MXPA00002214A - Separation of methanol and propylene oxide from a reaction mixture - Google Patents

Abstract

Propylene is separated from the methanol present as a solvent in a crude olefin epoxidation product 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 extractive solvent. The method is also useful for removing water and impurities such as acetaldehyde from the propylene oxide.

Description

SEPARATION OF METHANOL AND PROPYLENE OXIDE FROM A REACTION MIXTURE FIELD OF THE INVENTION This invention provides a method for recovering propylene oxide in purified form from the epoxidation reaction mixture which additionally contains methanol. These mixtures can be formed by epoxidizing propylene with hydrogen peroxide using a zeolite containing titanium as the catalyst and methanol as a solvent for the reaction. A polar solvent containing hydroxy group such as water or propylene glycol is used as an extractive solvent in a distillation column to increase the difference in volatility between propylene oxide and methanol, thereby allowing the propylene oxide to recover as a current top of the column. The extractive distillation can also be operated to reduce the level of other undesirable impurities such as water and acetaldehyde in the propylene oxide.

BACKGROUND OF THE INVENTION In recent years it has been proposed to produce propylene oxide from propylene using hydrogen peroxide as an oxidant and a zeolite containing titanium as a catalyst. Methanol is a preferred reaction solvent particularly for these purposes, since it tends to promote high catalytic activity and selectivity. Epoxidation processes of this type are described, for example, in U.S. Patent Nos. 5,591,875, 4,833,260, 5,621,122, 5,646,314, and 4,824,976, International Patent Publication No. 0732327, and Clerici et al., J. Catalysis 129 , 159-167 (1991), whose teachings are incorporated herein by reference in its entirety. Although these processes are capable of providing exceptionally high selectivity to propylene oxide, the crude reaction product obtained by this typically contains only about 2 to 10 weight percent propylene oxide with the remainder being predominantly methanol. However, propylene oxide and methanol usually have similar boiling points and thus it can be difficult to separate them quantitatively, especially when methanol is present in large excess. Minor amounts of certain side products such as acetaldehyde are inevitably formed during epoxidation and subsequent processing. Since a satisfactory propylene oxide for commercial purposes should contain less than 100 parts per million, and preferably less than 20 parts per million, of acetaldehyde, it is necessary to develop methods for reducing the level of the secondary product of acetaldehyde in these mixtures of reaction. In addition, epoxidation processes of this type form water as a secondary product, deriving water from the hydrogen peroxide oxidant. Depending on the method used to generate the hydrogen peroxide to be used in the epoxidation, water may also be present in the feed to the reactor. Although epoxidation processes catalyzed by titanium containing zeolites are very water tolerant, it will be necessary for most commercial purposes to obtain propylene oxide in substantially anhydrous form. Thus, it is apparent that there is a need to develop purification methods capable of efficiently removing methanol, water, and acetaldehyde simultaneously from a crude product of the epoxidation reaction when the propylene oxide is present at a relatively low concentration. COMPENDIUM OF THE INVENTION We have now found that a crude product of the epoxidation reaction composed of propylene oxide and methanol can be purified by the following process. The product of the crude epoxidation reaction is introduced into an intermediate section of an extractive distillation zone, while also introducing a polar solvent having a hydroxy functionality and a boiling point higher than methanol in a higher section of the zone. of extractive distillation. Propylene oxide is distilled above the extractive distillation zone, with a background current comprised of methanol and the polar solvent removed from a lower section. The process can be operated so that the water and at least a portion of the acetaldehyde present in the crude product of the epoxidation reaction is extracted into the polar solvent and removed in the bottom stream. DESCRIPTION OF THE DRAWING Figure 1, which will be explained in more detail later herein, schematically illustrates one embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION The crude product of the epoxidation reaction treated according to the process of this invention is typically obtained by epoxidizing propylene with hydrogen peroxide or an equivalent thereof in a reaction medium in which the methanol is used as the solvent. Any unreacted propylene that may be present can be largely or entirely removed by subjecting the crude product of the epoxidation reaction to an initial separation or fractionation using conventional distillation methods. For example, propylene can be removed from the top using a separating tank or something similar. The epoxidation catalyst, which may be, for example, a titanium containing zeolite such as titanium silicalite, is also preferably separated by filtration or other of these means from the crude product of the epoxidation reaction before it is processed according to the present invention. invention. Depending on the epoxidation conditions employed in the degree of removal of propylene in the initial separation step, the crude product of the epoxidation reaction will typically have a composition composed of the following components, in percent by weight: 2-10 propylene oxide methanol 50-85 acetaldehyde 0.01-0.1 water 10-30) propylene glycol 0.1-20 other glycols and heavy 0-1 propylene and / or propane 0.01-0.1 The concentration range of