TWI776891B - Process for the epoxidation of propene - Google Patents

Abstract

In a process for the epoxidation of propene by reacting propene with hydrogen peroxide in the presence of a methanol solvent and a titanium zeolite epoxidation catalyst, the steps of: separating from the reaction mixture a crude propene oxide and a solvent mixture comprising methanol, water and peroxides; subjecting the solvent mixture to a catalytic hydrogenation for hydrogenating peroxides, providing a hydrogenated solvent mixture comprising from 1 to 1000 mg/kg of acetaldehyde; separating the hydrogenated solvent in at least one distillation stage, adding an acid to the hydrogenated solvent mixture or to at least one distillation stage, providing a recovered methanol as an overhead product; passing the recovered methanol through a bed of an acidic ion exchange resin, providing a treated methanol; and recycling the treated methanol to the epoxidation reaction prevent deactivation of the epoxidation catalyst by recycling acetaldehyde with the methanol.

Description

Epoxidation method of propylene

The present invention relates to a process for the epoxidation of propylene with hydrogen peroxide in the presence of a titanium silicalite catalyst.

The epoxidation of propylene with hydrogen peroxide in the presence of a titanium siliceous catalyst is known from EP 0 100 119 A1. The reaction of propylene with hydrogen peroxide in the presence of titanium zeolite catalyst is usually carried out in methanol solvent to obtain high reaction rate and product selectivity. In addition to propylene oxide, the epoxidation reaction produces by-products such as formaldehyde, acetaldehyde, and hydroperoxides formed by the ring-opening reaction of hydrogen peroxide with propylene oxide.

The by-products acetaldehyde and propionaldehyde are difficult to separate from the propylene oxide product. WO 2004/048355 discloses a process for the removal of methanol and acetaldehyde from crude propylene oxide by extractive distillation in a single distillation column, wherein the additional feed contains unsubstituted NH at or above the feed point of the crude propylene oxide ₂ -radical and can react with acetaldehyde under distillation conditions. An aqueous hydrazine solution is preferred as the additional feed compound. Water is particularly preferred as the extraction solvent. This process provides high purity propylene oxide suitable for the manufacture of polyether polyols.

WO 03/093255 teaches the use of heterogeneous catalysts to hydrogenate, under certain conditions, a solvent stream recovered from the epoxidation of olefins with hydrogen peroxide, wherein unreacted hydrogen peroxide, formaldehyde, acetaldehyde, and those formed in the epoxidation reaction Hydroperoxides such as 1-hydroperoxy-2-propanol and 2-hydroperoxy-1-propanol are hydrogenated before recycling the solvent to the epoxidation reaction. WO 03/093255 teaches herein that substantial methyl formate, formaldehyde, acetaldehyde, dimethoxymethane and 1,1-dimethoxyethane lead to catalyst deactivation.

WO 2004/029032 teaches that in the presence of a titanium-containing zeolite catalyst, strong bases or cations of such bases with a pK _B of less than 4.5 below 100 wppm and at least 100 wppm of weak bases with a pK _B of at least 4.5 or The epoxidation of olefins is carried out with hydrogen peroxide in an aqueous reaction mixture of the cations of these bases. Limiting the amount of strong base reduces or prevents long-term deactivation of the catalyst, while the presence of a weak base improves selectivity to epoxides without affecting the long-term activity of the catalyst. Organic amines are strong layers with pK _B below 4.5, so introduction of these amines into the epoxidation step with the recycle stream must be avoided to maintain long-term activity and selectivity of the epoxidation catalyst.

WO 2004/048354 teaches the recovery of a solvent stream from a reaction mixture for epoxidizing an olefin using hydrogen peroxide in the presence of a titanium-containing zeolite catalyst, wherein the recovered solvent stream is treated to contain less than 50 wppm before being recycled to the epoxidation step Nitrogen in the form of organic nitrogen compounds to reduce catalyst deactivation when recycling the solvent. The solvent treatment is preferably an acid treatment. WO 2004/048354 teaches that acid treatment can be performed by adding a carboxylic or mineral acid to the solvent stream before or during the recovery of the solvent as an overhead product by distillation, or by treating the overhead product obtained by distillation with an acidic ion exchanger. conduct.

