US006034269A

Ulllted States Patent [19] [11] Patent Number: 6,034,269

Turner et al. [45] Date of Patent: Mar. 7, 2000

[54] PROCESS FOR PRODUCING PURE 3,665,033 5/1972 Ohlswager ........................ .. 260/524 R
CARBOXYLIC ACIDS 4,892,970 1/1990 Nowicki et 81.
5,110,984 5/1992 Janulis ......... ..

[75] Inventors: John Arthur Turner, North Yorkshire; , ,

itreich ett a11- --
enney e a . ........................ ..
423/478
EVEEEEHtSIh?FE dom 5,409,672 4/1995 Cetinkaya ............................. .. 422/189
g g 5,840,965 11/1998 Turner et a1. ......................... .. 562/412

[73] Assignee: E. I. du Pont de Nemours and FOREIGN PATENT DOCUMENTS

Company, Wilmington, Del.
O 041 784 12/1981 European Pat. Off. ...... .. CO7C 63/26
1 574 651 9/1980 United Kingdom ........ .. CO7C 51/265
[21] Appl. No.: 09/127,751
_ Primary ExaminerRosalynd Keys
[22] Flled: Aug 3 1998 Attorney, Agent, or FirmCharles E. Krukiel
[51] Int. Cl.7 ........................ .. C07C 51/255; C07C 51/42 [57] ABSTRACT
[52] US. Cl. ........................ .. 562/412; 562/414; 562/485;
562/486 A process for producing pure carboXylic acids by catalytic
[58] Field of Search ................................... .. 562/414, 485, liquid Phase OXidatiOH Of a Suitable Precursor in a Solvent in
562/486, 412 Which the oxidation reaction is carried out in a plug ?oW
reaction Zone at a high solventzprecursor ratio and reaction
[56] References Cited conditions suf?cient to maintain the pure acid in solution as
it is formed, and product produced from such process.
U.S. PATENT DOCUMENTS
3,507,913 4/1970 Motoo et al. ......................... .. 260/524 5 Claims, N0 Drawings
 6,034,269
1 2
PROCESS FOR PRODUCING PURE 0.5% to 3.0% W/W and optionally preheating the feed
CARBOXYLIC ACIDS stream to a temperature in the range of from 120 C. to
180 C.;
BACKGROUND OF THE INVENTION
(c) continuously and simultaneously feeding the feed
The present invention relates to an improved process for stream and a precursor to a plug ?oW reaction Zone to
producing pure carboxylic acids by catalytic liquid phase form a reaction medium in Which the solvent:precursor
oxidation of a suitable precursor in a solvent, and, more ratio is at least about 30:1 and the resulting carboxylic
particularly, to a process for producing highly pure tereph acid is maintained in solution as it is formed;
thalic acid according to such process by conducting the (d) systematically reducing the pressure of the reaction
oxidation reaction in a plug ?oW reaction Zone at a high 10
medium from step (c) While cooling it to a temperature
solvent:precursor ratio, temperature and pressure suf?cient in the range of from 120 C. to 180 C. thereby
to maintain the terephthalic acid in solution as it is formed. precipitating pure acid crystals to form a slurry of pure
Thereafter, pure terephthalic acid is systematically crystal crystals in the reaction medium;
liZed from the resulting reaction medium and recovered as (e) optionally concentrating the slurry; and
pure crystals Without the need for separate puri?cation. 15
(f) recovering the pure acid crystals from the slurry.
Pure terephthalic acid, an important raW material used in The pure acid crystals can be recovered from the reaction
the production of poly(ethylene terephthalate), i.e., PET, for medium, also referred to herein as mother liquor, as a Wet
conversion into ?bers, ?lms and containers, is commercially cake by ?ltration and Washing, and then conveyed directly
produced by purifying crude- or technical-grade terephthalic to a next reaction step, e.g., esteri?cation, or the crystals can
20
acid. Practically all technical-grade terephthalic acid is pro be dried and conveyed to storage.
