Note: Descriptions are shown in the official language in which they were submitted.
`` 10'79Z~f~
Background of Invention
The unique catalysis provided by a combination of one or
more transistion metal oxidation catalysts and a source of
bromine has been taught by United States Patent No. 2,833,816
and the commonly derived foreign patent counterparts as general-
ly applicable for the preparation of aromatic carboxylic acids
at elevated temperatures in the range of 50 to 275C and under
pressures to maintain liquid phase conditions for the oxidation
with a source of molecular oxygen of aromatic hydrocarbons
having at least one substituent oxidizable to a carboxylic acid
substituent. Said patents also taught the use of C2-C8
carboxylic acids as useful for such catalytic liquid phase
oxidations.
Such catalytic liquid phase oxidation has been developed
into the predominant world-wide commercial production of lSO-
phthalic acid, terephthalic acid and trimellitic acid by the
respective air oxidations of m-xylene, p-xylene and pseudocumene
in the presence of acetic acid. Such catalytic liquid phasè
oxidation has also been applied to the commercial production
of benzoic acid, but of lower magnitude, by the air oxidation
of toluene in the presence of benzoic acid as solvant.
The use of acetic acid as solvent in such commercial ~
production of the benzene di- and tricarboxylic acid products, ;~`
although providing a most economically advantageous process, ;~-
does have the disability of co-oxidation of acetic acid to the
extent of from 80 to 160 kilograms per metric ton (MT) of pro-
duct produced. Acetic acid, the most refractory of the C2-C
aliphatic acid class, is oxidatively converted in such processes `
to oxides of carbon, water and methyl acetate.
Temperature control of such oxidations conducted in the
presence of acetic acid solvent presents no commercial operating
~b. ' ~ '
.. : - . . . .. . . . . .
-" 1079Z96
problem because of the relatively close boiling points of acetic
acid and by-product water and the water concentrations, 3 to 18
weight percent of solvent, involved. Such acetic acid-water
solvent system for practical consideration is a one component
system of miscible liquids and has from phase rule relationships
but one degree of freedom. Thus, for constant volume operation,
setting operating pressure as a constant provides a constant
operating temperature.
In 1963, a continuous process was disclosed for the pre-
paration of benzene di- and tricarboxylic acids, especially tere-
phthalic acid, by the catalytic liquid phase air oxidation of
the appropriate methyl-substituted benzene in the presence of
benzoic acid, in place of acetic acid, as reaction solvent and
in the presence of the earlier disclosed unique combination of
one or more transition metal oxidation catalysts and a source
of bromine. British Patent Specification No. 1,088,183, pub-
lished 25 October 1967, is directed to and contains details of
such oxidations conducted in the presence of benzoic acid
solvent. According to said patent, benzoic acid was selected
over the other C2-C8 monocarboxylic acids because its aeration
I` at temperatures in the 50-275C range produced a medium less
i corrosive to metals available for fabrication of oxidation
vessels than aerated acetic acid at such temperatures, it was
less volative than acetic acid, and by-product water vapor
was more readily separated from benzoic acid vapor than from
~ acetic acid vapor at the reaction site. For example, selective
'i condensation of benzoic acid vapors from a vapor mixture thereof
with by-product water can be accomplished by simple partial
i condensation but separation of a mixture of acetic acid and by-
product water vapors require fractionation.
Further benefits from the use of benzoic acid solvent,
according to the British Patent, come from the use of high, 170
-- 3 --
' ~: ''' :', :
~ - 1079~96
to 275C, oxidation temperatures which provide high oxidation
rates; the use of high, 21 to 35 kg/cm , operating gauge pres-
sures which provide high oxygen concentrations in the liquid
phase reaction medium; the required removal of by-product water
as it is formed; and the independence of operating temperature
on such operating pressure.
The above British Patent provides three continuous air
oxidations of p-xylene to terephthalic acid wherein its yields
of 85-95 mole percent (% of theory) and purity of 99-99.9 wèight
~ 10 percent are illustrated.
- However, in spite of such promised high yields and
purity of terephthalic acid prepared in the presence of liquid
benzoic acid solvent, we have found a major problem associated
~; with the conduct of the continuous process of said British
, Patent.
i Said problem was found to occur during initiation of
the p-xylene oxidation at the operating gauge pressure of 21
to 35 kg/cm2 while removing by-product water as it was formed.
