Note: Descriptions are shown in the official language in which they were submitted.
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CONTINUOUS PROCEDURE FOR OBTAINING PURE TERF.PHTHALIC ACID AND GLYCOL
STARTING FROM POLYETHYLENE TEREPHTHALATE WASTE _ __
Abstract of the invention
A procedure is described to obtain pure terephthalic acid and
glycol, which consists in carrying out, in a continuous manner and
. in the presence of decolorizing carbon~ an aqueous and neutral
hydrolysis of polyethylene terephthalate waste, under high temper-
ature and pressure conditions, continuously filtering the product
at the hydrolysis temperature and crystallizing the terephthalic
r acid, while the mother liquors are subjected t.o distillation in
order to obtain the glycol.
Basis of the inventiorl
Field of the invention
The invention uncler consideration is related to a continuous
method which permits obtaining pure terephthalic acid and glycol
and more specificallyr it is related with a procedure which utilizes
as a raw material polyethylene terephthalate waste and in which it
is not necessary to first separate the crude or impure terephthalic
acid in order to purify it in a subsequent stage by means of one
oE the known procedures. In the method of the invention under
consideration, the product is obtained pure and can be utilized
as a raw material once again in -the preparatlon of the polyethy-
lene terephthalate utilized in the manufacture of polyester
fibers and plastics.
Description of the prior art
Neutral. hydrolysls of polyethylene terephthalate at a
high temperature and pressure is a known operation (see Ludewig,
~. and Ramm, ~ German Economic Patent 14,854 (1956) and
Littmann, E. On the preparation of terephthalic acid or its
dimethyl or diglycol ester from polyethylene terephthalate.
Abh. d. Deutsch. Akad. d. Wiss. Kl. F. Chem, Geolo~. und
Biochemie I tl963) 401-~11).
Neither of the methods described in the aforemention-
ed references mentions the addition of decolorizing carbon in
the hydrolysis stage.
On the other hand, the methods mentioned there are
discontinuous methods in which, moreover, the terephthalic acid
is obtained impure so that once it is separated, it will have
to be subjected to some purifying procedure.
In this way, the methods to which reference has been
made require a subsequent purifying stage which may be the
dissolution of the product in sodium hydroxide and subsequently,
a reprecipitation by means of acidification~ either with treat-
ment of the solution obtained or without an intermedi.ate treat~
ment, or by having recourse to transformation to the dimethyl
ester, which is subsequently sublimed.
~ummary of the invention
The method of the invention under consideration has
as its object the obtaining of terephthalic acid and glycol
in which the purification is undertaken simultaneously with
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the hydrolysis.
~ nother object of the invention under consideration is
to provide a method for the preparation of pure terephthalic
acid and glycol in which the number of stages of the process is
reduced.
An advantage of the method of the invention under
consideration is that upon reducing the number of stages of the
process - that is, upon eliminating the subsequent stage of
purifying the terephthalic acid - the cost of the product is
lowered.
Another advantage of the invention under considera-
tion is that it permits the utilization of the polyethylene
terephthalate in a simple manner.
Yet another advantage oE the method of this invention
is that due to the simplicity of the equi~ment utilized, it can
be carried out with relative ease.
~ ccording to the present invention, there is now
provided a continuous method for obtaining pure terephthalic
acid and glycol on the basis of polyethylene terephthalic waste,
characterized by the fact that it comprises the following
stages: subject to hydrolysis, in the presence of a certain
amount of decolorizing carbon, a quantit~ of polyethylene tere-
phthalate waste with water, at a high pressure and at a
tem~erature which varies between 200C and 300C; filter the
hydrolyzate also under conditions of high temperature and
pressure; crystallize the terepht~alic acid of the filtered
solution by means of cooling in the crystallization receptor at
a final temperature which varies between -10C and 200C;
filter or centrifuge to separate the filtrate crystals; wash and
dry the pure terephthalic acid crystals; and, on the other hand,
distill ~he filtrate so as to obtain pure glycol.
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Preferably, the quantity of water used is equa~ or
greater than that required to keep the -terephthalic acid dis-
solved under hydrolysis conditions.
