METHOD OF PRODUCING DIAMINES AND POLYAMINES OF THE DIPHENYLMETHANE SERIES

Abstract

The invention relates to a method for producing diamines and polyamines of the diphenylmethane series, by condensing aniline and formaldehyde followed by an acid-catalysed rearrangement at different production capacities with alteration of the content of diamines of the diphenylmethane series (altering the binuclear content). Adapting the molar ratio of the total used aniline to the total used formaldehyde and adapting the reaction temperature allows the rearrangement reaction to be fully completed despite the change in dwell time inevitably associated with a change in production capacity, and allows the formation of undesired by-products to be avoided as far as possible; the intended modification to binuclear content is likewise achieved.

Claims

1. A process for preparing di- and polyamines of the diphenylmethane series from aniline (1) and formaldehyde (2) in a production plant (10 000), where the molar ratio of total aniline used (1) to total formaldehyde used (2), n(1)/n(2), is always not less than 1.6, comprising: (A-I) reacting aniline (1) and formaldehyde (2) in the absence of an acidic catalyst to obtain a reaction mixture (4) comprising an aminal (3), and then at least partly separating an aqueous phase (6) from the reaction mixture (4) to obtain an organic phase (5) comprising the aminal (3); (A-II) contacting the organic phase (5) which comprises the aminal obtained in step (A-I) with an acidic catalyst (7) in a reactor cascade (3000) composed of i reactors connected in series (3000-1, 3000-2, . . . , 3000-i), where i is a natural number from 2 to 10, wherein the first reactor (3000-1) in flow direction is operated at a temperature T3000-1 in the range from 25.0 C. to 65.0 C. and is charged with stream (5) and acidic catalyst (7) and optionally with further aniline (1) and/or further formaldehyde (2), and every reactor downstream in flow direction (3000-2, . . . , 3000-i) is operated at a temperature of more than 2.0 C. above T.sub.3000-1 and is charged with the reaction mixture obtained in the reactor immediately upstream; (B) isolating the di- and polyamines of the diphenylmethane series from the reaction mixture (8-i) obtained from step (A-II) in the last reactor (3000-i) by a process comprising: (B-I) adding a stoichiometric excess of base (9), based on the total amount of acidic catalyst used (7), to the reaction mixture (8-i) obtained in the last reactor (3000-i) in step (A-II) to obtain a reaction mixture (10); and (B-II) separating the reaction mixture (10) obtained in step (B-I) into an organic phase (11) comprising di- and polyamines of the diphenylmethane series and an aqueous phase (12); wherein in the event of a change in the production capacity from a starting state A with a mass flow rate ml of total aniline used in the starting state of m1(A)0, a mass flow rate m.sub.2 of total formaldehyde used in the starting state of m.sub.2(A)=X(A).Math.m.sub.2(N), where X(A) is a dimensionless number>0 and 1 and m.sub.2(N) denotes the nameplate load of the production plant (10 000), a molar ratio n(1)/n(2) of total aniline used (1) to total formaldehyde used (2) in the starting state of n(1)/n(2)(A), a molar ratio n(7)/n(1) of total acidic catalyst used to total aniline used