WO2010119484A1 - Method for recovering crystals from a crystallization slurry - Google Patents

Abstract

In conventional recovery of pure terephthalic acid crystals, separation while maintaining the final crystallization tank slurry at the highest possible temperature (maximum 205°C) and high pressure is preferred from the point of view of ease of separation of p-toluyl acid, and expensive high temperature and high pressure plant is needed for the separator (filter, centrifuge) and the system therefor (removal of the wet separation cake, separating liquid/washing liquid receiving tanks and pressurized circulation gas system, etc.). Disclosed is an effective method for filtration and washing, which is a method for recovering crystals from a crystallization slurry comprising a solvent and precipitated crystals by pressure filtration, wherein the content of impurities in the recovered crystals is decreased by employing a filter system using comparatively low pressure. In pressure filtration at no more than ca. 6 kg/cm2G, the filtration cake is washed by employing a washing solution superheated until the vapour pressure thereof exceeds the pressure of the filter system, the solution being supplied at least as a steaming washing solution. Washing is carried out by supplying a washing solution superheated until the proportion of vapour produced exceeds ca. 2%, and preferably a washing solution superheated until this proportion is at least ca. 4%.

Description

Method for recovering crystals from crystallization slurry

The present invention relates to a crystal recovery method for improving the purity of recovered crystals when recovering the crystals by continuous filtration in which the steps of filtration, washing, liquid removal and peeling are repeatedly performed under pressure from the crystal precipitation slurry.

Crude terephthalic acid is dissolved in high-temperature water, hydrotreated in the presence of a reduction catalyst, 4-CBA (4-carboxybenzaldehyde) contained in the crude terephthalic acid is reduced to p-toluic acid, Widely commercialized is a method for producing purified high-purity terephthalic acid by collecting crystals by solid-liquid separation from a crystallization slurry produced by reducing the pressure stepwise through a plurality of crystallization tanks connected in series. Has been done.

This is because p-toluic acid contained in the purified aqueous solution of terephthalic acid has higher solubility and molecular weight than p-toluic acid in the process of precipitation of terephthalic acid crystals and separation and recovery of the precipitated crystals. This is because it is easily separated from the terephthalic acid crystal because of its crystal structure. For this reason, a purification method has been adopted in which 4-CBA is hydroreduced into p-toluic acid with good separability and then terephthalic acid is crystallized and recovered.

These methods include Patent Documents 1, 2, and 3 (Japanese Patent Publication Nos. 47-49049 and 53-24057) that improve the separability of p-toluic acid in a crystallization step for producing terephthalic acid crystals from an aqueous terephthalic acid solution. And Japanese Patent Application Laid-Open No. 11-228492, and Patent Documents 4 and 5 (Special Tables) that improve the separation of p-toluic acid in the recovery step of recovering the crystals of terephthalic acid by solid-liquid separation of the produced crystals No. 6-506461, No. 7-507291, etc.) have been proposed.

Among them, the separation property of p-toluic acid in the recovery step is that terephthalic acid crystals are separated at high temperature from the difference in solubility between terephthalic acid and p-toluic acid (Table 1 of Patent Document 1 is shown in FIG. 5). Since the separability from p-toluic acid is better, the temperature and pressure of the crystallization slurry in the final crystallization tank are maintained high, and the crystallization slurry is reduced without lowering the temperature (pressure). The terephthalic acid crystals are recovered by solid-liquid separation.

Therefore, in JP 6-506461, purified terephthalic acid slurry in the temperature range of about 38 to 205 ° C, the filtration system (recovery system) pressure ranges from atmospheric pressure to 235 psig (0 to 16.5 kg / cm ² G) The filter cake is put into a high-pressure filter cell-type filtration zone in which the filter cake is formed by forming a water reservoir layer that covers the filter cake in the filter cell with wash water at a temperature of about 38 to 205 ° C. We have proposed a microfiltration / washing system that reliably replaces and cleans p-toluic acid-containing filtrate remaining in the filter.

