This invention relates to devices for use in transferring particulate material from an elevated pressure zone to a lower pressure zone. The need to effect such transfer can arise for instance in solid-liquid separation processes carried out at elevated pressure. A commonly used method for effecting the transfer involves slurrying the separated particulate solid material with liquid so that the resulting slurry can in effect be treated as a liquid and transferred from the high pressure zone to the low pressure zone via a control valve. Although this is a very effective method of engineering the transfer, in addition to slurrying the solid prior to the transfer, it is also necessary to subject the slurry to a further solid-liquid separation process following the transfer.

An alternative method is to effect the transfer without slurrying the particulate solid material in the "dry" state. In practice, the "dry" solid is usually in the form a moist mass since, inevitably, a residual amount of liquid is still present in the solid material following the solid-liquid separation at elevated pressure. In one known arrangement, the moist mass is introduced into successive compartments of a multi-vaned rotary valve arrangement which is rotated to move each compartment between positions in which it registers with the higher and lower pressure zones respectively. Such an arrangement is described in the literature - see for instance Pages 252 and 253 of the textbook "Industrial Filtration of Liquids" by D B Purchas (1967 edition, Chemical and Process Engineering Series published by Leonard Hill). Another example of effecting the transfer without slurrying the particulate material is the BHS-Fest rotary drum pressure filter in which, following filtration and washing, the "dry" solid material is transferred to a section of the drum housing which is open to the lower pressure zone. The above-mentioned textbook also gives details of the BHS-Fest pressure filter.

Where the solid-liquid separation takes place under elevated temperature conditions, the problem of effecting the transfer from the higher to the lower pressure zone is exacerbated since moving parts of the pressure-isolating device have to be manufactured to tight tolerances in order to maintain pressure isolation and this leads to design and materials problems in terms of coping with temperature differentials.

According to one aspect of the present invention there is provided a method for effecting transfer of particulate material to a lower pressure zone from an elevated pressure zone, in which elevated pressure zone the particulate material is contacted with a fluid at elevated temperature and subjected to a solid-liquid separation process, said method comprising effecting the transfer of the particulate material obtained from said solid-liquid separation process by means of a pressure-isolating device and heating said device to compensate for temperature differentials that would otherwise be produced within the device by contact with the hot particulate material passing through the device.

Preferably the device is heated by means of said fluid, which may be liquid or gaseous.

In this manner, it is possible to ensure that thermal expansion effects are minimised without necessarily having to employ special materials for this purpose. In addition, where the properties of the particulate material are affected by temperature change on contact with internal surfaces of the device, such variations in properties can be minimised. The device may be heated by said fluid before and/or after the fluid is contacted with said particulate material in the elevated pressure zone.

The invention has particular application to the production of terephthalic acid and may be employed in the transfer of crude and/or pure terephthalic acid crystals from a zone under elevated pressure and temperature conditions to a zone at lower temperature and pressure conditions.

In one embodiment of the invention, the solid-liquid separation process typically involves a washing step in which a washing medium is contacted with the particulate material and is then separated from the particulate material whereby the level of contamination present in the particulate material is reduced. One instance of this is disclosed in our copending International Application No. PCT/GB/01019 which is concerned with the production of purified terephthalic acid in which, following hydrogenation of an aqueous solution of crude terephthalic acid and crystallisation of purified terephthalic acid, a slurry containing the purified terephthalic acid is subjected to an integrated filtration and washing process under elevated pressure and temperature conditions before a moist mass of the purified terephthalic acid is transferred to a lower pressure zone by means of a pressure-isolating device such as a rotary valve.

In such an embodiment, said device is preferably heated at least in part by means of the liquid with which the solid material is washed in said elevated pressure zone, said wash liquid being brought into heat exchange relation with said device before and/or after it is contacted with the particulate material in the higher pressure zone. Usually it is preferred that the wash liquid is brought into heat exchange relation with said device prior to contacting the wash liquid with the particulate material as the wash liquid, following such contact, is likely to contain some solids matter which may be undesirable in a heat exchange arrangement and may therefore have to be removed by filtration prior to heat exchange.

In accordance with a second aspect of the present invention there is provided a process for the production of terephthalic acid in which a mass of terephthalic acid crystals is washed with a wash liquid in a zone at elevated temperature and pressure resulting in a moist mass of washed terephthalic acid crystals and in which said moist mass is transferred to a lower pressure zone without reslurrying the same, such transfer being effected by means of a pressure-isolating device which is heated using said wash liquid. The terephthalic acid referred to in said second aspect of the invention may be crude terephthalic acid or purified terephthalic acid.

