Process for the synthesis of isophorone in the liquid phase, including recycling of the by-products

Abstract

The present invention relates to a continuous process for the synthesis of isophorone in the liquid phase by alkaline self-condensation of acetone, comprising the following successive steps: a) continuously injecting, through a tubular reactor (R), a stream of an aqueous alkali hydroxide solution and a stream of an organic solution comprising acetone and by-products recycled in step g), followed by b) condensation reaction of the acetone in the tubular reactor (R), then c) distillation of the reaction mixture from the tubular reactor (R), then d) separation of the concentrated raw reaction product from the distillation of step c), resulting in an alkaline aqueous phase and an organic phase containing isophorone, then e) distillation of the organic phase containing the isophorone recovered in the preceding step, then f) distillation of the polycondensation sub-products from the bottom of the column (D2) from the preceding distillation, then g) recycling, to the tubular reactor (R), of the stream from the column head of the preceding distillation (D3), said stream comprising xylitones and / or isoxylitones.

Description

[0001] Liquid-phase isophorone synthesis process with by-product recycling

[0002] Technical field

[0003] The present invention relates to a process for the continuous synthesis of isophorone by alkaline self-condensation of acetone in the liquid phase.

[0004] Technical background

[0005] Isophorone (or 3,5,5-trimethylcyclohex-2-enone) is an a,p-unsaturated cyclic ketone that is increasingly used as a synthesis intermediate, notably for the manufacture of isophorone diamine used as a hardener for epoxy resins, isophorone diisocyanate used as a polyurethane monomer, 3,5-xylenol used as a precursor to PCMX (an antimicrobial agent), keto-isophorone, which is an intermediate in the synthesis of 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 agrochemistry for the formulation of emulsifiable concentrates of pesticides.

[0006] Isophorone is classically obtained by catalytic self-condensation of 3 acetone molecules, according to the following reaction:

[0007] The reaction is carried out in the liquid phase or in the gas phase.

[0008] The gas phase processes described in the literature mainly use solid heterogeneous catalysts, whereas the liquid phase processes use homogeneous or heterogeneous catalytic systems.

[0009] The synthesis of isophorone by condensation of acetone in the liquid phase is carried out almost exclusively under alkaline conditions at high temperature and under high pressure; alkaline catalysis is most often carried out by using an aqueous solution of sodium hydroxide or potassium hydroxide.

[0010] Due to the low solubility of the mineral base in acetone, methods are being sought to promote contact between the acetone and the catalyst. Thus, it is known from document US 2,344,226 to carry out the synthesis in a stirred reactor. Document FR 1 238954 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.

[0011] 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, the catalyst concentrations are less than 1% by weight, or even around 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, EP 2 837 618.

[0012] 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 so 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.

[0013] Due to the drastic reaction conditions used, the self-condensation reaction of acetone is accompanied by the formation of polycondensation by-products consisting of a number of acetone molecules greater than or equal to 4. To limit the production of these heavy derivatives, the synthesis is carried out with limited conversion to acetone. In addition to isophorone and polycondensation derivatives, the reaction crude also contains a more or less significant quantity of synthesis intermediates, mainly mesityl oxide.

[0014] US 2,344,226 describes the recycling of mesityl oxide into the reactor with unconverted acetone. US 2,351,352 describes a process in which mesityl oxide is separately retrograded 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).

[0015] Document US 2,419,051 describes the partial retrogradation of polycondensation products into acetone and isophorone by alkaline hydrolysis in a stirred reactor. This retrogradation is also described in documents FR 1 316 514, FR 1 316 515, EP 2 649 032, EP 2 707 352, EP 2 837 618 via a reactive hydrolysis distillation column.

[0016] 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 oxide and aluminum (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).

[0017] 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 of the empirical formula C3nH(4n+2)O (with n > 4), mainly xylitones and isoxylitones C11HigO and compounds C15H22O.

[0018] For liquid phase syntheses, even with limited conversion, the selectivity to isophorone at the outlet of stirred or tubular reactors is at best 75%.

[0019] 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%.

[0020] But such reactive distillation equipment, which has to operate under high pressures generally between 30 and 50 bars and requires specific materials capable of withstanding the alkaline conditions of the reaction medium and temperatures above 200°C, is particularly expensive.

