Note: Descriptions are shown in the official language in which they were submitted.
~ Yokoyama and Empie Case 1
9~8
This invention relates to the hydrogenation of molten
aldehydic dimethyl terephthalate (DMT) to reduce the aldehydic
content thereof.
DMT is one of the principal chemicals from which the now
very common polyethylene terephthalate fiber is made. To obtain
polyethylene terephthalate fiber of high quality, highly stringent
quality specifications have been imposed on DMT. In one such
; specification for a fiber grade DMT the maximum permitted content
of aldehyde-ester (4-carbomethoxybenzaldehyde) and other alde-
hydes is only 20 p.p.m. (parts by weight per million parts by
weight of DMT).
One process for the production of DMT comprises oxidizing
p-xylene in the liquid phase with molecular oxygen to form p-toluic
acid, esterifying the acid with methanol to form methyl p-toluate,
oxidizing the methyl p-toluate with molecular oxygen to monomethyl
terephthalate, and esterifying the monomethyl terephthalate with
methanol to form DMT. As commercially practiced at the present
time, the process is continuous with the oxidation steps being
carried out together in the same reactor or reactors (oxidation
stage) and the esterification steps being performed together in a
separate reactor or in separate reactors (esterification stage).
The reaction mixture from the esterification stage comprises DMT,
intermediate oxidation products and by-products. These inter-
mediate oxidation products and by-products include aldehyde-ester
and other aldehydic impurities. In treating the reaction mixture
to isolate DMT, a methyl p-toluate fraction is separated by dis-
tillation and sent to the oxidation stage, and then a crude DMT
fraction is separated by distillation. To minimize aldehydic im-
purities in the crude DMT fraction, an effort is made to remove
; 30 these impurities in the methyl p-toluate fraction. However, the
boiling points of all these aldehydic impurities are very close to
that of DMT. Consequently, there is a significant concentration
of DMT in the methyl p-toluate fraction and, therefore, recycle of
significant amounts of DMT through the oxidation stage, which
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~4~L8
reduces substantially the DMI' production rate. Moreover, thereis still such a high concentration of aldehydic impurities in the
crude DMT fraction, concentrations of the order of 5000-10,000
p.p.m. being not uncommon, it is necessary to reduce the aldehydic
content to at least the desired level. Heretofore in connection
with the above-described process, this has been done by a series
of crystallization steps.
However, British Patent Specification 955,516 discloses
a~process in which DMT containing aldehydic impurities is subjec-
ted to catalytic hydrogenation to reduce the aldehydic impuritiesto compounds which either can be tolerated in fiber grade poly-
ethylene terephthalate, or can be more easily separated from DMT.
While that patent specification in its generic disclosures does
not disclose the details of the process other than that the hydro-
genation catalyst must be selective (that is, one that promotes
hydrogenation of the formyl group or aldehyde moiety, but not aro-
matic rings or the carbomethoxy group), and that copper-chromium
oxide or Raney copper meets this requirement, the working examples
of the patent specification indicate a process based on convention-
al laboratory procedure. In such process the hydrogenation cata-
lyst and a quantity of aldehydic DMT are charged to an autoclave
of the rocking or shaker type, or of the stirrer type, hydrogen is
introduced into the autoclave and established at an initial hydro-
gen pressure as low as 10 kg./sq. cm. to as high as 125 kg./sq.cm.
The autoclave contents are established at a temperature as low as
150C. to as high as 250C. and maintained at this temperature for
2-3 hours. While these conditions may result in substantial re-
duction of aldehydic impurities without subsequent aromatic ring
reduction, experimental evidence has been obtained which indicates
30 the rate of hydrogenation of the aldehydic moiety is slow and the
catalyst loses activity too quickly to be commercially practical.
When using other hydrogenation catalysts under otherwise the same
conditions, the result can be significant ring hydrogenation and
consequent appearance of dimethyl hexahydroterephthalate which is
~(~4Z5~8
to be avoided.
The problem to which this invention provides a solution
is how to catalytically hydrogenate the aldehydic content of molt-
en aldehydic DMT without substantial formation of dimethyl hexa-
hydroterephthalate when using a catalyst that at the temperature
of molten DMT normally tends to promote ring hydrogenation.
This invention is based on the discovery that when molt-
en aldehydic DMT is subjected to catalytic hydrogenation with a
catalyst that tends to promote hydrogenation of the DMT aromatic
ring at the temperature of molten DMT, hydrogenation of the alde-
hydic content tends to take place at a faster rate than ring hydro-
genation. One embodiment of the invention is based on the further
discovery that, when palladium is the catalyst and the aldehydic
content is at a concentration above a certain concentration (here-
in called threshold concentration), which is dependent on the mode
of catalyst contact, hydrogen availability, and the like, hydro-
genation of the aldehydic content proceeds without significant
ring hydrogenation until the aldehydic content concentration falls
below the threshold concentration.
