Liquid-phase isophorone synthesis process with by-product recycling

Abstract

The present invention relates to a continuous liquid-phase synthesis process for isophorone by alkaline autocondensation of acetone, comprising the following successive steps: a) continuous injection through a tubular reactor (R) of a stream of an aqueous solution of alkali hydroxide and a stream of an organic solution comprising acetone and by-products recycled via step g); b) condensation reaction of acetone within the tubular reactor (R); c) distillation of the reaction mixture from the tubular reactor (R); d) separation of the concentrated crude reaction from the distillation of step c) leading to an alkaline aqueous phase and an organic phase containing isophorone; e) distillation of the organic phase containing isophorone recovered in the previous step; f) distillation of the polycondensation by-products from the column foot (D2) from the previous distillation.then g) recycling of the stream from the column head of the previous distillation (D3) including xylitones and / or isoxylitones to the tubular reactor (R). [Fig. 1]

Description

Title of the invention: Process for the synthesis of isophorone in liquid phase with recycling of by-products. Technical field

[0001] The present invention relates to a process for the continuous synthesis of isophorone by alkaline autocondensation of acetone in liquid phase. Technical background

[0002] Isophorone (or 3,5,5-trimethylcyclohex-2-enone) is an α,[3-unsaturated] cyclic ketone increasingly used as a synthetic intermediate, notably for the manufacture of isophorone diamine used as a hardener for epoxy resins, isophorone diisocyanate used as a monomer in polyurethanes, 3,5-xylenol used as a precursor to PCMX (an antimicrobial agent), keto-isophorone, which is 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-point solvent for many natural and synthetic resins, used in the paints, inks, and varnishes industry. Isophorone is also a solvent used in agrochemicals for the formulation of emulsifiable pesticide concentrates.

[0003] Isophorone is classically obtained by catalytic autocondensation of 3 molecules of acetone, according to the following reaction: Oh

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

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

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

[0007] Due to the low solubility of the mineral base in acetone, processes are sought to promote contact between the acetone and the catalyst. Thus, it is known from US patent 2,344,226 that the synthesis can be carried out in a stirred reactor.

[0008]

[0009]

[0010] Document FR 1 238 954 discloses a synthesis using a tubular reactor with internal packing. Document US 2,399,976 discloses a synthesis using a tubular reactor equipped with a recirculation system. Documents CN 102367223 and CN 102516051 disclose a synthesis using a premixing system, such as a static mixer. It is also known from document FR 1042057 to replace the alkaline aqueous solution with an alkaline alcoholic solution. The synthesis can be carried out continuously in a tubular reactor without special mixing equipment by using very low weight concentrations of sodium hydroxide or potassium hydroxide. Generally, catalyst concentrations are less than 1% by weight, or even as low as 0.1% by weight relative to the total weight of the reaction mixture. This low concentration allows for single-phase mixing. These processes are described in documents FR 1 316 515, DD145096, EP 2 649 032, EP 2 707 352, and EP 2 837 618. The synthesis can also be carried out by reactive distillation via the injection of acetone and an aqueous solution of sodium hydroxide or potassium hydroxide into a reactive distillation column in such a way as to maintain a low concentration of sodium hydroxide or potassium hydroxide (< 0.1% by weight relative to the total weight of the reaction mixture) and to react the acetone countercurrently with the sodium hydroxide or potassium hydroxide. This process is described in documents FR 1 315 788, FR 2 271 191 and FR 2 328 686. Due to the stringent reaction conditions employed, the autocondensation of acetone is accompanied by the formation of polycondensation byproducts composed of four or more acetone molecules. To limit the production of these heavy derivatives, the synthesis is carried out with limited conversion to acetone. In addition to isophorone and the polycondensation derivatives, the crude reaction mixture also contains a more or less significant amount of synthetic intermediates, primarily mesityl oxide. O

[0011] US patent 2,344,226 describes the recycling of mesityl oxide in the reactor with unconverted acetone. US patent 2,351,352 describes a process in which mesityl oxide is separately downgraded to acetone by hydrolysis. in the presence of an alkaline aqueous solution in a reactive distillation column. In the specific case of the synthesis of isophorone via reactive distillation, mesityl oxide is retrograded in situ to acetone (FR 1 316 515).

