CA2273958A1 - Production of aromatic carboxylic acids - Google Patents

Abstract

An effluent gas containing organic components including an aliphatic carboxylic acid and methyl bromide, gases includings nitrogen, oxygen, carbon monoxide and carbon dioxide, and water, particularly when derived from the production of an aromatic carboxylic acid such as terephthalic acid, is processed to eliminate substantially all of its aliphatic carboxylic acid content and is then subjected to high temperature combustion, optionally in the presence of an oxidation catalyst and/or a combustion assistant, to effect complete conversion of its methyl bromide content to HBr and/or Br2. The resulting gas stream is passed to an energy conversion device such as a gas turbine and is then scrubbed to remove HBr and/or Br2.

Description

PRODUCTION OF AROMATIC CARBOXYLIC ACIDS
This invention relates to a process for the production of an aromatic carboxylic acid, especially dicarboxylic acids such as terephthalic acid and isophthalic acid, by the liquid-phase oxidation of a precursor (e.g. p-xylene) of said carboxylic acid.
In a widely practised method of producing terephthalic acid, p-xylene is oxidised under elevated temperature and pressure conditions in a liquid phase reaction using air or other source of oxygen, the oxidation being carried out in a reaction solvent comprising a C2 - C6 monocarboxylic acid) such as acetic acid, in the presence of a catalyst system comprising one or more heavy metal compounds and one or more promoter compounds including bromine. Water is present in the reaction solvent and is formed as a result of the oxidation reaction. The oxidation reaction is accompanied by evolution of a reaction offgas which generally comprises inter alia nitrogen, unreacted oxygen, carbon dioxide, carbon monoxide and methyl bromide.
Because the reaction is exothermic, usual practice is to remove the heat of reaction by allowing the monocarboxylic acid solvent to vaporise resulting in a gaseous reactor overheads stream containing monocarboxylic acid and water. The overheads stream may be processed in various ways. For instance, one method involves passing the overheads stream to a condensing system in which a large proportion of the monocarboxylic acid and water is condensed, the condensate in part being returned to the reactor as reflux and in part processed further for recycle of monocarboxylic acid to the oxidation reaction - see for example US Patent No.
4777287. The non-condensed components of the gaseous overheads stream are vented from the condensing system.
In another method, the gaseous overheads stream may be passed directly to a distillation column in which substantially all of the monocarboxylic acid is separated and recovered as a bottoms product while the tops product comprises steam and other components - see for example GB Patent No. 1373230. In both methods, an offgas is obtained which may be processed further to recover energy for use in the production of the aromatic carboxylic acid. JP-A-55-9517 discloses a method for processing the offgas to recover energy.
The present invention is concerned with the treatment of the offgas derived from the oxidation reactor.
According to a first aspect of the present invention there is provided a process for the production of an aromatic carboxylic acid comprising:
oxidising a precursor of the aromatic carboxylic acid in a liquid-phase C2 -monocarboxylic acid solvent containing water and in the presence of a catalyst system containing one or more heavy metals and bromine;

withdrawing from the reaction an overheads gaseous stream at elevated pressure containing inter alia water, monocarboxylic acid and gaseous by-products including methyl bromide and feeding the high pressure gaseous stream to means for removing said monocarboxylic acid from the overheads stream to produce a high pressure monocarboxylic acid-depleted gaseous stream containing inter alia water and methyl bromide;
effecting high temperature combustion of the high pressure monocarboxylic acid-depleted gaseous stream so as to convert the methyl bromide to bromine and/or hydrogen bromide: and passing the treated gas containing bromine and/or hydrogen bromide to an energy recovery system.
Preferably the pressure and temperature conditions are controlled so as to prevent condensation of bromine and/or hydrogen bromide on passage through the energy recovery system.
Conveniently following combustion, preferably by catalytic combustion, the gas stream is passed to the energy recovery system without scrubbing the gas stream.
Thus, instead of removing the potentially corrosive bromine compounds prior to passage of the treated gas stream through the energy recovery system e.g.
by scrubbing the gas stream, the temperature of the treated gas stream is maintained and corrosion is suppressed by control of the temperature and pressure conditions to ensure the potentially corrosive bromine compounds) remain in the gaseous phase during passage through the energy recovery system. In this way, full advantage is taken of the increase in temperature imparted to the gas stream in the course of high temperature combustion without the necessity of fabricating the energy recovery system using expensive highly corrosion-resistant materials. Thus, for example, the energy recovery system (eg a gas turbine) may be fabricated using more conventional materials such as high chrome or austenitic stainless steels.
Prior to introduction into the energy recovery system, at least part of the gas stream may be used to preheat the gas stream upstream of the combustion step.
Preferably following passage through the energy recovery system, the treated gas is treated to remove substantially all of the bromine species and/or hydrogen bromide.
Preferably the monocarboxylic acid is removed from the overheads gas stream to such an extent that the water content of the resulting gas stream exceeds its monocarboxylic acid content.
Typically at least 90 wt%, more preferably at least 95 wt%, of said monocarboxylic acid is removed from the overheads stream.

