EP0944418A1

EP0944418A1 - Suspension crystallization method

Info

Publication number: EP0944418A1
Application number: EP97952811A
Authority: EP
Inventors: Rüdiger DROPE; Dieter Grenner; Hartmut Hetzel; Hans-Peter Schal; Gerhard Wegener
Original assignee: Bayer AG
Current assignee: Bayer AG
Priority date: 1996-12-10
Filing date: 1997-11-27
Publication date: 1999-09-29
Also published as: WO1998025680A1; BR9714138A; JP2001506539A; TW386987B; AU5655798A; DE19651215C1

Abstract

The invention concerns a method of separating organic compounds by single- or multi-stage suspension crystallization, each stage consisting of a crystallizer, optionally a crystallizer vessel, and a filter (7) for separating the liquid phase from the crystal mass. The liquid phase (8) separated in the filter (7) is returned to the respective crystallization stage and the crystal mass (9, 11) is fed to the subsequent stage or out of the final stage into the washing column (20) for the complete separation of the mother liquor, the liquid depleted of isomer to be crystallized (21) being channelled out of the respective stage and the initial compound mixture (6a, 6b) being passed behind the final crystallization stage. The method is characterized in that at least some of the mother liquor of the crystal mass extracted from the final crystallization stage is gradually replaced by the initial compound mixture.

Description

Suspension crystallization process

The present invention relates to a method for separating crystallizable organic compounds from a liquid compound mixture by suspension crystallization

Because of their synthesis process, organic compounds are generally obtained in the form of a compound mixture which, in addition to the desired organic compounds, has significant or predominant proportions of other compounds such as solvents, starting compounds, by-products and / or isomers of the organic

Owns connection. In many cases, the desired organic compound can be separated from the mixture by crystallization. A characteristic of many crystallizable organic compounds is the low solubility of foreign substances, so that crystallization is often the means of choice for the separation.

Common prior art methods include crystallization from solution or static crystallization from the compound mixture itself, in which the compound to be isolated is deposited on fixed, cooled surfaces, then separated from the mother liquor and melted again

Both crystallization processes require a large amount of energy, since the mass to be cooled or reheated for the crystallization considerably exceeds the mass of the partners actually to be separated

Energy-efficient and with a better separation effect than static crystallization processes can generally be carried out in suspension crystallization processes if so-called washing columns according to US Pat. Nos. 3,777,892 and 3,872,009 are used for the separation and as the melting crystal phase

In principle, such suspension crystallization processes are in particular for water-based systems, such as the removal of water from food liquids from US Pat. Nos. 4,004,886, 4,316,368, 4,430,104 and 4,459 144 Known suspension systems for cleaning chemicals in several stages have also been proposed according to US Pat. No. 4,787,985

Common to all of these processes is that they (in the case of multi-stage crystallization per stage) have a crystallization container in which crystals are frozen out by means of heat removal, a recrystallization container in which larger crystals grow at the expense of the smaller ones, and a filter in which the crystals are removed from the mother liquor be separated, have at least the filter of the last stage is designed as a washing column

The washing column has the task of completely separating the resulting pure crystal phase from the mother liquor.The crystal slurry is introduced into the column from below, compacted by a tamper designed as a sieve plate, the mother liquor flowing through the sieve plate below and a compacted one above the sieve plate Crystal block forms Above the block, crystals freed from the mother liquor are scraped off, melted and discharged from the block by means of a scraper. A liquid pressure of the melted crystals is maintained above the block that a small amount of the liquid is washed out by the crystal block adhering mother liquor is printed. The purity of the separated crystal phase depends to a large extent on the temperature difference between the melting temperature of the crystals and the temperature of the pulp fed to the washing column Such washing columns can advantageously only be operated up to temperature differences of 5 to max. 10 ° C. between the Kstastaususpension supplied and the melted pure phase

Mixtures of organic compounds, on the other hand, occur in mixture ratios which, insofar as they are accessible at all to the separation by crystallization, cause considerably larger temperature differences between the Kπstall suspension and the melting temperature of the pure compound to be separated off due to the liquidus curve Although it is possible to increase the concentration in the liquid phase of the crystal suspension in the last stage before the wash column by multi-stage crystallization processes to such an extent that a liquidus temperature of the mother liquor is reached which approximately corresponds to the liquidus temperature of the starting mixture, but this requires a considerable outlay in terms of plant technology due to the the advantages of suspension crystallization compared to static processes can be compensated.

Organic compounds generally have very low solubility in one another, so that single-stage suspension crystallization processes are suitable for the separation.

