Note: Descriptions are shown in the official language in which they were submitted.
CA 022021~3 1997-04-08
PROCESS FOR SELECTIVE SEPARATION OF MORPHOLINE
The present invention is concerned with a process for the
selective separation of morpholine from an aqueous solution
cont~;ning morpholine, N-methylmorpholine and N-
methylmorpholine-N-oxide. In particular, the present
invention is concerned with a process for the regeneration of
an aqueous process liquid of the amine-oxide process
containing morpholine, N-methylmorpholine and N-
methylmorpholine-N-oxide.
For some decades there has been searched for processes for
the production of cellulose moulded bodies able to substitute
the viscose process, today widely employed. As an alternative
which is interesting for its reduced environmental impact
among other reasons, it has been found to dissolve cellulose
without derivatisation in an organic solvent and extrude from
this solution moulded bodies, e.g. fibres, films and other
moulded bodies. Fibres thus extruded have received by BISFA
(The International Bureau for the Standardization of man made
fibers) the generic name Lyocell. By an organic solvent,
BISFA understands a mixture of an organic chemical and water.
It has turned out that as an organic solvent, a mixture of a
tertiary amine-oxide and water is particularly appropriate
for the production of cellulose moulded bodies. As the amine-
oxide, primarily N-methylmorpholine-N-oxide (NMMO) is used.
Other amine-oxides are described e.g. in EP-A - 0 s53 070. A
process for the production of mouldable cellulose solutions
is known e.g. from EP-A - 0 356 419. For the purposes of the
present specification and the present claims, the production
o~ cellulose moulded bodies using tertiary amine-oxides
generally is referred to as amine-oxide process.
In EP-A - 0 356 419, an amine-oxide process for the
production of spinnable cellulose solutions is described,
wherein as a starting material among other substances a
suspension of cellulose in liquid, aqueous N-
methylmorpholine-N-oxide (NMMO) is used. This process
CA 022021~3 1997-04-08
consists in transforming the suspension in a thin-film
treatment apparatus in one single step and continuously into
a mouldable solution. Finally, the mouldable solution is spun
into filaments by a forming tool such as a spinneret and the
filaments are passed through a precipitation bath.
In the precipitation bath the cellulose is precipitated. The
tertiary amine-oxide is accumulated in the precipitation
bath. The precipitation bath may contain up to 30 weight% o~
amine-oxide. For economic reasons of the amine-oxide process
it is of vital importance to recover the amine-oxide as
completely as possible and reuse it for the production of a
mouldable cellulose solution. Thus is is necessary to recover
NMMO from the precipitation bath.
A process for the recovery of NMMO from diluted aqueous
solutions is known from DD-A - 274 435. According to this
process, the aqueous solution is passed over exchanger
columns filled with styrene/divinylbenzene copolymer
conta;n;ng SO3H groups until it reaches its maximum equimolar
exhaustion. Subsequently the NMMO is displaced by equimolar
amounts of sodium hydroxide solution and the exchanger
columns are regenerated by means of acid.
In addition to the amine-oxide however, the degradation
products of the amine-oxide are also accumulated in the
precipitation bath. These degradation products may be
intensively coloured, thus deteriorating the quality of the
cellulose moulded bodies produced. On the other hand, other
substances may represent an additional safety risk, since
under certain conditions the amine-oxide tends to show highly
exothermic decomposition reactions and these decomposition
reactions may be induced or accelerated by certain
substances. These substances have to be removed from the
precipitation bath which is to be regenerated before the NMMO
is concentrated and separated.
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After removing these unwanted substances, water is withdrawn
from the purified precipitation bath which optionally is
combined with other process liquids of the amine-oxide
process such as vapour condensates formed during the
production of the cellulose solution. This may be carried out
for instance by means of evaporation. The residue of this
evaporation contains highly concentrated aqueous amine-oxide
which is recycled again into the amine-oxide process. The
vapours of the evaporation consist mainly of water, wherein
considerable amounts of N-methylmorpholine, the main
degradation product of NMM0, are also dissolved. Moreover,
the vapours contain also NMM0 and morpholine. Typically, the
vapours contain up to loo mg of NMMO, 240 mg of N-
methylmorpholine and up to 30 mg of morpholine per litre.
