DD139977A3 - METHOD OF PROCESS WATER RECOVERY IN THE RELAXATION EXPRESSION CRYSTALLIZATION OF TEREPHTHALIC ACID - Google Patents

Abstract

The invention relates to a process for terephthalic acid purification by relaxation crystallization to effective Condensation and fine cleaning of fumes for recovery as Process water. It has been found that by a combination of Absorption effects in a direct-chemical and pre-quenching step Heat exchange one drop system and two subsequent Quenching, which the material and heat exchange both in organized spray regimes as well as post-purification in the film regime realize a highly effective condensation and condensate cleaning can be achieved. In addition to the reuse of the exhaust steam as Solvent for the terephthalic acid purification process may be the Terephthalic acid contained in the boil off condensate can be recovered.

Description

Field of application of the invention

The invention relates to a process in terephthalic acid purification by flash evaporation crystallization for effective condensation and fine cleaning of evaporation for recovery as process water,

Characteristic of the known technical solutions

The crystallization of terephthalic acid from its aqueous solution according to the known purification methods can ZpBo according to DE-OS I5 935 39, 40, 41 continuously or after DD-AP 115 I09 be carried out continuously "30- 30 % of the original solvent in the form of water vapor-free 'This water vapor varies depending on the method used simultaneously in a plurality of print areas with temperatures from 370 to 570 K at, whereby preferably in the intermittent method of the W _a strong sserdampfmengenstrom fluctuates "

In addition to entrained partially crystalline terephthalic acid, the water vapor also contains a portion of the accompanying components to be separated off and impurities such as p-toluic acid, benzoic acid, phthalic acid and very small amounts of 4-carboxylbenzaldehyde due to the water vapor volatilities of these substances.

-ar

The steam is either discharged into the atmosphere according to conventional methods or, more or less completely condensed and more or less discarded.

Publications dealing with the condensation and treatment of these particular fumes are not known. ^;

The use of conventional material and Vförmeaustauschmethoden for partial or "complete condensation turns out for the special case ^a ls ungee.ignetj since returning such, containing the impurities water in the process of enriching the same would result in the final product <>'It could therefore only a small proportion of the stream is reused. Specific quality requirements are imposed on a high-purity terephthalic acid suitable for fiber production. Thus, the chemical compounds 4-carboxylbenzaldehyde, p-toluic acid and benzoic acid may only be present in traces in the end products. High demands are also made on the terephthalic acid moiety the ash content and the content of colored aromatic compounds. For this reason, the water used to purify crude terephthalic acid as a solvent must be nearly deionized

Aim of the invention

The aim of the invention is to achieve a complete condensation of the fumes, to return most of the condensate as process water and to avoid the negative effects of pressure and flow fluctuations of the water vapor as feed stream ¹ .

Since rlegu ng of the-nature of the invention

It is the object of the invention to develop a method with which it is possible to substantially improve both the condensation and the purification of the process vapors and to make the course of the process trouble-free and economical, as well as the environmental; To avoid contamination by preventing the escape of polluted vapors into the atmosphere

It has been found that a combination of absorption effects in a Vorquenchstufe with direct mass and heat exchange in the drop system and two subsequent Q, uenchstufen ₅ which realize the mass and heat exchange both in the organized spray regime and the final purification in the film regime, a highly effective condensation and condensate cleaning can be achieved. "

The solution to the problem outlined is achieved according to the invention in a special three-stage process.

In the first stage, the exhaust steam of the various crystallizer pressure stages is collected and thereby relieved to a pressure close to atmospheric pressure (100-120 kPa). V The steam thus superheated is subjected to a simple quenching in the cocurrent process, which according to the invention with steam velocities of 25 -. 50 m / s is carried out and has only a minimum mixing distance «; According to the invention, the residence time should be 0.5-5 s in the mixing zone «;

This makes it possible to generate a regime for a short time, where superheated steam is present alongside quench water;

- · - 210 700

The amount of quench water to be used should be controlled so that it is sufficient for the saturation of the steam. This object is achieved according to the invention when the steam temperature after quenching is 375-390 K. It is expedient to carry out this process step in the steam feed line directly in front of the condensing apparatus. The steam Y thus treated is fed to the condensing apparatus by passing it on a baffle plate with deceleration Surprisingly, it has now been found that the quench water thus collected and collected in the bottom of the column contains most of the terephthalic acid contained in the steam carried by the crystallizers, but only minimal amounts of steam also contained unwanted by-products »'

The precipitation process on the baffle plate can be assisted by slight water irrigation in the flash zone of the column. " This irrigation also counteracts high terephthalic acid amounts of any clogging in the swamp. The bottom product is expediently used by being pumped in the circulation. "The further treatment of the steam takes place in the second stage of the process, the steam initially being increased again so that it is greater than 1-5 to 50 Ws The processes in the second Quenchatufe are characterized in that the high-velocity gas entrains the liquid in the form of very fine droplets, the heat and mass transfer in DC in the combination of dispersing and settling takes place in specially trained Kontak- tiereinrichtungen particular high-speed contact stages . ' The intensive mixing of contaminated vapors with the washing liquid ensures

For short residence times, the required partial condensation and gas purification. ' The high gas velocities in phase DC the pressure surges occurring in the course of the process can be processed without adverse remixing or mechanical damage to ^one ''

According to the invention, the amount of vapor condensing in the second stage should amount to less than 20% of the total amount of vapor since in a condensing apparatus operated according to the invention the terephthalic acid remaining up to this point and the unwanted by-products contained therein condense almost completely and thus in the outgoing stream of this stage at least 80 % are includedV

