Method for controlling a stand-by stage of a process for preparing propylene oxide

Abstract

The present invention relates in a first aspect to a method for controlling a stand-by stage of a process for preparing propylene oxide, the process for preparing propylene oxide comprising a normal run stage with propene epoxidation and a stand-by stage without propene epoxidation, the stand-by stage comprising steps (a) to (g), wherein the method for controlling the stand-by stage comprises: (i) defining, based on a molar amount of hydrogen peroxide provided into the epoxidation zone during a normal run stage MAH2O2(nr) a molar ratio MApropene(lbs-sb) / MAH2O2(nr) in the range of from 0.02 to 1.03; (ii) at least periodically determining MAPropene(lbs- sb); and (ii.a) if MAPropene(lbs-sb) / MAH2O2(nr) is > 1.03, adjusting the distillation in f) so that a molar ratio MAPropene(lbs-sb) / MAH2O2(nr) in the range of from 0.02 to 1.03 is achieved, (ii.b) if MAPropene(lbs-sb) / MAH2O2(nr) is in the range of from 0.02 to 1.03, maintaining the distillation in f). A second aspect of the invention is directed to a process for preparing propylene oxide, the process for preparing propylene oxide comprising at least a normal run stage with propene epoxidation and at least a stand-by stage without propene epoxidation, the process further preferably comprising intermediate shut-down stage(s) after a normal run stage and before a stand-by stage and / or start-up stage(s) after a stand-by stage and before a normal run stage. A third aspect of the invention is directed to propylene oxide obtained or obtainable from the process of the second aspect.

Description

[0001] Method for controlling a stand-by stage of a process for preparing propylene oxide

[0002] The present invention relates in a first aspect to a method for controlling a stand-by stage of a process for preparing propylene oxide, the process for preparing propylene oxide comprising a normal run stage with propene epoxidation and a stand-by stage without propene epoxidation, the stand-by stage comprising steps (a) to (g), wherein the method for controlling the stand-by stage comprises: (I) defining, based on a molar amount of hydrogen peroxide provided into the epoxidation zone during a normal run stage MAn202(nr) a molar ratio MAPropene(lbs-sb) / MAH202(nr) in the range of from 0.02 to 1.03; (ii) at least periodically determining MAprOpene(lbs- sb);and (ii.a) if MAprOpene(lbs-sb) / MAH202(nr) is > 1 .03, adjusting the distillation in f) so that a molar ratio MApropene(lbs-sb) / MAn202(nr) in the range of from 0.02 to 1.03 is achieved, (ii.b) if MAprOpene(lbs-sb) / MAH202(nr) is in the range of from 0.02 to 1 .03, maintaining the distillation in f). A second aspect of the invention is directed to a process for preparing propylene oxide, the process for preparing propylene oxide comprising at least a normal run stage with propene epoxidation and at least a stand-by stage without propene epoxidation, the process further preferably comprising intermediate shut-down stage(s) after a normal run stage and before a stand-by stage and / or start-up stage(s) after a stand-by stage and before a normal run stage. A third aspect of the invention is directed to propylene oxide obtained or obtainable from the process of the second aspect.

[0003] State of the art

[0004] The HPPO (Hydrogen Peroxide to Propylene Oxide) process is a method that is used to produce propylene oxide (PO). It involves using an aqueous hydrogen peroxide (H2O2) solution to selectively epoxidize propylene to PO in the presence of a heterogeneous catalyst, which is a Ti-silicate based zeolite. There are several HPPO processes known in the industry and reported in literature. One class of processes uses methanol as a solvent and TS-1 (MFI) type zeolites as catalysts, while another class of processes uses acetonitrile as a solvent and Ti-MWW (MMW) type zeolites as catalysts (see, for example, WO 2022 / 268859 A1).

[0005] HPPO processes are generally processes, which, in order to be economically, are performed with recirculation of non-spent reactants and solvents. In HPPO processes based on acetonitrile and Ti-MWW type zeolites as catalyst, propylene oxide is separated after epoxidation and solvent recovery further downstream is often performed including a phase separation induced by the addition of propylene to a stream consisting mainly of acetonitrile and water, which is obtained after removal of propylene oxide; see, for example, WO 2011 / 006990 A1.

[0006] After the phase separation, the organic phase consists mainly of propylene, acetonitrile and a little water. An advantageous feature of this process management is that one can use a single distillation tower downstream of the phase separation to remove part of the propylene at the top of the tower and recycle it back to the phase separation. The conditions in said distillation tower are generally chosen in such a way that the bottom mixture contains besides acetonitrile also the required amount of propylene to return the stream directly to the reaction section. In the epoxidation reaction, normally a slight molar excess of propylene with respect to H2O2 is required, for example, an excess of 0.3 is sometimes used. This means that after the epoxidation reactor(s) an excess of gaseous propylene remains, which is unconverted and has to be processed to be recycled, wherein a compression of this stream is mandatory before phase separation.

[0007] As long as the HPPO process runs continuously at steady state and without interruptions, malfunctions and / or breakdowns, the above-described process is indeed the most advantageous in terms of simplicity of equipment and in terms of energy consumption.

[0008] However, there are always problems or circumstances, which require a shutdown of the process. In such a case the first measure is to stop feeding H2O2 to the process. Additionally, the H2O2 (and PO) still contained in the reactor have to be flushed out. However, if one uses the recycled acetonitrile stream to flush the reactor, the amount of propylene downstream of the reactor will massively increase (in the case above, from 0.3 to 1 .3. which is more than a factor of four). This would overwhelm the compressor used for compression of the gaseous propylene containing stream, which cannot cope with such an increased amount of feed stream.

[0009] One could of course store in a large tank enough acetonitrile or acetonitrile / water mixture, to flush the reactor. However, this would not only lead to an enormous increase of hold-up in the plant which would have to be removed at another point of the process and stored separately. It is also not trivial to make sure that the material for flushing is available with a high degree of reliability, because flushing the reactor is a safety relevant feature.

[0010] Therefore, there was a need to provide a new method, which would keep as many as possible of the advantages of the above-described process, requires as few as possible new pieces of equipment, keeps the energy consumption as low as possible, but still allows to deal with the operability problems described above in a simple way.

[0011] A first aspect of the invention thus relates to a method for controlling a stand-by stage of a process for preparing propylene oxide, the process for preparing propylene oxide comprising a normal run stage with propene epoxidation and a stand-by stage without propene epoxidation, the stand-by stage comprising: a) providing propene, water and acetonitrile into an epoxidation zone comprising a heterogeneous epoxidation catalyst, thereby obtaining a mixture comprising propene, water and acetonitrile; b) conducting the mixture comprising propene, water and acetonitrile through the epoxidation zone; c) removing an effluent stream S1 from the epoxidation zone, the effluent stream S1 comprising propene, water and acetonitrile; d) separating the effluent stream S1 removed in c), optionally including one or more further work-up steps, obtaining at least a stream comprising propene S3COnd and a stream S5 comprising acetonitrile and water; e) mixing the stream S5 comprising acetonitrile and water obtained in d) and at least a stream comprising propene under phase separation conditions, thereby obtaining a liquid aqueous phase LAP comprising water and acetonitrile and a liquid organic phase LOP comprising acetonitrile and propene, f) subjecting the liquid organic phase LOP comprising acetonitrile and propene obtained in e) to distillation, thereby obtaining a gaseous top stream S9 comprising propene and a liquid bottoms stream S10 comprising acetonitrile and propene, with a molar amount of propene in the liquid bottoms stream S10 MApropene(lbS-Sb), g) recycling the liquid bottoms stream S10 comprising acetonitrile and propene obtained in f) into a); wherein the method for controlling the stand-by stage comprises:

[0012] (I) defining, based on a molar amount of hydrogen peroxide provided into the epoxidation zone during a normal run stage MAn202(nr) a molar ratio MAProPene(lbs-sb) / MAH202(nr) in the range of from 0.02 to 1.03;

[0013] (ii) at least periodically determining MApropene(lbs-sb); and

[0014] (ii.a) if MApropene(lbs-sb) / MAH202(nr) is > 1 .03, adjusting the distillation in f) so that a molar ratio MApropene(lbs-sb) / MAn202(nr) in the range of from 0.02 to 1.03 is achieved,

[0015] (ii.b) if MApropene(lbs-sb) / MAH202(nr) is in the range of from 0.02 to 1 .03, maintaining the distillation in f).

[0016] Surprisingly, it was found that is possible by using the above-described method to shift the position where propylene is dosed into the plant and thus allow for the plant to be easily shifted from a production mode to a stand-by mode without incurring in a severe penalty (energy wise in the production mode, in the best case it's only 2% extra steam consumption).

[0017] According to step e), the stream S5 comprising acetonitrile and water obtained in d) is mixed with at least a stream comprising propene under phase separation conditions, thereby obtaining a liquid aqueous phase LAP comprising water and acetonitrile and a liquid organic phase LOP comprising acetonitrile and propene.

[0018] Preferably, the stream comprising propene, which is mixed with the stream comprising acetonitrile and water in e), comprises at least a part of the propene from the stream S3COnd comprising propene obtained in d), at least a part of the propene obtained in f).

[0019] Step e) is preferably conducted in a separation unit, preferably a decanter, which is preferably operated at a temperature in the range of from 5 to 30°C, more preferably in the range of from 10 to 20°C and / or at a pressure in the range of from 5 to 30 bar, preferably in the range of from 10 to 25 bar. The separation unit, preferably the decanter, is also called D1.

[0020] It is preferred that step e) comprises e.1) adding at least a part of the gaseous top stream S9 comprising propene obtained in f) to the stream comprising propene S3COnd obtained in d), thereby obtaining a mixed propene containing stream; e.2) mixing the stream S5 comprising acetonitrile and water obtained in d) and the mixed propene containing stream of e.1) under phase separation conditions, thereby obtaining a liquid aqueous phase LAP comprising water and a liquid organic phase LOP comprising acetonitrile and propene.

[0021] Preferably, the liquid aqueous phase LAP comprising water obtained in e) or e.2) comprises at least 85 weight-% water and at the outmost 15 weight-% of acetonitrile and propene, based on the total weight of the liquid aqueous phase being 100 weight-%, more preferably at least 88 weight-% water and at the outmost 10 weight-% of acetonitrile and at the outmost 1 weight-% of propene, based on the total weight of the liquid aqueous phase being 100 weight-%.

[0022] Preferably, the liquid organic phase LOP comprising acetonitrile obtained in e) or e.2) comprises at the outmost 10 weight-% of water and at least 80 weight-% of acetonitrile and propene, based on the total weight of the liquid organic phase being 100 weight-%, more preferably at the outmost 6 weight-% of water and at least 45 weight-% of acetonitrile and at least 30 weight-% of propene, based on the total weight of the liquid organic phase being 100 weight-%.

[0023] Distillation f) - rectification column RC(1) (T6)

[0024] According to step f), the liquid organic phase LOP comprising acetonitrile and propene obtained in e) is subjected to distillation, thereby obtaining a gaseous top stream S9 comprising propene and a liquid bottoms stream S10 comprising acetonitrile and propene, with a molar amount of propene in the liquid bottoms stream S10 MApropene(lbS-Sb).

[0025] The distillation in f) is preferably done by distillation in a at least one rectification column RC(1 ), wherein a liquid stream S10 comprising acetonitrile, propene and water is removed from the bottom of RC(1 ) and a gaseous top stream S9 comprising propene is removed from the top of RC(1). The distillation tower of step f), especially the rectification column RC(1) of step f), is also called T6. Preferably, RC(1) has in the range of from 5 to 100 theoretical stages, more preferably in the range of from 1 to 10 theoretical stages. Preferably, RC(1) is operated at a top pressure pRc<i)(top) in the range of from 5 to 50 bar, more preferably in the range of from 10 to 30 bar. Preferably, RC(1) is operated at a bottoms temperature TRC(i)(bottoms) in the range of from 70 to 140 °C, more preferably in the range of from 80 to 120 °C . It is preferred that RC(1 ) is operated with a reflux ratio in the range of from 0.1 : 10 to 10:1 , more preferably in the range of from 1 :2 to 1 :7, more preferably in the range of from 1 :3 to 1 :5.

