Process for the continuous thermal cleavage of carbamic acid esters and preparation of isocyanates from the products thereof

A carbamic acid ester corresponding to the formula R.sup.1 --NH--CO--OR.sup.2 is thermally cleaved to form isocyanate R.sup.1 --NCO and alcohol R.sup.2 --OH fractions. This cleavage is accomplished by boiling the carbamic acid ester, condensing the vapor given off in a first fractionation column, and condensing the vapor from the first fractionation column in a second fractionation column. The boiling of the carbamic acid ester is carried out in a manner such that the average dwell time in the reaction vessel is from 1 to 20 hours, the temperature is from 160.degree. to 260.degree. C. and the pressure is from 0.001 to 2 bar. The isocyanate fraction obtained by this cleavage process may be used as a starting material for a transurethanation reaction in which a lower boiling isocyanate R.sup.3 --NCO is produced. The radicals R.sup.1, R.sup.2, and R.sup.3 are defined herein.

BACKGROUND OF THE INVENTION 
This invention relates to an improved process for the continuous thermal 
cleavage of N-monosubstituted carbamic acid alkyl esters and a process for 
the preparation of isocyanates which at normal pressure have a boiling 
point at least 50.degree. C. below that of the isocyanate obtained from 
the thermal cleavage. 
The thermal cleavage of N-monosubstituted carbamic acid alkyl esters has 
long been known. As demonstrated by the work of A. W. Hoffmann (Berichte 
der Deutschen Chemischen Gesellschaft, Year 1870, page 653 et seq) and M. 
Metayer (Bull. Soc. Chim. France, Year 1951, page 802 et seq.), these 
cleavage reactions are reversible, i.e. when the hot reaction mixtures 
cool, the isocyanates recombine with the alcohols to form carbamic acid 
esters. Special measures are therefore required if the isocyanates and 
alcohols obtained from the thermal cleavage of carbamic acid esters are to 
be recovered separately. 
U.S. Pat. No. 2,409,712 describes a process in which the recombination of 
isocyanates and alcohols after the thermal cleavage of carbamic acid 
esters is prevented by immediate separation of the cleavage products. Such 
separation may be accomplished by introduction of the gases of thermolysis 
into a cyclohexane-water mixture or by rapid distillation. Although this 
process is suitable for the discontinuous preparation of isocyanates on a 
laboratory scale, it is not suitable for a commercial process because 
immediate separation of the cleavage products is extremely difficult from 
a technical standpoint. Moreover, the process described in this patent 
provides only moderate yields of isocyanate, as can be seen from the 
examples given therein. 
It is also known that when N-monosubstituted carbamic acid esters are 
subjected to heat, they may undergo partial or complete irreversible 
decomposition. As the investigations of H. Schiff (Berichte der Deutschen 
Chemischen Gesellschaft, Year 1870, page 649 et seq.) and of E. Dyer and 
G. C. Wright (J. Amer. Chem. Soc. Volume 81, Year 1959, page 2138 et seq.) 
have shown the decomposition products may include substituted ureas, 
biurets, carbodiimides, isocyanurates, secondary amines, olefines and/or 
carbon dioxide. These decomposition reactions not only reduce the 
isocyanate yield but may also interfere with processing equipment. For 
example, difficulty soluble ureas or isocyanaurates may cause blockages in 
pipes. Carbon dioxide and gaseous olefins may heavily charge the 
distillation columns with gas. Lastly, basic materials which form as 
by-products may catalyze irreversible decomposition reactions of carbamic 
acid esters. 
Various processes have been developed in an effort to suppress the 
decompositions which accompany thermal cleavage. One approach is to reduce 
the amount of heat used in the cleavage reaction. Such processes are, 
however, disadvantageous in that the thermal cleavage must generally be 
carried out in the presence of a catalyst since the volume/time yields 
would otherwise be too low. In any event, the cleavage of carbamic acid 
esters into isocyanates and alcohols is by its nature a process in which 
the application of at least a minimum amount of heat is unavoidable, 
whether catalysts are used or not. 
Processes for the preparation of isocyanates by thermal cleavage of 
carbamic acid esters in the presence of basic catalysts have been 
described in U.S. Pat. Nos. 2,713,591; 2,692,275 and 2,727,020 and in 
Japanese Patent Application No. 54-88201 (1979). Use of basic catalysts 
may, however, lead to increased irreversible decomposition reactions of 
carbamic acid esters. (See e.g., J. Appl. Polym. Sci., Volume 16, Year 
1972, page 1213). Processes using basic catalysts can therefore result in 
acceptable isocyanate yields only if the carbamic acid esters used are 
protected against decomposition by means of suitable substituents. 
Another possible method for suppressing side reactions in the thermal 
cleavage of carbamic acid esters is dilution of the carbamic acid esters 
and/or the gases of thermolysis with inert diluents. In the processes 
described in U.S. Pat. No. 3,919,279, German Offenlegungsschrift No. 
2,635,490 and Japanese Patent Applications 54-39002 (1979) and 54-88222 
(1979), thermal cleavage of carbamic acid esters is carried out in inert 
solvents, optionally in the presence of certain catalysts. In the 
processes described in German Auslegeschriften Nos. 2,421,503 and 
2,526,193, carrier gases are used in addition to inert solvents, 
optionally in the form of vaporized low boiling solvents. 
The use of solvents in the thermal cleavage of carbamic acid esters does, 
however, present serious difficulties. The solvent must be stable under 
the conditions of thermolysis and it must also be inert with respect to 
isocyanates. The solvent must also be readily miscible with carbamic acid 
esters and have a vapor pressure at the temperatures employed low enough 
that it will remain substantially in the liquid phase during thermolysis. 
These requirements severely limit the choice of solvents. Suitable 
inexpensive solvents are difficult to find, particularly for the cleavage 
of carbamic acid esters which have a high molecular weight. Moreover, the 
use of solvents reduces the volume/time yields of isocyanates. Yet another 
disadvantage is that when high boiling solvents are used, it is difficult 
to separate the pure components (residues of isocyanate and carbamic acid 
ester, and solvent) from the residue in the liquid reaction mixtures by 
distillation. (See e.g. German Auslegeschrift No. 2,530,001). Further, the 
working-up and storing of inert diluents entails considerable additional 
capital expenditure. 
U.S. Pat. Nos. 3,734,941 and 3,870,739 describe processes in which carbamic 
acid esters are split at high temperatures (400.degree. to 600.degree. C. 
and 350.degree. to 550.degree. C.) in the gaseous phase. One disadvantage 
of such a process is that the dwell times of the gases in the high 
temperature range must be short to avoid extensive decomposition of the 
carbamic acid esters and/or the isocyanates formed due to the high 
temperature which would otherwise occur in spite of the dilution by the 
gaseous phase. Short dwell times, however, result in correspondingly low 
yields of isocyanates. Moreover, this process entails considerable 
technical difficulty since gases are difficult to heat and cool within a 
short time due to their low thermal conductivity. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a process for the 
thermal cleavage of carbamic acid esters. 
It is another object of the present invention to provide a process for the 
thermal cleavage of carbamic acid esters and for the separation of the 
thus-produced fractions. 
It is also an object of the present invention to provide a continuous 
process for the thermal cleavage of carbamic acid esters into an 
isocyanate and an alcohol fraction and for the separation of these 
fractions. 
It is a further object of the present invention to provide a technically 
practical process for the thermal cleavage of N-monosubstituted carbamic 
acid alkyl esters into fractions containing isocyanate and alcohol which 
process does not require use of a solvent, of a catalyst or of extremely 
high temperatures. 
It is yet another object of the present invention to provide a process in 
which the isocyanate fraction obtained from the thermal cleavage of a 
carbamic acid ester may be used to produce another isocyanate having a 
boiling point at least 50.degree. C. below that of the isocyanate produced 
by thermal cleavage. 
These and other objects which will be apparent to those skilled in the art 
are accomplished by a thermal cleavage process in which a carbamic acid 
ester is continuously introduced into a reaction vessel in which the ester 
is boiled to partially cleave that ester into isocyanate and alcohol. 
The gas given off from the reaction vessel is then partially condensed in a 
first fractionation column. The gaseous product from this first 
fractionation column is then partially condensed in a second fractionation 
column. The higher boiling of the isocyanate and alcohol fractions is 
condensed in the second fractionation column while the lower boiling 
fraction is given off as a gas. The thus-produced isocyanate fraction may 
then be used to produce a lower boiling isocyanate.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention thus relates to a process for the continuous thermal 
cleavage of carbamic acid esters which have a boiling point (at 
atmospheric pressure) of at least 200.degree. C., corresponding to the 
general formula 
EQU R.sup.1 --NH--CO--OR.sup.2 
wherein 
R.sup.1 represents an aliphatic hydrocarbon group having a total of from 1 
to 18 carbon atoms, which may be olefinically unsaturated and/or may carry 
inert substituents; a cycloaliphatic hydrocarbon group having a total of 
from 3 to 18 carbon atoms which may be olefinically unsaturated and/or may 
carry inert substituents; an araliphatic hydrocarbon group having 7 to 18 
carbon atoms which may carry inert substituents; or an aromatic 
hydrocarbon group having from 6 to 18 carbon atoms which may carry inert 
substituents; and 
R.sup.1 represents a group such as may be obtained by removal of the 
hydroxyl group from a primary or secondary aliphatic, cycloaliphatic or 
araliphatic alcohol whose boiling point (at atmospheric pressure) is at 
least 50.degree. C. below or above the boiling point of the isocyanate 
R.sup.1 --NCO corresponding to the group R.sup.1. 
The invention also relates to the separation of the cleavage products by 
distillation into a fraction containing an isocyanate of the formula 
R.sup.1 --NCO and a fraction containing an alcohol of the formula R.sup.2 
--OH. 
