[Title of the Invention]

PRODUCTION PROCESS OF PROPYLENE OXIDE AND PRODUCTION APPARATUS THEREFOR

[Technical Field]

[0001]

The present invention relates to a production process of propylene oxide and a production apparatus thereof. Particularly, the present invention relates to a production process of propylene oxide by reacting hydrogen peroxide and propylene to form propylene oxide and a production apparatus thereof. [Background Art]

[0002]

As a production process of propylene oxide, a process is known wherein propylene, hydrogen and oxygen are reacted in a liquid phase in the presence of solid catalyst particles to produce propylene oxide. For example, Japanese Patent Kokai Publication No. 2009-256301 discloses a production process of propylene oxide wherein propylene and a mixture gas which contains oxygen, hydrogen and nitrogen for dilution are supplied to a mixture which contains a mixture solvent of acetonitrile and water, a solid titanium catalyst and a palladium supporting catalyst, and these are reacted so as to produce propylene oxide.

[Citation List]

[Patent Literature]

[0003]

Japanese Patent Kokai Publication No. 2009-256301

[Summary of Invention]

[Technical Problem]

[0004]

It is generally said that the above mentioned production process of propylene oxide is superior in its productivity. In this process, the solid titanium patalyst and the palladium supporting catalyst are used in a mixture, and durabilities of these two catalysts may be different. In such situation, when one of the catalysts reaches its durability limit, it is impossible or not easy to only said one catalyst is recovered so as to replace or regenerate the same, so that such process is not necessarily preferable from a viewpoint of catalyst usage efficiency.

[Solution to Problem]

[0005]

Thus, the object that the present invention achieves is to provide a new production process of propylene oxide and a new production apparatus thereof which are superior in the productivity of propylene oxide.

[0006]

It has been found that the above mentioned object is achieved by a process of producing propylene oxide wherein a first reaction mixture comprising hydrogen peroxide is obtained in a first reaction step, the first reaction mixture and propylene are reacted in a second reaction step to obtain a second reaction mixture comprising propylene oxide, the second reaction mixture is returned to the first reaction step, then the first reaction step is again carried out so as to obtain the first reaction mixture, then the second reaction step is gain carried out, and the second reaction mixture obtained by such second reaction step is again returned to the first reaction step, and so on. That is, it has been found that the object is achieved by a production process of propylene oxide with repeating the first reaction step, the second reaction step and the return to the first reaction step while withdrawing a portion of the first reaction mixture and/or a portion of the second reaction mixture followed by recovering propylene oxide contained in such portion(s).

[0007]

Thus, in the first aspect, the present invention provides a production process of propylene oxide comprising:

(1) a first reaction step of obtaining a first reaction mixture which comprises hydrogen peroxide by reacting hydrogen and oxygen in a solvent in the presence of a palladium catalyst in a first reactor;

(2) a second reaction step of obtaining a second reaction mixture which comprises propylene oxide by reacting the first reaction mixture and propylene in the presence of a solid titanium catalyst in a second reactor; and

(3) a return step of returning the second reaction mixture to the first reactor,

characterized in that the steps (1) to (3) are repeated.

[0008]

In a preferable embodiment, a portion of the first reaction mixture is withdrawn to the outside of the system while the above mentioned steps (1) to (3) are repeated, and propylene oxide contained therein is recovered. In other embodiment, a portion of the second reaction mixture which is returned to the first reactor is withdrawn to the outside of the system while the above mentioned steps (1) to (3) are repeated, and propylene oxide contained therein is recovered. In a further embodiment, both of a portion of the first reaction mixture and a portion of the second reaction mixture are withdrawn to the outside of the system, and propylene oxide contained therein is recovered. Such recovery may be carried out in a post-treatment step which includes any appropriate method. Also, in a further embodiment, the solvent is a mixture solvent which comprises acetonitrile and water.

