PRODUCTION PLANT FOR PRODUCING A CHEMICAL PRODUCT BY REACTING H-FUNCTIONAL REACTANTS WITH PHOSGENE, AND METHOD FOR OPERATING SAME

Abstract

The invention relates to a method for operating a production plant for producing a chemical product (1) by reacting a H-functional reactant (2) with phosgene (3) during an interruption in production when taking at least one plant part of the production plant out of operation, wherein low-oxygen and oxygen-rich phosgene-containing exhaust gas flows are directed separately from one another in different phosgene decomposition directions and separately from one another—at spatially different points—into a combustion device, wherein plant parts that have not been taken out of operation are operated in a closed-circuit operating mode. The invention also relates to a production plant for producing a chemical product by reacting H-functional reactants with phosgene, which is suitable for being operated with the method according to the invention.

Claims

1. A method of operating a production plant for preparation of a chemical product by reacting an H-functional reactant with phosgene in the event of a production stoppage, wherein the production plant has the following plant components: A) a reaction section suitable for reacting the H-functional reactant with phosgene, the reaction section comprising: A.I) a mixing zone for mixing the H-functional reactant and phosgene to give a reaction mixture, and A.II) a reaction zone connected to the mixing zone for reacting the reaction mixture obtained in A.I) to form a liquid phase comprising the chemical product and phosgene and a first phosgene-containing process offgas stream; B) a workup section connected to the reaction section, the workup section comprising: B.I) a separation unit for separating the liquid phase obtained in A.II) into a second phosgene-containing process offgas stream and into a liquid phase comprising the chemical product; C) an offgas workup section suitable for workup of phosgene-containing offgas streams obtained during the preparation of the chemical product and during the production stoppage, the offgas workup section comprising a first phosgene breakdown unit and a second phosgene breakdown unit, where the first phosgene breakdown unit and the second phosgene breakdown unit are configured to receive inflow of phosgene-containing offgas streams independently of one another; and D) an incineration unit (6000) suitable for incineration of the worked-up offgas obtained in the offgas workup section (3000); wherein phosgene is used in a stoichiometric excess relative to all active hydrogen atoms of the H-functional reactant during the preparation of the chemical product in the reaction section, wherein the first phosgene-containing process offgas stream obtained from A.II) in the reaction zone and the second phosgene-containing process offgas stream obtained from B.I) in the separation unit, each optionally after passing through further workup steps, are sent to the first phosgene breakdown unit, the method comprising switching off the supply of the H-functional reactant to temporarily interrupt production of the chemical product, wherein, during the temporary interruption, at least one plant component of A) and/or B) is shut down and, in at least one of the plant components that have not been shut down, an output stream therefrom is conducted (i) into the same plant component or (ii) into an upstream or downstream plant component and then, optionally via further plant components that have not been shut down, recycled into the same plant component thereby putting such plant component in a circulation mode, wherein process offgas is obtained in the at least one plant component put in circulation mode and oxygen-containing offgas is obtained in the at least one plant component that has been shut down; process offgas from the at least one plant component that was put in circulation mode is conducted into the first phosgene breakdown unit, wherein the first phosgene breakdown unit remains in operation even during the production interruption; oxygen-containing offgas from the at least one plant component that was shut down is supplied to the second phosgene breakdown unit, wherein the second phosgene breakdown unit is in operation at least during the production interruption, and process offgas that was freed of phosgene from the first phosgene breakdown unit and oxygen-containing offgas that was freed of phosgene from the second phosgene breakdown unit are supplied separately to and combusted in the incineration unit at spatially separate points.

2. The method of claim 1, in which the chemical product is an organic carbonate.

3. The method of claim 1, in which the chemical product is an isocyanate.

4. The method of claim 3, in which the separation unit from B.I) comprises: a first distillation apparatus for separating the liquid phase into a liquid stream comprising solvent and isocyanate and a third phosgene-containing process offgas stream comprising phosgene and hydrogen chloride; a second distillation apparatus for separating the liquid stream comprising solvent and isocyanate into a process offgas stream comprising solvent and a liquid stream comprising isocyanate; a third distillation apparatus for separating the process offgas stream-comprising solvent into a liquid stream comprising solvent and a fourth phosgene-containing process offgas stream.

