Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
3~3~
This invention relates to an improved process, and
apparatus, for converting polyester wastes to compounds useful
in preparing the polyester and, in a preferred embodiment,
relates to ester-exchange conversion of ethylene glycol-degraded
polyethylene terephthalate to dimethyl terephthalate.
High molecular weight polyesters of terephthalic acid
and aliphatic dihydric alcohols are well known in the art. Poly-
ethylene terephthalate is a commercially preferred polyester of
this class due to its exceptional physical and chemical proper-
10 ties.
Polyethylene terephthalate is typically prepared bycontacting an organic ester of terephthalic acid, such as
dimethyl terephthalate, with ethylene glycol in the presence of
an ester exchange catalyst to form dihydroxyethyl terephthalate
monomer, and then polymerizing the monomer to high molecular
weight using condensation polymerization techniques. Details
of this process are disclosed in ~.S. Patent 2,465,319 to
Whinfield and Dickson. Various inert additives, such as slip
additives, are generally added during the process to adapt the
20 polyester for its intended commercial use as a packaging film,
fiber, electrical insulator, molded article, etc.
Considerable waste is generated as the polyester is
manufactured into commercial form. For instance, edge trim,
slitting trim, and re~ect material is accumulated as polyethylene
terephthalate is extruded, biaxially stretched, and slit into
film widths desired by customer industries. The industry has
~ proposed several processes for reclaiming these wastes in order
- to conserve resources and eliminate ecological problems
associated with waste disposal.
; 30 One proposal, according to German OLS 1,247,291, has
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been to (1) degrade the polyester wastes with the glycol used
in making the polyester to prepare dihydroxyalkyl/terephthalate,
followed by (2) reacting the degraded wastes wlth a monohydric
alcohol to convert the terephthallc acid values to dialkyl tere-
phthalate. The diallcyl terephthalate, when recovered~ would be
recycled to prepare fresh polyester,
Another proposal, according to East German Patent
69,500, discloses a waste recovery procedure whereln (1) poly-
ethylene terephthalate wast is subJected successively to degra-
10 dation with the glycol used in its preparation and ester inter-
change of the resulting product with monohydric alcohols, the
degradation and ester exchange being effected in the presence
of known ester exchange catalysts under superatmospheric pres-
' sure and at an elevated temperature to prepared solution con-
taining dimethyl terephthalate. After the release of the super-
atmospheric pressure, the solution is cooled to crystallize
dimethyl terephthalate which is then recovered using a centri-
fuge.
Although the glycol-degradation step disclosed in the
20 German patents is satisfactory, the methanol ester exchange and
recovery steps are not readily adapted for continuous commercial
operation. Cooling of the solution to crystallize dimethyI
terephthalate causes a substantial heat loss since excess
methanol contained in the solution must be reheated for recycle.
Also, the recovered dimethyl terephthalate contains occluded
contaminants which detract from the properties of polyester
made therefrom, inert additives present in the wastes, and some
of the ester exchange catalyst. Presence of these materials
complicates quality control in the manufacture of polyester made
30 from recovered dimethyl terephthalate. Moreover, crystallization
~v~
recovery techniques are better adapted to a batch process than
the more desirable continuous process.
Thus, there is a need for an improved process for
preparing dialkyl terephthalate ~rom glycol-degraded polyester
wastes. Especially desirable is a process which can readily be
integrated in a continuous commercial polyester waste recovery
operation.
The present invention provides a continuous process
for the preparation of dialkyl terephthalate from polyester
10 wastes. A glycol-degraded waste is generally selected since the
process is conducted at superatmospheric pressure and it is
generally more convenient in a continuous process under pressure,
to employ solution feed than a solid or slurry feed. Glycol-
degradation converts the polyester to a liquid which is readily
fed to the process under pressure.
, Suitable methods for degrading polyesters with glycols
are well known in the art. Generally the same glycol is selected
to degrade the polyester as was used in preparing the polyester;
i.e., ethylene glycol is normally selected to degrade polyethyl-
20 ene terephthalate. The quantity of glycol employed is sufficient
to degrade the waste into a liquid solution containing glycol
- ester of depolymerized waste.
