EP0000463A1 - Method for separating toluene diisocyanate from fabrication process residues - Google Patents

Abstract

Process for the separation without decomposition, fully continuous and automatic, of the TDI from the heavy manufacturing products, consisting in evaporating the TDI in a stirred and scraped evaporator, under a vacuum of 1 to 50 mm Hg at a temperature of 210 to 260 ° C, with a minimum residence time of 15 min in the evaporator, and to continuously extract the residues.

Description

The present invention relates to a process for the separation of toluene diisocyanate (TDI) from manufacturing residues.

During the production of TDI, 10 to 15% of heavy liquid compounds are formed. These heavy compounds are concentrated by distillation of TDI to the typical concentration of 30 to 40% TDI and 60 to 70% of heavy compounds. Due to the increased viscosity of the mixture, it is difficult to continue the distillation and obtain higher concentrations of heavy compounds.

Consequently, other techniques must be used to recover the TDI from the mixture and to collect the heavy compounds in the form of combustible residues no longer containing TDI, in order to avoid both pollution by toxic vapors and costly product losses. .

Various methods have been proposed for this operation, for example that described in American patent 3,405,040 where the mixture TDI / heavy compounds is sent in oil at 220 ° C under a pressure of 20 mm Hg. The TDI undergoes a flash evaporation and the heavy compounds precipitate in the form of solid residues which are separated from the oil by wringing and then burned.

Another process described in French patent 2,091,813 recommends the extraction of heavy compounds with a solvent, which is then recovered by distillation.

However, these methods have the drawbacks of being discontinuous and costly in terms of raw material and energy.

Other methods attempt to eliminate these drawbacks by proposing the direct separation of TDI and heavy compounds by evaporation under vacuum of TDI and direct transformation of heavy compounds into solids.

However, these methods have the drawbacks of being either discontinuous, such as the method described in US Pat. No. 3,457,291, or difficult to implement, such as the method of BF 2,320,938.

All direct separation processes have the characteristic of very quickly obtaining a thin layer of solid residues either on a hot wall or on a bed of hot solid residues so as to avoid the risks of polymerization of TDI and heavy compounds, under the effect of temperature and residence time, and the evolution of carbon dioxide or other gases resulting from the decomposition of amines reactive with TDI, thus risking the formation of foams within the mass. However, scraped thin-layer evaporation devices are expensive and very difficult to operate due to the short residence time of the products, a difficulty indicated in the previously cited BF 2,091,813.

The Applicant has developed an entirely continuous process for separating TDI from heavy materials, and for extracting these residues in the form of a solid, which has the advantage of being simple, easy to use and reliable. , without foaming or decomposition, the residues containing only low TDI contents.

The separation takes place in a stirred and scraped type evaporator, such as that described in BF 2,039,628.

The conditions of pressure, temperature and residence time have been studied so as to avoid the decomposition of TDI into gases and polymers or various products, and the formation of foam which would make the use of such devices impossible.

It is proven that from 220/250 ° C the decomposition of TDI into C0 ₂ and various gases is significant and rapid, and that the release of gas within the viscous mass causes foaming. However, the mixture must be heated enough for most of the TDI to be removed.

The Applicant has provided a solution to this problem which consists in continuously heating the TDI / heavy product mixture on a wall at a maximum temperature of 250 ° C., in evaporating the TDI at a fairly low temperature (100 to 130 ° C. under a pressure of 5 to 20 torr), stirring the mixture for a time sufficient to promote mass transfer. The mixture is gradually brought throughout the apparatus to a temperature of approximately 220 to 260 ° C., while it passes from the viscous state to the pasty state then to the solid state.

The agitation must be sufficient to ensure the heat exchange, the progression of the products in the evaporator, the scraping of the solids and the breaking of the foams possibly formed.

The volume of the device must be such that it allows sufficient residence time to ensure the exchange of materials (from 15 to 45 minutes).

The outgoing solids contain less than 3% TDI. Their temperature can then be without risk of foaming brought to a higher temperature (280 ° C) at the end of the device to evacuate the last traces of TDI.

The dry, porous and fragmented residues flow easily, either in a vacuum recipe system operating in an airlock, or better still are extruded by heated screw, continuously through a die ensuring vacuum tightness.

Solid residues in the form of powder or compact granules are taken up by pneumatic transport and sent to an incinerator, while the TDI is condensed in a conventional manner.

The process of the invention has the advantages of being able to operate continuously in an entirely automatic and to guarantee all non-pollution conditions. In addition, it requires little thermal or mechanical energy and is therefore very economical.

A device of 20 to 25 m2 and 1,600 liters, followed by an extruder, is sufficient to ensure the continuous separation of heavy and TDI from a unit of 30,000 tonnes / year of TDI.

The following examples illustrate the invention without, however, limiting it.

EXAMPLE 1.

A mixture composed of 43% TDI and 57% heavy compounds from the bottom of the concentration column is introduced into the apparatus at a rate of 60 kg / h m2. The device has a heated surface of 0.86 m2 and a total volume of 20 liters and has 2 self-cleaning stirring screws rotating at different speeds. Under a pressure of 12 mm Hg, the mixture is gradually brought to 215 ° C., and is transformed into a powder containing 2.8% of TDI after a residence time of 15 min.

The TDI is condensed and collected, and the solid residues are extruded by pressure through a die in the form of granules of 6 mm in diameter and 15 mm in length, at a rate of 30 kg / h.

EXAMPLES 2 to 4.

By operating as in Example 1, but with different feed rates and residence times, the following results were obtained:

Example 3 shows that a minimum residence time, which we have set at 15 min, is required if we want to obtain a low TDI content in the residues.

Claims (4)

1 °) Process of separation without decomposition, entirely continuous and automatic of the TDI of the heavy manufacturing products, consisting in evaporating the TDI in a stirred and scraped evaporator, at a temperature starting around 70-175 ° C, under a pressure of 1 to 50 mm Hg then brought gradually to a temperature of 210 to 250 ° C, under a pressure of 1 to 50 mm Hg, with a minimum residence time of 15 minutes in the evaporator, and to continuously extract the residues. 2 °) Method according to claim 1, wherein the pressure is preferably chosen in the range 5 to 20 mm Hg. 3 °) Method according to claim 1, wherein the heating temperature is preferably chosen in the range 220-230 ⁰ C. 4 °) A method according to claim 1, wherein the evaporation residue is removed by an extrusion system.