阅读说明：本技术 丙烯的环氧化方法 (Process for epoxidation of propene ) 是由 王志军 M·帕斯卡利 M·伯恩哈德 于 2018-05-07 设计创作，主要内容包括：在通过在甲醇溶剂和钛沸石环氧化催化剂的存在下使丙烯与过氧化氢反应的丙烯的环氧化方法中,以下步骤：从反应混合物中分离粗环氧丙烷和包含甲醇、水和过氧化物的溶剂混合物；使溶剂混合物进行使过氧化物氢化的催化氢化,从而提供包含1-1000mg/kg乙醛的氢化的溶剂混合物；在至少一个蒸馏段中分离出氢化的溶剂,将酸加入到氢化的溶剂混合物或至少一个蒸馏段,从而提供作为塔顶产物的回收的甲醇；使回收的甲醇通过酸性离子交换树脂的床,从而提供处理过的甲醇；以及将处理过的甲醇再循环至环氧化反应,以通过用甲醇再循环乙醛来防止环氧化催化剂的失活。(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 crude propylene oxide and a solvent mixture comprising methanol, water and peroxide from the reaction mixture; subjecting the solvent mixture to a catalytic hydrogenation that hydrogenates the peroxide, thereby providing a hydrogenated solvent mixture comprising 1 to 1000mg/kg acetaldehyde; separating the hydrogenated solvent in at least one distillation section and adding an acid to the hydrogenated solvent mixture or to at least one distillation section to provide recovered methanol as an overhead product; passing the recovered methanol through a bed of an acidic ion exchange resin to provide treated methanol; and recycling the treated methanol to the epoxidation reaction to prevent deactivation of the epoxidation catalyst by recycling acetaldehyde with methanol.)

1. A process for the epoxidation of propene comprising the steps of:

a) reacting propene with hydrogen peroxide in the presence of a methanol solvent and a titanium zeolite epoxidation catalyst to provide a reaction mixture,

b) separating crude propene oxide and a solvent mixture comprising methanol, water and peroxide from the reaction mixture of step a),

c) subjecting the solvent mixture separated in step b) to a catalytic hydrogenation which hydrogenates the peroxide to provide a hydrogenated solvent mixture comprising from 1 to 1000mg/kg acetaldehyde,

d) separating the hydrogenated solvent mixture of step c) in at least one distillation section, adding an acid to the hydrogenated solvent mixture of step c) or at least one distillation section, providing recovered methanol as an overhead product,

e) passing said recovered methanol of step d) through an acidic ion exchange resin bed to provide treated methanol, an

f) Recycling said treated methanol of step e) to step a).

2. The process of claim 1, wherein acid is added in step d) in an amount to provide a nitrogen content in the recovered methanol as organic nitrogen compounds of less than 250ppm by weight.

3. The process according to claim 1 or 2, wherein sulfuric acid is added in step d).

4. The process of any one of claims 1-3, wherein the acidic ion exchange resin used in step e) comprises sulfonic acid groups.

5. The method of any one of claims 1-4, wherein the apparent pH of the treated methanol obtained in step e) is monitored, and the acidic ion exchange resin is replaced or regenerated when the apparent pH of the treated methanol exceeds a threshold value.

6. The method of claim 5, wherein the threshold value is 2-4 pH units higher than the apparent pH of treated methanol obtained with fresh or regenerated acidic ion exchange resin.

7. The process according to any one of claims 1 to 6, wherein ammonia is added in step a) in a weight ratio of from 0.0001 to 0.003 of ammonia to the initial amount of hydrogen peroxide.

8. The process according to any one of claims 1 to 7, wherein steps a) to f) are carried out continuously.

9. The process according to any one of claims 1 to 8, wherein in step b) a crude propene oxide comprising 15-97 wt.% propene oxide, 2-84 wt.% methanol, and acetaldehyde is separated off, the crude propene oxide is subjected to an extractive distillation in an extractive distillation column which uses an aqueous extractive solvent and which will comprise NH₂Feeding to an extractive distillation column a reactive compound which is reactive with acetaldehyde and which is reactive with acetaldehyde under the extractive distillation conditions, together with or separately from the feed stream to the extractive distillation column at a feed point above the feed point of the crude propene oxide, thereby providing purified propene oxide as an overhead product and a bottom product comprising water and methanol, and subjecting the bottom product comprising water and methanol to the catalytic hydrogenation of step c) to acetaldehyde and a product comprising NH₂The reaction product of said reactive compound of group is hydrogenated.