Crude terephthalic acid (CTA) may be produced by the liquid phase oxidation of p-xylene in an aliphatic carboxylic acid solvent (typically acetic acid) in the presence of a catalyst system

comprising heavy metals, such as cobalt and manganese, and bromine as a promoter. Following separation from the reaction medium and washing, the terephthalic acid derived from the oxidation reaction may be utilised directly in the production of polyesters where the level of contamination can be tolerated in the end product. In this event, the moist mass of CTA crystals may be transferred from the elevated pressure and temperature zone to a lower pressure temperature zone.

However, for some polyester end products, the terephthalic acid must be first purified. Such purification typically comprises adding an aqueous medium to the CTA to form a slurry thereof which is then heated to dissolve the CTA in the medium to provide an aqueous solution of terephthalic acid. This solution is then passed to a reduction step in which the solution is contacted with hydrogen under reducing conditions in the presence of a heterogeneous catalyst to reduce chemically organic impurities, for example 4-carboxybenzaldehyde (4-CBA). The hydrogenated solution is passed to pressure let-down vessels in which pure terephthalic acid crystals form to provide a slurry of PTA in aqueous medium. PTA can be recovered from the aqueous medium by crystallisation followed by solid-liquid separation and washing.

In both instances, the wash liquid used in washing CTA or PTA may, in accordance with said second aspect of the present invention, be employed in heating the pressure-isolating device for effecting transfer of the washed CTA or PTA crystals from a zone under elevated pressure and pressure conditions to a zone at lower pressure and temperature conditions. The washing step is preferably carried out as part of an integrated separation and washing process in which a slurry of CTA or PTA crystals in a reaction medium (usually acetic acid in the case of CTA and water in the case of PTA) is first filtered to leave a moist mass of TA crystals and in which the moist mass is then washed to displace residual reaction medium, the filtration and washing steps being carried out in the same item equipment, preferably a pressure filter such as a belt filter.

Instead of, or in addition to, using the wash liquid to effect heating of the pressure-isolating device, the fluid may be constituted by another component which may be gaseous. For instance, where the separation is effected using a pressure filtration system, the pressure on the upstream side of the filter cake may be established by means of a gaseous fluid which is the same as a liquor with which the particulate material is combined (eg as a slurry) and this fluid may be used to effect heating of the pressure-isolating device.

Thus, in accordance with a further aspect of the present invention there is provided a method of separating a particulate solid material (usually in the form of crystals) from a solute-containing liquor, said method comprising: filtering the slurry by means of a filter medium; establishing a pressure differential between the upstream and downstream sides of the filter medium by supplying a pressurised fluid to the upstream side of the filter medium, said pressurised fluid being constituted at least in part by a solvent component of the liquor in the

vapour phase whereby flashing of said solvent component within the liquor is substantially suppressed when contacted by said pressurised fluid; transferring the filter particulate material from the upstream side of the filter medium to a lower pressure zone by means of a pressure-isolating device; and heating said device by means of said fluid used pressurise the upstream side of the filter medium. By employing a pressurised fluid based on the solvent component actually present in said liquor or said liquid, it is possible to conduct the filtration in such a way that the filter cake is not chilled or at least only cooled to a somewhat lesser extent than is the case where the pressurised fluid comprises nitrogen or other inert gas. It is highly desirable that the partial pressure of said solvent component in or forming the pressurised fluid on the downstream side of the filter medium is maintained substantially at or above the vapour pressure of the solvent component actually present in the slurry fed to the upstream side of the filter medium.

The pressurised fluid may consist substantially wholly of said solvent component in the vapour phase but we do not exclude the possibility of incorporating in the pressurised fluid a non-condensible gas (usually one which is inert with respect to the liquor/crystals system, eg nitrogen). By non-condensable, we mean a gas which is only condenses at temperatures somewhat below normal room temperature and pressure.

Where a mixture of the solvent component and a gas are employed, the composition should desirably be such that the partial pressure of the solvent component in the pressurised fluid is substantially at or above the vapour pressure of the solvent component in the slurry at the feed condition of the latter.

Usually the solvent component will comprise the major fraction (by volume) of said pressurised fluid. Typically the filter medium undergoes movement during the filtration process; such movement may be continuous or otherwise (for instance, it may be indexed motion).

Preferably the filtration is carried out in such a way that the temperature differential across the filter cake is essentially zero, and typically no more than 1 to 2°C.

Preferably the method also includes at least one stage of washing the filter cake while jn situ on the filter medium.

Where said crystals to be recovered are of terephthalic acid, the solvent comprises water or a carboxylic acid such as acetic acid.

Thus, for instance where the method of the present invention is employed for the filtration of the product stream resulting from from the liquid phase oxidation of para-xylene in acetic acid as solvent, the product stream comprises a slurry of relatively crude terephthalic acid in a liquor containing mainly acetic acid. In accordance with said further aspect of the present invention, subsequent filtration of the slurry may be effected using acetic acid vapour as the pressurised fluid. The acetic acid may, but need not necessarily, be derived from the acetic acid evaporated

from the liquor during a crystallisation process (which may be conventional) carried out following withdrawal of the acetic acid/terephthaiic acid slurry from the oxidation reactor. In this case therefore, following the crystallisation process in which further terephthalic acid crystallises out from the liquor, the slurry is passed to a filtration zone in which the method according to the invention is carried out using, as the pressurised fluid, acetic acid vapour derived from the crystallisation process.

