Process and apparatus for continuous epoxidation in liquid phase of propene
Patent Information
- Application Number
- AE202602367
- Authority / Receiving Office
- AE · AE
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2024-01-17
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Abstract
Description
PROCESS AND APPARATUS FOR CONTINUOUS EPOXIDATION IN LIQUID PHASE OF PROPENE DESCRIPTIONSUMMARYIn a process for the epoxidation of propene with hydrogen peroxide in the liquid phase in the presence of a titanium silicalite catalyst which involves the use of a plant comprising two twin catalytic reactors, operationally interchangeable, one in operation and one in regeneration, said catalyst is frequently regenerated alternately without interrupting the operation of the plant by continuously washing with methanol solvent recycled from the recovery section of the plant, in the presence of a weak basic additive at a mild controlled temperature not exceeding 4°C above the reaction exit temperature. The regenerating methanol containing the weak basic additive coming from the regenerating reactor is then continuously fed to the reactor in operation. This regeneration procedure will maintain a substantially constant propene oxide yield and stable operating conditions of the plant for a long period of time.FIELD OF INVENTIONThe present invention concerns an improved propene epoxidation process and system capable of maintaining a high yield of propene oxide for a long period of time, using an efficient and simple method to regenerate the catalyst, without interrupting the operation of the plant.More specifically, the present invention concerns a reaction system of the process for the continuous preparation of an olefinic oxide by direct oxidation of an olefin with hydrogen peroxide, which process comprises in its overall the following main steps:(a) feeding into a reaction / regeneration system, comprising reaction units containing an epoxidation catalyst, an olefin, hydrogen peroxide, a weak basic additive and a reaction solvent;(b) feeding the product of the reaction system into a distillation system, preferably comprising a distillation section comprising one or more columns in which olefinic oxide is recovered and purified, the excess of olefin is recovered and recycled to the reaction system and more specifically one flash column for each reactor of the reaction system, to obtain a head product comprising olefinic oxide and unreacted olefin, and a tail product comprising unreacted hydrogen peroxide, reaction by-products, water and reaction solvent;(c) feeding the tail product from (b) to a solvent recovery system to obtain purified solvent, which is recycled into the reaction system (a) and a tail product comprising the reaction water and water introduced with the hydrogen peroxide, the reaction by-products and traces of solvent, which is discarded. According to the invention, propene and hydrogen peroxide are reacted in a liquid phase in the presence of a heterogeneous epoxidation catalyst in a reaction plant system comprising at least two twin reactors, one in operation phase and one in catalyst regeneration phase, which are connected to a common circulation external loop with a heat exchanger, and a recirculation pump, means being furthermore provided to ensure a regular passage of the liquid flow.Advantageously, performing a continuous washing of deactivated catalyst bed by reaction solvent at mild temperature condition not exceeding 4°C above the reaction exit temperature, in presence of a weak basic additive, the activity of the catalyst is very well recovered and there is no need to raise temperature and pressure during the reaction to maintain a constant yield of propene oxide but just switch the process reaction to the reactor where the washing was completed, when needed.In this way the catalyst is not stressed, avoiding both the costly regeneration processes and also expensive refilling of the catalyst, burdensome in term of time.The solution provided by the invention has the advantages of simplifying the regeneration process of the catalyst, realizing long-period continuous and stable operation in the production process of propene oxide, reducing the loss caused by equipment downtime and having simple technology process and lower plant investment cost.BACKGROUND OF THE INVENTIONOver the last several decades, different types of insoluble substances have been found to be highly active and selective catalysts for transforming olefins such as propene to epoxides such as propene oxide using active oxygen species. One class of such catalysts includes the titanium silicalites such as TS-1 and other zeolites having titanium atoms in their framework structures, which work well where the oxidant is hydrogen peroxide and the olefin is relatively small. See, for example, U.S. Pat. No. 4410501.Although heterogeneous epoxidation catalysts typically exhibit high activity and selectivity when freshly prepared, gradual deactivation takes place simultaneous with epoxidation. This problem is particularly acute in a large scale continuous commercial operation where, for economic reasons, an epoxidation process must be capable of being operated over an extended period of time while maintaining high yields of epoxide. The catalyst employed is a titanium silicalite catalyst. However, a serious problem associated with this process is that the activity of the titanium silicalite catalyst is rapidly reduced due to deposition of organics on the catalyst. This fact has been justified by the presence of the propene oxide condensation products that are produced as byproducts in the propene oxide synthesis, such as dimers of propene oxide and methoxypropanols. As a matter of fact, the molecules of these compounds, formed in the micropores of the zeolites, cannot diffuse completely outside from the channels and part of them resides there. In addition to the product propene oxide, the epoxidation of propene is accompanied by the formation of by-products such as propylene glycol, propylene glycol monomethyl ethers, propylene glycol isomethyl ether, and the condensation products from reaction of propene with the above by-products as macromolecular alcohol ethers. The aggregation of these macromolecular condensed alcohol ethers blocks the internal pores of the catalyst, resulting in a decrease in the number of active sites of the molecular sieve, resulting in deactivation of the catalyst.Thus, it is essential to have means for regenerating the catalyst in order to use it repeatedly. Although regeneration methods for such catalysts are known, it would be highly advantageous to develop procedures whereby the interval between deep regenerations is extended for as long as possible.Regeneration requires that epoxidation is interrupted for some period of time sufficient to perform catalyst reactivation, thereby reducing the effective annual capacity of a commercial plant. The deactivated catalyst could alternatively be replaced with fresh catalyst, but the same practical disadvantages will result to be the same as with regeneration. Additionally, catalysts of this type tend to be relatively costly and it would be desirable to minimize the quantity of fresh catalyst which is needed to be supplied to the plant.The aim of the present invention is to provide an improved process and plant for the continuous epoxidation of propene with hydrogen peroxide in the presence of a titanium silicalite catalyst wherein, according