10. The method of claim 9, wherein the reactive compound is selected from the group consisting of hydrazine, hydrazine monohydrate, and hydrazine salts.

11. The method of claim 9, wherein the reactive compound is selected from the group consisting of 1, 2-diaminoethane, 1, 2-diaminopropane, and 1, 3-diaminopropane.

12. The method of claim 11, wherein the reactive compound is 1,2 diaminoethane.

13. The process of any one of claims 9 to 12, wherein the reactive compound is fed to the extractive distillation column mixed with the extraction solvent.

14. The process according to any one of claims 9 to 13, wherein the bottom product provided by the extractive distillation is combined with the solvent mixture separated in step b) before it is subjected to the catalytic hydrogenation of step c).

15. The process of any one of claims 9 to 14, wherein the molar ratio of the reactive compound to acetaldehyde is from 0.5 to 10.

16. The process according to any one of claims 9 to 15, wherein the mass ratio of the extraction solvent relative to the amount of methanol contained in the crude propylene oxide fed is from 0.01 to 1.

Technical Field

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

Background

Epoxidation of propene with hydrogen peroxide in the presence of a titanium silicalite catalyst is known from EP 0100119 a 1. The reaction of propylene with hydrogen peroxide in the presence of a titanium zeolite catalyst is typically carried out in a methanol solvent to achieve high reaction rates and product selectivity. In addition to propylene oxide, epoxidation reactions produce by-products such as formaldehyde, acetaldehyde, and hydroperoxides formed from the ring-opening reaction of hydrogen peroxide with propylene oxide.

The byproducts acetaldehyde and propionaldehyde are difficult to separate from the propylene oxide product. WO 2004/048355 discloses a process for removing methanol and acetaldehyde from crude propene oxide in a single distillation column by extractive distillation, which comprises unsubstituted NH₂The compounds which are radical and which are capable of reacting with acetaldehyde under distillation conditions are additionally fed at or above the feed point of the crude propene oxide. Preferably, an aqueous hydrazine solution is used as the further feed compound. Water is particularly preferred as the extraction solvent. The process provides a high purity propylene oxide suitable for use in the preparation of polyether polyols.

WO03/093255 teaches hydrogenating a solvent stream recovered from the epoxidation of an olefin with hydrogen peroxide with a heterogeneous catalyst under conditions wherein unreacted hydrogen peroxide, formaldehyde, acetaldehyde and hydroperoxides (such as 1-hydroperoxide-2-propanol and 2-hydroperoxide-1-propanol formed in the epoxidation reaction) are hydrogenated before recycling the solvent to the epoxidation reaction. WO03/093255 teaches that in this context the impurities methyl formate, formaldehyde, acetaldehyde, dimethoxymethane and 1,1 dimethoxyethane lead to catalyst deactivation.

WO2004/029032 teaches the epoxidation of an olefin with hydrogen peroxide in the presence of a titanium-containing zeolite catalyst in an aqueous reaction mixture comprising strong bases having a pKB of less than 100wppm or cations of such bases having a pKB of less than 4.5wppm and weak bases having a pKB of at least 100wppm or cations of such bases having a pKB of at least 4.5. Limiting the amount of strong base reduces or prevents long-term deactivation of the catalyst, while the presence of weak base improves epoxide selectivity without affecting the long-term activity of the catalyst. Organic amines are strong bases with a pKB typically less than 4.5, and therefore, in order to maintain the long-term activity and selectivity of the epoxidation catalyst, the introduction of such amines into the epoxidation step with a recycle solvent stream must be avoided.