duced by catalytic, liquid phase air oxidation of paraxylene. According to another aspect of the invention,
Commercial processes use acetic acid as a solvent and a
precipitation, i.e., crystallization, of the pure acid crystals
multivalent heavy metal or metals as catalyst. Cobalt and from the reaction medium is accomplished in a de?ned
manganese are the most Widely used heavy metal catalysts, sequence by ?rst reducing the pressure of the reaction
and bromine is used as a reneWable source of free radicals
medium to a value in the range of from 1,000 to 3,000 kPa
in the process. Whereby unreacted oxygen, Water, acetic acid and volatile
Acetic acid, air (molecular oxygen), paraxylene and cata by-products, e.g., carbon oxides, vaporiZe, and the vapor is
lyst are fed continuously into an oxidation reactor that is vented from the reaction medium, and thereafter (ii) reduc
maintained at from 175 C. to 225 C. and 15003000 kPa 30 ing the pressure of the reaction medium in one or more
(i.e., 1530 atm). The feed acetic acid:paraxylene ratio is additional steps to a value in the range of about 300 kPa
typically less than 5:1. Air is added in amounts in excess of While cooling the reaction medium to a temperature of about
stoichiometric requirements to minimiZe formation of 150 C.
by-products. The oxidation reaction is exothermic, and heat The present invention, according to another aspect, is a
is typically removed by alloWing the acetic acid solvent to neW composition of matter consisting essentially of substan
boil. The corresponding vapor is condensed and most of the tially pure terephthalic acid in the form of discrete rhomboid
condensate is re?uxed to the reactor. TWo moles of Water are crystals Which are produced by the process of:
formed per mole of paraxylene reacted, and the residence (a) forming a feed stream comprising acetic acid and an
time is typically 30 minutes to 2 hours, depending on the oxidation catalyst at a pressure in the range of from
process. 40 2,000 to 10,000 kPa;
The effluent from the reactor is a slurry of crude tereph
(b) dissolving gaseous oxygen in the feed stream to
thalic acid crystals Which are recovered by ?ltration,
achieve an oxygen concentration in the range of from
Washed, dried and conveyed to storage. They are thereafter
0.5% to 3.0% W/W and optionally preheating the feed
fed to a separate puri?cation step. The main impurity is
stream to a temperature in the range of from 120 C. to
4-carboxybenZaldehyde (4-CBA), Which is incompletely 45 180 C.;
oxidiZed paraxylene. Although the purity of crude-grade
terephthalic acid is typically greater than 99%, it is not pure (c) continuously and simultaneously feeding the feed
enough for the PET made from it to reach the required stream and paraxylene to a plug ?oW reaction Zone to
degree of polymeriZation. form terephthalic acid Within a reaction medium in
Which the acetic acid:paraxylene ratio is at least about
SUMMARY OF THE INVENTION 30:1 and the terephthalic acid thus formed is main
The present invention is an improved continuous process tained in solution;
for producing pure carboxylic acids by catalytic liquid phase (d) systematically reducing the pressure of the reaction
oxidation of a suitable corresponding precursor in a solvent medium from step (c) While cooling it to a temperature
comprising an aliphatic carboxylic acid and optionally Water 55
in the range of from 120 C. to 180 C. thereby
Which substantially reduces reactor residence time and pro precipitating substantially pure terephthalic acid crys
vides for precipitation of pure acid crystals directly from the tals to form a slurry;
resulting reaction medium in a systematic, i.e., de?ned, (e) optionally concentrating the slurry; and
crystalliZation sequence Which is separate from the oxida (f) recovering the substantially pure terephthalic acid
tion reaction. In the case of terephthalic acid, for example, 60 crystals from the slurry. Pure terephthalic acid crystals
the process of the invention eliminates the need for separate produced according to the invention are distinctly
puri?cation of crude TA crystals. The process comprises: angular, e.g., rhomboid, in structure, and thereby differ
(a) forming a feed stream comprising solvent and oxida from TA crystals produced according to the prior art,
tion catalyst at a pressure in the range of from 2,000 to Which tend to be rounded agglomerates of many
10,000 kPa; 65 smaller crystals.
(b) dissolving gaseous oxygen in the feed stream to The process of the invention results in the production of
achieve an oxygen concentration in the range of from highly pure carboxylic acid crystals from a single stage plug
 6,034,269
3 4
How oxidation reaction sequence, including crystallization The invention, hoWever, is intended to embrace any reactor
and product recovery, i.e., Without the need for a separate con?guration Which approximates a plug ?oW reaction Zone
additional puri?cation stage. of a type suitable for carrying out the oxidation reaction
according to the process of the invention, i.e., carrying out
DETAILED DESCRIPTION the oxidation reaction in a non-boiling liquid phase. The
The present invention is an improved continuous process residence time of the reaction medium Within the reaction
for catalytic liquid phase oxidation of a suitable precursor, Zone is relatively short, i.e., on the order of 5 minutes or less,
such as paraxylene, in the presence of an aliphatic carboxy Which takes into account the discovery that the reaction is
lic acid solvent, particularly acetic acid, to produce highly selective and proceeds very rapidly under the process con
pure carboxylic acid. In the case of terephthalic acid (TA), 10 ditions of the invention. In practice, it has been observed that
the pure crystals are in the form of discrete distinctly angular the oxidation reaction under plug ?oW runs to desired
crystals of a rhomboid structure as distinguished from TA conversion in from 0.5 to 2.5 minutes.