propylene glycol shown above includes the embodiment of the invention wherein the propylene glycol is used as the polar solvent in the extractive distillation step and the propylene glycol recovered in the bottom stream is incompletely separated from the methanol before recycling the methanol for use in epoxidation. Typically, the amount of propylene glycol actually generated as a secondary product of the epoxidation will represent less than 1 weight percent of the crude product of the epoxidation reaction. If a glycol other than propylene glycol or other heavy component is used as a polar solvent in the embodiment described above, then the concentration of the polar solvent in the crude product of the epoxidation reaction will be greater than the range shown in the above table. The extractive distillation is carried out conveniently in any suitable distillation column or tower suitable for the distillation of propylene oxide and methanol. For best results, the extractive distillation zone should contain at least 10 theoretical floors and ordinarily contain 20 to 60 theoretical floors. The maximum number of theoretical floors is limited only by economic considerations. A single distillation column or tower is usually preferred for economic reasons, but the use of multiple distillation columns to carry out the same result is not excluded. The polar solvent to be delivered to the extractive distillation zone is a compound that contains one or more hydroxy functional groups (-0H) and has a boiling point higher than that of methanol. In addition to water, convenient classes of polar solvents include, but are not limited to, glycols and glycol ethers. Glycols containing from 2 to 6 carbon atoms and oligomers thereof (eg, dimers, trimers and tetramers) are generally convenient for use. Exemplary glycols include ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, 1,4-butanediol, 2-methyl-1,3-propanediol, neopentyl glycol and the like and oligomers thereof. Illustrative glycol ethers include the alkyl ethers of 1 to 6 carbon atoms of propylene glycol, ethylene glycol and oligomers thereof such as di- and tripropylene glycol and di- and triethylene glycol. Polar solvent mixtures (for example aqueous propylene glycol) could also be used. The amount of polar solvent supplied to the extractive distillation zone should be sufficient to achieve the desired degree of separation of methanol and other impurities such as acetaldehyde and water of propylene oxide. This amount will vary, of course, depending on several factors, including the identity of the polar solvent, but will generally be at least about 5 percent, based on the weight of the crude product of the epoxidation reaction. Typically, the polar solvent is fed to the extractive distillation zone at an expense (based on weight) that is 5 to 35 percent of the expense of the crude product of the epoxidation reaction. When propylene glycol is used as the extractive solvent, the amount of propylene glycol introduced is preferably 10 to 30 weight percent of the crude product of the epoxidation reaction. The optimum amount will depend on the concentration of propylene glycol in the crude product of the epoxidation reaction (for example, more propylene glycol in the reaction product will generally mean that less additional propylene glycol will be necessary to introduce and vice versa). The feed point for the polar solvents should be between the feed point of the crude product of the epoxidation reaction and the point at which the upper stream comprising the propylene oxide is extracted from the extractive distillation zone. This will help to minimize the contamination of propylene oxide by the polar solvent. Preferably, the polar solvent is introduced to the extractive distillation zone at a point not less than two theoretical levels below the upper stream of the extraction point. The point at which the crude product of the epoxidation reaction is introduced is the intermediate section of the extractive distillation zone, preferably from about 40 percent to 70 percent of the distance, in terms of theoretical floors, from the bottom towards the upper part of the extractive distillation zone. A convenient ratio of reflux / distillate (molar) is important to achieve optimal results and will generally be in the range of 5: 1 to 15: 1. The pressure under which the extractive distillation is conveniently operated is around atmospheric pressure, for example, from about 8 psia (55 Pa) to about 50 psia (345 kPa). The temperature of the bottoms (reboiler) will of course be variant with the pressure but will typically be in the range of 90 ° C to 120 ° C. The extractive distillation conditions are selected so as to provide a higher stream composed predominantly of propylene oxide. In a preferred embodiment, at least 95 percent by weight (more preferably at least 98 percent by weight) of the top stream is propylene oxide. The overhead stream will preferably contain less than 0.5 percent by weight of water (more preferably less than 0.05 percent by weight of water) and less than 1 percent by weight of methanol (more preferably less than 0.2 percent by weight of methanol) . The bottoms streams extracted from the extractive distillation will contain all or essentially all (eg, 99 +%) of the polar solvent (including any water present in the crude product of the epoxidation reaction) and a preponderance of the methanol (eg, 99%). +%) originally present in the crude product of the epoxidation reaction. Depending on the selected extractive distillation conditions, various amounts of acetaldehyde will be removed in the bottom stream (e.g., 50 to 98 percent) while still achieving the desired separation of propylene oxide from methanol. For example, adjustments in the reflux ratio, the number of theoretical trays, the upper cutoff point and the extractive solvent flow will change the acetaldehyde distribution.

