METHOD FOR PRODUCING ISOCYANATES

Abstract

The present invention relates to a process for preparing an isocyanate comprising the steps of (A) reacting an amine with a stoichiometric excess of phosgene using an aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X, wherein X=1 or 2, (a) as a diluent during the reaction and/or (b) as a means for cooling the reaction mixture formed from the reaction of the amine with phosgene (so-called quench) to obtain (optionally after a decompression) a liquid product mixture comprising the isocyanate and the aromatic solvent employed and a gaseous product mixture containing phosgene and hydrogen chloride; followed by (B) isolating the isocyanate from the liquid product mixture obtained in step (A) comprising the step of a final distillation in which the isolated isocyanate is obtained as a product stream, wherein in the final distillation or in a distillation step upstream of the final distillation at least one stream comprising aromatic solvent of formula C.sub.6H.sub.6-YCl.sub.Y, wherein Y=X+1, is (at intervals or continuously) discharged such that the isolated isocyanate has a mass fraction, based on its total mass, of aromatic solvent of formula C.sub.6H.sub.6-YCl.sub.Y in the range from 0.0 ppm to 9.9 ppm, preferably 0.0 ppm to 5.0 ppm, particularly preferably 0.0 ppm to 3.0 ppm.

Claims

1. A process for preparing an isocyanate comprising: (A) reacting an amine with a stoichiometric excess of phosgene using an aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X, wherein X=1 or 2, (a) as a diluent during the reaction and/or (b) as a means for cooling the reaction mixture formed from the reaction of the amine with phosgene to obtain a liquid product mixture comprising the isocyanate, the aromatic solvent, and a gaseous product mixture containing phosgene and hydrogen chloride; and (B) isolating the isocyanate from the liquid product mixture obtained in step (A) by a process comprising a final distillation in which the isolated isocyanate is obtained as a product stream, wherein in the final distillation or in a distillation step upstream of the final distillation at least one stream comprising aromatic solvent of formula C.sub.6H.sub.6-YCl.sub.Y, wherein Y=X+1, is discharged such that the isolated isocyanate has a mass fraction, based on its total mass, of aromatic solvent of formula C.sub.6H.sub.6-YCl.sub.Y of 0.0 ppm to 9.9 ppm.

2. The process as claimed in claim 1, wherein the liquid product mixture obtained in step (A) is, in step (B), prior to the final distillation, passed through a solvent distillation for separating aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X.

3. The process as claimed in claim 1, wherein in step (B) the final distillation: (i) is performed in a dividing wall column to obtain the product stream of isocyanate in a sidestream takeoff from the dividing wall column and aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X at the top of the dividing wall column, or (ii) is performed in two serially arranged non-dividing wall distillation columns to obtain aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X at the top of the first distillation column and the product stream of isocyanate as distillate from the second distillation column.

4. The process as claimed in claim 2, wherein, in step (B), the final distillation: (i) is performed in a dividing wall column to obtain the product stream of isocyanate in a sidestream takeoff from the dividing wall column and aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X at the top of the dividing wall column, or (ii) is performed in two serially arranged non-dividing wall distillation columns to obtain aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X at the top of the first distillation column and the product stream of isocyanate as distillate from the second distillation column.

5. The process as claimed in claim 4, wherein the aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X separated in the solvent distillation is obtained in a mixture with aromatic solvent of formula C.sub.6H.sub.6-YCl.sub.Y, wherein a first portion of the mixture obtained in the solvent distillation is recycled into step (A) and a second portion of the mixture obtained in the solvent distillation is not recycled into step (A) but is discharged, or wherein the aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X separated in the solvent distillation is obtained in a mixture with aromatic solvent of formula C.sub.6H.sub.6-YCl.sub.Y, wherein a first portion of the mixture obtained in the solvent distillation is recycled into step (A) and a second portion of the mixture obtained in the solvent distillation is purified in a further distillation, wherein aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X is separated from the second portion of the mixture obtained in the solvent distillation and subsequently recycled into step (A), wherein the portion of the second portion of the mixture obtained in the solvent distillation remaining after the separation of the aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X from the second portion of the mixture obtained in the solvent distillation is discharged, or wherein the aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X separated in the solvent distillation is obtained in a mixture with aromatic solvent of formula C.sub.6H.sub.6-YCl.sub.Y, wherein the mixture obtained in the solvent distillation is purified in a further distillation, wherein aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X is separated from the mixture obtained in the solvent distillation and subsequently recycled into step (A), wherein the portion of the mixture obtained in the solvent distillation remaining after the separation of the aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X from the mixture obtained in the solvent distillation is discharged.