In summary, this invention, considered broadly, provides
a process for hydrogenating molten aldehydic DMT with a hydrogen-
ation catalyst that tends to promote ring hydrogenation at the
temperature of the molten DMT. According to the inventive process
molten aldehydic DMT is contacted with molecular hydrogen and a
catalytic quantity of said catalyst for a period of time suffici-
ent for a substantial proportion of the aldehydic content of the
DMT to be hydrogenated with minimal ring hydrogenation.
DMT melts at 140.65C. Accordingly, in the process of
this invention the temperature of the aldehydic DMT is established
30 and maintained at about 141-240C. Higher temperatures are not
recommended because of the danger of pyrolysis of the DMT.
The quantity of molecular hydrogen brought into contact
with the molten aldehydic DMT is dependent on the concentration of
the aldehydic impurities, in the case of palladium as the catalyst
_4_
~04'r~9~
the threshold concentration, the dimethyl hexahydroterephthalate
content of the reaction mixture resulting from the catalyst con-
tacting step, whether the molecular hydrogen and molten aldehydic
DMT are brought together at the same time as the molten aldehydic
DMT is contacted with the catalyst or they are brought together
prior to the molten aldehydic DMT being contacted with the catalyst,
and the way in which the catalyst contacting is carried out (batch
process or continuous process, upflow fixed catalyst bed reactor
or downflow fixed catalyst bed reactor, etc.). No generalization
can be made other than the quantity of hydrogen should be such
that when the mixture is contacted with the catalyst there will be
enough available to hydrogenate a substantial proportion of the
aldehydic impurities. In this connection, except when palladium
is the catalyst and when the aldehydic content is above the thre-
shold concentration, increasing the quantity of molecular hydro-
gen in contact with the DMT, as by increasing the hydrogen partial
pressure, enhances the rate of formation of dimethyl hexahydro-
terephthalate. In a preferred embodiment of the process of this
invention, whiah embodiment is a continuous process in which
molecular hydrogen is dissolved in the molten aldehydic DMT prior
to the catalytic contacting step, a recommended quantity of molec-
ular hydrogen admixed with the DMT when it is at 155C. and con-
tains, for example, 500 p.p.m. of aldehyde-ester, which in most
instances involving palladium as the catalyst is above the thresh-
old concentration, is that which is equivalent to the amount of
molecular hydrogen in solution in molten DMT at a hydrogen pres-
sure of 3.5 kilograms per square centimeter gauge at 155C.
The hydrogenation catalyst employed in the second step
of the process of this invention can be any catalyst for hydrogen-
ating aldehydes. Representative of such a catalyst are palladium,nickel (including Raney nickel), ruthenium, rhodium, platinum,
cobalt, iron, molybdenum, silver, palladium chloride, nickel oxide,
cobalt nitrate, and the like. Preferred are palladium and nickel.
The catalyst can be supported or unsupported.
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~425~18
The period of time in which the catalyst is in contact
with the molecular hydrogen and molten aldehydic DMT is selected
as above stated. It should be insufficient for substantial ring
hydrogenation. While the selection depends to a certain extent on
what is desired, in a preferred embodiment of this invention in
which palladium is the hydrogenation catalyst and the aldehyde
content of DMT is substantially above the threshold concentration,
the period of time is selected so that hydrogenation of the alde-
hydic impurities does not reduce the concentration of the aldehyde
moiety below the threshold concentration.
As already indicated above, the contacting step of this
invention can be carried out by admixing the molten aldehydic DMT,
molecular hydrogen and catalyst sequentially or at the same time.
The adm1xing also can be carried out by various ways and means.
In one embodiment, a batch process embodiment, the hyd-
; rogenation catalyst and molten aldehydic DMT with or without dis-
solved molecular hydrogen are introduced into an autoclave. If
the molten aldehydic DMT is charged with no or an insufficient
quantity of molecular hydrogen, the autoclave is pressurized with
molecular hydrogen or a molecular hydrogen-bearing gas. In either
case the autoclave contents are agitated or stirred at 141-240C.
for a period of time sufficient to substantially hydrogenate the
aldehydic impurities, but insufficient to significantly hydrogen-
ate the DMT. At the end of this period of time the autoclave is
opened and the resulting reaction mixture is separated from the
hydrogenation catalyst.
