Process for the synthesis of isophorone in liquid phase with by-product recycling.

Abstract

The present invention comprises the following consecutive steps: a) continuously injecting a stream of aqueous alkali metal hydroxide solution and a stream of organic solution comprising acetone and the by-products recycled via step g) through a tubular reactor (R), followed by b) condensation of acetone in a tubular reactor (R), followed by c) distilling the reaction mixture obtained from the tubular reactor (R), followed by d) separating the concentrated crude reaction mixture obtained from the distillation of step c) to obtain an alkaline aqueous phase and an organic phase containing isophorone, followed by e) distilling the organic phase containing isophorone recovered in the preceding step, followed by f) distilling the polycondensation by-products from the bottom of column (D2) obtained from the preceding distillation, followed by g) recycling the stream obtained from the top of the distillation column (D3) of the preceding step, which contains xylitol and / or isoxylitol, to the tubular reactor (R); This invention relates to a continuous process for the liquid phase synthesis of isophorone by alkaline self-condensation of acetone, comprising:

Description

[Technical Field]

[0001] The present invention relates to a process for the continuous synthesis of isophorone by alkaline self-condensation of acetone in the liquid phase. [Background technology]

[0002] Isophorone (or 3,5,5-trimethylcyclohex-2-enone) is an α,β-unsaturated cyclic ketone that is increasingly used as a synthetic intermediate, particularly for the production of isophorone diamine, used as a curing agent for epoxy resins; isophorone diisocyanate, used as a polyurethane monomer; 3,5-xylenol, used as a precursor to PCMX (an antibacterial agent); keto-isophorone, a synthetic intermediate for vitamin E; and 3,5,5-trimethylcyclohexanol, used as a precursor to homosalate (a UV absorber). Isophorone is also an excellent high-boiling solvent for many natural and synthetic resins used in the paint, ink, and varnish industries. Isophorone is also a solvent used in pesticides for the formulation of emulsifiable pesticide concentrates.

[0003] Isophorone is conventionally obtained by catalytic self-condensation of three molecules of acetone according to the following reaction: [ka]

[0004] The reaction is carried out in the liquid phase or the gas phase.

[0005] The gas phase processes described in the literature essentially use solid heterogeneous catalysts, whereas the liquid phase processes use homogeneous or heterogeneous catalyst systems.

[0006] The synthesis of isophorone by condensation of acetone in the liquid phase is carried out almost exclusively under alkaline conditions at high temperature and pressure, with alkaline catalysis most often achieved by using aqueous solutions of sodium hydroxide or potassium hydroxide.

[0007] Due to the low solubility of inorganic bases in acetone, a process aimed at promoting the contact of acetone with the catalyst is needed. Thus, from US 2,344,226, it is known to carry out the synthesis in a stirred reactor. FR 1,238,954 discloses the synthesis using a tubular reactor with internal packing. US 2,399,976 discloses the synthesis using a tubular reactor equipped with a recirculation system. CN 102367223 and CN 102516051 disclose the synthesis using a premixing system such as a static mixer. FR 1,042,057 also discloses the replacement of the aqueous alkaline solution with an alkaline alcohol solution.

[0008] The synthesis can be carried out continuously in a tubular reactor without special mixing equipment by using very low concentrations (by weight) of sodium hydroxide or potassium hydroxide. Typically, the catalyst concentration is less than 1 wt. % or even as low as 0.1 wt. % based on the total weight of the reaction mixture. This low concentration allows for single-phase mixing. These processes are described in documents FR1316515, DD145096, EP2649032, EP2707352, and EP2837618.

[0009] The synthesis can also be carried out by reactive distillation by injecting acetone and an aqueous solution of sodium hydroxide or potassium hydroxide into a reactive distillation column, maintaining a low concentration of sodium hydroxide or potassium hydroxide (less than 0.1% by weight relative to the total weight of the reaction mixture) and reacting the acetone countercurrent with the sodium hydroxide or potassium hydroxide. This process is described in documents FR1315788, FR2271191, and FR2328686.

[0010] Due to the extreme reaction conditions used, the acetone self-condensation reaction is accompanied by the formation of polycondensation by-products formed from four or more acetone molecules. To limit the production of these heavy derivatives, the synthesis is carried out with limited acetone conversion. In addition to isophorone and polycondensation derivatives, the crude reaction mixture also contains larger or smaller amounts of synthesis intermediates, the majority of which is mesityl oxide. [ka] [ka]

[0011] Document US 2,344,226 describes the recycle of mesityl oxide in the reactor using unconverted acetone. Document US 2,351,352 describes a process in which mesityl oxide is separately converted back to acetone by hydrolysis in the presence of aqueous alkaline solution in a reactive distillation column. In the specific case of isophorone synthesis by reactive distillation, mesityl oxide is converted back to acetone in situ (FR 1316515).