This invention provides, in a process for the
preparation of a dialkyl ester of an aromatic diacid from poly-
ester waste by:
(1) contacting the waste with an excess of a mono-
- hydric alcohol and an ester exchange catalyst
at an elevated temperature and superatmospheric
pressure to prepare solution containing the
dialkyl ester, and
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(2) withdrawing the dialkyl ester solution,
the improvement wherein the process is
cont~nuous and
(a) the contacting step (1) is conducted in a
partially filled, closed reaction vessel
at a pressure substantiayly the same as the
partial vapor pressure of the monohydric
alcohol at the elevated temperature.
(b) a sequestering agent is introduced to the
solution at superatmospheric pressure to
deactivate the ester exchange catalyst,
(c) solution containing the deactivated catalyst
is separated and further heated to vaporize
a portion of the excess monohydric alcohol
therefrom and form solution rich in the
dialkyl ester,
(d) monohydric alcohol vapors of step (c) are
returned to step (a), and
(e) solution rich in the dialkyl ester is withdrawn
from step (c).
The process is particularly useful for the recovery of dimethyl
terephthalate from glycol-degraded polyethylene terephthalate
waste by selecting methanol as the monohydric alcohol.
In a preferred embodiment, a vapor barrier of the
monohydric alcohol is maintained between the heated separated
solution of step (c) and solution not yet separated for heat-
ing, and monohydric vapors of step (c) are returned to the
solution of step (a) by passage through the vapor barrier.
Also provided is apparatus particularly useful for
30 carrying out the process comprising:
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(1) a closed reaction vessel ~ith an upper reactlon
zone and a lower reboiler zone, said reaction
zone having an entrance port for reactants and
said reboiler zone havlng an exit port ~or
reaction product;
(2) a perforated barrier plate mounted in the
reaction vessel to separate said reaction zone
from said reboiler zone;
(3) liquid level sensing means mounted in said
reaction zone to detect the level of liquid
reactants therein;
(4) a conduit by-passing the perforated barrier
plate to transport liquid from said reaction
zone to said reboiler zone;
(5) control means mounted in said by-passconduit
responsive to said reaction zone liquid level
sensing means to maintain liquid in the
'~ reaction zone at a predetermined level;
(6) liquid level sensing means mounted in the
, 20 reboiler zone to detect the level of liquid
- therein;
- (7) a conduit to remove liquid product from the ~-
reboiler zone having mounted therein control
means responsive to said reboiler liquid level
sensing means, maintaining liquid in the
reboiler zone at a predetermined level; and
~ (8) heating means to maintain liquid in said
. reboiler zone at a higher temperature than
.~ liquid in said reaction zone.
' 30 The drawing is a vertical view, in partial section, of
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a preferred chemical reactor use~ul for convertin~ the glycol-
degraded waste to dialkyl terephthalate.
The invention will now be further described with
respect to the conversion o~ glycol-degraded polyethylene tere-
phthalate waste to dialkyl terephthalate
Preparation of the glycol-degraded waste is conve-
niently accomplished using a two stage process as disclosed in
U.S. Patent 3~257,335 to Whitfield et al, In the first stage,
particulate polyester and ethylene glycol are continuously fed
10 and reacted at atmospheric pressure to partially depolymerize the
waste and form a liquid solution. Solution is continuously with-
drawn from the first stage and further reacted with ethylene
glycol under pressure in the second stage to obtain the desired
~- degree of depolymerization.
The quantity of glycol employed in the Whitfield et al.
process, or other glycolysis processes, is sufficient to degrade
- the polyester into a solution, but small enough that substanti-
ally all the glycol is consumed during degradation of the poly-
ester. ~arger quantities of glycol could be used, but are not
20 desirable since excess glycol must ultimately be separated from
the dialkyl terephthalate product and excess glycol can reduce
the efficiency of the ester exchange reaction employed in
- practicing the invention.
- The glycol-degraded waste containg dihydroxyethyl
terephthalate and low molecular weight oligomers thereof,
diethylene glycol resulting from the condensation of ethylene
glycol, residual ethylene glycol, and various impurities and
additives present in the wastes. As used herein, the term
"oligomers" refers to partially degraded polyethylene tere-
30 phthalate capped, at the po~nts of chain scission, pr~mar~ly
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with ethylene glycol units. Generally the oli~omers have 2, 3,
4, 5~ etc,, polyethylene terephthalate units.
