Epoxidation process of propylene

Abstract

In a method for continuously epoxidizing propylene with hydrogen peroxide, the method comprises: step a), continuously reacting propylene with hydrogen peroxide in the presence of a titanium zeolite epoxidation catalyst to provide a liquid reaction mixture containing propylene oxide; determining the amount of molecular oxygen formed in step a); regenerating the titanium zeolite catalyst when the molar ratio of the molecular oxygen formed in step a) to the hydrogen peroxide fed to step a) exceeds a preset threshold; and repeating step a) with the regenerated titanium zeolite.

Description

Technical Field

[0001] The present invention relates to a continuous process for the epoxidation of propylene with hydrogen peroxide in the presence of a titanium zeolite epoxidation catalyst. Background Art

[0002] The epoxidation of propylene with hydrogen peroxide in the presence of titanium silicalite catalysts is known from EP 0 100 119 A1 and has found technical application in the so-called HPPO process.

[0003] When the epoxidation is continued, the titanium zeolite catalyst gradually loses its catalytic activity. The desired hydrogen peroxide conversion can be maintained by gradually increasing the reaction temperature to compensate for the loss of catalyst activity.

[0004] As described in MG Clerici et al., Journal of Catalysis, Vol. 129, pages 159 to 167, JP-A 03-114 53, EP 1 190 770 A and WO 2005 / 000827, the deactivated titanium zeolite catalyst can be regenerated by washing with a solvent at high temperature; or as described in EP 743 094 A, EP 790 075 A, WO 98 / 55228 and WO 98 / 55430, the deactivated titanium zeolite catalyst can be regenerated by thermal regeneration.

[0005] The prior art teaches regenerating titanium zeolite catalysts when the catalyst activity or selectivity to propylene oxide falls below a desired level. Summary of the invention

[0006] The inventors of the present invention have now found that if the catalyst is regenerated when the proportion of hydrogen peroxide decomposed into oxygen in the reaction step exceeds a certain limit, which may be before the catalyst activity has fallen to a level at which catalyst regeneration would be required in prior art processes, an optimization of a low average consumption of raw materials required for epoxidation of propylene with hydrogen peroxide and a short proportion of reactor downtime due to catalyst regeneration can be achieved compared to the reactor used for epoxidation.

[0007] The subject of the present invention is therefore a process for the continuous epoxidation of propylene with hydrogen peroxide, comprising:

[0008] a) continuously reacting propylene with hydrogen peroxide in the presence of a titanium zeolite epoxidation catalyst to provide a liquid reaction mixture comprising propylene oxide;

[0009] b) determining the amount of molecular oxygen formed in step a);

[0010] c) regenerating the titanium zeolite catalyst when the molar ratio of molecular oxygen formed in step a) to hydrogen peroxide fed into step a) exceeds a preset threshold; and

[0011] d) repeating step a) using the titanium zeolite regenerated in step c). DETAILED DESCRIPTION

[0012] In step a) of the process of the present invention, propylene is continuously reacted with hydrogen peroxide in the presence of a titanium zeolite epoxidation catalyst to provide a liquid reaction mixture comprising propylene oxide.

[0013] Propylene is preferably used in a molar excess relative 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, most preferably 3:1 to 5:1. In a preferred embodiment, propylene is used in an excess sufficient to maintain a further propylene-rich liquid phase throughout step a). Propylene may contain propane, preferably in a molar ratio of propane to propylene of 0.001 to 0.15, and more preferably 0.08 to 0.12.

[0014] The hydrogen peroxide may be used as an aqueous solution, preferably containing 30 to 75 wt% hydrogen peroxide, and most preferably 40 to 70 wt% hydrogen peroxide.The aqueous solution of hydrogen peroxide is preferably prepared by the anthraquinone process.