acid crystals produced according to knoWn oxidation/ The oxidation reaction is exothermic to the extent of
puri?cation processes. As used herein in describing carboxy 12.6><106 J/kg of paraxylene reacted. Typically, this heat has
lic acid crystals produced according to the process of the 15 been removed by alloWing the acetic acid solvent to boil,
invention, the terms pure, highly pure, and substan With the resulting vapor being condensed and the condensate
tially pure are used interchangeably and mean such acid in varying amounts being re?uxed to the reactor. According
crystals having a purity of at least 99.5% by Wt., although the to the present invention, hoWever, the choice of solvent,
purity can be as high as 99.9% by Wt. and even higher, e.g., solvent:precursor ratio, temperature and pressure cooperate
99.95% by Wt. 20 to maintain the reaction medium, particularly oxygen and
The process Will be described as it relates to the produc TA, in a non-boiling liquid phase as the reaction medium
tion of highly pure terephthalic acid, although it is applicable passes through the plug ?oW reaction Zone. Furthermore, in
to the production of a range of pure benZenepolycarboxylic operation the heat of reaction need not necessarily be
acids, such as, phthalic acid, isophthalic acid, etc., and removed from the reaction Zone.
25
mixtures thereof. The process is carried out by ?rst forming The pure TA is recovered from the reaction medium in a
a feed stream comprising solvent, i.e., an aliphatic carboxy crystalliZation process Which is separate from the plug ?oW
lic acid, Which is typically acetic acid, or a non-aliphatic oxidation reaction. CrystalliZation involves systematically
organic solvent such as benZoic acid, and an oxidation reducing the pressure and temperature of the reaction
catalyst at an elevated pressure in the range of from 2,000 medium Whereby TA precipitates from the reaction medium
30
kPa up to 10,000 kPa. In practice, the feed stream Will as substantially pure crystals While impurities and other
usually contain some amount of Water. The term solvent, reaction by-products remain in solution. The reaction
as used in describing the process of the invention, therefore, medium exits the plug ?oW reaction Zone at a temperature in
means the total amount of Water, if present, Which can be the range of from 1800 C. to as high as 2500 C. and a
at a concentration of from 3% by Wt. up to as high as 30% pressure in the range of from 2,000 kPa to 10,000 kPa and
by Wt., and (ii) aliphatic carboxylic acid, or non-aliphatic is passed to a separate crystalliZation step. CrystalliZation
organic acid. can be accomplished by reducing the pressure of the reaction
Molecular oxygen is dissolved in the feed stream to medium to about 300 kPa in a single step or in several
achieve a concentration of dissolved oxygen of from 0.5% graduated steps While reducing the temperature to a value in
to 3.0% W/W, and the feed stream can then be heated to a the range of about 150 C.
40
temperature in the range of from 120 C. up to 180 C. In a preferred embodiment of the invention, crystalliZa
before being introduced into the reaction Zone. The source of tion of the pure TA crystals is accomplished in tWo basic
oxygen can be pure oxygen, air, or any convenient oxygen stages. In a ?rst stage, the pressure of the reaction medium
containing gas. is reduced to a value in the range of from 1,000 to 3,000 kPa,
In practice, the feed stream is fed into a plug ?oW reactor 45
e.g., 2,000 kPa, Whereby unreacted oxygen, Water, acetic
simultaneously and continuously With paraxylene and cata acid and volatile by-products, e.g., carbon oxides, vaporiZe
lyst to thereby form a reaction medium in Which the result and the vapor is vented from the reaction medium.
ing solventzparaxylene ratio is at least about 30:1, although Thereafter, in a second stage, the pressure of the reaction
the solventzparaxylene ratio can be as high as 200:1 With the medium is further reduced in one or more additional steps
process achieving satisfactory results. In a preferred 50
While the reaction medium is cooled to around 150 C.