[0012] Document US 2,419,051 describes the partial conversion of polycondensation products to acetone and isophorone by alkaline hydrolysis in a stirred reactor, which is also described in documents FR 1 316 514, FR 1 316 515, EP 2 649 032, EP 2 707 352 and EP 2 837 618 via a hydrolysis reaction distillation column.

[0013] The synthesis of IPHO by condensation of acetone in the gas phase is carried out at high temperatures (200-400 °C) over a fixed bed of solid catalysts such as, for example, calcium oxide and / or calcium hydroxide (FR850334), mixed magnesium-aluminum oxide catalysts (EP0640387), calcium aluminate (US2,393,510), alkali metal-doped zeolites or magnesium oxides (JP9151152, JP9151153, JP9169687, JP9169688) or hydrotalcites (CN106423124, CN106423125, CN106423126).

[0014] Whether using a liquid-phase or gas-phase process, the high reaction temperatures required for the synthesis of isophorone result in reaction selectivity that is primarily driven by C 12 H 18 Oxyliton and isoxyliton and C 15 H 22 O compounds, including empirical formula C 3n H (4n+2) O(n≧4) is limited by the formation of polycondensation by-products.

[0015] In the case of liquid phase synthesis, even with limited conversion, the selectivity to isophorone at the outlet of the stirred or tubular reactor is at most 75%.

[0016] Only the reactive distillation synthesis process with partial reversion of the polycondensation products in situ and the process combining a tubular reactor and a reactive distillation column for hydrolysis of the heavy products make it possible to achieve an overall isophorone selectivity of 85% to 91%.

[0017] However, such reactive distillation equipment is particularly expensive because it must operate under high pressure, generally 30-50 bar, and requires specific materials that can withstand the alkaline conditions of the reaction medium and temperatures exceeding 200 °C.

[0018] Unlike liquid-phase processes, gas-phase processes can operate at atmospheric pressure, but have the major drawback of decreasing reaction performance as the catalyst ages due to contamination by polycondensation by-products and coking caused by the use of high reaction temperatures. Thus, industrial production is heavily affected by the frequent need to regenerate or change catalyst beds. Summary of the Invention [Problem to be solved by the invention]

[0019] Therefore, there is a need for an alternative process that has stable productivity and is cheaper in terms of investment than processes that use reactive distillation(s). [Means for solving the problem]

[0020] The present invention comprises the following consecutive steps: a) continuously injecting a stream of aqueous alkali metal hydroxide solution (A) and a stream of organic solution comprising acetone and the by-products recycled via step g) through a tubular reactor (R), followed by b) a step of subjecting acetone to a condensation reaction in a tubular reactor (R), the reactor containing an emulsion mainly comprising an alkali metal hydroxide aqueous phase (B), the concentration of the alkali metal hydroxide in the aqueous phase (B) in the tubular reactor being 50 g / l or more; and then c) distilling the reaction mixture obtained from the tubular reactor (R), followed by d) separating the concentrated crude reaction mixture obtained from the distillation of step c) to obtain an alkaline aqueous phase and an organic phase containing isophorone; Preferably, at the end of separation d), a step of neutralizing the recovered organic phase, followed by e) distillation of the organic phase containing isophorone recovered in the preceding step in order to extract the polycondensation by-products mainly at the bottom of column (D2) and to recover a stream containing mainly isophorone at the top of column (D2), followed by f) distilling the polycondensation by-products from the bottom of column (D2) obtained from the preceding distillation, followed by g) recycling the stream obtained from the top of the distillation column (D3) of the preceding step, which contains xylitol and / or isoxylitol, to the tubular reactor (R); This invention relates to a continuous process for the liquid phase synthesis of isophorone by alkaline self-condensation of acetone, comprising:

[0021] Other advantageous features of the process according to the invention are specified below:

[0022] the aqueous alkali metal hydroxide solution (A) is an aqueous solution of sodium hydroxide or potassium hydroxide,

[0023] the concentration of alkali metal hydroxide in the aqueous phase (B) present in the tubular reactor is between 50 and 200 g / l, preferably between 80 and 150 g / l and more preferentially between 100 and 150 g / l,

