WO2020049111A1

WO2020049111A1 - Process for producing isoprenol

Info

Publication number: WO2020049111A1
Application number: PCT/EP2019/073723
Authority: WO
Inventors: Ralf Dirk LOEFFLER; Peter HEIDEBRECHT; Andreas Woelfert; Julia HOFINGER; Christian Kunkelmann; Georg Seeber
Original assignee: Basf Se
Priority date: 2018-09-05
Filing date: 2019-09-05
Publication date: 2020-03-12
Also published as: CN112638850A; EP3847147A1

Abstract

A process for producing isoprenol, comprising: mixing and injecting a formaldehyde source and isobutylene into a reactor through a plurality of nozzles operated in parallel and reacting the formaldehyde source and isobutylene under supercritical conditions; wherein the reactor comprises a vertically disposed vessel, a sidewall, an upper portion and a lower portion; and wherein the formaldehyde source and isobutylene are injected into a mixing chamber of the reactor disposed in the upper portion and a fluid comprising formaldehyde and/or isobutylene and/or isoprenol is passed from the mixing chamber into a post-reaction chamber disposed in the lower portion; and providing draft tubes arranged essentially concentrically underneath each of the nozzles in the mixing chamber, the draft tubes providing downcomer conduits within the draft tubes and a riser conduit outside of the draft tubes, so that the formaldehyde source and isobutylene injected through the nozzles travel generally downward in the downcomer conduits, a fluid comprising formaldehyde and/or isobutylene and/or isoprenol is then diverted in a generally upward direction in the riser conduit, and the fluid is back-mixed with the injected formaldehyde source and isobutylene. The process allows for isoprenol to be obtained with high selectivity and in high yields. The invention also relates to a reactor comprising: a vertically disposed vessel, a sidewall, an upper portion and a lower portion, a perforated plate separating a mixing chamber of the reactor disposed in the upper portion and a post-reaction chamber disposed in the lower portion; a plurality of nozzles for injecting a fluid into the mixing chamber in an essentially downward direction; draft tubes arranged essentially concentrically underneath each of the nozzles in the mixing chamber, the draft tubes providing downcomer conduits within the draft tubes and a riser conduit outside of the draft tubes, so that the fluid injected through the nozzles travels generally downward in the downcomer conduits, a reacted fluid is then diverted in a generally upward direction in the riser conduit, and the fluid is back-mixed with the injected fluid; and a fluid outlet in the post-reaction chamber.

Description

Process for producing isoprenol

Isoprenol, or 3-methyl-3-buten-1-ol, is an important intermediate for pharmaceuticals and aroma compounds, with a yearly global production of several thousand tons. Isoprenol is commercially synthesized by reacting formaldehyde with isobutylene (2-methylpropene). The process is described, e.g., in DE 100 64 751 A1.

While isobutylene is gaseous at standard temperature and pressure, formaldehyde is usually provided as an aqueous solution. Reacting a gas and a liquid requires relatively complicated reactor setups in order to ensure the necessary magnitude of the surface area between the two phases for a reaction with a high yield. In two-phase reactions, local oversaturation of the reaction partners is highly probable. Adversely, high concentrations of formaldehyde lead to the formation of formiates and/or polyols, which are believed to further react to coke, an undesired side product.

Further, high temperatures are required to obtain a high isoprenol yield in uncatalyzed reactions of formaldehyde with isobutylene, as described in GB 1 205 397. However, formaldehyde is known to oxidize to formic acid more readily at high temperatures, leading to a decrease in process efficiency. RU 2 403 969 C2 describes a process for preparing isoprene, wherein isobutylene, formaldehyde and an acid catalyst are reacted at a supercritical isobutylene pressure. WO 2008/037693 A1 describes a process for producing citral, wherein a first step of the process comprises obtaining isoprenol from the reaction of isobutylene and formaldehyde under, e.g., supercritical conditions.

The problem underlying the present invention can be seen in providing a process wherein isoprenol is obtained with high selectivity.

