(19)
(11)EP 2 759 537 A1

(12)EUROPEAN PATENT APPLICATION
published in accordance with Art. 153(4) EPC

(43)Date of publication:
30.07.2014 Bulletin 2014/31

(21)Application number: 13781304.4

(22)Date of filing:  27.04.2013
(51)International Patent Classification (IPC): 
C07D 233/76(2006.01)
B01F 5/00(2006.01)
C07D 233/72(2006.01)
B01D 3/00(2006.01)
(86)International application number:
PCT/CN2013/074858
(87)International publication number:
WO 2013/159741 (31.10.2013 Gazette  2013/44)
(84)Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR

(30)Priority: 28.04.2012 CN 201210130404

(71)Applicant: Chongqing Unisplendour Tianhua Methionine Co., Ltd
Fengdu County, Chongqing 408200 (CN)

(72)Inventors:
  • LUO, Yucheng
    Chongqing 408200 (CN)
  • HUANG, Baoyu
    Chongqing 408200 (CN)
  • YANG, Ping
    Chongqing 408200 (CN)
  • YANG, Yong
    Chongqing 408200 (CN)
  • CHANG, Kerang
    Chongqing 408200 (CN)
  • WEI, Tianlu
    Chongqing 408200 (CN)
  • LIU, Yaling
    Chongqing 408200 (CN)
  • LIU, Daowei
    Chongqing 408200 (CN)
  • ZHU, Nengjun
    Chongqing 408200 (CN)

(74)Representative: ABG Patentes, S.L. 
Avenida de Burgos, 16D Edificio Euromor
28036 Madrid
28036 Madrid (ES)

  


(54)DEVICE AND METHOD FOR PREPARING HYDANTOIN


(57) Disclosed is a device for preparing hydantoin. The device comprises three stages of static mixing reactors connected in series and a stripping column. A feed port of the stripping column is in communication with a discharge port of the third-stage static mixing reactor. Also disclosed is a method for preparing hydantoin by using the foregoing device. The method comprises: feeding 3-(methylthio)propionaldehyde, sodium cyanide, and an aqueous solution of excessive carbon dioxide and ammonia to the three stages of static mixing reactors connected in series to undergo a continuous three-stage reaction, the pressure in the three stages of static mixing reactors connected in series being controlled to be 1.4-2.6 MPa, the temperature in the first-stage reactor rising with a gradient from 40 °C to 80 °C, the temperature in the second-stage reactor rising with a gradient from 80 °C to 120 °C, and the temperature in the third-stage reactor rising with a gradient from 120 °C to 160 °C; and feeding the liquid flowing out from the discharge port of the third-stage reactor to the atmospheric pressure vapor stripping column, and separating and recycling un-reacted carbon dioxide and ammonia, the liquid flowing out from the bottom of the column being an aqueous solution of hydantoin, and mixed gas discharged from the top of the column being used for recycling and preparing an aqueous solution of carbon dioxide and ammonia. The device and method of the present invention not only can easily achieve gradient temperature rising, high-efficiency mixing, complete reaction, high reaction speed and a small quantity of by-products, but also can achieve convenient and efficient refinement of hydantoin, with a high quality and yield of hydantoin.




Description

FIELD OF THE INVENTION



[0001] The present invention belongs to the field of chemical industry, and relates to an improved device and method for preparing a chemical product.

BACKGROUND OF THE INVENTION



[0002] Methionine is one of the basic units for constituting protein and the only sulfur-containing amino acid among the essential amino acids. It is a raw material for the synthesis of a protein and cystine besides participating in transmethylation, metabolism of phosphorus and synthesis of adrenaline, choline and creatine in animal bodies. Methionine is widely applied in the fields of medicine, food, feedstuff, cosmetics and the like, and is most consumable as a feedstuff additive. However, currently almost all methionine in our country is imported. The domestic production is very low and far from meeting the needs.