5. The method of claim 4, in which the separation unit from B.I) also comprises an absorption apparatus in which the first phosgene-containing process offgas stream, the third phosgene-containing process offgas stream and the fourth phosgene-containing process offgas stream are cleaned by absorption in a solvent to obtain a liquid stream comprising solvent and phosgene and a gaseous process offgas stream comprising hydrogen chloride and solvent, wherein the phosgene-containing process-offgas stream and the third phosgene-containing process offgas stream are combined and the combined phosgene-containing process offgas stream and the fourth phosgene-containing process offgas stream are each condensed and then introduced in liquid form into the absorption apparatus.

6. The method of claim 5 in which the workup section from B) comprises, in addition to the separation unit from B.I), B.II) a separation unit for separation of hydrogen chloride from the second phosgene-containing process offgas stream in which the second phosgene-containing process offgas stream is depleted of hydrogen chloride, wherein a fifth gaseous phosgene-containing process offgas stream comprising solvent and any gaseous secondary components is obtained, wherein the fifth phosgene-containing process offgas stream is sent to the first phosgene breakdown unit.

7. The method of claim 6, in which the separation of the hydrogen chloride in the separation unit is performed by absorption of hydrogen chloride in water or hydrochloric acid at a concentration in the range from 0.50% by mass to 15.0% by mass, based on the total mass of the hydrochloric acid, to obtain a hydrochloric acid-containing stream in addition to the fifth phosgene-containing process offgas stream comprising solvent and any gaseous secondary components.

8. The method of claim 63, in which the workup section from B) comprises, in addition to the separation unit from B.I.) and the separation unit (2600) from B.II), B.III) a distillation unit for workup of the liquid phase comprising the isocyanate, wherein the distillation unit is optionally preceded upstream by a unit for removing polymeric isocyanate fractions.

9. The method of claim 8, wherein the unit for removing polymeric isocyanate fractions is present, in which circulation modes are established during the production interruption, the circulation modes comprising: a first circulation mode proceeding from the top of the second distillation apparatus via the third distillation apparatus back into the second distillation apparatus; a second circulation mode proceeding from the bottom of the second distillation apparatus via the unit for removing polymeric isocyanate fractions back into the second distillation apparatus; a third circulation mode proceeding from the first distillation apparatus (2100) and back thereto, and a fourth circulation mode proceeding from the separation unit for separating hydrogen chloride and back thereto, and wherein the reaction section from A) is shut down.

10. The method of claim 1, in which the production plant comprises a phosgene preparation section comprising an apparatus for preparing phosgene from carbon monoxide and chlorine, wherein the preparation of phosgene is shut down when production is interrupted, optionally with a time delay after the supply of the H-functional reactant has been switched off.

11. The method of claim 1, in which the plant components of the workup section from B) are operated at least partially at reduced pressure relative to ambient pressure, wherein the reduced pressure is generated by vacuum generation plants in which process offgas streams that are supplied to the first phosgene breakdown unit are obtained.

12. The method of claim 11, in which the first phosgene breakdown unit has at least two separately operated phosgene breakdown plant components, wherein, during the preparation of the chemical product, one of the two phosgene breakdown plant components is supplied solely with the process offgas streams from the vacuum generation plants, while the other phosgene breakdown plant component is supplied with all other process offgas streams, wherein, during a production stoppage, the vacuum generation plants are shut down and remain connected to the phosgene breakdown plant component.

13. The method of claim 1, in which, during the production of the chemical product, oxygen-containing offgas streams are obtained, wherein the second phosgene breakdown unit for oxygen-containing offgas streams is operated during the production of the chemical product and the second phosgene breakdown unit is supplied with the oxygen-containing offgas streams.

14. The method of claim 13, in which the second phosgene breakdown unit comprises at least two phosgene breakdown plant components connected in parallel, wherein one of the phosgene breakdown plant components is supplied solely with the oxygen-containing gas streams obtained during the production of the chemical product, while the other phosgene breakdown plant component is supplied with the oxygen-rich offgas streams obtained in at least one plant component shut down in the event of a production stoppage.