In the accompanying drawing (Figure 1) a representative system for carrying out the extractive distillation process of this invention is illustrated diagrammatically. Thus, referring to the drawing, reference numeral 1 designates the line for feeding the crude epoxidation reaction product to be treated to an extractive distillation zone 2. The heat can be supplied to the column or tower that it comprises the extractive distillation zone by means of a reboiler. A polar solvent is supplied as the extractive distillation solvent through line 3. The bottom stream comprising methanol and other substances that are less volatile than propylene oxide under extractive distillation conditions such as water and at least a portion of the acetaldehyde are removed through line 4. The purified propylene oxide is removed as a higher stream via line 5 in the form of steam. Further purification of the overhead stream can be carried out. For example, if the upper stream still contains unacceptably high levels of propane, propylene, acetaldehyde or other relatively light impurities, it can be introduced into an intermediate section of the fractionator 6 and subjected to fractional distillation. The light impurities are removed from the top through line 7, while a repurified propylene oxide bottom stream is removed via line 8. An advantage of the present process is that the fractionator 6 can be in size significantly reduced (and thus cost) compared to the fractionator that would be required if the reaction mixture were subjected directly to fractional distillation instead of an initial extractive distillation. Another advantage is that the fractionation of acetaldehyde and propylene oxide is carried out much more easily in the absence of methanol; the present process thus facilitates the complete removal of acetaldehyde by providing a higher stream that essentially lacks methanol. After the removal of the volatile components in the fractionator 6, the propylene oxide obtained as the bottom stream can, if desired, undergo further purification steps. For example, for many commercial applications the concentrations of impurities such as methanol, water and other low molecular weight oxygenates should be reduced to very low levels. This can be carried out with any of the conventional methods known in the art such as, for example, the extractive distillation methods described in the US Pat.

