In this document, the term “reactor” generally defines one reactor (A) or else two or more mutually interconnected reactors that act as one “reactor” (A). The reactor (A) additionally comprises a reactor heating element in order to heat the reaction mixture. The reactor heating element is, for example, an immersion heater in the reactor (A), a tube system comprising coiled tubes or half-coiled tubes disposed on the outer jacket surface of the reactor and/or within the reactor (A), an electrical heating system disposed on the outer jacket surface of the reactor (A) and/or within the reactor (A), an evaporator outside the reactor (A), where the reaction mixture flows at least partly through the evaporator, or a jacketed design of the outer reactor wall, in which a fluid separated from the reaction mixture, such as a liquid, a gas and/or a heating steam, is under temperature control and hence a predetermined heating temperature is set, whereby the reaction mixture in the reactor (A) is heated. Two or more reactor heating elements may generally be used to heat the reaction mixture in the reactor (A). For instance, a jacketed design of the outer reactor wall and an evaporator outside the reactor (A) may be used to heat the reaction mixture simultaneously or at least partly with a time delay. In addition, the reactor (A) comprises a column (B) on top, which preferably separates off water by distillation. The column (B) itself is a distillation column with internals. Such internals are trays, such as bubble cap trays, perforated trays, especially dual-flow trays, random packings, structured packings or the like. In a further preferred embodiment, the reactor (A) is integrated into the rectification column (E), such that the esterification can take place in the bottom of the rectification column (E), i.e. in the reaction zone (E1).

In this document, the term “mass flow ratio” between a real numerical value A1 and a real numerical value B1 is equivalent to the division ratio with A1 in the numerator and B1 in the denominator. Thus, the following formula is applicable:

Mass
⁢

flow
⁢

ratio

=

A
⁢
1

B
⁢
1

.

The term “external water” in this document is understood to mean a water that originates from outside the process and is introduced into the process by an inlet in a mixer (H). The external water is preferably a demineralized water, more preferably a fully demineralized water. The external water preferably has a pH in the range from 4.5 to 10.5, more preferably in the range from 6.5 to 10.0. Preferably, the external water includes only a low level of electrolyte components, if any, preferably less than 0.1% by weight of electrolyte components.

The acidic esterification catalyst, i.e. acidic catalyst, used is preferably para-toluenesulfonic acid. The content thereof in the reaction zone (E1) or in the reactor (A) based on the reaction mixture present therein is appropriately 0.1% to 10.0% by weight, preferably 0.1% to 6.0% by weight. Other organic sulfonic acids such as methanesulfonic acid, benzenesulfonic acid, dodecylbenzenesulfonic acid and/or sulfuric acid are likewise usable. The amount thereof is equimolar to that of para-toluenesulfonic acid. Corresponding mixtures are also possible. The content of catalytically active acid in the bottom of the rectification column (E) based on the mixture present therein may advantageously be between 2.5% and 50.0% by weight of para-toluenesulfonic acid or an equivalent amount of another organic sulfonic acid and/or sulfuric acid.

In a preferred configuration of the process, the resulting mixture (10) from the mixer (H) is at a temperature in the range from 20 to 100° C., preferably in the range from 70 to 95° C., at the outlet of the mixer (H). This results in the advantage that both the aqueous phase and the organic phase need not be heated later on in the process and hence energy can be saved.

In a preferred configuration of the process, the high boiler bottoms output (23) has a concentration of less than 10% by weight, preferably less than 5% by weight, of water. This results in the advantage that less water has to be distilled off in the rectification column (E) and hence less energy is consumed for heating of the rectification column (E), as a result of which there is also a lower level of secondary components formed.

In a preferred configuration of the process, a substream of the aqueous extract is returned to the reactor (A) or to the reaction zone (E1), where the mass flow ratio between the substream of the aqueous extract and the total mass flow rate of the aqueous extract (12) is in the range from 0.1 to 1.0, preferably in the range from 0.8 to 1.0. This results in the advantage that the catalyst can be returned efficiently without increasing the water content in the plant.