15. A production plant for preparation of a chemical product by reacting an H-functional reactant with phosgene, comprising the following plant components: A) a reaction section suitable for reacting the H-functional reactant with phosgene, the reaction section comprising: A.I) a mixing zone configured to mix the H-functional reactant and phosgene to give a reaction mixture, and A.II) a reaction zone connected to the mixing zone configured to react the reaction mixture to form a liquid phase comprising the chemical product and phosgene and a first phosgene-containing process offgas stream; B) a workup section connected to the reaction section and comprising: B.I) a separation unit configured to separate the liquid phase into a second phosgene-containing process offgas stream and into a liquid phase comprising the chemical product, C) an offgas workup section configured for workup of phosgene-containing offgas streams obtained during the preparation of the chemical product and during the production stoppage, comprising a first phosgene breakdown unit and a second phosgene breakdown unit, where the first and second phosgene breakdown unit are configured to receive inflow of phosgene-containing offgas streams independently of one another; D) an incineration unit configured for incineration of the worked-up offgas obtained in the offgas workup section, wherein the incineration unit is connected to the offgas workup section so that the offgas streams obtained in the first phosgene breakdown unit and in the second phosgene breakdown unit are sent separately to the incineration unit, wherein the production plant is configured such that, in the event of a production stoppage arising from shutting down the supply of the H-functional reactant and shutting down at least one plant component, an output stream from at least one plant component that has not been shut down is conducted (i) into the same plant component or (ii) into an upstream or downstream plant component and then, optionally via further plant components that have not been shut down, recycled into the same plant component thereby putting such plant component in circulation mode, wherein the production plant is further configured such that process offgas obtained in the at least one plant component put in circulation mode can be conducted into the first phosgene breakdown unit and oxygen-containing offgas obtained in the at least one plant component that has been shut down can be conducted into the second phosgene breakdown unit,  without mixing of process offgas and oxygen-containing offgas with one another.

16. The method of claim 2, in which the organic carbonate comprises a polycarbonate.

17. The method of claim 3, in which the isocyanate comprises tolylene diisocyanate or a mixture of methylene diphenylene diisocyanate and polymethylene polyphenylene polyisocyanate.

18. The method of claim 6 wherein the second phosgene-containing process offgas stream depleted of hydrogen chloride passes through a vapor condenser before the fifth gaseous phosgene-containing process offgas stream comprising solvent and any gaseous secondary components is obtained.

Description

EXAMPLES

[0145] A. General Conditions

[0146] A.I General Conditions for the Preparation of a Mixture of Methylene Diphenylene Diisocyanate and Polymethylene Polyphenylene Polyisocyanate (Collectively MDI) by Phosgenation of a Mixture of Methylene Diphenylene Diamine and Polymethylene Polyphenylene Polyamine (Collectively MDA) in Regular Operation—Cf. Also FIG. 3

[0147] The procedure is in principle as described in WO 2017/050776 A1 (pages 35 and 36), with the following differences or further details of the method regime that are not mentioned explicitly in WO 2017/050776 A1: [0148] 20.4 t/h of MDA as starting material are mixed in 55.0 t/h of monochlorobenzene (MCB) to give a 27.1% MDA solution. [0149] 100 t of phosgene solution per hour are mixed with the MDA solution. [0150] The bottom product (100) obtained is 25.45 t/h of MDI. [0151] The hydrogen chloride absorption (method stage B.II); method stage VII in WO 2017/050776 A1) is operated with the additional units (2630), (2640), (2650) and (2660) elucidated above in connection with FIG. 3. [0152] The phosgene breakdown unit consists of two phosgene destruction towers connected in series, each filled with 14 m.sup.3 of activated carbon (Norit RB4C), that are operated at a reduced pressure of 930 mbar (absolute) and in which phosgene is broken down by reaction with water (water condensation and dilute hydrochloric acid). Between the phosgene breakdown unit (3011) and the offgas incineration (6000) there is a solvent adsorption stage (3021; not shown in FIG. 3) as a further constituent of the offgas workup (C, 3000) in which residual solvent (4) is removed from the process offgas stream by adsorption on activated carbon. [0153] Offgases are sucked into the offgas workup by means of ventilators that generates a reduced pressure of 930 mbar (absolute). Such ventilators are beyond the phosgene breakdown and beyond the solvent adsorption.