[0026] Preferably, the temperature of the liquid organic phase LOP obtained in e) or e.2) is increased so that a liquid part-stream LOPiiqUid and a gaseous part-stream LOPgaseoUs are obtained, which are introduced into RC(1 ) in the upper part of RC(1 ), wherein preferably, the feeding point of the gaseous part-stream LOPgaseoUs into RC(1 ) is above the feeding point of the liquid part-stream LOPiiqUid. rectification column RC(2) (T7)

[0027] According to step e) or step e.2), a liquid organic phase LOP comprising acetonitrile and propene is obtained. Preferably, the liquid aqueous phase LAP comprising water and acetonitrile obtained in e) or e.2) is subjected to distillation in at least one rectification column RC(2), wherein a top stream enriched in acetonitrile compared to the liquid aqueous phase comprising water and acetonitrile obtained in e) or e.2) is obtained and a bottom stream enriched in water compared to the liquid aqueous phase comprising water and acetonitrile obtained in e) or e.2) is obtained. RC(2) is also called T7.

[0028] Preferably, RC(2) has in the range of from 5 to 100 theoretical stages, preferably in the range of from 1 to 10 theoretical stages. Preferably, RC(2) is operated at a top pressure ppc(2)(top) in the range of from 0.5 to 5 bar, preferably in the range of from 1 to 3 bar. Preferably, RC(2) is operated at a bottoms temperature Tpc(2)(bottoms) in the range of from 90 to 150 °C, preferably in the range of from 100 to 140 °C.

[0029] It has to be noted that for a stand-by stage, the bottom stream obtained from RC(2), which is enriched in water compared to the liquid aqueous phase comprising water and acetonitrile obtained in e) or e.2), is preferably not removed from the process but is rather returned into the process, preferably to a stage prior to step e).

[0030] Option 1 - modified rectification column RC(1) (T6)

[0031] According to step f), the liquid organic phase LOP comprising acetonitrile and propene obtained in e) is subjected to distillation, thereby obtaining a gaseous top stream S9 comprising propene and a liquid bottoms stream S10 comprising acetonitrile and propene, with a molar amount of propene in the liquid bottoms stream S10 MApropene(lbs-sb), and, for controlling the stand-by stage, MAprOpene(lbs-sb) is at least periodically determining and according to (ii.a), if MAprOpene(lbs-sb) / MAH202(nr) is > 1.03, the distillation in f) is adjusted so that a molar ratio MApropene(lbs-sb) / MAn202(nr) in the range of from 0.02 to 1.03 is achieved.

[0032] It is preferred in one option that adjusting the distillation in f) is done in that RC(1 ) is operated at a bottom temperature TRC(i)(bottoms)-2, wherein TRC(1) (bottoms)-2 > TRC(1) (bottoms), more preferably 1.5 x Tpc(i)(bottoms) < Tpc(i)(bottoms)-2 < 2.0 x TRc<i)(bottoms). Regarding said option, RC(1) has preferably in the range of from 8 to 20 theroretical stages, preferably in the range of from 10 to 15 theoretical stages. Preferably, the mixture comprising propene, water and acetonitrile of a) has a temperature Tfeed and the temperature of liquid bottoms stream S5 comprising acetonitrile and propene obtained in f), before being recycled according to g), is adjusted to be in the range of from 0.8 x Tfeed to 1.2 x Tfeed

[0033] Option 2 - further reboiler for RC(2) According to step f), the liquid organic phase LOP comprising acetonitrile and propene obtained in e) is subjected to distillation, thereby obtaining a gaseous top stream S9 comprising propene and a liquid bottoms stream S10 comprising acetonitrile and propene, with a molar amount of propene in the liquid bottoms stream S10 MApropene(lbs-sb), and, for controlling the stand-by stage, MAprOpene(lbs-sb) is at least periodically determining and according to (ii.a), if MAprOpene(lbs-sb) / MAH202(nr) is > 1.03, the distillation in f) is adjusted so that a molar ratio MApropene(lbs-sb) / MAn202(nr) in the range of from 0.02 to 1.03 is achieved.

[0034] It is preferred in a further option that adjusting the distillation in f) is done in that a part of the of the liquid bottoms stream S10 of RC(1 ) having a temperature T2, is passed through a reboiler, preferably an intermediate reboiler, of RC(2), wherein the liquid bottoms stream S10 of RC(1 ) after having passed said reboiler of RC(2) has a temperature T3 with T2>Ts.

[0035] Option 3 - heating feed of RC(2)

[0036] It is preferred in a further option that the liquid bottoms streams S10 of RC(1 ) having temperature T3 is passed through a reboiler, preferably a feed reboiler of RC(2), wherein the liquid bottoms stream S10 of RC(1 ) after having passed said feed reboiler has a temperature T4 with Ts>T4.

[0037] Option 4 - Heating feed of RC(1)

[0038] According to step f), the liquid organic phase LOP comprising acetonitrile and propene obtained in e) is subjected to distillation, thereby obtaining a gaseous top stream S9 comprising propene and a liquid bottoms stream S10 comprising acetonitrile and propene, with a molar amount of propene in the liquid bottoms stream S10 MApropene(lbs-sb), and, for controlling the stand-by stage, MAprOpene(lbs-sb) is at least periodically determining and according to (ii.a), if MAprOpene(lbs-sb) / MAH202(nr) is > 1.03, the distillation in f) is adjusted so that a molar ratio MApropene(lbs-sb) / MAn202(nr) in the range of from 0.02 to 1.03 is achieved.

[0039] It is preferred in a further option that RC(1) has in the range of from 1 to 15 theoretical stages, preferably in the range of from 5 to 10 theoretical stages. For said further option, it is preferred that adjusting the distillation in f) is done in that the liquid bottoms stream S10 from RC(1) having a temperature T1 is passed through a feed reboiler of RC(1 ), wherein the temperature of the liquid organic phase LOP obtained in e) or e.2) is increased so that liquid part-stream LOPiiqUid and gaseous part-stream LOPgaSeous are obtained, wherein the liquid part-stream LOPiiquid is introduced into RC(1 ), and wherein the liquid bottoms stream S10 from RC(1 ), after having passed the feed reboiler of RC(1) has a temperature T4 with T1 > T4.

[0040] Option 5 - with additional rectification column RC(1)-2 and compressor According to step f), the liquid organic phase LOP comprising acetonitrile and propene obtained in e) is subjected to distillation, thereby obtaining a gaseous top stream S9 comprising propene and a liquid bottoms stream S10 comprising acetonitrile and propene, with a molar amount of propene in the liquid bottoms stream S10 MApropene(lbs-sb), and, for controlling the stand-by stage, MAprOpene(lbs-sb) is at least periodically determining and according to (ii.a), if MAprOpene(lbs-sb) / MAH202(nr) is > 1.03, the distillation in f) is adjusted so that a molar ratio MApropene(lbs-sb) / MAn202(nr) in the range of from 0.02 to 1.03 is achieved.

[0041] It is preferred in a further option that the distillation in f) is done by distillation in a at least two rectification column RC(1 ) and RC(1 )-2, wherein the liquid bottoms stream S10 from rectification column RC(1 ) is, optionally after temperature increase and pressure decrease, fed into a further rectification column RC(1 )-2, and from RC(1 )-2 a gaseous top stream comprising propylene and a liquid stream S10-1 comprising acetonitrile, propylene and water are obtained, wherein the liquid bottoms stream S10-1 from RC(1 )-2 is depleted of propylene compared to the liquid stream S10 comprising acetonitrile, propylene and water from RC(1 ). Preferably, the liquid bottoms stream S10 from rectification column RC(1 ) having temperature Ti is heat exchanged with the liquid bottoms stream S10- 1 from RC(1 )-2, the liquid bottoms stream S10 from rectification column RC(1 ) after said heat exchange having a temperature T5 with T1 < T5, and a pressure p, wherein said stream S10 is subsequently at least once decompressed, resulting in a liquid stream S10decomPhaving reduced pressure pred with pred < p and a gaseous stream, wherein liquid stream S10decomPis depleted of propene compared to stream S10 and is fed into the further rectification column RC(1 )-2, and the gaseous stream resulting from decompression is enriched in propene compared to stream S10. Preferably, the gaseous stream enriched in propene obtained from decompression of stream S10 and the gaseous top stream comprising propylene obtained from RC(1 )-2 are combined, resulting in a combined stream comprising propene, which, optionally after pressure increase in at least one compressor, is introduced into the lower part of RC(1 ).

[0042] Step d) (T1)

[0043] Preferably, the propene provided in a) comprises propene and propane, then the mixture obtained in b) and the effluent stream S1 removed in c) comprises propane, and the step d) preferably comprises d.1 ) separating propene and propane from the effluent stream S1 removed in c) by washing and distillation, thereby obtaining a stream S2 enriched in propylene oxide, water and acetonitrile compared to the effluent stream removed in c); and a stream S3 enriched in propene and propane compared to the effluent stream removed in c).

[0044] Step d.1 ) is preferably done in at least one distillation tower, wherein the stream S2 enriched in propylene oxide, water and acetonitrile compared to the effluent stream S1 removed in c) is preferably removed as a bottoms stream, more preferably as a liquid bottoms stream from said distillation tower. The stream S3 enriched in propene and propane compared to the effluent stream S1 removed in c) obtained in d.1 ) is preferably obtained as top stream from said distillation tower. Preferably, the distillation tower has in the range of from 10 to 30 theoretical stages and is preferably operated at a top pressure in the range of from 0.5 to 1 .5 bar and / or, preferably and, a bottom temperature in the range of from 40 to 90°C, preferably in the range of from 50 to 90ooC, more preferably in the range of from 60 to 80°C. The distillation tower of d.1) is also called T1. Either liquid acetonitrile or a mixture comprising acetonitrile and water is added in the upper part of the distillation tower as washing liquid, wherein the acetonitrile is preferably at least partially taken from a downstream process step. Preferably, the weight ratio of the amount of acetonitrile fed to the distillation tower relative to the weight of the effluent stream S1 removed in c) and fed to d.1) is in the range of from 1 :1 to 4:1. Adding liquid acetonitrile or a liquid mixture of acetonitrile with water to the upper part of the distillation tower as washing liquid allows an external reflux, which serves as entraining agent, which, among others, prevents propylene oxide from being separated via the top of the distillation tower. The entraining agent is taken from a separate tank or at least partially taken from a downstream process step. The temperature of the external reflux is generally in the range of from 2 to 20 °C, preferably in the range of from 5 to 15 °C. Preferably, at least 90 volume-% of the stream S3 enriched in propene and propane obtained from d) consist of propene, propane and oxygen. Depending on the oxygen content of said stream, a work-up stage can be used after d.1 ) and before e.2), wherein the oxygen content is reduced preferably in the presence of a suitable catalyst with hydrogen, wherein prior to oxygen reduction, one or more optional workup-steps can be comprised such as compression and partial condensation, wherein a non-condensed part is then subjected to hydrogenation. Preferably, prior to (d.1 ), the effluent stream S1 is depressurized, preferably to a pressure of from 0.5 to 2.8 bar, more preferably of from 0.6 to 2.5 bar, more preferably of from 0.8 to 1 .5 bar. Generally, there is no specific restriction how the effluent stream is depressurized. Preferably, the effluent stream is depressurized into a flash drum. Preferably, from depressurizing the effluent stream, a gaseous stream and a liquid stream are obtained, wherein the gaseous and liquid streams are preferably passed separately to the distillation unit employed according to (d.1 ), preferably to the same theoretical tray of the distillation tower employed according to (d.1 ).