In the process of the present invention, a carbamic acid ester which is to 
be split is continuously introduced into a reaction vessel equipped with a 
fractionation column. The ester in the reaction vessel is kept boiling for 
a period such that the average dwell time is from 1 to 20 hours at a 
temperature within the range of 160.degree. to 260.degree. C. and a 
pressure of from 0.001 to 2 bar. Under these conditions, the carbamic acid 
ester undergoes partial cleavage and a product mixture containing carbamic 
acid ester, isocyanate and alcohol is continuously evaporated. The 
thus-produced vapor is then partially condensed in a first fractionation 
column. The condensate from this column will generally contain substantial 
amounts of undecomposed carbamic acid ester which are recycled to the 
reaction vessel. The gaseous product mixture escaping above the first 
fractionation column is then partially condensed in a second fractionation 
column to form a condensate containing (i) residues of carbamic acid ester 
and (ii) either the isocyanate R.sup.1 --NCO boiling at a higher 
temperature than the alcohol R.sup.2 --OH or the alcohol boiling at a 
higher temperature than the isocyanate. The lower boiling of the alcohol 
and the isocyanate escapes from the second fractionation column in gaseous 
form and may be in admixture with small portions of carbamic acid ester. 
The present invention also relates to a process for the production of a 
monoisocyanate corresponding to the formula R.sup.3 --NCO from the mixture 
containing isocyanate of the formula R.sup.1 --NCO and carbamic acid ester 
of the formula R.sup.1 --NH--CO--OR.sup.2 obtained as condensate of the 
second fractionation column in the above-described cleavage process and 
from a carbamic acid ester of the general formula R.sup.3 
--NH--CO--OR.sup.2. The monoisocyanate R.sup.3 --NCO has a boiling point 
(at atmospheric pressure) at least 50.degree. C. below the boiling point 
of the isocyanate R.sup.1 --NCO and the radical R.sup.3 has the meaning 
indicated above for R.sup.1 except for this restriction with respect to 
boiling point. Specifically, the condensate from the second fractionation 
column in the above-described cleavage process and the carbamic acid ester 
of the formula R.sup.3 --NH--CO--OR.sup.2 are continuously reacted in a 
molar ratio of carbamic acid ester R.sup.3 --NH--CO--OR.sup.2 to 
isocyanate R.sup.1 --NCO of from 1:1 to 1:10 in a reaction vessel or a 
series of reaction vessels. This reaction is carried out at a temperature 
of from 50.degree. to 200.degree. C. The pressure is adjusted so that the 
reaction mixture boils. The gaseous product mixture given off during this 
boiling includes isocyanate R.sup.3 --NCO and may also have small amounts 
of the isocyanate R.sup.1 --NCO and of the carbamic acid ester R.sup.3 
--NH--CO--OR.sup.2 present. This gaseous product mixture is continuously 
removed from the reaction vessel or vessels and the isocyanate R.sup.3 
--NCO may be separated from this product mixture in virtually pure form by 
distillation. Any distillation residue may then be returned to the 
reaction vessel or vessels. A liquid product mixture enriched in carbamic 
acid ester of the formula R.sup.1 --NH--CO--OR.sup.2 may be continuously 
removed from the reaction vessel or from the last reaction vessel of the 
series and recycled to a reaction vessel in which that ester is subjected 
to thermal cleavage. Prior to introduction to the vessel in which the 
thermal cleavage is carried out, it is advantageous to subject the 
carbamic acid ester containing mixtures to a stripping distillation in 
which the isocyanate R.sup.1 --NCO present therein is partially or 
completely separated and the carbamic acid ester R.sup.3 
--NH--CO--OR.sup.2 present is separated. The thus-separated isocyanate and 
carbamic acid ester may then be returned to the reaction vessel or vessels 
in which the isocyanate R.sup.3 --NCO is formed. The carbamic acid esters 
which may be used as starting materials for the process according to the 
invention have a boiling point (at atmospheric pressure) of at least 
200.degree. C. and correspond to the general formula 
EQU R.sup.1 --NH--CO--OR.sup.2 
wherein 
R.sup.1 and R.sup.2 have the meaning indicated above. 
Particularly suitable carbamic acid esters are those corresponding to the 
above formula in which the group R.sup.2 is the residue of an alcohol 
which has a boiling point at normal pressure at least 70.degree. C. above 
or below the boiling point of the isocyanate R.sup.1 --NCO. In the process 
of the present invention, those carbamic acid esters corresponding to the 
above formula in which the boiling points of the cleavage products R.sup.1 
--NCO and R.sup.2 --OH differ from each other by at least 50.degree. C., 
(preferably at least 70.degree. C. at normal pressure) should be used. 
Particularly preferred carbamic acid esters for the process according to 
the invention are those corresponding to the above general formula in 
which the hydrocarbon group R.sup.2 contains from 1 to 6 carbon atoms if 
the hydrocarbon group R.sup.1 contains from 6 to 18 carbon atoms and those 
in which the group R.sup.2 contains from 6 to 14 carbon atoms when the 
group R.sup.1 contains from 1 to 5 carbon atoms. 
Suitable carbamic acid esters which may be used as starting compounds for 
the process of the present invention include: N-methylcarbamic acid-hexyl 
ester, -(1-methyl-pentyl)-ester, -(2-ethylbutyl)-ester, 
-2(2-isopropoxy-ethyl)-ester; N-ethylcarbamic acid-hexylester, 
-cyclohexylester, -(1-methyl-pentyl)-ester, -(2-butoxy-ethyl)-ester; 
N-propylcarbamic acid -heptyl ester, -(1-methylheptyl)-ester, 
-(2-ethyl-hexyl)-ester, -(2-acetoxyethyl)-ester; N-isopropylcarbamic 
acid-hexylester, -(2-butoxy-ethyl)-ester, -heptylester, 
-(2-ethylhexyl)-ester; N-(2-methoxy-ethyl)-carbamic acid-octylester, 
-(2-(2-ethoxy-ethoxy)-ethyl)-ester, -(2-phenyl-ethyl)-ester, -decylester; 
N-(2-cyanoethyl)-carbamic acid-ethylester, -propylester, -butylester, 
-(2-methoxy-ethyl)-ester; N-butylcarbamic acid-octyl ester, 
-(2-(2-methoxy-ethoxy)ethyl)-ester, -(2-(2-ethoxy-ethoxy)-ethyl)-ester, 
-(2-phenyl-ethyl)-ester; N-tert.-butylcarbamic acid-hexylester, 
-cyclohexylester, -(2-ethyl-butyl)ester, -(2-acetoxy-ethyl)-ester; 
N-pentylcarbamic acid-methyl ester, -(2-(2-ethoxy-ethoxy)-ethyl)ester, 
-(2-phenyl-ethyl)-ester, -decylester; N-neopentylcarbamic acid-methyl 
ester, -(2-ethylhexyl)-ester, -octylester, -(2-phenyl-ethyl)-ester; 
N-hexylcarbamic acid-methyl ester, -ethylester, -isopropylester, 
-decylester; N-(2-ethyl-hexyl)carbamic acid-ethyl ester, -propylester, 
-isopropylester, -(2-methyl-propyl)-ester; N-octylcarbamic acid-methyl 
ester, -isopropylester, -(1-methylpropyl)-ester, -butylester; 
N-heptadecylcarbamic acid-ethylester, -isopropylester, -butylester, 
-(2-ethoxy-ethyl)-ester; N-allylcarbamic acid-cyclohexyl ester, 
-(2-butoxy-ethyl)-ester, -(1-methylheptyl)-ester, -(2-ethyl-hexyl)-ester; 
N-(3-methylallyl)-carbamic acid-(2-butoxy-ethyl)-ester, -heptylester, 
-(2-ethylhexyl)-ester, -octylester; N-cyclopentylcarbamic acid-methyl 
ester, -ethylester, -(2-phenyl-ethyl)-ester, -decylester; 
N-cyclohexylcarbamic acid-methylester, -ethylester, -isopropylester, 
-(2-methyl-propyl)-ester; N-(cyclohexyl-cyclohexyl)-carbamic 
acid-ethylester, -isopropylester, -butylester, -(2-ethoxyethyl)-ester; 
N-(2-methylhex-1-enyl)-carbamic acid methyl ester, -ethylester, 
-propylester, -(1-methyl-propyl)-ester; N-benzylcarbamic acid-methyl 
ester, -ethylester, -propylester, -isopropylester; 
N-(2-phenyl-ethyl)-carbamic acid-ethylester, -butylester, 
-(2-methoxy-ethyl)ester, -(3-methylbutyl)-ester; N-phenylcarbamic 
acid-methylester, -ethylester, -propylester, -isopropylester; 
N-(4-chlorophenyl)-carbamic acid-ethylester, -propylester, -butylester, 
-(2-methoxy-ethyl)ester; N-(3,4-dichloro-phenyl)carbamic acid-ethylester, 
-butylester, -(2-methyl-propyl)-ester, -(2-ethoxy-ethyl)-ester; 
N-3-tolyl-carbamic acid-methylester, -ethylester; -isopropylester, 
-(2-methylpropyl)-ester; N-(3-chloro-4-methyl-phenyl)-carbamic 
acid-ethylester, -butylester, -(2-methoxy-ethyl)ester, 
-(3-methyl-butyl)-ester; N-(4-cyclohexylphenyl)-carbamic acid-ethylester, 
-butylester, -pentylester, -(2-ethoxy-ethyl)-ester; 
N-(3-trifluoromethyl-phenyl)-carbamic acid-methylester, -ethylester, 
-propylester, -isopropylester; N-(4-benzyl-phenyl)-carbamic 
acid-ethylester, -butylester, -(2-ethoxy-ethyl)-ester, -hexylester; 
N-(3-cyanophenyl)-carbamic acid-methylester, -isopropylester, 
-(2-methoxy-ethyl)-ester, -pentylester; 
N-(4-methoxycarbonyl-phenyl)-carbamic acid-ethylester, -propylester, 
-butylester, -(2-ethoxy-ethyl)-ester, N-1-naphthyl-carbamic 
acid-methylester, -(2-methyl-propyl)-ester, -pentylester, -hexylester. 
The carbamic acid esters which are suitable starting compounds for the 
process according to the invention may be prepared by known chemical 
methods. Such known methods include: (1) reaction of the corresponding 
primary amines with chloroformic acid ester; (2) carbonylation of the 
corresponding nitro compounds in the presence of alcohols; and (3) the 
reaction of N,N'-disubstituted ureas with alcohols. The carbamic acid 
esters may, of course, also be prepared by any other method desired. 
