WO1998018749A1

WO1998018749A1 - Production of aromatic carboxylic acids

Info

Publication number: WO1998018749A1
Application number: PCT/GB1997/002774
Authority: WO
Inventors: Peter Anthony Donaldson; Ian Peter Milne; Andrew Martin Hollins; Geoffrey William Usher; Michael William Gardner; Philip Nigel Ward
Original assignee: E.I. Du Pont De Nemours And Company
Priority date: 1996-10-25
Filing date: 1997-10-08
Publication date: 1998-05-07
Also published as: KR20000052790A; BR9712567A; EP0937026A1; TR199900884T2; CN1235593A; AU4629997A

Abstract

In the production of aromatic carboxylic acids such as terephthalic acid by the liquid phase oxidation of a precursor thereof, a slurry of the carboxylic acid crystals in an aliphatic carboxylic acid-based mother liquor is withdrawn from the oxidation reactor (50), cooled and passed to a solids-liquid separation unit (66) to separate mother liquor from the aromatic carboxylic acid crystals. Following cooling, the slurry is managed to secure reduction in the extent to which small particles present in the slurry interfere with the solids-liquid separation process.

Description

PRODUCTION OF AROMATIC CARBOXYLIC ACIDS

This invention relates to the production of aromatic carboxylic acids such as terephthalic acid and isophthalic acid.

Such acids may be produced by liquid phase oxidation of a precursor of the aromatic carboxylic acid (e.g. paraxylene in the case of terephthalic acid). The oxidation step often results in the production of a crude aromatic carboxylic acid which is not sufficiently pure for use in the production of end products such as polyester fibres, films and bottles since the impurities tend to discolour the end product to an unacceptable extent. It is therefore common practice to subject the crude aromatic carboxylic acid to a further purification process in order to secure an acceptable product for use in polyester production.

Terephthalic acid for example is produced commercially by oxidising p-xylene with oxygen in a liquid phase which comprises a lower aliphatic carboxylic acid solvent, such as acetic acid, and a dissolved heavy metal catalyst system (usually cobalt and manganese and a bromine promoter), the oxidation being carried out at elevated temperature and pressure consistent with maintaining the solvent in the liquid phase. A slurry of terephthalic acid in the solvent is withdrawn from the reactor and, after passing through one or more crystallisation zones in which pressure is reduced with accompanying cooling of the slurry to promote crystallisation of further terephthalic acid, the slurry is subjected to a solids-liquid separation process resulting in crude terephthalic acid crystals. Crude terephthalic acid obtained by oxidation of p-xylene usually contains relatively large amounts of various impurities including so-called colour bodies and 4-carboxybenzaldehyde (hereinafter referred to simply as "4-CBA"), the amount of 4-CBA impurity present typically being in the range of 50 to 10,000 ppm by weight.

The solids-liquid separation step usually involves the production of a filter cake comprising crude terephthalic acid and residual solvent, and a mother liquor filtrate comprising mainly solvent containing dissolved species and some undissolved solids in the form of small terephthalic acid particles which pass through the filter medium. A major fraction of the mother liquor filtrate, together with the suspended terephthalic acid particles, is recycled to the oxidation reactor and a minor fraction of the filtrate is purged to maintain the level of impurities in the oxidation reactor within predetermined limits. Conventional practice is to wash the terephthalic acid crystals recovered from the solids-liquid separation with fresh acetic acid, deliquor the washed crystals and then pass the crystals through a drier to drive off residual acetic acid. The washed and dried crystals may then be purified by dissolution in water and hydrogenation at elevated temperature and pressure. In recent times, the above process has been simplified by carrying out the filtration and washing steps by means of a filtration unit in which the slurry is initially filtered and then undergoes a combined washing and solvent interchange process involving displacing acetic acid from the crystals by water - see for instance our prior European Patent Application No. 502628. This approach avoids the conventional acetic acid washing and drying steps; instead the crude terephthalic acid is contacted with water resulting in replacement of acetic acid with water thereby allowing the water-containing crude terephthalic acid crystals to be supplied to the purification step without an intervening drying step as was necessary in the past when washing was carried out using acetic acid.

The combined filtration and solvent interchange/washing process may be carried out using an integrated solids separation and water washing unit (such as a belt filter, a rotary filter, a pressure drum filter or a centrifuge with washing facilities incorporated in the centrifuge), e.g. of the types described in International Patent Application No. WO 93/24440. For large throughputs, in following conventional filtration techniques as practised in the manufacture of terephthalic acid, we have found that it is necessary to employ more than one solids separation and washing unit in parallel with each other in order to maintain continuous production.

JP-A-06321856 discloses a method for producing highly pure terephthalic acid by hydrogenation of crude terephthalic acid containing no more than 7wt% of particles of particle size less than 44 micron obtained by suitable (but undisclosed) adjustment to the crystallization conditions or drying conditions at the time of the production/purification of the crude terephthalic acid, or by combined methods such as sieving or air elutriation. It is stated that the fine particles of less than 44 micron removed by sieving or the like can be disposed of, but it is said to be more economic to recycle them to the p-xylene oxidation stage.

GB-A-1436279 discloses a process for the production of terephthalic acid in which the slurry withdrawn from the oxidation reactor is subjected to solids-liquid separation at high temperature. As stated in GB-A-1436279: "Since most of the terephthalic acid separated by filtration at this high temperature is crystallized at the oxidation reaction temperature in the oxidation reaction apparatus, the particle sizes of the crystals are uniform, and the crystals are of high purity without containing particles of finer particle diameters. They can be directly dried to obtain a final product. Alternatively, they can be dried after washing with acetic acid, to obtain a final product".

To quote further from GB-A-1436279, "a very simple step of filtering the liquid oxidation reaction product immediately makes it possible to prevent organic impurities such as oxidation intermediates or oxidation by-products from precipitating, and adhering to the terephthalic acid or forming a crystallization nucleus having an abundance of impurities. There is no need at all to use any control device for recovering terephthalic acid of high purity from the reaction product. This can lead to the reduction of the content of organic impurities, especially 4-formylbenzoic acid, contained in the precipitated terephthalic acid to about 100 ppm, and also to the inhibition of precipitating fine particles of terephthalic acid, to enable the production of terephthalic acid crystals of uniform particle sizes having an average particle diameter of about 100 to 160 microns. Therefore, by the present invention, high purity terephthalic acid can be prepared without purification by, for example, recrystallization or oxidation. There is a further advantage that since the particle diameters of the terephthalic acid are uniform and large, it lends itself to easier separability and easier drying".

