Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HOECHST AKTIENGESEL~SCHAET HOE 90/F 331 Dr. K/rh
Description
Process for the preparation of trioxane
The invention relates to a process for th~ preparation of
S trioxane from homo- and/or copo]L~mers of formaldehyds in
the presence o~ acid cataly~ts.
The pxeparation of trioxane from aqueous foxmaldehyde
solutions is de6cribed in various in~tances in the
literature ~cf. Walker, Formaldehyde, Reinhold Publ. New
York, 3rd edition, 1964, pages 198-199).
Trioxane (TO~ formed from aqueous formaldehyde at
relatively high temperatures in the presence of acid
catalysts and is removed from the reaction mixture by
distillation. ~he synthesis vapor is usually worked up in
a rectifyiny column mounted on the reactor ~US Paent
2,304,080). The trioxane-rich phase i8 ~ubjected to
extraction and/or another separation process. To achieve
high space-time yields, the reaction is carried out in a
forced eirculation evaporator (Canadian Pa~ent
1,125,774). The copolymers obtained by trio~ane copoly-
merization are of a high quality and belong to the field
of industrial plastics.
Industrial plastics are processed ~y extrusion Dr in~ec-
tion molding, product waste being formed, which must be
disposed of. Sprues and burrs are produced during
processing by in~ection molding. Extruded goods are
mostly machined, up to 50% of the material not
infrequently being obtained a8 waste. Material~ which do
not always conform to type are al80 sometime~ formed
during the production proce~s a~d mu t be dispo~ed of,
Incineration and landfill have to date been available as
methods of disposal. A recycling process is therefore to
be preferred.
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A process for generating gaseous formaldehyde from
polyoxymethylene, such as paraformaldehyde, is known (US-
A 3,883,309). In this process, a dispersion of polymeric
formaldehyde is decomposed the~ally at lOD - 300C to
give formaldehyde, and thi~ eparated off in gaseous
form. This process is extremely complicated and requires
considerable technical effor~ to prevent blockages due to
paraformaldehyde. ~f copolymers are used, contamination
of the formaldehyde by comonome:rs cannot be exclud~d.
The object of the present invention waæ to ~ind a
process which, inter alia, enables polyacetal to be
racycled without displaying the difficulties mentioned,
producing a material which can be used again directly for
copolymerization.
This has been achieved by a process for the continuous
preparation of trioxane from formaldehyde, in which an
ace~al polymer is broken down in the presence of w~ter
and an acid catalyst and the formaldehyde formed is
converted into trioxane in the same process step.
High trioxane concentrations in the synthesis vapor which
correspond to those of the maxLmum equilibrium which can
be achieved using formaldehyde solutions are obtained by
the process according to the invention.
The preparation of trioxane according to the invention is
carried out by conversion of homo- and~or copolymers of
formaldehyde and if appropriate ~yclic formals, for
example in the form of a recycling material, in the
presenc~ of w~ter and acid catalys The amount of water
added is measured so that a calculated formaldehyde
content of 50 - 90, preferably 65 - 85~ by weight is
established in the reactor. Addition of foam suppressants
may be helpful. Mineral acids, strong organic acids or an
amount of another acid catalyst of corresponding cata-
lytic activity are used as the catalyst. Acid catalysts,
which in general must be less volatile than the reaction
.
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mixture, which have proved to be usable are, in particu-
lar, sulfuric acid, phosphoric acid, p-toluene~ulf4nic
acid or ~trongly acid ion exchangers, for example poly-
styrene e~changers containing sulfonic acid groups. ~he
amount of catalyst is not critic:al and is as a rule 1 to
60, preferably 10 - 50~ by weight, ba~ed on the reaction
mixture~
Homo- and copolymers of fQrmaldehyde and/or cyclic
formals, preferably copolymers~ in ~omminuted form are
employed as ~he recycling material. The recycling mater
ials can also contain dyestuffs, pigments, stabilizers
and other customary additives. The recycling material i~
metered into the reactor separately, for example via a
~ransfer channel for solids, or together Wit}l wa~er. The
additives, for exæmple stabilizers, do not interfexe with
the reaction. ~he comonomers are also re-formed in the
reactor and are worked up together with the trio~ane.