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

[0013] The synthesis of IPHO by condensation of acetone in the gas phase is carried out at high temperature (200-400 °C) through a fixed bed of solid catalyst(s) such as, for example: calcium oxide and / or calcium hydroxide (FR 850 334), a mixed catalyst of magnesium and aluminum oxide (EP 0 640 387), a calcium aluminate (US 2,393,510), zeolites or magnesium oxide doped with alkali metals (JP 9151152, JP 9151153, JP 9169687, JP 9169688) or hydrotalcites (CN 106423124, CN 106423125, CN 106423126).

[0014] Regardless of the liquid or gas phase process used, due to the high reaction temperatures required for the synthesis of isophorone, the selectivity of the reaction is limited by the formation of polycondensation by-products with the empirical formula C3nH(4n+2)O (with n > 4), of which mainly are the xylitones and isoxylitones Ci2Hi8O and the compounds C15H22O.

[0015] For liquid phase syntheses, even with limited conversion, the selectivity in isophorone at the outlet of stirred or tubular reactors is at best 75%.

[0016] Only reactive distillation synthesis processes, which are accompanied by partial retrogradation of the polycondensation products in situ, as well as processes coupling a tubular reactor and a reactive distillation column for hydrolysis of heavy products, make it possible to achieve overall isophorone selectivities of between 85 and 91%.

[0017] But such reactive distillation equipment, which must operate under high pressures generally between 30 and 50 bars and require specific materials capable of withstanding the alkaline conditions of the reaction medium and temperatures above 200 °C, is particularly expensive.

[0018] Unlike liquid-phase processes, gas-phase processes can be operated at atmospheric pressure, but they have the major drawback of a decrease in reaction performance as the catalyst ages, due to fouling by polycondensation byproducts and coking caused by the use of high reaction temperatures. Industrial production is therefore significantly impacted by the frequent need to regenerate or replace the catalytic bed.

[0019] There is therefore a need for a selective process, with stable productivity and less costly in terms of investment than processes implementing one (or more) reactive distillation(s). Brief description of the invention

[0020] The present invention relates to a continuous liquid-phase synthesis process for isophorone by alkaline autocondensation of acetone, comprising the following successive steps: a) continuous injection through a tubular reactor (R) of a stream of an aqueous solution of alkali hydroxide (A) and a stream of an organic solution comprising acetone and by-products recycled via step g) then b) condensation reaction of acetone within the tubular reactor (R), the reactor containing an emulsion comprising predominantly an aqueous phase of alkali hydroxide (B), the concentration of alkali hydroxide of the aqueous phase (B) within the tubular reactor being greater than or equal to 50 g / L then c) distillation of the reaction mixture from the tubular reactor (R), then d) separation of the concentrated crude reaction product from the distillation of the step c) leading to an alkaline aqueous phase and an organic phase containing isophorone, preferably neutralization of the organic phase recovered after separation d) then e) distillation of the organic phase containing the isophorone recovered in the previous step in order to extract mainly the polycondensation by-products at the bottom of column (D2) and to recover at the top of column (D2) a stream comprising mainly the isophorone, then f) distillation of the polycondensation by-products from the column foot (D2) resulting from the previous distillation, then g) recycling of the stream from the column head of the previous distillation (D3) including xylitones and / or isoxylitones to the tubular reactor (R).

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

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

[0023] -the concentration of alkali 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 preferably between 100 and 150 g / L;

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

[0025] -the ratio of the weight flow rate of the fed aqueous alkali hydroxide solution (A) (Qalali hydroxide) and the weight flow rate of the fed organic flow (Qorga) is between 0.25 and 1.0, preferably between 0.4 and 0.8 and more preferably between 0.5 and 0.7;

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

[0027] -the process includes a distillation step h) of the stream recovered at the top of the distillation column (D2) of step e);

[0028] -the process includes a distillation step i) of the stream recovered at the bottom of the distillation column (D4) of step h);

[0029] -the process includes a step of recycling the stream recovered at the bottom of the distillation column (D5) of the previous step to the distillation column (D2) of step e);

[0030] -the process includes a step of decanting the stream recovered at the top of the distillation column (D4) of step h), then a step of recycling the organic phase to the tubular reactor (R).

