WO2016059127A1

WO2016059127A1 - Integrated purified terephthalic acid production and polyester polymerization plant with condensate recycling

Info

Publication number: WO2016059127A1
Application number: PCT/EP2015/073810
Authority: WO
Inventors: Alexander Stuart Coote; Robert Edward Neate
Original assignee: Invista Technologies S.À R.L.
Priority date: 2014-10-16
Filing date: 2015-10-14
Publication date: 2016-04-21
Also published as: CN105985515A; GB201418362D0; CN204644246U

Abstract

The present invention provides a method for the production of purified terephthalic acid crystals in an integrated terephthalic acid – polyester plant comprising the steps of: i) providing a stream of purified terephthalic acid crystals in an aqueous medium; ii) separating the purified terephthalic acid crystals from the aqueous medium in a filtration zone; and iii) washing the purified terephthalic acid crystals with an aqueous wash liquid, characterised in that at least a portion of the aqueous wash liquid is derived from an aqueous condensate liquor from the polyester plant.

Description

INTEGRATED PURIFIED TEREPHTHALIC ACID PRODUCTION AND POLYESTER

POLYMERIZATION PLANT WITH CONDENSATE RECYCLING

TECHNICAL FIELD

The present invention relates to the operation of a combined terephthalic acid - polyester plant. BACKGROUND ART

Aromatic dicarboxylic acids are commonly manufactured by the catalytic oxidation of a hydrocarbon precursor in an organic solvent. An example is terephthalic acid (TA), which is widely used in the manufacture of polyesters, such as poly(ethylene terephthalate) (PET). The TA required as a reactant for PET production is known as "purified terephthalic acid" (PTA) and generally contains over 99.97 wt%, preferably over 99.99 wt%, of terephthalic acid, and less than 25 ppm 4- carboxybenzaldehyde (4-CBA). On the commercial scale, PTA suitable for use in PET production is generally prepared in a two-stage process. First, p-xylene is oxidized (e.g. using air) in the presence of a metal catalyst (e.g. a cobalt and/or manganese salt or compound) to provide "crude terephthalic acid" (CTA), as described in, for example, US 2,833,816. Second, the CTA produced by this oxidation reaction is then purified, as it is typically contaminated by impurities such as 4-CBA, p-toluic acid, and various coloured impurities that impart a yellowish colour to the TA. Purification of the CTA typically requires at least one chemical transformation (e.g. hydrogenation) in addition to at least one physical procedure (e.g. crystallization, washing, etc.) to yield PTA. PET is thus formed by an esterification reaction between ethylene glycol and PTA.

PET is generally considered to be a commodity item, with several tens of millions of tonnes being produced annually, and it is therefore desirable for manufacturers to reduce their costs to maximise the economy and efficiency of its production. This can be achieved both by reducing capital costs (e.g. equipment costs) and variable costs (e.g. costs associated with waste disposal, use of starting materials, organic solvent, heating fuel and demineralised water).

A combined PTA - PET plant may use demineralised water in a number of process steps, including the washing procedure for the preparation of PTA mentioned above. In addition, a combined PTA - PET plant produces a number of aqueous waste streams, which are not only costly to treat and dispose of but may also be detrimental to the environment. United Kingdom Patent Application No. 1507465.1 , which is incorporated by reference in its entirety, discloses methods for purifying aqueous waste streams from an integrated PTA - PET plant and recycling the purified water back into the integrated PTA - PET plant, in particular to form a slurry of CTA prior to its purification. It is an object of the present invention to provide a more economic and efficient process for the combined manufacture of terephthalic acid and polyester and, in particular, to minimise the demineralised water usage of a combined terephthalic acid - polyester plant and/or the aqueous waste produced by the combined terephthalic acid - polyester plant, thus reducing variable costs and the impact on the environment. Further objects will be apparent from the description below. DISCLOSURE OF THE INVENTION