The process of the present invention is particularly advantageous in that 
thermal cleavage results in optimum yields of the cleavage products (i.e., 
isocyanate and alcohol) and minimum quantities of by-products if the 
carbamic acid esters are continuously fed into a reaction vessel, heated 
to the cleavage temperature in this vessel for comparatively long dwell 
times, and care is taken to ensure that by adjustment of a suitable 
pressure the cleavage products are continuously removed from the reaction 
mixture in the gaseous form together with any undecomposed carbamic acid 
ester. To achieve maximum yields, this gaseous product mixture should be 
partially condensed in a fractionation column in a manner such that the 
condensate discharged from the column and returned to the reaction vessel 
consists substantially of undecomposed carbamic acid ester and that the 
gaseous product mixture escaping at the head of this fractionation column 
is partially condensed in a second fractionation column in a manner such 
that the condensate obtained from it is a mixture of residues of carbamic 
acid ester and the higher boiling of the isocyanate and alcohol fractions. 
The alcohol or isocyanate fraction which boils at the lower temperature 
escapes in gaseous form from the head of the second fractionation column. 
It must be regarded as extremely surprising that both the thermal cleavage 
of carbamic acid esters and the subsequent separation of the cleavage 
products (i.e., isocyanate and alcohol) by the process of the present 
invention can be carried out with high yields and minimal formation of 
by-products. Such results are particularly surprising in view of Examples 
2, 12 and 13 (Comparison Examples) of German Auslegeschrift No. 2,421,503 
which show that the heating of N-monosubstituted carbamic acid alkyl 
esters to 200.degree.-260.degree. C. for 3 hours or for one hour, 
respectively, results in the formation of substantial amounts of unusable 
by-products. 
The effectiveness with which the gaseous mixtures containing carbamic acid 
ester, isocyanate and alcohol can be separated in the process of the 
present invention is also surprising. This is evidenced by the fact that 
when one uses an efficient distillation column (optionally with means for 
removal by a side stream) instead of the two fractionation columns to 
separate the products, a far smaller quantity of cleavage products is 
obtained (sometimes none at all) and the product obtained consists 
completely or virtually completely of carbamic acid ester. 
The process of the present invention is described in more detail with 
reference to FIG. 1. 
FIG. 1 illustrates one example of an apparatus suitable for carrying out 
the thermal cleavage process of the present invention. The process of the 
present invention is not, however, limited to use of the apparatus 
illustrated in FIG. 1. 
In FIG. 1, A denotes a reaction vessel equipped with heating jacket, B and 
C each represent cooling coils used as fractionation columns. When the 
process of the present invention is carried out in the apparatus 
illustrated in FIG. 1, the carbamic acid ester is continuously fed into 
the reaction vessel A through pipe (101) and heated therein. A mixture 
containing carbamic acid ester, isocyanate and alcohol is continuously 
removed in gaseous form from the reaction vessel A through the pipe (102), 
to the fractionation column B in which the mixture is partially condensed. 
The condensate, consisting mainly of carbamic acid ester, is returned to 
the reaction vessel A through the pipe (103). The gaseous product mixture 
escaping from the head of the fractionation column B through pipe (104) is 
introduced into the fractionation column C in which that mixture is 
partially condensed. A condensate consisting substantially of residues of 
carbamic acid ester and of isocyanate boiling at a higher temperature than 
the alcohol or of alcohol boiling at a higher temperature than the 
isocyanate is continuously removed through pipe (105). The gaseous product 
escaping at the head of the fractionation column C through pipe (106) 
consists substantially of alcohol boiling at a lower temperature than the 
isocyanate or of isocyanate boiling at a lower temperature than the 
alcohol, optionally mixed with proportionally small amounts of carbamic 
acid ester. 
It is not an essential feature of the process according to the invention 
that the two fractionation columns shown in FIG. 1 be separated and 
connected through a pipe. It may even be advantageous to arrange them in a 
single apparatus one above the other, the condensate from the upper 
fractionation column being advantageously collected on a tray situated 
between the two fractionation columns. 
Nor is it important for the process of the present invention that the 
gaseous and liquid streams of product should flow in two separate pipes 
from reaction vessel A to fractionation column B and conversely. The two 
product streams may also be passed through a single pipe of suitably large 
cross-section. The fractionation column B could, of course, be directly 
attached to the reaction vessel A so that no pipes need be provided to 
connect the two apparatuses. 
When carrying out the process according to the present invention, the 
temperature of the reaction mixture in reaction vessel A should generally 
be from 160.degree. to 260.degree. C., preferably from 180.degree. to 
240.degree. C. It is advantageous to adjust the reaction temperature to 
obtain maximum volume/time yields of cleavage products with minimum 
formation of unusable by-products. This optimum reaction temperature 
varies for different carbamic acid esters. This optimum temperature which 
depends on the nature of the groups R.sup.1 and R.sup.2, may be readily 
determined in each case by techniques known to those in the art. The 
optimum reaction temperature also depends upon the nature and quantity of 
the catalyst and/or stabilizers added. Cleavage of the carbamic acid 
esters may, of course, also be carried out at a temperature other than the 
optimum reaction temperature within the temperature range indicated above. 
When carrying out the process of the present invention, the pressure in the 
reaction vessel A may be adjusted so that the reaction mixture boils. This 
pressure is from 0.001 to 2 bar, preferably from 0.01 to 1 bar. The amount 
of pressure is dependent upon the reaction temperature, the vapor pressure 
of the carbamic acid ester to be split, and the cleavage products (i.e., 
isocyanate and alcohol). 
The pressure in the fractionation columns should generally be as high as or 
slightly lower (due to pressure loss in the pipes and apparatus) than 
reaction vessel A. If desired, however, the fractionation columns B and C 
may be adjusted to a lower pressure than that of reaction vessel A. 
When carrying out the process of the present invention, the average dwell 
time in reaction vessel A of the carbamic acid ester being split is from 1 
to 20 hours, preferably from 3 to 10 hours. The average dwell time may be 
adjusted to different values within certain limits, but the resulting rate 
of cleavage is then altered correspondingly. The dwell time should 
preferably be chosen so that maximum volume/time yields of cleavage 
products, isocyanate and alcohol, are achieved with minimum formation of 
unusable by-products. This optimum dwell time depends upon the groups 
R.sup.1 and R.sup.2 of the carbamic acid ester which is to be split, the 
reaction temperature, and the nature and quantity of any catalyst and/or 
stabilizer added. This optimum dwell time, like the optimum reaction 
temperature, may be determined for each carbamic acid ester by techniques 
known to those in the art. The thermal cleavage of carbamic acid esters by 
the process according to the invention may, of course, also be carried out 
within the range indicated in a manner such that the average dwell time is 
other than the optimum. 
It is not essential to the process of the present invention that a 
proportionately large quantity of undecomposed carbamic acid ester should 
be removed in gaseous form from the reaction vessel A, condensed in the 
first fractionation column and returned to reaction vessel A. In fact, it 
is generally advantageous to keep the quantity of undecomposed carbamic 
acid ester recirculated as low as possible since recirculation entails 
expenditure of significant amounts of energy. The only essential condition 
with respect to the amount of carbamic acid ester employed is that the 
quantity of gaseous product mixture removed from the reaction vessel A 
should be sufficient to ensure that at least a small proportion of this 
gaseous mixture should be able to condense in fractionation column B to 
form a liquid containing substantial amounts of carbamic acid ester. The 
quantity of condensate formed in fractionation column B is generally from 
5 to 80 wt. %, preferably from 10 to 50 wt. % (based on the total quantity 
of vapors leaving the reaction vessel A, but not including vapors of any 
high boiling auxiliary solvents used and condensed in fractionation column 
B). The quantity of condensate formed may easily be adjusted within the 
ranges mentioned above by using an appropriate temperature and pressure 
(i.e., one within the above-mentioned range) in the reaction vessel A and 
by the cooling power of the fractionation column B. The cooling fluid in 
the fractionation column B should be between the boiling point of the 
carbamic acid ester being cleaved and the boiling point of the higher 
boiling of the isocyanate and the alcohol at the pressure employed. If the 
cooling fluid of the fractionation column is maintained at such a 
temperature, at least 70 wt. % (preferably at least 85 wt. %) of the 
carbamic acid ester leaving the reaction vessel in a gaseous form and at 
most 35 wt. % (preferably not more than 10 wt. %) of the higher boiling 
of the alcohol and the isocyanate cleavage products leaving the reaction 
vessel in a gaseous form will condense in the fractionation column B. 
The vapor leaving the fractionation column B, is a mixture of alcohol, 
isocyanate and small quantities of carbamic acid ester. This gaseous 
mixture is separated in fractionation column C into a condensate made up 
of small quantities of carbamic acid ester and the higher boiling of the 
isocyanate and the alcohol cleavage products. A gaseous phase composed 
primarily of the lower boiling of the alcohol and the isocyanate cleavage 
products is given off from the fractionation column. It is advantageous to 
maintain the cooling fluid in the fractionation column C at a temperature 
between the boiling point of the isocyanate and of the alcohol cleavage 
products at the pressure employed. The cooling fluid in fractionation 
columns B and C may, however, also be adjusted to temperatures 
substantially lower than those described above. If such lower temperatures 
are employed, the partial condensation of the vapors fed into the 
fractionation columns may be achieved by controlled overloading of the 
heat exchangers. 
The fractionation columns used in the process of the present invention are 
generally heat exchangers operated with either liquid or gaseous cooling 
fluids such as water, oil acting as heat carrier, or air. 
The process of the present invention may be accompanied by the formation of 
a small quantity of high boiling by-products which accumulate in the 
reaction vessel A. These by-products, which will be referred to 
hereinafter as residue, may be separated from the reaction mixture by 
various methods known to those in the art. One possible method for 
removing the residue consists of stopping the supply of fresh carbamic 
acid ester into the reaction vessel A when the concentration of residue in 
the reaction mixture has become too high, removing the volatile 
constituents of the reaction mixture from the reaction vessel A by 
distillation, and discharging the residue left behind. The residue may 
also be flushed out of the reaction mixture continuously if this appears 
to be necessary or desirable. This flushing may be achieved, for example, 
by continuously removing liquid reaction mixture from reaction vessel A, 
freeing it from residue by a stripping distillation and then returning the 
residue-free liquid into reaction vessel A. The separation of residue may, 
of course, also be carried out by filtration. 
