METHOD FOR OBTAINING PURE CYCLOHEXYL(METH)ACRYLATE BY DISTILLATION

Abstract

Process for isolating pure cyclohexyl (meth)acrylate from a mixture comprising cyclohexyl (meth)acrylate, cyclohexanol and entrainer, by feeding or introducing the mixture is fed as side feed stream into a rectification column having a bottom vaporizer, separation-active internals and a stripping section and an enrichment section, where the column has from 4 to 50 theoretical plates, the side feed stream is introduced in the region commencing at least one theoretical plate above the bottommost theoretical plate and ending at least one theoretical plate below the uppermost theoretical plate, the pressure at the top of the column is in the range from 10 mbar to 5 bar and the reflux ratio is from 1:0.2 to 1:10, and the bottom output is fed to a further distillation unit in which the cyclohexyl (meth)acrylate is separated off at the top or at a side offtake.

Claims

1. A process for isolating pure cyclohexyl (meth)acrylate from a mixture comprising cyclohexyl (meth)acrylate, cyclohexanol and entrainer by distillation, wherein the mixture is fed as side feed stream into a rectification column having a bottom vaporizer, separation-active internals and a stripping section and an enrichment section, where the column has from 4 to 50 theoretical plates, the side feed stream is introduced in the region commencing at least one theoretical plate above the bottommost theoretical plate and ending at least one theoretical plate below the uppermost theoretical plate, the pressure at the top of the column is in the range from 10 mbar to 5 bar and the reflux ratio is from 1:0.2 to 1:10, and the bottom output is fed to a further distillation unit in which the cyclohexyl (meth)acrylate is separated off at the top or at a side offtake.

2. The process according to claim 1, wherein the entrainer is cyclohexane.

3. The process according to claim 1, wherein the separation-active internals are random packing elements, ordered packing or trays.

4. The process according to claim 1, wherein the separation-active internals bring about a specific pressure drop in the range from 0.05 to 10 mbar per theoretical plate.

5. The process according to claim 1, wherein the random packing elements are high-performance random packing elements.

6. The process according to claim 1, wherein the residence time in the bottom vaporizer and the associated piping system is in the range from 1 to 60 minutes.

7. The process according to claim 1, wherein the residence time in the bottom vaporizer and the associated piping system is in the range from 10 to 30 minutes.

8. The process according to claim 1, wherein the bottom vaporizer is a thin film evaporator or falling film evaporator.

9. The process according to claim 1, wherein the vaporization in the bottom vaporizer is effected at a specific heat input in the range from 1 to 100 kW/m.sup.2.

10. The process according to claim 1, wherein the vaporization in the bottom vaporizer is effected at a specific heat input in the range from 2 to 50 kW/m.sup.2.

11. The process according to claim 1, wherein the vaporization in the bottom vaporizer is effected at a specific heat input in the range from 5 to 30 kW/m.sup.2.

12. The process according to claim 1, wherein the column has from 5 to 20 theoretical plates and the side feed stream is introduced in the region commencing at least one theoretical plate above the bottommost theoretical plate and ending at least one theoretical plate below the uppermost theoretical plate.

13. The process according to claim 1, wherein the pressure at the top of the column is in the range from 10 mbar to 200 mbar.

14. The process according to claim 1, wherein the reflux ratio is from 1:0.2 to 1:1.

15. The process according to claim 1, wherein the mixture fed in at the side of the rectification column has the following composition: from 50 to 98% by weight of cyclohexyl (meth)acrylate, from 0.5 to 20% by weight of cyclohexanol, from 0.5 to 20% by weight of entrainer, from 0.25 to 5% by weight of relatively high boilers [relative to cyclohexyl (meth)acrylate], from 0.25 to 5% by weight of further relatively low boilers [relative to cyclohexyl (meth)acrylate].

16. The process according to claim 1, wherein the process is for isolating pure cyclohexyl acrylate having a purity of >98% by weight from a mixture comprising cyclohexyl acrylate, cyclohexanol and entrainer.