The problem is solved by provision of a process for producing isoprenol, comprising:

- mixing and injecting a formaldehyde source and isobutylene into a reactor through a plurality of nozzles operated in parallel and reacting the formaldehyde source and isobutylene under supercritical conditions; wherein the reactor comprises a vertically disposed vessel, a sidewall, an upper portion and a lower portion; and wherein the formaldehyde source and isobutylene are injected into a mixing chamber of the reactor disposed in the upper portion and a fluid comprising formaldehyde and/or isobutylene and/or isoprenol is passed from the mixing chamber into a post-reaction chamber disposed in the lower portion; and preferably

- providing draft tubes arranged essentially concentrically underneath each of the nozzles in the mixing chamber, the draft tubes providing downcomer conduits within the draft tubes and a riser conduit outside of the draft tubes, so that the formaldehyde source and isobutylene injected through the nozzles travel generally downward in the downcomer conduits, a fluid comprising formaldehyde and/or isobutylene and/or isoprenol is then diverted in a generally upward direction in the riser conduit, and the fluid is back-mixed with the injected formaldehyde source and isobutylene.

Further provided is a reactor comprising:

- a vertically disposed vessel, a sidewall, an upper portion and a lower portion, a perforated plate separating a mixing chamber of the reactor disposed in the upper portion and a post-reaction chamber disposed in the lower portion;

- a plurality of nozzles for injecting a fluid into the mixing chamber in an essentially downward direction;

- draft tubes arranged essentially concentrically underneath each of the nozzles in the mixing chamber, the draft tubes providing downcomer conduits within the draft tubes and a riser conduit outside of the draft tubes, so that the fluid injected through the nozzles travels generally downward in the downcomer conduits, a reacted fluid is then diverted in a generally upward direction in the riser conduit, and the fluid is back-mixed with the injected fluid; and

- a fluid outlet in the post-reaction chamber.

The following discussion with regard to the process of the invention is intended to also relate to the reactor of the invention, where applicable, and vice versa.

Supercritical conditions exist when a substance or mixture of substances is subjected to temperature and pressure exceeding the thermodynamic critical point of the substance or mixture. In this state, there is no differentiation between the liquid and gas phases and the fluid is referred to as a dense gas in which the saturated vapor and saturated liquid states are identical. It has been found that under supercritical conditions the reactivity of isobutylene towards formaldehyde is sufficiently high to allow for a smooth reaction even in the absence of an extraneous solvent or catalyst. This leads to a decreased amount of side products.

It is considered that supercritical isobutylene acts as a supercritical solvent which solubilizes at least part of the formaldehyde source and/or extracts formaldehyde out of the formaldehyde source. This allows for formaldehyde to be mixed with isobutylene efficiently, alleviating the problems generally associated with mixing multiple phases. For instance, local oversaturation of formaldehyde may be avoided. Thus, while ideally the entire reaction mixture, including the formaldehyde source and isobutylene, becomes a homogeneous single phase in the supercritical or near-supercritical region, it is also envisaged that part of the formaldehyde source exists as a second liquid phase. Also, the isoprenol formed in the reaction may dissolve in the near- or supercritical reaction mixture, or cause a separate liquid phase to form.

The phase diagrams of the reaction components are readily available to those skilled in the art. Further, the critical point of a compound or a mixture may be determined by calculating or experimentally obtaining a phase diagram of the compound or mixture. From the above it is evident that the pressure and temperature of the process are required to be sufficient to maintain at least the isobutylene in a supercritical state. The critical temperature Tc and pressure Pc of isobutylene are Tc = 417.9 K and Pc = 40.00 bar (Tsonopoulos, C.; Ambrose, D., Vapor-Liquid Critical Properties of Elements and Compounds. 6. Unsaturated Aliphatic Hydrocarbons, J. Chem. Eng. Data, 1996, 41 , 645-656).

In order to achieve supercritical conditions, formaldehyde and isobutylene are preferably reacted at a temperature of at least 220 °C, for example in the range of 220 to 290 °C, and an absolute pressure of at least 200 bar, more preferably at a temperature of at least 250 °C, for example in the range of 250 to 280 °C, and an absolute pressure of at least 220 bar, and most preferably at a temperature of at least 260 °C, for example in the range of 260 to 275 °C, and an absolute pressure of at least 250 bar. All pressures cited herein are absolute pressures, unless noted otherwise. As the process is directed to producing and recovering isoprenol, it is understood that it is essential to avoid reaction conditions which are conducive to further reaction of the obtained isoprenol.

For example, the presence of a Bronsted acid compound, such as a Bronsted acid catalyst, may induce the elimination of water from isoprenol, yielding isoprene and water. It is thus preferable that the process of the invention be performed essentially free of compounds inducing further reactions of isoprenol, such as Bronsted acid compounds. Of course, this does not relate to the starting materials and product of the process.