[0009]

In the present specification, hydrogen and oxygen are consumed in the first reaction step so that hydrogen peroxide is produced, and hydrogen peroxide and propylene are consumed in the second reaction step so that propylene oxide and water are produced. It is of course that all of the reaction raw materials are not converted to the aimed reaction product(s) stoichiometrically in either of the reaction steps.

[0010]

The first reaction mixture is a mixture as a liquid phase which is obtained in the first reactor as a result of the first reaction step, and generally comprises, in addition to the solvent, hydrogen peroxide as a reaction product, reaction raw materials which have not reacted in the first reaction step (usually, gases (oxygen, hydrogen and optionally nitrogen) dissolved and/or dispersed therein). Also, the second reaction mixture further comprises, in addition to the components which are contained in the first reaction mixture, unreacted propylene and propylene oxide as a reaction product. It is noted that the first reaction mixture further comprises propylene oxide produced and unreacted propylene in the second reaction step since the second reaction mixture is returned to the first reactor.

[0011]

As readily seen, since the first reaction mixture is reacted in the second reactor and the second reaction mixture is returned to the first reactor, the first reaction mixture and the second reaction mixture contain substantially the same components after repeating the steps (1) to (3) as described above. However, since hydrogen peroxide is produced in the first reaction step and consumed in the second reaction step, the compositions of the first reaction mixture and the second reaction mixture are different. That is, when the composition after second reaction step with that before the second reaction step, an amount of hydrogen peroxide is decreased and an amount of propylene is also decreased while an amount of propylene oxide is increased.

[0012]

In the second aspect, the present invention provides a production apparatus of propylene oxide comprising:

(A) a first reactor which reacts hydrogen and oxygen in the presence of a palladium catalyst in a solvent so that a first reaction mixture which comprises hydrogen peroxide is produced;

(B) a second reactor which reacts the first reaction mixture and propylene in the presence of a solid titanium catalyst so that a second reaction mixture which comprises propylene oxide is produced; and

(C) a return line which returns the second reaction mixture to the first reactor.

Such apparatus is preferably used for carrying out the production process of propylene oxide according to the present invention.

[Ad antageous Effects of Invention]

[0013]

While in the conventional production process of propylene oxide, oxygen, hydrogen and propylene are reacted in a single reactor so as to produce propylene oxide, the hydrogen peroxide production step and the propylene oxide production step are carried out in the separate steps (or reactors) in the production process (or production apparatus) of propylene oxide according to present invention.

[0014]

Thus, each reaction can be carried out under conditions which are preferably selected for such reaction. As a result, it becomes possible to further improve the productivity of the propylene oxide production. In addition, the palladium catalyst and the solid titanium catalyst are used in the separate reactors, so that catalyst replacement or catalyst regeneration becomes very easy as to each of the spent catalysts.

[Brief Description of Drawings]

[0015]

Fig. 1 schematically shows a flow sheet of one embodiment of the production process of propylene oxide according to the present invention.

Fig. 2 schematically shows a flow sheet of other embodiment of the production process of propylene oxide according to the present invention.

Fig. 3 schematically shows a flow sheet of a further embodiment of the production process of propylene oxide according to the present invention.

Fig. 4 schematically shows a flow sheet of one embodiment of the production process of propylene oxide according to the present invention.

Fig. 5 schematically shows a flow sheet of one embodiment of the production process of propylene oxide according to the present invention.

Fig. 6 schematically shows a flow sheet of one embodiment of the production process of propylene oxide according to the present invention.

Fig. 7 schematically shows a flow sheet of one embodiment of the production process of propylene oxide according to the present invention.

Fig. 8 schematically shows a flow sheet of one embodiment of the production process of propylene oxide according to the present invention.

Fig. 9 schematically shows a flow sheet of one embodiment of the production process of propylene oxide according to the present invention.

