EP2970082A1

EP2970082A1 - Methods for producing free flowing dicarboxylic acid crystalline crops

Info

Publication number: EP2970082A1
Application number: EP14710549.8A
Authority: EP
Inventors: Christian Dienes; Matthias Rauls; Oliver Bey; Mathias WEICKERT
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2013-03-15
Filing date: 2014-03-14
Publication date: 2016-01-20
Also published as: CN105189435A; KR20150131355A; BR112015023302A2; JP2016516022A; US20160023982A1; WO2014140276A1

Abstract

The invention relates to methods for producing free flowing dicarboxylic acid crystalline crops from an aqueous solution or suspension of the dicarboxylic acid in a crystallizer.

Description

The invention relates to processes for the preparation of free-flowing dicarboxylic acid crystallizates from an aqueous solution or suspension of the dicarboxylic acid in a crystallizer.

Dicarboxylic acids and especially adipic acid are important monomers for the preparation of polymers, in particular of polyamides.

Dicarboxylic acids can be z. Example by oxidation of cyclic alcohols, cyclic ketones or mixtures of these alcohols and ketones with oxidizing agents such as concentrated nitric acid or air. The quantitatively most important dicarboxylic acid is adipic acid, which is obtained industrially in two reaction steps from cyclohexane. In the first step, cyclohexane is oxidized with air to a cyclohexanol / cyclohexanone mixture (anolone mixture). After removal of unreacted cyclohexane, the anolone mixture is oxidized with concentrated nitric acid.

The industrial-scale oxidation of the cyclohexanol / cyclohexanone mixtures is carried out with excess 40-70% by weight, in particular concentrated (60% strength by weight) nitric acid at from 40 to 90 ° C. and atmospheric pressure. The catalysts used are copper and vanadium salts. In addition to the target product, the reaction product contains adipic acid (93-96 mol%) as by-products, succinic acid and glutaric acid (4-5 mol%) and small amounts of monocarboxylic acids, such as, for example, As acetic acid. The reaction effluent is passed into an outflow column, into which air is blown from below. The exhaust gas withdrawn at the top of the column is worked up to nitric acid. The bottom product is then dehydrated in a second column to the extent that the nitric acid content increases to 56 wt .-%. About 90% by weight thereof is recycled to the oxidation reactor, about 10% by weight purified by crystallization from water, see DE-A-1 238 000.

It is also possible to oxidize cyclohexanol and / or cyclohexanone with air. More recently, the production of adipic acid from adipic acid precursors has gained importance, which can be obtained from renewable raw materials such as sugar. It is known to obtain adipic acid by hydrogenating muconic acid (2,4-hexadiene dicarboxylic acid). Furthermore, adipic acid is accessible from glucaric acid. Dicarboxylic acids having preferably two to twelve carbon atoms, such as. As oxalic, malonic, succinic, maleic, glutaric, adipic, pimelic, suberic, sebacic, decanedicarboxylic and dodecanedicarboxylic are crystalline compounds with melting points between 98 ° C and 185 ° C. When heated, they can decay, such. B. Malonic acid in acetic acid and carbon dioxide. But you can also, such. For example, succinic a cyclic anhydride or z. B. adipic acid form a cyclic ketone (cyclopentanone).

Their purification by distillation must therefore take place because of their lack of thermal stability in a vacuum. In many cases, the purification is therefore carried out instead by crystallization. Water is often used as the solvent. The solubility of the dicarboxylic acids in water decreases with increasing molecular weight.

In order to ensure easy processability and handling, the dicarboxylic acids are usually crystallized into crystal powders (crystallizates). In this case, however, the crystals should not have too low an average crystal size distribution in order to reduce or avoid, for example, dust formation during handling.

However, such crystallizates often have the property to cake on prolonged storage in the heap to larger crystallites. Larger transport and storage containers such as big bags or silos can therefore often be emptied only with considerable mechanical effort for loosening caked Kristallisates. This circumstance causes, for example, in the further processing of adipic an undesirable additional time and cost. Adipic acid usually crystallizes from pure solutions in the form of thin sheets, which have a large contact area and thus allow good adhesion between adjacent crystals due to attractive interactions between the individual contact surfaces. Adipic acid crystals are described, for example, in RJ Davey et al, J. Chem. Soc. Faraday Trans. 88 (23), 3461-3466 (1992). It is also shown that the surface of pure adipic acid crystals is essentially determined by the {100} -oriented crystallographic surfaces, the physical properties of which result from the hydrophilic carboxyl groups lying there. When two such {100} faces are brought into contact with each other, they can immediately adhere weakly to each other through the formation of hydrogen bonds. Such facets are generally saturated with a "monofilm" of water, and the contacting of two such surfaces saturated with water substantially enhances their cohesiveness. "The formation of such crystalline bridges is responsible for the caking of the crystallizate described above A further disadvantage of such adipic acid crystallizates is that the formed crystal platelets are very thin Thin thin platelets break very easily in the course of the manufacturing or processing process and thereby produce a generally unwanted fine fraction On the other hand, the fine fraction leads to dust formation during processing, which can cause product losses and, if necessary, costly measures associated with a deterioration of the Rieselverhaltens to ensure safety at work. In the prior art, a number of physical and chemical processes are described, which allow an inhibition of the Verbackungsvorgangs. For example, during storage of adipic acid in a product silo silo continuously flows through small amounts of a dry gas. Since with this gas stream always existing traces of moisture are largely dissipated, the formation of intergranular bridges remains substantially, and a caking can be largely prevented. However, this method has the disadvantage that it can be difficult to apply to transport containers, especially not on big bags. Another method to prevent the strong intergranular adhesion is the coating of the crystals with hydrophobing agents. Thus, for example, DE-A 1, 618,796 describes several possibilities, such as by hydrophobic application of monocarboxylic acids to adipic acid crystals whose surface can be prevented and the formation of intercrystalline bridges. A disadvantage of this method is that the adipic acid must be added between 20 and 100 ppm of fatty acids that remain in the product, making it unsuitable for applications with high purity requirements. In addition, this method requires an additional process step in the production of adipic acid.