[0015] The reaction of propylene with hydrogen peroxide can be carried out in the absence or presence of a solvent, and is preferably carried out in the presence of a solvent. All solvents that are not oxidized by hydrogen peroxide or are only oxidized by hydrogen peroxide to a small extent under the selected reaction conditions and are dissolved in water in an amount greater than 10% by weight are suitable. Solvents that are completely miscible with water are preferred. Particularly suitable solvents are alcohols, such as methanol, ethanol or tert-butyl alcohol; glycols, such as ethylene glycol, 1,2-propylene glycol or 1,3-propylene glycol; cyclic ethers, such as tetrahydrofuran, dioxane or propylene oxide; glycol ethers, such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether or propylene glycol monomethyl ether; ketones, such as acetone or 2-butanone; and nitriles, such as acetonitrile and propionitrile. When titanium silicalite is used as an epoxidation catalyst, propylene is preferably reacted with hydrogen peroxide in a methanol solvent. The methanol solvent can be industrial grade methanol, a solvent stream recovered in the treatment of the epoxidation reaction mixture, or a mixture of the two. The methanol solvent may contain a small amount of other solvents, such as ethanol, and the amount of such other solvents is preferably less than 2% by weight. The methanol solvent may also contain water, preferably 2% to 13% by weight of water. The solvent is preferably used for epoxidation in a weight ratio of 0.5 to 20 relative to the combined weight of water and hydrogen peroxide.

[0016] The epoxidation catalyst used in step a) preferably comprises a titanium zeolite having titanium atoms at silicon lattice sites. Preferably, a titanium silicate zeolite catalyst, preferably a titanium silicate zeolite catalyst having an MFI or MEL crystal structure, is used. Most preferably, the titanium silicate zeolite-1 catalyst having an MFI structure known from EP 0 100 119 A1 is used. The titanium silicate zeolite catalyst is preferably used as a shaped catalyst in the form of granules, extrudates or moldings. For the shaping process, the catalyst may comprise from 1% to 99% of a binder or carrier material, and all binder and carrier materials that do not react with hydrogen peroxide or propylene oxide under the reaction conditions employed for the epoxidation are suitable, and silica is preferably used as the binder. Extrudates having a diameter of 1 mm to 5 mm are preferably used as the shaped catalyst. The amount of catalyst employed can vary within a wide range, and the amount of catalyst employed is preferably selected so as to achieve a hydrogen peroxide consumption of more than 90%, preferably more than 95%, within 1 minute to 5 hours under the epoxidation reaction conditions employed.

[0017] The epoxidation reaction of step a) is preferably carried out at a temperature of from 20 °C to 80 °C, more preferably at a temperature of from 25 °C to 60 °C. The epoxidation reaction is preferably carried out at a pressure above the vapor pressure of propylene at the reaction temperature in order to maintain propylene dissolved in the solvent or present as a separate liquid phase. The pressure in step a) is preferably from 1.9 MPa to 5.0 MPa, more preferably from 2.1 MPa to 3.6 MPa and most preferably from 2.4 MPa to 2.8 MPa. The use of an excess of propylene at high pressure provides a high reaction rate and a high conversion of hydrogen peroxide, and at the same time provides a high selectivity to propylene oxide.

[0018] As described in EP 0 230 949 A2, the epoxidation reaction is preferably carried out in the presence of added ammonia to increase the selectivity to the epoxide. Ammonia is preferably added in a weight ratio of ammonia to the initial amount of hydrogen peroxide of from 0.0001 to 0.003.

[0019] The epoxidation reaction of step a) is preferably carried out in a fixed bed reactor by passing a mixture comprising propylene, hydrogen peroxide and a solvent through a fixed bed comprising a shaped titanium zeolite catalyst. The fixed bed reactor is preferably a tube bundle reactor and the catalyst fixed bed is arranged inside the reactor tube. The fixed bed reactor is preferably equipped with a cooling device and cooled with a liquid cooling medium. The temperature distribution along the length of the catalyst fixed bed is preferably adjusted so that the reaction temperature distribution along 70% to 98%, preferably 80% to 95% of the length of the catalyst fixed bed is maintained in a range of less than 5°C, preferably 0.5°C to 3°C. The temperature of the cooling medium fed to the cooling device is preferably adjusted to a value of 3°C 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 a downward flow mode, preferably in a downward flow mode at an apparent velocity of 1m / h to 100m / h, more preferably 5m / h to 50m / h, most preferably 5m / h to 30m / h. The apparent velocity is defined as the ratio of the volume flow rate / cross section of the catalyst bed. Furthermore, it is preferred to allow the reaction mixture to -1 Until 20h -1 , preferably 1.3h -1 Until 15h -1 The catalyst bed is passed through at a liquid hourly space velocity (LHSV) of 1.5 wt % through the catalyst bed. It is particularly preferred that the catalyst bed is maintained in a trickle bed state during the epoxidation reaction. Suitable conditions for maintaining a trickle bed state during the epoxidation reaction are disclosed in WO02 / 085873, page 8, line 23 to page 9, line 15. Most preferably, the epoxidation reaction is carried out by maintaining a fixed bed of catalyst in a trickle bed state at a pressure close to the vapor pressure of propylene at the reaction temperature, using an excess of propylene providing a reaction mixture comprising 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 so as to be able to operate the epoxidation process continuously while regenerating the epoxidation catalyst.

