Note: Descriptions are shown in the official language in which they were submitted.
CA 02248288 1998-08-12
WO 97/30963 PCT/GB97/00439
PRODUCTION OF AROMATIC POLYCARBOXYLIC ACIDS
This invention relates to the production of aromatic carboxylic acids,
especially
dicarboxylic acids such as terephthalic acid.
Terephthalic acid for example is produced commercially by oxidising p-xylene
with oxygen in a liquid phase which comprises a lower aliphatic carboxylic
acid
solvent, such as acetic acid, and a dissolved heavy metal catalyst system
(usually
cobalt and manganese and a bromine promoter). A slurry of terephthaiic acid in
the
solvent is withdrawn from the reactor and is subjected to a solids-liquid
separation
process resulting in crude terephthalic acid crystals which may be
subsequently
processed further and a mother liquor filtrate which, in addition to catalyst
and
promoter used in the oxidation reaction, contains dissolved terephthalic acid
and
various by-products and impurities. These by-products and impurities arise
from
various sources such as minor impurities in the p-xylene feed stock to the
reaction,
incomplete oxidation of p-xylene resulting in partially oxidised products and
by-products arising from the competing side reactions in the oxidation of p-
xylene to
terephthalic acid.
It is common practice to recycle a large proportion of the recovered mother
liquor to the oxidation reaction in order to return catalyst and promoter to
the
oxidation reaction while purging a smaller proportion to a solvent recovery
system
so as to maintain the level of impurities and by-products in the reaction
within
tolerable limits. In the solvent recovery system, the mother liquor purge is
subjected
to evaporation to remove a substantial proportion of the aliphatic acid
solvent and
water present (which can be returned to the oxidation reaction) leaving a
concentrate containing terephthalic acid and impurities/by-products together
with
some of the heavy metal catalyst present in the original mother liquor
filtrate. The
concentrate (the residues) may then be disposed of or, if economically
justifiable,
treated in order to recover valuable components for recycling, e.g. catalyst
metals.
Typical downstream treatments of the residues include catalyst recovery,
incineration and anaerobic/aerobic biological treatment to reduce chemical
oxygen
demand (COD).
The present invention is concerned with a process for the production of an
aromatic carboxylic acid comprising oxidising a precursor of the aromatic
carboxylic
acid in an aqueous liquid phase medium comprising a lower aliphatic carboxylic
acid
and in the presence of a heavy metal catalyst system, withdrawing from the
reaction
mixture a slurry of the aromatic carboxylic acid in mother liquor comprising
mainly
the aliphatic acid, subjecting the slurry to a solids-liquid separation to
recover
crystals of aromatic carboxylic acid, recycling a first fraction of the
resulting mother
liquor to the oxidation reaction, concentrating a second fraction of the
separated
1
CA 02248288 2005-07-12
mother liquor to remove aliphatic acid, and disposing of or processing the
concentrated residue. Such a process is hereinafter referred to as being 'of
the kind
specified".
According to the present invention there is provided a process of the kind
Speoified in which the second fraction of mother liQuor is subjected to solids-
liquid
separation prior to concentration th~roof.
As in conventional practice, the first fraction will usually be substantially
large
than the second fraction.
By subjecting the second mother liquor fraction to solids-liquid separation
prior
to concentration thereof, we have sound that a small but useful emouni at the
aromatic carboxylic acid~can be recovered from the mother liquor since some
aromatic carboxylic acid in the form of fine partides lends to pass through
the
solids-liquid separation with the mother liquor. The solids-liquid separation
treatment
Is conveniently carried out so as to remove et least a major proportion of the
fines
present in the second mother liquor fraction.
Typically the silt range of particles removed is 10 to 100 microns. The
aromatic carboxylic acid concentration in mother liquor is typically 0.5 to
1.5% by
weight and 50 to 99% of these solids are recovered.
Although reference is made to first and second fractions, ws da net exclude
2D the possibility of the mother liquor being split into more than two
fractions provided
that at least one fraction is returned to the oxidation reaction end at lesst
one
traction is treated to remove said fines.
~y rernovinfl the aromatic carboxylic acid in this manner, the organic content
of the second mother liquor fraction can de reduced significantly thereby
reducing
the load on downstream equipment used to cor~centrata the second mother liquor
fraction and treat or dispose of the resulting residue.