[0045] Distillation towers T4, T5 & (C1) T2, T3

[0046] It is preferred that step d) further comprises d.2) separating the stream S2 enriched in propylene oxide, water and acetonitrile obtained in d.1 ) by one or more distillations, thereby obtaining a stream S4, which comprises more than 99 wt.-% of propylene oxide based on the total weight of the stream S4, a streamS5 comprising acetonitrile and water and a stream S6 comprising propene; and / or, preferably and, d.3) separating propane from the stream S3 enriched in propene and propane obtained in d.1 ) in a separation zone, comprising subjecting the stream enriched in propene and propane obtained in d.1) to washing conditions in a scrubber, wherein a solvent mixture comprising acetonitrile and water is added as entraining agent, obtaining a liquid bottoms stream S7, which comprises acetonitrile, water and at least 70 weight-% of the propene comprised in the steam S3 enriched in propene and propane obtained in d.1 ); and a gaseous top stream S8, which comprises at least 5 weight-% of the propane comprised in the stream S3 enriched in propene and propane obtained in d.1).

[0047] - Distillation towers T4, T5

[0048] Preferably, step d.2) comprises d.2. a) feeding the stream S2 enriched in propylene oxide, water and acetonitrile obtained in d.1) into a distillation tower, obtaining, preferably as bottom stream, a stream S5 comprising acetonitrile and water and being depleted of propylene oxide compared to the stream obtained in d) and a stream, preferably a top stream, enriched in propylene oxide compared to the stream obtained in d.1 ); d.2b) feeding the stream enriched in propylene oxide obtained in d.2a) into a further distillation tower, obtaining a stream S4 enriched in in propylene oxide compared to the stream obtained in d.2a), preferably as a side stream, said stream enriched in in propylene oxide comprising more than 99 wt.-%, preferably more than 99.9 wt.-%, more preferably more than 99.99 wt.-%, of propylene oxide based on the total weight of the stream S4.

[0049] Preferably, the distillation tower of step d.2. a) has in the range of from 50 to 150, preferably in the range of from 80 to 120 theoretical stages, is preferably operated at a top pressure in the range of from 0.1 to 2 bar, preferably in the range of from 0.2 to 1 bar, a bottom temperature in the range of from 40 to 80°C, preferably in the range of from 50 to 70°C and / or at a reflux ratio in the range of from 0.5 to 1 .5, preferably in the range of from 0.6 to 1 .0. Preferably, the distillation tower of step d.2.b) has in the range of from 10 to 100, preferably in the range of from 40 to 70 theoretical stages, is preferably operated at a top pressure in the range of from 1 to 5 bar, preferably in the range of from 2.5 to 3.5 bar, a bottom temperature in the range of from 50 to 90°C, preferably in the range of from 45 to 80°C. Preferably, the distillation tower of step d.2. a) is also called distillation tower T4 and the distillation tower of step d.2.b) is called distillation tower T5. Preferably, also a low boiler stream, a so called stream LB, is taken from the top of T4.

[0050] Preferably, the distillation tower of step d.2.b) is operated with reflux, preferably with a reflux ratio in the range of from 0.5 : 1 to 1 : 1 , more preferably in the range of from 0.7 : 1 to 1 : 1 , more preferably in the range of from 0.9 : 1 to 1 : 1 . When operated with reflux, the distillation tower of step d.2.b) is operated as a non-reboiled stripping unit. In step e.1 .a), preferably a further stream is removed from the distillation tower, preferably from a region at about half the height of the distillation tower, preferably in the range of from 35 to 55% of the height of the distillation tower, said further stream comprising propylene oxide, acetonitrile, water and one or more side products formed, preferably at least propionaldehyde. Said further stream is removed from the distillation tower in a ratio to the stream, preferably the top stream, enriched in propylene oxide obtained in step d.2. a), in the range of from 0.01:100 to 0.05:100. Further details regarding step d.2. a) and the further stream are disclosed in WO 2018 / 015435 A1 , especially in the parts thereof related to side stream S1 b. Said further stream is also called stream IB.

[0051] Compression unit C1

[0052] It is preferred that step d.3) comprises d.3.a) feeding stream S3 enriched in propene and propane obtained in d) and having a pressure ps3 into a compression unit, wherein the pressure is increased to ps3comPwith ps3comP> Ps3, followed by partial condensation, obtaining a condensed part stream S3COnd and a non-condensed part stream S3gase0Us of S3, condensed part stream S3COnd comprising propene and propane and the non-condensed part stream S3gaseous comprising propene and propane.

[0053] The compression unit used in d.3.a) has at least one compression stage, preferably in the range of from 1 to 10 compression stage(s), more preferably in the range of from 3 to 5 compression stages. The compression unit used in step d.3.a) is also called compression unit C1.

[0054] Distillation towers T2, T3

[0055] It is preferred that step d.3) further comprises d.3.b) feeding the non-condensed part stream S3gase0Us obtained in d.3.a) into a scrubber, optionally with addition of an inert gas, and subjecting S3gase0Us to washing conditions in the scrubber, wherein a solvent mixture comprising acetonitrile and water is introduced as entraining agent into the scrubber, preferably to the upper part of the scrubber; thereby obtaining a liquid bottoms stream S7, which comprises acetonitrile, water and propene; d.3.c) feeding the bottom stream comprising propene and acetonitrile into a distillation tower, from which a gaseous top stream S8 and a liquid bottoms stream S9 are obtained, wherein liquid bottoms stream S9 comprises acetonitrile and gaseous top stream S8 comprises at least 5 weight-% of the propane comprised in the stream S3 enriched in propene and propane obtained in d.1).

[0056] Additive

[0057] It is preferred that in step a), additionally an additive is provided to the epoxidation zone, wherein the additive comprises at least one potassium salt selected from the group consisting of at least one inorganic potassium salt, at least one organic potassium salt, and mixtures of at least one inorganic potassium salt and at least one organic potassium salt; preferably selected from the group consisting of at least one inorganic potassium salt selected from the group consisting of potassium hydroxide, potassium chloride, potassium nitrate, potassium hydrogen phosphate, potassium dihydrogen phosphate, at least one organic potassium salt selected from the group consisting of potassium formate, potassium acetate, potassium carbonate, and potassium hydrogen carbonate, and mixtures of at least one of the at least one inorganic potassium salts and at least one of the at least one organic potassium salts, more preferably, the additive is selected from the group consisting of potassium dihydrogen phosphate, dipotassium hydrogen phosphate potassium formate and mixtures of two or more thereof; more preferably the additive comprises at least potassium formate, more preferably the additive is potassium formate. hydrogen peroxide

[0058] According to the present invention, it is preferred to employ hydrogen peroxide (H2O2) which is obtained as crude hydrogen peroxide solution by extraction of a mixture which results from a process known as anthraquinone process by means of which virtually the entire world production of hydrogen peroxide is produced (see, e.g., Ullmann's Encyclopedia of Industrial Chemistry, 5thedition, volume A 13 (1989) pages 443-466) wherein a solution of an anthraquinone is used containing an alkyl group preferably having of from 2 to 10 carbon atoms, more preferably at least 5 carbon atoms such as 5 carbon atoms or 6 carbon atoms and where the solvent used usually consists of a mixture of two different solvents, whereby preferably none of the solvents is a nitrogen containing substance. This solution of the anthraquinone is usually referred to as the working solution. In this process, the hydrogen peroxide formed in the course of the anthraquinone process is generally separated by extraction from the respective working solution after a hydrogenation / re-oxidation cycle. Said extraction can be performed preferably with essentially pure water, and the crude aqueous hydrogen peroxide solution is obtained. While it is generally possible to further purify the thus obtained crude aqueous hydrogen peroxide solution by distillation, it is preferred, according to the present invention, to use such crude aqueous hydrogen peroxide solution which has not been subjected to purification by distillation. Further, it is generally possible to subject the crude aqueous hydrogen peroxide solution to a further extraction stage wherein a suitable extracting agent, preferably an organic solvent is used. More preferably, the organic solvent used for this further extraction stage is the same solvent, which is used in the anthraquinone process. Preferably the extraction is performed using just one of the solvents in the working solution and most preferably using just the most nonpolar solvent of the working solution. In case the crude aqueous hydrogen peroxide solution is subjected to such further extraction stage, a so-called crude washed hydrogen peroxide solution is obtained. The production of a crude solution is described, for example, in European patent application EP 1 122 249 A1. As to the term "essentially pure water", reference is made to paragraph 10, page 3 of EP 1 122 249 A1 which is incorporated by reference. The hydrogen peroxide can also be treated to remove trace metals, for example, as described in the WO 2015 / 049327 A1 before use. Extraction without subsequent distillation of an aqueous hydrogen peroxide solution results in concentrations in the range of from 40 to 50 weight-% H2O2 based on the total weight of the aqueous solution being 100 weight-%. For achieving higher concentrations, distillation is required.

[0059] Epoxidation zone Generally, there are no specific restrictions regarding the design of the epoxidation zone provided that it is suitable for carrying out a, preferably continuous, epoxidation reaction.

[0060] Preferably, the epoxidation zone according to (b) comprises a first epoxidation subzone consisting of one or more epoxidation reactors A. The term "first epoxidation subzone" as used in this context of the present invention relates to the epoxidation subzone into which the components provided in (a) are passed, wherein the epoxidation zone may comprise further epoxidation subzones which are arranged downstream of the first epoxidation subzone. If the first epoxidation subzone consisting of two or more epoxidation reactors A, it is preferred that the two or more epoxidation reactors A are arranged in parallel.

[0061] The epoxidation conditions according to (b) comprise an epoxidation temperature TN, wherein TNis the temperature of a heat transfer medium used for adjusting the temperature of the reaction mixture in the first epoxidation reaction subzone, wherein it is preferred that said temperature is adjusted by passing the heat transfer medium through a jacket of the one or more epoxidation reactors A, wherein TNis preferably the temperature of the heat transfer medium prior to adjusting the temperature of the reaction mixture, preferably the temperature of the heat transfer medium at the entrance of the jacket of the one or more epoxidation reactors A. If the first epoxidation subzone comprises two or more epoxidation reactors A, the epoxidation temperature TNrelates to the epoxidation temperature TNof a given reactor A in operation of first epoxidation subzone. Preferably, TNis a temperature n the range of from 10 to 80°C, more preferably in the range of from 20 to 70°C, more preferably in the range of from 25 to 55°C. The epoxidation reactor(s) of A are also called reactors R1 (preferably tube bundle reactors R1 a, R1b, ,...R1x, with x being an integer and being at least 100, preferably at least 1,000, more preferably being selected from the range of from 1,000 to 100,000, more preferably from the range of from 10,000 to 50,000.

[0062] Preferably, the epoxidation conditions according to (b) comprise a first epoxidation reaction pressure in the range of from 14 to 100 bar, more preferably in the range of from 15 to 35 bar, more preferably in the range of from 15 to 30 bar. The first epoxidation reaction pressure is defined as the absolute pressure at the exit of the first epoxidation subzone. If the first epoxidation subzone comprises two or more epoxidation reactors A, the first epoxidation reaction pressure relates to the absolute pressures at the exit of a given reactor A in operation of first epoxidation subzone.

[0063] According to a first preferred embodiment of the present invention, the epoxidation zone according to (b) consists of the first epoxidation subzone.