[0021] Unlike liquid-phase processes, gas-phase processes can be operated at atmospheric pressure, but they have the major disadvantage of a decrease in reaction performance as the catalyst ages, due to fouling by polycondensation by-products and coking caused by the use of high reaction temperatures. Industrial production is therefore strongly impacted by a frequent need to regenerate or change the catalyst bed.

[0022] There is therefore a need for a selective process, with stable productivity and less costly investments than processes using one (or more) reactive distillation(s).

[0023] Brief description of the invention

[0024] The present invention relates to a continuous process for the liquid-phase synthesis of isophorone by alkaline self-condensation 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 the acetone within the tubular reactor (R), the reactor containing an emulsion comprising mainly an aqueous phase of alkali hydroxide (B), the alkali hydroxide concentration 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 reaction crude 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 at the end of separation d) then e) distillation of the organic phase containing the isophorone recovered in the previous step in order to extract mainly at the bottom of the column (D2) the polycondensation by-products and to recover at the top of the column (D2) a stream mainly comprising isophorone, then f) distillation of the polycondensation by-products from the bottom of the column (D2) resulting from the previous distillation, then g) recycling of the stream from the top of the column of the previous distillation (D3) comprising xylitones and / or isoxylitones to the tubular reactor (R).,

[0025] Other advantageous characteristics of the method according to the invention are specified below:

[0026] - the aqueous solution of alkali hydroxide (A) is an aqueous solution of sodium hydroxide or potassium hydroxide;

[0027] -the alkali hydroxide concentration of 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;

[0028] -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;

[0029] -the ratio of the weight flow rate of the flow of aqueous alkali hydroxide solution (A) supplied (Qalkali hydroxide) and the weight flow rate of the organic flow supplied (Qorga) is between 0.25 and 1.0, preferably between 0.4 and 0.8 and more preferably between 0.5 and 0.7;

[0030] -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;

[0031] -the process comprises a distillation step h) of the stream recovered at the top of the distillation column (D2) of step e);

[0032] -the process comprises a distillation step i) of the stream recovered at the bottom of the distillation column (D4) of step h);

[0033] -the method comprises a step of recycling the flow recovered at the bottom of the distillation column (D5) from the previous step to the distillation column (D2) of step e);

[0034] -the process comprises a step of decanting the flow 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).

[0035] 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 comprising synthesis intermediates, such as mesityl oxide and fractions comprising retrogradable by-products makes it possible to achieve these productivity and selectivity thresholds.

[0036] Brief description of the figure

[0037] Figure 1 is a diagram of the device implementing the claimed method. Detailed description

[0038] Other features, aspects, objects and advantages of the present invention will become even more apparent from the following description.

[0039] It is specified that the expressions “from ... to ...” and “between ... and ....” used in this description must be understood as including each of the limits mentioned.

[0040] The process according to the invention comprises the seven consecutive steps mentioned above: steps a) to g). This process may include additional purification steps.

[0041] Step a): injection of flows

[0042] 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 flow of an organic solution comprising acetone and by-products recycled via step g).

[0043] Aqueous solution of alkali hydroxide (A)

[0044] The alkali hydroxide used is preferably sodium hydroxide or potassium hydroxide, and more preferably sodium hydroxide in the form of an aqueous sodium hydroxide solution.

[0045] Preferably, the alkali hydroxide of aqueous solution (A) is identical to the alkali hydroxide of aqueous solution (B).

[0046] The alkali hydroxide concentration 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.

[0047] Preferably, 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.

[0048] 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.

[0049] If the alkali hydroxide is potassium hydroxide, then the concentration of the aqueous potassium hydroxide solution 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.

[0050] Preferably, the installation is started by first loading the reactor with an aqueous solution of alkali hydroxide (B).

[0051] The organic solution

[0052] The organic stream includes acetone, recycled by-products and possibly recycled reaction intermediates.

[0053] By "recycled by-products" we mean the polycondensation by-products that are retrogradable under the conditions of isophorone synthesis, namely xylitones and / or isoxylitones (C11H10O). Isoxylitones and xylitones include several isomers including the molecules below:

[0054] These different CI2HI8O isomers are formed by condensation of isophorone with acetone or by self-condensation of mesityl oxide:

[0055] In order to optimize the selectivity for isophorone, 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%.

[0056] As developed below, the organic stream may also include recycled acetone, originating from one or more distillations of the process.

