Title of Invention

PROCESS FOR PRODUCING AROMATIC CARBOXYLIC ACID

Abstract A method for producing an aromatic carboxylic acid, which includes (A) an oxidation step of oxidizing an alkylaromatic compound under a pressure higher than an ordinary pressure, in an acetic acid solvent, in the presence of a catalyst, at 140 to 230˚C, to form an aromatic carboxylic acid, (B) a solid-liquid separation step of subjecting a slurry containing crystals of the above aromatic carboxylic acid formed to a solid-liquid separation into the aromatic carboxylic acid and a mother liquor under a pressure higher than an ordinary pressure, (C) a mother liquor recycling step of dividing the above mother liquor into two portions, while retaining a pressure higher than an ordinary pressure, and feeding the one of the two portions back to the above oxidation step (A), and (D) a concentration step of concentrating the other portion being divided in the above mother liquor recycling step (C), wherein in the above concentration step (D), the pressure of the mother liquor is released to a pressure not higher than the vapor pressure thereof at the temperature thereof, to thereby vaporize a part of the mother liquor, concentrate the mother liquor, and precipitate the aromatic carboxylic acid, and then the precipitated aromatic carboxylic acid is recovered. The above method allows the efficient utilization of the energy being held by the separated mother liquor and also the recovery of a useful component.
Full Text TECHNICAL FIELD
This invention relates to a process for producing an aromatic carboxylic acid.
BACKGROUND ART
Usually, in a process for producing terephthalic acid as an aromatic carboxylic acid, for example as shown in Fig. 2, firstly as an oxidation reaction step, paraxylene a as an alkylaromatic compound is oxidized by air b in an acetic acid solvent in a reactor 1 in the presence of a catalyst to form terephthalic acid. The formed terephthalic acid is a slurry having a part thereof dissolved in the solvent, and this reaction slurry c is introduced into a crystallizer 2 and subjected to pressure-release evaporation thereby to further precipitate terephthalic acid to obtain a crystallized slurry d. This crystallized slurry d is subjected to a solid-liquid separation step by a solid-liquid separator 3 and separated into a terephthalic acid cake e and a separated mother liquor f . The terephthalic acid cake e is dried by a drying device 4 to obtain terephthalic acid crystals g.
The separated mother liquor f can be re -used as a
recycled mother liquor h for a solvent in the above oxidation reaction step. However, if impurities contained in the separated mother liquor f are accumulated in the system, they are likely to influence the quality of the resulting terephthalic acid crystals g. Therefore, in order to suppress accumulation of impurities, a part of the separated mother liquor f is purged out of the system as a purged mother liquor i. However, in this purged mother liquor i, not only the impurities, but also the oxidation catalyst and the solvent component are contained, and it is necessary to recover such useful components.
Usually, firstly, the solvent component in the purged mother liquor i is evaporated by a solvent-evaporator 5 to concentrate high boiling point components such as the oxidation catalyst and impurities, and from such a concentrated residue j, the above oxidation catalyst is recovered by treatment such as extraction. Whereas, the evaporated solvent vapor k is subjected to removal of water 1 formed by oxidation, by means of a dehydrator 6 by e.g. distillation, and recovered acetic acid m thereby obtained is re-used as a solvent for the above oxidation reaction step or as a cleaning liquid in the above solid-liquid separation step. Here, the purged mother liquor i will be accompanied by a solid content such as terephthalic acid or its oxidized intermediate by e.g. leakage from the solid-liquid separator 3. If the
purged mother liquor accompanied by the solid content is included in the residue by the concentration treatment, recovery of such a solid content will be difficult, thus leading to a decrease in the productivity.
In this connection, Patent Document 1 discloses a method wherein a separated mother liquor f is divided into two portions, and one portion is returned as a recycled mother liquor h to the above oxidation reaction step, while the other portion is subjected to concentration treatment as a purged mother liquor i, and wherein prior to such concentration treatment, a solid content contained in the purged mother liquor i is preliminarily subjected to solid-liquid separation.
Patent Documents 2 and 3 disclose a method wherein a reaction slurry c is subjected directly to solid-liquid separation without via crystallizer 2, or even in a case where it is subjected to pressure-release evaporation in the crystallizer 2, solid-liquid separation is carried out in a state where a high temperature and high pressure condition is maintained without releasing the pressure to atmospheric pressure. It is thereby possible to simplify the step of drying the downstream separated terephthalic acid cake e, or to reduce the heat energy consumed in the step of purifying the dried terephthalic acid crystals g, such being desirable.
However, the high temperature and high pressure separated mother liquor obtained by solid-liquid
separation under a high temperature and high pressure condition, has a high solubility as compared with a separated mother liquor obtained by solid-liquid separation under a lower temperature and lower pressure condition. Therefore, useful components such as terephthalic acid and its oxidized intermediate are dissolved in a large amount, and if the concentration operation is carried out all at once by purging such a separated mother liquor as it is under a high temperature and high pressure condition to obtain a residue, useful components such as terephthalic acid and its oxidized intermediate, dissolved in the separated mother liquor under a high temperature and high pressure condition will be included in the residue, thus leading to a decrease in the productivity. On the other hand, with respect to the recycled mother liquor, if it is supplied to the reactor as cooled by pressure-release evaporation, a large amount of energy will be required to heat the reaction product to a prescribed temperature for the oxidation reaction, which will be a factor to lower the efficiency in the recovery of the oxidation reaction energy.
Patent Document 1: JP-A-2000-504741
Patent Document 2: JP-A-2001-139514
Patent Document 3: JP-A-2002-336687
DISCLOSURE OF THE INVENTION
OBJECT TO BE ACCOMPLISED BY THE INVENTION
It is an object of the present invention to provide a process for producing an aromatic carboxylic acid from an alkylaromatic compound, wherein in the treatment of a separated mother liquor obtained by solid-liquid separation of the oxidation reaction slurry in a high temperature and high pressure state under a high temperature and high pressure condition, the energy of the separated mother liquor is efficiently utilized for the concentration treatment, whereby useful components dissolved in the mother liquor are recovered, and an aromatic carboxylic acid is produced.