At such oxidation initiation conditions the operating temper- ~ -
ature could not be controlled and the benzoic acid and/or p-
xylene or its partial oxidation products were quite rapidly ;
over oxidized to a carbonaceous residue. The lack of stirring
of the liquid phase composition, the illustrative examples
in the British Patent did not use a stirred-tank type oxidation
vessel, could lead to inefficient dispersion of air and
distribution of heat of reaction in the liquid phase and thus
. .
I provide localized "hot spots", the isolated high oxidation
. ~ . .
rate resulting from highly localized conditions of high temper-
ature and oxygen concentration. But unstirred tubular reaction
vessels had been successfully used for other oxidations; for
example, the preparation of benzoic acid by air oxidation of
_ 4 _
~, ,.
,~ ' . ~ .. . .
10'79Z96 :
,~
toluene in the presence of benzoic acid solution of a combina-
tion of one or more transition metal and a source of bromine,
without encountering such sudden charring of reactant, product
or benzoic acid solvent. Hence, the lack of stirring was not
the controlling effect with respect to the problem of rapid char
formation from the inability to control temperature during
initiation of p-xylene oxidation.
We found that the effects causing the rapid charring
during oxidation initiation to be associated with the required
high operating gauge pressure of 21 to 35 kg/cm2 and removal
of by-product water as it is formed. This discovery resulted
from our investigation of the effect of retention or lack thereof
'~ of small amounts of water in the benzoic acid solvents on the
control of initial operating temperature of the 21 to 35 kg/cm2
operating gauge pressure required according to the process of
the British Patent.
By experimentation it was found that by going from 5
, weight percent to 0 weight percent water concentration of liquid
benzoic acid under a gauge pressure of 25-35 kg/cm2 and an - -
l~20 initial temperature of 205C a temperature increase of as much
;~I as 110C was observed without change of pressure.
Substantiation of such effects causing the rapid charring
can be understood from the results of the following investigations.
Benzoic acid (boiling point of 249C at 760 mm Hg) with a water
content of 5 weight percent was heated to an initial temperature
~ of 205C under a gauge pressure set at 21 to 35 kg/cm2. The
!~ temperature of the liquid benzoic acid was measured as its water
content went from the initial 5% down to 0%. By going from 5%
to 0% water the temperature of the liquid benzoic acid increased
as much as 110C without causing an increase in the set gauge
pressure. This indicated a unique temperature sensitivity
- 5 -
?
.' , -
,: ~
1079Z96
with respect to water content of liquid benzoic acid solvent
and accounted for the rapid drastic over oxidation to a charred
product during initiation of p-xylene oxidation at a gauge
pressure of 21 to 35 kg/cm2 following the required operating
conditions of the British Patent.
Based on our discovery from the dramatic temperature in-
creases found during our foregoing experiments we concluded
that removing by-product water as rapidly as formed to maintain
a near 0 water content and conducting at least the oxidation
initiation under a pressure as high as 21 to 35 kg/cm could
not lead to successful temperature control necessary for ~
commercial operation of the product of iso- or terephthalic -
acid from the air oxidation of m- or p-xylene in the presence
of liquid benzoic acid as reaction solvent.
Another problem arose beyond initiation of the continuous
oxidation of m- or p-xylene with air in the presence of liquid
, benzoic acid as a solvent according to the operating conditions
of the British Patent. After the oxidation had been success-
fully initiated in the stirred liquid phase with good tempera-
, 20 ture control provided by the proper initial and retained water
`, content of the liquid benzoic acid, there were still wide
:! :
fluctuations in react1on temperature caused by changes in the
amount of water removed. This and the foregoing experimental
results led us to conclude that the 21 to 35 kg/cm operating
pressure imposed by the British Patent was too high and the
required removal of by-product water as rapidly as formed was
unnecessary and would not provide a commercially acceptable
~, continuous process.
Statement of the Invention
We have found that control of operating temperature for
both initiation and lined-out operation for a commercially
-- 6 --
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.
.: . : :. .: .
.
1079296
' ' .
acceptable continuous air oxidation of m- or p-xylene can be
' achieved. Thus the present invention provides a process of
,~- preparing iso- or terephthalic acid by the liquid phase oxidation
;` of m- or p-xylene with air in an oxidation zone at an elevated
~ temperature of up to 275C in the presence of a monocarboxylic
,;
~, acid solution of catalysis components comprising a source of
, bromine and one or more transition metal oxidation catalyst
,~ .:
maintained as a liquid at said temperature by elevated oxidation
zone pressure.
