CN117736114A - Continuous production method for synthesizing isocyanate by salified phosgene method - Google Patents
Continuous production method for synthesizing isocyanate by salified phosgene method Download PDFInfo
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- YGYAWVDWMABLBF-UHFFFAOYSA-N Phosgene Chemical compound ClC(Cl)=O YGYAWVDWMABLBF-UHFFFAOYSA-N 0.000 title claims abstract description 88
- 238000000034 method Methods 0.000 title claims abstract description 73
- 239000012948 isocyanate Substances 0.000 title claims abstract description 58
- 150000002513 isocyanates Chemical class 0.000 title claims abstract description 58
- 238000010924 continuous production Methods 0.000 title claims abstract description 34
- 230000002194 synthesizing effect Effects 0.000 title claims abstract description 27
- 150000001412 amines Chemical class 0.000 claims abstract description 160
- 239000007788 liquid Substances 0.000 claims abstract description 129
- 238000006243 chemical reaction Methods 0.000 claims abstract description 106
- 150000003839 salts Chemical class 0.000 claims abstract description 94
- 239000007789 gas Substances 0.000 claims abstract description 75
- 238000004062 sedimentation Methods 0.000 claims abstract description 71
- VEXZGXHMUGYJMC-UHFFFAOYSA-N Hydrochloric acid Chemical compound Cl VEXZGXHMUGYJMC-UHFFFAOYSA-N 0.000 claims abstract description 58
- 229910000041 hydrogen chloride Inorganic materials 0.000 claims abstract description 53
- IXCSERBJSXMMFS-UHFFFAOYSA-N hydrogen chloride Substances Cl.Cl IXCSERBJSXMMFS-UHFFFAOYSA-N 0.000 claims abstract description 53
- 238000000926 separation method Methods 0.000 claims abstract description 26
- -1 amine hydrochloride Chemical class 0.000 claims abstract description 22
- 238000007599 discharging Methods 0.000 claims abstract description 15
- 238000002844 melting Methods 0.000 claims description 25
- 230000008018 melting Effects 0.000 claims description 25
- 239000000463 material Substances 0.000 claims description 22
- 239000002002 slurry Substances 0.000 claims description 20
- MVPPADPHJFYWMZ-UHFFFAOYSA-N chlorobenzene Chemical compound ClC1=CC=CC=C1 MVPPADPHJFYWMZ-UHFFFAOYSA-N 0.000 claims description 12
- 238000010791 quenching Methods 0.000 claims description 12
- 238000001816 cooling Methods 0.000 claims description 10
- YXFVVABEGXRONW-UHFFFAOYSA-N Toluene Chemical compound CC1=CC=CC=C1 YXFVVABEGXRONW-UHFFFAOYSA-N 0.000 claims description 9
- NAQMVNRVTILPCV-UHFFFAOYSA-N hexane-1,6-diamine Chemical compound NCCCCCCN NAQMVNRVTILPCV-UHFFFAOYSA-N 0.000 claims description 9
- 230000000171 quenching effect Effects 0.000 claims description 9
- 239000006228 supernatant Substances 0.000 claims description 9
- VHRGRCVQAFMJIZ-UHFFFAOYSA-N cadaverine Chemical compound NCCCCCN VHRGRCVQAFMJIZ-UHFFFAOYSA-N 0.000 claims description 8
- 238000009833 condensation Methods 0.000 claims description 6
- 230000005494 condensation Effects 0.000 claims description 6
- 230000014759 maintenance of location Effects 0.000 claims description 6
- 238000010992 reflux Methods 0.000 claims description 4
- 239000002904 solvent Substances 0.000 claims description 4
- OCJBOOLMMGQPQU-UHFFFAOYSA-N 1,4-dichlorobenzene Chemical compound ClC1=CC=C(Cl)C=C1 OCJBOOLMMGQPQU-UHFFFAOYSA-N 0.000 claims description 3
- CBCKQZAAMUWICA-UHFFFAOYSA-N 1,4-phenylenediamine Chemical compound NC1=CC=C(N)C=C1 CBCKQZAAMUWICA-UHFFFAOYSA-N 0.000 claims description 3
- VOZKAJLKRJDJLL-UHFFFAOYSA-N 2,4-diaminotoluene Chemical compound CC1=CC=C(N)C=C1N VOZKAJLKRJDJLL-UHFFFAOYSA-N 0.000 claims description 3
- RNLHGQLZWXBQNY-UHFFFAOYSA-N 3-(aminomethyl)-3,5,5-trimethylcyclohexan-1-amine Chemical compound CC1(C)CC(N)CC(C)(CN)C1 RNLHGQLZWXBQNY-UHFFFAOYSA-N 0.000 claims description 3
- 229940117389 dichlorobenzene Drugs 0.000 claims description 3
- ZZTCPWRAHWXWCH-UHFFFAOYSA-N diphenylmethanediamine Chemical compound C=1C=CC=CC=1C(N)(N)C1=CC=CC=C1 ZZTCPWRAHWXWCH-UHFFFAOYSA-N 0.000 claims description 3
- KQSABULTKYLFEV-UHFFFAOYSA-N naphthalene-1,5-diamine Chemical compound C1=CC=C2C(N)=CC=CC2=C1N KQSABULTKYLFEV-UHFFFAOYSA-N 0.000 claims description 3
- 239000002245 particle Substances 0.000 abstract description 30
- 239000000243 solution Substances 0.000 description 55
- 230000015572 biosynthetic process Effects 0.000 description 24
- 238000001514 detection method Methods 0.000 description 12
- 230000008569 process Effects 0.000 description 9
- 238000003786 synthesis reaction Methods 0.000 description 8
- RFFLAFLAYFXFSW-UHFFFAOYSA-N 1,2-dichlorobenzene Chemical compound ClC1=CC=CC=C1Cl RFFLAFLAYFXFSW-UHFFFAOYSA-N 0.000 description 6
- 239000002826 coolant Substances 0.000 description 6
- 239000012535 impurity Substances 0.000 description 5
- 238000004519 manufacturing process Methods 0.000 description 3
- 239000000047 product Substances 0.000 description 3
- 230000009286 beneficial effect Effects 0.000 description 2
- 239000011261 inert gas Substances 0.000 description 2
- 230000004048 modification Effects 0.000 description 2
- 238000012986 modification Methods 0.000 description 2
- 238000006552 photochemical reaction Methods 0.000 description 2
- 238000007086 side reaction Methods 0.000 description 2
- PCHXZXKMYCGVFA-UHFFFAOYSA-N 1,3-diazetidine-2,4-dione Chemical compound O=C1NC(=O)N1 PCHXZXKMYCGVFA-UHFFFAOYSA-N 0.000 description 1
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- XSQUKJJJFZCRTK-UHFFFAOYSA-N Urea Chemical compound NC(N)=O XSQUKJJJFZCRTK-UHFFFAOYSA-N 0.000 description 1
- 238000010521 absorption reaction Methods 0.000 description 1
- OHJMTUPIZMNBFR-UHFFFAOYSA-N biuret Chemical compound NC(=O)NC(N)=O OHJMTUPIZMNBFR-UHFFFAOYSA-N 0.000 description 1
- 239000004202 carbamide Substances 0.000 description 1
- 150000001718 carbodiimides Chemical class 0.000 description 1
- 239000007795 chemical reaction product Substances 0.000 description 1
- 239000011248 coating agent Substances 0.000 description 1
- 238000000576 coating method Methods 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 230000002349 favourable effect Effects 0.000 description 1