The inventors of the present invention have now found that if the hydrogenation does not convert all acetaldehyde and acetaldehyde salts, as described in WO 03/093255, the methanol solvent stream recovered from the epoxidation of propylene is hydrogenated and then recovered by distillation in the form of The removal of organic nitrogen compounds from the hydrogenated solvent stream before or during the addition of acid to the hydrogenated solvent stream prior to or during the methanol overhead will provide recovered methanol that may contain more acetaldehyde than the hydrogenated solvent stream contains. Recycle of methanol recovered in this way to the epoxidation reaction results in deactivation of the catalyst. The inventors of the present invention have further found that, contrary to what would be expected from the teaching of WO 03/093255 on catalyst deactivation by 1,1-dimethoxyethane, by passing the methanol recovered in this way through acidic ion exchange Treating the methanol with a bed of reagents will convert most of the acetaldehyde to 1,1-dimethoxyethane, and recycling the methanol treated in this way prevents deactivation of the epoxidation catalyst.

Therefore, the object of the present invention is a process for the epoxidation of propylene, comprising the following steps: a) providing a reaction mixture by reacting propylene with hydrogen peroxide in the presence of a methanol solvent and a titanium zeolite epoxidation catalyst, b) from step a ) to separate crude propylene oxide and a solvent mixture comprising methanol, water and peroxide, c) catalytic hydrogenation of the solvent mixture isolated in step b) to hydrogenate the peroxide to provide a mixture comprising 1 to 1000 mg/kg of acetaldehyde in a hydrogenated solvent mixture, d) separating the hydrogenated solvent mixture of step c) in at least one distillation stage, adding acid to the hydrogenated solvent mixture of step c) or to at least one distillation stage, providing recovered methanol as overhead product, e) applying step The recovered methanol of d) is passed through an acidic ion exchange resin bed to provide treated methanol, and f) the treated methanol of step e) is recycled to step a).

In step a) of the method of the present invention, propylene is reacted with hydrogen peroxide in the presence of methanol solvent and a titanium zeolite epoxidation catalyst to provide a reaction mixture.

Propylene is preferably used in molar excess to hydrogen peroxide, preferably in a molar ratio of propylene to hydrogen peroxide of 1.1:1 to 30:1, more preferably 2:1 to 10:1 and most preferably 3:1 1 to 5:1. In a preferred embodiment, propylene is used in an excess sufficient to maintain the additional propylene-rich liquid phase during step a). The propylene may contain propane, preferably with a propane to propylene molar ratio of 0.001 to 0.15, and more preferably 0.08 to 0.12.

Hydrogen peroxide can be used in the form of an aqueous solution, preferably containing 30 to 75% by weight, and most preferably 40 to 70% by weight of hydrogen peroxide. The aqueous hydrogen peroxide solution is preferably prepared by the anthraquinone method.

The methanol solvent can be technical grade methanol, a solvent stream recovered during processing of the epoxidation reaction mixture, or a mixture of the two. The methanol solvent may contain small amounts of other solvents, such as ethanol, preferably in amounts less than 2% by weight. The methanol solvent may also contain water, preferably 2 to 8% by weight of water. The methanol solvent is preferably used in the epoxidation in a weight ratio of 0.5 to 20 relative to the combined weight of the water and hydrogen peroxide solution.

The epoxidation catalyst used in step a) preferably comprises titanium zeolite containing titanium atoms at the silicon lattice sites. Preferably, a titanium siliceous rock catalyst is used, preferably having an MFI or MEL crystal structure. The best system uses a titanium siliceous rock-1 catalyst with MFI structure, as known from EP 0 100 119 A1. The titanium siliceous rock catalyst is preferably used as a shaped catalyst in the form of pellets, extrudates or shaped bodies. For the molding method, the catalyst may contain from 1 to 99% of a binder or carrier material, all of which are free from hydrogen peroxide or epoxy under the reaction conditions used in the epoxidation. For propane reaction, silica is the preferred binder. Preferably, extrudates with a diameter of 1 to 5 mm are used as the forming catalyst. The amount of catalyst used can vary within wide limits and is preferably selected to achieve a consumption of more than 90%, preferably more than 95%, of peroxide within 1 minute to 5 hours under the epoxidation reaction conditions used hydrogen.

The epoxidation reaction in step a) is preferably carried out at a temperature of 20 to 80°C, more preferably 25 to 60°C. The epoxidation reaction is preferably carried out at a pressure higher than the vapor pressure of the propylene at the reaction temperature to maintain the propylene dissolved in the solvent or in a separate liquid phase. The pressure in step a) is preferably 1.9 to 5.0 MPa, more preferably 2.1 to 3.6 MPa, and most preferably 2.4 to 2.8 MPa. The use of excess propylene at high pressure provides high reaction rates and hydrogen peroxide conversion while providing high selectivity to propylene oxide.