US Pat. No. 5,296,639 describes a process for the purification of adipic acid during crystallization, in which the crystal morphology is modified in such a way that the uptake of impurities during crystallization is reduced. For example, caproic acid or selected surfactants such as sodium dodecyl sulfate, sodium dodecylsulfonate or sodium dodecylbenzenesulfonate are added for this purpose. A disadvantage of this method is that the additives typically have to be added in concentrations of more than 100 ppm up to 3% in order to achieve the desired effect. As a result, the product is usually inadmissibly contaminated. In addition, the use of surfactants still has the disadvantage that in the case of an accumulation by internal recycling of the solvent (usually water) in systems for foaming, so that an application in concrete technical processes in the rule is difficult or even über- home is not possible.

From EP-A-0 968 167 it is known to subject solutions of dicarboxylic acids with the addition of at least one anionic polyelectrolyte having a molecular weight of at least 2000 as a crystallization aid to crystallization. The resulting dicarboxylic acid crystals are significantly more compact with a larger average diameter.

However, the presence of additives is considered to be fundamentally disadvantageous. They only act if they adsorb on the interface and so inevitably appear in the dicarboxylic acid crystals as impurities.

The object of the present invention is to provide a process for the preparation of free-flowing dicarboxylic acid crystallizates from an aqueous suspension of the dicarboxylic acid in a crystallizer, wherein the co-use of additives can be dispensed with and Nevertheless, good storage and free-flowing crystals are obtained, which are not prone to caking.

The crystals should preferably not only be larger than the known crystals. Moreover, they should not be in the form of thin crystallographic surfaces oriented in the {100} direction, but in a more favorable, compact crystal form.

The crystals should have a good trickling and not lose their flowability during prolonged storage. In addition, they should have no tendency to form fines and high purity.

The object is achieved by a process for the preparation of free-flowing Dicarbonsäurekristallisaten from an aqueous solution or suspension of dicarboxylic acid in a crystallizer, characterized in that a stirred container is used as a crystallizer, a vertical, cylindrical tank with side walls and a bottom , Means for supplying and discharging the solution or suspension, a coaxially arranged in the cylindrical tank guide tube and a coaxial arranged on the tank bottom blade stirrer with rotating coaxial shaft and Rührerblättern which promotes the solution or suspension in the radial direction, so that a Adjusting the flow of the solution or suspension in the manner of a loop reactor, wherein the peripheral speed of Blattrührers 0.5 to 6 m / s and the power input into the solution or suspension through the blade stirrer is 0.01 to 5 kW / m ³ .

The term "solution or suspension" includes solutions, suspensions, and mixed solutions / suspensions.The terms "solution" and "suspension" preferably include these three meanings, and "suspension" thus also means solutions.

The term "free-flowing" denotes dicarboxylic acid crystallizates which do not have the form of thin leaves described at the outset, but represent three-dimensional crystal agglomerates which are characterized by an irregular surface structure and therefore do not tend to cake, and which are free-flowing even after prolonged storage.

The term "aqueous solution or suspension" denotes a solution or suspension in which the majority of the solvent or suspension medium consists of water, preferably the suspension medium contains at least 60% by weight of water, more preferably at least 80% by weight of water. in particular at least 95% by weight of water. Particular preference is given to using only water as the solvent or suspension medium.

The term "flow of the suspension in the manner of a loop reactor" means that the suspension flows within the guide tube in the axial direction, between the guide tube and side walls of the cylindrical tank also flows in the axial direction with opposite orientation and at the bottom of the cylindrical tank and above the As a result, based on a cross-sectional view of the cylindrical tank in the form of a vertical section a loop-shaped flow of the suspension, which thus flows in the manner of a loop reactor. The term "flow of the suspension in the manner of a loop reactor" thus refers to an annular flow of the suspension in a loop, is introduced into the suspension and discharged from the suspension.There is no tubular loop, but the loop is between cylindrical tank and With respect to the vertical direction of the cylindrical tank, there is an axial flow direction through the vertical cylindrical tank except at the top and bottom reversal points is limited in the direction of the tank bottom so that substantially or exclusively results in a radial conveying direction on the tank bottom. The distance between the stirrer bottom edge and the bottom of the tank is chosen as small as possible. It has been found according to the invention that, when the blade stirrer is arranged on the tank bottom, advantageous crystal forms are formed, provided that the peripheral speed of the blade stirrer is 0.5 to 6 m / s and the power input into the solution or suspension by the blade stirrer is 0.01 to 5 kW / m ³ . This ensures on the one hand that a stable flow is obtained in which all suspended particles are moved up from the bottom of the tank, kept in motion and thus can not bake at the bottom of the tank.