[0020] In a side reaction of the reaction of propylene with hydrogen peroxide, part of the hydrogen peroxide fed to step a) decomposes into molecular oxygen according to the following equation:

[0021] 2H 2 O 2 →2H 2 O+O 2

[0022] The molecular oxygen formed by the decomposition of hydrogen peroxide may remain dissolved in the liquid reaction mixture formed in step a). Some or all of the molecular oxygen formed by the decomposition of hydrogen peroxide may also be transferred from the liquid reaction mixture into the gas phase present in step a). The molecular oxygen dissolved in the liquid reaction mixture formed in step a) may also be released into the gas phase during the treatment of the liquid reaction mixture.

[0023] In step b) of the process of the invention, the amount of molecular oxygen formed in step a) is determined. The amount of molecular oxygen can be determined by any method known in the prior art for determining the concentration of molecular oxygen. The amount of molecular oxygen present in the liquid phase, for example the liquid reaction mixture formed in step a), can be determined by gas chromatography from a sample of the liquid phase. Alternatively, the oxygen concentration in the liquid phase can be measured using a sensor for measuring dissolved oxygen, such as a polarographic or galvanic electrochemical sensor or a sensor that detects fluorescence quenched by triplet oxygen. The amount of molecular oxygen present in the gas phase can be determined by gas chromatography from a sample of the gas phase, by Raman spectroscopy or by measuring magnetic susceptibility. These methods can be combined for determining the amount of molecular oxygen formed in step a).

[0024] In a preferred embodiment, an inert gas stream is introduced in step a) to prevent the formation of an oxygen-rich, flammable gas phase. A purge gas stream comprising oxygen is obtained from step a), and step b) comprises measuring the flow rate and oxygen content of the purge gas stream. The oxygen content measured for the purge gas stream can be used to adjust the amount of inert gas introduced in step a) so as to prevent the formation of an oxygen-rich, flammable gas phase with a minimum amount of inert gas. The inert gas is preferably selected from nitrogen, argon, oxygen-depleted air, methane and mixtures thereof, and most preferably nitrogen. Before measuring the flow rate and oxygen content of the purge gas stream, the solvent and propylene oxide product used in step a) can be removed from the purge gas stream by condensation.

[0025] In another preferred embodiment, during the treatment of the reaction mixture, preferably by decompressing the reaction mixture or by distilling propylene from the reaction mixture, a treatment gas stream containing oxygen is separated from the liquid reaction mixture of step a). Then, step b) comprises measuring the flow rate and oxygen content of the treatment gas stream. Inert gas can be introduced during the treatment to prevent the treatment gas stream from becoming oxygen-enriched and flammable. The same inert gas as in the embodiment described in the previous paragraph can be used. Before measuring the flow rate and oxygen content of the treatment gas stream, the solvent and propylene oxide product used in step a) can be removed from the treatment gas stream by condensation. The embodiment described in the embodiment described in the previous paragraph can be combined by merging the purge gas stream with the treatment gas stream and measuring the flow rate and oxygen content of the resulting combined gas stream, which allows the amount of molecular oxygen formed in step a) to be determined with a single measurement.