~i 10 This process is characterized by conducting the air
oxidation of m- or p-xylene in a semi-continuous or continuous
- manner in a stirred oxidation zone containing said catalysis
.,; . .
si wherein the transition metal component is manganese or a com-
bination of manganese with one or both of cobalt and cerium in
a solvent system consisting essentially of liquid benzoic
,',:! acid and water and at an oxidation zone temperature maintained . .
substantially constant at a selected temperature within the ~ :
temperature range of 175 to 235C by maintaining (a) the solvent
system components within the range of 85 to 97 weight percent
.,~
benzoic acid and 15 to 3 weight percent water, (b) the oxida-
`~l tion zone gauge pressure within the range of 6 to 25 kg/cm2,~'s
and (c) the removal of by-product water as vapor by cooling
the exhaust from the oxidation zone to condense benzoic acid
"~ from reflux thereto and varying the amount of water returned
,' to limit fluctuation of oxidation zone temperature to +5C from
the selected temperature wherein said oxidation zone the weight
ratio of such solvent system to said xylene is in the range of
2 to 10:1.0, said components of catalysis in the solvent system
are present in the amounts of 0.2 to 1.5 weight percent total
metal and 0.2 to 1.5 weight percent bromine based on the xylene
l with a weight ratio of bromine to total metal in the range of
..
: ~ 7 -
,, . -,.. , . , ~. .... . ~ : ~
---" 1079296
0.5 to 2.5 weight parts of bromine for each part by weight
total metal and the ratio of air to xylene fed to said zone
provide an exhaust gas therefrom containing 3 to 10 volume
percent oxygen. ~ -
In one aspect such a process is described wherein p-
xylene is oxidized with air and the transition metal component
of catalysis is provided by manganese and cobalt in the Mn/Co -
weight ratio of from 1:1 to 6:1 and the sum of Mn and Co metal
weights are within the 0.5 to 1.5 weight percent of p-xylene.
In a preferred embodiment such a process is provided wherein ~
the solvent system consists essentially of 90% benzoic acid - -
and 10% water, the weight ratio of the solvent system to p-xylene
is 2 to 7:1, the components of catalyst are in the concentrations
of 0.015 to 0.1% cobalt, 0.08 to 0.2% manganese and 0.02 to
0.3% bromine based on said solvent, the oxidation zone tempera- ~;
ture is from 205 to 226.5C, the oxidation zone gauge pressure ~ -
is from 14-24.6 kg/cm2, and the oxygen content of the exhaust ~
;~ gas is from 6-10 volume percent. -
In a further aspect such an invention is provided wherein
p-xylene is oxidized with air, the solvent system consists
essentially of 90% benzoic acid and 10% water, the weight ratio
of solvent system to p-xylene is 3-5:1.0, and the transition
metal component of catalysis is manganese and the ratio of
bromine to manganese is from 0.8 to 1.5:1.0 and the manganese
concentration is from 0.15 to 0.2 weight percent of solvent.
In a preferred embodiment the operating temperature
fluctuation limited to not more than -~ 5C from a selected
constant pressure by varying the rate of water condensate re-
turned to the oxidation zone.
For such continuous oxidation the unique catalysis of a
combination of one or more transition metal oxidation catalyst
and a source of bromine is provided by dissolving suitable souces
.. : ~ ,. ..,, ~, . . .
.. . . . . . . .. .
` 10792~6
of the components in the solvent system.
The present inventive continuous process is conducted in
a stirred oxidation zone to provide efficient dispersion of air
in and distribution of heat of reaction through the liquid phase
in the oxidation zone.
The heat of reaction will cause vaporization of a little
benzoic acid and mainly water from the oxidation zone. Little
or no xylene is vaporized because it is oxidized to products
which do not tend to be vaporized at the operating conditions.
Satisfactory temperature control is achieved for substantially
constant operating conditions by regulating the water content
of the liquid reflux (mainly water) to the oxidation zone after ;
~ the condensation of said vapors to remove heat of reaction.
; The water content of the liquid reflux, generally 90-95 weight
percent, can be regulated by the operating temperature of the
reflux condenser. Change in oxidation temperature from a
selected constant operating temperature can be corrected by
varying the rate of addition of water (since the condensate
; has only 5-10 weight ~ benzoic acid) returned to the oxidation~20 zone. For example, the rate of water return by way of the re-
flux liquid is decreased or increased in response to a decrease
or increase of oxidation zone temperature from the selected
constant temperature. Means for such variation of water con- -
tent of liquid reflux and rate of water return to the oxidation
zone are hereafter described. Such means can keep the change
of oxidation zone temperature within 5C above or below the
selected constant temperature for the oxidation zone.
