TECHNICAL FIELD

The present invention relates to a process and apparatus for the production of an aromatic dicarboxylic acid.

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.

PTA is generally considered to be a commodity item, with several million tonnes being produced annually, and it is therefore desirable for manufacturers to reduce their costs to maximise the economy and efficiency of PTA 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).

The oxidation reaction described above produces an off-gas from which organic solvent and water are typically recovered in a distillation stage and a condensing stage. The progress of the oxidation reaction may be monitored to ensure that it is proceeding in both a safe and an efficient manner by analysis of this off-gas. For instance, the oxygen content of the off-gas may be monitored to check that the off-gas composition is outside flammable limits, and the carbon dioxide and/or the carbon monoxide content of the off-gas may be monitored to check that the oxidation reaction is proceeding efficiently (e.g. the burn of the organic solvent and organic compounds employed in the oxidation reaction is low). Conventional processes analyse the off-gas as it exits the condensing stage. However, in such processes, because the off-gas must traverse both the distillation stage and the condensing stage before its composition is analysed, there is a delay in detecting, and thus responding to, any anomalous performance of the oxidation reaction. It is an object of the present invention to provide a more economic and efficient process for the manufacture of aromatic dicarboxylic acids and, in particular, to provide a process in which anomalous performance of the oxidation reaction is detected more quickly than in the conventional process described above. Further objects will be apparent from the description below.

DISCLOSURE OF THE INVENTION

The present invention provides a process for the production of an aromatic dicarboxylic acid comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent, wherein said process comprises:

(a) separating off-gas generated by said oxidation reaction into an organic solvent-rich liquid stream and a water-rich vapour stream, wherein the water-rich vapour stream comprises organic compounds and non-condensable gases, and wherein said separating is performed in a pressurised distillation stage comprising a first pressurised distillation column and a second pressurised distillation column, and

(b) condensing said water-rich vapour stream exiting the pressurised distillation stage into a condensate stream and an overhead gas stream in a condensing stage,

characterised in that a first water-rich vapour stream is withdrawn from said first pressurised distillation column and passed to said second pressurised distillation column and a first organic solvent-rich liquid stream is withdrawn from the first pressurised distillation column and returned to the oxidation reaction, wherein said process further comprises the step of monitoring the oxidation reaction by analysing the composition of said first water-rich vapour stream.

The present invention further provides a process of monitoring and/or controlling the catalytic oxidation of a hydrocarbon precursor to an aromatic dicarboxylic acid in an organic solvent, wherein said process comprises:

(a) separating off-gas generated by said oxidation reaction into an organic solvent-rich liquid stream and a water-rich vapour stream, wherein the water-rich vapour stream comprises organic compounds and non-condensable gases, and wherein said separating is performed in a pressurised distillation stage comprising a first pressurised distillation column and a second pressurised distillation column, and

(b) condensing said water-rich vapour stream exiting the pressurised distillation stage into a condensate stream and an overhead gas stream in a condensing stage,

characterised in that a first water-rich vapour stream is withdrawn from said first pressurised distillation column and passed to said second pressurised distillation column and a first organic solvent-rich liquid stream is withdrawn from the first pressurised distillation column and returned to the oxidation reaction, wherein said process further comprises the step of analysing the composition of said first water-rich vapour stream.

