Solvent refining method for isocyanate prepared by phosgene method and devices used in same

Abstract

A solvent refining method for isocyanate prepared by the phosgene method and multistage absorbing towers used in same. Solvent to be refined which contains water, iron, and/or phosgene, hydrogen chloride and other materials with color is dealt by the present method and multistage absorbing towers, which can effectively prevent a drying agent from absorbing water and hardening, partial overheating in the tower and generating channeling. Meanwhile, the pressure drop is effectively lowered. In addition, the content of water is 50 ppm, the content of iron is 5 ppm, the content of phosgene and hydrogen chloride is 20 ppm, PtCo chroma is 20 in the refined solvent. Therefore, the refined solvent can be used as the solvent for preparing isocyanate in the phosgene method and remarkably improve an L color of isocyanate.

Claims

1. A method for refining solvents generated in the preparation of isocyanates by the phosgene method, comprising treating the solvents to be refined with a multistage absorption tower comprising a support packing section at the bottom, a packing absorbing section in the middle and a support packing section on the top from bottom to top, wherein the packing absorption section in the middle comprises N absorbing layers, and N is an integer from 3 to 8; from the first absorbing layer to the Nth absorbing layer, the layers are arranged from bottom to top; from the first absorbing layer to the (N1)th absorbing layer, each layer of these layers is composed of an upper liquid distributing device that connects an external feeding pipe and a lower absorbing packing layer; and the absorbing packing layer of every of the absorbing layers is formed by the uniform mixing of desiccants and adsorbents.

2. The method according to claim 1, wherein the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the first absorbing layer is 1:1-5:1, and the desiccants represent 4-15% of the weight of all the desiccants in the multistage absorption tower; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the Nth absorbing layer is 5:1-12:1, and the desiccants represent 4-15% of the weight of all the desiccants in the multistage absorption tower; and the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of every of the second absorbing layer to the (N1)th absorbing layer is 5:1-15:1, and the desiccants of every of the absorbing packing layers represent 15-92% of the weight of all the desiccants in the multistage absorption tower.

3. The method according to claim 1, wherein the desiccants are alkaline desiccants, which are selected from the group consisting of calcium oxide, sodium hydroxide, potassium hydroxide, and a mixture of two or more thereof; the BET specific surface area of the desiccants is 1500-4500 m.sup.2/g; the average particle size is 0.5-10 m; and the mechanical strength is 85-99%.

4. The method according to claim 1, wherein the adsorbents are macroporous resin adsorbents or activated carbon; the macroporous resin adsorbents are selected from the group consisting of nonpolar macroporous adsorption resins of styrene polymer, nonpolar macroporous adsorption resins of divinylbenzene polymer, and a mixture of two or more thereof the BET specific surface area of the adsorbents is 2500-5000 m.sup.2/g; the pore size of mesoporous is 2-10 nm; and the mechanical strength is 85-98%.

5. The method according to claim 1, wherein the support packings of the support packing section on top and the support packing section at the bottom are selected from the group consisting of gravels, molecular sieves, activated carbon, and a mixture of two or more thereof; the weight of the support packings in the support packing section on top and the weight of the support packings in the support packing section at the bottom are the same; and the weight ratio of the support packings in the support packing section on top to all the desiccants in the multistage absorption tower is 1:20-1:3.

6. The method according to claim 1, wherein the ratio of the volume flow rate of the total feed rate of the solvents to be refined to the total weight of all the desiccants and the adsorbents in the multistage absorption tower is 1:200-1:600 m.sup.3/kg/h.

7. The method according to claim 6, wherein the feeding volume of the external feeding pipe of the first absorbing layer represents 1/15- of, the total feeding volume of the solvents to be refined, the feeding amount of the external feeding pipe of the (N1)th absorbing layer represents 1/15- of the total feeding volume of the solvents to be refined, and the feeding amount of the external feeding pipe of every of the absorbing layers, from the second absorbing layer to the (N2)th absorbing layer, represents 1/15-of the total feeding amount of the solvents to be refined.