On the other hand, not too much shear is introduced into the suspension, so that the formation of crystal agglomerates is made possible.

The parameters according to the invention ensure a sufficiently large flow rate and a sufficiently low shear introduction into the suspension. Any suitable dicarboxylic acids which form a suspension in water can be used in the process according to the invention. Preferably, the dicarboxylic acid is selected from C2-i2 dicarboxylic acids, preferably C4-8 dicarboxylic acids. The dicarboxylic acid is preferably aliphatic, linear and terminal or aromatic. Examples of suitable dicarboxylic acids are terephthalic acid or isophthalic acid and oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, sebacic acid, decanedicarboxylic acid and dodecanedicarboxylic acid. The dicarboxylic acids may be saturated, unsaturated or branched. In addition, the dicarboxylic acids further functional groups such. B. alkyl radicals having 1 to 5 carbon atoms, hydroxyl, keto or halogen radicals.

For the pure crystallization of dicarboxylic acids, preference is given to using terephthalic acid, adipic acid or succinic acid. Particularly preferred is adipic acid, in particular was prepared by oxidation of cyclohexanol, cyclohexanone or mixtures thereof with nitric acid. After removal of excess nitric acid, nitrogen oxides and water, it is preferably subjected to at least one crude crystallization, preferably two crude crystallizations. The adipic acid content of this prepurified adipic acid is preferably 95-99% by weight, preferably 97-98% by weight (based on dry substance).

The cyclohexanol or cyclohexanone used for the oxidation with nitric acid can be prepared by oxidation of cyclohexane with air or oxygen, by hydrogenation of phenol or by hydration of cyclohexene.

Also suitable is adipic acid, which was prepared by single-stage oxidation of cyclohexane with air or by saponification of adipic diesters.

Finally, adipic acid can also be used, starting from renewable raw materials, eg. B. from muconic acid or glucaric acid, is accessible.

The dicarboxylic acid suspensions used in the process are preferably obtained according to the following steps: a) oxidation of cyclic alcohols, ketones or mixtures thereof with nitric acid, oxygen or air to give corresponding dicarboxylic acids, b) working up the oxidation effluent, using water and unused oxidizing agent separated from the dicarboxylic acids formed completely or partially, c) at least one coarse crystallization of the dicarboxylic acids from water as the solvent.

Crude adipic acid, prepared on the said routes, is preferably also subjected to at least one crude crystallization from water before the pure crystallization.

Crystallization generally takes place when a supersaturated solution of the dicarboxylic acid is present in a solvent. A supersaturated solution of a dicarboxylic acid can be produced in various ways: Once it is possible to bring about the supersaturation of the solution by evaporation of solvent, at atmospheric pressure or in vacuo.

On the other hand, it is possible to produce a solution of the dicarboxylic acid at elevated temperature, which then gives, by cooling, a supersaturated solution which leads to crystallization.

It is also possible to combine both measures so that the solution is cooled by evaporation of solvent in vacuo to a temperature below the original solution temperature. The pure crystallization of the dicarboxylic acids, in particular of adipic acid, preferably takes place from water as solvent. It can be carried out batchwise, preferably continuously.

The dicarboxylic acids used for the pure crystallization, in particular adipic acid, preferably have a dicarboxylic acid content of 90-99 wt .-% after at least one crude crystallization. The crystallization is preferably carried out at temperatures of 30 to 90 ° C, preferably 40 to 80 ° C, particularly preferably 50 to 70 ° C. In this case, the concentration of the aqueous dicarboxylic acid solution which is fed to the crystallizer, preferably 20 to 70 wt .-%, preferably 30 to 60 wt .-%. The residence time of the dicarboxylic acid suspension in the crystallizer is preferably 0.25 to 8 hours, preferably 0.5 to 4 hours, more preferably 1 to 3 hours.

It may be advantageous in the crystallizer at the beginning of the crystallization z. B. to submit an adipic acid solution containing undissolved adipic acid suspended as seed crystals. The temperature in the crystallizer is z. B. maintained at 50 to 70 ° C. Then, crystal-free adipic acid solution is supplied. Adipic acid suspension is withdrawn continuously via a product take-off, so that the level of the crystallizer remains constant. Once the originally submitted adipic acid suspension has been replaced, a new steady state equilibrium has been established.

The crystallizers used according to the invention are known in a general form from the prior art. For this purpose, reference may be made, for example, to WO 2004/058377.

WO 2004/058377 A1 (DuPont, Priority Dec. 16, 2002) describes an apparatus which is suitable for the production of particles by precipitation or crystallization (FIG. 1 in FIG

WO 2004/058377). This apparatus can be designed to crystallize biological products such as proteins and enzymes, small organic molecules such as pharmaceuticals, fine chemicals and inorganic materials such as mineral salts (page 1, lines 8 to 15). The apparatus shown in Figure 1 consists of a crystallisation apparatus comprising: a) a container,

b) a radial stirrer optionally containing a top and a bottom plate,

c) a guide tube containing a plurality of baffles, which are arranged so that between the guide tube and a side wall of the container, a channel is formed, wherein the guide tube a

Has diameter that is about 0.7 times the container diameter. The apparatus can produce crystals of different sizes ranging from about 0.5 to about 3000 microns (page 6, lines 24 to 30).

The crystallization of dicarboxylic acids, in particular of adipic acid, is not mentioned in WO 2004/058377 A1. Mention is made in a broad list only fatty acids that represent monocarboxylic acids and no dicarboxylic acids.