Similarly where, in accordance with widespread practice, terephthalic acid (however obtained) is purified by dissolving the same in water and subjecting the solution to hydrogenation, the purified terephthalic acid is subsequently recovered by filtration using water vapour as the pressurised fluid, ie. in the form of steam. Again the steam may, but need not necessarily, be derived from a crystallisation process following the purification process.

The terephthalic acid to be purified (and treated in accordance with any of the aspects of the invention defined above) will usually be derived from the liquid phase oxidation of para-xylene. However, it may be derived from other sources, eg. by treatment of recycled terephthalic acid obtained from polyalkylene terephthalate products such as bottles.

The purification process may, if desired, by preceded by oxidation (pre-oxidation) of the crude terephthalic acid obtained from the liquid phase oxidation of para-xylene, such pre-oxidation being carried out with the crude TA in aqueous solution using an oxidising agent, such as air, molecular oxygen or other agent which need not necessarily be in the gaseous phase, and in such a way as to oxidise impurities and, in particular, to oxidise 4-carboxybenzaldehyde to terephthalic acid.

Terephthalic acid in the moist state, eg containing up to 2 to 20% by weight moisture, tends to be very sticky. The pressure-isolating device is therefore preferably a positive displacement device which is arranged to positively eject the moist mass of material into the lower pressure zone and may take various forms - eg a ram-type pump, a screw feed device or a progressive feed device such as a progressive cavity pump of the type used to pump cold pastes of high solids contents. Alternatively, the pressure-isolating device may comprise a rotary valve in which the flashing of the material within the cavity as the valve cavity moves from the high pressure side to the lower pressure side serves to discharge, or assist discharge, of the material from the valve on the lower pressure side.

The pressure-isolating device may comprise a lock hopper arrangement in which a positive displacement device such as a ram pump, screw feeder or progressive cavity feeder is effective to move the material to be transferred through the lock hopper arrangement.

According to another aspect of the present invention there is provided a process for the purification of crude terephthalic acid, comprising:

(a) dissolving the crude terephthalic acid in an aqueous medium to produce a terephthalic acid-containing solution;

(b) contacting the terephthalic acid solution with hydrogen under reducing conditions and at elevated temperature and pressure to reduce chemically at least some of the impurity present in the crude terephthalic acid;

(c) effecting crystallisation to obtain a slurry comprising pure terephthalic acid crystals in aqueous medium ;

(d) in a first zone at elevated pressure and temperature effecting separation of the PTA crystals from the aqueous medium by filtration of the slurry to remove aqueous medium through a filter surface to obtain a moist mass of crystals, said elevated pressure being established by means of a fluid in its vapour phase; (e) transferring said moist mass of crystals to a second zone at elevated pressure and temperature without reslurrying said mass and supplying aqueous wash liquid to said mass while effecting filtration whereby the wash liquid is displaced through the mass and through a filter surface in the second zone, said elevated pressure in the second zone being established by means of a fluid in its vapour phase; (f) transferring the washed crystals to a third zone at lower pressure without reslurrying the crystals, such transfer being effected by means of a pressure-isolating device; and (g) using the aqueous medium obtained in step (d) and/or the wash liquid employed in step (e) and/or said pressurising fluid to heat the pressure-isolating device.

Preferably the wash liquid is used to heat the pressure-isolating device, the wash liquid being brought into heat exchange relation with the device prior to being used in step (e).

In at least the first and second zones, preferably a differential pressure is maintained across the filter surface such that, on the lower pressure side of the filter surface, the pressure is substantially the same as or greater than the pressure prevailing following step (c).

Step (c) will normally be carried out in a series of crystalliser stages, in which the pressure and temperature is reduced progressively. Thus, said pressure differential will be such that, on the lower pressure side of the filter surface, the pressure is at least equal to the pressure prevailing in the final crystalliser stage, which will normally be superatmospheric, eg within the range 1.5 to 15 bara, more preferably 3 to 10 bara.

Preferably the pressure differential will be such that, on the lower side of the filter surface in each of said zones, the pressure is at least equal to the pressure prevailing following step (c). Advantageously steps (d) and (e) are carried out by discharging said slurry on to a filter material which is movable to transport the terephthalic acid through said first zone in which filtration of said aqueous medium from the slurry is effected, to the second zone in which said mass of crystals is washed by displacement of said aqueous wash liquid through said mass. In this way, reslurry of the terephthalic acid is avoided and by effecting filtration through a filter surface in such a way that the lower pressure side of the filter surface is at a pressure no less than said superatmospheric pressure, liquid removal from the terephthalic acid can be effected substantially without accompanying flashing thereby reducing the tendency for soluble impurities

to precipitate and contaminate the mass of purified terephthalic acid. In addition, any tendency for material to precipitate and foul the filter medium is reduced.