- VKYKSIONXSXAKP-UHFFFAOYSA-N hexamethylenetetramine Chemical compound C1N(C2)CN3CN1CN2C3 VKYKSIONXSXAKP-UHFFFAOYSA-N 0.000 description 1
- 230000006872 improvement Effects 0.000 description 1
- 229920000642 polymer Polymers 0.000 description 1
- 150000003141 primary amines Chemical class 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 230000033764 rhythmic process Effects 0.000 description 1
- 239000012266 salt solution Substances 0.000 description 1
- 238000001308 synthesis method Methods 0.000 description 1
Abstract
The invention provides a continuous production method for synthesizing isocyanate by a salified phosgene method. The method comprises the following steps: s1, continuously introducing an amine-containing solution and hydrogen chloride into a salifying kettle to carry out salifying reaction, salifying part of amine in the amine-containing solution to obtain salifying liquid containing amine hydrochloride and unreacted amine, and continuously discharging the salifying liquid; s2, continuously introducing salifying liquid into a salifying-photochemical kettle to react with photochemical tail gas, salifying unreacted amine, performing preliminary phosgenation on amine hydrochloride to obtain photochemical-salifying liquid, and continuously discharging photochemical-salifying liquid; s3, continuously introducing photochemical salt forming liquid and phosgene into a photochemical kettle for phosgenation reaction to obtain photochemical liquid and photochemical tail gas, and returning the photochemical tail gas into the salt forming-photochemical kettle; s4, carrying out sedimentation separation on the photochemical liquid to obtain isocyanate. The invention solves the problems of low salification rate or uneven hydrochloride particles and excessive consumption of hydrogen chloride and phosgene when synthesizing isocyanate by a salification phosgene method.
Description
Technical Field
The invention relates to the technical field of isocyanate synthesis, in particular to a continuous production method for synthesizing isocyanate by a salified phosgene method.
Background
The salified phosgene method is used as a synthesis method of isocyanate, has the advantages of protecting primary amine active groups and avoiding side reactions with product isocyanate to generate indissolvable impurities such as urea, and the like, but in the actual production process, some difficult problems can be unavoidable, such as: 1) The excessive concentration of the salifying liquid can cause excessive viscosity, so that the material transportation is inconvenient, and the salifying rate is lower due to the fact that the amine is wrapped by salt particles; too low a salt-forming solution concentration may result in insufficient equipment capacity and excessive solvent usage. 2) Phosgenation tail gas contains a large amount of hydrogen chloride and part of unreacted phosgene, and the part of tail gas usually needs an additional set of equipment for absorption separation or rectification separation, so that the cost is high. 3) The salt particles have uneven particle size and overlarge viscosity, so that the consumption of phosgene for photochemical reaction is high.
CN111825572a provides a method for preparing isocyanate by salt-forming-atomizing phosgene method by dispersing and atomizing salt-forming liquid using inert gas and then phosgenating. The method can alleviate a series of problems caused by overlarge viscosity of the salt solution to a certain extent, but the excessive consumption of a large amount of inert gas causes overlarge amount of tail gas in the system, so that the pressure of a tail gas treatment system is overlarge, and the excessive non-condensable gas also causes the reduction of the phosgene utilization rate, so that the method has the disadvantage of being low in cost.
Both CN113181859A and CN115286535A disclose salifying reactors for solving the problems of incomplete salifying, oversized salt particles and the like, but the two reactors are provided with step-by-step devices with smaller opening diameters, and are easy to be blocked by the salt particles, so that the service cycle of the reactor is shorter.
In view of this, the present application is specifically proposed.
Disclosure of Invention
The invention mainly aims to provide a continuous production method for synthesizing isocyanate by a salified phosgene method, which aims to solve the problems of low salified rate or uneven size of hydrochloride particles and excessively high consumption of hydrogen chloride and phosgene in the synthesis of isocyanate by the salified phosgene method in the prior art.
In order to achieve the above object, according to one aspect of the present invention, there is provided a continuous production method for synthesizing isocyanate by a salt-forming phosgene method, comprising the steps of: s1, continuously introducing an amine-containing solution and hydrogen chloride into a salifying kettle to carry out salifying reaction, salifying part of amine in the amine-containing solution to obtain salifying liquid containing amine hydrochloride and unreacted amine, and continuously discharging the salifying liquid; s2, continuously introducing salifying liquid into a salifying-photochemical kettle to react with photochemical tail gas, salifying unreacted amine, performing preliminary phosgenation on amine hydrochloride to obtain photochemical-salifying liquid, and continuously discharging photochemical-salifying liquid; s3, continuously introducing photochemical salt forming liquid and phosgene into a photochemical kettle for phosgenation reaction to obtain photochemical liquid and photochemical tail gas, and returning the photochemical tail gas into the salt forming-photochemical kettle; s4, carrying out sedimentation separation on the photochemical liquid to obtain isocyanate.
Further, in step S1, the molar ratio of amine in the amine-containing solution to hydrogen chloride is 1:0.3-1.8, preferably 1:0.6-1.2.
