DD238042A1 - METHOD FOR CONTINUOUS PRODUCTION OF MDA / POLYAMINE MIXTURES - Google Patents

Abstract

The invention relates to a process for the preparation of high-quality aromatic isocyanates by phosgenation of amines of the diphenylmethane series, by acid condensation of aniline and formaldehyde, which are used in the production of rigid foam. The object according to the invention is achieved by continuously adding 2 equivalents of formaldehyde to the aniline hydrochloride stream by phosgenation of methylenedianilines and their higher-nuclear oligomers, which are distinguished by a low content of N-methylation products, in a continuous manner and after the first formaldehyde addition, the product stream is cooled to a temperature 50C.

Description

Explanation of the essence of the invention

The invention has for its object to develop a continuous process for the preparation of high-quality aromatic isocyanates, which is also by use of a relatively low aniline content (molar ratio aniline: formaldehyde S 2: 1) by a low content of N-methyl compounds, a high content of 4,4'-isomer, as well as a variable according to the intended use content of methylene diisocyanate and 2,4 'isomers. According to the invention the object is achieved in that the continuous production of high-quality aromatic isocyanates by phosgenation of methylenedianilines and their higher-nuclear oligomers, which are formed by condensation of aniline and formaldehyde under acidic conditions and characterized by a low content of N-methylation products is carried out in such a way in that formaldehyde is added continuously to the aniline hydrochloride stream in 2 installments.

Thereafter, a certain adiabatic increase in temperature of the reaction mixture is allowed and cooled before the second addition of formaldehyde to a temperature <50 ⁰ C. The quantitative proportion of the first formaldehyde addition is 30 to 50% of the total amount of formaldehyde.

By the method according to the invention, in contrast to the methods of the prior art, both the formation of N-methyl compounds in the acid-catalyzed aniline-formaldehyde condensation is kept very low and at the same time prevents a deposition of polymeric precondensates in the heat exchangers. After the usual complete rearrangement of present in the reaction mixture Aminobenzylanilin bonds to primary amines, the neutralization of the hydrochloric acid reaction mixture, washing and subsequent removal by distillation of aniline and residual water to obtain an MDA / polyamine mixture with a low content of N-methyl-MDA or higher-nuclear oligomers and dichin or its derivatives. The MDA / polyamine mixture thus produced can be easily, d. H. without appreciable solid deposits in an inert solvent, such as monochlorobenzene, phosgenate and has respect to the content of chlorine-containing compounds to a significantly improved quality. The process according to the invention can be used for all aniline / formaldehyde molar ratios of 1.4 to 4: 1, preferably 1.6 to 2.0: 1. The molar ratio of acid: aniline can be varied according to the desired isomer ratio between 0.3 and 0.9, wherein preferably hydrochloric acid is used as the acidic catalyst. The advantages of the method according to the invention are that

- Minimized the content of N-methylation products in the MDA / polyamine mixture and thus the isocyanate quality is substantially improved with respect to the content of chlorine-containing compounds

- Deposits are avoided both in the production of MDA / polyamine and in the isocyanate, whereby the term of a large-scale plant is increased

- By the continuous driving a good space-time yield is achieved.

The invention will be explained in more detail below with reference to two exemplary embodiments and two comparative examples.

Embodiment 1

A product stream of an aniline-containing aniline hydrochloride solution is continuously produced from 31.5% strength hydrochloric acid and water-saturated aniline, in which there is always a molar ratio of acid to aniline of 0.5 to 0.55. With a temperature of 40 ⁰ C, the aniline hydrochloride solution passes to a mixing device in which under turbulence, a homogeneous mixing with a 37% aqueous and held at a temperature of 30 ⁰ C held formaldehyde solution. The volume flows of the aniline hydrochloride solution and of the formaldehyde solution are mixed in a ratio of 3.7: 0.5 (molar ratio of aniline: formaldehyde 3.6: 1) After adiabatic reaching a reaction temperature of 70 ^° C., the mixture is immediately cooled to 40 ^° C. by means of a heat exchanger and again mixing in a mixer the reaction mixture with the remaining formalin (37% aqueous solution, 30 ° C _/ total molar ratio aniline: formaldehyde 1.8: 1) under turbulence.