In a preferred configuration of the process, there is added to the high boiler substream (7) of the discharged high boiler bottoms output (23) in the mixer (H) such a mass flow rate of the substream of the aqueous phase (18) from the phase separator (G) and/or of the substream of the aqueous phase (26) from the phase separator (D) that a phase ratio between the aqueous extract (12) to be obtained and the organic raffinate (11) to be obtained in the range from 0.08 to 0.5 kg/kg, preferably in the range from 0.1 to 0.3 kg/kg, is achieved. This results in the advantage that a biphasic separation of the two phases is assured, as small amount as possible of catalyst has to be discharged from the process, and the water content is minimized in the esterification and in the distillation.

A mass flow ratio between the substream of the aqueous phase (18) and the high boiler substream (7) of the discharged high boiler bottoms output (23) or a mass flow ratio between the substream of the aqueous phase (26) and the high boiler substream (7) of the discharged high boiler bottoms output (23) of 0.5 kg/kg or less is sufficient for the successful recovery of the acidic catalyst and lowers the energy requirement of the process, especially for evaporation of water, compared to the use of greater amounts of water. A mass flow ratio between the substream of the aqueous phase (18) and the high boiler substream (7) of the discharged high boiler bottoms output (23) or a mass flow ratio between the substream of the aqueous phase (26) and the high boiler substream (7) of the discharged high boiler bottoms output (23) of at least 0.08 kg/kg serves in turn for effective extraction and phase separation in the extraction phase separator (I).

In a preferred configuration of the process, at least a substream of the organic raffinate (11) is fed to a cleavage reactor (J). In particular, the cleavage reactor (J) is connected downstream of the mixer (H) and the extraction phase separator (I). This results in the advantage that what is fed to the cleavage reactor (J) is the organic components after the phase separation, i.e. the organic raffinate (11), which means that the bottoms mixture in the cleavage reactor (J) which is removed via a conduit 21 is anhydrous and hence less corrosive. In addition, the recycling of by-products formed in the cleavage reactor (G) with the extract into the reactor (A) or into the reaction zone (E1) is avoided by this arrangement. The phase separation in the extraction phase separator (1) is also improved by this arrangement, for reasons including the fact that the viscosity of the continuous phase, i.e. of the organic raffinate (11), is lower and the difference in density between the two phases, i.e. the organic raffinate (11) and the aqueous extract (12), is higher by comparison with an extraction downstream of the cleavage reactor (J).

In a preferred configuration of the process, a substream of the organic raffinate (11) is fed to a cleavage reactor (J), where the mass flow ratio between the substream of the organic raffinate (11) and the total mass flow rate of the organic raffinate is in the range from 0.1 to 1.0, preferably in the range from 0.95 to 1.0. This results in the advantage that the high boilers can be cleaved and the cleavage products such as (meth)acrylic acid and n-butanol can be used again in the process as reactants.

In a preferred configuration of the process, the esterification takes place at a temperature in the range from 90 to 130° C., preferably in the range from 95 to 105° C., and at an absolute pressure in the range from 0.8 to 2.0 bar, preferably in the range from 1.0 to 1.5 bar. This results in the advantage that the conversion is at a maximum and the formation of secondary components is kept to a minimum.

In a preferred configuration of the process, a substream (8) of the high boiler bottoms output (23) is fed to the cleavage reactor (J) in a mass flow ratio to the high boiler substream (7) of the high boiler bottoms output (23) in the range from 0.0 to 10.0, preferably in the range from 0.1 to 1.0. This results in the advantage that there is sufficient catalyst for the cleavage reaction in the cleavage reactor (J).

In a preferred configuration of the process, a substream (9) of the high boiler bottoms output (23) is fed to the reactor (A) in a mass flow ratio to the total high boiler bottoms output (23) in the range from 0.1 to 0.99, preferably in the range from 0.85 to 0.95. This results in the advantage that the catalyst is returned and reused.