Further, the amine in the amine-containing solution is one or more of toluenediamine, 1, 6-hexamethylenediamine, 1, 5-pentanediamine, isophoronediamine, p-phenylenediamine, 1, 5-naphthalenediamine and diphenylmethane diamine; preferably, when the melting point of the amine in the amine-containing solution is higher than 80 ℃, the molar ratio of the amine in the amine-containing solution to the hydrogen chloride is 1:0.9-1.2; when the melting point of the amine in the amine-containing solution is lower than 80 ℃, the molar ratio of the amine in the amine-containing solution to the hydrogen chloride is 1:0.6-0.9; preferably, when the melting point of the amine in the amine-containing solution is higher than 80 ℃, the reaction temperature in the salifying kettle is 60 to 130 ℃, more preferably 80 to 100 ℃; when the melting point of the amine in the amine-containing solution is lower than 80 ℃, the reaction temperature in the salifying kettle is 0-60 ℃, more preferably 0-30 ℃; preferably, the residence time of the amine-containing solution in the salifying kettle is 0.5-2 h; more preferably, when the melting point of the amine in the amine-containing solution is higher than 80 ℃, the residence time of the amine-containing solution in the salifying kettle is 1.2-2 h; when the melting point of the amine in the amine-containing solution is lower than 80 ℃, the residence time of the amine-containing solution in the salifying kettle is 0.5-1.2 h; preferably, the mass concentration of the amine-containing solution is 10-40%, and the solvent is one or more of chlorobenzene, dichlorobenzene and toluene.
Further, the molar ratio of the amine in the amine-containing solution to the phosgene introduced in the step S3 is 1:2-10.
Further, the molar ratio of the amine in the amine-containing solution to the phosgene introduced in the step S3 is 1:3-6.
In the step S4, the photochemical liquid is subjected to sedimentation separation in a sedimentation kettle, and the obtained supernatant is isocyanate; the sedimentation separation also obtains lower layer sedimentation slurry at the bottom of the sedimentation kettle, and the method further comprises the following steps: and returning the lower layer sedimentation slurry to the step S2, and introducing the lower layer sedimentation slurry into a salification-photochemical kettle for internal reaction.
Further, the ratio of the introduced volume flow of the salifying liquid to the returned lower layer sedimentation slurry is 1:0.05-0.1.
Further, in step S2, the reaction temperature in the salifying-photochemical kettle is 30-130 ℃, preferably 40-80 ℃; and/or, in step S3, the reaction temperature in the photochemical kettle is 80-170 ℃, preferably 110-160 ℃; and/or the retention time of the materials in the salifying-photochemical kettle is 0.5-2 h; and/or the retention time of the materials in the photochemical kettle is 2-10 h; and/or settling separation time is 3-8 h.
Further, the photochemical kettle is provided with a photochemical tail gas outlet, a condensation reflux device is arranged at the photochemical tail gas outlet, and the temperature of a condensation medium is between-30 and 5 ℃.
Further, before the photochemical liquid is subjected to sedimentation separation, the step S4 further comprises the steps of cooling and quenching the photochemical liquid in advance; preferably, the temperature of the quench is 20 to 80 ℃, more preferably 40 to 60 ℃.
By adopting the technical scheme of the invention, the viscosity of the feed liquid in the salifying process can be reduced, the problems of low salifying rate and uneven size of hydrochloride particles caused by overlarge viscosity of the salifying liquid are avoided, the phosgene utilization rate and the photochemical reaction efficiency are obviously improved, and the consumption of hydrogen chloride and phosgene is reduced.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the invention. In the drawings:
FIG. 1 shows a flow diagram of a continuous production process for synthesizing isocyanate by a salt-forming phosgene method according to one embodiment of the present invention;
wherein the above figures include the following reference numerals:
10. a salifying kettle; 20. salifying-photochemical kettle; 30. a photochemical kettle; 40. and (5) a sedimentation kettle.
Detailed Description
It should be noted that, in the case of no conflict, the embodiments and features in the embodiments may be combined with each other. The invention will be described in detail below with reference to the drawings in connection with embodiments.
As described in the background section, the prior art has the problems of low salt formation rate or uneven size of hydrochloride particles and excessive consumption of hydrogen chloride and phosgene when synthesizing isocyanate by using a salt-forming phosgene method. In order to solve the problem, the invention provides a continuous production method for synthesizing isocyanate by a salified phosgene method, which comprises the following steps of: s1, continuously introducing an amine-containing solution a and hydrogen chloride b into a salifying kettle 10 for salifying reaction, salifying part of amine in the amine-containing solution to obtain salifying liquid c containing amine hydrochloride and unreacted amine, and continuously discharging the salifying liquid c; s2, continuously introducing salifying liquid c into a salifying-photochemical kettle 20 to react with photochemical tail gas d to salify unreacted amine, performing preliminary phosgenation on amine hydrochloride to obtain photochemical-salifying liquid e, and continuously discharging photochemical-salifying liquid e; s3, continuously introducing photochemical salt forming liquid e and phosgene f into a photochemical kettle 30 for phosgenation reaction to obtain photochemical liquid g and photochemical tail gas d, and returning the photochemical tail gas d into the salt forming photochemical kettle; s4, carrying out sedimentation separation on the photochemical liquid to obtain isocyanate h.