The present invention provides a method for the production of purified terephthalic acid crystals in an integrated terephthalic acid - polyester plant comprising the steps of:

i) providing a stream of purified terephthalic acid crystals in an aqueous medium;

ii) separating the purified terephthalic acid crystals from the aqueous medium in a filtration zone; and

iii) washing the purified terephthalic acid crystals with an aqueous wash liquid,

characterised in that at least a portion of the aqueous wash liquid is derived from an aqueous condensate liquor from the polyester plant. The present inventors have found that an aqueous wash liquid in which at least a portion is derived, directly or indirectly, from an aqueous condensate liquor from the polyester plant is suitable for washing purified terephthalic acid crystals, i.e. the purified terephthalic acid crystals washed in this aqueous wash liquid are suitable for use on the PET plant. Accordingly, the aqueous effluent produced by the integrated PTA - PET plant is reduced as the aqueous condensate liquor is recycled rather than being discharged, and the use of demineralised water in making up the aqueous wash liquid may be reduced, thus reducing variable costs. In addition, the aqueous condensate liquor typically leaves the polyester plant at a temperature of about 120 ^eC and thus may serve as a heat source on the terephthalic acid plant, further reducing variable costs. DETAILED DESCRIPTION OF THE INVENTION

Various embodiments of the invention are described herein. It will be recognised that features specified in each embodiment may be combined with other specified features to provide further embodiments. It will be appreciated that the general operation of an integrated terephthalic acid - polyester plant and process is well known. For example, as discussed above, terephthalic acid suitable for use in poly(ethylene terephthalate) production (i.e. purified terephthalic acid) is generally prepared in a two- stage process. First, p-xylene is oxidized (e.g. using air) in the presence of a metal catalyst (e.g. a cobalt and/or manganese salt or compound) to provide crude terephthalic acid. Second, the crude terephthalic acid produced by this oxidation reaction is then purified to remove impurities, such as 4-CBA and p-toluic acid, to yield purified terephthalic acid. Purification of crude terephthalic acid typically requires at least one chemical transformation (e.g. hydrogenation) in addition to at least one physical procedure (e.g. crystallization, washing, etc.). Poly(ethylene terephthalate) is formed by an esterification reaction between ethylene glycol and the purified terephthalic acid. Production of terephthalic acid

The terephthalic acid is typically produced by a process comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent. The hydrocarbon precursor is a compound that may be oxidised to form the terephthalic acid. Thus, the hydrocarbon precursor is typically benzene substituted with groups such as Ci-₆alkyl, formyl, or acetyl in the positions of the carboxylic acid substituents in terephthalic acid. Preferred hydrocarbon precursors are Ci-₆alkyl-substituted benzene, in particular p-xylene. The organic solvent is typically an aliphatic carboxylic acid, such as acetic acid, or a mixture of such aliphatic carboxylic acid(s) and water. The oxidation reaction may be carried out under any conditions wherein oxygen is available, e.g. the reaction can be carried out in air. The reaction catalyst typically comprises soluble forms of cobalt and/or manganese (e.g. their acetates), with a source of bromine, such as hydrogen bromide, used as a promoter. The temperature of the oxidation reaction is typically in the range of about 100-250 ^eC, preferably about 150-220 ^eC. Any conventional pressure may be used for the reaction, suitably to maintain the reaction mixture in a liquid state. An oxidation stage performs the function of catalytically oxidizing the hydrocarbon precursor in the organic solvent, thus forming a product stream and the vent gas. The product stream is typically transferred to a crystallisation stage to form a first slurry of crude terephthalic acid crystals and an overhead vapour. The first slurry of crude terephthalic acid crystals is typically passed to a separation stage in which a mother liquor is separated from the crude terephthalic acid crystals, which may then be mixed with an aqueous liquid to form a second slurry of crude terephthalic acid crystals. This second slurry of crude terephthalic acid crystals is typically transferred to a purification plant, heated and subjected to hydrogenation, before being cooled to form a slurry of purified terephthalic acid crystals. The vent gas from the oxidation stage is typically separated in a distillation stage into an organic solvent-rich stream and a water-rich vapour stream. The organic solvent-rich stream from the distillation stage typically comprises 80-95 % w/w organic solvent and is typically returned to the oxidation stage. The water-rich vapour stream from the distillation stage typically comprises 0.1 - 5.0 % w/w organic solvent and is typically condensed to form a condensate stream and an overhead gas in a condensing stage. A portion of the condensate stream is typically used as a source of the aqueous liquid used to form the second slurry of crude terephthalic acid crystals mentioned above. In the present invention, the condensate stream preferably forms a portion of the wash fluid for the purified terephthalic acid crystals from the purification plant.