The present invention therefore monitors the oxidation reaction by analysing the composition of the off-gas from the oxidation reaction at an earlier position in its path than in the conventional process, namely after the off-gas has traversed the first pressurised distillation column in the distillation stage. Accordingly, any anomalous performance of the oxidation reaction can be detected, and any necessary corrective measures applied, earlier than in the conventional process, thus improving the safety and/or the efficiency of the process. In an alternative configuration, the processes of the invention may comprise the step of analysing the off-gas generated by the oxidation reaction instead of the step of analysing the first water-rich vapour stream. However, this configuration is less preferred because the off-gas generated by the oxidation reaction may contain solids that may foul the sampling and conditioning systems used to perform the analysing step. Accordingly, it is preferred that the processes of the invention comprise the step of analysing the composition of the first water-rich vapour stream because this stream has been scrubbed clean of solids by the first pressurised distillation column yet still allows earlier detection of any anomalous performance of the oxidation reaction than in the conventional process. It is further preferred that the processes of the invention comprise determining the oxygen, and/or carbon dioxide, and/or carbon monoxide content of said first water-rich vapour stream. As mentioned above, the oxygen content of the off-gas may be monitored to check that the off-gas composition is outside flammable limits, whilst the carbon dioxide and/or the carbon monoxide content may be monitored to check that the oxidation reaction is proceeding efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic of a process according to the invention.

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 a process and apparatus for the production of an aromatic dicarboxylic acid by the catalytic oxidation of a hydrocarbon precursor in an organic solvent is well known. For example, as discussed above, terephthalic acid suitable for use in PET 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.).

Production of an aromatic dicarboxylic acid

The aromatic dicarboxylic acid produced in the process and apparatus of the present invention is preferably selected from terephthalic acid, orthophthalic acid and isophthalic acid. The aromatic dicarboxylic acid is preferably terephthalic acid. The hydrocarbon precursor is a compound that may be oxidised to form the aromatic dicarboxylic acid. Thus, the hydrocarbon precursor is typically benzene or naphthalene substituted with groups such as Ci-6alkyl, formyl, or acetyl in the positions of the carboxylic acid substituents in the desired end product. 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 °C, preferably about 150-220 °C. Any conventional pressure may be used for the reaction, suitably to maintain the reaction mixture in a liquid state.

An oxidation stage, which typically comprises an oxidation reactor, performs the function of catalytically oxidizing the hydrocarbon precursor in the organic solvent, thus forming a product stream and a vent gas. The product stream is typically transferred to a crystallisation stage to form a first slurry of crude aromatic dicarboxylic acid crystals and an overhead vapour. The first slurry of crude aromatic dicarboxylic acid crystals is typically passed to a separation stage in which a mother liquor is separated from the crude aromatic dicarboxylic acid crystals, which may then be mixed with an aqueous liquid to form a second slurry of crude aromatic dicarboxylic acid crystals. This second slurry of crude aromatic dicarboxylic acid crystals is typically transferred to a purification plant, heated and subjected to hydrogenation, before being cooled to form a slurry of purified aromatic dicarboxylic acid crystals.

The vent gas from the oxidation stage is separated in the distillation stage into an organic solvent- rich liquid stream and a water-rich vapour stream. The organic solvent-rich liquid stream from the distillation stage typically comprises 80-95 % w/w organic solvent and is returned to the oxidation stage. The water-rich vapour stream from the distillation stage comprises organic compounds, such as organic solvent (typically 0.1 -5.0 % w/w), hydrocarbon precursor, aromatic monocarboxylic acids (e.g. p-toluic acid and/or benzoic acid), derivatives of the organic solvent (e.g. methyl acetate), methyl bromide and/or methanol, and non-condensable gases, such as oxygen, nitrogen, carbon monoxide and/or carbon dioxide, and is condensed to form a condensate stream and an overhead gas in the condensing stage, which typically comprises one or more condensers. A portion of the condensate stream is typically used as a source of the aqueous liquid used to form the second slurry of crude aromatic dicarboxylic acid crystals mentioned above. A portion of the condensate stream also typically forms a source of wash fluid for the purified aromatic dicarboxylic acid crystals from the purification plant. Pressurised distillation stage