[0024] the concentration of alkali metal hydroxide in the aqueous solution (A) in the feed zone is between 5 and 40 g / l, preferably between 10 and 40 g / l and more preferentially between 15 and 35 g / l,

[0025] the ratio of the mass flow rate of the aqueous alkali metal hydroxide stream (A) fed (Q alkali metal hydroxide) to the mass flow rate of the organic stream fed (Q organic) is between 0.25 and 1.0, preferably between 0.4 and 0.8, more preferentially between 0.5 and 0.7,

[0026] the reaction temperature in the tubular reactor is between 180°C and 250°C, preferably between 200°C and 230°C, and / or the absolute pressure in the tubular reactor is between 30 and 50 bar, preferably between 35 and 45 bar, and even more preferentially between 38 and 42 bar,

[0027] the process comprises a step h) of distilling the stream recovered at the top of the distillation column (D2) of step e),

[0028] the process comprises a step i) of distilling the stream recovered at the bottom of the distillation column (D4) of step h),

[0029] the process comprises a step of recycling the stream recovered at the bottom of the distillation column (D5) of the preceding step to the distillation column (D2) of step e),

[0030] The process comprises a step of decanting the stream recovered at the top of the distillation column (D4) of step h), after which the organic phase is recycled to the tubular reactor (R).

[0031] The process according to the invention achieves productivity and selectivity levels similar to those typically obtained with reactive distillation, but has the advantage of avoiding the use of this type of equipment. In particular, the systematic recycling of fractions containing synthetic intermediates such as mesityl oxide and fractions containing reversible by-products makes it possible to reach these productivity and selectivity thresholds. [Brief explanation of the drawings]

[0032] [Figure 1] FIG. 1 shows a schematic diagram of an apparatus for carrying out the claimed process. DETAILED DESCRIPTION OF THE INVENTION

[0033] Other features, aspects, subjects and advantages of the present invention will become even more clearly apparent upon reading the following description.

[0034] It is expressly stated that the expressions "from to" and "between and" used in this specification are to be understood as including each of the boundaries referred to.

[0035] The process according to the present invention comprises the seven consecutive steps described above: steps a) to g). This process may comprise additional purification steps.

[0036] Step a): Flow injection

[0037] The synthesis comprises: a stream of aqueous alkali metal hydroxide solution (A), and a stream of organic solution comprising acetone and by-products recycled via step g). is carried out by continuously injecting it through a tubular reactor (R).

[0038] Alkali metal hydroxide aqueous solution (A)

[0039] The alkali metal hydroxide used is preferably sodium hydroxide or potassium hydroxide, more preferentially sodium hydroxide in the form of an aqueous sodium hydroxide solution.

[0040] Preferably, the alkali metal hydroxide in aqueous solution (A) is the same as the alkali metal hydroxide in aqueous solution (B).

[0041] The concentration of alkali metal hydroxide in the reactor depends on the concentration of the aqueous alkali metal hydroxide solution (A) and the ratio of the flow rates of the aqueous alkaline solution (A) and the organic solution in the feed zone of the reactor.

[0042] Preferably, the concentration of alkali metal hydroxide in the aqueous solution (A) in the feeding zone is between 5 and 40 g / l, preferably between 10 and 40 g / l, more preferentially between 15 and 35 g / l.

[0043] When said alkali metal hydroxide is sodium hydroxide, the concentration of sodium hydroxide in said aqueous solution in the feeding zone is advantageously between 5 and 30 g / l, preferably between 10 and 30 g / l and more preferentially between 15 and 25 g / l.

[0044] When said alkaline hydroxide is potassium hydroxide, the concentration of potassium hydroxide in said aqueous solution in the feeding zone is advantageously between 5 and 40 g / l, preferably between 10 and 40 g / l and more preferentially between 20 and 35 g / l.

[0045] Preferably, the start-up of the unit is carried out by pre-charging the reactor with aqueous alkali metal hydroxide solution (B).

[0046] organic solution

[0047] The organic stream comprises acetone, recycled by-products, and optionally recycled reaction intermediates.

[0048] The term "recycled by-products" refers to polycondensation by-products that can be reversed under isophorone synthesis conditions, i.e., xylitol and / or isoxylitol (C 12 H 18 O). Isoxiliton and xylitol include several isomers, including the following molecules: [ka]

[0049] These various C 12 H 18 The O isomer is formed by the condensation of isophorone with acetone or by the self-condensation of mesityl oxide. [ka] [ka]

[0050] To optimize the selectivity for isophorone, the acetone condensation reaction is carried out with an acetone conversion limited to less than 50%, preferably less than 30%, and more preferentially between 15% and 25%.