[0003] Hydantoin, with the chemical name of 5-(2-(methylthio)ethyl)hydantoin, is a key intermediate for preparing DL-methionine. Hydantoin is hydrolyzed with an alkali to generate DL-methionine salt which is subsequently neutralized with an acid to obtain DL-methionine. At present, the methods for preparing hydantoin at home and abroad comprise a one-step method and a two-step method. The one-step method refers to feeding acraldehyde, methyl mercaptan, cyanide and hydantoin-forming reagents together to prepare hydantoin, and it is a heterogeneous reaction, low in reaction speed and prone to such side reactions as polymerization. The two-step method comprises first preparing 3-(methylthio)propionaldehyde (TPMA) from acraldehyde and methyl mercaptan, and reacting 3-(methylthio)propionaldehyde, cyanide, carbon dioxide and ammonia to obtain hydantoin. In industrial production using the two-step method, synthetic quality of 3-(methylthio)propionaldehyde in the first step can be well controlled, however in the second step, the reaction for synthesis of hydantoin is performed at high temperature and high pressure, thus it is difficult to control the reaction conditions and liable to cause side reactions such as degradation of organic raw materials and polymerization, so as to induce decrease in yield of hydantoin and instability of process.

DESCRIPTION OF THE INVENTION



[0004] In view of this, the object of the present invention is to provide a device and a method for preparing hydantoin, which enables easy control on reaction temperature, well control of byproduct generation and improvement in the quality and yield of hydantoin.

[0005] In order to achieve the above purpose, the present invention provides the following technical solutions:

1. A device for preparing hydantoin, comprising three stages of static mixing reactors connected in series and a stripping column, wherein a discharge port of a first-stage static mixing reactor is in communication with a feed port of a second-stage static mixing reactor, a discharge port of the second-stage static mixing reactor is in communication with a feed port of a third-stage static mixing reactor, and a discharge port of the third-stage static mixing reactor is in communication with a feed port of the stripping column.
Preferably, the top of the stripping column is provided with a gas-liquid separator, such as a cyclone separator. The device can well remove the liquid carried in the recycled gas.
Preferably, a relief valve is further arranged in the passage from the discharge port of the third-stage static mixing reactor to the feed port of the stripping column. The pressure of the high-pressure fluid flowing out from the discharge port of the third-stage static mixing reactor can be conveniently released to a certain extent using the device, and then fed to the stripping column.

2. A method for preparing hydantoin using the above-described device, comprising the following steps: feeding 3-(methylthio)propionaldehyde, sodium cyanide, and an aqueous solution of excessive carbon dioxide and ammonia to the three stages of static mixing reactors connected in series to undergo a continuous three-stage reaction, the pressure in the three stages of static mixing reactors connected in series being controlled to be 1.4-2.6 MPa, wherein the temperature in the first-stage static mixing reactor rising with a gradient from 40 °C to 80 °C, the temperature in the second-stage static mixing reactor rising with a gradient from 80 °C to 120 °C, and the temperature in the third-stage static mixing reactor rising with a gradient from 120 °C to 160 °C; and feeding the effluent from the discharge port of the third-stage static mixing reactor to the atmospheric pressure steam stripping column for separating and recovering un-reacted carbon dioxide and ammonia, the effluent from the bottom of the column being an aqueous solution of hydantoin, and the mixed gas discharged from the top of the column being used for recovering and preparing the aqueous solution of carbon dioxide and ammonia and being recycled in the preparation of hydantoin.



[0006] The reaction equation involved is



[0007] Preferably, the pressure in the three stages of static mixing reactors connected in series being controlled to be 1.6-1.8 MPa, and the total residence time of the reaction is 16-20 min. Under the preferred conditions, high reaction speed, short period, less byproducts and high hydantoin yield can be achieved, and the requirements for the materials of the device and the connection parts are not high.

[0008] Preferably, the aqueous solution of carbon dioxide and ammonia, sodium cyanide, and 3-(methylthio)propionaldehyde are sequentially fed, i.e. sodium cyanide is first added to the aqueous solution of carbon dioxide and ammonia, and then 3-(methylthio)propionaldehyde is added. If 3-(methylthio)propionaldehyde is first added to the aqueous solution of carbon dioxide and ammonia, the obtained mixture has the risk of polymerization and it is liable to cause decrease in hydantoin yield or process fluctuation and introduction of impurities to the product.

[0009] Preferably, 3-(methylthio)propionaldehyde and sodium cyanide are fed in a molar ratio of 1:1.02-1.03; in the aqueous solution of carbon dioxide and ammonia, carbon dioxide is not less than 12% w/v, ammonia is not less than 8% w/v, the molar ratio of carbon dioxide to ammonia is 1:1.5-1.9; and carbon dioxide and 3-(methylthio)propionaldehyde are fed in a molar ratio of 1.8-2.2:1. Under the above preferred conditions, 3-(methylthio)propionaldehyde reacts completely and is not prone to formation of polymer, so as to enable less byproducts and high yield of hydantoin.

