MXPA06015144A - Separation of propylene oxide from a mixture comprising propylene oxide and methanol - Google Patents

Abstract

A method of separating propylene oxide from a mixture (M) comprising 5 to 50 percent by weight propylene oxide and 50 to 85 percent by weight methanol, said method comprising (i) introducing said mixture (M) into an extractive distillation column;(ii) additionally introducing a polar solvent into said extractive distillation column;(iii) distilling propylene oxide overhead from said extractive distillation column at a bottoms temperature of from 40 to 70 C and at a pressure of from 300 to 750 mbar.

Description

SEPARATION OF PROPYLENE OXIDE FROM A MIXTURE COMPRISING PROPYLENE OXIDE AND METHANOL FIELD OF THE INVENTION The present invention relates to a method of separating propylene oxide from a mixture (M) comprising from 5 to 50% percent by weight of propylene oxide and from 50 to 85% by weight of methanol. This method comprises (i) introducing said mixture (M) into an extractive distillation column; (ii) additionally introducing a polar solvent into said extractive distillation column; and (iii) distilling the propylene oxide leaving the top of said extractive distillation column at a bottom temperature of 40 to 70 ° C and a pressure of 300 to 750 mbar. According to a preferred embodiment of the present invention , the mixture (M) is formed by a reaction of propene with hydrogen peroxide in methanol as a solvent and in the presence of a fixed-bed catalyst of a titanium zeolite. In addition, the present invention also provides a method of preparing propylene oxide, wherein said reaction comprises the reaction of propene with hydrogen peroxide in methanol as a solvent and in the presence of a fixed-bed catalyst of a titanium zeolite. This The epoxidation reaction results either directly or after at least one treatment step, in a mixture (M) comprising from 5 to 50 weight percent of propylene oxide and from 50 to 85 weight percent of methanol and the method of the present invention further comprises (i) introducing said mixture (M) into an extractive distillation column; (ii) additionally introducing a polar solvent into said extractive distillation column; and (iii) distilling the propylene oxide leaving the top of said extractive distillation column at a bottom temperature of 40 to 70 ° C and at a pressure of 300 to 750 mbar. BACKGROUND OF THE INVENTION At atmospheric pressure or superatmospheric pressures, essentially in the range of 1 to 5 bar, the propylene oxide and methanol can be separated by distillation only when a distillation column having a very large number of theoretical plates and is used. when at the same time a very high reflux ratio is established due to the azeotrope entrainment. These mixtures comprising propylene oxide and methanol result, for example, from the epoxidation processes where the Propene reacts with a hydroperoxide such as hydrogen peroxide in the presence of methanol as the solvent. US Patent 5,849,938 discloses a process wherein the propene is separated from methanol in an epoxidation product of crude olefins by means of an extractive distillation wherein a relatively heavy polar solvent having hydroxy groups such as water or propylene glycol is used as the Extraction solvent, propylene glycol is particularly preferred. According to this prior art document, the ordinary distillation column has from 20 to 60 theoretical plates and the reflux / distillate ratio is generally in the range of 5 to 15. According to the examples, a typical ratio is 9. Typical bottom temperatures are in the range of 90 to 120 ° C, the pressure under which the distillation is carried out is 0.55 to 3.44 bar. According to the example, a preferred bottom pressure of the distillation column is 2.76 bar and therefore it is above the standard pressure. As typical propylene oxide fractions, the fractions are obtained comprising 300 to 1500 ppm methanol. The fractions of the bottoms obtained according to the examples comprise more than 6,300 ppm of propylene oxide. U.S. Patent 6,500,311 Bl discloses a process where a separation of methanol and propylene. As the extraction solvent, a non-polar solvent, mainly a C7-C9 hydrocarbon such as n-octane, is used. It is an object of the present invention to provide a method of separating propylene oxide from methanol, which, compared to the processes described in the prior art, has an improved energy balance and additionally, leads to higher currents and bottom currents to have a lower degree of impurity with respect to methanol and propylene oxide respectively . It is a further object of the present invention to provide a method of separating propylene oxide from methanol in which an inexpensive extraction solvent is employed which simultaneously permits milder distillation conditions than those described in the prior art. It is another object of the present invention to provide a method of producing ethylene oxide in the course in which propylene oxide is separated from methanol where this separation has the aforementioned advantages thus giving the method to energetically produce the oxide. of propylene and also with respect to the purity of the advantageous distillation fractions on the prior art. BRIEF DESCRIPTION OF THE INVENTION The present invention relates to a method of separating propylene oxide from a mixture (M) comprising 5 to 50 weight percent propylene oxide and 50 to 85 weight percent methanol, said method comprises (i) introducing said mixture (M) into an extractive distillation column; (ii) additionally introducing a polar solvent into said extractive distillation column; (iii) distilling the propylene oxide leaving the top of said extractive distillation column at a bottom temperature of 40 to 70 ° C and at a pressure of 300 to 750 mbar. The present invention relates to a method for separating propylene oxide from a mixture (M) comprising 5 to 15 weight percent propylene oxide, 50 to 85 weight percent methanol and 10 to 25 weight percent. water weight, said method comprises (i) introducing said mixture (M) into an extractive distillation column; (ii) additionally introducing water into said extractive distillation column in an amount of 2 weight percent of the mixture (M) or less; (iii) distilling the propylene oxide that exits from the top of said extractive distillation column to a bottom temperature of 40 to 70 ° C and a pressure of 300 to 750 mbar. The present invention relates to a method for separating propylene oxide from a mixture (M) comprising 5 to 15 weight percent propylene oxide, 50 to 85 weight percent methanol and 10 to 25 weight percent. water weight, said method comprises (i) introducing said mixture (M) into an extractive distillation column; (ii) additionally introducing water in said distillation; (iii) distilling the propylene oxide leaving the top of said extractive distillation column at a bottom temperature of 40 to 70 ° C and at a pressure of 300 to 750 mbar. wherein the distillate is partially refluxed in said extractive distillation column and wherein the proportion of mass reflux to distillate is less than or equal to 4. The present invention relates to a method of separating propylene oxide from a mixture (M) comprising 5 to 15 weight percent propylene oxide, 50 to 85 weight percent methanol and 10 to 25 weight percent water, said method comprises (i) introducing said mixture (M) into an extractive distillation column; (ii) additionally introducing water into said extractive distillation; (iii) distilling the propylene oxide leaving the top of said extractive distillation column at a bottom temperature of 40 to 70 ° C and at a pressure of 300 to 750 mbar. (iv) extracting