The epoxidation reaction is preferably carried out with the addition of ammonia to improve the epoxide selectivity, as described in EP 0 230 949 A2. The amount of ammonia added is preferably such that the weight ratio of ammonia to the initial amount of hydrogen peroxide is 0.0001 to 0.003.

The epoxidation reaction of step a) is preferably carried out in a fixed bed reactor by passing a mixture comprising propylene, hydrogen peroxide and methanol solvent through a fixed bed comprising shaped titanium zeolite catalyst. The fixed bed reactor is preferably a tube bundle reactor, and the catalyst fixed bed is arranged inside the reaction tube. The fixed bed reactor is preferably equipped with cooling means and cooled using a liquid cooling medium. The temperature profile along the length of the fixed catalyst bed is preferably adjusted to maintain the reaction temperature in the range below 5°C along 70 to 98%, preferably 80 to 95% of the length of the fixed catalyst bed , preferably in the range of 0.5 to 3°C. The temperature of the cooling medium fed to the cooling means is preferably adjusted to a value 3 to 13°C lower than the maximum temperature in the catalyst fixed bed. The epoxidation reaction mixture is preferably passed through the catalyst bed in downflow mode, preferably at a surface velocity of 1 to 100 m/h, more preferably 5 to 50 m/h, most preferably 5 to 30 m/h . The superficial velocity is defined as the ratio of volume flow rate/cross-section of the catalyst bed. Additionally, it is preferred to pass the reaction mixture through the catalyst bed at a liquid space velocity (LHSV) per hour of 1 to 20 h ^-1 , preferably 1.3 to 15 h ^-1 . It is particularly preferred to maintain the catalyst bed in a trickle bed state during the epoxidation reaction. Suitable conditions for maintaining the trickle bed state during the epoxidation reaction are disclosed in WO 02/085873, page 8, line 23 to page 9, line 15. The best epoxidation reaction is to use a catalyst fixed bed maintained in a trickle bed state, and at a pressure close to the vapor pressure of propylene at the reaction temperature, the use of two liquid phases - a solvent-rich phase and a propylene-rich liquid are provided. Phase-excess propylene of the reaction mixture was carried out. Two or more fixed bed reactors can be operated in parallel or in series to enable continuous operation of the epoxidation process while regenerating the epoxidation catalyst. Regeneration of the epoxidized catalyst can be carried out by calcination, by use of heated gas, preferably oxygen-containing gas, or by solvent washing, preferably by periodic regeneration as described in WO 2005/000827. The regeneration of the epoxidation catalyst is preferably carried out without removing it from the propylene reactor. Different methods of regeneration can be combined.

In step b) of the process according to the invention, crude propylene oxide is separated from the reaction mixture of step a) and a solvent mixture comprising methanol, water and peroxide is separated from the reaction mixture of step a). Isolation of the crude propylene oxide and solvent mixture from the reaction mixture can be carried out by methods known from the prior art. Preferably, the separation of the solvent mixture from the reaction mixture is performed to provide a solvent mixture comprising less than 5% by weight of propylene and less than 2% by weight of propylene oxide.

Preferably, the reaction mixture is depressurized and the propylene vapor formed by decompression is recompressed and cooled to recover propylene by condensation. The compressed propylene vapor is preferably fed to a propylene distillation column and separated into an overhead product containing unreacted propylene and a bottoms product containing compounds boiling higher than propylene, such as propylene oxide and methanol solvents. The overhead product containing unreacted propylene can be recycled to the epoxidation reaction. The bottom product can be combined with the liquid mixture remaining after depressurization. The liquid mixture remaining after depressurization is preferably separated by distillation in a pre-separation column to provide crude propylene oxide comprising propylene oxide, methanol and residual propylene as overhead product and methanol, water and peroxide A solvent mixture of the compounds was used as the bottom product. The pre-separation column is preferably operated to provide an overhead product comprising 20 to 60% of the methanol contained in the liquid phase of the final depressurization step. The pre-separator preferably has 5 to 20 theoretical separation stages in the stripping section and less than 3 theoretical separation stages in the rectification section, and is preferably in a no reflux and no rectification section are operated to minimize the residence time of propylene oxide in the front separation column. The pre-separation column is preferably operated at a pressure of 0.16 to 0.3 MPa. Propylene oxide and methanol are condensed from the overhead product of the pre-separator column, and propylene is preferably stripped from the condensate obtained in the propylene stripper, which provides a substantially propylene-free propylene oxide and methanol-containing product. Bottom logistics.