[0010] Preferably, the feeding ratio of 3-(methylthio)propionaldehyde to sodium cyanide is controlled by using a mass flowmeter. The feeding ratio of 3-(methylthio)propionaldehyde to sodium cyanide has a large influence on the quality and yield of hydantoin. The existing technology adopts volumetric flowmeter to control the feeding ratio of both, which easily brings about feeding error due to volume change induced by temperature change. Serious fluctuation of feeding ratio in industrial production would lead to frequent interruption of production and low continuous production level. By replacing the volumetric flowmeter with a mass flowmeter, the feeding ratio of 3-(methylthio)propionaldehyde to sodium cyanide can be controlled accurately, so as to reduce generation of byproducts as much as possible, increase yield of hydantoin, improve continuous production level, and reduce production cost.

[0011] Preferably, steam at a pressure of 0.2-0.8 Mpa is introduced into the steam stripping column. If the pressure of the water vapor is too high, the gas recycled from the top of the column will carry much liquid, which would result in loss of partial hydantoin and decrease in hydantoin yield; moreover, it will lead to poor quality of the aqueous solution of carbon dioxide and ammonia that is prepared from the recycled gas, thereby affecting subsequent preparation of hydantoin. On the other hand, if the pressure of the water vapor is too low, the ammonium dissolved in the solution cannot be recycled sufficiently. Under the preferred conditions, carbon dioxide and ammonia can be recycled sufficiently and loss of hydantoin is substantially avoided.

[0012] Preferably, the effluent from the discharge port of the third-stage static mixing reactor is subjected to pressure release with the relief valve to 0.8-1.0 MPa and fed to the stripping column. If the reaction mixture is fed to the atmospheric pressure stripping column in a high-temperature and high-pressure form, corrosion to the column body will readily occur. On the other hand, if the reaction mixture is fed to the atmospheric pressure stripping column after proper pressure release, it can not only allow the stripping operation with water vapor to proceed well, but also reduce corrosion of the column body and prolong service life of the equipment.

[0013] The advantageous effects of the present invention comprise: by using the three stages of static mixing reactors connected in series instead of the single reactor in the prior art to synthesize hydantoin, the present invention can easily achieve gradient temperature rising, high-efficiency mixing, and complete reaction, and it not only realizes high reaction speed but also well controls generation of byproducts and improves the quality and yield of hydantoin. In addition, the present invention employs atmospheric pressure water steam stripping column to separate and recycle un-reacted carbon dioxide and ammonia contained in the reaction mixture, without introducing new impurities, so that not only convenient and efficient refinement of hydantoin is achieved, but also the mixed gas of water vapor, carbon dioxide and ammonia gas discharged from the top of the column can be used for recovering and preparing the aqueous solution of carbon dioxide and ammonia and be recycled in the preparation of hydantoin. The preparation device and method of the present invention are applicable to large-scale continuous industrial production, and have good application prospect.

DESCRIPTIONS OF THE FIGURES



[0014] In order to make the object, technical solutions and advantages of the present invention clearer, the present invention is further depicted in detail in combination with the following figure, wherein:

Fig. 1 shows a schematic diagram of the device and the process flow of the method of the present invention.


DESCRIPTION OF THE PREFERRED EMBODIMENTS



[0015] With reference to the figure, preferred examples of the present invention are elaborated.

[0016] Fig. 1 shows a schematic diagram of the device of the present invention. The device for preparing hydantoin comprises three stages of static mixing reactors connected in series and a stripping column, wherein a discharge port of a first-stage static mixing reactor is in communication with a feed port of a second-stage static mixing reactor, a discharge port of the second-stage static mixing reactor is in communication with a feed port of a third-stage static mixing reactor, and a discharge port of the third-stage static mixing reactor is in communication with a feed port of the stripping column.

[0017] As a preferred example of the device of the present invention, a gas-liquid separator is further arranged on the top of the stripping column; and a relief valve is further arranged in the passage from the discharge port of the third-stage static mixing reactor to the feed port of the stripping column.