When synthesizing isocyanate by salt-forming phosgene method, amine and hydrogen chloride need to be subjected to salt-forming reaction to form amine hydrochloride, and then to phosgenation reaction with phosgene (carbonyl chloride). However, the amine for preparing isocyanate is generally poor in solubility, and the reaction with hydrogen chloride is very rapid, the salifying reaction is rapidly changed from a homogeneous reaction system to a heterogeneous reaction system, so that the viscosity of the system is rapidly increased, the uniformity of the particle size of salified particles is poor, and the problem of reduced salifying rate caused by the coating of the amine by the salt particles occurs. Through a great deal of researches, the inventor provides the stepwise salification process, the viscosity of the system is effectively controlled, and salification particles are uniform, so that the salification rate is improved. In particular, the method comprises the steps of,
according to the invention, an amine-containing solution and hydrogen chloride are continuously introduced into a salifying kettle for salifying reaction, and only part of amine is salified in the process to obtain salifying liquid containing amine hydrochloride and unreacted amine. In the process, only part of amine forms salt, so that the viscosity of the system is greatly reduced, the uniformity of salified particles is ensured, and the amine is not easy to coat and still presents a free state. The main reaction of the salifying kettle:
R-NH 2 +HCl→R-NH 2 ·HCl
secondly, the residual unreacted amine enters a salifying-photochemical kettle along with salifying liquid to react with photochemical tail gas in the subsequent phosgenation reaction process. The photochemical tail gas contains a large amount of hydrogen chloride and a small amount of phosgene, and unreacted amine can be further reacted to form a salt with respect to the hydrogen chloride (reaction 1 below). The system viscosity is effectively controlled by stepwise salifying the amine, so that the material is convenient to transfer, continuous production can be realized, the total salt rate of the amine is relatively high, and salifying particles are uniform. In addition, a small amount of phosgene is carried in the photochemical tail gas, and amine hydrochloride in the salifying liquid, unreacted amine and amine hydrochloride generated by hydrogen chloride in the tail gas can be subjected to preliminary phosgenation reaction (reaction 3 below) with the part of phosgene, so that the amine hydrochloride in the salifying-photochemical kettle is ensured to be in a dynamic balance process, and the viscosity of the system is maintained in a lower state. Inevitably, unreacted amine and phosgene also react to form impurity components (reaction 2 below), but this reaction 2 is effectively suppressed due to the high concentration of hydrogen chloride in the salification-photochemical kettle and the state of dynamic equilibrium of amine hydrochloride. Based on the method, the photochemical tail gas can be effectively utilized to participate in the reaction in the salifying-photochemical kettle, the tail gas utilization rate is obviously improved, the higher purity of isocyanate can be maintained, and the method plays a very important role in reducing the consumption of hydrogen chloride and phosgene and protecting the environment of the whole isocyanate production.
Reaction 1: R-NH 2 +HCl→R-NH 2 ·HCl
Reaction 2: R-NH 2 +COCl2→R-NHCOCl+HCl
Reaction 3: R-NH 2 ·HCl+COCl2→R-NCO+3HCl
After the reaction in the salification-photochemical kettle, the amine hydrochloride in the photochemical-salification liquid can be fully phosgenated with the newly introduced phosgene in the photochemical kettle (R-NH 2 HCl+COCl2→R-NCO+3HCl) to obtain photochemical liquid, and separating by sedimentation to obtain isocyanate. It should be noted that the salt particle size is not the important cause of poor photochemical rate, and the invention controls the system viscosity in the step-by-step salt formation process, so that the particle size of the salt-forming particles is more uniform step by step, and the efficient implementation of the phosgenation reaction in the photochemical kettle is ensured.
In order to better control the stepwise salification rhythm and thus make the system proceed stably and efficiently in a lower viscosity state, in a preferred embodiment, in step S1, the molar ratio of amine in the amine-containing solution to hydrogen chloride is 1:0.3-1.8, preferably 1:0.6-1.2. Compared with the traditional method for adding excessive hydrogen chloride, the invention adopts the molar ratio of the amine and the hydrogen chloride, so that the salification degree in the salification kettle is more suitable, the viscosity is more stable, the total salt rate of two steps is higher, and the salt particles are more uniform.
The amines employed in the present invention may be of the type commonly used in the art including, but not limited to, one or more of toluene diamine, 1, 6-hexane diamine, 1, 5-pentane diamine, isophorone diamine, p-phenylene diamine, 1, 5-naphthalene diamine, diphenyl methane diamine. Different amines have different melting points and can lead to different degrees of viscosity during salt formation. For the purpose of further maintaining a lower viscosity state, improving the salt formation rate, the uniformity of salt particles and the phosgenation reaction efficiency, in a preferred embodiment, when the melting point of the amine in the amine-containing solution is higher than 80 ℃, the molar ratio of the amine in the amine-containing solution to hydrogen chloride is 1:0.9-1.2; when the melting point of the amine in the amine-containing solution is lower than 80 ℃, the molar ratio of the amine in the amine-containing solution to the hydrogen chloride is 1:0.6-0.9.
In the actual production process, preferably, when the melting point of the amine in the amine-containing solution is higher than 80 ℃, the reaction temperature in the salifying kettle is 60-130 ℃, more preferably 80-100 ℃; when the melting point of the amine in the amine-containing solution is lower than 80 ℃, the reaction temperature in the salifying kettle is 0-60 ℃, more preferably 0-30 ℃; preferably, the residence time of the amine-containing solution in the salifying kettle is 0.5-2 h; more preferably, when the melting point of the amine in the amine-containing solution is higher than 80 ℃, the residence time of the amine-containing solution in the salifying kettle is 1.2-2 h; when the melting point of the amine in the amine-containing solution is lower than 80 ℃, the residence time of the amine-containing solution in the salifying kettle is 0.5-1.2 h. By controlling the reaction temperature and the residence time, the beneficial effects of fractional salinization can be further improved, and the continuous reaction is more stable. Preferably, the mass concentration of the amine-containing solution is 10-40%, and the solvent is one or more of chlorobenzene, dichlorobenzene and toluene.
In view of the reaction rate of phosgene in the continuous production process and the suitability of phosgene in the tail gas, in a preferred embodiment, the molar ratio of the amine in the amine-containing solution to the phosgene introduced in step S3 is 1:2 to 10, preferably 1:3 to 6. The molar ratio of phosgene to amine is controlled in the above range, which is favorable for the dynamic balance of each reaction in a salifying kettle, a salifying-photochemical kettle and a photochemical kettle, and has better promotion effect on the stability and the material balance of continuous production.
During the phosgenation reaction, isocyanate is formed after the amine hydrochloride reacts with phosgene, but it is inevitable that some salified particles do not react, and some small amount of polymer impurities like isocyanate biuret, uretdione, carbodiimide, etc. are inevitably generated during the reaction. In a preferred embodiment, as shown in fig. 1, in the step S4, the photochemical solution is subjected to sedimentation separation in the sedimentation tank 40, and the obtained supernatant is isocyanate h; the sedimentation separation also obtains a lower layer sedimentation slurry i at the bottom of the sedimentation kettle, and the method further comprises the following steps: and returning the lower layer sedimentation slurry i to the step S2, and introducing the lower layer sedimentation slurry i into a salifying-photochemical kettle for internal reaction. Through sedimentation separation, isocyanate is positioned in the clear liquid for separation, unreacted salified particles and a small amount of impurities are positioned at the bottom of the sedimentation kettle, and the unreacted salified particles and a small amount of impurities are returned to the salification-photochemical kettle, so that the continuous reaction of the unreacted salified particles is facilitated, and the conversion rate and the product yield are further improved. Meanwhile, it is emphasized that the return of the lower-layer settled slurry to the salifying-photochemical kettle is also advantageous for discharging the total reaction tail gas j and further improving the purity of the product, compared with the direct return of the lower-layer settled slurry to the photochemical kettle.