[Description of Embodiments]

[0016]

Next, the production process of propylene oxide according to the present invention will be explained in detail with reference to the drawings. It is noted that in Figs. 1 to 9, the same references are used to mean substantially the same elements. Also, any of the drawings exemplifies an example wherein the first reactor is shown as a stirred tank type reactor and the second reactor is shown as a fixed bed catalyst type reactor. However, both of the first reactor and the second reactor may be of stirred tank type reactors, both of the first reactor and the second reactor may be of a fixed bed catalyst type reactors, or the first reactor may be a fixed bed catalyst type reactor while the second reactor may be a stirred tank type reactor. Referring to Fig. 1 first, in the production process of propylene oxide according to the present invention, the first reaction step is carried out in a first reactor 10, the second reaction step is carried out in a second reactor 12 and the return step is carried out with a return line 14 to the first reactor 10 from the second reactor 12.

[0017]

In the first reaction step, a reaction is carried out wherein oxygen and hydrogen as a mixture gas 18 (optionally diluted with nitrogen) are reacted to produce hydrogen peroxide in the presence of the palladium catalyst which is in a suspended condition in a solvent 16. Such reaction per se is known. As to its details, for example Japanese Patent Kokai Publication Nos. 1998-324507 and 1998-330103 can be referred to. Further, in the process disclosed in the above Patent Literature 1, such reaction can be carried out without using the solid titanium catalyst as well as propylene.

[0018]

In the second reaction step, a reaction is carried out wherein propylene and hydrogen peroxide are reacted so that propylene is epoxidized to produce propylene oxide in the presence of the solid titanium catalyst. Such reaction per se is known. As to its details, for example Japanese Patent Kokai Publication Nos. 2005-262164 and 2010- 159245 can be referred to. Further, in the process disclosed in the above Patent Literature 1, such reaction can be carried out without using the palladium catalyst as well as the mixture gas (oxygen and hydrogen).

[0019]

A solvent 16, for example a mixture solvent of aceton itrile/water is supplied beforehand to the first reactor 10, and the palladium catalyst is in a suspended condition in the solvent. The mixture gas 18 is supplied to the first reactor 10 in such catalyst condition, followed by being subjected to a liquid phase reaction under the predetermined conditions so as to produce hydrogen peroxide. As a result of that, the first reaction mixture 20 is present in the first reactor 10.

[0020]

As readily seen, since the first reaction mixture 20 comprises, in addition to the solvent, hydrogen peroxide as a product, oxygen and hydrogen as reaction raw materials, and optional nitrogen for dilution, and the palladium catalyst is suspended in such first reaction mixture. It is noted that the first reactor 10 may be any appropriate reaction apparatus, and for example it is preferably a stirred tank type reactor which is an autoclave equipped with a stirrer as shown. [0021]

In other embodiment, the first rector may be a fixed bed type reactor which includes the palladium catalyst as a fixed bed, and it is for example a tube reactor. In this embodiment, the mixture gas and the solvent are supplied to the first reactor, and the first reaction step is carried out there.

[0022]

It is noted that the first reactor may be of any type, and may be a single reactor in one embodiment. In other embodiment, it may be a multi-stage reactor which comprises a plurality of unit reactors connected in series. In such embodiment, a reaction raw material which comprises hydrogen, oxygen and optional nitrogen is supplied to the most upstream unit reactor of the plurality of the unit reactors which form the multi-stage reactor, and the reaction is carried out in the most upstream unit reactor. Then, the reaction mixture obtained in the most upstream unit reactor is supplied to its next unit reactor, where the reaction is further promoted, and then the reaction mixture obtained in such next unit reactor is supplied to its further next unit reactor, where the reaction is further promoted. By repeating such supply of the reaction mixture from one unit reactor to its next unit reactor where the reaction is further progressed as described, a conversion of hydrogen/oxygen can be increased. It is noted that if necessary, the first reactor may be constructed such that the reaction raw material may be additionally supplied through a line as required which connects the unit reactors arranged adjacently.