WO 2004/058377 enables the production of larger crystals with a narrower size distribution or finer crystals with narrower size distribution. Increasing the RPM value often results in finer particles, adjusting the addition rate of the feed to change the particle size (page 21, lines 22-25).

The size of the crystals can be varied by changing the chemical composition of the liquid streams, the stirrer speed (RPM) and the ratio of the different liquid flows to each other.

WO 2004/058377 A1, however, gives no indication as to how to proceed in the crystallization of dicarboxylic acids and in particular of adipic acid in order to achieve the object of the invention.

The embodiment of the crystallizer used in the invention will be explained in more detail below.

The parameters, definitions and abbreviations used in the following are illustrated in the attached FIG. 2.

FIG. 2 shows the schematic structure of a crystallizer used according to the invention in a cross-sectional view. The parameters listed below are given in FIG. FIG. 2 relates to a crystallizer used according to the invention with a radial stirrer arranged near the bottom. The numbers 1 to 7 given on the right in FIG. 2 have the following meanings:

1 container

2 radial stirrer

3 draft tube

4 draft tube breaker

5 annular space breaker

6 dosing points for solution

7 deduction

FIG. 3 shows a schematic representation of a horizontal cross-sectional view of the crystallizer according to FIG. 2. Thus, the individual agitator blades of the radial agitator and the Order of the radial stirrer to recognize in the crystallizer. The reference numbers 2 to 7, which are recorded on the right-hand edge of FIG. 3, have the meanings given above.

A possible embodiment of an inclined blade stirrer can be taken from FIGS. 4 and 5.

FIG. 4 shows a crystallizer not used according to the invention, in which a inclined blade stirrer (axial stirrer) is not arranged close to the ground, but in the guide tube with a significant ground clearance. FIG. 4 shows a schematic cross-sectional view of the crystallizer, and FIG. 5 shows a horizontal cross section through the crystallizer at the level of the

Inclined-. Therefore, FIG. 5 shows the arrangement of the oblique blade stirrer in the crystallizer.

The inclined blade stirrer per se can also be used in the process according to the invention. The reference numerals 1 to 7 used in Figures 4 and 5 at the right edge have the following meaning:

1 container

2 axial stirrer (inclined blade stirrer)

3 draft tube

4 draft tube breaker

5 annular space breaker

6 metering point for solution

7 deduction

Preferred relations according to the invention between the individual parameters can be seen from FIG. 2, which is true to scale.

FIG. 1 also shows photographic images of adipic acid crystals obtained by the process according to the invention (FIG. 1, top) or customary adipic acid crystals obtained by a process not according to the invention (FIG. 1, bottom).

As a tank or container, a cylindrical basic shape is selected according to the invention, which is closed with a bottom and preferably with a lid. As bottom and lid forms are preferably dished bottoms, semi-elliptical soils and basket bottom floors use. But it can also be flat and conical bottoms for the lower end. Especially for the Anglo-American area, the molds for floor and lid can also be manufactured according to the ASME standard (American Society of Mechanical Engineering). Important importance is the slenderness H / D of the container, which is given by the ratio between the total container height H (including the bottom and the lid) and the container diameter D. Values for H / D between 1 and 6 are preferably suitable for the stirred tank or container, preferably from 2 to 4. Depending on the specific process conditions, the degree of filling of the container is defined, which is defined dimensionless by the reference of the liquid level Hf to the total container height H. A common range for Hf / H is preferably between 0.5 and 0.9, preferably from 0.6 to 0.8.

In the container, a plug-in tube, also called guide tube, is arranged concentrically or coaxially, which serves for the defined alignment of the circulation flow generated by the stirrer. An important design criterion with which the process parameters can be favorably influenced is the ratio di_ / D between the guide tube diameter di_ and the container diameter D. Smaller diameter ratios promote a dispersing function, larger diameter ratios are more likely to counteract a gentle treatment of the material to be stirred. Diameter ratios di_ / D are preferably between 0.2 and 0.8, preferably from 0.3 to 0.7.

The distance between the lowest point of the container bottom and the lower edge of the Einsteckrohres Ahi_, i is chosen to be large enough to operate the arranged in this space stirrer with a safe distance to the ground or to the plug-in tube. At the same time, this influences the deflection of the fluid flow between the plug-in tube and the annular space and thus the pressure loss in the loop. For technical applications, the ratio between the ground clearance and the container diameter Ahi_, i / D is preferably

0.1 to 0.6, preferably 0.15 to 0.5.

For a stable operation of the Leitrohrapparates also the overlap of the upper edge of the Einsteckrohres with liquid Ahi_, 2 is advantageous to achieve a favorable flow deflection and to keep the resulting pressure loss low. A well-designed covering can also favor the intake of floating solids or foams. The covering Ahi_, 2 / D based on the container diameter is preferably selected between 0.05 and 0.5, preferably between 0.1 and 0.3. According to the design of the container height H, the level Hf, the Leitrohrbodenabstandes Ahi_, i and the Leitrohr- coverage Ahi_, 2 results in the length of the Einsteckrohres li_.

