METHOD AND DEVICE FOR SEPARATING A SUBSTANCE OUT OF A SOLUTION

Abstract

The present invention relates to a method for separating off a substance from a solution, in which electromagnetic radiation is radiated into the solution, an intensity of the electromagnetic radiation which has been scattered by crystals located in the solution is detected, the detected intensity is compared with a desired intensity (I.sub.S) and the temperature of the solution is regulated depending on the difference between the detected intensity and the desired intensity (I.sub.S) in such a way that the amount of this difference is reduced. If the amount of the difference between the detected intensity and the desired intensity (I.sub.S) is less than a limiting value, a crystallization method is started in which crystals of the substance are obtained which are then separated off.

Claims

1.-21. (canceled)

22. A method for separating off a substance by crystallization from a solution of the substance, in which a suspension of seed crystals is produced and, if a desired amount of seed crystals is present, a crystallization method is started, in which crystals of the substance are obtained which are then separated off, where, for producing the desired amount of seed crystals: electromagnetic radiation is radiated into the solution, where the electromagnetic radiation radiated into the solution has the form of a beam, the aperture angle of which is greater than 5 degrees, for establishing a desired amount of seed crystals an intensity of the electromagnetic radiation which has been scattered by crystals located in the solution is detected, the detected intensity is compared with a desired intensity (I.sub.S), the temperature of the solution is regulated depending on the difference between the detected intensity and the desired intensity (I.sub.S) in such a way that the amount of this difference is reduced, if the amount of the difference between the detected intensity and the desired intensity (I.sub.S) is less than a limiting value, the desired amount of seed crystals for the crystallization method is present.

23. The method according to claim 22, wherein the desired intensity (I.sub.S) is determined by reference measurements in which, for the solution, the relationship between the crystal size and/or the crystal morphology at the end of the crystallization method and the detected intensity at the start of the crystallization method is determined and from this the desired intensity (I.sub.S) is selected as the intensity which is assigned to the desired crystal size and/or crystal morphology.

24. The method according to claim 22, wherein the solution or some of the solution is brought in a crystallization vessel to a temperature which is lower than a defined starting temperature value (T.sub.A), which is below the anticipated saturation temperature of the solution, and the solution is then heated until the amount of difference between the detected intensity and the desired intensity (I.sub.S) is less than the limiting value.

25. The method according to claim 22, wherein the starting temperature value (T.sub.A) is determined from the desired intensity (I.sub.S) by selecting the starting intensity (I.sub.A) assigned to the starting temperature value (T.sub.A) as the x-fold intensity of the desired intensity (I.sub.S), where the value x is in a range from 1.2 to 10, and the temperature of the solution is regulated in such a way until the intensity is greater than the starting intensity (I.sub.A).

26. The method according to claim 22, wherein the electromagnetic radiation of one wavelength range or two or more wavelength ranges which is/are wider than 20 nm is radiated into the solution.

27. The method according to claim 22, wherein the electromagnetic radiation radiated into the solution has the form of a beam, the minimum cross section of which is greater than 0.1 mm.

28. The method according to claim 22, wherein the infrared radiation is radiated into the solution and the intensity of infrared radiation is detected.

29. The method according to claim 22, wherein the electromagnetic radiation is radiated into the solution by means of a scattered-light probe (9) and the intensity of the back-scattered electromagnetic radiation is detected by means of the scattered-light probe (9).

30. The method according to claim 22, wherein the incident direction of the radiated electromagnetic radiation is essentially parallel to the detection direction, from which the intensity of the back-scattered electromagnetic radiation is detected.

31. The method according to claim 22, wherein the solution is introduced into a crystallization vessel (1) at a temperature which is below the starting temperature value (T.sub.A), if the scattered-light probe (9) is located within the introduced solution, the electromagnetic radiation is radiated into the solution by means of the scattered-light probe (9) and the intensity of the electromagnetic radiation which has been scattered by the crystals located in the solution is detected, the temperature of the solution upon further introduction of the solution into the crystallization vessel (1) is regulated such that the amount of difference between the detected intensity and the desired intensity (I.sub.S) is less than the limiting value.

