Method for preparing isoprenal and / or prenal

Abstract

In a process for preparing isoprenal and / or prenal by contacting a gaseous reactant stream containing isoprenol with a silver-containing heterogeneous catalyst in the presence of molecular oxygen to obtain a product stream containing isoprenal and / or prenal, the weight ratio of formaldehyde to isoprenol in the reactant stream is maintained below 0.04, the process maintains high conversion and selectivity for extended periods of time, and avoids catalyst plugging and pressure loss.

Description

[Technical field]

[0001] The present invention relates to the preparation of 3-methyl-3-buten-1-al (isoprenal) or 3-methylbut-2-enal (prenal) from 3-methyl-3-buten-1-ol (isoprenol) by oxidative dehydrogenation. More specifically, the present invention relates to a process for the preparation of isoprenal and / or prenal by contacting a gaseous reactant stream containing isoprenol with a silver-containing heterogeneous catalyst in the presence of molecular oxygen to obtain an (iso)prenal-containing product stream. [Background technology]

[0002] Prenal and isoprenal are highly desirable intermediates for the synthesis of flavors, vitamins, and carotenoids. Their preparation by oxidative or nonoxidative dehydration of isoprenol over suitable catalysts is known to those skilled in the art and has been widely described in the literature.

[0003] WO 2009 / 115492 relates to the use of a supported catalyst containing a noble metal for the oxidative dehydration of isoprenol. The supported catalyst comprises, for example, silver on steatite spheres, with a silver content of 6% by weight and a particle size of 0.18-0.22 cm. It is synthesized by flame spraying or application of a complex solution of ethylenediamine and silver oxalate, followed by drying in a stream of air.

[0004] EP 2448669 relates to supported catalysts containing precious metals, such as silver. The silver metal-containing catalyst is obtained by depositing colloidal silver on steatite spheres.

[0005] WO 2012 / 146436 relates to silver-coated steatite sphere catalysts for the oxidative dehydrogenation of 3-methyl-3-buten-1-ol to 3-methyl-3-buten-3-al.

[0006] WO 2012 / 146528 describes a process for the preparation of C1-C10 aldehydes by oxidative dehydrogenation of the corresponding alcohols in the presence of shaped catalyst bodies obtained by three-dimensional deformation and / or arrangement in space of silver-containing fibers and / or filaments.

[0007] WO 2018 / 153736 describes a silver-containing catalyst for the preparation of aldehydes and ketones, in particular formaldehyde, by oxidative dehydrogenation of methanol. The catalyst is a two-layer system. The first catalyst layer has a unit weight of 0.3 to 10 kg / m 2 The first layer is made of silver-containing material in the form of a bundle, net, or mesh with a wire diameter of 30-200 μm. The second layer is made of silver-containing material in the form of granules with a particle size of 0.5-5 mm. The three-dimensional deformation and / or arrangement of the silver-containing fibers or threads in space can occur in an irregular or regular manner. Granules are granular materials made of small, usually irregularly shaped particles, e.g., silver crystals.

[0008] WO 2020 / 099390 relates to the use of a catalyst bed comprising a metallic silver catalyst body for the oxidative dehydrogenation of organic compounds under exothermic conditions, in particular for the preparation of olefinically unsaturated carbonyl compounds from olefinically unsaturated alcohols.

[0009] CN Patent No. 103769162 relates to a composite metal catalyst used in the oxidation of unsaturated alcohols and its preparation method. CN Patent No. 108404944 relates to a preparation method of vanadium silver molybdenum phosphate catalyst and the use of this catalyst for the preparation of prenal.

[0010] The oxidative dehydrogenation of olefinically unsaturated alcohols to olefinically unsaturated aldehydes is highly exothermic. The reaction is difficult to control because the reaction rate is highly dependent on the reaction temperature, reactants, and products that are unstable under the reaction conditions. This can lead to the accumulation of coke formation, clogging of tubes, and increased pressure loss over time. To ensure continuous operation, it is necessary to periodically regenerate the catalyst by burning the coke deposits using an oxygen-containing gas stream.

[0011] Despite frequent burning off of coke and other deposits on the catalyst, both conversion and selectivity can decrease over many years of operation, and the catalyst must be replaced. When such a reactor is opened, carbonaceous deposits are found and some of the reactor tubes are found to be clogged.

[0012] US Patent No. 6,013,843 discloses a method for the continuous industrial production of aldehydes by oxidative dehydrogenation of isoprenol over a catalyst consisting of copper, silver, and / or gold on an inert support, which method includes evaporating alcohol, mixing the alcohol vapor with an oxygen-containing gas, first passing the resulting oxygen-containing alcohol vapor through a catalyst bed at a temperature higher than the dew point of the alcohol but lower than the reaction initiation temperature, and then reacting the oxygen-containing alcohol vapor to form the corresponding aldehyde. Clogging in the reaction tubes is believed to be due to a very small amount of by-products in the starting alcohol, which is replenished by recycling unconverted alcohol. The reaction that destroys the catalyst is said to be avoided if the oxygen-containing alcohol vapor is first passed through the supported catalyst at a temperature lower than the initiation temperature.

