MXPA00001292A - Propylene oxide purification - Google Patents

Abstract

Propylene oxide obtained by an epoxidation process which uses methanol as a solvent may be effectively treated to remove acetaldehyde by subjecting the crude epoxidation reaction product to fractional distillation. The methanol solvent is utilized during such distillation to lower the relative volatility of the acetaldehyde impurity, thereby making it possible to obtain a bottoms fraction containing substantially all the acetaldehyde. Purified propylene oxide having a reduced acetaldehyde concentration is removed as an overhead stream. Water may also be effectively separated from the propylene oxide using this procedure.

Description

PU RI FICATION OF PROPYLENE OXIDE FIELD OF THE INVENTION This invention provides a method for recovering propylene oxide in purified form from an epoxidation reaction mixture further comprised by methanol and polluting amounts of acetaldehyde impurity. Such mixtures can be formed by epoxidizing propylene with hydrogen peroxide using a titanium silicalite as a catalyst and methanol as a reaction solvent. Substantially all the acetaldehyde can be removed by fractionation of the epoxidation reaction mixture in a distillation column, where a methanol concentration is maintained in the distillation column enough to suppress the volatility of the acetaldehyde, with the end to minimize the amount of acetaldehyde present in the upper stream.

BACKGROUND OF THE INVENTION In recent years, the production of propylene oxide from propylene has been proposed, using hydrogen peroxide as an oxidant and a titanium containing zeolite, as a zeolite as a catalyst. Methanol is a particularly preferred reaction solvent for such purposes, since it tends to promote high activity and catalyst selectivity. Epoxidation processes of this type are described, for example, in US Pat. Nos. 5, 591, 875, 4, 833, 260, 5, 621, 1 22, 5,646, 31 4 and 4, 824, 976, E P Pub. No. 0732327, and Clerici et al. , J. Catalysis 129, 159-167 (1991). Although such processes are capable of providing an exceptionally high selectivity to propylene oxide, minor amounts of certain by-products, such as acetaldehyde, inevitably form. Since a satisfactory propylene oxide for commercial purposes should contain less than 1 00 ppm, and preferably less than 20 ppm of acetaldehyde, it is necessary to develop methods for substantially removing all of the acetaldehyde by-product from such reaction mixtures. In addition, epoxidation processes of this type form water as a co-product, the water being derived from the hydrogen peroxide oxidant. Depending on the method used to generate the hydrogen peroxide to be used in the epoxidation reaction, water may also be present in the feed to the reactor. Although epoxidation processes catalyzed by titanium containing zeolites are remarkably water tolerant, it will be necessary for most commercial purposes to obtain propylene oxide in substantially anhydrous form. Consequently, an efficient method is needed to remove water from the propylene oxide produced by such an epoxidation process. Distillation methods for preparing propylene oxide are described in US 371 5284 and EP 06751 1 9A.

BRIEF DESCRIPTION OF THE INVENTION The prior art methods for separating acetaldehyde from propylene oxide, which were developed mainly in connection with epoxidation processes based on organic hydroperoxide, where either ethylbenzene or t-butyl alcohol is used as Reaction solvent, generally have charged the crude propylene oxide, after removing substantially all the unreacted propylene, to a distillation column and removed propylene oxide and all the minor boiling materials, including acetaldehyde, as a superior product. Subsequent fractioning of the top product in a second distillation column is then employed to separate acetaldehyde as a top product from a bottom fraction containing the propylene oxide. We have now found that the substantial amount of methanol present as a reaction solvent in a crude epoxidation product of a hydrogen peroxide / titanium silicalite process can be used to excel in achieving the desired separation of acetaldehyde from propylene oxide. . The presence of methanol at high concentrations substantially reduces the volatility of acetaldehyde relative to propylene oxide. A) Yes, instead of taking superior acetaldehyde as in conventional purification schemes, the present invention operates by maintaining a methanol concentration in the liquid phase within the sufficiently large distillation column, in order to suppress the volatility of acetaldehyde and to force all or substantially all of the acetaldehyde towards a bottom stream. The purified propylene oxide containing a reduced level of acetaldehyde is taken from the top. The upper stream will also have a substantially lower water content as compared to that of the crude epoxidation reaction product.