WO2004/048354 teaches the recovery of a solvent stream from a reaction mixture of the epoxidation of an olefin with hydrogen peroxide in the presence of a titanium-containing zeolite catalyst, wherein the recovered solvent stream is treated to contain less than 50wppm of nitrogen in the form of organic nitrogen compounds prior to recycling the recovered solvent stream to the epoxidation step in order to reduce deactivation of the catalyst upon recycling of the solvent. The solvent treatment is preferably an acid treatment. WO2004/048354 teaches that the acid treatment can be carried out by adding a carboxylic acid or a mineral acid to the solvent stream before or during distillative recovery of the solvent as the overhead product or by treating the overhead product obtained by distillation with an acidic ion exchanger.

Disclosure of Invention

The inventors of the present invention have now found that subjecting a methanol solvent stream recovered from the epoxidation of propene to hydrogenation as described in WO03/093255, followed by removal of organic nitrogen compounds from the hydrogenated solvent stream by adding an acid to the hydrogenated solvent stream prior to or during distillative recovery of methanol as an overhead product, will provide recovered methanol that may contain more acetaldehyde than the hydrogenated solvent stream contains if the hydrogenation does not convert all acetaldehyde and acetaldehyde acetals. Recycling the methanol recovered in this manner to the epoxidation reaction leads to catalyst deactivation. The inventors of the present invention have further found that treating the recovered methanol by additionally passing it through a bed of acidic ion exchanger in this manner will convert most of the acetaldehyde to 1, 1-dimethoxyethane and recycling the methanol recovered in this manner prevents deactivation of the epoxidation catalyst, in contrast to what was expected from the teaching of WO03/093255 for 1, 1-dimethoxyethane to cause catalyst deactivation.

The subject of the present invention is therefore a process for the epoxidation of propene comprising the following steps:

a) reacting propene with hydrogen peroxide in the presence of a methanol solvent and a titanium zeolite epoxidation catalyst to provide a reaction mixture,

b) separating crude propylene oxide and a solvent mixture comprising methanol, water and peroxide from the reaction mixture of step a),

c) subjecting the solvent mixture separated in step b) to a catalytic hydrogenation which hydrogenates the peroxide to provide a hydrogenated solvent mixture comprising from 1 to 1000mg/kg acetaldehyde,

d) separating the hydrogenated solvent mixture of step c) in at least one distillation section, adding an acid to the hydrogenated solvent mixture of step c) or to at least one distillation section, providing recovered methanol as an overhead product,

e) passing the recovered methanol of step d) through a bed of acidic ion exchange resin to provide treated methanol, an

f) Recycling the treated methanol of step e) to step a).

Drawings

Fig. 1 shows the acetaldehyde concentrations in the hydrogenated solvent mixture (a) and in the methanol recovered by distillation before (B) and after (C) were treated with ion exchange resins as determined in example 2.

Detailed Description

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

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

Hydrogen peroxide may be used as an aqueous solution, preferably containing 30-75 wt% hydrogen peroxide, and most preferably 40-70 wt%. The aqueous hydrogen peroxide solution is preferably prepared by the anthraquinone process.

The methanol solvent may be technical grade methanol, a solvent stream recovered from the work-up of the epoxidation reaction mixture, or a mixture of both. The methanol solvent may contain small amounts of other solvents such as ethanol, preferably in an amount of less than 2 wt%. The methanol solvent may also comprise water, preferably 2-8 wt% 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 water and hydrogen peroxide.

The epoxidation catalyst used in step a) preferably comprises a titanium zeolite containing titanium atoms in silicon lattice positions. Preferably, a titanium silicalite catalyst is used, preferably having an MFI or MEL crystal structure. Most preferably, a titanium silicalite-1 catalyst with MFI structure known from EP 0100119 a1 is used. The titanium silicalite catalyst is preferably used as a shaped catalyst in the form of granules, extrudates or shaped bodies. For the shaping process, the catalyst may contain from 1 to 99% of a binder or carrier material, all binders and carrier materials which do not react with hydrogen peroxide or propylene oxide under the reaction conditions used for the epoxidation being suitable, silica being preferred as binder. Extrudates having a diameter of from 1 to 5mm are preferably used as shaped catalysts. The amount of catalyst used may vary within wide limits and is preferably selected such that a hydrogen peroxide consumption of more than 90%, preferably more than 95%, is achieved within 1 minute to 5 hours under the epoxidation reaction conditions used.