MEANS TO ACCOMPLISH THE OBJECT
The present inventors have conducted an extensive study to accomplish the above object and as a result, have found it possible to accomplish the object by carrying out concentration treatment by evaporating the purged mother liquor by releasing the pressure to a level of not higher than the vapor pressure of the mother liquor at the temperature of the mother liquor thereby to precipitate the aromatic carboxylic acid, and then recovering the precipitated aromatic carboxylic acid, and thus have accomplished the present invention. Namely, the gist of the present invention resides in the following 1 to 10.
I. A process for producing an aromatic carboxylic acid, which comprises:
an oxidation step of oxidizing an alkylaromatic
compound under a pressure higher than atmospheric
pressure in an acetic acid solvent in the presence of a
catalyst at from 140 to 230°C to form an aromatic
carboxylic acid,
a solid-liquid separation step of subjecting a
slurry containing crystals of the above aromatic
carboxylic acid formed to solid-liquid separation into
the aromatic carboxylic acid and a mother liquor under a
pressure higher than atmospheric pressure,
a mother liquor recycling step of dividing the
above mother liquor into two portions while the pressure
is maintained to be higher than atmospheric pressure, and
returning one of the two portions to the above oxidation
step (A), and
a concentration step of concentrating the other
portion of the mother liquor divided in the above mother
liquor recycling step (C),
wherein in the above concentration step (D), the concentration treatment is carried out by evaporating the mother liquor by releasing the pressure to a level of not higher than the vapor pressure of the mother liquor at the temperature of the mother liquor thereby to precipitate the aromatic carboxylic acid, and then the precipitated aromatic carboxylic acid is recovered. 2. The process for producing an aromatic carboxylic acid according to the above 1, wherein the concentration step
(D) comprises two stages of concentration, such that the mother liquor is sent to the first stage concentration treatment under a pressure higher than atmospheric pressure and evaporated by releasing the pressure to a level of not higher than the vapor pressure of the mother liquor at the temperature of the mother liquor to carry out the first stage concentration treatment thereby to precipitate the aromatic carboxylic acid, then the precipitated aromatic carboxylic acid is recovered, and the mother liquor after recovering the aromatic carboxylic acid is subjected to the second stage concentration treatment to evaporate and recover the solvent component.
The process for producing an aromatic carboxylic acid
according to the above 1 or 2, wherein the recovered
aromatic carboxylic acid is returned to the oxidation
step (A).
A process for producing an aromatic carboxylic acid,
which comprises:

an oxidation step of oxidizing an alkylaromatic
compound under a pressure higher than atmospheric
pressure in an acetic acid solvent in the presence of a
catalyst to form an aromatic carboxylic acid,
a solid-liquid separation step of subjecting a
slurry containing crystals of the above aromatic
carboxylic acid formed to solid-liquid separation into
the aromatic carboxylic acid and a mother liquor under a
pressure higher than atmospheric pressure,
a mother liquor recycling step of dividing the
above mother liquor into two portions while the pressure
is maintained to be higher than atmospheric pressure, and
returning one of the two portions to the above oxidation
step (A), and
a concentration step of concentrating the other
portion of the mother liquor divided in the above mother
liquor recycling step (C),
wherein the concentration step (D) comprises two stages of concentration, such that the mother liquor is sent to the first stage concentration treatment under a pressure higher than atmospheric pressure and evaporated by releasing the pressure to a level of not higher than the vapor pressure of the mother liquor at the temperature of the mother liquor to carry out the first stage concentration treatment thereby to precipitate the aromatic carboxylic acid, then the precipitated aromatic carboxylic acid is recovered and returned to the step (A), and the mother liquor after recovering the aromatic carboxylic acid is subjected to the second stage concentration treatment to evaporate and recover the solvent component.
5. The process for producing an aromatic carboxylic acid according to any one of the above 1 to 4, wherein in the mother liquor recycling step (C), said one portion of the mother liquor divided is returned to the oxidation step
(A) while the pressure is maintained to be higher than atmospheric pressure.
The process for producing an aromatic carboxylic acid
according to any one of the above 1 to 5, wherein the
solvent component evaporated by the first stage
concentration treatment and/or the second stage
concentration treatment, is directly or indirectly
returned to the step (A).
The process for producing an aromatic carboxylic acid
according to any one of the above 1 to 6, wherein the
solid-liquid separation step (B) includes a step of
cleaning with a cleaning liquid the aromatic carboxylic
acid obtained by the solid-liquid separation, and the
above solid-liquid separation step and the above cleaning
step are carried out in a unified solid-liquid separation
apparatus.
The process for producing an aromatic carboxylic acid
according to the above 7, wherein the solid-liquid
separation apparatus is a screen-bowl centrifuge.
The process for producing an aromatic carboxylic acid
according to the above 7 or 8, wherein the cleaning
liquid is acetic acid or water.
10. The process for producing an aromatic carboxylic
acid according to any one of the above 1 to 9, wherein
the alkylaromatic compound is paraxylene, and the
aromatic carboxylic acid is terephthalic acid.
EFFECTS OF THE INVENTION
By this invention, in the process for producing an aromatic carboxylic acid, at the time of concentrating the purged mother liquor, the energy of the mother liquor is utilized for a part of concentration, whereby it is possible to increase the recovery rate of useful components contained in the purged mother liquor. Further, with respect to the recycled mother liquor, it is recycled to the oxidation reactor in a state where the high temperature and high pressure condition is maintained, whereby the energy required for raising the temperature will be reduced, and effective recovery of the oxidation reaction energy will be possible.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a flow chart for the production of aromatic carboxylic acid crystals according to this invention.