In order to stabilize the continuous production, in a preferred embodiment, the ratio of the salt-forming liquid to the returned lower-layer sedimentation slurry is 1:0.05-0.1.
In a specific implementation process, a plurality of parallel settling tanks 40, such as two settling tanks 40, are preferably arranged, and the photochemical liquid can firstly enter one settling tank 40 and reach a preset liquid level of the settling tank after a certain time, and at the moment, the photochemical liquid pipeline is switched to discharge the photochemical liquid into the other settling tank 40.
In a preferred embodiment, in step S2, the reaction temperature in the salification-photochemical kettle is 30 to 130 ℃, preferably 40 to 80 ℃; and/or, in step S3, the reaction temperature in the photochemical kettle is 80-170 ℃, preferably 110-160 ℃; and/or the retention time of the materials in the salifying-photochemical kettle is 0.5-2 h; and/or the retention time of the materials in the photochemical kettle is 2-10 h; and/or settling separation time is 3-8 h. The reaction temperature and the material residence time of each kettle are controlled within the above ranges, which is beneficial to further improving the stability and the reaction efficiency of the continuous reaction.
Preferably, the photochemical kettle is provided with a photochemical tail gas outlet, a condensation reflux device is arranged at the photochemical tail gas outlet, and the temperature of a condensation medium is between-30 and 5 ℃. By using the condensing reflux device, phosgene in the photochemical tail gas can be refluxed to the photochemical kettle as much as possible to participate in the reaction, and the phosgene proportion in the photochemical tail gas is reduced, so that the balance of each reaction in the salt forming-photochemical kettle is better positioned, and the progress of the side reaction 2 is restrained as much as possible.
In order to make the reaction product more stable, in a preferred embodiment, step S4 further comprises, prior to subjecting the photochemical liquid to the sedimentation separation, pre-cooling and quenching the photochemical liquid; preferably, the temperature of the cooling quench is 20 to 80 ℃, more preferably 40 to 60 ℃.
The present application is described in further detail below in conjunction with specific embodiments, which should not be construed as limiting the scope of the claims.
Example 1
In this example, a continuous production method for synthesizing isocyanate by a salt-forming phosgene method is provided, as shown in fig. 1, which comprises the following steps:
s1, continuously introducing an o-dichlorobenzene solution with the mass concentration of 10 percent (the melting point is approximately equal to 138 ℃) and hydrogen chloride gas into a salifying kettle with the volume of 50L for salifying reaction, controlling the reaction temperature in the salifying kettle to be 100 ℃, controlling the flow rate of an amine-containing solution to be 16.25L/h, controlling the volume flow rate of the hydrogen chloride gas to be 525.78L/h, controlling the molar ratio of amine to hydrogen chloride to be 1:1.2, and controlling the material residence time in the salifying kettle to be 2h; the salification liquid obtained by salification reaction contains amine hydrochloride and unreacted amine, and is continuously discharged (the viscosity is about 83 mpa.s); through detection, the salt formation rate of the amine in the salt formation kettle is 48%;
s2, continuously introducing salt forming liquid into a salt forming-photochemical kettle with the volume of 60L to react with photochemical tail gas, wherein the flow rate of the salt forming liquid is 17.89L/h, the volume flow rate of the photochemical tail gas is 3945.15L/h, the reaction temperature in the salt forming-photochemical kettle is 80 ℃, and the residence time is 2h, so as to obtain photochemical-salt forming liquid; the combined salt ratio of the amines in the salt forming kettle and the salt forming-photochemical kettle is 96.8%, and the median particle diameter d50=9.33 microns;
s3, continuously introducing photochemical salt forming liquid into a photochemical kettle with the volume of 180L to carry out phosgenation reaction with fresh phosgene, controlling the flow rate of the photochemical salt forming liquid to be 20.16L/h, the volume flow rate of the phosgene to be 1753L/h, the molar ratio of amine in the amine-containing solution to the phosgene to be 1:4, and the reaction temperature in the photochemical kettle to be 140 ℃ and the material residence time to be 6h; continuously discharging the obtained photochemical liquid, and returning photochemical tail gas to the step S2; a condensing device with a cooling medium of minus 20 ℃ is arranged at the photochemical tail gas outlet so that most phosgene in the photochemical tail gas flows back to the photochemical kettle for reaction;
s4, preparing two switchable sedimentation kettles, cooling and quenching photochemical liquid at 55 ℃, continuously introducing the photochemical liquid into one sedimentation kettle, and switching photochemical liquid pipelines after a certain time reaches a set liquid level to enable the photochemical liquid to be discharged into the other sedimentation kettle. After 4.5 hours of sedimentation separation in a sedimentation kettle, the supernatant containing isocyanate is discharged, and the lower sedimentation slurry at the bottom of the kettle returns to the salifying-photochemical kettle in the step S2 according to the flow of 0.63L/h for continuous reaction.
The detection calculation shows that the synthesis yield of the isocyanate in the continuous production is 98.9%.