8. The method according to claim 7, wherein the residence time of the solvents to be refined in the multistage absorption tower is 0.25-8 hrs.

9. The method according to claim 6, wherein the refined solvents partially reflux and enter into the multistage absorption tower after mixing with the solvents to be refined, and the reflux ratio is 0.5-4; and the pressure drop between the external feeding pipe of the first absorbing layer of the multistage absorption tower and the outlet on top of the tower is 5-40 kPa.

10. The method according to claim 1, wherein the solvents to be refined are the waste solvents that contains impurities generated during the preparation of isocyanates by the phosgene method or the fresh solvents that should be added to the reaction system during preparation of isocyanates because of the consumption of solvents, and the content of water, iron and the color number in the fresh solvents fail to reach standards; the solvents are selected from the group consisting of o-dichlorobenzene, chlorobenzene, toluene, and a mixture of two or more thereof; the content of water in the solvents to be refined is 150-600 ppm; the content of iron component is 40-300 ppm; the content of phosgene and hydrogen chloride is 0-10000 ppm; the PtCo color number is 30-100; the content of water in the refined solvents is 50 ppm; the content of iron component is 5 ppm; the content of phosgene and hydrogen chloride is 20 ppm; and PtCo color number is 20.

11. A multistage absorption tower used for refining solvents generated in the preparation of isocyanates by the phosgene method, comprising: a support packing section at the bottom; a packing absorbing section in the middle; and a support packing section on top from bottom to top; wherein the packing absorbing section in the middle comprises N absorbing layers, and N is an integer from 3 to 8; from the first absorbing layer to the Nth absorbing layer, the layers are arranged from bottom to top; from the first layer to the (N1)th absorbing layer, each layer of these layers is composed of an upper liquid distributing device that connects an external feeding pipe and a lower absorbing packing layer; and the absorbing packing layer of every of the absorbing layers is formed by the uniform mixing of desiccants and adsorbents.

12. The multistage absorption tower according to claim 11, wherein the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the first absorbing layer is 1:1-5:1, and the desiccants represent 4-15% of the weight of all the desiccants in the multistage absorption tower; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the Nth absorbing layer is 5:1-12:1, and the desiccants represent 4-15% of the weight of all the desiccants in the multistage absorption tower; and the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of every of the second absorbing layer to the (N1)th absorbing layer is 5:1-15:1, and the desiccants of every of the absorbing packing layers represent 15-92% of the weight of all the desiccants in the multistage absorption tower.

13. The multistage absorbing tower according to claim 11, wherein the desiccants are alkaline desiccant, which are selected from the group consisting of calcium oxide, sodium hydroxide, potassium hydroxide, and a mixture of two or more thereof; the BET specific surface area of the desiccants is 1500-4500 m.sup.2/g; the average particle size is 0.5-10m; and the mechanical strength is 85-99%.

14. The multistage absorbing tower according to claim 11, wherein the adsorbents are macroporous resin adsorbents or activated carbon; the macroporous resin adsorbents are selected from the group consisting of nonpolar macroporous adsorption resins of styrene polymer, nonpolar macroporous adsorption resins of divinylbenzene polymer, and a mixture of two or more thereof; the BET specific surface area of the adsorbents is 2500-5000 m.sup.2/g; the pore size of mesoporous is 2-10 nm, and the mechanical strength is 85-98%.

15. The multistage absorbing tower according to claim 11, wherein the support packings of the support packing section on top and the support packing section at the bottom are selected from the group consisting of gravels, molecular sieves, activated carbon, and a mixture of two or more thereof; the weight of the support packings in the support packing section on top and the weight of the support packing in the support packing section at the bottom are the same; and the weight ratio of the support packing in the support packing section on top to all the desiccants in the multistage absorption tower is 1:20-1:3.