Conveniently, these vapours are concentrated, e.g. by means
of reverse osmosis. The aqueous solution obtained contains
typically up to 4 g of NMMO, up to 10 g of N-methylmorpholine
and up to approximately 1 g of morpholine.
From EP-A - 0 402 347 it is known to separate amines from
waste waters of cellulose processing by means of a cation
exchanger. The cation exchanger carries carboxyl groups as
functional groups. Afterwards, the cation exchanger charged
with the amines is treated with an aqueous solution of a weak
acid having a pKa value of more than 3,0 to eluate the
amines. The eluate is regenerated by means of distillation,
part of the weak acid being separated from the amines and
optionally also recovered. By means of this process, up to
94% of N-methylmorpholine and morpholine in aqueous solutions
containing both amines are removed from the waste water. The
separated amines are disposed by combustion.
Moreover it is known to separate morpholine, N-
methylmorpholine and NMMO together from waste waters by means
of a cation exchanger (C. Grilc and N. Zitko, Recovery of
Morpholine; Chem. Biochem. Eng. Q. 6 (4), 189-193 (1992)).
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EP-A - 0 468 951 describes a process for the separation of
amine-oxides from aqueous solutions, particularly waste
waters, produced in the cellulose process. According to this
known process, the waste waters are contacted with a cation
exchanger comprising carboxyl groups as functional groups to
charge the cation exchanger with the amine-oxides, whereafter
the charged cation exchanger is washed and the amine-oxides
are treated with an aqueous solution of a weak acid having a
pKa value of more than 3,0 to eluate the amine-oxides. This
process also aims at eliminating the amine-oxides completely
from the waste waters so as to dispose them in an
environmentally friendly way.
In the amine-oxide process however, the NMMO losses should be
kept as low as possible. Also, N-methylmorpholine should be
oxidized again to NMMO and recovered. Oxidation may be
carried out for instance using a peroxidic oxidant.
A process for the preparative production of tertiary amine-
oxides by means of oxidation of tertiary amines is known e.g.
from EP-A - 0 092 862. According to this process, the amine-
oxide is oxidized under pressure with molecular oxygen in an
aqueous solvent, said solvent having a pH value approximately
equal or higher than the pKa value of the tertiary amine.
DD-A - 259 863 is concerned with the production of aqueous
NMMO solutions by means of oxidation of N-methylmorpholine
with H22 and by passing the reaction solution over one or
more exchanger columns filled with styrene/divinylbenzene
copolymer containing sulphonate groups, as well as by
adjusting a pH value of the solution to values ranging from 8
to 5 by addition of phosphoric acid.
In an oxidation it is disadvantageous that morpholine present
in the process liquid introduced as a contamination together
with the tertiary amines is partially transformed into toxic
N-nitrosomorpholine, which is accumulated unwantedly in the
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NMMO cycle. Additionally, other nitrosoamines are also formed
in the oxidation reactions.
Oxidation of N-methylmorpholine with H22 to NMMO is known
e.g. from EP-A - 0 254 803. From DE-A - 4 140 259, the
production of NMMO by a process wherein the formation of
nitrosoamines is restricted by scavenging primary and
secondary amines, for instance by means of acid halides, is
known. EP-A - 0 320 690 describes the production of-amine-
oxides substantially free from nitrosoamines by oxidation
with peroxides in the presence of a combination of
CO2/ascorbic acid acting as a nitrosoamine inhibitor. From
EP-A - 0 401 503, oxidation with H22 in water and a co-
solvent, preferably a carboxylic acid ester, is known.