The variation of water rate of such returned liquid should
not decrease the water content of the solvent system below 3
weight percent because at such condition the uncontrolled wide
temperature fluctuation conditions again occur. The variation
,'
_ g _
--'` 10792~6
of such returned liquid should not increase the water content
of the solvent system above about 15 weight percent, e.g. to
18 weight percent, because such amount of water deactivates the
system of catalysis partially or completely to make the oxida-
tion rate commercially unattractive.
For the purposes of this invention the m- or p-xylene
oxidation is carried out with a weight ratio of the benzoic
acid-water solvent system to xylene in the range of 2:1 to 10
Also, the system of catalysis is suitably provided by a source
of bromine in combination with one or more of manganese, cobalt, ~ ~
or cerium as transition metal oxidation catalyst. Also, it is ~ -
preferred to use an amount of air which provides from 2 up to
10 volume percent oxygen in the exhaust gas (benzoic acid-free
basis) to minimize the amount of partial oxidation products and
color body impurities in the iso- or terephthalic acid product
recovered.
Operation of the present inventive process is conducted
on a continuous basis for the combination of 6-25 kg/cm2 gauge
pressure and the particular benzoic acid-water solvent system
to proyide for efficient control of substantially constant tem-
perature in the oxidation zone in the operating temperature
~ ~ .
range of 175-235C.
The continuous operation of the present inventive process ;;
does not have relatively high xylene concentration at start-up; -
i.e., initiation of oxidation, as will be understood from the
start-up procedure for continuous operation. Said start-up
differs from batchwise operation in that there is initially
charged to the stirred oxidation zone the components of the -
unique catalysis and solvent system. The resulting solution is
stirred and heated to a temperature at which oxidation is initi-
ated, e.g., 160-170C, but preferably to operating temperature
.
~1 .
-- 10 --
,
--``` 1079Z~
of 175-235C. When such heating is only to oxidation initiation
temperature, then m- or p-xylene is pumped into the stirred
liquid in the oxidation zone and air is injected into said
stirred liquid at about 1000-1500 N liters per kilogram of
xylene until the oxidation zone temperature reaches the oper-
ating temperature selected from 175-235C. Thereafter the
xylene is pumped into the oxidation zone and air rate is in-
creased to the range of from about 3800 to about 5900 N liters
per kilogram of xylene to provide the 3-10 volume percent oxygen
in the exhaust gas (benzoic acid and water-free basis). After
the weight ratio of originally charged solvent system to the
total xylene charged reaches the selected ratio in the range
of 3-10:1.0, the solution of catalyst components in the benzoic
acid-water solvent system is pumped in at a rate with respect
to continued xylene pumping to preserve the selected solvent
to xylene weight ratio as the injection of air is continued at
said 3800-5900 N l/kg xylene providing such 3-10 volume percent
oxygen in the exhaust gas. The time for such initiation to
complete continuous feed operation is relatively short but
does provide time to adjust the operation of the condenser
to which the exhaust gas is conducted for removing heat of
reaction and adjust the rate of water condensate returned with
liquid condensate reflux to the stirred oxidation zone. Upon
reaching the operating aerated liquid volume of the oxidation
zone, the fluid oxidation reaction mixture is then withdrawn
from the oxidation zone to supply feed to the separation of iso~
or terephthalic acid product from liquid benzoic acid-water sol-
vent system. --
Although such operation from initiation through complete
continuous operation introducing reactants and solvent system
solution of catalysis and withdrawing fluid reaction mixture
- -- 11 --
~79296
is not preferred, it does serve as a useful start-up procedure ;
to gain experience with the unique temperature sensitivity -
with respect to water content of the solvent system having the
most pronounced effect on successful control of operating
temperature.~ - -
After such experience is gained then the start-up of
the present inventive process can be simplified by resorting to
the preferred start-up conditions. For the preferred start-up,
the solvent system solution of components of catalyst needed to -~
reach operating volume of the reactor is charged and stirred
and heated to the selected operating temperature under the
selected operating pressure. Thereafter the xylene is pumped ~-
in and air is injected into the stirred solvent at continuous ;~
operating rates with the condenser reflux system operating in
response to the temperature of the oxidation zone. Removal of
fluid is started when the total xylene charged provides, with
respect to initial solvent system charged, the selected weight
ratio of solvent system to xylene.