According to one aspect of the present invention there is provided a process for the production of an aromatic carboxylic acid in which aromatic carboxylic acid and mother liquor in which the aromatic carboxylic acid is suspended and/or dissolved is cooled to produce a slurry of aromatic carboxylic acid crystals in mother liquor and in which following cooling the slurry is supplied to at least one solids-liquid separation unit to recover said crystals in the form of a filter cake, characterised by management of the slurry following such cooling so as to reduce the extent to which small particles in the slurry interfere with the solids-liquid separation process.

In this way, it is possible to enhance the filterability of the cake formed from the slurry thereby increasing the capacity of the solids-liquid separation unit(s) and hence the rate of filtration and making possible the use of either fewer and/or more compact solids-liquid separation units to cope with a given product throughput.

The process as defined above and hereinafter may be applied to the slurry derived from an oxidation stage for producing the aromatic carboxylic acid or to the slurry derived from a purification stage in which the aromatic carboxylic acid (obtained from the oxidation stage or from another source such as recovery from polyesters manufactured using the aromatic carboxylic acid) is treated to reduce the level of impurities present. Typically, such purification is effected by hydrogenation of an aqueous solution of the aromatic carboxylic acid in the presence of a suitable catalyst.

Typically, the major impurity content of the crude aromatic carboxylic acid precipitate in the slurry prior to solids-liquid separation is at least 200 ppm, e.g. 200 to 10,000 ppm. For instance, in the case of crude terephthalic acid, the major impurity is 4-CBA which will typically be present in an amount ranging from 200 ppm to 10,000 ppm, more usually from 500 ppm to 10,000 ppm and often 1 ,000 to 10,000 ppm. In contrast with JP-A-06321856, in the process of the present invention management of small particles occurs independently of particle growth that may take place during any crystallisation process carried out prior to the solids-liquid separation and in such a way as to reduce the extent that small particles in the slurry interfere with content of the solids-liquid separation.

By "independently of any particle growth effects", we mean that the small particle management referred to is separate from any small particle growth effects that may be induced by the process of crystallisation.

In contrast with GB-A-1436279 which seeks to inhibit the formation of small particles by carrying out solids-liquid separation of the terephthalic acid slurry under high temperature conditions, the process of the present invention involves cooling of the mother liquor containing suspended and/or dissolved aromatic carboxylic acid so as to promote recovery of crystals with consequent formation of small particles and co-precipitation of impurities. Thus, GB-A-1436279 can be said to attempt management of small particles by inhibiting their formation in the first place whereas the process of the present invention effects management of small particles after they have formed. Moreover, in practice, a significant quantity of small particles are inevitably formed within the reactor and will therefore be present in the slurry withdrawn from the reactor. GB-A-1436279 does not disclose any means of managing small particles already present in the slurry passing from the reactor to the high temperature solids-liquid separation step. Additionally because the solids-liquid separation step is carried out at high temperature without any intermediate cooling of the slurry, the equipment for carrying out the solids-liquid separation is necessarily mechanically complex because of the high temperature and pressure conditions it is required to operate under. In one form of the present invention at least 1 %, preferably at least 2%, by weight of the aromatic carboxylic acid contained in the slurry withdrawn from the oxidation reaction zone per unit time is separated, predominantly in the form of small particles, from the remaining aromatic carboxylic acid slurry downstream of the cooling/precipitation process so as to reduce the small particle content of the filter cake formed in the solids-liquid separation unit(s), and said separated fraction is conveniently recycled at least in part to the oxidation zone.

In this way, it is possible to ensure that a significant fraction of the small particle content of the slurry does not interfere with the solids-liquid separation process.

Preferably the amount of aromatic carboxylic acid separated in this way is up to 10%, more preferably 3 to 8% and most preferably 3 to 6%, by weight of the aromatic carboxylic acid contained in the slurry withdrawn from the oxidation reaction zone per unit time. By "aromatic carboxylic acid contained in the slurry withdrawn from the oxidation reactor" we mean the total content of said aromatic carboxylic acid in the slurry, including dissolved and solid phase forms of said aromatic carboxylic acid. The term "small particles" refers to particles which are no greater than 36 microns in size when measured using a Coulter LS 130 Laser diffraction and PIDS Particle Size Analyser fitted with the Fluid Module, as supplied by Coulter Electronics Limited of Northwell Drive, Luton, Bedfordshire, England.

Typically between 50 and 90wt% of the separated fraction comprises particles which are no greater than 36 microns in size. The slurry following cooling, e.g. by passage through one or more crystalliser vessels, typically contains at least 3% by weight of small particles, i.e. particles which are no greater than 36 microns in size.

It has been observed that smaller particles tend to contain a higher proportion of impurities (e.g. 4-CBA, paratoluic acid and colour bodies in the case of terephthalic acid) than larger particles. Recycling of a more substantial fraction of aromatic carboxylic acid containing predominantly small particles consequently affords potential for secondary oxidation to take place on a larger scale than hitherto. This in turn makes it feasible to re-optimise the oxidation reaction towards milder operating conditions, e.g. lower pressure and temperature. Milder oxidation conditions imply larger amounts of 4-CBA and other partial oxidation species. The process of the present invention affords potential for compensation of the resulting increase in partial oxidation species in the crude aromatic carboyxlic acid withdrawn from the reactor, i.e. the increased amount of such species recycled to the oxidation reaction along with the recycled aromatic carboxylic acid is exposed to secondary oxidation thereby allowing the possibility of complete oxidation of such species to the desired aromatic carboxylic acid. In addition, reintroduction of small particles into the oxidation reaction affords opportunity for particle growth. By recycling a larger proportion of the smaller particles to the reaction zone than has occurred hitherto, including those particles which are greater than 1 micron is size, the increased opportunity for growth of small particles within the reaction zone may give rise to the possibility of using crystalliser vessels of smaller capacity or even fewer crystallisers since any particle growth that may take place in the crystallisation process is supplemented by particle growth in the reactor.

The cooling of the mother liquor containing suspended and/or dissolved aromatic carboxylic acid envisaged by the present invention may be effected by passing the mother liquor slurry through one or more stages of crystallisation with accompanying reduction in pressure and temperature and flashing or controlled evaporation of solvent, e.g. the slurry may be introduced into one or more stirred crystalliser vessels. However, the present invention is not limited to cooling by passage through one or more crystalliser vessels. In order to secure precipitation of aromatic acid crystals, other forms of cooling may be employed in addition to or instead of crystallisation by flashing or evaporation of the solvent component of the mother liquor, e.g. cooling by the addition of cooler liquid as described further below. Once the slurry cooling phase however effected has been completed, the slurry can then be managed so as to reduce the extent to which small particles forming prior to and in the course of the cooling phase interfere with the solids-liquid separation process.