The process is thus generally performed in the presence of less than 0.5 mol-%, preferably less than 0.1 mol-%, more preferably less than 0.01 mol-% and most preferably less than 0.001 mol-% of compounds capable of inducing further reactions of isoprenol, especially Bronsted acid compounds. A Bronsted acid compound is understood to be any compound with a pH value of less than 7, as measured at 25 °C and 1 bar absolute.

Preferably, a molar excess of isobutylene is reacted with formaldehyde. The term “molar excess of isobutylene” is understood to mean a molar excess of isobutylene over formaldehyde. This leads to a relatively low concentration of formaldehyde in the reaction mixture, thereby reducing the potential of a local oversaturation of formaldehyde. Thus, the amount of obtained side products is lowered. Further, it was found that the larger the molar ratio of isobutylene to formaldehyde, the lower the tendency of the reaction mixture to form multiple phases.

In a preferred embodiment, the molar ratio of isobutylene to formaldehyde under supercritical conditions is at least 7:1 , especially preferred at least 10:1 and most preferably at least 12:1 , for example at least 12.5:1.

Formaldehyde may be provided as a liquid, for example as a solution of paraformaldehyde. Preferably, the formaldehyde source is an aqueous formaldehyde solution. When formaldehyde is provided as an aqueous solution, the solution preferably comprises at least 15 wt.-%, more preferably at least 25 wt.-% and most preferably at least 35 wt.-%, for example at least 45 wt.-% or at least 50 wt.-%, of formaldehyde, based on the total weight of the aqueous solution of formaldehyde.

In one embodiment, the process is performed in the presence of a Bronsted base compound. The Bronsted base compound is useful for neutralizing acids that may be present in the starting materials or may be formed in side reactions during the process.

Examples of suitable Bronsted base compounds are hydroxides, carbonates and bicarbonates of alkali metals and alkaline earth metals, ammonia or organic amines, and salts of acids which are weaker than formic acid. It is particularly advantageous to use ammonia or organic amines, such as ethylamine, trimethylamine, hexamethylenetetramine (urotropin), aniline, pyridine, or piperidine. Substances exhibiting a buffer action, such as hexamethylenetetramine (urotropin), are particularly suitable.

The Bronsted base compound is favorably present in amounts of 0.001 to 10 wt.-%, particularly 0.01 to 1 wt.-%, based on the total weight of reaction mixture. It is advantageous to use weak bases and, in order to avoid secondary reactions which may take place in a strongly alkaline range, to ensure that the starting mixture of the reactants, after the Bronsted base compound has been added, has a pH value of from 7 to 1 1 , advantageously from 7 to 10, particularly from 7.5 to 9, as determined at 20 °C and 1 bar absolute. If anhydrous starting materials are used, the pH value of the starting mixture is determined after dilution with an equal amount by weight of water.

The Bronsted base compound may be fed into the reactor at one or more suitable locations. When the formaldehyde source is an aqueous formaldehyde solution, the Bronsted base compound is preferably fed into the reactor dissolved in the aqueous formaldehyde solution. In another embodiment, the Bronsted base compound is separately fed into the reactor as an aqueous solution.

The process of the invention may favorably be carried out in the absence of an extraneous solvent, or in the presence of only a small amount of extraneous solvent, allowing for a more efficient process since less or no extraneous solvent has to be separated from the product and recycled. Extraneous solvents are in this context of course understood to not comprise the components necessarily introduced by the formaldehyde source and isobutylene themselves.

In one embodiment, the formaldehyde source and isobutylene are reacted in the essential absence of an extraneous solvent, in particular in the essential absence of a non-aqueous extraneous solvent. This means that the total amount of extraneous solvents present during the reaction of the formaldehyde source and isobutylene is preferably less than 5.0 wt.-%, more preferably less than 3.5 wt.-% and most preferably less than 1.5 wt.-%, for example less than 1.0 wt.-% or less than 0.5 wt.-%, based on the total weight of the reaction mixture.

In another embodiment, the formaldehyde source and isobutylene are reacted in the presence of an extraneous solvent, in particular in the presence of a non-aqueous extraneous solvent, such as a hydrocarbon, an ether, an ester, or an alcohol. Preferably, the extraneous solvent is an ether or an alcohol.