In a preferred configuration of the process, the high boiler bottoms output (23) has a water content in the range from 0.1% to 10.0% by weight, further preferably from 0.1% to less than 10.0% by weight, further preferably from 0.1% to less than 5% by weight, especially from 0.1% to 4.5% by weight. This results in the advantage that not too much water is present in the process and hence less energy is required and a lower level of secondary components is formed. At the same time, however, monophasic withdrawal of the high boiler bottoms output (23) and then, after addition of water in the mixer (H), a biphasic separation of the resulting mixture (10) is assured.

In a preferred configuration of the process, the high boiler bottoms output (23) has a content of oligomers and/or polymers in the range from 1% to 80% by weight, further preferably from 10% to 65% by weight, especially from 20% to 60% by weight. Oligomers and/or polymers are understood to mean molecules having a mass-average molar mass of more than 1000 g/mol. A content of oligomers and/or polymers of 80% by weight or less in the high boiler bottoms output (23) results in the advantage of a lower viscosity, such that the subsequent phase separation in the extraction phase separator (I) is improved. The mass flow ratio between the substream of the aqueous phase (18) and the high boiler substream (7) of the discharged high boiler bottoms output (23) or between the substream of the aqueous phase (26) and the high boiler substream (7) of the discharged high boiler bottoms output (23) can be reduced further.

In a preferred configuration of the process, the high boiler bottoms output (23) has a catalyst content in the range from 0.1% to 10.0% by weight. This results in the advantage that not too much catalyst has to be removed, and hence the process can proceed with greater energy efficiency and less catalyst can be used in the process.

In a preferred configuration of the process, the acidic catalyst comprises in the range from 0% to 100% by weight, preferably in the range from 80% to 100% by weight, more preferably in the range from 95% to 100% by weight, of p-toluenesulfonic acid. This results in the advantage that the esterification proceeds very efficiently because this catalyst has high selectivity, high reactivity and a high lifetime in this process.

In a further configuration of the process, the high boiler bottoms output (23) is monophasic. In this case, a substream of the aqueous phase (18) from the phase separator (G) and/or a substream of the aqueous phase (26) from the phase separator (D) is added to the mixer (H) in such an amount that the resulting mixture (10) is biphasic.

In a further configuration of the process, an external water is additionally added to the mixer (H). This is advantageous especially if the mass flow of the aqueous phase (18) and the phase separator (G) and/or the mass flow of the aqueous phase (26) from the phase separator (D) into the mixer (H) should be insufficient for a biphasic separation of the high boiler substream (7) of the high boiler bottoms output (23).

In a further configuration of the process, a substream of the organic phase (14) from the phase separator (G) is returned to the rectification column (E) with a reflux ratio based on the organic phase in the range from 0.1 to 1.0, and a substream of the aqueous phase (16) from the phase separator (G) with a reflux ratio based on the aqueous phase in the range from 1 to 10. This results in the advantage that less organic phase has to be discharged from the process and less aqueous phase has to be removed, and both the esterification and the removal in the rectification column (E) proceed in an energy-efficient manner. Moreover, a lower level of secondary components is formed.

In a further configuration of the process, the acidic catalyst is present in a concentration in the range from 0.1% to 10% by weight in the reaction zone (E1) of the rectification column (E) or in the resulting reaction output of the reactor (A). This results in the advantage that not too much catalyst has to be removed, and hence the process can proceed with greater energy efficiency and less catalyst can be used in the process.

In a further configuration of the process, a substream of the aqueous extract (12) is fed to the reactor (A) in a mass flow ratio to the total high boiler bottoms output (23) in the range from 0.01 to 0.5, preferably in the range from 0.01 to 0.3.

The invention will be elucidated in detail hereinafter with reference to the drawings. The drawings should be considered to be schematic diagrams. They do not constitute a limitation of the invention, for example with regard to specific dimensions or design variants.