In particular, the reduced amount of organic r»aterial in the feed to the
concentrating equipment such as an evaporator leads to an improvement in
performance of such equipment thereby allov~ing use of smatter eQuipment or
3t) affording potential uprating of existing plant. Luring concentration of
the second
mother liquor fraction, the aliphatic acid component (e8 acetic acid) is
driven off.
Because the evaporator ties to handle less solids content of a different
compositional nature when the process of the present invention iS employed, it
becomes possible to recover more of the aliphatic acid component stoic the
amount
of liquid necessary to maintain fluidity in the evaporator bottoms Is reduced.
Typically, acetic acid IEVeIs in reslurHed evaporator bottoms fall from
between 10 to
15°i° fa 3 to 7°~. In this context, it will be understood
that it is irt~portant to maintain
z
CA 02248288 2006-03-03
fluidity of the evaporator bottoms otherwise a mass having poor flow
characteristics
would result with consequent difficulties in handling.
Where the concentrated residue is subjected to incineration the load on such
equipment is likewise reduced by the reduction in the organic content of the
second
mother liquor fraction, making downsizing of the incinerator possible.
Similarly,
where the concentrated residue is subjected to further processing such as
catalyst
recovery or biological treatment, the load on the equipment employed is
reduced.
Where the concentrated residue is simply sent to landfill, the amount to be
disposed
is reduced.
Moreover, the process of the invention allows the recovery of aromatic
carboxylic acid which would otherwise be lost in a waste stream.
The aromatic carboxylic acid recovered from the second fraction may be
recycled to the oxidation reaction, for example by resturrying it in solvent
and
returning the resulting slurry to the oxidalion process. Thus, for example the
aromatic carboxylic acid may be resiurried with mother liquor comprising said
first
fraction and/or with solvent recovered during concentration of the second
fraction.
Alternatively, the aromatic carboxylic acid recovered from the second fraction
may
be transferred to the next stage of processing along with terephthalic acid
recovered
from the slurry in the solids-liquid separation. Such further stage of
processing may
comprise purification for example by hydrogenation of an aqueous solution of
crude
aromatic carboxylic acid as disclosed for instance in European Patens
Applications
Nos. 498591 and 502628 or International Patent Applications Nos. W093/24440
and
W094117892.
In another embodiment, at least some of the aromatic carboxylic acid
recovered from the second fraction may be introduced into a crystallisation
process
following the oxidation reaction and preceding the primary solids-liquid
separation
(separating crude terephthalic acid and the mother liquor) in order to secure
growth
of the fines to a larger particle size. The crystallisation process serves to
induce
further precipitation of the aromatic carboxylic acid from solution in the
mother
liquor by reducing pressure and temperature in one or more crystallisation
vessels.
Where the recovered aromatic carboxylic acid is recycled to the oxidation
reaction it may be combined with aromatic carboxylic acid recovered from
elsewhere
in the process and !he combined amounts of aromatic carboxylic acid may be
reslurried with mother liquor andlor recovered solvent for recycle to the
reaction as
a slurry. For example, in our prior European Patent Application No. 498591, we
disclose a process for the purification of crude aromatic carboxylic acid in
which,
following purification and separation of the purified aromatic carboxylic acid
crystals
3
CA 02248288 1998-08-12
WO 97/30963 PCT/GB97/00439
from aqueous mother liquor, the mother liquor is further treated to
precipitate less
pure aromatic carboxylic acid which is recycled to the oxidation process.
Thus, in
the present invention, less pure aromatic carboxylic acid derived from a
purification
process for terephthalic acid or other appropriate aromatic carboxylic acid
may be
combined with aromatic carboxylic acrd recovered from the second fraction of
the
oxidation mother liquor and returned to the oxidation reaction.
The solvent, eg acetic acid, recovered from the concentration of the second
mother liquor fraction may be transferred to facilities for storage, disposal
and/or
further processing or it may be recycled to the oxidation reaction. The
concentrated
residue may be disposed of, far example by incineration or landfill, or
subjected to
further processing to recover valuable constituents thereof.