According to FR-A - 8 808 039, oxidation is carried out while
adding CO2, and according to US-A - 5,216,154, oxidation to
NMMO is carried out in a pure C02 atmosphere.
In the state of the art, the forming of nitrosoamine either
is not prohibited, or it is achieved by removing the starting
products of the N-nitrosomorpholine or by employing additives
to slow down the formation rate of the N-nitrosomorpholine.
Particularly in an amine-oxide process comprising a closed
cycle, the addition of various chemicals such as acid halides
or ascorbic acid or CO2 to the process causes problems in the
purification of the process liquids, since the degradation
products introduced together with the added chemicals have to
be removed from the process. For many chemicals, it is also
necessary to consider safety aspects such as the risk of
exothermic reactions. Thus, neither of the described
processes is appropriate for the regeneration of process
liquids of the amine-oxide process.
Thus it is the object of the present invention to provide a
process for the selective separation of morpholine from
various process liquids of the amine-oxide process wherein
substantially only morpholine is separated and NMMO and N-
methylmorpholine will remain in the process liquid.
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The process according to the invention for selective
separation of morpholine from an aqueous solution cont~in;ng
morpholine, N-methylmorpholine and NMMO is characterized by
the following steps:
(A) passing the aqueous solution over a cation exchanger
capable of adsorbing morpholine in such an amount until
it cannot be charged substantially with morpholine any
more and an eluate substantially free from morpholine
but cont~in;ng N-methylmorpholine and N-
methylmorpholine-N-oxide is obtained, and
(B) regenerating the cation exchanger charged with
morpholine and reusing it in step (A).
The present invention is based upon the finding that a cation
exchanger evidently has a higher activity for morpholine than
for N-methylmorpholine and NMMO and that due to this higher
activity a high yield of N-methylmorpholine and NMMO has
already been eluated the moment morpholine starts to break
through, thus allowing a distinct separation of the
morpholine. In detail, the separation is carried out such
that first each of the three components, i.e. morpholine, N-
methylmorpholine and NMMO, are adsorbed at the fresh cation
exchanger. When the cation exchanger is charged with these
three components, NMMO starts to break through, since on the
one hand the following NMMO cannot be adsorbed any more and
on the other the following morpholine and N-methylmorpholine
displace the NMMO already adsorbed. This means that the
eluate contains at that point actually only NMMO.
When substantially all the NMMO is displaced at the cation
exchanger, N-methylmorpholine also appears in the eluate,
being displaced by the following morpholine. At that point,
the eluate contains NMMO and N-methylmorpholine. Only when
substantially no N-methylmorpholine is adsorbed at the cation
exchanger and the capacity of the cation exchanger is
exhausted, morpholine starts to break through and the
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eluation has to be interrupted and the cation exchanger
regenerated. This can be done for instance using diluted
mineral acids.
The cation exchanger employed in the process according to the
invention exhibits preferably carboxyl groups and/or
sulphonic acid groups.
A preferred embodiment of the process according to the
invention is characterized by the further step of
(C) subjecting the eluate obtained in step (A) to an
oxidation treatment optionally after water has been
removed to oxidize N-methylmorpholine to N-
methylmorpholine-N-oxide.
This embodiment of the process according to the invention
completely prevents the new formation of the toxic N-
nitrosomorpholine, since the eluate contains actually no
morpholine. Thus the oxidized eluate contains only the usual
reduced basic level of N-nitrosomorpholine which occurs in
the amine-oxide process.
Conveniently, oxidation is carried out by means of a
peroxidic oxidant. As the peroxidic oxidant, in the process
according to the invention preferably H22 is used. The H22
is employed preferably as an aqueous solution having 30-50
weight% of H202. The H22 is best employed in an amount of
from 0,8 to 2 mole per mole of N-methylmorpholine.
Another preferred embodiment of the process according to the
invention is characterized in that the aqueous solution is
exposed to ultraviolet radiation having substantially a
wavelength of 254 nm during or subsequently to the oxidation
treatment. The ultraviolet radiation is best emitted by a
mercury low-pressure lamp.