[0031] The process according to the invention has the advantage of achieving levels of productivity and selectivity similar to those generally obtained with reactive distillations, but without resorting to this type of equipment. Indeed, the systematic recycling of fractions containing synthesis intermediates, such as mesityl oxide, and of fractions containing retrogradable by-products makes it possible to reach these productivity and selectivity thresholds. Brief description of the figure

[0032] [Fig. 1] is a diagram of the device implementing the claimed method. Detailed description

[0033] Other features, aspects, objects and advantages of the present invention will become even clearer upon reading the following description.

[0034] It is specified that the expressions "from ... to ..." and "between ... and ..." used in this description should be understood as including each of the limits mentioned.

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

[0036] Step a): Injection of the flows

[0037] The synthesis is carried out by continuous injection through a tubular reactor (R): -of a flow of an aqueous solution of alkali hydroxide (A) and -of a stream of an organic solution comprising acetone and recycled by-products via step g).

[0038] The aqueous solution of alkali hydroxide (A)

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

[0040] Preferably, the alkali hydroxide of the aqueous solution (A) is identical to the alkali hydroxide of the aqueous solution (B).

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

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

[0043] If the alkali hydroxide is sodium hydroxide, then the concentration of the aqueous sodium hydroxide solution in the feed is advantageously between 5 and 30 g / L, preferably between 10 and 30 g / L, and more preferably between 15 and 25 g / L.

[0044] If the alkali hydroxide is potassium hydroxide, then the concentration of the aqueous solution of potassium hydroxide in the feed is advantageously between 5 and 40 g / L, preferably between 10 and 40 g / L, and more preferably between 20 and 35 g / L.

[0045] Preferably, the start-up of the installation is carried out by first loading the reactor with an aqueous solution of alkali hydroxide (B).

[0046] The organic solution

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

[0048] By "recycled by-products" is meant the retrogradable polycondensation by-products under the isophorone synthesis conditions, namely xylitones and / or isoxylitones (Ci2Hi8O). Isoxylitones and xylitones comprise several isomers, including the molecules listed below:

[0049] These different isomers Ci2H[8O] are formed by condensation of isophorone with acetone or by autocondensation of mesityl oxide: + h2o

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

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

[0052] Besides fresh acetone and recycled acetone, the organic stream may include recycled reaction intermediates, such as mesityl oxide.

[0053] The ratio of the weight flow rate of the fed aqueous alkali hydroxide solution (A) (Qalali hydroxide) and the weight flow rate of the fed organic flow (Qorga) is advantageously between 0.25 and 1.0, preferably between 0.4 and 0.8 and more preferably between 0.5 and 0.7.

[0054] When the alkali hydroxide is sodium hydroxide, the ratio of the weight flow rate of the fed aqueous sodium hydroxide solution flow (QNaOH) and the weight flow rate of the fed organic flow (Qorga) is advantageously between 0.25 and 1.0, preferably between 0.4 and 0.8 and more preferably between 0.5 and 0.7.

[0055] When the alkali hydroxide is potassium hydroxide, the ratio of the weight flow rate of the fed aqueous potassium hydroxide solution flow (QKOH) and the weight flow rate of the fed organic flow (Qorga) is advantageously between 0.25 and 1.0, preferably between 0.4 and 0.8 and more preferably between 0.5 and 0.7.

[0056] Before entering the reactor, the streams can be preheated beforehand using heat exchangers.

[0057] Step b): reaction

[0058] The condensation reaction of acetone takes place within the tubular reactor (R), the reactor containing an emulsion comprising mainly an aqueous phase of alkali hydroxide (B), the concentration of alkali hydroxide of the aqueous phase (B) within the tubular reactor being greater than or equal to 50 g / L.

[0059] Preferably, the reactor is vertical.