One embodiment of small particle management scheme in accordance with the invention comprises passing the slurry through one or more particle classification devices prior to introducing it into the solids-liquid separation unit, the particle classification device(s) being arranged to partition the slurry into separate fractions, a thickened slurry fraction or fractions for introduction into the solids-liquid separation unit and a low solids content liquor fraction or fractions containing a greater proportion of the original small particle population than the thickened fraction(s). The particle classification is conveniently effected centrifugally, e.g. by means of one or more hydrocyclones producing the thickened fraction(s) as the underflow and the low solids content liquor fraction(s) as the overflow.

Where more than one particle classification device such as a hydrocyclone is employed, the devices may be arranged in series or in parallel. In this embodiment, cooling to effect precipitation of aromatic carboxylic acid crystals may be effected by quenching of the slurry by addition of cooler liquid (e.g. acetic acid in the case of a crude terephthalic acid slurry where the mother liquor comprises mainly acetic acid) to the slurry. Such quenching may with advantage be effected prior to completion of the crystallisation process where employed. For example, where the crystallisation process involves two or more stages of crystallisation by flashing or evaporation of solvent from the slurry, quenching may be effected immediately upstream of the final stage of crystallisation. As mentioned above, cooling need not necessarily be effected by passing the liquor/slurry through one or more stages involving flashing or evaporation of solvent from the slurry; for instance, cooling may be accomplished by quenching without accompanying crystallisation by flashing or evaporation of solvent and may be carried out irrespective of the particular form of small particle management employed.

Instead of, or in addition to, screening out small particles upstream of the solids-liquid separation unit(s) by means of one or more particle classification devices such as hydrocyclones, in another form of the invention the small particle containing fraction of aromatic carboxylic acid is separated from the bulk within the solids-liquid separation unit(s). Thus, in the case where the solids-liquid separation unit comprises a filter medium, the filter medium may be designed deliberately to allow greater slippage of small particles than has occurred hitherto - for example by using a relatively high porosity filter medium so that a greater proportion of the small particle population passes through the filter medium with the filtrate, thereby favouring the production of a more porous filter cake and promoting rapid filtration.

According to this aspect of the invention there is provided a process for the production of an aromatic carboxylic acid in which aromatic carboxylic acid and mother liquor in which the aromatic carboxylic acid is suspended and/or dissolved is cooled to produce a slurry of aromatic carboxylic acid crystals in mother liquor and in which following cooling the slurry is supplied to at least one solids-liquid separation unit to recover said crystals in the form of a filter cake, characterised by management of the slurry within the solids-liquid separation unit(s) so as to reduce the extent to which small particles in the slurry interfere with the solids-liquid separation process.

The solids-liquid unit preferably comprises a filter medium which is sufficiently open with respect to small particles that at least 1 %, preferably at least 2%, by weight of the aromatic carboxylic acid contained in the slurry withdrawn from the oxidation reaction zone per unit time is separated, predominantly in the form of small particles (including those greater than sub-micron size), from the remaining aromatic carboxylic acid slurry in the course of carrying out the filtration thereby reducing the small particle content of the filter cake.

Various types of filter devices may be employed, particularly those in which the filtration is effected or facilitated by developing a pressure differential across the filter medium. For example, the filter device may comprise a pressurised rotary filter of the type in which the filter medium is in the form of a suitably supported cylindrical band of filter material mounted for rotation about the cylinder axis in a housing which is pressurised with a gas, e.g. nitrogen, or vapour, e.g. steam, so that a pressure differential is established across the filter medium with the higher pressure prevailing exeteriorly of the band. In operation the band is partly immersed in the slurry so that as it rotates about the cylinder axis, the exterior surface of the band repeatedly travels through a pool of the slurry and, by virtue of the pressure differential, crystals accumulate and form a filter cake on the higher pressure side of the filter medium while mother liquor and smaller particles pass through the filter medium for collection at the lower pressure side of the filter medium.

A pressurised rotary filter of this type is described in our prior International Patent Application No. WO 93/24440. Other types of filter device employing a continuous or non-continuous band of filter material are also described in International Patent Application No. WO 93/24440 and may be used in this aspect of the present invention by equipping them with a filter material which is sufficiently open to allow slippage of a significant proportion of the small particle population in the slurry undergoing filtration. For instance, instead of a pressurised rotary filter of the type in which the liquid component of the slurry is displaced through the filter cake and the filter medium by means of a pressure differential established by pressurised gas or vapour, a multi-celled pressure drum filter of the BHS-Fest type may be employed in which the filter medium is in the form of a discontinuous band constituted collectively by sections of filter cloth material fitted into the individual cells of the drum and in which the pressure necessary to achieve such displacement is developed hydraulically, for instance by using liquid, e.g. water or fresh solvent (e.g. acetic acid or other monocarboxylic acid), to displace the liquid component of the slurry and a significant proportion of the small particle population through the filter cake developed in each of the filter cells whereby filtration is enhanced by the elimination of small particles from the filter cake. Another form of filter device which may be employed (also described in International Patent Application No. WO 93/24440) is a pressurised belt filter. Preferably, irrespective of the type of filter device used, the solids-liquid separation unit incorporates filter cake washing means whereby a suitable wash liquor is displaced through the filter cake to wash out substantially all of the residual mother liquor in the filter cake thereby reducing the level of impurities within the filter cake. The wash liquor is conveniently water, especially in the case where the filtration and washing is applied to the crude aromatic carboxylic acid obtained following liquid phase oxidation. Alternatively, the wash liquor may comprise fresh solvent corresponding to the monocarboxylic acid solvent (e.g. acetic acid) employed in the liquid phase oxidation reaction or it may comprise any other suitable wash liquor such as a xylene (e.g. p-xylene in the case of terephthalic acid production) or an acetic acid ester as described in UK Patent 2295149.

Washing within the solids-liquid separation unit (whether in the form of pressurised rotary filter, a multi-celled pressure drum or a belt filter) may be carried out in a number of stages, preferably in countercurrent fashion as described in International Patent Application No. WO 93/24440, to further enhance removal of impurities from the filter cake.

Small particle management may also be effected (additionally or alternatively) by diverting part of the slurry to a separate solids-liquid separation zone in which at least a proportion of the small particles present in the diverted slurry fraction is removed, the remaining solids then passing into the main solids-liquid separation zone.