The filter material is suitably a metal gauze, or a cloth comprising a suitable temperature resistant plastics material. The filter surface is suitably in the form of a band, preferably a continuous band which is moved continuously or intermittently to convey material comprising terephthalic acid through the first and second zones.

Usually the CTA employed in the production process of the invention is derived from the oxidation of paraxylene in a liquid reaction medium containing acetic acid to produce a slurry of CTA in the reaction medium. However, we do not exclude the possibility of the CTA being derived by another route, eg by chemical treatment of a polyalkylene terephthalate. The liquid reaction medium normally incorporates a catalyst, for example a cobalt/manganese/bromide catalyst system which is soluble in the reaction medium. Suitably the oxidation is carried out in the presence of an oxygen source for example air, at a pressure of 5 to 30 bars absolute, and preferably an oxygen concentration of 0 to 8% by volume in the gas leaving the reactor and at a temperature of 150 to 250°C. It is suitably a continuous process, and is preferably carried out in a stirred reactor. The reaction is exothermic and the heat of the reaction may conveniently be removed by evaporation of water and acetic acid from the reaction medium.

The water and acetic acid evaporated from the reaction medium is preferably distilled to produce acetic acid having a lower water content. Acetic acid having a lower water content obtained in this way may be passed to the oxidation step and the water recovered from distillation may be used as the aqueous medium for dissolving CTA and/or as the aqueous wash liquid for washing PTA.

Following the oxidation step, the CTA produced may then be separated from the reaction medium conventionally by centrifugal separation and drying but it is more expedient to exchange the reaction medium, preferably continuously, for an aqueous medium to provide a terephthalic acid stream comprising CTA in aqueous medium in a process such as that disclosed in our prior co-pending EP-A-502628.

If desired, following combination of the CTA with aqueous medium to produce the terephthalic acid-containing solution, the resulting solution may be fed directly to the hydrogenation step or altematively, may be treated prior to it being fed to the hydrogenation step. Such treatment may comprise subjecting the aqueous terephthalic acid-containing solution to oxidation to increase the conversion of terephthalic acid precursor compounds, especially 4-CBA, into terephthalic acid. Such oxidation may be effected in the aqueous phase using air or gaseous oxygen or a non-gaseous oxidising agent may be employed. Suitably the heterogeneous catalyst employed in the purification of the crude terephthalic acid product is a supported noble metal catalyst, for example platinum, rhodium and/or preferably palladium on an inert, for example carbon, support. The reduction is suitably carried out by passing the terephthalic acid solution comprising terephthalic acid and impurities, for example

4-carboxybenzaldehyde, through a flooded bed of catalyst at a temperature of 250 to 350°C in the presence of hydrogen. The solution suitably comprises 20 to 50% by weight of terephthalic acid.

The terephthalic acid solution, after reduction, is suitably cooled in a crystallisation process to a temperature in the range 100 to 220°C, typically 135 to 180°C, and a pressure of 3 to 10 bara, to produce solid purified terephthalic acid product.

Desirably, at least part of the aqueous medium removed in the first zone and at least part of the aqueous wash removed through the filter surface in the second zone is recovered and combined, directly or indirectly, with the CTA. The aqueous wash and/or medium thus desirably constitute at least a part of the aqueous medium with which the CTA is combined. If both the aqueous medium and the aqueous wash are recycled, they may be mixed together to form a single stream prior to combination with the CTA. The aqueous medium and the aqueous wash may be treated, for example by distillation and/or evaporation to produce substantially pure water or at least partially eliminate para-toluic acid, either individually before mixing, or as a single stream after mixing prior to being combined with the CTA as desired. Such treatment of the aqueous medium and/or wash may also comprise cooling, preferably to a temperature in the range 15 to 100°C, or evaporation of the aqueous medium and/or wash to produce a less pure precipitate and a residual mother liquor which are then suitably separated. Suitably, where the CTA is produced by an oxidation plant integrated with the purification plant, such less pure precipitate is returned to oxidation step of the oxidation plant. The mother liquor may be treated further and/or used as aqueous medium to be combined with the CTA.

The second zone desirably comprises a single stage wash in which the wash liquid passes through the filter surface only once either as a single stream or, following splitting of the wash liquid, as a plurality of streams. If desired the second zone may comprise a succession of wash stages wherein the wash liquid is passed through the filter surface more than once. The succession of wash stages may be co-current but is preferably counter-current in which, in each stage (other than the last), the incoming aqueous wash passed through the wet mass of crystals and the filter surface is the aqueous wash which has passed through the wet mass and the filter surface in the succeeding stage. In the last stage the aqueous wash liquid is preferably fresh incoming water. The wash liquid is preferably introduced at a temperature which is substantially the same as the temperature of said mass as it enters the second zone so as to avoid problems with flashing or quenching (with the consequent risk of precipitating impurities).