In accordance with the present invention, the glycol-
degraded wastes ls contacted with an excess o~ a monohydric
alcohol and an ester exchange catalyst at an elevated temperature
and superatmospheric pressure substantially the same as the par-
tial vapor pressure of the monohydric alcohol at the selected
elevated temperature to convert the dihydro~yethyl terephthalate
and oligomers present in the degraded waste to dialkyl tere-
10 phthalate. The monohydric alcohol is selected in accordancewith the desired dialkyl terephthalate; i.e., methanol is select-
, ed to prepare dimethyl terephthalate, a preferred feed for making
polyethylene terephthalate, and the process is hereinafter
described with respect to methanol.
The ester exchange is a reversible reaction re~uiringa stoichiometric excess of methanol, an ester exchange catalyst,
and elevated temperature to drive the reaction to acceptable
, yields of dimethyl terephthalate within a reasonable holding
time. The weight ratio of methanol to glycol-degraded waste
20 should generally be at least 2 to 1, and preferably at least 3
to 1, to minimize holding time. For instance, more than 90% of
the terephthalate units present in the glycol-degraded waste are
converted to dimethyl terephthalate when employing a weight ratio
. of about 3 to 1 and a holding period up to 60 minutes. It will
be understood that the required holding time to achieve a desired
.
-- conversion of glycol-degraded waste to dimethyl terephthalate is
influenced by the selected catalyst, catalyst concentration, and
operating temperature.
It has been mentioned that the ester exchange reaction
30 is conducted at an elevated temperature and at a superatmospheric
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pressure substantially the same as the ~artial vapor pressure of
methanol at the elevated temperature The elevated temperature
is typically above 180C., but wlll generally be less than about
300C. since the vapor pressure of methanol at higher tempera-
tures unduly complicates construction of the ester exchange
reaction vessel and supporting equipment. The superatmospheric
pressure should be high enough to maintain methanol in the
liquid state during the ester exchange, but should not be sub-
stantially higher for reasons which will be evident hereinafter.
10 Preferred operating temperatures are within the range of about
190 to 230C.
Useful ester exchange catalysts are well known in the
art and include, for example, catalysts disclosed in U.S.Patent
2,465,319 to Whinfield et al. Representative catalysts include
metal salts Or acetic acid, such as zinc and manganese acetate,
and organic amines, such as triethyl and tributyl amine. The
catalyst is generally used as a solution for ease in pumping to
the reaction vessel, which is maintained under pressure.
Optimum quantities of a desired catalyst are readily determined
20 for given ester exchange conditions by varying the quantity
added and monitoring the degree of conversion.
When the ester exchange reaction has reached the
desired degree of completion, an appropriate sequestering agent
is added to the solution to deactivate the ester exchange
: catalyst. Addition of the sequestering agent is generally accom- -
- plished while the solution is under superatmospheric pressure
`` so that the catalyst is deactivated before excess methanol is
- removed from the solution by flashing. Otherwise the yield of
dimethyl terephthalate is reduced.
The sequestering agent will generally be used as a
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solution, pr~ferably in the monohydrlc alcohol used in preparing
the dialkyl terephthalate, for ease in pumping to the pressur-
ized solution. When introduced to the ester exchange vessel,
the sequesterlng agent is added to a region of the vessel where
the ester exchange has reached the desired degree of completion
and at which the sequestering agent does not migrate to prema-
; turely deactivate the ester exchange catalyst. Alternatively~
a separate vessel can be provided for sequestering of the ester
exchange catalyst.
10Useful catalyst sequestering agents are known in the
art and include, but are not limited to, phosphoric acid;
phosphorous acid, aryl, alkyl, cycloalkyl, and aralkyl phosphite
or phosphate esters; aliphatic and aromatic carboxylic acids
such as oxalic acid, citric acid, tartaric acid and terephthalic
acid; the tetra sodium salt of ethylene diamine tetraacetic acl~d;
and phenyl phosphinlc acid. The amount of the selected seques-
tering agent used should be sufficient to effectively deactivate
- the catalyst since àctive catalyst will promote undesired ester
: exchange in following operation, reducing the yield of~dimethyl
20 terephthalate
The hot solution, after introduction of the sequester-
ing agent, contains dimethyl terephthalate, small quantities of
unreacted dihydroxyethyl terephthalate and oligomer, small quan-
tities of hydroxyethyl methyl terephthalate mixed esters result-
ing from incomplete ester exchange, catalyst residues, inert
` material introduced with the waste, ethylene glycol, diethylene
glycol and excess methanol from the ester exchange reaction.