[0051] As developed below, the organic stream may also include recycled acetone resulting from one or more distillations of the process.

[0052] In addition to fresh and recycled acetone, the organic stream may also contain recycled reaction intermediates such as mesityl oxide.

[0053] The ratio of the mass flow rate of the aqueous alkali metal hydroxide stream (A) fed (Q alkali metal hydroxide) to the mass flow rate of the organic stream fed (Q organic) is advantageously between 0.25 and 1.0, preferably between 0.4 and 0.8, more preferentially between 0.5 and 0.7.

[0054] When said alkali metal hydroxide is sodium hydroxide, the ratio of the mass flow rate of the aqueous sodium hydroxide stream fed (QNaOH) to the mass flow rate of the organic stream fed (Qorganic) is advantageously between 0.25 and 1.0, preferably between 0.4 and 0.8, more preferentially between 0.5 and 0.7.

[0055] When said alkali metal hydroxide is potassium hydroxide, the ratio of the mass flow rate of the aqueous alkali metal hydroxide stream fed (QKOH) to the mass flow rate of the organic stream fed (Qorganic) is advantageously between 0.25 and 1.0, preferably between 0.4 and 0.8, and more preferentially between 0.5 and 0.7.

[0056] A heat exchanger may be used to preheat the stream before it enters the reactor.

[0057] Step b): Reaction

[0058] The acetone condensation reaction is carried out in a tubular reactor (R), which contains an emulsion mainly comprising an aqueous alkali metal hydroxide phase (B), the concentration of alkali metal hydroxide in the aqueous phase (B) in the tubular reactor being 50 g / l or more.

[0059] Preferably, the reactor is vertical.

[0060] The emulsion present in the reactor is an aqueous phase (B) which is the continuous phase of the emulsion and which comprises water, acetone and an alkali metal hydroxide; an organic phase that is the dispersed phase of the emulsion, preferably in the form of droplets, and that contains acetone, isophorone, any synthesis intermediates and polycondensation by-products; Includes.

[0061] The emulsion contains predominantly the aqueous alkali metal hydroxide phase, and for purposes of the present invention, the term "predominantly" means that the aqueous alkali metal hydroxide phase is greater than 50% by volume relative to the total volume of emulsion present in the reactor.

[0062] The concentration of alkali metal hydroxide in the aqueous phase (B) in said tubular reactor is greater than 50 g / l.

[0063] In contrast to the sodium hydroxide or potassium hydroxide catalyzed liquid phase processes described in the literature, which use conditions to have as homogeneous a reaction phase as possible, the process according to the invention carries out the reaction in a heterogeneous medium, which allows for an increased selectivity for isophorone.

[0064] The heterogeneous reaction medium is formed from a continuous aqueous phase concentrated in the alkali metal hydroxide through which the organic phase passes in the form of droplets, preferably rising droplets, and the much lower solubility of isophorone in this concentrated alkaline aqueous phase compared to acetone therefore makes it possible to limit the formation of the polycondensation by-products.

[0065] Thus, the reaction medium is heterogeneous: it comprises a majority aqueous alkali metal hydroxide phase and a minor organic phase containing acetone, isophorone, and any of the recycled organic by-products and any of the synthetic intermediates.

[0066] The reaction temperature in the tubular reactor may be 180°C to 250°C, preferably 200°C to 230°C, and may be under an absolute pressure of 30 to 50 bar, preferably 35 to 45 bar, and even more preferentially 38 to 42 bar.

[0067] The concentration of alkali metal hydroxide in the aqueous phase (B) present in said tubular reactor may be between 50 and 200 g / l, preferably between 80 and 150 g / l and more preferentially between 100 and 150 g / l.

[0068] When said alkali metal hydroxide is sodium hydroxide, the concentration of sodium hydroxide in the aqueous phase (B) present in said tubular reactor is preferably between 50 and 200 g / l, preferably between 80 and 150 g / l, more preferentially between 100 and 120 g / l.

[0069] When said alkaline hydroxide is potassium hydroxide, the concentration of potassium hydroxide in the aqueous phase (B) present in said tubular reactor may be between 50 and 200 g / l, preferably between 80 and 150 g / l, more preferentially between 125 and 150 g / l.

[0070] The tubular reactor R may, if appropriate, consist of several tubular reactors fed in parallel.

