Purification of propylene oxide by extractive distillation

Abstract: An extractive distillation agent comprising ethylene carbonate, propylene carbonate or a mixture thereof is fed to an extractive distillation column used for the distillation of propylene oxide contaminated with water to obtain an overhead distillate fraction consisting of essentially anhydrous propylene oxide, and a heavier bottoms distillation fraction containing substantially all of the extraction distillation solvent and water introduced into the distillation column.

Description: This invention relates to the purification of propylene oxide. More particularly, this invention relates to a distillation process for removing contaminating quantities of water and acetone from an impure propylene oxide feedstock containing water and other oxygen-containing contaminants. Still more particularly, this invention relates to a method wherein an impure propylene oxide feedstock contaminated with from about 0.01 to about 2 wt.% of water and from about 50 to about 4,000 ppm of methanol is purified in an extractive distillation column using ethylene carbonate, propylene carbonate, or a mixture thereof, as an extractive distillation agent. It is known to react a hydroperoxide feedstock such as tertiary butyl hydroperoxide with propylene in the presence of an epoxidation catalyst in order to provide a reaction product comprising propylene oxide, an alcohol corresponding to the hydroperoxide feedstock, a solvent, and impurities (see, for example, Kollar U. S. Patent No. 3,350,422, Kollar U. S. Patent No. 3,351,635 and Sorgenti U. S. Patent No. 3,666,777. The propylene oxide that is prepared in this manner is contaminated with water, as is propylene oxide prepared by other processes. Propylene oxide prepared by a peroxidation process of the type exemplified by Kollar U. S. Patent No. 3,350,422 will also be contaminated with other oxygen-containing impurities such as methanol, acetone, methyl acetate, acetaldehyde, methyl formate and propionaldehyde. It has heretofore been proposed to purify propylene oxide by extractive distillation and a variety of extractive distillation solvents have been proposed for this purpose. However, there are problems. In Kageyama et al. U. S. Patent No. 3,838,020, which is directed to a process for purifying propylene oxide utilizing two extractive distillation columns and two extractive distillation solvent systems (e.g., a first distillation solvent comprising 1,3-butylene glycol, 1,4-butylene glycol, isobutylene glycol, glycerine and mixtures thereof and a second extractive distillation solvent comprising dioxane, butyl acetate, 2-ethylhexanol and mixtures thereof) points out that it is difficult to remove water and other impurities from propylene oxide by extractive distillation with a single solvent and that a single solvent will normally remove either water or organic impurities, but not both. Among the extractive distillation solvents that have been proposed are materials such as alkanes, alkenes and naphthenes containing 8 to 20 carbon atoms (Schmidt U. S. Patent No. 3,843,488) aromatic hydrocarbons containing 6 to 12 carbon atoms (Schmidt et al. U. S. Patent No. 3,909.366), and primary amines (Seifert et al. U. S. Patent No. 4,369,096). Robeson et al. U. S. Patent No. 2,622,060 discloses a process for the purification of propylene oxide contaminated with impurities, including methanol, by subjecting the impure propylene oxide to distillation in the presence of an extractive distillation agent comprising an aqueous solution of an alkali. The inventors report in Example 1 of their patent a method wherein 500 parts by weight of a crude propylene oxide fraction was extractively distilled in accordance with their invention to obtain 325 parts by weight of a product containing about 99.6 wt.% of propylene oxide. Thus, a significant loss of propylene oxide occurred during the process. Washall U. S. Patent No. 3,578,568 discloses a process for removing oxygen-containing impurities such as acetone, acetaldehyde and methanol from impure propylene oxide using a glycol such as ethylene glycol or propylene glycol as an extractive distillation agent. Hoory and Newman U. S. Patent No. 3,632,482 is directed to a propylene oxide recovery process by extractive distillation using an alcohol-ketone-hydrocarbon solvent. The invention relates to a method for the purification of crude propylene oxide contained in a mixture produced by the epoxidation of propylene with an organic hydroperoxide and calls for extractive distillation of the crude propylene oxide in a plurality of successive extractive distillation zones with the aid of a solvent mixture consisting essentially of hydrocarbons, alcohols, and/or ketones corresponding to the organic hydroperoxide employed in