Fig. 2 is a flow chart of the conventional production of terephthalic acid crystals.
MEANINGS OF SYMBOLS 1: reactor 2: crystallizer 3: solid-liquid separator 4: drying device 5: solvent-evaporator
6: dehydrator
11: reactor
lla: condenser
12: solid-liquid separator
13: cleaning device
14: solid-liquid separation and cleaning system
15: drying device
16: pressure-release evaporator
17: solid content recovery device
18: solvent-evaporator
19: dehydrator
a: paraxylene
b: air
c: reaction slurry
d: crystallized slurry
e: terephthalic acid cake
f: separated mother liquor
g: terephthalic acid crystals
h: recycled mother liquor
i: purged mother liquor
j: residue
k: solvent vapor
1: water formed by oxidation
m: recovered acetic acid
A: alkylaromatic compound
B: molecular oxygen containing gas
C: aromatic carboxylic acid slurry
E: aromatic carboxylic acid cake
F: mother liquor
G: aromatic carboxylic acid crystals
H: recycled mother liquor
I: purged mother liquor
J: residue
L: water formed by oxidation
M: recovered acetic acid
N: reaction gas
O: condensed liquid
P: exhaust gas
Q: cleaning liquid
R: cleaned cake
S: cleaning waste liquid
T: solvent vapor
U: concentrated liquid
V: separated mother liquor
W: solid material
X: solvent vapor
BEST MODE FOR CARRYING OUT THE INVENTION
Now, this invention will be described in detail with reference to Fig. I.
This invention provides a process for producing an aromatic carboxylic acid, which comprises (A) an oxidation step, (B) a solid-liquid separation step, (C) a mother liquor recycling step and (D) a concentration
step.
In the above oxidation step (A) , an alkylaromatic compound (A) is oxidized in a liquid phase in a reaction solvent at from 140 to 230°C to obtain an aromatic carboxylic acid slurry C. In a case where the aromatic carboxylic acid to be produced is terephthalic acid, the above alkylaromatic compound A is paraxylene. In such a case, it is preferred to oxidize at least 90 wt%, more preferably at least 95 wt%, of paraxylene as the above alkylaromatic compound A, to terephthalic acid.
Further, as the reaction medium for the above oxidation step (A), acetic acid is employed. In a case where the above terephthalic acid is to be produced, the amount of the acetic acid solvent to be used is preferably from 2 to 6 times by weight, more preferably from 2 to 4 times by weight, to paraxylene. If the amount of acetic acid is too small, the temperature of the reaction slurry tends to be too high that a trouble such as clogging is likely to be led. If it is too large, the amount of the solvent in the system to the production amount of the product tends to be large, and the installation is required to be large, such being economically undesirable. As such acetic acid solvent, it is possible to reuse a recycled mother liquor H obtained in the after-mentioned solid-liquid separation step (B) or acetic acid M recovered from the solvent vapor obtained in the concentration step (D) . At that
time, in order to prevent an increase of water formed by the oxidation reaction, in the solvent, it is preferred to subject the solvent vapor obtained in the concentration step (D) to dehydration treatment by e.g. distillation.
Further, the acetic acid solvent in the present invention is preferably a mixture of acetic acid and water and is usually a mixture having from I to 20 parts by weight, preferably from 5 to 15 parts by weight, of water mixed to 100 parts by weight of acetic acid.
As the alkylaromatic compound A in the present invention, an aromatic compound having an alkyl substituent or a partially oxidized alkyl substituent may be used. Such an alkyl aromatic compound may be monocyclic or polycyclic. The above alkyl substituent may, for example, be a Ci-4 alkyl group such as a methyl group, an ethyl group, a n-propyl group, or an isopropyl group. Further, as the partially oxidized alkyl group, an aldehyde group, an acyl group, a carboxyl group or a hydroxyalkyl group may, for example, be mentioned.
Specific examples of the aromatic compound having an alkyl substituent i.e. the alkyl-substituted aromatic hydrocarbon include, for example, a di- or poly-alkyl benzene having from 2 to 4 Cj-4 alkyl groups, such as ni­di isopropyl benzene, p-diisopropyl benzene, m-cymene, p-cymene, m-xylene, p-xylene, a trimethyl benzene or a tetramethyl benzene; a. di- or poly-alkyl naphthalene
having from 2 to 4 C^ alkyl groups, such as a dimethyl naphthalene, a diethyl naphthalene or a diisooctyl naphthalene; and a poly-alkyl biphenyl having from 2 to 4 Cx-4 alkyl groups such as a dimethyl biphenyl .
Whereas, the aromatic compound having a partially oxidized alkyl substituent, is a compound having an alkyl group in the above compound partially oxidized to the above aldehyde group, the acyl group, the carboxyl group or the hydroxyalkyl group. As a specific example, 3-methyl benzene aldehyde, 4-methyl benzene aldehyde, m-toluic acid, p-toluic acid, 3-formyl benzoic acid, 4-formyl benzoic acid or 2-methyl-6-formyl naphthalene may, for example, be mentioned. These compounds may be used alone or in combination as a mixture of two or more of them.
The alkylaromatic compound A may preferably be m-xylene or p-xylene, more preferably p-xylene.
In the above oxidation step (A), the alkylaromatic compound A may usually be oxidized by a molecular oxygen-containing gas B. The molecular oxygen-containing gas B may, for example, be a gas containing molecular oxygen, such as air, oxygen diluted with an inert gas or oxygen-enriched air. Among them, air is practically preferably employed.
In the present invention, the aromatic carboxylic acid may, for example, be an aromatic dicarboxylic acid such as terephthalic acid, isophthalic acid, 2, 6-
naphthalene dicarboxylic acid or 4 , 4 '-biphenyl dicarboxylic acid; an aromatic tricarboxylic acid such as trimellitic acid or trimesic acid; or an aromatic polycarboxylic acid such as pyromellitic acid.