Production of polyester

The polyester is preferably poly(ethylene terephthalate). Poly(ethylene terephthalate) is typically produced by the esterification of ethylene glycol and purified terephthalic acid at a pressure of about 2-6 barA and a temperature of about 200-300 ^eC. The by-product stream (or streams, depending on the plant configuration) from the esterification reaction is typically fed to a distillation column to separate excess ethylene glycol, which can be recycled and reused, from water and by-product organic compounds. The aqueous stream from the top of the distillation column may further be sent to a stripping column to reduce the level of by-product organic compounds in it. Accordingly, the resultant aqueous condensate liquor from the PET plant may comprise one or more organic compounds selected from acetaldehyde; 2-methyl-1 ,3-dioxolane; 1 ,4-dioxane; ethylene glycol; and acetic acid. More specifically, the aqueous condensate liquor may comprise 0.01 -3 wt.% acetaldehyde; 0.01 -3 wt.% 2-methyl-1 ,3-dioxolane; 0.01 -0.5 wt.% 1 ,4-dioxane; 0.01 -2 wt.% ethylene glycol; and/or 0.01 -1 wt.% acetic acid. Even more specifically, the aqueous condensate liquor may comprise 0.5-2 wt.% acetaldehyde; 0.5-2 wt.% 2-methyl-1 ,3-dioxolane; 0.1 -0.25 wt.% 1 ,4-dioxane; 0.1 -1 wt.% ethylene glycol; and/or 0.1 -0.75 wt.% acetic acid. The balance of the aqueous condensate liquor is typically water, i.e. the aqueous condensate liquor may comprise at least 90.5 wt.% water, or at least 92 wt.% water, or at least 95 wt.% water. In conventional integrated terephthalic acid - polyester plants, this aqueous condensate liquor was treated, recycled, and discharged independently from the terephthalic acid process waste water.

Aqueous wash liquid

In the present invention, at least a portion of the aqueous wash liquid is derived from the aqueous condensate liquor. For instance, at least 1 wt.%, at least 2 wt.%, at least 5 wt.%, at least 10 wt.%, at least 20 wt.%, at least 30 wt.%, or at least 40 wt.% of the aqueous wash liquid is derived from the aqueous condensate liquor. For instance, 1 -75 wt.%, 2-50 wt.%, 5-40 wt.%, 10-30 wt.%, or about 15 wt.% of the aqueous wash liquid is derived from the aqueous condensate liquor. The rest of the wash fluid is typically derived from the condensate stream from the condensing stage, although other aqueous process streams and, if required, demineralised water can be used to make up the aqueous wash liquid if required. Accordingly, deriving the aqueous wash liquid from the aqueous condensate liquor may reduce or eliminate the use of demineralised water in making up the aqueous wash liquid. The portion of the aqueous wash liquid derived from the aqueous condensate liquor may be derived directly or indirectly from the aqueous condensate liquor. As used herein, "directly" means that the aqueous condensate liquor is not subjected to any intermediate treatment to alter its composition prior to being used to make up the aqueous wash liquid. It will be appreciated that because the aqueous wash liquid may be (and, preferably, is) also made up from the condensate stream (and, possibly, other streams), its composition is not the same as the aqueous condensate liquor. Accordingly, in this embodiment, the aqueous wash liquid comprises the organic compounds that were present in the aqueous condensate liquor, i.e. the aqueous wash liquid comprises one or more organic compounds selected from acetaldehyde; 2-methyl-1 ,3-dioxolane; 1 ,4-dioxane; ethylene glycol; and acetic acid. Deriving the aqueous wash liquid directly from the aqueous condensate liquor is advantageous as it avoids the capital and variable costs associated with treating the aqueous condensate liquor to remove the organic components mentioned above. The present inventors have surprisingly found that aqueous wash liquid that comprises the organic compounds mentioned above is suitable for washing PTA crystals.