[0057] In addition to fresh and recycled acetone, the organic stream may include recycled reaction intermediates, such as mesityl oxide.

[0058] The ratio of the weight flow rate of the flow of aqueous alkali hydroxide solution (A) supplied (Qalkali hydroxide) and the weight flow rate of the organic flow supplied (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.

[0059] When the alkali hydroxide is sodium hydroxide, the ratio of the weight flow rate of the supplied aqueous sodium hydroxide solution stream (QNaOH) to the weight flow rate of the supplied organic stream (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.

[0060] When the alkali hydroxide is potassium hydroxide, the ratio of the weight flow rate of the supplied aqueous potassium hydroxide solution stream (QKOH) to the weight flow rate of the supplied organic stream (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.

[0061] Before entering the reactor, the streams can be preheated using heat exchangers.

[0062] Step b): reaction

[0063] The acetone condensation reaction takes place within the tubular reactor (R), the reactor containing an emulsion comprising mainly an aqueous phase of alkali hydroxide (B), the alkali hydroxide concentration of the aqueous phase (B) within the tubular reactor being greater than or equal to 50 g / L. Preferably, the reactor is vertical.

[0064] The emulsion present within the reactor includes:

[0065] - an aqueous phase (B), which is the continuous phase of the emulsion and which comprises water, acetone and alkali hydroxide, and

[0066] - 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.

[0067] The emulsion contains predominantly the aqueous alkali hydroxide phase. For the purposes of the present invention, the term "predominantly" means that the aqueous alkali hydroxide phase represents more than 50% by volume relative to the total volume of the emulsion present in the reactor.

[0068] The alkali hydroxide concentration of the aqueous phase (B) within the tubular reactor is greater than 50 g / L.

[0069] Unlike the liquid phase processes catalyzed with sodium hydroxide or potassium hydroxide described in the literature, which implement conditions allowing for the most homogeneous reaction phase possible, the process according to the invention operates a reaction in a heterogeneous medium. This makes it possible to increase the selectivity for isophorone.

[0070] The heterogeneous reaction medium is formed by the continuous aqueous phase concentrated in alkali hydroxide crossed by the organic phase in the form of droplets, preferably ascending. The solubility of isophorone, much lower than that of acetone in this concentrated alkaline aqueous phase, thus limits the formation of polycondensation by-products.

[0071] The reaction medium is thus heterogeneous. It comprises a majority aqueous alkali hydroxide phase and a minority organic phase comprising acetone, isophorone and possibly recycled organic by-products, and possibly synthesis intermediates.

[0072] The reaction temperature within the tubular reactor may be between 180 and 250°C, and preferably between 200 and 230°C, and under an absolute pressure of between 30 and 50 bars, preferably between 35 and 45 bars, and even more preferably between 38 and 42 bars.

[0073] The alkali hydroxide concentration of the aqueous phase (B) present in the tubular reactor may be between 50 and 200 g / L, preferably between 80 and 150 g / L and more preferably between 100 and 150 g / L.

[0074] When the alkali hydroxide is sodium hydroxide, the sodium hydroxide concentration of 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.

[0075] When the alkali hydroxide is potassium hydroxide, the potassium hydroxide concentration of the aqueous phase (B) present within the tubular reactor may be between 50 and 200 g / L, preferably between 80 and 150 g / L and more preferably between 125 and 150 g / L.

[0076] The tubular reactor R may, if necessary, consist of several tubular reactors supplied in parallel.

[0077] The reaction mixture is recovered at the outlet of the tubular reactor and directed to a distillation column.

[0078] Distillations

[0079] 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.

[0080] Advantageously, the distillations are carried out under reduced pressure.

[0081] 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.

[0082] Preferably, the process according to the invention does not involve reactive distillation.

[0083] Step c): Distillation 1

[0084] The reaction mixture, recovered at the reactor outlet, is distilled through a D1 column. The unconverted acetone is recovered at the top of the column and the concentrated reaction crude is withdrawn at the bottom of the column. Distillation is preferably carried out at atmospheric pressure.

[0085] Possible additional recycling

[0086] The acetone recovered at the top of column D1 is advantageously recycled to the tubular reactor R, in whole or in part.

[0087] Step d): Separation

[0088] The concentrated reaction crude drawn off at the bottom of distillation column D1 is separated, preferably by decantation. The alkaline aqueous phase is separated from the organic phase, rich in isophorone, preferably using a decanter.