16. The method according to claim 1, where N is an integer from 4-6.

17. The method according to claim 2, wherein the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the first absorbing layer is 2:1-4:1, and the desiccants represent 5-10% of the weight of all the desiccants in the multistage absorption tower; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the Nth absorbing layer is 6:1-10:1, and the desiccants represent 5-10% of the weight of all the desiccants in the multistage absorption tower; and the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of every of the second absorbing layer to the (N1)th absorbing layer is 6:1-10:1, and the desiccants of every of the absorbing packing layers represent 20-45% of the weight of all the desiccants in the multistage absorption tower.

18. The method according to claim 6, wherein the ratio of the volume flow rate of the total feed rate of the solvents to be refined to the total weight of all the desiccants and the adsorbents in the multistage absorption tower is 1:300-1:500 m.sup.3/kg/h.

19. The method according to claim 7, wherein the feeding volume of the external feeding pipe of the first absorbing layer represents 1/10-of the total feeding volume of the solvents to be refined, the feeding amount of the external feeding pipe of the (N1)th absorbing layer represents 1/10-of the total feeding volume of the solvents to be refined, and the feeding amount of the external feeding pipe of every of the absorbing layers, from the second absorbing layer to the (N2)th absorbing layer, represents 1/10- 7/10of the total feeding amount of the solvents to be refined.

20. The method according to claim 9, wherein the reflux ratio is 1-3; and the pressure drop between the external feeding pipe of the first absorbing layer of the multistage absorption tower and the outlet on top of the tower is 10-25 kPa.

21. The multistage absorption tower according to claim 11, wherein N is an integer from 4 to 6.

22. The multistage absorption tower according to claim 12, wherein the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the first absorbing layer is 2:1-4:1, and the desiccants represent 5-10% of the weight of all the desiccants in the multistage absorption tower; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the Nth absorbing layer is 6:1-10:1, and the desiccants represent 5-10% of the weight of all the desiccants in the multistage absorption tower; and the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of every of the second absorbing layer to the (N1)th absorbing layer is 6:1-10:1, and the desiccants of every of the absorbing packing layers represent 20-45% of the weight of all the desiccants in the multistage absorption tower.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is the schematic diagram of the multistage absorption tower used in examples 1-5 according to the present invention.

DETAILED DESCRIPTION

(2) The present invention will be further illustrated by the following examples, but the present invention is not limited to the following examples. When the detailed experiment conditions for the experiment method are not mentioned in the following examples, the conditions follow the conventional conditions.

(3) In the following examples:

(4) The L color of isocyanates is measured by the L value, a value and b value of the CIE color system known by those skilled in the art;

(5) The content of iron component in the solvents is determined by the content of iron atoms measured at 248.33 nm by atomic absorption emission spectrometer;

(6) The content of water in the solvents is measured by the automatic Karl fischer moisture meter;

(7) The content of phosgene and hydrogen chloride is measured by the reaction of sodium hydroxide with phosgene and hydrogen chloride, then adding silver nitrate and measuring the content of the residual silver ions by automatic potentiometric titrator, and then converting the content of the residual silver ions to the content of phosgene and hydrogen chloride in the solvents;

(8) The coconut shell activated carbon is commercially available from Chengde Jing Da Activated Carbon Manufacturing Company; the macroporous resin D101 is commercially available from Xian Lan Xiao Science and Technology Ltd.