in the starting state of n(7)/n(1)(A) and a proportion by mass .sub.MMDA, based on the total mass of di- and polyamines of the diphenylmethane series, of diamines of the diphenylmethane series of .sub.MMDA(A) to an end state E with a mass flow rate ml of total aniline used of in the end state m.sub.1(E)0, a mass flow rate m.sub.2 of total formaldehyde used in the end state of m.sub.2(E)=X(E).Math.m.sub.2(N), where X(E) is a dimensionless number>0 and 1, a molar ratio n(1)/n(2) of total aniline used (1) to total formaldehyde used (2) in the end state of n(1)/n(2)(E), a molar ratio n(7)/n(1) of total acidic catalyst used to total aniline used in the end state of n(7)/n(1)(E) and a target proportion by mass .sub.MMDA, based on the total mass of di- and polyamines of the diphenylmethane series, of diamines of the diphenylmethane series for the end state of .sub.MMDA(E) by a quantity X=IX(E)X(A)|, with X0.10, wherein the process comprises at least one change in production capacity that commences at a time t.sub.1 and concludes at a time t.sub.2, wherein .sub.MMDA is also altered in such a way that, for the target value of .sub.MMDA(E) for the end state, either1.05.Math..sub.MMDA(A) or 0.95.Math..sub.MMDA(A), and wherein:
0.90.Math.n(7)/n(1)(A)n(7)/n(1)(E)1.10.Math.n(7)/n(1)(A); characterized in that, in the period from ti to t.sub.2, the transition state T, with a molar ratio of total aniline used (1) to total formaldehyde used (2) of n(1)/n(2)(T) and a molar ratio of total acidic catalyst used to total aniline used of n(7)/n(1)(T), (i) the temperature in the first reactor (3000-1) in flow direction from step (A-II) is adjusted to a value that differs from the temperature in that reactor during the starting state A by not more than 10.0 C.; (ii-1) in the case that m.sub.2(E)>m.sub.2(A), the temperature in at least one of the reactors downstream in flow direction (3000-2, . . . , 3000-i), by comparison with the starting state A, is increased by more than 2.0 C. in such a way that the target end temperature is reached no later than at time t2, and in all reactors (3000-2, . . . , 3000-i) in which the temperature is not increased it is kept the same within a range of variation of 2.0 C.; (ii-2) in the case that m.sub.2(E)<m.sub.2(A), the temperature in at least one of the reactors downstream in flow direction (3000-2, . . . , 3000-i), by comparison with the starting state A, is lowered by more than 2.0 C. in such a way that the target end temperature is reached no later than at time t2, and in all reactors (3000-2, . . . , 3000-i) in which the temperature is not lowered it is kept the same within a range of variation of 2.0 C.; (iii-1) in the case that .sub.MMDA(E)1.05.Math..sub.MMDA(A), n(1)/n(2)(T) is adjusted such that, at time t.sub.2:
1.05.Math.n(1)/n(2)(A)n(1)/n(2)(T)2.00.Math.n(1)/n(2)(A); where it is always the case throughout the transition state prior to attainment of time t.sub.2 that:
0.80.Math.n(1)/n(2)(A)n(1)/n(2)(T)2.50.Math.n(1)/n(2)(A); (iii-2) in the case that .sub.MMDA(E)0.95.Math..sub.MMDA(A), n(1)/n(2)(T) is adjusted such that, at time t.sub.2:
0.50.Math.n(1)/n(2)(A)n(1)/n(2)(T)0.95.Math.n(1)/n(2)(A) where it is always the case throughout the transition state prior to attainment of time t.sub.2 that:
0.40.Math.n(1)/n(2)(A)n(1)/n(2)(T)2.50.Math.n(1)/n(2)(A).