Diversion in this manner is conveniently implemented at the location of slurry feed on to the filter medium. Thus instead of, or in addition to, employing a relatively coarse filter medium in order to slip small particles deliberately, the arrangement may be such that when the slurry is deposited onto a moving filter medium the slurry is permitted to spread upstream of the feed location whereby that part of the slurry which spreads upstream may reside temporarily in an area in which at least some degree of drainage of the mother liquor occurs before the diverted slurry fraction is transported back to the feed location and downstream thereof. As the slurry in practice tends to be relatively dilute (i.e. it has a high liquor content - typically 2 parts liquor to 1 part solids by weight), the slurry fraction spreading upstream of the feed location tends to be even more dilute and tends to contain smaller (and hence lighter) particles. By allowing slurry ingress to the upstream section of the filter medium, some degree of particle classification can be secured thereby facilitating elimination of a proportion of those particles which are sufficiently small to pass through the filter material with the advantage that, in this region of the filter medium, there will be no significant development of filter cake which would otherwise tend to capture and hence retain smaller particles rather than allow them to pass through with the mother liquor filtrate. This arrangement is conveniently implemented using a belt filter.

It will be understood that this aspect of the invention, involving small particle management within the solids-liquid unit(s), may be used in conjunction with, or instead of, the previously described technique in which small particle management is implemented by means of one or more particle classification devices upstream of the solids-liquid separation unit(s) and may also be used in conjunction with cooling of the slurry by the addition of cooler liquid to the slurry.

In another implementation of the invention, small particle management may be effected by providing a settling zone in or upstream of the solids-liquid separation unit in which the slurry is allowed to develop a supernatant fraction which will tend to contain suspended fine particles. This supernatant fraction can then be withdrawn thereby reducing the small particle population of the crystals progressing through the solids-liquid separation unit.

According to another aspect of the invention there is provided a process for filtering a slurry comprising depositing the slurry on to a moving band of filter material at a feed location, allowing the slurry to spread upstream of the feed location, collecting from a region upstream of the feed location a first filtrate which is relatively rich in small particles and collecting from a region downstream of the feed location a filtrate having a reduced content of small particles. In this manner, it is possible to secure selective filtration in such a way that a substantial proportion of the small particle population present in the slurry is removed ahead of filter cake formation on the filter band thereby reducing the potential for entrapment of small particles in the filter cake which can -otherwise reduce filtration efficiency. Moreover, by utilising part of the filter band upstream of the feed location, the effective area of filtration can be extended.

According to further aspect of the invention there is provided a solids-liquid separation unit comprising a moving filter belt, a slurry feeder for depositing slurry on to the upper run of the filter belt at a feed location, and means for collecting filtrate which passes through the filter belt at locations downstream of the feed location, the arrangement being such that slurry supplied to the filter belt is allowed to spread over the filter belt in a region upstream of the feed location and means is provided for collecting filtrate passing through the filter belt in said upstream region.

In one embodiment of the invention means is provided upstream of the feed location for contouring the filter belt to a channel shape, the contouring means being of openwork structure so that slurry deposited on the filter belt is free to spread upstream of the feed location and beyond the contouring means. The filter belt is typically constrained to follow a path such that upstream of the contouring means the belt is inclined and downstream of the contouring means it is substantially horizontal or, if inclined, much less so than the belt when upstream of the contouring means.

The collecting means associated with the upstream region may be stationary and arranged to collect filtrate which passes through the filter belt at locations upstream of the contouring means.

Means may be provided for applying a pressure differential across the filter belt downstream of the feed location. Such means may comprise a series of filtrate collecting vessels mounted for reciprocation relative to the direction of filter belt travel and means for applying suction to the interiors of the collection vessels over part of the reciprocation cycle.

As disclosed in our prior EP-A-502628 (the entire disclosure of which is incorporated herein), the filter belt may be mounted within a housing which is pressurised so that the pressure on the slurry side of the filter belt exceeds atmospheric pressure, e.g. so as to operate at under the pressure conditions specified in EP-A-502628.

A further aspect of the present invention is concerned with small particle management in such a way that mother liquor recovered from the slurry in the course of solids-liquid separation comprises separate fractions containing differing proportions of small particles.

As indicated previously, it is common practice to recycle a substantial fraction of the recovered mother liquor to the oxidation reaction in order to return catalyst and promoter to the oxidation reaction while purging a second, usually smaller, fraction to a solvent recovery system so as to maintain the level of impurities and by-products in the reaction within tolerable limits. In the solvent recovery system, the mother liquor purge is subjected to evaporation to remove a substantial proportion of the aliphatic acid solvent and water present (which can be returned to the oxidation reaction) leaving a concentrate containing the aromatic carboyxlic acid and impurities/by-products together with some of the heavy metal catalyst present in the original mother liquor filtrate. The concentrate (the residues) may then be disposed of or, if economically justifiable, treated in order to recover valuable components for recycling, e.g . catalyst metals. Typical downstream treatments of the residues include catalyst recovery, incineration and anaerobic/aerobic biological treatment to reduce chemical oxygen demand (COD).

According to this further aspect of the present invention there is provided a process for the production of an aromatic carboxylic acid in which a precursor of said aromatic carboxylic acid in admixture with an aliphatic carboxylic acid solvent is reacted in an oxidation reaction zone with oxygen or other oxidising agent, the aromatic carboxylic acid so produced is withdrawn from the reaction zone as a slurry of aromatic carboxylic acid crystals in mother liquor, separation of the aromatic carboxylic acid from the slurry is effected in such a way that two mother liquor fractions are obtained, a first fraction containing a higher amount of small particles of the aromatic carboxylic acid and a second fraction containing a lower amount of said small particles, and the first fraction is recycled to the oxidation reaction zone while at least part of the second fraction is purged to a solvent recovery system.

Solvent recovered in the solvent recovery system may be recycled to the oxidation zone and the residue obtained in the course of solvent recovery may be disposed of or treated in the manner referred to above.

Where the process employs a particle classification device or devices, the first fraction may be derived from the overflow from such device(s) while the second fraction may be constituted by the mother liquor filtrate obtained from the solids-liquid separation unit(s). In an alternative implementation of this aspect of the invention, the first and second mother liquor fractions may be obtained by carrying out filtration of the slurry on a moving filter medium and collecting filtrate from different points along the path of travel of the filter medium. The first fraction may be collected as filtrate from one or more upstream locations relative to the path of travel of the filter medium while the second fraction may be collected as filtrate from one or more downstream locations. Thus, for example, the first fraction may be collected in a zone or zones in which the depth of filter cake is less developed or has not developed at all to any significant extent while the second fraction may be collected in a zone or zones in which the depth of filter cake is more, or fully, developed.

The moving filter medium may be continuous, for example in the form of an endless band of filter material as used for instance in belt filters and rotary filters of the type comprising a cylindrical filter band. Alternatively the moving filter medium may be discontinuous as in a multi-celled drum filter such as a BHS-Fest drum filter as described above.