The primary solids-liquid separation will usually be preceded by a
crystallisation process, i.e. the slurry obtained from the oxidation reaction
will
usually be passed through one or more stages of crystallisation to promote
further
precipitation of the aromatic carboxylic acid from solution thereby increasing
the
yield of aromatic carboxylic acid crystals obtained from the solids-liquid
separation.
The primary solids-liquid separation may be carried out using conventional
techniques but preferably it is carried out in a pressurised integrated
filtration and
washing apparatus as disclosed in European Patent Application No. 502628 or
International Patent Application No. W094/17892 preferably with a
countercurrent
wash using water as the wash liquor so that the liquid content of the filtered
and
washed aromatic carboxylic acids obtained comprises water, i.e. the solvent
used in
the purification stage of processing where this is employed. Preferably the
integrated filtration and washing apparatus comprises belt filtration
equipment
operating under elevated pressure conditions. Alternative forms of filtration
and
washing apparatus include rotary filters such as rotary filters in which a
pressure
differential is established between the slurry and filtrate sides of a
cylindrical filter
medium to effect displacement of mother liquor and wash water through the
fitter
cake, and pressure drum filters in which the mother liquor is displaced from
the filter
cake by water under hydraulic pressure (as in for example a BHS-Fest
pressurised
drum filter).
The technique used to effect solids-liquid separation of the second mother
liquor fraction is selected to ensure adequate recovery of the aromatic
carboxylic
acid fines present. A preferred technique is filtration, conveniently using a
filter in
which build up of a filter cake on a filter medium occurs in such a way that
the filter
cake is itself instrumental in filtering the solids-containing liquid. Various
forms of
filtration processes are suitable, e.g. a pressure vessel equipped with a
candle filter
or filters.
4
CA 02248288 1998-08-12
WO 97/30963 PCT/GB97/00439
After concentration of the second mother liquor fraction, the resulting
residues
may be processed using conventional techniques such as incineration and/or
biological treatment (anaerobic and/or aerobic).
However, in a preferred aspect of the invention, the concentrated residue
including a substantial part of its organic content is dissolved in an aqueous
medium
and the metal catalyst components are precipitated from the solution.
The metal catalyst components may be precipitated from said solution by the
inclusion of metal salt-forming anions in the solution.
Typically the aromatic carboxylic acid is one which has very low solubility in
water, viz. less than 1 % by weight at 25°C, e.g. terephthalic acid.
Preferably substantially the whole of the residue is dissolved in said aqueous
medium. Conveniently the catalyst metals are precipitated from the aqueous
medium
by the addition of carbonate and/or bicarbonate ions to the medium. By
dissolving
substantially the whole of the residue, the catalyst metal yield can be
increased
since catalyst metals occluded, chemically or otherwise, with the organics are
taken
into solution and can then be precipitated, for example as carbonates and/or
bicarbonates.
Because aromatic carboxylic acid is removed from the second mother liquor
fraction prior to concentration, not only can the organic content of the
second
mother liquor fraction be reduced significantly with consequent reduction of
the load
on downstream equipment used to concentrate the second mother liquor fraction
but
the organic content of the resulting concentrated residue is significantly
reduced by
elimination of the aromatic carboxylic acid which may have low solubility in
water
thus making it feasible to carry out residue treatment involving dissolution
of
substantially the whole of the residue in said aqueous medium.
After precipitation of the catalyst metals, the residual liquor may be
processed
using conventional techniques such as biological treatment (anaerobic and/or
aerobic).
Precipitation of the metal catalysts is conveniently effected by inclusion in
the
aqueous medium of the carbonate and/or bicarbonate reaction product obtained
following contacting a metal or ammonium hydroxide with a carbon
dioxide-containing offgas derived from the oxidation reaction in which said
polycarboxylic acid is produced.
Preferably the aqueous medium comprises, as at least a major component
thereof, an organic material-containing mother liquor derived from the
hydrogenation
of an aqueous solution of the poiycarboxylic acid. In contrast with prior
catalyst
recovery schemes in which water is used to extract the desired metals from the
residue, the process of the present invention involves the solubilisation of
5
CA 02248288 1998-08-12
WO 97!30963 PCT/GB97/00439
substantially the whole of the residue before precipitating the catalyst
metals, thus
making it feasible to use an organics containing aqueous medium for
dissolution of
the metals and organic content of the residue.