BRIEF DESCRIPTION OF FIGURES

FIG. 1: A first embodiment of a process of the invention for continuously preparing n-butyl (meth)acrylate, in which a reactor A with a downstream rectification column E is used. In this case, a catalyst-comprising aqueous extract is recycled into the reactor A and/or into the rectification column E.

FIG. 2: A second embodiment of a process of the invention for continuously preparing n-butyl (meth)acrylate, in which a reaction zone E1 is integrated in the bottom region of a rectification column E. In this case, a catalyst-comprising aqueous extract is recycled into the rectification column E.

FIG. 3: A third embodiment of a process for continuously preparing n-butyl (meth)acrylate as a comparative example, in which a reactor A with a downstream rectification column E is used. In this case, there is no recycling of a catalyst-comprising aqueous extract into the reactor A and/or into the rectification column E.

LIST OF REFERENCE NUMBERS USED

- 1 conduit for feeding in the reactant stream
- 2 residual vapor from the condenser C
- 3 conduit for recycling of the organic phase from the liquid-liquid phase separator D
- 4 conduit for discharging the organic phase from the liquid-liquid phase separator D
- 5 conduit for feeding the aqueous phase from the liquid-liquid phase separator D to the liquid-liquid phase separator G, or aqueous phase from the liquid-liquid phase separator D to the liquid-liquid phase separator G
- 6 conduit for feeding the resulting reaction output into the rectification column E
- 7 conduit for the substream of the high boiler bottoms output 23 to the mixer H or high boiler substream of the high boiler bottoms output 23 to the mixer H
- 8 conduit for the substream of the high boiler bottoms output 23 to the cleavage reactor J
  - or substream of the high boiler bottoms output 23 to the cleavage reactor J
- 9 conduit for recycling of a substream of the high boiler bottoms output 23 to the reactor
- A or substream of the high boiler bottoms output 23 to the reactor A
- 10 resulting mixture from the mixer H
- 11 conduit for feeding organic raffinate from the extraction phase separator I to the cleavage reactor J or organic raffinate
- 12 conduit for recycling the aqueous extract from the extraction phase separator I to the reactor or to the rectification column E or aqueous extract
- 13 residual vapor from the condenser F
- 14 conduit for recycling the organic phase from the liquid-liquid phase separator G to the rectification column E, or organic phase from the liquid-liquid phase separator G
- 15 conduit for discharging a substream of the organic phase from the liquid-liquid phase separator G or crude product stream
- 16 conduit for recycling the aqueous phase from the liquid-liquid phase separator G to the rectification column E or aqueous phase from the liquid-liquid phase separator G
- 17 conduit for discharging a substream of the aqueous phase from the liquid-liquid phase separator G
- 18 conduit for feeding a substream of the aqueous phase from the liquid-liquid phase separator G to the mixer H, or aqueous phase from the liquid-liquid phase separator G
- 21 conduit for discharging the high boiler bottoms output of the cleavage reactor J
- 22 conduit for the discharge of the high boilers from the cleavage reactor J
- 23 conduit for the high boiler bottoms output of the rectification column E or high boiler bottoms output of the rectification column E
- 24 conduit for feeding the polymerization inhibitor stream to the condenser C
- 25 conduit for feeding the polymerization inhibitor stream to the condenser F
- 26 conduit for feeding the aqueous phase from the liquid-liquid phase separator D to the mixer H, or aqueous phase from the liquid-liquid phase separator D
- A reactor
- B column
- C condenser
- D phase separator
- E rectification column
- E1 reactor integrated into the rectification column
- F condenser
- G phase separator
- H mixer
- I extraction phase separator
- J cleavage reactor

FIG. 1 shows a schematic of a process flow diagram of a chemical engineering process according to a first embodiment of the process of the invention, in which a substream of the aqueous phase 18 from a liquid-liquid phase separator G is added to a mixer H.