[0064] According to a second preferred embodiment of the present invention, the epoxidation zone according to (b) additionally comprises a second epoxidation subzone consisting of one or more epoxidation reactors B wherein, if the second epoxidation subzone comprises two or more epoxidation reactors B, the two or more epoxidation reactors B are arranged in parallel, wherein the second epoxidation subzone is arranged downstream of the first epoxidation subzone. In this case, it is preferred that in (b), the effluent stream obtained from the first epoxidation subzone, optionally after a suitable intermediate treatment, is passed into at least one of the epoxidation reactors B. The epoxidation reactors B are also called reactors R2. It is possible, for example, that, while the effluent stream obtained from the first epoxidation subzone, optionally after a suitable intermediate treatment, is passed into at least one of the epoxidation reactors B, at least one of the reactors B is taken out of operation, for example for maintenance purposes and / or for regenerating the catalyst comprised in the at least one of the reactors B. If the second epoxidation subzone comprises two or more epoxidation reactors B, the reactors in operation are operated essentially identically so that in every epoxidation reactor B in operation, a given epoxidation condition is in the same range in every reactor. Generally, it is conceivable that in addition to the first epoxidation subzone and the second epoxidation subzone, the epoxidation zone according to (b) comprises at least one further epoxidation subzone arranged downstream of the second epoxidation subzone. Preferably, according to the second preferred embodiment of the present invention, the epoxidation zone according to (b) consists of the first epoxidation subzone and the second epoxidation subzone.

[0065] Preferably, the epoxidation conditions according to (b) comprise a second epoxidation reaction pressure in the range of from 14 to 100 bar, preferably in the range of from 14.5 to 32 bar, more preferably in the range of from 15 to 25 bar. The second epoxidation reaction pressure is defined as the absolute pressure at the exit of the second epoxidation subzone. If the second epoxidation subzone comprises two or more epoxidation reactors B, the second epoxidation reaction pressure relates to the absolute pressures at the exit of a given reactor B in operation of second epoxidation subzone.

[0066] Preferably, the temperature of the reaction mixture in the second epoxidation reaction subzone is not adjusted by passing a heat transfer medium through a jacket of the one or more epoxidation reactors B. More preferably, the second epoxidation subzone is an essentially adiabatic epoxidation subzone. More preferably, the second epoxidation subzone is an adiabatic epoxidation subzone.

[0067] Epoxidation catalyst

[0068] According to (b), the mixture comprising propene, water and acetonitrile is contacted in the epoxidation zone with the heterogeneous epoxidation catalyst. The heterogeneous epoxidation catalyst comprising a titanium zeolite. Preferably, titanium zeolite comprised in the epoxidation catalyst is a titanium zeolite having ABW, AGO, AEI, AEL, AEN, AET, AFG, AFI, AFN, AFO, AFR, AFS, AFT, AFX, AFY, AHT, ANA, APC, APD, AST, ASV, ATN, ATO, ATS, ATT, ATV, AWO, AWW, BCT, BEA, BEG, BIK, BOG, BPH, BRE, CAN, CAS, CDO, CFI, CGF, CGS, CHA, CHI, CLO, CON, CZP, DAO, DDR, DFO, DFT, DOH, DON, EAB, EDI, EMT, EPI, ERI, ESV, ETR, EDO, FAU, FER, FRA, GIS, GIU, GME, GON, GOO, HEU, IFR, ISV, ITE, ITH, ITW, IWR, IWW, JBW, KFI, LAU, LEV, LIO, LOS, LOV, LTA, LTL, LTN, MAR, MAZ, MEI, MEL, MEP, MER, MMFI, MFS, MON, MOR, MSO, MTF, MTN, MTT, MTW, MWW, NAB, NAT, NEES, NON, NPO, OBW, OFF, OSI, OSO, PAR, PAU, PHI, PON, RHO, RON, RRO, RSN, RTE, RTH, RUT, RWR, RWY, SAO, SAS, SAT, SAV, SBE, SBS, SBT, SFE, SFF, SFG, SFH, SFN SFO, SGT, SOD, SSY, STF, STI, STT, TER, THO, TON, TSO, UEI, UFI, UOZ, USI, UTL, VET, VFI, VNI, VSV, WEI, WEN, YUG, ZON framework structure or a mixed structure of two or more of these framework structures, preferably a titanium zeolite having an MFI framework structure, an MEL framework structure, an MWW framework structure, an ITQ framework structure, a BEA framework structure, a MOR framework structure, or a mixed structure of two or more of these framework structures, preferably an MFI framework structure, or an MWW framework structure.

[0069] The epoxidation catalyst comprising a titanium zeolite can be employed in every conceivable form. Preferably, the catalyst comprising the titanium zeolite is employed as a molding comprising the titanium zeolite. More preferably, the catalyst comprising the titanium zeolite is present in the epoxidation zone as a molding, preferably as fluidized-bed catalyst or a fixed-bed catalyst, more preferably as a fixed-bed catalyst.

[0070] Preferably, the titanium zeolite comprised in the epoxidation catalyst is a titanium zeolite having MWW framework structure, which preferably comprises at least one of Al, B, Zr, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Zn, Ga, Ge, In, Sn, Pb, Pd, Pt, Au, preferably at least one of B, Zr, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Zn, Ga, Ge, In, Sn, Pb, Pd, Pt, Au, more preferably Zn.

[0071] Preferably, the titanium zeolite is an aluminum-free zeolitic material of MWW framework structure, containing titanium, preferably in an amount of from 0.5 to 5 weight-%, more preferably from 1 to 2 weight-%, calculated as elemental titanium and based on the total weight of the titanium containing zeolite, and containing zinc, preferably in an amount of from 0.5 to 5 weight-%, preferably from 1 to 2 weight-%, calculated as elemental zinc and based on the total weight of the titanium containing zeolite. The term "aluminum-free” in the context of the present invention refers to an embodiment according to which the aluminum content of the zeolitic material is 0.05 weight- ppm at most, preferably 0.03 weight-ppm at most, more preferably 0.02 weight-ppm at most, based on the total weight of zeolitic material. The weight-%-values refer to an embodiment according to which the zeolitic material is in dry state, preferably after drying for at least ten hours at 80 °C at a pressure of less than 1013.25 hPa.

[0072] More preferably, the titanium zeolite comprised in the epoxidation catalyst is a titanium zeolite of MWW framework structure, preferably being aluminum-free and comprising zinc.

[0073] Normal run stage

[0074] As indicated above, the process for preparing propylene oxide comprises stand-by stage without propene epoxidation and a normal run stage with propene epoxidation.

[0075] It is preferred that the normal run stage with propene epoxidation comprises:

[0076] A) providing propene, hydrogen peroxide, water and acetonitrile into the epoxidation zone comprising a heterogeneous epoxidation catalyst, wherein the molar amount of propene provided into the epoxidation zone is MApropene(nr) and wherein the molar amount of hydrogen peroxide provided into the epoxidation zone is MAH202(nr), with a molar ratio of MAprOpene(nr) : MAn202(nr) in the range of from 1.1 to 2.0, thereby obtaining a reaction mixture;

[0077] B) subjecting the reaction mixture comprising propene, hydrogen peroxide, water and acetonitrile obtained in A) to epoxidation reaction conditions in the epoxidation zone, thereby obtaining a mixture comprising propylene oxide, propene, water and acetonitrile;

[0078] C) removing an effluent stream from the epoxidation zone, the effluent stream comprising propylene oxide, propene, water and acetonitrile;

[0079] D) separating the effluent stream removed in C), optionally including one or more further work-ups, obtaining at least a stream comprising acetonitrile and water and a stream comprising propene;

[0080] E) mixing the stream comprising acetonitrile and water and at least the stream comprising propene obtained in D) and optionally a further stream comprising propylene, under phase separation conditions, thereby obtaining a liquid aqueous phase comprising water and a liquid organic phase comprising acetonitrile and propene;

[0081] F) subjecting the liquid phase comprising the propene obtained in E) to distillation, thereby obtaining a liquid bottoms stream comprising acetonitrile, propene and water and a gaseous top stream comprising propene, with a molar amount of propene in the liquid bottoms stream MAprOpene(lbs-nr);

[0082] G) recycling the liquid bottoms stream comprising acetonitrile and propene obtained in E) into A).

[0083] Preferably, the molar amount of propene provided in A) MAprOpene(nr) is the sum of the molar amount of propene in the liquid bottoms stream MAprOpene(lbs-nr) obtained in F) and recycled in G) into A) and the molar amount of propene freshly provided in A) MAprOpene(fresh):

[0084] MApropene(nr) — MApropene(lbS-nr) + MApropene(frOSh).

[0085] It is preferred that propene is freshly provided in A) so that the molar ratio of MAprOpene(fresh) to MAn202(nr) is

[0086] 0.97 < MAprOpene(fresh) / MAH202(nr) < 1 .03; and the molar ratio of MAprOpene(lbs-nr) to MAn202(nr) is

[0087] 0.04< MApropene(lbs-nr) / MAH202(nr) < 1 .03, with the provision that 1.1 < MAprOpene(nr) / MAH202(nr) < 2.0.

[0088] 2ndaspect - Combined process with normal run stage, stand-by stage and intermediate staqefs)

[0089] The present invention relates in a second aspect to a process for preparing propylene oxide, the process for preparing propylene oxide comprising at least a normal run stage with propene epoxidation and at least a standby stage without propene epoxidation, wherein the normal run stage comprises steps A) to G) as indicated above in the section related to the first aspect of the invention, and the stand-by stage comprises steps a) to g) and I) to ii) as indicated above in the section related to the first aspect of the invention, the process further preferably comprising intermediate shut-down stage(s) after a normal run stage and before a stand-by stage and / or start-up stage(s) after a stand-by stage and before a normal run stage.

[0090] All details, embodiments and preferred embodiments described herein above in the section related to the method of the first aspect of the invention also apply to the process of the second aspect of the invention.

[0091] 3rdaspect - Propylene oxide

[0092] In a third aspect, the invention is directed to propylene oxide obtained or obtainable from the process of the second aspect of the invention.

[0093] All details, embodiments and preferred embodiments described herein above in the section related to the method of the first aspect of the invention and in the section related to the process of the second aspect of the invention apply also to the propylene oxide according to the third aspect of the invention.

[0094] The present invention is further illustrated by the following set of embodiments and combinations of embodiments resulting from the dependencies and back-references as indicated. In particular, it is noted that in each instance where a range of embodiments is mentioned, for example in the context of a term such as "The method of any one of embodiments 1 to 4", every embodiment in this range is meant to be explicitly disclosed for the skilled person, i.e. the wording of this term is to be understood by the skilled person as being synonymous to "The method of any one of embodiments 1, 2, 3 and 4". Further, it is explicitly noted that the following set of embodiments represents a suitably structured part of the general description directed to preferred aspects of the present invention, and, thus, suitably supports, but does not represent the claims of the present invention.