17. The process according to claim 1, wherein the process is for isolating pure cyclohexyl methacrylate having a purity of >98% by weight from a mixture comprising cyclohexyl methacrylate, cyclohexanol and entrainer.

Description

[0049] The invention will be illustrated in more detail below with the aid of a drawing (FIG. 1) and examples.

[0050] FIG. 1 shows the schematic depiction of a preferred rectification column for carrying out the process of the invention. The feed stream is conveyed via the feed conduit 1 into the column 2. The column is filled with random packing elements or ordered packing (11). The vapor stream 3 obtained at the top of the column is partially condensed in the condenser 4, which is optionally supplemented by an after-condenser, and divided into the reflux stream 12 and the distillate stream 6. The uncondensed fraction from the condenser 4 contains the low-boiling impurities and is taken off in vapor form as stream 5. At the lower end of the column, the liquid 10 is partially vaporized in a bottom vaporizer 8 and recirculated via the pipe 7 into the column. A main stream 9 which comprises the high-boiling impurities is taken off. The vaporizer 8 can be configured as a natural convection vaporizer or as a force circulation vaporizer; in the latter case, a circulation pump for the liquid stream 10 is additionally required. It is particularly advantageous, in terms of avoiding undesirable polymerization reactions, to use a falling film evaporator or thin film evaporator instead of the force circulation vaporizer since very short residence times are possible using evaporators of this type.

[0051] To reduce the residence time of the liquid in the vaporizer, it is advantageous to configure the bottom space including the bottom conduit in such a way that a very small liquid volume is present.

[0052] All pressures indicated are absolute pressures.

[0053] A ‘relatively low boiler’ [relative to cyclohexyl (meth)acrylate] is a material whose boiling point is lower than the boiling point of cyclohexyl acrylate or cyclohexyl methacrylate at the same pressure.

[0054] A ‘relatively high boiler’ [relative to cyclohexyl (meth)acrylate] is a material whose boiling point is higher than the boiling point of cyclohexyl (meth)acrylate or cyclohexyl methacrylate at the same pressure.

EXAMPLES

1) Comparative Example

[0055] A crude cyclohexyl methacrylate was introduced at 883 kg/h and with a temperature of 97° C. into the column at the upper end of the random packing element bed made up of 25 mm Pall rings.

[0056] The composition of the crude cyclohexyl methacrylate was determined by means of analyses:

TABLE-US-00001 cyclohexyl methacrylate 95.2% by weight cyclohexanol  2.8% by weight cyclohexane  1.0% by weight
further relatively low boilers, relatively high boilers: 1.0% by weight

[0057] The column was operated at a pressure at the top of 16 mbar and a pressure at the bottom of 37 mbar. At the top of the column, the distillate was condensed and collected in a container. This was continuously drained by means of level regulation, so that a continual reflux stream of distillate of 134 kg/h flows into the first esterification stage.

[0058] The composition of the distillate was determined by means of analyses and was:

TABLE-US-00002 cyclohexyl methacrylate 82% by weight cyclohexanol 11% by weight cyclohexane  5% by weight further relatively low boilers  2% by weight

[0059] The bottoms from the column run through a falling film evaporator which is operated in countercurrent. The falling film evaporator was heated with 93 kW. The bottom product flowing out from the falling film evaporator at 746 kg/h had a temperature of 107° C.

[0060] The composition was determined by means of analyses:

TABLE-US-00003 cyclohexyl methacrylate  98% by weight cyclohexanol 1.3% by weight relatively high boilers 0.7% by weight

[0061] The commercial specifications for cyclohexyl methacrylate were adhered to with 98% by weight of cyclohexyl methacrylate. The distillation yield of cyclohexyl methacrylate from this column was more than 86%.