[0023]

Next, a portion of the first reaction mixture 20 produced in the first reactor 10 is supplied to the second reactor 12 through a first reaction mixture line 22. Since the palladium catalyst is contained in the first reaction mixture 20, the first reaction mixture is withdrawn from the first reactor through a filter 24 which removes the palladium catalyst. For example, the filter such as a cross flow filter or the like may be arranged in the first reactor 10. In other embodiment, a line is provided which circulates the first reaction mixture 20 in the first reactor 10, and a filter for solid-liquid separation such as a cross flow filter, a centrifugal separator or the like is provided in such line so as to obtain the first reaction mixture of which the palladium catalyst has been removed.

[0024]

The second reactor 12 is for example a fixed bed type reactor as shown in which the titanium catalyst is charged beforehand, and propylene 26 and the first reaction mixture 20 from the first reactor 10 are supplied to the second reactor 12. In the shown embodiment, propylene 26 is mixed with the first reaction mixture 20 before supplied to the second reactor12, and then they are supplied to the second reactor 12. In other embodiment, a step (or an apparatus) of mixing the first reaction mixture 20 and propylene 26 may be provided before the second reaction step (or the second reactor 12), and then thus resulted mixture is supplied to the second reaction step (or the second reactor 12).

[0025]

In other embodiment, the second reactor may be a tank type reactor such as a stirred tank type reactor. In this embodiment, propylene is reacted under the condition in which the solid titanium catalyst is dispersed in the tank type reactor which contains the first reaction mixture.

[0026]

It is noted that the second reactor may be of any type, and may be a single reactor in one embodiment. In other embodiment, it may be a multi-stage reactor which comprises a plurality of unit reactors connected in series. In such embodiment, propylene and the first reaction mixture are supplied to the most upstream unit reactor of the plurality of the unit reactors which form the multi-stage reactor, and the reaction is carried out in the most upstream unit reactor. Then, the reaction mixture obtained in the most upstream unit reactor is supplied to its next reactor, where the reaction is further promoted, and then the reaction mixture obtained in such next unit reactor is supplied to its further next unit reactor, where the reaction is further promoted. By repeating such supply of the reaction mixture from one unit reactor to its next unit reactor where the reaction is further progressed as described, so that a conversion of propylene can be increased. It is noted that if necessary, the second reactor may be constructed such that propylene may be additionally supplied through a line which connects the unit reactors arranged adjacently.

[0027]

In the second reactor 12, hydrogen peroxide in the first reaction mixture 20 reacts with propylene in the liquid phase in the presence of the solid titanium catalyst to produce propylene oxide, as a result of which the second reaction mixture is obtained. As readily seen, the second reaction mixture comprises oxygen, hydrogen, optional nitrogen for dilution, hydrogen peroxide, propylene and propylene oxide.

[0028]

Such second reaction mixture is withdrawn from the second reactor 12, and returned to the first reactor 10 through the return line 14 so as to carry out the return step while a portion of the first reaction mixture 20 is withdrawn outside the system for example from the first reactor 10 through a first reaction mixture system outside discharge line 30. In the production process of propylene oxide according to the present invention, a ratio of an amount of the solvent in the first reaction mixture which is withdrawn from the first reactor 10 through the first reaction mixture 22 to an amount of the solvent in the first reaction mixture which is withdrawn outside the system through the first reaction mixture system outside discharge line 30, that is a ratio of an amount of the solvent which is circulated through the system to an amount of the solvent which is withdrawn outside the system is preferably in the range between 1 and 12000, more preferably in the range between 10 and 5000 and most preferably in the range between 50 and 500. With such large circulated amount, it is possible to obtain the first reaction mixture and the second reaction mixture which contain propylene oxide at a high concentration even though the concentration of hydrogen peroxide in the first reaction mixture is low. It is noted that a portion of the second reaction mixture may be withdrawn from the return line 14 through a second reaction mixture system outside discharge line 30' as shown in Fig. 2, and then propylene oxide which is produced in the second reactor from such portion may be obtained. [0029]

It is noted that when the second reaction mixture is returned through the return line 14, it may be returned indirectly to the first reactor 10 instead of returning directly to the first reactor 10 as shown. For example, the return line 14 is connected to a supply line 40 which supplies the reaction raw material to the first reactor 10, and the second reaction mixture is returned to the first reactor 10 through the s u pply line 40 as shown in-Fig. 3.