embodiment, the solvent:paraxyene ratio is in the range of Pressure reduction can be accomplished by any suitable
65:1. means, such as, for example, by passing the reaction
The process is carried out in the presence of an oxidation medium through a pressure reducing valve or a liquid
catalyst Which can be homogeneous or heterogeneous and turbine.
selected from one or more heavy metal compounds, such as, 55 Pure TA crystals precipitate from the reaction medium and
for example, cobalt and/or manganese compounds. In form a dilute slurry of from 1% to 6% W/W concentration. It
addition, the catalyst can also include an oxidation promoter has been discovered that by reducing the temperature of the
such as bromine or acetaldehyde. The selection of catalyst reaction medium according to the present process to a value
and oxidation promoter and their use and handling through in the range of 150 C. for crystalliZation, primary
out the process according to the invention is Within conven 60 impurities, such as 4-CBA and undesirable color bodies,
tional practice. The catalyst/oxidation promoter components Which Would otherWise precipitate With the TA, remain in
are added to the feed stream in liquid form as a solution solution. Thus, it is possible according to the invention to
before the feed stream is introduced into the reaction Zone, produce pure TA crystals via catalytic liquid phase oxidation
and they largely remain in solution throughout the process. of paraxylene Without the need for a separate puri?cation
The term plug ?oW reactor is used herein to de?ne a 65 step.
typically tubular reaction Zone in Which radial mixing of the For economy in operation, the dilute slurry can be
reactants occurs as they ?oW through the tube or conduit. thickened, i.e., concentrated, by any suitable means to a
 6,034,269
5 6
higher acid crystal concentration up to about 60% W/W. 1.28 minutes residence time, single pass paraxylene conver
Thereafter, the pure TA crystals can be recovered from the sion to TA fell to about 84%. Signi?cantly, hoWever, par
slurry by ?ltration, Washed, and optionally dried and sent to axylene conversion to 4-CBA (the intermediate that tends to
storage. The reaction medium Which remains after pure TA co-precipitate With TA in conventional processes) is of the
crystals are recovered, i.e., the mother liquor, can be order of 1% or beloW throughout. High single pass conver
recycled and fed as a component of the feed stream to the sion of paraxylene to TA is not essential to the process so
oxidation reaction. long as the 4-CBA concentration in the reaction stream
leaving the reactor is loW. During the crystalliZation step, TA
In practice, the feed stream for operating the process on precursors substantially remain in solution in the reaction
a continuous basis Will comprise recycled mother liquor medium, i.e., dissolved in the mother liquor, and thereby
Which is supplemented With fresh aliphatic carboxylic acid 10
they can be recycled to the plug ?oW oxidation reactor.
(e.g., acetic acid) and fresh liquid catalyst make-up to Table 1: Plug FloW Reactor Oxidation Results
account for chemical and physical losses from the original In all experiments, the folloWing parameters Were ?xed
feed stream. The feed stream Will be pressuriZed and (all compositions are W/W);
oxygenated, and then fed simultaneously With liquid par Solvent:Water 5%, acetic acid 95%
axylene into the plug ?oW reaction Zone at an inlet tem 15
Paraxylene:0.5% W/W (200:1 solventzparaxylene ratio)
perature of about 120 C. to 150 C. and elevated pressure Catalyst:Co 632 ppm, Mn 632 ppm, Br 1264 ppm+Zr 96
to achieve a solventzparaxylene ratio of about 65:1 for the ppm
resulting reaction medium With the result that the reaction Oil Bath Temperature:210 C.
proceeds rapidly (i.e., from 0.5 to 2.5 minutes reactor
residence time) Without boiling, and TA remains in solution 20
as it is formed. Although the reaction can be adiabatic,
cooling means can be employed to recover and reuse heat Reaction Reactor Solution ppm W/W
directly from the reaction.