United States of America number 5,000,825 (Shih et al.). The methanol in the bottom stream can advantageously be recycled for use in the reaction solvent in an olefin epoxidation process, preferably after at least partial separation of the polar solvent by distillation elements or the like. The separated polar solvent can also be recycled for later use in the present extractive distillation process. For example, the bottom stream can be fed through line 4 to an intermediate section of fractionator 9 and subjected to fractional distillation. Methanol and other components of the bottom stream that are more volatile than the polar solvent are removed from the top of the fractionator 9 via line 10. The polar solvent recovered in the bottom stream returns to the extractive distillation zone 2 to through the line 3. The extractive distillation process described in the Application with serial number, presented on August 15, 1997 (attorney reference number 01-2467A) can be used to remove at least a portion of any acetaldehyde from methanol before reusing it in the epoxidation if desired. Although it is possible to completely separate the polar solvent from methanol by distillation, it will often be economically advantageous to distill only a portion of the bottom stream (with the remaining portion directly feeding back into the epoxidation reactor) since the energy costs can thereby be reduced. The result will be that an appreciable amount of polar solvent will be recycled to the epoxidation reactor along with the methanol. Ordinarily, however, this will not significantly affect the epoxidation results obtained since the polar solvents useful in the present purification process will be substantially inert under typical epoxidation conditions. However, it will generally be desirable that the weight ratio of the methanol to the polar solvent in the epoxidation is at least 2: 1, more preferably at least 3: 1. EXAMPLES Example 1 This Example illustrates one embodiment of the process claimed herein wherein a crude product of the epoxidation reaction that has been subjected to initial flash distillation to remove most of the unreacted propylene and having the following composition is subjected to distillation extractive using propylene glycol as the polar solvent: Component Weight% Propylene oxide 6.8 Methanol 61.3 Water 24.2 Acetaldehyde 0.039 Propylene glycol 6.7 Propylene and propane 0.055 Other heavy components 0.906 The crude product of the epoxidation reaction is fed to an extractive distillation tower containing 50 Theoretical floors (including reboiler), the feeding point is 19 stories from the top of the tower. The propylene glycol is fed on the fourth floor from the top of the tower at a flow rate that is 13.4 percent of the crude product flow rate of the epoxidation reaction based on the weight. A purified propylene oxide product is taken as the upper distillate. The extractive solvent of propylene glycol is removed from the bottom step (reboiler), together with all, or almost all, methanol and water as well as other relatively heavy components. The tower is operated at a reflux (reflux to distillate) ratio of 9. The pressure in the column condenser is set at 30 psia (207 kPa) and the column operated with a pressure drop of 0.2 psi (1 kPa) per floor, resulting in a bottom pressure of approximately 40 psia (276 kPa). The bottom temperature (reboiler) is 104 ° C and the upper temperature (condenser) is 51 ° C. Under these conditions, 99.8 percent of the propylene oxide in the crude epoxidation reaction product fed to the tower is recovered in the upper distillate and 99.995 percent of the methanol, 97.9 percent of the acetaldehyde, and all the water is recovered in the background current. The compositions of the product streams are as follows: Component D estimated F ounds (% weight) (% weight) Propylene oxide 99.6 0.014 Methanol 0.03 57.6 Water 0 22.7 Acetaldehyde 0.012 0.036 Propylene glycol 0 18.8 Propane and propylene 0.9 0 Other components heavy 0 0.85 Example 2 This Example illustrates an alternative mode of the claimed process. A crude product of the epoxidation reaction that has been subjected to initial flash distillation to remove most of the unreacted propylene and which has the following composition is subjected to extractive distillation using propylene glycol as the extractive solvent: Component% weight Propylene oxide 7.14 Methanol 50.8 Water 26 Acetaldehyde 0.0406 Propylene glycol 15.1 Propylene and propane 0.0576 Other heavy components 0.862 The crude product of the epoxidation reaction is fed to an extractive distillation tower containing 25 theoretical floors (including reboiler), the feeding point is 13 stories from the top of the tower.

The propylene glycol is fed into the third floor from the top at a flow rate which is 28 percent of the flow rate of the crude product of the epoxidation reaction fed based on the weight. A purified propylene oxide product is taken as upper distillate. The extractive solvent of propylene glycol is removed from the lower floor (reboiler), along with all or almost all the water and methanol as well as other components. The tower is operated at the reflow (reflux to distillate) ratio of 9. The pressure in the column condenser is set at 30 psia (207 kPa) and the column is operated with a pressure drop of 0.2 psi (1 kPa) per tray so that the bottom pressure is approximately 37 psia (255 kPa). In this way there is at the bottom a temperature (reboiler) of 105 ° C and in the upper part (condenser) the temperature is 51 ° C. Under these conditions, 99.5 percent of the propylene oxide in the crude product of the epoxidation reaction fed to the column is recovered in the upper distillate and 99.98 percent of the methanol, 100 percent of the water and 89.5 percent of the acetaldehyde is recover in the background current. The compositions of the columns of the product are as follows: Component Distilled F or n d or s (% weight) (% weight) Propylene oxide 98.84 0.03 Methanol 0.15 42 Water 0 21.5 Acetaldehyde 0.06 0.03 Propylene glycol 0 35.7 Propylene and Propane 0.95 0 Other components peeled 0 0.74