32. The method according to claim 22, wherein the material comprises at least one ligand of the formula (I) ##STR00030## where Ar.sup.1, Ar.sup.2, Ar.sup.3, Ar.sup.4, independently of one another, are chosen from C.sub.6-C.sub.15-aryl radicals or C.sub.2-C.sub.15-heteroaryl radicals, which, if appropriate, can in each case carry 1 to 7 identical or different substituents chosen from C.sub.1-C.sub.6-alkyl, C.sub.1-C.sub.6-perfluoroalkyl, C.sub.1-C.sub.6-alkoxy, C.sub.7-C.sub.12-aralkyl, halogen, SiR.sup.5aR.sup.6aR.sup.7a, optionally substituted C.sub.6-C.sub.10-aryl, NR.sup.8aR.sup.9a, SR.sup.10a, NO.sub.2, R.sup.1, R.sup.2, R.sup.3, R.sup.4, independently of one another, are chosen from hydrogen, C.sub.1-C.sub.6-alkyl, C.sub.1-C.sub.6-perfluoroalkyl, C.sub.1-C.sub.6-alkoxy, C.sub.7-C.sub.12-aralkyl, halogen, SiR.sup.5bR.sup.6bR.sup.7b, optionally substituted C.sub.6-C.sub.10-aryl, NR.sup.8bR.sup.9b, SR.sup.10b, NO.sub.2 and where R.sup.1 or R.sup.2 and/or R.sup.3 or R.sup.4, together with A, can form an aromatic or nonaromatic cycle, and A is a straight-chain or branched and/or cyclic hydrocarbon radical having 1 to 25 carbon atoms which may be saturated or mono- or polyunsaturated and/or partially aromatic and can, if appropriate, have one or more identical or different heteroatoms chosen from O, S, NR.sup.11, and/or one or more identical or different functional groups chosen from the functional groups C(O), S(O), S(O).sub.2 and can, if appropriate, carry one or more identical or different substituents chosen from the substituents C.sub.1-C.sub.6-alkyl, C.sub.1-C.sub.6-perfluoroalkyl, C.sub.1-C.sub.6-alkoxy, C.sub.1-C.sub.10-acyloxy, C.sub.7-C.sub.12-aralkyl, halogen, —SiR.sup.5cR.sup.6cR.sup.7c, optionally substituted C.sub.6-C.sub.10-aryl, substituted or unsubstituted C.sub.2-C.sub.10-hetaryl, NR.sup.8cR.sup.9c, SR.sup.10c, NO.sub.2, C.sub.1-C.sub.12-acyl, C.sub.1-C.sub.10-carboxyl, or is a C.sub.6-C.sub.15-aryl radical or a C.sub.2-C.sub.15-heteroaryl radical which can, if appropriate, in each case carry 1 to 5 substituents chosen from C.sub.1-C.sub.6-alkyl, C.sub.1-C.sub.6-perfluoroalkyl, C.sub.1-C.sub.6-alkoxy, C.sub.7-C.sub.12-aralkyl, halogen, SiR.sup.5dR.sup.6dR.sup.7d, substituted or unsubstituted C.sub.6-C.sub.10-aryl, NR.sup.8dR.sup.9d, SR.sup.10d, NO.sub.2, or is a functional group or a heteroatom chosen from the group —O—, —S—, —N(R.sup.11)—, —S(O)—, —C(O)—, —S(O).sub.2—, —P(R.sup.11)—, —(R.sup.11)P(O)— and —Si(R.sup.12R.sup.13), where the radicals R.sup.5a, R.sup.6a, R.sup.7a, R.sup.8a, R.sup.9a, R.sup.10a to R.sup.5d, R.sup.6d, R.sup.7d, R.sup.8d, R.sup.9d, R.sup.10d, R.sup.11, R.sup.12 and R.sup.13 are in each case independently of one another chosen from C.sub.1-C.sub.6-alkyl, C.sub.7-C.sub.12-aralkyl and/or substituted or unsubstituted C.sub.6-C.sub.10-aryl and where the radicals R.sup.8a and R.sup.9a, R.sup.8b and R.sup.9b, R.sup.8c and R.sup.9c, R.sup.8d and R.sup.9d, independently of one another, can in each case together also form a cyclic hydrocarbon radical having 2 to 8 carbon atoms which can have one or more identical or different heteroatoms chosen from the group O, S, NR.sup.11a, and R.sup.11a can have the meanings given for R.sup.11, in free and/or complex-bound form.

33. The method according to claim 32, wherein the solution or some of the solution is brought in a crystallization vessel (1) to a temperature which is less than 95° C.

34. A method for obtaining a substance from a solution by means of crystallization, in which the solution is introduced into a first crystallization vessel (1) and the substance is separated off by means of crystallization in the first crystallization vessel (1) by the method according to claim 22 and in which, while carrying out the crystallization method in the first crystallization vessel (1), the solution is introduced into a second crystallization vessel (1′) and the substance is separated off by means of crystallization in the second crystallization vessel (1′) by the process according to claim 22.