[0060] The emulsion present within the reactor comprises:

[0061] -an aqueous phase (B), which is the continuous phase of the emulsion and which comprises water, acetone and alkali hydroxide, and

[0062] -an organic phase, which is the dispersed phase of the emulsion; it is preferably in the form of droplets and it comprises acetone, isophorone, possibly synthesis intermediates and polycondensation by-products.

[0063] The emulsion contains predominantly the aqueous phase of alkali hydroxide. By "predominantly", for the purposes of the present invention, it is understood that the aqueous phase of alkali hydroxide represents more than 50% by volume of the total volume of the emulsion present in the reactor.

[0064] The concentration of alkali hydroxide in the aqueous phase (B) within the tubular reactor is greater than 50 g / L.

[0065] Unlike liquid-phase processes catalyzed with sodium hydroxide or potassium hydroxide described in the literature, which employ conditions to obtain the most homogeneous reaction phase possible, the process according to the invention carries out a reaction in a heterogeneous medium. This increases the isophorone selectivity.

[0066] The heterogeneous reaction medium consists of a continuous aqueous phase concentrated in alkali hydroxide through which the organic phase passes in the form of droplets, preferably ascending. The much lower solubility of isophorone than that of acetone in this concentrated alkaline aqueous phase thus limits the formation of polycondensation byproducts.

[0067] The reaction medium is thus heterogeneous. It comprises a major aqueous phase of alkali hydroxide and a minor organic phase comprising acetone, isophorone and possibly recycled organic by-products, and possibly synthesis intermediates.

[0068] The reaction temperature within the tubular reactor can be between 180 and 250 °C, and preferably between 200 and 230 °C, and under an absolute pressure between 30 and 50 bars, preferably between 35 and 45 bars, and even more preferably between 38 and 42 bars.

[0069] The concentration of alkali hydroxide in the aqueous phase (B) present in the tubular reactor can be between 50 and 200 g / L, preferably between 80 and 150 g / L and more preferably between 100 and 150 g / L.

[0070] When the alkali hydroxide is sodium hydroxide, the concentration of sodium hydroxide in the aqueous phase (B) present within the tubular reactor is preferably between 50 and 200 g / L, preferably between 80 and 150 g / L and more preferably between 100 and 120 g / L.

[0071] When the alkali hydroxide is potassium hydroxide, the concentration of potassium hydroxide in the aqueous phase (B) present within the tubular reactor can be between 50 and 200 g / L, preferably between 80 and 150 g / L and more preferably between 125 and 150 g / L.

[0072] The tubular reactor R may, where appropriate, consist of several tubular reactors fed in parallel.

[0073] The reaction mixture is recovered at the outlet of the tubular reactor and conveyed to a distillation column.

[0074] Distillations

[0075] In the process according to the invention, the distillation columns preferably comprise a boiler at the bottom of the column and a condenser at the top of the column. The columns may be tray columns or packed columns.

[0076] Advantageously, the distillations are carried out under reduced pressure.

[0077] Distillation under reduced pressure corresponds to distillation carried out under an absolute pressure of less than 1013 mbar, preferably less than 250 mbar and more preferably between 10 and 100 mbar.

[0078] Preferably, the process according to the invention does not involve reactive distillation.

[0079] Step c): Distillation 1

[0080] The reaction mixture, recovered from the reactor outlet, is distilled through a DI column. The unconverted acetone is recovered at the top of the column and the concentrated crude reaction product is drawn off at the bottom. Preferably, the distillation is carried out at atmospheric pressure.

[0081] Possible Recycling of Elementary Supp

[0082] The acetone recovered at the top of column DI is advantageously recycled to the tubular reactor R, in whole or in part.

[0083] Step d): Separation

[0084] The concentrated crude reaction product withdrawn from the bottom of the DI distillation column is separated, preferably by decantation. The alkaline aqueous phase is separated from the organic phase, rich in isophorone, preferably using a decanter.

[0085] Possible Additional Recycling

[0086] The alkaline aqueous phase recovered in the separation step d) is advantageously recycled to the reaction step b), in whole or in part and preferably in part.