[0154] A.II. General Conditions for the Preparation of Phosgene (Process Stage 0)—Cf. Also FIG. 3

[0155] The procedure is in principle as described in WO 2017/050776 A1 (pages 36 and 37; referred to in the corresponding figure as process stage IX), with the following differences or further details of the method regime that are not mentioned explicitly in WO 2017/050776 A1: [0156] Starting materials used are 4400 m.sup.3 (STP)/h of chlorine (310) and 4650 m.sup.3 (STP)/h of carbon monoxide (300). [0157] The shell and tube phosgene generators each contain 10 tonnes of activated carbon (Norit RB4C). [0158] 42.0 t/h of phosgene are conducted into the phosgene dissolution tank (1030). [0159] The top product (320) from the second phosgene liquefier, 150 m.sup.3/h of excess carbon monoxide and traces of phosgene (0.50% of the total amount of offgas), before being introduced into the offgas workup (3000), are subjected to preliminary cleaning in an absorption column (“phosgene scrubber” (4010)) which is operated with cold solvent (MCB) at −17° C., scrubbing some of the phosgene out of the gas stream. The phosgene-containing MCB solution thus obtained is sent to the phosgene absorption (2500) as liquid stream (340). [0160] What are called ammonia screw compressors are used for refrigeration in the phosgene liquefiers. In this way, the MCB solvent that finds use in the reaction and the MCB coolant with which the coolers for the condensation of the offgas streams are supplied are cooled down to −17° C.

B. Examples

Example 1 (Comparative Example): Brief Shutdown of an MDI Production Plant Comprising a Complete Shutdown of the Production Plant (Except for the Offgas Workup (C, 3000) and the Offgas Incineration (D, 6000) and Auxiliary Systems Such as the Nitrogen Supply), Performance of a Repair Measure and Restarting of the Production Plant

[0161] The production plant was operated as described under A. In a departure from FIG. 3, all of the offgases, process offgases and oxygen-rich offgases (in regular operation the latter are merely streams from sampling points that occur from time to time) were conducted through one and the same phosgene breakdown unit (3010). A leak occurred in the vapor conduit for the process offgas stream (70) from a phosgenation tower (1200) into the phosgene absorption (2500), which was fixed by proceeding as follows:

[0162] Shutdown of the Production Plant (Except for the Offgas Workup (C, 3000) and the Offgas Incineration (D, 6000) and Auxiliary Systems Such as the Nitrogen Supply) [0163] 1. The supply of MDA was stopped, which resulted directly in a drop in the temperature in the mixing unit (1100). The feeds of MCB from the tank (1040) and phosgene solution from the tank (1030) were maintained for another 3 minutes, then the two streams were likewise switched off. The reaction solution that remained in the phosgenation tower was emptied via an emergency outlet into an emergency discharge vessel (not shown in FIG. 3). The gas space in the emergency discharge vessel was connected to the phosgene absorption (B.I), 2500). The pressure in the apparatuses connected to the emergency discharge vessel fell to 980 mbar (absolute). The leak results in introduction of oxygen into the offgas which is guided through the phosgene absorption (B.I), 2500), the hydrogen chloride adsorption (B.II, 2600) into the offgas workup (C), 3000). [0164] 2. After switching off the MDA supply, the phosgene preparation (0, 4000, 4010) was stopped and all inlets and outlets into it were closed. This took 3 minutes. [0165] 3. A nitrogen conduit was run to the vapor conduit affected. The assembly took 15 minutes. The blowing-in of nitrogen reduced the oxygen content of the offgas, and the pressure rose slightly to 990 mbar (absolute). [0166] 4. The vapor conduit affected was purged with MCB with introduction of nitrogen, then adhering MCB was driven out by further introduction of nitrogen. The MCB used for purging was conducted into the emergency discharge vessel. This operation took 3 hours. [0167] 5. After the emergency shutdown of the reaction under number 1, the dephosgenation column (B.I), 2100), the phosgene absorption (B.I), 2500), the solvent distillation (B.I), 2200), the solvent purification (B.I), 2300) and the MDI distillation (B.III), 2400, 2410) were shut down, which took 10 minutes. The apparatuses mentioned were filled with solvent and MDI, except for the phosgene absorption (2500) that was filled with phosgene and MCB. The heaters of the apparatuses mentioned were switched off, such that they cooled down. The gas pathway to the phosgene absorption (B.I), 2500) remained open. [0168] 6. The hydrogen chloride absorption (B.II), 2600) remained in operation at first. The hydrogen chloride content of the gas phase (70) that was high in regular operation dropped rapidly. Within 5 minutes, the incoming amount of hydrogen chloride fell to such an extent that the hydrogen chloride absorption (B.II), 2600) formed only weak hydrochloric acid that was guided into a hydrochloric acid tank for weak hydrochloric acid (2620) that was provided for the purpose. 30 minutes after the reaction section (A), 1000) had been shut down, the hydrogen chloride absorber (2600) was shut down. The pathway to the hydrochloric acid tank (2610) was closed. The pathway to the dilute acid tank (2620) remained open. The gas pathway from the phosgene absorption (B.I), 2500) through the hydrogen chloride absorption (B.II), 2600) into the offgas workup (C), 3000) remained open. The phosgene breakdown (C, 3010) and the solvent adsorption (C), 3020) remained in operation. [0169] 7. After the hydrogen chloride absorption (B.II), 2600) had been shut down, the vacuum system of the distillation apparatuses (2200), (2300), (2410) and (2400) was shut down. Subsequently, nitrogen was added to these distillation apparatuses so as to establish ambient pressure. These operations took a total of 1 hour. [0170] 8. The refrigeration for cooling of MCB was shut down within 30 minutes. [0171] 9. The entire offgas workup (C, 3000) remained in operation. After the vacuum system had been shut down (number 7), offgas present in the phosgenation tower, in the phosgene absorption, in the hydrogen chloride absorption and in the respective gas conduits (vapor conduits) was sucked out by means of the ventilator present until a pressure of 930 mbar (absolute) was established, which took 8 hours.