producing the propylene oxide. In the preferred embodiment of their invention, the extractive distillation agent is a recycle fraction from a three column distillation sequence wherein the bottoms from the third distillation column are flashed to obtain an overhead composed of hydrocarbons, alcohols and/or ketones which is recycled as an extractive distillation agent to the three distillation columns involved in the propylene oxide purification sequence. Burns et al. U. S. Patent No. 3,715,284 discloses a process for the purification of impure propylene oxide using acetone or a mixture of acetone with methanol which is introduced into a distillation column either below or together with the impure propylene oxide. Schmidt U. S. Patent No. 3,881,996 is directed to a distillation sequence employing at least three and optionally four columns for the purification of crude propylene oxide, one of the columns optionally being an extractive distillation column wherein a hydrocarbon such as octane is used as the extractive distillation agent. Schmidt U. S. Patent No. 4,140,588 discloses a process for the purification of propylene oxide contaminated with methanol and acetone using water as an extractive distillation agent, the water being introduced into the distillation column above the point of introduction of the crude propylene oxide feed. Schmidt states at column 2, lines 50-55 that: "Propylene oxide, however, has a substantial solubility in water and is readily hydrolyzed to propylene glycol (PG) in the presence of large amounts of water"--i.e., in the reboiler section of the tower. U. S. Patent No. 4,971,661 discloses the use of an aqueous acetone extraction to remove methanol from propylene oxide. U. S. Patent No. 3,578,568 discloses the use of glycols or glycol ethers in an extractive distillation to remove oxygen containing impurities such as acetone, acetaldehyde, and methanol. It is claimed that the concentration of the solvent in the vapor space in the extractive distillation zone of the distillation tower is preferably between 15 and 50 mole percent of the total vapor. Compared to U. S. Patent No. 3,578,568, this invention uses considerably lower concentrations of solvent in the extractive distillation zone to remove water and oxygen-containing impurities such as acetone. Since the concentration of the extractive distillation agent is lower, the size and heat requirements of the associated solvent regenerator are reduced. U. S. Patent No. 3,607,669 discloses the use of a C₈ to C₁₂ hydrocarbon to separate propylene oxide from water. Shih et al. U. S. Patent No. 5,000,825 discloses the purification of monoepoxides such as propylene oxide that are contaminated with oxygenated impurities such as water, low molecular weight alcohols, low molecular weight ketones, low molecular weight aldehydes and the like by the extractive distillation of the contaminated monoepoxide using a lower glycol containing 2 to 4 carbon atoms. Examples of lower glycols that are given in the patent include ethylene glycol, 1,2-propane diol, 1,3-propane diol, 1,4-butane diol, 1,2-butane diol, 1,3-butane diol and 2,3-butane diol. It is stated that higher diols or higher glycol ethers do not provide sufficient selectivity for the removal of such impurities and are not included as the extractive distillation solvents suitable for use in the invention. Mitchell et al. U. S. Patent No. 2,550,847 is directed to a process for separating purified propylene oxide from a crude propylene oxide product contaminated with acetaldehyde, methyl formate, methanol, etc., by treating the crude mixture with an aqueous basic substance followed by recovery of the purified propylene oxide by any suitable means such as by decantation. Mitchell et al. reported a recovery of a product containing 78 to 82 wt.% of propylene oxide which, they stated, could be increased in purity to about 95 to 99% by fractional distillation. An object of the present invention is to provide a distillation process for removing contaminating quantities of impurities including oxygen-containing impurities such as methanol, acetone and water from an impure propylene oxide feedstock. The present invention provides an extractive distillation process for the removal of oxygenated contaminants, including water, acetone and methanol from an impure propylene oxide feedstock characterised by introducing the said impure propylene oxide feedstock into the lower half of a distillation column, introducing an extractive distillation agent comprising ethylene carbonate, propylene carbonate or a mixture thereof above the impure propylene oxide feed point at a ratio of feedstock to extractive distillation agent of from 1:1 to 20:1, withdrawing from said distillation column an overhead distillate fraction comprising essentially