[0071] The reaction mixture is collected at the outlet of the tubular reactor and conveyed to a distillation column.

[0072] distillation

[0073] In the process according to the invention, the distillation column is preferably equipped with a boiler at the bottom of the column and a condenser at the top of the column. The column may be a tray column or a packed column.

[0074] Advantageously, the distillation is carried out under reduced pressure.

[0075] Distillation under reduced pressure corresponds to a distillation carried out at an absolute pressure of less than 1013 mbar, preferably less than 250 mbar, more preferentially between 10 and 100 mbar.

[0076] Preferably, the process according to the present invention does not involve reactive distillation.

[0077] Step c): Distillation 1

[0078] The reaction mixture recovered at the reactor outlet is distilled through column D1. Unconverted acetone is recovered at the top of the column, and the concentrated crude reaction mixture is withdrawn at the bottom of the column. Preferably, the distillation is carried out at atmospheric pressure.

[0079] Optional additional recirculation

[0080] The acetone recovered at the top of column D1 is advantageously recycled, in whole or in part, to tubular reactor R.

[0081] Step d): Separation

[0082] The concentrated crude reaction mixture withdrawn at the bottom of the distillation column D1 is preferably separated by decantation, and the alkaline aqueous phase is preferably separated from the isophorone-rich organic phase by a decanter.

[0083] Optional additional recirculation

[0084] The alkaline aqueous phase recovered in separation step d) is advantageously recycled completely or partly, preferably partly, to reaction step b).

[0085] Optional neutralization of the organic phase

[0086] The alkali metal hydroxides present in the isophorone-rich organic phase recovered in separation step d) may be neutralized. This neutralization can be carried out by any technique known to those skilled in the art, but preferably by a mineral acid that provides a buffering effect. Preferably, phosphoric acid is used.

[0087] Step e): Distillation 2

[0088] The isophorone-containing organic phase recovered in separation step d) and then optionally neutralized is preferably distilled under reduced pressure in order to extract said polycondensation by-products mainly at the bottom of column D2 and to recover a stream comprising mainly isophorone at the top of column D2.

[0089] Step f): Distillation 3

[0090] The fraction comprising polycondensation by-products recovered at the bottom of column D2 in the preceding distillation step is preferably distilled under reduced pressure. The fraction obtained from the top of distillation column D3 preferably comprises mainly xylitol and / or isoxylitol, the fraction at the bottom of said column preferably being of the empirical formula C 15 H 22 It mainly contains polycondensation by-products of O.

[0091] C 15 H 22 The polycondensation by-products include several isomers, including the following molecules: [ka]

[0092] These C 15 H 22 The O derivative can be obtained by condensation of isophorone with mesityl oxide or by C 12 H 18 It is formed by the condensation of oxylitol or isoxylitol with acetone. [ka] [ka]

[0093] Heavy reaction by-product S at the bottom of column D3 L These C 15 H 22 O By-products and their higher homologues C 3n H (4n+2) Includes O(n≧6).

[0094] Process g): Recirculation

[0095] The fraction obtained from the top of column D3 of the preceding distillation step, preferably containing mainly xylitol and / or isoxylitol, is recycled to the tubular reactor R. This fraction is added to the continuous stream of organic phase fed to said reactor.

[0096] Optional bleaching treatment

[0097] The stream recovered at the top of column D2 may be subjected to a decolorization treatment.

[0098] This decolorization treatment consists in converting certain reaction intermediates and / or by-products containing conjugated unsaturated and carbonyl-containing hydrocarbon chains that are difficult to separate from isophorone by distillation. Therefore, their residual presence can cause the yellowish coloration of isophorone. This treatment can be carried out by any method known to those skilled in the art for oxidizing or reducing olefinic bonds or for polycondensing the responsible by-products.

[0099] Preferably, the decolorization treatment comprises a step of reaction with an acid: the isophorone stream extracted at the top of column D2 is subjected to a continuous heat treatment in the presence of a catalytic amount of a strong mineral acid, such as sulfuric acid.

[0100] Residual sulfuric acid can then be advantageously neutralized by adding a strong inorganic base such as the aqueous alkaline solution described above, preferably that obtained from separation step d).

[0101] Optional Distillation 4 and Distillation 5: Step h) and Step i)

[0102] The process according to the invention may also comprise a step h) of distillation of the stream recovered at the top of the distillation column (D2) of step e). This stream recovered at the top of the column (D2) can be sent, after an optional decolorization step, to a subsequent distillation column.