[0089] Possible additional recycling

[0090] The alkaline aqueous phase recovered in separation step d) is advantageously recycled to reaction step b), in whole or in part and preferably in part.

[0091] Possible neutralization of the organic phase

[0092] Any alkali hydroxide present in the isophorone-rich organic phase recovered in separation step d) may be neutralized. This neutralization may be carried out by any technique known to those skilled in the art, but preferably by means of a mineral acid providing a buffer effect. Preferably, phosphoric acid is used.

[0093] Step e): Distillation 2 The organic phase containing the isophorone recovered in separation step d), then optionally neutralized, is distilled, preferably under reduced pressure, 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 mainly comprising isophorone.

[0094] Step f): Distillation 3

[0095] The fraction comprising the polycondensation by-products recovered at the bottom of the column from the previous distillation step D2 is preferably distilled under reduced pressure. The fraction from the top of the column from the previous distillation step D3 preferably comprises mainly xylitones and / or isoxylitones and the fraction at the bottom of the column preferably comprises mainly polycondensation by-products of empirical formula C15H22O.

[0096] The polycondensation by-products C15H22O include several isomers including the molecules below:

[0097] These C15H22O derivatives are formed by condensation of isophorone with mesityl oxide or by condensation of xylitones or isoxylitones C^HigO with acetone:

[0098] Heavy reaction by-products S L at the bottom of column D3 include these by-products C15H22O as well as their higher counterparts C3nH(4n+2)O (with n > 6).

[0099] Step g): Recycling

[0100] The fraction from the top of the column of the previous distillation D3, preferably comprising mainly 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.

[0101] Possible bleaching treatment

[0102] The flow recovered at the top of column D2 can be subjected to a decolorization treatment.

[0103] This bleaching treatment consists of transforming certain reaction intermediates and / or by-products comprising unsaturated and conjugated carbonyl hydrocarbon 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 the olefinic bonds or for polycondensing the incriminated by-products.

[0104] Preferably, the decolorization treatment comprises a reaction step with an acid. The isophorone stream extracted at the top of column D2 is subjected to a continuous hot treatment in the presence of a catalytic amount of a strong mineral acid, such as sulfuric acid.

[0105] The residual sulfuric acid can then be advantageously neutralized by adding a strong mineral base such as the aqueous alkaline solution, preferably that resulting from separation step d).

[0106] Possible distillations 4 and 5: steps h) and i)

[0107] The process according to the invention may comprise a distillation step h) of the flow recovered at the top of the distillation column (D2) from step e). This flow recovered at the top of the column (D2) may be sent to a subsequent distillation column, possibly after a decolorization step.

[0108] The stream, comprising mainly isophorone and recovered at the top of distillation column D2, can undergo distillation, preferably under reduced pressure, in a column D4 allowing light impurities to be extracted at the top of the column, such as residual acetone and water, mesityl oxide and 1,3,5-trimethylbenzene.

[0109] The stream extracted 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 S E and an organic phase comprising mainly mesityl oxide and isophorone.

[0110] This organic phase comprising mainly mesityl oxide and isophorone can be recycled to the reaction stage.

[0111] The process according to the invention may comprise a distillation step i) of the flow recovered at the bottom of the distillation column (D4) from step h). The fraction recovered at the bottom of the column D4 may feed a fifth distillation column, which makes it possible to obtain at the top of the column isophorone with a purity greater than 99% and at the bottom of the column residual polycondensation by-products.

[0112] These residual polycondensation by-products can be recycled into the D2 distillation column.

[0113] Description of the figure

[0114] Figure 1 represents an embodiment of steps a) to g) of the method according to the invention.

[0115] Acetone is introduced via line 1 into heat exchanger E1. The aqueous alkali hydroxide solution is introduced via line 2 into heat exchanger E2. The preheated streams are recovered in line 3 and introduced into tubular reactor R.

[0116] The reaction mixture from reactor R is introduced into distillation column DI via line 4.

[0117] The acetone that has not been converted within the reactor is recovered at the top of column D1. This fraction is recycled via line 5 to line 1. The concentrated reaction crude is recovered at the bottom of column DI and is brought to decanter d1 via line 6.

[0118] 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.

[0119] The organic phase from decanter dl is brought to neutralizer N via line 7. The neutralized organic phase is brought to distillation column D2 via line 10.