anhydrous propylene oxide contaminated with reduced quantities of acetone and methanol, and a bottoms distillation fraction containing substantially all of the ethylene carbonate, propylene carbonate or a mixture thereof, water, acetone and a portion of the methanol introduced into said distillation column. Preferably the propylene oxide is charged to the lower half of a distillation column containing at least 25 theoretical plates and the extractive distillation agent is added at a point at least 4 theoretical plates above the said impure propylene oxide feed point. Most preferably distillation conditions comprise a pressure of 69 to 276 kPa (10 to 40 psia), a reflux ratio of from 1:1 to 5:1, a reboiler temperature within the range of 100° to 250°C and a top temperature of 20° to 80°C. When propylene is reacted in liquid phase with an organic hydroperoxide such as tertiary butyl hydroperoxide in solution in a solvent such as tertiary butyl alcohol in the presence of a soluble epoxidation catalyst such as a molybdenum epoxidation catalyst, a reaction mixture is formed comprising propylene oxide, an alcohol corresponding to the organic hydroperoxide feedstock and impurities including water and other oxygenated impurities such as methyl formate, acetaldehyde, acetone and methanol. Propylene oxide is a hygroscopic substance, so that water is removed only with difficulty. It is important to remove as much of the water as possible, however, because the water present in the propylene oxide will tend to react with the propylene oxide to form propylene glycol. It is also important to reduce the level of other oxygenated contaminants such as methanol to the lowest reasonably attainable level. In accordance with conventional practice, an epoxidation reaction product formed by the molybdenum-catalyzed reaction of propylene with tertiary butyl hydroperoxide in solution in tertiary butyl alcohol is separated into the principle components by distillation so as to form distillation fractions including a propylene distillation fraction, a propylene oxide distillation fraction, a tertiary butyl alcohol distillation fraction and a heavy distillation fraction containing the molybdenum catalyst and other products and by-products of the epoxidation reaction. However, the distillation fractions that are thus-obtained are characterized by the inclusion of impurities and, normally, must be further treated if commercially acceptable products are to be obtained. This is especially true for a propylene oxide distillation fraction contaminated with water and oxygenated contaminants including acetone. It has been surprisingly discovered in accordance with the present invention that substantially all of the water initially present in a contaminated propylene oxide feedstock can be removed therefrom when the propylene oxide feedstock is extractively distilled in the presence of an extractive distillation agent consisting essentially of ethylene carbonate, propylene carbonate, or a mixture thereof. Even more surprising is our discovery that substantially all of the acetone and some of the methanol present in the contaminated feedstock can also be removed from the propylene oxide when using ethylene carbonate, propylene carbonate, or a mixture thereof, as the extractive distillation agent. The invention will be further described with reference to the accompanying drawing. The drawing is a schematic flow sheet with conventional parts omitted showing the general recovery sequence that is used in accordance with the present invention in purifying propylene oxide. In the drawing, for convenience, the present invention is illustrated in connection with a process wherein the propylene oxide is prepared by the epoxidation of propylene with tertiary butyl hydroperoxide in solution in tertiary butyl alcohol to provide a reaction product comprising propylene oxide and additional tertiary butyl alcohol. Turning now to the drawing, there is shown a schematic flow sheet illustrating a preferred method of practicing the process of the present invention. In the drawing, conventional parts such as valves, pumps, temperature sensors, pressure sensors, heaters, coolers and flow control regulation apparatus have been omitted. In accordance with a preferred embodiment of the present invention, propylene oxide is separated in a preliminary distillation zone (not shown) from other components of an epoxidation reaction mixture in order to provide an impure propylene oxide fraction contaminated with oxygen-containing impurities such as acetone, methanol and water. The impure propylene oxide feedstock that is thus obtained in the preliminary distillation zone is then purified in a propylene oxide purification distillation zone, which in accordance with the preferred embodiment of the present invention, comprises two distillation columns, each of which is equipped with an appropriate reflux condensing means and an appropriate reboiler heating means. In accordance with the present invention, an impure propylene oxide fraction contaminated with from about 50 to 4,000 ppm of methanol, from about 0.01 to 2 wt.