|Title of Invention||
A PROCESS FOR THE PRODUCTION OF PURIFIED TEREPHTHALIC ACID (PTA) FROM CRUDE TEREPHTHALIC ACID (CTA)
|Abstract||An improved process for the production of purified terephthalic acid (PTA) 5 from crude terephthalic acid (CTA) comprising mixing the CTA with an aqueous medium to form an aqueous CTA slurry, and heating the slurry to dissolve substantially all of the CTA in the aqueous medium by means of a hot liquid stream being introduced into the CTA slurry stream wherein the vapor pressure of the hot liquid stream is lower than the pressure of the CTA slurry.|
|Full Text||FORM 2
THE PATENTS ACT 1970
[39 OF 1970]
[See Section 10]
"PURE STAGE CONDENSATE INJECTION"
E.I. DU PONT DE NEMOURS AND COMPANY, a Corporation organized and existing under the laws of the State of Delaware, U.S.A., of 1007 Market Street, Wilmington, Delaware 19898, United States of America
The following specification particularly describes and ascertain the nature of the invention and the manner in which it is to be performed :-
TITLE PURE STAGE CONDENSATE INJECTION
BACKGROUND OF THE INVENTION
This invention relates to an improved process for the production of purified 5 terephthalic acid, and, more particularly, to a process improvement whereby a hot liquid stream is used to heat the crude terephthalic acid in aqueous medium slurry with the result that raw material and energy usages for the process arc reduced.
Purified terephthalic acid (PTA) may be obtained by purification of crude terephthalic acid (CTA) which is conventionally produced by the oxidation of p-
10 xylene in the presence of oxygen and a catalyst in a liquid phase comprising, for example, acetic acid. CTA typically contains impurities, such as catalyst residues and by-products of the oxidation of p-xylene, such as benzoic acid, p-toluic acid, trimeliitic acid and especially 4-carboxybenzaldehyde (4-CBA). CTA may also be produced by the hydrolysis of polyaikylene terephthalate, for example, polyethylene
The purification process conventionally involves mixing the CTA with an aqueous medium which, at relatively low temperatures, provides a slurry of CTA. The slurry of CTA is heated to dissolve the crude terephthalic acid crystals in the aqueous medium and thereby provide an aqueous terephthalic acid solution which
20 vvill also contain organic impurities, for example, 4-CBA. This aqueous solution is then contacted at elevated pressure and temperature with hydrogen, preferably in the presence of a catalyst, under reducing conditions to reduce chemically at least a proportion of the impurities, following hydrogen treatment, the terephthalic acid solution is subjected to a crystallization process in which the pressure and
25 temperature of the solution are decreased, resulting in the formation of PTA crystals, which are then recovered. Energy is required to heat the slurry of CTA in aqueous medium to at least a temperature at which the terephthalic acid dissolves in the said medium. Hitherto this energy has been derived, at least in part, by indirect heat transfer from a vapor or liquid stream derived from another stage of me terephthalic
30 ■ acid production process, usually by using a series of shell and tube heat exchangers. Japanese Patent No. 3 05 8155 discloses one proposal for using steam derived from the crystallization process in these heat exchangers.
Provision of such heat exchangers in a terephthalic acid production process for use with vapor or liquid heating streams can incur significant capital expense. 35 Furthermore, heat is transferred across a surface and inevitably heat loss is
encountered in such heat exchangers. There are further problems associated with using such heat exchangers in a terephthalic acid production process. They may suffer from fouling due to the fact that the process stream is a slurry of a solid in a liquid phase. European Patent No. 642491 discloses one proposal for overcoming 5 the problems of heat exchangers by introducing vapor directly into the CTA slurry to heat the slurry. This method is limited for introducing liquid only streams into the CTA slurry because of vibration and damage to equipment. It is well known that the said liquid streams can undergo further processing to derive vapor streams for use as described above, but this processing also incurs unwanted additional capital expense 10 and further degrades the energy and raw material value of the liquid stream.