[0018] Fig. 1 shows a schematic diagram for the process flow of the method of the present invention. An aqueous solution of carbon dioxide and ammonia (containing 13% by mass of carbon dioxide and 8% by mass of ammonia), a sodium cyanide solution, and 3-(methylthio)propionaldehyde are continuously added to the first-stage static mixing reactor, wherein the aqueous solution of carbon dioxide and ammonia is directly pumped by a centrifuge pump, the molar ratio of carbon dioxide to 3-(methylthio)propionaldehyde is 2:1, the sodium cyanide solution and 3-(methylthio)propionaldehyde are delivered with a high-precision metering pump, and the feeding ratio of sodium cyanide to 3-(methylthio)propionaldehyde is controlled accurately with a mass flowmeter. The materials are mixed with high efficiency in the three stages of static mixing reactors connected in series and undergo a continuous three-stage reaction at a certain pressure and a temperature rising with gradient, wherein the temperature in the first-stage static mixing reactor rises with a gradient from 40 °C to 80 °C, the temperature in the second-stage static mixing reactor rises with a gradient from 80 °C to 120 °C, and the temperature in the third-stage static mixing reactor rises with a gradient from 120 °C to 160 °C. The effluent from the discharge port of the third-stage static mixing reactor is subjected to pressure release with the relief valve to 0.8-1.0 MPa and then fed to the stripping column to separate and recycle un-reacted carbon dioxide and ammonia. The top of the stripping column is provided with a gas-liquid separator. The top of the column has a pressure of atmospheric pressure and a temperature of 101 °C, and the interior of the column is introduced with water vapor at a pressure of 0.5 MPa. The mixed gas discharged from the top of the column is used for recovering and preparing the aqueous solution of carbon dioxide and ammonia and is recycled in the preparation of hydantoin. The effluent from the bottom of the column is aqueous solution of hydantoin. The qualified product is a light yellow, clear and transparent solution. The solution additionally contains sodium carbonate as a byproduct, and can be hydrolyzed directly with an alkali to prepare DL-methionine salt. Table 1 shows the specific conditions and results of examples.
Table 1: Conditions for preparation of hydantoin and results
ExamplesNaCN/TPMA molar ratioReaction pressure (Mpa)Total residence time of reaction (min)Content of hydantoin in product (%)yield (%)Product appearance
1 1.055 1.6 16 10.36 73.25 Yellow, oily material deposited
2 1.050 1.6 16 11.43 80.63 Orange, clear and transparent
3 1.030 1.6 16 12.82 90.68 Light yellow, clear and transparent
4 1.025 1.6 16 12.99 93.72 Light yellow, clear and transparent
5 1.020 1.6 16 12.65 89.44 Light yellow, clear and transparent
6 1.000 1.6 16 11.41 80.63 lemony yellow, turbid
7 0.980 1.6 16 9.74 68.85 lemony yellow, turbid
8 1.021 1.6 6 9.59 69.44 Light yellow, oily material deposited
9 1.023 1.6 8 10.38 75.14 Light yellow, oily material deposited
10 1.022 1.6 10 11.31 81.89 Light yellow, oily material deposited
11 1.024 1.6 12 12.18 88.15 Light yellow, clear and transparent
12 1.025 1.6 16 13.01 94.20 Light yellow, clear and transparent
13 1.026 1.6 18 12.96 93.84 Light yellow, clear and transparent
14 1.027 1.6 20 12.86 93.10 Light yellow, clear and transparent
15 1.028 0.6 16 9.47 68.35 Light yellow, oily material deposited
16 1.028 0.9 16 10.38 74.93 Light yellow, oily material deposited
17 1.029 1.2 16 11.26 81.27 Light yellow, oily material deposited
18 1.030 1.4 16 12.25 88.45 Light yellow, clear and transparent
19 1.024 1.6 16 12.97 93.66 Light yellow, clear and transparent
20 1.025 2.0 16 13.11 94.65 Light yellow, clear and transparent
21 1.020 2.3 16 12.89 93.10 Light yellow, clear and transparent
22 1.025 2.6 16 13.09 94.52 Light yellow, clear and transparent
23 1.032 1.6 16 12.66 90.12 Light yellow, clear and transparent
24 1.025 1.6 16 13.17 93.74 Light yellow, clear and transparent
25 1.024 1.6 16 13.06 93.02 Light yellow, clear and transparent
26 1.022 1.6 16 12.90 91.82 Light yellow, clear and transparent
27 1.027 1.6 16 13.27 94.49 Light yellow, clear and transparent
28 1.025 1.6 16 13.02 92.69 Light yellow, clear and transparent
29 1.030 1.6 16 12.73 90.64 Light yellow, clear and transparent
30 1.028 1.6 16 13.09 93.23 Light yellow, clear and transparent
31 1.022 1.8 20 12.31 88.02 Light yellow, clear and transparent
32 1.024 1.8 20 12.48 89.25 Light yellow, clear and transparent
33 1.025 1.8 20 13.01 94.25 Light yellow, clear and transparent
34 1.026 1.8 20 12.96 93.92 Light yellow, clear and transparent
35 1.027 1.8 20 12.86 93.28 Light yellow, clear and transparent
36* 1.023 1.6 16 11.52 82.1 Light yellow, oily material deposited
Note: * refers to reaction performed merely using a one-stage static mixing reactor with the reaction temperature rising with gradient from 40 °C to 160 °C.