Solubilisation of substantially all of the residue in the aqueous medium may
be
effected by inclusion of an alkaline agent added to the aqueous medium prior
to
andlor in the course of combining the residue with the aqueous medium. The
agent
may comprise ammonium hydroxide or a metal hydroxide, such as sodium
hydroxide. Alternatively , pH may be increased initially by the inclusion of
carbonate
and/or bicarbonate ions in the aqueous medium either as the sole alkaline
agent or
in combination with another alkaline agent or agents such as ammonium
hydroxide
or a metal hydroxide. The inclusion of carbonate and/or bicarbonate ions in
the
aqueous medium (by addition to the aqueous medium prior to or after contacting
the
same with the residue) is considered advantageous since the metal recovered in
the
form of carbonates andlor bicarbonates is of a higher quality, apparently
because
less oxide contamination occurs compared with use of hydroxide only in the
initial
solubilisation of the residue. For this reason, the process of the invention
may with
advantage be carried out using carbonate andlor bicarbonate ions as the major
or
sole alkaline agent in effecting initial solubilisation of the residue.
Typically the alkaline agent is introduced to raise the pH sufficiently,
preferably at least to 5.5, to neutralise the acidic content of the residue
(and
aqueous medium where the latter contains acidic components as in the case
where it
is constituted by the mother liquor derived from said hydrogenation reaction)
partially and dissolve the same. Subsequently carbonate and/or bicarbonate
reaction
ions are added to raise the pH further to precipitate the catalyst metals and
secure a
pH compatible with downstream processing of the liquor remaining following
separation of the solids. For instance, the pH is conveniently increased to
about 6.5
to about 9, preferably about 7 to 8, by addition of said carbonate and/or
bicarbonate
reaction product. Where the downstream processing includes anaerobic treatment
of
the liquor, the pH of the liquor obtained following precipitation of the
metals rnay be
adjusted to 6.5 to 8, preferably about 7, by the addition of further mother
liquor
derived from the hydrogenation reaction.
Preferably the dissolution of the residue and subsequent precipitation of the
metal catalyst components is effected by initially adding hydroxide to
increase pH to
a level such that the subsequent addition of the carbonate and/or bicarbonate
is not
accompanied by any substantial evolution of carbon dioxide. Suppression of
carbon
dioxide evolution is advantageous in order to avoid stripping volatiles such
as acetic
acid from the aqueous medium since the evolved gases/vapours would then need
to
be treated before disposal. Also, suppression of carbon dioxide evolution
avoids
6
CA 02248288 1998-08-12
WO 97/30963 PCT/GB97/00439
operating and/or design problems, e.g. foaming and level control, in dealing
with
such evolution during the addition of the carbonatelbicarbonate ions.
Following precipitation and separation of the catalyst metal carbonates and/or
bicarbonates, the liquor is conveniently subjected to anaerobic treatment or
wet
oxidation, optionally followed by aerobic treatment.
The invention will now be described by way of example only with reference to
the accompanying drawings in which:
Figure 1 is a flow sheet illustrating the handling of mother liquor recycle in
a process
for the production of terephthalic acid; and
Figure 2 is a flow sheet illustrating a catalyst recovery system for use in
conjunction
with the process illustrated in Figure 1.
In the embodiment of Figure 1, terephthalic acid is produced in a reactor 10
by
reacting p-xylene with oxygen (eg. air or oxygen enriched air) in acetic acid
solvent
containing some water and a dissolved catalyst system comprising heavy metals,
usually cobalt and manganese, and bromine as a promoter. The p-xylene, acetic
acid and catalyst may be supplied to the reactor via a feed mix drum 12 in
which
these components are mixed with recycled mother liquor from mother liquor drum
14. The oxygen/air is introduced separately into the reactor 10 via a feed
line or
lines (not shown). Further details of the reaction are given in our prior
European
Patent Applications Nos. 498591 and 502628. Typically the reaction is carried
out at
a temperature of 170-230°C and a pressure of several kg/cm2 to 100
kg/cmz, eg.
8-30 kglcmz.
The terephthalic acid is withdrawn from the reactor 10 in the form of a slurry
of
terephthalic acid crystals in mother liquor comprising acetic acid and some
water.