This solution is continuously separated from solution containing
active ester exchange catalyst and, without release of the
30 superatmosp~eric pressure, is further heated in a partially
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filled vessel to continuously evolve methanol vap~rs there~rom.
The evolving vapors are continuously returned to solution con-
taining active ester exchange catalyst.
To simpllfy recycle of the evolving vapors~ a methanol
vapor barrier is established between the partially filled vessel
and solutlon containing active ester exchange catalyst, and the
methanol vapors are recycled by passage through the vapor barrier.
Preferably the recycled vapors pass through the liquid solution,
both in the region where the sequestering agent is added and in
10 the region where the ester exchange reaction is being conducted,
to agitate the solution and minimize or eliminate mechanical
stirring of the solution.
- A vertical ester exchange column, wherein reactants
are introduced at the top and solution containing deactivated
catalyst is withdrawn from the bottom~ can be conveniently
employed for this purpose. In this case, the withdrawn solution
is heated to a temperature at which the evolved vapors have a
pressure at least equal to the total of liquid head and super-
atmospheric pressure maintained in the ester exchange column,
20 and the evolved vapors are recycled through a vapor barrier
maintained at the bottom of the column. But the heating period
should not be so long, nor should the heating temperature be so
high, that unacceptable quantities of dimethyl terephthalate are
lost by reaction with other constituents present in the solution;
e.g., by reaction with constituents having glycol end groups.
Especially favorable results have been achieved by
heating the separated solution to a temperature at which a
; majority of the excess methanol is evolved. The heating results
in evolved methanol vapors having a pressure high enough to
30 overcome resistance to upward vapor flow presented by the liquid
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he~d, superatmospheric ~ressure, and any trays in the ester
exchange column as well as resistance to flow of the vapor
barrier. For example, up to 70% by weight, and higher, of
methanol present in the ~ithdrawn solution can be recycled in
this manner while holding dimethyl terephthalate losses to 3%
or less by heating the withdrawn solution to a temperature 20
to 30C. higher than that employed during ester exchange for a
short period of time, such as a 10 minute holding time.
The internal recycle provided by this process has the
10 advantages of removing a large quantity of the excess methanol
- in an economical manner using minimum process equipment. The
recycled vapors also provide agitation during the ester exchange
reaction, improving the efficiency of the reaction. Moreover,
the effluent leaving this continuous process is rich in dimethyl
terephthalate and can be continuously processed for recovery of
dimethyl terephthalate. For instance, remaining methanol can
be flashed from the effluent by reducing the superatmospheric
` pressure. The effluent can then be fed to a centrifuge, filter,
` or sedimentation tank to remove solids, and dimethyl terephthalate
20 can be recovered from the solution by a continuous distillation.
Alternatively, dimethyl terephthalate can be recovered using
crystallization and/or sublimation techniques if desired.
While the process has been described with respect to
the conversion of glycol-degraded polyethylene terephthalate to
dialkyl terephthalate, it will be understood that the process
.'~ .
can also be used to recover dialkyl esters of aromatic diacids
~ from other polyesters of aliphatic diols and aromatic diacids.
; ~or instance, the process can be used to convert glycol-degraded
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polytetramethylene terephthalate or isophthalate, poly 1,4-cyclo-
30 hexylene dimethylene terephthalate or isophthalate, or polyethylene
,
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naphthalate into dialkyl es~ers of the respective aromatic
diacids, such as dimethyl terephthalate, isophthalate, or
naphthalate. ~hile ethylene glycol will normally be selected to
degrade these polyesters, other glycols can be selected if
desired.
The process will now be further described with reference
to preferred apparatus illustrated in the drawing comprising
partially filled, closed reaction vessel having an upper reaction
zone for the preparation Or dimethyl terephthalate from glycol-
10 degraded polyethylene terephthalate~ and a lower reboiler zonefor evaporation of methanol vapors from the dimethyl terephthalate
solution prepared in the reaction zone.