[0030]

A further embodiment of the production process of propylene oxide according to the present invention is shown in Fig. 4. In the shown embodiment, the first reactor 10 comprises a plurality of unit reactors, specifically three unit reactors 10-1. 10-2 and 10-3, and connecting lines 22-1 and 22-1 are arranged between the two adjacent unit reactors. The reaction raw material 18 which contains hydrogen, oxygen and optional nitrogen is supplied to the first unit reactor 10-1, where the reaction proceeds to produce a reaction mixture, which is in turn supplied through the connecting line 22-1 to the second unit reactor 10-2, where the reaction further proceeds to produce a reaction mixture, which is in turn supplied through the connecting line 22-2 to the third unit reactor 10-3, where the reaction further proceeds to produce a reaction mixture, which is discharged through the connecting line 22-3 from the third unit reactor 10-3.

[0031]

It is noted that the connecting line 22-3 corresponds to the first reaction mixture line 22, and the first reaction mixture 20 which is discharged from the third unit reactor 10-3 and therefore which is discharged from the first reactor 10 is supplied to the second reactor 12. In this embodiment, the reaction raw material which contains hydrogen, oxygen and optional nitrogen may be added to at least one of the connecting lines. Such embodiment is shown in Fig. 5, wherein raw material addition lines 18-1 and 18-2 are connected to the connecting lines 22-1 and 22-2, respectively. The addition of the reaction raw material as described means a divisional feed of the reaction raw material, which makes possible to control a reaction speed of the reaction raw material in each of the unit reactors, so that a reaction amount of oxygen and hydrogen can be readily controlled.

[0032]

A further embodiment of the production process of propylene oxide according to the present invention is shown in Fig. 6. In the shown embodiment, a portion of the first reaction mixture which is to be supplied to the second reactor 12 is returned to the first reactor 10. In the embodiment shown in Fig. 6(a), the first reaction mixture is returned directly to the first reactor 10 through a circulating line 42 comprises, and in the embodiment shown in Fig. 6(b), the first reaction mixture is returned indirectly to the first reactor 10 through a circulating line 44 which is connected to the supply line 40. By returning a portion of the first reaction mixture to the first reactor 10 as described above, a portion of the first reaction mixture is circulated, so that an amount of the catalyst used in the first reactor 10 can be suppressed. In other words, the conversion in the first reactor is increased so that the catalyst use efficiency is improved.

[0033]

A further embodiment of the production process of propylene oxide according to the present invention is shown in Fig. 7. In the shown embodiment, the second reactor 12 comprises a plurality of unit reactors, specifically three unit reactors 12-1. 12-2 and 12-3, and connecting lines 23-1 and 23-1 are arranged between the two adjacent unit reactors. The first reaction mixture and propylene are supplied to the first unit reactor 12-1, where the reaction proceeds to produce a reaction mixture, which is in turn supplied through the connecting line 23-1 to the second unit reactor 12-2, where the reaction further proceeds to produce a reaction mixture, which is in turn supplied through the connecting line 23-2 to the third unit reactor 12-3, where the reaction further proceeds to produce a reaction mixture, which is discharged from the third unit reactor 12-3. In this embodiment, the reaction raw material which contains propylene may be added to at least one of the connecting lines. Such embodiment is shown in Fig. 8, wherein propylene addition lines 26-1 and 26-2 are connected to the connecting lines 23-1 and 23-2, respectively. The addition of the reaction raw material as described makes possible to control an amount of propylene to be supplied in each of the unit reactors, so that an amount of reaction heat can be readily controlled .