[0026] In another preferred embodiment, the liquid reaction mixture of step a) is stripped with an inert gas to provide a stripping gas stream, and step b) comprises measuring the flow rate and oxygen content of the stripping gas stream. The inert gas is preferably selected from nitrogen, argon, oxygen-depleted air, methane and mixtures thereof, and most preferably nitrogen. The stripping is preferably carried out in a stripping tower, more preferably in a countercurrent stripping tower, to provide a stripping liquid reaction mixture that is substantially free of oxygen and preferably contains less than 300 ppm by weight of oxygen. The stripping is preferably carried out at a temperature below the boiling point of propylene at the pressure used in the stripping step, so as to prevent propylene from distilling off during the stripping. The amount of inert gas is preferably selected to provide an oxygen concentration in the stripping gas stream of 0.1 wt % to 10 wt %, preferably 0.5 wt % to 8 wt %. Preferably, the oxygen concentration in the stripping gas stream is continuously measured, and the measured value is used to control the amount of inert gas used for stripping, so as to achieve substantially complete stripping of oxygen with a minimum amount of stripping gas. Stripping oxygen from the liquid reaction mixture of step a) facilitates further processing of the reaction mixture, since no additional safety measures are required to prevent the formation of flammable gas mixtures in downstream processing of the stripped liquid reaction mixture. In a further preferred embodiment, an additional inert gas stream is introduced in step a), a purge gas stream comprising oxygen is obtained from step a), said purge gas stream is combined with said stripping gas stream, and step b) comprises measuring the flow rate and oxygen content of the resulting combined gas stream.

[0027] In a preferred embodiment, the reaction of step a) is carried out at a pressure of 0.5 MPa to 50 MPa, preferably 1.9 MPa to 5.0 MPa, and step b) comprises measuring the oxygen content in a vapor sample obtained by decompressing the liquid reaction mixture of step a). This allows the amount of oxygen formed in step a) to be measured without measuring the oxygen content of the pressurized liquid, and prevents oxygen measurement errors caused by residual hydrogen peroxide and organic peroxide byproducts contained in the liquid reaction mixture. The amount of oxygen contained in the liquid reaction mixture can be correctly measured by one of the following two preferred embodiments. In the first embodiment, step b) includes: step b1) taking out a sample of the liquid reaction mixture and determining the weight of the sample; step b2) depressurizing the sample to provide a depressurized liquid sample and a vapor sample, and determining the weight of the depressurized liquid sample, the vapor sample or both samples; step b3) determining the oxygen content in the vapor sample; and the following step of calculating the oxygen content in the liquid reaction mixture based on the oxygen content determined in step b3), the weight of the depressurized liquid sample, the vapor sample or both determined in step b2), and the weight of the sample of the liquid reaction mixture determined in step b1). In a second embodiment, step b) includes: step b1), taking out a sample flow of the liquid reaction mixture and determining the mass flow of the sample flow; step b2), depressurizing the sample flow to provide a depressurized liquid sample flow and a vapor sample flow, and determining the mass flow of the depressurized liquid sample flow, the vapor sample flow or both; step b3), determining the oxygen content in the vapor sample flow; and the following step, calculating the oxygen content in the liquid reaction mixture from the oxygen content determined in step b3), the mass flow of the depressurized liquid sample flow, the vapor sample flow or both determined in step b2), and the mass flow of the sample flow of the liquid reaction mixture determined in step b1).

[0028] In step c) of the process of the present invention, the titanium zeolite catalyst is regenerated when the molar ratio of molecular oxygen formed in step a) to hydrogen peroxide fed to step a) exceeds a preset threshold value. The threshold value is preferably set to a value of 0.01 to 0.1, more preferably a value of 0.04 to 0.08.

[0029] The titanium zeolite catalyst can be regenerated by calcination, by treatment with a heated gas, preferably a gas containing oxygen, by treatment with an oxidant, preferably hydrogen peroxide, in the absence of propylene, or by washing with a solvent. The titanium zeolite catalyst is preferably regenerated by washing the titanium zeolite catalyst with a solvent, preferably at a temperature of 80°C to 250°C, more preferably 100°C to 200°C. The solvent used to regenerate the titanium zeolite catalyst is preferably selected from alcohols, glycols, cyclic ethers, glycol ethers, ketones, nitriles and combinations thereof, and preferably comprises methanol, most preferably methanol in an amount of 50% to 100% by weight. Regeneration is preferably performed with a methanol solvent as described in WO2005 / 000827. When a solvent is used in step a), the titanium zeolite catalyst is preferably regenerated by washing the titanium zeolite catalyst with the same solvent as used in step a). When a fixed bed reactor is used in step a) of the process, the regeneration of the titanium zeolite catalyst is preferably performed without removing the catalyst from the fixed bed reactor.