Conveniently the filter medium is one which has a relatively high porosity as described hereinbefore. Usually the oxidation reaction is carried out with the aromatic carboxylic acid precursor in admixture with an aliphatic monocarboxylic acid as solvent, e.g. a C2 to C6 monocarboxylic acid such as acetic acid, and in the presence of a catalyst, typically a dissolved heavy metal catalyst system including a promoter. For example, the heavy metal catalyst system may contain manganese and cobalt together with bromine as a promoter. Other metals may be present in the catalyst system, e.g. hafnium, zirconium or cerium. The oxidation reaction is typically carried out at a temperature within the range 150 to 250°C and a pressure in the range 6 to 30 bara.

For instance, in another aspect of the invention, which may incorporate any one or more of the other aspects and/or other features of the invention disclosed herein, the process for the production of an aromatic carboxylic acid such as terephthalic acid may be carried out under mild oxidising conditions at a relatively low temperature, for instance 195°C or lower (preferably 175 to 195°C, e.g. 185°C), and the cooling/precipitation phase may involve the use of a single crystallisation stage involving evaporation/flashing of solvent and/or the use of a cooler quenching liquid (usually comprising the solvent used in the oxidation reaction). To compensate for the increased impurity levels that accompany mild oxidation conditions, cooling of the slurry may be limited so that its temperature on being fed to the solids-liquid separation unit(s) is for instance 130°C or higher (preferably 130 to 175°C, e.g. 160°C or higher) whereby a greater proportion of the impurities remain in solution and pass through the filtration medium for recycle to the oxidation reaction, with appropriate purging to maintain the level of impurities within tolerable limits.

The source of oxygen may for instance be air, oxygen-enriched air, pure oxygen or oxygen combined with an inert gas. Also we do not exclude the possibility that the oxidation may employ an oxidising agent other than gaseous oxygen, e.g. a dissolved oxidising agent compatible with the reaction.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic view of plant for production of crude terephthalic acid in which small particle management is effected by means of a hydrocyclone and, optionally, quenching of the crude terephthalic acid slurry;

Figure 2 is a schematic view of a pressurised belt filter system for use in one form of the present invention;

Figure 3 is an enlarged fragmentary view showing the beach area and slurry feed poisition of the system shown in Figure 2; and

Figure 4 is a schematic view showing one form of belt forming device for use in the embodiment of Figure 2. Although the present invention is applicable to the manufacture of various aromatic aromatic carboxylic acids such as terephthalic acid, isophthalic acid, trimellitic acid, naphthalene dicarboxylic acid etc., it will suffice for an understanding of the invention to limit description to the production of terephthalic acid.

A detailed description of a process for the preparation of crude terephthalic acid and purification thereof to produce purified terephthalic acid is given in prior

EP-A-502628 and WO-A-93/24440, the entire disclosures of which are incorporated herein by this reference. As described in EP-A-502628 and WO-A-93/24440, a slurry of mother liquor and crude or purified terephthalic acid is passed to an integrated solids-liquid separation and solids washing unit in which the slurry is initially deliquored to remove a substantial proportion of the mother liquor, and a filter cake comprising terephthalic acid crystals and residual mother liquor is washed with water and deliquored to produce a washed filter cake containing residual water. In the case of crude terephthalic acid, the mother liquor comprises aliphatic monocarboxylic acid such as acetic acid while in the case of purified terephthalic acid the mother liquor comprises water and dissolved impurities such as paratoluic acid. As described in

WO-A-93/24440, the integrated solids-liquid and solids washing unit as used in the production of the crude and purified forms of terephthalic acid may take various forms, including pressurised belt filters (e.g. of the Pannevis type), pressurised rotary filters of the type having a cylindrical filter band which passes through a pool of slurry, pressure drum filters of the multi-cell type (e.g. BHS-Fest pressure drum filters) and centrifuges fitted with washing facilities. In practice, the throughput of terephthalic acid may be such that more than one unit is required to handle the slurry obtained from the oxidation or purification reaction.

Referring now to Figure 1 of the drawings, reference numeral 50 depicts a reactor in which paraxylene, in acetic acid solvent containing a dissolved manganese/cobalt/bromine catalyst system, is oxidised by air to produce terephthalic acid. The paraxylene, solvent and dissolved catalyst is supplied to the reactor via line 52 and air is supplied via line 54. The reaction is typically carried out at a temperature in the range of 150 to 250°C and a pressure in the range 6 to 30 bara. The heat of reaction is removed by an overheads condenser system 56, a major part of the condensate, comprising mainly acetic acid, being refluxed via line 58 and the remainder being passed via line 60 to a dehydration column 62 in which acetic acid with a small water content is produced as bottoms. The acetic acid/water bottoms product, typically at a temperature of about 95°C, is supplied via line 65 to mother liquor drum 64 which also receives mother liquor from one or more solids-liquid and filter cake washing units 66. The mother liquor collected in drum 64 is returned via line 67 to the reactor 50 either directly or after admixture with the fresh reactor feed supplied via line 52.

The product of the reaction, terephthalic acid is withdrawn from the reactor via line 68 as a slurry comprising crude terephthalic acid crystals in mother liquor which contains dissolved terephthalic acid. Typically the temperature of the slurry withdrawn from the reactor is about 170 to 210°C. The slurry is then cooled, for instance by being fed to a crystallisation section 70 which comprises one or more crystalliser vessels. In the crystallisation section, pressure and temperature are reduced to induce precipitation of further terephthalic acid with accompanying flashing off of liquor and venting of gases via line 73. Typically following crystallisation, the temperature and pressure of the slurry are in the ranges of about 90 to 160°C and about 0.5 to 6 bara respectively. The resulting slurry usually contains 15 to 60% by weight terepthalic acid. Conventionally the slurry obtained following completion of the coolingcrystallisation process is fed to a solids-liquid separation unit 66 in which washing of the filter cake may also be effected, wash liquor being supplied via line 72. However, in contrast with conventional practice, small particle management is applied following the cooling process. In the illustrated embodiment, this is achieved by passing the slurry from the crystalliser section 70 via line 71 to a hydrocyclone 74 which produces an overflow comprising mother liquor and small particles and an underflow comprising thickened slurry with a low small particle content. The underflow is fed via line 76 to the solids-liquid separation unit 66 while the overflow is fed via line 78 to the mother liquor drum 64 for recycle to the oxidation reactor. The hydrocyclone may also serve to eliminate gas present in the slurry withdrawn from the reactor.

The unit 66 may be an integrated solids separation and water washing unit (such as a belt filter, a rotary filter, a pressure drum filter or a centrifuge with washing facilities incorporated in the centrifuge), e.g. of the types described in International Patent Application No. WO 93/24440. The mother liquor filtrate derived from the unit 66 is passed to the mother liquor drum 64 via line 80 and a purge is drawn off via line 82 for supply to a solvent recovery system. The crude terephthalic acid recovered from the solids-liquid separation unit 66 may be purified for example in the manner described hereinbefore or it may be used without purification for the production of polyester in its relatively crude form.