In the continuous process embodiments of this invention,
the molten aldehydic DMT and molecular hydrogen are passed either
cocurrently or countercurrently through a bed of hydrogenation
catalyst. The distance of travel of the DMT through the bed and
the rate of flow of DMT through the bed and the rate of flow of
DMT through the bed are selected so that at least a substantial
portion of the aldehydic impurities are hydrogenated without ring
hydrogenation of DMT to a substantial extent. In one of these em-
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~ o~z~
bodiments the catalyst bed is established and maintained in a down-
flow reactor. In another embodiment the catalyst bed is estab-
lished and maintained in an upflow reactor.
The initial aldehyde concentration in the molten aldehydic
DMT to be treated according to this invention can be high enough
that the amount of molecular hydrogen initially present or the
amount of contact between the catalyst, molecular hydrogen and
catalyst is not enough to hydrogenate the desired proportion or
substantially all of the aldehydic impurities. In such event, the
contacting step is again repeated until the aldehyde concentration
has been reduced at least to the desired l~vel. In the continuous
process embodiments of this invention repetition of the contacting
step can be accomplished in a number of ways. In one embodiment a
portion of ~he reaction mixture flowing from the catalyst bed is
; recirculated through the bed while the remainder of the reaction
mixture is removed as product. In another embodiment the reaction
mixture from the catalyst bed is introduced into another catalyst
bed into which more molecular hydrogen is introduced, if not
enough molecular hydrogen is present in the reaction mixture leav-
ing the first catalyst bed.
The best mode now contemplated of carrying out the inven-
tion is illustrated by the drawing which forms a material part of
these disclosures, and which depicts diagrammatically a preferred
specific continuous process embodiment of this invention. This
invention is not limited to this embodiment.
In the specific process illustrated by the drawing, mol-
ten DMT containing aldehydic impurities is conducted by way of a
feed conduit 10 equipped with a flow rate control and shutoff valve
12 into an infeed leg of a mixing tee 14. Gaseous molecular hy-
drogen is introduced by way of a hydrogen supply conduit 16 fittedwith a flow rate control and shutoff valve 18 into a second in-
feed leg of the mixing tee 14. Inside the mixing tee 14 the mol-
ecular hydrogen is admixed with the molten DMT and its content of
aldehydic impurities, and the resulting solution is transferred
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. 1q)42~8
~Dm the output leg o~ the mixing tee 14 through the transfer con-
duit 20 to the inlet of the catalyst contact reactor 22.
The catalyst contact reactor 22 is of the upflow type.
Within its interior is a bed 24 of particles of hydrogenation cat-
alyst. The reactor comprises a surrounding jacket 26 with an in-
let in the region of the top end thereof in communication with a
heat transfer fluid feed conduit 28 equipped with a flo~ rate
control and shutoff valve 29 and with an outlet in the region of
the bottom end thereof in communication with a heat transfer fluid
discharge conduit 30. When desired, under normal operative condi-
tions a heat transfer fluid such as, for example, steam, is intro-
duced into the jacket by way of the feed conduit 28, and spent
heat transfer fluid is withdrawn from the jacket 26 by way of the
; discharge conduit 30. By adjustment of the valve 29, the flow rate
of heat transfer fluid through the jacket 26 and thus the tempera-
ture of the contents o the catalyst contact reactor is controlled.
The solution of molten aldehydic DMT and molecular hyd-
rogen introduced into the inlet at the bottom of the catalyst con-
tact reactor 22 passes upwardly through the hydrogenation catalyst
~ 20 bed 24. Reaction between the dissolved molecular hydrogen and al-
; dehydic impurities is thereby promoted, resulting in the formation
of alcohols and reduction in the concentration of aldehydic impur-
ities. The resulting reaction mixture reaching the top of the
catalyst contact reactor 22 is withdrawn therefrom by way of dis-
charge conduit 32 which is equipped with a flow rate control and
shut-off valve 33.
The height of the catalyst bed 24 and the flow rate of
the solution of molecular hydrogen and impure molten DMT through
the catalyst bed 24 are selected to give a residence time of the
solution in the catalyst bed 24 sufficient for hydrogenation of a
substantial portion of the aldehydic impurities with minimal or no
ring hydrogenation. The rate of flow of solution through the cat-
alyst bed 24 is regulated by means of the discharge conduit valve
13, the impbre molten DMT feed conduit valve 12, or both of these
.