a bottom stream from said extractive distillation column, said bottom stream comprising 100 ppm propylene oxide or less, and separating an overhead stream from said distillation column, said top stream comprising 100 ppm methanol or less . The present invention relates to a method for separating propylene oxide from a mixture (M) comprising 5 to 15 weight percent propylene oxide, 50 to 85 weight percent methanol and 10 to 25 weight percent. water weight, said method comprises (i) introducing said mixture (M) into an extractive distillation column; (ii) additionally introducing water as steam at a pressure of not more than 2 bar in said extractive distillation column; (iii) distilling the propylene oxide leaving the top of said extractive distillation column at a bottom temperature of 40 to 70 ° C and at a pressure of 300 to 750 mbar. The present invention relates to a method of removing propylene oxide from a mixture (M) comprising 5 to 15 weight percent propylene oxide, 50 to 85 weight percent methanol and 10 to 25 weight percent in that of water, said method comprises (i) introducing said mixture (M) into an extractive distillation column; (Ii) additionally introducing water into said extractive distillation column in an amount of 2 percent by weight of the mixture or less; (iii) distilling the propylene oxide leaving the top of said extractive distillation column at a bottom temperature of 40 to 70 ° C and at a pressure of 300 to 500 mbar. (iv) extracting a bottom stream from said extractive distillation column, said bottom stream comprising 100 ppm propylene oxide or less, and separating an overhead stream from said extractive distillation column, said top stream comprising 10 ppm methanol or less.

The present invention relates to a method of preparing propylene oxide, said reaction comprises the reaction of propene with a hydroperoxide in methanol as a solvent, said reaction results in a mixture (M) comprising 5 to 15 weight percent of oxide of propylene, 50 to 85 percent by weight of methanol and 10 to 25 percent by weight of water or results in a mixture that is treated to give said mixture (M), said method comprising (i) introducing said mixture (M) in an extractive distillation column; (ii) additionally introducing water as steam at a pressure of not more than 2 bar in said extractive distillation column in an amount of 0.45 to 1 weight percent of the mixture (M); (iii) distilling the propylene oxide leaving the top of said extractive distillation column as an overhead stream at a pressure of 450 to 500 mbar and at a bottom temperature of from 50 to 60 ° C; (iv) extracting a bottom stream from said extractive distillation column, said bottom stream comprising 100 ppm propylene oxide or less, and separating an overhead stream from said distillation column, said top stream comprising 50 ppm methanol or less.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is provided which provides a diagram showing a preferred embodiment of the present invention, FIG. 2 providing a diagram showing a prior art process, FIG. 3 providing a diagram showing another process of prior art. DETAILED DESCRIPTION OF THE INVENTION According to the present invention, the propylene oxide is separated from a mixture (M) comprising 5 to 50 weight percent propylene oxide and 50 to 85 weight percent methanol. In a preferred embodiment of the present invention, the mixture (M) comprises from 5 to 15 weight percent, preferably from 6 to 12 weight percent and particularly preferably from 8 to 10.5 weight percent propylene oxide and from 55 to 85 weight percent, preferably from 60 to 80 weight percent and particularly preferably from 65 to 75 weight percent methanol. With respect to the method of the present invention, the mixture (M) may comprise one or more additional compounds. According to these compounds, there are no specific restrictions on the condition in which the propane oxide can be distilled out of the top of the column of extractive distillation in order to separate the propylene oxide from the methanol included in (M). According to a preferred embodiment, the mixture (M) additionally comprises water, more preferably water in an amount of more than 25, more preferably from 1 to 25, more preferably from 2 to 25, more preferably from 3 to 25 percent in weight, more preferably from 4 to 25, more preferably from 5 to 25, more preferably from 6 to 25, more preferably from 7 to 25, more preferably from 8 to 25, more preferably from 9 to 25 and even more preferably from 10 to 25 25 percent by weight of water, based on the total weight of the mixture (M). Therefore, the mixture (M) for example can comprise from 10 to 25 or from 10 to 20 or from 10 to 15 or from 15 to 25 or from 15 to 20 or from 20 to 25 weight percent of water, in based on the total weight of the mixture (M). According to a preferred embodiment wherein (M) comprises 10 to 25 weight percent water, (M) preferably comprises 5 to 45, more preferably 5 to 40, more preferably 5 to 35, more preferably 5 to 30, more preferably from 5 to 25, more preferably from 5 to 20 and even more preferably from 5 to 15 weight percent of propylene oxide, based on the total weight of the mixture (M). Therefore, the present invention also provides a method as described above wherein the mixture (M) comprises from 50 to 85 percent by weight of methanol, from 5 to 15 percent by weight of propylene oxide, and from 10 to 25 percent by weight of water, based on the total weight of the mixture (M) ). In addition to methanol and propylene oxide and, preferably water, the mixture () can comprise at least one more compound. According to a preferred embodiment of the present invention, the mixture (M) results directly or indirectly from a process where the propylene oxide is prepared by the reaction of propene with a hydroperoxide in the presence of methanol as a solvent. Therefore, the mixture (M) can additionally comprise propene which did not react and / or hydroperoxide which did not react and / or at least one by-product of said epoxidation reaction such as propylene glycol and / or acetaldehyde. The reaction mixture obtained from said epoxidation reaction can be introduced directly into (i) as mixture (M) if the content of (M) with respect to methanol, propylene oxide and preferably water is within the ranges mentioned above. According to an especially preferred embodiment of the present invention, the reaction mixture obtained from said epoxidation reaction is treated before the introduction in (i) of the inventive method. The treatment of the reaction mixture obtained from said epoxidation reaction can be carried out in every conceivable manner in the condition where a mixture (M) which can be introduced into (i) is obtained. Said treatment may comprise the separation and / or addition of at least one compound of and / or to the mixture obtained from the epoxidation reaction. Preferably, at least one compound is separated from the mixture obtained from the epoxidation reaction. According to an even more preferred embodiment of the present invention, at least one compound is separated from the mixture obtained during the treatment of the epoxidation reaction, said at least one compound having a boiling point lower than that of the oxide of propylene, methanol and preferably water. Depending on the reaction conditions applied and the reagents used for the epoxidation reaction, these low-boiling components can be, for example, unreacted propene, and / or propane which is then introduced, for example, into the reaction of epoxidation in case, for example, chemical grade propene is used as reagent having a propene: propene volume ratio of from about 99.5: 0.5 to 94: 6.