Purified propylene oxide is preferably separated from the bottoms stream of the propylene stripper in extractive distillation using water as the extraction solvent. Extractive distillation is preferably operated with an additional feed of reactive compounds containing unsubstituted _NH2 groups and capable of reacting with acetaldehyde during the extractive distillation, as described in WO 2004/048335. Extractive distillation using reactive compounds provides high purity propylene oxide containing less than 50 ppm carbonyl compounds.

In step c) of the process of the present invention, the solvent mixture isolated in step b) is subjected to catalytic hydrogenation to hydrogenate the peroxide contained in the solvent mixture. The reaction conditions for the catalytic hydrogenation are selected to provide a hydrogenated solvent mixture comprising 1 to 1000 mg/kg of acetaldehyde.

The catalytic hydrogenation is preferably carried out at a hydrogen partial pressure of 0.5 to 30 MPa, more preferably 1 to 25 MPa and most preferably 1 to 5 MPa. The temperature is preferably in the range of 80 to 180°C, more preferably 90 to 150°C. The catalytic hydrogenation is carried out in the presence of a hydrogenation catalyst, preferably a heterogeneous hydrogenation catalyst. Reye's Nickel and Reye's Cobalt can be used as hydrogenation catalysts. Preferably, a supported metal catalyst comprising one or more metals selected from the group consisting of Ru, Rh, Pd, Pt, Ag, Ir, Fe, Cu, Ni and Co. The metal is preferably platinum, palladium, iridium, ruthenium or nickel, and most preferably ruthenium or nickel. The catalyst support can be any solid that is inert and does not deteriorate under hydrogenation conditions. Suitable as catalyst supports are activated carbon, the oxides SiO ₂ , TiO ₂ , ZrO ₂ and Al ₂ O ₃ , and mixed oxides comprising at least two of silicon, aluminum, titanium and zirconium. Activated carbon is preferably used as the catalyst support of the supported metal catalyst. The catalyst support is preferably shaped into spheres, pellets, lozenges, granules or extrudates. Preferred are extrudates with a diameter of 0.5 to 5 mm, especially 1 to 3 mm, and a length of 1 to 10 mm. The supported metal catalyst preferably contains 0.01 to 60% by weight of metal. The supported precious metal catalyst preferably contains 0.1 to 5% of metal. Supported nickel and cobalt catalysts preferably contain 10 to 60% metal. Supported metal catalysts can be prepared by methods known in the art, preferably by impregnating the catalyst support with a metal salt and then reducing the metal salt to the catalytically active metal. Suitable supported metal catalysts are commercially available, for example, from Clariant under the tradename ^NISAT® and from Evonik Industries under the tradename ^Noblyst® .

Catalytic hydrogenation to convert unreacted hydrogen peroxide into water and the by-product peroxides 1-hydroperoxy-2-propanol and 2-hydroperoxy-1-propanol formed in step a) 1,2-propanediol and prevents by-products from decomposition of peroxides in subsequent processing stages. Catalytic hydrogenation is preferably carried out to provide hydrogen peroxide conversion of a hydrogenated solvent mixture containing less than 0.1 wt% hydrogen peroxide.

Hydrogenation also converts the aldehyde and ketone by-products to the corresponding alcohols, the extent of which depends on the catalyst and reaction conditions used. The conversion of acetaldehyde hydrogenation to ethanol can be adjusted by varying the reaction time and the hydrogen partial pressure and temperature used in the catalytic hydrogenation to provide a hydrogenated solvent mixture containing 1 to 1000 mg/kg of acetaldehyde. Depending on the reaction conditions, part of the acetal of the aldehyde with methanol and 1,2-propanediol may also be hydrogenated. However, hydrogenated solvent mixtures containing 1 to 1000 mg/kg of acetaldehyde typically contain significant amounts of 1,1-dimethoxyethane and 2,4-dimethyl-1,3-dioxolane, which are Acetal of acetaldehyde with methanol and 1,2-propanediol.

In step d) of the process of the invention, the hydrogenated solvent mixture of step c) is separated in at least one distillation stage to provide recovered methanol as overhead product. The acid is added to at least one of the hydrogenated solvent mixture of step c) or to the distillation stage. When the acid is added to the distillation stage, it is preferably added at a feed point above the feed point of the hydrogenated solvent mixture and below the top of the column. The acid can also be added to the reflux stream of the distillation column. More preferably, the hydrogenated solvent mixture is separated in two subsequent distillation stages to provide recovered methanol as the overhead product from both stages, and the acid is fed to the hydrogenated solvent mixture before feeding the first distillation stage. Feed the hydrogenated solvent mixture. The two distillation stages are preferably operated at higher pressure in the second stage and use the overhead vapor from the second stage to heat the bottoms evaporator of the first stage to save energy. The addition of acid in step d) reduces volatile organic amines in the recovered methanol and prevents deactivation of the epoxidation catalyst by organic amines when the recovered methanol is recycled to step a).

