Note: Descriptions are shown in the official language in which they were submitted.
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Le A 29 279-F.C.
Process for the preparation of residue-free 2,4/2,6-
diaminotoluene mixture
The present invention relates to a process for the
continuous separation of residue from raw m-diaminotoluene
mixtures (m-TDA; 2,4/2,6-diaminotoluene mixture) for the
preparation of residue-free 2,4/2,6-diaminotoluene mixtures
(m-TDA~, and use of the residue-free m-TDA mixtures.
When m-TDA is industrially produced on a large scale by
dinitration of toluene followed by hydrogenation to the
tolylenediamines, higher-boiling by-products, inter alia
diphenylmethanes, diphenylamines, acridines and phenazines,
naturally arise.
The m-tolylenediamine mixture produced from toluene by
dinitration followed by hydrogenation contains between 0.3%
and 2 % of higher molecular weight incidental constituents,
hereinafter designated TDA residue, depending on the
process used for its preparation. Three processes are
known for further processing the residue-containing m-TDA
mixture after the more volatile components - solvent and
water - and the ortho-isomers (2,3- and 3,4-
diaminotoluenes) have been separated by distillation.
1. The residue-containing m-TDA mixture is directly
phosgenated to m-tolylene diisocyanate (TDI). The
higher molecular weight compounds also phosgenated in
the process are discharged as a TDI residue in the
working-up process.
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Advantage: - a process step is saved
Disadvantages: - the process can be applied only to
liquid-phase phosgenation
- residue in the m-TDA leads in the
next process to increased formation
of higher molecular weight
components, at the expense of yield
- difficulties in residue removal
from the TDI process.
2. The residue-containing m-TDA mixture is distilled out
over a column in distillation boilers heated with steam
at 30 bar. A proportion of m-TDA > 5% r~m~ins in the
bottom to ensure that the bottom is fluid under these
conditions (230 C).
Advantage: - the residue-free m-TDA can be
utilised in gas phase phosgenation
- and as a marketable product.
Disadvantages: - lower yield than in the process
discussed under head 1,
- disposal of the TDA residue mixture
(highly viscous) by combustion,
with loss of m-TDA.
3. Continuous residue separation by residual evaporation
of the residue-containing m-TDA in thin-film
evaporators and discharge of the residue having a m-TDA
content > 10% as "plasticiser".
Advantage: - the residue-free m-TDA can be
utilised in gas phase phosgenation
and as a~marketable product.
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Disadvantages: - lower yield than in the process
discussed under head 1,
- disposal of the TDA residue mixture
(highly viscous) by combustion,
with loss of m-TDA.
When the residue is separated by distillation in accordance
with the processes under heads 2 and 3, the residue occurs
as a highly viscous mass which - depending on the m-TDA
content of the residue - vitrifies at temperatures below
150 C,. The m-TDA which remains in the residue detracts
from the process yield.
The object of the invention was therefore to provide a
process which enables residue-free m-TDA to be prepared and
separated completely from the raw m-TDA or the residue,
without the disadvantages mentioned above.
This object was achieved by means of the process according
to the invention.
The subject of the invention is a process for the
preparation of residue-free m-tolylenediamine by nitrating
toluene, hydrogenating the nitro compounds, separating the
water and the solvent by distillation, and removing the
ortho-isomers from the tolylenediamine mixture by
distillation, followed by distillation of the remainder,
which is characterised in that the distillation is
performed such that a quantity of 2,4- and 2,6-
tolylenediamine (m-tolylenediamine, m-TDA) is distilled
from the remainder such that a bottom having a residue
concentration of from 30 to 70 wt-% remains, the latter
bottom is mixed with an auxiliary substance which is inert-
to tolylenediamines and has a boiling point of > 290 C, in
a quantity such as to adjust a residue concentration of
from 28 to 50 wt-%, the latter bottom which is mixed with
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an auxiliary substance undergoes a further single- or
multi-stage distillation until a residual concentration of
2,4- and 2,6-tolylenediamine of from 1 to 5 wt-% is
obtained in the bottom, and the remaining residual bottom,
which is still liquid even at low temperatures, is
discharged.
The advantage of the process according to the invention is
that, without incurring heavy technical costs, virtually
all the m-TDA can be obtained free of residue, and that
moreover the r~m~;n;ng residue itself can be discharged at
low temperatures (approx. 100 C) without difficulty.
One or more compounds from the group below are preferably
utilised as auxiliary substances:
- distillation residues from ethylene glycol preparation
(EDR)
- distillation residues from propylene glycol preparation
(PDR)
- high-boiling polyethers based on ethylene oxide
- high-boiling polyethers based on propylene oxide
- high-boiling polyethers based on mixed polymers of
ethylene oxide and propylene oxide
- high-boiling polyethers based on polytetrahydrofuran
- high-boiling polyethers based on mixed polymers of
aromatic hydroxyl compounds
- high-boiling polyethers based on cycloaliphatic oxo
compounds
- sugar, for example molasses
- high-boiling alkyl aromatics.
The process according to the invention is preferably
performed continuously. The raw m-TDA (tolylenediamine
mixture after separation of the more volatile components
(water, solvent) and of the ortho-isomers) normally
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contains from 0.3 to 2 wt-% residue. In a first step a
quantity of m-TDA (2,4/2,6-tolylenediamine) is distilled
off such that the concentration of the residue is
subsequently from 30 to 70 wt-%. This concentrated raw m-
TDA is mixed with an auxiliary substance and undergoes asecond distillation. This is conducted preferably at
temperatures of between 160 C and 260 C, particularly
preferably between 190 and 220 C, and at pressures of
between 2 and 100 mbar, particularly preferably between 5
and 40 mbar. The concentration of m-TDA in the bottom
after distillation is preferably now only from 1 to 5 wt-%.
The bottom is discharged and preferably has a viscosity at
100 C of < 150 mPas.
The in~ention is explained in greater detail with the aid
of the Example which follows.
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Example
152.733 kg of raw m-TDA having a residue content of 1.5%
are fed continuously into a distillation column which is
operated at 206 C and 5 mbar. A distillate of 148.649 kg
m-TDA is obtained (yield calculated on m-TDA: 98.81%).
The residue concentrate leaving the distillation column
(4,0842 kg; residue content: 56.1%) is mixed with
5.325 kg of residue from ethylene glycol distillation and
is fed continuously into a second column which is operated
at 206 C and 5 mbar. A distillate of 1,724 kg of m-TDA is
obtained (yield calculated on m-TDA: 1.15%). The total
m-TDA yield is 99.95%.
The residue leaving the column still contains 0.05% m-TDA
(calculated on total m-TDA) and has a viscosity at 80 C of
50 mPas.