[0120] Distillation under reduced pressure using distillation column D2 makes it possible to recover at the top of column D2 a stream comprising mainly isophorone, which is transferred via line 11 to distillation column D4. At the bottom of column D2, the recovered fraction comprising the polycondensation by-products is transferred via line 12 to distillation column D3.

[0121] The distillation under reduced pressure of the polycondensation by-products in the distillation column D3 makes it possible to recover at the top of the column D3 a stream preferably comprising xylitones and / or isoxylitones. This fraction is recycled via line 13 to line 1.

[0122] Heavy reaction by-products S L are recovered at the bottom of column D3.

[0123] The flow recovered at the top of column D2, comprising mainly isophorone, feeds distillation column D4. Distillation under reduced pressure in column D4 allows light impurities such as residual acetone and water, mesityl oxide and 1,3,5-trimethylbenzene to be extracted at the top of the column. This fraction is discharged via line 15 to decanter d2. The aqueous phase from decanter d2 is sent via line 17 to a wastewater treatment S Eand the organic phase comprising mainly mesityl oxide and isophorone is recycled via line 16 to line 1.

[0124] 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 at the bottom of column D5 contains residual polycondensation by-products, which are recycled via line 20 into distillation column D2.

[0125] Therefore, the organic phase flow feeding reactor R comprises fresh acetone, the light fraction coming from distillation column D1 and recycled via line 5, the light fraction coming from distillation column D3 and recycled via line 13 and the organic phase coming from decanter d2 and recycled via line 16.

[0126] The alkaline aqueous solution (A) feeding reactor R comprises an aqueous solution of fresh alkali hydroxide and the aqueous phase from decanter dl and recycled via line 9.

[0127] The following examples illustrate the present invention, but are in no way limiting. Examples

[0128] Example No. 1:

[0129] The synthesis of isophorone is carried out in a vertical tubular reactor made of 316 L stainless steel with a volume of 815 mL and an L / D ratio of 3, equipped with a lateral tubing positioned at L / 2 allowing a sample to be taken from the middle of the reactor.

[0130] The alkaline aqueous phase and the organic phase consisting of fresh acetone, recycled acetone and, where appropriate, recycled mesityl oxide and recycled polycondensation products are respectively preheated through 2 electric heat exchangers in order to reach the desired reaction temperature within the reactor.

[0131] The tubular reactor is pre-filled with an aqueous sodium hydroxide solution, also pre-heated, and with a sodium hydroxide concentration by weight of 10%.

[0132] The reagents are supplied by means of piston pumps.

[0133] Conversions and selectivities are established after continuous operation of at least 24 hours in order to guarantee stabilized reaction conditions within the reactor.

[0134] 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 supplied.

[0135] In Tables 1 and 2 below, OM stands for mesityl oxide; C12 stands for by-products of C12 reactions, i.e., xylitones and / or isoxylitones; C15 stands for by-products of C15 reactions, such as C15H22O; Cig stands for by-products of Cig reactions; ACE stands for acetone; IPHO stands for isophorone, and the selectivities are defined as follows:

[0136] S P = selectivity to product P compared to converted acetone

[0137] SIPHO = 100 x 3 x (number of moles of IPHO formed) / number of moles of ACE converted

[0138] SQM = 100 x 2 x (number of moles of OM formed) / number of moles of ACE converted

[0139] SC12 = 100 x 4 x (number of moles of C12H18O formed) / number of moles of ACE converted

[0140] SC 15 = 100 x 5 x (number of moles of C15H22O formed) / number of moles of ACE converted

[0141] SC 18 = 100 x 6 x (number of moles of Cigl- eO formed) / number of moles of ACE converted with: number of moles of P formed = (number of moles of P at reactor outlet - number of moles of P recycled to reactor feed)

[0142] Conversions and selectivities are calculated on the basis of the mass compositions of the crude mixtures leaving the reaction zone; the compositions are determined by gas chromatographic analyses.

[0143] Table 1

[0144] Test 1 is a comparative test, it illustrates a process without recycling.

[0145] Test 2 is also a comparative test, it only recycles the fractions containing mesityl oxide.

[0146] Test 3 is according to the invention, it recycles only the fractions comprising xylitones and / or isoxylitones.

[0147] Test 4 is according to the invention, it recycles all of the fractions comprising mesityl oxide and xylitones and / or isoxylitones.