The wash liquid is suitably at least in part water separated from acetic acid in the aforesaid distillation step following evaporation of acetic acid and water from the oxidation step, if present, or derived from other water streams within the process, for example from treatment of the aqueous wash and/or medium. This is advantageous as it reduces further the intake of fresh water and disposal of water in the process.

Typically the pressure differential across the filter surface in each of said first and second zones is at least 0.05 bar with the side of the filter surface on which the mass of terephthalic acid crystals is located being at a higher pressure than the other side of the filter. Preferably the pressure differential is 0.1 to 10 bar, more preferably, 0.2 to 3 bar and especially 0.2 to 1 bar, for example 0.3 bar.

The actual pressure on the lower pressure side of the filter is maintained at such a pressure that the aqueous wash liquid in the second zone and, if applicable, the aqueous medium in the first zone which are removed through the filter surface, remain substantially in the liquid phase. The higher pressure side of the filter surface is preferably maintained at elevated pressure, desirably at 2 to 15 bara and especially 3 to 10 bara and is desirably above the pressure of the preceding pressure-reducing step in the process.

The terephthalic acid slurry is suitably introduced into the first zone at a temperature of at least 60°C and preferably 100 to 200°C, especially 120 to 180°C.

Suitably the slurry is deposited in such a way that the saturation pressure of the feed is less than the absolute pressure on the lower (downstream) side of the filter medium.

Deposition of the terephthalic acid stream at elevated temperature and pressure is advantageous as improved filtration is possible due to the aqueous medium being less viscous at elevated temperature. Furthermore there is less co-crystallisation of impurities for example p-toluic acid, with the terephthalic acid product at elevated temperature. Thus a higher purity terephthalic acid product is obtained and there is a correspondingly higher level of impurities for example p-toluic acid in the aqueous medium which is desirably recycled within the process. The elevated temperature may also permit heat recovery and hence provide a reduction in variable costs.

The invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 is flow sheet of a terephthalic acid production process in accordance with the present invention;

Figure 2 is a schematic view of a belt filter system for use in the process of Figure 1 and which integrates separation and washing stages; Figure 3 is a schematic view of an arrangement for effecting transfer of terephthalic acid from a zone at elevated pressure and temperature to a zone at a lower pressure and temperature; and Figure 4 is a schematic view of a belt filter system in which pressurisation is established by means of steam.

Referring to Figure 1 , reactor A is fed with paraxylene and acetic acid containing a dissolved catalyst comprising cobalt, manganese and bromine ions by line 1 and with air via line 2. Product from the reactor A is passed to crystallisation section B by line 3. The temperature within the reactor A is controlled by evaporating a mixture of acetic acid and water from the reactor to a condensing system C via line 4 and is typically above 150°C. Most of the condensate

is returned to the reactor A via line 5 with πoncondensibles venting via line 6. To control the water content of the reactor A, part of the condensate is removed from the condensing system via line 7 and passed to the distillation column D.

In the crystallisation section B the temperature is dropped to approximately 80°C to 150°C and the slurry containing crystalline terephthalic acid in reaction mother liquor (mainly acetic acid) thereby produced is passed to a separation stage E. Acetic acid and/or water may be recovered from crystallisation section B and passed to the distillation column D via line 8 and/or to the reactor A via line 9a. The crystallisation section B typically comprises a series of crystalliser vessels in which the pressure and temperature of the slurry withdrawn from the oxidation reactor A is progressively reduced. In a typical embodiment of the invention, the resulting slurry of crude terephthalic acid in acetic acid-based mother liquor is reduced to a pressure in the range 0.5 to 2 bara and a temperature in the range of 90 to 130°C.

Separation stage E may be a conventional centrifuge and drier arrangement but is preferably an integrated separation and washing stage in which a continuous solvent exchange process is carried out, eg. as disclosed in our prior EP-A-502628. Reaction mother liquor recovered from stage E is returned in part via lines 9 and 9a to the reactor A optionally by first mixing with the fresh catalyst, paraxylene and acetic acid contained in line 1. Any remaining reaction mother liquor and any wash liquid is suitably passed to an evaporation stage F in which water and acetic acid vapour is removed by line 10, condensed and passed to reactor A via line 10a or optionally passed to distillation column D via line 10b. A purge of by-products and catalyst is withdrawn via stream 11.

The solids material, ie crude terephthalic acid (CTA), recovered from the separation stage E may in some circumstances be acceptable for use in the production of certain polyester end-products and may, in this event, be extracted for polyester production without further purification. In this case, the moist CTA filter cake will be transferred from the elevated pressure and temperature conditions prevailing in the integrated separation and washing stage to a lower pressure, eg. atmospheric pressure.