[0087] Possible neutralization of the organic phase

[0088] Any alkali hydroxide present in the isophorone-rich organic phase recovered in separation step d) can be neutralized. This neutralization can be carried out by any technique known to those skilled in the art, but preferably by means of a mineral acid providing a buffering effect. Preferably, phosphoric acid is used.

[0089] Step e): Distillation 2

[0090] The organic phase containing the isophorone recovered in the separation step d), and then possibly neutralized, is distilled, preferably under reduced pressure, in order to extract mainly at the bottom of column D2 the polycondensation by-products and to recover at the top of column D2 a stream comprising mainly the isophorone.

[0091] Step f): Distillation 3

[0092] The fraction containing the polycondensation by-products recovered from the bottom of the column in the previous distillation step D2 is preferably distilled under reduced pressure. The fraction from the top of the column in the previous distillation D3 preferably comprises mainly xylitones and / or isoxylitones, and the fraction at the bottom of the column preferably comprises mainly polycondensation by-products with the empirical formula Ci5H22O.

[0093] The polycondensation by-products Ci5H22O include several isomers, including the molecules listed below:

[0094] These Ci5H22O derivatives are formed by condensation of isophorone with mesityl oxide or by condensation of xylitones or isoxylitones Ci2Hi8O with acetone: O

[0095] The heavy reaction by-products SLen column foot D3 include these by-products Ci5H22O as well as their higher homologues C3nH(4n+2)O (with n > 6).

[0096] Step g): Recycling

[0097] The fraction from the top of the column of the previous distillation D3 comprising, preferably predominantly, xylitones and / or isoxylitones, is recycled to the tubular reactor R. This fraction is added to the continuous flow of the organic phase fed into the reactor.

[0098] Possible bleaching treatment

[0099] The flux recovered at the top of column D2 can be subjected to a decolorization treatment.

[0100] This decolorization treatment consists of transforming certain reaction intermediates and / or by-products comprising unsaturated hydrocarbon and conjugated carbonyl chains that are difficult to separate from isophorone by distillation. Their residual presence could therefore generate a yellowish coloration of the 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 by-products in question.

[0101] Preferably, the decolorization treatment includes a reaction step with an acid. The isophorone stream extracted at the top of column D2 is subjected to continuous hot treatment in the presence of a catalytic amount of a strong mineral acid, such as sulfuric acid.

[0102] The residual sulfuric acid can then advantageously be neutralized by adding a strong mineral base such as the alkaline aqueous solution, preferably that from the separation step d).

[0103] Possible distillations 4 and 5: steps h) and i)

[0104] The process according to the invention may include a distillation step h) of the stream recovered at the top of the distillation column (D2) of step e). This stream recovered at the top of column (D2) may be sent to a subsequent distillation column, optionally after a decolorization step.

[0105] The stream, comprising mainly isophorone and recovered at the top of the distillation column D2, can undergo distillation, preferably under reduced pressure, in a column D4 allowing the extraction at the top of the column of light impurities, such as residual acetone and water, mesityl oxide and 1,3,5-trimethylbenzene.

[0106] The extracted flow at the top of column D4 can be decanted through the decanter in order to separate an aqueous phase, which is sent to a wastewater treatment SE and an organic phase comprising mainly mesityl oxide and isophorone.

[0107] This organic phase, consisting mainly of mesityl oxide and isophorone, can be recycled in the reaction step.

[0108] The process according to the invention may include a distillation step i) of the stream recovered at the bottom of the distillation column (D4) of step h). The fraction recovered at the bottom of column D4 can feed a fifth distillation column, which makes it possible to obtain at the top of the column isophorone of a purity greater than 99% and at the bottom of the column residual polycondensation by-products.

[0109] These residual polycondensation by-products can be recycled in the distillation column D2. Description of the figure

[0110] [Fig-1] represents an embodiment of steps a) to g) of the process according to the invention.

[0111] Acetone is introduced via line 1 into heat exchanger E1. The aqueous solution of alkali hydroxide is introduced via line 2 into heat exchanger E2. The preheated streams are recovered in a line 3 and introduced into tubular reactor R.

[0112] The reaction mixture from reactor R is introduced into the distillation column DI via the line 4.