Separation of the comonomers is prior art. The reaction
is carried out according to the invention in a known
circulatory reactor with an evaporator. Evaporator6 which
are suitable for this are, for example, forced circula-
tion e~aporators, alling film e~aporators, ri ing film
evaporators or thin film evaporators with forced circula-
tion. Such systems are described, for example, in
~5 Ullmann, Volume 1 (1951~, 3rd edition, pages 533 - 537.
Forced circulation e~aporators are particularly suitable.
However, these reactors mu t con~ain baffles to prevent
the solid particles being sucked up by the pump vf the
forced circulation reactor. Basket-liXe baffles or sieves
in front of the pumps are suitable for t~is purpo e.
The residence time of the reaction mixture in the reac-
tion system i 5 - 240 minukes, preferably 15 - 60
minutes. ~he temperature of the reaction mixture i9 50 to
150C, preferably 95 to 130C, depending on the pressure.
The reaction mixture, which oonsists of trioxane, orm-
aldehyde and water, and if ~ppropriate cyclic formals, is
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removed from the reac~ion system by distillakion with the
aid of the evaporator. The reaction hexe can be carried
out under normal pressure, under reduced pressure, for
examiple under 300 to 1000 mbar, or under increased
pressure, for example 1 to 4 bar. It is preferably
carried out under l to 2 bar.
The synthesis vapor which leaves the reaction sy~tem i8
enriched in the customary manner, either as the vapor or
as the condensate, by means of rectification, as des-
cribed in British Patent 1,012,372.
The txioxane-rich fraction obtained, which al80 contain~
cyclic formals if appropriate, can be purified, for
example, by extraction with a water-immiscible solvent
for trioxane (and if appropriate for the cyclic formals),
such as methylene chloride, and subsequent ~eutralization
and fractional distillation or crystallization. O~her
known iseparation processes can also be used for this
purpose (Process Economics Program Stanford Institute
Report 23 (1967) 181 or DE-OS 1,570,335). The product
stre~ms freed from the trioxane~ which still contain
chiefly formaldehyde and water, can be recycled continu-
ously into the reaction tank.
The proce~s accordin~ to the invention, which can be
carried out continuously or discontinuouisly, makes
possible a trioxane synthesis with a minimum energy
requirement, since th manufacturing costs for providing
highly concentrated formaldehyde are dispensed with. The
highly concentrated formaldehyde is replaced here by the
recycling material employed. ~he proce s represents a
contribution to waste disposal and environmental
protection, since waste substances are p~s~ed to the
production proces~ and incineration plant~ and landfill
sites via which the recycling material previouRly had to
be disposed of are tharefore relievad.
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~xample~:
250 g of water, 250 g of concentrated sulfuric acid and
500 g of recycling material in granule form (copolymer of
trioxane and 3~ by weight of dioxolane) were initially
introduced into a ~ 1 four-nec}ced flask with a ~tirrer.
~he mi~ture was heated to the boiling point and ~he
distillate was condensed in a quench cooler. i~ ifurther
39 g of granules and 21 g of water (corresponding to a
65% ~trength formaldehyde solution) were introduced
separately into ~he flask at interval~ oif in each case 15
minutes. The total reaction tLme was 5 hours. The com-
position of the distillate was analyzed hourly. The mean
values of the experiments are sho~m in the table. The
dioxolane content corresponds to the comonomer content in
the granules employed. The trioxane content in the
examples according to the inven~ion corresponds to that
obtained from aqueous formaldehyde in the conventional
~rioxane process according to the prior art (comparison
example).
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Table
Conditions: Initially introduced into the flask: 250 ~ of
concentrated ~ulfu:ric acid; 250 g of water;
500 g of ~ranules
Temperature: 104C, running time S hours
~eed every ~ hour: 39 g of granules, 21 g of
water
Example Through-TOX CH20 Dioxolane Water
putin the in the in the as the
g~h dis- dis- dis~illate dif fer-
tillate tillate ence~
- % 9~ % %
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201 25.7 39.8 3.1 31.~
1~ 2 223 19 . 7 40 . 1 2 . 4 37 . 8
Comparison~ 250 20 . 6 41. 9 --- 37 . 5
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* Formaldehyde feed concentrations 63 . 596 ~ sulfuric acid
content in the reactor bottom product 10%
** to make 10 0 ~6 -
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