to a peculiar feature of the invention, the reaction methanol solvent stream and a weak basic additive mixture is firstly used to perform the catalyst bed regeneration of a twin reactor and then it is continuously fed to the reactor in operation, through a circulation external loop comprising a heat exchanger and a recirculation pump. In this way the twin reactor catalyst bed is continuously regenerated by washing it at mild condition of temperature. The epoxidation reaction is carried out at essentially constant hydrogen peroxide conversion (negative deviation in the range 0.1-5%), before switching to the regenerated reactor. PRIOR ARTThe epoxidation of propene with hydrogen peroxide and a titanium silicalite catalyst is known from EP0100118B1 and from M. G. Clerici, et al., Journal of Catalysis, Vol. 129, pages 159 to 167. M. G. Clerici, et al., disclose that in this reaction catalytic activity decreases with time on stream and that titanium silicalite may be regenerated by washing with solvents somewhat above reaction temperature. Suitable solvents are methanol or the solvent that is used in the reaction. It is also stated that fresh and recovered catalysts have similar activity and physical-chemical properties. However, no information is given on the deactivation behavior of catalysts that were regenerated by washing with solvents.US6063941A discloses a process for the regeneration of titanium silicalite catalysts in which the regeneration is carried out with a treatment of oxidizing agent such as hydrogen peroxide, organic peroxides or ozone in a liquid phase at pH range of 4-7. However, it has been observed that the regeneration of the catalyst is temporary and the catalyst loses its regained activity very rapidly.US6617465B2 discloses a process for the catalytic epoxidation of olefins by means of hydrogen peroxide and a titanium zeolite catalyst, wherein the epoxidation reaction is carried out in a reaction system through which the reaction mixture flows continuously and the regeneration of deactivated catalyst is carried out by means of hydrogen peroxide in the presence of the olefin while continuing the epoxidation reaction. In this case catalyst regeneration is performed by hydrogen peroxide solution and olefine mixture and there is no evidence of efficiency of the washing step, nor examples reported; anyhow the reaction system to be used becomes elaborate and not so easy to manage. The hydrogen peroxide solution may react with olefine over the catalyst or decompose to oxygen and water.US6169050B1 discloses a process in which the deactivated catalyst is first washed with solvents, removed from the reactor system and then regenerated by thermal treatment in presence of gaseous stream that includes one or more components chosen from nitrogen, oxygen and water. However, the process involves repeated removal of the catalyst from the reactor for such regeneration, thus making the overall process cycle laborious, time consuming and expensive in terms of catalyst losses.CN101747297 (B) discloses catalyst regeneration at a pressure of 100 bar or less at 100 to 300 °C for 1 to 100 hours, and a mass hourly space velocity of the solvent is 0.1-100 hour<-1>. It is a method for continuously producing epichlorohydrin, which comprises introducing 3-chloropropene, hydrogen peroxide and a solvent into a plurality of fixed bed reactors equipped with a titanium silicalite catalyst to make 3-chloropropene and hydrogen peroxide is subjected to an epoxidation reaction. The regeneration of catalyst requires high temperature washing by different solvents, there is no evidence of efficiency of the washing step, nor examples reported; anyhow the reaction system is related to epichlorohydrin production.US6790969B2 and US6958304B2 disclose a method for regeneration of a titanium silicate catalyst used for propene epoxidation. In this process, the catalyst regeneration is achieved through steps such as washing with a suitable solvent, drying the catalyst in the temperature range of 50 to 250°C, heating the catalyst further to a temperature of 450-600°C, and regenerating the catalyst with a gas stream containing either nitrogen, oxygen, hydrogen, carbon monoxide or carbon dioxide gas.The process involves repeated removal of the catalyst from the reactor for such regeneration, thus making the overall process cycle laborious, time consuming and expensive in terms of catalyst losses.WO9818555A1 describes a process for regenerating a titanium zeolite catalyst which is used in the epoxidation of olefins by means of hydrogen peroxide. According to one embodiment, a solution of the oxidant which is also used for the epoxidation is employed as regeneration medium. For example, if appropriate after addition of hydrogen peroxide, the reaction medium leaving the epoxidation reactor can be used as regeneration medium.In this case catalyst regeneration is performed by hydrogen peroxide solution and it requires dedicated equipment; anyhow the hydrogen peroxide solution may decompose to oxygen and water during regeneration at high temperature (90-100°C).Most of known regeneration processes have the disadvantage that the catalyst displays a step increase in activity after the regeneration and that there is an increased formation of by-products formed by subsequent reactions of propene oxide along the reaction period. This results in decreased yields and in problems in the operation of a continuous production plant due to the fluctuations in heat evolution and the concentrations of by-products.JPH03114536A describes the regeneration of a titanium silicalite catalyst that was deactivated in an epoxidation reaction by washing with a solvent at a temperature that is 5 to 150°C higher than the temperature used in the epoxidation reaction. Examples 4 and 5 disclose the regeneration of a catalyst that was used for the epoxidation of allyl chloride by washing with methanol at 70°C and 85°C. No information is given on the deactivation behaviour of catalysts that were regenerated by washing with solvents.EP1190770B1 discloses a method for regenerating a titanium silicalite that has been used as an epoxidation catalyst by washing with a solvent containing a source of ammonium and / or alkali metal ions at a temperature of at least 150°C. The presence of ammonium or alkali metal ions in the washing solvent has the effect that regenerated catalysts reach high epoxide selectivity more quickly after recommencement of epoxidation. However, no information is given on the deactivation behaviour of catalysts that were regenerated by washing with a solvent. EP1190770B1 also teaches that, if propene is epoxidized and the reaction temperature is raised to compensate for the deactivation of the catalyst while a constant pressure is maintained, such temperature rise will lead to a decrease in propene oxide selectivity. In order to keep the propene oxide selectivity constant, the pressure has to be raised during the temperature rise in such a manner as to maintain a constant propene concentration in the reaction mixture. The epoxidation process disclosed in EP1190770B1 therefore has the disadvantage that a constant product selectivity can only be achieved if the reaction mixture is permanently monitored for the propene content. US6878836B2 / EP1489074 A1 describes a process for the continuous epoxidation of propene comprising reacting propene with a source of hydrogen peroxide in the presence of a titanium silicalite catalyst