[0095] 1 . A method for controlling a stand-by stage of a process for preparing propylene oxide, the process for preparing propylene oxide comprising a normal run stage with propene epoxidation and a stand-by stage without propene epoxidation, the stand-by stage comprising: a) providing propene, water and acetonitrile into an epoxidation zone comprising a heterogeneous epoxidation catalyst, thereby obtaining a mixture comprising propene, water and acetonitrile; b) conducting the mixture comprising propene, water and acetonitrile through the epoxidation zone; c) removing an effluent stream S1 from the epoxidation zone, the effluent stream S1 comprising propene, water and acetonitrile; d) separating the effluent stream S1 removed in c), optionally including one or more further work-up steps, obtaining at least a stream comprising propene S3COnd and a stream S5 comprising acetonitrile and water; e) mixing the stream S5 comprising acetonitrile and water obtained in d) and at least a stream comprising propene under phase separation conditions, thereby obtaining a liquid aqueous phase LAP comprising water and acetonitrile and a liquid organic phase LOP comprising acetonitrile and propene, f) subjecting the liquid organic phase LOP comprising acetonitrile and propene obtained in e) to distillation, thereby obtaining a gaseous top stream S9 comprising propene and a liquid bottoms stream S10 comprising acetonitrile and propene, with a molar amount of propene in the liquid bottoms stream S10 MAprOpene(lbs-sb); g) recycling the liquid bottoms stream S10 comprising acetonitrile and propene obtained in f) into a); wherein the method for controlling the stand-by stage comprises:

[0096] (I) defining, based on a molar amount of hydrogen peroxide provided into the epoxidation zone during a normal run stage MAn202(nr) a molar ratio MAProPene(lbs-sb) / MAH202(nr) in the range of from 0.02 to 1.03;

[0097] (ii) at least periodically determining MApropene(lbs-sb); and

[0098] (ii.a) if MApropene(lbs-sb) / MAH202(nr) is > 1 .03, adjusting the distillation in f) so that a molar ratio MApropene(lbs-sb) / MAn202(nr) in the range of from 0.02 to 1 .03 is achieved,

[0099] (ii.b) if MApropene(lbs-sb) / MAH202(nr) is in the range of from 0.02 to 1 .03, maintaining the distillation in f).

[0100] 2. The method of embodiment 1 , wherein the stream comprising propene, which is mixed with the stream comprising acetonitrile and water in e), comprises at least a part of the propene from the stream S3COnd comprising propene obtained in d), at least a part of the propene obtained in f).

[0101] 3. The method of embodiment 1 or 2, wherein e) comprises e.1 ) adding at least a part of the gaseous top stream S9 comprising propene obtained in f) to the stream comprising propene S3COnd obtained in d), thereby obtaining a mixed propene containing stream; e.2) mixing the stream S5 comprising acetonitrile and water obtained in d) and the mixed propene containing stream of e.1 ) under phase separation conditions, thereby obtaining a liquid aqueous phase LAP comprising water and a liquid organic phase LOP comprising acetonitrile and propene.

[0102] 4. The method of any one of embodiments 1 to 3, wherein the liquid aqueous phase LAP comprising water obtained in e) or e.2) comprises at least 85 weight-% water and at the outmost 15 weight- % of acetonitrile and propene, based on the total weight of the liquid aqueous phase being 100 weight-%, preferably at least 88 weight-% water and at the outmost 10 weight-% of acetonitrile and at the outmost 1 weight-% of propene, based on the total weight of the liquid aqueous phase being 100 weight-%. 5. The method of any one of embodiments 1 to 4, wherein the liquid organic phase LOP comprising acetonitrile obtained in e) or e.2) comprises at the outmost 10 weight-% of water and at least 80 weight-% of acetonitrile and propene, based on the total weight of the liquid organic phase being 100 weight-%, preferably at the outmost 6 weight-% of water and at least 45 weight-% of acetonitrile and at least 30 weight-% of propene, based on the total weight of the liquid organic phase being 100 weight-%.

[0103] 6. The method of any one of embodiments 1 to 5, wherein the distillation in f) is done by distillation in a at least one rectification column RC(1), wherein a liquid stream S10 comprising acetonitrile, propene and water is removed from the bottom of RC(1 ) and a gaseous top stream S9 comprising propene is removed from the top of RC(1).

[0104] 7. The method of embodiment s, wherein RC(1) has in the range of from 5 to 100 theoretical stages, preferably in the range of from 1 to 10 theoretical stages.

[0105] 8. The method of embodiment 6 or 7, wherein RC(1 ) is operated at a top pressure PRC(top) in the range of from 5 to 50 bar, preferably in the range of from 10 to 30 bar.

[0106] 9. The method of any one of embodiments 6 to 8, wherein RC(1) is operated at a bottoms temperature TRc<i)(bottoms) in the range of from 70 to 140 °C, preferably in the range of from 80 to 120 °C .

[0107] 10. The method of any one of embodiments 6 to 9, wherein RC(1) is operated with a reflux ratio in the range of from 0.1 :10 to 10:1 , preferably in the range of from 1 :2 to 1 :7, more preferably in the range of from 1 :3 to 1 :5.

[0108] 11. The method of any one of embodiments 6 to 10, wherein the temperature of the liquid organic phase LOP obtained in e) or e.2) is increased so that a liquid part-stream LOPiiqUid and a gaseous part-stream are obtained, which are introduced into RC(1) in the upper part of RC(1 ), wherein preferably, the feeding point of the gaseous part-stream LOPgaSeous into RC(1) is above the feeding point of the liquid part-stream LOPiiqUid.

[0109] 12. The method of any one of embodiments 6 to 11 , wherein the liquid aqueous phase LAP comprising water and acetonitrile obtained in e) or e.2) is subjected to distillation in at least one rectification column RC(2), wherein a top stream enriched in acetonitrile compared to the liquid aqueous phase comprising water and acetonitrile obtained in e) or e.2) is obtained and a bottom stream enriched in water compared to the liquid aqueous phase comprising water and acetonitrile obtained in e) or e.2) is obtained.

[0110] 13. The method of embodiment 12, wherein RC(2) has in the range of from 5 to 100 theoretical stages, preferably in the range of from 1 to 10 theoretical stages. 14. The method of embodiment 12 or 13, wherein RC(2) is operated at a top pressure pRc<2)(top) in the range of from 0.5 to 5 bar, preferably in the range of from 1 to 3 bar.

[0111] 15. The method of any one of embodiments 12 to 14, wherein RC(2) is operated at a bottoms temperature TRC(2)(bottoms) in the range of from 90 to 150 °C, preferably in the range of from 100 to 140 °C.

[0112] 16. The method of any one of embodiments 6 to 11, wherein adjusting the distillation in f) is done in that RC(1) is operated at a bottom temperature TRC(i)(bottoms)-2, wherein TRC(1) (bottoms)-2 > TRC(1) (bottoms), preferably 1.5 x TRc<i)(bottoms) < TRc<i)(bottoms)-2 < 2.0 x TRc<i)(bottoms).

[0113] 17. The method of embodiment 16, wherein RC(1) has in the range of from 8 to 20 theroretical stages, preferably in the range of from 10 to 15 theoretical stages.

[0114] 18. The method of embodiment 16 or 17, wherein the mixture comprising propene, water and acetonitrile of a) has a temperature Tfeed and the temperature of liquid bottoms stream S5 comprising acetonitrile and propene obtained in f), before being recycled according to g), is adjusted to be in the range of from 0.8 x Tfeed to 1 .2 X Tfeed

[0115] 19. The method of any one of embodiments 6 to 15, wherein adjusting the distillation in f) is done in that a part of the of the liquid bottoms stream S10 of RC(1) having a temperature T2, is passed through a reboiler, preferably an intermediate reboiler, of RC(2), wherein the liquid bottoms stream S10 of RC(1) after having passed said reboiler of RC(2) has a temperature T3 with T2>Ts.

[0116] 20. The method of embodiment 19, wherein liquid bottoms streams S10 of RC(1) having temperature T3 is passed through a reboiler, preferably a feed reboiler of RC(2), wherein the liquid bottoms stream S10 of RC(1) after having passed said feed reboiler has a temperature T4 with Ts>T4.

[0117] 21. The method of any one of embodiments 6 to 11, wherein RC(1) has in the range of from 1 to 15 theoretical stages, preferably in the range of from 5 to 10 theoretical stages.

[0118] 22. The method embodiment 21 , wherein adjusting the distillation in f) is done in that the liquid bottoms stream S10 from RC(1) having a temperature T1 is passed through a feed reboiler of RC(1 ), wherein the temperature of the liquid organic phase LOP obtained in e) or e.2) is increased so that liquid part-stream LOPiiquid and gaseous part-stream LOPgaSeous are obtained, wherein the liquid part-stream LOPiiqUid is introduced into RC(1 ), and wherein the liquid bottoms stream S10 from RC(1 ), after having passed the feed reboiler of RC(1) has a temperature T4 with T1 > T4. The method of any one of embodiments 6 to 11 , wherein the distillation in f) is done by distillation in a at least two rectification column RC(1) and RC(1 )-2, wherein the liquid bottoms stream S10 from rectification column RC(1) is, optionally after temperature increase and pressure decrease, fed into a further rectification column RC(1 )-2, and from RC(1)-2 a gaseous top stream comprising propylene and a liquid stream S10-1 comprising acetonitrile, propylene and water are obtained, wherein the liquid bottoms stream S10-1 from RC(1)-2 is depleted of propylene compared to the liquid stream S10 comprising acetonitrile, propylene and water from RC(1). The method of embodiment 23, wherein the liquid bottoms stream SlO from rectification column RC(1) having temperature Ti is heat exchanged with the liquid bottoms stream S10-1 from RC(1 )-2, the liquid bottoms stream S10 from rectification column RC(1) after said heat exchange having a temperature T5 with T1 < T5, and a pressure p, wherein said stream S10 is subsequently at least once decompressed, resulting in a liquid stream S10decomPhaving reduced pressure pred with pred < p and a gaseous stream, wherein liquid stream S10decomPis depleted of propene compared to stream S10 and is fed into the further rectification column RC(1 )-2, and the gaseous stream resulting from decompression is enriched in propene compared to stream S10. The method of embodiment 23 or 24, wherein the gaseous stream enriched in propene obtained from decompression of stream S10 and the gaseous top stream comprising propylene obtained from RC(1)-2 are combined, resulting in a combined stream comprising propene, which, optionally after pressure increase in at least one compressor, is introduced into the lower part of RC(1). The method of any one of embodiments 1 to 25, wherein the propene provided in a) comprises propene and propane, the mixture obtained in b) and the effluent stream S1 removed in c) comprising propane, the step d) comprising d.1 ) separating propene and propane from the effluent stream S1 removed in c) by washing and distillation, thereby obtaining a stream S2 enriched in propylene oxide, water and acetonitrile compared to the effluent stream removed in c); and a stream S3 enriched in propene and propane compared to the effluent stream removed in c). The method of any one of embodiments 1 to 26, step d) further comprising d.2) separating the stream S2 enriched in propylene oxide, water and acetonitrile obtained in d.1 ) by one or more distillations, thereby obtaining a stream S4, which comprises more than 99 wt.-% of propylene oxide based on the total weight of the stream S4, a streamS5 comprising acetonitrile and water and a stream S6 comprising propene; and / or, preferably and, d.3) separating propane from the stream S3 enriched in propene and propane obtained in d.1 ) in a separation zone, comprising subjecting the stream enriched in propene and propane obtained in d.1 ) to washing conditions in a scrubber, wherein a solvent mixture comprising acetonitrile and water is added as entraining agent, obtaining a liquid bottoms stream S7, which comprises acetonitrile, water and at least 70 weight-% of the propene comprised in the steam S3 enriched in propene and propane obtained in d.1 ); and a gaseous top stream S8, which comprises at least 5 weight-% of the propane comprised in the stream S3 enriched in propene and propane obtained in d.1). The method of embodiment 27, wherein d.2) comprises d.2.a) feeding the stream S2 enriched in propylene oxide, water and acetonitrile obtained in d.1) into a distillation tower, obtaining, preferably as bottom stream, a stream S5 comprising acetonitrile and water and being depleted of propylene oxide compared to the stream obtained in d) and a stream, preferably a top stream, enriched in propylene oxide compared to the stream obtained in d.1 ); d.2b) feeding the stream enriched in propylene oxide obtained in d.2a) into a further distillation tower, obtaining a stream S4 enriched in in propylene oxide compared to the stream obtained in d.2a), preferably as a side stream, said stream enriched in in propylene oxide comprising more than 99 wt.-%, preferably more than 99.9 wt.-%, more preferably more than 99.99 wt.-%, of propylene oxide based on the total weight of the stream S4. The method of embodiment 27, wherein d.3) comprises d.3.a) feeding stream S3 enriched in propene and propane obtained in d) and having a pressure ps3 into a compression unit, wherein the pressure is increased to ps3comPwith ps3comP> Ps3, followed by partial condensation, obtaining a condensed part stream S3COnd and a non-condensed part stream S3gase0Us of S3, condensed part stream S3COnd comprising propene and propane and the non-condensed part stream S3gase0Us comprising propene and propane. The method of embodiment 29, wherein d.3) further comprises d.3.b) feeding the non-condensed part stream S3gase0Us obtained in d.3.a) into a scrubber, optionally with addition of an inert gas, and subjecting S3gase0Us to washing conditions in the scrubber, wherein a solvent mixture comprising acetonitrile and water is introduced as entraining agent into the scrubber, preferably to the upper part of the scrubber; thereby obtaining a liquid bottoms stream S7, which comprises acetonitrile, water and propene; d.3.c) feeding the bottom stream comprising propene and acetonitrile into a distillation tower, from which a gaseous top stream S8 and a liquid bottoms stream S9 are obtained, wherein liquid bottoms stream S9 comprises acetonitrile and gaseous top stream S8 comprises at least 5 weight-% of the propane comprised in the stream S3 enriched in propene and propane obtained in d.1). The method of embodiment 1 to 30, wherein the normal run stage with propene epoxidation comprises:

[0119] A) providing propene, hydrogen peroxide, water and acetonitrile into the epoxidation zone comprising a heterogeneous epoxidation catalyst, wherein the molar amount of propene provided into the epoxidation zone is MAprOpene(nr) and wherein the molar amount of hydrogen peroxide provided into the epoxidation zone is MAn202(nr), with a molar ratio of MAprOpene(nr) : MAn202(nr) in the range of from 1.1 to 2.0, thereby obtaining a reaction mixture;

[0120] B) subjecting the reaction mixture comprising propene, hydrogen peroxide, water and acetonitrile obtained in A) to epoxidation reaction conditions in the epoxidation zone, thereby obtaining a mixture comprising propylene oxide, propene, water and acetonitrile;

[0121] C) removing an effluent stream from the epoxidation zone, the effluent stream comprising propylene oxide, propene, water and acetonitrile;

[0122] D) separating the effluent stream removed in C), optionally including one or more further work-ups, obtaining at least a stream comprising acetonitrile and water and a stream comprising propene;

[0123] E) mixing the stream comprising acetonitrile and water and at least the stream comprising propene obtained in D) and optionally a further stream comprising propylene, under phase separation conditions, thereby obtaining a liquid aqueous phase comprising water and a liquid organic phase comprising acetonitrile and propene;

[0124] F) subjecting the liquid phase comprising the propene obtained in E) to distillation, thereby obtaining a liquid bottoms stream comprising acetonitrile, propene and water and a gaseous top stream comprising propene, with a molar amount of propene in the liquid bottoms stream MAprOpene(lbs-nr);

[0125] G) recycling the liquid bottoms stream comprising acetonitrile and propene obtained in E) into A). The method of embodiment 31 , wherein the molar amount of propene provided in A) MAprOpene(nr) is the sum of the molar amount of propene in the liquid bottoms stream MAprOpene(lbs-nr) obtained in F) and recycled in G) into A) and the molar amount of propene freshly provided in A) MAprOpene(fresh)

[0126] MApropene(nr) — MApropene(lbS-nr) + MApropene(frOSh). The method of embodiment 32, wherein propene is freshly provided in A) so that the molar ratio of MAprOpene(fresh) to MAn202(nr) is

[0127] 0.97 < MAprOpene(fresh) / MAH202(nr) < 1 .03; and the molar ratio of MAprOpene(lbs-nr) to MAn202(nr) is 0.04< MApropene(lbs-nr) / MAH202(nr) < 1 .03, with the provision that 1.1 < MAprOpene(nr) / MAH202(nr) < 2.0. 34. A process for preparing propylene oxide, the process for preparing propylene oxide comprising at least a normal run stage with propene epoxidation and at least a stand-by stage without propene epoxidation, wherein the normal run stage comprises steps A) to G) of any one of embodiments 31 to 33 , and the stand-by stage comprises steps a) to g) and I) to ii) of any one of embodiments 1 to 30, the process further preferably comprising intermediate shut-down stage(s) after a normal run stage and before a stand-by stage and / or start-up stage(s) after a stand-by stage and before a normal run stage.

[0128] 35. Propylene oxide obtained or obtainable from the process of embodiment 34.

[0129] The present invention is further illustrated by the following reference examples, comparative examples, and examples.

[0130] Examples

[0131] For the purpose of illustrating the invention the different process options were simulated using the Aspen software suite, AspenONE, version V14 (company AspenTech) and physical data known in literature, taken from the Dortmunder Datenbank (DDB, version 2024). All the simulations were performed for production of 50 t / h of pure propylene oxide (PO, i.e. PO conforming to generally agreed specification for technical produced premium quality PO according to Chinese standard GB / T14491 -2015).

[0132] In all the cases the feed to the main reactor was identical. The state-of-the-art process was basically as described in WO 2017 / 140774A1 and WO2018 / 197234 A1. The conventional PP-splitter described sometimes in the state- of-the-art, while still perfectly possible, has been replaced by extractive distillation using acetonitrile as the entrained. Such PP-splitters are known from literature, and are known to be energetically advantageous, but are seldom used, because the pure propylene will always be contaminated to some extent with the extracting agent. But in this case, the extracting agent was identical with the solvent used in the process and thus the presence of traces of acetonitrile in the recovered propylene were not a problem. For this reason, it was chosen to use the extractive distillation to separate propylene from propane.

[0133] The specific values of pressure, temperature, composition, etc. the values used in the simulation used to illustrate the invention are by no means to be understood as limiting the invention. Reference is made to the process scheme in Fig. 1 (comparative) and Fig. 2 (inventive). When giving the composition of feed streams only the main components were given.

[0134] In the examples, low-pressure steam means a steam with a pressure of 4 bar gauge and a temperature of 152°C, medium-pressure steam means a steam with a pressure of 16 bar gauge and 201 °C and high-pressure steam means a steam with a pressure of 48 bar gauge and a temperature of 280°C. Comparative Example 1 - Process description according to a modified state of the art

[0135] The process is graphically shown in Fig. 1. A pre-mixed feed containing mainly propylene, hydrogen peroxide (HP, H2O2), water and additive (potassium formate, KFo) entered the main reactor R1 . Reactor R1 was a tube bundle reactor, which was filled with a fixed bed of an epoxidation catalyst, which was a molding comprising a ZnTIMWW prepared as described in WO 2018 / 197234 A1 , especially Reference Example 2. The composition of the liquid feed was chosen in such a way that the mixture under the conditions at which it was fed to the reactor was liquid and only one liquid phase was present. The feed preferably flowed in the reactor from bottom to top, to minimize the risk of gas-bubble entrapment in case of excessive HP decomposition.

[0136] A typical feed temperature was 30°C and the pressure was chosen in such a way, that even if up to 4% of hydrogen peroxide (HP, H2O2) decomposed to O2 this would remain in solution. Usually, a pressure of 23 bar was used. The weight-based ratio of propylene to H2O2 in the feed stream to the main reactor was controlled. Since technical propylene always contains some propane, there was also some propane in this stream with a ratio of propylene to propane of 98:2. The composition of the feed stream was set as follows in wt.-%: acetonitrile (64.1), hydrogen peroxide (7.2), propylene (12.7), water (14.9), potassium formate (0.023), propane (0.24). The main reactor itself was not modelled, but fixed conversions and selectivities were used. The selectivities were also fixed to match approximately what was observed experimentally.

[0137] The following selectivities based on H2O2 were used in all cases: Propylene oxide (PO) (97.45 wt.-%), mono propylene glycol (MPG) (0.6 wt.-%), O2 (0.55 wt.-%), hydroperoxypropanols (0.58 wt.-%, sum of both isomers), acetaldehyde (0.38 wt.-%), propionaldehyde (0.017 wt.-%), acetone (0.008 wt.-%). The difference to 100 wt.-% were polar high boilers, which left the system with the wastewater stream. These minor by-products were not relevant to the present invention. The conversion of H2O2 was set to 95%, based on the amount of H2O2 fed to R1 being 100%. The output of all the main reactor(s) R1 were mixed and after a temperature adjustment to 42°C fed to the finishing reactor R2 (pressure 22 bar). The composition of the feed to R2 was as follows in weight-%: acetonitrile (64.1), hydrogen peroxide (0.36), propylene (4.4), water (18.4), propylene oxide (11.3), propane (0.24), propylene glycol (0.16), oxygen (0.018).

[0138] The finishing reactor R2 was an adiabatic fixed bed reactor which was filled with the same catalyst as the main reactor. The finishing reactor again converted 95% of the H2O2 contained in its feed. The total conversion of H2O2 after both reactors was thus 99.75% based on the amount of H2O2 fed to R1 being 100%. The stream leaving the secondary reactor had the following composition in weight-%: acetonitrile (64.1), hydrogen peroxide (0.018), propylene (3.9), water (18.6), propylene oxide (11.9), propane (0.24), propylene glycol (0.16), oxygen (0.018). This temperature of the stream S1 leaving the secondary reactor was adjusted to 47°C and the pressure was reduced to 1.3 bar. This stream S1 was then mixed with other streams (liquid from the interstage condensers of C1 (marked as (2) in Fig. 1), non-condensables at the top of T4 (marked as (3) in Fig. 1), non-condensables at the top of T7 (marked as (4) in Fig. 1) and non-condensables at the top of T6 (marked as (6) in Fig. 1)). The mixed stream was then fed to the lower section of a combined washing / distillation tower T1, which was operated at a top pressure of 1 bar and a bottom temperature of 69°C. The composition of the feed stream was in weight-%: acetonitrile (63.9), hydrogen peroxide (0.018), propylene (4.0), water (18.6), propylene oxide (12.0), propane (0.25), propylene glycol (0.16), oxygen (0.025). As a washing liquid, a stream taken downstream of the tower T4 was cooled to about 10°C and fed to the upper part of the tower. The composition of the stream, which was taken as a washing liquid, in wt.-% was: acetonitrile (76.4), hydrogen peroxide (0.02), water (22.2), propylene glycol (0.19). The gaseous product at the top of tower T 1 had the following composition in wt.-%: acetonitrile (4.7), propylene (88.2), water (0.4), propane (5.6), oxygen (0.6).

[0139] The gaseous product (stream S3) from the top of tower T 1 was then fed to a multi-stage compressor C 1 , where it was compressed to 16.7 bar. Any condensate collected in the intermediate cooling between stages was returned to the feed of tower T 1 (marked as (2) in Fig.1 ). After the last compressor stage, the stream had a pressure of 16.7 bar and a temperature of 108°C. After cooling, the stream partly condensed. The condensed stream (stream S3cond), which contained in wt.-% propylene (87.9), propane (6.1), acetonitrile (5.0), was ted to the entrance of the decanter D1.

[0140] The condensation temperature was chosen in such a way that the non-condensed stream S3gase0Us, which contained propylene and O2 stayed well away from the explosible range. For the simulation, a condensation temperature of 38°C was set, leading to the following composition of the gas stream in wt.-%: propylene (89.7), propane (5.8), oxygen (2.0).

[0141] This stream S3gase0Us then went to the propene / propane separation section which used an extractive distillation setup (towers T2 and T3) with acetonitrile as solvent. The gas stream was first diluted with an inert gas, in this case a mixture of indicated in wt.-% N2 (80) and methane (20). The mixed stream was fed to the lower part of absorption tower T2, which was operated at 14 bar and a bottoms temperature of 80°C. To the top of the tower acetonitrile recycled from tower T2 was fed as extracting agent. The stream at the top of tower T2 (3470) had the following composition in weight-%: propane (13.7), propylene (3.2), 02 (9.5 vol%), N2 (54.3), methane (14.2), acetonitrile (1.0).