[0172] Procedure for the Repair Measure

[0173] To fix the leak in the seal in the vapor conduit in question in the phosgenation tower, a seal was changed. This was done with maintenance of the reduced pressure of 930 mbar (absolute). After installation of the new seal, the vapor conduit upstream of the phosgene absorption was closed with a valve for isolation from the reduced pressure and performance of a leak test with nitrogen. Thereafter, the reaction section (A, 1000), the phosgene absorber (B.I, 2500), the hydrogen chloride absorber (B.III), 2600) and the offgas workup (C), 3000) (except for the solvent adsorption) were purged with nitrogen in order to remove the very last traces of oxygen. The time taken for the repair was 3 hours, and that for the purging with nitrogen a further 6 hours.

[0174] The change of the faulty seal took a total of 18 hours.

[0175] Restarting of the Production Plant

[0176] The procedure was as follows: [0177] 1. MCB was pumped out of the solvent tank (1040) into the emptied reaction section. After 6 hours, there was a sufficient liquid level (MCB ran out of the phosgenation tower across into the dephosgenation column). The dephosgenation column was then put into operation. [0178] 2. Refrigeration and vacuum generation were put back into operation, which took 4 hours. [0179] 3. The solvent distillation column (B.I), 2200) and the solvent purification (B.I), 2300) were put into operation successively in that sequence. [0180] 4. After starting the solvent purification (B.I), 2300), solvent from solvent tank (1040) was conducted via the reaction section (A, 1000) and the dephosgenation column (B.I), 2100) into the solvent distillation column (B.I), 2200), with the heating of the reaction zone and the dephosgenation column switched on. Then the solvent distillation was ready for operation and ran in a circuit via the solvent purification, the solvent tank, the reaction section and the dephosgenator. This operation took 8 hours. [0181] 5. With the startup of the solvent distillation and purification, a solvent-containing gaseous phase was obtained (stream 130 in regular operation), which was condensed and sent to the phosgene absorption. The phosgene-free bottom stream from the solvent purification (stream 120 in regular operation) was pumped into the solvent tank (1040). These operations took 4 hours. [0182] 6. As soon as the condensed solvent-containing gas stream arrived from the solvent purification, the cooling systems for the condensation of the process offgas streams (70) and (90) that were obtained in regular operation were made ready for use in the phosgene absorption (B.I), 2500) and the outflow of the phosgene absorption to the phosgene dissolution tank (1030) was adjusted. Subsequently, the pathway was opened from the phosgene dissolution tank (1030) to the mixing unit (1100) and the phosgenation tower (1200). [0183] 7. The phosgene production (0, 4000, 4010) was started up within 45 minutes, and the phosgene concentration in the phosgene dissolution tank (1030) was then concentrated successively to the desired value (35%) within 6 hours. [0184] 8. The solvent-filled phosgenation tower (1200) was heated up to 105° C. with the aid of a heat transfer agent. As soon as the phosgene solution concentration reached 25%, the MDA feed was opened. During the startup, a stoichiometric excess of phosgene to MDA of 140% was established; the production capacity was 15% of the desired value of 25.45 t/h of MDI. The MDA flow rate was increased to 25% of the target production capacity after 1 h. On attainment of this load, a stoichiometric excess of phosgene to MDA of 100% was established. The MDA concentration in solvent was then adjusted to 28%; the concentration of phosgene in the phosgene solution (30) had now reached 35%. [0185] 9. As soon as the first crude MDI, solvent and phosgene