Example 2
In this example, a continuous production method for synthesizing isocyanate by a salt-forming phosgene method is provided, as shown in fig. 1, which comprises the following steps:
s1, continuously introducing an o-dichlorobenzene solution with the mass concentration of 10 percent (the melting point is approximately equal to 138 ℃) and hydrogen chloride gas into a salt forming kettle with the volume of 50L for salt forming reaction, controlling the reaction temperature in the salt forming kettle to be 80 ℃, controlling the flow rate of an amine-containing solution to be 16.25L/h, controlling the volume flow rate of the hydrogen chloride gas to be 328L/h, controlling the molar ratio of amine to hydrogen chloride to be 1:0.75, and controlling the material residence time in the salt forming kettle to be 1.2h; the salification liquid obtained by salification reaction contains amine hydrochloride and unreacted amine, and is continuously discharged (viscosity is about 79 mpa.s); through detection, the salt formation rate of the amine in the salt formation kettle is 41%;
s2, continuously introducing salt forming liquid into a salt forming-photochemical kettle with the volume of 60L to react with photochemical tail gas, wherein the flow rate of the salt forming liquid is 17.91L/h, the volume flow rate of the photochemical tail gas is 4293L/h, and the reaction temperature in the salt forming-photochemical kettle is 40 ℃ and the residence time is 2h to obtain photochemical-salt forming liquid; the combined salt ratio of the amines in the salt forming kettle and the salt forming-photochemical kettle is 96.3%, and the median particle diameter d50=9.25 microns;
s3, continuously introducing photochemical salt forming liquid into a photochemical kettle with the volume of 180L to carry out phosgenation reaction with fresh phosgene, controlling the flow rate of the photochemical salt forming liquid to be 21.22L/h, the volume flow rate of the phosgene to be 2629L/h, the molar ratio of amine in an amine-containing solution to the phosgene to be 1:6, and controlling the reaction temperature in the photochemical kettle to be 130 ℃ and the material residence time to be 6h; continuously discharging the obtained photochemical liquid, and returning photochemical tail gas to the step S2; a condensing device with a cooling medium of minus 30 ℃ is arranged at the photochemical tail gas outlet so that most phosgene in the photochemical tail gas flows back to the photochemical kettle for reaction;
s4, preparing two switchable sedimentation kettles, cooling and quenching photochemical liquid at 40 ℃, continuously introducing the photochemical liquid into one sedimentation kettle, and switching photochemical liquid pipelines after a certain time reaches a set liquid level to enable the photochemical liquid to be discharged into the other sedimentation kettle. After 8 hours of sedimentation separation in a sedimentation kettle, the supernatant containing isocyanate is discharged, and the lower sedimentation slurry at the bottom of the kettle returns to the salifying-photochemical kettle in the step S2 according to the flow of 0.35L/h for continuous reaction.
The detection calculation shows that the synthesis yield of the isocyanate in the continuous production is 98.5%.
Example 3
In this example, a continuous production method for synthesizing isocyanate by a salt-forming phosgene method is provided, as shown in fig. 1, which comprises the following steps:
s1, continuously introducing an o-dichlorobenzene solution with the mass concentration of 20 percent (the melting point is approximately equal to 138 ℃) and hydrogen chloride gas into a salifying kettle with the volume of 50L for salifying reaction, controlling the reaction temperature in the salifying kettle to be 130 ℃, controlling the flow rate of an amine-containing solution to be 16.25L/h, controlling the volume flow rate of the hydrogen chloride gas to be 876.3L/h, controlling the molar ratio of amine to hydrogen chloride to be 1:1.0, and controlling the material residence time in the salifying kettle to be 0.5h; the salification liquid obtained by salification reaction contains amine hydrochloride and unreacted amine, and is continuously discharged (the viscosity is about 116 mpa.s); through detection, the salt formation rate of the amine in the salt formation kettle is 52.5%;
s2, continuously introducing salt forming liquid into a salt forming-photochemical kettle with the volume of 50L to react with photochemical tail gas, wherein the flow rate of the salt forming liquid is 18.58L/h, the volume flow rate of the photochemical tail gas is 5082L/h, and the reaction temperature in the salt forming-photochemical kettle is 60 ℃ and the residence time is 1.0h to obtain photochemical-salt forming liquid; the combined salt ratio of the amines in the salt forming kettle and the salt forming-photochemical kettle was 92.5%, and the median particle diameter d50=11.63 micrometers;
s3, continuously introducing photochemical salt forming liquid into a photochemical kettle with the volume of 150L to carry out phosgenation reaction with fresh phosgene, controlling the flow rate of the photochemical salt forming liquid to be 23.68L/h, the volume flow rate of the phosgene to be 2629L/h, the molar ratio of amine in an amine-containing solution to the phosgene to be 1:3, and controlling the reaction temperature in the photochemical kettle to be 160 ℃ and the material residence time to be 3h; continuously discharging the obtained photochemical liquid, and returning photochemical tail gas to the step S2; a condensing device with a cooling medium of 5 ℃ is arranged at the photochemical tail gas outlet so that most phosgene in the photochemical tail gas flows back to the photochemical kettle for reaction;
s4, preparing two switchable sedimentation kettles, cooling and quenching photochemical liquid at 60 ℃, continuously introducing the photochemical liquid into one sedimentation kettle, and switching photochemical liquid pipelines after a certain time reaches a set liquid level to enable the photochemical liquid to be discharged into the other sedimentation kettle. After 3h of sedimentation separation in a sedimentation kettle, the supernatant containing isocyanate is discharged, and the lower sedimentation slurry at the bottom of the kettle returns to the salifying-photochemical kettle in step S2 according to the flow of 1.35L/h for continuous reaction.
The detection calculation shows that the synthesis yield of the isocyanate in the continuous production is 96.2%.
Example 4
In this example, a continuous production method for synthesizing isocyanate by a salt-forming phosgene method is provided, as shown in fig. 1, which comprises the following steps:
s1, continuously introducing a chlorobenzene solution with the mass concentration of 40% of 1, 6-hexamethylenediamine (with the melting point of about 42-46 ℃) and hydrogen chloride gas into a salifying kettle with the volume of 50L for salifying reaction, controlling the reaction temperature in the salifying kettle to be 30 ℃, controlling the flow rate of an amine-containing solution to be 16.25L/h, controlling the volume flow rate of the hydrogen chloride gas to be 1240L/h, controlling the molar ratio of amine to hydrogen chloride to be 1:0.9, and controlling the material residence time in the salifying kettle to be 1.2h; the salification liquid obtained by salification reaction contains amine hydrochloride and unreacted amine, and is continuously discharged (the viscosity is about 128 mpa.s); through detection, the salt formation rate of the amine in the salt formation kettle is 45%;
s2, continuously introducing salt forming liquid into a salt forming-photochemical kettle with the volume of 60L to react with photochemical tail gas, wherein the flow rate of the salt forming liquid is 21.63L/h, the volume flow rate of the photochemical tail gas is 9610L/h, and the reaction temperature in the salt forming-photochemical kettle is 50 ℃ and the residence time is 1.5h to obtain photochemical-salt forming liquid; the combined salt ratio of the amines in the salt forming kettle and the salt forming-photochemical kettle was detected to be 96.6%, and the median particle diameter d50=9.12 micrometers;
s3, continuously introducing photochemical salt forming liquid into a photochemical kettle with the volume of 200L to carry out phosgenation reaction with fresh phosgene, controlling the flow rate of the photochemical salt forming liquid to be 25.63L/h, the volume flow rate of the phosgene to be 8265.6L/h, the molar ratio of amine in an amine-containing solution to the phosgene to be 1:6, and controlling the reaction temperature in the photochemical kettle to be 110 ℃ and the material residence time to be 6h; continuously discharging the obtained photochemical liquid, and returning photochemical tail gas to the step S2; a condensing device with a cooling medium of minus 30 ℃ is arranged at the photochemical tail gas outlet so that most phosgene in the photochemical tail gas flows back to the photochemical kettle for reaction;
s4, preparing two switchable sedimentation kettles, cooling and quenching photochemical liquid at 40 ℃, continuously introducing the photochemical liquid into one sedimentation kettle, and switching photochemical liquid pipelines after a certain time reaches a set liquid level to enable the photochemical liquid to be discharged into the other sedimentation kettle. After 4.5h of sedimentation separation in a sedimentation kettle, the supernatant containing isocyanate is discharged, and the lower sedimentation slurry at the bottom of the kettle returns to the salifying-photochemical kettle in step S2 according to the flow of 1.5L/h for continuous reaction.