2. The process of claim 1, in which the temperature in the reactors of the reactor cascade 3000 increases from reactor 3000-1 to reactor 3000-i in all states of operation (A, T, E).

3. The process of claim 1, in which it is always the case that T.sub.3000-1 is set to a value in the range from 25.0 C. to 65.0 C. and the temperature in each of the reactors downstream in flow direction (3000-2, . . . , 3000-i) is set to a value in the range from 35.0 C. to 200.0 C.

4. The process of claim 3, in which it is always the case that T.sub.3000-1 is set to a value in the range from 30.0 C. to 60.0 C. and the temperature in each of the reactors downstream in flow direction (3000-2, . . . , 3000-i) is set to a value in the range from 50.0 C. to 180.0 C.

5. The process of claim 1, in which the acidic catalyst (7) is a mineral acid.

6. The process of claim 1, in which step (B) further comprises: (B-III) washing the organic phase (11) with washing liquid (13); (B-IV) separating the mixture (14) obtained in step (B-III) into an organic phase (16) comprising di- and polyamines of the diphenylmethane series and an aqueous phase (15); and (B-V) distilling the organic phase (16) from step (B-IV) to obtain the di- and polyamines of the diphenylmethane series (18), with removal of a stream (17) comprising water and aniline.

7. The process of claim 6, additionally comprising: (C) recycling stream (17), optionally after workup, into step (A-I) and/or, if the optional addition of further aniline (1) in step (A-II) is conducted, into step (A-II).

8. The process of claim 1, in which, in the case that .sub.MMDA(E)1.05.Math..sub.MMDA(A), m.sub.2(E) is lowered relative to m.sub.2(A).

9. The process of claim 1, in which, in the case that .sub.MMDA(E)0.95.Math..sub.MMDA(A), m.sub.2(E) is increased relative to m.sub.2(A).

10. The process of claim 1, in which the molar ratio of total aniline used (1) to total formaldehyde used (2), n(1)/n(2), in all states of operation (A, T, E) is adjusted to a value of 1.6 to 20.

11. The process of claim 1, in which the values of n(1)/n(2), n(7)/n(1) and of the temperature of each reactor j of the reactor cascade (3000), T.sub.3000-j, that exist in each case at time t.sub.2 are retained for the duration of the production with the formaldehyde mass flow rate m.sub.2(E).

12. The process of claim 1, in which, throughout the transition state T prior to attainment of time t.sub.2, the following is true of the molar ratio of total acid catalyst used to total aniline used n(7)/n(1)(T):
0. 85.Math.n(7)/n(1)(A)n(7)/n(1)(T)1.15.Math.n(7)/n(1)(A).

13. The process of claim 10, in which at time t.sub.2:
n(7)/n(1)(E)=n(7)/n(1)(A).

14. The process of claim 1, in which the target end value for the molar ratio of total aniline used to total formaldehyde used, n(1)/n(2)(E) is established by continuously adjusting the value of m.sub.2 and at least one of the values of m.sub.1 and/or m.sub.7 until time t.sub.2.

15. The process of claim 1, in which the period from t.sub.1 to t.sub.2 lasts from 1.00 minute to 120 minutes.

Description

EXAMPLES

[0131] The results outlined in the examples for the bicyclic content, the isomer composition and the content of N-methyl-4,4-MDA are based on calculations. The calculations are based partly on theoretical models and partly on process data collected in real operational experiments, the statistical evaluation of which created a mathematical correlation of running parameters and result (e.g. bicyclic content). The content of N-formyl-4,4-MDA is reported on the basis of operational experience values. All percentages and ppm values reported are proportions by mass based on the total mass of the respective stream of matter. The proportions by mass in the real operational experiments that gave the basis for the theoretical model were ascertained by HPLC.

[0132] Reactor temperatures are based on the temperature of the respective process product at the exit from the reactor.

[0133] The MDA prepared, in all examples, has a residual aniline content in the range from 50 ppm to 100 ppm and a water content in the range from 200 ppm to 300 ppm.

[0134] A. Reduction in load proceeding from production with nameplate load [0135] I. Starting state: description of the conditions chosen for the preparation of MDA at nameplate load

[0136] In a continuous reaction process, 23.20 t/h of feed aniline (containing 90.0% by mass of aniline, 1) and 9.60 t/h of 32% aqueous formaldehyde solution (corresponding to a molar ratio of aniline (1):formaldehyde (2) of 2.25:1) are mixed and converted to the aminal (3) at a temperature of 90.0 C. and a pressure of 1.40 bar (absolute) in a stirred reaction tank (1000). The reaction tank is provided with a cooler having a cooling circuit pump. The reaction mixture leaving the reaction tank is guided into a phase separation apparatus (aminal separator, 2000) (step (A-I)).