6. The process as claimed in claim 2, wherein the aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X separated in the solvent distillation is obtained in a mixture with aromatic solvent of formula C.sub.6H.sub.6-YCl.sub.Y, wherein a first portion of the mixture obtained in the solvent distillation is recycled into step (A) and a second portion of the mixture obtained in the solvent distillation is not recycled into step (A) but is discharged, or wherein the aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X separated in the solvent distillation is obtained in a mixture with aromatic solvent of formula C.sub.6H.sub.6-YCl.sub.Y, wherein a first portion of the mixture obtained in the solvent distillation is recycled into step (A) and a second portion of the mixture obtained in the solvent distillation is purified in a further distillation, wherein aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X is separated from the second portion of the mixture obtained in the solvent distillation and subsequently recycled into step (A), wherein the portion of the second portion of the mixture obtained in the solvent distillation remaining after the separation of the aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X from the second portion of the mixture obtained in the solvent distillation is discharged, or wherein the aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X separated in the solvent distillation is obtained in a mixture with aromatic solvent of formula C.sub.6H.sub.6-YCl.sub.Y, wherein the mixture obtained in the solvent distillation is purified in a further distillation, wherein aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X is separated from the mixture obtained in the solvent distillation and subsequently recycled into step (A), wherein the portion of the mixture obtained in the solvent distillation remaining after the separation of the aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X from the mixture obtained in the solvent distillation is discharged.

7. The process as claimed in claim 3, wherein the aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X obtained at the top of the dividing wall column or at the top of the first distillation column is obtained in a mixture with aromatic solvent of formula C.sub.6H.sub.6-YCl.sub.Y.

8. The process as claimed in claim 7, wherein the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is discharged, or wherein the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is purified in a further distillation, wherein aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X is separated from the mixture obtained at the top of the dividing wall column or at the top of the first distillation column and then recycled into step (A), wherein the portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column remaining after the separation of the aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X from the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is discharged. or wherein a first portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is recycled into step (A) and a second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is not recycled into step (A) but is discharged, or wherein a first portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is recycled into step (A) and a second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is purified in a further distillation, wherein aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X is separated from the second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column and then recycled into step (A), wherein the portion of the second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column remaining after the separation of the aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X from the second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is discharged.

9. The process as claimed in claim 7, comprising in step (B) a step of scrubbing the gaseous product mixture obtained in step (A) with aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X for separating isocyanate, wherein a first portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is recycled into the scrubbing step and a second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is not recycled into the scrubbing step but is discharged. or wherein a first portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is recycled into the scrubbing step and a second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is purified in a further distillation, wherein aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X is separated from the second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column and then recycled into the scrubbing step or into step (A), wherein the portion of the second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column remaining after the separation of the aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X from the second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is discharged, or wherein the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is purified in a further distillation, wherein aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X is separated from the mixture obtained at the top of the dividing wall column or at the top of the first distillation column and then recycled into the scrubbing step or into step (A), wherein the portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column remaining after the separation of the aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X from the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is discharged.

10. The process as claimed in claim 1, wherein pipe conduits which are used for connecting a tank container for receiving the liquid product mixture from step (A) with distillation means for performing step (B) and/or for connecting these distillation means to one another are fabricated from stainless steel of type 2.4610, 1.4529 or 1.4539.

11. The process as claimed in claim 1, wherein the final distillation is supplied with a product mixture comprising aromatic solvent of formula C.sub.6H.sub.6-XCl.sub.X in a mass fraction based on its total mass of 8% to 49%.