[0103] The stream mainly containing isophorone and recovered at the top of distillation column D2 is subjected to distillation, preferably under reduced pressure, in column D4, allowing residual acetone and light impurities such as water, mesityl oxide and 1,3,5-trimethylbenzene to be extracted at the top of said column.

[0104] The stream extracted at the top of column D4 can be decanted through the decanter to separate the aqueous phase, which is then passed to the wastewater treatment SE , and the organic phase containing mainly mesityl oxide and isophorone.

[0105] This organic phase, which mainly contains mesityl oxide and isophorone, can be recycled to the reaction step.

[0106] The process according to the invention may also comprise a step i) of distilling the stream recovered at the bottom of the distillation column (D4) of step h). The fraction recovered at the bottom of column D4 can be fed to a fifth distillation column, making it possible to obtain isophorone at the top of said column with a purity of more than 99% and residual polycondensation by-products at the bottom of said column.

[0107] These residual polycondensation by-products can be recycled to distillation column D2.

[0108] BRIEF DESCRIPTION OF THE DRAWINGS

[0109] FIG. 1 represents one embodiment of steps a) to g) of the process according to the invention.

[0110] Acetone is introduced into heat exchanger E1 via line 1. The aqueous alkali metal hydroxide solution is introduced into heat exchanger E2 via line 2. The preheated stream is recovered in line 3 and introduced into tubular reactor R.

[0111] The reaction mixture obtained from the reactor R is introduced into a distillation column D1 via a line 4.

[0112] The acetone not converted in the reactor is recovered at the top of column D1, this fraction being recycled via line 5 to line 1.

[0113] The concentrated crude reaction mixture is recovered at the bottom of column D1 and conveyed via line 6 to decanter d1.

[0114] Decanter d1 separates the aqueous phase from the organic phase. The alkaline aqueous phase is removed via line 8 and then line 9. A purge p is introduced to remove water co-produced by the condensation reaction. Line 9 recycles the alkaline aqueous phase to line 2.

[0115] The organic phase obtained from the decanter d1 is conveyed via line 7 to the neutralizer N. The neutralized organic phase is conveyed via line 10 to the distillation column D2.

[0116] Distillation under reduced pressure using distillation column D2 makes it possible to recover at the top of column D2 a stream mainly comprising isophorone, which is transferred via line 11 to distillation column D4. At the bottom of column D2, the recovered fraction containing said polycondensation by-products is transferred via line 12 to distillation column D3.

[0117] Distillation under reduced pressure of the polycondensation by-products in distillation column D3 makes it possible to recover at the top of column D3 a stream preferably comprising xylitol and / or isoxylitol, this fraction being recycled to line 1 via line 13.

[0118] The heavy reaction by-product S L is recovered at the bottom of column D3.

[0119] The stream recovered at the top of column D2 and containing mainly isophorone is fed to distillation column D4. Distillation under reduced pressure in column D4 makes it possible to extract residual acetone and light impurities such as water, mesityl oxide and 1,3,5-trimethylbenzene at the top of said column. This fraction is discharged via line 15 into decanter d2. The aqueous phase obtained from decanter d2 is sent via line 17 to wastewater treatment S E and the organic phase containing mainly mesityl oxide and isophorone is recycled via line 16 to line 1.

[0120] The fraction recovered at the bottom of column D4 is fed to column D5 via line 18. The fraction recovered at the top of column D5 contains isophorone at a purity of more than 99%. The fraction withdrawn at the bottom of column D5 contains residual polycondensation by-products, which are recycled to distillation column D2 via line 20.

[0121] The organic phase stream fed to reactor R therefore contains fresh acetone, the light ends from distillation column D1 recycled via line 5, the light ends from distillation column D3 recycled via line 13, and the organic phase obtained from decanter d2 recycled via line 16.

[0122] The aqueous alkaline solution (A) fed to the reactor R contains fresh aqueous alkali metal hydroxide solution and the aqueous phase obtained from the decanter d1 and recycled via line 9.

[0123] The following examples illustrate the present invention but do not limit it in any way.

[0124] (Example)

[0125] Example 1: The synthesis of isophorone was carried out in a vertical tubular reactor made of 316L stainless steel with a volume of 815 ml and an L / D ratio of 3, equipped with a side arm positioned at L / 2 to withdraw samples in the center of the reactor.