According to an even more preferred embodiment of the present invention, the unreacted propene is separated from the reaction mixture obtained from the epoxidation reaction in at least one distillation column, and the high boiling fraction whose respective content with reference to methanol, propylene oxide and water is within the ranges mentioned above is introduced as (M) in (i) of the method of the present invention. Therefore, the present invention also provides a method of preparing propylene oxide, said reaction comprising propene with a hydroperoxide in methanol as a solvent, said reaction resulting in a mixture (M) comprising 5 to 15 weight percent of oxide of propylene, 50 to 85 weight percent methanol, and 10 to 25 weight percent water, or preferably results in a mixture comprising propylene oxide, methanol, water, unreacted propene and optionally propane, said mixture which it is treated to give said mixture (M) comprising 5 to 15 weight percent propylene oxide, 50 to 85 weight percent methanol, and 10 to 25 weight percent water, and said mixture which is subjected to in addition to at least steps (i) to (iii) as described and described hereinafter. According to this embodiment of the present invention, the treatment preferably comprises the separation of propene and, if present, preferably also of propane and / or acetaldehyde, by distillation to give a mixture (M), preferably comprising not more than 500 ppm, preferably not more than 400 ppm and especially preferably not more than 350 ppm of propene and comprising not more than 50 ppm, preferably not more than 25 ppm and especially preferably not more than 10 ppm propane, and preferably not more than 200 ppm, more preferably not more than 150 ppm and especially preferably not more than 100 ppm of acetaldehyde. Accordingly, a mixture (M) is obtained, especially preferably comprising not more than 350 ppm of propene and not more than 10 ppm of propane and not more than 100 ppm of acetaldehyde. According to an even more preferred embodiment of the present invention, the mixture (M) introduced in (i) comprises not more than 1 weight percent, more preferably not more than 0.75 weight percent and especially preferably not more than 0.65. percent by weight of high boiling compounds such as methoxypropanoles and / or hydroperoxide. In the context of the present invention, the term "hydroperoxide" refers to a compound of the formula ROOH. The details regarding the preparation of the hydroperoxides and with respect to the hydroperoxides that can used, inter alia, in the method of the present invention can be found in DE-A-198 35 907 the respective content of which is incorporated in the context of the present invention as a reference. Examples of hydroperoxides that can be used for the purposes of the present invention are, inter alia, tert-butyl hydroperoxide, ethylbenzene hydroperoxide, ter-amyl hydroperoxide, eumenohydroperoxide, cyclohexylhydroperoxide, methylcyclohexylhydroperoxide, tetrahydronaphthalene hydroperoxide, isobutylbenzene hydroperoxide, ethoxynaphthalene hydroperoxide, peracids such as peracetic peroxide and hydrogen peroxide. Mixtures of two or more hydroperoxides can also be used according to the present invention. Preference is given to using hydrogen peroxide as a hydroperoxide in the method of the present invention, and additional preference is given to the use of an aqueous solution of hydrogen peroxide. More preferably, the aqueous solution of hydrogen peroxide comprises hydrogen peroxide in a concentration in the range of from 1 to 90, more preferably from 10 to 70 and especially more preferably from 30 to 50% by weight, based on the total weight of the solution. It is also possible to use a mixture of two or more different hydroperoxides.