12. The process as claimed in claim 1, wherein the discharged stream comprising aromatic solvent of formula C.sub.6H.sub.6-YCl.sub.Y contains said solvent in a mass fraction based on its total mass of 1.0% to 10%.

Description

EXAMPLES

[0170] Reported amounts in percent and ppm are mass fractions based on the total mass of the respective material stream.

[0171] Examples 1 to 4 describe the workup (Step (B)) of a TDI product mixture obtained in a gas phase reaction (Step (A)) according to variant 4 (no separate dephosgenation but separate separation of the majority of the solvent in a solvent column upstream of the final distillation; final distillation in a dividing wall column). The solvent C.sub.6H.sub.6-XCl.sub.X employed in the gas phase reaction as a means for cooling the reaction mixture was ortho-dichlorobenzene (ODB). The ODB-containing stream distilled off overhead in the dividing wall column was, after condensation, partially recycled into the dividing wall column as reflux and partially recycled into the process (namely into the scrubbing of the gaseous reaction product remaining in step (A) after the quench).

Example 1 (Comparative, without Discharging of a Stream Comprising Aromatic Solvent of Formula C.SUB.6.H.SUB.6-Y.Cl.SUB.Y.)

[0172] The purified TDI withdrawn as a sidestream from the dividing wall column had a trichlorobenzene (TCB) concentration of 11 ppm. The low boiler stream withdrawn overhead contained 8.6% TDI, 88.6% ODB and 2.2% TCB. More highly chlorinated chlorobenzenes such as tetra-, penta- and hexachlorobenzene were not detectable with the employed gas chromatographic analytical method.

Example 2 (Inventive)

[0173] To achieve depletion of TCB, 1.9 t of the stream containing 8.6% TDI, 88.6% ODB and 2.2% TCB from the portion destined for recycling into the process were discharged and sent for external incineration on a one-off basis. This corresponds to the inventive discharging of a stream comprising aromatic solvent of formula C.sub.6H.sub.6-YCl.sub.Y. The solvent recirculation circuit was depleted in TCB by a total of 42 kg.

[0174] This one-off depletion of TCB in the overall process had the result that the TCB concentration in the TDI sidestream from the dividing wall column fell from 11 ppm to 6 ppm over 12 h.

Example 3 (Comparative, without Discharging of a Stream Comprising Aromatic Solvent of Formula C.SUB.6.H.SUB.6-Y.Cl.SUB.Y.; Process Simulation (VTPlan)

[0175] The composition of the tops stream of the TDI dividing wall column is as follows: [0176] 92.8% ODB, [0177] 5.1% TDI, [0178] 2.1% TCB.

[0179] The sidestream from the TDI dividing wall column (TDI product) contains 15 ppm TCB.

Example 4 (Inventive; Simulation as in Example 3)

[0180] 300 kg/h of the TCB-rich tops stream from the portion destined for recycling into the process are diverted and passed into a downstream distillation column. This is a packed column with 24 theoretical plates operated at 70 mbar head pressure. ODB having a TCB content below 1 ppm is separated as tops product in this downstream distillation column. The thus-purified ODB is then likewise recycled into the process. As bottoms product from this downstream distillation column, 14.5 kg/h of a TCB-rich stream comprising 3.0% TCB, 96.9% TDI and 0.1% ODB are withdrawn, discharged and sent for external waste recovery. This corresponds to the discharging of a stream comprising aromatic solvent of formula C.sub.6H.sub.6-YCl.sub.Y according to the invention.

[0181] As a result of the continuous discharge, workup and recycling of purified solvent ODB, the composition of the tops stream from the dividing wall column changes as follows: [0182] 95.3% ODB, [0183] 4.56% TDI, [0184] 0.14% TCB.

[0185] This continuous removal of trichlorobenzene from the process reduces the contamination of trichlorobenzene in the TDI product from 15 ppm previously (example 3) to below 3 ppm.