[0137] After the phase separation to remove the aqueous phase (6), the organic phase (5) is admixed in a mixing nozzle with 30% aqueous hydrochloric acid (7) (protonation level 10%, i.e. 0.10 mol of HCl is added per mole of amino groups) and run into the first rearrangement reactor (3000-1). The first rearrangement reactor (called vacuum tank) is operated at 50.0 C., which is ensured by means of evaporative cooling in a reflux condenser at a pressure of 104 mbar (absolute). The reflux condenser is charged with 0.50 t/h of fresh aniline. The rearrangement reaction is conducted to completion in a reactor cascade composed of a total of seven reactors at 50.0 C. to 156.0 C. (i.e. 50.0 C. in reactor 3000-1/60.0 C. in reactor 3000-2/83.0 C. in reactor 3000-3/ 104.0 C. in reactor 3000-4/119.0 C. in reactor 3000-5/148.0 C. in reactor 3000-6/156.0 C. in reactor 3000-7) (step (A-II)).

[0138] On completion of reaction, the reaction mixture (8-i) obtained is admixed with 32% sodium hydroxide solution in a molar ratio of 1.10:1 sodium hydroxide to HCl and reacted in a stirred neutralization vessel (4000) (step (B-I)). The temperature here is 115.0 C. The absolute pressure is 1.40 bar. The neutralized reaction mixture (10) is then separated in a neutralization separator (5000) into an aqueous lower phase (12), which is guided to a wastewater collection vessel, and into an organic phase (11) (step (B-II)).

[0139] The organic upper phase (11) is guided to the washing and washed with condensate (13) in a stirred washing vessel (6000) (step (B-III)). After the washing water (15) has been separated from the biphasic mixture (14) obtained in the washing vessel (6000) in a washing water separator (7000, step (B-IV)), the crude MDA (16) thus obtained is freed of water and aniline (removed together as stream 17) by distillation, and 17.00 t/h of MDA (18) were obtained as bottom product (step (B-V)).

[0140] MDA prepared in this way has an average composition of 45.2% 4,4-MDA, 5.5% 2,4-MDA, 0.3% 2,2-MDA, i.e. a total bicyclic content of 51.0% and also 0.3% N-methyl-4,4-MDA and 0.3% N-formyl-4,4-MDA, the remainder to 100% consisting essentially of higher homologs (PMDA) and isomers thereof. [0141] II. Target end state: production at half-load

Example 1 (Comparative Example)

[0142] reduction in load of the MDA plant from nameplate load to half-load (=50% of nameplate load), where the n(1)/n(2) ratio and the n(7)/n(1) ratio are kept the same, the aniline, formalin, hydrochloric acid and sodium hydroxide solution feedstocks are reduced simultaneously and the temperatures in the vacuum tank (3000-1), the downstream reactors and the exit temperature of the crude MDA solution in the last rearrangement reactor (3000-7) remain the same

[0143] The MDA plant, as described above under A.I, is operated at a production capacity of 17.00 t/h of MDA. Now the bicyclic content is to be increased to about 62%, combined with halving of the load owing to lower product demand. For this purpose, at the same time, the feed rates of aniline and formalin to the aminal reactor (1000) are adjusted to the new production load within 120 minutes. The formalin rate is reduced to 4.80 t/h. The aniline feed rate is reduced to 11.35 t/h. At the same time, the flow rate of hydrochloric acid into the mixing nozzle in the feed to the first rearrangement reactor (3000-1) is halved. The first rearrangement reactor is still operated at 50.0 C., which is ensured by means of the evaporative cooling in the reflux condenser at 104 mbar (absolute). The reflux condenser is still charged with 0.50 t/h of fresh aniline. The rearrangement reaction is conducted to completion in a reactor cascade at 50.0 C. to 156.0 C. (50.0 C. in reactor 3000-1/60.0 C. in reactor 3000-2/83.0 C. in reactor 3000-3/104.0 C. in reactor 3000-4/119.0 C. in reactor 3000-5/148.0 C. in reactor 3000-6/156.0 C. in reactor 3000-7). On completion of reaction, the reaction mixture obtained, as described in the general conditions for preparation of MDA, is neutralized with sodium hydroxide solution, with reduction of the amount of sodium hydroxide solution within the same time window as formalin and aniline, and then worked up to give MDA (18).

