FORM-2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2006
COMPLETE
Specification
(See Section 10 and Rule 13)
IMPROVED PROCESS FOR SEPARATING CARBOXYLIC ACID FROM WATER IN AZEOTROPIC DISTILLATION
RELIANCE INDUSTRIES LIMITED
an Indian Company
of 3rd Floor, Maker Chamber - IV,
222, Nariman Point, Mumbai - 400 021,
Maharashtra, India
Inventors:
a. UKILTAMAGNA
b. GARIMELLA PADMAVATHI
c. SAGOTRA PANKAJ
d. MATHEW THOMAS
e. JASRA RAKSHVIR.
The following specification particularly describes the invention and the manner in which
it is to be performed.

FIELD OF DISCLOSURE
The present disclosure relates to a process for the production of terephthalic acid by liquid phase oxidation process. Particularly, the present disclosure relates to a process for the recovery of an aliphatic carboxylic acid from water from a stream containing the aliphatic carboxylic acid, water, a hydrocarbon, and an entrainer.
BACKGROUND
Terephthalic acid is an aromatic carboxylic acid which is an important intermediate in the production of polyester polymers which are typically used in fibre production and in the manufacture of bottles. Terephthalic acid is commercially produced by liquid phase partial oxidation of paraxylene (p-xylene) using an oxygen containing gas in a solvent comprising lower C2 to C6 aliphatic monocarboxylic acid, usually acetic acid, in the presence of dissolved heavy metal catalyst system incorporating a promoter such as bromine. The reaction is carried out in a stirred single stage or multiple vertical stage reactor at a temperature between 150 to 250 °C and a pressure of about 6 to 30 bar. P-xylene and the acid solvent are continuously introduced at the top of the reactor and oxygen-containing gas is preferably introduced into the lowest stage or into each stage. The oxygen dissolves in the solvent and reacts with the p-xylene to form terephthalic acid in high yield, at least 95%.
Isothermal reaction conditions are maintained in the oxidation vessel by allowing evaporation of the solvent, together with water produced in the reaction. Carbon oxides are also formed by over oxidation of the p-xylene and oxidation of the acetic acid solvent. These products rise to the top of the reactor vessel and are continuously withdrawn there from. The withdrawn vapor stream is condensed and the condensate is then subjected to distillation to separate the water from the acetic acid. The dehydrated acetic

acid is then recycled at least in part to the reactor vessel. A principal drawback associated with this conventional process is that gaseous hydrocarbon derivative by-products, such as methyl acetate, and other hydrocarbon components including some of the precursor (p-xylene) and alkyl aromatic impurities like toluene are carried in significant quantities through the vapor phase.
Traditionally, fractional distillation has been used for separating the water from the acetic acid since the oxidation process produces significant waste heat which is available for use as reboil heat for the distillation column. However, with the development of low pressure processes for the manufacturing of terephthalic acid and need for more efficient heat recovery systems, azeotropic distillation has been identified as an advantageous and cost-effective method over fractional distillation. Heterogeneous azeotropic distillation is used for separating the water from the acetic acid, and the precursor is recovered as a purge and recycled to the oxidation reactor vessel. The method involves adding an additional agent called an entrainer to the acetic acid-water system. The entrainer affects the volatility of one of the constituents (azeotrope), and when the mixture is distilled the azeotrope constituent (water) vaporizes giving acetic acid having low water content from the base of the distillation column. The azeotropic distillation provides low reflux ratio and hence reduced heat energy is required for the distillation. The reflux ratio is depended on the entrainer selected for the distillation. The entrainer used in an acetic acid-water system is typically alkyl acetate such as isobutyl acetate, n-butyl acetate, n-propyl acetate, and the like.
Some of the entrainer is also removed along with the purge stream of the precursor. Toluene is highly miscible in alkyl acetate and at least a portion