35. A method for working up an aluminum-containing reaction product from the production of isopulegol by cyclizing citronellal, comprising i) isopulegol, ii) at least one ligand of the formula (I), ##STR00031## where Ar.sup.1, Ar.sup.2, Ar.sup.3, Ar.sup.4, independently of one another, are chosen from C.sub.6-C.sub.15-aryl radicals or C.sub.2-C.sub.15-heteroaryl radicals, which, if appropriate, can in each case carry 1 to 7 identical or different substituents chosen from C.sub.1-C.sub.6-alkyl, C.sub.1-C.sub.6-perfluoroalkyl, C.sub.1-C.sub.6-alkoxy, C.sub.7-C.sub.12-aralkyl, halogen, SiR.sup.5aR.sup.6aR.sup.7a, optionally substituted C.sub.6-C.sub.10-aryl, NR.sup.8aR.sup.9a, SR.sup.10a, NO.sub.2, R.sup.1, R.sup.2, R.sup.3, R.sup.4, independently of one another, are chosen from hydrogen, C.sub.1-C.sub.6-alkyl, C.sub.1-C.sub.6-perfluoroalkyl, C.sub.1-C.sub.6-alkoxy, C.sub.7-C.sub.12-aralkyl, halogen, SiR.sup.5bR.sup.6bR.sup.7b, optionally substituted C.sub.6-C.sub.10-aryl, NR.sup.8bR.sup.9b, SR.sup.10b, NO.sub.2 and where R.sup.1 or R.sup.2 and/or R.sup.3 or R.sup.4, together with A, can form an aromatic or nonaromatic cycle, and A is a straight-chain or branched and/or cyclic hydrocarbon radical having 1 to 25 carbon atoms which may be saturated or mono- or polyunsaturated and/or partially aromatic and can, if appropriate, have one or more identical or different heteroatoms chosen from O, S, NR.sup.11, and/or one or more identical or different functional groups chosen from the functional groups C(O), S(O), S(O).sub.2 and can, if appropriate, carry one or more identical or different substituents chosen from the substituents C.sub.1-C.sub.6-alkyl, C.sub.1-C.sub.6-perfluoroalkyl, C.sub.1-C.sub.6-alkoxy, C.sub.1-C.sub.10-acyloxy, C.sub.7-C.sub.12-aralkyl, halogen, —SiR.sup.5cR.sup.6cR.sup.7c, optionally substituted C.sub.6-C.sub.10-aryl, substituted or unsubstituted C.sub.2-C.sub.10-hetaryl, NR.sup.8cR.sup.9c, SR.sup.10c, NO.sub.2, C.sub.1-C.sub.12-acyl, C.sub.1-C.sub.10-carboxyl, or is a C.sub.6-C.sub.15-aryl radical or a C.sub.2-C.sub.15-heteroaryl radical which can, if appropriate, in each case carry 1 to 5 substituents chosen from C.sub.1-C.sub.6-alkyl, C.sub.1-C.sub.6-perfluoroalkyl, C.sub.1-C.sub.6-alkoxy, C.sub.7-C.sub.12-aralkyl, halogen, SiR.sup.5dR.sup.6dR.sup.7d, substituted or unsubstituted C.sub.6-C.sub.10-aryl, NR.sup.8dR.sup.9d, SR.sup.10d, NO.sub.2, or is a functional group or a heteroatom chosen from the group —O—, —S—, —N(R.sup.11)—, —S(O)—, —C(O)—, —S(O).sub.2—, —P(R.sup.11)—, —(R.sup.11)P(O)— and —Si(R.sup.12R.sup.13), where the radicals R.sup.5a, R.sup.6a, R.sup.7a, R.sup.8a, R.sup.9a, R.sup.10a to R.sup.5d, R.sup.6d, R.sup.7d, R.sup.8d, R.sup.9d, R.sup.10d, R.sup.11, R.sup.12 and R.sup.13 are in each case independently of one another chosen from C.sub.1-C.sub.6-alkyl, C.sub.7-C.sub.12-aralkyl and/or substituted or unsubstituted C.sub.6-C.sub.10-aryl and where the radicals R.sup.8a and R.sup.9a, R.sup.8b and R.sup.9b, R.sup.8e and R.sup.9c, R.sup.8d and R.sup.9d, independently of one another, can in each case together also form a cyclic hydrocarbon radical having 2 to 8 carbon atoms which can have one or more identical or different heteroatoms chosen from the group O, S, NR.sup.11a, and R.sup.11a can have the meanings given for R.sup.11, in free and/or complex-bound form, in which a) the reaction product is subjected to distillative separation to obtain an isopulegol-enriched top product and an isopulegol-depleted bottom product, b) the isopulegol-depleted bottom product is brought into close contact with an aqueous base to give an aluminum-containing aqueous phase and an organic phase comprising the majority of the ligands of the formula (I), c) the ligand of the formula (I) is separated off from the organic phase according to the method according to claim 22.