[0142] The bottoms of tower T2 contained the propylene dissolved in acetonitrile and had the following composition in weight-%: acetonitrile (89.7), propylene (9.5), propane (0.3). The stream S7 was then fed to tower T3, which was operated at 12.5 bar and a bottoms temperature of 185°C to recover the propylene over top and the acetonitrile as the bottoms stream. The bottoms stream S9 contained acetonitrile and a trace of propylene. Before it was fed back to tower T2 make-up acetonitrile was added as required to compensate for the acetonitrile leaving the section with the overhead product.

[0143] The overhead product of tower T3 (stream S8) was then cooled down and condensed at 28°C. The uncondensable stream, which still contained in wt.-% mainly propylene (95.0), with some propane (2.7) and CO2 (1 .5) was taken off and was disposed by incineration to avoid build-up of CO2. The condensed product separated into two liquid phases, of which the lower phase consisted in wt.-% mostly of water (99.0). This was disposed in the wastewater treatment system. The upper phase was partly returned to the tower as reflux (reflux ratio = 1 .3) and the remainder was fed to the entrance of the decanter D1 . The composition of this stream was as follows in wt.-%: propylene (96.0), propane (3.0), acetonitrile (0.5). Going back to tower T1 , the bottoms product of this tower, stream S2, had the following composition in wt.-%: acetonitrile (69.0), water (20.0), propylene oxide (9.7), propylene glycol (0.2) and traces of other by-products like acetaldehyde, propionaldehyde, acetone, formaldehyde hydroperoxypropanols, and others in very small amounts (each was less than 0.1). This stream was mixed with the bottoms product of tower T5 (see below) and fed to tower T4. Tower T4 had 99 theoretical separation stages and was operated at a pressure of 0.5 bar and a bottom temperature of 61 °C and a reflux ratio of 0.8. Feeding point was at stage 55 counted from the top. At stage 43 (counted from the top) there was a side draw to remove a very small stream of intermediate boilers (shown as IB in Fig. 1). This stream had the following composition in wt.-%: propylene oxide (77.7), propionaldehyde (17.8), acetonitrile (3.3), water (1.2) and traces of acetone. This stream was discarded.

[0144] The top stream of tower T4 was feed to a vapor recompression, which also provided most of the energy for the sump evaporator. The top product had the following composition in wt.-%: propylene oxide (99.79), acetaldehyde (0.15) and propylene (0.04).

[0145] This stream was fed to tower T5 with 54 theoretical separation stages, operated at 2.9 bar and a bottoms temperature of 68.5°C. Feed point was at stage 21 (counted from the top) and pure propylene oxide was taken as a side draw at stage 52. The obtained propylene oxide in stream S4 had a purity of 99.9985%. The bottoms product, which still contains mostly pure PO was fed back to tower T4. The top product of tower T5 (marked as LB for light boilers in Fig. 1), which consisted in wt.-% of propylene oxide (27.2), propylene (13.4), acetaldehyde (50.3) and formaldehyde (8.8) was disposed by incineration.

[0146] The bottoms product of tower T4, stream S5, had the following composition in wt.-%: acetonitrile (76.4), water

[0147] (22.2), propylene glycol (0.2), hydroperoxypropanols (0.1), hydrogen peroxide (ca. 200 ppm) but essentially no propylene oxide. If required a part of this stream (usually less than 10%) can be detoured and passed to a purification tower to remove unpolar high boilers, like those usually contained as an impurity in technical hydrogen peroxide solutions and stemming from the working solution used in the H2O2 process. Examples of such compounds are for instance trioctyl phosphate, tetrabutyl urea, 2-methylcyclohexyl acetate, diisobutyl carbinol, C9-C10 aromatics hydrocarbons, or degradation and oxidation products therefrom. For this simulation, this tower was not included, and it is also not shown in Fig. 1 . Part of this stream was taken off and cooled to 10°C to be used as washing solvent in tower T1 (marked as (1) in Fig. 1). The remainder was combined with the liquid stream at the top of tower T7 (marked as (5) in Fig. 1) and fed to the decanter D1. The condensate stream after compressor C1, the recovered propylene from the top of tower T3, the liquid top product of tower T6 and the fresh polymer grade propylene, all of which consisted mainly of propylene were mixed to obtain a stream with the following composition in wt.-%: propylene (74.1), propane (22.4), acetonitrile (1.2) and water (0.4). The stream was also fed to the decanter D1 . Decanter D1 was operated at 15°C and 16 bar. The mixed feed of decanter D1 separated into two liquid phases which were removed separately from the decanted.

[0148] The lower liquid aqueous phase LAP taken from decanter D1 had the following composition in wt.-%: water

[0149] (78.3), acetonitrile (19.9), propylene glycol (0.4), hydroperoxypropanols (0.25) and other polar by-products. This stream LAP was fed to tower T7, which was operated at 1 .5 bar and a sump temperature of 116°C. In this tower acetonitrile was recovered at the top as azeotrope with water with the following composition in wt.-%: acetonitrile (78.5), water (19.7) and returned to the decanter D1 (marked as (5) in Fig. 1). Uncondensable material at the top of the tower (marked as (4) in Fig. 1) is returned to the feed side of tower T 1 . The bottoms product of T7 contained in wt.-% water (98.7), propylene glycol (0.6), hydroperoxypropanol (0.3) and potassium formate (0.14), along with other polar by-products. This stream was cooled down and sent to the wastewater treatment station. The upper liquid organic phase LOP from decanter D1 had the following composition: propylene (35.6), propane (10.7), acetonitrile (49.4), water (2.6). This was fed to tower T6, which had 5 theoretical separation stages, and was operated at 19 bar. The bottoms product of this tower, stream S10, was the recycled acetonitrile stream, which was fed back to the reaction stage. The conditions in the tower (i.e. the energy input to the sump and associated sump temperature) were chosen in such a way that the amount of propylene required for the reaction remained dissolved in the sump stream. When the sump temperature of T6 was set to 100.5°C the sump, i.e. stream S10, had the following composition in wt.-%: acetonitrile (78.7), water (3.9), propylene (15.6). Enough fresh acetonitrile was added through the stream marked ACN in Fig. 1 to compensate for the losses of acetonitrile in streams leaving the process. This stream S10 was then recycled back to the reactor, if necessary, acetonitrile as solvent make-up was added and 40% hydrogen peroxide solution and a 2.5% aqueous solution of potassium formate (w / w) were added to generate the reactor feed described at the beginning.

[0150] In steady state operation this way of operating the process was energy efficient and only required one dosing for fresh propylene even when several main reactors are used. However, when there was a disturbance to the system, feeds had to be stopped, especially the H2O2 feed. In such a case it was also required that the reactors be flushed of reactive components like H2O2 or propylene oxide for safety reasons and not to damage the catalyst. However, with the above-described setup problems arose: If the H2O2 feed had to be stopped, for what reason ever, it was not possible to also shut off the feed of propylene, because this was coming back to the reactor with the acetonitrile, which was required to flush the reactor free of H2O2 and propylene oxide. But if the H2O2 was not present to consume the propylene, it went completely unconverted to tower T 1 and to compressor C1, which inevitably tripped, due to receiving a feed stream which was, depending on the propylene excess used, a multiple of the feed amount for which it was designed. This then caused the complete to plant to trip and restarting it took a long time and led to production losses due to downtime.

[0151] For this comparative example the specific steam consumption per ton of PO produced was 1 .83 tons of low- pressure steam and 0.25 tons of high-pressure steam (in sum 2.08 tons of steam / ton of PO).

[0152] Reference Example 1

[0153] The process was carried out as described above for Comparative Example 1 and is basically shown in Fig. 2, with the following deviations:

[0154] The first change was in tower T6 and its mode of operation. Instead of leaving in the sump the entire amount of propylene required for the main reactors, only the amount of propylene, which could be processed by compressor C1 was left in the bottoms stream. This usually (but not necessarily) corresponded approximately to the excess of propylene used in the reaction. There were several ways to achieve this, and these were explained below in the specific examples.

[0155] The second change was that fresh propylene was not added upflow of the decanter D1 as described above, but it was dosed upflow of the reactors. Preferably each of the R1 reactors had its own propylene, H2O2 and potassium formate dosing unit. This allowed more flexibility to the process, because each reactor could now be run independently at its own optimal load according to its state of deactivation.

[0156] If now a disturbance occurred and the feed of H2O2 had to be interrupted, the feeds of propylene and potassium formate could also be closed. The solvent loop could continue running to flush the reactors of reactive components. But since the amount of propylene carried by this stream fitted to the regular load of compressor C1 , the compressor stayed within its load limits and did not trip. This meant the entire separation train could continue to operate in a stand-by mode by setting towers T4 and T5 to full reflux and closing the offtake at the sump of T7. This way, once the problem had been resolved the plant could quickly be restarted.

[0157] Example 1 - Operation mode 1

[0158] The process was carried out as in Reference Example 1 , wherein a tower T6 with 13 theoretical separation stages was used (option 1 - modified rectification column RC(1 ), T6).

[0159] The operation pressure remained the same (19 bar) and the composition of the feed stream and the overhead stream also remained unchanged. To reduce the amount of propylene in the sump of tower T6, and thus consequently in stream S10, the sump temperature was raised to 167°C so that more propylene then in Comparative Example 1 left tower T6 via the top. To minimize the energy consumption an intermediate reboiler was installed at tray 8 counted from the top (which was halfway between the tower feed point and the tower bottoms) and this was heated by heat exchange with the hot sump stream. After the heat exchange in the intermediate reboiler, the sump stream still had a temperature of 75°C and could still be used to pre-heat the feed stream of tower T6. After this heat exchange the temperature of the sump stream was adjust to 30°C, complemented with make-up acetonitrile as required so as to have a composition in wt.-%: acetonitrile (88.6), water (4.4), propylene (5.4), and returned to the main reactors. After addition of fresh propylene, H2O2 and formate, the composition of the stream entering the reactors R1 was the same as in Comparative Example 1 .

[0160] Using this set up the specific steam consumption per ton of PC produced was now 1 .76 tons of low-pressure steam and 0.78 tons of high-pressure steam (in sum 2.54 tons of steam / ton of PC).

[0161] Example 2 - Operation mode 2

[0162] The process was carried out as in Reference Example 1 , wherein tower T6 had now only 8 theoretical separation stages but was still operated at 19 bar (option 4 - heating feed of REC(1), T6). The tower feed stream was first pre-evaporated at a temperature of 45°C to give a liquid and a gaseous stream. The liquid stream was then heat exchanged with the hot tower sump leading to a further partial evaporation. Both gaseous streams were jointly fed to the tower at stage 4. The liquid stream, which had now a temperature of 88°C was also fed to the tower at the same point as the steam.

[0163] After pre-heating the feed the hot sump stream of tower T6, i.e. stream S10, had a temperature of 67°C. It was then cooled down to the required temperature of 30°C, complemented with make-up acetonitrile as required so as to have a composition in wt.-%: acetonitrile (88.6), water (4.4), propylene (5.4), and returned to the main reactors. After addition of fresh propylene, H2O2 and potassium formate, the composition of the stream entering the reactors R1 was the same as in Comparative Example 1 .

[0164] Using this setup the specific consumption of steam per ton of PC produced was now 1 .23 tons of low-pressure steam and 0.98 tons of high-pressure steam (in total 2.21 tons of steam).