"/~lves. ~4Z~
- In the preferred practice of the process depicted in the
drawing, periodically the reaction mixture discharged from the
catalyst contact reactor 22 by way of conduit 32 is analyzed for
aldehyde ester content or aldehydic impurities content and for
dimethyl hexahydroterephthalate content. On the basis of these
; periodic analyses, the rate of introduction of molecular hydrogen
to the mixing tee 14 (the hydrogen pressure at the tee 14), the
rate of flow of impure molten DMT through the mixing tee 14, or
both, and the rate of discharge from the reactor of reaction mix-
ture are adjusted by the respective valves to minimize the aldehyde-
; ester or aldehydic impurities content and the dimethyl hexahydro-
terephthalic content.
In an especially preferred embodiment of the process de-
picted in the drawing, palladium is employed as the hydrogenation
catalyst, and the aldehydic content of the molten aldehydic DMT is
substantially above the threshold concentration. In this embodi-
ment the infeed rate of the molten aldehydic DMT, the molecular
hydrogen flow rate or hydrogen pressure at the tee 14, and rate of
discharge of reaation mixture from the reactor 22 are established
and maintained so that hydrogenation of the aIdehydic impurities
reduces the aldehydic impurities concentration to, but not beyond,
the threshold concentration. In this connection in the laboratory
in an upflow reactor in which the catalyst was 0.5% by weight pal-
ladium on carbon, the molecular hydrogen was dissolved as in the
process of the drawing at 1-9 kg./sq. cm. and the aldehyde-ester
threshold concentration at 155C. was about 150 p.p.m. On the
other hand, in a laboratory cocurrent downf 1QW system in which the
flow of molten aldehydic DMT and molecular hydrogen gas was down-
wardly together through a catalyst bed of 1% by weight palladiumon carbon at 155C. and hydrogen pressure of 0.2-4 kg./sq. cm. the
threshold concentration was about 350 p.p.m. Hence, bearing in
mind the extremely low aldehyde-ester requirement for at least one
fiber grade DMT, this especially preferred embodiment of the process
_g _
~4Z9~3
picted in the drawing is intended for that part of the purifi-
cation section ofthe DMT production plant having a stream of mol-
ten aldehydic DMT with an aldehyde-ester content of 200-2000 p.p.m.,
which is followed by one or more purification steps such as dis-
tillation, crystallization and the like.
Hydrogenation of the aldehydic impurities converts them ko
alcohols which ln a number of instances can be tolerated in the
production of fibex grade polyethylene terephthalate. For DMT
destined for such instances, the molten reaction mixture discharged
from the catalyst contaat reactor 22 when it is the last DMT treat-
ment step in a DM~ production plant can be conveyed directly to
storage or to the polyethylene terephthalate polymer producing
plant, or solidified, formed into divided particles and stored or
packaged. In those other instances in which alcoholic impurities
and even trace concentrations of dimethyl hexahydroterephthalate
are not wanted, the molten reaction mixture discharged from the
catalyst contact rea~tor 22 is conveyed to a separation station
and treated to separate therefrom the alcoholic content as well as
dimethyl hexahydroterephthalate. An example of such a separation
treatment is distillation. The alcohols and dimethyl hexahydro-
terephthalate generally have boiling points substantially differ-
ent from that of DMT. Thus, the boiling point of the alcohol cor-
responding to the aldehyde ester is higher than that of DMT, while
the boiling point of dimethyl hexahydroterephthalate is lower than
that of DMT. Consequently, DMT, free of these alcohols and dime-
thyl hexahydroterephthalate, can be isolated by distillation from
the reaction mixture discharged from the catalyst contact reactor
22.
The following are typical conditions for the practice of
the process depicted in the drawing, "v" being volume, "w" being
weight, p.p.m. being w per million w of DMT, and "m" being a unit
of linear measure, the relationship of v to w to m being as the
liter to the kilogram to the centimeter.
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91~
Aldehyde ester concentration of DMT
in conduit 10 = 530 p.p.m.
Flow rate of molten DMT containing
aldehydic impurities through
conduit 10 = 0.80 v/hr.
Hydrogen pressure at mixing tee 14 = 1.9 kg./sq. cm.
Catalyst = 0.5~ Palladium oncarbon
Diameter of catalyst bed = 2.5 m
Height of catalyst bed = 27 m
10 Temperature of catalyst contact
reactor 22 contents = 155C.
Aldehyde ester content of reaction
mixture in conduit 32 = 100 p.p.m.
Dime~hyl hexahydroterephthalate
content of reaction mixture in
conduit 32 = 20 p.p.m.
Thus, this invention provides ways and means for cataly-
tically hydrogenating the aldehydic content of molten aldehydic
DMT with minimal ring hydrogenation.
,
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