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This embodiment of the process according to the invention is
based on the finding that N-nitrosomorpholine can be
destroyed by exposure to ultraviolet radiation having an
intensity maximum of 254 nm. Therefore, when exposure to
ultraviolet radiation is carried out during or subsequently
to the oxidation treatment, destruction of the N-
nitrosomorpholine present at a basic level will occur, and
thus it becomes possible to significantly reduce the basic
level of this toxic substance.
It has been shown that it is more advantageous to first
separate morpholine by means of the cation exchanger and then
proceed to oxidize the eluate, since thus the exposure time
to ultraviolet radiation and the exposure intensity necessary
to destroy the N-nitrosomorpholine are significantly reduced.
When morpholine is not separated before oxidation, an amount
of N-nitrosomorpholine equivalent to the morpholine content
will be formed again, requiring a significantly higher
exposure time and rate for destruction.
The exposure rate may range e.g. from 200 to 500 mJ/cm2,
depending on the design of the lamp and the process
conditions, particularly the temperature.
General methods for the quantitative analysis of
nitrosoamines which use a W exposure and a subsequent
determination of the nitrites formed are known (D.E.G.
Shuker, S.R. Tannenbaum, Anal. Chem., 1983, 55, 2152-2155; M.
Rhighezza, M.H. Murello, A.M. Siouffi, J. Chromat., 1987,
410, 145-155; J.J. Conboy, J.H. Hotchkiss, Analyst, 1989,
114, 155-159; B. Buchele, L. Hoffmann, J. Lang,
Fresen.J.Anal.Chem., 1990, 336, 328-333). These analytic
methods however do not deal with the destruction of N-
nitrosomorpholine.
For exposure according to the invention using a low-pressure
lamp, the lamp may be hung into a container containing the
process liquid which is to be treated. However the lamp may
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also be arranged in another way. Moreover, exposure may be
carried out for instance during a continuous pumping over of
the solution to be exposed in a thin-film W-reactor.
The process according to the invention is particularly
appropriate for regeneration of a process liquid from the
amine-oxide process.
Another preferred embodiment of the process according to the
invention comprises the following steps:
(1) passing the above vapours concentrated for instance by
means of reverse osmosis over a cation exchanger capable
of selectively adsorbing morpholine and safeguarding that
the pH value lies in a range of from 6,0 to 9,0,
thereafter
(2) combining the eluate obtained from the cation exchanger
with purified precipitation bath of the amine-oxide
process, said precipitation bath containing 10-30 weight%
of NMMO and
(3) treating the eluate combined with the precipitation bath
in an evaporation reactor with the peroxidic oxidant to
oxidize N-methylmorpholine and to concentrate, obtaining
concentrated, aqueous NMMO which is recycled again into
the amine-oxide process and vapours which are condensed
and employed in step (1).
By means of the following Examples, the invention will be
explained in more detail. The abbreviations NMOR, NMMO, NMM
and M used in the following to denote N-nitrosomorpholine, N-
methylmorpholine-N-oxide, N-methylmorpholine and morpholine
respectively.
Example 1
A process liquid from the amine-oxide process, i.e. a residue
of a reverse osmosis, was passed over a weak acidic cation
exchanger (polyacryl back bone having carboxyl groups as
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--10--
functional groups; Dowex CC-2 made by The Dow Chemical
Company). The residue had a pH of 9,9 and the following
composition:
NMMO: 1661 ppm
NMM: 2377 ppm
M: 1376 ppm
30 ml of cation exchanger were used in a column having a
diameter of 2,5 cm and a height of approximately 5,5 cm. The
residue was passed over the cation exchanger at a flow rate
of 4 bed volumes per hour. The eluates were collected in
intervals of 5 bed volumes each, and afterwards the pH value
and the NMMO, NMM and M concentrations were determined. The
cation exchanger started to swell after 10 bed volumes, and
the swelling went on continuously until the charging stopped,
amounting to 150% after 200 bed volumes.