It may be advantageous to provide baffles in the draft tube and / or between the draft tube and side walls of the cylindrical tank. The baffles are important to break the tangential twist of the fluid flow caused by the impeller rotation and redirect it in an axial flow direction. In the first place, the arrangement of the baffles on the pressure side of the stirrer should be carried out, which, similar to pumps, is the side to which the stirrer conveys the fluid. In the case of the radial stirrer arranged between the container bottom and the plug-in tube, this concerns the area of the annular space between the guide tube wall and the container / tank wall. For an axial stirrer arranged concentrically in the guide tube, the conveying direction of which is directed upwards in the guide tube, the pressure side of the guide tube interior is above the propeller stirrer. If an axial stirrer conveys the fluid down the guide tube, the pressure side is the area between the bottom of the stirrer and the bottom of the container. The reinforcement can then be arranged both in the diameter range of the guide tube and / or in the region of the annular space.

In the fluidic tests it has been shown that even a suction-side reinforcement improves the axial flow orientation and thus the conveying effect. In the case of the arranged between the container bottom and guide tube radial agitator, the suction side is the region of the Leitrohrinnenraumes above the stirrer, as well as for the concentric arranged in the guide tube axial stirrer, which promotes downward in the guide tube. If the flow direction of an axial stirrer in the guide tube is directed upwards, the suction side is the area below the stirrer.

In addition to the positioning, the dimension and the number of baffles influence the reinforcement effect. The number of baffles per pressure and suction side is preferably from 3 to 12, preferably from 4 to 8.

The width of the baffles bs / i_ in the guide tube is dimensionless relative to the guide tube diameter di_. Preferably, for bs / i_ / di_ between 0.05 and 0.5, preferably from 0.1 to 0.35. The distance between the flow baffles arranged in the guide tube is AS, L = 0.02 x dL between the longitudinal side of the baffle and the inside of the baffle tube as standard, but values from 0 x dL to 0.1 x dL are also possible. The length of the baffles in the guide tube IS, L can be max. correspond to the length of the plug-in tube IL and should preferably be at least half the diameter of the guide tube dL to ensure a sufficient reinforcement effect.

The ground clearance of the Leitrohrstrombrecher AhS, L is, in particular for the execution with the ground-level blade stirrer, preferably minimally the distance of Leitrohrunterkante to the container bottom AhL, 1 and max. AhL, 1 + half the stirrer outer diameter dR / 2.

The width of the baffles in the annulus bS, R can be max. the annulus width (D-dL) / 2 amount, which results from the clear distance between the guide tube and container wall. For a sufficient reinforcement effect, the ratio of bS, R to the container diameter D should preferably have a value of bS, R / D> = 0.05 × D, preferably bS, R / D> = 0.1 × D. The distance of the annular space baffle to the inner wall of the container AS, R1 is 0.02 x D by default, but may also have values from 0 x D to 0.1 x D. Likewise, the distance of the annular space-stream breaker to Leitrohrinnenwand AS, R2 be dimensioned by default with 0.02 x D, depending on the version but also from 0 x D to 0.1 x D.

The length of the Ringraumstrombrecher IS, R is based on achieving a sufficient reinforcement effect and should be similar to the Leitrohrstrombrecher minimal half container diameter D. The maximum length results from the conclusion of the annular space with the Leitrohroberkante AhL, 3. The distance between the annular space flow breaker and the container bottom AhS, R can preferably be designed with dimensions between 0 × D and 1 × D, preferably from 0.02 × D to 0.5 × D. According to the invention, it has been found that the size and above all form (morphology) of the dicarboxylic acid crystals can be drastically improved by the speed or peripheral speed of the stirrer (RPM) and the power input, preferably also by the type of stirrer used.

In order to bring about a turbulent mixing of the aqueous dicarboxylic acid solutions and suspensions in the crystallizer, the crystallizer content should be stirred intensively.

The blade stirrer used in the crystallizer used according to the present invention has a rotating shaft and stirrer blades attached thereto, which may have any desired angle of attack with respect to the agitator shaft. Corresponding stirrer geometries are known per se and are explained in more detail, for example, in the publications listed below. Each stirrer becomes a radial stirrer when placed according to the invention near the ground.

EP-A-1 208 905 describes a stirred vessel for producing a suspension of solids in a liquid having a uniform concentration, the agitator having turbine blades on a rotating shaft. Textbooks and publications on stirrer design are z. B. K. Kipke, "Power consumption and flow optimized in Leitrohrpropellern", Maschinenmarkt, Würzburg 88 (1982) 52, Ekato Handbook of Stirring Technology "(1990); F. Liepe et al., "Rührwerke", 1st edition 1998, self-published University of Applied Sciences Kothen The stirrer in Figure 1 of WO 2004/058377 may have any shape, provided that this allows the necessary circulation of liquid .. Suitable are "radial flow impeller", at the head and / or at the bottom of the guide tube have an axial "flow propeller" or a "marine propeller", a "double propeller" or a "multi propeller". The stirrer is preferably a "radial flow agitator", preferably a "radial flow impeller" with at least one blade, a bottom plate and optionally a cover plate (page 10, line 27 to page 11, line 11).

The number of stirrer blades, the size of the vanes, the vane angle of attack and the number of stirrer revolutions per minute (RPM) can be varied by means of these parameters of the mixture (page 1 1, lines 17 to 25).

Preferably, the "impeller" comprises the configurations shown in Figures 2, 3, 4 and 5.