[0144] 40 hours after commencement of the change in load, the MDA in the feed to the MDA tank has a composition of 44.7% 4,4-MDA, 5.2% 2,4-MDA, 0.2% 2,2-MDA, i.e. a total bicyclic content of 50.1%, and also 0.3% N-methyl-4,4-MDA and 0.3% N-formyl-4,4-MDA, the remainder to 100% consisting essentially of higher homologs (PMDA) and isomers thereof. The MDA has an elevated proportion of unwanted by-products with acridine and acridane structure. In this regard, see also section C further down. The bicyclic content is essentially the same as that in the starting state.

Example 2 (Inventive)

[0145] reduction in load of the MDA plant from nameplate load to reduced load, where the n(1)/n(2) ratio is increased, the aniline and formalin feedstocks are reduced simultaneously and where the n(7)/n(1) ratio is kept the same, hydrochloric acid and sodium hydroxide solution are reduced simultaneously with the aniline and formalin feedstocks, the temperature in the vacuum tank (3000-1) remains the same and the exit temperature of the crude MDA solution in the last rearrangement reactor (3000-7) is lowered

[0146] The MDA plant, as described above under A.I, is operated at a production capacity of 17.00 t/h of MDA. Now the bicyclic content is to be increased to about 62%, combined with a reduction in the load to slightly less than half the load in the starting state owing to lower product demand. For this purpose, at the same time, the feed rates of aniline and formalin to the aminal reactor (1000) are adjusted to the new production load within 120 minutes. The formalin rate is reduced to 4.60 t/h. The aniline feed rate is reduced to 14.60 t/h. The flow rate of hydrochloric acid into the mixing nozzle in the feed to the first rearrangement reactor is adjusted within the same period as aniline and formalin with retention of the protonation level of 10%. The first rearrangement reactor (3000-1) is still operated at 50.0 C., which is ensured by means of the evaporative cooling in the reflux condenser at 104 mbar (absolute). The reflux condenser is still charged with 0.50 t/h of fresh aniline. The rearrangement reaction is conducted to completion in a reactor cascade at 50.0 C. to 146.0 C. (50.0 C. in reactor 3000-1/60.0 C. in reactor 3000-2/81.0 C. in reactor 3000-3 /95.0 C. in reactor 3000-4/115.0 C. in reactor 3000-5/143.0 C. in reactor 3000-6/146.0 C. in reactor 3000-7) (step (A-II)). On completion of reaction, the reaction mixture obtained, as described in the general conditions for preparation of MDA, is neutralized with sodium hydroxide solution, with reduction of the amount of sodium hydroxide solution within the same time window as formalin and aniline and HCl with retention of the molar ratio of 1.10:1 sodium hydroxide solution to HCl, and then worked up to give the desired MDA type, obtaining 8.50 t/h of MDA (18) as the bottom product from the distillation.

[0147] 40 hours after commencement of the change in load, the MDA in the feed to the MDA tank has a composition of 54.3% 4,4-MDA, 7.5% 2,4-MDA, 0.3% 2,2-MDA, i.e. a total bicyclic content of 62.1% and also 0.2% N-methyl-4,4-MDA and 0.2% N-formyl-4,4-MDA, the remainder to 100% consisting essentially of higher homologs (PMDA) and isomers thereof. The product differs corresponds to the desired target product. An elevated proportion of unwanted by-products with acridine and acridane structure is not formed.

[0148] Table 1 below compares the results from section A.