Example Time (min) ptolald ptol 4 CBA
The invention Will noW be described With reference to the
25
folloWing examples. 1 1.28 228 687 6
2 1.78 55 411 51
EXAMPLES 3 2.28 132 312 42
4 2.31 99 192 38
5 3.29 15 82 6
Example 1 6 4.86 1.7 27 <0.1
Plug FloW Reactor Oxidations 30
Experiments Were performed using a plug ?oW reactor
system comprising tWo feed vessels, a reaction coil and tWo Example 2
product collection vessels. Simultaneous ?oW from the feed Crystallization
vessels through the reaction coil into either one of the A solution of 2% W/W terephthalic acid (TA), 125 ppm
product collection vessels Was established by differential gas 35 4-CBA, 175 ppm ptol and other oxidation intermediates in
pressure and appropriate valve operations. 5% W/W Water, 95% W/W acetic acid solvent Was prepared at
The ?rst vessel Was charged With a knoWn compositon of elevated temperature (210 C.) and at a pressure suf?ciently
paraxylene in acetic acid/Water solvent. The second feed high to maintain a liquid phase. The solution Was passed,
vessel Was charged With a knoWn composition of catalyst in continuously, through a pressure reducing valve into a
acetic acid/Water solvent. Air Was introduced into both feed 40 crystalliZer vessel Whose pressure and temperature Was
vessels, through dip pipes, at a pressure to ensure that the controlled such that TA precipitated from solution. The
desired amount of oxygen (in excess of the stoichiometric slurry produced in the crystalliZer Was passed forWard to
paraxylene requirement) Went into solution. The feed ves further crystalliZation vessels in Which the pressure and
sels and reaction coil Were immersed in an oil bath to preheat temperature Were systematically reduced to ambient
the feeds to the required reaction temperature. Simultaneous 45 conditions, and further TA precipitated.
?oW from the feed vessels through the reaction coil Was then During the course of the experiment, crystals from the
established into the ?rst product collection vessel labeled as ?rst crystalliZer (Hot Filtered TA) Were recovered and
off spec. After a predetermined time, the product stream analyZed for 4-CBA and paratoluic acid (ptol) content and
from the reaction coil Was sWitched to the second product median particle siZe (using a Coulter LS230 Laser Diffrac
collection vessel. After a further predetermined time, the 50 tion psd analyZer). Crystals from the doWnstream vessels
product stream Was sWitched back to the off-spec collec (Cold Filtered TA) Were also recovered and analyZed for
tion vessel. At the end of the experiment, all vessels Were reference purposes.
cooled, vented, Washed out and drained. The solid and liquid In Table 2, the Hot Filtered TA, 4-CBA and ptol contents
contents of the sample product collection vessel Were and median particle siZes are reported for experiments Where
recovered, Weighed and analyZed, and the composition of 55 the ?rst crystalliZer temperature, residence time and stirrer
the reaction solution leaving the reaction coil Was calcu speed Were varied. For reference, one analysis of Cold
lated. Filtered TA is also included. Examples 7, 8 and 9 shoW that,
In Table 1, the concentrations of the TA precursors in the Hot Filtered TA, 4-CBA and ptol contents fell as the
paratolualdehyde (ptolald), paratoluic acid (ptol) and ?ltration temperature Was reduced from 196 to 148 C. The
4-carboxybenZaldehyde (4-CBA) are reported for experi 60 data also shoWs that median particle siZe inceases With
ments Where the reaction residence time Was varied. At this reducing temperature. In a separate experiment, Examples
scale, the reactions ran under nearly isothermal conditions, 10 and 11 shoW that, in the Hot Filtered TA, reduction in
close to the oil bath temperature of 210 C. throughout. ?ltration temperature from 151 to 126 C. causes 4-CBA
The Examples demonstrate the effect of residence time on level to increase, While ptol level and median particle siZe
precursor concentrations. At 4.86 minutes residence time, 65 decline.
single pass paraxylene conversion to TA Was greater than When vieWed together, Examples 7 through 11 indicate an
99.5% (precursors less than 0.5% of feed paraxylene). At optimum crystalliZer temperature, With respect collectively
 6,034,269
7 8
to intermediates incorporation and median particle siZe, in kPa Whereby unreacted oxygen, Water, precursor and vola
the region 140 to 160 C., particularly around 150 C. tile by-products vaporiZe and the vapor is vented from the
Examples 12 and 13 shoW that increasing ?rst crystalliZer reaction medium, and thereafter (ii) reducing the pressure of
residence time from 9 to 18 minutes bene?ts both interme the reaction medium in one or more additional steps to a
diates incorporation and median particle siZe. Examples 14 value in the range of 300 kPa While cooling the reaction
and 15, When vieWed alongside Example 9, shoW that medium to a temperature of about 150 C.
increasing ?rst crystalliZer agitator speed, from 270 to 1000 3. The process of claim 1 or claim 2 Which includes the
rpm, does not have a strong in?uence on median particle additional step of recycling the reaction medium remaining
siZe, but tends to reduce intermediates incorporation. from step as a component of the feed stream to the
reactor.