The epoxidation reaction of step a) is preferably carried out at a temperature of from 20 to 80 c, more preferably from 25 to 60 c. The epoxidation reaction is preferably carried out at a reaction temperature and at a pressure above the vapor pressure of propylene in order to maintain the propylene dissolved in the solvent or present as a separate liquid phase. The pressure in step a) is preferably from 1.9 to 5.0MPa, more preferably from 2.1 to 3.6MPa, most preferably from 2.4 to 2.8 MPa. The use of excess propylene at high pressure provides high reaction rates and hydrogen peroxide conversion, and at the same time provides high selectivity to propylene oxide.

The epoxidation reaction is preferably carried out with addition of ammonia to increase epoxide selectivity as described in EP 0230949 a 2. The ammonia is preferably added in a weight ratio of 0.0001 to 0.003 ammonia to the initial amount of hydrogen peroxide.

The epoxidation reaction of step a) is preferably carried out in a fixed bed reactor by passing a mixture comprising propene, hydrogen peroxide and a methanol solvent through a fixed bed comprising the shaped titanium zeolite catalyst. The fixed-bed reactor is preferably a tube bundle reactor, and the fixed catalyst bed is arranged inside the reactor tubes. The fixed bed reactor is preferably equipped with cooling means and cooled with a liquid cooling medium. The temperature profile along the length of the fixed catalyst bed is preferably adjusted so that 70 to 98%, preferably 80 to 95%, of the reaction temperature along the length of the fixed catalyst bed is maintained in the range of less than 5 c, preferably in the range of 0.5 to 3 c. The temperature of the cooling medium supplied to the cooling device is preferably adjusted to a value 3-13 c lower than the highest temperature in the fixed catalyst bed. The epoxidation reaction mixture is preferably passed through the catalyst bed in a downflow mode, preferably at a superficial velocity of from 1 to 100m/h, more preferably from 5 to 50m/h, most preferably from 5 to 30 m/h. Superficial velocity is defined as the ratio of the volumetric flow rate/cross-section of the catalyst bed. In addition, the reaction mixture is preferably allowed to stand for 1 to 20 hours^-1Preferably 1.3 to 15h^-1Is passed through the catalyst bed. 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 on page 8, line 23 to page 9, line 15 of WO 02/085873. The epoxidation reaction is most preferably carried out using an excess of propane supplied to contain the catalyst in a trickle bed regime at reaction temperature and at a pressure close to the vapor pressure of propeneA reaction mixture of two liquid phases, a solvent-rich phase and a propylene-rich liquid phase. Two or more fixed bed reactors may be operated in parallel or in series to enable continuous operation of the epoxidation process while regenerating the epoxidation catalyst. The regeneration of the epoxidation catalyst may be carried out by calcination, by treatment with a heated gas, preferably an oxygen-containing gas, or by washing with a solvent, 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 fixed bed reactor. Different regeneration methods may be combined.

In step b) of the process of the present invention, the crude propene 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). The separation of the crude propylene oxide and solvent mixture from the reaction mixture can be carried out by methods known in the art. The separation of the solvent mixture from the reaction mixture is preferably carried out to provide a solvent mixture comprising less than 5 wt% propylene and less than 2 wt% propylene oxide.