In addition, in Table 7-7507291, the pressure on the low pressure side of the filtration surface is filtered and separated and washed under a high pressure (pressure on the filtration surface side) so that the pressure on the low pressure side of the crystallization slurry is not substantially lower than the generation pressure of the crystallization slurry. A method for producing terephthalic acid has been proposed, but the final crystallization stage pressure is crystallized at 1.5 to 15 bar, and the pressure on the filtration side is preferably 2 to 15 bar, ie high temperature filtration. Has proposed.

The cleaning solution used to remove the p-toluic acid-containing filtrate remaining in the terephthalic acid crystal filter cake should avoid the problem of flushing or cooling, which can cause the risk of precipitation impurities. And are introduced at substantially the same temperature. However, none of the above proposals proposes filtration separation of the crystallization slurry at high temperature and high pressure in order to recover the crystals of purified terephthalic acid. The actual situation is that cleaning is performed using this cleaning solution.

In continuous filtration of terephthalic acid crystal precipitation (crystallization) slurry, from the viewpoint of reducing clogging due to crystal precipitation on the filter medium (filter cloth), the pressure on the low pressure side of the filtration surface is supersaturated with the filtrate. A pressure filtration system in which filtration is performed by pressurizing the high pressure side of the filtration surface (slurry side pressure) to a pressure exceeding the vapor pressure of the filtration slurry so as to maintain the temperature and pressure that are not to be used is a preferable method of the continuous filtration method. (Patent Document 6: Japanese Patent Laid-Open No. 1-299618).

In addition, as a method for preventing clogging of the filter medium (filter cloth) when collecting the crystals, the temperature and pressure of the crystallization final tank are maintained and the crystals are separated by centrifugal sedimentation (centrifugal settling method without using a filter medium). After recovery, the separated crystals (wet cake) are reslurried with a washing solution (water), and then centrifuged again (sedimentation). The first-stage separated crystals are replaced by washing by slurrying and terephthalic acid by a two-stage centrifuge method. Crystals are also collected (Patent Document 1).

Japanese Patent Publication No.47-49049 Japanese Patent Publication No.53-24057 JP 11-228492 A JP-T 6-506461 JP 7-507291 JP Japanese Unexamined Patent Publication No. 1-299618

Based on the above background, in the continuous separation method of recovering crystals from purified terephthalic acid crystal slurry, the separation method of filtration with reduced p-toluic acid content in recovered terephthalic acid with improved separation of p-toluic acid In general, a separation method of high temperature and high pressure (pressurization) is adopted.
Therefore, in the recovery of purified terephthalic acid crystals, it is preferable from the separation of p-toluic acid that the final crystallization tank slurry is separated as high as possible (up to 205 ° C) and at a high pressure. High-cost and high-pressure equipment has been required as a centrifuge) and its system (such as separation wet cake take-out, separation liquid / wash liquid receiving tank and pressurized circulation gas system).

However, high-temperature separation of purified terephthalic acid crystallization slurry increases the amount of terephthalic acid dissolved in the separation liquid with temperature (see FIG. 5). There is a problem that the post-treatment load of the dissolved separation liquid increases.

Also, the two-stage separation method by centrifugation has problems such as requiring a large amount of washing water to reslurry with 3 to 5 times the water of the separated crystals (Japanese Patent Publication No. 47-49049).

As described above, in the production of high-purity terephthalic acid, even if the aqueous solution of crude terephthalic acid containing 4-CBA is converted to p-toluic acid, which is easily soluble in water, by hydrotreating, In the process of crystal washing in the crystallization step, the recovery step, and the recovery step, which are processes, it has been carried out with the above-mentioned equipment and conditional problems.

The present invention is based on the above-mentioned actual situation of the production technology of high-purity terephthalic acid. In the recovery of crystals from the crystallization slurry of purified terephthalic acid, a relatively low-pressure filtration system is used to recover impurities contained in a normal amount of washing liquid. The present invention provides a filtration and washing method in which separability of (p-toluic acid) becomes better.

Therefore, the maximum (operation) pressure of the filtration system (filter) is 6 kg / cm ² G, and the terephthalic acid crystallization slurry has a relatively low loss of terephthalic acid to the filtrate, 160 ° C (water vapor pressure: 5.3 kg) The purpose of the present invention was to improve the separability of p-toluic acid by using a filter and a system for the temperature below / cm ² G).