As used herein, "indirectly" means that the aqueous condensate liquor is subjected to intermediate treatment to alter its composition prior to being used to make up the aqueous wash liquid, e.g. the aqueous condensate liquor may be treated to remove some or all of the organic compounds mentioned above. For instance, the aqueous condensate liquor may be subjected to treatment process(es) such as the "extraction process" and/or "reverse osmosis" described in WO 2014/070766 A1 , which is incorporated herein in its entirety by reference. The aqueous condensate liquor may also be combined with other streams from elsewhere in the combined plant, such as a purification plant mother liquor (PPML). The PPML may comprise the mother liquor from which PTA crystals are separated and, optionally, aqueous wash fluid that has been used to wash the PTA crystals. Accordingly, in this embodiment, the aqueous condensate liquor is transferred to an extraction stage to form an aqueous stream and an organic stream. Thus, in this embodiment, at least a portion of the aqueous wash liquid is derived from the aqueous stream. Optionally, the aqueous condensate liquor is combined with a PPML prior to its transfer to the extraction stage. Alternatively, the aqueous condensate liquor is not combined with a PPML. The extraction stage may comprise the steps of combining the aqueous condensate liquor (and, optionally, the PPML) with a solution comprising an organic entrainer to form a mixture having a temperature of at least about 20 °C less than the azeotropic temperature of the mixture and separating the mixture into an organic stream (which may comprise residual aromatic carboxylic acids) and an aqueous stream. The extraction stage may further comprise the steps of heating the aqueous stream, optionally via heat exchange with effluent from a recovery column to form a heated aqueous stream, and feeding at least a portion of the heated aqueous stream to the recovery column. The recovery column is designed to strip out residual organic compounds from the aqueous stream (e.g. it may be a stripping column comprising a plurality of stages known in the art as theoretical equilibrium stages). The extraction stage may also comprise the steps of heating the organic stream, optionally via heat exchange with effluent from a distillation column, to form a heated organic stream, and feeding the heated organic stream to the distillation column. The aqueous stream removed from the extraction stage (i.e. from the recovery column), may be subjected to further treatment steps, including contacting the aqueous stream with an alkali to form a pH adjusted stream; contacting the pH adjusted stream with a filter to form a treated stream; and contacting the treated stream with a reverse osmosis unit to form an aqueous liquid that can be used to make up the aqueous wash liquid. Thus, in this embodiment, at least a portion of the aqueous wash liquid is derived from the aqueous liquid.

Filtration zone

The filtration zone may comprise at least one filtration device, such as a rotary filter. A rotary filter typically comprises a drum that is rotatable within a housing that is divided around its circumference into a series of zones in each of which a treatment stage may be applied to the filter cake, e.g. a separation stage, a wash stage etc. Thus, the rotary filter may comprise a first zone for separating the PTA crystals from an aqueous medium in which the PTA crystals are suspended by filtration of the slurry to remove the aqueous medium through a filter surface in the first zone to obtain a wet mass of PTA crystals. The rotary filter may then operate by transferring the wet mass of PTA crystals to a second zone (without reslurrying the wet mass of PTA crystals) and supplying the aqueous wash liquid to the wet mass of PTA crystals whilst effecting filtration. Thus, the aqueous wash liquid may be displaced through the wet mass of PTA crystals and through the filter surface in the second zone, thus washing the wet mass of PTA crystals. Accordingly, the washing of the purified terephthalic acid crystals with an aqueous wash liquid may take place in the filtration zone.

Claims

A method for the production of purified terephthalic acid crystals in an integrated terephthalic acid - polyester plant comprising the steps of:

characterised in that at least a portion of the aqueous wash liquid is derived from an aqueous condensate liquor from the polyester plant.

The method of claim 1 , wherein the aqueous condensate liquor comprises one or more organic compounds selected from acetaldehyde; 2-methyl-1 ,3-dioxolane; 1 ,4-dioxane; ethylene glycol; and acetic acid.

The method of claim 2, wherein the aqueous condensate liquor comprises 0.01 -3 wt.% acetaldehyde; 0.01 -3 wt.% 2-methyl-1 ,3-dioxolane; 0.01 -0.5 wt.% 1 ,4-dioxane; 0.01 -2 wt.% ethylene glycol; and/or 0.01 -1 wt.% acetic acid.

The method of any preceding claim, wherein at least a portion of the aqueous wash liquid is derived directly from the aqueous condensate liquor.

The method of any one of claims 1 -3, further comprising the step of:

iv) transferring the aqueous condensate liquor to an extraction stage to form an aqueous stream and an organic stream,

wherein at least a portion of the aqueous wash liquid is derived from the aqueous stream.

The method of claim 5, wherein the aqueous condensate liquor is combined with a purification plant mother liquor prior to being transferred to the extraction stage in step iv).

The method of claim 5 or claim 6 further comprising the steps of

v) contacting the aqueous stream with an alkali to form a pH adjusted stream;

vi) contacting the pH adjusted stream with a filter to form a treated stream; and

vii) contacting the treated stream with a reverse osmosis unit to form an aqueous liquid, wherein at least a portion of the aqueous wash liquid is derived from the aqueous liquid.