TABLE-US-00001 TABLE 1 End state End state Half-load Reduced load Comparison of the Starting state Example 1 Example 2 examples from section A Nameplate load (comp.) (inv.) Aniline (90%) in reactor 1000 [t/h] 23.20 11.35 14.60 Aniline in reactor 3000-1 [t/h] 0.50 0.50 0.50 Formalin (32%) in reactor 1000 [t/h] 9.60 4.80 4.60 n(1)/n(2) 2.25 2.25 2.99 [n(1)/n(2)(T)(t = t.sub.2)]/[n(1)/n(2)(A)] 1.00 1.33 Protonation level [%] 10 10 10 n(7)/n(1) 0.10 0.10 0.10 Temp. gradient in reactor cascade 50.0 .fwdarw. 156.0 50.0 .fwdarw. 156.0 50.0 .fwdarw. 146.0 3000 [ C.] T(3000-1) [ C.] 50.0 50.0 50.0 T(3000-2) [ C.] 60.0 60.0 60.0 T(3000-3) [ C.] 83.0 83.0 81.0 T(3000-4) [ C.] 104.0 104.0 95.0 T(3000-5) [ C.] 119.0 119.0 115.0 T(3000-6) [ C.] 148.0 148.0 143.0 T(3000-7) [ C.] 156.0 156.0 146.0 Production capacity [t/h] 17.00 8.50 8.50 4,4-MDA [%] 45.2 44.7 54.3 2,4-MDA [%] 5.5 5.2 7.5 2,2-MDA [%] 0.3 0.2 0.3 N-methyl-MDA [%] 0.3 0.3 0.2 N-formyl-MDA [%] 0.3 0.3 0.2 Bicyclic content [%] 51.0 50.1 62.1 Comment Elevated As desired, proportion of product has an unwanted by- increased bicyclic products with content and a acridine and similar by-product acridane spectrum to the structure; bicyclic product in the content virtually starting state unchanged

[0149] B. Increase in Load Proceeding from Production at Half-Load [0150] I. Starting state: description of the conditions chosen for the preparation of MDA at half-load

[0151] (N.B. There are of course various options for operating a production plant 10 000 at half-load.) The conditions set in example 2 are one option; for examples 3 and 4 which follow, another option was chosen for the half-load starting state.)

[0152] The reaction is operated as described above under A.I for nameplate load with the following differences:

[0153] 11.35 t/h of feed aniline (containing 90.0% by mass of aniline);

[0154] 4.80 t/h of 32% aqueous formaldehyde solution (i.e. the molar ratio of aniline:formaldehyde is 2.25:1);

[0155] 50.0 C. in reactor 3000-1/60.0 C. in reactor 3000-2/81.0 C. in reactor 3000-3/95.0 C. in reactor 3000-4/116.0 C. in reactor 3000-5/144.0 C. in reactor 3000-6/146.0 C. in reactor 3000-7;

[0156] Bottom product of 8.50 t/h of MDA (18).

[0157] MDA prepared in this way has an average composition of 46.3% 4,4-MDA, 5.0% 2,4-MDA, 0.2% 2,2-MDA, i.e. a total bicyclic content of 51.5% and also 0.3% N-methyl-4,4-MDA and 0.3% N-formyl-4,4-MDA, the remainder to 100% consisting essentially of higher homologs (PMDA) and isomers thereof. [0158] II. Target end state: production at nameplate load

Example 3 (Comparative Example)

[0159] increase in load of the MDA plant from half-load to nameplate load (nameplate load based on an MDA having 62% bicyclic content), where the n(1)/n(2) ratio is increased and the n(7)/n(1) ratio is kept the same, the aniline, formalin, hydrochloric acid and sodium hydroxide solution feedstocks are increased simultaneously and the temperatures in the vacuum tank (3000-1), the downstream reactors and the exit temperature of the crude MDA solution in the last rearrangement reactor (3000-7) remain the same

[0160] The MDA plant, as described above under B.I, is operated at a production capacity of 8.50 t/h of MDA. Now the bicyclic content is to be increased to about 62%, combined with an increase in the load to the nameplate load possible for such a product of m.sub.2=7.20 t/h owing to higher product demand. For this purpose, at the same time, the feed rates of aniline and formalin to the aminal reactor are adjusted to the new production load within 120 minutes. The formalin rate is increased to 7.20 t/h. The aniline feed rate is increased to 23.20 t/h. The flow rate of hydrochloric acid into the mixing nozzle in the feed to the first rearrangement reactor is increased within the same period as aniline and formalin with retention of the protonation level of 10% (i.e. with retention of the n(7)/n(1) ratio). The first rearrangement reactor (3000-1) is still operated at 50.0 C., which is ensured by means of the evaporative cooling in the reflux condenser at 104 mbar (absolute). The reflux condenser is charged with 0.50 t/h of fresh aniline. The rearrangement reaction is still conducted to completion in the reactor cascade at 50.0 C. to 146 C. (50.0 C. in reactor 3000-1/60.0 C. in reactor 3000-2/82.0 C. in reactor 3000-3/94.0 C. in reactor 3000-4/116.0 C. in reactor 3000-5/144.0 C. in reactor 3000-6/146.0 C. in reactor 3000-7).