of toluene forms an azeotrope with water and the entrainer. The purge stream can be recycled to the oxidation reactor in which case a portion of the entrainer containing the toluene will pass into the overhead vapor stream and following condensation along with the water, the acetic acid and the precursor will again be passed to the distillation column. Although conversion of the entrainer and toluene into other compounds will occur in the oxidation reactor vessel, these compounds do not affect the quality of the terephthalic acid to any significant extent; nonetheless, this will result in loss of the entrainer which will require continual fresh addition of the entrainer to the distillation column. However, the amount of toluene impurity will increase and it is found that after a certain quantity the presence of toluene affects the efficiency of the azeotropic distillation process.
Dehydration of acetic acid by azeotropic distillation is disclosed in US Pat. No. 5,980,696 and CA Pat. No. 2,247,291. The US Pat. No. 5,980,696 claims an azeotropic distillation process which comprises feeding to the azeotropic distillation column a stream having water content in the range of 20 - 40 % by weight based on the combined weight of the aliphatic carboxylic acid and water in the feed stream, so that the bottom product is substantially free of the entrainer and contains water in the range of 2 - 12 % by weight based on the combined weight of the aliphatic carboxylic acid and water in the bottoms product. The process further discusses the detrimental effect of p-xylene on the performance of the distillation column, where p-xylene which has an ability to form an azeotrope with water, and thereby accumulate in the distillation column. This process further discloses reducing the toluene impurity content by purging p-xylene from the column at a location just above the point of introduction of the feed stream to the azeotropic distillation column. The process does not provide separate draw-offs for the p-xylene and the toluene. Also, no treatment is provided to

remove toluene and p-xylene impurity from the entrainer. The CA Pat. No. 2,247,291 discloses a process for reducing the loss of the entrainer as a result of purging of the precursor from the distillation column. A purge stream containing the hydrocarbon, the carboxylic acid, the entrainer and water is withdrawn from the azeotropic zone of the distillation column and the entrainer is recovered by subjecting the purge stream to stripping in a stripping column to obtain a bottom product including the hydrocarbon as a major constituent and a top product containing the entrainer as the major constituent. The process does not disclose the removal of toluene impurities from the entrainer and the entrainer will carry these impurities in the top product thereof.
Thus, the problem associated with toluene and p-xylene accumulation, in the azeotropic distillation column used for separating acetic acid and water, which subsequently affects the efficiency of the azeotropic distillation column, has not been overcome in the past.
OBJECTS
It is therefore an object of the present disclosure to provide a simple, effective and economical process for dehydrating an aliphatic carboxylic acid by azeotropic distillation. The process is typically provided in combination with liquid phase oxidation processes used in the production of an aromatic carboxylic acid such as terephthalic acid. The alkyl aromatic impurities coming from the precursor of the liquid phase oxidation processes are maintained at a desired concentration in the azeotropic distillation column so as to improve the operation of the azeotropic distillation column and enhance the efficiency of the entrainer.

Another object of the present disclosure is to provide an azeotropic distillation process for dehydrating an aliphatic carboxylic acid by which aliphatic carboxylic acid with minimal water content is obtained.
Yet another object of the present disclosure is to provide an azeotropic distillation process for dehydrating an aliphatic carboxylic acid by which loss of the entrainer is minimized.
These objects and other advantages of the present disclosure will be more apparent from the following description.
SUMMARY
In accordance with the present disclosure, there is provided a process for the azeotropic distillation of a feed stream (A) comprising water, at least one aliphatic carboxylic acid, at least one alkyl carboxylate, and at least one alkyl aromatic compound, said process comprising the following steps:
• azeotropically, distilling said stream (A) in a first azeotropic distillation column (102), to obtain: (a) a relatively water-lean aliphatic carboxylic acid stream (C) from a location proximal to the operative lower end of the column (102) (b) at least one acid-lean vapor stream (P, D) withdrawn from at least one location in the distillation column (102) above the location for drawing stream (C), said stream(s) (P, D) comprising said alkyl carboxylate(s), said alkyl aromatic compound(s), and water;
• condensing selectively at least one of the streams (P, D) in a first condenser (108) to obtain a condensate stream comprising said alkyl carboxylate(s) and said alkyl aromatic compound(s);
• separating said condensate stream in a phase separator (114) to obtain an organic phase (114a) comprising said alkyl carboxylate(s) and said

alkyl aromatic compound(s) and an aqueous phase (114b) comprising the water; and • streaming said organic phase (114a), based on the concentration of said alkyl aromatic compound(s) in said organic phase (114a), to perform at least one operation selected from returning at least a portion of said organic phase (114a) to said first distillation column (102) as reflux (E) and feeding at least a portion of said organic phase (114a) to a second azeotropic distillation column (112) in which, said alkyl aromatic compound(s) are removed from said alkyl carboxylate(s), to obtain a purified alkyl carboxylate(s) stream (K) and vapor stream (H) comprising said alkyl aromatic compound(s).
Typically, said aliphatic carboxylic acid is acetic acid.
Typically, said alkyl carboxylate is at least one alkyl carboxylate selected from the group consisting of methyl acetate, butyl formate, amyl formate, n-propyl acetate, n-butyl acetate, iso-butyl acetate, allyl acetate, n-propyl propionate, iso-propyl propionate, n-butyl propionate, iso-butyl propionate, and mixtures thereof.
Typically, said alkyl carboxylate is n-propyl acetate.
Typically, wherein said alkyl aromatic compound is at least one alkyl aromatic compound selected from p-xylene and toluene.
Typically, said alkyl aromatic compound is toluene.
Typically, the process comprises withdrawing said acid-lean vapor stream (D) from the operative top of said first azeotropic distillation column (102)