Ethers are preferably selected from dialkyl ethers, such as diethyl ether, diisopropyl ether, ethyl butyl ether, and methyl tert-butyl ether (MTBE); and cyclic ethers, such as dioxane. An especially preferred ether is methyl tert-butyl ether (MTBE).

Alcohols are preferably selected from C3-Cio-alcohols, for example straight-chain and branched-chain aliphatic alcohols, such as n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, tert-butyl alcohol, isobutyl alcohol, sec-butyl alcohol, n-amyl alcohol, isoamyl alcohol, tert-amyl alcohol, hexyl alcohol, methyl-2-butanol, 3-methyl-3-pentanol, 2-ethylhexyl alcohol, heptyl alcohol, octyl alcohol, nonyl alcohol and decanyl alcohol; cycloaliphatic alcohols, such as cyclohexyl alcohol, methylcyclohexyl alcohol, cyclopentyl alcohol; alicyclic alcohols; and aromatic alcohols such as benzyl alcohol.

Preferred C3-Cio-alcohols are isopropyl alcohol, isobutyl alcohol, sec-butyl alcohol, tert-butyl alcohol, isoamyl alcohol, and tert-amyl alcohol. More preferred C3-C10- alcohols are isobutyl alcohol and tert-butyl alcohol. An especially preferred C3-C10- alcohol is tert-butyl alcohol.

The extraneous solvent may also be a mixture of organic solvents. The total amount of extraneous solvent present may be in the range of 2 to 20 moles, more preferably 3 to 10 moles, per mole of formaldehyde.

In a preferred embodiment, the formaldehyde source and isobutylene are reacted in the essential absence of a catalyst. This means that the total amount of catalysts, both heterogeneous and homogeneous, present during the reaction of formaldehyde and isobutylene is preferably less than 1.0 wt.-%, more preferably less than 0.5 wt.-% and most preferably less than 0.1 wt.-%, for example less than 0.05 wt.-% or less than 0.01 wt.-%, based on the total weight of the reaction mixture. Reacting formaldehyde and isobutylene in the essential absence of catalysts allows for a commercially favorable process and a simplified workup of the reaction product, as the obtained isoprenol does not have to be separated from catalyst and spent catalyst.

Adequate fluid mixing is fundamental to the claimed process. The process involves mixing and injecting the formaldehyde source and isobutylene into a reactor through a plurality of nozzles operated in parallel. Preferably, the plurality of nozzles is arranged in the lid of a reactor. In order to allow for a high degree of mixing and to avoid dead zones, the nozzles should be flush with the lid.

Upon injection and mixing with the reactor contents, the formaldehyde source and isobutylene form a homogeneous single phase in the supercritical region. Isobutylene is either a supercritical fluid prior to its injection into the reactor, or is a liquefied or gaseous phase near its supercritical state before the injection into the reactor.

The formaldehyde source and isobutylene are injected through a plurality of nozzles operated in parallel. The parallel arrangement of a plurality of nozzles allows for an increased throughput while retaining adequate fluid mixing of the reaction mixture than a single nozzle. Preferably, a multitude of nozzles are arranged around a central nozzle. For example, three or more nozzles, such as four, five, or most preferably six nozzles, are arranged around a central nozzle. Preferably, three or more nozzles, such as four, five, or most preferably six nozzles, are arranged concentrically around a central nozzle.

The nozzles may be one-component or two-component nozzles. In a one-component nozzle, only one fluid is injected via the nozzle. One-component nozzles exhibit the advantage of having a simple structure. In a two-component nozzle, two fluids are separately injected via the nozzle and mixed only after exiting the nozzle.

In a preferred embodiment, the nozzles are two-component nozzles. Such a two- component nozzle may for example be designed so as to provide an annular jet of the formaldehyde source around a central isobutene jet. The injection velocities of these two jets may be the same or different. Preferably, the injection velocities are different, so as to provide a high degree of turbulence and hence, a high degree of mixing.

It is especially preferable that a two-component nozzle is designed so as to provide an annular jet of the formaldehyde source around a central isobutene jet, and that the injection velocities of these two jets are different. In this embodiment, the jet of the formaldehyde source has a large shear surface towards both the central isobutene jet and the reaction mixture in the reactor, allowing for favorable fast mixing of formaldehyde with isobutylene.