Example 1

(9) The desiccants are sodium hydroxide, the total amount of the desiccants in the absorption tower is 500 kg, the BET specific surface area of the desiccants is 2500 m.sup.2/g, the average particle size is 1 m, and the mechanical strength is 90%; the adsorbents are coconut shell activated carbon, the BET specific surface area is 3000 m.sup.2/g, the pore size of the mesoporous is 2 nm, and the mechanical strength is 90%. The support packings on top and bottom of the absorption tower are 3A type molecular sieves, and the total amount is all 50 kg. There are N absorbing layers and N is 4; from the first absorbing layer to the third absorbing layer, each layer of these layers is composed of an upper liquid distributing device that connects an external feeding pipe and a lower absorbing packing layer; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the first absorbing layer is 2:1, and the desiccants represents 5% of the weight of all the desiccants in the multistage absorption tower; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of every of the second and third absorbing layer is 10:1, and the desiccants of the absorbing packing layer of every of the second and third absorbing layers represents 45% of the weight of all the desiccants in the multistage absorption tower; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the fourth absorbing layer is 10:1, and the desiccants represents 5% of the weight of all the desiccants in the multistage absorption tower. The desiccants and the adsorbents of the absorbing packing layer of every of the absorbing layers are mixed uniformly and then are filled. The pressure drop between the external feeding pipe of the first absorbing layer of the multistage absorbing tower and the outlet on top of the tower is 10 kPa.

(10) The solvent to be refined is the waste chlorobenzene solvent generated during the abnormal startup and shutdown of the MDI manufacturing device, the ratio of the volume flow rate of the total feed rate of the solvent to the total weight of all the desiccants and the adsorbents in the multistage absorption tower is 1:300 m.sup.3/kg/h; the feed rate of the external feeding pipe of the first absorbing layer represents of the volume of the total feed rate of the solvent to be refined, the feed rate of the external feeding pipe of the second absorbing layer represents 7/10of the volume of the total feed rate of the solvent to be refined, and the feed rate of the external feeding pipe of the third absorbing layer represents 1/10of the volume of the total feed rate of the solvent to be refined. The residence time of the solvent to be refined in the multistage absorption tower is 2 hrs, and the reflux ratio of the refined solvent to the solvent to be refined is 1.

(11) The content of water in the solvent to be refined is 300 ppm, the content of iron component is 50 ppm, the content of phosgene and hydrogen chloride is 5000 ppm, and the CoPt color number is 40. The analysis results of the refined solvent are shown in table 1. The refined solvent is reused as the MDI manufacturing solvent, the method of manufacturing is referred to the method of manufacturing MDI in the patent document CN1254724A, and the analysis results of the obtained MDI product are shown in table 1.

Example 2

(12) The desiccants are potassium hydroxide, the total amount of the desiccants in the absorption tower is 1000 kg, the BET specific surface area of the desiccants is 3000 m.sup.2/g, the average particle size is 3 m, and the mechanical strength is 95%; the adsorbents are coconut shell activated carbon, the BET specific surface area is 3500 m.sup.2/g, the pore size of the mesoporous is 4 nm, the mechanical strength is 93%. The support packings on top and bottom of the absorption tower is 5A type molecular sieves, and the total amount is all 80 kg. There are N absorbing layers and N is five; from the first layer to the fourth layer, each layer of these layers is composed of an upper liquid distributing device that connects an external feeding pipe and a lower absorbing packing layer; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the first absorbing layer is 3:1, and the desiccants represents 7% of the weight of all the desiccants in the multistage absorption tower; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of every of the second, third and fourth absorbing layers is 9:1, the desiccants of the absorbing packing layer of every of the second, third and fourth absorbing layers represents 29% of the weight of all the desiccants in the multistage absorption tower; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the fifth absorbing layer is 6:1, and the desiccants represents 6% of the weight of all the desiccants in the multistage absorption tower. The desiccants and the adsorbents of the absorbing packing layer of every of the absorbing layers are mixed uniformly and then are filled. The pressure drop between the external feeding pipe of the first absorbing layer of the multistage absorption tower and the outlet on the top of the tower is 16 kPa.

(13) The solvent to be refined is the waste dichlorobenzene solvent generated during the overhaul of the TDI manufacturing device, the ratio of the volume flow rate of the total feed rate of the solvent to the total weight of all the desiccants and the adsorbents in the multistage absorption tower is 1:400 m.sup.3/kg/h; the feed rate of the external feeding pipe of the first absorbing layer represents of the volume of the total feed rate of the solvent to be refined, the feed rate of the external feeding pipe of every of the second and third absorbing layers represents of the volume of the total feed rate of the solvent to be refined respectively, and the feed rate of the external feeding pipe of the fourth absorbing layer represents of the volume of the total feed rate of the solvent to be refined. The residence time of the solvent to be refined in the multistage absorption tower is 3 hrs, and the reflux ratio of the refined solvent to the solvent to be refined is 1.5.