The process of the present invention is preferably carried out without the 
aid of auxiliary solvents although it is possible in principle to carry 
out the thermal decomposition in reaction vessel A in the presence of 
inert liquids. Inert liquids are those which are virtually non-volatile at 
the given temperature and pressure conditions or which condense to a large 
extent in the fractionation column B under the given temperature and 
pressure conditions. Such inert liquids may be used to plasticize high 
melting residues. Suitable liquids of this type include aromatic and 
araliphatic hydrocarbons having at least 10 carbon atoms and optionally 
carrying inert substituents, diarylethers, diarylsulphones and 
triarylphosphates. 
The thermal cleavage of carbamic acid esters by the process of the present 
invention may be accelerated by use of suitable catalysts such as Lewis 
acids (see Houben-Weyl, Methoden der Organischen Chemie, Volume 4, part 2, 
page 6), e.g. BF.sub.3, BCl.sub.3, B(OC.sub.2 H.sub.5).sub.3, B(OC.sub.4 
H.sub.9).sub.3, AlCl.sub.3, AlBr.sub.3, SnCl.sub.4, dibutyl tin oxide, 
SbCl.sub.5, TiCl.sub.4, TiBr.sub.4, FeCl.sub.3, cobalt octoate, 
ZnCl.sub.2, zinc octoate or CuCl. Mixtures of several such compounds may 
also be used as catalysts. The catalyst, if used at all, should generally 
be added to the reaction mixture at a concentration of from 0.001 to 2 wt. 
%, preferably from 0.01 to 1 wt. %. If a catalyst is used, the same 
cleavage rate that is obtained without a catalyst can generally be 
achieved with a shorter dwell time and/or a lower reaction temperature. 
When carrying out the process of the present invention, the formation of 
unwanted by-products can be reduced by the addition of stabilizers. 
Examples of suitable stabilizers include carboxylic acid chlorides such as 
acetyl chloride, butyric acid chloride, stearic acid chloride, adipic acid 
dichloride, benzoyl chloride, phthalic acid dichloride and terephthalic 
acid dichloride; and/or sulfonic acid chlorides such as methanesulfonic 
acid chloride, benzenesulfonic acid chloride and p-toluene-sulfonic acid 
chloride; and/or sulfonic acid esters such as methanesulfonic acid butyl 
ester, octanesulfonic acid ethyl ester, benzenesulfonic acid methyl ester, 
p-toluene sulfonic acid ethyl ester and 4-ethoxycarbonylbenzene sulfonic 
acid ethyl ester; and/or alkylating compounds such as n-hexylchloride, 
n-hexyliodide, n-octylbromide, dimethylsulfate and diethylsulfate. 
Mixtures of several compounds may also be used as a stabilizer. The 
stabilizer, if used, should be added to the reaction mixture at a total 
concentration of from 0.001 to 2 wt. %, preferably from 0.01 to 1 wt. %. 
The monoisocyanate R.sup.1 --NCO prepared by the process of the present 
invention can be isolated from the fractions in which it is present by 
distillation and thus obtained in pure form. These 
monoisocyanate-containing fractions are removed as gaseous product mixture 
from the head of the fractionation column C if the alcohol R.sup.2 --OH 
obtained by cleavage boils at a higher temperature than the isocyanate 
R.sup.1 --NCO or as condensate from the fractionation column C if the 
isocyanate R.sup.1 --NCO prepared by cleavage boils at a higher 
temperature than the alcohol R.sup.2 --OH. These fractions contain 
substantial amounts of the isocyanate R.sup.1 --NCO, minor quantities of 
carbamic acid ester R.sup.1 --NH--CO--OR.sup.2 and optionally small 
quantities of allophanate R.sup.1 --NH--CO--NR.sup.1 --CO--OR.sup.2 which 
may be formed by molecular addition of the isocyanate R.sup.1 --NCO to the 
carbamic acid ester R.sup.1 --NH--CO--OR.sup.2. Distillative separation of 
the fractions containing the isocyanate may be carried out by methods 
known to those in the art such as use of distillation columns. 
The distillation residue obtained when isolating the isocyanate R.sup.1 
--NCO in pure form is made up of carbamic acid ester and may also include 
residues of isocyanate. It may therefore be advantageous to feed these 
residues to the reaction vessel A where they may be subjected to the 
cleavage process. 
The alcohol R.sup.2 --OH may also be obtained in pure form by distillation 
from the fractions containing the alcohol R.sup.2 --OH prepared by the 
process of the present invention. These alcohol-containing fractions, 
which are removed by fractionation column C either as condensate or as 
gaseous product mixture, consist primarily of the alcohol R.sup.2 --OH and 
minor quantities of carbamic acid ester R.sup.1 --NH--CO--OR.sup.2. 
Separation of these fractions by distillation may be carried out by 
methods known in the art, for example by use of separating columns. The 
distillation residues obtained, which are mainly carbamic acid ester and 
possibly residues of alcohol, may be returned to reaction vessel A to be 
again subjected to cleavage. 
The process of the present invention for the preparation of monoisocyanates 
(including the recycling of carbamic acid ester containing residues) will 
be described with reference to FIG. 2 which illustrates an apparatus 
suitable for the preparation of isocyanates R.sup.1 --NCO. The process 
according to the invention is, however, in no way restricted to the use of 
the specific apparatus illustrated in FIG. 2. In FIG. 2, A represents a 
reaction vessel equipped with immersion evaporator; B and C each represent 
nests of tubes used as fractionation columns; D represents a discharge 
tray; E and F represent distillation columns; and G represents a 
distillation vessel equipped with immersion evaporator. 
In the apparatus represented in FIG. 2, the carbamic acid ester is 
continuously fed into reaction vessel A through pipe (201) and heated 
therein. A gaseous mixture is continuously removed from reaction vessel A 
through pipe (202) and delivered to the fractionation column B to be 
partially condensed therein. The condensate is returned to reaction vessel 
A through pipe (203) while the gaseous mixture leaving the head of the 
fractionation column B enters the fractionation column C where it is 
partially condensed. The condensate reaching the discharge tray D of the 
fractionation column C flows through pipe (204) into distillation column E 
where it is separated by distillation. The head product removed through 
pipe (206) is either pure isocyanate boiling at a higher temperature than 
the alcohol or pure alcohol boiling at a higher temperature than the 
isocyanate. The sump product discharged through pipe (207) is returned to 
the reaction vessel A. The gaseous head product removed from fractionation 
column C through pipe (205) is transferred to the distillation column F 
where it is separated by distillation. The lower boiling of the alcohol 
and the isocyanate is removed in pure form from column F through pipe 
(208). The pump product of column F is returned to reaction vessel A 
through pipe (209). At the same time, liquid product mixture is 
continuously removed from the sump of the reaction vessel A through the 
pipe (210) to be subjected to a stripping distillation in distillation 
vessel G. The distillate obtained in distillation vessel G is returned to 
reaction vessel A through pipe (211) while the residue is continuously 
removed from the bottom of the distillation vessel G through pipe (212). 
As has already been explained above, if the process of the present 
invention is carried out using carbamic acid esters whose alcohol 
component boils at a lower temperature than the isocyanate component, the 
condensate obtained from fractionation column C is a mixture if isocyanate 
boiling at a higher temperature than the alcohol and minor quantities of 
carbamic acid ester. This mixture is not only suitable as a starting 
material for the preparation of the isocyanate R.sup.1 --NCO in pure form, 
but may also be used as a starting material for the preparation of 
isocyanates R.sup.3 --NCO. The isocyanates R.sup.3 --NCO at normal 
pressure have a boiling point at least 50.degree. C. lower than the 
boiling point of the isocyanate R.sup.1 --NCO. Apart from this restriction 
with respect to boiling point, R.sup.3 may have the same meaning as 
R.sup.1. 
When the above-described mixtures obtained as condensate of fractionation 
column C in the process of the present invention are used as a starting 
material for the preparation of the isocyanate R.sup.3 --NCO in accordance 
with the present invention, these mixtures are reacted with carbamic acid 
esters of the formula R.sup.3 --NH--CO--OR.sup.2 to undergo 
transurethanation. The amounts in which the reactants are used in this 
reaction are such that from 1 to 10, preferably from 1.1 to 3 mol of 
isocyanate R.sup.1 --NCO are present for each mol of carbamic acid ester 
R.sup.3 --NH--CO--OR.sup.2. The transurethanation is carried out at a 
temperature in the range of from 50.degree. to 200.degree. C., preferably 
from 80.degree. to 180.degree. C., under pressure conditions such that the 
reaction mixture boils. The gaseous product mixture formed is mainly 
isocyanate R.sup.3 --NCO, possibly small quantities of isocyanate R.sup.1 
--NCO and possibly small quantities of carbamic acid ester R.sup.3 
--NH--CO--OR.sup.2. The isocyanate R.sup.3 --NCO may be obtained in pure 
form from this mixture by distillation. The carbamic acid ester R.sup.1 
--NH--CO--OR.sup.2 formed in the reaction of the isocyanate R.sup.1 --NCO 
put into the process may be continuously returned to reaction vessel A. 
Although the preparation of isocyanates by such transurethanation reactions 
is known in principle (see German Pat. No. 1,207,378), the isocyanates 
used and exemplified in the process of this prior publication are higher 
functional polyisocyanates such as tolylene diisocyanate or 
polyisocyanates of the diphenylmethane series which must first be prepared 
by phosgenation of the corresponding amines. Such higher functionality 
polyisocyanates constitute valuable intermediate products for the 
production of polyurethanes. Further, the carbamic acid esters of higher 
boiling polyisocyanates formed in the process according to German Pat. No. 