[0013] There is a continuing need for methods that maintain high conversion and selectivity over long periods of time and avoid catalyst plugging and pressure loss. Summary of the Invention [Means for solving the problem]

[0014] The present invention relates to the preparation of isoprenal and / or prenal by contacting a gaseous reactant stream containing isoprenol with a silver-containing heterogeneous catalyst in the presence of molecular oxygen to obtain a product stream containing isoprenal and / or prenal, characterized in that the weight ratio of formaldehyde to isoprenol in the reactant stream is maintained below 0.04, preferably below 0.03, in particular below 0.02 or below 0.01.In an even more preferred embodiment, the weight ratio of formaldehyde to isoprenol is maintained below 0.002 or below 0.001.

[0015] Hereinafter, the abbreviation "(iso)prenal" is used to denote prenal, isoprenal, or a mixture of both.

[0016] In accordance with the present invention, the weight ratio of formaldehyde to isoprenol in the reactant stream is maintained at or below a certain level. However, if the weight ratio of formaldehyde to isoprenol in the reactant stream is reduced beyond a certain point, a point of rapid reduction is reached. Removal of formaldehyde requires additional equipment and operating costs. An economic balance must be struck between the improvements from reducing the ratio and the costs of maintaining such a ratio. Thus, the weight ratio of formaldehyde to isoprenol is preferably greater than or equal to 0.0005, or in some cases greater than or equal to 0.005.

[0017] It has now been found that the plugging of the reactor and the increase in pressure drop are significantly affected by the presence of formaldehyde in the reactant stream. Catalyst fouling reactions by condensation and polymerization are believed to be the main reactions responsible for the formation of carbon or coke on the catalyst. The formation of carbon is believed to involve the thermal condensation of formaldehyde or of formaldehyde with the olefinic hydrocarbons isoprenol and (iso)prenal. In the presence of the catalyst, the primary condensation products undergo dehydrogenation and polymerization type reactions and tend to undergo further dehydrogenation and decomposition until they deposit on the catalyst and form carbonaceous deposits.

[0018] The presence of formaldehyde in the reactant stream comes from two main sources. Formaldehyde can be present in the fresh feed stream sent to the reactor, i.e. as an impurity originating from the isoprenol production process. In industrial practice, isoprenol is synthesized from isobutene and formaldehyde. Any formaldehyde that cannot be separated in the purification steps after isoprenol synthesis ends up in the reactant stream.

[0019] In addition, formaldehyde is also produced in situ. A portion of the isoprenol is decomposed back to isobutene and formaldehyde. Since most continuous industrial processes operate with a recycle of unconverted isoprenol at a single pass conversion level of 50-60%, formaldehyde may be present in the unconverted isoprenol recycle stream if no steps are taken to purify the stream containing unreacted isoprenol. It has now been found that the unconverted isoprenol recycle stream is the largest source of formaldehyde contamination in the reactant stream. Therefore, the purity of the isoprenol recycle stream from the reactor is crucial to solving the objectives of the present invention.

[0020] As with conventional processes, the process according to the invention is usually carried out at partial conversion, for example 30-70%, preferably 50-60%. The unreacted isoprenol stream is separated from the product stream. The unreacted isoprenol stream is recycled, i.e. mixed with a fresh feed stream containing isoprenol to provide the reactant stream. The unreacted isoprenol stream contains isoprenol as the main component, but may also contain prenal, isoprenal, isoamyl alcohol, isovaleraldehyde, isovaleric acid, prenol, formaldehyde. It may also contain traces of other C3 and C2 aldehydes and acids.

[0021] According to the present invention, reducing the weight ratio of formaldehyde to isoprenol in the reactant stream can be accomplished in several different ways: In one embodiment, formaldehyde is removed from the unreacted isoprenol stream before it is mixed with the fresh feed stream.

[0022] In one embodiment, the unreacted isoprenol stream is mixed with a fresh feed stream and formaldehyde is removed from the combined stream.

[0023] Alternatively, it is possible to mix an unreacted isoprenol stream with an amount of fresh feed stream that is sufficiently purified to obtain the desired weight ratio of formaldehyde to isoprenol in the mixed stream.

[0024] In one embodiment, a fresh feed stream comprising isoprenol is obtained from a process of reacting isobutene with formaldehyde and is purified until the weight ratio of formaldehyde to isoprenol is less than 0.04, preferably less than 0.03, in particular less than 0.02 or less than 0.01. In a further preferred embodiment, the fresh feed stream is purified until the weight ratio of formaldehyde to isoprenol is less than 0.002 or less than 0.001.

[0025] Formaldehyde can be removed from the isoprenol stream by conventional separation methods such as distillation, selective adsorption, and / or selective reaction.

[0026] The removal of formaldehyde by distillation may involve the use of a single distillation column or a series of distillation columns. The columns or columns used may be conventional distillation columns. Suitable types of distillation columns include packed columns, e.g., columns with random or structured packing, plate columns (i.e., tray columns), and mixed columns containing both packing and trays.

[0027] Suitable tray columns may include internals through which the liquid phase flows. Suitable internals include sieve trays, bubble cap trays, valve trays, tunnel trays, and Thormann® trays, particularly bubble cap trays, valve trays, tunnel trays, and Thormann® trays.

[0028] Randomly packed columns can be packed with a variety of shaped bodies. Heat and mass transfer is improved by increasing the surface area with shaped bodies, which usually have sizes in the range of 25-80 mm. Suitable shaped bodies include Raschig rings (hollow cylinders), Lessing rings, Pall rings, Hiflow rings, and Intalox saddles. The packing material can be provided in the column in a regular or irregular manner (as bulk material, i.e., loosely packed). Suitable materials include glass, ceramic, metal, and plastic.