The process of the present invention is preferably applied to the production of an aromatic dicarboxylic acid or an aromatic carboxylic acid hardly soluble in a reaction solvent, particularly preferably applied to the production of terephthalic acid employing p-xylene as the starting material.
The catalyst to be used at the time of oxidizing the above aIkylaromatic compound A is not particularly limited so long as it is one having an ability to oxidize the alkylaromatic compound A and convert it to an aromatic carboxylic acid. Usually, a heavy metal compound is used, and as the case requires, a bromine compound may be used as a catalyst-assisting agent. The heavy metal in the heavy metal compound may, for example, be cobalt, manganese, nickel, chromium, zirconium, copper, lead, hafnium or cerium. They may be used alone or in combination. Particularly, it is preferred to employ cobalt and manganese in combination. As such a heavy metal compound, an acetate, a nitrate, an acetylacetonate, a naphthenate, a stearate or a bromide may, for example, be mentioned. Particularly preferred is an acetate or a bromide.
Further, the bromine compound may, for example, be
an inorganic bromine compound such as molecular bromine, hydrogen bromide, sodium bromide, potassium bromide, cobalt bromide or manganese bromide, or an organic bromine compound such as methyl bromide, methylene bromide, bromoform, benzyl bromide, bromomethyltoluene, dibromoethane, tribromoethane or tetrabromoethane. These bromine compounds may be used alone or in combination as a mixture of two or more of them.
As the catalyst to be used at the time of oxidizing the alkylaromatic compound A, specifically, a combination of cobalt, manganese and bromine may be mentioned, and particurarly preferred is a combination of cobalt acetate, manganese acetate and hydrogen bromide.
In the present invention, the catalyst made of a combination of the above heavy metal compound and the bromine compound is preferably one which contains bromine atoms within a range of from 0.05 to 10 mol, preferably from 0.1 to 5 mol, per mol of the heavy metal atoms. Such a catalyst is used usually within a range of from 10 to 10,000 ppm, preferably from 100 to 3,000 ppm, as the heavy metal concentration in the reaction solvent.
The reaction temperature for oxidation of the alkylaromatic compound A in the reactor 11 in the drawing is usually from 140 to 230°C, preferably from 150 to 210°C, more preferably from 170 to 200°C. If the reaction temperature is too low, the reaction rate decreases, and if the reaction temperature is too high, by the
combustion of the acetic acid solvent, the amount of loss tends to increase, such being undesirable. The reaction pressure is required to be at least a pressure under which the mixture can be maintained to be in a liquid phase at least at the reaction temperature and is required to be a pressure higher than atmospheric pressure. Specifically, it is preferably from 0.2 to 5 MPa (absolute pressure), more preferably from 0.4 to 3 MPa (absolute pressure).
The reaction is usually carried out continuously, and the reaction time (the average residence time) is preferably from 30 to 300 minutes, particularly preferably from 40 to 150 minutes. If the reaction time is too short, the reaction tends to be inadequate, and the desired product quality may not be obtained, and if it is too long, combustion of the acetic acid increases, such being uneconomical. Further, such is not economical also from the viewpoint that the capacity of the reactor 11 is obliged to be large.
The concentration of water in the reaction solvent may be adjusted, for example, by purging, out of the system, a part of the condensed liquid O obtained by condensing a reaction gas N formed in the reactor 11.
The above reactor 11 is preferably a tank equipped with a stirrer, but may not necessarily be equipped with a stirrer and may be one of a bubble columm type. At a lower potion of the reactor 11, an inlet for the
molecular oxygen-containing gas B is provided, and as the case requires, a condenser lla may be provided at an upper potion of the reactor 11. The molecular oxygen-containing gas B supplied from this lower potion is, after utilized for the oxidation reaction of the alkylaromatic compound A, withdrawn from the reactor 11 as a reaction gas E accompanied by a large amount of acetic acid vapor. Then, in the condenser lla, the condensed liquid O composed mainly of acetic acid is condensed and separated, and the rest is discharged as an exhaust gas P. Usually, the condensed liquid O contains water, and a part thereof will be purged out of the system to adjust the water content in the system, and the rest will be refluxed to the reactor 11. Further, the exhaust gas P may be divided into two streams, so that one stream is discharged out of the system, and the other stream may be continuously recycled to the reactor 11.
Further, in the present invention, in the oxidation step (A), after the oxidation reaction in the reactor 11, additional treatment may be carried out as the case requires. This additional treatment is meant for additional oxidation treatment (hereinafter referred to as "low temperature additional oxidation") of a reaction mixture obtained by the above oxidation reaction being the first stage (hereinafter referred to as "the first reaction zone"), in a second reaction zone maintained at a temperature lower than the above first reaction zone
and, in the case where the alkylaromatic compound A is paraxylene, usually at a temperature lower by from 5 to 20°C than the above first reaction zone, without supplying the alkylaromatic compound A. The pressure at the time of this low temperature additional oxidation is required to be at least a pressure under which the internal mixture can maintain the liquid phase at least at the reaction temperature, and is preferably from 0.2 to 5 MPa (absolute pressure) . Further, this low temperature additional oxidation reaction is preferably carried out continuously, and the reaction time is preferably from 5 to 120 minutes. Further, such a low temperature additional oxidation may be carried out twice or more. Further, as the case requires, in a third reaction zone, an additional oxidation treatment (hereinafter referred to as "high temperature additional oxidation") may be carried out at a temperature higher than the above first and second reaction zones.
As the molecular oxygen-containing gas B to be supplied to carry out the above low temperature additional oxidation or high temperature additional oxidation, air, oxygen diluted with an inert gas, oxygen-enriched air or the like may be employed in the same manner as in the above first reaction zone. Practically, air is preferably employed. Further, the amount to be supplied is preferably from about 1/10,000 to 1/5, more preferably from 1/100 to 1/10, of the amount to be
supplied to the oxidation reaction to be carried out in the first reaction zone. Further, a reactor of the same type as for the above first reaction zone may be used for the above second reaction zone for such a low temperature additional oxidation reaction or for the above third reaction zone for the above high temperature additional oxidation.