36. A method for producing isopulegol of the formula (IV) comprising ##STR00032## α) the cyclization of citronellal of the formula (V) ##STR00033## in the presence of a catalyst which is obtainable by reacting a bis(diarylphenol) ligand of the formula (I) as defined in claim 31, with an aluminum compound of the formula (II),
(R.sup.14).sub.3-pAlH.sub.p  (II) where Al is aluminum, R.sup.14 is a branched or unbranched alkyl radical having 1 to 5 carbon atoms and p is 0 or an integer from 1 to 3, and/or with an aluminum compound of the formula (III),
MAlH.sub.4  (III) where Al is aluminum and M is lithium, sodium or potassium, β) the recovery of the bis(diarylphenol) ligand of the formula (I) after the reaction has taken place by a) subjecting the reaction product obtained in step a) to distillative separation to obtain an isopulegol-enriched top product and an isopulegol-depleted bottom product, b) bringing the isopulegol-depleted bottom product into close contact with an aqueous base to give an aluminum-containing aqueous phase and an organic phase comprising the majority of the ligands of the formula (I) and c) separating off the ligand of the formula (I) from the organic phase according to the method according to claim 22.

37. A method for producing menthol comprising the steps: A) production of isopulegol of the formula (IV) according to claim 36 and B) hydrogenation of the ethylenic double bond of the isopulegol obtained in this way.

38. A device for separating off a substance by crystallization from a solution of the substance with a crystallization vessel (1) which comprises an opening for introducing the solution, a heating device (5, 7) for changing the temperature of the solution to be introduced and/or introduced, a temperature sensor (6, 8) for measuring the temperature of the solution to be introduced and/or introduced, a scattered-light probe (9) which is arranged within the crystallization vessel (1) and with which electromagnetic radiation can be radiated into the solution, the aperture angle of which is greater than 5 degrees, and for setting a desired quantity of seed crystals an intensity of the electromagnetic radiation which has been scattered by crystals located in the solution can be detected, a regulating unit (10), which is data-coupled with the temperature sensor (6, 8), the scattered-light probe (9) and the heating device (5, 7), with which the temperature of the solution in the crystallization vessel (1) can be regulated such that the amount of the difference between the detected intensity and a desired intensity (I.sub.S) is reduced and, if the amount in the difference between the detected intensity and the desired intensity (I.sub.S) is less than a limiting value, the desired amount of seed crystals for a crystallization method is present and then the crystallization method can be actuated, via which crystals of the substance are obtained, and a separation unit (11) for separating off the resulting crystals.

39. The device according to claim 38, wherein the electromagnetic radiation irradiated from the scattered-light probe (9) uses one wavelength range or two or more wavelength ranges which is/are wider than 20 nm.

40. The device according to claim 38, wherein the beam that can be produced from the scattered-light probe (9) has a minimum cross section greater than 0.1 mm.

Description

[0231] Embodiments of the invention are illustrated below with reference to the drawings.

[0232] FIG. 1 shows the design of a first embodiment of the device according to the invention for separating off a substance from a solution,

[0233] FIG. 2 shows the design of the scattered-light probe which is used in the embodiment shown in FIG. 1,

[0234] FIG. 3 shows a diagram in which an example of the temperature course and of the detected intensity is plotted for an embodiment of the method according to the invention,

[0235] FIG. 4 shows a diagram which illustrates the relationship between the detected intensity and the temperature for an embodiment of the method according to the invention, and

[0236] FIG. 5 shows a further embodiment of the device according to the invention.

[0237] With reference to FIG. 1, the design of the first embodiment of the device according to the invention for separating off a substance from a solution is explained:

[0238] The device comprises a crystallization vessel 1 which has a feed line 2 and a discharge line 3. The solution is introduced into the crystallization vessel 1 via the feed line 2. So that the introduced solution remains firstly in the crystallization vessel 1, an electronically controllable valve 4 is provided in the discharge line 3 which is initially closed. After the crystallization method has been carried out in the crystallization vessel 1, the suspension with the crystals is discharged from the crystallization vessel 1 via the discharge line 3.

[0239] A heating device 5 is provided in the feed line 2 or alternatively in a storage vessel. By means of this heating device 5 it is possible to regulate the temperature of the solution which is introduced into the crystallization vessel 1 via the feed line 2. For the temperature regulation, a temperature sensor 6 is furthermore provided in the feed line 2. Furthermore, a heating device 7 and a temperature sensor 8 are also provided in the crystallization vessel 1, by means of which the temperature of the solution that is located in the crystallization vessel 1 is measured and regulated.