WO 97/27168 PCTlGB97/00104 The means for removing monocarboxylic acid from the overheads stream conveniently includes a separation column in which water/monocarboxyfic acid distillation is effected, e.g a packed or trayed distillation or rectification column capable of effecting a separation whereby at least 95%, more preferably about 98%
and most preferably at least about 99%, by weight of the monocarboxylic acid solvent is removed from the gaseous overheads stream from the oxidation reaction.
The separation column is preferably operated at the same, or close to the, pressure at which the oxidation reaction is conducted. The separation column may be mounted separately from the oxidation reactor with suitable pipework connecting the reactor to the column for the supply of the overheads stream to the latter.
Alternatively the separation column may be mounted directly above and integrated with the oxidation reactor.
Thus, in a preferred embodiment of the present invention, the high pressure gaseous overheads stream derived from the oxidation reactor vapor (which typically contains, in addition to water and monocarboxylic acid, residual oxgyen, by-product gases such as methyl bromide and methyl acetate formed as a result of the oxidation, carbon dioxide, carbon monoxide and nitrogen) is passed to the separation column to remove most of the monocarboxylic acid solvent.
Consequently, the offgas stream discharged from the separation column will be at high pressure and will contain, in addition to a significant amount of water in the form of steam, residual oxgyen, by-product gases such as such as methyl bromide (typically present in an amount in the range of 25 to 125 ppm) and methyl acetate formed as a result of the oxidation, carbon dioxide, carbon monoxide and nitrogen.
As such therefore, the high pressure offgas stream discharged from the separation column constitutes a significant source of energy which can be extracted by suitable means such as an expander.
Typically the residual oxygen content of gaseous overheads stream withdrawn from the oxidation reactor constitutes about 3 to 8% by volume of the non-condensibles present in the overheads stream. Preferably the liquid phase oxidation process is carried out in such a way that the amount of residual oxygen in the offgas is sufficient under normal operating conditions to avoid the need to supply further oxygen under pressure to the offgas treatment process while ensuring that there is substantially no risk of oxygen starvation in the offgas treatment without giving rise to potentially hazardous conditions that can obtain if the gaseous overheads stream contains excessive oxygen.
Where necessary, scrubbing of the high pressure monocarboxylic acid-depleted gaseous stream with liquor may be effected upstream of the combustion zone in order to recover at least in part any volatile precursor or precursors of said aromatic carboxylic acid which would otherwise be entrained in the gaseous stream passing to the combustion step. Where the means for removing said monocarboxylic acid from the overheads stream comprises a separation column, such scrubbing may be effected within the separation column, conveniently using water which has been recovered from the treated gas after passage through the energy recovery system and may optionally be preheated upstream of said scrubbing.
The combustion step (preferably catalytic combustion) of the present invention serves to eliminate combustible components and carbon monoxide present in the high pressure offgas stream before it is directed to the energy recovery means and contributes to elevation of the offgas temperature prior to the energy recovery system, thereby increasing power recovery and avoiding dewing of corrosive bromine species e.g. HBr in the system. Additional temperature elevation may be provided by indirect heating via steam or a fired heater, direct firing of fuel and/or injection of a support fuel such as methyl acetate, methanol, methane, propane, butane, etc.
In the combustion step, the offgas stream is preferably contacted with a suitable catalyst so that at least a substantial proportion of the combustible components and carbon monoxide is converted to environmentally acceptable forms.
The combustion step also results in conversion of methyl bromide to bromine and/or hydrogen bromide which can be highly corrosive but, in accordance with the invention, are passed through the energy recovery system under controlled conditions instead of being scrubbed from the offgas thus avoiding the inevitable substantial reduction in the temperature which would be necessary if scrubbing of the offgas is to be undertaken before passage through the energy recovery means coupled with the necessary reheating prior to passing the scrubbed offgas stream to the energy recovery system. Moreover, in the process of the present invention, it is possible to avoid pre-heating of the combusted offgas that might otherwise be necessary in order to revaporise any condensed water to steam before passage through the energy recovery means.
After passage through the energy recovery means and conveniently after removal of substantially all of the bromine (preferably in such a way as to retain all or at least a substantial part of the steam content in the offgas stream), the offgas stream may be condensed to recover water for recycle within the overall process for the production of the aromatic carboxylic acid. The recovered water may for instance be used as:
a reflux in the separation column for separating the monocarboxylic acid from the high pressure gaseous overheads stream; and/or solvent for dissolution of crude carboxylic acid in preparation for purification of the aromatic carboxylic acid by hydrogenation, for example in the manner disclosed in our prior EP-A-498591 and/or EP-A-502628.
Thus, for example. following passage through the energy recovery system, the offgas stream may be desuperheated to a temperature corresponding to or close to the dewpoint of the stream and scrubbed to remove the bromine and HBr components (e.g. using an aqueus caustic soda) while retaining the water content within the offgas stream. The scrubbed offgas stream at the dewpoint temperature may then be condensed to remove its water content.
Although it is preferred to remove the bromine and HBr components from the offgas stream before recovery of the water vapour content of the gas stream, we do not exclude the possibility of recovering the water by condensation followed by subsequent removal of the bromine and HBr components by scrubbing.
Typically the offgas stream obtained following separation of the monocarboxylic acid in said separation column is at a pressure in the range of 5 to tiara (for instance between 10 and 16 tiara) and a temperature of the order of to 200°C (e.g. about 177°C and 14 tiara). Prior to combustion thereof, the offgas stream from the separation column is conveniently heated directly or indirectly (eg by means of high pressure steam, heating oil, heat exchange between the offgas 20 stream upstream and downstream of the combustion step, passage through a fuel-fired heater or by direct firing of fuel into the gas stream) to an elevated temperature, usually in the range of 250 to 450°C (typically about 300°C).
Depending on the exotherm available from the combustion step, it may be appropriate to introduce a combustion assistant into the combustion zone. The 25 combustion assistant is preferably pre-mixed with the gas stream prior to entry into the combustion zone. One form of device for effecting good mixing of the combustion assistant with the offgas stream is disclosed in our prior PCT
Published Patent Application No. WO 94/23813, the disclosure of which is incorporated herein by this reference.
The combustion assistant is preferably, but need not necessarily be, one including one or more oxygen atoms per molecule. Various assistants may be used, eg methanol, methyl acetate, hydrogen, natural gas, methane, propane, butane or mixtures thereof. Where methyl acetate is used, it is conveniently derived from the terephthalic acid production process as it is generated as a by-product of the liquid phase oxidation of p-xylene in acetic acid solvent. Where methane is used) it may be derived from an anaerobic process far the treatment of effluent produced in the manufacture of the aromatic carboxylic acid, e.g. terephthalic acid. if desired, additional air may be introduced into the combustion zone to promote oxidation.