Where however the terephthalic acid is required to be of greater purity, the CTA is transferred to reslurry stage G. In reslurry stage G, the CTA crystals are reslurried with water recovered from the distillation column D via lines 12, 12a and/or other water which may be recycle mother liquor via stream 13, recycle mother liquor via stream 14 and/or demineralised water via stream 15. The slurry produced in this stage is heated in section H to a temperature of for example 250 ^β C to 350°C to form an aqueous solution of CTA which is passed to reactor J in which it is reacted with hydrogen over a fixed bed palladium catalyst thus reducing impurities in the solution and then crystallised in crystallisation section K. The temperature to which the solution is cooled in the crystallisation section K and the rapidity of cooling is adjusted to produce the appropriate purity of the desired terephthalic acid product. The crystallisation section K typically comprises a series of crystalliser vessels in which the pressure and temperature of the

hydrogenated solution is progressively reduced. In a typical embodiment of the invention, the resulting slurry of purified terephthalic acid in aqueous mother liquor is reduced to a pressure in the range 3 to 10 bara and a temperature in the range of 135 to 180 ^β C.

The slurry from the final stage of the crystallisation section K is transferred at the pressure and temperature prevailing in the final crystalliser stage to section L in which an integrated separation and washing process is carried out. Thus, in stage L, PTA crystals are separated from the aqueous mother liquor and the separated PTA product is washed with water which may be derived from column D via lines 12, 12b, recovery stage M via line 17 and/or fresh water via line 18 and is recovered, following washing, via line 19. The aqueous mother liquor from the separation in stage L is passed to recovery stage M via line 13a and/or to reslurry stage G via line 13.

In stage M the aqueous mother liquor is evaporated or further cooled so as to permit the recovery of further solids in the form of a less pure precipitate of terephthalic acid which is passed back to reactor A via stream 20. In stage M the temperature of the liquor may be reduced by flashing steam from it at, for example, atmospheric pressure. Such steam may be further purified for example by distillation in column D via line 22 and used if desired as wash in stage L, used elsewhere in the process or purged. The remaining liquor may be cooled or evaporated further and solids separated from it and recycled to reactor A via line 20 as desired.

The mother liquor recovered from stage M may be in part passed back to the distillation column D via line 22 and processed as described later, may be returned to the reslurry stage G via stream 14 and/or be purged via stream 21. Preferably, if the aqueous mother liquor is evaporated, the evaporated water is returned to the reslurry stage G via line 14.

The distillation column D fractionally distils a mixture of water and acetic acid evaporated from the reaction medium in condenser system C and is modified if required for use for the treatment of mother liquor separated from stages F and M . The column D comprises three zones; an upper zone comprising for example 5 theoretical stages, a middle zone comprising for example 45 theoretical stages and a lower zone comprising for example 5 theoretical stages. Part of the mixture of acetic acid and water evaporated derived from the reactor A is passed via stream 7 optionally together with stream 8 and/or 10b to between the middle and lower zones of the column D. Mother liquor from the precipitation of terephthalic acid may be passed into the column D between the upper and middle zones via stream 22. Acetic acid and heavy material are passed from the base of the column D via stream 23 to reactor A. Water is condensed in the condenser and may be re-used in the process via stream 12.

Referring to Figure 2, there is shown one embodiment for implementing an integrated separation and washing process suitable for use in each of stages E and L, in the form of a continuous band or belt filter unit such as a Pannevis filter of the form generally described in Filtration and Separation (Page 176 et seq, March/April 1979). The filter unit comprises an endless filter belt or band 100 driven by rollers around which the belt or band extends at each end,

the belt being enclosed in a pressure tight housing 101. The belt 100 comprises generally horizontally disposed upper and lower runs 100a and 100b. The interior of the housing 101 is pressurised with a suitable gas such as nitrogen in the case of the filtration unit employed in stage E or nitrogen or steam in the case of the filtration unit employed in stage L. Dotted lines S and T show the locations of a first zone on the left, a second zone between the lines S and T and a third zone to the right of line T. The slurry of terephthalic acid in mother liquor (ie. acetic acid in the case of stage E or aqueous medium in the case of stage L) is introduced to the first zone via line 103 onto the band and mother liquor drains through the band into collector pan 104 from which it is removed via line 105 to leave a first wet deposit of terephthalic acid crystals which is then passed to a second (middle) zone (in the direction of arrow A). In the second zone, aqueous wash liquid, for example water, is introduced via line 106 and passed through the band to collector pan 107 to produce a second wet deposit. The aqueous wash is removed via line 108. The second wet deposit then passes to a third zone (in the direction of arrow B) in which it is removed from the band, collected in a receiver 109, recovered and then slurried with aqueous medium in the case of CTA or dried in the case of PTA.