[0029] Structured packing is an extension of ordered packing, having a regular shaped structure, which can reduce the pressure drop of the gas flow. Suitable types of structured packing include fabric packing and metal sheet packing.

[0030] Formaldehyde removal by selective adsorption involves contacting the stream with an adsorbent that exhibits selectivity for low molecular weight aldehydes, particularly formaldehyde. Useful adsorbent materials should provide high selectivity and high adsorption capacity. An additional very important requirement is that the adsorbent material should not catalyze or participate in chemical reactions that could reduce the recovery of (iso)prenal and / or render the adsorbent inactive. Adsorbents include ion exchange resins, mesoporous solids, activated carbon, and zeolites.

[0031] Removal of formaldehyde by selective reaction involves subjecting the stream to reaction conditions which selectively react the formaldehyde into products which are less likely to deactivate and plug the catalyst, or which can be more easily separated from the stream than formaldehyde.

[0032] In one embodiment, the unreacted isoprenol stream is mixed with a crude isoprenol stream and formaldehyde is removed from the combined stream. The crude isoprenol stream is generally a product stream of an isoprenol production process from which unreacted isobutylene has been removed. This means that the removal of formaldehyde occurs in a purification unit after isoprenol synthesis. A preferred method for recovering formaldehyde from a crude isoprenol stream mixed with an unreacted isoprenol stream is described in more detail below.

[0033] In yet another embodiment, the reactant stream comprises a mixture of an unreacted isoprenol stream, isoprenol from other sources, and a fresh feed stream from the reaction of isoprenol with formaldehyde. Other sources of isoprenol are processes other than the reaction of isoprenol with formaldehyde where isoprenol is obtained as a by-product or end product (e.g., isoprenol isomerization, or isoprenol from commercial sources).

[0034] The presence of formaldehyde in the reactant stream reduces both catalyst activity and selectivity, and causes increased pressure drop and plugging of the reactor. In addition to formaldehyde, other impurities that may be present in the reactant stream may also cause reduced catalyst activity and selectivity. Preferably, the equipment or operations used to maintain a constant weight ratio of formaldehyde to isoprenol in the reactant stream are also effective to remove most of these impurities. In a preferred embodiment, the concentration in the reactant stream of at least one of the following impurities, and in particular the concentration of all of the following impurities, is kept below the indicated limit values:

[0035] [Table 1]

[0036] Adherence to these limitations is especially important when the reactant stream includes isoprenol streams from other sources.

[0037] Oxidation The process involves contacting a gaseous reactant stream containing isoprenol with a silver-containing heterogeneous catalyst in the presence of molecular oxygen to obtain an (iso)prenal-containing product stream.

[0038] Typically, the process includes the following steps: a) evaporating isoprenol; b) mixing the isoprenol vapor with an oxygen-containing gas; c) contacting the gaseous mixture with a silver-containing heterogeneous catalyst, for example by passing the gaseous mixture through a bed of the silver-containing heterogeneous catalyst, and reacting the gaseous mixture to form (iso)prenal.

[0039] The silver-containing heterogeneous catalyst is not particularly limited. It may comprise silver coated on an inert support such as steatite spheres. Alternatively, the heterogeneous catalyst is formed from silver-containing fibers and / or filaments. In a further embodiment, the heterogeneous catalyst is composed entirely of metallic silver catalyst bodies.

[0040] To produce a catalyst having silver coated on an inert support, silver metal is preferably applied to the inert material by flame spraying, although other methods, such as impregnation or plasma spraying, are suitable as long as a wear-resistant shell is obtained, which should otherwise be as smooth as possible.

[0041] If necessary, the catalyst can be diluted with an inert material that is not coated with the active composition.Suitable inert materials that are also useful as carrier materials include ceramic materials such as aluminum oxide, silicon dioxide, magnesium oxide, silicon carbide, and especially steatite.However, the catalyst filler must contain at least 10% active material particles.

[0042] Suitable shapes for the catalyst include spheres as well as other shapes such as ellipsoids, cylinders, or rings. The diameter d of a sphere, or the maximum diameter of other shapes, may be in the range of 0.1 to 1.5 cm, depending on the inner diameter of the tube bundle.

[0043] The catalyst particles are held on, for example, a silver or stainless steel mesh in a typical upright reactor.

[0044] The heterogeneous catalyst is placed in a chemical reactor (reactor) suitable for continuous gas-phase reactions. The reactor usually has at least two openings, at least one for the passage of the compounds therein and at least one for the exit of the products from the reactor. In a particular embodiment, the catalyst bed is placed in a tubular reactor, preferably in the reaction tubes of a tube bundle reactor. A tubular reactor suitable for carrying out catalytic gas-phase reactions usually comprises a catalyst tube bundle traversed by the reaction gas, filled with a catalyst bed and around which flows a heat transfer medium contained in a surrounding reactor jacket. The heat transfer medium is preferably a salt bath, usually a molten mixture of various salts such as alkali nitrates and / or nitrites.