The combustion step is carried out with regard to ensuring that, during the subsequent expansion on passage through the energy recovery system, bromine and HBr derived from the methyl bromide constituent of the effluent gas stream remain in the gas phase thereby avoiding dew point corrosion conditions in the energy recovery system. Usually the combustion step is carried out in the presence of a catalyst and the temperature of the treated offgas stream exiting the oxidation zone will be in the range from about 250 to about 700°C, e.g. 350 to 700°C, and will depend on whether or not the gas stream is preheated before introduction into the catalytic oxidation zone and whether or not a combustion assistant is employed. For instance, the catalytic oxidation may be conducted in such a way that the temperature of the treated gas exiting the catalytic oxidation zone is of the order of 400°C or greater. Where no combustion assistant is used, or where the combustion assistant is one which is easily o«elatively easily oxidised (e.g. methanol, methyl acetate or hydrogen) the exit temperature may be in the range of about 250 to about 550°C, typically about 350 to 500°C (e.g. about 480°C).
With a combustion assistant which is less readily oxidisable (e.g. methane, propane or butane) the exit temperature will usually be higher, i.e. about 400 to about 700°C, typically 550 to 700°C, e.g. of the order of 630°C.
Although catalytic combustion is preferred, we do not exclude the possibility of carrying out the combustion step in the absence of a catalyst and using a supply of fuel. In this case, the temperature of the gas stream following combustion will typically be in excess of 700°C, usually in excess of 800°C, and conveniently the liquid phase oxidation process is operated so that the oxygen content of the gaseous overheads stream withdrawn from the oxidation reactor is increased. eg in excess of 5% by volume relative to the non-condensible components in the overheads stream, thereby avoiding the need for separate supply of pressurised oxygen to the combustion step under normal operating conditions of the liquid phase oxidation process.
In general, the combustion process will be carried out using operating conditions (eg. temperature, space velocity, catalyst composition) selected to ensure that methyl bromide is substantially completely converted to HBr and Br2, the aim being to minimise or avoid the production of underconversion brominated aromatic compounds which have high dew points. In addition, pressure and temperature conditions are controlled so as to prevent condensation of the HBr and/or Br2 compound (s) on passage through the energy recovery system.
The energy recovery system may produce an output in mechanical or electrical form and may for instance be used to power other equipment in the production plant such as a compressor forming part of the system for feeding air, oxygen-enriched r air, oxygen-containing gas or oxygen to the reactor in which the liquid phase oxidation is carried out.
According to a further aspect of the present invention there is provided in a process for the production of an aromatic carboxylic acid such as terephthalic acid, which process comprises oxidising a precursor of said aromatic carboxylic acid (e.g.
paraxylene) in a reaction medium comprising a monocarboxylic acid (e.g. acetic acid) to produce a slurry of crude aromatic carboxylic acid in said aliphatic acid, recovering the crude acid from said slurry, dissolving the recovered crude acid in water, purifying the crude acid by a reaction comprising contacting the solution with hydrogen, and separating the purified acid from the mother liquor component of said solution, the steps of withdrawing from the reaction zone a high pressure gaseous overheads stream containing inter olio monocarboxyiic acid, water and methyl bromide, separating substantially all of the monocarboxylic acid from said overheads stream in a separation column to which aqueous mother liquor obtained from said purification reaction is also supplied whereby a high pressure offgas stream is obtained which contains inter olio water derived from both the oxidation reaction and the purification reaction and methyl bromide obtained from the oxidation reaction, oxidising said high pressure offgas stream, optionally in the presence of a catalyst andlor a combustion assistant) whereby the methyl bromide is converted to bromine andlor hydrogen bromide, passing the treated gas containing bromine and/or HBr in the vapour phase through an energy recovery system under temperature and pressure conditions such that condensation of the bromine andlor HBr is substantially prevented, and removing substantially all of the bromine from the treated gas following passage through the energy recovery system.
Usually following such treatment to remove bromine, the treated gas has a bromine content of less than 4 ppm vol/vol.
The aqueous mother liquor separated from the purified aromatic acid, e.g by means of filtration equipment such as that disclosed in our prior International Patent Application No. WO 93/24440, contains various impurities, reaction intermediates and also said aromatic carboxylic acid in suspended and dissolved forms. By recycling the aqueous mother liquor to the separation column, such impurities, reaction intermediates and dissolved aromatic carboxylic acid content can be separated from the water content of the aqueous mother liquor and returned to the oxidation reactor as bottoms product from the column along with the monocarboxylic acid recovered by means of the column.
Usually a major fraction of the aqueous mother liquor will be recycled to the separation column in this way, preferably as reflux.