Irrespective of whether the above additional oxidation treatment is applied or not, the aromatic calboxylic acid slurry C as the obtained reaction mixture may be subjected to crystallization treatment under a pressure higher than atmospheric pressure, as the case requires, prior to the recovery of the aromatic carboxylic acid crystals in the next solid-liquid separation step (B) . This aromatic carboxylic acid slurry C is evaporated by releasing the pressure to a pressure lower than at the time of the above oxidation step, whereby it is cooled, and the aromatic carboxylic acid dissolved in the solvent will further be precipitated, and the amount of recovery of the aromatic carboxylic acid crystals will be increased. However, when it is cooled by crystallization, the temperature for the separation operation obtainable in the above solid-liquid separation step (B) will decrease. As a result, the temperature of the separated mother liquor or the separated cake will also be low, whereby the effects of the present invention tends to be small. In order to
carry out the after-mentioned solid-liquid separation under a pressure higher than the atmospheric pressure, it is necessary to maintain the final aromatic carboxylic acid slurry C under a pressure higher than atmospheric pressure at a high temperature, specifically at a high temperature of from 110°C to the oxidation reaction temperature.
In the above solid-liquid separation step (B), by the solid-liquid separator 12, the above aromatic carboxylic acid slurry C is subjected to solid-liquid separation into an aromatic carboxylic acid cake E and a mother liquor F under a pressure higher than atmospheric pressure. Among those separated in the above solid-liquid separator 12, the above mother liquor F will be sent to the above mother liquor recycling step (C) while the pressure is maintained to be higher than atmospheric pressure. Here, the pressure higher than atmospheric pressure is a pressure higher than the vapor pressure of the mother liquor at the temperature of the mother liquor of the above aromatic carboxylic acid slurry C. By maintaining the pressure in such a manner, the mother liquor will be evaporated, and the temperature level of the above aromatic carboxylic acid slurry C can be substantively maintained without being cooled.
In the present invention, the pressure at the time of solid-liquid separation of the slurry containing aromatic carboxylic acid crystals formed in the solid-
liquid separation step (B) into the aromatic carboxylic acid and the mother liquor, is required to be a pressure higher than atmospheric pressure, and specifically, it is preferably from 0.2 to 1.5 MPa (absolute pressure), more preferably from 0.3 to 1.2 MPa (absolute pressure). If the pressure is too low, useful components dissolved in the separated mother liquor tend to be less, and the effects of employing the present invention tend to be less. Further, the upper limit is the pressure of the slurry obtained from the oxidation step (A).
The separated aromatic carboxylic acid cake E is preferably cleaned with a cleaning liquid Q to remove impurities or by-products in a cleaning device 13 prior to drying in a drying device 15. Here, as the cleaning liquid Q, acetic acid or water may, for example, be used.
Further, it is more preferred that the step for solid-liquid separation by the solid-liquid separator 12 and the step for cleaning by the cleaning device 13 are carried out in one solid-liquid separation cleaning system 14 as shown by a dotted line potion in Fig. 1, whereby the process will be simplified. As a solid-liquid separation cleaning system 14 wherein the two steps can be carried out in such a unified manner, a screen bowl centrifuge, a rotatory vacuum filter or a horizontal belt filter may, for example, be mentioned. Particularly preferred is a screen bowl centrifuge which is excellent in heat resistance even in a high
temperature region close to the temperature in the oxidation step (A).
The obtained cleaned cake R is dried in the drying device IS, whereby the attached liquid remaining in the cake is removed to obtain aromatic carboxylic acid crystals G. Here, the drying device 15 may be constituted by a plurality of devices. Among them, as a device to remove at least a part of the above attached liquid, it is preferred to employ a device wherein the liquid attached to such a cake is subjected to pressure-release evaporation by an internal energy thereof by bringing such a cake and the liquid attached thereto to a lower pressure state. Here, pressure release evaporation is such that a liquid in a high pressure state is transferred abruptly to a low pressure state where the temperature before the transfer becomes at least the boiling point under the pressure after the transfer, whereby a part will be cooled to a temperature not higher than the boiling point under the pressure after the transfer and/or a part is evaporated by the internal energy thereof. It is preferred that by such pressure release evaporation, the above attached liquid can be evaporated as much as possible. However, in a case where drying is inadequate by the pressure release evaporation only, further heating may be required.
Further, the cleaning waste liquid S after cleaning the above aromatic carboxylic acid cake E contains the
acetic acid solvent, the above aromatic carboxylic acid, etc. Therefore, it is preferably returned to the above reactor 11 alone or together with the recycled mother liquor H, whereby the overall yield of the production process will be good.
In the above recycling step (C), the mother liquor F is divided into a recycled mother liquor H and a purged mother liquor I while the pressure is maintained to be higher than atmospheric pressure. The expression "while the pressure is maintained to be higher than atmospheric pressure" means a pressure level where the operation pressure in the solid-liquid separation step (B) is substantially maintained. The ratio for division into the recycled mother liquor H and the purged mother liquor I may optionally be adjusted depending upon the situation of the overall production process, but it is usually a ratio such that the recycling rate (weight of recycled mother liquor x 100/ (weight of recycled mother liquor + weight of purged mother liquor) will be from 40 to 95%, preferably from 60 to 90%. Further, the recycled mother liquor H and the purged mother liquor I preferably maintain the pressure higher than atmospheric pressure, more preferably substantially maintain the operation pressure in the solid-liquid separation step (B).