[0045] The process is preferably carried out in a reactor such as that described in EP 0881206 B1, which consists of multiple short tubular reactors placed in a salt bath. Because the reagents and products of the process are thermally unstable, it is preferable to have relatively short reactor tubes to minimize residence time. It is also preferable to have relatively thin reactor tubes to maximize salt bath cooling and thereby minimize hot spot temperatures associated with the highly exothermic nature of the reaction. If the process was run without salt bath cooling, the high temperatures obtained under adiabatic conditions would adversely affect selectivity.

[0046] Depending on the desired reactor capacity, the tube bundle reactor used typically has 5-60 000 tubes, preferably 500-50 000 tubes, in particular 1 000-45 000 tubes. For experimental purposes, a single tube can be used.

[0047] In yet another embodiment, the tubular reactor includes a plurality of tubes disposed between tube sheets. The term "tube sheet" refers to a round, flat part of a plate or sheet with holes drilled to receive the tubes or pipes in precise locations and patterns relative to each other. The "tube sheet" is used to support and isolate the tubes in the tubular reactor.

[0048] In a preferred embodiment, the reaction tube has an inner diameter preferably in the range of 0.25 cm to 5.0 cm, more preferably in the range of 0.5 cm to 4.0 cm, especially in the range of 1.0 cm to 1.5 cm. Preferably, the reaction tube has a length of at least 5 cm, preferably in the range of 10 to 60 cm, especially in the range of 20 to 40 cm.

[0049] Preferably, the filling height of the catalyst bed in the reactor tubes is in the range of 12 mm to 500 mm, in particular 50 mm to 500 mm. The filling height depends on the length and the inner diameter of the reactor tubes. The height of the catalyst bed is preferably slightly shorter than the length of the tubes. Preferably, it extends only to the part of the tube that is in good heat exchange with the external cooling medium. In a preferred embodiment, the part of the reactor tubes towards the inlet and / or outlet of the individual tubes can be filled with shaped bodies of a substantially inert material, such as steatite. In a preferred embodiment, these inert materials have the same similar shape as the catalyst particles. Preferably, all reactor tubes are filled in a similar manner to each other with regard to the pressure drop, the volume of the catalyst bed and the position of the catalyst bed, as well as the inert material, if present. The filling of the individual tubes can be simplified by using pre-packaged samples of catalyst, where the amount of catalyst or inert material per bag is defined. For quality control, the pressure drop of the individual tubes can be monitored and documented.

[0050] The residence time of the mixed gas in the reaction tube is preferably within a range of 0.0005 to 2 seconds, more preferably within a range of 0.1 to 1.5 seconds. The composition of the reaction gas will be described in detail later.

[0051] The dehydrogenation is preferably carried out at a pressure in the range of 0.5-2 bar (absolute), preferably at atmospheric pressure or at a slightly higher pressure to create a downstream pressure drop. Particularly preferably, the reactor is operated in the range of 1.150-1.350 bar (absolute). The pressure drop along the reactor tube is preferably maintained in the range of 5-100 mbar.

[0052] The dehydrogenation is preferably carried out at a temperature in the range of 300°C to 500°C, more preferably at a temperature in the range of 350°C to 450°C.

[0053] Preferably, the concentrations of flammable molecules, oxygen and inert gases are defined by controlling the feed stream composition in a way that avoids entering the explosive region in the process. Preferably, this is achieved by flowing a "fat" composition with a concentration of flammable molecules above the explosive limit. In a preferred embodiment, oxygen is provided by using air, rather than pure or enriched oxygen. Nitrogen can be used for partial replacement of air during start-up to avoid operation in the explosive region. Once in steady state operation, the nitrogen is slowly exchanged for air.

[0054] The product stream is worked up in a conventional manner. For example, the hot reaction gas is quenched with a solvent such as water or, preferably, directly in the condensed product mixture as it leaves the reactor. The liquid phase of the reactor effluent is separated. The organic phase is distilled to separate the (iso)prenal from the unreacted isoprenol.

[0055] reproduction Typically, regeneration cycles are performed periodically to remove accumulated coke from the silver-containing heterogeneous catalyst. The regeneration cycles can be initiated when the pressure drop increases above a threshold value, or can be initiated at any time interval (e.g., once a week). The regeneration cycle consists of pumping dilution air or air into the reactor for a predetermined time, e.g., 6 to 24 hours, while increasing the temperature of the salt bath to, e.g., 400-450°C to burn off the coke.

[0056] Integration of formaldehyde removal and isoprenol synthesis The selective recovery of formaldehyde from aqueous alcohol solutions is very difficult. This difficulty arises from the fact that monomeric formaldehyde (and polymeric formaldehyde) forms both hydrates with water and hemiformals with alcohols such as isoprenol. The hydrates and hemiformals, which have different degrees of polymerization of formaldehyde, have mixed boiling points. The stability of and the equilibrium between the hydrates and hemiformals are temperature dependent. Formals formed in the upper regions of the distillation column can decompose in the hot bottom of the column, further complicating the separation operation.

[0057] Recently, it has been reported that formaldehyde can be substantially completely separated from isoprenol in a distillation train involving a first distillation at a lower temperature where the equilibrium shifts to the hemiformal side of formaldehyde and isoprenol, so that essentially all of the formaldehyde remains at the bottom of the distillation column, and a second distillation at a higher temperature where the hemiformal is cleaved into formaldehyde and isoprenol, so that the formaldehyde can be easily separated from the isoprenol. See M. Dyga, A. Keller, and H. Hasse, Industrial & Engineering Chemistry Research 2021, 60, 11, 4471-4483.