[0148] Tests 3 and 4 use a succession of 5 distillation columns and recycle the fractions containing C12 impurities, which are retrogradable, and the C15 impurities.

[0149] Table 2 below shows the results obtained.

[0150] Table 2

[0151] The results of the selectivities in mesityl oxide (MO) and xylitones and / or isoxylitones (total C12) allow us to note that the recycling of mesityl oxide and xylitones and / or isoxylitones can lead to the total suppression of their respective productions.

[0152] Example 3 shows that recycling only the fraction containing xylitones and / or isoxylitones (total C12) allows an increase in isophorone selectivity, an increase in the conversion rate and an increase in productivity.

[0153] Example 4 shows that the selectivity to isophorone (IPHO) of 74.4% without recycling thus increases to 85.6% with recycling of mesityl oxide (MO) and xylitones and / or isoxylitones (total C12).

[0154] The negative selectivity for 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 did the recycling prevent the formation of mesityl oxide, but also allowed the excess recycled product to be downgraded compared to the reaction equilibrium.

[0155] These tests show the decisive impact of the recycling of mesityl oxide and xylitones and isoxylitones CiîHigO on the selectivity and productivity of isophorone.

[0156] Example #2:

[0157] The operating procedure followed is similar to that of example no. 1 but using a vertical tubular reactor in 316 L stainless steel with a volume of 940 mL with an L / D ratio of 18.5 and equipped with 3 lateral pipes positioned at L / 3, L / 2 and 2L / 3 allowing samples to be taken at a third of the length of the reactor, in the middle of the reactor and at 2 thirds of the length of the reactor.

[0158] Table 3 below shows the operating conditions of the tests:

[0159] Table 3

[0160] Test 5 is a comparative test, it illustrates a process without recycling.

[0161] Test 6 is according to the invention. The process followed is that illustrated in Figure 1, it uses a succession of 5 distillation columns and it recycles the fractions containing C12 impurities, which are retrogradable and the C15 impurities.

[0162] Table 4 below shows the results obtained.

[0163] Table 4

[0164] As in Example 1, sufficiently extensive recycling of xylitones and / or isoxylitones makes it possible to completely eliminate their formation. Note that negative selectivities for mesityl oxide and CuHigO express the fact that the quantities of mesityl oxide and CuHigO at the outlet of the tubular reactor are lower than those at the inlet; this indicates that not only has recycling prevented the formation of mesityl oxide and CuHigO but has also made it possible to downgrade the excess of recycled products compared to the reaction equilibrium.

[0165] Furthermore, it should be noted that these recyclings do not increase the formation of heavy higher condensation by-products CI S H26O. 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 C12).

[0166] 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

Claims 1. Continuous process for the liquid phase synthesis of isophorone by alkaline autocondensation of acetone comprising the following successive steps: a) continuous injection through a tubular reactor (R): -a flow 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 the acetone within the tubular reactor (R), the reactor containing an emulsion comprising mainly an aqueous phase of alkali hydroxide (B), the concentration of alkali hydroxide in 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 reaction crude 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 at the end of separation d) then e) distillation of the organic phase containing the isophorone recovered in the previous step in order to extract mainly at the bottom of the column (D2) the polycondensation by-products and to recover at the top of the column (D2) a stream mainly comprising isophorone, then f) distillation of the polycondensation by-products from the bottom of the column (D2) resulting from the previous distillation, then g) recycling of the stream from the top of the column of the previous distillation (D3) comprising xylitones and / or isoxylitones to the tubular reactor (R)., 2. Method according to claim 1, characterized in that the aqueous alkali hydroxide solution (A) is an aqueous solution of sodium hydroxide or potassium hydroxide.

3. Method 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, preferably between 80 and 150 g / L and more preferably between 100 and 150 g / L.

4. Method 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. Method according to any one of the preceding claims, characterized in that the ratio of the weight flow rate of the flow of aqueous alkali hydroxide solution (A) supplied (Qalkali hydroxide) and the weight flow rate of the organic flow supplied (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. Process 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. Method according to any one of the preceding claims, characterized in that it comprises a distillation step h) of the flow recovered at the top of the distillation column (D2) of step e).

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

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

10. Method according to any one of claims 7 to 9, characterized in that it comprises a step of decanting the flow 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).

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