[0126] The alkaline aqueous phase and the organic phase, consisting of fresh acetone, recycled acetone, and, where appropriate, recycled mesityl oxide and recycled polycondensation product, are preheated through two electric heat exchangers, respectively, to reach the desired reaction temperature in the reactor.

[0127] The tubular reactor is pre-filled with aqueous sodium hydroxide solution, also preheated, with a sodium hydroxide concentration of 10% by weight.

[0128] The reagents are supplied by piston pumps.

[0129] Conversion and selectivity are established after at least 24 hours of continuous operation to ensure stabilized reaction conditions in the reactor.

[0130] Table 1 below shows the operating conditions for the tests carried out at an absolute pressure of 40 bar in the tubular reactor, with a sodium hydroxide concentration of 20 g / l in the aqueous solution fed.

[0131] In the following Tables 1 and 2, MO represents mesityl oxide, and C 12 is C 12 represents the reaction by-products, i.e., xylitol and / or isoxylitol, and C 15 is C 15 H 22 C such as O 15 represents the reaction by-product, C 18 is C 18 where ACE represents acetone and IPHO represents isophorone, and selectivity is defined as:

[0132] S P = selectivity of product P relative to converted acetone S IPHO = 100 × 3 × (moles of IPHO formed) / moles of ACE converted S MO = 100 × 2 × (moles of MO formed) / moles of ACE converted SC 12 = 100 × 4 × (formed C 12 H 18 (moles of O) / moles of ACE converted SC 15 = 100 × 5 × (formed C 15 H 22 (moles of O) / moles of ACE converted SC 18 = 100 × 6 × (formed C 18 H 26 (moles of O) / moles of ACE converted where moles of P formed = (moles of P at reactor outlet - moles of P recycled to reactor feed).

[0133] Conversions and selectivities are calculated based on the composition (by weight) of the crude mixture at the outlet of the reaction zone, the composition being determined by gas chromatographic analysis. [Table 1]

[0134] Test 1 is a comparative test and shows the process without recycle.

[0135] Test 2 is also a comparative test in which only the fraction containing mesityl oxide is recycled.

[0136] Test 3 is according to the invention and involves recycling only the fraction containing xylitol and / or isoxylitol.

[0137] Test 4 is according to the invention and recycles all fractions containing mesityl oxide and xylitol and / or isoxylitol.

[0138] Tests 3 and 4 used a series of five distillation columns and were reversible C 12 Impurities and C 15 The impurity-containing fraction is recycled.

[0139] The results obtained are shown in Table 2 below. [Table 2]

[0140] Mesityl oxide (MO) and xylitol and / or isoxylitol (C 12合計 The selectivity results of (2) make it possible to observe that the recycling of mesityl oxide and xylitol and / or isoxylitol can lead to the complete suppression of the respective formations.

[0141] Example 3 is a compound containing xylitol and / or isoxylitol (C 12合計 It is shown that recycling a single fraction containing 2,4-dichloroisothiazolinone (2,4-dichloroisothiazolinone) allows for increased selectivity to isophorone, increased conversion and increased productivity.

[0142] Example 4 shows that the selectivity to isophorone (IPHO) of 74.4% without recycle was thus significantly higher than that of mesityl oxide (MO) and xylitol and / or isoxylitol (C 12合計 ) is shown to increase to 85.6% by recycling.

[0143] The negative selectivity towards mesityl oxide reflects the fact that the amount of mesityl oxide at the outlet of the tubular reactor was lower than at the inlet, indicating that recycle not only prevented the formation of mesityl oxide but also allowed excess recycle product to be restored to the reaction equilibrium.

[0144] These studies demonstrate the effectiveness of mesityl oxide and C on isophorone selectivity and productivity. 12 H 18 The crucial effect of recycling oxylitol and isoxylitol is shown.

[0145] Example 2: The following procedure is similar to that of Example 1, but uses a 316L stainless steel vertical tubular reactor with a volume of 940 ml and an L / D ratio of 18.5, equipped with three side arms positioned at L / 3, L / 2, and 2L / 3 to withdraw samples at 1 / 3 of the reactor length, in the middle of the reactor, and at 2 / 3 of the reactor length.

[0146] Table 3 below shows the operating conditions for the test. [Table 3]

[0147] Test 5 is a comparative test and shows the process without recycle.

[0148] Test 6 is in accordance with the present invention. The process followed is that shown in Figure 1, which uses a series of five distillation columns and a reversible C 12 Impurities and C 15 The impurity-containing fraction is recycled.