In order to afford additional control of the small particle content of the slurry, the slurry obtained from the reactor via line 68 is cooled by introduction of a suitable liquid into the slurry. The cooler liquid used for quenching purposes is derived from any suitable source, e.g. acetic acid from the bottoms of the dehydration column 62, and is introduced into the slurry at a location upstream of the final crystalliser vessel of the crystallisation section 70. Thus, as shown, the quench liquid is introduced via line 84 at a location upstream of pressure letdown valve 81 of the crystallisation train; however, it is to be understood that the quench liquid may instead, or additionally, be introduced at any suitable location within the crystallisation train 70. For instance, the quenching liquid may be introduced into the slurry immediately upstream of the pressure letdown valve preceding the final crystalliser vessel. The introduction of quench liquid in this manner has been found to reduce the amount of fine particles present in the slurry and, in addition, because of the dilution effect allows more 4-CBA and other impurities to remain in solution thereby reducing contamination levels in the recovered terephthalic acid or allowing operation of the oxidation reactor with milder oxidation conditions (which leads to higher 4-CBA production). While we do not wish to be bound by any particular theory, it is thought that a reduction in the small particle population can be secured by the use of liquid quenching because flashing is less violent and, moreover, when liquid quenching precedes the crystallisation section, the increased dilution of the slurry fed to the crystalliser leads to a reduction in particle-to-particle attrition during crystallisation. In an alternative embodiment of the invention, cooling of the mother liquor containing suspended and/or dissolved aromatic carboxylic acid may be effected by addition of colder liquid, particularly acetic acid, without passing the liquor/slurry through one or more stages in which cooling is effected with accompanying flashing or evaporation of acetic acid and water from the mother liquor. Thus, it is envisaged that cooling may be effected largely by addition of cooler acetic acid from a suitable source to reduce the temperature of the liquor/slurry from reaction temperature to the temperature at which solids-liquid filtration is carried out by means of a belt or other band type filter under elevated pressure and temperature conditions.

In the embodiment of Figure 1 , small particle management is effected upstream of the solids-liquid separation unit. In alternative embodiments, small particle management is effected by means of the filter medium itself. Conventionally in the manufacture of aromatic carboxylic acids such as terephthalic acid, filtration of the slurries obtained in the course of the process is carried out with the aim of minimising slippage of crystals through the filter medium. This is implemented in practice by appropriate selection of the mesh size of the filter material. Small particle management by means of the filter medium may be effected, in this aspect of the present invention, by employing a relatively high porosity filter material, conveniently a suitable double layer cloth woven for example from PEEK (polyetheretherketone), so that a higher proportion of solids slippage with the mother liquor filtrate is deliberately engineered in contrast with previous practice where solids slippage has been regarded as anathema. Rather than having fewer larger holes, the filter medium is desirably one which has a large number of holes per unit area more or less corresponding in size to the maximum particle size which is to be allowed to slip. The use of high porosity filter cloths in the filtration of typical terephthalic acid slurries allows slippage of solids with the mother liquor, with small particles dominating the slippage. Filter cakes developed on such filter cloths have been observed to have enhanced liquor drainage rates and because drain rate is the main parameter influencing machine throughput, higher throughputs can be achieved as a result of allowing slippage of the small particle population in the slurry. This in turn leads to potential reduction in the number of filtration/washing units needed to handle the throughput in a commercial scale aromatic carboxylic acid production plant and hence reduced capital costs. In practice, the small particle content of the slurry feed to the filtration/washing unit(s) can vary with the consequence that the fines are high and the filtration rates are low. The use of a filter cloth with a high porosity cloth allows more consistent filtration rates to be achieved even though the fines content of the slurry may vary. The filter cloth employed may comprise a monofilament or multifilament weave and may be calendared or uncalendared, the latter being preferred since the calendaring of the filter cloths appears to reduce solids slip and reduce filtration rate.

Where the small particles are recovered by allowing deliberate slippage through the filter cloth, the filtrate may be segregated into portions having higher and lower small particle contents by weight. The portion containing a higher proportion of small particles may be recycled to the oxidation reactor while the portion having the lower small particle content may be purged, e.g. via a residue treatment facility which may include catalyst recovery. For example, where the filtration is carried out using a filter band (continuous or otherwise) the filtrate collected at upstream locations in the direction of travel of the band will tend to be richer in small particles than the filtrate collected at downstream locations. This difference in small particle content arises because, as the filter cake develops on the belt, it plays an increasing role in filtration of small particles with the consequence that the mother liquor filtrate collected at the downstream locations tends to be "cleaner" than that collected at the upstream locations. Mother liquor segregation into higher and lower small particle portions can therefore be achieved by maintaining mother liquor collected from one or more upstream locations separate from that collected from one or more of the downstream locations. Because the fraction of mother liquor collected from the downstream location(s) has a lower small particle content, its subsequent handling in a solvent recovery system is simplified since the proportion of terephthalic acid present in the form of small particles is reduced.

Referring now to Figures 2 to 4, a belt filter system is illustrated which is suitable for use in effecting solids-liquid separation and filter cake washing in the course of processing crude or purified terephthalic acid . As shown, the belt filter system comprises a continuous belt 10 of a suitable filter material having a generally horizontal upper run 12 on which the filter cake 14 is formed and a lower return run 16. The filter belt 10 is housed in a substantially pressure tight housing (not shown) and is driven by a motor which may also be enclosed within the housing or located externally of the housing. As described in a WO-A-93/24440, the interior of the housing is pressurised by means of a gaseous fluid. The upper run 12 is initially downwardly inclined and contacts a forming device 18 at the feed end of the machine which shapes the belt across its width so as to form a channel shape with upturned edges conforming with the profile of a set of suction boxes 20 mounted beneath the upper run 12. The boxes 20 are mounted for movement to and fro relative to the direction V of belt travel.

Filtration is effected by the creation of a pressure differential between the pressurised interior of the housing and the interior of the boxes 20. As described for instance in EP-A-51 1710 and EP-A-587223, the upper run 12 of the filter belt is sucked onto the suction boxes 20 during a suction cycle so that the suction boxes move with the upper run 12 and collect filtrate as the belt and hence the filter cake 14 advances from left to right in Figure 2. At the end of their travel, the suction is discontinued and the boxes are retracted for commencement of a further suction cycle. As well as profiling the filter belt across its width, the forming device 18 also serves to bring the upper run 12 of the belt into close proximity with the suction boxes so that the necessary suction can be applied to the slurry and filter cake transported by the upper run 12.