The reaction zone is a right cylinder section of the
closed vessel, 40, and is provided with an entrance line, 42,
mounted in the upper portion thereof for the introduction of
reactants, and an in~ection nozzle, 46, mounted in the lower
portion thereof for the introduction of catalyst sequestering
agent. A hemispherical top, 41, is provided having a port for
the continuous removal of vapors, primarily methanol, from the
20 vessel. The vapors are continuously fed by line 43 to a
condenser, wherein the vapors are condensed. Liquid from the
condenser is continuously returned to the vessel by line 44.
` A purge line (not shown) is installed in condenser
vapor line 43. The purge line has a pressure relief value to
control column temperature and pressure, providing smooth con-
- tinuous operation in the case of feed fluctuations and prevent-
ing undue buildup of inert gases in the condenser recycle loop
and vapor space above the reaction zone. A typical control valve
setting is 500 psi ~35.2 kg/cm2]. The column pressure, and
temperatuxe, equilibrates at the control valve setting, resulting
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in a small fl~w of vapors in the purge 11ne when the feed
fluctuates or inerts accumulate.
Perforated trays, 45, baffles, or o~her structures
havin~ a suitable deslg~ to impede downward flow of reactants
and to promote liquid-vapor contact, without being plugged by
solids present in the glycol-degraded wastes, are mounted in
the reaction zone. Preferred perforated trays, shown in the
enlargement, have lips which extend below the tray, and the holes
are sized small enough to impede downward flow of liquids through
the holes but large enough to permit upward flow and bubbling of
vapors through the holes. In this design tne lips serve to trap
vapor beneath the trays, restricting the upward flow of vapor to
passage through the perforated tray. Trays having holes of about
0.25 inch diameter on a 1.25 inch triangular spacing are suitable
under the operating conditlons described hereinafter for the
preparation of dimethyl terephthalate.
The reboiler zone forms the bottom section of the
closed vessel. This zone is constructed to have a large surface
area for transfer of heat into liquid contained therein, and to
contain a limited volume of liquid so that residence time in the
reboiler zone is short enough to minimize reactions between
dimethyl terephthalate and other constituents of the liquid.
In the embodiment shown in the drawing, the reboiler
zone is defined by a right cylinder section 47 of smaller dia-
meter than the reaction vessel 40, a truncated conical section
48 which connects the walls of the reaction and reboiler zones,
and a hemispherical bottom section 49 having an e~it port which
communicates with line 50 for removal of product from the closed
vessel. The reboiler zone is internally heated by immersed
heated tubes (not shown).
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A per~orated barrier plate 51 is mounted in the closed
vessel below the lowest tray 45 and extends across the entire
cross-section area of the vessel. This barrier plate divides
the vessel into the reaction and reboiler zone. The perforations
are sized small enou~h to prevent any substantial liquid L low
through the barrier plate, but large enough to permit upward flow
of vapors fro~ the reboiler to the reaction zone. A barrier
plate having 0.25 inch [o~64 cm.] diameter holes on a 2.25 inch
[5.7 cms.] triangular spacing is suitable under the operating
10 conditions described herein for the preparation of dimethyl tere-
phthalate.
A by-pass line,52, is provided which communicates with
an exit port located in the reaction zone above the barrier plate
and with an entrance port located below the liquld level in the
reboiler zone. Mounted in the by-pass line is a control valve,
53, responsive to a float, 54, mounted in the reaction zone to
malntain a constant liquid level in the reaction zone. The by-
pass line presents a sufficient liquid head to prevent reverse-
flow of liquids from the reboiler to the reaction zone. If
20 necessary, a pump or other means can be installed in the by-pass
line to ensure that liquid only flows from the reaction zone to
the reboiler zone. A predetermined liquid level is maintained
- - in the reboiler zone by a control valve, 55, mounted in line 50
for the withdrawal of liquid. This valve is responsive to a
level sensing device, 56, such as a manometer, which detects
the reboiler liquid level.
In operation, premixed ethylene-glycol degraded poly-
ethylene terephthalate waste, methanol, and zinc acetate are
heated to 190 to 230iC. and then pumped to the vessel through
30 line 42. The feed contains a sufficient amount of methanol to
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; mai~tain a stoichiometric excess ir the reaction zone, generally
a weight ratio of at least 2 to 1, preferably at least 3 to 1,
methanol ~o glycol-degraded wast;e. The feed contains about 200
ppm by weight of zinc acetate catalyst, based on weight of the
glycol-degraded waste.