The NMMO, NMM and M concentrations (ppm) were determined by
means of HPLC (column: Hypersil Si 150 x 4 mm; 50C; eluant:
52% of acetonitrile far W, Fisions Scientific Equipment no.
A/0627/17; 48% of 10 mmole KH2PO4 (Merck no. 4873), adjusted
to pH 6,7 with NaOH; isocratically lml/min; detector: W 192
nm). The quantification of each of the components was carried
out by calibrating an external 3 point gauging. The results
are shown in the following Table.
Table
Bed NMMO NMM M pH
volume
Start 1661 2377 1376 9,9
1 n.d. n.d. 4,0
1 n.d. n.d. 3,9
332 2 n.d. 5,1
2350 1 n.d. 5,8
2409 241 n.d. 7,4
2064 276 2 7,3
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2026 1210 3 8,1
1943 1517 5 8,1
1850 2516 6 8,4
1805 2736 6 8,4
100 1671 3461 5 8,5
110 1632 4031 5 8,6
120 1594 4050 6 8,6
130 1594 3919 6 8,6
140 1596 4132 6 8,6
150 1597 4063 7 8,6
160 1596 3939 13 8,6
170 1588 4060 85 8,6
180 1605 3441 459 8,8
190 1625 2723 1422 9,3
200 1620 2390 1875 9,3
210 1646 2390 1748 9,2
n.d. = not detectable
As can be seen from the Table, M may be distinctly separated
from NMM and NMMO:
In the beginning, each of the three components, i.e. NMMO,
NMM and M, is retained by the cation exchanger, and the pH
drops from 9,9 to approximately 4,0.
From the 20th bed volume on, NMMO starts to eluate, while NMM
and M are retained, so that up to the 40th bed volume the
eluate actually contains only NMMO. The pH increases to 5,8.
The eluation of NMMO is probably due to a displacement of the
NMMO already adsorbed by the cation exchanger by following
NMM and M.
From the 40th bed volume on, NMM also starts to eluate, while
M is still retained. The pH increases further to
approximately 8-9. Obviously the adsorbed NMM is displaced by
the following M at the ion exchanger.
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- 12 -
Surprisingly, M is eluated only from approximately the 17Oth
bed volume on, i.e. at a point when at least 85 weight% of
the NMMO and the NMM have already been recovered. From this
point on, the pH increases again to approximately g, 3. Thus
the cation exchanger is wholly charged with morpholine and
has to be regenerated after 170 bed volumens.
The eluate collected up to the 170th bed volume is actually
free from M and may be used for the oxidation treatment to
produce NMMO.
Example 2
An aqueous solution cont~in;ng 25 ~g of NMOR, 2530 mg of
NMMO, 3923 mg of NMM and 30 mg of M was mixed with 30% H22
(mole NMM/mole H22 = 1/1,2) to oxidize NMM to NMMO and
exposed to radiation of a mercury low-pressure lamp in a W
reactor (type EK-36, no. 79000, made by Katadyn) (wavelength:
254 nm). The temperature of the process liquid was 50C.
The NMOR concentration was determined by means of HPLC
(column: Hypersil ODS 250 X 4 mm; 50 C; eluant: A = 0,6% of
acetonitrile; B = 49,7% of H2O; gradient 1 ml/min.; 10 min. -
100% A; 7 min - 100% B; detector: W 238 nm).
Within the first 90 minutes, the NMOR concentration increased
to 45 ~g/l, which is due to a fast reaction of the M present
in the solution. Afterwards, the NMOR concentration decreased
again rapidly. After 6 hours, there was no evidence of NMOR.
After a total oxidation time of 20 hours, the solution
contained 5386 mg of NMMO/litre. This amounts to a yield of
62% of the theory.