A reactant mass flow comprising n-butanol, acrylic acid, PTZ and p-toluenesulfonic acid as esterification catalyst is fed to a reactor A through a conduit 1. A column B disposed above the reactor A separates a vapor mixture flowing out of the reactor A by means of its separating internals. A downstream condenser C, which may optionally be supplemented by a postcooler, at least partly condenses the vapor stream arising from the column B. A solution of the polymerization inhibitor PTZ is added to the condenser C via a conduit 24. The uncondensed fraction from the condenser C comprises low-boiling impurities and is drawn off in vaporous form via a conduit 2. The condensed vapor stream flows as condensate into a liquid-liquid phase separator D. The condensate separates therein into an aqueous phase and an organic phase. The aqueous phase comprising mainly water is guided through a conduit 5 from the liquid-liquid phase separator D to a liquid-liquid phase separator G. A further substream of the aqueous phase may in principle also serve as extractant and be fed through a conduit 26 from the liquid-liquid phase separator D to the mixer H.

A resulting reaction output comprising n-butyl acrylate, unconverted reactants and higher-boiling impurities inter alia is drawn off from the lower portion of the reactor A via a conduit 6 and fed to a rectification column E having separating internals. Water, n-butyl acrylate and alcohol inter alia are separated off in the rectification column E. The vapors exiting from the column are fed to a condenser F, which may optionally be supplemented by a postcooler, and partly condensed therein. A solution of the polymerization inhibitor PTZ is added to the condenser F via a conduit 25. The uncondensed fraction from the condenser F comprises low-boiling impurities and is drawn off in vaporous form via a conduit 13, whereas the resulting condensate flows into the liquid-liquid phase separator G. The condensate separates therein into an organic phase and an aqueous phase.

An organic phase comprising mainly n-butyl acrylate and n-butanol is partly returned as reflux via a conduit 14 back to the rectification column E, and the remainder is discharged via a conduit 15 for further workup. The aqueous phase comprising mainly water is partly returned as reflux via a conduit 16 back to the rectification column E, and partly conducted to a mixer H as extractant via a conduit 18, and the remainder is discharged via a conduit 17 for further workup.

Mainly higher-boiling impurities and the catalyst used are drawn off from the lower portion of the rectification column E via a conduit 23 and returned to the cleavage reactor J, to the mixer H and back to the reactor A via the downstream conduits 7, 8 and 9. In the mixer H, as extractant, the aqueous phase from the liquid-liquid phase separator G via the conduit 18 and/or the aqueous phase via the conduit 26 is mixed with the mass flow from the conduit 7 and fed to an extraction phase separator I via a conduit 10.

In the extraction phase separator I, the resultant mixture separates into an organic raffinate and a catalyst-comprising aqueous extract. The aqueous extract, comprising mainly water, is fed to the reactor A via a conduit 12. The organic raffinate is fed at least partly to a cleavage reactor J via a conduit 11 for further workup. The high boiler bottoms output is also fed at least partly to the cleavage reactor J via a conduit 8 for further workup. The bottoms output from the cleavage reactor J is discharged from the process via a conduit 21, whereas the gaseous substances are drawn off at the top of the cleavage reactor J via a conduit 22. Subsequently, the gaseous substances can be condensed and finally returned back to the reactor A.

FIG. 2 shows a schematic of a process flow diagram of a chemical engineering process according to a second, alternative embodiment of the process of the invention, in which a substream of the aqueous phase 18 is added to a mixer H from a liquid-liquid phase separator G, wherein the reaction zone E1 is integrated into the bottom of the rectification column E in this embodiment by comparison with the first embodiment.

FIG. 3 shows a schematic of a process flow diagram of a chemical engineering process as a comparative example, wherein there is no mixer H and no extraction phase separator I by comparison with the first embodiment, which means that no recycling of the aqueous extract from the extraction phase separator I to the reactor A is enabled.