and said acid-lean vapor stream (P) from a location along the operative length of said first azeotropic distillation column (102).
Typically, the process comprises adding a fresh portion of said alkyl carboxylate(s) to said organic phase (114a) in said phase separator (114).
Typically, said reflux (E) is fed at a location proximal to the operative top of said first azeotropic distillation column (102).
Typically, the process comprises feeding at least a portion of said purified alkyl carboxylate(s) stream (K) to said first azeotropic distillation column (102).
Typically, said alcohol entrainer is a lower alcohol, preferably methanol.
Typically, the process comprises condensing said alcohol entrainer vapor stream (H) in a second condenser (116) to obtain a condensate stream containing said alcohol entrainer and said alkyl aromatic compound(s).
Typically, the process comprises collecting said condensate stream containing said alcohol entrainer and said alkyl aromatic compound(s) in a collection tank (118) from where at least a portion of said condensate stream is returned as reflux (I) at a location proximal to the operative top of said second azeotropic distillation column (112).
Typically, the process comprises adding said alcohol entrainer to said organic phase (114a) prior to subjecting to azeotropic distillation in said second azeotropic distillation column (112).
Typically, said purified alkyl carboxylate(s) stream (K) is substantially-free of said alkyl aromatic compound(s).

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
The disclosure will now be described with the help of the accompanying drawing, in which,
FIGURE 1 illustrates a schematic view of the azeotropic distillation system in accordance with the present disclosure.
DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWING
The disclosure will now be described with reference to the accompanying drawing which does not limit the scope and ambit of the disclosure. The description provided is purely by way of example and illustration.
The present disclosure envisages an improved process for the azeotropic distillation of aliphatic carboxylic acid-water system. The present process is typically provided in combination with liquid phase oxidation processes for the production of an aromatic carboxylic acid such as terephthalic acid. Terephthalic acid is produced by oxidizing an alkyl aromatic precursor such as p-xylene in an aqueous liquid phase medium of a lower aliphatic carboxylic acid like acetic acid and in the presence of heavy metal catalyst incorporating a bromine promoter. The oxidation process results in the production of a vapor stream comprising the aliphatic carboxylic acid, water, the precursor and impurities like ester derivatives of the carboxylic acid such as methyl acetate. This vapor stream is condensed to obtain a liquid stream which is subjected to azeotropic distillation using an alkyl carboxylate entrainer to produce a relatively water-lean aliphatic carboxylic acid stream which is returned as reflux for the oxidation process. The precursor (p-xylene) comprises some toluene as an impurity. This impurity is carried to the azeotropic distillation column through the liquid stream and since

toluene is highly miscible in the alkyl carboxylate entrainer it remains with the entrainer and a portion of this toluene is carried by the entrainer reflux stream to the distillation column. This way, over a period the toluene and p-xylene impurities accumulate in the distillation column affecting the efficiency of the distillation column. The present process aims at preventing the accumulation of these alkyl aromatics including toluene and p-xylene in the azeotropic distillation column, thereby improving the operation of the distillation column and enhancing the efficiency of the entrainer. In accordance with the present disclosure, a bleed stream from the entrainer reflux stream to the distillation column is treated by azeotropic distillation using a second entrainer to remove the toluene impurity.
A schematic view of the azeotropic distillation system in accordance with the present disclosure is illustrated in the FIGURE 1 of the accompanying drawings. The azeotropic distillation system comprises: a first azeotropic distillation column 102, a first reboiler 104, a first condenser 108, a phase separator 114, a pump 106, a second azeotropic distillation column 112, a feed vessel and second reboiler 110 in communication with the second azeotropic distillation column 112, a second condenser 116 and a collection tank 118. The azeotropic distillation columns 102 & 112 are packed columns. A feed stream (A) comprising 40 to 80% of an aliphatic carboxylic acid such as acetic acid, and remaining mostly water, with at least one compound from 0 to 1% of an ester derivative of the aliphatic carboxylic acid such as methyl acetate, an alkyl carboxylate entrainer, typically n-propyl acetate, and alkyl aromatics including 0 to 1% p-xylene and less than 1 ppm toluene, is fed to the first azeotropic distillation column 102. A reflux stream (E) mainly containing the entrainer is fed at the operative top of the distillation column. The feed stream (A) is azeotropically distilled to produce a relatively water lean-aceticVacid stream (C) containing at least 90