The present invention provides means by which these problems may be avoided or reduced and enables liquid streams derived from other parts of the terephthalic acid production process to be introduced directly in the CTA slurry.
SUMMARY OF THE INVENTION
15 The present invention relates to a process for the production of purified
terephthalic acid, and, more particularly, to a process improvement whereby a hot liquid stream used to heat the CTA aqueous slurry is first cooled to a temperature such that the vapor pressure of the hot liquid stream is Lower than the pressure of the CTA slurry at the point of mixing, with the result that energy from the hot liquid
20 stream can be recovered without substantial loss or risk of equipment damage.
Cooling of the hot liquid stream may be carried out in an indirect heat exchanger; in the subcooling section of a vapor heat exchanger; by mixing with a colder water stream; or by any combination of the above.
Preferably, the CTA slurry is caused to flow in the form of a stream, and the 25 hot liquid is introduced into the flowing stream. The CTA slurry stream may be heated in a plurality of stages to maximize the energy recovery of the hot liquid streams available.
The hot liquid stream is available from condensing vapor from another stage in the terephthalic acid production process, for example, the crystallization stage of 30 the process, but it may also be obtained from a source outside the process, for example, from available utility steam formed by heating water in a boiler.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described by way of example with reference to the
Fig 1 is a simplified schematic diagram of the relevant portion of the terephthalic acid production process with the hot liquid stream being cooled by an indirect heat exchanger.
Fig 2 is the same simplified diagram of the process but with the hot liquid 5 stream being cooled in the subcooling section of a vapor heat exchanger.
DETAILED DESCRIPTION OF THE INVENTION
In the production of purified terephthalic acid (PTA) from crude terephthalic acid (CTA), the commercial process typically comprises the steps of (i) mixing the
10 CTA with an aqueous medium to form an aqueous CTA slurry, (ii) heating the slurry to dissolve substantially all of the CTA in the aqueous medium and thereby produce a solution of CTA, (iii) hydrogenating the aqueous CTA solution to reduce chemically at least some of the organic impurities originally present in the CTA and to form an aqueous PTA stream, and (iv) decreasing the pressure and temperature of
15 the aqueous PTA stream to effect crystallization of PTA, said heating of the CTA slurry being effected by a variety of methods. This invention provides a means of utilizing hot liquid streams within the terephthalic acid production process for heating said CTA slurry by cooling the liquid stream prior to mixing with the CTA slurry stream, i.e., cooling to a temperature such that the vapor pressure o f the hot
20 liquid stream is below the pressure of said CTA slurry stream at the point of mixing.
The control of cooling of the hot liquid stream can be accomplished by methods well known in the process industry. Similarly, methods of introducing the liquid stream into the CTA slurry stream are well known-Referring now to Fig 1, CTA crystals (line 1) are mixed with an aqueous 25 stream (line 2), and the resultant slurry is pumped tlirough line 3 to Hydrogenation Reactor 5 where the CTA aqueous solution is hydrogenated to remove at least some of the impurities, particularly 4-CBA, to form an aqueous PTA stream 6 which is reduced in temperature and pressure through a series of crystallization stages 7 to form PTA crystals in aqueous slurry S.
30 Slurry stream 3 is heated by a variety of methods shown schematically as a
single heat exchanger 4 but which may include a plurality of heat exchangers heated by vapor or liquid streams supplied from utility supplies or derived from streams in other parts of the terephthalic acid process, such as crystal ligation. Additionally heating may include, in part, vapor injection, using vapor derived as above.
Hot liquid, line 11, is collected and fed to cooling device 12. The hot liquid is derived from condensation of a vapor stTeam which has previously been used in a heat exchanger in another part of the terephthalic acid production process. The illustrated cooling device is a heat exchanger, the details of which are well known in 5 the process industry and so will not be described in further detail herein. The
cooling stream in this illustrated device may be utility cooling water or some other stream derived from within the terephthalic acid process, for example, aqueous CTA slurry line 3 described above. The cooling device is given by way of example only; other cooling processes well known in the industry are applicable, for example, 10 direct mixing of a cool liquid stream.
