Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1079Z~7
~his invention relates to a process for producing
terephthalic acid from p-xylene by oxidation. ;~
~here is a process already known, the so-called
"SD process", in which p-xylene in acetic acid as a -~
solvent is oxidized with molecular oxygen in the
presence of a heavy metal oxidation catalyst system. ~ -
~he terephthalic acid produced is isolated from the
slurry product b~ centrifugal separatlon and the mother~
liquor separated is usually subjected to distillation
...
to separate water and a residue containing the catalyæt
' and recover acetic acid for reuse. Because such mother
liquor contains valuable materials, for example, t~e
unreacted p-xylene, the catalyst and the oxidation
intermediates, it is desirable from the commercial ~-
point of view to recycle the mother liquor to the
~ oxidation step. However, the mother liquor also
l contains small amounts of other materials which
adversely affect the reaction, and, therefore, if the -
mother liquor is recycled, then undesirable side
reactions will occur to impair the quality of th~o -
terephthalic acid product. ~hus, in practice, re-
cycling the mother liquor has hitherto been impossible
i~- without causing disadvantageæ as mentioned above.
Under these circumstances, we made intensive
efforts to fi~d a process for producing terephthalic - -
acid wherein the mother li~uor can be recycled to the
reactor without cauæing deterioration of the reaction
product, and, in consequence have found that, if the
slurry product obtained in the oxidation reaction is
30 directly subjected to a post-oxidation treatment without
:
- 2 - ~ ~
1079Z97
separation of terephthalic acid, the mother liquor can
be recycled to the oxidation reactor without causing
any adverse effect on the oxidation reaction to obtain
high quality terephthalic acid. This invention has
been accomplished on the basis of this knowledge.
Accordingly, this invention provides a continuous
process for producing terephthalic acid by oxidation of
p-xylene in acetic acid as a solvent with molecular
oxygen in the presence of an oxidation catal~st consisting
essentially of a cobalt compound, a manganese compound and
hydrogen bromide at a temperature of 150 to 250C under a
pressure of atmospheric to 200 atm., which comprises
(a) introducing at least a portion of the condensable
gas discharged from an oxidation reactor to a distillation
column to effect removal of water and recovery of the
solvent,
(b) effecting post-oxidation of the terephthalic acid
slurry recovered from the oxidation reactor without
addition of any p-xylene at a temperature ranging from
the oxidation temperature to a temperature 50C lower
than the oxidation temperature,
(c) recovering terephthalic acid by centrifugal
separation from the slurry obtained in the post-
oxidation step (b), and
(d) recycling the solvent recovered in the step (a)
and the mother liquor obtained by separating the
terephthalic acid in the step (c) into the oxidation
reactor.
This invention will be explained in detail.
~he process for producing terephthalic acid to
-- 3 --
1079Z~7
which this invention is applicable may be any process
in which p-xylene in acetic acid is reacted in a liquid
phase with molecular oxygen in the presence of a catalyst
containing heavy metals and bromine, so far as the above-
mentioned requirements (a) to (d) are satisfied. Atypical example of such process i8 the so-called SD
process the details of which are disclosed i~ U~P
2,833,816.
The amount of the solvent to be used is usually
0.5 to 20, preferably 1 to 10, parts per part by weight
of the p-xylene. The solvent may contain water or a
reaction promoter, such as an aldehyde, a ketone, an
alcohol or paraaldehyde.
The catalyst employed in this invention is a
known Co-Mn-Br ternary catalyst system. Cobalt and -~
manganese compounds suitable for use in the catalyst
system are disclosed in USP 2,833,816; for example, as
a cobalt compound, cobalt acetate, cobalt bromide or
cobalt naphthenate and, as a manganese compound, ~-
t 20 man~anese acetate, manganese bromide or manganese
naphthenate. It is essential that a portio~ or the
whole of the bromine compound be hydrogen bromide. - -
When cobalt bromide and/or manganese bromide are used,
the amount of hydrogen bromide in the catalyst system
-- 25 can be reduced since they contain bromine. -
In the prior art oxidation of p-xylene with -
molecular oxygen, sodium bromide is a typical bromine
source in the catalyst. Due to the loss of a part of
the bromine during the reaction the addition of a
bromine compound is required in order to maintain the
., - , . . .
1079Z97
predetermined bromide concentration in the reaction
system. However, the addition of sodium bromide increases
the concentration of sodium ion in the mother liquor-which
is recycled, since an amount of sodium ion corresponding
to that of the lost bromine remains. Such excess sodium
ion adversely affects the oxidation reaction. ~herefore,
according to this invention, the use of hydrogen bromide
as the bromine source is significant with respect to
recycling the mother liquor to the oxidation step without
adversely affecting the reaction. The amounts of catalyst
components to be used are 200 to 5000 ppm, preferably
,j .