[0030] Different regeneration methods may be combined. Preferably, the catalyst is regenerated by washing with a solvent until the time span during which the catalyst can be used after regeneration has fallen below a desired level. The catalyst is then regenerated by calcination or by treatment with a heated gas. Preferably, when the time span during which the catalyst can be used between two regenerations by washing the catalyst with an organic solvent is below a preset threshold, the titanium zeolite epoxidation catalyst is regenerated by heat treatment with a gas stream containing oxygen at 80°C to 600°C, more preferably at 250°C to 550°C.

[0031] In step d) of the process of the present invention, the reaction of step a) is repeated with the titanium zeolite catalyst regenerated in step c). The reaction of step a) is preferably repeated under the same reaction conditions. The order of steps a) to d) may be repeated as long as the catalyst regains sufficient catalytic activity by regeneration and the regeneration causes the decomposition of hydrogen peroxide to fall to a level where the molar ratio of molecular oxygen formed in step a) to hydrogen peroxide fed to step a) is below a preset threshold. When regeneration fails to restore the catalytic activity to the desired level or fails to reduce the decomposition of hydrogen peroxide to below the threshold, the titanium zeolite catalyst needs to be replaced.

[0032] Monitoring the amount of molecular oxygen formed in the continuous reaction of propylene with hydrogen peroxide and regenerating the titanium catalyst for epoxidation when the molecular oxygen formed exceeds a threshold value both allow maintaining a high yield of propylene oxide based on hydrogen peroxide and allow adjusting an optimal balance between a low average consumption of raw materials required for the production of propylene oxide and a short reactor downtime caused by catalyst regeneration.

[0033] In a preferred embodiment of the process of the present invention, in step a), the conversion of hydrogen peroxide is maintained within a preset range by increasing the reaction temperature to compensate for the loss of activity caused by catalyst deactivation. When the reaction temperature exceeds a predetermined threshold, the titanium zeolite catalyst is regenerated; and step a) is repeated with the regenerated catalyst. In this embodiment, two different criteria for initiating catalyst regeneration are applied, namely if the molecular oxygen formed by the decomposition of hydrogen peroxide exceeds a threshold or if the reaction temperature required to maintain the desired hydrogen peroxide conversion exceeds a threshold, the titanium zeolite catalyst is regenerated. This embodiment has the advantages of maintaining a substantially constant production capacity of propylene oxide and maintaining a high yield of propylene oxide based on hydrogen peroxide. The conversion of hydrogen peroxide is preferably maintained in the range of 90% to 99.9%, and more preferably is maintained constant with a deviation of less than 2% from a preset value. Preferably, the titanium zeolite catalyst is regenerated when the reaction temperature exceeds a preset value of 50°C to 70°C.

[0034] Preferably, a combination of regenerating the titanium zeolite catalyst by washing with a solvent and regenerating the titanium zeolite catalyst by heat treatment is used. When regeneration by washing with a solvent is repeated, the time span of the regenerated catalyst can be used before another regeneration must be performed after regeneration by washing with a solvent will gradually decrease. Therefore, preferably, after regeneration by washing with a solvent, the time span of the regenerated catalyst can be used has been shortened to 10% to 50%, more preferably 30% to 50%, when the time span of fresh catalyst can be used until regeneration becomes necessary, regeneration is performed by heat treatment. Then, preferably at 80°C to 600°C, more preferably 250°C to 500°C, regeneration is performed by heat treatment with a gas stream containing oxygen. After regeneration by heat treatment, the regenerated catalyst can be used over a longer time span until the catalytic activity loss requires regeneration again. Then, when the time span over which the regenerated catalyst can be used is shortened again to a value of 10% to 50%, preferably 30% to 50%, of the time span over which the catalyst regenerated by thermal treatment can be used, further regeneration can be carried out through a series of multiple regenerations by washing with a solvent followed by thermal regeneration.