By maintaining the recycled mother liquor G and the cleaning waste liquid S under the operation pressure in the solid-liquid separation step (B), the recycled mother
liquor H will not be substantively cooled by pressure-release evaporation and can be returned to the reactor 11 in the oxidation step (A) while the operation temperature in the solid-liquid separation step (B) is maintained. Further, it is thereby possible to save the energy for pressurizing and heating again to satisfy a prescribed oxidation reaction condition, whereby the energy generated in the oxidation reaction (oxidation reaction heat) can be effectively recovered from the distillate vapor and reused. Specifically, in a case where steam is generated by using the heat of condensation obtained by the condenser lla, it is possible to increase the amount of steam generated.
Sine the above solid-liquid separation step (B) is carried out under a high temperature and high pressure operation condition, in the mother liquor F, the above aromatic carboxylic acid and other useful components are dissolved in a large amount as compared with a case where solid-liquid separation is carried out under a normal temperature and normal pressure condition. Therefore, the purged mother liquor I divided from such a mother liquor F is subjected to the concentration step (D), wherein the aromatic carboxylic acid is precipitated and recovered. If the purged mother liquor I is subjected to the concentration operation all at once to obtain the residue, the above aromatic carboxylic acid and other useful components thus dissolved therein, are likely to
be included in this residue, and they will be hardly recovered. Thus, the productivity will be lowered. Therefore, a method is preferred wherein the concentration step comprises two stages of concentration such that in the first stage concentration treatment, the aromatic carboxylic acid is precipitated, and the precipitated aromatic carboxylic acid is recovered, and the mother liquor after recovering the aromatic carboxylic acid is subjected to second stage concentration treatment, whereby the solvent component is evaporated and recovered.
In the above concentration step (D), firstly, the purged mother liquor I is subjected to pressure release evaporation to carry out the first stage concentration treatment to evaporate a part of the solvent component, and then a solid substance precipitated by the above first stage concentration treatment is recovered. Further, the mother liquor having the aromatic carboxylic acid separated by this recovery, is subjected to the second stage concentration treatment to evaporate the remaining solvent component and recover the solvent. Here, it is preferred to maintain the operation pressure in the solid-liquid separation step (B) until the mother liquor F is divided into the purged mother liquor I and the recycled mother liquor, whereby the temperature of the separated mother liquor is maintained at the operation temperature in the solid-liquid separation step
(B) .
Firstly, in the above first stage concentration step, the purged mother liquor I having the recycled mother liquor H removed from the above mother liquor F, is introduced into the pressure-release evaporator 16 set under a pressure of not higher than the vapor pressure of the purged mother liquor I at the liquid temperature of the mother liquor and subjected to pressure-release evaporation to carry out the first stage concentration and cooling treatment thereby to evaporate a part of the solvent component as a solvent vapor T and to carry out concentration and cooling to obtain a concentrated liquid U. The cooled temperature corresponds to the boiling point of the mother liquor under the pressure after the pressure release. The amount of the solvent to be evaporated by the first stage concentration is usually from 10 to 50 wt%, preferably from 20 to 40 wt%. If the amount to be evaporated is too large, not only the useful components but also impurities adversely influencing the quality of the product, tend to be precipitated, such being undesirable. On the other hand, if it is too small, the recovery of the useful components will be less, and the effects will be inadequate.
Further, the operation pressure of the pressure-release evaporator 16 at that time (the operation pressure for the first stage concentration) is preferably a pressure of not higher than the vapor pressure of the
purged mother liquor I at the liquid temperature of the mother liquor in order to carry out the concentration efficiently. Specifically, it is from 0.05 MPa (absolute pressure) to a pressure of not higher than the vapor pressure of the purged mother liquor I at the liquid temperature of the mother liquor, preferably from 0.08 MPa (absolute pressure) to a pressure of not higher than the vapor pressure of the purged mother liquor I at the liquid temperature of the mother liquor. If the pressure in the pressure release operation is too low, by the concentration and cooling, not only the useful components but also impurities adversely influencing the quality of the product tend to be precipitated, such being undesirable. On the other hand, if the pressure is too high, the recovery of the useful components will be less, and the effects will be inadequate. In this manner, by maintaining the state pressurized in the above oxidation step (A) with the purged mother liquor and utilizing the pressurized state at the time of the concentration, it is possible to evaporate the solvent in the purged mother liquor I without carrying out heating or the like, thereby to carry out the concentration efficiently.
Here, the solvent vapor T evaporated at the time of such concentration is one composed mainly of acetic acid as the solvent component. To use this acetic acid again as a solvent, the solvent vapor T is cooled and returned to a liquid, whereupon it may be returned directly to the
oxidation step (A) or after being treated by a hydrator 19 such as by distillation, indirectly returned to the oxidation step (A), as recovered acetic acid M. Further, it may be used as a cleaning liquid Q.
The concentrated liquid U obtained by the above first stage concentration contains a precipitate formed by the pressure release evaporation. To recover this precipitate, in the solid content recovery step, it is separated from a separated mother liquor V by a solid content recovery device 17 to obtain a solid material W containing the precipitate formed by the above pressure-release evaporation. As such a solid content recovery device 17, a filtration machine, a centrifugal separator, a cyclone or a thickener may, for example, be mentioned.
Such solid material W comprises useful components such as the above aromatic carboxylic acid and its intermediate, and by returning the solid material to the reactor 11 in the oxidation step (A), the productivity can be improved.