200 to 600 ppm, in terms of Co, 10 to 1000 ppm, preferably
100 to 600 ppm, in terms of Mn and 400 to 10,000 ppm,
preferably 600 to 2000 ppm, in terms of Br, on the basis
of the solvent. When the amount of catalyst used is
within the above range, the oxidation reaction is
;~ successfully carried out while the mother liquor is
i recycled.
The temperature at which the oxidation is carried
out may be, in general, from 150 to 250C and, where the
amount of catalyst used is within the above range, the
temperature is from 170 to 230C, preferably 205 to
, 225C. ~he pressure under which the oxidation is carried
out is atmospheric to 200 atm., preferably up to 100 atm.
~ 25 The gas containing molecular oxygen to be supplied
; to the liquid phase containing p-xylene is usually air
and the amount thereof to be supplied is from 1 to 100
moles, preferably 3 to 100 moles of oxygen, per mole of
the material to be oxidized.
In the oxidation reactor, at least 95% by weight
~ 1079~:97
of p-xylene is oxidized, preferably more than 98%,
especially more than 99%.
The reaction product slurry obtained from the
oxidation reaction is then subjected to post-oxidation
treatment without isolating terephthalic acid. The
purpose of this treatment is to further oxidize the
oxidation intermediates present in the slurry and the -
oxygen content of a molecular oxygen-containing gas
to be supplied to this treatment may be lower than that
of the oxidation reaction. Usually, a portion of the
exhaust gas from the oxidation reactor is conveniently
used. ~he temperature range within which the post- ~-
oxidation is carried out is from the oxidation
temperature to a temperature 50C, preferably 30C,
lower than that of the oxidation reaction. In the
post-oxidation, no further catalyst may be required.
~he post-oxidation may be effected in a separate
post-oxidation vessel or in a cooling crystallizer
in which the post-oxidation and crystallization are
concurrently carried out while cooling.
~ he slurry which has been subjected to the
post-oxidation is cooled to effect crystallization,
in the usual manner,and separated into the terephthalic
acid and the mother liquor by a centrifugal separator.
It i8 essential according to this invention that the
mother liquor be recycled as it is to the oxidation
reactor. ~he amount of mother li~uor to be recycled
is 20 to 80%, preferably 50 to 80%~ by weight.
It i8 preferred that the remaining mother liquor
~0 not being recycled be introduced into a distillation
-- 6
1~79Z97
column in which water iæ removed and acetic acid is
recovered. ~he recovered acetic acid may be reused in
the oxidation step, if desired.
From the distillation residue, metallic catalyst
components may be recovered by extracting the residue
' with water, followed by adding a carbonate compound to
precipitate carbonates of metallic components, washing
the precipitate with water and dissolving it in acetic ~ ~ -
acid. ~he acetates thus recovered can be introduced
into the oxidation step.
In order to recycle the mother liquor as it is
to the oxidation reactor according to this invention,
it is necessary to remove water which is formed during
the reaction. ~ecause too high a water concentration
adversely affects the reaction, it is preferred that,
! in general, the water concentration of the mother liquor
!~ in the oxidation reaction mixture be maintained at less
than 20% by weight. According to this invention, the
water concentration is controlled in such a way that at
i 20 least a portion of the condensable gas from the reactor
~ is removed and introduced into a distillation column,
''1
: wherein water is removed and the solvent is recovered
for reuse. lt is not always necessary to install such
distillation column separately from the reactor, and
the distillation column used ma~ be of a type such that
the column is directly connected with the top of the
reactor so that water and a non-condensable gas are
removed from the top of the column and a condensable
gas is returned to the reactor. In general, the
condensable gas discharged from the reactor is
-- 7 --
'
` 1079Z97
condensed in the condenser, and most of the condensate
is returned to the reactor and only a portion thereof is
introduced into the distillation column.
~he distillation is usually carried out under
atmospheric pressure and at a bottom temperature of
about 124C and at a top temperature of about 100C.
~he solvent recovered is recycled to the oxidation
reactor.
The continuous proceæs according to this invention
will be explained by referring to an accompanying drawing
which is a block diagram illustrating one embodiment for
the production of terephthalic acid. Supplied to
oxidation reactor I are p-x~lene through pipe 10, a
catalyst-containing solvent through pipe 12 and air
through pipe 14 to effect oxidation reaction.
I The slurry is transferred through pipe 16 into --
post-oxidation reactor II to which diluted air is blown
.
through pipe 18 to effect the post-oxidation treatment.