The slurry is then subjected to crystallisation in one or more crystallisation
vessels
(not shown) by reducing pressure and temperature so as to precipitate further
terephthalic acid. Following the crystallisation process, the slurry is
typically at a
temperature of the order of 70 to 200°C. The slurry next undergoes an
integrated
solids-liquid separation process in which the crystals are separated from the
mother
liquor by filtration and are washed using water or acetic acid as the wash
medium.
The solids-liquid separation process is carried out in unit 18 under pressure
using a
filter medium across which a pressure differential is produced to effect
displacement
of the mother liquor and the wash liquor through the filter cake, comprising
terephthalic acid crystals, which develops on the filter medium. The pressure
differential may be produced by pressurising the upstream side of the filter
medium
with a gas or vapour or by hydraulically pressurising the slurry and the wash
liqour.
The integrated filtration and washing process may be carried out using for
example a
belt filter as disclosed in European Patent Application No. 502628 under
conditions
7
CA 02248288 1998-08-12
WO 97/30963 PCT/GB97/00439
described therein or using a rotary suction filter or a pressure drum filter
such as a
BHS-Fest drum filter or a centrifuge. In the illustrated embodiment, the
filtration and
washing process is shown carried out in a rotary filter unit, if desired with
countercurrent washing of the filter cake with water. The filter cake
comprising
terephthalic acid crystals is removed from the unit 18 via line discharge
outlet 20 for
further processing, e.g. preparation for use in polyester production without
any
further purification or purification to reduce the level of impurities therein
followed
by subsequent use in the production of polyesters, e.g. as disclosed in our
prior
International Patent Application No. W093/24440.
The mother liquor filtrate derived from the solids-liquid separation unit 18
via
line 22 largely consists of acetic acid (typically 85 - 95% by weight) and
water
{typically 5 - 15% by weight). The mother liquor also contains soluble organic
by-products and intermediates produced in the reaction, reaction catalyst and
residual terephthalic acid. Also with this type of filter, the wash liquor
often mixes
with the mother liquor stream. The recovered mother liquor is fed to a
separator 24
in which the liquor is separated from the gas used to provide the pressure
difference
for the filtration and washing unit 18 (e.g. nitrogen). The gas is recovered
via line 26
and the mother liquor via line 28. The mother liquor is split into two
fractions, one of
which is recycled via line 30 and mother liquor drum 14 back to the reactor
and the
second of which is purged from the process via line 32 in order to maintain
the level
of impurities in the system within acceptable limits. The mother liquor
recycle
fraction is typically in the range 0.7 to 0.99 (e.g. 0.7 to 0.95) and the
purge fraction
is correspondingly 0.3 to 0.01 (e.g. 0.3 to 0.05).
In contrast with conventional practice, before being subjected to
concentration,
the purge stream 32 is fed to a filter unit 34 which may comprise one or more
candle-type filters within a pressure vessel. The candle-type filter unit 34
is
designed so as to separate terephthalic acid fines. A particularly suitable
filter unit
for this duty is a Cricketfilter (Model 1200W-25/1500-100) which is a
synthesis of
conventional leaf filters and candle filters and is manufactured by Amafilter
b.v., P
O Box 396, 1800 AJ Alkmaar, Kwakelkade 28, 1823 CL. Alkmaar, Holland.
Effective
filtration was secured by using a PTFE filter cloth material in the
Cricketfilter.
The filtered mother liquor purge is fed via line 36 to a stripper stillpot 38
in
which a substantial part of the solvent (acetic acid) is boiled off and fed to
an acetic
acid recovery process (distillation column) via line 40. The residual liquor
is fed to
evaporator 42 for concentration. In evaporator 42, further acetic acid is
driven off
via line 44 for feed to acetic acid recovery in such a way as to leave the
evaporator
. bottoms in a fluid state for supply of the resulting residue to a waste
treatment
system via line 46. In the process of the present invention, an increased
amount of
8
CA 02248288 2006-03-03
acetic acid can be recovered compared with the level of recovery achieved in
the
absence of filtering the mother liquor purge fraction. For instance, whereas
99.515%
of the acetic acid was recovered in the absence of such filtration, in
practice it has
been found possible to Increase this figure to 99.758°~ as a result of
filtering the
mother liquor purge fraction. As mentioned previously, the residue obtained
from the
evaporator may be.disposed of or may be subjected to treatment by means of
incineration or other treatment such as catalyst recovery or biological
processing.