% acetic acid, at the operative bottom of the column 102. A portion of the bottom stream (B) is recirculated to the column 102 via the first reboiler 104. A relatively acetic acid-lean water vapor stream (D) containing at least one compound selected from the ester derivative of the aliphatic carboxylic acid, the alkyl carboxylate entrainer, and at least a portion of the alkyl aromatics is removed as an overhead vapor stream. Further, a portion of the alkyl aromatics may be purged from the first azeotropic distillation column 102 at a location along the operative length of the first azeotropic distillation column 102 above the relatively water-lean acetic acid stream (C).
The vapor stream (D) is condensed in the first condenser 108 to produce an acetic acid-lean condensate stream containing at least one compound selected from the ester derivative of the aliphatic carboxylic acid, the alkyl carboxylate entrainer, and at least a portion of the alkyl aromatics. The liquid stream is received in the phase separator 114. In the phase separator 114, an organic phase 114a is separated from an aqueous phase 114b. The organic phase 114a contains the entrainer, the alkyl aromatics, and a portion of the ester derivative and the aqueous phase 114b contains water and a portion of the ester derivative of the carboxylic acid. Fresh entrainer is added to the organic phase 114a in the phase separator 114. At least a portion of the organic phase 114a is returned to the first distillation column 102 proximal to the operative top of the column through reflux line (E) via the pump 106. The aqueous phase 114b is removed via discharge line (F).
Over a period of time, the toluene concentration can rise to more than 1% in the first azeotropic distillation column 102 and can increase to as high as 50% by weight in the entrainer reflux stream (E). This toluene is difficult to separate from the entrainer and therefore decreases the efficiency of the entrainer and affects the operation of the first azeotropic distillation column

102. It is thus necessary to suppress the concentration of toluene to less than 25% by weight in the entrainer reflux stream (E). To prevent the accumulation of toluene in the reflux line (E) to the first distillation column 102, selectively a portion of the organic phase 114a is passed to the second azeotropic distillation column 112 via the feed vessel and the second reboiler 110 as an organic stream (G). An alcoholic entrainer is used in the second azeotropic distillation column 112 and is typically methanol. Toluene being highly miscible in the alkyl carboxylate entrainer is difficult to remove by steam stripping or solvent extraction. However, when the organic stream (G) containing the alkyl carboxylate entrainer and the alkyl aromatics (toluene) is azeotropically treated by using methanol entrainer, the toluene is easily extracted with methanol. A purified alkyl carboxylate entrainer stream (K) is obtained from the operative bottom of the second azeotropic distillation column 112. At least a portion of the treated entrainer stream (K) is pumped by pumping means 120 to the reflux line (E) for feeding to the first azeotropic distillation column 102. The overhead vapor stream (H) from the second distillation column 112, containing the methanol and the toluene, is condensed in the second condenser 116 and the condensate is collected in the collection tank 118. A portion of the condensed stream can be fed as reflux (I) at the operative top of the second azeotropic distillation column 112. A portion of the condensed stream is disposed through line (J). Fresh methanol can be fed in the organic stream (G) prior to azeotropic distillation. The purified alkyl carboxylate entrainer stream (K) is substantially free of toluene. In accordance with the present disclosure, the concentration of the toluene in the alkyl carboxylate entrainer in the organic phase 114a is monitored. When the concentration rises to a critical level, the second azeotropic distillation column 112 is put in operation. The second azeotropic distillation column 112 is operated till the concentration of the toluene is reduced to a level suitable for the reflux stream (E).

TECHNICAL ADVANTAGES
An azeotropic distillation process for dehydrating an aliphatic carboxylic acid, as described in the present disclosure has several technical advantages including but not limited to the realization of: the disclosure provides a simple, effective and economical method for dehydrating the aliphatic carboxylic acid by azeotropic distillation; the process is typically provided in combination with liquid phase oxidation processes used in the production of an aromatic carboxylic acid such as terephthalic acid, where the alkyl aromatic impurities coming from the precursor are maintained at a desired concentration in the azeotropic distillation column so as to improve the operation of the azeotropic distillation column and enhance the efficiency of the entrainer. The process of the present disclosure provides aliphatic carboxylic acid with minimal water content and minimizes the entrainer losses.
Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
The use of the expression "at least" or "at least one" suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the invention to achieve one or more of the desired objects or results.
Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of

providing a context for the invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the invention as it existed anywhere before the priority date of this application.
The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the invention, unless there is a statement in the specification specific to the contrary. In view of the wide variety of embodiments to which the principles of the present invention can be applied, it should be understood that the illustrated embodiments are exemplary only. While considerable emphasis has been placed herein on the particular features of this invention, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principle of the invention.
These and other modifications in the nature of the invention or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