[0161] After the reaction, the reaction mixture obtained is admixed with 32% sodium hydroxide solution in a molar ratio of 1.10:1 sodium hydroxide solution to HCl and reacted in a stirred neutralization vessel, increasing the amount of sodium hydroxide solution within the same time window as formalin, aniline and HCl with retention of the molar ratios. The further workup is effected as described above under A.I. At the end of the transition state, 13.3 t/h of a bottom product are obtained.

[0162] Result: MDA thus prepared has not been completely rearranged and still contains partly rearranged products such as aminobenzylanilines, which lead to quality problems in the subsequent phosgenation to give MDI, for example distinctly elevated color values in the resulting MDI product. The MDA thus prepared, and also the resulting MDI, are off-spec product.

Example 4 (Inventive)

[0163] Increase in load of the MDA plant from half-load to nameplate load (nameplate load based on an MDA having 62% bicyclic content), where the n(1)/n(2) ratio is increased, the aniline and formalin feedstocks are increased simultaneously, and where the n(7)/n(1) ratio is kept the same, hydrochloric acid and sodium hydroxide solution are increased simultaneously, the temperature in the vacuum tank (3000-1) is kept the same and the exit temperature of the crude MDA solution in the last rearrangement reactor (3000-7) is increased

[0164] The MDA plant, as described above under B.I, is operated at a production capacity of 8.50 t/h of MDA. Now the bicyclic content of the product is to be increased to about 62%, and this altered product is to be produced with its specific maximum possible production capacity owing to higher product demand. For this purpose, at the same time, the feed rates of aniline and formalin to the aminal reactor are adjusted to the new production load within 120 minutes. The formalin rate is increased to 7.20 t/h. The aniline feed rate is increased to 23.20 t/h. The flow rate of hydrochloric acid into the mixing nozzle in the feed to the first rearrangement reactor is adjusted within the same period as aniline and formalin with retention of the protonation level of 10%. The first rearrangement reactor (3000-1) is still operated at 50.0 C., which is ensured by means of the evaporative cooling in the reflux condenser at 104 mbar (absolute). The reflux condenser is charged with 0.50 t/h of fresh aniline. The rearrangement reaction is conducted to completion in the reactor cascade at 50.0 C. to 156 C. (50.0 C. in reactor 3000-1/60.0 C. in reactor 3000-2/83.0 C. in reactor 3000-3/104.0 C. in reactor 3000-4/119.0 C. in reactor 3000-5/148.0 C. in reactor 3000-6/156.0 C. in reactor 3000-7) (step A-II)).

[0165] After the reaction, the reaction mixture obtained is admixed with 32% sodium hydroxide solution in a molar ratio of 1.10:1 sodium hydroxide solution to HCl and reacted in a stirred neutralization vessel, increasing the amount of sodium hydroxide solution within the same time window as formalin, aniline and hydrochloric acid. The temperature is 115.0 C. The absolute pressure is 1.40 bar. The further workup is effected as described further up under A.I. At the end of the transition state, the bottom product obtained is 13.30 t/h of MDA (18).

[0166] 20 hours after commencement of the change in load, the MDA in the feed to the MDA tank has a composition of 53.6% 4,4-MDA, 7.8% 2,4-MDA, 0.4% 2,2-MDA, i.e. a total bicyclic content of 61.8% and also 0.2% N-methyl-4,4-MDA and 0.3% N-formyl-4,4-MDA, the remainder to 100% consisting essentially of higher homologs (PMDA) and isomers thereof. The product differs only insignificantly from the MDA stream (18) which is obtained on average in the starting state as described in B.I. An elevated proportion of unwanted by-products with acridine and acridane structure is not formed.