[0019] It can be seen from examples 1-7 that, although the molar ratio of sodium cyanide to 3-(methylthio)propionaldehyde in reaction is 1, the feeding ratio of sodium cyanide to 3-(methylthio)propionaldehyde should be more than 1 since sodium cyanide decomposes into ammonia and sodium formate at high temperature. However, if sodium cyanide is excessive too much, 3-(methylthio)propionaldehyde tends to form dimers and trimers in alkaline ammonia bicarbonate solution so that the product contains deposited oily materials. Therefore, the charge molar ratio of sodium cyanide to 3-(methylthio)propionaldehyde is preferably 1.02-1.03. Referring to examples 8-14, if the residence time of reaction is too short, the temperature rise of the material from the feed port of the reactor to the outlet thereof is too fast, and it easily induces polymerization of 3-(methylthio)propionaldehyde and decomposition of sodium cyanide so as to decrease yield of hydantoin. Hence, the residence time of the reaction is preferably 12-20 min, and more preferably 16-20 min. The reaction pressure in the synthesis of hydantoin is controlled for the purpose of ensuring the whole reaction is performed in liquid phase to increase the reaction speed. With reference to examples 15-22, if the pressure is excessively low, carbon dioxide and ammonia may be present in gaseous form, so that the reaction speed is low, the reaction is incomplete in the residence time, the yield of hydantoin is affected and 3-(methylthio)propionaldehyde tends to form dimers and trimers to make the product contain deposited oil materials; and an excessively high pressure will propose high requirements for the materials of regulating valve and the like in industrial production. Consequently, the reaction pressure is preferably 1.4-2.6 MPa, and more preferably 1.6-1.8 MPa, From examples 23-35, it can be seen that, when production is continuously carried out for 8 times and 5 times by using a mass flowmeter under the preferred conditions of feeding ratio, residence time of reaction and reaction pressure, stable feeding ratio of sodium cyanide to 3-(methylthio)propionaldehyde, high reaction speed, less byproducts, high hydantoin yield, good process stability and high continuous production level are achieved.

[0020] The present invention compares the reactions effects of the three stages of static mixing reactors connected in series and a one-stage static mixing reactor under the same conditions (example 36). The result shows that, the reaction preformed using a single static mixing reactor at a temperature rising with gradient from 40 to 160 °C is liable to cause side reactions such as degradation of organic raw materials and polymerization due to the temperature rising with larger gradient and the temperature not easy to control. Moreover, a single static mixing reactor cannot ensure adequate residence time of reaction, leading to incomplete reaction, lower hydantoin yield (82.1 %) and un-reacted oily 3-(methylthio)propionaldehyde appearing in the product.

[0021] The high-pressure effluent from the discharge port of the third-stage static mixing reactor is subjected to pressure release with the relief valve to 0.8-1.0 MPa and then fed to the stripping column. The un-reacted carbon dioxide gas and ammonia gas contained in the reaction mixture are recycled by flash evaporation with low-pressure water vapor of different pressures to realize refinement of hydantoin, with the effects shown in tables 2-4.
Table 2: Each component and content thereof before and after stripping with 0.5 MPa water vapor (kg/h)
Components0.5MPa water vaporFeed pressureMaterials fed to the columnLiquid from the bottom of the columnGas from the top of the column
CO2 - 0.8 MPa 315.6 - 315.6
NH3 - 0.8 MPa 352.1 48.4 303.7
hydantoin - 0.8 MPa 1511.5 1511.5 -
Na2CO3 - 0.8 MPa 516.0 516.0 -
H2O 976.3 - 7253.3 7620 609.6
Total mass flow 976.3 - 9948.5 9695.9 1228.9
Table 3: Each component and content thereof before and after stripping with 0.2 MPa water vapor (kg/h)
Components0.2 MPa water vaporFeed pressureMaterials fed to the columnLiquid from the bottom of the columnGas from the top of the column
CO2 - 0.8 MPa 315.6 - 315.6
NH3 - 0.8 MPa 352.1 68.4 283.7
hydantoin - 0.8 MPa 1511.5 1511.5 -
Na2CO3 - 0.8 MPa 516.0 516.0 -
H2O 976.3 - 7253.3 7670.2 559.4
Total mass flow 976.3 - 9948.5 9766.1 1158.7
Table 4: Each component and content thereof before and after stripping with 0.8 MPa water vapor (kg/h)
Components0.8 MPa water vaporFeed pressureMaterials fed to the columnLiquid from the bottom of the columnGas from the top of the column
CO2 - 1.0 MPa 315.6 - 315.6
NH3 - 1.0 MPa 352.1 48.4 303.7
hydantoin - 1.0 MPa 1511.5 1503.3 8.2
Na2CO3 - 1.0 MPa 516.0 509.2 6.8
H2O 976.3 - 7253.3 7521.1 708.5
Total mass flow 976.3 - 9948.5 9582 1342.8