The water introduced via line 106 may be derived from line 12b, 17 and/or 18 (stage L) or line 12c (stage E) as shown in Figure 1 or any other suitable source. Suction is applied to the filter cake formed on the upper run 100a via the pans 104 and 107 and the pans are coupled together for reciprocating movement as a unit in a direction parallel to the direction of travel of the belt. During travel of the pans from left to right, suction is applied to draw liquid through the upper run 100a and during return travel of the pans, suction is terminated. The pressure differential across the filter belt, between the region above the filter cake and the downstream side of the filter medium (ie. the interior of the pans) is typically of the order of 0.6 bar. In the case of stage L, the pressure residing on the downstream side of the filter medium is at least as great as that prevailing in the final stage of the crystallisation section K. The wash liquid applied to the filter cake passes through the cake by virtue of the pressure differential between the upstream side of the filter cake and the downstream side of the filter medium and is applied in such a way that the wash liquid displaces residual mother liquor within the cake without undergoing channelling. It will be appreciated that the wash liquid may be applied in a single stage or it may comprise a series of stages with wash liquid applied at a number of locations along the path of travel of the filter belt. In this event, the wash liquid may be applied as a number of parallel streams or it may be applied in a serial manner, either co-current or counter-current. The mother liquor and wash liquid recovered from each filter belt unit are utilised in the manner described with reference to Figure 1. Thus, for example, in the case of the filtration unit forming stage L, the mother liquor and recovered wash liquid may be combined and then subjected to cooling or evaporation to produce a less pure terephthalic acid precipitate which is recycled to the reactor A. The residual mother liquor/wash liquid may be further processed for example to reduce the para-toluic acid content

thereof and then recycled to the reslurry stage G for use in dissolution of CTA prior to the hydrogenation reaction in reactor J.

The stages E and L may be implemented by other forms of filtration unit capable of operating under pressure; for instance, the filtration units may be constituted by pressure drum filters, eg multi-cell pressure drum filters of the type well-known in the filtration art or rotary cylindrical filters.

As the separation and washing processes are carried out under pressure somewhat in excess of atmospheric pressure, where it is desired to transfer the washed crude or pure terephthalic acid to equipment operating at lower pressure (eg at atmospheric pressure), in accordance with the present invention, this is effected by transferring the filter cake in its moist condition from higher pressure to lower pressure by means of a positive displacement device, which also serves to isolate the higher and lower pressure zones from each other. For instance, a PTA filter cake removed from the filtration unit of Figure 2 into the receiver 109 may be transferred to drying equipment at lower pressure by means of various devices such as variable pitch screws or a rotary valve arrangement.

Figure 3 illustrates schematically one embodiment for effecting transfer of crude or pure terephthalic acid from the elevated pressure and temperature conditions prevailing in the filtration and washing stage 200 (which may be constituted by a pressure filtration system such as a Pannevis filter system) to a rotary drier 202 operating at atmospheric pressure. The CTA or PTA slurry is fed to the filtration and washing stage via line 204 and mother liquor is removed via line 206 for recycling etc as described previously. Following initial filtration of the slurry, the moist mass of terephthalic acid crystals is contacted with a wash liquid (usually clean water) introduced at point 208. The wash liquid supplied from source 210 is pre-heated, for instance by heat exchange with steam from source 211 in heat exchanger 212, so that it is at substantially the same temperature as the terephthalic acid to be washed. The washed terephthalic acid undergoes filtration during the washing process and a moist mass of crystals results which is fed to the drier 202 by a rotary valve or a positive displacement device 214 such as a ram pump or a progressive cavity feed pump. Progressive cavity feed pumps are well-known, one manufacturer of such pumps being Robbins & Meyers Limited of Chandlers Ford, Hampshire, UK. The device 214 is arranged to isolate the elevated pressure conditions prevailing in the filtration and washing stage from the lower pressure (eg atmospheric pressure) at which the drier 202 operates. The device 214 is required to handle a moist mass of terephthalic acid crystals at relatively high temperature and is therefore potentially subject to temperature differentials which may prove troublesome because of thermal expansion effects. Clearances and tolerances between those components of the device which move relative to one another in operation are critical to the performance of the device especially in terms of ensuring adequate pressure isolation between the higher and lower pressure zones. Moreover, it is essential that the device should not sieze up in operation as a result of thermal expansion effects.