Claims (18)

1. A method for purifying a crude product of the epoxidation reaction composed of 2 to 10 weight percent propylene oxide, 50 to 85 weight percent methanol and 10 to 30 weight percent water, the method comprising steps of: (a) introducing the crude product of the epoxidation reaction into an intermediate section of an extractive distillation zone; (b) introducing a polar solvent having a hydroxy functionality and a boiling point higher than that of methanol in a higher section of the extractive distillation zone; (c) distilling the higher propylene oxide from the extractive distillation zone; and (d) recovering a bottom stream composed of methanol and the polar solvent from a lower section of the extractive distillation zone.

2. The method of claim 1 wherein the polar solvent is selected from the group consisting of water, glycols, glycol ethers, and mixtures thereof.

3. The method of claim 1 wherein the amount of polar solvent introduced into the extractive distillation zone is at least 5 percent by weight of the crude mixture of the epoxidation reaction.

4. The method of claim 1 wherein the crude product of the epoxidation reaction is further composed of acetaldehyde. The method of claim 4 wherein the amount of polar solvent is effective to render the acetaldehyde less volatile than propylene oxide and cause at least a portion of the acetaldehyde to be removed as part of the bottom stream. The method of claim 1 wherein the upper section of the extractive distillation zone into which the polar solvent is introduced is at least 2 theoretical plates below the point at which the propylene oxide is extracted from the distillation zone extractive The method of claim 1 wherein the extractive distillation zone contains from 20 to 60 theoretical plates. The method of claim 1 wherein the propylene oxide distilled at the top contains less than 0.5 weight percent water. The method of claim 1 wherein the bottom stream contains at least 99.9 percent of the methanol originally present in the crude product of the epoxidation reaction. 10. A method of purifying a crude epoxidation reaction product comprising 2 to 10 weight percent propylene oxide, 50 to 85 weight percent methanol and 10 to 30 weight percent water, the method comprising the steps: (a) introducing the crude epoxidation reaction product into an intermediate section of an extractive distillation zone containing from 20 to 60 theoretical plates at a point which is 40 to 70 percent distance in theoretical plates of the bottom of the extractive distillation zone; (b) introducing a polar solvent selected from the group consisting of water, glycols, glycol ethers and mixtures thereof in a top section of the extractive distillation zone, in an amount which is at least 5 weight percent of the product crude epoxidation reaction; (c) distilling the propylene oxide at the top of the extractive distillation zone, wherein the propylene oxide thus distilled contains less than 0.05 weight percent water and less than 0.2 weight percent methanol; and (d) recovering a bottom stream composed of methanol and the polar solvent from a lower section of the extractive distillation zone. 11. The method of claim 10 wherein the polar solvent is propylene glycol. The method of claim 11 wherein the crude product of the epoxidation reaction is further composed of acetaldehyde. 13. The method of claim 12 wherein the 50 to 98 percent of the acetaldehyde in the crude epoxidation reaction product is removed as part of the stream background . 14. The method of claim 10 wherein a reflux-distillate ratio of from 5: 1 to 15: 1 is used. The method of claim 10 wherein the extractive distillation zone is operated at a reaction of from 8 to 50 psia (55 to 345 kPa) and a background temperature of 90 ° C to 120 ° C. 16. The method of claim 10 wherein the bottom stream is subjected to fractional distillation wherein the methanol is taken as an upper part and the polar solvent is recovered in a second bottom stream. The method of claim 16 wherein the methanol taken from the top is recycled for use in an olefin epoxidation process and the second bottom stream is recycled for use in step (b). 18. The method of claim 10 wherein the crude product of the epoxidation reaction is formed by epoxidizing propylene with hydrogen peroxide using a zeolite containing titanium as a catalyst and methanol as a solvent for the reaction.