[0240] Finally, a scattered-light probe 9, which is explained in detail later, is located within the crystallization vessel 1. The valve 4, the heating devices 5 and 7, the temperature sensors 6 and 8, as well as the scattered-light probe 9 are data-coupled with a regulating unit 10. In this way, the measurement values of the temperature sensors 6 and 8 and the measurement values of the scattered-light probe 9 are conveyed to the regulating unit 10. Furthermore, the regulating unit 10 controls the light emission of the scattered-light probe 9, as is explained later, and the heating or cooling output of the heating devices 5 and 7. Furthermore, the valve 4 can be opened and closed by means of the regulating unit 10.

[0241] The discharge line 3 through which the suspension is removed from the crystallization vessel 1 is connected to a separation unit 11. The separation unit 11 can be configured as a filter device known per se.

[0242] With reference to FIG. 2, the scattered-light probe 9 is described in detail below:

[0243] The scattered-light probe 9 comprises a tube 12 in which the waveguides L1 and L3 are located. At the end of the tube 12, which dips into the crystallization vessel 1, the waveguide L1 has a decoupling area and the waveguide L3 has a coupling area.

[0244] In the scattered-light probe 9, a radiation source 14 or an emitter for electromagnetic radiation is provided. The electromagnetic radiation emitted by the radiation source 14 is coupled via a coupling area in the waveguide L1, via which the electromagnetic radiation is conveyed to the decoupling area of the waveguide L1. The electromagnetic radiation generated by the radiation source 14 is thus radiated as radiation S1 into the solution located in the crystallization vessel 1.

[0245] The beam generated by the radiation S1 has, upon entering into the solution or suspension, a cross section greater than 0.39 mm. Furthermore, the beam with an angle of about +/−12° is divergent, i.e. the aperture angle of the beam is 24°.

[0246] The waveguides L1 and L3 are passed through the opening 13 of the scattered-light probe 9 in a parallel and liquid-tight manner. They are in particular configured such that the direction of the radiation S1 radiated into the solution or suspension is parallel to the detection direction for the radiation S2 which has been scattered at the crystals and which is coupled into the waveguide L3. The tube 12 of the scattered-light probe 9 is dipped into the crystallization vessel 1 such that, in the event of a clear solution for which no crystals are present in the solution, no radiation arrives in the waveguide L3 which has a wavelength in which the radiation source 14 emits radiation if radiation is emitted into the clear solution via the waveguide L1.

[0247] The beam of the incident radiation S2 which enters the waveguide L3 is also divergent with the same aperture angle, meaning that the emitting and receiving range of the scattered-light probe 9 is spatially overlapping. This gives rise to two adjoining cones which intersect spatially. This gives rise to a very large measurement volume, which is important particularly in the case of very low particle concentrations.

[0248] The scattered-light probe 9 has no disk as termination between the waveguides L1 and L3 on the one hand and the solution or suspension on the other hand. The optical offset of the scattered-light probe 9 therefore approaches zero.

[0249] In the embodiment described here, the radiation source 14 generates infrared radiation in a wavelength range from 800 nm to 900 nm. The electromagnetic radiation radiated into the solution is scattered onto the surfaces of crystals which are located in the solution. Some of the electromagnetic radiation S2 back-scattered at the crystals is conveyed via the coupling area of the waveguide L3 from this to a detector 15. The detector 15 is configured such that it can measure the intensity of the electromagnetic radiation in the wavelength range in which the radiation source 14 emits electromagnetic radiation.

[0250] The detector 15 has a receiving electronic unit which permits a very wide intensification range from 2 mW/V to 20 picoW/V. This means that the receiving electronic unit produces a voltage of 1 V for an incident light intensity of 20 picoW, i.e. at a light intensity of about 150 μLux/cm.sup.2. Consequently, the detector 15 is extremely sensitive.

[0251] A diversion is also provided in the waveguide L1. Some of the radiation generated by the radiation source 14 and coupled in the waveguide L1 is diverted to a waveguide L2 and passed to the detector 15. The radiation diverted via the waveguide L2 and passed to the detector 15 serves as reference radiation.

[0252] In the detector 15, a voltage level is generated which is directly related to the light intensity back-scattered by the crystals in the solution. The reference voltage level generated by the detector 15, which is brought about by the reference radiation, takes into consideration here the intensity of the radiation S1 radiated into the solution. The voltage level of the detector 15 is corrected in an evaluation unit, taking into consideration the reference voltage level of the detector 15, and transferred to the regulating unit 10.

[0253] The scattered-light probe 9 used can for example be a variation of the photometric measuring device, as described in EP 0 472 899 A1. The scattered-light probe described in this specification can be used both for a transmission measurement and for a back scatter measurement. In the present case, the back scatter measurements would be taken into consideration.

[0254] In a further variant, the scattered-light probe 9 comprises a rod probe which is dipped into the crystallization vessel 1. The detector 15 is then connected to the rod probe via waveguides and arranged outside of the crystallization vessel 1.