The mother liquor recovered from the purification reaction may be treated, e.g.
by cooling or evaporation, to recover therefrom crystals of less pure aromatic carboxylic acid, and at least part of the mother liquor supplied to the separation column may comprise the secondary mother liquor obtained following such treatment. Alternatively, since the separation will tend to be carried out at relatively high temperature, primary mother liquor recovered from the purification reaction may be recycled to the separation column without cooling the same to any signficant extent although it may be filtered before introduction into the separation column in order to remove any suspended fines.
Conveniently the crude acid is separated from said monocarboxylic acid by replacing the monocarboxylic acid in said slurry with water to produce a wet deposit of crude acid containing water for use in the subsequent purification of the crude acid (i.e. by hydrogenation of an aqueous solution formed from said wet deposit), replacement of the monocarboxylic acid with water being effected by means of an integrated separation and water washing filter preferably operating under elevated pressure conditions.
The integrated separation and water washing filter may comprise a gas pressurised belt filter, a gas pressurised rotary cylindrical filter, a hydraulically pressurised multi-celled pressure drum filter or a centrifuge provided with washing facilities. In each instance, the washing operation may be carried out in stages, preferably in countercurrent fashion so that the filter cake is washed with water of increasing purity as it advances downstream from the location at which separation of the crystals from the mother liquor takes place.
Typically the filter operates wish a pressure differential in the range of 0.1 to 15 tiara (preferably between 0.3 and 7 tiara), preferably such that the pressure on the lower pressure side thereof is no less than one tiara although we do not exclude the possibility of the lower pressure side being at sub-atmospheric pressure.
Another aspect of the invention is concerned with the removal from the gas stream of bromine and/or hydrogen bromide following high temperature particularly with the aim of processing the gas stream to remove the bromine components so that any discharge to atmosphere is substantially free of such components.
Such processing may for instance be effected by desuperheating the gas stream using water and contacting the gas stream with a suitable aqueous scrubbing media in a scrubbing section to remove the Br~ and HBr. HBr for instance may be removed by countercurrent contact with HBr solution or it may be removed simply by contact with water, e.g. a water spray, white at the same time desuperheating the treated gas.
Contacting the treated gas with HBr solution is for instance appropriate where the aim is to recover HBr for reuse as part of the catalyst system employed in the I ._ oxidation reactor. Where this is not required, water may be used. If used, it is preferred that sufficient water is employed to irrigate the pipeline transporting the water treated gas downstream and thereby prevent corrosion problems. Br, may be removed by countercurrent contact with an aqueous solution of components such as sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, sodium bromide, sodium formate, sodium sulphite, urea or mixtures containing any combination of two or more of these compounds (e.g. sodium hydroxide and sodium sulphite).
The oxidation reaction for the production of the aromatic carboxylic acid and its subsequent purification may be carried out in accordance with the teachings in our prior EP-A-498591 and EP-A-502628, the entire disclosures of which are incorporated herein by this reference.
The aromatic carboxylic acid may be terephthalic acid, in which the case the precursor thereof which is oxidised to produce terephthalic acid will be p-xylene.
Alternatively the aromatic carboxylic acid may be isophthalic acid (precursor m-xylene) or 2,6-naphthalene-dicarboxylic acid (precursor 2,6-dimethylnaphthalene).
The oxidation reaction is typically carried out at a temperature in the range of about 120°C to about 240°C and a pressure which is at least sufficient to maintain the reaction mixture under liquid phase conditions, typically 5 tiara to 30 tiara.
The promoter component of the catalyst system contains bromine and is typically in the form of hydrogen bromide, molecular bromine, sodium bromide and/or suitable organic bromide compounds well known to those in the art. The heavy metal components of the catalyst system usually in the form of cobalt and/or manganese compounds, for example cobalt and/or manganese carbonate.
According to a further aspect of the present invention there is provided in a process for the production of an aromatic carboxylic acid such as terephthalic acid, which process comprises oxidising a precursor of said aromatic carboxylic acid (e.g.
paraxylene) in a reaction medium comprising an aliphatic monocarboxylic acid (e.g.
acetic acid) to produce a slurry of crude aromatic carboxylic acid in said aliphatic acid, and replacing the aliphatic monocarboxylic acid in said slurry with water to produce a wet deposit of crude aromatic carboxylic acid containing water for use in the subsequent purification of the crude carboxylic acid (e.g. by hydrogenation of an aqueous solution formed from said wet deposit), replacement of the aliphatic monocarboxylic acid with water being effected by means of an integrated separation and water washing filter operating under elevated pressure conditions, the steps of processing a high pressure gaseous overheads stream comprising water, non-condensible gases such as nitrogen and carbon oxides, and organic compounds including aliphatic monocarboxylic acid and methyl bromide, evaporated in the course of the oxidation reaction to produce a high pressure gaseous effluent stream containing water and organic compounds such that the water content of the effluent stream exceeds the organic compound content, subjecting the high pressure gaseous effluent stream to high temperature combustion, optionally in the presence of a catalyst andlor a combustion assistant, with accompanying conversion of methyl bromide to bromine and/or hydrogen bromide, and removing bromine and/or HBr from the treated gaseous effluent.
Preferably the high pressure gaseous effluent stream is substantially free of said aliphatic monocarboxylic acid. Thus, for example, processing of the overheads stream conveniently includes the step of separating water and aliphatic monocarboxylic acid in a separation column to allow recovered aliphatic acid to be recycled to the reactor. Conveniently, aqueous mother liquor derived from purification of the crude aromatic carboxylic acid is also supplied to the distillation column, preferably as reflex, whereby impurities, reaction intermediates and aromatic carboxylic acid present in the aqueous mother liquor are passed to the oxidation reaction together with aliphatic monocarboxylic acid and the water content of the gaseous effluent stream includes water derived from said aqueous mother liquor.
It is to be understood that where the context admits features of the invention as defined in the consistory clauses which follow said first aspect of the invention are also applicable to the other aspects of the invention defined herein.
The invention will now be described by way of example only with reference to the accompanying drawings, in which:
Figure 1 is a schematic flow diagram illustrating one embodiment of the invention as applied to the treatment of an effluent gas stream derived from plant for the production of terephthalic acid: and Figure 2 illustrates a scrubbing unit for reducing the brominelhydrogen bromide content of the effluent gas.