The epoxidation reaction of the mixture (M) obtained directly or indirectly can be carried out in the presence of each convenient catalyst or a convenient combination of two or more catalysts. Particularly preferred, a zeolite containing titanium is employed, wherein the zeolites known to the artisan as "titanium silicates" (TS) are particularly preferred. Such titanium containing zeolites, particularly those having a crystalline structure of the MFI type, in addition to the forms for producing them are described, for example, in WO 98/55228, EP-A-0 311 983, or EP-A-0 405 978. The respective content of these documents is hereby incorporated by reference. In addition to Si and Ti, said zeolite materials may contain additional elements, such as aluminum, zirconium, tin, iron, cobalt, nickel, gallium, boron, or small amounts of fluorine. It is possible that the titanium in the zeolite is partially or completely replaced by vanadium, zirconium, or niobium, or any mixture of two or more of these components. Zeolites containing titanium and having an MFI structure are known to produce a characteristic pattern in X-ray diffraction. In addition, these materials exhibit an infrared (IR) vibration band at approximately 960 cm "1. Therefore, it is possible to distinguish the zeolites that contain titanium of crystalline or amorphous Ti02 phases or of alkali metal titanates. In another preferred embodiment, the at least one zeolite catalyst comprises at least one of the elements of titanium, germanium, tellurium, vanadium, chromium, niobium, zirconium. Particularly preferred are the zeolite catalysts having a pentasyl zeolite structure, in particular the structural types which may be, via X-ray diffraction, assigned to the structure types of ABW-, ACO-, AEI-, AEL-, AEN -, AET-, AFG-, AFI-, AFN-, AFO-, AFR-, AFS-, AFT-, AFX- AFY-, AHT-, ANA-, APC-, APD-, AST-, ATN-, ATO-, ATS-, ATT- ATV-, AWO-, A W-, ??? -, BIK-, BOG-, BPH-, BRE-, CAN-, CAS- CFI-, CGF-, CGS-, CHA-, CHI-, CLO-, CON-, CZP-, DAC-, DDR- DFO-, DFT-, DOH-, DON-, EAB-, EDI-, EMT-, EPI-, ERI-, ESV-, EUO-, FAU-, FER-, GIS-, GME-, G00-, HEU-, IFR-, ISV-, ITE-, JBW-, KFI-, LAU-, LEV-, IOL-, LOS-, LOV-, LTA-, LTL-, LTN-, MAZ-, MEI-, MEL-, MEP-, MER-, MFI-, MFS-, MON-, MOR-, MSO-, MTF-, MTN-, MTT-, MTW-, MWW-, NAT-, NES-, NON-, OFF-, OSI-, PAR-, PAU-, PHI-, RHO-, RON-, RSN-, RTE-, RTH-, RUT-, SAO-, SAT-, SBE-, SBS-, SBT-, SFF-, SGT-, SOD-, STF-, STI-, STT-, TER-, THO-, TON-, TSC-, VET-, VFI-, VNI-, VSV-, WIE-, WEN-, YUG-, ZON, as well as the mixed structures of at least two or more of the aforementioned structures. Furthermore, it is conceivable to use zeolite catalysts containing titanium with the structure ITQ-4, ITQ-9, SSZ-24, TTM-1, UTD-1, CIT-1 or CIT-5. Other zeolites containing titanium are of the structure types ZSM-48 or ZSM-12. The titanium containing zeolites of the MFI, MEL structure or the mixed MFI / MEL structures, in addition to the MWW, BEA or mixed structures thereof are preferred in the context of the present invention. In the context of the present invention, titanium-containing zeolite catalysts are most preferred, generally referring to "TS-1", "TS-2" or "TS-3", in addition to the zeolites which contain titanium that exhibit a structure that is isomorphic to beta zeolite. Although it is possible to carry out the reaction using a suspension catalyst, particular preference is given to a heterogeneous catalyst and even more preferably a fixed bed catalyst. Therefore, according to this preferred embodiment of the present invention, it is not necessary to separate the catalyst from the reaction mixture obtained from the epoxidation reaction. Therefore, the present invention also provides a method of separating propylene oxide from a mixture (M) comprising propylene oxide, methanol and preferably water, as described above, wherein the mixture () is directly or indirectly, after at least one treatment step, obtained from an epoxidation process wherein the propene reacts with a hydroperoxide, preferably hydrogen peroxide, in the presence of methanol as a solvent and in the presence of a fixed bed catalyst, preferably a fixed bed zeolite catalyst, more preferably a zeolite catalyst of fixed bed titanium, even more preferably a titanium silicate catalyst of the TS-1 fixed bed type, and wherein said catalyst does not have to be separated from the reaction mixture resulting from the epoxidation process. Accordingly, the present invention also provides a method of preparing propylene oxide, said reaction comprising reacting propene with a hydroperoxide, preferably hydrogen peroxide, in methanol as a solvent, said reaction resulting in a mixture (M) comprising 5 to 15 percent by weight of propylene oxide, 50 to 85 percent by weight of methanol, and 10 to 25 percent by weight of water, or preferably results in a mixture comprising propylene oxide, methanol, water, unreacted propene and optionally propane, said mixture which is treated to give said mixture comprises from 5 to 15 weight percent of propylene oxide, 50 to 85 weight percent of methanol and 10 to 25 weight percent of water and said mixture is subjected to further to at least steps (i) to (iii) as described above and as will be described below, wherein the epoxidation is carried out in the presence of a fixed bed catalyst, preferably a fixed bed zeolite catalyst, more preferably a fixed bed titanium zeolite catalyst, even more preferably a titanium silicate catalyst of the TS-1 type of fixed bed, and wherein said catalyst does not need to be separated from the reaction mixture resulting from the epoxidation process. In (i) of the present invention, any convenient extractive distillation column can be used. Preferably, the column has up to 80 theoretical plates such as 10 to 80 or 20 to 80 or 30 to 80 or 40 to 80 or 50 to 80 or 60 to 80 or preferably more than 60 to 80 such as from 61 to 80 or from 65 to 80 or from 70 to 80 or from 75 to 80. Preferably, the column has more than 60 theoretical plates such as from 61 to 65 theoretical plates. According to the present invention two or more columns can be used where two or more columns can be connected in series and / or two or more columns can be arranged in parallel, preferably a column is used. According to (ii) of the present invention, at least one polar solvent is added. As for the chemical nature of the at least one polar solvent, there are no specific limitations as to the condition at which extractive distillation is possible under the conditions of (iii). From According to the present