and a methanol solvent, and periodically regenerating the catalyst in a regeneration step by washing with a methanol solvent and wherein epoxidation is carried out for periods of at least 300 h between two regeneration steps. After regeneration of the catalyst the reaction temperature for the epoxidation is raised by 0.025 K / h or less to compensate for catalyst deactivation and to maintain an essentially constant hydrogen peroxide conversion. The catalyst is regenerated at a temperature from 100°C to 200°C for a period of 0.5 to 48 hours. With the process according to the invention a high selectivity can also be maintained while the reaction temperature is raised to compensate for catalyst deactivation without the need for raising the pressure as for the process of EP1190770B1. This facilitates process control because there is no need to measure the amount of propene in the liquid phase of the reaction mixture, whereas the process of EP1190770B1 requires such measurement to adjust the pressure for maintaining a constant propene concentration.CN102260226(B) discloses an olefin epoxidation method, which is characterized by comprising the following steps of: performing epoxidation on olefin and hydrogen peroxide in the presence of a titanium-silicon molecular sieve catalyst and a reaction solvent under the conditions of epoxidation; when the titanium-silicon molecular sieve catalyst is deactivated, stopping feeding the olefin and the hydrogen peroxide, regenerating the titanium-silicon molecular sieve catalyst; after the regeneration, restoring to feed the olefin and the hydrogen peroxide, and continuously performing the olefin epoxidation under the conditions of epoxidation; and when the titanium-silicon molecular sieve catalyst is deactivated, repeating the regeneration, wherein the regeneration comprises the following steps of: a) keeping the temperature of a reactor unchanged, and making a washing regeneration solvent contact and wash the deactivated titanium-silicon molecular sieve catalyst for 1 to 24 hours;b) raise the washing regeneration temperature at a heating rate of 0.2 to 20°C / min, so that the temperature of the washing regeneration solvent is 0.85 to 1 related to its saturated vapor pressure, and wash at that temperature the deactivated titanium-silicon molecular sieve catalyst for at least 1 hour;c) the catalyst is cooled to the epoxidation reaction temperature at a temperature reduction rate of 1-10°C / min. In Examples 1 and Example 2 (evaluation method of the catalytic activity of the titanium-silicon molecular sieve catalyst in the epoxidation of 3-chloropropene and hydrogen peroxide in a fixed bed reactor) step b) temperature is respectively 225°C (0.97 related to saturation temperature) and 210°C (0.94 related to saturation temperature).In the method provided in CN102260226 (B), in the epoxidation reaction, an alkaline additive may be added to the feed. The alkaline additive may be a weak base, including inorganic weak bases and organic weak bases. The inorganic weak base generally refers to ammonium hydroxide, which generally exists in the form of an aqueous solution. In the regeneration treatment process, the epoxidation reaction time between two catalyst washing regenerations is usually greater than 100 h, preferably not more than 600 h.So the CN102260226 (B) regeneration method is a solvent-on-line washing to regenerate the titanium-silicon molecular sieve catalyst deactivated in the fixed-bed reactor. The regenerated catalyst can completely recover the catalytic activity. The washing regeneration can be repeated multiple times. The epoxidation reaction time of the regenerated catalyst under the preferred epoxidation reaction conditions is greater than 150h, which can significantly reduce the number of catalyst washings and regenerations, greatly increase the use time of the epoxidation reaction of the catalyst, and reduce the cost of the catalyst.Therefore, there is the need to improve the process for continuous epoxidation of propene with hydrogen peroxide and a titanium silicalite catalyst with a catalyst regeneration by washing with a solvent in such a way, that: (1) the regenerated catalyst can be used for long period of time without removing it from the reactor;(2) it maintains very high level of hydrogen peroxide conversion and selectivity to propene oxide; (3) at the same time it overcomes the disadvantages known from EP1190770B1, EP1489074 A1 / US6878836B2 and from CN102260226 (B), like the needs to wash the catalyst at severe condition (more than 100°C in one or more steps) and to increase reaction temperature and / or pressure to maintain constant hydrogen peroxide conversion.Catalyst regeneration procedures known in the art include calcination, solvent washing, and / or treatment with various reagents. Where the catalyst is deployed in fixed bed form, it is highly desirable to practice a regeneration technique where the catalyst is reactivated in place (i.e. without removal from the epoxidation reactor). This will, generally speaking, further enhance the overall efficiency of the process by minimizing the amount of time the process is off-line since discharging the catalyst from the reactor and then recharging it is typically quite time-consuming.SUBJECT OF THE INVENTIONThe invention is related to a process for the continuous epoxidation of propene with hydrogen peroxide in the presence of a titanium silicalite catalyst, a methanol solvent and ammonia. The epoxidation reaction takes place in a fixed bed, the fixed-bed reactor includes, but is not limited to, a jacketed tube reactor, a tubular reactor, an external circulation fixed bed reactor, and a fixed bed catalytic distillation column reactor. External circulation fixed bed reactors are preferably used. In addition to the product propene oxide, the epoxidation of propene is accompanied by the formation of by-products such as propylene glycol, propylene glycol monomethyl ether, propylene glycol isomethyl ether, and the condensation products from reaction of propene with the above by-products as macromolecular alcohol ethers. The aggregation of these macromolecular condensed alcohol ethers blocks the internal pores of the catalyst, resulting in a decrease in the number of active sites of the molecular sieve, resulting in deactivation of the catalyst. Catalyst deactivation is then the formation of high molecular weight species deposits that block catalyst pores or otherwise prevent access to catalytic sites. The activity of the titanium silicate catalyst is rapidly reduced due to deposition of those heavy organics on the catalyst pores. The molecules of these organic by-products are growing in the micropores of the zeolites and they cannot diffuse outside from the pore channels.It is essential to wash very often the catalyst to avoid the formation of long organics chains during a long reaction period. In this way the catalyst does not get deeply dirty and it is easier to regenerate it at mild washing conditions.Surprisingly, the Applicant found that applying a continuous washing by reaction solvent in presence of ammonia to a parallel twin reactor catalyst bed connected to the first active reactor through a single external loop, maintaining constant the first reactor outlet temperature during the whole reaction time, between 40 to 65°C, and the temperature of said recycled reaction solvent at a mild temperature condition not higher than 4°C above reaction temperature, there is no