Referring to Figure 1 , the effluent gas stream entering the treatment plant via line 10 is derived from a distillation column or rectifier D associated with a reactor R
for the production of terephthalic acid by liquid phase oxidation of p-xylene, for example by means of the process disclosed in our prior EP-A-498591 and/or EP-A-502628. In the process disclosed in these patent applications, catalysed liquid phase oxidation of paraxylene is carried out in a solvent comprising acetic acid to produce terephthalic acid) the catalyst system comprising heavy metals such as cobalt and manganese and bromine promoter. The temperature of the liquid phase reaction is controlled by withdrawing a vapour phase overheads stream from the reactor comprising the acetic acid, water, gaseous by-products including methyl bromide and methyl acetate, and gases such as nitrogen, carbon monoxide) carbon dioxide and unreacted oxygen. Following processing involving removal of a large proportion of the acetic acid, an offgas or gaseous effluent stream is obtained which is at elevated pressure and typically contains 20 to 125 ppm methyl bromide, depending on the reactor operating temperature and pressure.
Processing of the overheads stream in the illustrated embodiment comprises passing the gaseous stream to the column D which serves to effect the separation of acetic acid from water. Column D is operated so that the heavy components such as acetic acid are recovered as a liquid phase bottoms product while the light components such as water, gaseous by-products such as methyl bromide and methyl acetate and gases such as nitrogen, carbon monoxide, carbon dioxide and oxygen, are recovered as a gaseous phase tops product constituting the high pressure gaseous offgas stream which is to be treated. Water in the resulting offgas stream is in the form of steam. Thus, for example, while the overheads stream withdrawn from the oxidation reactor may contain acetic acid and water in the proportion of 70 to 95% acetic acid to 30 to 5% water (on a weighs to weight basis) , the high pressure offgas stream recovered from the column D will typically contain acetic acid and water in the proportion of 0.5 to 1.0 wt% acetic to 99.5 to 99.0 wt% water as steam.
The steam content will typically be in the range of about 40 to 70% by weight of total offgas.
The acetic acid-rich bottoms product from the column D is returned to the oxidation reactor R via line 11. This bottoms product will comprise acetic acid and some water and may also contain other organics such as precursors, e.g.
paratoluic acid, of terephthalic acid routed to the column D in the manner described below. The tops product) namely the high pressure offgas comprising steam as a primary component, is removed via line 10.
A feature of the process disclosed in EP-A-498591 and EP-A-502628 is the handling of the aqueous mother liquor produced in the purification process.
The purification process involves the hydrogenation of an aqueous solution of crude terephthalic acid obtained from the oxidation of paraxylene, crystallisation of purified terephthalic acid and separation of the purified crystals from the aqueous mother liquor. The resulting aqueous mother liquor contains impurities and intermediates such as paratofuic acid and in prior processes was treated as a waste.
EP-A-498591 and EP-A-502628 teach recycle of at least part of this primary mother liquor to the distillation column associated with the oxidation reactor, conveniently as reflux, in such a way that high boiling point impurities such as paratoluic acid are recovered in the acetic acid-rich bottom product withdrawn from the column for recycle to the oxidation reactor. The mother liquor recovered from the purification process may be recycled to the distillation column with or without treatment.
Such treatment, where employed, may comprise subjecting it to cooling or evaporation to precipitate further, but less pure, terephthalic acid and feeding the resulting secondary mother liquor (as a reflux feed) to the distillation column for separating water and acetic acid. The less pure terephthalic acid precipitate is also recycled to the oxidation reactor, e.g. by slurrying it in acetic acid derived from the distillation column. Use of the purification mother liquor in this way may also be employed in the present invention, with the mother liquor (treated or untreated) being supplied to the distillation column D, preferably as reflux.
The effluent treatment disclosed herein may for instance be used in conjunction with aromatic polycarboxylic acid production plant wherein the crude acid crystals and the purified acid crystals are separated from the primarily aliphatic monocarboxylic acid mother liquor and primarily aqueous mother liquor respectively and are subjected to washing with water by means of an integrated solids separation and water washing apparatus such as those described in our prior published International Patent Applications Nos. WO 93124440 and WO 94/17982 (the entire disclosures of which are incorporated herein by this reference) so that the mother liquor is replaced by water as a result of washing. Thus, for example the integrated solids separation and water washing apparatus may comprise a belt filter unit operated with the slurry side under superatmospheric conditions, or a pressurised rotary cylindrical filter unit operated with the slurry side under superatmospheric conditions, or a pressure drum filter unit (e.g. a BHS-Fest pressure filter drum formed with a plurality of slurry receiving cells in which the mother liquor is displaced from filter cake by water under hydraulic pressure supplied to the cells).
The processes for the filtration and washing of crude terephthalic acid, its subsequent purification) recovery and washing and the recycle of the purification mother liquor are fully described in EP-A-498591 , EP-A-502628, WO 93/24440 and WO 94117982. Detailed description herein is consequently unnecessary and these processes are represented in Figure 1 by reference P with the recycled mother liquor (treated or untreated) being depicted by reference 13.
The effluent gas stream is typically at a pressure of the order of 10 to 16 tiara and a temperature of the order of 170 to 190°C. The gas stream is preheated in heat exchanger 12 using for example high pressure steam as the heat source.
Typically the temperature of the gas stream following such heat exchange is of the order of 250 to 450°C, preferably 300 to 400°C. The gas stream then enters a mixer 14 into which a combustion assistant is also introduced via line 16, the combined gas stream and combustion assistant then being fed to a catalytic combustion unit with a space velocity of the order of 10' to 5 x 10° h~', preferably 5 x 103 to 2 x 10° h '.
A convenient combustion assistant is methyl acetate which is produced as a by-product in the terephthalic acid production process. Various other combustion assistants may be used instead or in addition, especially those which contain oxygen - eg methanol. The amount of combustion assistant introduced is cnrh fh~t rho temperature of the combusted gas stream exiting the catalytic combustion unit 18 is of the order of 400°C or greater, typically of the order of 480°C. A feedback arrangement comprising valve 20 in line 16, temperature sensor 22 and appropriate control equipment is used to regulate the supply of combustion assistant to the mixer 14 so that the desired temperature is maintained at the exit of the unit 18.
