organic layer 9 was 5.63. Then, after adding acetic acid and sulfurous acid gas to the organic layer 9 (pH 4.32), this was distilled to obtain purified hydrogen cyanide (step (E)). The concentration of hydrogen cyanide contained in the organic layer 9 was about 8.3% by mass.

[0043] Each facility for performing step (C) and step (D) was made of carbon steel, and in other steps, the facilities and members in contact with hydrogen cyanide were made of stainless steel (SUS304).

[0044] Even when the carbon steel absorption tower for performing step (C) was used for one year (manufacturing up to 10,000 t / year of purified hydrogen cyanide), no progress of corrosion was observed, and no blockage or the like caused by polymerization of hydrogen cyanide or the like was observed in the absorption tower and the lines before and after it. From this, it can be said that by performing deammoniation treatment after mixing crude hydrogen cyanide gas and crude acrylonitrile gas, in the production facility of purified hydrogen cyanide, the places that require the use of expensive materials with high corrosion resistance can be reduced, and moreover, polymerization inhibition of hydrogen cyanide can be efficiently performed. Therefore, according to the production method of the present invention, compared with the case of purifying crude hydrogen cyanide alone, the equipment cost can be suppressed, and purified hydrogen cyanide can be obtained safely and stably.

Explanation of symbols

[0045] 1 Crude hydrogen cyanide gas 2 Crude acrylonitrile gas 3 Crude mixed gas 4 Aqueous sulfuric acid solution 5 Deammoniated gas 6 Water 7 Crude mixture 8 Aqueous layer 9 Organic layer 11 Purified hydrogen cyanide 12 Hydrogen cyanide removal treatment liquid 21 Hydrogen cyanide product 22 Acrylonitrile product 23 Aqueous sulfuric acid solution 24 Off-gas 25 Water 26 Acid 31 Separated crude hydrogen cyanide 32 Separated crude acrylonitrile 100 Quench column 200 Absorption column 300 Recovery column 400 Decanter 500 Head column

Claims

**Claim 1**: A process (A) of mixing crude hydrogen cyanide gas obtained by ammoxidation of methanol with crude acrylonitrile gas obtained by ammoxidation to obtain a crude mixed gas; a process (B) of mixing the crude mixed gas with an aqueous sulfuric acid solution, neutralizing and removing unreacted ammonia contained in the crude mixed gas to obtain a deammoniated gas; a process (C) of absorbing the deammoniated gas in water to obtain a crude mixed liquid; a process (D) of separating an aqueous layer from the crude mixed liquid to obtain an organic layer containing hydrogen cyanide and acrylonitrile; and a process (E) of distilling the organic layer to obtain purified hydrogen cyanide. A method for producing purified hydrogen cyanide comprising these steps. **Claim 2** The method for producing purified hydrogen cyanide according to claim 1, wherein the crude mixed gas contains 2.5 to 9.0 moles of acrylonitrile per 1 mole of hydrogen cyanide. **Claim 3** The method for producing purified hydrogen cyanide according to claim 1 or 2, wherein the concentration of hydrogen cyanide in the crude mixed liquid is 0.1 to 20.0% by mass. **Claim 4** The method for producing purified hydrogen cyanide according to any one of claims 1 to 3, wherein in the process (B), the crude mixed gas and the aqueous sulfuric acid solution are mixed by flowing the crude mixed gas into the aqueous sulfuric acid solution. **Claim 5** The method for producing purified hydrogen cyanide according to any one of claims 1 to 4, wherein the process (C) is carried out using equipment made of carbon steel. **Claim 6** The method for producing purified hydrogen cyanide according to any one of claims 1 to 5, wherein the process (D) is carried out using equipment made of carbon steel. **Claim 7** The method for producing purified hydrogen cyanide according to any one of claims 1 to 6, wherein the crude acrylonitrile gas is obtained by ammoxidation of a synthetic raw material containing propylene.

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