Hot liquid stream 11 is cooled 12 to a temperature such that the vapor pressure of the hot liquid stream is below the pressure of said CTA slurry stream at the point of mixing. Cooled liquid stream 13 is introduced into CTA slurry stream 3 to effect heating of said stream. Any convenient method for mixing the two streams
15 may be used. Mixing methods of the type -which may be used in carrying out the invention are well known to the industry and will not be described further herein. By way of illustration, line 13 is shown as a single point of introduction into line 3. In practice, however, mixing may be accomplished via a plurality of points on line 3. Further, in order to maximize the heating effect of CTA slurry stream 3 and recover
20 as much energy as possible from available hot liquid streams i I, a plurality of hot liquid processing systems (including line 11, cooling device 12 and cooled liquid line 13) may be used to feed a plurality of introduction points on line 3.
Referring to Fig. 2, the main terephthalic acid production process is the same as described for Fig. 1, above. In this embodiment, liquid introduced via line 13 into
25 CTA slurry line 3 is cooled in a sabcooling section of vapor heat exchanger 15, the details of which are well known in the industry and will not be described further herein. Vapor line 14 for heat exchanger J 5 may be derived from a number of sources, including utility steam or vapor from other parts of the terephthalic acid process. By way of illustration, vapor heat exchanger 15 in Fig. 2 is shown as a
30 heating device on CTA slurry line 3, but equally the heat exchanger may be a device used in another part of the terephthalic acid process.
Vapor stream 15 is condensed and subcooled to a temperature such that the vapor pressure of liquid stream 13 is below the pressure of CTA slurry stream 3 at the point of mixing. The now cooled liquid stream 13 is introduced into CTA slurry 35 stream 3 to effect heating of said stream. As described in connection with Fig. 1, a variety of methods of mixing the two streams may be used, a plurality of mixing
points may be used and a plurality of cooled liquid streams 13 may be used deriving from a piuraiity of heat exchange devices 15 and vapor streams 14.
1. In a process for the production of purified terephthalic acid (PTA) from crude terephthalic acid (CTA) which contains at least one organic impurity and 5 comprises the steps of:
a) mixing the CTA with an aqueous medium to form an aqueous slurry;
b) heating the aqueous slurry to dissolve substantially all of the CTA in the aqueous medium;
c) passing the resulting solution with hydrogen over a hydrogenation
10 catalyst to reduce chemically at least some of the said organic impurity
thereby forming an aqueous PTA stream;
d) decreasing the temperature and pressure of the aqueous PTA stream to
effect crystallization of PTA;
the improvement comprising heating said CTA slurry formed in step b) at least in 15 part by introducing thereinto a hot liquid stream at a temperature such that the vapor pressure of the hot liquid stream is lower then the pressure of said CTA slurry at the point of mixing.
2. The process as claimed in Claim 1in which the CTA slurry is caused
to flow in the form of a stream, and said hot liquid is introduced into the flowing
20 CTA slurry stream.
3. A process as claimed in Claim 2 in which the CTA slurry stream is heated by said hot liquid in a plurality of heating stages.
4. A process as claimed in Claim 3 in which said hot liquid is cooled in a plurality of cooling stages before being introduced into said CTA slurry stream.
25 5. A process as claimed in Claim 4 in which said hot liquid is cooled by
indirect exchange with said aqueous CTA slurry.
6. A process for the production of purified terephthalic acid substantially as herein described with reference to the accompanying drawings.
Dated this 10th
day of August, 2001.
OF REMFRY & SAGAR
AGENT FOR THE APPLICANTS
|Indian Patent Application Number||788/MUM/2001|
|PG Journal Number||14/2011|
|Date of Filing||10-Aug-2001|
|Name of Patentee||E.I. DUPONT DE NEMOURS AND COMPANY|
|Applicant Address||1007 MARKET STREET, WILMINGTON, DELAWARE 19898, UNITED STATES OF AMERICA|
|PCT International Classification Number||C07C63/26|
|PCT International Application Number||N/A|
|PCT International Filing date|