As described in EP-A-502628 and WO-A-93/24440, following contacting the slurry with the moving filter material, the following steps take place: Initially, after deposition on the slurry on the upper run via a feed device 22 which directs the slurry in a direction counter to the direction V of belt travel, the slurry is transported through a first zone A in which mother liquor (e.g. acetic acid or water) is removed through the filter belt and collected in the suction boxes, the slurry. This results in the development of the filter cake on the upper run 12. As belt travel continues, the filter cake enters a second zone B in which wash liquor (usually water) is applied to, and drawn through, the filter cake thereby washing the terephthalic acid particles forming the cake while also displacing residual mother liquor through the cake. The washing process may involve countercurrent washing as disclosed in EP-A-502628 and WO-A-93/24440. The wash liquor and residual mother liquor is collected in the boxes 20 and subsequently removed for further treatment/use in the manner described in EP-A-502628 and WO-A-93/24440.

Continued belt travel moves the filter cake into a third zone C where the filter cake is discharged from the filter belt with assistance if necessary, e.g. washing off with liquid (e.g. water), ejection by passing pressurised gas through the filter belt at the location of discharge or by means of a mechanical device such as a scraper, doctor blade, cheesewire etc.

In conventional belt filter designs, steps are taken to prevent feed slurry ingress into the area of the belt upstream of the forming device 18 (herein referred to as the "beach" area). The beach area being upstream of the forming device does not permit effective suction to be applied to the upper run of the belt in this area. Nevertheless, to facilitate small particle management in accordance with the present invention, the embodiment of Figure 2 is deliberately designed so as to allow the slurry ingress to the beach area upstream of the belt forming device 18, the belt forming device 18 being of a design which permits this, e.g. an openwork design as shown in Figure 3.

As the slurry S in practice tends to be relatively dilute (i.e. it has a high liquor content - typically 2 parts liquor to 1 part solids by weight), the slurry fraction S' (see Figure 3) entering the beach area tends to be even more dilute and tends to contain smaller (and hence lighter) particles. By allowing slurry ingress to the beach area, some degree of particle classification can be secured. As a consequence, in the beach area drainage of mother liquor from the slurry takes place by virtue of the gravity head available and is accompanied by the elimination of a proportion of those particles which are sufficiently small to pass through the filter material. The mother liquor and small particles removed in this way may be collected in a fixed collecting pan 19 and recycled (via line 21 , see Figure 3) for example back to the oxidation reactor or employed as a means of at least partially effecting a mother liquor purge from the process. The solids and residual mother liquor remaining on the filter belt are transported past the forming device for subsequent processing in the zones A, B and C. Because some degree of small particle elimination is effected upstream of the filtration zone, the number of small particles available to fill the interstices of the filter cake developed in zone A is reduced thereby securing a more porous filter cake with consequent reduction in the resistance to flow of mother liquor and, subsequently, wash liquor through the cake. Moreover, because use is made of the filter belt upstream of the belt forming device 18, the effective area available for filtration is extended.

As described above, the mother liquor is drained from the beach area under gravity. However, we do not exclude the possibility of applying suction to the beach area in order to draw mother liquor and small particles through the filter belt. For example, a fixed collection pan to which suction is applied may be located beneath the upper run 12 in the vicinity of the beach area. EXAMPLE 1

A belt filter was configured as shown in Figure 2 and was supplied with a slurry feed representative of the slurry obtained from the oxidation of paraxylene to produce terephthalic acid following by crystallisation. The slurry feed to the belt filter averaged 2,746 kg/hour and had an average solids content 35.3%w/w of which, on average, 6.9%w/w constituted fines (i.e. sub 36 micron size particles). Part of the feed slurry entered the beach area of the filter belt and a sample of the filtrate drained from this area was found to have a solids concentration of 4.14 wt% with a size distribution as follows: 10 wt% < 8 microns

25 wt% < 19.5 microns 50 wt% < 36 microns 75 wt% < 58.2 microns 90 wt% < 88.8 microns From measurements made of the wash liquor flow rate, the main filtrate flow and filter cake moisture, the average solids rate passing through the beach area was calculated to be 10.9 kg/hour compared with a calculated solids rate of 969 kg/hour in the feed slurry. The percentage of solids recovered essentially as fines through the beach is therefore 1 .1 % of the total solids supplied via the feed slurry. In this manner, a significant proportion of fines is excluded thus allowing a marked increase in the filtration and washing rate of the material from which the filter cake is developed. EXAMPLE 2

The effect of fines elimination from a slurry to be filtered was investigated by comparing a 'normal' feed and 'fines deficient' feed slurries of terephthalic acid crystals. The fines content of a 2 kg sample of filter feed was significantly reduced by elutriation and recovered. The fines fraction and the remaining bulk fraction were separately collected and dried. The particle size distribution of the two fractions was then determined using a Coulter LS130 Laser Diffraction Sizer and the results are set out in Table 1 .

From the two fractions two samples were prepared by re-slurrying the solids in production plant mother liquor (essentially 93% acetic acid, 6% water with the remainder made up of reaction catalyst, by-products and reaction intermediates). This gave slurries containing 40% by weight of solids with the following properties: Sample 1 The fines deficient fraction, containing 6.8% by weight of sub 36 micron particles.

Sample 2 The fines deficient fraction plus an extra 10% by weight of the fines fraction, yielding a sample with 15.2% by weight of sub 36 micron particles.

To measure filtration rates the samples were poured into a 100 cm² Buchner filtration unit, allowed 10 seconds for solids to settle, then filtered for a further 120 seconds under a vacuum of 10 inches of mercury. The filtration medium (cloth) used was a heat set non-calendered PEEK material. Experiments were carried out in duplicate and the average filtration rates in Table 2 were observed.

Table 2

"Normalisation to a cake depth of 35 mm As can be seen from Table 2, the addition of 10% of the fines fraction (essentially doubling the proportion of sub 36 micron particles) has lead to an approximate halving of the filtration rate.

Claims

1 . A process for the production of an aromatic carboxylic acid in which aromatic carboxylic acid and mother liquor in which the aromatic carboxylic acid is suspended and/or dissolved is cooled to produce a slurry of aromatic carboxylic acid crystals in mother liquor and in which following cooling the slurry is supplied to at least one solids-liquid separation unit to recover said crystals in the form of a filter cake, characterised by management of the slurry following such cooling so as to reduce the extent to which small particles in the slurry interfere with the solids-liquid separation process.

2. A process as claimed in Claim 1 in which at least 1 % by weight (more preferably at least 1 .5 wt%) of the aromatic carboxylic acid contained in the slurry withdrawn from the oxidation reaction zone per unit time is separated, predominantly in the form of small particles, from the remaining aromatic carboxylic acid slurry downstream of the cooling/precipitation process so as to reduce the small particle content of the filter cake formed in the solids-liquid separation unit(s), the separated fraction optionally being recycled at least in part to the reaction for the production of the aromatic carboxylic acid.