In other words, the high-temperature and high-pressure filtration system is a conventional vacuum rotary cylinder filter (rotary vacuum filter) and a vacuum belt filter type filter with a pressure of 6 kg / cm ² G (withstand pressure 9 kg / cm ² G). It is designed to be installed in a casing (high pressure vessel) and can be filtered. Withstand pressure devices with a maximum working pressure of 6 kg / cm ² G for relatively low pressure specifications, even for incidental equipment and accessories accompanying filtration. The equipment cost is suppressed by using the

On the other hand, at the final crystallization tank temperature (filtration separation temperature) of 160 ° C of the purified terephthalic acid crystallization slurry, terephthalic acid loss (0.33gr / 100grH2O) due to dissolution in the filtrate during filtration separation is about 1 of the recovered terephthalic acid crystal amount. Since the amount was less than% by weight, crystals were recovered from the terephthalic acid crystallization slurry in which the maximum operation temperature of the final crystallization tank was set to 160 ° C.

In addition, since the differential pressure between the high pressure side and low pressure side of the filtration surface in the filter is usually 0.2 to 0.9 kg / cm ² (about 0.5 kg / cm ² ), the maximum is 160 ° C (water vapor pressure: 5.3 kg / A pressure filtration system with an operating pressure of up to 6 kg / cm ² G can be carried out on the crystallization slurry of cm ² G), which is a preferred method for continuous filtration with reduced clogging of the filter medium (filter cloth).

Further, in the recovery step of the purified terephthalic acid crystal, it is preferable that the filtration separation temperature does not decrease from the separability of p-toluic acid, but in the method of the present invention, at least the solubility ratio (p-toluic acid / terephthalic acid) in FIG. It was intended to perform filtration separation at a temperature of 30 or higher (130 ° C. or higher).

Therefore, in the method of the present invention, the pressure in the filtration system (filter, filter cake take-out mechanism, filtrate receiving tank, washing liquid receiving tank and pressurized gas circulation mechanism) is about 2.5 to 6 kg / cm ² G (withstand pressure 9 kg / cm ² G). Using a pressure filtration system within the pressure range of), a purified terephthalic acid crystallization slurry of about 130-160 ° C. can be obtained as an effect of separating impurities, which becomes a higher temperature recovery step.

The present invention relates to a method for recovering crystals by continuous filtration in which a slurry consisting of a solvent and precipitated crystals is repeatedly filtered, washed, and peeled in order.
In the above filtration step, the filtration of the slurry is performed by pressurizing the high pressure side of the filtration surface (slurry side) using the circulating solvent vapor-containing gas,
In the above-described washing step, the crystal separated by filtration is washed with a washing liquid accompanied by vapor generation using a washing liquid heated to a vapor pressure exceeding the pressure on the high pressure side of the filtration surface.

Further, the present invention provides the crystal recovery method described above, wherein in the filtration step, a terephthalic acid aqueous solution in which crude terephthalic acid is dissolved in high-temperature and high-pressure water is reduced with hydrogen in the presence of a reduction catalyst. The crystal slurry in which the terephthalic acid crystals were precipitated through a plurality of serially connected final crystallization tanks, in which the pressure was lowered stepwise, was applied at a pressure of 2.5 to 6 kg / cm ² G on the high pressure side of the filtration surface. And filter.

In the washing step, the cleaning liquid (water) heated to a vapor pressure exceeding the pressure on the high pressure side of the filtration surface is supplied, and the crystals separated by filtration with the washing liquid accompanied by the generation of vapor are washed.

Therefore, cleaning water heated to a vapor pressure of about 2.7-7Kg / cm ² G (about 140-170 ° C) or higher, which exceeds the high-pressure side pressure, is supplied and separated by washing water with steam generation. The crystal is washed.

The present invention also provides cleaning water heated to a vapor pressure of about 7 Kg / cm ² G (about 170 ° C.) or higher in the recovery of crystals at about 151 ° C. (about 4 Kg / cm ² G) of the described crystallization slurry. By washing the crystals separated by filtration, a low reduction effect of contained impurities (p-toluic acid) appears, and further by using wash water heated to about 9Kg / cm ² G (about 180 ° C) or more. Is characterized by becoming more prominent.