2) Experiments on the Thermal Stability of a Feed Product (Side Feed of the Rectification Column)

[0062] Experiments on the thermal stability of a feed product, here a mixture of

94.2% by weight of cyclohexyl methacrylate,
2.5% by weight of cyclohexanol, 1.0% by weight of cyclohexene,
1.4% by weight of relatively high boilers and 0.9% by weight of further relatively low boilers,
at atmospheric pressure, three different temperatures (120, 130, 140° C.) and a residence time of one hour have shown that the formation of dimers and oligomers which are responsible for impairing the product quality of the target ester increases with increasing temperature.

[0063] In the following, the increase in the cyclohexyl methacrylate dimer of the formula

##STR00001##

is shown versus a temperature increase (gas-chromatographic analysis);

TABLE-US-00004 Starting material: 0.52 percent by area 120° C. 0.56 percent by area 130° C. 0.64 percent by area 140° C. 0.69 percent by area

3) Example According to the Invention

[0064] The mode of operation is shown with the aid of data from a thermodynamic simulation of an overall plant for preparing cyclohexyl methacrylate.

[0065] The thermodynamic simulation of the process was carried out using the software Aspen Plus® (Aspen for short). Aspen is comprehensive simulation software which is used for modeling, simulation and optimization of chemical processes and plants in industry. Aspen has comprehensive modeling data banks for modeling the basic operations and also materials data banks for the materials properties of many different substances. The properties of mixtures are calculated by Aspen from the materials data of the pure substances by means of various thermodynamic models.

[0066] The thermodynamic simulation of the overall plant led to the following results:

[0067] A crude cyclohexyl methacrylate having a temperature of 97° C. is introduced at 883 kg/h into a rectification column having 12 theoretical plates at the 5th theoretical plate.

[0068] The composition of the inflowing crude cyclohexyl methacrylate is:

TABLE-US-00005 cyclohexyl methacrylate 94.7% by weight cyclohexanol  2.7% by weight cyclohexane   1% by weight further relatively low boilers  0.7% by weight relatively high boilers  0.9% by weight

[0069] The column is operated at a pressure at the top of 16 mbar and a pressure at the bottom of 37 mbar. The specific pressure drop in the column is 1.75 mbar/theoretical plate. The temperature at the top is 61° C. At the top of the column, the distillate is condensed. The reflux stream of distillate in the first esterification stage amounts to 40 kg/h. The column is operated at a reflux ratio of 3.1.

[0070] The composition of the distillate is:

TABLE-US-00006 cyclohexyl methacrylate 10% by weight cyclohexanol 59% by weight cyclohexane 15% by weight further relatively low boilers 16% by weight

[0071] The vaporizer is operated at 31 kW. The bottom product flowing out from the vaporizer at 840 kg/h has a temperature of 107° C.

[0072] The composition of the bottom output is:

TABLE-US-00007 cyclohexyl methacrylate  99% by weight cyclohexanol 0.1% by weight relatively high boilers 0.9% by weight

[0073] The distillation of crude cyclohexyl (meth)acrylate can be carried out by means of the process of the invention at the same daily capacity of, for example, 14.0 metric tons while adhering to the required specifications with an energy cost saving of 69% compared to a conventional distillation process.

[0074] The reduction in the recirculation to the esterification from 134 kg/h to 40 kg/h enables the daily capacity of an overall plant for preparing cyclohexyl (meth)acrylate to be increased, for example, by about 100 kg/h, corresponding to a capacity increase of about 14%.

[0075] As a result of the lowering of the temperature at the bottom of the vaporizer from 113° C. to 107° C., the formation of unknown secondary components which are responsible for the fouling in the vaporizer is considerably reduced.

[0076] Since the time of operation of an overall plant for preparing cyclohexyl (meth)acrylate is limited by the time of operation of the vaporizer as a result of fouling, the time of operation of an overall plant can be increased by the reduction in secondary component formation due to the reduced temperature at the bottom in the low boiler distillation. This leads to a higher annual capacity of the overall plant.