1,207,378 must be regarded as valueless waste product. In short, the 
process according to German Pat. No. 1,207,378 has the disadvantages of 
requiring the use of valuable products and the disposal of unusable 
reaction products. In contrast, the process of the present invention 
enables low boiling monoisocyanates to be prepared by an inexpensive and 
economical process without the formation of valueless by-products. The 
isocyanate R.sup.1 --NCO required to produce the isocyanate R.sup.3 --NCO 
in accordance with the present invention is continuously obtained from the 
carbamic acid ester R.sup.1 --NH--CO--OR.sup.2 by the cleavage process and 
at the same time carbamic acid ester is continuously being re-formed by 
the transurethanation reaction and may be reused. Consequently, the use of 
the products of the cleavage reaction of the present invention in such a 
transurethanation in which carbamic acid ester R.sup.3 --CO--OR.sup.2 is 
split into isocyanate R.sup.3 --NCO and alcohol R.sup.2 --OH, is actually 
use of the isocyanate R.sup.1 --NCO as an auxiliary agent in circulation. 
Since the carbamic acid ester R.sup.1 --NH--CO--OR.sup.2 is continuously 
formed again in the combined cleavage/transurethanation reactions of the 
present invention, it need only be replaced to the extent that losses 
occur (e.g., due to the formation of residues). Another advantage of the 
combination of cleavage and transurethanation reactions in the present 
invention is that such a process may be used for the conversion of those 
carbamic acid esters R.sup.3 --NH--CO--OR.sup.2 into isocyanate and 
alcohol which are difficult or impossible to split by direct heat 
cleavage. An example of such a difficult to split isocyanate is one in 
which the boiling points of the isocyanate and alcohol obtained as 
cleavage products are similar or identical to those which distill without 
decomposition below 200.degree. C. 
When the cleavage process of the present invention is used in combination 
with the transurethanation reaction, the carbamic acid esters R.sup.1 
--NH--CO--OR.sup.2 used should be of the kind in which the isocyanate 
component R.sup.1 --NCO has a boiling point (at atmospheric pressure) at 
least 50.degree. C. above the boiling point of isocyanate R.sup.3 --NCO 
and of the alcohol R.sup.2 --OH. The chemical reactions which take place 
in this combination of reactions may be represented by the following 
equations: 
EQU R.sup.1 --NH--CO--O--R.sup.2 .fwdarw.R.sup.1 --NCO+HO--R.sup.2(1) 
EQU R.sup.1 --NCO+R.sup.3 --NH--CO--O--R.sup.2 .fwdarw.R.sup.3 --NCO+R.sup.1 
--NH--CO--O--R.sup.2 (2) 
EQU R.sup.3 --NH--CO--O--R.sup.2 .fwdarw.R.sup.3 --NCO+HO--R.sup.2(3) 
It can be seen from these equations that carbamic acid esters which have 
the same alcohol component R.sup.2 --OH may be used in both the cleavage 
(equation (1)) and transurethanation (equation (2)) reactions of the 
present invention. As was discussed above, the transurethanation of the 
present invention may be carried out by using the condensate of 
fractionation column C which condensate contains the isocyanate R.sup.1 
--NCO and minor quantities of carbamic acid ester R.sup.1 
--NH--CO--OR.sup.2. This carbamic acid ester, present in the condensate is 
inert under the transurethanation reaction conditions, is returned to 
reaction vessel A together with the carbamic acid ester of the same 
composition formed during the transurethanation reaction. When the 
isocyanate R.sup.1 --NCO is used in excess, based on the quantity of 
carbamic acid ester R.sup.3 --NH--CO--O--R.sup.2, the product mixtures 
formed in the transurethanation still contain excess isocyanate R.sup.1 
--NCO in addition to the volatile isocyanate R.sup.3 --NCO and the 
carbamic acid ester R.sup.1 --NH--CO--OR.sup.2. After removal of the 
isocyanate R.sup.3 --NCO by distillation, this excess isocyanate R.sup.1 
--NCO may also be removed from the carbamic acid ester R.sup.1 
--NH--CO--OR.sup.2 by distillation and may be used again in the thermal 
cleavage of the present invention, optionally together with the condensate 
from the fractionation column C. 
Examples of carbamic acid esters R.sup.3 --NH--CO--OR.sup.2 suitable for 
the use in the present invention include: N-methyl-carbamic 
acid-methylester, -ethylester; N-ethyl-carbamic acid-methylester, 
-isopropylester; N-propylcarbamic acid-ethylester, -isopropylester; 
N-isopropylcarbamic acid-methylester, -ethylester; N-butylcarbamic 
acid-ethylester, -butylester; N-(2-methyl-propyl)-carbamic 
acid-isopropyl-ester, -butylester; N-(1-methylpropyl)-carbamic 
acid-methylester, -propylester; N-pentylcarbamic acid-butylester, 
-(2-methoxyethyl)-ester; N-(ethoxycarbonyl-methyl)-carbamic 
acid-ethylester, -hexylester; N-allylcarbamic acid-ethylester, 
-isopropylester; N-cyclobutylcarbamic acid-methylester, -butylester; 
N-benzylcarbamic acid-(2-methoxy-ethyl)-ester, -(2-ethoxy-ethyl)-ester; 
N-(3-nitro-phenyl)-carbamic acid-ethylester, -butylester. 
Although the transurethanation reaction which takes place in the present 
invention may be carried out in the absence of a catalyst, it is 
frequently advantageous to accelerate the reaction with suitable 
catalysts. Examples of suitable catalysts include the Lewis acids already 
mentioned above with respect to the thermal cleavage process. Particularly 
suitable catalysts are boric acid trialkylesters having 1 to 18 carbon 
atoms in the alkyl groups, especially those of the formula 
B(OR.sup.2).sub.3 (i.e., boric acid esters in which the alcohol component 
corresponds to the alcohol component of the carbamic acid ester). 
If a Lewis acid is used as a catalyst, it may be used in the process as a 
solid bed catalyst, optionally on an inert carrier material, or it may be 
homogeneously dissolved in the liquid reaction mixtures. In homogeneous 
catalysis, the catalyst content in the reaction mixture should generally 
be from 0.01 to 10 wt. %, preferably from 0.1 to 8 wt. %. When volatile 
catalysts are used, it is advantageous to separate these catalysts by 
distillation from the liquid phase of the transurethanation reaction 
mixture before the reaction mixture or a portion thereof is returned to 
reaction vessel A. These catalysts separated by distillation may, of 
course, be used in subsequent transurethanation reactions. 
The transurethanation reaction which takes place between the carbamic acid 
ester R.sup.3 --NH--CO--OR.sup.2 and the condensate from fractionation 
column B containing the isocyanate R.sup.1 --NCO and the removal of 
gaseous isocyanate R.sup.3 --NCO may be carried out in a single reaction 
vessel. It is generally advisable, however, to carry out the reaction in a 
series of reaction vessels particularly if the boiling points of the 
isocyanates differ by little more than 50.degree. C. The reaction 
temperatures of the individual reaction vessels in such a series may 
differ within the ranges mentioned above. The optimum reaction 
temperatures depend upon the nature of the starting materials and the 
nature and quantity of any catalyst used. These temperatures may be 
readily determined by techniques known to those in the art. The reaction 
may, of course, also be carried out at temperatures other than the optimum 
reaction temperatures provided that such other temperatures are within the 
above-described temperature ranges. 
As was discussed above, the transurethanation reaction of the present 
invention should take place under pressure conditions at which the 
reaction mixtures boil. The pressure required is dependent upon the nature 
of the reaction products and upon the reaction temperature and should 
generally be within the range of from 0.001 to 2 bar, preferably from 0.01 
to 1 bar. When a series of reaction vessels is employed, the pressures in 
the individual reaction vessels may be adjusted to differing values if 
desired. It is generally advantageous, however, to adjust the reaction 
vessels in such a series to the same pressure and, if necessary, to employ 
differing reaction temperatures. 
The average dwell time of the reaction mixtures in the reaction vessel or 
reaction vessels also depends upon the nature of the starting materials 
used, the nature and quantity of any catalysts used, and the pressure and 
temperature conditions. The average dwell time may therefore vary within 
wide limits although it is generally from 0.1 to 10 hours, preferably from 
0.5 to 5 hours. 
FIG. 3 shows an apparatus in which the thermal cleavage process combined 
with the transurethanation of the present invention may be carried out 
continuously. The transurethanation reaction need not, however, be carried 
out in the apparatus illustrated in FIG. 3. In FIG. 3, the letters (A), 
(B), (C), (D), (F) and (G) have the same meaning as in FIG. 2 with the 
exception that the reaction vessel (A) in FIG. 3 is heated by means of a 
circulation evaporator. E, E' and E" denote a cascade of reaction vessels 
equipped with immersion evaporators. H denotes a distillation vessel 
equipped with immersion evaporator and J denotes a distillation column. 
When the thermal cleavage is carried out simultaneously with the 
transurethanation reaction of the condensate obtained in fractionation 
column C, the cleavage of the carbamic acid ester R.sup.1 
--NH--CO--OR.sup.2 initially takes place in a manner analogous to the 
method described above with respect to FIG. 2. Therefore, in FIG. 3, the 
apparatus parts (A), (B), (C), (D), (F) and (G) perform the same function 
and the pipes (301) to (305) and (308) to (312) correspond to the pipes 
(201) to (205) and (208) to (212) of FIG. 2 in their function and in the 
streams of product transported by them. 
In the transurethanation of the present invention using the condensates 
from fractionation column C removed from the discharge tray D, the said 
condensate is introduced into reaction vessel E through pipe (304) while 
carbamic acid ester R.sup.3 --NH--CO--OR.sup.2 is introduced into reaction 
vessel E through pipe (306). The pipes (313) and (315) connect reaction 
vessel E to reaction vessels E' and E" in cascade formation. The pressure 
in reaction vessels E, E' and E" should be adjusted in each case so that 
the reaction mixtures heated to the given reaction temperatures boil. 
Gaseous product mixture is removed from the reaction vessels through pipes 
(307), (314) and (316) into the pipe (320) and transferred from into the 
distillation column J from which pure isocyanate R.sup.3 --NCO is 
continuously removed at the top through pipe (321) while the sump is 
returned to reaction vessel E through pipe (322). At the same time, liquid 
product mixture is continuously removed from reaction vessels E and E' 
through pipes (313) and (315), respectively, to be transferred to the next 
reaction vessel. Product mixture enriched with carbamic acid ester R.sup.1 
--NH--CO--OR.sup.2 is continuously removed from the sump of the reaction 
vessel E" through pipe (317) to be transferred to the distillation vessel 
H where it is stripped by distillation. The gaseous product mixtures 
thereby obtained are returned to reaction vessel E through pipe (318) 
while liquid product mixture is removed from the sump through pipe (319) 
to be returned to the reaction vessel A. 