[0149] The results obtained are shown in Table 4 below. [Table 4]

[0150] As in Example 1, a sufficiently substantial recycling of xylitol and / or isoxylitol makes it possible to completely eliminate its formation. 12 H 18 The negative selectivity to O is due to the presence of mesityl oxide and C at the outlet of the tubular reactor. 12 H 18 Note that this represents the fact that the amount of O is lower than the amount at the inlet, which means that the recycle is 12 H 18 This indicates that the reaction not only prevented the formation of O but also allowed excess recycled products to be restored to the reaction equilibrium.

[0151] Furthermore, these recirculation operations require higher C 18 H 26 Note that this does not increase the formation of polycondensation by-products.

[0152] Therefore, the selectivity to isophorone of 75.3% without recycle was achieved by the addition of mesityl oxide (MO) and xylitol and / or isoxylitol (C 12合計 ) is recycled to 93.0%.

[0153] Thus, the savings for acetone consumed under the conditions of Test 6 compared to acetone consumed under the conditions of Test 5 is 0.32 kg of acetone per kg of isophorone produced.

Claims

1. The following consecutive steps: a)—a stream of aqueous alkali metal hydroxide solution (A), and a stream of organic solution comprising acetone and by-products recycled via step g). continuously injecting the mixture through a tubular reactor (R), and then b) a step of subjecting acetone to a condensation reaction in the tubular reactor (R), wherein the reactor contains an emulsion mainly containing an alkali metal hydroxide aqueous phase (B), and the concentration of the alkali metal hydroxide in the aqueous phase (B) in the tubular reactor is 50 g / l or more; and then c) distilling the reaction mixture obtained from the tubular reactor (R), and then d) separating the concentrated crude reaction mixture obtained from the distillation of step c) to obtain an alkaline aqueous phase and an organic phase containing isophorone; Preferably, a step of neutralizing the organic phase recovered at the end of the separation d) followed by e) distillation of the organic phase containing isophorone recovered in the preceding step in order to extract the polycondensation by-products mainly at the bottom of column (D2) and to recover a stream mainly comprising isophorone at the top of said column (D2), followed by f) distilling the polycondensation by-products from the bottom of the column (D2) obtained from the preceding distillation, followed by g) recycling the stream obtained from the top of the distillation column (D3) of the preceding step, which contains xylitol and / or isoxylitol, to said tubular reactor (R); A continuous process for the liquid-phase synthesis of isophorone by alkaline self-condensation of acetone, including:

2. 2. The process according to claim 1, characterized in that the aqueous alkali metal hydroxide solution (A) is an aqueous solution of sodium hydroxide or potassium hydroxide.

3. 3. Process according to claim 1 or claim 2, characterized in that the concentration of alkali metal hydroxide in the aqueous phase (B) present in the tubular reactor is between 50 and 200 g / l, preferably between 80 and 150 g / l and more preferentially between 100 and 150 g / l.

4. 4. The process according to any one of claims 1 to 3, characterized in that the concentration of alkali metal hydroxide in said aqueous solution (A) in the feeding zone is between 5 and 40 g / l, preferably between 10 and 40 g / l, more preferentially between 15 and 35 g / l.

5. 5. A process according to any one of claims 1 to 4, characterized in that the ratio of the mass flow rate of the aqueous alkali metal hydroxide stream (A) fed (Q alkali metal hydroxide) to the mass flow rate of the organic stream fed (Q organic) is between 0.25 and 1.0, preferably between 0.4 and 0.8, more preferentially between 0.5 and 0.

7.

6. 6. The process according to any one of claims 1 to 5, characterized in that the reaction temperature in the tubular reactor is between 180°C and 250°C, preferably between 200°C and 230°C, and the absolute pressure in the tubular reactor is between 30 and 50 bar, preferably between 35 and 45 bar, even more preferentially between 38 and 42 bar.

7. 7. The process according to any one of claims 1 to 6, characterized in that it comprises a step h) of distilling the stream recovered at the top of the distillation column (D2) of step e).

8. 8. Process according to claim 7, characterized in that it comprises a step i) of distilling the stream recovered at the bottom of the distillation column (D4) of step h).

9. 9. The process according to claim 8, characterized in that it comprises a step of recycling the stream recovered at the bottom of the distillation column (D5) of the preceding step to the distillation column (D2) of step e).

10. 10. The process according to any one of claims 7 to 9, characterized in that it comprises a step of decanting the stream recovered at the top of the distillation column (D4) of step h), after which the organic phase is recycled to the tubular reactor (R).

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