[0113] The acetone that has not been converted within the reactor is recovered at the top of column DI. This fraction is recycled via line 5 to line 1.

[0114] The concentrated crude reaction product is recovered at the bottom of column DI and is brought to the decanter dl via line 6.

[0115] The decanter dl separates the aqueous phase from the organic phase. The alkaline aqueous phase is removed via line 8, then line 9. A purge p is introduced to remove the water co-produced by the condensation reaction. Line 9 recycles the alkaline aqueous phase to line 2.

[0116] The organic phase from the decanter dl is brought to the neutralizer N via the line 7. The neutralized organic phase is brought to the distillation column D2 via the line 10.

[0117] Distillation under reduced pressure using distillation column D2 allows the recovery at the top of column D2 of a stream consisting mainly of isophorone, which is transferred via line 11 to distillation column D4. At the bottom of column D2, the recovered fraction containing the polycondensation by-products is transferred via line 12 to distillation column D3.

[0118] Distillation under reduced pressure of the polycondensation by-products within the distillation column D3 allows for the recovery at the top of column D3 of a stream preferably comprising xylitones and / or isoxylitones. This fraction is recycled via line 13 to line 1.

[0119] The heavy reaction by-products SL are recovered at the bottom of column D3.

[0120] The stream recovered at the top of column D2, consisting mainly of isophorone, feeds the distillation column D4. Distillation under reduced pressure in column D4 allows the extraction at the top of the column of light impurities such as residual acetone and water, mesityl oxide, and 1,3,5-trimethylbenzene. This fraction is discharged via line 15 to the decanter d2. The aqueous phase The product from the settling tank d2 is sent via pipe 17 to a wastewater treatment SE and the organic phase consisting mainly of mesityl oxide and isophorone is recycled via pipe 16 to pipe 1.

[0121] The fraction recovered at the bottom of column D4 feeds column D5 via line 18. The fraction recovered at the top of column D5 contains isophorone with a purity greater than 99%. The fraction withdrawn from the bottom of column D5 contains residual polycondensation by-products, which are recycled via line 20 into the distillation column D2.

[0122] Consequently, the organic phase stream feeding reactor R comprises fresh acetone, the light fraction from distillation column DI and recycled via line 5, the light fraction from distillation column D3 and recycled via line 13 and the organic phase from decanter d2 and recycled via line 16.

[0123] The alkaline aqueous solution (A) feeding reactor R comprises a fresh aqueous solution of alkali hydroxide and the aqueous phase from the decanter dl and recycled via line 9.

[0124] The following examples are intended to illustrate the present invention, but are in no way limiting. Examples Example no. 1:

[0125] The synthesis of isophorone is carried out in a vertical tubular reactor made of 316L stainless steel with a volume of 815 mL with an L / D ratio of 3, equipped with a lateral tube positioned at L / 2 allowing a sample to be taken from the middle of the reactor.

[0126] The alkaline aqueous phase and the organic phase consisting of fresh acetone, recycled acetone and where applicable recycled mesityl oxide and recycled polycondensation products are respectively preheated through 2 electric heat exchangers so as to reach the desired reaction temperature within the reactor.

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

[0128] The reagents are supplied by means of piston pumps.

[0129] The conversions and selectivities are established after a continuous run of at least 24 h in order to guarantee stabilized reaction conditions within the reactor.

[0130] Table 1 below indicates the operating conditions of the tests carried out under absolute pressure of 40 bars in the tubular reactor with a sodium hydroxide concentration of 20 g / L of the aqueous solution fed.

[0131] In Tables 1 and 2 below, OM stands for mesityl oxide; Ci2 denotes by-products of Ci2 reactions, i.e., xylitones and / or isoxylitones; Ci5 denotes by-products of Ci5 reactions, such as Ci5H22O; C[8 denotes by-products of Ci8 reactions; ACE stands for acetone; IPHO stands for isophorone, and the selectivities are defined as follows:

[0132] SP = selectivity in product P with respect to converted acetone

[0133] Sh>ho = 100 x 3 x (number of moles of IPHO formed) / number of moles of ACE converted

[0134] Som = 100 x 2 x (number of moles of MO formed) / number of moles of ACE converted