EXAMPLES

The examples of the process that follow are simulated by thermodynamic simulations. For this purpose, the Aspen Plus® software (Aspen) is used, which can be found at the website https://www.aspentech.com (retrieved 15.07.2022). Aspen is an extensive simulation software package which is used for modeling, simulation and optimization of chemical processes and plants in industry. Aspen has extensive model databases for modelling of basic operations and substance databases for the physical properties of many different substances. The properties of mixtures are calculated by Aspen with the aid of different thermodynamic models from the physical data of the pure substances.

Comparative Example 1

A thermodynamic simulation of the overall plant according to FIG. 3 is conducted by Aspen and gives the following results:

A reactant stream is fed via a conduit 1 to a reactor A with a mass flow rate of 1000 kg/h, where the reactant stream is a mixture having the following composition:

- n-Butanol: 582.2 kg/h
- Acrylic acid: 413.9 kg/h
- p-Toluenesulfonic acid: 3.9 kg/h.

A column B disposed above the reactor A separates a vapor mixture flowing out of the reactor A by means of its separating internals. A downstream condenser C at least partly condenses the vapor stream formed from column B.

A solution of the polymerization inhibitor is added via the conduit 24 to a condenser C with a mass flow rate of 2 kg/h, said solution of the polymerization inhibitor having the following composition:

- n-Butyl acrylate: 98% by weight
- Phenothiazine: 2% by weight.

The uncondensed fraction from the condenser C comprises low-boiling impurities and is drawn off in vaporous form via a conduit 2.

The esterification is conducted in the reactor A at a temperature of 105° C., a pressure of 470 mbar and a dwell time of 2 h.

27 kg/h of the organic phase is drawn off as organic distillate via a conduit 4 from a liquid-liquid phase separator D downstream of the condenser C, and 798 kg/h is returned to the column B as reflux via a conduit 3.

The organic phase as organic distillate has the following composition:

- Water: 19.6% by weight
- n-Butanol: 77.3% by weight
- n-Butyl acrylate: 1.5% by weight
- Acrylic acid: <0.01% by weight
- n-Butyl acetate: 1.2% by weight
- Phenothiazine: <0.01% by weight
- Unknown: 0.4% by weight.

145 kg/h of the aqueous phase is guided via a conduit 5 from the liquid-liquid phase separator D to a liquid-liquid phase separator G. The aqueous phase has the following composition:

- Water: 94.0% by weight
- n-Butanol: 5.9% by weight
- Unknown: 0.1% by weight.

In the lower portion of the reactor A, a resulting reaction output is drawn off via the conduit 6 with a mass flow rate of 1415 kg/h with the following composition, and fed to a rectification column E:

- Water: 0.2% by weight
- n-Butanol: 10.2% by weight
- n-Butyl acrylate: 68.0% by weight
- Acrylic acid: 5.0% by weight
- n-Butyl acetate: 0.0% to 0.1% by weight
- p-Toluenesulfonic acid 1.6% by weight
- Phenothiazine 0.1% by weight
- Unknown: 14.7% to 14.8% by weight.

In a rectification column E equipped with 13 theoretical plates, water, n-butanol and n-butyl acrylate are drawn off from the rectification column E in vaporous form overhead, condensed in a condenser F and then fed to a liquid-liquid phase separator G. In the top region of the column, the pressure is 1059 mbar and the temperature is 95° C.

A solution of the polymerization inhibitor is added via the conduit 25 to a condenser F with a mass flow rate of 2 kg/h, said solution of the polymerization inhibitor having the following composition:

- n-Butyl acrylate: 98% by weight
- Phenothiazine: 2% by weight.

803 kg/h of the organic phase is drawn off via the conduit 15 from the liquid-liquid phase separator G as organic distillate for further purification. A substream with a mass flow rate of 365 kg/h is returned to the rectification column E as reflux via the conduit 14.

The organic phase has the following composition:

- Water: 2.7% by weight
- n-Butanol: 16.8% by weight
- n-Butyl acrylate: 80.3% by weight
- Acrylic acid: <0.01% by weight
- n-Butyl acetate: 0.1% by weight
- Phenothiazine: <0.01% by weight
- Unknown: 0.1% by weight.