This is characterized in that the rate of steam generation from the superheated washing water corresponds to about 2% or more, more preferably about 4% or more.
However, when the steam pressure of the superheated washing water is about 16.5Kg / cm ² G (about 205 ° C, steam generation rate 9.6%) or more, the reduction effect of contained impurities (p-toluic acid) tends not to change. A high-pressure equipment burden is required.

Therefore, in the washing step in the recovery of the crystals by continuous pressure filtration that performs the filtration, washing, and peeling steps from the crystallization slurry, an overheated washing liquid that has a vapor pressure exceeding the high pressure on the filtration surface is supplied, and about The filter-separated crystals are washed with a washing liquid with 2 to 10% vapor generation.

Furthermore, the present invention provides a crystallization slurry obtained by a crystallization method in which the pressure of the purified terephthalic acid aqueous solution is gradually reduced and simultaneously the temperature is lowered, and the temperature in the final crystallization tank (about 130 to 160 ° C) is increased. In a method of maintaining and supplying this crystallization slurry to a continuous filter of a filtration system pressurized to about 2.5 to 6 kg / cm ² G and recovering purified terephthalic acid crystals by filtration separation, A cleaning liquid (water) heated to a vapor pressure exceeding the pressure is supplied to a filter washing zone, and the cake separated by filtration is washed while at least generating steam. This makes p-toluic acid more separable than crystals recovered by filtration and washing when supplying a cleaning liquid (washing water that does not generate steam) equal to or lower than the conventional crystallization slurry temperature. The content of p-toluic acid in the recovered terephthalic acid crystal can be improved.

In the conventional filtration separation method, the p-toluic acid-containing liquid remaining in the cake after filtration is thoroughly replaced with a washing solution and washed with washing water at a temperature substantially equal to or lower than that of the separation cake. In Patent Document 1, which has been considered preferable, there are impurities (p-toluic acid) that remain due to the physical and chemical affinity of the crystal, such as adhesion and adsorption of the cake layer to the crystal surface, and are eliminated only by substitution washing. There seems to be a limit amount that cannot be obtained. The present inventors have intensively studied to further reduce these limit amounts, and as a result have reached the present invention.

That is, in the present invention, the amount of impurities adhering to and adsorbing on the cake layer crystal surface in accordance with the concentration of the filtrate impurity (p-toluic acid) in the filtration separation is at least from the temperature at the time of filtration separation by the introduced superheated water. It is cleaned with a steam-liquid mixed phase cleaning liquid with high temperature steam. In this cleaning, the flow rate of the cleaning solution is increased by steam, the crystal surface is activated by high temperature, and the desorption and desorption of impurities are promoted to dissolve and eliminate, thus reducing p-toluic acid. Appears. As a result, detachment was promoted even in the water content of the recovered recovered terephthalic acid crystal cake, and the wet rate tended to decrease.

In the above processing operation of the present invention, a pressure filtration system capable of pressurization to 2.5 to 6 kg / cm ² G can be used, and the cleaning liquid heated to a vapor pressure that exceeds the system pressure is supplied. This is achieved by using equipment that can be used to supply at least a cleaning liquid with steam generation to the cleaning zone of the filter. In addition, by supplying and cleaning the cleaning solution that has been heated to a vapor generation rate exceeding about 2%, preferably about 4% or more, the effect of reducing contained impurities (p-toluic acid) is more prominent. Become.

Therefore, the improvement in the separation property of p-toluic acid due to the improvement to the superheated cleaning liquid accompanied by the generation of steam can achieve the same effect as the filtration / separation effect at higher pressure (high temperature) in the conventional method, The pressure strengthening measures of the filtration system equipment for p-toluic acid content could be suppressed.

In order to carry out the method of the present invention, in the crystallization step of the purified terephthalic acid aqueous solution, stepwise flash crystallization proposed in Patent Document 2 (Japanese Patent Publication No. 53-24057), Japanese Patent Application Laid-Open No. 2006-96710, etc. A crystallization slurry is produced by the method, and in the crystal recovery step, pressure filtration according to proposals of Patent Document 6 (Japanese Patent Laid-Open No. 1-299618), Patent Document 5 (Special Table of Hei 7-507291), etc. is performed, and a washing step By supplying the superheated cleaning liquid (water) accompanied by the generation of steam from the above-described system for supplying the superheated liquid (water), the production of high purity terephthalic acid by the method of the present invention can be made more effective.