(14) The content of water in the solvent to be refined is 200 ppm, the content of iron component is 150 ppm, the content of phosgene and hydrogen chloride is 4000 ppm, and the CoPt color number is 60; the analysis results of the refined solvent are shown in table 1. The refined solvent is reused as the TDI manufacturing solvent, the method of manufacturing is referred to the method of manufacturing TDI in the patent document CN101205199A, and the analysis results of the obtained TDI product are shown in table 1.

Example 3

(15) The desiccants are sodium hydroxide, the total amount of the desiccants in the absorbing tower is 800 kg, the BET specific surface area of the desiccants is 4000 m.sup.2/g, the average particle size is 5 m, and the mechanical strength is 98%; the adsorbents are divinyl benzene polymer based nonpolar macroporous resinD101, the BET specific surface area is 4500 m.sup.2/g, the pore size of the mesoporous is 5 nm, and the mechanical strength is 95%; the support packings on top and bottom of the absorption tower are 10A type molecular sieves, the total amount is all 90 kg. There are N absorbing layers and N is six; from the first layer to the fifth layer, each layer of these layers is composed of an upper liquid distributing device that connects an external feeding pipe and a lower absorbing packing layer; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the first absorbing layer is 4:1, and the desiccants represents 10% of the weight of all the desiccants in the multistage absorption tower; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of every of the second, third, fourth and fifth absorbing layers is 5:1, and the desiccants of the absorbing packing layer of every of the second, third, fourth and fifth absorbing layers represents 20.5% of the weight of all the desiccants in the multistage absorption tower; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the sixth absorbing layer is 5:1, and the desiccants represents 8% of the weight of all the desiccants in the multistage absorption tower. The desiccants and the adsorbents of the absorbing packing layer of every of the absorbing layers are mixed uniformly and then are filled. The pressure drop between the external feeding pipe of the first absorbing layer of the multistage absorption tower and the outlet on the top of the tower is 25 kPa.

(16) The solvent to be refined is the fresh chlorobenzene solvent to be added to the HDI manufacturing device, the ratio of the volume flow rate of the total feed rate of the solvent to the total weight of all the desiccants and the adsorbents in the multistage absorption tower is 1:500 m.sup.3/kg/h; the feed rate of the external feeding pipe of the first absorbing layer represents of the volume of the total feed rate of the solvent to be refined, the feed rates of the external feeding pipe of the second, third and fourth absorbing layers respectively represents of the volume of the total feed rate of the solvent to be refined, and the feed rate of the external feeding pipe of the fifth absorbing layer represents of the volume of the total feed rate of the solvent to be refined. The residence time of the solvent to be refined in the multistage absorption tower is 4 hrs, and the reflux ratio of the refined solvent to the solvent to be refined is 3.

(17) The content of water in the solvent to be refined is 150 ppm, the content of iron component is 200 ppm, the content of phosgene and hydrogen chloride is 0 ppm, and the CoPt color number is 80. The analysis results of the refined solvent are shown in table 1. The refined solvent is reused as the HDI manufacturing solvent, the method of manufacturing is referred to the method of manufacturing HDI in the patent document CN101429139A, and the analysis results of the obtained HDI product are shown in table 1.