[0034]

A further embodiment of the production process of propylene oxide according to the present invention is shown in Fig. 9. In the shown embodiment, a portion of the second reaction mixture which is to be supplied to the first reactor is returned to the second reactor. In the embodiment shown in Fig. 9(a), the second reaction mixture is returned directly to the second reactor 12 through a circulating line 46 comprises, and in the embodiment shown in Fig. 9(b), the second reaction mixture is returned indirectly to the second reactor 12 through a circulating line 46 which is connected to the first reaction mixture line 22. By returning a portion of the second reaction mixture to the second reactor 12 as described above, a portion of the second reaction mixture is circulated, so that an amount of the catalyst used in the second reactor can be suppressed. In other words, the conversion in the second reactor is increased so that the catalyst use efficiency is improved.

[0035]

While repeating the above mentioned first reaction step and second reaction step in this order, the mixture gas 18 and propylene 26 are supplied to the first reactor 10 and the second reactor 12, respectively depending on an amount of the first reaction mixture which is discharged outside the system. In addition, since the solvent is discharged outside the system with being accompanied by the first reaction mixture, makeup solvent 28 is supplied to the first reactor 10. The mixture gas 18, propylene 26 and the makeup solvent 28 are supplied and a portion 30 of the first reaction mixture is discharged so as to achieve and keep a steady state. In other embodiment, in place of or in addition to the supply to the gas mixture 18, the makeup solvent 28 may be supplied to the first reaction-mixture line 22 which supplies the first reaction mixture.

[0036]

As readily seen for those skilled in the art, in order to achieve and keep the steady state, the supply of the mixture gas 18 to the first reaction step, the supply of a portion of the first reaction mixture 20 from the first reaction step to the second reaction step, the supply of propylene 26 and the return of a portion of the second reaction mixture from the second reaction step to the first reactor 10 are preferably carried out continuously. Thus, it is preferable that the steps (1) to (3) of the production process of propylene oxide according to the present invention are carried out continuously. In this embodiment, it is also preferable that the supply from the outside into the system and the withdrawal from the system to the outside are carried out continuously.

[0037]

However, it is not necessarily required to completely continuously carry out as described above, and intermittent operation may be possible such that compositions of the first reaction mixture and the second reaction mixture fluctuate within predetermined ranges. In a further embodiment, at least a part of the supply and at least a part of the discharge may be carried out continuously while the remaining parts may be carried out intermittently.

[0038]

In the production process of propylene oxide according to the present invention, the liquid phase of the first reactor 10, namely the first reaction mixture 20 comprises acetonitrile and water as the solvent, hydrogen peroxide produced in the first reaction step, hydrogen peroxide unreacted in and returned from the second reaction step, propylene oxide produced in the second reaction step and supplied to the first reactor 10 and propylene unreacted in the second reaction step and returned to the first reactor 10. It is of course that a portion of the first reaction mixture is also the same which is discharged from the first reactor 10 to the outside of the system through the first reaction mixture system outside discharge line 30. In order to prevent the palladium catalyst getting outside by being accompanied with the first reaction mixture to be discharged, the first reaction mixture is discharged through a filter 32.

[0039]

It is noted as described above that the first reaction mixture 20 contains in addition to propylene oxide, oxygen, hydrogen and optional nitrogen which are dissolved or dispersed in the first reaction mixture as the other components. Therefore, in order to recover propylene oxide as an aimed product from the first reaction mixture which is discharged through the first reaction mixture system outside discharge line 30, the other components are removed in a post-treatment step (not shown). The post- treatment step may be any appropriate processing, and for example the gaseous components (hydrogen, oxygen and nitrogen) may be removed first by depressurizing the system. Hydrogen peroxide may be removed similarly. Propylene and the solvent may be removed by distillation under pressure so as to recover propylene oxide. The second reaction mixture contains substantially the same components as those of the first reaction mixture while its composition is different from that of the first reaction mixture. Therefore, even when the a portion of the second reaction mixture which is returned from the second reactor 12 to the first reactor 10 is discharged through the second reaction mixture system outside discharge line 30', aimed propylene oxide may be recovered from said portion of the second reaction mixture. Such recovery may be carried out in any appropriate manner similarly to the above post- treatment.