The detection calculation shows that the synthesis yield of the isocyanate in the continuous production is 98.7%.
Example 5
In this example, a continuous production method for synthesizing isocyanate by a salt-forming phosgene method is provided, as shown in fig. 1, which comprises the following steps:
s1, continuously introducing a chlorobenzene solution with the mass concentration of 20% of 1, 6-hexamethylenediamine (with the melting point of about 42-46 ℃) and hydrogen chloride gas into a salifying kettle with the volume of 50L for salifying reaction, controlling the reaction temperature in the salifying kettle to be 0 ℃, controlling the flow rate of an amine-containing solution to be 39.93L/h, controlling the volume flow rate of the hydrogen chloride gas to be 1016L/h, controlling the molar ratio of amine to hydrogen chloride to be 1:0.6, and controlling the material residence time in the salifying kettle to be 0.5h; the salification liquid obtained by salification reaction contains amine hydrochloride and unreacted amine, and is continuously discharged (the viscosity is about 58 mpa.s); through detection, the salt formation rate of the amine in the salt formation kettle is 44%;
s2, continuously introducing salt forming liquid into a salt forming-photochemical kettle with the volume of 150L to react with photochemical tail gas, wherein the flow rate of the salt forming liquid is 43.38L/h, the volume flow rate of the photochemical tail gas is 15108L/h, the reaction temperature in the salt forming-photochemical kettle is 30 ℃, and the residence time is 2h, so as to obtain photochemical-salt forming liquid; the combined salt ratio of the amines in the salt forming kettle and the salt forming-photochemical kettle is 96.0%, and the median particle diameter d50=9.08 microns;
s3, continuously introducing photochemical salt forming liquid into a photochemical kettle with the volume of 400L to carry out phosgenation reaction with fresh phosgene, controlling the flow rate of the photochemical salt forming liquid to be 46.95L/h, the volume flow rate of the phosgene to be 10160L/h, the molar ratio of amine in an amine-containing solution to the phosgene to be 1:6, and controlling the reaction temperature in the photochemical kettle to be 80 ℃ and the material residence time to be 6h; continuously discharging the obtained photochemical liquid, and returning photochemical tail gas to the step S2; a condensing device with a cooling medium of minus 20 ℃ is arranged at the photochemical tail gas outlet so that most phosgene in the photochemical tail gas flows back to the photochemical kettle for reaction;
s4, preparing two switchable sedimentation kettles, cooling and quenching photochemical liquid at 55 ℃, continuously introducing the photochemical liquid into one sedimentation kettle, and switching photochemical liquid pipelines after a certain time reaches a set liquid level to enable the photochemical liquid to be discharged into the other sedimentation kettle. After 4.5 hours of sedimentation separation in a sedimentation kettle, the supernatant containing isocyanate is discharged, and the lower sedimentation slurry at the bottom of the kettle returns to the salifying-photochemical kettle in step S2 according to the flow of 1.83L/h for continuous reaction.
The detection calculation shows that the synthesis yield of the isocyanate in the continuous production is 97.6%.
Example 6
In this example, a continuous production method for synthesizing isocyanate by a salt-forming phosgene method is provided, as shown in fig. 1, which comprises the following steps:
s1, continuously introducing a chlorobenzene solution with the mass concentration of 30% of 1, 5-pentanediamine (the melting point is approximately equal to 9 ℃) and hydrogen chloride gas into a salifying kettle with the volume of 50L for salifying reaction, controlling the reaction temperature in the salifying kettle to be 30 ℃, controlling the flow rate of an amine-containing solution to be 16.25L/h, controlling the volume flow rate of the hydrogen chloride gas to be 1531L/h, controlling the molar ratio of amine to hydrogen chloride to be 1:1.3, and controlling the material residence time in the salifying kettle to be 3h; the salification liquid obtained by salification reaction contains amine hydrochloride and unreacted amine, and is continuously discharged (the viscosity is about 63 mpa.s); through detection, the salt formation rate of the amine in the salt formation kettle is 38%;
s2, continuously introducing salt forming liquid into a salt forming-photochemical kettle with the volume of 60L to react with photochemical tail gas, wherein the flow rate of the salt forming liquid is 17.55L/h, the volume flow rate of the photochemical tail gas is 11541L/h, the reaction temperature in the salt forming-photochemical kettle is 30 ℃, and the residence time is 2h, so as to obtain photochemical-salt forming liquid; the combined salt ratio of the amines in the salt forming kettle and the salt forming-photochemical kettle is 88.3%, and the median particle diameter d50=11.38 micrometers;
s3, continuously introducing photochemical salt forming liquid into a photochemical kettle with the volume of 150L to carry out phosgenation reaction with fresh phosgene, controlling the flow rate of the photochemical salt forming liquid to be 19.35L/h, the volume flow rate of the phosgene to be 7066L/h, the molar ratio of amine in an amine-containing solution to the phosgene to be 1:6, and controlling the reaction temperature in the photochemical kettle to be 80 ℃ and the material residence time to be 4h; continuously discharging the obtained photochemical liquid, and returning photochemical tail gas to the step S2; a condensing device with a cooling medium of minus 20 ℃ is arranged at the photochemical tail gas outlet so that most phosgene in the photochemical tail gas flows back to the photochemical kettle for reaction;
s4, preparing two switchable sedimentation kettles, cooling and quenching photochemical liquid at 55 ℃, continuously introducing the photochemical liquid into one sedimentation kettle, and switching photochemical liquid pipelines after a certain time reaches a set liquid level to enable the photochemical liquid to be discharged into the other sedimentation kettle. After 4.5 hours of sedimentation separation in a sedimentation kettle, the supernatant containing isocyanate is discharged, and the lower sedimentation slurry at the bottom of the kettle returns to the salifying-photochemical kettle in the step S2 according to the flow of 0.93L/h for continuous reaction.