In a preferred embodiment, the ratio of the injection velocity of isobutylene to the injection velocity of formaldehyde is in the range of 4:1 to 6:1 , preferably 4.5:1 to 5.5:1 , such as 5:1. For example, the injection velocity of isobutylene may be about 70 to 100 m/s, preferably 80 to 90 m/s. The injection velocity of the formaldehyde source may be about 10 to 30 m/s, preferably 15 to 20 m/s.

In a preferred embodiment, isobutylene is pre-heated outside of the two-component nozzle, for example via electrical heaters and/or in a heat exchanger, and the formaldehyde source is heated in the nozzle upon thermal contact with the heated isobutylene. Upon injection into the reactor, the formaldehyde source is further heated upon direct contact with the reaction mixture. Thus, by controlling the temperature of the heated isobutylene, the temperature of the reaction mixture may be influenced. The electrical heaters and the heat exchanger are preferably located outside of the reactor. In one embodiment, the formaldehyde source is slightly pre-heated prior to being heated in the nozzle, for example to a temperature of 60 to 100 °C, such as 75 to 90 °C, for example about 80 °C or about 85 °C. Heating the formaldehyde source to higher temperatures only immediately prior to injection into the reactor avoids formation of side products and a decrease in process efficiency. While initial rapid and intense mixing of the formaldehyde source and isobutylene is desirable, it may be advantageous to continue and complete the reaction under conditions of limited backmixing. Thus, the reactor comprises a vertically disposed vessel, a sidewall, an upper portion and a lower portion. The formaldehyde source and isobutylene are injected into a mixing chamber of the reactor disposed in the upper portion and a fluid comprising formaldehyde and/or isobutylene and/or isoprenol is passed from the mixing chamber into a post-reaction chamber disposed in the lower portion. In the post-reaction chamber, backmixing is limited.

One way of enhancing the mixing in the mixing chamber is through the use of draft tubes. In draft tube mixing systems, a usually cylindrical tube open at both ends is disposed vertically inside the tank creating a cylindrical space inside the draft tube and a space outside the draft tube.

According to the present invention, draft tubes are arranged essentially concentrically underneath each of the nozzles, the draft tubes providing downcomer conduits within the draft tubes and a riser conduit outside of the draft tubes, so that the formaldehyde source and isobutylene injected through the nozzles travel generally downward in the downcomer conduits, a fluid comprising formaldehyde and/or isobutylene and/or isoprenol is then diverted in a generally upward direction in the riser conduit, and the fluid is back-mixed with the injected formaldehyde source and isobutylene. Part of the fluid is passed from the mixing chamber to the post-reaction chamber.

The draft tubes extend into the upper part of the mixing chamber and allow fluid flow communication between the riser conduit and the downcomer conduit via the upper end of the draft tube; and extend into the lower part of the mixing chamber and provide fluid flow communication between the riser conduit and the downcomer conduit via the lower end of the draft tube. The draft tubes are fixed in the reactor via, e.g., one or more girders or cross beams. Favorably, the means for fixing the draft tubes in the reactor stabilize the draft tubes while offering a minimum of flow resistance. For example, it is preferable that the volume of the means for fixing the draft tubes in the reactor is as small as possible.

In order to allow for an even distribution of fluid flow throughout the mixing zone, the spacing between the sidewall of the reactor and the draft tubes may be adjusted. For instance, when the space between the sidewall of the reactor and the draft tubes is too large, most of the fluid may flow along the walls, while only a small amount of fluid flows through the central part of the reactor. This may lead to an uneven degree of mixing within the riser conduit. The positioning of the nozzles and accordingly the draft tubes thus directly affects the mixing efficiency in the reactor.

The optimum spacing between the sidewalls of the reactor and the draft tubes depends on a variety of parameters, such as the reaction volume, the length of the draft tubes and the circulation ratio of the reactor. It is preferable that the spacing allows the circulating reaction mixture to be drawn into the upper end of the draft tubes at an essentially uniform rate from all sides.

In a further preferred embodiment, openings are provided in the wall (or lateral area) of the draft tubes. The openings allow cross flow of fluid through the draft tube wall without traveling through either end of the draft tube. The openings thus advantageously reduce the loss of pressure upon entry of the fluid flow into the draft tube. Further, the openings induce turbulence and thus improve the mixing efficiency of the reactor.