% of acetone and about 0.01 to 2 wt.% of water and other oxygen-containing impurities is charged by way of a line 120 leading to a distillation column 100 which, in accordance with the present invention, will preferably be a column containing at least about 10 theoretical plates and more preferably, from about 30 to 100 theoretical plates. The column 100 is suitably operated under distillation conditions including a pressure of about 69 to 276 kPa (10 to 40 psia), a reflux ratio of from 2:1 to 10:1, a reboiler temperature within the range of about 100° to 250°C (e.g., 210°C.) and a top temperature of about 20° to 80°C. (e.g., 20°C.). The impure propylene oxide is preferably charged to the distillation column 100 in the lower half thereof. An extractive distillation agent consisting essentially of ethylene carbonate, propylene carbonate or a mixture thereof is charged to the upper half of the distillation column 100 by an extractive distillation charge line 106. Essentially anhydrous purified propylene oxide containing about 100 ppm or less of water is removed from the column 100 as a light distillation fraction 112, the purified propylene oxide in the line 112 containing significantly reduced amounts of acetone, such as about 0.1 to 100 ppm of acetone and a reduced amount of methanol. A heavier fraction 110 is withdrawn from the distillation column 100 which contains substantially all of the extractive distillation agent charged by the line 106 and also substantially all of the water, acetone and other oxygen-containing impurities introduced into the column 100 with the impure propylene oxide 120. The heavier distillation fraction 110 from the column 100 comprising water, methanol, acetone, tertiary butyl alcohol and other impurities and extractive distillation agent is charged to a second distillation column 200 wherein light impurities such as methanol, acetone, water, etc., are separated overhead as a distillation fraction 204 that is discharged from the system for any suitable use, such as for use as a steam boiler feedstock or for recovery. A heavier distillation fraction 106 is discharged from the distillation column 200 comprising alkylene carbonate which is recycled to distillation column 100 by line 106. Ethylene carbonate is a compound having the formula: Ethylene carbonate is a liquid having a boiling point of 248°C., a vapor pressure of <1 mm at 40°C., a flash point of 290°F. (143°C.) and a specific gravity of 1.3218 (39/4°C.). Technical grade ethylene carbonate contains water (e.g., generally about 0.005 to about 0.3 wt.%), and most normally 0.01-0.1% water. Propylene carbonate is a compound having the formula: Propylene carbonate is a liquid having a boiling point of 241.7°C., a vapor pressure of <1 mm at 20°C., a flash point of 270°F. (132°C.) and a specific gravity of 1.2057 (20/4°C.). Industrial grades of ethylene carbonate and propylene carbonate and mixtures thereof contain water. Therefore, if industrial grades of ethylene carbonate and propylene carbonate were introduced directly into the column 100, a substantial amount of undesired contaminating water would also be introduced. In accordance with the present invention, the ethylene carbonate and propylene carbonate, either as the original charge, or as make-up solvent, is introduced into the system by a branch line 230 leading to the charge line 110 for the second distillation column 200 so that any water introduced into the system with the fresh ethylene carbonate or propylene carbonate, or a mixture thereof, will be separated therefrom in the column 200 and withdrawn from the column 200 through the line 204. Water and oxygen-containing impurities such as acetone and methanol are difficult to remove from propylene oxide by standard distillation. The use of the invention as presently described improves the separation of these impurities from propylene oxide. EXAMPLES The invention will be further illustrated by the following non-limiting examples. Where parts are mentioned, they are parts by weight. Comparative distillation runs were made without the use of an extractive distillation solvent and with propylene carbonate and a mixture of ethylene carbonate with propylene carbonate as the solvents. The results are tabulated below. The runs were conducted in a 25 actual plate pilot plant distillation column at an operating pressure of 96 to 124 kPa(14-18 psia). The runs were made on a semi-batch basis wherein an actual known mass of impure propylene oxide was