When the thermal cleavage process is carried out in combination with the 
transurethanation reaction according to the invention with the apparatus 
illustrated in FIG. 3, pure isocyanate R.sup.3 --NCO (by way of pipe 321) 
and pure alcohol R.sup.2 --OH (by way of pipe 308) are continuously 
obtained from carbamic acid ester R.sup.3 --NH--CO--OR.sup.2 (from pipe 
306). When the apparatus of FIG. 3 is in continuous operation, the 
quantity of carbamic acid ester R.sup.1 --NH--CO--OR.sup.2 which must be 
supplied to the system by way of (301) is that corresponding to the 
quantity of by-products which are formed by side reactions and removed 
through (312). This quantity of by-products removed through (312) 
generally amounts to at the most 10 wt. % (based on the sum of process 
products removed through (308) and (321)). 
It is not essential to the transurethanation reaction of the present 
invention that the separation by distillation of isocyanate R.sup.3 --NCO 
from the gaseous product mixture removed from the reaction vessel E or 
reaction vessels E, should be carried out by means of a separately 
arranged distillation column. This separation may also be carried out by 
means of a fractionation column. The distillation column or the 
fractionation column may also be directly attached to the reaction vessel 
E so that the distillation reflux returns directly to reaction vessel E. 
When a series of reaction vessels E is employed, the gaseous product 
mixtures removed from each reaction vessel may, of course, be separated by 
distillation, for example by means of fractionation columns or 
distillation columns directly attached to the reaction vessels. 
The number of reaction vessels which may be combined to form a series is, 
of course, not critical. The liquid product mixture removed from the sump 
of the reaction vessel E or from the last reaction vessel of a series may 
be broken down by distillation. Such distillation is illustrated in FIG. 
3, where the gaseous head product obtained from distillation vessel H is a 
gas phase enriched with isocyanates R.sup.1 --NCO. This gas phase rich in 
R.sup.1 --NCO is returned to the reaction vessel E while the sump of 
distillation vessel H contains carbamic acid ester R.sup.1 
--NH--CO--O--R.sup.2 which is returned to the cleavage reaction A. The 
distillative separation of the sump of the reaction vessel E or of the 
last reaction vessel may, however, be omitted if in the course of the 
reaction only a slight excess of isocyanate R.sup.1 --NCO has been used 
because the sump will be virtually pure carbamic acid ester R.sup.1 
--NH--CO--OR.sup.2. 
When a series of reaction vessels E is used, it is not essential that in 
the transurethanation reaction the sump products from the column "J" 
and/or the distillates of the stripping distillation should be completely 
returned to the first reaction vessel E of the series as illustrated in 
FIG. 3. These streams of product may also be partly or completely returned 
to another reaction vessel E or into several other reaction vessels E. 
It is essential to the transurethanation reaction of the present invention 
that during the course of the reaction, from 1 to 10, preferably from 1.1 
to 3 mol of isocyanate R.sup.1 --NCO should be present for each mol of 
carbamic acid ester R.sup.3 --NH--CO--O--R.sup.2. The total quantity of 
the isocyanate R.sup.1 --NCO is the sum of the isocyanate in the 
condensate of the fractionation column and any isocyanate R.sup.1 --NCO 
recycled as described above. 
In the transurethanation reaction of the present invention, particularly 
when preparing low-boiling alkyl isocyanates R.sup.3 --NCO (such as methyl 
isocyanate), it may be advantageous to add a certain proportion (e.g., 30% 
by weight of the reaction mixture) of inert solvent to the reaction 
mixtures in reaction vessel E or in the reaction vessels E. The solvent 
should preferably be chosen so that its boiling point is between the 
boiling points of the isocyanates R.sup.1 --NCO and R.sup.3 --NCO. Such a 
solvent acts as a distillation aid in the process and promotes boiling of 
the reaction mixtures to form fractions containing isocyanate R.sup.3 
--NCO. Any residue of such solvent in the liquid reaction mixtures 
introduced into reaction vessel A should be removed by distillation before 
the mixtures are fed into reaction vessel A (e.g., by stripping 
distillation). 
The monoisocyanates which may be prepared by the process according to the 
invention are valuable starting materials for the production of plant 
protective agents or of pharmaceuticals. 
The invention is further illustrated, but is not intended to be limited by 
the following examples in which all parts and percentages are by weight 
unless otherwise specified. 
The following Examples illustrate the process according to the invention 
and should not be construed as limitations thereof. 
EXAMPLES 
Examples 1-17 
The apparatus used in Examples 1 to 17 was similar to that illustrated in 
FIG. 1. The apparatus used in these Examples consisted of a 100 liter tank 
(reaction vessel A) with stirrer and heating jacket to which two cooling 
coils (B and C) used as fractionation columns were connected through heat 
insulated pipes. The cooling coils were charged with thermostatically 
controlled oil which acted as heat carrier. The volume of substance in 
reaction vessel A was adjusted to 80 liters and was kept constant by the 
rate of product feed through pipe (101), the heating power (A) and the 
cooling power of (B). The pressure in the fractionation columns was 
virtually the same as in reaction vessel A. 
Any catalysts and/or stabilizer used was mixed with the carbamic acid ester 
introduced into reaction vessel A. 
In Examples 1 to 13, the fractions containing isocyanate were obtained as 
partial condensates through pipe (105) from fractionation column C, and in 
Examples 14 to 17 they were obtained as gaseous mixtures through pipe 
(106) at the head of the fractionation column C. 
Example 1 (see FIG. 1) 
20.2 kg/h of molten N-cyclohexyl-carbamic acid ethyl ester were 
continuously introduced through pipe (101) into reaction vessel A. The 
reaction temperature in reaction vessel A was 225.degree. C., the reaction 
pressure 1.0 bar. The gaseous product mixture escaping from the reactor 
through pipe (102) was partially condensed in fractionation column B which 
was supplied with oil adjusted to 180.degree. C. The product mixture 
returned through pipe (103) to reaction vessel A contained 95.0 wt. % 
N-cyclohexyl-carbamic acid ethyl ester. Gaseous product mixture leaving 
through pipe (104) at the head of fractionation column B was introduced 
into fractionation column C which was supplied with oil adjusted to 
95.degree. C., and partially condensed there. 15.3 kg per hour of 
condensate containing 66.4% by weight of cyclohexylisocyanate were 
continuously removed through pipe (105) from fractionation column C while 
4.5 kg/h of gaseous product mixture containing 86.3% by weight of ethanol 
continuously escaped through pipe (106) at the head of fractionation 
column C. The time of continuous production was 16 hours. The selectivity 
of thermal cleavage for the production of cyclohexyl isocyanate was 96 mol 
%. 
Examples 2 through 17 were carried out in the same manner as in Example 1. 
The process parameters and results are summarized in Table 1. 
Example 2 
R.sup.1 --NH--CO--OR.sup.2 : N-phenyl-carbamic acid-ethyl ester 
Catalyst: none 
Stabilizer: none 
Example 3 
R.sup.1 --NH--CO--OR.sup.2 : N-phenyl-carbamic acid-ethyl ester 
Catalyst: Di-n-butyl-tin dichloride 
Stabilizer: none 
Example 4 
R.sup.1 --NH--CO--OR.sup.2 : N-phenyl-carbamic acid-ethyl ester 
Catalyst: none 
Stabilizer: 4-toluenesulfonic acid-methyl ester 
Example 5 
R.sup.1 --NH--CO--O--R.sup.2 : N-phenyl-carbamic acid-isopropylester 
Catalyst: none 
Stabilizer: none 
Example 6 
R.sup.1 --NH--CO--OR.sup.2 : N-3-tolyl-carbamic acid-ethylester 
Catalyst: none 
Stabilizer: 4-toluenesulfonic acid-methylester 
Example 7 
R.sup.1 --NH--CO--OR.sup.2 : N-3-tolyl-carbamic acid-isopropylester 
Catalyst: none 
Stabilizer: none 
Example 8 
R.sup.1 --NH--CO--OR.sup.2 : N-3-(trifluoromethyl)-phenyl-carbamic 
acid-ethylester 
Catalyst: none 
Stabilizer: none 
Example 9 
R.sup.1 --NH--CO--OR.sup.2 : N-4-chlorophenyl-carbamic acid-ethylester 
Catalyst: none 
Stabilizer: none 
Example 10 
R.sup.1 --NH--CO--OR.sup.2 : N-4-chlorophenyl-carbamic acid-ethylester 
Catalyst: tin-(II) chloride 
Stabilizer: phthalic acid dichloride 
Example 11 
R.sup.1 --NH--CO--OR.sup.2 : N-3,4-dichlorophenyl-carbamic acid-ethyl ester 
Catalyst: none 
Stabilizer: 4-toluenesulfonic acid-methyl ester 
Example 12 
R.sup.1 --NH--CO--OR.sup.2 : N-3,4-dichlorophenyl-carbamic acid-butyl ester 
Catalyst: none 
Stabilizer: 4-toluenesulfonic acid chloride 
Example 13 
R.sup.1 --NH--CO--OR.sup.2 : N-3,4-dicylorophenyl-carbamic acid-pentylester 
Catalyst: none 
Stabilizer: 4-toluenesulfonic acid-methylester 
Example 14 
R.sup.1 --NH--CO--OR.sup.2 : N-isopropyl-carbamic acid-cyclohexyl-ester 
Catalyst: none 
Stabilizer: none 
Example 15 
R.sup.1 --NH--CO--OR.sup.2 : N-isopropyl-carbamic acid-cyclohexyl-ester 
Catalyst: zinc octoate (8 wt. % Zn) 
Stabilizer: none 
Example 16 
R.sup.1 --NH--CO--OR.sup.2 : N-isopropyl-carbamic 
acid-(2-ethyl-hexyl)-ester 
Catalyst: none 
Stabilizer: none 
Example 17 
R.sup.1 --NH--CO--OR.sup.2 : N-isopropyl-carbamic 
acid-(2-ethyl-hexyl)-ester 
Catalyst: zinc oxide 
Stabilizer: 4-toluenesulfonic acid-methyl ester 
TABLE 1 
__________________________________________________________________________ 
Posi- 
tion* 
Example 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 
__________________________________________________________________________ 
(A) 
Reaction 
temp. (.degree.C.) 