need to raise temperature and pressure during the reaction to maintain a constant yield of propene oxide but just switch the process reaction from the reactor in use to the washed twin reactor.The methanol that is used for regeneration comes from the methanol distillation column located in the solvent recovery system, where all its latent heat and part of the sensitive heat is recovered in the process up to the defined washing temperature and where it is pumped at the required pressure to enter the regenerating reactor located in the reaction system. At the exit of the reactor in regeneration, the washing methanol is mixed in the reaction mixture.So, the process becomes so very simple: as soon as the conversion decreases below a defined value in the reactor in operation, then the switch is made between the reactor in operation and the regenerated twin reactor. Conversion of hydrogen peroxide and selectivity to propene oxide remain very similar to those of the fresh catalyst.Furthermore, washing the catalyst in presence of ammonia and reaction done in the presence of ammonia, the washing efficiency is very high even with methanol at mild temperature, equal or very close to reaction temperature. So, if often washed, the catalyst does not get strongly deactivated and there is no need to raise temperature and pressure during the reaction, to maintain a constant hydrogen peroxide conversion.The use of reactors arranged as disclosed in the description of the present invention permits an easy and frequent changeover of the reactor without interruptions of epoxide production.The time between changeovers is not defined “a priori”, since it depends on the behaviour of the reaction in terms of conversion and selectivity.SUMMARY OF THE INVENTIONFollowing a series of experimental tests, the Applicant has developed an improved process and its relating plant for the production of propene oxide in a reaction system in which a heterogeneous catalyst is equally distributed in two twin reactors, operationally interchangeable, which process includes the following phases:(a) start the epoxidation reaction by bringing propene into contact with hydrogen peroxide and a reaction solvent in a first reactor, in a liquid phase using a heterogeneous catalyst at constant temperature and pressure, appropriately selected to obtain at least a minimum yield desired propene oxide in the presence of ammonia;(b) apply a continuous wash using the feed flow of the reaction solvent to regenerate the catalytic bed of the second reactor, twin of the first, at mild temperature conditions, close to the reaction temperature, in the presence of ammonia; (c) feeding the stream of the reaction solvent and ammonia coming from the washing of the second twin reactor, which is in the regeneration phase, to the first reactor in which the reaction is taking place and is in productive operation;(d) switch the regeneration operation and the reaction operation between the first and second reactors when the reaction yield falls below a target range.The time between regenerations depends on the behaviour of the reaction in terms of conversion and selectivity: advantageously, it is not necessary to increase temperature and pressure during the reaction to maintain a constant yield of propene oxide, but it is enough to simply change the process reaction from the reactor in operation to the regenerated twin reactor.The epoxidation reaction occurs in a fixed bed: the fixed bed reactor includes, but is not limited to, a jacketed tube reactor, a tubular reactor, an external circulation fixed bed reactor, and a fixed bed catalytic distillation column reactor. External circulation fixed bed reactors are preferably used.The process according to the invention presents numerous advantages which should be underlined:- it is a simple and almost cost-free catalyst regeneration process, also in terms of utilities consumption and investment costs;- the epoxidation reaction is carried out at constant temperature and pressure;- it allows for long-term continuous and stable operation in the propene oxide production process, and keeps the consumption of utilities stable, avoiding peaks in consumption;- it allows both to reduce losses caused by equipment downtime and to have a simple technological process and convenient process implementation.The process is in fact very simple: as soon as the conversion drops below a defined value in the operating reactor, the transition is made between the operating reactor and the regenerated twin reactor. The hydrogen peroxide conversion and selectivity to propene oxide remain very similar to that of the fresh catalyst.Surprisingly, by washing the catalyst in the presence of ammonia and the reaction carried out in the presence of ammonia, the washing efficiency is very high even with methanol at a mild temperature, equal to or very close to the reaction temperature.Therefore the catalyst, if washed often, is not strongly deactivated and it is not necessary to increase temperature and pressure during the reaction, to keep the conversion of hydrogen peroxide constant.The use of reactors arranged as described in the body of the invention allows for easy and frequent change of reactor without interruption of epoxide production. The changeover between the two reactors may be done by automatic switching valves well known in the art. A peculiar characteristic of the invention is that the time between one step and another is not defined "a priori", since it depends on the behavior of the reaction in terms of conversion and selectivity.Further characteristics and advantages of the invention will be evident from the attached figure 1, which shows, by way of non-limiting example only, a block diagram of the system according to the invention itself.DETAILED DESCRIPTION OF THE INVENTIONThe invention is directed to an improved process and its relating plant for the continuous epoxidation of propene with hydrogen peroxide in the presence of a titanium silicalite catalyst, ammonia aqueous solution and a methanol solvent, in two twin reactors, containing the same catalyst amount, which are connected to a common circulation external loop, wherein:(1) continuous washing is applied by feeding methanol reaction solvent to a twin reactor catalyst bed at mild temperature condition not exceeding 4°C above the reaction exit temperature, in presence of ammonia, without the need to raise temperature and pressure during the reaction to maintain a constant yield of propene oxide but just switching the process reaction from the reactor in use to the washed twin reactor;(2) time between regenerations depends on the behaviour of the reaction in terms of conversion and selectivity.For economic reasons it is preferred for an industrial scale process to use propene not in a pure form but as a technical mixture with propane that as a rule contains 1 to 15 vol.% of propane. Propene may be fed as a liquid as well as in gaseous form into the epoxidation process.Hydrogen peroxide is used in the form of an aqueous solution with a hydrogen peroxide content of 1 to 90 wt.%, preferably 10 to 80 wt.% and particularly preferably 30 to 45 wt.%. The hydrogen peroxide may be used in the form of a commercially available, stabilized solution. Also suitable sources of peroxide are unstabilized, aqueous hydrogen peroxide solutions such as are obtained in the anthraquinone process for producing hydrogen peroxide. Hydrogen peroxide solutions in methanol which are obtained by reacting hydrogen and oxygen in the presence of a noble metal catalyst in a methanol solvent may also be used.Titanium silicalites are crystalline, titanium-containing zeolites preferably with the composition xTiO2.