The exit from the reactor 65 ° C hot reaction mixture is heated to 85 ° C and maintained at a temperature of 85 ° C to 95 ⁰ C and an average residence time of 120 min. After leaving the Endreaktionszone the product is cooled to 70 ⁰ C, neutralized with excess sodium hydroxide solution, the organic phase separated, washed with water virtually free of chloride and then removed the excess aniline and dissolved water by gentle distillation. The resulting MDA / polyamine mixture is characterized by the following analytical parameters:

MDA 3-core 4-core 5 core (determined by GPC, Angabein

51.5% 24.3% 12.2% 12.0% by weight)

MDA isomer distribution (determined by GC,

2,4 ', 4,4'-N-methyl-MDA, anabout per cent by weight)

3.4% 96.0% 0.6%

1.1% by weight of dichlorine (by UV measurement)

320 mPas viscosity / 70 ° C

This MDA / polyamine mixture is dissolved in monochlorobenzene and phosgenated. In the phosgenation, a bright chlorobenzene reaction solution is obtained which is free from deposits. After working up by distillation, the following analytical values are determined:

0.30 weight percent hydrolyzable chlorine

0.04 weight percent acidity

185mPasViskosität / 25 ° C

Embodiment 2

Embodiment 1 is repeated with the only modification that the volume flows of the aniline hydrochloride solution and the formaldehyde solution in a ratio of 3.7: 0.3 (molar ratio of aniline: formaldehyde 6: 1) are mixed. To

Cooling by means of a heat exchanger, the reaction mixture is mixed with the remaining formalin (total molar ratio of aniline: formaldehyde 1.8: 1). After the corresponding process steps, an MDA / polyamine mixture is obtained

following analysis values:

MDA 3-core 4-core 5-10 core by GPC, Angabein

52.9 24.8 11.9 10.4% by weight)

Viscosity at 7O ⁰ C.

295mPas

MDA isomer distribution (determined by GC,

2,4 ', 4,4'-N'-methyl'MDA is an amount by weight)

2.9 96.6 0.5

Dichin (by UV measurement, specified in

1.0% by weight)

After dissolution of this MDA / polyamine mixture in monochlorobenzene and phosgenation to obtain a bright solution that is free of deposits. After working up, the following characteristics are obtained in the analytical characterization:

0.28 weight percent hydrolyzable chlorine 0.04 weight percent acidity 180mPas viscosity / 25 ° C

Comparative Example 1

Analogously to the exemplary embodiments, an aqueous aniline hydrochloride stream containing 31.5% strength hydrochloric acid and water-saturated aniline is continuously produced with a molar ratio of acid to aniline of 0.50 to 0.55. This Anilinhydrochloridlösung (temperature 40 ⁰ C) is mixed by means of a mixer turbulent with a 37% aqueous formaldehyde solution (temperature 30 ⁰ C). The volume flows of the aniline hydrochloride solution and the formaldehyde solution are also mixed in a ratio of 3.7: 0.5 (molar ratio of aniline: FM A 3.6: 1). In contrast to the embodiment, the reaction mixture is not cooled after reaching the reaction temperature of 69 ⁰ C, but immediately with the remaining formalin C 37% aqueous solution, 30 ⁰ C, total molar ratio aniline: FMA 1.8: 1) in a mixing device under turbulence mixed. It turns this case a reaction temperature of 98 ⁰ C a. The product obtained at this temperature from the reactor of the initial reaction stage is treated analogously to the exemplary embodiments. An MDA / polyamine mixture with the following analytical parameters is obtained:

(determined by GPC, by weight)

(determined by GC, indication in weight percent)

(by UV measurement, in percent by weight)

The resulting MDI / polyisocyanate is characterized by the following characteristics: 0.88% by weight of hydrolyzable chlorine 0.10% by weight of acidity 175mPas viscosity / 25 ° C

Comparative Example 2

The procedure described in Comparative Example 1 is used with the modification that the formalin in three rates (each 33 Ѵз percent of the total) is added to the Anilinhydrochloridstrom without intermediate cooling. After the respective additions of formalin, the following reaction temperatures are established: 60.degree. C., 85.degree. C. and ^100.degree. C. The resulting product is treated further analogously to the exemplary embodiment and is characterized by the following analytical parameters:

MDA 3-core 4-core 5-10 core (determined by GPC, Angabein

53.4, 22.9, 13.9, 9.8 percent by weight)

Viscosity at 70 ^° C.

280mPas

MDA isomer distribution (by GC, indicated in

2,4 '4,4' МММ weight percentage)

1.4 96.1 2.4

Dichin: 3.6 (by UV measurement, Angabein

By weight)

After the phosgenation, an MDI / polyisocyanate with the following characteristics is obtained: 0.81% by weight of hydroly

Adjustable chlorine

0.09% by weight acidity

155 mPas viscosity / 25 ° C

MDA 3 core 4-core 5-10 core 52.1% 24.1% 12.0% 11.8% Viscosity at 70 ^° C. 300mPas MDA-isomer: 2,4 '4,4' МММ 3.0% 94.1% 2.9% Dichin: 4.1%

Claims (2)