invention, hydrocarbons, especially C7-C9 hydrocarbons, such as n-octane are not used as extraction solvents, either alone or in combination with a polar solvent such as water. Preferred polar solvents are water, alcohols having one or more hydroxy groups such as one, two or three hydroxy groups, preferably monools and diols or ethers, preferably ether compounds having at least one hydroxy group, preferably a hydroxy group such as 1-methoxy-2-propanol and / or 2-methoxy-1-propanol. Water is especially preferred where, for example, demineralized water, potable water, convenient industrial water, convenient waste water, especially conveniently treated waste water, convenient process water, or a mixture of two or more of the same can be used. . The water introduced in the process of the present invention must be essentially free of organic material, especially essentially free of methanol. According to one embodiment of the present invention, the water introduced in (ii) is a process water of a convenient process such as a process carried out in the epoxidation plant in which the method of the present invention is conducted. According to one aspect of the present invention, the process water is taken from a process in the epoxidation plant where the methanol as the solvent of the epoxidation reaction and the water are separated from one another as in the case of step (v) described below. Preferably the water is taken from the bottom of at least one distillation column in which methanol is removed as a solvent from the epoxidation reaction and water. More preferably, the water resulting from said separation process, optionally after one or more additional purification steps, is introduced into (ii), and the methanol resulting from said separation process, optionally after one or more stages of purification, it is recirculated as a solvent in the epoxidation reaction. Thus, the present invention also relates to a method as described above, wherein an integrated process is implemented by treating a mixture comprising methanol and water by separating methanol and water from one another and recirculating the separated water , optionally after one or more additional purification steps, preferably without any additional purification step, in (ii) and optionally recirculating the separated methanol, optionally after one or more additional purification steps, as a solvent in the epoxidation reaction to from which results the mixture (M) introduced in (i). Therefore, the present invention also provides a method of separating propylene oxide from a mixture (M) as described above wherein the water is introduced as a polar solvent in said extractive distillation column in (ii). According to a more preferred embodiment, no other solvent is introduced except water as a polar solvent in (ii). According to another preferred embodiment, propylene glycol is not used as the polar solvent. The preferred modality according to which water is used and not propylene glycol as a polar solvent, shows, among other advantages, that water is available cheaply compared to propylene glycol and can be disposed of without having disadvantageous ecological impacts. Therefore, in case propylene glycol is used as a polar solvent, it is necessary to treat and recycle propylene glycol to produce the process ecologically and economically efficient. However, the treatment necessarily includes at least one additional process step which is superfluous in case water is used as a polar solvent. According to one embodiment of the present invention, the at least one polar solvent is introduced into the extractive distillation column approximately in the theoretical plate 15, more preferably approximately in the theoretical plate 10, below the upper end of the distillation column extractive The at least one polar solvent, preferably water, can be introduced into the column as liquid or as steam or as a liquid in addition to as steam. If two or more solvents are used, at least one solvent can be introduced as a liquid and at least one other solvent can be introduced as steam. According to a preferred embodiment, water is used as a polar solvent and it is introduced into the extractive distillation column as liquid and / or vapor, more preferably as steam. Even more preferably, the steam that is introduced into (ii) has a pressure of not more than 2 bar, preferably of not more than 1 bar, more preferably of not more than 900 mbar and especially preferably not more than 800 mbar . As regards the amount of polar solvent introduced into the extractive distillation column according to (ii) there are no specific limitations. Preferably, the polar solvent, water in particular, is introduced in an amount of not more than 2 weight percent, based on the total weight of the mixture (M). More preferably, the polar solvent is introduced in an amount of not more than 1.8, more preferably not more than 1.6, more preferably not more than 1.4, more preferably not more than 1.2, and even more preferably not more than 1 per percent in weight, based on total weight of the mixture (M). Other preferred amounts of the polar solvent are at least 0.2, more preferably at least 0.25, more preferably at least 0.3, and even more preferably at least 0.4 weight percent, based on the weight of the mixture ( M) Therefore, preferred ranges are, for example, from 0.2 to 2, more preferably from 0.3 to 1.6, more preferably from 0.4 to 1.2 and even more preferably from 0.45 to 1 weight percent, based on the weight of the mixture ( M) Therefore, the present invention also provides a method of separating propylene oxide from a mixture (), as described above, wherein in (ii) at least one polar solvent, water in particular, is preferably introduced as steam at a pressure of preferably not more than 2 bar, in an amount of from 0.45 to 1 weight percent, based on the weight of the mixture (M). Preferred mass ratios of the ethylene oxide comprised in (M): extraction solvent added in (ii) are from 0.6: 1 to 70: 1, more preferably from 1: 1 to 20: 1 and especially preferably from 3: 1 to 8: 1 such as for example, from 4: 1 to 7: 1 or from 5: 1 to 7: 1 or from 6: 1 to 7: 1. The distillation in (iii) is carried out at a bottom temperature of from 40 to 70 ° C at a pressure in the range of 300 to 750 mbar. Other preferred ranges of the pressure at which The distillation carried out is from 300 to 700, more preferably from 300 to 650, more preferably from 300 to 600, more preferably from 300 to 550, and even more preferably from 300 to 500 mbar, or from 350 to 750, more