[0058] In one embodiment, the unreacted isoprenol stream is mixed with a crude isoprenol stream containing isoprenol, water, and formaldehyde. And removing formaldehyde from the mixed stream comprises: (i) passing the combined stream to a first light end separation column operating at a pressure less than or equal to 1.5 bara to obtain a first bottoms stream comprising isoprenol and formaldehyde and a first distillation stream comprising water and light boilers; (ii) passing the first bottoms stream to a second light end cut column operated at a pressure of at least 2 bara to obtain a second distillation stream comprising aqueous formaldehyde and a second bottoms stream comprising isoprenol; and (iii) passing the second bottoms stream to a finishing tower to obtain a bottoms stream containing high boilers and a reactant stream as a distillation stream; Includes.

[0059] Formaldehyde is reacted with isobutylene to obtain a reaction mixture. Unreacted isobutylene is removed from the reaction mixture in an isobutylene distillation column to obtain a crude isoprenol stream. The crude isoprenol is obtained as a bottoms stream from the isobutylene distillation column. Isobutylene is obtained as an isobutylene distillation stream. The isobutylene distillation stream is preferably recycled to the reaction of formaldehyde with isobutylene.

[0060] In order to allow a first distillation at a temperature below the dissociation temperature of isoprenol-formaldehyde and a second distillation at a temperature above the dissociation temperature of isoprenol-formaldehyde, the invention envisages two low-boiler columns operated at different pressures. Thus, at the relatively low pressure prevailing in the first low-boiler column, a first distillate is obtained that contains water and low boilers that are essentially free of formaldehyde. At the relatively high pressure prevailing in the second low-boiler column, substantially all of the formaldehyde is separated from the isoprenol. Thus, the process of the invention makes it possible to obtain isoprenol that is essentially free of formaldehyde.

[0061] The term "essentially free of formaldehyde" is understood to indicate that no significant amount of formaldehyde is present in the resulting pure isoprenol. Thus, the resulting pure isoprenol preferably contains less than 0.5% by weight, more preferably less than 0.1% by weight, of formaldehyde.

[0062] The crude isoprenol stream comprises isoprenol, water and formaldehyde. Preferably, the crude isoprenol stream comprises 50-75% by weight, more preferably 60-65% by weight, of isoprenol. Preferably, the crude isoprenol stream comprises 15-40% by weight, more preferably 22-35% by weight, of water. Preferably, the crude isoprenol stream comprises 1-5% by weight, more preferably 2-3% by weight, of formaldehyde.

[0063] Preferably, the crude isoprenol stream is a liquid stream. The liquid stream can be a single-phase liquid stream or a two-phase liquid stream.

[0064] The crude isoprenol is sent to a first light-end column operated at a pressure of 1.5 bara or less. Any high pressure crude isoprenol stream is preferably released prior to being sent to the first light-end column. The crude isoprenol stream is preferably fed as a side stream to the first light-end column to define a rectifying section above the feed location and a stripping section below the feed location.

[0065] The first low boiler separation column provides a first bottoms stream comprising isoprenol and formaldehyde, and a first distillation stream comprising water and low boilers. The term "low boilers" is understood to refer to organic compounds (other than formaldehyde) that have a boiling point at atmospheric pressure lower than that of isoprenol, and thus a boiling point below about 130° C. The most common low boilers are methanol and / or isoprenyl formate, which are formed as by-products during the process.

[0066] In a preferred embodiment, the first light end cut column is operated at a pressure of 1.2 bara or less, preferably 0.5 bara or less. The bottom temperature of the first light end cut column is preferably in the range of 80 to 135°C, more preferably 90 to 115°C, and most preferably 95 to 105°C. The top temperature of the first light end cut column is preferably in the range of 45 to 105°C, more preferably 55 to 80°C.

[0067] In a particularly preferred embodiment, the first light end cut column is operated at a pressure in the range of 0.2 to 0.5 bara, a bottom temperature in the range of 90 to 115°C, and a top temperature in the range of 55 to 80°C.

[0068] The first light end separation column preferably has a number of theoretical plates of 15 to 65, more preferably a number of theoretical plates of 25 to 40. In particular, the stripping section of the first light end separation column preferably has a number of theoretical plates of 10 to 25. The rectification section of the first light end separation column preferably has a number of theoretical plates of 5 to 40.

[0069] The first bottoms stream preferably comprises 75 to 95 wt. % isoprenol, more preferably 80 to 90 wt. % isoprenol.

[0070] The first distillate is typically removed in gaseous form at the top of the first light-cutting column and condensed to obtain a two-phase liquid stream. The two-phase liquid stream is preferably phase separated in a separation vessel to obtain an aqueous phase and an organic phase. The aqueous phase is preferably passed to a wastewater stripping column, described below. The organic phase is preferably partially returned to the top of the first light-cutting column as a reflux stream. Another part of the organic phase is preferably discarded from the process to avoid the accumulation of water-insoluble low boilers in the first light-cutting column.

[0071] In a preferred embodiment, at least a portion of the first distillation stream is sent to a wastewater stripping column to separate low boilers and entrained isoprenol from the water. Preferably, the portion of the first distillation stream sent to the wastewater stripping column is the aqueous phase obtained by condensation and phase separation of the first distillation stream, as discussed above.