[0035] Example

[0036] The continuous epoxidation of propylene was carried out in a test reactor using a reaction tube with a cooling jacket. The reaction tube had an inner diameter of 30.5 mm and contained a catalyst fixed bed of an extruded titanium silicalite catalyst of 10 L. A mixture of 40 wt % propylene, 7.7 wt % hydrogen peroxide, 3.3 wt % water, 49 wt % methanol and 77 wt ppm ammonia was fed to the top of the reaction tube and passed through a catalyst fixed bed in a trickle flow mode. The pressure in the reactor was maintained at 2.6 MPa by introducing nitrogen and the temperature in the reactor was regulated by controlling the coolant flow to maintain a substantially constant conversion rate of 97% to 98% of hydrogen peroxide, compensating for the loss of activity of the catalyst by increasing the reaction temperature. The average residence time of the reaction mixture in the reactor was less than 5 minutes. The liquid reaction mixture leaving the reaction tube was decompressed to ambient pressure. The flow rate and oxygen content of the gas phase formed during decompression were measured, and the proportion of hydrogen peroxide decomposed into oxygen was calculated by these measured values. The propylene oxide and hydrogen peroxide of the liquid phase remaining after the decompression were analyzed, and the yield of propylene oxide and the conversion rate of hydrogen peroxide were calculated based on the analysis results.

[0037] After 2335 hours of operation, the epoxidation reaction was interrupted by stopping the feed and the catalyst was regenerated in the reactor by passing methanol through the catalyst fixed bed at a rate of 6 kg / h, increasing the reactor temperature at a rate of 10 K / h to 150° C., maintaining it at a temperature of 150° C. for 24 hours, and then reducing the reactor temperature at a rate of 10 K / h to about 25° C. Then, epoxidation was restarted as described above and continued for a further 1155 hours. The regeneration was repeated and epoxidation was restarted again and continued for a further 1094 hours. Then, the catalyst was regenerated for the third time as described above and then thermally treated with an oxygen-containing gas. The thermal treatment was carried out by draining off the methanol solvent; passing a nitrogen stream of 68 l / h and an air stream of 22 l / h through the catalyst bed to establish an oxygen concentration of 5% by volume; heating the reactor at a rate of 10 K / h to a temperature of 280° C.; and maintaining the temperature until the oxygen consumption caused by the thermal regeneration had ceased and the oxygen concentration of the gas leaving the reactor had risen to the level of the gas entering the reactor. The reactor was then cooled to room temperature at a rate of 10 K / h, maintaining the gas flow. Thereafter, the gas supply was stopped and, before restarting the epoxidation for the third time, the catalyst was re-wetted by passing water through the catalyst fixed bed and then gradually increasing the methanol content in the liquid passing through the catalyst fixed bed to the level used for epoxidation, as described in WO 2016 / 016070.

[0038] Table 1 summarizes the changes in propylene oxide yield, hydrogen peroxide decomposition and average temperature in the catalyst fixed bed during the epoxidation reaction. In the first 150 hours to 200 hours after starting or restarting the epoxidation reaction, hydrogen peroxide decomposition decreases and propylene oxide yield generally increases. From then on, hydrogen peroxide decomposition increases slowly and propylene oxide yield slowly decreases until regeneration becomes necessary.

[0039] Table 1

[0040]

Claims

1. A method for the continuous epoxidation of propylene with hydrogen peroxide, wherein include: a) continuously reacting propylene with hydrogen peroxide in the presence of a titanium zeolite epoxidation catalyst to provide a liquid reaction mixture comprising propylene oxide; b) determining the amount of molecular oxygen formed in step a); c) regenerating the titanium zeolite catalyst when the molar ratio of molecular oxygen formed in step a) to hydrogen peroxide fed to step a) exceeds a preset threshold, wherein the threshold is 0.04 to 0.08; and d) Repeating step a) using the titanium zeolite regenerated in step c).

2. The method according to claim 1, wherein an inert gas stream is introduced in step a), a purge gas stream comprising oxygen is obtained from step a), and step b) comprises measuring the flow rate and oxygen content of the purge gas stream.