The openings may be perforations of many different shapes including rectangles and circles and are of a size and position that allows the desired level of cross flow through the draft tube wall without significantly degrading the axial flow of fluid through the draft tube. From a viewpoint of ease of manufacture, the openings are preferably slits. In the broader embodiment of the invention, the specific shape and pattern of the openings is not particularly limited and will most likely be chosen according to ease (or cost) of manufacture. Simple geometric shapes including rectangles, squares, circles, and ovals are some preferred examples. The openings may comprise turbulence- enhancing geometric variations, such as spikes.

The positioning of the openings is also not particularly limited, although columns or arrays of cut-outs may be more practical than random positions depending on the method of manufacture. Furthermore, the openings (e.g. columns and/or rows of cut- out sections) may be positioned entirely around the circumference of the draft tube or may only be positioned on a portion thereof, for example approximately 60%, 50%, or 25% thereof. Preferably, the openings are positioned in the upper part of the wall of the draft tubes.

The size and number of openings can also vary considerably. In some embodiments of the invention there may be many smaller openings, or there could just as easily be fewer larger openings to produce the same amount of open area.

It has been found that a further improvement is brought about by providing deflector means above of the draft tubes, preferably above each of the draft tubes. The deflector means are suitably centered above the draft tubes. The deflector means are designed so as to deflect the fluid travelling upward in the riser conduit in a downward direction via the deflector means.

The deflector means suitably comprise a surface which is concave relative to the upper end of the draft tube. In a preferred embodiment, the deflector means have a partial toroidal surface. It is especially preferred that the deflector means are provided in the shape of the upper portion of a ring torus bisected in a plane parallel to the toroidal direction. This shape allows for an especially efficient deflection of the fluid travelling upward in the riser conduit. The deflector means may allow for a stabilization of the injected stream of formaldehyde source and isobutylene. This is especially relevant when the flow rate of the fluid travelling upward in the riser conduit is not uniform across the cross section of the reactor, which may lead to an eccentricity of the injected stream of formaldehyde source and isobutylene. Such an eccentricity may cause a decrease in circulation rate if left unattended.

In a preferred embodiment, the shape of the deflector means constitutes the upper portion of a ring torus bisected in a plane parallel to the toroidal direction wherein the ring torus is bisected at at least 50% of its height, such as at least 55% or 65% of its height. Thus, the upper portion of the ring torus is the same size or smaller than the lower portion of the ring torus. In another preferred embodiment, the shape of the deflector means constitutes the upper portion of a ring torus bisected in a plane parallel to the toroidal direction wherein the ring torus is bisected at at most 85% of its height, for example 80% of its height. In these ranges, the entry of the deflector is angled especially suitable for fluid deflection. Preferably, the deflector means are not attached to the reactor lid, but are arranged so that a gap between the reactor lid and the deflector means sufficiently large enough to avoid dead zones exists. For example, the gap between the reactor lid and the deflector means is at least 15 mm, such as at least 20 mm. In a preferred embodiment, the deflector means are attached to the draft tubes above which they are arranged.

The mixing chamber can be seen as an internal loop reactor. The mixing can be quantified in terms of the circulation ratio, which is defined as the total mass flow through the mixing tubes divided by the feed mass flow rate. The higher the circulation ratio, the more homogeneous the formaldehyde concentration in the mixing chamber, and the lower the amount of side products obtained due to a local formaldehyde oversaturation. With a circulation ratio of more than 10:1 , the system favorably approaches continuous stirred-tank reactor (CSTR) behavior. Preferably, the circulation ratio is at least 15:1 , preferably more than 20:1.

Part of the fluid is passed from the mixing chamber to the post-reaction chamber. In the post-reaction chamber, backmixing is limited. Preferably, backmixing is limited to the degree that the residence time distribution in the post-reaction chamber is approximately that of a plug flow reactor. The residence time distribution governs the dynamic equilibrium between the desired reaction and undesired further reactions and thus the point of maximum yield. When the residence time distribution in the post- reaction chamber is approximately that of a plug flow reactor, conversion of formaldehyde and isobutylene is increased.

Adjusting the ratio of the volume of the mixing chamber to the volume of the post- reaction chamber may also influence the residence time distribution and thus the yield of the reaction. Preferably, the ratio of the volume of the mixing chamber to the volume of the post-reaction chamber is from 1 :20 to 1 :1 , preferably from 1 :10 to 1 :3.