Example 4

(18) The desiccants are calcium oxide, the total amount of the desiccants in the absorbing tower is 1200 kg, the BET specific surface area of the desiccants is 4200 m.sup.2/g, the average particle size is 4.5 m, and the mechanical strength is 95%; the adsorbents are nonpolar macroporous adsorption resin of styrene polymerLX-60, the BET specific surface area is 4800 m.sup.2/g, the pore size of the mesoporous is 4.5 nm, and the mechanical strength is 93%. The support packings on top and bottom of the absorption tower is 5A type molecular sieves, and the total amount is all 120 kg. There are N absorbing layers and N is six; from the first layer to the fifth layer, each layer of these layers is composed of an upper liquid distributing device that connects an external feeding pipe and a lower absorbing packing layer; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the first absorbing layer is 4:1, and the desiccants represents 10% of the weight of all the desiccants in the multistage absorption tower; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of every of the second, third, fourth and fifth absorbing layers is 5:1, and the desiccants of the absorbing packing layer of every of the second, third, fourth and fifth absorbing layers represents 20.5% of the weight of all the desiccants in the multistage absorption tower; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the sixth absorbing layer is 5:1, and the desiccants represents 8% of the weight of all the desiccants in the multistage absorption tower. The desiccants and the adsorbents of the absorbing packing layer of every of the absorbing layers are mixed uniformly and then are filled. The pressure drop between the external feeding pipe of the first absorbing layer of the multistage absorption tower and the outlet on the top of the tower is 25 kPa.

(19) The solvent to be refined is the fresh toluene solvent to be added to the MDI manufacturing device, the ratio of the volume flow rate of the total feed rate of the solvent to the total weight of all the desiccants and the adsorbents in the multistage absorption tower is 1:400 m.sup.3/kg/h; the feed rate of the external feeding pipe of the first absorbing layer represents of the volume of the total feed rate of the solvent to be refined, the feed rates of the external feeding pipe of the second, third and fourth absorbing layers respectively represents of the volume of the total feed rate of the solvent to be refined, and the feed rate of the external feeding pipe of the fifth absorbing layer represents of the volume of the total feed rate of the solvent to be refined. The residence time of the solvent to be refined in the multistage absorption tower is 3.5 hrs, and the reflux ratio of the refined solvent to the solvent to be refined is 2.

(20) The content of water in the solvent to be refined is 250 ppm, the content of iron component is 120 ppm, the content of phosgene and hydrogen chloride is 0 ppm, and the CoPt color number is 60. The analysis results of the refined solvent are shown in table 1. The refined solvent is reused as the MDI manufacturing solvent, the method of manufacturing is referred to the method of manufacturing MDI in the patent document CN1254724A, and the analysis results of the obtained MDI product are shown in table 1.

Example 5

(21) The desiccants are potassium hydroxide, the total amount of the desiccants in the absorption tower is 900 kg, the BET specific surface area of the desiccants is 3200 m.sup.2/g, the average particle size is 5 m, and the mechanical strength is 95%; the adsorbents is nonpolar macroporous absorption resin of styrene polymerLX-20, the BET specific surface area is 5000 m.sup.2/g, the pore size of the mesoporous is 4 nm, and the mechanical strength is 95%; the support packings on top and bottom of the absorption tower is 3A type molecular sieves, and the total amount is all 90 kg. There are N absorbing layers and N is five; from the first layer to the fourth layer, each layer of these layers is composed of an upper liquid distributing device that connects an external feeding pipe and a lower absorbing packing layer; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the first absorbing layer is 3:1, and the desiccants represents 7% of the weight of all the desiccants in the multistage absorption tower; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of every of the second, third and fourth absorbing layers is 9:1, and the desiccants of the absorbing packing layer of every of the second, third and fourth absorbing layers represents 29% of the weight of all the desiccants in the multistage absorption tower; the weight ratio of the desiccants to the adsorbents of the absorbing packing layer of the fifth absorbing layer is 6:1, and the desiccants represents 6% of the weight of all the desiccants in the multistage absorption tower. The desiccants and the adsorbents of the absorbing packing layer of every of the absorbing layers are mixed uniformly and then are filled. The pressure drop between the external feeding pipe of the first absorbing layer of the multistage absorption tower and the outlet on the top of the tower is 20 kPa.