The continuous operation of the present inventive process
' 20 can be conducted in an oxidation vessel having a stirrer, a
Z reflux condenser operated at a temperature of about the melting -
` point of benzoic acid (121.7C) to condense benzoic acid as a
liquid for reflux to the stirred oxidation zone and a side arm- -
type condenser operated to condense water vapor for removal of
by-product water. A pressure control valvè can be used between
said condensers or preferably after the water vapor condenser,
for example at the exit therefrom. Uncondensed gases can be
~I discharged from said condensate collecting vessel through the
pressure reducing valve. The reaction vessel should be fitted
with temperature and pressure measuring devices and means for
charging reactants, solvent solution of components of catalysis,
- 12 -
- 10~;~9296
and withdrawing fluid oxidation effluent. With such combina-
tion of apparatus elements control of water content of the benzoic
acid-water solvent system can be readily monitored and control-
led within the 3-15 weight percent water content on the basis
of a water material balance. Only a metered amount of water
condensate is discarded which is equivalent to the amount of
by-product water produced and the remainder of the condensate
is pumped back into the reaction zone to control the oxidatibn
zone at constant pressure.
Product iso- or terephthalic acid, relatively insoluble -
in the solvent system, can be separated from the fluid oxida-
tion effluent by any solid-liquid separation means such as by ;
filtxation or centrifugation, at a temperature at which benzoic
acid in said effluent remains liquld. Since such fluid effluent
is at a temperature of 175-235C (well above the temperature of
benzoic acid solidification), and a gauge pressure of 6-25 kg/cm2,
such product separation can be accomplished by decompression with
attendant cooling of such effluent. Such decompression can be
to ambient or subatmospheric pressure. The cooled effluent but
~20 still fluid effluent is pumped to said solid-liquid separation.
; Preferably such effluent is decompressed to subatmospheric pres-
sure to avoid any flash evaporation in the means for solid-liquid
separation.
The separated crystalline product can be washed with hot,
fresh liquid benzoic acid and then with xylene, toluene or a
combination thereof to remove adhering benzoid acid. The washed
product is dried and the xylene or xylene-toluene mixture removed
by drying is recovered for reuse. The xylene and/or toluene
wash of product can be made in the means for solid-liquid
separation but is preferably accomplished by suspending the
benzoic acid washed solid product in the aromatic hydrocarbon.
- 13 -
.
-`` 107~3Z~ ~
The foregoing separating and washing of iso- or terephthalic ;
acid product provide the benefits of higher product purity
because the product is recovered at a higher temperature than
would be possible when acetic acid was the solvent. Such
higher temperature separation leaves more of the impurity oxida-
tion intermediates in the mother liquor. Further impurity re-
moval is enhanced by the xylene and/or toluene washing because
such aromatic hydrocarbons are better solvents for the impurities
than is acetic acid.
Additional benefits resulting, in general, from the pre-
sent inventive process are reduced combustion of the organic
component of the solvent system; cleaner vent streams; less
potential corrosion in the oxidation reactor, transfer lines and
product recovery apparatus; and lower cost of oxidation reactor -
because of lower reaction pressure.
With respect to combustion of benzoic acid component of
the solvent system, this is reduced to 50 percent of that exper-
ienced with acetic acid solvent for the same weight ratios of
solvent to xylene and same operating temperatures. Moreover,
the benzoic acid combustion products are only water and oxides ~
of carbon thus eliminating venting of the ester (methylacetate) -
or separating it from solvent for recycle use as is needed when
acetic acid is used as reaction solvent. Also, no appreciable
amount of solvent vapor is vented with non-condensibles as is
the case when acetic acid is the reaction solvent. This leads
in the practice of the present inventive process to cleaner
discarded gases.
Aerated, wet (3-15~ water) benzoic acid is substantially
less corrosive at operating temperatures even when containing
thebromine component of catalysis than aerated, wet (5-10% water)
acetic acid containing such bromine component. Such substantially
- 14 -
. ~ .~ . . : ,, :
1079Z~6
less corrosivity associated with the benzoic acid-water solvent
permits the use of less expensive stainless steels of the SS316
type for process apparatus, especially the condenser, rather
than the rather expensive metals such as titanium used when
acetic acid is the solvent.