[0072] In the wastewater stripping column, low boilers are obtained as a low boiler distillate stream and wastewater is obtained as a bottoms stream. Both the low boiler distillate stream and the wastewater bottoms stream are removed from the process and each stream can be sent for further processing.

[0073] Furthermore, isoprenol is preferably obtained as a side stream in the wastewater stripping column. The isoprenol side stream is typically a two-phase stream and preferably contains 15-40 wt. %, more preferably 25-35 wt. % isoprenol. The isoprenol side stream is preferably recycled to the first light end cut column.

[0074] The low boiler distillation stream preferably contains 75 to 95% by weight, more preferably 80 to 85% by weight, of low boilers.

[0075] The wastewater bottoms stream preferably contains less than 1.2 wt. % organic matter, more preferably less than 0.6 wt. % organic matter. The wastewater bottoms stream typically contains formaldehyde at a concentration of 0.05 to 1.5 wt. % formaldehyde, for example 0.3 to 0.9 wt. % formaldehyde.

[0076] The wastewater stripping column is preferably operated at a pressure of 1.5 bara or less, preferably 1.1 bara or less. The bottom temperature of the wastewater stripping column is preferably in the range of 95 to 110° C., more preferably in the range of 97 to 103° C. The top temperature of the wastewater stripping column is preferably in the range of 65 to 100° C., more preferably in the range of 75 to 85° C.

[0077] In a particularly preferred embodiment, the wastewater stripping column is operated at a pressure in the range of 0.95 to 1.1 bara, a bottoms temperature in the range of 97 to 103°C, and an overhead temperature in the range of 75 to 85°C.

[0078] The wastewater stripping column preferably has 6-30 theoretical plates, more preferably 10-20 theoretical plates.

[0079] The first bottoms stream obtained in the first light ends column is sent to a second light ends column operating at a pressure of at least 2 bara. The first bottoms stream is preferably fed as a side stream to the second light ends column and defines a rectifying section above the feed location and a stripping section below the feed location.

[0080] In the second low boiler separation column, a second distillation stream comprising or consisting essentially of aqueous formaldehyde and a second bottoms stream comprising isoprenol are obtained. The second bottoms stream further comprises high boilers. The term "high boilers" is understood to refer to organic compounds having a boiling point at atmospheric pressure higher than that of isoprenol, i.e., higher than about 130° C. The most common high boilers are diols and / or oligomers formed as by-products during the process.

[0081] In a preferred embodiment, the second light end cut column is operated at a pressure of 2.5 bara or more, preferably 2.8 bara or more, and most preferably 2.9 bara or more. The bottom temperature of the second light end cut column is preferably in the range of 160 to 200°C, more preferably 170 to 185°C, and most preferably 175 to 180°C. The top temperature of the second light end cut column is preferably in the range of 115 to 160°C, and more preferably in the range of 125 to 145°C.

[0082] In a particularly preferred embodiment, the second light end cut column is operated at a pressure in the range of 2.9 to 3.5 bara, a bottom temperature in the range of 175 to 180°C, and a top temperature in the range of 130 to 140°C.

[0083] The second light end separation column preferably has a number of theoretical plates of 20 to 60, more preferably 35 to 60. In particular, the stripping section of the first light end separation column preferably has a number of theoretical plates of 25 to 45. The rectification section of the first light end separation column preferably has a number of theoretical plates of 7 to 20.

[0084] At the top of the second light end cut column, an off-gas is typically obtained which mainly contains nitrogen and may contain traces of isoprenol, formic acid, water, formaldehyde, and / or decomposition gases.

[0085] The second bottoms stream preferably contains 82-96 wt. %, more preferably 87-91 wt. % isoprenol. The relatively high pressure of the second light-ends cut column allows for a high degree of separation of formaldehyde and isoprenol. Thus, the second bottoms stream preferably contains at most 0.5 wt. %, more preferably at most 0.1 wt. % formaldehyde.

[0086] The second distillation stream is an aqueous stream and preferably comprises 25-60% by weight, more preferably 40-50% by weight, especially 45-50% by weight of formaldehyde. The second distillation stream preferably comprises at most 15% by weight, more preferably at most 5% by weight of isoprenol.

[0087] Due to the broad condensation curve of the vapors appearing at the top of the second light end column, it is advantageous to use a condenser with liquid recycling. Direct condensation in the quench with liquid circulation is particularly advantageous. Therefore, in a preferred embodiment of the process, a quench section is provided in the vapor flow direction downstream of the rectification section of the second light end column. The term "vapor flow direction" relates to the direction of flow of the gas components in the separation column, i.e. upwards towards the top of the column. The quench section is preferably provided in the second light end column above the rectification section.

[0088] Direct condensation in the quench also reduces fouling caused by various condensation and polymerization mechanisms of formaldehyde that may occur at points of localized high formaldehyde concentrations. To avoid the risk of fouling in the second light end column and downstream processes, especially in the off-gas of the second light end column, the concentration of formaldehyde in the second distillate is preferably 60 wt.% or less, more preferably 55 wt.% or less, in particular 50 wt.% or less.

[0089] At the lower end of the quench section, the aqueous liquid is recovered. If a quench section is provided in the second light-ends cut column, the aqueous liquid can be recovered, for example, in recovery trays above the rectification section and below the quench section.