Preferably, the reaction mixture is subjected to pressure reduction, and the propylene vapor formed from the pressure reduction 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 comprising unreacted propylene and a bottom product containing compounds having a boiling point higher than that of propylene, such as propylene oxide and methanol solvent. The overhead product containing unreacted propylene may be recycled to the epoxidation reaction. The bottoms product may be combined with the liquid mixture remaining after depressurization. The liquid mixture remaining after depressurization is preferably separated by distillation in a preseparation column to provide crude propylene oxide comprising propylene oxide, methanol and residual propene as overhead product and a solvent mixture comprising methanol, water and peroxide as bottom product. The pre-separation column is preferably operated to provide an overhead product comprising 20-60% methanol contained in the liquid phase of the last depressurization step. The pre-separation column 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 most preferably operated without reflux and without rectification section to minimize the residence time of propylene oxide in the pre-separation column. The pre-separation column is preferably operated at a pressure of from 0.16 to 0.3 MPa. Propylene oxide and methanol are condensed from the overhead product of the pre-separation column, and propylene is preferably stripped from the resulting condensate in a propylene stripper column which provides a substantially propylene-free bottoms stream comprising propylene oxide and methanol.

The purified propylene oxide is preferably separated from the bottom stream of the propylene stripper in an extractive distillation using water as the extractive solvent. The extractive distillation preferably contains unsubstituted NH in the additional feed₂And can be operated in the case of reactive compounds which react with acetaldehyde during extractive distillation, as described in WO 2004/048335. Extractive distillation using reactive compounds provides high purity propylene oxide containing less than 50ppm carbonyl compounds.

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

The catalytic hydrogenation is preferably carried out at a hydrogen partial pressure of from 0.5 to 30MPa, more preferably from 1 to 25MPa, most preferably from 1 to 5 MPa. The temperature is preferably in the range of 80 to 180 deg.C, more preferably 90 to 150 deg.C. The catalytic hydrogenation is carried out in the presence of a hydrogenation catalyst, preferably a heterogeneous hydrogenation catalyst. Raney nickel and Raney cobalt may be used as hydrogenation catalysts. Preferably, a supported metal catalyst is used comprising one or more metals selected from the group consisting of Ru, Rh, Pd, Pt, Ag, Ir, Fe, Cu, Ni and Co on a catalyst support. The metal is preferably platinum, palladium, iridium, ruthenium or nickel, most preferably ruthenium or nickel. The catalyst support may be any solid that is inert and does not deteriorate under hydrogenation conditions. Suitable as catalyst carriers are activated carbon, the oxide SiO₂、TiO₂、ZrO₂And Al₂O₃And mixed oxides comprising at least two of silicon, aluminum, titanium, and zirconium. Activated carbon is preferably used as a catalyst support for supported metal catalysts. The catalyst support is preferably shaped as spheres, pellets, tablets, granules or extrudates. Preference is given to extrudates having a diameter of from 0.5 to 5mm, in particular from 1 to 3mm, and a length of from 1 to 10 mm. The supported metal catalyst preferably contains 0.01 to 60% by weight of a metal. The supported noble metal catalyst preferably comprises 0.1 to 5% of metal. The supported nickel and cobalt catalyst preferably contains 10-60% metal. The supported metal catalysts may 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 selected from, for example, Clariant and

trade name, and from Evonik Industries and

trade names are commercially available.

The catalytic hydrogenation converts unreacted hydrogen peroxide into water and the by-products peroxide 1-hydroperoxy-2-propanol and 2-hydroperoxy-1-propanol formed in step a) into 1, 2-propanediol and prevents the formation of by-products by peroxide decomposition in the subsequent work-up stage. The catalytic hydrogenation is preferably carried out to conversion of hydrogen peroxide to provide a hydrogenation solvent mixture comprising less than 0.1 wt% hydrogen peroxide.

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

In step d) of the process of the present invention, the hydrogenated solvent mixture of step c) is separated in at least one distillation section to provide recovered methanol as an overhead product. Adding an acid to the hydrogenated solvent mixture or at least one distillation section of step c). When the acid is added to the distillation section, it is preferably added at a feed point above the feed point of the hydrogenated solvent mixture and below the top of the column. Acid may also be added to the reflux stream of the distillation column. Most preferably, the hydrogenated solvent mixture is separated in two subsequent distillation stages, thereby providing recovered methanol as overhead product from both stages, the acid being fed to the hydrogenated solvent mixture before being fed to the first distillation stage. The two distillation sections are preferably operated at the higher pressure in the second section and the overhead vapor from the second section is used to heat the bottom evaporator of the first section to save energy. The addition of acid in step d) reduces the volatile organic amine content of the recovered methanol and prevents deactivation of the epoxidation catalyst by the organic amine when the recovered methanol is recycled to step a).