The acid is preferably added in an amount to provide less than 250 ppm (by weight) of nitrogen in the form of organic nitrogen compounds in the recovered methanol, more preferably in an amount to provide less than 50 ppm (by weight) of organic nitrogen in the form of organic nitrogen compounds Nitrogen in compound form. The acid may be a mineral acid, such as nitric, sulfuric, hydrochloric, phosphoric, or perchloric acid; a sulfonic acid, such as methanesulfonic acid; or a carboxylic acid, such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid, hexane acid, heptanoic acid, octanoic acid, nonanoic acid, capric acid, undecanoic acid, dodecanoic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid or fumaric acid acid. Preferred are sulfuric acid and phosphoric acid, and most preferred is sulfuric acid. The amount of nitrogen in the form of organic nitrogen compounds can be determined as the difference between the total amount of nitrogen and the amount of nitrogen in the form of inorganic nitrogen compounds. The total amount of nitrogen can be determined by the Kjeldahl method as described in DIN 53625. The recovered methanol is often free of inorganic compounds other than ammonia, so the amount of nitrogen in the form of inorganic nitrogen compounds can be determined by ion chromatography of acidified samples that detect ammonium ions.

The acid is preferably added in an amount to provide an apparent pH of 1.7 to 5.0, more preferably 1.8 to 4.0, in the bottom product left after methanol recovery. The term apparent pH as used herein refers to the value measured at 20°C using a pH meter equipped with a pH-sensitive glass electrode calibrated with an aqueous buffer solution. The apparent pH is maintained within these ranges to provide recovered methanol with low alkylamine content while reducing or preventing acid corrosion of the distillation equipment.

The recovered methanol provided in step d) may contain more methanol than step c) due to acid-catalyzed hydrolysis of 1,1-dimethoxyethane and 2,4-dimethyl-1,3-dioxolane during distillation The hydrogenated solvent mixture has a higher concentration of acetaldehyde.

In step e) of the process of the present invention, the recovered methanol of step d) is passed through a bed of acidic ion exchange resin to provide treated methanol. Both strongly acidic ion exchange resins and weakly acidic ion exchange resins can be used. The preferred ones are strongly acidic ion exchange resins containing SO ₃ H groups and weakly acidic ion exchange resins containing COOH groups. The best ones are strongly acidic ion exchange resins containing sulfonic acid groups. The acidic ion exchange resin is preferably based on organic polymers (such as cross-linked polystyrene), or organic-inorganic hybrid polymers (such as polysiloxanes). The acidic ion exchange resin can be a gel-type solid or a microporous solid. Preferably, the two ion exchanger beds are arranged side by side to allow regeneration of the ion exchange resin without interruption of methanol treatment. Preferably, the apparent pH of the treated methanol is monitored, and the acidic ion exchange resin is replaced or regenerated when the apparent pH of the treated methanol exceeds a threshold value. The threshold value is preferably selected from 2 to 4 pH units above the apparent pH of the treated methanol obtained with fresh or regenerated acidic ion exchange resin.

The treated methanol provided in step e) generally contains a lower concentration of acetaldehyde and a higher concentration of 1 , 1 , as the recovered methanol of step d) due to the acetalization of acetaldehyde with methanol catalyzed by an acidic ion exchange resin. 1-Dimethoxyethane.

In step f) of the process of the invention, the treated methanol of step e) is recycled to step a). The inventors of the present invention have found that, contrary to the teaching of WO 03/093255 regarding catalyst deactivation by 1,1-dimethoxyethane, the treated methanol provided in step e) is recycled to the epoxidation Step a) does not result in significant deactivation of the titanium zeolite epoxidation catalyst, however, recycling the recovered methanol from step d) without acid ion exchange resin treatment does cause deactivation of the titanium zeolite epoxidation catalyst.

Steps a) to f) of the process of the invention are preferably carried out continuously, preferably using continuously operating reactors in steps a) and c) and using rectification columns in the separation steps b) and d).