arrived in the dephosgenation column (B.I), 2100) from the overflow from the phosgenation tower (1200) of the reaction section, it was adjusted, at a pressure of 1.6 bar (absolute), to a target temperature in the distillation bottoms of 157° C. in the dephosgenator bottom and was thus in operation. [0186] 10. As soon as the first hydrogen chloride found its way via the phosgene absorption (B.I), 2500) into the hydrogen chloride absorption (B.II), 2600) after the start of the phosgenation, the hydrochloric acid concentration in the outflow from the hydrochloric acid absorber (2600) was adjusted to 31%. The pathway of the hydrochloric acid to the hydrochloric acid tank (2610) was opened and the pathway to the dilute acid tank (2620) was closed. Then the hydrogen chloride absorption was in operation. This operation took 2 hours and ran in parallel with the startup of the phosgenation. [0187] 11. As soon as the pressure at the top of the second solvent distillation column (B.I), 2300; for reasons of drawing simplicity, FIG. 3 shows just one column) had settled out at 70 mbar (absolute) and a bottom temperature of 120° C. had been attained, the bottoms from this second solvent distillation column were switched to the inflow of the distillation apparatus for removal of polymeric isocyanate fractions (B.I), 2410). The polymeric bottom product was pumped into an MDI product tank. The monomeric MDI that was obtained at the top of the column was purified in further columns (B.I), 2400). [0188] 12. Then the MDI plant was running at 25% of the target production capacity, and the load was gradually increased further. The target production capacity of 25.45 t/h of MDI was not established until on-spec product was obtained in the distillation (B.III), 2410, 2400). This took 12 hours.

[0189] Assessment of the Energy and Auxiliaries Required and Time Taken for the Running Down and Starting Up of the Plant Including the Fixing of the Leak:

[0190] The total time taken for the measure until the production plant was again providing on-spec product with the target capacity of 25.45 t/h of MDI was 64 hours. This reduced the production volume by 1629 tonnes of MDI. The nitrogen consumption during the repair measure (3 hours) and during the inertization of the plant with nitrogen (6 hours) was 450 m.sup.3 (STP). The natural gas consumption for the offgas incineration (D), 6000) during the measure was 8450 m.sup.3 (STP).

Example 2 (Inventive): Brief Shutdown of an MDI Production Plant, Comprising a Partial Shutdown of the Production Plant, the Establishment of Circulation Modes for Plant Components that have not been Shut Down, the Performance of a Repair Measure and Restarting of the Production Plant

[0191] The production plant was operated as described under A. As shown in FIG. 3, there were two (identically constructed) phosgene breakdown units (3011, 3012). The first phosgene breakdown unit (3011) serves exclusively to accommodate process offgas. The second phosgene breakdown unit (3012) is used solely for oxygen-containing offgas streams. There is no solvent adsorption stage connected between the second phosgene breakdown unit (3012) and the offgas incineration. In regular operation, these are merely streams from sampling points that are obtained from time to time; but the unit (3012) was permanently kept ready for operation. A pipeline grid made of electrically conductive polyethylene having closable connections for movable hoses was available in all regions of the production plant in order to conduct oxygen-rich offgases to this second phosgene breakdown unit (3012). Process offgas (210-X) from the first phosgene breakdown unit (3011) and oxygen-containing offgas (410-X) from the second phosgene breakdown unit (3012) are conducted separately from one another into the offgas incineration (D, 6000) and incinerated therein.