The acid is preferably added in an amount to provide less than 250ppm by weight of nitrogen in the form of organic nitrogen compounds in the recovered methanol, more preferably less than 50ppm by weight of nitrogen in the form of organic nitrogen compounds. The acid may be an inorganic acid such as nitric acid, sulfuric acid, hydrochloric acid, phosphoric acid or perchloric acid; sulfonic acids such as methanesulfonic acid; or a carboxylic acid such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, undecanoic acid, dodecanoic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, or fumaric acid. Sulfuric acid and phosphoric acid are preferred, with sulfuric acid being most preferred. The amount of nitrogen in the form of an organic nitrogen compound may be determined as the difference between the total amount of nitrogen and the amount of nitrogen in the form of an inorganic nitrogen compound. The total amount of nitrogen can be determined by the Kjeldahl method as described in DIN 53625. The recovered methanol usually does not contain inorganic compounds other than ammonia, and therefore the amount of nitrogen in the form of inorganic nitrogen compounds can be determined by ion chromatography of acidified samples detecting 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 bottoms product remaining after methanol recovery. The term apparent pH value here refers to the value measured at 20 ℃ with a pH meter equipped with a pH sensitive glass electrode calibrated with an aqueous buffer. Maintaining the apparent pH within these ranges provides recovered methanol with low levels of alkylamines while reducing or preventing acid corrosion of distillation equipment.

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

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. Preferably containing SO₃Strongly acidic ion exchange resins with H groups and weakly acidic ion exchange resins with COOH groups. Most preferred are strongly acidic ion exchange resins containing sulfonic acid groups. The acidic ion exchange resin is preferably based on an organic polymer such as crosslinked polystyrene, or an organic-inorganic hybrid polymer such as polysiloxane. The acidic ion exchange resin may be a gel-type solid or a macroporous solid. Preferably, the two ion exchange beds are arranged in parallel to allow regeneration of the ion exchange resin without interrupting the methanol process. 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 higher than the apparent pH of the treated methanol obtained with fresh or regenerated acidic ion exchange resin.

The treated methanol provided in step e) will typically contain a lower concentration of acetaldehyde and a higher concentration of 1, 1-dimethoxyethane than the recovered methanol of step d) due to acetalization of acetaldehyde with methanol catalyzed by acidic ion exchange resins.

In step f) of the process according to the invention, the treated methanol of step e) is recycled to step a). The inventors of the present invention have found that the recycle of treated methanol provided in step e) to the epoxidation step a) does not result in significant deactivation of the titanium zeolite epoxidation catalyst, contrary to what would be expected from the teaching of WO03/093255 of 1, 1-dimethoxyethane resulting in deactivation of the catalyst, whereas the recycle of recovered methanol of step d) without treatment with an acidic ion exchange resin does result in 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 operated reactors in steps a) and c) and continuously operated rectification columns in the separation steps b) and d).

In a preferred embodiment of the process according to the invention, a crude propene oxide comprising from 15 to 97% by weight of propene oxide, from 2 to 84% by weight of methanol and acetaldehyde is separated off in step b) and is subjected to extractive distillation in an extractive distillation column. An aqueous extraction solvent is used, and will contain NH₂The reactive compound which is reactive with acetaldehyde and is capable of reacting with acetaldehyde under extractive distillation conditions is fed to the extractive distillation column with the feed stream or separately to the extractive distillation column at a feed point above the feed point of the crude propylene oxide. The extractive distillation provides purified propylene oxide as overhead product and a bottom product comprising water and methanol, and this bottom product comprising water and methanol is subjected to the catalytic hydrogenation of step c) to hydrogenate the product of acetaldehyde with a product containing NH₂The reaction of the reactive compound of the group produces a reaction product.

The crude propene oxide, comprising from 15 to 97% by weight of propene oxide, from 2 to 84% by weight of methanol, and acetaldehyde, can be separated in step b) by a sequence of depressurization, distillation in a preseparator and stripping of propene in a propene stripper as described further above.