preferably from 400 to 750, more preferably from 450 to 750, more preferably from 450 to 700, more preferably from 450 to 650, more preferably from 450 to 600, more preferably from 450 to 550 and especially preferably from 450 to 500 mbar. Therefore, the present invention also provides a method for separating propylene oxide from a mixture (M) as described above, wherein in (iii), the distillation is carried out at a pressure of from 300 to 500 mbar , in particular from 450 to 500 mbar. The term "pressure at which distillation is carried out" as used in the context of the present invention refers to the pressure in the dome of the column in which the distillation is carried out. Particularly preferred distillation column bottom temperatures are, for example, from 40 to 70 or from 40 to 65 or from 40 to 60 or from 45 to 70 or from 45 to 65 or from 45 to 60 or from 50 to 70 or from 50 to 65 or from 50 to 60 ° C. Combinations of pressure ranges and temperature ranges of specially preferred backgrounds are, for example, 300 to 750 mbar and 40 to 70 ° C or 300 to 500 mbar and 40 to 60 ° C or 450 to 500 mbar and 50 to 60 ° C. Thus, the present invention also provides a method of separating propylene oxide from a mixture (M) as described above, wherein in (iii) the distillation is carried out at a pressure of 300 to 500 mbar and at a temperature of from 40 to 60 ° C, more preferably at a pressure of from 450 to 500 mbar and at a temperature of from 50 to 60 ° C. Therefore, the present invention provides a method of separating propylene oxide from a mixture (M) by extractive distillation, preferably using water as the extraction solvent in an amount of not more than 2 weight percent based on the weight of the (M), wherein the extractive distillation is carried out at low pressures of 750 mbar and below, preferably from 300 to 750 mbar, more preferably from 300 to 500 mbar and especially preferably from 450 to 500 mbar and simultaneously at low temperatures of 70 ° C and below, preferably 40 to 70 ° C, more preferably 40 to 60 ° C and even more preferably 50 to 60 ° C such as at about 51, 52, 53, 54, 55, 56, 57, 58, 59 ° C. As extractive distillation column, it is essentially possible to use any column. Preference is given particular to a distillation column configured as a packed column, more preferably a packed column containing ordered packing. Such a packed column has a high separation efficiency per pack meter and has only a very low pressure drop. While the ordered package mentioned can essentially be of any type, preference is given to a package having a specific surface area in the range of 100 to 750 m2 / m3. It is possible to use metal sheet packaging, for example from Montz (type Bl 100 to Bl 500) or from Sulzer ChemTech (Type BX or CY). The unit m2 / m3 refers to the geometric surface area of the material that forms the packaging per cubic meter of packaging. According to the present invention, the fraction of propylene oxide formed separately from methanol and water is preferably distilled from the top. The distilled propylene oxide fraction leaving the top in (iii) preferably comprises at least 99.0, more preferably 99.5, more preferably at least 99.6, and most preferably at least 99.7 weight percent oxide of propylene, based on the total weight of the propylene oxide fraction. The fraction of propylene oxide of distillate leaving the top in (iii) preferably comprises no more than 500 μm, more preferably no more than 200 ppm, more preferably no more than 100 ppm, more preferably no more than 50 ppm, more preferably no more than 20 ppm and even more preferably no more than 10 ppm methanol, based to the total weight of the propylene oxide fraction. The fraction of distilled propylene oxide exiting from the top in (iii) preferably comprises not more than 200 ppm, more preferably not more than 100 ppm, more preferably not more than 50 ppm, more preferably not more than 25 ppm, and even more preferably not more than 20 ppm of water, based on the total weight of the propylene oxide fraction. The fraction of distilled propylene oxide exiting from the top in (iii) preferably comprises no more than 0.5, more preferably no more than 0.3, and even more preferably no more than 0.25 weight percent propene and propane, based on to the total weight of the propylene oxide fraction. ? the conditions of the extractive distillation according to the present invention, the high boiling fraction at the bottom of the column comprises, in addition to water and methanol, no more than 100 ppm, preferably no more than 75 and especially preferably no more than 50 ppm propylene oxide, based on the weight of the high boiling fraction.

To the conditions of the extractive distillation according to the present invention where propylene glycol is not used but preferably water as the polar extraction solvent in (iii) the high boiling fraction comprises, in addition to water and methanol, not more than 1, preferably not more than 0.5 and especially preferably not more than 0.2 weight percent propylene glycol, based on the weight of the high boiling fraction. According to an especially preferred embodiment of the present invention, the distilled propylene oxide fraction exiting from the top in (iii) is partially refluxed in the extractive distillation column. According to an even more preferred embodiment, the mass ratio of reflux: distillate is less than 5, more preferably less than or equal to 4.5 and especially more preferably less than or equal to 4 such as about 3.5 or about 3.6 or about 3.7 or about 3.8 or about 3.9 or 4. Therefore, the present invention also provides a method of separating propylene oxide from a mixture (M) as described above, wherein the obtained distillate leaving the top of (iii) ) is partially refluxed in said distillation column extractive and where the reflux to distillate ratio is less than or equal to 4., the extractive distillation process of the present invention combines the advantages of low distillation pressures, low distillation temperatures and, simultaneously, a low proportion of reflux: distillate. The distilled propylene oxide fraction leaving the top can be used as such or be subjected to at least one further treatment step in the event that the content of this fraction with respect to components such as propene, propane and / or or acetaldehyde is too high for the purpose for which it means propylene oxide. Such a treatment may comprise, for example, a fractional distillation where the low-boiling components are removed at the top and a purified