In a preferred embodiment of the method of the present invention, crude propylene oxide comprising 15 to 97% by weight of propylene oxide, 2 to 84% by weight of methanol, and acetaldehyde is isolated in step b) and extracted in step b). The crude propylene oxide is subjected to extractive distillation in a distillation column. Using an aqueous extraction solvent and feeding reactive _compounds containing NH groups and capable of reacting with acetaldehyde under extractive distillation conditions and a feed stream to an extractive distillation column or separately at a feed point above the crude propylene oxide to the extractive distillation column. Extractive distillation provides purified propylene oxide as an overhead product and a bottom product comprising water and methanol, and the catalytic hydrogenation of step c) is carried out on the bottom product comprising water and methanol to hydrogenate the reaction of acetaldehyde with the NH group _- containing The reaction product obtained from the reaction of the compound.

Crude propylene oxide comprising 15 to 97% by weight of propylene oxide, 2 to 84% by weight of methanol, and acetaldehyde can be distilled in a pre-separator column in step b) by decompression as described further above, and The propylene stripping sequence is separated in the propylene stripper.

The extractive distillation of the crude propylene oxide is carried out in an extractive distillation column. The extractive distillation column may be a tray column containing individual trays, such as sieve trays or bubble cap trays. Extractive distillation columns can also be packed columns, and both random packing as well as structured packing, such as metal mesh packing, can be used. Extractive distillation columns can also combine sections with individual plates and sections with packing. Extractive distillation columns typically also include at least one overhead condenser and at least one column reboiler. The extractive distillation column preferably has at least two feed points: a feed point A in the middle section of the extractive distillation column for feeding crude propylene oxide and a feed point above feed point A for feeding the aqueous extraction solvent. Feed point B. The feed point defines three sections of the extractive distillation column: a stripping section between the bottom of the column and feed point A, an extraction section between feed point A and feed point B, and a stripping section between feed point A and feed point B Point B and the rectification section at the top of the extractive distillation column. Preferably, a separation efficiency of 10 to 30 theoretical stages is used in the stripping section, a separation efficiency of 15 to 40 theoretical stages in the extraction section, and a separation efficiency of 20 to 60 theoretical stages in the rectification section (ie. , feed point B is preferably located in a distillation column 15 to 40 theoretical separation stages above feed point A and 20 to 60 theoretical separation stages below the top of the extractive distillation column).

The aqueous extraction solvent preferably contains more than 80% by weight water, more preferably more than 90% by weight water. Preferably, the aqueous extraction solvent does not contain other solvents except water. The preferred feed amount of the extraction solvent provides a mass ratio of the extraction solvent to the amount of methanol contained in the crude propylene oxide feed of 0.01 to 1, more preferably 0.03 to 0.2. Use of this amount of aqueous extraction solvent provides efficient methanol extraction and a propylene oxide product with low methanol content while preventing hydrolysis of the propylene oxide in the extractive distillation column.

In addition to the aqueous extraction solvent, reactive _compounds containing NH groups and capable of reacting with acetaldehyde under extractive distillation conditions and the feed stream to be fed to the extraction column are fed to the extractive distillation column or separately above the crude The feed point for propylene oxide is fed to the extractive distillation column. The reactive compound is preferably fed to the extractive distillation column in admixture with the extraction solvent. The amount of reactive compound fed to the distillation column is preferably selected so that the molar ratio of reactive compound relative to acetaldehyde is in the range of 0.5 to 10. The use of such amounts of reactive compounds provides efficient conversion of carbonyls to high boilers, as well as propylene oxide products with low levels of acetaldehyde and other carbonyls. At the same time, by-product formation due to the reaction of reactive compounds with propylene oxide can be kept low.

In preferred embodiments, the reactive compound has the structure R1 ^- Y ^- _NH2 , wherein Y is oxygen or NR2 and R1 and R2 are independently ^of each other ^hydrogen , alkyl or aryl. A preferred compound having the structure R1 ^- Y- _NH2 is hydrazine. Hydrazine hydrate and  salt can be used instead of hydrazine. The amount of reactive compound fed to the distillation column is preferably selected so that the molar ratio of reactive compound to acetaldehyde is in the range of 0.5 to 2.

In another preferred embodiment, the reactive compound is a diaminoalkane having 2 to 6 carbon atoms, preferably 1,2-diaminoethane, 1,2-diaminopropane or 1 , 3-diaminopropane, and most preferably 1,2-diaminoethane. The amount of reactive compound fed to the distillation column is preferably selected so that the molar ratio of reactive compound relative to acetaldehyde is in the range of 0.5 to 10, more preferably in the range of 3 to 8. Hydrogenation from acetaldehyde with reactive _compounds containing NH groups in a subsequent step of the bottoms of hydroextractive distillation using diaminoalkanes as reactive compounds compared to reactive compounds with structure R1 ^- Y- _NH2 The resulting reaction product of the reaction reduces the formation of volatile amines.