The object of the present invention is to provide a method in which the required number of crystallization stages in the suspension crystallization can be reduced.

The present invention relates to a process for the separation of organic compounds by single-stage or multi-stage suspension crystallization, each stage consisting of a crystallizer, optionally a recrystallization container and a filter for separating liquid phase from the crystal slurry, the liquid phase separated off in the filter in the respective crystallization Stage is recycled, the crystal slurry is transferred to the subsequent stage or from the last stage to the washing column for complete separation of the mother liquor, and the liquid phase depleted in the compound to be crystallized is discharged from the first stage, the starting compound mixture being behind the last crystallization stage is supplied, which is characterized in that the mother liquor of the crystal slurry from the last crystallization stage is at least partially gradually replaced by the starting compound mixture.

The exchange can take place in stages in at least two stages.

The exchange is preferably carried out gradually continuously, ie continuously in infinitesimal steps. This can be done in a flow channel with sieves on both sides. Designed walls take place, with the starting compound mixture flowing in through one sieve and, on the other side, mother liquor being drawn off through the other sieve.

According to a particularly preferred embodiment, the flow channel has an annular flow channel which is formed by concentric sieve tubes. A preferred exchange element consists of three concentric tubes, the two inner tubes being designed as sieves. The crystal slurry emerging from the last crystallization stage flows through the middle one Tube, the starting compound mixture is introduced into the outer tube and from the innermost tube

Mother liquor is removed.

In a particularly preferred manner, it is provided that the feed of the starting compound mixture and the drain of mother liquor can be exchanged via changeover valves. In this way, the direction of flow through the sieve bores can be reversed and any blockages can be effectively prevented

The exchange is preferably carried out in such a way that the mother liquor demanding the crystal slurry is undersaturated with the crystallized compound, ie part of the crystal phase in the mother liquor disappears. The undersaturation is preferably chosen so that 2 to 5% of the crystal phase is dissolved again. This prevents it that caking of crystals occurs in the exchange organ

The mother liquor drawn off by the exchange is fed back to the crystallizer in single-stage crystallization processes. In the case of multi-stage crystallization processes, the exchange mother liquor is preferably returned to the previous crystallization stage. It is particularly preferably used for the corresponding exchange of mother liquor in the previous stage

The process according to the invention is preferably carried out with only one crystallization stage The process according to the invention is particularly preferably used for the isomer separation of isocyanates, in particular for the isomerically pure recovery of 2,4-tolylene diisocyanate or 4,4'-diphenylmethane diisocyanate from the isomer mixtures with 2,6-tolylene diisocyanate or 2,4'-diphenylmethane diisocyanate

Toluene diisocyanate is obtained as an isomer mixture containing 80% 2,4-TDI and 20% 2,6-TDI. The liquidus temperature of the mixture is 13.5 ° C. The melting point of the pure 2,4-TDI is 22 ° C. Due to the different reaction behavior when using TDI in the polyaddition reaction for the production of polyurethanes, isomerically pure 2,4-TDI becomes on the one hand and on the other

Isomer mixture of 65 to 69% 2,4-TDI and 35 to 31% 2,6-TDI used differently. The liquidus temperature of the isomer mixture with 65% 2,6-TDI is around 6.3 ° C. The crystal slurry of the single-stage suspension crystallization process accordingly has a temperature of 6.3 ° C. For effective separation of the pure crystal phase in a wash column, a crystal suspension with at least 12 ° C had to be added to the wash column, taking into account the melting temperature of the pure 2,4-TDI phase. According to the liquidus curve, this corresponds to a content of 77% 2,4- TDI in the mother liquor. Taking into account the additional requirement of a pumpable mixture, which preferably has a crystal content of not more than 30%, such a concentration can only be achieved by adding the starting isomer mixture to the crystal slurry only at the expense of uneconomically high jerkiness of at least 50% of the crystal phase, or by multi-stage crystallization

By continuous-gradual exchange of mother liquor according to the invention by the starting isomer mixture, on the other hand, it is readily possible to provide the required mother liquor concentration and temperature for the washing column only at the expense of a technically advantageous jerkless rate of 2.5 to 5% , 4-TDI depleted phase with 67% 2,4-TDI as the product, it is sufficient to use only a part of the

Exchanging the starting isomer mixture with the mother liquor and mixing the rest of the starting isomer mixture with the crystal slurry 69% 2,4-TDI is desired, it is sufficient to carry out the exchange of the starting isomer mixture and mother liquor in two stages, part of the starting isomer mixture and the crystal slurry being continuously mixed and then discharged with a filter mother liquor before the in another container the remaining starting isomer mixture is mixed with the crystal slurry The excess mother liquor from the last exchange stage can be discharged via a separate filter or directly in the wash column