[0255] The measurement can furthermore also be carried out with a detector 15 without referencing the radiation source 14. However, for the long-term stability of the measurement, the referencing with a second detector is advantageous. The correction of the scattered signal that is detected by the detector 15 then takes place by reference to the reference signal that is detected by a further detector in an evaluation unit which then generates a corrected scattered signal and conveys it to the regulating unit 10.

[0256] Either one or more waveguides can serve as emitters and receivers in the rod probe. The fiber geometry does not necessarily have to be realized with parallel emitting and receiving fibers, although this is preferred. Furthermore, a solution with disk before the fiber ends with deviating geometry could also be used, although then, on account of the internal reflections, this leads to a considerably higher signal offset and consequently to a significantly lower sensitivity of the measuring system especially in the case of very low particle concentrations.

[0257] The scattered-light probe 9 does not detect the complete scattered radiation. On account of multiple scattering, transmission and absorption and on account of the spatially limited receiving cone (aperture), the scattered-light probe 9 detects only a fraction of the scattered radiation proportional to the particle surface.

[0258] Further details of the device according to the invention as well as an embodiment of the method according to the invention are explained in the detail below:

[0259] In the described embodiment, the aim is to separate off the ligand Ia.sub.2-3 described at the start, which is dissolved in phenylcyclohexane. The solution was obtained as bottom product from the cyclization of citronellal in the presence of a (bis(diarylphenoxy))aluminum catalyst. This solution gives rise to the problem that the complex chemical method carried out beforehand leads to the precise composition of the solution and the establishing concentrations of dissolving and nondissolving secondary components not being known exactly and therefore the saturation temperature at which the ligand crystallizes can fluctuate greatly.

[0260] According to the invention, reference measurements are therefore carried out beforehand. The reference measurements can advantageously also be carried out in the laboratory. Here, the solution is introduced into the crystallization vessel 1 at a temperature which is a few 10 K below the expected saturation temperature. For example, the solution is introduced at a temperature of 80° C. This temperature is adjusted by means of the regulating unit 10, the heating devices 5 and 7 and the temperature sensors 6 and 8. At this temperature, a very large amount of crystals of the ligand is in the solution. However, the crystal size and morphology of the crystals is unsuitable for subsequent filtration in the separation unit 11. The temperature of the solution which is introduced into the crystallization vessel 1 is now raised by means of the heating device 5. At the same time, by means of the scattered-light probe 9 electromagnetic radiation is radiated into the solution. The temperature of the solution is then continuously determined by the regulating unit 10 by means of the temperature sensor 8. In addition, the intensity of the back-scattered electromagnetic radiation is ascertained by reference to the voltage level conveyed by the evaluation unit. During the increase in temperature, the signal for the intensity of the back-scattered electromagnetic radiation decreases since the crystals dissolve and the crystal surface available for the back-scattering is thus reduced.

[0261] As a result of the reference measurements, the intensity of the detected electromagnetic radiation is determined at which an amount of seed crystals of the ligand or a crystal surface of this seed crystal is present which is ideal for a subsequently carried out crystallization method in which the solution is cooled again and crystals are supposed to form which have a crystal size and morphology ideal for the subsequent separation. In the case of the reference measurements, the temperature of the solution is therefore increased until the intensity of the back-scattered electromagnetic radiation has dropped to a certain value. Consequently, a cooling crystallization method is started in a manner known per se in which, with a certain cooling curve, the solution is cooled again such that crystals of the ligand are formed. The crystals are then filtered out in the separation unit 11, and the size and morphology of these crystals is investigated.

[0262] The reference measurements are now carried out for a large number of intensities, for which the subsequent crystallization method is always carried out in the same way. The reference measurement is then determined at which the crystal size and morphology ideal for the separation have been generated. The intensity of the back-scattered electromagnetic radiation at the start of the crystallization method of this reference measurement, i.e. the minimum intensity of the back-scattered electromagnetic radiation at this reference measurement, is defined as desired intensity I.sub.S. At this desired intensity Is, the size of the crystal area which is formed by the seed crystals of the ligand is ideal for the subsequently carried out crystallization method.

[0263] Furthermore, a starting temperature value T.sub.A is stipulated beforehand at which the solution is introduced into the crystallization vessel 1 at the start of the method. This starting temperature value T.sub.A is clearly below the temperature value T.sub.K which corresponds to the desired intensity Is, i.e. the starting temperature for the crystallization method. In the present example, the starting temperature value T.sub.A is about 90° C. This starting temperature value T.sub.A can moreover also be determined from the desired intensity I.sub.S by selecting the starting intensity I.sub.A assigned to the starting temperature value T.sub.A for the back-scattered electromagnetic radiation as the x-fold intensity of the desired intensity I.sub.S. The value x here can be in a range from 1.2 to 10. In the present case, the value x is 6.5.