The present invention may also be defined as the
continuous process for obtai.ning pure terephthalic acid and
glycol from polyethylene terephtha:Late waste compri.sing the
steps: subjecting polyethylene terephthalate waste to neutral,
aqueous hydrolysis in ahydrol~sis zone at a temperature of
between 200C and 300C and superatmospheric pressure of at
least 15 atmospheres~, said hydrolysis being conducted in the
presence of decolorizing carbon in an amount of between 0.1 to
30 weight percent of said waste; removing supernatant liquid
from said hydrolysis zone and filtering said liquid under
conditions of high temperature and pressure to recover an
aqueous solution of terephthalic acid and glycol passing said
aqueous solution of terephthalic acid and glycol to a crystal-
lization zone and cooling said solution to a temperature of
between -10C and 200C in said crystallization zone to there-
by cause crystallization of terephthalic acid from said solu-
tion; separating by filtering or centrifuge said terephthalicacid crystals from the liquid in said crystallization zone and
washing and drying said crystals to provide pure terephthalic
acid; passing the liquid from said crystallization zone to a
distillation zone and distilling said liquid to provide pure
glycol.
Detailed description of the invention
According to the preferred me-thod of this invention,
the polyethylene terephthalate waste is supplied in the form
of granules or yarns or in any other form suitable for handling,
to the melting chamber of a screw-type extruder so that the
extruder, in turn, feeds it, in a continuous manner, to a
hydrolyzer. Alternately, the polymer can also be melted and
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injected continuously into the hydrolyzer in this state by means
of a high-pressure pump.
In a simultaneous manner, water is injected into said
hydrolyzex in a suitable proportion - that is, between 3 to 20
times the amount of the polymer. The water can be lntroduced
cold or hot. ~n turn, the decolorizing carbon is added in a
proportion between 0.1% and 30% (with respect to the quantity
of the treated waste) r feeding it to the hydrolyzer either with
the polymer or suspended in water. Also, a large amount can be
introduced in bulk intermittently into the hydrolyzer every so
often. The hydrolyzer utilized can be of any type - that is,
it can be cylindrical vertical, horizontal or inc:Lined, or it
can be the U-shaped type, either ~ertical, horizontal or
inclined.
The hydrolyzer, in turn, should be equipped with a
source of heating which can be a jacket into which a heating
fluid is introduced; this fluid can be diphenyl-diphenyl oxide
or a hot oil or steam. The heating source can also be an
interior or e~terior coil, using one of the fluids previously
mentioned.
Heating can also be provided by means of li~e steam
at high pressure, which is introduced directly into the
hydrolyzer.
Preferably, the hydrolyzer should be equipped with
devices which impart a slight agitation.
When the equipment is put into operation, the polymer
which has not yet been hydrolyzed will be found in the lower
part. This is also true for the greater part of the carbon,
whereas, on the other hand, the supernatant liquid will be an
aqueous solution of terephthalic acid and glycol with some
proportion o~ the decolorizing carbon utilized, which will be
suspended in the solution.
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This solution is subjected to a filtering operation
under the temperature and pressure conditions in which the
hydrolysis is carried out, with the object of eliminating the
carbon and all the impurities adsorbed by it. This filtration
can be carried out in many diverse ways but the use of a heated
cartridge filter is particularly preferred; this is located
next to the hydrolyzer - that is, said filtration can be under-
taken by means of cartridges which are installed directly
within the hydrolyzer.
Once the solution is filtered, the pressure to which
it is subjected is released and it is sent to a continuous
crystallizer which may be at atmospheric pressure, at a
pressure higher than atmospheric pressure, or at a pressure
lower than atmospheric pressure.
This crystallizer can be equipped ~ith a cooling
jacket or a coil, or cooling may be attained by simply venting
the pressure.
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residual polar solvent (e.g. methanol) used for the
desorption operation is driven out of the column by
heating and preferably a volatile aliphatic hydrocarbon,
e.g. hexane either in the vapour phase or as a superheated
5 liquid. The formation of azeotropic mixtures is also
helpful in this case. Herein the first step is called
"desorption" and the second step "reactivation".
Pretreatment, adsorption and desorption are
preferably carried out at temperatures from 15 to 75C,
10 more preferably not in excess of 60C, but pretreatment in
particular may be carried out at temperatures from 30 to
110C and adsorption and desorption at 40 to 80C.
Adsorption is preferably effected in particular at 30 to
60C, particularly approximately between 20 and 50%.
Reactivation is preferably carried out between 50 to
170C. Substantially less bleaching aid or active carbon
is used in the pre-treatment step than adsorption agent,
from 0.5 to 5% by weight of the fat being adequate for the
former, but with preferably a fat:adsorbent ratio from 10:1
20 to 1:1 by weight of the fat. Preferably both the pre-
treatment and adsorption steps are applied to the same
solution of fat.
Suitable bleaching earths for use in the invention
A include activated Fuller's earth, for example Tonsil and
25 Fulmont, Lucilitet Kieselsaure of Degussa. Granulated or
non-granulated active carbon, e.g. Norit~ may alternatively
or in addition be used. Bleaching earths are usually
acid-activated natural earths of structures typified by
Montmorillonit and Bentonit. Acid treatment increases
30 their propensity for adsorption of highly polar organic
com~vunds including highly polar pigments, e.g.
chlorophyll. These earths are not suitable for the
zdsorpti~n of bulk amount~ of diglycerides etc, but very
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Note 30 The greater the temperature, the higher will be the
rate of the reaction, with the pressure in the equipment being
greater at the same time.