The invention is explained in more detail below with reference to the attached figures

1 shows a single-stage crystallization process according to the invention with a two-stage exchange

2 shows a single-stage crystallization process with gradual and gradual exchange

3 shows a two-stage crystallization process according to the invention

4 shows a two-stage crystallization with mother liquor exchange in both stages

5 shows an exchange organ preferred according to the invention

1 schematically shows a crystallizer 1, in which heat is removed from the crystal suspension therein via the cooling jacket 2. The cooling jacket 2 is cooled via the cow unit 3. The crystal suspension is fed via line 4 to an exchange container 5, which functions as the first exchange stage for the mother liquor. Via line 6a, part of the starting compound mixture, preferably about 40-60%, is fed to the exchange container 5. Filter 7 is used to exchange exchange mother liquor via line 8 returned to the crystallizer 1. A crystal suspension is withdrawn via line 9 from the exchange container 5, the mother liquor of which has a higher concentration ah of the compound to be crystallized than that Crystal suspension emerging from crystallization vessel 1 via line 4 The mass flow through line 8 is controlled in such a way that a crystal suspension with good pumpability is present in line 9, ie that the crystal suspension in line 9 has a solids content of 25-35% Starting substance mixture is supplied at a temperature such that after mixing with the crystal suspension supplied via line 4, the mixture is slightly undersaturated. The remaining part of the starting compound mixture is introduced via line 6b into the mixing container 10, into which the crystal suspension from the Exchange container 5 is introduced. The mixing container 10 serves as a second stage for the gradual mother liquor exchange. A filter can be provided at the outlet of the mixing container 10, through which excess mother liquor is separated and returned to the crystallizer 1. In the embodiment shown, the function of the filter is taken over by the washing column 20. The crystal suspension is transferred via line 11 and the hollow axis of the reciprocating piston pump 12 Introduced into the washing column 20 above the sieve plate 13. The reciprocating pump 12 periodically moves the sieve plate 13 upwards, the mother liquor flowing down through the sieve plate 13. Above the sieve plate 13, a compact crystal block is formed due to the movement of the sieve plate the scraper 14 scraped off pure crystals from the crystal block, at least partially melted in the heat exchanger 15 and discharged via the valve 16 in the direction of arrow 17 as a pure crystallizable compound. A part of the melted pure crystallizable compound is returned to the head of the washing column 20 via the pump 18 as a transport medium for the scraped-off crystals. Appropriate adjustment of the valve 16 maintains a pressure in the head of the washing column such that a part of the pure crystallizable compound is used as Washing liquid is printed against the compacted crystal block

The mother liquor separated in the washing column is returned to the crystallizer 1 via line 19. Mother liquor depleted in the crystallizable compound is discharged from the crystallizer via filter 22 and line 21. The device according to FIG. 1 is designed to separate 2,4-TDI from an 80% 2,4-TDI. holding starting isomer mixture to obtain a depleted isomer mixture still containing 69% 2,4-TDI, as described above. In FIG. 2, the exchange container 5 is replaced by the exchange column 5 for continuously gradual mother liquor exchange compared to FIG. 1. The exchange column 5 is described in more detail below. The device shown in FIG. 2 is suitable for the recovery of 2,4-TDI from a starting isomer mixture containing 80% 2,4-TDI with simultaneous recovery of a depleted isomer mixture with 65% 2,4-TDI (discharge via line 21). The entire starting isomer mixture is preferably introduced into the exchange column via line 6a

In the two-stage crystallization process according to FIG. 3, the exchanged mother liquor flowing out of the exchange column 5 is returned to the first stage (I) via line 8 and mixed in the mixing container 40 with the crystal suspensions emerging from the crystallizer 31 in the first stage. Container 40 exiting mother liquor through line 41 becomes excess 47 in the filter

(in terms of pumpability) mother liquor separated and returned to the crystallizer 31. The crystal suspension is fed to the crystallizer 1 of the first stage. The compound mixture depleted of the crystallizable compound is discharged from the crystallizer 31 of the second stage via line 51

The method according to FIG. 3 is particularly suitable for the separation of isomers of diphenylmethane diisocyanate (MDI). Due to the manufacturing process, MDI is obtained as a starting isomer mixture with 90% 4,4'-MDI and 10% 2,4'-MDI, although minor amounts of 2,2'-MDI may still be present. The liquidus temperature of the starting isomer mixture is approx. 36 ° C . The pure 4,4'-MDI has one