The above separated mother liquor V is, in the second stage concentration treatment, introduced into a solvent evaporator 18 and subjected to heat treatment to further evaporate the remaining solvent component for concentration. The operation here may be carried out by means of a conventional method. For example, by means of a heat medium such as steam, the solvent component is
evaporated. Usually, the above separated mother liquor V is introduced into a solvent evaporator 18 composed of a tank, and the solvent component is evaporated by using steam from outside, as the heat source, under an operation pressure of from 0.03 MPa (absolute pressure) to 0.15 MPa (absolute pressure), thereby to increase the concentration of high boiling point components such as the catalyst component, byproducts, impurities, etc. from 10 to 40 times. The concentrated liquid is withdrawn from the solvent evaporator 18 continuously or intermittently, and the solvent component may further be evaporated and recovered. The solvent vapor X thereby evaporated contains, as the main component, acetic acid being the solvent component, and therefore, in the same manner as the above mentioned solvent vapor T, it may be cooled and returned to a liquid and then may be returned directly to the oxidation step (A) , or indirectly to the oxidation step (A) after being treated by a dehydrater 19 such as by distillation to remove water L formed by oxidation. Further, it may likewise used as a cleaning liquid Q. Further, the above dehydration treatment may be carried out by combining the above solvent vapor T and the above solvent vapor X.
The residue J not evaporated also in the above second step concentration treatment, contains a byproduct of the above aromatic carboxylic acid, the catalyst used in the above reactor 11, etc. It is preferred that such
a residue J is subjected to an appropriate treatment such as extraction to recover the catalyst component, and the recovered catalyst is subjected to regeneration treatment in a catalyst regeneration step for reuse.
According to this invention, in the production process for an aromatic carboxylic acid, at the time of concentrating the purged mother liquor, by utilizing the energy of the mother liquor for a part of concentration, it is possible to increase the recovery rate of useful components contained in the purged mother liquor. Further, by recycling the high temperature and high pressure mother liquor to the oxidation step to oxidize the alkylaromatic compound to the aromatic carboxylic acid, the energy to obtain a predetermined reaction condition can be reused, whereby the energy generated in the oxidation reaction can effectively be utilized. EXAMPLES
Now, the present invention will be described in detail with reference to Examples, but the present invention is by no means restricted by the following Examples. EXAMPLE 1
To 1 part by weight of p-xylene, 3.35 parts by weight of an acetic acid solution (water content: 14 wt%) containing catalysts (an acetic acid solution of cobalt acetate and manganese acetate, and hydrogen bromide), 6.24 parts by weight of a separated mother liquor
recycled from a later stage solid-liquid separation step and 3.57 Nm3 of air, were continuously supplied to an agitation tank, and an oxidation reaction was carried out while the liquid level was adjusted so that the residence time would be one hour at an operation temperature of 190°C under an operation pressure of 1.23 MPa (absolute pressure). Further, the distillate vapor was cooled by multi stage condensers finally to 40°C, and the operation was carried out so that the oxygen concentration in the exhaust gas was adjusted to be 2.5 vol%. Further, the condensed liquids obtained from the respective condensers were put together and refluxed to the oxidation reactor, and a part thereof was withdrawn so that the water content in the mother liquor as the withdrawn reaction slurry became 10 wt%. As a result, crude terephthalic acid in the slurry withdrawn from the oxidation reactor was 2.05 parts by weight; the mother liquor was 3.79 parts by weight; the cobalt/manganese/bromine concentrations in the reaction mother liquor were 300/300/1,000 ppm; and the slurry concentration was 35 wt%.
5.84 Parts by weight of the slurry withdrawn from the oxidation reactor was continuously supplied together with 0.08 Nm3 of air to an agitation tank, and a low temperature additional oxidation reaction was carried out while the liquid level was adjusted so that the residence time would be 15 minutes at an operation temperature of
181°C under an operation pressure of 1.15 MPa (absolute pressure). Further, the distillate vapor was cooled by multi stage condensers finally to 40°C, and the operation was carried out so that the oxygen concentration in the exhaust gas was adjusted to be 6 vol%. Further, the condensed liquids obtained from the respective condensers were put together and refluxed to the low temperature additional oxidation reactor.
The slurry withdrawn from the low temperature additional oxidation reactor was supplied to a screen bowl centrifuge and subjected to solid-liquid separation. Here, the operation pressure was 1.18 MPa (absolute pressure). The centrifugally separated crude terephthalic acid cake was washed with 2.99 parts by weight of acetic acid at the screen potion in the screen bowl centrifuge, and then the pressure was released all at once from the discharge valve to atmospheric pressure to evaporate the liquid adhered to the cake for drying to obtain 1.56 parts by weight of crude terephthalic acid crystals.
On the other hand, the separated mother liquor recovered by the solid-liquid separation was divided while the solid-liquid separation operation pressure was maintained to obtain 20% of a purged mother liquor, and 80% of the rest of the separated mother liquor was recycled to the oxidation reactor together with the washing waste water formed by the above washing as 6.24
parts by weight of a recycled mother liquor (mother liquor recycling rate: 80%).
0.91 part by weight of the purged mother liquor was supplied to the first concentration tank in such a state that it had a pressure of 1.18 MPa (absolute pressure) and a temperature of 183°C, and 0.35 part by weight of the solvent component in the mother liquor was evaporated by releasing the pressure to 0.05 MPa (absolute pressure) by means of a steam ejector. As a result, in the first condensation tank, the purged mother liquor was cooled to 90°C, and 0.01 part by weight of terephthalic acid dissolved in the purged mother liquor was precipitated. The slurry withdrawn from this first condensation tank was separated by a centrifugal separator to recover the solid content, whereby it was possible to recover 94% of terephthalic acid dissolved in the purged mother liquor. The solvent vapor distilled from the first condensation tank was condensed by a condenser, and 0.35 part by weight of the obtained condensed liquid was supplied to the oxidation reactor.
0.56 part by weight of the separated liquid after recovery of the solid content was supplied to the second condensation tank and externally heated by steam to carry out condensation treatment so that the solvent component was distilled and the concentration of impurities and catalyst as high boiling point components could be concentrated 20 times. The solvent vapor distilled from
the second condensation tank was supplied to the dehydration distillation column, and the acetic acid solvent was recovered. On the other hand, the concentrated liquid was intermittently withdrawn and further externally heated by means of a heating medium oil to evaporate the solvent remaining in the condensed liquid and to recover it for the second condensation tank thereby to obtain the residue.