[0165] Example 3 - Operation mode 3

[0166] Examples 1 and 2 (operation modes 1 and 2) required the use of high-pressure steam to heat the sump of tower T6. Since this was the only point of the process where high pressure steam was required, an alternative operation mode was devised which does not require the use of high-pressure stream. This operation mode used an additional tower T6.1 and a compressor C2 (option 5 - with additional rectification column RC(1)-2 (T6.1) and compressor).

[0167] In this operation mode the sump stream of the conventional tower T6, i.e. stream S10, containing acetonitrile in weight-% (78.8), water (4.1) and propylene (13.9) at 91 °C and 19 bar was heated to 117°C by heat exchange with the hot sump of an additional tower T6.1. Stream S10 was then decompressed to 16.2 bar, where part of the stream evaporated. The liquid was further decompressed to 13.5 bar and again partly evaporated. The liquid was then fed to the top of tower T6.1 , which contained only the stripping section. The tower T6.1 was operated at 13.5 bar and a bottoms temperature of 132°C. The sump stream S10-1 from tower T6.1 had the desired composition in weight- % of acetonitrile (88.6), water (4.4), propylene (5.4). It was first heat exchanged with the hot sump of tower T6.1 as mentioned above, before stream S10-1 was recycled to R1 . The gaseous products from the two pressure reduction steps and the top product of tower T6.1 , which consisted mainly of propylene, were then mixed and fed to compressor C2 to increase the pressure to 20 bar. The compressed stream was then re-fed to the lower section of tower T6. Stream S10-1 had to be cooled down to the required temperature of 30°C, complemented with make-up acetonitrile as required so as to have a composition in wt.-%: acetonitrile (88.6), water (4.4), propylene (5.4), and returned to the main reactors. After addition of fresh propylene, H2O2 and potassium formate, the composition of the stream entering the reactors R1 was the same as in Comparative Example 1.

[0168] Using this setup the specific consumption of steam per ton of PC produced was now 2.02 tons of low-pressure steam, 0.11 tons medium-pressure steam (in total 2.13 tons of steam) and additionally 4.82 kWh of electricity for the additional compressor. Summary:

[0169] Table 1

[0170] Results of Comparative and Inventive Examples in view of energetic demand

[0171] The above Inventive Examples 1, 2 and 3 and the Comparative Example 1 show, that it is possible with very few significant modifications to shift the position where propylene is dosed into the plant (omitting a dosing upstream of the decanter D1 and allowing a controlled dosing upstream of the reactor R1) and thus allow for the plant to be easily shifted from a production mode to a stand-by mode without incurring in a severe penalty (energywise in the production mode, in the best case it was only 2% extra steam consumption).

[0172] Short description of the Figure

[0173] Fig. 1 shows a comparative process with propylene addition prior to decanter D1 as used in Comparative Example 1 .

[0174] Fig. 2 shows an overall process without addition of propylene prior to decanter D1 as used in Inventive Examples 1-3, wherein the individual modifications of Inventive Examples 1, 2 and 3 are not graphically depicted.

[0175] Cited Literature

[0176] - WO 2022 / 268859 A1

[0177] - WO 2011 / 006990 A1

[0178] - WO 2018 / 015435 A1

[0179] - EP 1 122 249 A1

[0180] - EP 1 122 249 A1

[0181] - WO 2015 / 049327 A1 - WO 2017 / 140774 A1

Claims

1. Claims1 . A method for controlling a stand-by stage of a process for preparing propylene oxide, the process for preparing propylene oxide comprising a normal run stage with propene epoxidation and a stand-by stage without propene epoxidation, the stand-by stage comprising: a) providing propene, water and acetonitrile into an epoxidation zone comprising a heterogeneous epoxidation catalyst, thereby obtaining a mixture comprising propene, water and acetonitrile; b) conducting the mixture comprising propene, water and acetonitrile through the epoxidation zone; c) removing an effluent stream S1 from the epoxidation zone, the effluent stream S1 comprising propene, water and acetonitrile; d) separating the effluent stream S1 removed in c), optionally including one or more further work-up steps, obtaining at least a stream comprising propene S3COnd and a stream S5 comprising acetonitrile and water; e) mixing the stream S5 comprising acetonitrile and water obtained in d) and at least a stream comprising propene under phase separation conditions, thereby obtaining a liquid aqueous phase LAP comprising water and acetonitrile and a liquid organic phase LOP comprising acetonitrile and propene, f) subjecting the liquid organic phase LOP comprising acetonitrile and propene obtained in e) to distillation, thereby obtaining a gaseous top stream S9 comprising propene and a liquid bottoms stream S10 comprising acetonitrile and propene, with a molar amount of propene in the liquid bottoms stream S10 MAprOpene(lbs-sb); g) recycling the liquid bottoms stream S10 comprising acetonitrile and propene obtained in f) into a); wherein the method for controlling the stand-by stage comprises:(i) defining, based on a molar amount of hydrogen peroxide provided into the epoxidation zone during a normal run stage MAn202(nr) a molar ratio MAProPene(lbs-sb) / MAH202(nr) in the range of from 0.02 to 1.03;(ii) at least periodically determining MApropene(lbs-sb); and(ii.a) if MApropene(lbs-sb) / MAH202(nr) is > 1 .03, adjusting the distillation in f) so that a molar ratio MApropene(lbs-sb) / MAn202(nr) in the range of from 0.02 to 1.03 is achieved,(ii.b) if MApropene(lbs-sb) / MAH202(nr) is in the range of from 0.02 to 1 .03, maintaining the distillation in f).

2. The method of claim 1 , wherein the stream comprising propene, which is mixed with the stream comprising acetonitrile and water in e), comprises at least a part of the propene from the stream S3COnd comprising propene obtained in d), at least a part of the propene obtained in f).

3. The method of claim 1 or 2, wherein e) comprisese.1 ) adding at least a part of the gaseous top stream S9 comprising propene obtained in f) to the stream comprising propene S3COnd obtained in d), thereby obtaining a mixed propene containing stream; e.2) mixing the stream S5 comprising acetonitrile and water obtained in d) and the mixed propene containing stream of e.1) under phase separation conditions, thereby obtaining a liquid aqueous phase LAP comprising water and a liquid organic phase LOP comprising acetonitrile and propene.

4. The method of any one of claims 1 to 3, wherein the distillation in f) is done by distillation in a at least one rectification column RC(1), wherein a liquid stream S10 comprising acetonitrile, propene and water is removed from the bottom of RC(1) and a gaseous top stream S9 comprising propene is removed from the top of RC(1).

5. The method of claim 4, wherein RC(1) is operated at a bottoms temperature TRC(i)(bottoms) in the range of from 70 to 140 °C, preferably in the range of from 80 to 120 °C .

6. The method of claim 4, wherein the liquid aqueous phase LAP comprising water and acetonitrile obtained in e) or e.2) is subjected to distillation in at least one rectification column RC(2), wherein a top stream enriched in acetonitrile compared to the liquid aqueous phase comprising water and acetonitrile obtained in e) or e.2) is obtained and a bottom stream enriched in water compared to the liquid aqueous phase comprising water and acetonitrile obtained in e) or e.2) is obtained.

7. The method of claim 4, wherein adjusting the distillation in f) is done in that RC(1 ) is operated at a bottom temperature TRC(i)(bottoms)-2, wherein TRC(1) (bottoms)-2 > TRC(1) (bottoms), preferably 1.5 x TRC(i)(bottoms) < TRC(i)(bottoms)-2 < 2.0 x TRc<i)(bottoms).

8. The method of claim 4, wherein adjusting the distillation in f) is done in that a part of the of the liquid bottoms stream S10 of RC(1 ) having a temperature T2, is passed through a reboiler, preferably an intermediate reboiler, of RC(2), wherein the liquid bottoms stream S10 of RC(1 ) after having passed said reboiler of RC(2) has a temperature T3 with T2>Ts.

9. The method of claim 8, wherein liquid bottoms streams S10 of RC(1 ) having temperature T3 is passed through a reboiler, preferably a feed reboiler of RC(2), wherein the liquid bottoms stream S10 of RC(1 ) after having passed said feed reboiler has a temperature T4 with Ts>T4.

10. The method of claim 4, wherein RC(1 ) has in the range of from 1 to 15 theoretical stages, preferably in the range of from 5 to 10 theoretical stages.11 . The method of claim 4, wherein the distillation in f) is done by distillation in a at least two rectification column RC(1 ) and RC(1 )-2, wherein the liquid bottoms stream S10 from rectification column RC(1) is, optionally after temperature increase and pressure decrease, fed into a further rectification column RC(1)- 2, and from RC(1)-2 a gaseous top stream comprising propylene and a liquid stream S10-1 comprising acetonitrile, propylene and water are obtained, wherein the liquid bottoms stream S10-1 from RC(1)-2 is depleted of propylene compared to the liquid stream S10 comprising acetonitrile, propylene and water from RC(1).

12. The method of claim 1 to 11 , wherein the normal run stage with propene epoxidation comprises:A) providing propene, hydrogen peroxide, water and acetonitrile into the epoxidation zone comprising a heterogeneous epoxidation catalyst, wherein the molar amount of propene provided into the epoxidation zone is MAprOpene(nr) and wherein the molar amount of hydrogen peroxide provided into the epoxidation zone is MAn202(nr), with a molar ratio of MAprOpene(nr) : MAn202(nr) in the range of from 1.1 to 2.0, thereby obtaining a reaction mixture;B) subjecting the reaction mixture comprising propene, hydrogen peroxide, water and acetonitrile obtained in A) to epoxidation reaction conditions in the epoxidation zone, thereby obtaining a mixture comprising propylene oxide, propene, water and acetonitrile;C) removing an effluent stream from the epoxidation zone, the effluent stream comprising propylene oxide, propene, water and acetonitrile;D) separating the effluent stream removed in C), optionally including one or more further work-ups, obtaining at least a stream comprising acetonitrile and water and a stream comprising propene;E) mixing the stream comprising acetonitrile and water and at least the stream comprising propene obtained in D) and optionally a further stream comprising propylene, under phase separation conditions, thereby obtaining a liquid aqueous phase comprising water and a liquid organic phase comprising acetonitrile and propene;F) subjecting the liquid phase comprising the propene obtained in E) to distillation, thereby obtaining a liquid bottoms stream comprising acetonitrile, propene and water and a gaseous top stream comprising propene, with a molar amount of propene in the liquid bottoms stream MAprOpene(lbs-nr);G) recycling the liquid bottoms stream comprising acetonitrile and propene obtained in E) into A).

13. The method of claim 12, wherein the molar amount of propene provided in A) MAprOpene(nr) is the sum of the molar amount of propene in the liquid bottoms stream MAprOpene(lbs-nr) obtained in F) and recycled in G) into A) and the molar amount of propene freshly provided in A) MAprOpene(fresh):MApropene(nr) — MApropene(lbS-nr) + MApropene(frOSh), and / or, preferably and, wherein propene is preferably freshly provided in A) so that the molar ratio of MAprOpene(fresh) to MAn202(nr) is 0.97 < MAprOpene(fresh) / MAH202(nr) < 1 .03; andthe molar ratio of MAprOpene(lbs-nr) to MAn202(nr) is0.04< MApropene(lbs-nr) / MAH202(nr) < 1 .03, with the provision that 1.1 < MAprOpene(nr) / MAH202(nr) < 2.

0.

14. A process for preparing propylene oxide, the process for preparing propylene oxide comprising at least a normal run stage with propene epoxidation and at least a stand-by stage without propene epoxidation, wherein the normal run stage comprises steps A) to G) of claim 12 or 13, and the stand-by stage comprises steps a) to g) and i) to ii) of any one of claims 1 to 11 , the process further preferably comprising intermediate shut-down stage(s) after a normal run stage and before a stand-by stage and / or start-up stage(s) after a stand-by stage and before a normal run stage.

15. Propylene oxide obtained or obtainable from the process of claim 14.

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