Pressure of the reaction zone is maintained substan-
tially at the partial vapor pressure of methanol in the reaction
- zone to prevent any significant quantity of methanol feed from
evaporating. The pressure is controlled by a valve in the purge
10 line as discussed hereinbefore. In the reaction zone dimethyl
terephthalate and ethylene glycol are formed by ester exchange
- between methanol and the glycol-degraded waste.
The liquid reaction solution slowly passes downward
through the reaction zone, by passage through spaces between the
perforated trays. At a point in the lower region o~ the reaction
zone where the ester exchange has reached the desired degree of
completion (e.g., when about 90% or more of the terephthalate
values in the feed solution have been converted to dimethyl
terephthalate), the solution comes into contact with a catalyst
20 sequestering agent, typically phosphoric acid, introduced through
line _. At this point the catalyst is deactivated and methanol
can be removed from the solution without significantly reversing
the ester exchange reaction.
' After the catalyst has been deactivated, the hot
< solution is withdrawn from the reaction zone and introduced to
the reboiler zone through line 52. The reboiler zone is heated
to a temperature sufficiently higher than that of the reaction
zone to evolve methanol vapor having a pressure high enough to
overcome resistance to upward vapor flow presented by the liquid
30 head, superatmospheric pressure, and trays in the ester exchange
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column. Methanol vapors cont~nuously pass ~hrough the barrier
plate and bubble upward through liquld in the reaction zone,
continuously agitating the liquid~ and into the vapor space at
the top of the vessel. Ascending vapors, as they pass through
the reaction zone, collect beneath the trays, 45, and are redis-
persed as bubbles by passing through the tray perforations.
Ethylene glycol and other vapors evolved in the reboiler
also pass through the barrier plate but are condensed as they
rise through the reaction zone. Condensation primarily occurs in
10 the lower region of the reaction zone and does not effect the
ester exchange dimethyl terephthalate yield to any significant
extent.
Pressure in the vapor space at the top of the vessel is
maintained substantially at the partial vapor pressure of methanol
at the reaction zone temperature so that significant quantities
of methanol are only evolved in the reboiler zone; i.e., methanol
is evaporated after the ester exchange catalyst has been deacti-
vated. Vapors at the top of the column, primarily methanol, exit
the vessel through line 43, are condensed, and are returned to the
20 vessel by line 44.
The reaction zone is typically maintained at 200C.
and at a pressure of about 500 to 550 psia [35.2-38.7 kg/cm2],
with liquids in the reboiler zone being heated to 220 to 230C.
Under these conditions, and employing about a 60 minute holding
time in the reaction zone and up to a 10 minute holding time in
the reboiler zone, dimethyl terephthalate yields up to about 87%
or more of the theoretical yield are obtained while reducing the
methanol content in the solution by up to about 70% or more.
Hot solution withdrawn from the reboiler zone by line
30 50 contains dimethyl terephthalate, residual methanol, diethylene
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glycol, ethylene glycol, catalyst re3idues, solids introduced
with the wastes~ and small quantitie~ of uncompletely reacted
wastes and conden~ation by-products. Dl~ethyl terephthalate is
conveniently recovered from the ~olution by, in ~eguence, dis-
tilling off the residual methanol, removing the sollds using
sedlmentation, filtration or centrifugation technlgues, distill-
ing off the allphatic components~ and distilling of~ dimethyl
terephthalate from the remaining ~olution. The proce~s disclosed
and claimed in copending coas~igned application Serial No.
220 163 can be used for recovery purposes.
The process o~ this invention reduces the methanol feed
requirements ~ince it provides for internal methanol recycle,
and conserves heat since the methanol i8 recycled at an elevated
temperature. Moreover, the process i~ flexible in that it can
accomodate waste containing various additives and is readily
integrated in a continuous waste recovery operation. The proces~
can be u~ed to recover terephthalate values from textile or film
wastes, as well as ~rom polyester artlcles returned ~or recycle.
For example, polyethylene terephthalate article~, ~uch as used
bottle~, can be collected and transported to a central locat~on
where they can be glycol-degraded to serve a~ feèd to the poly-
ethylene terephthalate manufacturing process.
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