Note 4. The pressure is a function of the ten~perature.
Note 5. The residence time required will depend on the temper-
ature used and on the morphology of the polymern
Note 6. The residence time in the crystallizer will depend
on the average size of the crystal desired, wi~h the crystal being
larger, the greàter the residence time.
In accordance with the data previously indicated, the method
of the invention can be carried out under conditions which are dif~
ferent up to a certain point, and it can be modified in accordance
with the level of production required.
Also, very different heating sources can be used, with the
needed changes also made in accordance with the morphology and the
quality of the waste which is utilized as a raw material.
There will also be variations in accordance with the charac-
teristics of purity and size of the crystal desired in the terephtha-
lic acid produced.
Furthe~nore, the economic aspect will also influence the
selection of the equipment to be employed.
Taking into cons~deration all the modifications which are com-
prised within the spirit and scope of the invention, the following
will give the preferxed en~odiment, which is merely for illustrative
purposes.
Detailed explanation of the preferrea embodiment
/ In this en~odiment, the method of the invention is carried out ~th the
/ followins equipment as illustrated in the drawing:
a) A twin-screw melter extruder which is discharged by means
of a multiple-orifice spray head directly into the upper part of
the hydrolyzer.
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b~ A vertical cylindrical hydrolyzer which has a verticai
plate in its interior which divides the hydrolyzer into two semicir-
cular sections~ Said plate does not have to extend along the whole
length but rather will leave a free space both in the upper as well
as the lower par~s. The supply nozzle which is connected with the
extruder is centered over one of the semicircles. On the upper part
of the other semicircle there is a bundle of filter cartridges
whose outlets are all connected with a common discharge tube. The
entry of water i5 on one side of the hydrolyzer, the side correspond
ing to the supply of the polymer. The bottom of the hydro~yzer
will be equipped with two connections - one which will be used
for the introduction of live steam, and the drainage of sai~ hydroly-
zer being undertaken through the other connection.
c~ A continuous crystallizer which is made up of a vertical
tank, a conical bottom and which is equipped with an agitatorO The
crystallizer is connected to the discharge tube of the filter car-
tridges on the hydrolyzer by means of a line and an automatic valve
which permits the passage of all the liquid in addition to ~ small
quantity of steam which has an agitation function in the hy-lrolyzer.
Said crystallizer also includes an automatic level control ~ischarge
device~
d) A continuous centrifuge with a washing device which receives
the discharge ~rom the crystallizer and whichr in turn, continuously
discharges toward the drier.
e) A drier of the vibrating screen type.
f) A distillation column for the recovery of the glycol~ on
the basis of mother liquors.
What has been described in the preceding is related to the
equipment which is particularly preferred ~or carrying out the method~
which consists of the following stages~
The hydrolyzer is loaded with a suitable quantity of water in
a continuous manner by means of a metering pump. High-pressure
saturated steam is also injected con-tinuously from the lower part
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of the hydroly~er. The waste polymer, together with decolorizing
carbon is fed continuously to the hyclrolyzer by means of the ex-
truder previously described.
The discharge from the hydrolyzer i5 adjusted in such a manner
that it is slightly agitated by means of the excess steam.
Once the hydrolysis has been undertaken, the product is sub-
jected to filtration in order to be subsequently crystallized. One
should indicate that the agitation in the crystallizer has to be
the minimum so as to keep the crystals suspended.
The crystals obtained are subjected to a drying stage, which
is carried out in a drier which has the flow of air and the temper-
ature suitably adjusted so that neither is excessive.
The distillation of the filtrate, in turn, is carried out in
the customary manner in order to obtain the glycol pure.
For the case particularly described, the specific reaction con-
ditions are the following:
Water/polymer ratio 12
Carbon~polymer ratio 10%
Hydrolysis temperature 248C
Hydrolysis pressure 42 atmospheres
Residence time in the hydrolyzer 2 hours
Frequency of carbon discharge every 24 hours
Residence time in the crystallizer 4 hours
Drying temperature 80C
Although a particularly preferred embodiment has been described
and certain changes which can be carried out in the method of the
invention under consideration have been outlined, it is obvious
that experts in the branch will be able to make, in view of the
description under consideration, other modifications, without this
deviating from the spirit and scope of the lnvention so that the
preceding description should be considered only for the purposes
of illustration and~not in any limiting sense.
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