The extractive distillation of the crude propene oxide is carried out in an extractive distillation column. The extractive distillation column may be a tray column comprising discrete trays, such as sieve trays or bubble cap trays. The extractive distillation column may also be a packed column, and random packing as well as structured packing, such as wire mesh packing, may be used. The extractive distillation column may also combine a section with discrete trays and a section with packing. The extractive distillation column typically also includes 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 for feeding the crude propylene oxide in the middle section of the extractive distillation column and a feed point B located above the feed point a for feeding the aqueous extractive solvent. The feed points define 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 rectifying section between feed point B and the top of the extractive distillation column. Preference is given to using a distillation column having a separation efficiency of from 10 to 30 theoretical stages (the theoretical stages) in the stripping section, a separation efficiency of from 15 to 40 theoretical stages in the extraction section and a separation efficiency of from 20 to 60 theoretical stages in the rectification section, i.e. the feed point B is preferably from 15 to 40 theoretical separation stages above the feed point a and from 20 to 60 theoretical separation stages below the top of the extractive distillation column.

The aqueous extraction solvent preferably comprises more than 80 wt.% water, more preferably more than 90 wt.% water. Preferably, the aqueous extraction solvent comprises no additional solvent other than water. The extraction solvent is preferably fed in an amount providing a mass ratio of extraction solvent to the amount of methanol contained in the crude propene oxide feed of from 0.01 to 1, more preferably from 0.03 to 0.2. The use of such an amount of aqueous extraction solvent provides for efficient extraction of methanol and propylene oxide products having a low methanol content while avoiding hydrolysis of propylene oxide in the extractive distillation column.

Comprising NH in addition to the aqueous extraction solvent₂The reactive compound which is reactive towards acetaldehyde and which is capable of reacting with acetaldehyde under the conditions of extractive distillation is fed to the extractive distillation column either together with the feed stream to the distillation column or separately at a feed point above the feed point of the crude propylene oxide. The reactive compound is preferably fed to the extractive distillation column in admixture with the extraction solvent. The amount of reactive compounds fed to the distillation column is preferably chosen such that the molar ratio of reactive compounds relative to acetaldehyde is in the range of 0.5 to 10. The use of such amounts of reactive compounds provides for efficient conversion of carbonyl compounds to higher boiling compounds and provides a propylene oxide product having a low level of acetaldehyde and other carbonyl compounds. At the same time, the formation of by-products by reaction of the reactive compounds with propylene oxide can be kept at a low level.

At one endIn a preferred embodiment, the reactive compound has the structure R¹-Y-NH₂Wherein Y is oxygen or NR²And R is¹And R²Independently of one another, hydrogen, alkyl or aryl. Structure R¹-Y-NH₂The preferred compound of (a) is hydrazine. Hydrazine hydrate and hydrazine salts may be used instead of hydrazine. The amount of reactive compound fed to the distillation column is then preferably chosen such 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 then preferably chosen such that the molar ratio of reactive compound to acetaldehyde is in the range of 0.5 to 10, more preferably 3 to 8. And structure R¹-Y-NH₂When compared with the reaction of acetaldehyde with NH-containing compounds₂The use of diaminoalkanes as reactive compounds reduces the formation of volatile amines when the reaction product resulting from the reaction of the reactive compounds of the radicals is hydrogenated in a subsequent step of hydrogenation of the bottom product of the extractive distillation.

The bottom product provided by extractive distillation comprises water, methanol and water containing NH₂The reactive compound of the group is reacted to form a reaction product. Subjecting the bottom product to a catalytic hydrogenation of step c) consisting of reacting acetaldehyde with a catalyst containing NH₂The reaction of the reactive compounds of the group produces a reaction product which is hydrogenated. By the structure R¹-Y-NH₂The oximes and hydrazones formed by the reactive compounds of (a) will hydrogenate with hydrogenolysis of the oxygen-nitrogen bond or nitrogen-nitrogen bond. 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).