[0264] The method for separating off the ligand from the solution introduced via the feed line 2 is then carried out as follows following the determination of the desired intensity and the starting temperature value:

[0265] The solution is introduced via the feed line 2 with the starting temperature value T.sub.A. As soon as enough solution has been introduced into the crystallization vessel 1 that the scattered-light probe 9 is located within the solution, the intensity I of the electromagnetic radiation back-scattered at the crystals is determined by the regulating unit 10.

[0266] In FIG. 3, the course over time of the signal I of the scattered-light probe 9, which correlates with the intensity I of the back-scattered electromagnetic radiation, as well as the associated course over time of the temperature T of the solution is shown. The starting temperature value T.sub.A in this case is 89.13° C. The associated signal I.sub.A of the scattered-light probe 9 is 0.85 V. Conceptually, no distinction is made hereinbelow between the signal I of the scattered-light probe 9 and the intensity I of the back-scattered electromagnetic radiation since these are directly related.

[0267] The temperature of the solution introduced into the crystallization vessel 1 via the feed line 2 is now increased by means of the regulating unit 10. As is evident from FIG. 3, the temperature of the solution within the crystallization vessel 1 thus also increases. At the same time, the signal I of the scattered-light probe 9 drops since the crystals of the ligand dissolve. The temperature of the introduced solution is raised until the signal I of the scattered-light probe 9 is within a tolerance range around the desired intensity I.sub.S. In other words, this means that the amount of difference between the detected intensity I and the desired intensity I.sub.S is less than a limiting value. This limiting value can for example be 10% of the desired intensity I.sub.S.

[0268] In the ideal case, in the event of the complete filling of the crystallization vessel 1, the amount of difference between the detected intensity I and the desired intensity I.sub.S is less than this limiting value. If this is not the case, the temperature of the solution located within the crystallization vessel 1 is also finely adjusted via the heating device 7 and the regulating unit 10 until the amount of this difference is below this limiting value.

[0269] In this state of the solution located in the crystallization vessel 1, the ideal amount of seed crystals of the ligand determined in the reference measurements is present with the ideal crystal surface. The actual cooling crystallization method is now started. Regulated by the regulating unit 10, the solution is cooled firstly at a low cooling rate of about 3 K/h. After a certain time, i.e. when a certain amount of crystals of a certain size is present, the cooling rate can for example be increased to about 20 K/h. In this way, crystals of the ligand which have a crystal size and morphology ideal for the subsequent separation are formed within the shortest possible time. The suspension with the crystals is then supplied, by opening the valve 4 and via the discharge line 3, to the separation unit 11, in which the suspension is filtered and the ligand of the formula Ia.sub.2-3 can be obtained as a white solid.

[0270] FIG. 4 shows the relationship between the detected intensity I of the electromagnetic radiation scattered at the crystals and the temperature T, specifically for the measurement values shown in FIG. 3. The measurement values for arrow A show the dissolution of the crystals at the start of the method, i.e: before the actual crystallization method, and the measurement values along arrow K show the crystallization during the crystallization method which was started at the intensity I.sub.S and temperature T.sub.K.

[0271] A clear difference arises in the case of the curve for the dissolution, i.e. while raising the temperature to the desired intensity Is, and the curve for the subsequent crystallization, i.e. while lowering the temperature starting with the temperature value T.sub.K. At the same temperature, the intensity measured by the scattered-light probe 9 during the dissolution is substantially higher than during the crystallization. During the dissolution, the crystals of the ligand thus have a greater specific surface area. This means that they are very finely divided. These are thus small crystals. This is undesired for the subsequent separation of the crystals. During the subsequent crystallization, the intensity of the signal measured by the scattered-light probe 9, by contrast, is smaller by a factor of 2 to 3. At a certain temperature, however, the same mass of crystals is in solution. The lower intensity of the back-scattered radiation therefore indicates that the specific surface area of the crystals is smaller. It is evident from this that the crystals are larger, as is desired for the subsequent separation of the crystals.

[0272] A second embodiment of the device according to the invention and of the method according to the invention is explained below with reference to FIG. 5:

[0273] The device of the second embodiment comprises the device of the first embodiment shown in FIG. 1. The same parts are therefore labeled with the same reference numerals. Accordingly, reference is made to the above description of these parts. The device of the second embodiment shown in FIG. 5, however, has a further crystallization vessel 1′. Like the first crystallization vessel 1, the second crystallization vessel 1′ comprises a feed line 2′, a discharge line 3′ with a valve 4′. Provided in the feed line 2′ are a heating device 5′ and a temperature sensor 6′ for regulating the temperature of the solution which is introduced into the second crystallization vessel 1′. Provided for the second crystallization vessel 1′ are a heating device 7′ and a temperature sensor 8′, and a further scattered-light probe 9′. The valve 4′, the heating devices 5′ and 7′, the temperature sensors 6′ and 8′, and the scattered-light probe 9′ are data-coupled with the regulating unit 10.