[0135] SC12 = 100 x 4 x (number of moles of Ci2Hi8O formed) / number of moles of ACE converted

[0136] SC15 = 100 x 5 x (number of moles of Ci5H22O formed) / number of moles of ACE converted

[0137] SC[8 = 100 x 6 x (number of moles of Ci8H26O formed) / number of moles of ACE converted

[0138] with: number of moles of P formed = (number of moles of P at reactor outlet - number of moles of P recycled to reactor feed)

[0139] The conversions and selectivities are calculated on the basis of the mass compositions of the crude mixtures at the outlet of the reaction zone; the compositions are determined by gas chromatographic analyses. Flow rates of recycled products Weight ratio QnsOH aq / Qorga Average reaction temperature Cnboh in the reaction medium OM a+p C12 totals ^15 totals (g / h) (g / h) (g / h) °C (g / L) 1 comp - - - 0.66 216 122 2 comp 10.6 - - 0.68 216 122 3 0.5 19.2 5.5 0.73 216 93 4 11.1 35.6 10.3 0.67 215 102

[0140] Table 1

[0141] Test 1 is a comparative test, it illustrates a process without recycling.

[0142] Test 2 is also a comparative test; it only recycles fractions containing mesityl oxide.

[0143] Test 3 is according to the invention, it recycles only the fractions containing xylitones and / or isoxylitones.

[0144] Test 4 is according to the invention, it recycles all the fractions containing mesityl oxide and xylitones and / or isoxylitones.

[0145] Tests 3 and 4 use a succession of 5 distillation columns and it recycles the fractions containing impurities in Ci2, which are retrogradable and the impurities in C15.

[0146] Table 2 below shows the results obtained. ACE Selectivities / ACE Productivity Conversion IPHO IPHO OM, +p C12 totals ^15 totals Cl8 totals (%) (%) (%) (%) (%) (%) (kg.h'.L0 1 comp 19.3 74.4 11.0 6.1 7.3 1.1 0.086 2 comp 17.1 81.4 0.9 7.2 8.9 1.4 0.086 3 24.0 80.4 7.2 0.0 9.9 2.5 0.115 4 18.9 85.6 -0.5 0.9 12.6 1.0 0.091

[0147] Table 2

[0148] The results of the selectivities in mesityl oxide (MO) and xylitones and / or isoxylitones (total Ci2) show that the recycling of mesityl oxide and xylitones and / or isoxylitones can lead to the total suppression of their respective productions.

[0149] Example 3 shows that recycling only the fraction containing xylitones and / or isoxylitones (total Ci2) allows for an increase in isophorone selectivity, an increase in the conversion rate and an increase in productivity.

[0150] Example 4 shows that the isophorone selectivity (IPHO) of 74.4% without recycling thus increases to 85.6% with recycling of mesityl oxide (OM) and xylitones and / or isoxylitones (total Ci2)-

[0151] The negative selectivity in mesityl oxide expresses the fact that the amount of mesityl oxide at the outlet of the tubular reactor is less than that at the inlet; this indicates that not only has recycling prevented the formation of mesityl oxide, but has also made it possible to backgrade the excess of recycled product relative to the reaction equilibrium.

[0152] These tests demonstrate the decisive impact of recycling mesityl oxide and xylitones and isoxylitones Ci2Hi8O on isophorone selectivity and productivity. Example No. 2:

[0153] The operating procedure followed is analogous to that of example no. 1 but using a vertical tubular reactor made of 316L stainless steel with a volume of 940 mL with an L / D ratio of 18.5 and equipped with 3 lateral tubes positioned at L / 3, L / 2 and 2L / 3 allowing samples to be taken at one-third of the length of the reactor, in the middle of the reactor and at two-thirds of the length of the reactor.

[0154] Table 3 below indicates the operating conditions of the tests: Flow rates of recycled products Weight ratio QnsOH aq / Qorga Average reaction temperature CnbOH medium (h aut) reactor OM a+p ri? totals C1 5 totals (g / h) (g / h) (g / h) °C (g / L) 5 comp - - - 0.68 217 118(118) 6 24.2 34.0 4.0 0.68 216 106(112)

[0155] Table 3

[0156] Test 5 is a comparative test; it illustrates a process without recycling.