(1-x)SiO2, where x is from 0.0001 to 0.04 having a MFI or MEL crystalline structure. Such catalysts may be produced for example according to the process described in U.S. Pat. No. 4410501 relied on and incorporated herein by reference. The titanium silicalite catalyst is preferably employed as a shaped catalyst in the form of granules, extrudates or shaped bodies. For the forming process the catalyst may contain 1 to 99% of a binder or carrier material, all binders and carrier materials being suitable, provided that they do not react with hydrogen peroxide or with the epoxide under the reaction conditions employed for the epoxidation. Extrudates with a diameter of 1 to 5 mm are preferably used as fixed bed catalysts.The methanol solvent used in the epoxidation reaction preferably comprises more than 90 wt.% methanol and more preferably more than 97 wt.% methanol. The methanol solvent is preferably a technical grade methanol, a methanol solvent from plant recovery section of the epoxidation process or a mixture of both.Hydrogen peroxide, propene and the methanol solvent may be introduced into the epoxidation reactor as independent feeds or one or more of these feeds may be mixed prior to introduction into the reactor.In a preferred embodiment of the invention an additional weak base, preferably ammonia, is fed to the epoxidation reactor to control the selectivity of the catalyst. The weak base may be added separately or mixed to one of the above feeds to the reactor.In the epoxidation reaction propene is preferably employed in excess relative to the hydrogen peroxide in order to achieve a significant conversion of hydrogen peroxide, the molar ratio of propene to hydrogen peroxide preferably being chosen in the range from 1.1 to 30. The methanol solvent is preferably used in the epoxidation in a weight ratio of 0.5 to 0.99 relative to the amount of hydrogen peroxide solution. The amount of catalyst employed may be varied within wide limits and is preferably chosen so that a hydrogen peroxide conversion of more than 90%, preferably more than 95%, is achieved within 1 minute to 5 hours under the employed epoxidation reaction conditions.During the epoxidation the pressure within the reactor is maintained constant at a value chosen in the range of 5 to 50 bar absolute, preferably 10 to 35 bar absolute. The epoxidation of propene is typically carried out at a temperature of 30 to 80°C, preferably at 40 to 65°C. The selected pressure is chosen in such a way to guarantee that in all reaction points the propene is kept in liquid phase. The epoxidation reaction takes place in a fixed bed reactor, the fixed-bed reactor includes, but is not limited to, a jacketed tube reactor, a tubular reactor, an external-circulation fixed-bed reactor, and a fixed-bed reactor. The epoxidation is preferably carried out in a fixed bed reactor by feeding a mixture comprising propene, hydrogen peroxide, ammonia aqueous solution and methanol over the catalyst fixed bed. The fixed bed reactor is preferably equipped with cooling facilities in an external loop and cooled with a liquid cooling medium. In order to be able to operate the epoxidation process continuously when regenerating the epoxidation catalyst, a twin reactor is in production and both reactors may be connected to a single external loop.During the continuous oxidation of propene the titanium silicalite catalyst slowly loses catalytic activity. When the activity of the catalyst drops below the desired level the epoxidation reaction is switched to the parallel twin reactor and the catalyst is regenerated by washing with a methanol solvent in presence of ammonia at a temperature not higher than 4°C plus reaction outlet temperature. Regeneration is preferably performed at a constant temperature in a continuous manner, until the following reactor switching to the twin one.The methanol pumping facilities in the methanol recovery section of the plant and the ammonia solution pumps will provide the necessary pressure to feed the methanol and the ammonia solution into the regenerating reactor first and then into the operating reactor circulation loop. When a fixed bed catalyst is regenerated by feeding a methanol solvent stream in presence of ammonia through the catalyst fixed bed, the solvent which passes through the catalyst fixed bed in regeneration is then fed to the reaction mixture circulation loop and then proceeds at the inlet of the reactor in operation with epoxidation reaction. In between two regeneration steps the epoxidation reaction is preferably operated to maintain an essentially constant hydrogen peroxide conversion. In this context, essentially constant has the meaning that hydrogen peroxide conversion decreases by no more than 5% referred to target value at a given reactants feed. In order to maintain an essentially constant hydrogen peroxide conversion while the catalyst declines activity through deactivation until maximum 5% referred to target value, it is in place an automatic switching to the regenerated twin reactor where the conversion comes back to the target value.With reference to Fig.1, the invention provides a conceptual process flow scheme comprising a reactor in regeneration phase and a reactor in reaction phase operating as further described:(a) feeding into a reactor A1, in which it is contained the deactivated catalyst, a stream 1 of methanol and aqueous ammonia through lines 5 and 6 in up-flow mode to perform a regeneration of the catalyst; the washing stream from reactor A1 outlet is fed through line 7 and 8 to the reactors loop line 9. The line 9 is passed through the heat exchanger B for cooling by an adequate cooling medium; (b) feeding a stream 2 of propene and a stream 3 of hydrogen peroxide mixing them with the cooled stream 10 and pumping the resulting mixture by circulation pump C to a reactor A2, in which the regenerated catalyst is contained. Reactor A2 is a twin of reactor A1, the two reactors are connected to a common circulation external loop composed of line 9, heat exchanger B, line 10, recirculation pump C and line 11. In this way the streams 1 after reactor A1 washing, 2 and 3 are mixed in the common external loop and fed in up-flow mode to reactor A2 through lines 12 and 13. The reaction output stream 4 containing epoxidation reaction products leaves the reactor A2 through line 14 and line 15. The stream 4 is sent to the propene oxide recovery section. In order to maintain an essentially constant hydrogen peroxide conversion while the catalyst declines activity through deactivation until maximum 5% lower than target value, it is foreseen an automatic switching to the regenerated twin reactor by the use of adequate automatic valves.