In addition, the method of the present invention is not limited to the production of high-purity terephthalic acid. In addition to recovering crystals from the crude terephthalic acid crystallization slurry, recrystallization from the crystallization slurry by recrystallization, which is expected to reduce the impurity content by crystallization, is also proposed. It can be applied to filtration and washing for collecting crystals, and a favorable effect can be expected.

That is, in the pressure filtration separation method from the crystallization slurry, by supplying a cleaning liquid that is superheated until the vapor pressure exceeds the pressure on the high pressure side of the filtration surface, and at least washing the filter cake with a cleaning liquid that generates steam. It can be said that this is an effective method by reducing impurities in the recovered crystal.

According to the present invention, by using a pressure filtration system of about 2.5 to 6 kg / cm ² G to supply crystallization slurry at a predetermined temperature with superheated water accompanied by vapor generation and washing, the separability of impurities is achieved. Can be improved, and high-purity crystals can be recovered, contributing to the improvement of product value.

In addition, it is possible to obtain the same separation effect as increasing the filtration temperature (system pressure) by improving the washing water supply system without enhancing the pressure resistance of the filter and the filtration system. This will reduce the burden of equipment for increasing pressure. At the same time, it is possible to reduce the drying burden in the subsequent process by reducing the wet rate of the recovered cake.

On the other hand, when terephthalic acid is used for the crystallization slurry, the improvement of the purity of high-purity terephthalic acid (purified terephthalic acid) (reduction of p-toluic acid content) reduces the burden on the production of crude terephthalic acid. Can contribute. In other words, the effect of reducing the content of p-toluic acid (impurities) in high-purity terephthalic acid can be converted into an allowable amount for increasing the 4-CBA content of crude terephthalic acid. It will also contribute to mitigation.

The above effects are assumed to be obtained not only in the crystal recovery step of purified terephthalic acid but also in the crystal recovery process from the same crystallization slurry.

The flowchart of the filtration system of this invention Example. The schematic diagram of the section of a pressurization type rotary cylindrical filter similarly. Similarly, the characteristic diagram showing the relationship between the wet rate (wt%) of the terephthalic acid discharge cake and the p-toluic acid content (ppm) with respect to the washing water temperature. Similarly, a table showing the relationship between the wet rate (wt%) of the terephthalic acid discharge cake and the p-toluic acid content (ppm) with respect to the washing water temperature. Table showing solubility of terephthalic acid and p-toluic acid.

Schematic diagrams of an example of a filtration system and a filter for carrying out the method of the present invention are shown in FIGS. 1 and 2, respectively.

In the pressurized rotary cylindrical filter 2, the inside of the casing is pressurized to about 2.5 to 6 kg / cm ² G using a pressurized circulating gas (containing steam) of an inert gas. A cylindrical rotary filter 11 is rotatably installed inside the casing, and is filtered in turn while rotating clockwise from the filtration area 13 at the bottom to the upper cleaning / drainage area 14 and then the peeling area 15. -Collect the filter cake through each step of washing and peeling.

The crystallization slurry is supplied from the slurry supply tank 1 to the bottom 12 of the casing of the filter 2 by a pressure pump or the like, and is added in the filtration zone 13 at the bottom of the rotary filter 11 where the liquid level of the crystallization slurry is maintained. Filtered by pressure and suction. The surplus slurry for this filtration is discharged from the filter 2 through the overflow pipe and returned to the slurry supply tank 1 or the like, but is discharged to a lower pressure than the casing internal pressure, so that the casing internal pressure and the slurry temperature are maintained. The pressure shut-off and slurry discharge control are performed with the discharge valve.

On the other hand, the filter cake peeled by the inert gas pulse supply in the peeling zone 15 of the rotary filter material 11 is maintained in the pressure buffer zone M− such as a two-stage valve (slide valve) or a rotary valve in order to maintain the casing internal pressure. 2 is discharged and collected.