The post-oxidized slurry is transferred through pipe
20 to cr~stallizer III in which crystallization is
effected to precipitate thoroughly terephthalic acid,
which is separated in centrifugal separator IV and is
j recovered through pipe 22. ~he mother liquor is
recycled through pipes 24 and 26 to oxidation vessel
I, while a portion thereof is removed through pipe 28.
The condensable gas which is formed during the
oxidation reaction and discharged through pipe 30 from
the top of oxidation reactor I is condensed in condenser
32 and the most of the condensate is returned to the
oxidation reactor and the remainder is introduced through
-- 8 --
~.
1079Z~7
pipe 34 to distillation column V, in which water is
removed through pipe 36 from the top and the solvent
is recovered from the bottom and recycled through
pipe 26 to oxidation reactor I.
hs mentioned above, the solvent and the mother
liquor which have been recovered are rec~cled to
oxidation reactor I through pipes 24 and 26, while a
fresh catalyst and solvent are added through pipe l? ~ .:
to make up for what has been lost and removed whereby
the operation is carried out continuously.
According to this invention, if the mother
liquor of the slurry is recycled as it is to the
oxidation reactor, the reaction is not adversely
affected and high ~uality terephthalic acid can be
obtained. ~urther, various valuable components in
the mother liquor, such as the p-xylene, the oxidation
intermediates and the catalyst, are completely utilized
to make this process commercially attractive. The reason
why the reaction is not impaired when the mother liquor
i~ recycled is that, with the post-oxidation of the
oxidation product slurry, the materials which would
otherwise impair the reaction are substantially removed.
This invention will be explained in further detail
by means of examples; however, it should be understood
that this inve~tion is in no way limited by these e~amples.
~xample 1:
Into a 10 ~ capacity titanium autoclave reactDr I
equipped with a stirrer and an external heater was charged
a mixture of 3 ~ of acetic acid (water content: ~h by
weight), 4.43 g of cobalt acetate tetrahydrate, 4.68 g of
1079297
manganese acetate tetrahydrate and 6.79 g of hydrobromic
acid (47% aqueous solution), and then p-xylene was
supplied through pipe 10 at a rate of 750 g~hr and air
was supplied through pipe 14 at such a rate that the
oxygen concentration of an exhaust gas from the reactor
was 4 to 5% by volume, while the reaction conditions,
reaction temperature of 210C and reaction pressure of ;-24 kg/cm2, were maintained for 1.5 hours to effect
semicontinuous reaction. Then, a freshly prepared
mixture of acetic acid and a catalyst having the above - -composition was supplied through pipe 12 at a rate of
2,250 g/hr, while the slurry was being discharged
through pipe 16 at a rate of 3,600 g/hr to effect
,~ continuous oxidation reaction with the average
residence time of 60 minutes. ~he slurry discharged
from reactor I was supplied to a 10 ~ capacity titanium
,~ autoclave II (the post-oxidation vessel) equipped with
~ a stirrer and an external heater to which diluted air
j~ having an o~gen concentration of 14% by volume was
' ? supplied through pipe 18 at such a rate that the oxygen
concentration of an exhaust gas from the post-oxidation -
vessel was maintained at 3 to 4% by volume, with the -
reaction conditions of a temperature of 195C, a pressure
of 19 kg/cm2 and an average residence time of 40 minutes. ~-
The post-oxidized slurr~ was transferred to crystallizer ~
III in which crystallization was effected at 100C,and ; -
terephthalic acid and, the reaction mother liquor
were separated in oentrifugal separator IV.
After 3 hours from the start of the continuous
~0- oxidation reaction, a portion of the mother liquor to
-- 10 --
~ - ,
1079Zg7
which a fresh hydrogen bromide was added in an amount
corresponding to that lost during the reaction was
recycled to the reactor I through pipes 24 and 26 at
a rate of 1250 g/hr (the recycling being 50%) and the
remainder of mother liquor was removed through pipe 28.
~he amount of acetic acid solvent containing the
catalyst supplied through pipe 12 was decreased to
1100 g/hr. In order to prevent the increase in the
water content of the mother liquor in the reactor I
due to the recycling of the mother liquor, a portion
of the condensate condensed from a condensable gas
accompanying the exhaust gas from the reactor was
withdrawn after 3 hours from the start of the continuous
reaction through pipe 34, and acetic acid was supplied
through pipe 26 in such an amount as contained in the
condensate withdrawn to maintain the water concentration
of the mother liquor in the reactor at 18% by weight
(said acetic acid being one recovered from the dis-
tillation column in commercial operation, but in this
example the column was not used).
After continuing such mother liquor recycling
for 30 hours, the terephthalic acid obtained by solid-
liquid separation was suspended in acetic acid in an
amount 3 times that of terephthalic acid and agitation
was continued at a temperature of 80C for 20 minutes
to effect washing. The properties of the terephthalic
acid thus obtained are given in Table 1.