The present invention provides a method for purifying propylene oxide produced in an epoxidation process, wherein methanol is used as a solvent, comprising the steps of (a) feeding a reaction product of crude epoxidation comprised of propylene oxide, methanol and acetaldehyde to a fractionator; (b) subjecting the crude epoxidation reaction product to fractional distillation within the fractionator; (c) removing an overhead stream comprised of propylene oxide and methanol, and having a reduced level of acetaldehyde as compared to the crude epoxidation reaction product of the fractionator; and (d) removing a bottom stream comprised of methanol and acetaldehyde from the fractionator. Said fractional distillation is carried out under conditions such that the concentration of methanol within the fractionator is maintained at a sufficiently high level, in order to substantially prevent acetaldehyde from being present at the point where the top stream is withdrawn from the fractionator. More specifically, the present invention provides a method for purifying a crude epoxidation reaction product comprised of 2 to 10 percent by weight of propylene oxide, 60 to 85 percent of methanol, 10 to 25 percent of water and 0.01 to 0.1 weight percent acetaldehyde, comprising the steps of (a) feeding the crude epoxidation reaction product to a fractionator having from 20 to 60 theoretical vapor-liquid contact stages; (b) subjecting the crude epoxidation reaction product to fractional distillation within the fractionator at a pressure higher than 103.4 to 344.7 kPa, a bottom temperature of 80 ° C to 1 1 0 ° C, and a reflux: distillate ratio of 1 0: 1 to 30: 1; (c) removing an overhead stream comprised of propylene oxide and at least 2 percent by weight of methanol from the fractionator; and (d) removing a first bottom stream comprised of methanol, water and at least 99% of the acetaldehyde present in the crude epoxidation reaction product of the fractionator.

DESCRIPTION OF THE DRAWING The accompanying drawing (Figure 1) schematically illustrates a particular embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION A reaction product of crude epoxidation, from which substantially unprocessed propylene has been removed by previous distillation, such as an instantaneous distillation or other conventional distillation operation, is fed through line 2 to an intermediate of the first fractionator 1. Depending on the epoxidation conditions employed, the crude epoxidation reaction product generally has a composition comprised of the following components, in percent by weight: propylene oxide 2-10 methanol 60-85 acetaldehyde 0.01 -0.1 water 10-25 sub- open ring products of 0.1-1 propylene oxide (for example, propylene glycol) propylene and / or propane 0.01 -0.1 The first fractionator 1 may comprise a conventional distillation column or tower having an appropriate capacity to handle the desired volume of crude epoxidation reaction product within a given period and having a sufficient number of theoretical plates to achieve the necessary separation of methanol and acetaldehyde of propylene oxide. The use of a plate column is particularly advantageous. For economic reasons, the fractionator should normally comprise a simple distillation tower or column, although the use of multiple columns or towers is not excluded to achieve the same result. The fractionator 1 is operated at conditions such that a first higher stream of propylene oxide and a smaller amount of methanol can be removed through line 3 and substantially all of the acetaldehyde (preferably, at least 99%, more preferably, at less 99.9%) is removed in a first bottom stream through line 4. It is essential that such conditions can be selected to provide a methanol concentration within the first fractionator 1, which is effective in making acetaldehyde less volatile than propylene oxide. Sufficient methanol should be taken from the top of the first fractionator, so that there is an area above the feed tray where both concentrations of methanol and propylene oxide are high enough for acetaldehyde to be heavier (ie less volatile) ) that both methanol and propylene oxide. This will prevent the acetaldehyde from reaching the top of the first fractionator 1 and allows, under optimum conditions, that essentially all of the acetaldehyde in the feed to the first fractionator be recovered in the bottom stream. Normally, the process is operated in order to have at least 2 (more preferably, at least 4) percent by weight of methanol in the upper stream. To minimize the degree of further downstream processing, which is used, the first fractionator 1 is preferably operated at a higher pressure of about 103.4 to 344.7 kPa and at a bottom temperature of about 80 ° C to about 1 10 ° C. It is particularly advantageous to operate the first fractionator 1, so that the upper zone thereof is at a pressure of about 172.4 to 275.8 kPa and the lower zone thereof is at a temperature of about 93 ° C to 1 04 ° C. The first fractionator 1 generally contains from 20 to 60 (more preferably, 30 to 50) theoretical vapor-liquid containing stages. An adequate proportion of reflux: distillate is important to achieve optimal results; this reflux ratio should preferably vary between 10: 1 and 30.1 (more preferably, 15: 1 to 25: 1). The heat can be supplied to the fractionator by means of a reboiler. The first bottom stream will usually contain substantial amounts of methanol and water, in addition to acetaldehyde, along with other minor by-products, such as propylene glycol, which have lower volatilities under distillation conditions than propylene oxide. An additional advantage of the process of this invention is that it is capable of producing propylene oxide, which is substantially free of water, as the bottom stream will normally contain almost all (eg, at least 99%) of the water originally present in the water. reaction product of crude epoxidation. The bottom stream can be purified by conventional means, such as fractional or extractive distillation, to recover the methanol to be reused as the solvent in the epoxidation process. Where significant amounts of propylene remain in the first top stream, which will typically also contain about 90 to 98 weight percent propylene oxide, 2 to 6 weight percent methanol, 0 to 2 weight percent propylene, and less than 20 ppm (more preferably, less than 5 ppm) of acetaldehyde, a second fractionator can be used to remove said residual propylene. The first top stream can thus be charged to an intermediate zone of a second fractionator 5 through line 3. The fractionator 5, which is operated so that the propylene is removed as a second top stream from a top zone through line 6. The propylene oxide, which is essentially free of propylene, is removed as a second bottom fraction from a lower zone of a second fractionator 5 through line 7. The second fractionator 5 is operated advantageously at an upper zone pressure from about 965.3 to 2068.4 kPa and at a lower zone temperature from about 115 ° C to about 150 ° C. The particularly preferred operation involved a pressure of about 1103.2 to 1516.8 kPa in the upper zone thereof and a temperature of about 120 ° C to 140 ° C in the lower zone thereof. From about 5 to about 15 (most preferably, 5 to 10) theoretical vapor-liquid contact stages are preferably present in the second fractionator. The reflux to distillate ratio is suitable from 10: 1 to 50: 1, with 20: 1 to 40: 1 being the preferred range. Further purification of the propylene oxide present in the second bottom stream can be achieved, if desired, by subjecting said stream to extractive distillation. The extractive distillation of impure propylene oxide fractions is well known in the art, and is described, for example, in extensive detail in the following US patents: 3,337,425, 3,338,800, 3,464,897, 3,578,568, 3,843,488, 4,140,588, 4,971,661, 5,000,825 , 5,006,206, 5,116,465, 5,116,466, 5,116,467. 5,129,996, 5,133,839, 5,139,622, 5,145,561, 5,145,563, 5,154,803. 5,154,804, 5,160,587, 5,340,446, 5,453,160, 5,464,505 and 5,620,568.