(c) After the switch, the feeding to reactor A2, in which a deactivated catalyst is contained, is composed of a stream 1 of methanol and aqueous ammonia, through lines 16 and 13 in up-flow mode to perform a regeneration of the catalyst; after the regeneration, the washing stream from reactor A2 is fed through line 14 and line 19 to the reactors loop line 9. (d) After the switch, the feed to reactor A1, in which a washed catalyst is contained, is composed of a stream 2 of propene and a stream 3 of hydrogen peroxide, through the reactors loop line 10. In this way stream 1, after reactor A2 washing, and streams 2 and 3 are mixed in the common external loop and fed in up-flow mode to reactor A1 through line 17 and line 6.The reaction output stream 4, containing epoxidation reaction products, leaves the reactor A1 through line 7 and line 18. The stream 4 is sent to the propene oxide recovery section. So when the catalyst declines activity again, it is in place the automatic switch to the regenerated twin reactor and the reaction and regeneration cycles are repeated as dictated by the target conversion of hydrogen peroxide. The weak basic additive is suitably chosen from aqueous ammonia, ammonium acetate, ammonium formate or a system comprising a nitrogenous base and a salt thereof with an organic or mineral acid, as described in US7138534B2.The epoxidation catalyst which may be used in the process of the present invention is chosen from those generally known as titanium silicalites although other known epoxidation catalysts may be employed as desired.The catalyst may be used in the form of pellets, microspheres, an extrudate or other suitable physical forms.The use of a binder co-gel or of an inert support in combination with the catalyst may be advantageous. The inert support may typically consist of silica, alumina, silica-alumina, zeolites, active charcoal and other materials that are well known in the state of the art.The amount of catalyst used in the epoxidation process is chosen so as to allow the epoxidation reaction to proceed to completion in the shortest possible time.Generally, the amount of catalyst will be chosen as a function of various parameters, such as the reaction temperature, the reactivity and concentration of the olefin, the concentration of hydrogen peroxide, the type and composition of the solvents, the catalytic activity of selected catalyst and the type of reactor or of reaction system used.The reaction temperature used in the process of the present invention is generally between 20°C and 150°C, preferably between 35°C and 100°C and particularly preferably between 40°C and 65°C.The pressure at which the process is performed is at least the one which allows the olefin to be maintained in the liquid phase at the chosen reaction temperature.The regeneration temperature used in the washing process of the present invention is maximum 4°C plus outlet reaction temperature.The epoxidation reaction takes place in a fixed bed reactor, the fixed-bed reactor includes, but is not limited to, a jacketed tube reactor, a tubular reactor, an external circulation fixed bed reactor, and a fixed bed catalytic distillation column reactor. A fixed bed reactor with external circulation and cooling is preferably used.The oxidation reactor operates at a liquid space velocity calculated on the net liquid stream fed to the reaction circulation loop, in the range from 0.2 to 60 hr-1, preferably in the range of 10 to 60 hr-1. Preferably, the reactor is isotherm and with a recycle ratio between the circulating loop flow rate and the net feed flow rate in the range 1-500, preferably in the range 10-100.The following examples illustrate the process of the invention and the improvement over the prior art achieved by the process of the invention.EXAMPLES Titanium silicalite, produced according to the process described in U.S. Pat. No. 4410501 was used in the form of extrudates with 2 mm diameter shaped with silica sol as a binder. Hydrogen peroxide was used as a commercial solution at 35 wt% in water. Reaction products were analyzed by gas chromatography and hydrogen peroxide conversion was determined by redox titration. The hydrogen peroxide selectivity to propene oxide was calculated as the molar ratio of the amount of propene oxide formed relative to the amount of hydrogen peroxide converted.Epoxidation of propene was carried out continuously in a tubular reactor of 22 ml volume, a diameter of 10 mm and a length of 0.28 m, equipped with external loop. The reactor was operated in up-flow operation mode.Reaction temperature was controlled by using an aqueous coolant through a heat exchanger placed in external circulating loop, whereby the coolant temperature was controlled by a thermostat to keep constant reaction temperature. Reactor pressure was kept at 21 bar absolute with nitrogen gas and a pressure controller. The external loop flowrate was 100 millilitres / minute.The feed containers were charged with methanol (88 wt% in total mixture), hydrogen peroxide aqueous solution (4 wt% as pure hydrogen peroxide in total mixture), ammonia aqueous solution (100 wt ppm NH3 referred to total mixture). Mass flowrate of propene was adjusted to result in a propene feed concentration of 13 wt% in total reactor feed, with a net propene feed flow of 33.6 grams / hour. The regeneration of the catalyst is a washing with methanol, at the same flowrate as in reaction, at 54-65°C, depending on the examples and with NH3 or not, depending on the examples, and the external loop flowrate was 100 millilitres / minute of methanol. The duration of the washing for all the presented examples was 4 hours.See following examples:Examples 1a-1b-1cExamples 1a, 1b and 1c represent the reference test for the invention. Example 1a is obtained at a temperature of 50°C, with 13 grams of catalyst and 100 wt ppm of NH3 in reaction.Example 1b is obtained after washing at 54°C with methanol and 100 wt ppm NH3 the catalyst used in Example 1a. The reaction is obtained at a temperature of 50°C in presence of 100 wt ppm NH3. The conversion of hydrogen peroxide is substantially fully recovered.The example 1c is obtained after washing, at 54°C and with 100 wt ppm NH3, the catalyst used in Example 1b. The reaction is obtained at a temperature of 50°C in presence of 100 wt ppm NH3. The conversion of hydrogen peroxide is again substantially fully recovered. Table 1 – Test Results example 1a, 1b and 1c The examples 1a, 1b and 1c showed that the washing at 54°C and 100 wt ppm NH3 is effective, the activity of the catalyst is fully recovered (See Table 1). In the reported example 1a, 1b and 1c the selectivity to propene oxide was maximum 94.5%.Comparative examples 2a-2b-2cIn the following comparative examples 2a-2b-2c no ammonia additive was used for reaction and catalyst washing. Example 2a is obtained at a temperature of 55°C, with 7.5 grams of catalyst.Example 2b is obtained after washing at 55°C with methanol the catalyst used in Example 2a. The reaction is obtained at a temperature of 55°C. The conversion of hydrogen peroxide is not recovered after the washing.The example 2c is obtained after washing at 55°C the catalyst used in Example 2b. The reaction is obtained at a temperature of 55°C. The conversion of hydrogen peroxide is again not recovered after the washing.In any case in this 2a-2b-2c examples selectivity to propene oxide is very low (max 52%). After the washing steps, the hydrogen peroxide conversion is not recovering the fresh catalyst conversion (see Table 2).Table 2 – Test Results examples 2a, 2b and 2cIn this comparative case the washing at 55°C is not effective, the conversion decreases in the sequence of reactions and subsequent washes.Comparative examples 3a-3b-3cIn the following comparative examples 3a-3b-3c no ammonia additive was used for reaction and catalyst washing. Example 3a is obtained at a temperature of 55°C, with 7.5 grams of catalyst.Example 3b is obtained after washing at 65 °C with methanol the catalyst used in Example 3a. The reaction is obtained at