Periodically, the filter cake comprising terephthalic acid fines and some
solid
phase catalyst, is removed from unit 34 by backflushing with mother liquor,
fresh
acetic acid solvent or inert gas and is supplied to the mother liquor drum 14,
or
elsewhere, for recycle to the reactor 10. If desired, and as illustrated, the
filter cake
may first of all be resiurried in reslurry drum 48 using mother liquor or
fresh acetic
acid. During backflushing of the filter unit 34, the mother liquor supplied
via line 32
may be diverted temporarily to the mother liquor drum 14 or bypass the filter
and
pass directly to the stripper stillpot 38.
~A commercial plant producing terephthalic acid at the rate of 55 to 56 te/hr
and
operating with a mother liquor recycle of 85% was equipped with the
Cricketfiiter
unit mentioned above and a comparison was made between plant operation with
the
Cricketfiiter unit on-line and off-line. With the Cricketfilter unit off-fine,
the acetic
acid loss via the mother liquor purge was 34 kg/hr and the rate of organic
residue
production was 5.04 kg of residue per tonne of terephthalic acid produced.
With the
Cricketfilter on-line, the acetic acid loss reduced to 23 kg/hr while the
organic
residue production rate fell to 4.13 kg of residue per tonne of terephthalic
acid
produced by the plant. Also, with the Cricketficter on-line, the amount of
terephthalic
acid recovered from the mother liquor purge was about 70 kg/hr, which equates
to a
reduction in paraxylene feedsiock to the process of about 0.13°~ -
which represents
significant annual cost savings.
Referring to Figure 2, the residue obtained from the evaporator 42 is fed via
line 46 to a stirred tank 50 together with a 5°~ caustic soda solution
supplied via line
52 and an aqueous medium supplied via lines 54, 56. Although in Figure 2, the
various components are shown as being fed separately to the tank 50, the
residue
may be scurried up in a portion of the aqueous medium in a slurry receiver
upstream
of the tank 50. At least a major part of the aqueous medium is advantageously
constituted by mother liquor derived from plant for purifying crude
terephthalic acid
by hydrogenation of an aqueous solution of the crude acid in the presence of a
noble
metal catalyst such as platinum and/or palladium on an inert, e.g. carbon,
support.
Suitable plant for purifying crude terephthalic acid is described in EP-A-
498591,
EP-A-502628 and WO-A-93!24440.
9
CA 02248288 2006-03-03
As described in these prior patent publications, following
hydrogenation the solution is passed through a crystallisation train resulting
in a
slurry of purified terephthalic acid crystals in aqueous mother liquor and the
slurry is
filtered and washed. The mother liquor filtrate (primary mother liquor)
obtained may
be used as the aqueous medium supplied to the tank 50. Alternatively, the
primary
mother liquor may be subjected to cooling or evaporation to precipitate
further, but
less pure, terephthalic acid crystals which, following separation from the
secondary
mother liquor, may be slurried in acetic acid for recycle to the oxidation
reactor.~The
secondary mother liquor so obtained may then be used as the aqueous medium in
the catalyst recovery system. If desired, the aqueous medium may comprise both
primary and secondary mother liquor. The advantage of using the secondary
mother
liquor is that its organic content is reduced compared with the primary mother
liquor.
Typically the mother liquor supplied to the tank will comprise primarily water
but will
also contain small amounts of water, acetic acid, benzoic acid, paratoluic
acid,
terephthalic acid and manganese and cobalt acetates.
In the tank 50, at a temperature of about 60°C, 5°~ caustic
soda is added to
raises the pH to for example about 5.5 and the metals and organics are
dissolved.
The liquor obtained overflows into a precipitation tank 58 via a baffled
outlet to
prevent carryover of any solids still undergoing dissolution in tank 50.
Sodium
carbonate and/or bicarbonate obtained from a scrubber as described below is
also
supplied to the tank 58 via line 104, the rate of supply being such that the
pH is
raised to about 6.5 to about 9 leading to precipitation of the catalyst
metals;
primarily as carbonates and/or bicarbonates thereof. As mentioned previously,
some
precipitation of the metals as oxides (especially manganese oxides andlor
hydroxides) may also occur especially if caustic soda is used in tank 50. The
oxides
are considered to be contaminants and consequently it may be preferred to
substitute at least part of the caustic soda in tank 50 with sodium carbonate
and/or
bicarbonate derived from the same source as that supplied via line 104.