The rotating cylindrical filter medium 11 sucks the filtrate (differential pressure 0.2 to 0.9 kg / cm) while rotating the attached cake filtered by pressurizing (about 2.5 to 6 kg / cm ² G) in the filtration region 13 at the bottom. ² ) Eliminate and move to cleaning / dehydration zone 14. In the cleaning / drainage zone 14, cleaning liquid (water) 17a heated to a vapor pressure exceeding the casing internal pressure is introduced through the flash valve 17, and the introduced cleaning liquid 17a is subjected to pressurization of the circulating gas (about 2.5 to At 6 kg / cm ² G), at least with the generation of steam, it is sucked out together with the residual filtrate inside the cake.

As for the generation ratio of the cleaning liquid vapor, the effect (cleaning effect, cake wetting rate) appears by supplying the cleaning liquid that has been heated to a temperature accompanied by the generation of steam exceeding about 2% of the supplied cleaning liquid volume. The effect becomes favorable by the cleaning liquid having a temperature of the steam generation exceeding. However, the steam generation effect tends to be small in the overheated cleaning liquid at a temperature where the steam generation amount exceeds 10%. Therefore, it is effective to perform cleaning with a cleaning liquid that has been heated to a temperature at which 2 to 10% of the cleaning liquid generates steam.

It is considered that the above phenomenon is dissolved and eliminated by promoting the desorption and desorption of impurities due to the increase in the flow rate of the cleaning liquid due to the vapor and the activation of the crystal surface due to the high temperature. In addition, since steam with a generation rate of 2 to 10% of the cleaning liquid is highly viscous at high humidity, the effect of scraping impurities on the crystal surface is large, and steam with a generation rate of 10% or more has low viscosity at low humidity. Since it is small, it is considered that the effect of scraping impurities on the crystal surface is small.

Note that the cleaning liquid superheated through the heater E-2 to the liquid temperature having a vapor pressure exceeding the casing internal pressure on the flash valve 17 controlled to a constant pressure exceeding the casing internal pressure is in the casing at a pressure exceeding the flash valve control pressure. And supplied to the cake in the washing zone 14 while flushing through the flush valve 17.

Further, the rotating filter material 11 rotates and the adhering cake moves to the peeling zone 15, and a pulsed gas is sprayed from the back side of the filtration surface to a gas pressure higher than the casing internal pressure (+0.1 to 0.5 kg / cm ² ). Removes the filter cake. The exfoliated filter cake is transferred to a dryer which is usually under atmospheric pressure through a special valve that suppresses fluctuations in the casing internal pressure, and is discharged and produced as a dried product (terephthalic acid) powder.

Next, in FIG. 1, the filtrate and washing drained liquid withdrawn by suction are individually supplied to the filtrate receiving tank 4 and the washing drain receiving tank (not shown) together with the suction vapor, or to the same filtrate / washing drain receiving tank 4. In order to separate the non-condensable gas, it is temporarily retained and transferred to each processing step.

The vapor mixed gas (non-condensed gas) separated in the receiving tank 4 is slightly cooled by the cooler E-3 for temperature stabilization, and the condensed liquid is recovered in the receiving tank 4 by the gas-liquid separation tank 5. The vapor mixture gas component that could not be condensed is supplied to the suction side of the compressor M-4, and the pressure regulator PIC-2 for the discharge pressure of the compressor M-4 is used to supply the excess (inert gas) and the excess. It discharges and is supplied to the filter 2 as circulating gas.

Circulating gas from the compressor M-4 is adjusted to the system pressure by the pressure controller PIC-2 for pressurization of the filtration system, and sent to the internal pressure of the filter 2 and the pressure buffer area for discharging the peeled cake. . Meanwhile, the circulating gas is adjusted to a temperature equal to or higher than the supply slurry temperature (TIC-2) by the heater E-4. The supply of the circulating gas to the filter 2 is preferably blown into the area before the cleaning area 14 in order to suppress the evaporation of the filtrate.

The above method not only recovers crystals from purified terephthalic acid crystal slurry in the production of high-purity terephthalic acid, but also recovers crude terephthalic acid crystals from crystallization slurry obtained by oxidation reaction. It can be applied for reduction. In addition, the present invention can be applied to the reduction of impurities contained in the recovery of crystals from a crystallization slurry by a crystal generated by crystallization and a solvent.