Table 2 Crystallization Experiments Results 10
4. A process for producing pure terephthalic acid by
In all experiments the folloWing parameters Were ?xed
catalytic liquid phase oxidation of paraxylene in a solvent
(all compositions are W/W); selected from an aliphatic carboxylic acid and optionally
Solvent:Water 5%, acetic acid 95% including Water Which comprises:
Feed Solution Aromatics:TA 2%, 4CBA 125 ppm, ptol (a) forming a feed stream comprising solvent and oxida
175 ppm 15 tion catalyst at a pressure in the range of from 2,000 to
Feed Solution Temperature:210 C. 10,000 kPa;

First Cryst. First Cryst. 4CBA ptol Median

First Cryst. Stirrer Speed Temp. Content Content Particle Size
Ex. Res. Time (min) (rpm) ( C.) (ppm) (ppm) (micron)
7 12 1,000 196 2,360 345 59
8 12 1,000 176 1,040 218 114
9 12 1,000 148 670 89 134
10 18 1,500 151 710 138 96
11 18 1,500 126 1,060 117 86
12 18 1,000 173 980 150 106
13 9 1,000 179 1,140 217 96
14 12 270 152 930 123 139
15 12 500 150 790 106 135
Ref. 12 1,000 148 2,340 281 102
(Cold Filter) (Cold Filter) (Cold Filter)

What is claimed is: 35 (b) dissolving gaseous oxygen in the feed stream to
1. A process for producing a pure carboxylic acid by achieve an oxygen concentration in the range of from
catalytic liquid phase oxidation of a corresponding precursor 0.5% to 3.0% W/W and optionally preheating the feed
stream to a temperature in the range of from 120 C. to
in a solvent selected from an aliphatic carboxylic acid or a
180 C.;
non-aliphatic organic acid and optionally including Water 40 (c) continuously and simultaneously feeding the feed
Which comprises: stream and said paraxylene to a plug ?oW reaction Zone
(a) forming a feed stream comprising solvent and oxida to form terephthalic acid in a reaction medium in Which
the solvent:paraxylene ratio is at least about 30:1 and
tion catalyst at a pressure in the range of from 2,000 to the terephthalic acid is maintained in solution as it is
10,000 kPa; 45 formed;
(b) dissolving gaseous oxygen in the feed stream to (d) systematically reducing the pressure of the reaction
achieve an oxygen concentration in the range of from medium from step (c) While cooling it to a temperature
0.5% to 3.0% W/W and optionally preheating the feed in the range of from 120 C. to 180 C. thereby
stream to a temperature in the range of from 120 C. to precipitating terephthalic acid crystals to form a slurry;
180 C.; 50 (e) optionally concentrating the slurry; and
(c) continuously and simultaneously feeding the feed (f) recovering the terephthalic acid crystals from the
slurry.
stream and said precursor to a plug ?oW reaction Zone 5. The process of claim 4 in Which systematically reduc
to form a reaction medium in Which the solvent:pre ing the pressure of the reaction medium from step (c) is
cursor ratio is at least about 30:1 and resulting car 55 accomplished by ?rst reducing the pressure of the reac
boxylic acid is maintained in solution as it is formed; tion medium to a value in the range of from 1,000 to 3,000
(d) systematically reducing the pressure of the reaction kPa Whereby unreacted oxygen, Water, precursor and vola
medium from step (c) While cooling it to a temperature tile by-products vaporiZe and the vapor is vented from the
in the range of from 120 C. to 180 C. thereby reaction medium, and thereafter (ii) reducing the pressure of
precipitating carboxylic acid crystals to form a slurry; 60 the reaction medium in one or more additional steps to a
value in the range of 300 kPa While cooling the reaction
(e) optionally concentrating the slurry; and medium to a temperature of about 150 C., and the process
(f) recovering the carboxylic acid crystals from the slurry. includes the additional step of recycling the reaction
2. The process of claim 1 in Which systematically reduc medium remaining from step as a component of the feed
ing the pressure of the reaction medium from step (c) is 65 stream to the reactor.
accomplished by ?rst reducing the pressure of the reac
tion medium to a value in the range of from 1,000 to 3,000