[0274] Furthermore, an electronically controlled valve 16 is arranged in the feed line 2 for the first crystallization vessel 1; similarly an electronically controllable valve 17 is arranged in the feed line 2′ for the second crystallization vessel 1′. Valves 16 and 17 are also data-coupled with the regulating unit 10.

[0275] According to a second embodiment of the method according to the invention, the device shown in FIG. 5 is operated as follows:

[0276] As explained with reference to FIGS. 1 to 3, the solution is introduced into the first crystallization vessel 1 via the feed line 2. In this case, the valve 16 is opened and the valve 17 is closed, meaning that no solution passes into the second crystallization vessel 1′. Upon introducing the solution, the temperature is regulated as explained above such that the temperature of the solution in the first crystallization vessel 1, if this is filled completely, corresponds to the temperature value T.sub.K, which is assigned to the desired intensity I.sub.S at which the desired amount of seed crystals is present.

[0277] The valve 16 is then closed and, in the first crystallization vessel 1, the cooling crystallization starts, in which the temperature of the solution in the first crystallization vessel 1 is reduced. At the same time, by means of the heating device 5 and the temperature sensor 6, the temperature of the solution to be introduced is again brought to the starting temperature value T.sub.A. The valve 17 is then opened so that the solution is conveyed to the second crystallization vessel 1′. By means of the heating device 5′ and the temperature sensor 6′, the temperature of the solution conveyed to the second crystallization vessel 1′ is then regulated such that the intensity of the back-scattered radiation measured by the scattered-light probe 9′ is close to the desired intensity Is, as has already been described above for the first crystallization vessel 1. As soon as the amount of difference between the detected intensity and the desired intensity I.sub.S is less than the limiting value, the valve 17 is closed and, in the second crystallization vessel 1′, the cooling crystallization method is carried out as described above, in which the temperature of the solution is reduced so that crystals of the ligand are formed. While the crystallization process in the second crystallization vessel 1′ is carried out, the crystallization process in the first crystallization vessel 1 is concluded and the valve 4 is opened so that the suspension is fed to the separation unit 11 via the discharge line 3. The crystals of the ligand are isolated in the separation unit 11. During this, the valve 4 can be closed again, and the solution is again passed to the first crystallization vessel 1.

[0278] If the crystallization process is concluded in the second crystallization vessel 1′, the crystals that have been supplied to the separation unit 11 via the discharge line 3 have already been isolated from the suspension of the first crystallization vessel 1. The valve 4′ can now be opened such that the suspension with the crystals of the ligand can be supplied, from the second crystallization vessel 1′ and via the discharge line 3′, to the separation unit 11. There, the crystals of the ligand are then filtered out.

[0279] In this way, the device shown in FIG. 5 can be used to carry out the method described above with reference to FIG. 1 alternately in the two crystallization vessels 1 and 1′.

[0280] An embodiment of the method for working up an aluminum-containing reaction product from the preparation of isopulegol by cyclizing citronellal is described below:

[0281] The aluminum-containing reaction product is worked up, as described in WO 2008/025852 A1. In the last process step, the ligand of the formula Ia.sub.2-3 is obtained, as has been described above with reference to FIGS. 1 to 5.

[0282] A further embodiment of the invention relates to a method for producing isopulegol. In this embodiment, isopulegol is prepared as described in WO 2008/025852 A1. In contrast to the method described in this specification, however, the ligand is separated off from the organic phase according to an embodiment as has been described above with reference to FIGS. 1 to 5.

[0283] A yet further embodiment relates to a method for producing menthol. In this case, isopulegol is prepared as described above. Menthol is then prepared by hydrogenation of the ethylenic double bond of the isopulegol obtained in this way.

LIST OF REFERENCE NUMERALS

[0284] 1, 1′ Crystallization vessel [0285] 2, 2′ Feed line [0286] 3, 3′ Discharge line [0287] 4, 4′ Valve [0288] 5, 5′ Heating device [0289] 6, 6′ Temperature sensor [0290] 7, 7′ Heating device [0291] 8, 8′ Temperature sensor [0292] 9, 9′ Scattered-light probe [0293] 10 Regulating unit [0294] 11 Separation unit [0295] 12 Tube [0296] 14 Radiation source for electromagnetic radiation [0297] 15 Detector [0298] 16 Valve [0299] 17 Valve