The at least one wing of the inventively usable stirrer according to

WO 2004/058377 can, for. B. have any shape, as long as the content of the crystallization apparatus is pumped through the apparatus at the necessary speeds. Typically, the height of the stirrer blades is about one-sixth of the agitator diameter. The width of the at least one wing varies with the wing angle (page 12, lines 7 to 17). In space- Commonly, the stirrer blades can have any angle that will cause the necessary circulation in the apparatus. The wing angle in WO 2004/058377 A1 is typically about 45 to 65 degrees, preferably about 55 degrees (page 12, lines 7 to 17). The linear velocity of the slurry is about 0.1 to about 1.8 meters per second, preferably about 0.9 meters per second (page 19, lines 12 to 14).

From Römpp Chemielexikon, 10th Edition, Volume 5, page 3876, keyword "stirring" shows that a large number of different types of stirrers is used on a large scale, for example, for stirring in the low-viscosity range, high-speed types of stirrers such as propeller, beveled Disc stirrers or jet mixers are also suitable. [0002] Leaf stirrers are also suitable (not mentioned in the Römpp).

Due to the close-to-ground arrangement of the blade stirrer, each stirrer type used becomes a radial stirrer, since the suspension flow can only be conducted to the outside, since an axial direction of flow through the tank bottom is hindered. Thus, the terms according to the invention are to be understood "close to the ground" and "on the tank bottom".

Therefore, any suitable blade stirrers with rotating coaxial shaft and stirrer blades can be used. For example, the blade stirrer can be selected from radial stirrers, inclined blade stirrers, turbine stirrers, propeller stirrers, anchor stirrers, spiral stirrers and helical stirrers.

The design and arrangement of the agitator is important in terms of achieving optimal crystal morphology. For the adipic acid crystallization, preference is given to using a radial stirrer which is placed on the tank bottom / container bottom or between the container bottom and lower edge of the insert tube and thus below the guide tube. Preferably, a single-stage stirrer design is selected, however, the subdivision of the stirrer into several individual stages is also possible. Radial stirrers are characterized in that the stirrer blades are arranged perpendicular to the horizontal plane at an angle α of 90 °. Typical representatives are disc or blade stirrers. The number of stirrer blades nRb may preferably be from 2 to 16, preferably from 4 to 8. The stirrer height hR depends on the ground clearance of the lower Einsteckrohrunterkante AhL, 1 taking into account a sufficient ground clearance of the stirrer lower edge AhR, 1 itself and a sufficient distance between Rühreroberkante and guide tube AhR, 2. With reference to the container diameter D, the stirrer may preferably be designed with a height hR between 0.1 x D and 0.58 x D, preferably from 0.25 x D to 0.5 x D. With reference to the container diameter D, the values for AhR, 1 and AhR, 2 may preferably be between 0.01 x D and 0.3 x D, preferably from 0.03 x D to 0.2 x D. The distances AhR, 1 and AhR, 2 can differ from each other. If necessary, the shape of the stirrer blades can be adapted to the bottom shape to achieve a uniform distance AhR, 1.

The diameter of the radial agitator dR is largely determined by the diameter of the guide tube dL, but may also be designed smaller or larger than this. Interpretations for dR are usually 0.1 x D to 0.98 x D, preferably from 0.15 x D to 0.9 x D possible, preferably from 0.3 x D to 0.7 x D.

In addition to the use of a radial stirrer with an angle of attack of 90 ° to the horizontal and other stirrer shapes for the ground-level arrangement below the guide tube are conceivable. There are also angles of incidence of Rührerflügel possible, which are in a range between 25 ° and <90 ° to the horizontal. Under this geometry fall in particular so-called axial stirrer such. B. slant blade, turbine stirrer and propeller stirrer. Besides these, forms such as anchor stirrers, screw-shaped stirrers or helical stirrers are also suitable in principle. The direction of rotation of the stirrer drive can be directed from above viewed both clockwise and counterclockwise.

In addition to the described arrangement of the stirrer below the guide tube and the arrangement of a Axialrührers on the tank bottom in the guide tube is also possible to bring about the crystal formation as desired and influence. As a stirrer for this o. G. Axialrührer used, which are preferably characterized by an angle between 25 ° and 45 ° to the horizontal. The number of stirrer blades may preferably be 2 to 12, preferably 3 to 8. Again, the direction of rotation in the container top view seen in both clockwise and run against it. In interaction with the wing angle in (mathematical) positive or negative direction of rotation to the horizontal, this results in the conveying direction of the axial agitator up or down in the guide tube. The ground clearance of the stirrer lower edge AhR, 1 is preferably at least the distance of the lower Leitrohrunterkante to the container bottom AhL, 1 and at most the sum of AhL, 1 and half the diameter of the stirrer dR / 2.

For the formation of the adipic acid crystals, a certain volume-specific power input is to be applied by the agitator, which ensures complete suspension of the solid particles and a stable circulation flow between the guide tube and annulus. The volume-specific power input is to be understood below as the ratio P / V between the stirrer power P and the filling volume V of the container. Power inputs are from 0.01 W / l to 5 W / l, preferably from 0.05 W / l to 2 W / l, in particular from 0.1 to 0.5 W / l or kW / m ³ . Corresponding to the power input and the stirrer or container size, the circumferential speed with respect to the outside diameter of the stirrer results. The stirring speed necessary for the formation of the compact agglomerates according to the invention (peripheral speed of the blade stirrer) is 0.5 to 6 meters per second, preferably 1 to 5 meters per second, more preferably 1.2 to 4 meters per second, particularly preferably 1.5 to 3 , 5 meters per second.