3. A process according to claim 1 or 2, wherein a process gas stream comprising oxygen is separated from the liquid reaction mixture of step a) by depressurizing the reaction mixture or by distilling propylene from the reaction mixture, and step b) comprises measuring the flow rate and oxygen content of the process gas stream.

4. A method according to claim 3, wherein an inert gas stream is introduced in step a), a purge gas stream comprising oxygen is obtained from step a), the purge gas stream is combined with the process gas stream, and step b) comprises measuring the flow rate and oxygen content of the resulting combined gas stream.

5. The process according to claim 1 or 2, wherein the liquid reaction mixture of step a) is stripped with an inert gas to provide a stripping gas stream, and step b) comprises measuring the flow rate and oxygen content of the stripping gas stream.

6. A process according to claim 5, wherein an inert gas stream is introduced in step a), a purge gas stream comprising oxygen is obtained from step a), the purge gas stream is combined with the stripping gas stream, and step b) comprises measuring the flow rate and oxygen content of the resulting combined gas stream.

7. The method according to claim 1 or 2, wherein step a) is carried out at a pressure of 0.5 MPa to 50 MPa, and step b) include: b1) taking a sample of the liquid reaction mixture and measuring the weight of the sample; b2) depressurizing the sample to provide a liquid sample and a vapor sample, and determining the weight of the liquid sample, the vapor sample, or both; b3) determining the oxygen content in the steam sample; b4) calculating the oxygen content in the liquid reaction mixture from the oxygen content determined in step b3), the weight of the liquid sample, the vapor sample or both determined in step b2), and the weight of the sample of the liquid reaction mixture determined in step b1).

8. The method according to claim 1 or 2, wherein step a) is carried out at a pressure of 0.5 MPa to 50 MPa, and step b) include: b1) taking a sample flow of the liquid reaction mixture and determining the mass flow of the sample flow; b2) depressurizing the sample stream to provide a liquid sample stream and a vapor sample stream, and determining the mass flow of the liquid sample stream, the vapor sample stream, or both; b3) determining the oxygen content in the vapor sample flow; b4) calculating the oxygen content in the liquid reaction mixture from the oxygen content determined in step b3), the mass flow of the liquid sample flow, the vapor sample flow or both determined in step b2), and the mass flow of the sample of the liquid reaction mixture determined in step b1).

9. The method according to claim 1 or 2, wherein the amount of oxygen is determined by Raman spectroscopy, by gas chromatography, by measuring magnetic susceptibility or by a combination thereof.

10. The method according to claim 1 or 2, wherein in step a), the conversion rate of hydrogen peroxide is maintained within a preset range by increasing the reaction temperature to compensate for the activity loss caused by catalyst deactivation; when the reaction temperature exceeds a predetermined threshold, the titanium zeolite catalyst is regenerated; and step a) is repeated using the regenerated catalyst.

11. The method according to claim 10, wherein the conversion rate of hydrogen peroxide is maintained in the range of 90% to 99.9%, and when the reaction temperature exceeds a preset value in the range of 50°C to 70°C, the titanium zeolite catalyst is regenerated.

12. The method according to claim 11, wherein the conversion of hydrogen peroxide is kept constant with a deviation of less than 2% from a preset value.

13. The process according to claim 1 or 2, wherein the titanium zeolite epoxidation catalyst is regenerated by washing with a solvent.

14. The method of claim 13, wherein the titanium zeolite epoxidation catalyst is regenerated by washing with a solvent at a temperature of 80°C to 250°C.

15. The process of claim 13, wherein a solvent is used in step a) and the catalyst is regenerated by washing the catalyst with the solvent used in step a).

16. The method of claim 13, wherein the solvent is selected from the group consisting of alcohols, glycols, cyclic ethers, glycol ethers, ketones, nitriles, and combinations thereof.

17. The method of claim 16, wherein the solvent comprises methanol.

18. The method of claim 16, wherein the solvent comprises methanol in an amount of 50 wt% to 100 wt%.

19. The method of claim 13, wherein the titanium zeolite epoxidation catalyst is additionally regenerated by heat treatment with a gas stream containing oxygen at 80°C to 600°C if regeneration by washing with a solvent no longer restores catalyst activity to a desired time span.

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