propylene oxide fraction is removed as bottom stream or side stream. Subsequently, if necessary, this stream of funds may undergo at least one additional purification process. The stream of funds extracted from the extractive distillation column in (iii) can be used as such or after at least one treatment step in at least some other process or can be recycled in the method of the present invention. According to a preferred embodiment of In the present invention, the bottom stream is treated in one, two or more steps to give a mixture comprising at least 97 weight percent methanol, no more than 2 weight percent water, and no more than 50 weight percent. ppm of acetaldehyde, based on the total weight of said mixture and the methanol thus purified is recirculated in the method of the present invention, preferably as a solvent for the epoxidation reaction resulting in the mixture (M). Depending on the polar solvent used in (ii) as an extraction solvent, this solvent can be conveniently separated from the bottom stream and recycled in the method of the present invention, preferably as a polar solvent in (ii). A further advantage of the preferred method of the present invention according to which water is used as the polar solvent, the treatment of the bottoms stream obtained in (iii) in order to obtain the polar solvent purified for recirculation in (ii) is not necessary because water is available cheaply, contrary to for example, the propylene glycol described in US Pat. No. 5,849,938 as the preferred extraction solvent. Accordingly, the present invention provides a method of preparing propylene oxide, said reaction comprising reacting propene with a hydroperoxide, preferably hydrogen peroxide in methanol as solvent and preferably in the presence of a fixed bed titanium silicate catalyst, said reaction results in a mixture comprising propylene oxide, methanol, water, unreacted propene and optionally propane, said mixture being try to completely remove in an essential way the propene that did not react, to give a mixture (M) comprising 15 weight percent propylene oxide, 50 to 85 weight percent methanol and 10 to 25 weight percent water, said method further comprises (i) introducing said mixture (M) into an extractive distillation column; (ii) additionally introducing a polar solvent, preferably water, more preferably water as steam, wherein the vapor is introduced at a pressure of not more than 2 bar, preferably not more than 1 bar, more preferably not more than 900 bar and especially preferably not more than 800 mbar, in said extractive distillation column in an amount of not more than 2, preferably from 0.45 to 1 weight percent of the mixture (M); (iii) distilling the propylene oxide from the upper part of said extractive distillation as an overhead stream at a pressure of from 300 to 750, more preferably from 300 to 500, especially preferably from 450 to 500 mbar and to a bottom temperature from 40 to 70 ° C, preferably from 40 to 60 ° C and especially preferably from 50 to 60 ° C; (iv) extracting a bottom stream from said extractive distillation column, said bottom stream comprising 100 ppm propylene oxide or less, preferably 75 ppm propylene oxide or less, especially preferably 50 ppm propylene oxide or less, based on the total weight of the bottoms stream and separating a dome stream from said distillation column, said domed stream comprises 500 ppm of methanol or less, preferably 200 ppm of methanol or less, more preferably 100 ppm of methanol or less, more preferably 50 ppm of methanol or less, more preferably 20 ppm of methanol or less and especially preferably 10 ppm of methanol or less, based on the total weight of the dome stream, and (v) optionally recirculating through at least partially the methanol included in the bottom stream of (iv) as a solvent in the reaction where the propene reacts with the hydroperoxide. The following examples and figures are used to illustrate the present invention and do not signify limitations. DETAILED DESCRIPTION OF THE FIGURES Figure 1 shows a preferred embodiment according to the invention. A mixture (M) and an extraction solvent (1) they are introduced into a column (K200) of extractive distillation (stages (i) and (ii)). The propylene oxide is distilled off at the top of (K200) as dome current (step (iii)). A heat exchanger (W230) is used to condense the dome current from the extractive distillation column (K200). As cooling agent (3) used in the heat exchanger (W230), cold water and cooling water are used, respectively. To heat the evaporator or reboiler of the column (K200), a heat exchanger (W200) is used and a low pressure steam (2) is used as a heating source. The heat exchanger (W220) is used to preheat the mixture (M) before entering the column (200). Figure 2 shows a modality where polar solvent is not used for distillation. A mixture (M) is introduced into a distillation column (K100). The propylene oxide is distilled from the upper part of (K100) as dome current which is compressed in an electric compressor (C100) and the compressed vapor stream is condensed in a heat exchanger (W100) where at least part The heat of condensation is transferred to a reboiler used in the extractive distillation column (K100). The heat exchanger (W110) as shown in figure 2 is only used to start the process of distillation, that is, during a continuous distillation process this heat exchanger (W110) is not used. The cooled and condensed stream leaving the heat exchanger (W100) is then divided, and a portion of the stream is passed to a first heat exchanger (W130). The cooled stream leaving the heat exchanger (W130) is then passed to a second heat exchanger (W140) where the stream is further cooled and finally recirculated as reflux at the top of the column (K100). If necessary and / or desired, part of the energy stored in the bottom stream of the distillation column can be used in an additional heat exchanger (W120) where the mixture (M) is heated or preheated before being introduced into the column (K100). Figure 3 shows another modality where polar solvent is not used for distillation. In contrast to the process shown in Figure 2, the process according to Figure 3 includes an additional heat exchanger (W101). Depending on the amount of energy that will be extracted from the compressed steam stream and transferred to a reboiler used in the distillation column (100), it may be necessary to split the compressed steam stream, and pass part of the stream to the exchanger of heat (W 100) and a part to the heat exchanger (W101).