To allow the device to be fabricated with close clearances and tight tolerances, heating of the device is effected in order to maintain the device temperature substantially homogeneous. As shown in Figure 3, such heating is effected using the heated wash liquid. Thus, wash liquid fed from the source 210 is first heated by heater 212 and is fed by line 216 into heat exchange relation with the device 214 after which the liquid is fed by line 218 to the inlet 208. A valve 220 is connected between lines 216 and 218 and can be controlled to regulate the amount of wash liquid diverted for the purposes of heating the device 214. By using the wash liquid in this way, it is possible to ensure that the device is heated to substantially the same temperature as the hot solids material received from the filtration and washing stage 200 since the water, having been used to wash the terephthalic acid filter cake, must be at substantially the same temperature and pressure as the filter cake. The heat exchange between the wash liquid and the device may for instance be effected by enclosing at least part of the device in a jacket through which the wash liquid is circulated. Commercially available positive displacement devices such as progressive cavity feed pumps are often fabricated with a jacket for the purpose of effecting cooling of the device. Thus, in contrast with conventional practice, in the present invention the jacket would be used for heating the device.

In addition to being used to heat the device, the wash liquid may also be used to wash and/or lubricate seal faces and other components of the device which are subject to wear. In this event, advantage can be taken of the fact that the water pressure will be equal to or greater than the upstream cake pressure in that the pressure difference over the seal may be arranged to push solids into the device rather than letting them through the seal.

Instead of using the wash liquid to heat the device 214, in an alternative embodiment, the heating fluid may be constituted by a fluid in its vapour phase which is used to establish pressurisation of the upstream side of the filter medium. Referring to Figure 4, the fluid used to pressurise the upstream side of the filter medium is constituted by the solvent component present in the liquor, ie water where the slurry is derived from the purification of terephthalic acid. As shown in Figure 4, an integrated filtration and washing system comprises a housing 400 which encloses a continuous filter band 402 which is driven so that the upper run thereof travels from left to right as viewed in Figure 4. The upper run of the filter band 402 traverses three zones A, B and C. A slurry of terephthalic acid in aqueous liquor is introduced to the first zone via line 403 onto the band and aqueous liquor passes through the band 402 with the assistance of pressurised fluid, namely steam (which may be derived from the crystallisation section K or from elsewhere), the steam serving to develop a pressure differential between the space above the upper run of the band 402 and the space immediately beneath the upper run thereof. The aqueous liquor passing through the band 402 in zone A is collected in collector pans 404 located beneath the upper run of the band 402 and is removed from the pans 404 via lines 405 to leave a first wet deposit which is then passed to the second zone B. The aqueous liquor is fed via lines 405 to a filtrate pot 406.

In the zone B, aqueous wash, for example water, is introduced via line 408 and a series of sprays 410 and passes, with the assistance of the pressurised steam, through the band 402 to collector pans 412 to produce a second wet deposit. The aqueous wash is removed via lines 414 and fed to the filtrate pot 406. The second wet deposit then passes to zone C in which it is removed from the upper run of the band 402, collected at point 416, recovered and then optionally dried to produce substantially dry purified terephthalic acid. The water collecting in filtrate pot 406 is removed via line 420 and level controlled valve 422 for further processing as described above in relation to stage L.

Although all of the liquor collected following filtration is shown as being collected in a common collection vessel, ie filtrate pot 406, it will be appreciated that liquor collected at different locations along the filter medium may be collected and supplied to different collection points as required. Also, as previously mentioned, the liquor collected at a location adjacent the end of the travel path of the upper run may, instead of being routed to the filtrate pot 406, be used to wash the deposit at a location preceding it, and likewise the liquor collected from the latter location may be used to effect washing at the location preceding it, and so on.

The pressurised steam is circulated around a loop including compressor 424, valve 426 (controlled by differential pressure sensor 428), the steam passing from the upstream side of the band 402 through the filter cake and the filter medium to the (downstream side) underside of the upper run of the band from where it passes to the filtrate pot 406 along with the filtered aqueous medium. Make-up steam is added to the system as needed, and steam is purged from the system when necessary, by means of valves 430, 432 controlled by pressure sensor 434. In an alternative embodiment, purge and make-up of the steam for pressurising the system may be effected by means of a balance line connected to the crystalliser from which the slurry is obtained. In this case, the need for valves 430 and 432 and sensor 434 is obviated. In carrying out the filtration process as described above, the pressure and temperature at the upstream and downstream sides of the filter are maintained substantially the same, ie. isothermal operation, so as to substantially prevent the development of a supersaturation condition during the filtering operation, thereby preventing deposition of otherwise soluble constituents of the liquor and hence reduction in product quality and/or clogging of the filter medium. In practice, compression of the steam will introduce heat into the circulating steam and means may be provided for moderating or controlling the temperature. For instance, this may be achieved by passing the steam through a heat exchanger following compression or by controlled injection of water (depending on whether heating or cooling is required) into the recirculating vapour to adjust the temperature thereof so that the stream entering the filter system is within a desired temperature range. It will be understood that at least part of the steam undergoing recirculation in the embodiment of Figure 4 may be routed into heat exchange relation with the pressure-isolating device 214 of Figure 3 in order to effect heating thereof as described above.