In addition, the system pressure of the pressure filtration system used for those methods becomes the pressure which does not exceed 6 kg / cm < ² > G, and the method implemented under comparatively low pressure is taken.

Next, embodiments of the present invention will be described in more detail with reference to the following examples. This example is an example of the embodiment, and the present invention is not limited to this.

In this example, a crystallization terephthalic acid slurry is used as the crystallization slurry. Crystallized terephthalic acid slurry is a high-purity terephthalic acid production plant by hydrorefining crude terephthalic acid. A terephthalic acid aqueous solution in which crude terephthalic acid is dissolved in high-temperature, high-pressure water is reduced with hydrogen in the presence of a reduction catalyst. After the treatment, it is obtained from the final crystallization tank (slurry supply tank 1, 151 ° C.) in the hydrorefining process using a plurality of serially connected crystallization tanks that gradually reduce the pressure. A high temperature crystallization terephthalic acid slurry from the slurry supply tank 1 was supplied to a pressure filtration system, and the crystal recovery method was performed by filtering and separating the crystallization slurry under pressure.

In FIG. 1, the pressure filtration system uses a rotary cylindrical filter ² with a filtration area of 0.2 m ² mounted in a pressure-resistant 7 kg / cm ² G casing, a cake receiving tank 3, a filtration mother liquor / washing drainage / receiving tank 4, 1 is a flow system within a dotted line in FIG. 1 including a gas-liquid separation tank 5, a compressor M-4, a cleaning liquid heater E-2, and the like.

The terephthalic acid crystallization slurry from the slurry supply tank 1 (final crystallization tank) is supplied to the bottom of the filter 2 pressurized to 5.3 kg / cm ² G (steam-containing N 2 gas) by the compressor M-4. By suction of the compressor M-4 through the filtered mother liquor / cleaning drainage / receiving tank 4, pressure (suction) filtration is performed at the immersion part (bottom part 12 in FIG. 2) of the cylindrical rotary filter material 11.

The rotary filter medium 11 rotates clockwise at a speed of 2 RPM, and the filter cake adhering to the slurry immersion part is washed with a washing liquid 17a introduced from the upper part of the rotary filter medium 11 while sucking and draining the filtered mother liquor. After the cake is washed and sucked, and subsequently sucked and drained, the filter cake is peeled off by pulse pressure (5.5 kg / cm ² G) from the inside of the filter medium by the gas supply pulser M-1. The peeled filter cake is discharged to the cake receiving tank 3 under normal pressure by adjusting the pressure with a pressure buffer valve M-2 blocked by two slide valves.

Wash water is supplied under pressure through a flush valve 17 set to about 20 kg / cm ² G and injected into the washing zone of the filter 2. At that time, the heater E-2 is heated (TIC-1) to each cleaning water set temperature shown in FIG. 4, and the washing liquid 17a is flushed (released) in the washing zone 14 to wash the filter cake. Washing water whose washing water temperature exceeds 160 ° C. (water vapor pressure 5.3 kg / cm ² G) will wash the filter cake as washing water accompanied by at least the generation of water vapor at the same time as flushing.

The filtered (mother) liquid sucked and filtered in the filtration area 13 at the bottom of the rotary filter 11 and the washed waste liquid sucked and washed in the upper washing / drainage area 14 are simultaneously filtered mother liquor / washing waste liquid / receiving tank 4. And is discharged from the lower portion of the receiving tank 4 as a mixed solution.

The suction gas containing the vapor separated from the filtered mother liquor / cleaning drainage / reception tank 4 is cooled to about 145 ° C. from the top through the cooler (condenser) E-3, and the condensate is cooled in the gas-liquid separation tank 5. Collected. The vapor / gas component which has not been condensed is supplied as a circulating gas to the suction of the compressor M-4. Circulating gas from compressor M-4 is circulated after adjusting to 5.3 kg / cm ² G with pressure controller PIC-2 and 151 ° C with temperature controller TIC-2 to pressurize the filtration system. .

The cake in the cake receiving tank 3 was obtained by discharging the terephthalic acid cake under atmospheric pressure using a screw type discharge machine M-3 attached to the lower part. A sample of the discharged cake was collected, and the wet rate and the amount of p-toluic acid containing terephthalic acid crystals were measured.