For example, the second bottom stream (or, alternatively, the first top stream, particularly where the first top stream contains little or no unreacted propylene) is fed through line 7 to an intermediate zone of the column of extractive distillation 8, wherein it is in countercurrent contact with an extraction solvent, such as, a hydrocarbon (e.g., octane), heavy polar organic compound (e.g., propylene glycol), or other substance known in the art by be useful for that purpose. The high purity propylene oxide is removed from an upper zone of column 8 through line 9, while the extraction solvent containing relatively less volatile impurities, such as water, methanol and the like, are removed. as an extract stream from a lower zone of the column 8 through the line 10. The extract stream is fed to an intermediate zone of the separator 1 1, which is operated so that the components having boiling points by Above the propylene oxide are removed at the top via line 1 2. The extraction solvent is removed via line 1 3 as a bottom stream to recycle to an upper zone of the extractive distillation column 8 via line 14 Additive extractive solvent can be added to line 14 from line 15.

EXAMPLE In a first distillation column containing 40 theoretical stages (including reboiler), a reaction product of crude epoxidation, from which propylene is removed greatly without reacting, is introduced in stage 1 6 of the upper part of the spine. A total condenser is used so that the distillate is removed as a liquid. The feeding of the column is as follows: Component% by weight Propylene oxide 7.2 Water 16.9 Methanol 74.8 Acetaldehyde 0.04 Propylene & Propane 0.06 Open ring products 0.95 Other heavy components 0.05 The first distillation column is operated at a molar ratio (reflux to distillate) of 1 9.4. The pressure in the column condenser is adjusted to 206.8 kPa and the column is operated with a pressure drop of 1.4 kPa per tray, resulting in a lower pressure of approximately 262 kPa. The bottom temperature (reboiler) is 98 ° C and the temperature of the upper part (condenser) is 51 ° C. Under these conditions, 99.93% of the acetaldehyde is recovered in the feed to the column in the lower stream and 99.7% of the propylene oxide is recovered in the upper part. The compositions of these two product streams are as follows: Component% by weight of distillate% by weight of the bottom Propylene oxide 94.8 0.023 Water 0.0006 1 8.2 Methanol 4.4 80.5 Acetaldehyde 0.00035 0.044 Propylene & propane 0.8 0 ring products 0 1 .03 open Other components 0.2 heavy