charged to a round-bottomed flask that was attached to the bottom of the Oldershaw-type distillation column. The distillation column had a feed line to the top tray. A solvent feed tank on a scale was connected to the feed line. A swinging bucket type of reflux splitter was placed on the top of the distillation column and a condenser was placed in the conventional manner over the reflux splitter. Each run was begun by heating the propylene oxide in the round-bottomed flask until the propylene oxide was boiling up the entire length of the column. The column was maintained in total reflux as a controlled amount of solvent was allowed to flow into the column through thge feed line. The solvent was permitted to flow through the feed line for a short period of time before beginning to take material off. The reflux ratio was set at 15/5 at a controlled take off rate so as to maintain a consistent ratio of solvent feed rate to overhead take off rate. The run was continued until 80 wt.% of the original propylene oxide had been taken off as overhead. The overhead and bottoms were then analyzed for their composition. In the case where no solvent was used, the procedure was the same except that no solvent was fed through the feed line to the top tray. Example 1 No Solvent: Component Feed wt% Solvent wt% Overhead wt% Bottoms wt% Water 0.170 N/A 0.0600 1.390 MeOH 0.201 N/A 0.0969 N/An Acetone 0.511 N/A 0.0017 N/An Note: ND is not detected N/A is not applicable N/An is not analyzed Example 2 No Solvent: Component Feed wt% Solvent wt% Overhead wt% Bottoms wt% Water 0.170 N/A 0.0784 2.1 MeOH 0.201 N/A 0.1057 N/An Acetone 0.511 N/A 0.0006 N/An Note: ND is not detected N/A is not applicable N/An is not analyzed Example 3 EC/PC 50/50: Component Feed wt% Solvent wt% Overhead wt% Bottoms wt% Water 0.170 0.0266 0.0190 0.254 MeOH 0.190 N/An 0.0600 N/An Acetone 0.479 N/An 0.0012 N/An Solvent:PO Ratio = 0.96 Note: ND is not detected N/A is not applicable N/An is not analyzed Example 4 Propylene Carbonate: Component Feed wt% Solvent wt% Overhead wt% Bottoms wt% Water 0.170 0.0013 0.0293 0.334 MeOH 0.190 N/An 0.0827 N/An Acetone 0.479 N/An 0.0012 N/An Solvent:PO Ratio = 0.78 Note: ND is not detected N/A is not applicable N/An is not analyzed

Claims: An extractive distillation process for the removal of oxygenated contaminants, including water, acetone and methanol from an impure propylene oxide feedstock characterised by introducing the said impure propylene oxide feedstock into the lower half of a distillation column, introducing an extractive distillation agent comprising ethylene carbonate, propylene carbonate or a mixture thereof above the impure propylene oxide feed point at a ratio of feedstock to extractive distillation agent of from 1:1 to 20:1, withdrawing from said distillation column an overhead distillate fraction comprising essentially anhydrous propylene oxide contaminated with reduced quantities of acetone and methanol, and a bottoms distillation fraction containing substantially all of the ethylene carbonate, propylene carbonate or a mixture thereof, water, acetone and a portion of the methanol introduced into said distillation column. An extractive distillation process as claimed in Claim 1 wherein the said impure propylene oxide feedstock comprises propylene oxide contaminated with from 0.01 to 2 wt.% of water and from 50 to 4000 ppm of methanol. A process as claimed in Claim 1 or Claim 2 wherein the propylene oxide is charged to the lower half of a distillation column containing at least 25 theoretical plates and the extractive distillation agent is added at a point at least 4 theoretical plates above the said impure propylene oxide feed point. A process as claimed in any one of Claims 1 to 3 wherein the distillation conditions comprise a pressure of 69 to 276 kPa (10 to 40 psia), a reflux ratio of from 1:1 to 5:1, a reboiler temperature within the range of 100° to 250°C and a top temperature of 20° to 80°C. An extractive distillation process as claimed in any one of Claims 1 to 4 wherein the extractive distillation solvent is ethylene carbonate. An extractive distillation process as claimed in any one of Claims 1 to 4 wherein the extractive distillation solvent is propylene carbonate. An extractive distillation process as claimed in any one of Claims 1 to 4 wherein the extractive distillation solvent is a mixture of ethylene carbonate with propylene carbonate.

Inventors: CULBRETH WILLIAM KEMP III, MARQUIS EDWARD THOMAS, POTTRATZ DAVID GEORGE, SHEU YU-HWA EDWARD, SPERANZY GEORGE PHILLIP, CULBRETH, WILLIAM KEMP III, MARQUIS, EDWARD THOMAS, POTTRATZ, DAVID GEORGE, SHEU, YU-HWA EDWARD, SPERANZY, GEORGE PHILLIP
Applicants: TEXACO CHEMICAL, TEXACO CHEMICAL COMPANY
Application year: 1992
Main classification: C07D301
IPC codes: C07D303, C07D301, B01D3