[0040]

As readily seen from the above descriptions, one of the features of the production process of propylene oxide according to the present invention is characterized in that the second reaction mixture is returned to the first reactor 10. A production apparatus of propylene oxide according to the present invention which can be used for such production process comprises the return line 14. That is, the production apparatus of propylene oxide according to the present invention is characterized in that it comprises: (A) the first reactor which reacts hydrogen and oxygen in the presence of the palladium catalyst in the solvent to produce the first reaction mixture which comprises hydrogen peroxide;

(B) the second reactor which reacts the first reaction mixture and propylene in the presence of the solid titanium catalyst to produce the second reaction mixture which comprises propylene oxide; and

(C) the return line which returns the second reaction mixture to the first reactor.

[0041]

In the production process of propylene oxide according to the present invention as described above, the first reaction step which produces hydrogen peroxide is carried out in the first reactor, and the second reaction step which produces propylene oxide is carried out in the second reactor while the return step which returns the second reaction mixture to the first reactor is carried out, so that propylene oxide can be obtained effectively even when a concentration of hydrogen peroxide contained in the first reaction mixture which is supplied to the second reactor is low.

[0042]

Specifically, the production process of propylene oxide according to the present invention can be simulated based on the material balance as below:

Propylene oxide is produced substantially only in the second reactor. When the second reaction mixture which contains 10 % by weight of propylene oxide is discharged to the system outside through the second reaction mixture system outside discharge line 30' as shown in Fig. 2 at a rate of 1 kg/sec, propylene oxide has to be produced in the second reactor at a rate of 0.1 kg/sec. Such production rate corresponds to a propylene oxide production rate of 1.7 mol/sec. When _. it is assumed that substantially 100 % of hydrogen peroxide supplied to the second reactor is used for the oxidation of propylene, 1.7 mol/sec of hydrogen peroxide has to be supplied to the second reactor so as to produce 1.7 mol/sec of propylene oxide. For example, when the first reaction mixture is supplied to the second reactor 90 kg/sec, the concentration of hydrogen peroxide contained in the first reaction mixture is 0.064 % by weight (1.7x34/90000 = 0.00064). That is, it is sufficient that the hydrogen peroxide concentration in the first reactor is at most 0.1 % by weight.

[0043]

When the second reaction mixture which contains 10 % by weight of propylene oxide is discharged to the system outside at a rate of 1 kg/sec without carrying out the return step, propylene, oxide has to be produced in the second reactor at a rate of 0.1 kg/sec, namely 1.7 mol/sec. In this case, 0.9 kg/sec of the first reaction mixture has to be supplied the second reactor based on the material balance. Similarly to the above, when it is assumed that substantially 100 % of hydrogen peroxide supplied to the second reactor is used for the oxidation of propylene, 1.7 mol/sec of hydrogen peroxide has to be supplied to the second reactor so as to produce 1.7 mol/sec of propylene oxide. Therefore, the concentration of hydrogen peroxide contained in the first reaction mixture is 6.4 % by weight (1.7x34/900 = 0.064).

[0044]

That is, when compared with a case with carrying out the return step, the hydrogen peroxide concentration in the first reactor has to be considerably high when the return step is not carried out.

[Reference Signs List]

[0045]

10: first reactor, 10-1: first unit reactor,

10-2: second unit reactor, 10-3: third unit reactor,

12: second reactor, 12-1: first unit reactor,

12-2: second unit reactor, 12-3: third unit reactor,

14: return line, 16: solvent, 18: mixture gas,

18-1, 18-2: raw material addition line, : first reaction mixture, 22: first reaction mixture line,-1, 22-2, 23-1, 23-2: connecting line, 24: filter,

: propylene, 26-1, 26-2: propylene addition line,

: solvent makeup,

: first reaction mixture system outside discharge line, ': second reaction mixture system outside discharge line,: filter, 40: supply line, 42, 44, 46: circulating line.