The catalyst employed in the catalytic combustion unit 18 may comprise any suitable oxidation catalyst, usually in solid form, to secure substantially total conversion of methyl bromide to bromine and HBr while also securing, in combination with the combustion assistant (where needed), substantially total oxidation of other organics such as acetic acid, elimination of carbon monoxide and production of heat to produce the desired exit temperature. Typically the catalyst employed comprises a noble metal catalyst such as platinum and/or palladium supported on a suitable support which may be inert. The support may be ceramic or metallic in the form of a monolith or pellets. Suitable commercial catalysts are available from catalyst manufacturers such as Johnson Matthey (e.g. Halocat AH/HTB-10 or LHC catalyst), Allied Signal/Degussa (e.g. HDC-2 or T2-HDC
catalyst) and Engelhard (e.g. VOCAT 300H or VOCAT 450H catalyst).
Following catalytic combustion, the treated gas stream typically has a temperature of the order of 400 to 700°C and a pressure only marginally lower than the untreated gas stream, ie about 9.5 to 15.5 tiara in the case where the untreated gas stream has a pressure of the order of 10 to 16 tiara. The treated gas is then passed through expander 24 in which the energy content of the gas stream is converted into mechanical power which, via shaft 26, can be employed appropriately within the terephthafic acid production process, for instance as power input for an air compressor for feeding air under pressure to the oxidation reactor of the production process or for generation of electric power for distribution either within the plant or to other users. At the exit side of the expander 24, the gas stream temperature is typically of the order of 140 to 220°C (eg about 170 to 200°C) and its pressure is near atmospheric, eg about 1.2 tiara. The temperature and pressure conditions employed are such that the bromine and HBr derived from methyl bromide in the course of the catalytic combustion remain in the gas phase thereby avoiding any risk of dew point corrosion. In this way, cost penalties otherwise incurred through the use of scrubbing plant upstream of the expander 24 (with consequent reduction in energy available for extraction by means of the expander) or through the use of expensive materials of construction for the expander 24, are avoided.
Following energy recovery, the gas stream is processed to remove the bromine components so that any discharge to atmosphere is substantially free of such components. Such processing may for instance be effected by desuperheating the gas stream in unit 28 using water and contacting the gas stream with a suitable aqueous scrubbing media in a scrubbing section 30 to remove the Brz and HBr.
HBr for instance may be removed by countercurrent contact with HBr solution and Br2 may be removed by countercu«ent contact with an aqueous solution of components such as sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium bromide, sodium formate, sodium sulphite or mixtures containing any combination of two or more of these compounds (e.g. sodium hydroxide and sodium sulphite). The water used for desuperheating may also be employed in the scrubbing section. The cleaned offgas (typically containing 40 to 70% by weight water vapour) is then cooled, in a cooling water exchanger 32 for example, to recover a major proportion and preferably substantially all of the water therein for re-use on the oxidation andlor purification stages of the plant. Following condensation of the bulk of the water, the offgas stream comprises mainly nitrogen and may be discharged to atmosphere andlor used elsewhere in the production process, eg for inerting duties.
Although not shown in Figure 1 , the column D may include a scrubbing section to which water derived for example from the water recovered from the cleaned offgas is supplied via line 34 in order to improve the recovery of the more volatile precursors of terephthalic acid such as paratoiuic acid which may otherwise pass out of the columm with the offgas removed as tops product from the column. This scrubbing section is located above the point of introduction of the reflux stream (pure plant mother liquor) into the column D and the water supplied to this scrubbing section may be preheated in order to increase the steam content of the tops product.
Figure 2 illustrates one form of scrubbing unit 50 for use in scrubbing the effluent gas in order to achieve a bromine content in the discharged gas of less than 4 ppm vollvol, more preferably less than 2 ppm vollvol, with 1 ppm vollvol being readily achievable. The unit 50 comprises a vessel having two packed sections and 54. The packings employed may be any suitable type, e.g. Raschig rings, Pal( rings etc. A liquid collection tray 56 is located between the two sections 52, 54. The effluent gas (together with water employed to irrigate the pipeline), following treatment to remove HBr, is fed to an inlet 58 at the base of the vessel 50 where the gas and liquid entering the vessel impinge on a plate (not shown) within the vessel base to prevent the gaslliquid mixture impinging on that part of the vessel wall opposite the inlet 58. The gas rises through the vessel, traversing the packed sections 52, 54, and leaves the vessel via outlet 60 which may be a discharge to atmosphere.
The scrubbing liquid employed may be any sui#able liquid capable of removing bromine from the effluent gas, including the chemicals specified above. The scrubbing liquid is circulated around a loop including the upper section 52, exit line 62, pump 64 and inlet line 66 so that the liquid flows countercurrent to the direction of gas flow passing up through the vessel 50. A second recirculatory flow of scrubbing liquid is established in the lower part of the vessel 50, again in countercurrent relation to the gas flow, by means of outlet line 68, pump 70 and return line 72. Spent scrubbing liquid is purged from the system via line 74 and make-up liquid is supplied via line 76. The amount of scrubbing liquid pumped through the vessel per unit time will generally be far in excess of that being purged) e.g. a ratio of at least 20:1 , e.g. at least 30:1 (typically of the order of 40:1 }. A purge line 78 interconnects the outlet of pump 64 and line 72 so that scrubbing liquid collecting in the collection tray 56 is passed to the lower recirculatory liquid flow loop. In a modification, the purge line 78 may be omitted and transfer of the scrubbing liquid from the upper section 52 to the lower section 54 and hence into the lower recirculatory loop may be implemented by allowing overflow of the liquor collecting in the tray 56. A small amount of the scrubbing liquid is routed to the inlet 58 via line 80, for example from the pump 70, in order to prevent any risk of corrosion in the region of the inlet.
In a further modification, the HBr scrubbing process may be integrated with the Br2 process by incorporating a further packing section into the scrubbing tower beneath the sections 52 and 54 so that the gas stream initially passes through the HBr scrubbing section. In the HBr scrubbing section, the scrubbing liquor may be an aqueous solution of HBr flowing in a recirculatory loop as described above in relation to sections 52 and 54 with suitable purging and make-up of the loop.
The HBr purge may for instance be supplied to catalyst make up.
From the foregoing, it will be seen that the bromine containing gas is subjected to a two stage scrubbing treatment allowing the bromine to be substantially completely removed before the gas is discharged from the vessel. As mentioned previously, the scrubbing liquid may be any suitable liquid for effecting bromine removal, with alkali metal compounds being preferred. Where for example the liquid is caustic soda, this is converted to sodium carbonate and bicarbonate in the scrubbing vessel as a result of absorption into the hydroxide of carbon dioxide contained in the effluent gas. Instead of, or in addition to, caustic soda, the scrubbing liquid may comprise one or more of the chemicals previously mentioned, e.g. sodium sulphite or sodium formate or other suitable reducing agent, or other compounds such as potassium hydroxide or urea.