In one embodiment, a perforated plate separates the mixing chamber from the post- reaction chamber. The design of the perforated plate(s) is not particularly limited. Preferably, the perforated plate is a flat plate with perforations which are regularly distributed across its area. The size and shape of the perforations depends on the size of the reactor and the reaction volume. In one embodiment, the perforated plate comprises round perforations, for example with a diameter of 10 to 50 mm, such as 15 to 35 mm, for example 25 mm.

In order to avoid fluid passing between the perforated plate and the reactor wall, rather than through the perforations of the plate, a seal may be used. For example, a metallic seal may be attached to the edge of the perforated plate.

The post-reaction chamber may comprise internals for limiting backmixing, which generally reduce the circulation of the reaction mixture and allow for the fluid flow to approach a laminar flow. Different internals may be used for this purpose.

In one embodiment, the post-reaction chamber comprises at least one perforated plate. In this embodiment, the post-reaction chamber comprises segments (or “compartments”) with a defined reaction volume between each of the plates and between the plates and the bottom of the reactor. With an increasing number of successive segments, the residence time distribution of the post-reaction chamber favorably approaches the residence time distribution of a plug flow reactor. The number of segments in the post-reaction chamber is, for example, 2 to 25, such as 10 to 22 or 13 to 18, for example 15 or 16 segments.

The perforated plate(s) in the post-reaction chamber may be the same as the perforated plate separating the mixing chamber from the post-reaction chamber described above.

After passing through the post-reaction chamber, the reaction mixture is guided out of the reactor through a fluid outlet and may be subjected to further processing. Further processing usually comprises a process such as distillation, separation, or other purification, such as flashing off unreacted isobutylene, and optionally sub-processes such as passing the reaction mixture through a heat exchanger, so as to improve process efficiency. In a preferred embodiment, further processing comprises feeding the reaction mixture to a high-pressure separator and a countercurrent heat exchanger.

Due to the improved fluid hydrodynamic performance that avoids dead spaces, the reactor may be operated continuously over long periods of time, such as several months. During operation of the reactor, undesired side products such as coke may accumulate, especially in the post-reaction chamber. Should this occur, the performance will deteriorate and the pressure loss will increase to such an extent as to obstruct the operation. For these reasons periodic removal of accumulated side products is a necessity. These are favorably removed when the reactor is not in operation.

In a suitable cleaning operation, a cleaning nozzle is introduced in the reactor, especially in the post-reaction chamber, and a pressurized liquid is supplied to the cleaning nozzle. By gradually raising and/or lowering and rotating the cleaning nozzle through the inside of the reactor, especially in the post-reaction chamber, accumulated side products are removed.

Specifically, a process suitable for cleaning the reactor may comprise the following steps:

- removing a nozzle from the reactor lid so as to provide a first opening;

- introducing a fastener such as a rope, a cord or a cable through the first opening;

- providing a second opening in the perforated plate separating the mixing chamber from the post-reaction chamber;

- passing the fastener through the second opening;

- providing a third opening in the post-reaction chamber, preferably the bottom of the post-reaction chamber;

- introducing a nozzle suitable for high-pressure cleaning through the third opening, wherein the nozzle is attached to a fluid hose, preferably a water hose;

- fastening the nozzle to the fastener;

- applying fluid pressure, preferably water pressure, to the nozzle and opening the nozzle so as to provide a nozzle jet; and

- raising and lowering the nozzle within the post-reaction chamber by means of the fastener.

The nozzle jet removes residues from the inner wall of the post-reaction chamber. The nozzle typically rotates during operation due to the applied fluid pressure. The fluid outlet in the post-reaction chamber of the reactor may constitute the third opening.

The invention is illustrated in detail by the appended drawings. Fig. 1 is a schematic diagram of the side view of a reactor suitable for performing the process of the invention.

According to Fig. 1 , the reactor comprises a mixing chamber (101 ) and a post-reaction chamber (102), which are separated by a perforated plate (103). A formaldehyde source and isobutylene are injected into the mixing chamber (101) via a plurality of nozzles (104). The mixing chamber (101) comprises draft tubes (105) arranged essentially concentrically underneath each of the nozzles. The walls of the draft tubes comprise slits (106). Further provided are deflector means (107) in the shape of the upper portion of a ring torus bisected in a plane parallel to the toroidal direction. The lower part of the reactor is not fully shown for sake of conciseness and may comprise further perforated plates.