It was found that when using blade stirrers, large dicarboxylic acid crystals are formed in the form of compact agglomerates which, after drying, remain free-flowing for weeks. The inventive method has the advantage that it solves the task in a conventional apparatus solely by using an unconventional stirrer assembly and coordinated peripheral speed and tuned power input, but without the aid of additives. The compact crystals formed according to the invention remain free-flowing because they do not cake because of their shape as the normally resulting thin dicarboxylic acid leaflets to larger units.

If, instead of a blade stirrer, an inclined blade stirrer mounted centrally in the guide tube and having an angle of attack of more than 20 degrees with the same energy input as in the case of the blade stirrer, substantially smaller dicarboxylic acid crystals are formed in the form of thin platelets. These platelets caked within weeks to non-free-flowing lumps.

Small amounts of dicarboxylic acid suspension are withdrawn continuously via a draw-off tube, so that the level of the crystallizer remains constant. The dicarboxylic acid crystals are z. B. separated on a centrifuge and optionally dried.

To ensure a uniform withdrawal of all particle size fractions in continuous operation, the variant of the suspension withdrawal is preferably selected on the container bottom, other deduction variants such as in Leitrohrinnern or in the region of the annulus are in principle also possible.

If the trigger is located at the bottom of the tank, a radial or axial stirrer arrangement is selected between the tank bottom and the bottom edge of the plug-in pipe, or an axial stirrer conveying downwards in the draft pipe at the bottom of the tank. Installation conditions, the metered dicarboxylic acid solution is preferably metered into the annulus to avoid short circuit currents between Zudosierstelle and drain. Due to the upward flow in the annular space and the subsequent downward flow in the guide tube, the metered volume elements are forced to pass through the circulation area of the container at least once before they can leave the crystallizer. The number of mixing points in the annulus can be from 1 to 100, preferably it corresponds to the number of baffles in the annulus.

The vertical position of the metering points AhZ, R should preferably at least AhZ, R = AhR, 1 + hR / 2 and at most AhZ, R = AhL, 3 (completion of the annular space with the Leitrohroberkante), preferably between AhZ, R = AhR, 1 + hR and AhZ, R = AhR, 1 + 2 hR so that the feed can be reliably detected by the upward flow of the stirrer. The Zudosierstellen can be distributed both in the vertical direction over the annulus height and distributed in the horizontal direction over the circumference of the annulus. The position of the Zudosierstellen should preferably be positioned in a horizontal plane in the direction of rotation of the stirrer in front of the baffles in an angle ß between 1 and 45 °, preferably from 5 to 20 °. The invention is further illustrated by the following examples. Examples

Execution of the laboratory crystallization of adipic acid

A cylindrical glass laboratory crystallizer (DN300) with an internal height of about 350 mm and an internal diameter of 300 mm with a diameter of 206 mm (DN200) can be equipped with different stirrers. The crystallizer is charged with a 35% suspension of adipic acid in water and heated to 60.degree. Partial dissolution of the crystals results in a solids content of about 23%. These initially present crystals of 100-200μηη medium size serve as seed crystals. To carry out a continuous crystallization, a 35% strength, crystal-free solution of adipic acid at 82 ° C. is conveyed into this crystallizer from a temperature-controlled storage container. In order to keep the temperature in the crystallizer at a constant 60 ° C at the delivery rate of 1 1, 7 kg / h, it is cooled over its walls. By periodically subtracting small amounts of suspension via a bottom outlet valve, the level in the crystallizer is kept constant. After 10 hours of operation of the experiment, the originally contained suspension has been replaced to less than 1% of the starting amount; in terms of particle size and shape, a new steady state equilibrium has been established. The suspended crystals are separated by centrifuging for three minutes on a Siebbecherzentrifuge at 600g, freed by rapid spreading on an absorbent filter paper from adhering mother liquor and finally dried overnight at 60 ° C in a vacuum oven.

Embodiment 1

In the experiment described above, an eight-blade blade stirrer with a diameter of 183 mm and a blade height of 80 mm arranged at the bottom of the tank (about 20 mm above the tank bottom) is inserted and rotated with 1 12 RPM, corresponding to a peripheral speed of about 1. 07 m / s , The power input here is approx. 0.5W / L.

Figure 2 and Figure 3 show the crystallizer used in the process schematically.

The crystals obtained in the crystallizer shown in FIGS. 2 and 3 by the process according to the invention have a mean size of 1 .100 μm determined by laser diffraction and show the shape of compact agglomerates. When stored in a closed screw-cap glass (height 0.2 m), the crystals remain free-flowing for weeks. They are shown in Figure 1 above. Comparative Example 1

Instead of the blade stirrer, a five-blade inclined blade turbine (angle of attack 38 °) of 185 mm diameter is used at a height of about 80 mm above the tank bottom and inside the guide tube. At a speed of 12RPM as in Example 1, crystallization can not be performed because the pump power of the stirrer and the power input are insufficient to produce a stable flow and to suspend the seed crystals. In order to achieve the same power input of 0.5W / L as in Example 1, a speed of 300RPM is set, corresponding to a peripheral speed of about 2.9 meters per second.

Figure 4 and Figure 5 show a schematic representation of the crystallizer used in Comparative Example 1 and Schrägblattrührers. The crystals obtained in the experiment were again measured and examined.