According to the invention, 12% -15% of the total amount of vapor must be condensed and discarded in this stage if the remaining condensate of the remaining stages is to meet the purity requirements for recycling to the process. The special second-stage construction has the advantage according to the invention that in the case of the extreme process Volume fluctuations of the supplied steam, the speed changes due to this self-regulation and thus equalization of condensation in this stage result and thus does not require the intervention of the outside be available o ^;

Encrustation by the precipitating solids does not occur due to self-cleaning by the indicated high rates » ¹ The complete condensation of the remaining vapor occurs as a third stage in a countercurrent gridpack. This is possible because the remaining steam contains only insignificant amounts of unwanted by-products as a result of the pretreatment, and thus

Avoid crusts or blockages. In addition, the water cycle used for direct countercurrent cooling is dimensioned so that the solubility limit is not undercut in any case, /

The complete condensation in counter-current is effected in column internals with targeted gas deflection at elevated turbulence of vapor flow and vapor rates of 2 to 4 m / s _o Here, the running from top to bottom liquid film is dispergierto 'The combined film drops regime ensures an intensive heat and mass exchanges''

According to the invention, the first and third stage water streams obtained are combined and recycled to the process as solvent. In the process water thus formed, the content of undesirable by-products is less than 100 ppnw. The second-stage product is discarded

The method will be explained in more detail by means of an embodiment example. The apparatus used is shown in a drawing

Ausführunfffsbeispiel

The vapors blown off from different producers are supplied via a pipeline system and a manifold 1 to the pre-quenching stage 3 in the overheated state.

The water vapor stream 1 contains the following other substances:

Terephthalic acid 2200 mg / kg

p-toluic acid 'I30 "benzoic acid 60"

Phthalic acid 20 "

The Vorguenchstufe consists of a free pipe section with 5OO ran Durchmesserο From a water supply of Vorquenchstufe 2 is the Quenchwa'ssermenge of T ₉ O t / h supplied in cocurrent, so that the temperature of the entering into the column inlet 4 steam reduced to 378 K. ⁷ In the case of vapor velocities of about 35 m / s .The residence time sufficient to saturate the steam almost '© In the bottom of the column thus a vapor-liquid mixture enters 15 * 75 t / h of steam a · ¹ by the reduction of speed due to the cross-sectional widening and further energy reduction at the baffle plate 5, the crystalline solid, which consists exclusively of terephthalic acid, separated with the liquid in the bottom of the column, while the vapors rise up to the 2V stage, The separation process of the crystalline components is further by a spray nozzle system 6 supports, with the spray liquid the bottom product. 20 is taken behind the bottom product pump T9 and set at 45 m / hv

With a discharge from the sump of the 3 * stage at the chimney bottom TO of 396 nr / h and an overflow of a portion of this amount from the overflow vessel 14 of 13.8 m-vh through the conduit 18 results in a quantity of 14 nr / h of water having a temperature of 368 K and an average terephthalic acid crystal loading of 0.2 to 0.3% by mass as bottom product 20 of the T ₀ stage, this amount being based on a terephthalic acid purification process as solvent.

- can be performed. "In the bottom product 20 the following concentrations are determined:

Terephthalic acid · 2230 mg / kgp-toluic acid 30 "

Benzoic acid 10 "

Phthalic acid 7 "

The vapors ascending into the 2 ₀ stage are fed to high-speed scrapping stages 7, whereby the design of the soils sets a vapor velocity of 19 m / s. At this high velocity, a liquid flow 8 of 2.2 m-yh, which is taken behind the head cooler 16 as a partial flow with a temperature of 34-3 K, intensively mixed with the rising steamj which takes place effective heat exchange with condensation, which is adjusted so that again withdrawn over the outlet 9 2.2 m / h of liquid ₉ can be discarded due to the high level of pollution'o '

In detail, this liquid stream contains!

Terephthalic acid 11AO mg / kg

p-toluic acid 740 "

Benzoic acid. ' , 350 "

Phthalic acid 90. ".

The temperature is 371 K. ^!

The remaining vapors free of organic acids pass through the chimney tray 10 into the stage of condensation 11

210,700

Here, a countercurrent flow through a manifold 12, a liquid amount of 377 »8 m / h with a temperature of 3 ^ 3 K taken from the bottom of the 3V stage with a temperature of 366 K and the overflow vessel 14 _, which simultaneously Has function of the pump template, and has an atmospheric breathing 17, the circulation pump 15, the KreislaufIeitung 13 and the head cooler 16 is promoted _o ^;

After the last stage, before the atmospheric respiration 22, there is a demister 21 to remove liquid portions from any uncondensed residual vapors

Claims (1)

Inj. S_a nspruch. Process for the recovery of process water in the relaxation crystallization of terephthalic acid, characterized in that a quenching at a steam speed of 25 - SO ws and a pressure of 100 - 120 kPa with quench water is operated in cocurrent, wherein the residence time to the entry in a condensing apparatus (23) is 0.5 - 5 ^s , the pre-quenched steam stream with temperatures of 375 - 390 K in the condensation apparatus (23) with speed reduction on a baffle plate (5) is passed, the vapor phase through a spray nozzle system (6) And the terephthalic acid-containing condensate is withdrawn as bottoms product (20), the vapor phase is passed through high-speed contact stages (7) at 15-50 m / s and a condensate is obtained, which accounts for 12-20% of the feed vapor, at least 80%. the registered undesirable by-products such as p-toluic acid, benzoic acid, phthalic acid and 4-Carboxylbenzaldehyd enthä lt and partially discarded, the residual vapor phase in a countercurrent grid pack (11) is completely condensed with a separate water loop and the condensate of the 1 and 3 condensation stages at a concentration of 1 to 100 ppm of unwanted by-products is subjected to the terephthalic acid purification process Solvent is fed back _o ^: To do this / t page drawing ^