190 175 
217 
200 
210 195 
220 
195 
195 205 
205 
210 
240 225 
245 
220 
Reaction 
press. (bar) 
0.16 
0.10 
0.47 
0.27 
0.27 
0.10 
0.33 
0.07 
0.07 
0.04 
0.013 
0.013 
1.0 1.0 
1.0 
1.0 
(B) 
Oil inlet 
temp. (.degree.C.) 
148 120 
170 
155 
165 145 
131 
150 
150 142 
135 
142 
176 175 
190 
188 
(C) 
Oil inlet 
temp. (.degree.C.) 
40 40 68 50 60 35 65 40 40 25 25 70 100 100 
130 
130 
(101) 
Product 
feed kg/h 
13.5 
17.0 
17.6 
23.1 
19.9 
26.0 
17.2 
12.1 
14.5 
18.1 
21.2 
22.0 
9.1 9.4 
13.1 
16.8 
wt. % 
catalyst 
-- 0.1 
-- -- -- -- -- -- 0.05 
-- -- -- -- 0.01 
-- 0.1 
wt. % 
stabilizer 
-- -- 0.1 
-- 0.1 -- -- -- 0.1 0.1 
0.1 
0.1 
-- -- -- 0.05 
(103) 
wt. % R.sup.1 -- 
NH--CO--OR.sup.2 
98.3 
96.7 
98.8 
98.4 
96.3 
98.6 
98.1 
95.2 
93.4 
96.4 
97.1 
99.0 
93.8 
94.6 
97.5 
96.2 
(105) 
discharge 
kg/h 10.1 
12.9 
13.8 
16.4 
15.4 
21.6 
14.6 
9.1 
11.2 
14.7 
16.3 
17.0 
5.2 5.3 
8.3 
11.1 
wt. % 
R.sup.1 --NCO 
72.0 
65.7 
63.1 
62.8 
73.8 
60.8 
58.4 
84.0 
82.8 
79.8 
67.5 
49.7 
-- -- -- -- 
wt. % 
R.sup.2 --OH 
-- -- -- -- -- -- -- -- -- -- -- -- 77.5 
79.4 
79.1 
72.5 
(106) 
discharge 
(kg/h) 3.2 3.9 
3.6 
6.4 
4.3 4.3 
2.2 
2.5 
3.1 3.0 
4.7 
4.8 
3.7 3.9 
4.5 
5.5 
wt. % 
R.sup.2 --OH 
90.3 
85.9 
95.5 
83.8 
93.2 
95.9 
95.4 
97.3 
91.5 
98.9 
94.4 
84.3 
-- -- -- -- 
wt. % 
R.sup.1 --NCO 
-- -- -- -- -- -- -- -- -- -- -- -- 88.1 
89.6 
89.4 
91.7 
Duration of pro- 
duction period (h) 
59 14 72 10 45 32 11 58 41 60 8 11 10 12 30 28 
Selectivity 
(R.sup.1 --NCO) (mol-%) 
97 98 98 97 98 99 95 94 98 97 98 98 94 96 93 96 
__________________________________________________________________________ 
*As shown in FIG. 1 
Examples 18-23 
The apparatus used in Examples 18 to 23 is shown schematically in FIG. 2. 
Reaction vessel A consisted of a 100 liter tank equipped with immersion 
evaporator and stirrer. The volume of its contents was kept constant at 80 
liter. Two nests of cooling pipes arranged inside an apparatus were 
connected to the tank and used as fractionation columns B and C. Situated 
between these two condensers was a discharge tray D by means of which the 
condensate of the fractionation column C was collected, removed and then 
discharged into a separating distillation column E. The gaseous mixture 
leaving at the head of the fractionation column C was fed into a second 
distillation column F serving as separator, optionally after first 
undergoing an intermediate condensation. A 20 liter tank G equipped with 
immersion evaporator and stirrer was connected to reaction vessel A to 
flush out the residue. 
When carrying out the process in these examples, the pressure in the 
fractionation columns and in distillation vessel G was virtually equal to 
that in reaction vessel A. 
Any catalyst and/or stabilizer used was added to the carbamic acid ester 
introduced into reaction vessel A. 
In Examples 18 through 22, the fractions containing isocyanate were removed 
as condensates from fractionation column C and the pure isocyanates were 
obtained at the head of column E. In Example 23, the fraction containing 
isocyanate was removed at the head of fractionation column C and the pure 
isocyanate was obtained at the head of column F through pipe (208). 
Example 18 (see FIG. 2) 
10.2 kg/h of molten N-phenyl-carbamic acidethylester were introduced 
continuously into reaction vessel A through pipe (201). The reaction 
temperature in reaction vessel A was 190.degree. C., the reaction pressure 
0.17 bar. The gaseous product mixture leaving reaction vessel A through 
pipe (202) was partially condensed in fractionation column B which was 
supplied with oil adjusted to 148.degree. C. The partial condensate 
returning to reaction vessel A through pipe (203) contained 98.1 wt. % 
N-phenyl-carbamic acid-ethylester. The gaseous mixture escaping from 
fractionation column B was partially condensed in fractionation column C 
which was operated with oil adjusted to 40.degree. C. Condensate 
containing 72.2 wt. % phenylisocyanate was continuously removed through 
pipe (204) from the discharge tray D at the rate of 10.1 kg/h. The 
condensate was introduced into column E where it was fractionally 
distilled at a pressure of 0.012 bar and a sump temperature of 100.degree. 
C. 7.2 kg/h of pure phenylisocyanate were obtained at the head of the 
column from pipe (206) while sump product containing 96.9 wt. % 
N-phenyl-carbamic acid-ethylester was continuously removed from the column 
and returned to reaction vessel A through pipe (207). 3.1 kg/h of product 
mixture containing 90.6% by weight of ethanol were continuously removed 
from the head of fractionation column C through pipe (205). This mixture 
was fed into the column, where it was fractionally distilled at a pressure 
of 1.0 bar and a sump temperature of 100.degree. C. 2.8 kg/h of ethanol 
were obtained from the head of the column through pipe (208) while sump 
product containing 90.6 wt. % N-phenyl-carbamic acid-ethylester was 
continuously removed from the column and returned to reaction vessel A 
through pipe (209). 1.8 kg/h of liquid product mixture containing 94.4 wt. 
% N-phenyl-carbamic acid-ethylester were continuously removed from the 
sump of reaction vessel A and introduced into distillation vessel G where 
it was distilled at a pressure of 0.17 bar and a sump temperature of 
195.degree. C. Evaporating product mixture was returned to reaction vessel 
A through pipe (211) while residue liquid containing 56.7 wt. % 
N-phenyl-carbamic acid-ethylester was removed from the sump of the vessel 
through pipe (212) at the rate of 0.2 kg/h. The selectivity of thermal 
cleavage for the preparation of phenylisocyanate was found to be 99 mol-%. 
Examples 19 to 23 were carried out as in Example 18. The process parameters 
and results are summarized in Table 2. 
Example 19 
R.sup.1 --NH--CO--OR.sup.2 : N-phenyl-carbamic acid-isopropylester 
Catalyst: none 
Stabilizer: none 
Example 20 
R.sup.1 --NH--CO--OR.sup.2 : N-3-tolyl-carbamic acid-ethylester 
Catalyst: none 
Stabilizer: 4-toluenesulfonic acid-methylester 
Example 21 
R.sup.1 --NH--(O--OR.sup.2 : n-3-tolyl-carbamic acid-isopropyl-ester 
Catalyst: none 
Stabilizer: none 
Example 22 
R.sup.1 --NH--CO--OR.sup.2 : N-(3,4-dichloro-phenyl)-carbamic 
acid-butylester 
Catalyst: none 
Stabilizer: 4-toluenesulfonic acid-methylester 
Example 23 
R.sup.1 --NH--CO--OR.sup.2 : N-isopropyl-carbamic acid-(2-ethylhexyl)-ester 
Catalyst: di-n-butyl-tin oxide 
Stabilizer: 4-toluenesulfonic acid-ethylester 
TABLE 2 
______________________________________ 
Posi- 
tion* 
Example 19 20 21 22 23 
______________________________________ 
(A) Reaction temp. (.degree.C.) 
200 210 195 205 225 
Reaction press. (bar) 
0.27 0.27 0.10 0.013 
1.0 
(B) Oil inlet temp. (.degree.C.) 
155 165 145 135 190 
(C) Oil inlet temp. (.degree.C.) 
50 60 35 25 125 
(E) Sump temp. (.degree.C.) 
145 120 125 140 120 
Pressure (bar) 0.27 0.013 
0.013 
0.005 
0.02 
(F) Sump temp. (.degree.C.) 
110 100 110 100 95 
Pressure (bar) 1.0 1.0 1.0 1.0 1.0 
(G) Sump Temp. (.degree.C.) 
205 215 200 210 240 
Pressure (bar) 0.27 0.27 0.10 0.013 
1.0 
(201) 
Product feed (kg/h) 
15.7 15.7 19.0 15.9 13.6 
wt. % catalyst -- -- -- -- 0.05 
wt. % stabilizer -- 0.1 -- 0.1 0.1 
(203) 
wt. % 
R.sup.1 --NH--CO--OR.sup.2 
98.7 95.8 97.9 98.3 96.9 
(204) 
Product flow (kg/h) 
16.4 15.5 21.3 16.9 11.3 
wt. % R.sup.1 --NCO 
62.9 73.9 60.8 67.4 -- 
wt. % R.sup.2 --OH 
-- -- -- -- 74.7 
(205) 
Product flow (kg/h) 
5.9 4.4 6.1 4.6 5.7 
wt. % R.sup.2 --OH 
87.0 91.1 96.9 95.7 -- 
wt. % R.sup.1 --NCO 
-- -- -- -- 90.4 
(206) 
Removal of pure pro- 
duct (kg/h) 10.1 11.3 12.9 11.0 8.1 
(207) 
wt. % 
R.sup.1 --NH--CO--OR.sup.2 
96.4 95.2 99.3 93.5 89.3 
(208) 
Removal of pure 
product (kg/h) 5.1 4.0 5.9 4.4 5.1 
(209) 
wt. % 
R.sup.1 --NH--CO--OR.sup.2 
96.3 97.5 97.4 95.2 91.2 
(210) 
Product flow (kg/h) 
2.9 2.6 1.9 3.9 2.0 
wt. % 
R.sup.1 --NH--CO--OR.sup.2 
92.9 86.9 91.6 92.6 88.6 
(212) 
Product flow (kg/h) 
0.4 0.4 0.2 0.4 0.4 
wt. % 
R.sup.1 --NH--CO--OR.sup.2 
51.3 40.0 36.4 36.6 51.0 
Selectivity (R.sup.1 --NCO) (mol %) 
97 98 99 97 96 
______________________________________ 
*As shown in FIG. 2. 