The detection calculation shows that the synthesis yield of the isocyanate in the continuous production is 98.9%.
The above description is only of the preferred embodiments of the present invention and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. A continuous production method for synthesizing isocyanate by a salified phosgene method is characterized by comprising the following steps:
s1, continuously introducing an amine-containing solution and hydrogen chloride into a salifying kettle to carry out salifying reaction, salifying part of amine in the amine-containing solution to obtain salifying liquid containing amine hydrochloride and unreacted amine, and continuously discharging the salifying liquid;
s2, continuously introducing the salifying liquid into a salifying-photochemical kettle to react with photochemical tail gas, salifying the unreacted amine, performing preliminary phosgenation on the amine hydrochloride to obtain photochemical-salifying liquid, and continuously discharging the photochemical-salifying liquid;
s3, continuously introducing the photochemical salt forming liquid and phosgene into a photochemical kettle for phosgenation reaction to obtain photochemical liquid and photochemical tail gas, and returning the photochemical tail gas into the salt forming-photochemical kettle;
s4, carrying out sedimentation separation on the photochemical liquid to obtain isocyanate.
2. The continuous production method for synthesizing isocyanate by salified phosgene method according to claim 1, characterized by comprising the following steps
In S1, the molar ratio of the amine in the amine-containing solution to the hydrogen chloride is 1:0.3-1.8, preferably 1:0.6-1.2.
3. The continuous production method for synthesizing isocyanate by using a salified phosgene method according to claim 2, wherein the amine in the amine-containing solution is one or more of toluenediamine, 1, 6-hexamethylenediamine, 1, 5-pentanediamine, isophoronediamine, p-phenylenediamine, 1, 5-naphthalene diamine and diphenylmethane diamine;
preferably, when the melting point of the amine in the amine-containing solution is higher than 80 ℃, the molar ratio of the amine in the amine-containing solution to the hydrogen chloride is 1:0.9-1.2; when the melting point of the amine in the amine-containing solution is lower than 80 ℃, the molar ratio of the amine in the amine-containing solution to the hydrogen chloride is 1:0.6-0.9;
preferably, when the melting point of the amine in the amine-containing solution is higher than 80 ℃, the reaction temperature in the salifying kettle is 60 to 130 ℃, more preferably 80 to 100 ℃; when the melting point of the amine in the amine-containing solution is lower than 80 ℃, the reaction temperature in the salifying kettle is 0-60 ℃, more preferably 0-30 ℃;
preferably, the residence time of the amine-containing solution in the salifying kettle is 0.5-2 h; more preferably, when the melting point of the amine in the amine-containing solution is higher than 80 ℃, the residence time of the amine-containing solution in the salifying kettle is 1.2-2 h; when the melting point of the amine in the amine-containing solution is lower than 80 ℃, the residence time of the amine-containing solution in the salifying kettle is 0.5-1.2 h;
preferably, the mass concentration of the amine-containing solution is 10-40%, and the solvent is one or more of chlorobenzene, dichlorobenzene and toluene.
4. A continuous production method for synthesizing isocyanate by salified phosgene method according to any one of claims 1 to 3, wherein the molar ratio of amine in the amine-containing solution to the phosgene introduced in step S3 is 1:2-10.
5. The continuous production method for synthesizing isocyanate by using a salified phosgene method according to claim 4, wherein the molar ratio of amine in the amine-containing solution to the phosgene introduced in the step S3 is 1:3-6.
6. The continuous production method for synthesizing isocyanate by using a salified phosgene method according to any one of claims 1 to 5, wherein in the step S4, the photochemical liquid is subjected to the sedimentation separation in a sedimentation kettle, and the obtained supernatant is the isocyanate; the sedimentation separation also produces a lower layer of settled slurry at the bottom of the sedimentation tank, the method further comprising:
and returning the lower layer sedimentation slurry to the step S2, and introducing the lower layer sedimentation slurry into the salification-photochemical kettle for internal reaction.
7. The continuous production method for synthesizing isocyanate by salified phosgene method according to claim 6, wherein the ratio of the introduced volume flow of salified liquid to the returned lower layer sedimentation slurry is 1:0.05-0.1.
8. The continuous production method for synthesizing isocyanate by salified phosgene method according to any one of claims 1 to 7, wherein,
in the step S2, the reaction temperature in the salifying-photochemical kettle is 30-130 ℃, preferably 40-80 ℃; and/or the number of the groups of groups,
in the step S3, the reaction temperature in the photochemical kettle is 80-170 ℃, preferably 110-160 ℃; and/or the retention time of the materials in the salifying-photochemical kettle is 0.5-2 h; and/or the number of the groups of groups,
the retention time of the materials in the photochemical kettle is 2-10 h; and/or the number of the groups of groups,
the sedimentation separation time is 3-8 h.
9. The continuous production method for synthesizing isocyanate by salified phosgene method according to any one of claims 1 to 7, wherein the photochemical kettle is provided with a photochemical tail gas outlet, and a condensation reflux device is arranged at the photochemical tail gas outlet, and the temperature of a condensation medium is between-30 ℃ and 5 ℃.
10. The continuous production method for synthesizing isocyanate by a salt-forming phosgene method according to any one of claims 1 to 7, wherein step S4 further comprises, before subjecting the photochemical liquid to the sedimentation separation, quenching the photochemical liquid in advance; preferably, the temperature of the cooling quench is 20 to 80 ℃, more preferably 40 to 60 ℃.
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