E ploses A stream, the output of an epoxidation unit from which almost all of the light boiling components were separated, was subjected to different PO / MeOH separation units (examples 1 to 3). In all the examples, this stream has the composition according to Table 1: Table 1: composition by current EXAMPLE 1. Extractive distillation in vacuum with water The process of example 1 is carried out in an apparatus unit as shown in figure 1. In figure 1, a heat exchanger (W230) is shown in which the current dome column (K200) of extractive distillation is cooled using cold water and cooling water, respectively. To heat the evaporator of the column, a heat exchanger (W200) is used and steam is used (2) Low pressure as a heating source. The heat exchanger (W220) is used to preheat the mixture (M), that is, the feed according to table 1, before entering the column (K200). The stream described above (Table 1) is fed to an extractive distillation tower (200) containing 80 theoretical plates. Low pressure steam (2) is used to heat the column evaporator via the heat exchanger (W200). The condenser (W230) is operated with cold water (3) which is prepared in a unit (not shown in figure 1) of cold water. The column (K200) is operated under vacuum at 500 mbar and water is used as extraction solvent (1). The feeding points are the following: the feed stream of the column is fed in stage 45 of the upper part of the column, the water as extraction agent is fed in stage 12 of the upper part of the column, at a flow rate of 5.2% with respect to the propylene oxide contained in the feed stream. The propylene oxide is taken at the top of the tower. The tower is operated at a mass reflux ratio (reflux: distillate) of 3.9. The upper propylene oxide stream contains, together with the light boiling components, 10 ppm of MeOH, and 55 ppm of water. The current of bottoms contains 50 ppm of propylene oxide, MeOH, water and all other heavy boiling components. Example 2: extractive distillation with (a) water and (b) propylene glycol at 2 bar (comparative examples) The processes of example 2 are carried out in an apparatus unit as shown in figure 1. In figure 1 , a heat exchanger (W230) is shown in which the dome current of an extractive distillation column (K200) is cooled using cold water and cooling water, respectively. To heat the evaporator of the column, a heat exchanger (W200) is used and low pressure steam (2) is used as a heating source. The heat exchanger (W220) is used to preheat the mixture (M), that is, the feed according to table 1, before entering the column (K200). a) The stream described above (Table 1) is fed to an extractive distillation tower (K200) containing 80 theoretical stages. The column is operated at a pressure of 2 bar. Water is used as extraction solvent (1). Low pressure steam is used as an external heating source to heat the column evaporator. The condenser (W230) is operated with water from the cooling tower. The evaporator load is 31.5 MW, the load of the capacitor of 30.2 MW. The feeding points are the following: the feed stream of the column is fed in stage 50 of the upper part of the column, the water as extracting agent is fed in stage 12 of the upper part of the column, a a flow rate of 10.4% with respect to the propylene oxide contained in the feed stream. The tower is operated at a mass reflux (reflow: distillate) ratio of 6.1. The purified propylene oxide is taken at the top of the tower. The top propylene oxide stream contains, together with the light components, 10 ppm of MeOH, and 1,500 ppm of water. The bottom stream contains 50 ppm of propylene oxide, MeOH, water and all the other heavy components. b) The stream described above (Table 1) is fed to an extractive distillation tower (K200) containing 80 theoretical stages. The column (K200) is operated at a pressure of 2 bar. Propylene glycol is used as extraction solvent (1). Low pressure steam is used as an external heating source to heat the column evaporator. The condenser (W230) is operated with water from the cooling tower. The evaporator load is 36.5 MW, the condenser load is 34.5 MW. The feeding points are as follows: the feed stream of the column is fed in stage 60 of the upper part from the column, the propylene glycol as extraction solvent is fed in stage 2 of the upper part of the column, at a flow rate of 30% with respect to the propylene oxide contained in the feed stream. The tower is operated at a mass reflow ratio (reflux: distillate) of 7.3. The purified propylene oxide is taken at the top of the tower. The upper propylene oxide stream contains, together with the light components, 10 ppm of MeOH. The bottom stream contains 50 ppm of propylene oxide, MeOH, water, the added propylene glycol and all the other heavy components. Example 3: Fractional distillation without polar solvent including compression of the dome stream from the distillation column (Comparative example) The process of example 3 is carried out in an apparatus unit as shown in figure 3. The current described previously (Table 1) is fed to a distillation tower (K100) containing 80 theoretical stages, equipped with a compressor (C100) to compress the upper steam output stream of the distillation column. This current is used as a heating source for the reboiler of the distillation column. The column is operated under vacuum at 500 mbar. No solvent is used of extraction. The feed point of the feed current is in step 68 of the top of the column. The purified propylene oxide is taken at the top of the tower. The tower is operated at a mass reflux ratio (reflux: distillate) of 9.4. The reboiler load is 49.5 MW. The top propylene oxide stream contains together with the light 10 ppm MeOH. The bottom stream contains 50 ppm of propylene oxide, MeOH, water and all the other heavy components. The following Table 2 gives a perspective of the debed examples: Table 2: perspective of the debed examples.

Claims (10)

1. CLAIMS 1. A method for separating propylene oxide from a mixture (M) comprising 5 to 50 weight percent of propylene oxide and 50 to 85 weight percent of methanol, said method characterized in that it comprises (i) introducing said mixture () in an extractive distillation column; (ii) additionally introducing a polar solvent into said extractive distillation column; (iii) distilling propylene oxide leaving the top of said extractive distillation column at a bottom temperature from 40 to 70 ° C and at a pressure from 300 to 750 mbar. The method as claimed in claim 1, characterized in that said mixture (M) comprises 5 to 15 weight percent of propylene oxide and additionally comprises 10 to 25 weight percent of water. 3. The method as claimed in claim 1 or 2, characterized in that the polar solvent is introduced in an amount of 2 percent by weight of the mixture (M) or less. 4. The method as claimed in any of claims 1 to 3, characterized in that said extractive distillation column has up to 80 theoretical plates. 5. The method as claimed in any of claims 1 to 4, characterized in that water is used as the polar solvent. The method as claimed in claim 5, characterized in that the water is introduced as steam at a pressure of not more than 2 bar. The method as claimed in any of claims 1 to 6, characterized in that the distillate obtained by the upper part of (iii) is partially refluxed in said extractive distillation column and wherein the reflux to distillate ratio is less than or equal to 4. 8. The method as claimed in any of claims 1 to 7, characterized in that the distillate stream leaving the top comprises 100 ppm of methanol or less. The method as claimed in any of claims 1 to 8, characterized in that the bottom stream drawn from said extractive distillation column has a propylene oxide content of 100 ppm or less. The method as claimed in any of claims 1 to 9, characterized in that the mixture (M) is formed by the reaction of propene with hydrogen peroxide in methanol as a solvent and in the presence of a fixed-bed catalyst of zeolite of titanium.