(22) The solvent to be refined is the waste chlorobenzene solvent generated during the overhaul of the TDI manufacturing device, the ratio of the volume flow rate of the total feed rate of the solvent to the total weight of all the desiccants and the adsorbents in the multistage absorption tower is 1:420 m.sup.3/kg/h; the feed rate of the external feeding pipe of the first absorbing layer represents of the volume of the total feed rate of the solvent to be refined, the feed rate of the external feeding pipe of every of the second and the third absorbing layers represents of the volume of the total feed rate of the solvent to be refined respectively, and the feed rate of the external feeding pipe of the fourth absorbing layer represents of the volume of the total feed rate of the solvent to be refined. The residence time of the solvent to be refined in the multistage absorption tower is 4 hrs, and the reflux ratio of the refined solvent to the solvent to be refined is 1.5.

(23) The content of water in the solvent to be refined is 260 ppm, the content of iron component is 180 ppm, the content of phosgene and hydrogen chloride is 3000 ppm, and the CoPt color number is 50; the analysis results of the refined solvent are shown in table 1. The refined solvent is reused as the TDI manufacturing solvent, the method of manufacturing is referred to the method of manufacturing TDI in the patent document CN101205199A, and the analysis results of the obtained TDI product are shown in table 1.

Comparative Example 1

(24) The waste chlorobenzene solvent is processed by a vacuum distillation column. The operation pressure of the vacuum distillation column is 40 kpa, the flux ratio is 2:1, the temperature of the bottom of the column is controlled at 100-105 C., and the temperature of the top of the column is controlled at 60-65 C.

(25) The solvent to be refined is the waste chlorobenzene solvent generated during the abnormal startup and shutdown of the MDI manufacturing device, wherein the content of water is 300 ppm, the content of iron component is 50 ppm, the content of phosgene and hydrogen chloride is 5000 ppm, and the CoPt color number is 60; the analysis results of the refined solvent are shown in table 1. The refined solvent is reused as the MDI manufacturing solvent, the method of manufacturing is referred to the method of manufacturing MDI in the patent document CN1254724A, and the analysis results of the obtained MDI product are shown in table 1.

Comparative Example 2

(26) The waste dichlorobenzene solvent is processed by azeotropic rectification tower, and the entrainer is tetrahydrofuran; the entrainer and the solvent to be refined are added to the bottom of the azeotropic rectification tower together, the weight ratio of the entrainer and the solvent to be refined is 10:1, the reflux ratio during operation is 2, the operation pressure of the azeotropic rectification tower is 101.3 kpa, the temperature of the top of the tower is controlled at 65-69 C., and the temperature of the bottom of the tower is controlled at 138-140 C.

(27) The solvent to be refined is the waste dichlorobenzene solvent generated during the operation of the solvent after the overhaul of the TDI manufacturing device, wherein the content of water is 280 ppm, the content of iron component is 45 ppm, the content of phosgene and hydrogen chloride is 4800 ppm, and the CoPt color number is 55; the analysis results of the refined solvent are shown in table 1. The refined solvent is reused as the TDI manufacturing solvent, the method of manufacturing is referred to the method of manufacturing TDI in the patent document CN101205199A, and the analysis results of the obtained TDI product are shown in table 1.

(28) TABLE-US-00001 TABLE 1 Analysis results Analysis indicators of the refined solvent The The L content content Phosgene + color of of iron hydrogen Re- of the water component chloride Po-Co covery prod- (ppm) (ppm) (ppm) color Rate uct Example 1 47 1.5 18 5 99.8% 92 Example 2 39 2.7 14 15 99.6% 98 Example 3 29 2.2 0 20 99.7% 95 Example 4 45 1.8 0 5 99.9% 90 Example 5 35 2.4 8 10 99.5% 93 Comparative 115 9.9 89 45 89% 82 example 1 Comparative 108 8.5 82 40 90% 85 example 2