The entire disclosure of Japanese Patent Application No. 2004-029478 filed on February 5, 2004 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.







We Claim:
1. A process for producing an aromatic carboxylic acid, which comprises:
(A) an oxidation step of oxidizing an alkylaromatic compound under 0.2 to 5 MPa absolute pressure in an acetic acid solvent in the presence of a catalyst at from 140 to 230°C to form an aromatic carboxylic acid,
(B) a solid-liquid separation step of subjecting a slurry containing crystals of the above aromatic carboxylic acid formed to solid-liquid separation into the aromatic carboxylic acid and a mother liquor under a pressure 0.2 to 1.5 MPa absolute pressure,
(C) a mother liquor recycling step of dividing the above mother liquor into two portions while the pressure is maintained at a pressure level where the operation pressure in a solid-liquid separation step (B) is substantially maintained, and returning one of the two portions to the above oxidation step (A), and
(D) a concentration step of concentrating the other portion of the mother liquor divided in the above mother liquor recycling step (C),
wherein in the above concentration step (D) , the concentration treatment is carried out by evaporating the mother liquor by releasing the pressure to a level of not higher than the vapor pressure of the mother liquor at the temperature of the mother liquor thereby to precipitate the aromatic carboxylic acid, and then the precipitated aromatic carboxylic acid is recovered.
2. The process as claimed in claim 1, wherein the concentration step (D)
comprises two stages of concentration, such that the mother liquor is sent to the first
stage concentration treatment under a pressure higher than atmospheric pressure and
evaporated by releasing the pressure to a level of not higher than the vapor pressure of
the mother liquor at the temperature of the mother liquor to carry out the first stage
concentration treatment thereby to precipitate the aromatic carboxylic acid, then the
precipitated aromatic carboxylic acid is recovered, and the mother liquor after

recovering the aromatic carboxylic acid is subjected to the second stage concentration treatment to evaporate and recover the solvent component.
3. The process as claimed in claim 1 or 2, wherein the recovered aromatic
carboxylic acid is returned to the oxidation step (A).
4. The process for producing an aromatic carboxylic acid as claimed in claim 1,
wherein the concentration step (D) comprises two stages of concentration,
such that the mother liquor is sent to the first stage concentration treatment under a pressure higher than atmospheric pressure and evaporated by releasing the pressure to level of not higher than the vapor pressure of the mother liquor at the temperature of the mother liquor to carry out the first stage concentration treatment thereby to precipitate the aromatic carboxylic acid, then the precipitated aromatic carboxylic acid is recovered and returned to the step (A), and the mother liquor after recovering the aromatic carboxylic acid is subjected to the second stage concentration treatment to evaporate and recover the solvent component.
5. The process as claimed in any one of claims 1 to 4, wherein in the mother liquor recycling step (C), said one portion of the mother liquor divided is returned to the oxidation step (A) while the pressure is maintained to be higher than atmospheric pressure.
6. The process as claimed in any one of claims 1 to 5, wherein the solvent component evaporated by the first stage concentration treatment and/or the second stage concentration treatment, is directly or indirectly returned to the step (A).
7. The process as claimed in any one of claims 1 to 6, wherein the solid-liquid separation step (B) includes a step of cleaning with a cleaning liquid the aromatic carboxylic acid obtained by the solid-liquid separation, and the above solid-liquid separation step and the above cleaning step arc carried out in a unified solid-liquid separation apparatus.
8. The process as claimed in claim 7, wherein the solid-liquid separation apparatus is a screen-bowl centrifuge.


9. The process as claimed in claim 7 or 8, wherein the cleaning liquid is acetic acid or water.
10. The process for producing an aromatic carboxylic acid as claimed in any one of claims 1 to 9, wherein the alkylaromatic compound is paraxylene. and the aromatic carboxylic acid is terephthalic acid.
11. A process for producing an aromatic carboxylic acid substantially as herein described with reference to the foregoing examples and the accompanying drawings.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=sfBg/q6Q+KPX4rLNunYWKA==&loc=+mN2fYxnTC4l0fUd8W4CAA==


Patent Number 270334
Indian Patent Application Number 4412/DELNP/2006
PG Journal Number 51/2015
Publication Date 18-Dec-2015
Grant Date 12-Dec-2015
Date of Filing 31-Jul-2006
Name of Patentee MITSUBISHI CHEMICAL CORPORATION
Applicant Address 33-8, SHIBA 5-CHOME, MINATO-KU, TOKYO 1080014 JAPAN
Inventors:
# Inventor's Name Inventor's Address
1 NUMATA MOTOKI C/O MITSUBISHI CHEMICAL CORPORATION, 1-1, KUROSAKISHIROISHI, YAHATANISHI-KU, KITAKYUSHU-SHI, FUKUOKA 8060004 JAPAN
2 OGATA TOMOHIKO C/O MITSUBISHI CHEMICAL CORPORATION, 1-1, KUROSAKISHIROISHI, YAHATANISHI-KU, KITAKYUSHU-SHI, FUKUOKA 8060004 JAPAN
3 ISOGAI TAKAYUKI C/O MITSUBISHI CHEMICAL CORPORATION, 1-1, KUROSAKISHIROISHI, YAHATANISHI-KU, KITAKYUSHU-SHI, FUKUOKA 8060004 JAPAN
4 FUKUI KATSUHIKO C/O MITSUBISHI CHEMICAL CORPORATION, 1-1, KUROSAKISHIROISHI, YAHATANISHI-KU, KITAKYUSHU-SHI, FUKUOKA 8060004 JAPAN
PCT International Classification Number C07C 51/43
PCT International Application Number PCT/JP2005/001683
PCT International Filing date 2005-02-04
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 2004-029478 2004-02-05 Japan