Capital investment and operating cost for the commercial
practice of the present invention would be lower than when
acetic acid or anhydrous benzoic acid are used as reaction
solvents. Such lower costs result directly from the use of less
expensive metals for process apparatus, lower operating pres-
sure in the oxidation reaction vessel and its auxiliary con-
densers and condensate receiver, the elimination of exhaust gas
scrubbing, and the elimination of the later process steps of
crystallization (generally two or three stages when acetic acid
is the solvent), and solvent fractionation to recover solvent
for recycle to the oxidation. -~
i~ The present inventive process, unlike processes using
acetic acid solvent, is not retarded rate-wise by the presence
! of more than 5 weight percent water in the solvent system but
there is a reaction rate penalty about 20 weight percent water.
Specific Embodiments of the Inventive Process
As mentioned before the conditions essential for the con-
~ duct of the present inventive process are the combination of
i operating pressure at 6 24, preferably 14-20, kg/cm2 gauge with
.,: .
the solvent system of benzoic acid (85-97 wt%) and water (15-3
wt %) to achieve control of substantially constant operating
temperature selected from the range of 175-235C inclusive. The
illustrative examples will provide combinations of operating
pressure and solvent compositions to obtain good control of
substantially constant temperature (not more than + 5C) from
the average operating temperature from which those skilled in
this art can make a selection or devise other useful combinations.
;
- 15 -
.
1~7929~
Although the use of a stirred reaction zone is important
with respect to good air dispersion in the liquid reaction mix-
ture and heat removal therefrom, only the ordinary skili of
stirring design is involved. For example, the ordinary design
criteria of reactor and stirrer-blade geometry, agitation pat-
tern and stirrer power to keep product cyrstals suspended in the
solvent system need be taken into account to provide the neces-
sary stirring. Those criteria can be readily calculated on the
basis of published formulae or from empirical data readily ob-
tainable by simple experiments with small scale apparatus.
The useful weight ratio of solvent system to m- or p-
xylene is, as before stated, in the range of 2-10:1Ø The
illustrative examples will provide a basis for the process design ;
engineer to either choose therefrom a ratio for specific pro-
cess design purpose or select a different ratio to meet the
particular design devised.
Heat of reaction can be removed as hereinafter illustrated
by the addition of water to the reaction zone for evaporation
therefrom. The evaporation of S kilograms of water per kilogram
of xylene oxidized will remove the heat of reaction. Heat of ~ -
reaction can also be removed by known means of internal indirect
heat exchange with heat exchange fluid; for example, such fluid
flowing through a tube coil in the reaction zone. Also heat of
reaction can be removed by any one of the known external fluid
loop heat exchangers wherein liquid reaction mixture flows from
the oxidation zone to the external heat exchange loop for in-
direct heat exchange, for example to exchange heat with water and
generate steam, and the cooled reaction mixture is pumped back
into the stirred reaction zone. Many other known useful means
for removing heat of reaction can be used.
:' '.
'''". ''
- 16 -
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1~79Z96
Not previously mentioned are the parameters of components
of catalysis which are from 0.3 to 1.5 weight percent of total
metal and 0.3 to 1.5 weight percent bromine, both calculated as
the element, based on the xylene. Heat of reaction in the pre-
ferred mode will be removed by evaporation of reactor solvent,
which would be condensed by a three stage condenser. The H2O
of reactor would be removed from last stage of the condenser,
other condensed liquid returned to reactor based on the xylene.
The suitable source of Mn, Co and Ce can be salts of
such metals soluble in the benzoic acid-water solvent system.
Such salts include the carboxylates:acetates, proprionates,
butyrates, naphthenates and benzoates; the complexes acetyl-
acetonate and ethylenediamine tetraacetate as well as the
bromides.
, .
The source of bromine can be elemental bromine; ionic
forms of bromine including ammonium bromide; bromides of the
transition metal components of catalysis, hydrogen bromide per
se or as hydrobromic acid, sodium or potassium bromide or sodium
' or potassium bromates; and organic bromides including tetra-
bromoethane, dibromoethylene, benzylbromide, and bromobenzene.
Such sources of bromine are known from United States Patent No
'~1 2,833,816 and its commonly derived foreign counterpart patents
Illustrative Examples of Invention Conduct
The following thirteen examples illustrate the conduct of
the present inventive process by semi-continuous oxidation wherein
terephthalic acid (TA) is produced by air oxidation of p-xylene
(PX). The oxidation apparatus is a cylindrical oxidation vessel
having a stirrer to agitate its reaction zone which, from the
amount of solvent system used and product suspended therein after
all the p-xylene had been charged and aerated, comprises about
60 volume percent of the total volu~e of the vessel. The re-
~ .