Comparative Example 1:
Procedures similar to those of Example 1 were
repeated to carry out continuous oxidation excepting
1079Z~'7
that recycling the mother liquor and withdrawing a
portion of the condensate were omitted. ~he properties
of the terephthalic acid obtained are given in ~able 1. '' '
As is clear from ~,xample 1 and Comparative ~xample
1, the properties of terephthalic acid obtained by
effecting post-oxidation and recycling the mother liquor
to the oxidation reactor are comparable with those of
the terephthalic acid in Comparative Example 1 which
does not involve recycling the mother liquor and which
gives good results.
~xample 2:
, Procedures similar to those of ~xample 1 were
repeated excepting that the amount of acetic acid
,- containing the catalyst supplied through pipe 12 was
680 g/hr and the amount of mother liquor recycled
~,, through pipes 24 and 26 was 1730 g/hr (recycling being
7~/0 of the mother liquor separated). ~he properties
of the terephthalic acid obtained are given in Table 1.;
, ~rom the results, it is clear that in Example 2
the amount of mother liquor to be recycled can be
increased to 70y by weight without deteriorating the
,, .
., properties of the terephthalic acid compared with those
in Example 1 (recycling being 50%) and Comparative
' E~ample 1 (no recycling). ''
- 25 Comparative xample 2:
Procedures similar to those of Comparative
Example 1 were repeated to carry out continuous oxidation
reaction for 30 hours excepting that p-xylene was
supplied at 500 g~hr, the semicontinuous reaction time
was 2 hours, acetic acid containing the cata,lyst was
.
- 12 -
.JI.. : ~ '. '' - : . .
! . . ' ~ . .
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supplied in the continuous reaction at 1500 g/hr and
the average residence time in rea.ctor I was 90 minutes;
. and no post-oxidation was carried out. ~he properties
of the terephthalic acid obtained are given in Table 1.
i 5 Comparative EXample 3:
.~ Procedures similar to those of Comparative
! Example 2 were repeated excepting that, after 4 hours
from the start of the continuous reàction, the reaction
mother liquor was recycled through pipes 24 and 2~ at
a rate of 830 g/hr and the amount of acetic acid solvent
containing the catalyst supplied through pipe 12 was
decreased to 750 g/hr, while the water concentration of
the mother liquor in reactor I was maintained at 18% in
the same manner as in ~xample 1 and recycling the mother
liquor was continued for 30 hours. ~he properties of
the terephthalic acid thus obtained are given in Table 1.
As is clear from the results, the terephthalic
~ acid produced by recycling to the reactor the mother
- liquor which does not subject to post-oxidation (Com-
parative Example 3) contains much more 4-carboxybenz-
aldehyde of oxidation intermediate impurity and its
transmittance is lower in comparison with terephthalic
acid produced without recycling the mother liquor and
effecting post-oxidation (Comparative ~xample 2).
~xample 3:
Procedures similar to those of Example 1 were
repeated excepting that p-x~lene was supplied at a rate
of 500 g/hr, the semicontinuous reaction time was 2
hours, acetic acid containing the catalyst was supplied
in the continuous reaction at 1500 g~hr, the average
.
~ - - 13 -
1079Z97
residence time in the reactor I was 90 minuteæ, and,
a~ter 3 hours from the start of continuous reaction~ the
mother liquor was recycled at 830 g/hr and the acetic
acid containing the catalyst was supplied at 750 g/hr.
~he properties of the tereph~halic acid thus obtained
are given in ~able 1.
Comparative ~ample 4:
~ he reaction was carried out as in Example 3
but recycling the mother liquor and withdrawing a
portion of the condensate were omitted. The propertieæ
of the resulting terephthalic acid are given in ~able 1.
Table 1
- ~ .
I :'
¦ Comp. Comp.¦ Comp. Comp.
Ex. 1 ~x. 2l-~x. 1 ~. 2 ~x. 3 ~x. 3 ~x. 4
. _
4-CBA*
concentration
in the product 460 460 450 400 580 280 270 -~
terephthalic
acid (ppm)
:.. _ . .'
Transmittance
340 m~ 88 87.5 89 84 ?7 91. o 91. 5
400 m~ 98. 5 98.0 98.5 98-5 97- 5 99.0 99.o
~ .
Residence 60 60 60 9o 9o 9o 9o
. __ .
.
Post-oxidation done done done none none done done
~ , . ._ ,.
mother liquor done done none none done done none
__ . ~ . ~
Amount of
mother liquor 5o 7o _ _ 5o 5o _
(% by weight) l
~_ ~ _
*Note 4-CBA: 4-Carboxybenzaldehyde
- 14 -