The contents of the precipitation tank 58 are passed to solids-liquid
separator
unit 62 which may, for instance, comprise a ciarifier producing a solids-
containing
undertlow and a liquor overflow. The underfiow is pumped to a sludge. buffer
tank
(not shown) and subsequently passed to a fitter press to produce a relatively
dry
cake containing the catalyst metal carbonates andlor bicarbonates. The
catalyst
metals recovered in this way may be recycled via line 64 to the oxidation
reactor 10
as their carbonates and/or bicarbonates or, alternatively, before recycle they
may be
converted to for example acetates by reaction with acetic acid. The unit 82
may
alternatively comprise a centrifuge.
CA 02248288 1998-08-12
WO 97/30963 PCT/GB97/00439
The overflow 66 from the clarifier is mixed with additional mother liquor
supplied via lines 54, 68 and passes to a final neutralisation tank 70 to
which caustic
soda is added via line 72 in order to adjust the pH of the liquor prior to
feed to
downstream processing plant via line 74. The mother liquor supplied via line
54
typically corresponds to the amount which is to be purged from the
purification plant
to maintain the levels of impurities within acceptable limits, especially when
the
mother liquor is to be recycled in the manner disclosed in EP-A-498591,
EP-A-502628 and WO-A-93/24440. The purged mother liquor requires treatment
before disposal because of its COD and such treatment will usually entail
adjustment
of its pH.
It will be seen that the process described with reference to Figure 2 allows
the
purge to be employed as a vehicle for recovery of catalyst metals despite the
organic content of the mother liquor purge. Rather than pass the entire amount
of
mother liquor purge to the residue dissolution tank 50, it is preferably
divided into
two fractions, as indicated by lines 56 and 58, so that the equipment size and
cost in
these stages can be reduced. Another factor governing the amount of mother
liquor
employed in the dissolution stage(tank 50) is the evolution of COZ that occurs
in the
course of increasing pH in the dissolution stage. If the sodium (or other
alkali metal)
carbonate andlor bicarbonate is added at low pH levels, for a given amount of
the
liquor present, the amount of COZ that can remain in solution (and hence be
available as carbonate ions at the precipitation stage) is reduced compared
with
addition at higher pH levels.
Consequently to avoid loss of COZ from solution on introduction of the sodium
carbonate and/or bicarbonate, it is desirable to secure conditions which
suppress
COz evolution from the solution.This can be achieved by controlling pH (e.g. a
pH of
about 5.5 is suitable) and/or the level of dilution during the dissolution
process.
Whilst equipment size and cost is a factor which implies minimising the amount
of
mother liquor used in the dissolution stage, it will be generally desirable to
employ
sufficient mother liquor consistent with suppressing C02 evolution.
The neutralisation carried out in tank 70 will usually involve adjustment of
pH
within the range fi.5 to 8, preferably 7, for comparability with the
downstream
processing of the liquor. Such downstream processing may take various forms
such
as anaerobic treatment (e.g. using the UASB process - upflow anaerobic sludge
blanket) followed by aerobic treatment (e.g. activated sludge treatment), or
wet
oxidation using for example the known ZIMPRO or LOPROX processes.
The carbonate used in the treatment of the residue may be derived from a
scrubber for scrubbing effluent gas from plant for the production of
terephthalic acid
after treatment of the effluent gas by high temperature combustion, preferably
in the
11
, , CA 02248288 2006-03-03
presence of a catalyst and under elevated pressure, to convert methyl bromide
in
the effluent to bromine andlor hydrogen bromide, A process for the treatment
of the
. effluent gas, including scrubbing thereof, is described in our published
International
Patent Application No. WO 96139595.
The scrubbing liquid is preferably caustic soda, which is
converted to sodium carbonate and bicarbonate in the scrubbing vessel as a
result
of absorption into the hydroxide of carbon dioxide contained in the effluent
gas. The
sodium (bi)carbonate resulting, from the scrubbing process is then used in the
recovery of catalyst metals as described above thereby making efficient use of
the
scrubbing liquor.
72