The pressurised distillation stage comprises a first pressurised distillation column and a second pressurised distillation column, which are connected in series. The pressurised distillation stage may comprise further pressurised distillation columns, which are preferably connected in series to the first and second pressurised distillation columns. The pressurised distillation columns (also known as "rectifiers") typically comprise a plurality of stages known in the art as theoretical equilibrium stages, which can be provided by trays, such as sieve, valve or bubble cap trays, random or structured packing or other suitable structures that provide surfaces for mass transfer between gaseous and liquid phases within the column(s). A first water-rich vapour stream is withdrawn from the first pressurised distillation column and passed to the second pressurised distillation column and a first organic solvent-rich liquid stream is withdrawn from the first pressurised distillation column and returned to the oxidation reaction. The first water-rich vapour stream may be separated in the second pressurised distillation column into a first water-rich liquid stream and a second water-rich vapour stream. The first water-rich liquid stream may be passed to the first pressurised distillation column. The second water-rich vapour stream may be passed to the condensing stage.

Monitoring/controlling the oxidation reaction

The present invention may comprise determining the oxygen, and/or carbon dioxide, and/or carbon monoxide content of said first water-rich vapour stream to monitor the progress of the oxidation reaction. This may comprise partially condensing or otherwise conditioning a sample of the first water-rich vapour stream prior to analysing its composition (i.e. the composition of the cooled and separated vapour). Suitable analysis methods are known to the person skilled in the art and are described in, for example, The Engineering Equipment and Materials Users' Association (EEMUA)'s publication 138: Design and installation of on-line analyser systems, Edition 2, 2010 (ISBN 978 0 85931 174 8). It is preferred that the sampling of said first water-rich vapour stream (to obtain a sample upon which the analysing step may be carried out) is performed using a fast loop system, which minimises the delay in the sampling system and thus further reduces the delay in the detection of any anomalous performance of the oxidation reaction. The performance of the oxidation reaction may thus be modulated in a manner determined by the analysed composition of said first water-rich vapour stream, such as by the measured oxygen, and/or carbon dioxide, and/or carbon monoxide content(s).

For instance, the feed rate of the hydrocarbon precursor may be increased or decreased based on the measured oxygen content of said first water-rich vapour stream, e.g. the feed rate of the hydrocarbon precursor may be increased if the oxygen content of said first water-rich vapour stream exceeds a threshold level to avoid unnecessary use of air. Furthermore, the oxidation reactor may be shut down if the oxygen content of said first water-rich vapour stream exceeds a higher threshold level to avoid continued operation with a potentially flammable atmosphere inside the oxidation reactor and the pressurised distillation stage. Similarly, if the carbon dioxide content of said first water-rich vapour stream exceeds a threshold level, the oxidation reactor may be shut down to avoid continued burning of organic components occurring inside the reactor, which could lead to damage to the lining of the oxidation reactor and, potentially, loss of containment of the contents of the oxidation reactor.

The invention will be further described with reference to the figure.

Figure 1 is a schematic of a process according to a preferred embodiment of the present invention. Oxidation reactor 10 is charged with aqueous organic solvent (preferably aqueous acetic acid), reaction catalyst, air, and hydrocarbon precursor (preferably p-xylene) (inlets not shown). The reaction to form an aromatic dicarboxylic acid (preferably terephthalic acid) is preferably carried out at about 150-220 °C. Vent gas 10a is passed from oxidation reactor 10 to first pressurised distillation column 22 which, together with second pressurised distillation column 24, forms pressurised distillation stage 20. Organic solvent-rich liquid stream 22a is passed from first pressurised distillation column 22 to oxidation reactor 10. Water-rich vapour stream 22b is passed from first pressurised distillation column 22 to second pressurised distillation column 24. Analyser 26 is used to analyse the composition (preferably the oxygen, and/or carbon dioxide, and/or carbon monoxide content) of water-rich vapour stream 22b. Water-rich liquid stream 24a is passed from second pressurised distillation column 24 to first pressurised distillation column 22. Water-rich vapour stream 24b is passed from second pressurised distillation column 24 to condensing stage 30, which comprises one or more condensers.

Condensate stream 30a is removed for use elsewhere in the process for production of the aromatic dicarboxylic acid. Product stream 10b is transferred to a crystallisation stage (not shown).