[0192] A leak occurred in the vapor conduit for the process offgas stream (70) from a phosgenation tower (1200) into the phosgene absorption (2500), which was fixed by proceeding as follows: [0193] 1. The gas exit from the phosgenation tower (1200), for stream (70) in regular operation, was opened via the fixedly connected pipeline grid by opening a valve to the second phosgene breakdown unit (3012). Connections from the reaction section (A, 1000) to the other parts of the production plant that are used in regular operation were closed by the closing of the appropriate valves. The pressure in the phosgenation tower dropped to 930 mbar (absolute). At the leakage site (that was between the exit from the phosgenation tower (1200) and the valve—now closed—in the conduit for stream (70) in regular operation), a hose was connected, by means of which a connection to the second phosgene breakdown unit (3012) was established; here too, a pressure of 930 mbar (absolute) was established. [0194] 2. The supply of MDA was stopped, which resulted directly in a drop in the temperature in the mixing unit (1100). The supply of MCB from the tank (1040) was maintained for another 3 minutes; phosgene solution from the tank (1030) was supplied further for 30 seconds, then both streams were likewise shut down. The reaction solution that remained in the phosgenation tower was emptied via an emergency outlet into an emergency discharge vessel (not shown in FIG. 3), which took 15 minutes. This emergency discharge vessel was connected to the first phosgene breakdown unit (3011). After the phosgenation tower had been emptied, the offgas pathway to the first phosgene breakdown unit (3011), which runs via the phosgene absorption (B.I), 2500), was closed directly beyond the gas exit from the emergency discharge vessel. The emergency discharge vessel was now no longer connected to the phosgene absorption (B.I), 2500), but was connected to the second phosgene breakdown unit (3012) via a fixedly connected offgas conduit by opening a valve, in order to remove oxygen-containing offgas (400-X) thereto (see also FIG. 2, although the emergency discharge vessel was omitted for reasons of drawing simplicity). [0195] 3. After switching off the MDA supply, the phosgene preparation (0, 4000, 4010) was stopped and all inlets and outlets into it were closed. This took 3 minutes. [0196] 4. The vapor conduit affected was purged with MCB, then adhering MCB was driven out by introduction of nitrogen. The MCB used for purging was conducted into the emergency discharge vessel. [0197] 5. After the emergency shutdown of the reaction under numeral 1, the dephosgenation column (B.I), 2100), the solvent distillation (B.I), 2200), the solvent purification (B.I), 2300), the unit for polymer removal (B.III), 2410), the distillation apparatus (B.III), 2400) and the hydrogen chloride absorption (B.II), 2600) were put into circulation mode. This was accomplished as elucidated further up with reference to figures FIG. 4a and FIG. 4b. The gaseous exit streams from the dephosgenation (90-X) and the solvent distillation (130-X) flowed here through the phosgene absorption (B-I), 2500) and thence as stream (170-X) into the hydrogen chloride absorption (B.II), 2600). The phosgene absorption (B.I), 2500) remained here at its standard operating pressure (1.6 bar (absolute)); only feeding of fresh MCB from the solvent tank (1040) to the top of the phosgene absorption was closed. By contrast with the comparative example, the vacuum system of the distillation apparatuses (2200), (2300), (2410) and (2400) was kept in operation. [0198] 6. The refrigeration for cooling of MCB was likewise kept in operation. [0199] 7. The entire offgas workup (C, 3000) remained in operation. A total of 198 minutes elapsed from initiation of the emergency shutdown until the preparation of the plant for the maintenance measure was complete.

[0200] Procedure for the Repair Measure

[0201] To fix the leak in the seal in the vapor conduit in question in the phosgenation tower, a seal was changed. This was done with maintenance of the reduced pressure of 930 mbar (absolute). After installation of the new seal, the connection to the second phosgene breakdown unit (3012) was closed for isolation from the reduced pressure and to perform a leak test with nitrogen. Thereafter, the reaction section (A, 1000) was purged with nitrogen in order to remove the very last traces of oxygen. The time taken for the repair was 3 hours, and that for the purging with nitrogen 1 hour.