[0022] Finally, it needs to note that the above examples are merely for explaining rather than limiting the technical solutions of the present invention. Although the present invention has been depicted with reference to the preferred examples of the present invention, a person skilled in the art should understand that there are a variety of variants of the invention in form and detail that conform to the concept of the invention and fall within the scope defined by the claims attached.


Claims

1. A device for preparing hydantoin, comprising three stages of static mixing reactors connected in series and a stripping column, wherein a discharge port of a first-stage static mixing reactor is in communication with a feed port of a second-stage static mixing reactor, a discharge port of the second-stage static mixing reactor is in communication with a feed port of a third-stage static mixing reactor, and a discharge port of the third-stage static mixing reactor is in communication with a feed port of the stripping column.
 
2. The device for preparing hydantoin according to claim 1, wherein a gas-liquid separator is further arranged on the top of the stripping column.
 
3. The device for preparing hydantoin according to claim 1 or 2, wherein a relief valve is further arranged in the passage from the discharge port of the third-stage static mixing reactor to the feed port of the stripping column.
 
4. The method for preparing hydantoin by using the device according to any of claims 1 to 3, comprising feeding 3-(methylthio)propionaldehyde, sodium cyanide, and an aqueous solution of excessive carbon dioxide and ammonia to the three stages of static mixing reactors connected in series to undergo a continuous three-stage reaction, the pressure in the three stages of static mixing reactors connected in series being controlled to be 1.4-2.6 MPa, wherein the temperature in the first-stage static mixing reactor rising with a gradient from 40 °C to 80 °C, the temperature in the second-stage static mixing reactor rising with a gradient from 80 °C to 120 °C, and the temperature in the third-stage static mixing reactor rising with a gradient from 120 °C to 160 °C; and feeding the effluent from the discharge port of the third-stage static mixing reactor to the atmospheric pressure steam stripping column for separating and recovering un-reacted carbon dioxide and ammonia, the effluent from the bottom of the column being an aqueous solution of hydantoin, and the mixed gas discharged from the top of the column being used for recovering and preparing the aqueous solution of carbon dioxide and ammonia and being recycled in the preparation of hydantoin.
 
5. The method according to claim 4, wherein the pressure in the three stages of static mixing reactors connected in series is controlled to be 1.6-1.8 MPa, and the total residence time of the reaction is 16-20 min.
 
6. The method according to claim 4, wherein the aqueous solution of carbon dioxide and ammonia, sodium cyanide, and 3-(methylthio)propionaldehyde are sequentially fed.
 
7. The method according to claim 4, wherein 3-(methylthio)propionaldehyde and sodium cyanide are fed in a molar ratio of 1:1.02-1.03; in the aqueous solution of carbon dioxide and ammonia, carbon dioxide is not less than 12% w/v, ammonia is not less than 8% w/v, the molar ratio of carbon dioxide to ammonia is 1:1.5-1.9; and carbon dioxide and 3-(methylthio)propionaldehyde are fed in a molar ratio of 1.8-2.2:1.
 
8. The method according to claim 4 or 7, wherein the feeding ratio of 3-(methylthio)propionaldehyde to sodium cyanide is controlled by using a mass flowmeter.
 
9. The method according to claim 4, wherein steam at a pressure of 0.2-0.8 Mpa is introduced into the steam stripping column.
 
10. The method according to claim 4 or 9, wherein the effluent from the discharge port of the third-stage static mixing reactor is subjected to pressure release with the relief valve to 0.8-1.0 MPa and then fed to the stripping column.
 




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