Melting point from 40 ° C. The isomer mixture to be discharged, which is depleted in 4,4'-MDI, still has 55% by weight 4,4'-MDI with a liquidus temperature of 18 ° C. Accordingly, the crystal suspension leaving crystallizer 31 has a temperature of 18 ° C The second crystallization stage 1 is operated at a temperature of 27 ° C. This corresponds to a mother liquor which contains 72% 4,4'-MDI. The mother liquor is introduced into the exchange column 5 via line 4. The starting isomer mixture is added via line 6 90% 4,4'-MDI with one Temperature of 40 ° C initiated in the exchange column 5. An exchanged mother liquor with 76% 4,4'-MDI flows out via line 8. The crystal suspension emerging from exchange column 5 via line 9 has a mother liquor with 80% 4,4'-MDI corresponding to a liquidus temperature of 32 ° C. For every 100 parts of starting isomer mixture fed via line 6, 77.8 parts of pure 4,4'-

MDI and 22.2 parts of an isomer mixture, which still contains 55% of 4,4'-MDI, are discharged via line 51. By controlling the flow rates in line 8 and line 49, a solids content of 30% is set in lines 4, 9 and 11 The solids content of the crystal suspension emerging from the crystallizer 31 of the first stage is only 23%. In the embodiment of the invention according to FIG. 4, an exchange column 5 or 35 is provided in both stages I and II. As a result, the mother liquor concentration in the mixing container 40 and line 41 can be increased are, so that the solids content in lines 34 and 41 is increased, ie the mass flows in the isomer separation of MDI are reduced. Furthermore, with a device according to FIG. 4, compound mixtures can be separated in two stages, in which there is an even greater depletion of the mixture to be discharged via line 51 than described for the isomer separation of MDI. The lines 6b, 8b and 49 shown in dashed lines denote only subordinate flow rates that the Control or adaptation of the procedure to specific tasks to be solved

5 shows a mother liquor exchange tube 5 which can be used according to the invention and which consists of three concentric tubes 61, 62 and 63, the inner tubes 61 and 62 being designed as sieve tubes. The crystal suspension coming from the crystallizer flows through the ring channel 64 between the two sieve tubes 61 and 62 In the direction of arrow 6, the starting isomer mixture is introduced into the outer ring channel 65, which enters the ring channel 64 through the sieve tube 62.At the same time, mother liquor is displaced into the inner sieve tube 61, and is discharged via line 8, which can be switched via the simultaneously switchable valves 66 and 67 Direction of flow through the sieve tubes 61 and 62 are reversed, so that a blockage of the sieve openings by periodic switching or by a change in pressure detected in the lines 6 or 8 ^" triggered switching of a blockage of the Screen openings are counteracted. The mother liquor concentration change in the ring channel 64 caused by the changeover is made more uniform in the mixing container 10 which is connected downstream.

Claims

claims

1 Process for the separation of organic compounds by one- or multi-stage suspension installation, each stage consisting of a crystallizer, optionally a recrystallization container and a filter for separating the liquid phase from the installation porridge, the liquid phase separated in the filter being returned to the respective installation stage, the installation porridge is transferred into the subsequent stage or from the last stage into the washing column for complete separation of the mother liquor, and from the stage depleted of the isomer to be crystallized

Liquid is discharged, the starting compound mixture being fed downstream of the last catalyst stage, characterized in that the mother liquor of the stall paste from the last stage of the catalyst exchange is at least partially gradually replaced by the starting compound mixture

2 The method according to claim 1, characterized in that the exchange takes place in a flow channel with walls configured as screens on both sides, with the starting compound mixture flowing through the screen from the side and mother liquor being drawn off through the screen on the other side

3 The method of claim 2, that the flow channel has an annular cross-section which is formed by concentric sieve tubes

4 The method according to claim 2 or 3, characterized in that, in order to avoid the blockage of the sieves, changeover valves are provided for exchanging the sides of the supply of starting compound mixture and the removal of mother liquor

5 The method of claim 1, that as a starting compound mixture

Isomer mixture of organic compounds is used Process according to one of claims 1 to 5, characterized in that a mixture of 80% 2,4-tolylene diisocyanate and 20% 2,6-tolylene diisocyanate is used as the starting isomer mixture and only one crystallization stage is used to obtain pure 2,4-tolylene diisocyanate

Method according to one of claims 1 to 5, characterized in that a mixture of 90% 4,4'-diphenylmethane diisocyanate and 10% 2,4'-diphenylmethane diisocynate is used as the starting isomer mixture, and pure 4,4'-MDI and a Mixture containing 55% 4,4'-MDI is discharged using two crystallization stages