The amount of washing water carried out was about 0.7 times the amount of the discharged terephthalic acid crystals, and the washing water temperature and pressure are shown in the execution number of FIG. The calculated steam generation rate (evaporation rate), the wet rate of the terephthalic acid discharge cake and the p-toluic acid content in each run number are as shown in FIG. FIG. 3 is a characteristic diagram showing the relationship between the wet rate (wt%) of the terephthalic acid discharge cake and the p-toluic acid content (ppm) with respect to the temperature, pressure, and evaporation rate of the washing water in each example shown in FIG. is there.

As a result, it was found that the p-toluic acid content decreased as the washing water temperature increased, and that the p-toluic acid content was greatly reduced at a superheating temperature of 170 ° C. or higher, which is considered to be accompanied by steam generation.
In this implementation, a high-purity terephthalic acid plant having a p-toluic acid content of about 125 ppm was produced in a high-purity terephthalic acid production plant in which the crystallization slurry was branched and collected. Therefore, by the method of the present invention in which the washing water is heated to a washing water temperature of 190 ° C. (evaporation rate 6.3%) in Example 3 and the same effect as the reslurry washing method (high-purity terephthalic acid production plant) by two-stage centrifugation Was found to be obtained.

Moreover, the p-toluic acid content of 150 ppm or less, which is the production standard for the production of high-purity terephthalic acid, can be achieved at the washing water temperature of Example 2 (washing water temperature 180 ° C., evaporation rate 4.2%) or more. I understood.

1 Slurry supply tank (final crystallization tank)
2 Rotary Cylindrical Filter 3 Cake Receiving Tank 4 Filtrate / Washing Drain / Receiving Tank 5 Gas-Liquid Separation Tank 6 Solid-Liquid Separator 11 Filter Material 13 Filtration Zone 14 Washing / Draining Zone 15 Peeling Zone M-1 Gas Supply Pulser M -2 Pressure buffer valve M-3 Screw type discharger M-4 Compressor E-1, E-3 Cooler E-2, E-4 Heater.

Claims (5)

In a method of recovering crystals by continuous filtration in which the slurry consisting of a solvent and precipitated crystals is repeatedly filtered, washed, and peeled in order.
In the above filtration step, the filtration of the slurry is performed by pressurizing the high pressure side of the filtration surface (slurry side) using the circulating solvent vapor-containing gas,
Crystals from the crystallization slurry, characterized in that, in the cleaning step, a cleaning liquid superheated to a vapor pressure exceeding the pressure on the high pressure side of the filtration surface is supplied, and the crystals separated by filtration are washed with a cleaning liquid accompanied by vapor generation. Recovery method. 2. The method according to claim 1, wherein, in the filtration step, a terephthalic acid aqueous solution in which crude terephthalic acid is dissolved in high-temperature and high-pressure water is reduced with hydrogen in the presence of a reduction catalyst, and then the pressure is increased stepwise. It is characterized by filtering the crystal slurry on which terephthalic acid crystals are precipitated through a plurality of crystallization tanks connected in series under a pressure of 2.5 to 6 kg / cm ² G on the high pressure side of the filtration surface. A method for recovering crystals from the crystallization slurry. 3. The method according to claim 2, wherein in the cleaning step, the pressure on the high pressure side of the filtration surface is about 2.7 to 7 kg / cm ² G (about 140 to 170 ° C.) or more, exceeding the pressure of 2.5 to 6 kg / cm ² G. A method for recovering crystals from a crystallization slurry, comprising supplying superheated washing water and washing the crystals separated by filtration with washing water accompanied by vapor generation. 4. The method according to claim 2 or 3, wherein in the washing step, superheated washing water is supplied to a vapor pressure of about 7.0 to 16.5 kg / cm ² G, and the crystal is washed with washing water accompanied by generation of vapor. A method for recovering crystals from a crystallization slurry characterized by the following. The crystallization slurry according to any one of claims 1 to 4, wherein, in the cleaning step, the crystals are washed with a superheated washing liquid so that a steam generation rate is about 2% to 10%. To recover crystals from

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