[0167] Table 2 below compares the results from section B.

TABLE-US-00002 TABLE 2 End state End state Nameplate load of Nameplate load of Comparison of the Starting state the target product the target product examples from section B Half-load Example 3 (comp.) Example 4 (inv.) Aniline (90%) in reactor 1000 [t/h] 11.35 23.20 23.20 Aniline in reactor 3000-1 [t/h] 0.50 0.50 0.50 Formalin (32%) in reactor 1000 [t/h] 4.80 7.20 7.20 n(1)/n(2) 2.25 2.99 2.99 [n(1)/n(2)(T)(t = t.sub.2)]/[n(1)/n(2)(A)] 1.33 1.33 Protonation level [%] 10 10 10 n(7)/n(1) 0.10 0.10 0.10 Temp. gradient in reactor cascade 50.0 .fwdarw. 146.0 50.0 .fwdarw. 146.0 50.0 .fwdarw. 156.0 3000 [ C.] T(3000-1) [ C.] 50.0 50.0 50.0 T(3000-2) [ C.] 60.0 60.0 60.0 T(3000-3) [ C.] 81.0 82.0 83.0 T(3000-4) [ C.] 95.0 94.0 104.0 T(3000-5) [ C.] 116.0 116.0 119.0 T(3000-6) [ C.] 144.0 144.0 148.0 T(3000-7) [ C.] 146.0 146.0 156.0 Production capacity [t/h] 8.50 13.30 13.30 4,4-MDA [%] 46.3 53.6 2,4-MDA [%] 5.0 7.8 2,2-MDA [%] 0.2 0.4 N-methyl-MDA [%] 0.3 0.2 N-formyl-MDA [%] 0.3 0.3 Bicyclic content [%] 51.5 61.8 Comment MDA has not been As desired, completely product has an rearranged and still elevated bicyclic contains partly content and a rearranged similar by-product products such as spectrum to the aminobenzylanilines product in the which lead to starting state quality problems in the subsequent phosgenation to give MDI, for example distinctly elevated color values [0168] C. Fundamental experiments for formation of by-products with acridine and acridane structure

[0169] In a series of experiments, the amount of aqueous 30% hydrochloric acid required in each case to achieve the desired protonation level (see table 3 below) was added to a 2,2-MDA solution in aniline preheated to 100.0 C. The 2,2-MDA concentration in each of the individual experiments was 1.0% by mass; in addition, the solutions contained octadecane as an internal standard for gas chromatography (GC) analysis. The resulting mixture was transferred as quickly as possible by means of a peristaltic pump to a Blichi glass autoclave preheated to 120.0 C. and heated to the reaction temperature envisaged (see table 3). On attainment of the desired reaction temperature, the first sample was taken (time=zero). In order to monitor the progress of the reaction, further samples were taken after 30, 60, 120 and 240 minutes and analyzed by means of GC analysis.

[0170] Reaction conditions and experimental results are collated in table 3 below.

##STR00002##

TABLE-US-00003 TABLE 3 Laboratory experiments for formation of the acridane and acridine secondary components Sum total Reaction Protonation (acridine + temperature level Time acridane) Experiment [ C.] [%] [min] [ppm] 1 160 C. 10% 15 79 30 208 60 421 120 925 240 1775 2 170 C. 25% 0 977 30 2554 60 3950 120 5647 240 8894 3 160 C. 25% 0 364 30 930 60 1594 120 2681 240 4354 4 180 10% 0 606 30 1630 60 2613 120 3932 240 5294 5 170 10% 0 292 30 794 60 1533 120 2406 240 3932 6 160 5% 0 0 30 0 60 0 120 0 240 0 7 170 5% 0 0 30 0 60 0 120 0 240 218 8 180 5% 0 0 30 402 60 834 120 1355 240 2380

[0171] It is found that, with rising temperature and rising hydrochloric acid concentration, the formation of the acridine and acridane secondary components from 2,2-MDA occurs to an increased degree.