[0090] The aqueous liquid is partially recycled to the quench section through a circulation line and partially removed as a second distillate. Suitably, a portion of the aqueous liquid recycled to the quench section is recycled to an upper portion of the quench section. Circulation of the aqueous liquid is typically achieved by the use of a pump. As mentioned above, the second distillate can optionally be at least partially recycled to the reaction of formaldehyde with isobutylene.

[0091] A portion of the aqueous liquid is circulated through the quench section to allow for cooling of the vapors rising through the quench section and absorption of formaldehyde from the vapor into the aqueous liquid, thus quenching the formaldehyde from the vapors rising through the quench section.

[0092] Additionally, the aqueous liquid is partially returned to the rectification section of the second light ends column as a reflux stream, which can be accomplished by a reflux line, or the aqueous liquid can be partially returned to the rectification section as an overflow from a collection tray below the quench section.

[0093] The mass flow ratio of the reflux stream to the second distillate is preferably in the range of 2:1 to 10:1, more preferably in the range of 3:1 to 7:1.

[0094] In a preferred embodiment, the aqueous liquid is cooled before being recycled to the quench section. Preferably, the portion of the aqueous liquid removed as the second distillate is a partial stream of the cooled aqueous liquid.

[0095] The temperature of the aqueous liquid recovered at the bottom of the quenching section is preferably in the range of 80 to 140° C., more preferably 125 to 135° C. The temperature of the cooled aqueous liquid circulated to the quenching section is preferably 10 to 80° C. lower than the temperature of the aqueous liquid recovered at the bottom of the quenching section, which allows for an energetically advantageous process.

[0096] The hot aqueous liquid removed at the lower end of the quench section serves for heat integration: in a suitable embodiment, it is heat exchanged with a stream of crude isoprenol entering the first light cut column before being recycled to the quench section.

[0097] In one embodiment, a scrubbing section is provided downstream of the quench section in the vapor flow direction, and water is introduced into the upper part of the scrubbing section. Preferably, the scrubbing section is provided in the second low-boiler separation tower above the quench section. The scrubbing section can maintain the formaldehyde concentration in the second distillate below the above-mentioned critical concentration, thus avoiding the accumulation of paraformaldehyde, for example, in the off-gas line.

[0098] The mass flow ratio of the water introduced at the top of the scrub section to the first bottoms stream obtained in the first light end cut tower is typically in the range of 0.01:1 to 0.06:1, and more preferably in the range of 0.015:1 to 0.03:1.

[0099] The second bottoms stream is sent to a finishing tower where pure isoprenol is obtained as a distillation stream. High boilers are removed via the bottoms stream. Since the second bottoms stream is essentially free of formaldehyde, the separation operations in the finishing tower are significantly less complicated than in the case where formaldehyde separation in the low boilers section is less efficient.

[0100] A pure isoprenol distillate stream preferably comprises at least 97.0% by weight, more preferably 98.0% by weight, for example 98.1 to 99.5% by weight, of isoprenol.

[0101] The high boiler bottoms stream preferably comprises 90 to 99.9 wt.%, more preferably 99 to 99.8 wt.% high boilers. Preferably, the high boiler bottoms stream comprises less than 0.2 wt.% formaldehyde, for example less than 0.05 wt.% formaldehyde.

[0102] In a preferred embodiment, the finishing column is operated at a pressure of 0.5 bara or less, preferably 0.25 bara or less. The bottom temperature of the first light end cut column is preferably in the range of 130 to 190°C, more preferably in the range of 150 to 170°C. The top temperature of the finishing column is preferably in the range of 60 to 90°C, more preferably in the range of 65 to 85°C.

[0103] In a particularly preferred embodiment, the finishing column is operated at a pressure in the range of 0.05 to 0.2 bara, a bottoms temperature in the range of 150 to 170°C, and an overhead temperature in the range of 65 to 85°C.

[0104] The finishing tower preferably has 6 to 40 theoretical plates, more preferably 10 to 20 theoretical plates.

[0105] The invention is illustrated by the accompanying drawings and the following examples. [Brief description of the drawings]

[0106] [Figure 1] FIG. 1 shows the conversion and selectivity over time in an isoprenol oxidation reaction run. [Diagram 2] FIG. 2 illustrates schematics of Schemes A-D with various options for accommodating the recycle stream of unreacted isoprenol. [Diagram 3] FIG. 3 is a flow chart of a process according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0107] According to the present invention, maintaining a constant weight ratio of formaldehyde to isoprenol in the reactant stream can be achieved in a number of ways, as illustrated in Figure 2. Scheme A shows how recycle stream b is typically incorporated into prior art processes. Scheme B shows how recycle stream b can be sent back through an isoprenol separation step of an existing isoprenol production unit. Scheme C shows the entire reactor feed stream through an additional impurity separation step, e.g., a distillation step. Scheme D shows only the recycle stream through an additional impurity separation step, e.g., a distillation step. EXAMPLES

[0108] Example 1 The oxidation of isoprenol is studied in a mini-plant reactor. The catalyst bed consists of 30 cm of silver-coated (5 wt%) steatite spheres (2 mm diameter) packed in a stainless steel tube (12 mm inner diameter). The temperature of the cooling jacket is maintained at 380° C. An isoprenol load of 300 g / h is employed, together with 30 g / h water and 92 L / h air. The reactant stream is quenched through a water cooler. The condensate is analyzed offline by gas chromatography. The uncondensed gas stream is analyzed online by gas chromatography. The conversion and the selectivity of (iso)prenal are calculated using both analyses.