3. A process as claimed in Claim 2 in which at least 50 wt% of the separated fraction comprises particles which are up to 36 microns in size.

4. A process as claimed in any one of Claims 1 to 3 in which cooling of the slurry is effected by passing the slurry through one or more stages of crystallisation with accompanying reduction in pressure and temperature and flashing or controlled evaporation of solvent.

5. A process as claimed in any one of Claims 1 to 4 in which cooling of the slurry is effected at least in part by the addition of cooler liquid to the slurry.

6. A process as claimed in any one of Claims 1 to 5 in which small particle management comprises comprises passing the slurry through one or more particle classification devices prior to introducing it into the solids-liquid separation unit.

7. A process as claimed in Claim 6 in which the particle classification is effected centrifugally.

8. A process as claimed in any one of Claims 1 to 7 in which small particle management is effected within the solids-liquid separation unit(s).

9. A process as claimed in Claim 8 in which the or each solids-liquid separation unit comprises a high porosity filter medium.

10. A process as claimed in Claim 8 or 9 in which the solids-liquid unit comprises a filter medium which is sufficiently open with respect to small particles that at least 1 % by weight of the aromatic carboxylic acid contained in the slurry withdrawn from the oxidation reaction zone per unit time is separated, predominantly in the form of small particles, from the remaining aromatic carboxylic acid content of the slurry.

1 1 . A process as claimed in Claim 9 or 10 in which the solids-liquid separation unit is of the type in which the filtration is effected or facilitated by developing a pressure differential across the filter medium.

12. A process as claimed in any one of Claims 1 to 1 1 in which small particle management is effected by diverting part of the slurry to a separate solids-liquid separation zone in which at least a proportion of the small particles present in the diverted slurry fraction is removed, the remaining solids then passing into the main solids-liquid separation zone.

13. A process as claimed in Claim 12 in which the arrangement is such that when the slurry is deposited onto a moving filter medium the slurry is permitted to spread to a region upstream of the feed location and in which filtration takes place in such upstream region.

14. A process as claimed in Claim 12 in which such diversion is implemented at the location of slurry feed on to the filter medium.

15. A process as claimed in Claim 12 in which the solids-liquid separation is carried out using a moving filter medium, the slurry being fed to the filter medium at a feed location in such a way that part of the slurry is allowed to spread upstream of that location while the remainder is transported downstream by the moving filter medium to a location or locations at which suction is applied to filter the slurry, and in which that mother liquor is separated from that part of the slurry diverted upstream of the feed location.

16. A process as claimed in any one of Claims 1 to 13 in which small particle management is effected by providing a settling zone in or upstream of the solids-liquid separation unit in which the slurry is allowed to develop a small particle-rich supernatant fraction which is separated from the slurry thereby reducing the small particle population of the crystals progressing through the solids-liquid separation unit.

17. A process as claimed in any one of Claims 1 to 16 in which the mother liquor comprises a lower aliphatic carboxylic acid solvent used in the production of the aromatic carboxylic acid by the liquid phase oxidation of a precursor thereof or an aqueous medium used in the purification of the aromatic carboxylic acid.

18. A solids-liquid separation unit comprising a moving filter belt, a slurry feeder for depositing slurry on to the upper run of the filter belt at a feed location, and means for collecting filtrate which passes through the filter belt at locations downstream of the feed location, the arrangement being such that slurry supplied to the filter belt is allowed to spread over the filter belt in a region upstream of the feed location and means is provided for collecting filtrate passing through the filter belt in said upstream region.

19. A unit as claimed in Claim 18 including means upstream of the feed location for contouring the filter belt to a channel shape, the contouring means being of openwork structure so that slurry deposited on the filter belt is free to spread upstream of the feed location and beyond the contouring means.

20. A unit as claimed in Claim 19 in which the filter belt is constrained to follow a path such that upstream of the contouring means the belt is inclined downwardly towards the contouring means.

21 . A unit as claimed in Claim 18 or 19 in which the collecting means associated with the upstream region is stationary and arranged to collect filtrate which passes through the filter belt at locations upstream of the contouring means.

22. A unit as claimed in any one of Claims 18 to 21 including means for applying a pressure differential across the filter belt downstream of the feed location.

23. A process for the production of an aromatic carboxylic acid in which a precursor of said aromatic carboxylic acid in admixture with an aliphatic carboxylic acid solvent is reacted in an oxidation reaction zone with oxygen or other oxidising agent, the aromatic carboxylic acid so produced is withdrawn from the reaction zone as a slurry of aromatic carboxylic acid crystals in mother liquor, separation of the aromatic carboxylic acid from the slurry is effected in such a way that two mother liquor fractions are obtained, a first fraction containing a higher amount of small particles of the aromatic carboxylic acid and a second fraction containing a lower amount of said small particles, and the first fraction is recycled to the oxidation reaction zone while at least part of the second fraction is purged to a solvent recovery system.

24. A process as claimed in Claim 23 when combined with a process as claimed in any one of Claims 1 to 17.

25. A process as claimed in Claim 23 or 24 in which solvent recovered in the solvent recovery system is recycled to the oxidation zone.

26. A process as claimed in Claim 24 or 25 when dependent on Claim 6 or 7 in which the first fraction is derived from the overflow from such device(s) while the second fraction is constituted by mother liquor filtrate obtained from the solids-liquid separation unit(s).

27. A process as claimed in any one of Claims 23 to 25 in which the first and second mother liquor fractions are obtained by carrying out filtration of the slurry on a moving filter medium and collecting filtrate from different points along the path of travel of the filter medium.

28. A process as claimed in Claim 27 in which the first fraction is collected in a zone or zones in which the depth of filter cake is less developed or has not developed at all to any significant extent while the second fraction is collected in a zone or zones in which the depth of filter cake is more, or fully, developed.

29. A process as claimed in any one of Claims 13 to 16, 27 or 28 in which the moving filter medium is continuous.

30. A process for the production of an aromatic carboxylic acid by the oxidation of a precursor thereof in a liquid phase medium comprising said precursor and an aliphatic carboxylic acid solvent at a temperature no greater than 195°C, in which process the resulting slurry of aromatic carboxylic acid is cooled by means of a single crystallisation stage involving evaporation/flashing of solvent and/or by means of the addition of a cooler quenching liquid and is then fed to a solids-liquid separation unit, the cooling being carried out so that the slurry fed to the solids-liquid seperation unit is at a temperature of at least 130°C.

31 . A process for filtering a slurry comprising depositing the slurry on to a moving band of filter material at a feed location, allowing the slurry to spread upstream of the feed location, collecting from a region upstream of the feed location a first filtrate which is relatively rich in small particles and collecting from a region downstream of the feed location a filtrate having a reduced content of small particles.