83 kg/h of the aqueous phase is drawn off via conduit 17 from the liquid-liquid phase separator G as aqueous distillate. A substream with a mass flow rate of 724 kg/h of the aqueous phase is returned to the rectification column E as reflux via the conduit 16.

The aqueous phase has the following composition:

- Water: 96.7% by weight
- n-Butanol: 2.0% by weight
- n-Butyl acrylate: 1.3% by weight
- Unknown: <0.05% by weight.

In the lower portion of the rectification column E, a substream with a mass flow rate of 73 kg/h is conducted via a conduit 8 to a cleavage reactor J, and a substream with a mass flow rate of 585 kg/h via a conduit 9 back to the reactor A. The vapor mixture formed in the cleavage reactor J is removed via a conduit 22, whereas the bottoms mixture is removed via a conduit 21.

The bottoms mixture in the cleavage reactor J has the following composition:

- Water: 3.1% by weight
- n-Butanol: 4.3% by weight
- n-Butyl acrylate: 21.4% by weight
- Acrylic acid: 11.1% by weight
- p-Toluenesulfonic acid 3.4% by weight
- Phenothiazine: 0.2% by weight
- Unknown: 56.5% by weight.

A content of oligomers and/or polymers is included in the unknowns.

Example 1

In this example 1, an inventive embodiment of the process for preparing n-butyl (meth)acrylate according to FIG. 1 is simulated by the Aspen software. The simulation gives the following results:

A reactant stream is fed via a conduit 1 to a reactor A with a mass flow rate of 1000 kg/h, where the reactant stream has the following composition:

- n-Butanol: 584.3 kg/h
- Acrylic acid: 414.4 kg/h
- p-Toluenesulfonic acid: 1.3 kg/h.

The esterification in the reactor A is conducted at a temperature of 105° C., an absolute pressure of 470 mbar and a dwell time of 2 hours.

A column B disposed above the reactor A separates a vapor mixture flowing out of the reactor A by means of its separating internals. A downstream condenser C at least partly condenses the vapor stream formed from column B.

A solution of the polymerization inhibitor is added via the conduit 24 to the condenser C with a mass flow rate of 2 kg/h, said solution of the polymerization inhibitor having the following composition:

- n-Butyl acrylate: 98% by weight
- Phenothiazine: 2% by weight.

The uncondensed fraction from the condenser C comprises low-boiling impurities and is drawn off in vaporous form via a conduit 2.

27 kg/h of the organic phase is drawn off as organic distillate via a conduit 4 from a liquid-liquid phase separator D downstream of the condenser C, and 934 kg/h is returned to the column B as reflux via a conduit 3.

The organic phase has the following composition:

- Water: 19.5% by weight
- n-Butanol: 77.0% by weight
- n-Butyl acrylate: 1.7% by weight
- Acrylic acid: <0.01% by weight
- n-Butyl acetate: 1.4% by weight
- Phenothiazine: <0.01% by weight
- Unknown: 0.4% by weight.

An aqueous phase with a mass flow rate of 155 kg/h is conducted via the conduit 5 from the liquid-liquid phase separator D to a liquid-liquid phase separator G.

The aqueous phase has the following composition:

- Water: 94.0% by weight
- n-Butanol: 5.9% by weight
- Unknown: 0.1% by weight.

In the lower portion of the reactor A, a liquid mixture with a mass flow rate of 1422 kg/h is drawn off via the conduit 6 with the following composition, and fed to a rectification column E:

- Water: 0.2% by weight
- n-Butanol: 10.2% by weight
- n-Butyl acrylate: 68.0% by weight
- Acrylic acid: 5.0% by weight
- n-Butyl acetate: 0.0% to 0.1% by weight
- p-Toluenesulfonic acid 1.6% by weight
- Phenothiazine 0.2% by weight