Examples 24-25 
The use of the condensate obtained in fractionation column C for the 
preparation of isocyanates R.sup.3 --NCO is described in Examples 24 and 
25. The apparatus illustrated in FIG. 3 was used. 
The reaction vessel A was a 100 liter tank equipped with a circulation 
evaporator. The volume of liquid in this system was kept constant at 90 
liters. Two nests of cooling tubes arranged inside an apparatus and 
connected to this tank were used as fractionation columns B and C. Between 
these two condensers was situated a discharge tray D in which the 
condensate of fractionation column C could be collected. The reaction 
vessels E consisted of three tanks (E, E' and E") arranged in a cascade 
and equipped with stirrers and immersion evaporators. The volume of liquid 
was adjusted to 20 liters in each tank. Two 20 liter distillation vessels 
(G and H), each equipped with a stirrer and immersion evaporator, were 
used for a stripping distillation of the liquids taken from tank E" and 
for flushing out the residue. Two separating columns (F and J) were used 
for fractionating the gaseous product mixtures escaping from the head of 
the fractionation column C and from the reaction vessels E, E' and E". 
During the process, the pressure was virtually the same in reaction vessel 
A, distillation vessel G and fractionation columns B and C. 
Example 24 (see FIG. 3) 
N-methyl-carbamic acid-ethylester was reacted with fractions containing 
phenyl isocyanate in the presence of triethylborate as catalyst and 
chlorobenzene as distillation aid. 
Before the reaction was begun, N-phenylcarbamic acid-ethyl ester was 
introduced into the apparatus and converted into phenyl isocyanate by 
thermal cleavage to the extent necessary for the reaction, and 
triethylborate and chlorobenzene were introduced into reaction vessels E, 
E' and E". Only then was the reaction begun. 
The following procedure was carried out after equilibrium had been 
established: 
6.6 kg/h of N-methyl-carbamic acid-ethylester to which 0.05 wt. % 
triethylborate had been added to compensate for losses were continuously 
introduced through pipe (306) into reaction vessel E 10.5 kg/h of 
condensate from fractionation column C containing 72.9 wt. % 
phenylisocyanate were also continuously introduced into reaction vessel E 
through pipe (304). The pressure in all three reaction vessels E, E' and 
E" was 1.0 bar. The reaction temperature in vessel E was 135.degree. C., 
in vessel E' 145.degree. C. and in vessel E" 155.degree. C. Liquid was 
continuously transferred from vessel E to vessel E' through pipe (313) and 
liquid from vessel E' to vessel E" through pipe (315). The gaseous product 
mixtures escaping from vessels E, E' and E" through pipes (307, 314 and 
316) were combined in pipe (320) to form a mixture containing 42.6 wt. % 
methylisocyanate, 23.1 wt. % triethylborate, 6.4 wt. % phenylisocyanate 
and 6.1 wt. % N-methyl-carbamic acid-ethylester as well as some 
chlorobenzene. This mixture was continuously introduced into column J and 
fractionally distilled there at a pressure of 1.0 bar and a sump 
temperature of 110.degree. C. 3.6 kg/h of pure methyl isocyanate were 
obtained from pipe (321) at the head of the column while 5.0 kg/h of 
liquid product mixture containing 39.7 wt. % of triethylborate, 11.0 wt. % 
phenyl isocyanate and 10.5 wt. % N-methyl-carbamic acid-ethylester were 
continuously removed from the sump of the column through pipe (322) and 
returned to reaction vessel E. 22.7 kg/h of a liquid enriched with 
N-phenyl-carbamic acid-ethylester and containing 11.5 wt. % 
phenylisocyanate, 5.2 wt. % chlorobenzene, 4.9 wt. % triethylborate, 2.7 
wt. % N-methyl-carbamic acid-ethyl ester and 0.5 wt. % methylisocyanate 
were continuously removed from reaction vessel E" through pipe (317). This 
liquid was introduced into distillation vessel H where it was subjected to 
a stripping distillation at a pressure of 0.05 bar and a temperature of 
170.degree. C. The gaseous product mixture thereby formed was returned to 
reaction vessel E through pipe (318) while 13.5 kg/h of a liquid 
containing 99.4 wt. % N-phenyl-carbamic acid-ethylester were continuously 
removed from the sump of the vessel through pipe (319) and returned to 
reaction vessel A. The reaction temperature in reaction vessel A was 
190.degree. C. and the reaction pressure 0.17 bar. The gaseous product 
mixture leaving reaction vessel A through pipe (302) was partially 
condensed in fractionation column B which was supplied with oil adjusted 
to 145.degree. C. The condensate returning to reaction vessel A through 
pipe (303) contained 96.0 wt. % N-phenyl-carbamic acid-ethylester. The 
gaseous mixture passing through fractionation column B was partially 
condensed in fractionation column C which was supplied with oil adjusted 
to 40.degree. C. 3.4 kg/h of gaseous product mixture containing 89.0 wt. % 
ethanol were removed from the head of fractionation column C through pipe 
(305). This mixture was introduced into column F where it was fractionally 
distilled at a pressure of 1.0 bar and a sump temperature of 100.degree. 
C. 3.0 kg/h of ethanol were continuously obtained from the head of the 
column through pipe (308) while liquid containing 93.5 wt. % 
N-phenyl-carbamic acid-ethylester was continuously removed from the sump 
of the column and returned to reaction vessel A through pipe (309). To 
flush out the residue, 2.0 kg/h of liquid containing 89.3 wt. % 
N-phenyl-carbamic acid-ethylester were continuously removed from reaction 
vessel A through pipe (310) and introduced into distillation vessel G 
where it was subjected to a stripping distillation at a pressure of 0.17 
bar and a temperature of 220.degree. C. The gaseous product mixture which 
distilled off was returned to reaction vessel A through pipe (311) while 
0.3 kg/h of liquid containing 36.7 wt. % N-phenyl-carbamic acid-ethylester 
were continuously removed from the vessel through pipe (312). To 
compensate for product losses, 0.3 kg/h of N-phenyl-carbamic 
acid-ethylester were continuously introduced into reaction vessel A 
through pipe (301). 
The yield of methylisocyanate obtained when the process was carried out 
continuously was 99% of the theoretical yield, based on the quantity of 
N-methyl-carbamic acid-ethylester put into the process. 
EXAMPLE 25 
N-isopropyl-carbamic acid-n-butyl ester (R.sup.3 --NH--CO--OR.sup.2) was 
reacted with fractions containing cyclohexylisocyanate (R.sup.1 --NCO) in 
the presence of tri-n-butylborate as catalyst by the method described in 
Example 24. The process parameters and results are summarized in Table 3. 
TABLE 3 
______________________________________ 
Position (As indicated in FIG. 3) 
______________________________________ 
(A) Reaction pressure: 0.39 bar 
Reaction temperature: 225.degree. C. 
(B) Oil inlet temperature: 175.degree. C. 
(C) Oil inlet temperature: 100.degree. C. 
(E) Reaction pressure: 0.41 bar 
Reaction temperature: 140.degree. C. 
(E') Reaction pressure: 0.41 bar 
Reaction temperature: 150.degree. C. 
(E") Reaction pressure: 0.41 bar 
Reaction temperature: 160.degree. C. 
(F) Pressure: 0.39 bar 
Temperature in sump: 140.degree. C. 
(G) Pressure: 0.39 bar 
Temperature: 230.degree. C. 
(H) Pressure: 0.03 bar 
Temperature: 150.degree. C. 
(J) Pressure: 1.0 bar 
Temperature in sump: 130.degree. C. 
(301) R.sup.1 --NH--CO--OR.sup.2 Input: 0.5 kg/h 
(303) 96.3 wt. % R.sup.1 --NH--CO--OR.sup.2 
(304) Product flow: 12.7 kg/h 
69.9 wt. % R.sup.1 --NCO 
(305) 88.6 wt. % R.sup.2 --OH 
(306) Product input: 11.2 kg/h 
0.05 wt. % catalyst 
(308) R.sup.2 --OH discharge: 5.3 kg/h 
(309) Product flow: 0.9 kg/h 
78.5 wt. % R.sup.1 --NH--CO--OR.sup.2 
(310) Product flow: 2.9 kg/h 
87.8 wt. % R.sup.1 --NH--CO--OR.sup.2 
(312) Product flow: 0.5 kg/h 
43.4 wt. % R.sup.1 --NH--CO--OR.sup.2 
(317) Product flow: 26.8 kg/h 
10.4 wt. % R.sup.1 --NCO 
6.9 wt. % catalyst 
0.6 wt. % R.sup.3 --NH--CO--OR.sup.2 
0.2 wt. % R.sup.3 --NCO 
(319) Product flow: 18.0 kg/h 
99.1 wt. % R.sup.1 --NH--CO--OR.sup.2 
(320) 88.1 wt. % R.sup.3 --NCO 
10.6 wt. % R.sup.1 NCO 
(321) R.sup.3 --NCO discharge: 5.9 kg/h 
(322) Product flow: 0.9 kg/h 
10.4 wt. % R.sup.3 --NCO 
9.8 wt. % R.sup.3 --NH--CO--OR.sup.2 
R.sup.3 --NCO 
Yield: 99% of theoretical yield 
______________________________________