Claims (28)

1. A process for the production of an aromatic carboxylic acid comprising:
oxidising a precursor of the aromatic carboxylic acid in a liquid-phase C2-C6 monocarboxylic acid solvent containing water and in the presence of a catalyst system containing one or more heavy metals and bromine;
withdrawing from the reaction an overheads gaseous stream at elevated pressure containing inter alia water, monocarboxylic acid and gaseous by-products including methyl bromide and feeding the high pressure gaseous stream to means for removing said monocarboxylic acid from the overheads stream to produce a high pressure monocarboxylic acid-depleted gaseous stream containing inter alia water and methyl bromide;
effecting high temperature combustion of the high pressure monocarboxylic acid-depleted gaseous stream so as to convert the methyl bromide to bromine and/or hydrogen bromide; and passing the treated gas containing bromine and/or hydrogen bromide to an energy recovery system.

2. A process as claimed in Claim 1 in which the pressure and temperature conditions are controlled so as to prevent condensation of bromine and/or hydrogen bromide on passage through the energy recovery system and in which following high temperature combustion the gas stream is passed to the energy recovery system without scrubbing the gas stream.

3. A process as claimed in Claim 1 or 2 in which said monocarboxylic acid is removed to such an extent that the water content of the resulting gas stream exceeds its monocarboxylic acid content.

4. A process as claimed in any one of Claims 1 to 3 which at least 95 wt% of said monocarboxylic acid is removed from the overheads stream.

5. A process as claimed in any one of Claims 1 to 4 in which the means for removing said monocarboxylic acid from the overheads stream includes a separation column in which water/monocarboxylic acid distillation is effected.

6. A process as claimed in any one of Claims 1 to 5 in which the means for removing said monocarboxylic acid from the overheads stream comprises a separation column capable of effecting a separation whereby at least 95% by weight of the monocarboxylic acid solvent is removed from the gaseous overheads stream from the oxidation reaction.

7. A process as claimed in any one of Claims 1 to 6 in which after passage through the energy recovery means, substantially all of the bromine and/or HBr content of the treated gas is removed in such a way as to retain all or at least a substantial part of the steam content in the offgas stream.

8. A process as claimed in Claim 7 in which the bulk of water vapour in the treated gas following removal of bromine and/or HBr is condensed to recover water for recycle within the process for the production of the aromatic carboxylic acid.

9. A process as claimed in any one of the preceding claims in which prior to high temperature combustion the monocarboxylic acid-depleted offgas stream is heated directly or indirectly.

10. A process as claimed in any one of the preceding claims in which said combustion step is carried out in the presence of molecular oxygen and/or a combustion assistant comprising an oxygen containing organic compound and/or an oxidation catalyst.

11. A process as claimed in any one of the preceding claims in which said combustion step is carried out in the presence of a combustion assistant selected from the group comprising methanol, methyl acetate, hydrogen, natural gas, methane, propane, butane or mixtures thereof.

12. A process as claimed in Claim 10 or 11 in which the combustion assistant is introduced into the offgas stream upstream of the high temperature combustion zone.

13. A process as claimed in any one of Claims 10 to 12 in which the supply of combustion assistant is controlled in dependence upon the temperature of the gas exiting the high temperature combusion zone.