The reactor is designed so that the draft tubes (105) provide a downcomer conduit within the draft tubes (105) and a riser conduit outside of the draft tubes (105) when the formaldehyde source and isobutylene are injected through the nozzles (104). The deflector means (107) are designed so as to deflect the fluid travelling upward in the riser conduit in a downward direction so as to back-mix the fluid with the injected formaldehyde source and isobutylene.

The reactor comprises a fluid outlet in the post-reaction chamber (not shown in Fig. 1) through which a reaction product comprising isoprenol is guided out of the reactor and may be subjected to further processing.

Fig. 2 is a schematic diagram of the bottom view of the lid of the reactor of Fig. 1 and the cut along which the side view of the reactor is depicted in Fig. 1.

According to Fig. 2, the lid of the reactor (201) is provided with a plurality of nozzles (202), in this case, seven nozzles. Six nozzles are arranged concentrically around a central nozzle.

Fig. 3 is a schematic diagram of the top view of the deflector means shown in Fig. 1.

Claims

1. A process for producing isoprenol, comprising:

mixing and injecting a formaldehyde source and isobutylene into a reactor through a plurality of nozzles operated in parallel and reacting the

formaldehyde source and isobutylene under supercritical conditions; wherein the reactor comprises a vertically disposed vessel, a sidewall, an upper portion and a lower portion; and wherein the formaldehyde source and isobutylene are injected into a mixing chamber of the reactor disposed in the upper portion and a fluid comprising formaldehyde and/or isobutylene and/or isoprenol is passed from the mixing chamber into a post-reaction chamber disposed in the lower portion; and

providing draft tubes arranged essentially concentrically underneath each of the nozzles in the mixing chamber, the draft tubes providing downcomer conduits within the draft tubes and a riser conduit outside of the draft tubes, so that the formaldehyde source and isobutylene injected through the nozzles travel generally downward in the downcomer conduits, a fluid comprising formaldehyde and/or isobutylene and/or isoprenol is then diverted in a generally upward direction in the riser conduit, and the fluid is back-mixed with the injected formaldehyde source and isobutylene.

2. The process according to claim 1 , comprising reacting the formaldehyde source and isobutylene at a temperature of at least 220 °C and an absolute pressure of at least 200 bar.

3. The process according to claim 1 or 2, comprising reacting a molar excess of isobutylene with the formaldehyde source, calculated as formaldehyde.

4. The process according to any of the preceding claims, wherein the formaldehyde source is an aqueous formaldehyde solution.

5. The process according to any of the preceding claims, wherein the nozzles are two-component nozzles.

6. The process according to any of the preceding claims, wherein passing the fluid from the mixing chamber into the post-reaction chamber comprises passing the fluid through a perforated plate.

7. The process according to any of the preceding claims, wherein the ratio of the volume of the mixing chamber to the volume of the post-reaction chamber is from 1 :20 to 1 :1.

8. The process according to any of the preceding claims, comprising providing

openings, preferably slits, in the wall of the draft tubes.

9. The process according to any of the preceding claims, comprising providing

deflector means above the draft tubes and deflecting the fluid travelling upward in the riser conduit in a downward direction via the deflector means, the deflector means having a partial toroidal surface.

10. A reactor comprising:

a vertically disposed vessel, a sidewall, an upper portion and a lower portion, a perforated plate separating a mixing chamber of the reactor disposed in the upper portion and a post-reaction chamber disposed in the lower portion; a plurality of nozzles for injecting a fluid into the mixing chamber in an essentially downward direction;

draft tubes arranged essentially concentrically underneath each of the nozzles in the mixing chamber, the draft tubes providing downcomer conduits within the draft tubes and a riser conduit outside of the draft tubes, so that the fluid injected through the nozzles travels generally downward in the downcomer conduits, a reacted fluid is then diverted in a generally upward direction in the riser conduit, and the fluid is back-mixed with the injected fluid; and a fluid outlet in the post-reaction chamber.

11. The reactor according to claim 10, further comprising:

openings, preferably slits, in the wall of the draft tubes; and

deflector means arranged above the draft tubes, the deflector means deflecting the fluid travelling upward in the riser conduit in a downward direction, wherein the deflector means have a partial toroidal surface.