[0157] Test 6 is according to the invention. The procedure followed is that illustrated in [Fig. 1]. uses a succession of 5 distillation columns and recycles the fractions containing impurities into Ci2, which are retrogradable, and the impurities into Ci5.

[0158] Table 4 below shows the results obtained. ACE Selectivities / ACE Productivity IPHO IPHO OM a+p C12 totals C15 totals C18 totals (%) (%) (%) (%) (%) (%) (kg.h *.L 0 5 comp 19.8 75.3 10.8 6.5 7.1 0.3 0.133 6 17.4 93.0 -4.6 -3.5 6.8 0.2 0.137

[0159] Table 4

[0160] As in Example 1, sufficiently extensive recycling of xylitones and / or isoxylitones completely eliminates their formation. Note that negative selectivities for mesityl oxide and Ci2H[8O] indicate that the amounts of mesityl oxide and Ci2H18O exiting the tubular reactor are lower than those at the inlet; this indicates that recycling not only prevented the formation of mesityl oxide and Ci2H18O but also reversed the excess of recycled products relative to the reaction equilibrium.

[0161] Furthermore, it should be noted that these recyclings do not increase the formation of heavy higher condensation by-products Ci8H26O.

[0162] The isophorone selectivity of 75.3% without recycling thus increases to 93.0% with recycling of mesityl oxide (MO) and xylitones and / or isoxylitones (total Ci2)-

[0163] The saving on acetone consumed under the conditions of test 6 compared to acetone consumed under the conditions of test 5 is thus 0.32 kg of acetone per kg of isophorone produced.

Claims

Demands

1. A continuous liquid-phase synthesis process for isophorone by alkaline autocondensation of acetone comprising the following successive steps: a) continuous injection through a tubular reactor (R): - of a stream of an aqueous solution of alkali hydroxide (A), and - of a stream of an organic solution comprising acetone and by-products recycled via step g) then b) condensation reaction of acetone within the tubular reactor (R), the reactor containing an emulsion comprising predominantly an aqueous phase of alkali hydroxide (B), the concentration of alkali hydroxide of the aqueous phase (B) within the tubular reactor being greater than or equal to 50 g / L then c) distillation of the reaction mixture from the tubular reactor (R), then d) separation of the concentrated crude reaction product from the distillation of step c) leading to an alkaline aqueous phase and an organic phase containing isophorone,preferably neutralization of the organic phase recovered after separation d) then e) distillation of the organic phase containing the isophorone recovered in the previous step in order to extract mainly the polycondensation by-products from the bottom of column (D2) and to recover from the top of column (D2) a stream mainly comprising the isophorone, then f) distillation of the polycondensation by-products from the bottom of column (D2) from the previous distillation, then g) recycling of the stream from the top of column of the previous distillation (D3) comprising xylitones and / or isoxylitones to the tubular reactor (R).

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

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

4. A process according to any one of the preceding claims, characterized in that the concentration of the aqueous solution of alkali hydroxide (A) at the feed is between 5 and 40 g / L, preferably between 10 and 40 g / L, and more preferably between 15 and 35 g / L.

5. A method according to any one of the preceding claims, characterized in that the ratio of the weight flow rate of the fed aqueous alkali hydroxide solution (A) (Qalali hydroxide) and the weight flow rate of the fed organic stream (Qorga) is between 0.25 and 1.0, preferably between 0.4 and 0.8 and more preferably between 0.5 and 0.

7.

6. A method according to any one of the preceding claims, characterized in that the reaction temperature within the tubular reactor is between 180 and 250 °C, and preferably between 200 and 230 °C, and the absolute pressure within the tubular reactor is between 30 and 50 bars, preferably between 35 and 45 bars, and even more preferably between 38 and 42 bars.

7. A method according to any one of the preceding claims, characterized in that it comprises a distillation step h) of the stream recovered at the top of the distillation column (D2) of step e).

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

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

10. A 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), and then a step of recycling the organic phase to the tubular reactor (R).

Images