14. A process as claimed in any one of Claims 1 to 13 in which oxidation of said precursor produces a slurry of crude aromatic carboxylic acid in said monocarboxylic acid from which the crude acid is recovered, the recovered crude acid is dissolved in water and purified by a reaction comprising contacting the solution with hydrogen, the purified acid is separated from the aqueous mother liquor component of said solution and aqueous mother liquor obtained following separation of the purified aromatic acid is recycled to said means for removing said monocarboxylic acid from the overheads stream.

15. A process as claimed in Claim 14 in which the means for removing said monocarboxyiic acid from the overheads stream comprises a distillation column and said aqueous mother liquor is supplied to the distillation column as reflux.

16. A process as claimed in Claim 14 or 15 in which prior to supply to said means for removing said monocarboxylic acid from the overheads stream the aqueous mother liquor is treated to precipitate and separate at least part of the dissolved content thereof.

17. A process as claimed in any one of Claims 14 to 16 in which the crude acid is separated from said monocarboxylic acid by replacing the monocarboxylic acid in said slurry with water to produce a wet deposit of crude acid containing water for use in the subsequent purification of the crude acid.

18. A process as claimed in Claim 17 in which replacement of the monocarboxylic acid with water is effected by means of an integrated separation and water washing filter.

19. A process as claimed in Claim 18 in which the integrated separation and water washing filter is selected from the group comprising a gas pressurised belt filter, a gas pressurised rotary cylindrical filter, a hydraulically pressurised multi-celled pressure drum filter and a centrifuge provided with washing facilities.

20. A process as claimed in Claim 18 or 19 in which the washing operation is carried out in countercurrent fashion so that the filter cake is washed with water of increasing purity as it advances downstream from the location at which separation of the crystals from the mother liquor takes place.

21. A process as claimed in any one of the preceding claims comprising scrubbing the high pressure monocarboxylic acid-depleted gaseous stream with liquor upstream of the combustion zone in order to recover at least in part any volatile precursor or precursors of said aromatic carboxylic acid which would otherwise be entrained in the gaseous stream passing to the combustion step.

22. A process as claimed in any one of the preceding claims in which following passage through the energy recovery system the gas is treated to reduce its bromine content to less than 4 ppm vol/vol.

23. A process for the production of an aromatic carboxylic acid comprising oxidising a precursor of said aromatic carboxylic acid in a reaction medium comprising a monocarboxylic acid to produce a slurry of crude aromatic carboxylic acid in said monocarboxylic acid, recovering the crude acid from said slurry, dissolving the recovered crude acid in water, purifying the crude acid by a reaction comprising contacting the solution with hydrogen, and separating the purified acid from the mother liquor component of said solution, said process further comprising the steps of withdrawing from the reaction zone a high pressure gaseous overheads stream containing inter alia monocarboxylic acid, water and methyl bromide, processing the overheads stream to remove substantially all of the monocarboxylic acid, said processing including carrying out water/monocarboxylic acid separation in a distillation column to which aqueous mother liquor obtained from said purification reaction is also supplied, combusting the resulting high pressure monocarboxylic acid-depleted offgas stream at high temperature whereby the methyl bromide is converted to bromine and/or hydrogen bromide, passing the treated gas containing bromine and/or HBr in the vapour phase through an energy recovery system under temperature and pressure conditions such that condensation of the bromine and/or HBr is substantially prevented, and removing substantially all of the bromine from the treated gas following passage through the energy recovery system to reduce the bromine content in the treated gas to less than 4 ppm vol/vol.

24. A process as claimed in Claim 22 or 23 in which following passage through the energy recovery system the gas is treated to reduce its bromine content to less than 2 ppm vol/vol.

25. A process as claimed in any one of the preceding claims in which following passage through the energy recovery system the gas is scrubbed using a scrubbing medium comprising a combination of two or more of the group comprising sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, sodium bromide, sodium formate, sodium sulphite and urea.

26. A process for the production of an aromatic carboxylic acid such as terephthalic acid, which process comprises oxidising a precursor of said aromatic carboxylic acid in a reaction medium comprising an aliphatic monocarboxylic acid to produce a slurry of crude aromatic carboxylic acid in said aliphatic acid, and replacing the aliphatic monocarboxylic acid in said slurry with water to produce a wet deposit of crude aromatic carboxylic acid containing water for use in the subsequent purification of the crude carboxylic acid, replacement of the aliphatic monocarboxylic acid with water being effected by means of an integrated separation and water washing filter operating under elevated pressure conditions, the steps of processing a high pressure gaseous overheads stream comprising water, non-condensible gases including nitrogen, carbon monoxide and carbon dioxide, and organic compounds including aliphatic monocarboxylic acid and methyl bromide, evaporated in the course of the oxidation reaction to produce a high pressure gaseous effluent stream containing water and organic compounds such that the water content of the effluent stream exceeds the organic compound content, subjecting the high pressure gaseous effluent stream to high temperature combustion with accompanying conversion of methyl bromide to bromine and/or hydrogen bromide, and removing bromine and/or HBr from the treated gaseous effluent.

27. A process as claimed in Claim 26 in which the high pressure gaseous effluent stream is substantially free of said aliphatic monocarboxylic acid.

28. A process as claimed in Claim 26 or 27 in which the processing of the high pressure gaseous overheads stream includes water/monocarboxylic acid separation in a distillation column to which aqueous mother liquor derived from purification of the crude aromatic carboxylic acid is also supplied whereby higher boiling impurities present in the aqueous mother liquor are passed to the oxidation reaction together with aliphatic monocarboxylic acid and the water content of the gaseous effluent stream comprises water derived from said aqueous mother liquor.