Claims (9)

REIVI NDICATIONS

1 . A method for purifying a crude epoxidation reaction product comprised of 2 to 10 percent by weight of propylene oxide, 60 to 85 percent in methanol, 1 to 25 percent water and 0.01 to 0.1 percent by weight of acetaldehyde, comprising the steps of (a) feeding the crude epoxidation reaction product to a fractionator having from 20 to 60 theoretical vapor-liquid contact steps; (b) subjecting the crude epoxidation reaction product to fractional distillation within the fractionator at a higher pressure than

103. 4 to 344.7 kPa), a bottom temperature of 80 ° C to 1 1 0 ° C, and a reflux: distillate ratio of 10: 1 to 30: 1; (c) removing an overhead stream comprised of propylene oxide and at least 2 percent by weight of methanol from the fractionator; and (d) removing a first bottom stream comprised of methanol, water and at least 99% of the acetaldehyde present in the crude epoxidation reaction product of the fractionator. 2. The method of claim 1, wherein said epoxidation process utilizes a zeolite containing titanium as a catalyst.

3. The method of claim 1, wherein said epoxidation process uses hydrogen peroxide as an oxidant.

4. The method of claim 1, wherein the concentration of methanol in the crude epoxidation reaction product is 60 to 80 percent by weight

5. The method of claim 1, wherein the crude epoxidation reaction product initially contains propylene and the crude epoxidation reaction product before step (a), is subjected to an initial distillation wherein a substantial portion of the propylene is removed.

6. The method of claim 1, wherein the crude epoxidation product and the background stream are additionally comprised of water.

7. The method of claim 1, wherein the top stream is fed to a second fractionator, wherein the top stream is subjected to distillation, such that a second top stream comprised of propylene and a second bottom stream comprised of propylene oxide and substantially propylene free are removed separately from the second fractionator. The method of claim 7, wherein the second bottom stream is fed to an extractive distillation column, wherein the second bottom stream is subjected to extractive distillation with an extraction solvent, so that a third higher stream consisting essentially of propylene oxide and a third bottom stream comprised of the extraction solvent and impurities present in the second bottom stream, are removed separately from the extractive distillation column. 9. The method of claim 8, wherein the extraction solvent is selected from the group consisting of propylene glycol and octane. 1 0. A method for purifying propylene oxide produced in an epoxidation process, wherein methanol is used as a solvent, hydrogen peroxide is used as an oxidant, and a zeolite containing titanium is used as a catalyst, comprising steps of (a) feeding a crude epoxidation reaction product comprised of 2 to 10 percent by weight of propylene oxide, 60 to 85 percent by weight of methanol, 10 to 25 percent by weight of water and 0.01 to 0.1 percent by weight weight of acetaldehyde, to a first fractionator having from 20 to 60 theoretical vapor-liquid contact stages; (b) subjecting the crude epoxidation reaction product to fractional distillation within the fractionator at a pressure higher than 103.4 to 344.7 kPa, a bottom temperature of 80 ° C to 1 10 ° C, and a reflux: distillate ratio of 1 0: 1 to 30: 1; (c) removing a bottom stream comprised of methanol, water and at least 99% of the acetaldehyde present in the crude epoxidation reaction product of the fractionator. eleven . The method of claim 10, where the upper pressure is 172.4 to 275.8 kPa, the bottom temperature is from 93 ° C to 104 ° C, and the reflux: distillate ratio is from 15: 1 to 25: 1. The method of claim 1, wherein the upper stream is comprised of at least 4 percent by weight of methanol. The method of claim 1 2, wherein the bottom stream contains at least 99% of the water originally present in the crude epoxidation reaction product.