WE CLAIM;
1. A process for azeotropic distillation of a feed stream (A) comprising water, at least one aliphatic carboxylic acid, at least one alkyl carboxylate, and at least one alkyl aromatic compound, said process comprising the following steps:
i. azeotropically, distilling said stream (A) in a first azeotropic distillation column (102), to obtain: (a) a relatively water-lean aliphatic carboxyiic acid stream (C) from a location proximal to the operative Jower end of the column (102) (b) at least one acid-lean vapor stream (P, D) withdrawn from at least one location in the distillation column (102) above the location for drawing stream (C), said stream(s) (P, D) comprising said alkyl carboxylate(s), said alkyl aromatic compound(s), and water;
ii. condensing selectively at least one of the streams (P, D) in a first condenser (108) to obtain a condensate stream comprising said alkyl carboxylate(s) and said alkyl aromatic compound(s);
iii. separating said condensate stream in a phase separator (114) to obtain an organic phase (114a) comprising said alkyl carboxylate(s) and said alkyl aromatic compound(s) and an aqueous phase (114b) comprising the water; and
iv. streaming said organic phase (114a), based on the concentration of said alkyl aromatic compound(s) in said organic phase (114a), to perform at least one operation selected from returning at least a portion of said organic phase (114a) to said first distillation column (102) as reflux (E) and feeding at least a portion of said organic phase (114a) to a second azeotropic

distillation column (112) in which, said alkyl aromatic compound(s) are removed from said alkyl carboxylate(s), to obtain a purified alkyl carboxylate(s) stream (K) and vapor stream (H) comprising said alkyl aromatic compound(s).
2. The process as claimed in claim 1, wherein said aliphatic carboxylic acid is acetic acid.
3. The process as claimed in claim 1, wherein said alkyl carboxylate is at least one alkyl carboxylate selected from the group consisting of methyl acetate, butyl formate, amyl formate, n-propyl acetate, n-butyl acetate, iso-butyl acetate, allyl acetate, n-propyl propionate, iso-propyl propionate, n-butyl propionate, iso-butyl propionate, and mixtures thereof.
4. The process as claimed in claim 3, wherein said alkyl carboxylate is n-propyl acetate.
5. The process as claimed in claim 1, wherein said alkyl aromatic compound is at least one alkyl aromatic compound selected from p-xylene and toluene.
6. The process as claimed in claim 1, wherein said alkyl aromatic compound is toluene.
7. The process as claimed in claim 1, which comprises withdrawing said acid-lean vapor stream (D) from the operative top of said first azeotropic distillation column (102) and said acid-lean vapor stream (P) from a location along the operative length of said first azeotropic distillation column (102).

8. The process as claimed in claim 1, which comprises adding a fresh portion of said alkyl carboxylate(s) to said organic phase (114a) in said phase separator (114).
9. The process as claimed in claim 1, wherein said reflux (E) is fed at a location proximal to the operative top of said first azeotropic distillation column (102).
10. The process as claimed in claim 1, which comprises feeding at least a portion of said purified alkyl carboxylate(s) stream (K) to said first azeotropic distillation column (102).
11. The process as claimed in claim 1, wherein said alcohol entrainer is a lower alcohol, preferably methanol.
12. The process as claimed in claim 1, which comprises condensing said alcohol entrainer vapor stream (H) in a second condenser (116) to obtain a condensate stream containing said alcohol entrainer and said alkyl aromatic compound(s).
13. The process as claimed in claim 12, which comprises collecting said condensate stream containing said alcohol entrainer and said alkyl aromatic compound(s) in a collection tank (118) from where at least a portion of said condensate stream is returned as reflux (I) at a location proximal to the operative top of said second azeotropic distillation column (112).
14. The process as claimed in claim 1, which comprises adding said alcohol entrainer to said organic phase (114a) prior to subjecting to azeotropic distillation in said second azeotropic distillation column (112).

15. The process as claimed in claim 1, wherein said purified alkyl carboxylate(s) stream (K) is substantially-free of said alkyl aromatic compound(s).