- 17 -
_ 1079Z~6
maining 40 volume percent provides disengagement of gases and
vapors from the stirred fluid in the reaction zone. Said oxida- ;
tion vessel is also fitted with separate means for introducing
air into the lower portion of the oxidation zone, p-xylene and
water into the upper portion of the oxidation zone, discharging -
fluid oxidation effluent from the bottom of said vessel, dis-
charging the disengaged mixture of gases and vapors from the top
of the vessel, and means for sealing the vessel for its operation -
at pressures above ambient pressure. The top discharge means is
connected to a reflux condenser for condensing benzoic acid for
, its liquid reflux and uncondensed gases and vapors discharge to a
side arm type condenser operated to condense water. The water
vapor condenser is connected to a receiving vessel which collects
water condensate and discharges uncondensed gases (a mixture of
nitrogen, oxygen and oxides of carbon together with some water
vapor) from the top through a pressure control valved vent line.
A water freeze-out trap is between said pressure control valve
(adjustable) and apparatus for sampling and analyzing the gases
to be vented. The p-xylene is fed to the oxidation zone by a
; 20 metering pumped from a pressurized feed tank. Provision is also
made to charge water to the oxidation zone by means of a combina-
' tion of pressurized feed tank (3.5 kg/cm2 gauge above oxidationzone pressure), metering needle valve and rotameter but an addi-
.1 - . .. .
tional by-pass through a ball valve is also provided to add a
large amount of water to the reaction zone to quench the reaction
quickly in the event an otherwise uncontrollable sudden rise of
reaction temperature might occur. Adjustable means is provided -
`~ for heating or cooling the stirred liquid or fluid contents in
the oxidation zone system initially charged to the reaction
vessel and maintaining said solution at operating temperature
up to initial introduction of p-xylene and air and, if need be,
- 18 -
"
`-~ 1079Z9~
after introduction of p-xylene is complete. The reflux con-
denser is heated by steam, 7 kg/cm2 gauge pressure maximum,
introduced through an air-pressurized steam regulator, by
regulation of the air pressure, the control of the steam pres-
sure over the range of 0-7 kg/cm2 to the condenser, hence con-
trol of its operating temperature is achieved. Said air pres-
sure is controlled in turn in response to the temperature in
the reaction zone. For such operation of the condenser tempera-
ture in response to the oxidation zone temperature, the steam
pressure is increased when reaction temperature decreases and
steam pressure is decreased when reaction temperature increases.
The attendant increase in steam pressure reduces water content
of the solvent system and decrease in steam pressure increases
water content of the solvent system. In the illustrative
examples provided hereafter the described means for controlling
reaction temperature by control of reflux condenser temperature
(steam feed pressure) was sufficiently precise that it did not
become necessary to add water in large quenching amounts. How-
ever, when the exit vapor flow rates were low and temperature
control of vent gas was difficult, more precise control of tem-
perature was more facile with the injection of small amounts of
water.
-- 19 --
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79Z96
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-- 20 --
.;: , . . . ~ . . . .
- . .
-" 1079Z96
The p-xylene oxidations of following Examples 9-14
are conducted in the same manner and in the same equipment as
described with respect to Examples 1-8, except in Example 14
the reaction solvent is made up by combining 80 percent of
benzoic acid mother liquor from Example 13 with water, fresh
benzoic acid, and manganese acetate tetrahydrate. Hence, the
accumulation of intermediate products 4-CBA and p-toluic acid
and the increase in color of the recovered product.
- 21 -
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-- 22 --
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., , , . .. , , ~ ... . ... .. .. . . .
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1079Z96
The following p-xylene oxidations illustrate contin-
;
uous conduct of the present inventive oxidation process.
These continuous oxidations are conducted in the same type
of apparatus elements as described for the oxidations of the
preceding fourteen examples with the exception the apparatus
elements are larger. This is reflected in the greater amounts
of p-xylene fed per hour. The continuous oxidations are
started in the same manner as the semi-continuous and operated
with p-xylene pumping until the weight ratio of solvent
(initially charged) to p-xylene is reached. Thereafter a
solvent solution of the components of catalysis is also pumped
i into the reaction zone at a rate to maintain such solvent/-
!
xylene ratio and TRE is withdrawn at the rate to provide the
residence (hold) time, shown in Table III.