The bottom product provided by extractive distillation comprises water, methanol and the reaction product formed by the reaction of acetaldehyde with a reactive compound containing _NH2 groups. The bottom product is subjected to the catalytic hydrogenation of step c) to hydrogenate the reaction product obtained from the reaction of acetaldehyde with a reactive compound containing an _NH2 group. Oximes and hydrazones formed with reactive compounds having the structure R1 ^- Y- _NH2 will hydrogenate by hydrogenolysis of oxygen-nitrogen or nitrogen-nitrogen bonds. The imine formed from acetaldehyde and diaminoalkane will be hydrogenated to the corresponding amine. The bottom product provided by the extractive distillation is preferably combined with the solvent mixture separated in step b) before it is subjected to the catalytic hydrogenation of step c). EXAMPLES Example 1 Acetalization of Acetaldehyde and Methanol Catalyzed by Acidic Ion Exchange Resins

1500 ppm (by weight) of acetaldehyde and 1490 ppm (by weight) of propionaldehyde were added to methanol, and 300 ml of the resulting solution was stirred with 160 g of DOWEX ^™ Marathon ^™ C ion exchange resin at about 20° C. for 4 Hour. The resulting solution was analyzed by GC and contained 200 ppm of acetaldehyde, 230 ppm of propionaldehyde, 1930 ppm of 1,1-dimethoxyethane, and 1810 ppm of 1,1-dimethoxypropane. Example 2

The epoxidation of propylene is carried out by recovering the solvent mixture, hydrogenating the solvent mixture, recovering methanol by distillation by addition of acid, and treating the recovered methanol with an ion exchange resin before recycling to epoxidation.

Propylene is epoxidized in a cooled tube bundle reactor in which a fixed bed of catalyst on extruded titanium siliceous catalyst is arranged in the reactor tubes. A mixture comprising 40 wt% propylene, 7.7 wt% hydrogen peroxide, 3.3 wt% water, 49 wt% methanol and 80 ppm ammonia was fed to the top of the reactor and passed through the catalyst in trickle mode Fixed bed. The pressure in the reactor was maintained at 2.6 MPa by introducing nitrogen. The temperature in the reactor was maintained at a substantially fixed temperature in the range of 25 to 60°C, and the temperature was adjusted during the epoxidation reaction to maintain a substantially fixed 97% hydrogen peroxide conversion.

The reaction mixture leaving the reactor was depressurized to a pressure of 0.25 MPa, and the depressurized liquid was fed to a pre-separation column to provide an overhead product comprising propylene oxide, methanol, residual propylene and acetaldehyde and methanol comprising , water and the bottom product of unreacted hydrogen peroxide. Propylene oxide and methanol are condensed from the overhead product of the pre-separator column, and propylene is stripped from the condensate obtained from the propylene stripper to provide 23% by weight propylene oxide, 70% by weight propylene oxide. Crude propylene oxide as bottoms stream of methanol and 380 ppm of acetaldehyde. The crude propylene oxide was purified by extractive distillation using 55 g of a 0.8 wt % aqueous solution of hydrazine hydrate as an extraction solvent per kg of crude propylene oxide. Purified propylene oxide containing less than 5 ppm of methanol and acetaldehyde is obtained as the overhead product of the column.

The bottom product of the extractive distillation column was combined with the bottom product obtained from the pre-separation column and continuously hydrogenated in a trickle bed reactor with a nickel hydrogenation catalyst. The hydrogenation was carried out at 100 °C and 1.5 MPa with a WHSV of 4 h-1. The resulting hydrogenated solvent mixture contained, on average, 78 wt% methanol, 17 wt% water, and 80 ppm acetaldehyde, as determined by GC analysis.

The hydrogenated solvent mixture was depressurized and fed to stage 14 (counting from the top) of a first methanol distillation column with 20 theoretical stages operating continuously at 0.5 MPa. A first recovered methanol stream containing 96 wt% methanol and 4 wt% water was obtained as overhead product. The bottom product was fed to a second methanol distillation column with 30 theoretical stages operating continuously at 1.0 MPa. A second recovered methanol stream containing 96 wt% methanol and 4 wt% water was obtained as overhead product. Concentrated sulfuric acid was added to the hydrogenated solvent mixture, which was then fed to the first methanol distillation column at a rate that provided an apparent pH of 2.2 in the bottoms of the second methanol distillation. The overheads of the first and second methanol distillation columns are combined. The combined overhead product containing the average recovered methanol stream upstream of the ion exchanger had an acetaldehyde content of 100 ppm acetaldehyde.