[0202] The change of the faulty seal took a total of 7.3 hours.

[0203] Restarting of the Production Plant

[0204] The procedure was as follows: [0205] 1. MCB was pumped out of the solvent tank (1040) into the emptied reaction section. After 6 hours, there was a sufficient liquid level (MCB ran out of the phosgenation tower across into the dephosgenation column). The dephosgenation column was then put into operation. [0206] 2. The phosgene production (0, 4000, 4010) was started up, and the phosgene concentration in the phosgene dissolution tank (1030) was concentrated successively to the desired value (35%) within 6 hours. [0207] 3. The solvent-filled phosgenation tower (1200) was heated up to 105° C. with the aid of a heat transfer agent. As soon as the phosgene solution concentration reached 25%, the MDA feed was opened. During the startup, a stoichiometric excess of phosgene to MDA of 140% was established; the production capacity was 15% of the desired value of 25.45 t/h of MDI. The MDA flow rate was increased to 25% of the target production capacity after 1 h. On attainment of this load, a stoichiometric excess of phosgene to MDA of 100% was established. The MDA concentration in the solvent was then adjusted to 28%; the concentration of phosgene in the phosgene solution (30) had now reached 35%. [0208] 4. As soon as the first crude MDI, solvent and phosgene arrived in the dephosgenation column (B.I), 2100) from the overflow from the phosgenation tower (1200) of the reaction section, it was adjusted, at a pressure of 1.6 bar (absolute), to a target temperature in the distillation bottoms of 157° C. in the dephosgenator bottom and was thus in operation. [0209] 5. As soon as the first hydrogen chloride found its way via the phosgene absorption (B.I), 2500) into the hydrogen chloride absorption (B.II), 2600) after the start of the phosgenation, the hydrochloric acid concentration in the outflow from the hydrogen chloride absorber (2600) was adjusted to 31%. The pathway of the hydrochloric acid to the hydrochloric acid tank (2610) was opened and the pathway to the dilute acid tank (2620) was closed. Then the hydrogen chloride absorption was in operation. This operation took 2 hours and ran in parallel with the startup of the phosgenation. [0210] 6. As soon as the pressure at the top of the second solvent distillation column (B.I), 2300); for reasons of drawing simplicity, FIG. 3 shows just one column) had settled out at 70 mbar (absolute) and a bottom temperature of 120° C. had been attained, the bottoms from this second solvent distillation column were switched to the inflow of the distillation apparatus for removal of polymeric isocyanate fractions (B.I), 2410). The polymeric bottom product was pumped into an MDI product tank. The monomeric MDI that was obtained at the top of the column was purified in further columns (B.I), 2400) to give the desired composition of the isomers. [0211] 7. Then the MDI plant was running at 25% of the target production capacity, and the load was gradually increased further. The target production capacity 25.45 t/h of MDI was not established until on-spec product was obtained in the distillation (B.III), 2410, 2400). This took 8 hours.

[0212] Assessment of the Energy and Auxiliaries Required and Time Taken for the Running Down and Starting Up of the Plant Including the Fixing of the Leak:

[0213] The total time taken for the measure until the production plant was again providing on-spec product with the target capacity of 25.45 t/h was 29 hours and 18 minutes. This reduced the production volume by 746 tonnes of MDI. Nitrogen consumption during the repair measure (1 hour) and during the inertization of the plant with nitrogen (1 hour) was 100 m.sup.3 (STP). The natural gas consumption for the offgas incineration (D), 6000) during the measure was 3211 m.sup.3 (STP).

[0214] Conclusion: In inventive example 2 with two separately installed phosgene breakdown units (3011, 3012) and in circulation mode of the unaffected plant components, 61% primary energy (steam and power) and about 80% nitrogen less were consumed than in the event of a complete shutdown of the plant as in example 1 (comparative example). In addition, greatly improved productivity of the plant is found, since 908 tonnes more of MDI were producible because of the shorter time taken for the whole operation (running down, measure and startup).