At the end of the experiment, the crystals have a mean size of 450μηη and have the shape of thin platelets. In the same storage conditions, the bed is clearly cohesive and moderately baked after only 24 hours, strongly caked after a few weeks in the form of large, hard lumps. They are shown in Figure 1 below.

Discontinuous crystallization of succinic acid

The crystallizer described in Example 1 of the continuous crystallization of adipic acid and depicted in FIGS. 2 and 3 is now filled with 9.6 kg of succinic acid and 20.4 kg of water. Its 32% solution has a saturation temperature of 68.5 ° C. After dissolution of all crystals at 75 ° C, the crystallizer is cooled to 68.3 ° C and seeded at this temperature with 96g succinic acid crystals, cooled another 0.5K and stirred at this temperature for half an hour. The still thin suspension is then cooled in the course of 3.5 h from 68 ° C to 30 ° C, the cooling initially begins slowly with 2K / h, then slowly increased and finally reaches the final temperature with a maximum cooling rate of 16K / h , The separation and work-up of the suspension is identical to the previously described procedure for adipic acid.

Embodiment 2

In a first experiment, an eight-blade blade stirrer with a diameter of 183 mm and a blade height of 80 mm, which is arranged at the tank bottom (about 20 mm above the tank bottom), is inserted and rotated with 90 RPM. The power input here is only about 0.2W / L. The crystals have a mean size of 1,440 μm determined with a vibrating sieve and show the shape of compact agglomerates. When stored in a closed screw-cap glass (height 0.1 m), the crystals remain free-flowing for weeks. Comparative Example 2

Instead of the blade stirrer, a five-blade inclined blade turbine (angle of attack 38 °) of 185 mm diameter is used at a height of about 80 mm above the tank bottom and inside the guide tube, as shown in FIGS. 4 and 5. Again, it is not possible at all to achieve stable flow and suspension of all crystals at the low RPM of 90 RPM. To achieve the same power input as in the reference example of 0.2W / L, a speed of 245RPM is set. At the end of the experiment, the crystals have a laser diffraction average size of 700μηη and are in the shape of highly rounded platelets. Under the same storage conditions, the bed appears clearly cohesive after only 24 hours, moderately baked after a few weeks in the form of large lumps.

Claims

claims

A process for the preparation of free-flowing dicarboxylic acid crystals from an aqueous solution or suspension of the dicarboxylic acid in a crystallizer, characterized in that a stirred container is used as the crystallizer, comprising a vertical, cylindrical tank with side walls and a bottom, means for supplying and discharging the Solution or suspension, a coaxially arranged in the cylindrical tank guide tube and a coaxial arranged on the tank bottom blade stirrer with rotating coaxial shaft and agitator blades which promotes the solution or suspension in the radial direction, so that a flow of the solution or suspension in Art a loop reactor, wherein the peripheral speed of Blattrührers 0.5 to 6 m / s, and the power input into the solution or suspension through the blade stirrer is 0.01 to 5 kW / m ³ .

A method according to claim 1, characterized in that the peripheral speed of the Blattrührers 1 to 5 m / s, preferably 1, 5 to 3.5 m / s.

A method according to claim 1 or 2, characterized in that the power input into the solution or suspension 0.05 to 2 kW / m ³ , preferably 0.1 to 0.5 kW / m ³ .

Method according to one of claims 1 to 3, characterized in that the cylindrical tank has a ratio H / D of height H to diameter D between 1 and 6.

Method according to one of claims 1 to 4, characterized in that the ratio di_ / D between Leitrohrdurchmesser di_ and diameter D of the cylindrical tank is between 0.2 and 0.8.

Method according to one of claims 1 to 5, characterized in that the ratio Ahi_, i / D between the distance of the lower edge of the guide tube from the lowest point of the bottom of the cylindrical tanks Ahi_, i and the diameter D of the cylindrical tank 0.1 to 0 , 6 is.

Method according to one of claims 1 to 6, characterized in that the distance of the lower edge of the Blattrührers from the tank bottom AhR, 1, based on the diameter D of the cylindrical tank, 0.01 x D to 0.3 x D.

Method according to one of claims 1 to 7, characterized in that the blade stirrer is arranged below the guide tube.

Method according to one of claims 1 to 8, characterized in that the blade stirrer is selected from radial stirrers, Schrägblattrührern, turbine stirrers, Propellerrüh- rern, anchor stirrers, helical stirrers and helical stirrers.

10. The method according to any one of claims 1 to 9, characterized in that the dicarboxylic acid is selected from C2-i2-dicarboxylic acids, preferably C4-8 dicarboxylic acids.

1 1. A method according to claim 10, characterized in that the dicarboxylic acid is aliphatic, linear and terminal or aromatic.

12. The method according to claim 10, characterized in that adipic acid or succinic acid or terephthalic acid are used as dicarboxylic acids.

Method according to one of claims 1 to 12, characterized in that the dicarboxylic acid solutions or suspensions used in the process are obtained by the following steps: a) oxidation of cyclic alcohols, ketones or mixtures thereof with nitric acid, oxygen or air to corresponding dicarboxylic acids .

Working up of the oxidation effluent from step a), wherein water and unused oxidant are completely or partially separated from the dicarboxylic acids formed,

at least one coarse crystallization of the dicarboxylic acids from water as the solvent.

14. The method according to any one of claims 1 to 13, characterized in that the dicarboxylic acids of the dicarboxylic acid solutions or suspensions used in the process are produced or obtained from renewable raw materials.