The combined overhead product was passed through one of two side-by-side ion exchange columns containing DOWEX ^™ Marathon ^™ C ion exchange resin with an average residence time of 5 minutes. After ion exchange resin treatment, the recovered methanol contained an average of 15 ppm of acetaldehyde.

Figure 1 shows the concentration of acetaldehyde in the hydrogenated solvent mixture (A) and the concentration of methanol (B) recovered by distillation before treatment with ion exchange resin and after treatment with ion exchange resin over a time span of 140 hours Methanol concentration (C) recovered by distillation.

Figure 1 shows the acetaldehyde concentration in the hydrogenated solvent mixture (A) and the methanol concentration (B) recovered by distillation before treatment with ion exchange resin and by distillation after treatment with ion exchange resin, as determined in Example 2 Recovered methanol concentration (C).

Claims (16)

A method for epoxidation of propylene, comprising the steps of: a) providing a reaction mixture by reacting propylene and hydrogen peroxide in the presence of a methanol solvent and a titanium zeolite epoxidation catalyst, b) removing the reaction mixture from step a) Crude propylene oxide and a solvent mixture comprising methanol, water and peroxide are separated, c) catalytic hydrogenation of the solvent mixture separated in step b) is carried out to hydrogenate the peroxide to provide ethylene comprising 1 to 1000 mg/kg The hydrogenated solvent mixture of aldehydes, d) separating the hydrogenated solvent mixture of step c) in at least one distillation stage, adding acid to the hydrogenated solvent mixture of step c) or to at least one distillation stage, providing the recovered methanol as a column The top product, e) passing the recovered methanol of step d) through a bed of acidic ion exchange resin to provide treated methanol, wherein the acidic ion exchange resin catalyzes the acetalization of acetaldehyde with methanol, and f) passing the recovered methanol of step e) The treated methanol is recycled to step a). The method of claim 1, wherein in step d) the acid is added in an amount to provide nitrogen in the form of organic nitrogen compounds in an amount of less than 250 ppm by weight in the recovered methanol. The method of claim 1 or 2, wherein sulfuric acid is added in step d). The method of claim 1 of the claimed scope, wherein the acidic ion exchange resin used in step e) contains a sulfonic acid group. The method of claim 1, wherein the apparent pH of the treated methanol obtained in step e) is monitored, and the acid is displaced or regenerated when the apparent pH of the treated methanol exceeds a threshold value ion exchange resin. The method of claim 5, wherein the threshold value is 2 to 4 pH units higher than the apparent pH of the treated methanol obtained with fresh or regenerated acidic ion exchange resin. The method of claim 1, wherein in step a), ammonia is added at a weight ratio of ammonia to the initial amount of hydrogen peroxide in the range of 0.0001 to 0.003. The method of claim 1 of the claimed scope, wherein steps a) to f) are performed continuously. The method of claim 1, wherein in step b), the crude propylene oxide comprising 15 to 97% by weight of propylene oxide, 2 to 84% by weight of methanol and acetaldehyde is separated, and an aqueous extraction solvent is used. and feeding reactive _compounds containing NH groups and capable of reacting with acetaldehyde under extractive distillation conditions with a feed stream to an extractive distillation column or to a feed point, respectively, above the feed point of the crude propylene oxide. is fed to the extractive distillation column in which the crude propylene oxide is subjected to extractive distillation to provide purified propylene oxide as an overhead product and a bottom product comprising water and methanol, and the The bottom product is subjected to the catalytic hydrogenation of step c) to hydrogenate the reaction product of acetaldehyde with the reactive compound containing _NH2 groups. The method of claim 9, wherein the reactive compound is selected from the group consisting of hydrazine, hydrazine monohydrate and

Salt. The method of claim 9 of the claimed scope, wherein the reactive compound is selected from the group consisting of 1,2-diaminoethane, 1,2-diaminopropane and 1,3-diamine base propane. The method of claim 11 of the claimed scope, wherein the reactive compound is 1,2-diaminoethane. The method of claim 9, wherein the reactive compound is fed into the extractive distillation column in admixture with the extraction solvent. The method of claim 9, wherein the bottom product provided by the extractive distillation is combined with the solvent mixture separated in step b) before subjecting it to the catalytic hydrogenation of step c). The method of claim 9, wherein the reactive compound The molar ratio to acetaldehyde is 0.5 to 10. The method according to claim 9, wherein the mass ratio of the extraction solvent to the amount of methanol contained in the crude propylene oxide fed in is 0.01 to 1.

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