a temperature of 55°C. The conversion of hydrogen peroxide is not recovered after the washing.The example 3c is obtained after washing at 65°C the catalyst used in Example 3b. The reaction is obtained at a temperature of 55°C. The conversion of hydrogen peroxide is again not recovered after the washing.In any case in this 3a-3b-3c examples selectivity to propene oxide is very low (max 50%).After the washing steps, the hydrogen peroxide conversion is not recovering the fresh catalyst conversion (see Table 3). Table 3 – Test Results examples 3a, 3b and 3c Also in this comparative case the washing at 65°C is not effective, the conversion decreases in the sequence of reactions and subsequent washes.It can be concluded that frequent washing with methanol is effective at a temperature close to the reaction temperature if done in the presence of ammonia, with the reaction however conducted in the presence of ammonia.Reference Examples 1a-1b-1c show that, by selecting the solvent and temperature according to the invention, the time on stream of the catalyst in the epoxidation of propene can be considerably prolonged before replacement becomes necessary, because after washing the catalyst activity is completely recovered.The results also show that by using methanol and ammonia at a temperature according to the invention, a high catalyst yield to propene oxide is reached after the regeneration without the need for temperature and pressure increase during the reaction time.Further variations and modifications of the foregoing will be apparent to those skilled in the art and are intended to be encompassed by the claims appended hereto. Comparative example 4:Patent EP 1489074 A1 / US 6878836 B2can be considered to be one of the prior art closestto the present application and itrefers to a process for the continuous epoxidation of propene with hydrogen peroxide in the presence of a titanium silicalite catalyst and a methanol solvent, wherein the catalyst is periodically regenerated by washing with a methanol solvent. In order to be able to operate the epoxidation process continuously when changing and / or regenerating the epoxidation catalyst, two or more flow reactors may if desired also be operated in parallel or in series. Differently from the present application, in EP 1489074 A1 / US 6878836 B2the regeneration is carried out by washing with a methanol solvent at a temperature of at least 100°C. According to claim 1 and claim 4 of said patent application, the catalyst is regenerated at a temperature from 100°C to 200°C, and in fact in the inventive example 4, the catalyst is regenerated at 150 °C.In the present application, on the contrary, the regeneration is carried out by washing with a methanol solvent at a controlled mild temperature not exceeding 4°C above the reaction temperature. Since the reactor outlet temperature is selected in the range between 40°C and 65°C, the resulting maximum regeneration temperature is the mild 69°C, which is 31°C below the minimum regeneration temperature of the prior art EP 1489074 A1 / US 6878836 B2. The mild regeneration temperature not only allows energy savings, but it also minimizes the thermal stress on said catalyst thus increasing the lifetime of the catalyst and the reaction period before the next regeneration, since this feature is synergically linked with other aspects which are better described in the following comparison paragraphs. A second feature of EP 1489074 A1 / US 6878836 B2is the period between two regeneration steps: according to claim 1 of said application, the epoxidation is carried out for periods of at least 300 h between two regeneration steps, and according to claim 2 from 500 h to 8000 h between two regeneration steps, or, according to the description, when the activity of the catalyst drops below the desired level, and both those indications refer to the common general and practical sense of any industrial production, without any synergic effect with other aspects. In fact, according to all the examples (1-2-3-4) of said application, the epoxidation reaction is stopped and the catalyst is regenerated after 2500h, without any described criterion, but only a practical choice of maintaining the H2O2 conversion constant at 95% by increasing the temperature. More in detail, according to the examples, during the 2500h of epoxidation the reaction temperature was increased by adjusting the coolant temperature from 41°C to 50°C to maintain a H2O2 conversion constant at about 95%, leading to a lower selectivity from 88% to 85% at the end of 2500 h.In the present application, on the contrary, the reactor outlet temperature is kept constant during the reaction period of the first usage or after any regeneration step, and as the catalyst loses its activity over the reaction time and the conversion of H2O2 is decreased. The selectivity is increased, which is an advantage both for the downstream units and for the mild deactivation associated to shorter chain byproducts. The criterion for switching the operating reactor to regeneration is described in the present application, and it is when the conversion of hydrogen peroxide decreases by maximum of 5%. This criterion is not only a merely practical criterion to ensure minimal disruption and maintain consistent production standard, but it is essential to obtain a mild deactivation of the catalyst, thus ensuring that the regeneration is effective also at the mild conditions of maximum 69°C, which is a regeneration temperature very mild compared to the prior art, where the deeper deactivation of the catalyst over 2500h requires a consequent regeneration temperature of at least 100°C, namely 150 °C as described above and in the example 4 of said prior art.A third peculiarity of EP 1489074 A1 / US 6878836 B2is the need to increase the epoxidation reaction temperature after the regeneration of the catalyst, raising such temperature by 0.025 K / h or less to compensate for catalyst deactivation and maintain an essentially constant hydrogen peroxide conversion, as described in claim 3 of said application, and furthermore there are no indications about the temperature profile of the reactor or the need to maintain constant the inlet or the outlet temperature of the reactor. In fact, in all the examples (1-2-3-4) the epoxidation is carried out for 2500h increasing the reaction temperature even during the first usage before the first regeneration by adjusting the reactor coolant temperature from 41°C to 50°C to maintain a H2O2 conversion constant at about 95%, leading to a lower selectivity from 88% to 85% at the end of 2500 h.Furthermore, according to the inventive example 4 of the prior art, after regeneration the reactor coolant temperature had to be increased to 44 °C (at 0.011 K / h) to maintain a constant H2O2 conversion.In the present application, on the contrary, the selected outlet reaction temperature is kept constant during the epoxidation reaction period before and after regenerations. This peculiarity not only ensures more stable conditions and product yields of the plant, but it also proves the complete recovery of the catalyst activity after regeneration, without any need to increase the temperature. Moreover, the above-described concept synergically ensures both a longer catalyst lifetime as well as a mild deactivation condition, thus allowing an effective regeneration at mild conditions of maximum 69°C, while if the temperature is further increased, as done according to other