Invention claim: 1. A process for the continuous production of high-quality aromatic isocyanates by phosgenation of methylenedianilines and their higher-nuclear oligomers, which are formed by condensation of aniline and formaldehyde under acidic conditions and are characterized by a low content of N-methylation products, characterized in that the addition of the Formaldehyde is carried out continuously in 2 installments and after the first addition of formaldehyde, the product stream is cooled to a temperature <50 ⁰ C. 2. The method according to item 1, characterized in that the quantitative proportion of the first addition of formaldehyde is 30 to 50% of the total amount of formaldehyde. Field of application of the invention The invention relates to a process for the continuous production of high-quality aromatic isocyanates by phosgenation of amines of the diphenylmethane series, by acid condensation of aniline and formaldehyde, which find application in the production of rigid foams. Characteristic of the known technical solutions For the preparation of di- or higher-functional polyisocyanates, it is known to use methylenedianilines and their higher-nuclear oligomers (referred to below as MDA / polyamine mixtures). Processes for the large-scale production of the MDA / polyamine mixture have long been known and z. In DD-AP 89620, DD-AP 89723, DE-PS 1643363, DE-OS 2127263 or DD-AP 119528. In general, the procedure for preparing the MDA / polyamide mixture is such that aniline is initially mixed with acidic catalyst, preferably hydrochloric acid, and then formaldehyde is added. The reacted, generally strong hydrochloric acid reaction mixture must be worked up alkaline to obtain the reaction product, washed with water and unreacted aniline and dissolved water are removed by distillation. From the MDA / polyamine mixture is formed by known methods by phosgenation in an inert solvent, such as monochlorobenzene, an MDI / polyisocyanate mixture, which worked up in a known manner and thereby can also be separated into a polyisocyanate mixture and a mixture containing preferably diisocyanate. While the production of MDA / polyamine mixtures in the batch process is relatively well controlled, there are major problems associated with transfer to the continuous production process. In the continuous reaction between aniline and formalin in the hydrochloric acid medium, the initial reaction phase is particularly critical. This is to be understood as meaning the actual condensation reaction, ie the elimination of dehydration from the formaldehyde and the primary reaction taking place on the nitrogen of the aniline and the subsequent further reaction to condensation with aminobenzylaniline bonds. The conditions in the initial phase of the aniline-formaldehyde reaction, such as temperature control, aniline-formaldehyde ratio and mixing of the components already strongly determine the quality of the isocyanates used for the synthesis of polyurethane. Thus, the preparation of the MDA / polyamine is accompanied by undesirable side reactions to form secondary and tertiary amines in the form of monomeric N-methyl-MDA or higher-nuclear oligomers and 3,4-dihydro-3-phenyl-quinazoline (dichin) or its derivatives and their extent depends on the mentioned reaction parameters. It is important to control these reactions since the by-products they produce, after phosgenation, do not give isocyanate but consistent, hard or non-removable contaminants with certain amounts of fixed chlorine. These by-products interfere z. B. the desired function of catalysts in the polyurethane or Polyisocyanuratsynthese and deteriorate the physical properties of the polyurethane product. Various process developments are known which accordingly seek to improve the initial reaction phase in continuous MDA / polyamine production. Several methods assume a thorough mixing of the reactants and a temperature control at the beginning of the reaction. For example, in DE-PS 1643363 a complete homogeneous mixing of the feed components is required before the actual condensation reaction begins. The mixing should take place at -20 ⁰ C to 40 ⁰ C in the period <2see. Subsequently, the reaction is carried out in the temperature range from 3O ⁰ C to 80 ⁰ C practically without backmixing to a mixture of defined precondensates. A disadvantage of this method, however, is the strong deposition of polymeric precondensates in the heat exchangers, and a continuous process control is thus virtually impossible. Other methods claim a better control of the initial exothermic reaction by introducing the well-blended component stream into refrigeration cycles from already further reacted product. An example of this is the DD-AP 89723. Interestingly, however, showed in own operating experience according to this principle that a catching the initial exothermicity by maintaining a short residence time between component mixing and inflow into the cooling circuit is possible, but this occurs in addition to an increased deposition tendency of Intermediates of aniline-formaldehyde condensation on a significantly increased deposition of undesirable products in the phosgenation stage, which forces the continuous process to disruptive cleaning stoppages. In other words, the well-known effect of suppressing side reactions by minimizing the temperature in the initial reaction stage (see, for example, SYSpur Reporter 19, 143-155, 1981) can be achieved in a continuous process by combining aniline (in hydrochloric acid) and formalin in the practically relevant molar ratios of aniline! formaldehyde = 2: 1 can not be realized for technological reasons, since undesired deposits occur either during the condensation process or in the phosgenation process. Object of the invention The aim of the invention is an economical process for the continuous production of high-quality aromatic isocyanates, in which the quality of the isocyanates is significantly improved by reduced formation of by-products.