~i
.:
TABLE III
-~ p-XYLENE OXIDATION IN BENZOIC ACID-WATER SOLVENT
Example Comparative I and II 15
, . .
Materials and Conditions
Xylene Pump Rate, g/hr. 1510 1420 1050
Solvent water, wt. ~ 15 8 12
Co on Solvent, wt. % 0 0 0.03
Mn on Solvent, wt. % 0.20 0.20 0.09
Br on Solvent, wt. % 0.30 0.30 0.18
Solvent/p-xylene, wt. ratio 3.0 3.0 4.0
Operating temp., C. 217 217-226 227
Operating gauge, pres.,
kg/cm2 22.1 15.5 25.0
2 in exhaust, vol. % 6.9 5.5 5.5
Residence time, min. 45 45 50
Mole Co2/mole xylene 0.29 0.72 0.79
Total reaction effluent:
4-CBA, ppm. 15800 7400 630
p-Toluic acid, wt. % 6.55 2.65 0.037
p-Xylene, wt. % 0.086 0.0005 ND
Terephthalic acid, wt. % 8.48 18.0 22.7
Terephthalic acid yield,
Mol. % 48.8 79.3 93.8
Washed filter cake:
4-CBA, wt. % 1.49 0.71 0.053
:
,'' ::
- 23 _ ~
~, :
~ .
--`` 1079296
In the conduct of p-xylene oxidation according to
Comparative Example I, by-product water was not removed. That
is, the reflux condenser is operated at a temperature to return
all water and benzoic acid condensate to the oxidation zone. ~ ,
This would permit the benzoic acid-water solvent system to in-
crease in water concentration from 10% to 18~ by weight at
steady state operation. Such 18% water content is above the 15
upper limit permitting an acceptable rate of oxidation. Also
225 grams of p-xylene is added to the initial charge of solvent
system containing the components of catalysis and p-xylene pump-
ing is delayed until the time (calculated) for conversion of
the initially charged p-xylene by the introduction of air. Under
these modified operating conditions, temperature in the stirred -
oxidation zone could not be controlled at the planned 217C,
but rather the oxidation zone temperature cycled considerably
above and below said temperature. Also the production of oxides
of carbon and oxygen content of exhaust gas fluctuated sub-
stantially which indicates not only poor control of temperature
of reaction but indicates conditions of too high and too low
oxygen concentration in the oxidation zone even though the air
input was constant. The sum of effects adverse to control of
reactivity and constant temperature can cause build-up of
aromatic co and by-products to concentrations which
significantly reduce the desired rate of oxidation to
TA.
For the oxidation of Comparative Example II, the
temperature of the reflux condenser was increased to permit re-
moval of by-product water in a gas-vapor mixture at a temperature
of 121C and maintain a 10~ water concentration in the solvent
system. sut p-xylene (908 grams) is again precharged. However,
the temperature in the oxidation zone again cycles, a constant
- 24 -
: . , .: . ~ - :
. . . . , : . : : ..
10'79296
temperature of 218C could not be maintained and the oxygen
consumption, in general, is low. The oxidation zone tempera-
ture reached a maximum of 226.5C and at this point th ~oxy-
gen consumption increases sharply as indicated by an attendant
75~ drop in oxygen content of exhaust gas. Also there was an
accumulation of p-xylene condensate in the cold traps preceding
the exhaust gas sampling and analyzing apparatus. Thus a sub-
stantial amount of p-xylene was vaporized as it entered the
oxidation zone maintained at 14.06 kg/cm2 gauge pressure an~
did not oxidize. Close control of reaction temperature was not
' possible because the temperature of the reflux condenser could
not be closely maintained.
For the conduct of Example 15, the temperature of
' operation of the reflux condenser is maintained by water heated
with a regulatable flow of steam added to the hot water input
t to the condenser. In this way, a close control of the reflux
¦ condenser's temperature in the range of 121 + 0.5C. is achieved.
With this close control of the temperature of operation of ;
ll the reflux condenser and by the use of the indicated reaction
l 20 zone pressure, close control of reaction temperature at 226.5C
~T is accomplished, and oxygen consumption reached a good steady -
state.
The preparation of similarly high yields of good --
color quality and purity isophthalic acid in the 85-95% benzoic
acid and 15-5% water system can be obtained by substituting
m-xylene for p-xylene in the foregoing illustrative examples.
'
-.
,
'~''
- 25 -
.
.1 ....