patents, such as EP 1489074 A1, the catalyst loss of activity would be certainly faster and deeper. The mild regeneration conditions and the criterion of regeneration at maximum 5% decrease of H2O2 conversion in chain synergically ensure to avoid the need to increase the temperature of the epoxidation reaction after the regeneration step. Another feature of EP 1489074 A1 / US 6878836 B2 is the periodic regeneration of the catalyst through the usage of a methanol solvent, and such regeneration is carried out for a period of 0.5h to 48 h, according to claim 4 of said patent application. Furthermore, at least part of the methanol which is passed through the catalyst bed for the regeneration, is reused for the regeneration without prior purification as described in claim 7 of said application, and the methanol used for the regeneration can be obtained from the working up of the reaction mixture from the epoxidation reaction according to claim 10, and finally the methanol solvent used for the regeneration can be passed to a process stage for working up the reaction mixture of the epoxidation reaction, as described in claim 11. However, there are no indications about the quantification of the flowrate of such methanol stream to be used for the regeneration: only in the inventive example 4 the pure (100%) methanol used for regeneration is 0.35 kg / h, while the total epoxidation reaction flowrate used for the 2500h is 0.35 kg / h at 43%wt methanol, meaning 0.15 kg / h methanol, meaning that the methanol used in the example 4 for the regeneration is more than the double of the methanol used in the epoxidation reaction. Furthermore, according to prior art there are at least two reactors in parallel or in series to ensure a continuous epoxidation reaction of the plant when regenerating or changing the catalyst, but such regeneration operation is not integrated in the process.In the present application, on the contrary, the periodic regeneration washing through the use of a methanol stream is continuous, up to the next switching between operating and regenerating reactors, and the characteristics and routing of the methanol stream for the regeneration are clearly defined. In fact, as defined in the claims, the catalyst is continuously washed using methanol solvent recycled from the recovery section of the plant, in the presence of a weak basic additive, and the flow of methanol and weak basic additive used for regenerating the catalyst is continuously fed to the operating reactor where the epoxidation reaction takes place. The above-described feature is a complete integration of the regeneration of the catalyst in the process, since the reactor in regeneration is upstream in continuous series of the epoxidation reactor. The use of the same methanol, both in terms of flowrate and composition and even temperature, which would be used for the epoxidation reaction, but that it is simply passed through the catalyst in regeneration before being sent to the epoxidation reactor, allows a complete cost-free regeneration, and eliminates the need to treat the effluent of the regeneration step. Finally, the continuous washing ensures a more effective regeneration of the catalyst.
Claims
1) A process for the continuous epoxidation of propene in the liquid phase which involves the use of a reaction plant composed of two alternately operating twin catalytic reactors, the first in operation phase and the second in regeneration phase, in which in the first reactor propene is reacted with hydrogen peroxide in the presence of a titanium silicalite catalyst, a methanol solvent and a weak basic additive, at a reaction temperature between 20°C and 150°C and at an adequate pressure selected in the range of 5 to 50 bar absolute to maintain the reaction in the liquid phase, and the epoxidation reaction is switched from the first reactor to the second reactor, when the first reactor needs to be regenerated,characterized by the fact that: i. the outlet temperature of the first reactor, in operation phase, is kept constant during the entire reaction time, with an appropriately selected reactor outlet temperature in the range between 40 and 65°C; ii. the titanium silicalite catalyst is continuously washed in the second reactor in regeneration phase, using methanol solvent recycled from the recovery section of the plant, in the presence of a weak basic additive, at a controlled mild temperature not exceeding 4°C above the reaction temperature, thus providing effective regeneration of the catalyst contained therein; iii. the flow of methanol and weak basic additive used for washing the catalyst of the second reactor is continuously fed to the first operating reactor; iv. the epoxidation reaction is switched from the first operating reactor to the second washing reactor when the conversion of hydrogen peroxide decreases by a maximum of 5% and at the same time the washing operation is switched from the second reactor to the first reactor. 2) The process of claim 1, wherein the epoxidation is carried out in a fixed bed of catalyst by feeding a mixture comprising propene, a source of hydrogen peroxide and methanol, in the presence of a weak basic additive. 3) The process of claim 2, further comprising regenerating the catalyst by feeding said recycled methanol solvent from plant recovery section with the addition of a weak basic additive. 4) The process of claim 1, wherein the epoxidation catalyst is in a fixed bed and epoxidation is performed by feeding a mixture comprising propene, hydrogen peroxide and methanol in presence of a weak basic additive over the catalyst fixed bed with cooling to maintain reaction temperature. 5) The process of claim 4, wherein the fixed bed reactor is equipped with cooling facilities in an external loop and cooled with a liquid cooling medium. 6)The process of claim 5, wherein the ratio between circulating loop mass flowrate stream (9) and net reactor effluent mass flowrate stream (4) is in the range 1-500, preferably in the range 10-100. 7) The process of claim 1, wherein weak basic additive for reaction and regenerating washing is chosen from aqueous ammonia, ammonium acetate, ammonium formate or a system comprising a nitrogenous base and a salt thereof with an organic or mineral acid. 8) The process of claim 6 wherein the preferred weak basic additive to be used is aqueous ammonia. 9) The process of claim 1, wherein hydrogen peroxide is used in the form of an aqueous solution with a hydrogen peroxide content of 1 to 90 wt.%, preferably 10 to 80 wt.% and particularly preferably 30 to 45 wt.%. 10) The process of claim 1, wherein propene is employed in excess relative to the hydrogen peroxide in order to achieve a significant conversion of hydrogen peroxide, the molar ratio of propene to hydrogen peroxide preferably being chosen in the range from 1.1 to 30. 11) The process of claim 1, wherein methanol solvent is used in the epoxidation in a weight ratio of 0.5 to 0.99 relative to the amount of hydrogen peroxide solution. 12) The process of claim 1, wherein the catalyst reaction for the oxidation to propylene oxide is carried out at a liquid space velocity from 0.2 to 60 hr-1, preferably in the range of 10 to 60 hr-1. 13) The process of claim 1, where propene is not in a pure form but as a technical mixture with propane that as a rule may contain 0.5 to 15 vol.% of propane. 14) The process of claim 1, wherein the epoxidation pressure within the reactor is preferably 10 to 35 bar absolute.