[0109] Experiments were carried out using isoprenol with increasing formaldehyde concentrations of 0, 2, 3 and 4 wt%.

[0110] FIG. 1 shows the conversion and selectivity over time on run. By day 5, the formaldehyde concentration was increased to 2 wt. % and by day 14, to 3 wt. %. Regeneration cycles are marked with a plus sign at 50% conversion. A decrease in selectivity is observed with increasing formaldehyde concentration. In the early hours of run, a decrease in selectivity of about 1.5 percentage points is observed per wt. % of formaldehyde in the feed stream. As the formaldehyde concentration increased, regeneration cycles became more frequent. At 4 wt. % formaldehyde, even more frequent regeneration cycles were required.

[0111] Example 2 Isoprenal is prepared by the process shown diagrammatically in Figure 3. The reactor feed stream consists of a mixture of recycled isoprenol, isoprenol from various sources and freshly synthesized isoprenol from the reaction of isobutene with formaldehyde (Figure 3; 1, a, b, i). The weight ratio of formaldehyde to isoprenol in the mixed stream is maintained below 0.04. The content of other impurities complies with the limits mentioned above. The mixed feed stream is partially distilled off (Figure 3; 2, d). The remaining liquid stream contains high boilers and is removed from the reactor feed stream (Figure 3; o). An air feed stream is introduced (Figure 3; e) and mixed with the gaseous isoprenol.

[0112] Reactor 3 is a multi-tube reactor packed with a catalyst containing silver on steatite spheres. The reaction feed stream is converted in the reactor and immediately quenched with cooled reaction products (Fig. 3; 3). The cooled reaction stream consists of a gas stream and a liquid stream (Fig. 3; j, f). The gas stream first passes through an absorption tower where the remaining gaseous (iso)prenal is absorbed (Fig. 3; 6). The (iso)prenal enriched absorbent (Fig. 3; k) is subsequently stripped in a stripping unit (Fig. 3; 7). The stripped absorbent is recycled to the absorption tower and the removed (iso)prenal is returned to the liquid reaction stream of the reactor (Fig. 3; l). The gas stream (Fig. 3; m) is further processed. This gas stream consists of nitrogen, unconverted oxygen and any gaseous by-products of the reaction including CO, CO2, traces of formaldehyde, isobutene and hydrogen. The higher the conversion in the reactor, the higher the content of these products. The liquid reactor stream (g) contains unconverted isoprenol, prenal, isoprenal, and water, along with undesirable secondary components such as isoamyl alcohol, isovaleraldehyde, isovaleric acid, prenol, and formaldehyde. This stream is sent to a separation unit (Figure 3; 5). The separated (iso)prenal is sent for further use (Figure 3; h), where water and isoprenol, as well as other secondary components, are first phase separated. The aqueous phase is sent for further processing (Figure 3; n). The organic phase (Figure 3; i) is recycled to the liquid reactor feed stream.

[0113] The table below shows a typical composition of the liquid reaction product (Figure 3, step 4).

[0114] [Table 2]

[0115] The table below shows a typical composition of gas stream m (Figure 3, stream m).

[0116] [Table 3]

Claims

1. 1. A method for preparing isoprenal and / or prenal by contacting a gaseous reactant stream containing isoprenol with a silver-containing heterogeneous catalyst in the presence of molecular oxygen to obtain an isoprenal and / or prenal-containing product stream, wherein the weight ratio of formaldehyde to isoprenol in the reactant stream is maintained at less than 0.

04.

2. 10. The method of claim 1, comprising separating an unreacted isoprenol stream from the product stream and combining the unreacted isoprenol stream with a fresh feed stream comprising isoprenol to provide the reactant stream.

3. 10. The method of claim 1, wherein maintaining the weight ratio of formaldehyde to isoprenol in the reactant stream comprises removing formaldehyde from a stream comprising isoprenol by distillation, selective adsorption, and / or selective reaction.

4. 4. The method of claim 3, comprising removing formaldehyde from the unreacted isoprenol stream prior to combining the unreacted isoprenol stream with the fresh feed stream.

5. 4. The method of claim 3, comprising combining the unreacted isoprenol stream with the fresh feed stream and removing formaldehyde from the combined stream.

6. mixing the unreacted isoprenol stream with a crude isoprenol stream containing isoprenol, water, and formaldehyde, and removing formaldehyde from the combined stream; (i) passing the combined stream to a first light-end cut column operated at a pressure of 1.5 bara or less to obtain a first bottoms stream comprising isoprenol and formaldehyde and a first distillation stream comprising water and light boilers; (ii) passing the first bottoms stream to a second light-ends cut column operated at a pressure of at least 2 bara to obtain a second distillation stream comprising aqueous formaldehyde and a second bottoms stream comprising isoprenol; and (iii) sending the second bottoms stream to a finishing tower to obtain a bottoms stream containing high boilers and a reactant stream as a distillate stream; The method of claim 5 , comprising:

7. 10. The method of claim 1, wherein the heterogeneous catalyst comprises silver coated on an inert support or a fully metallic silver catalyst body.

8. 8. The method of claim 3, wherein the fresh feed stream comprising isoprenol originates from a process of reacting isobutene with formaldehyde and is purified to a weight ratio of formaldehyde to isoprenol of less than 0.04.

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