Purification of propylene oxide by extractive distillation

Abstract: An extractive distillation agent consisting essentially of dipropylene glycol is fed to an extractive distillation column used for the distillation of propylene oxide contaminated with water, acetone and methanol to obtain an overhead distillate fraction consisting of essentially anhydrous propylene oxide contaminated with reduced quantities of acetone and methanol, and a heavier bottoms distillation fraction containing substantially all of the dipropylene glycol, water and acetone and some of the methanol 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 impurities including oxygen-containing impurities such as methanol, acetone and water from an impure propylene oxide feedstock. Still more particularly, this invention relates to a method wherein an impure propylene oxide feedstock contaminated with from about 50 to about 4,000 ppm of methanol, from about 0.01 to about 2 wt.% of water and from about 0.01 to about 2 wt.% of acetone is purified in an extractive distillation column using dipropylene glycol 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.) It is also known to separate the reaction product by distillation in order to obtain a plurality of fractions including, for example, a propylene recycle fraction, a propylene oxide product fraction and an alcohol fraction. It is also known that methanol, acetone and water are common contaminants for propylene oxide which are removed only with difficulty. For example, 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. 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. In a process unrelated to the purification of propylene oxide, Goddin et al. in U. S. Patent No. 2,751,337 disclose a process for separating acetone from a mixture of acetone with methanol and methyl acetate utilizing water as an extractive distillation agent. Hamlin et al. in U. S. Patent No. 3,409,513 disclose the hydro-extractive distillation of mixtures comprising acetone, lower aliphatic alcohols and esters of lower aliphatic alcohols with carboxylic acids. It is pointed out by the patentees that acetone, methyl acetate and methanol form an azeotrope boiling at 55.5-56.5°C. Hamlin et al. propose to recover partially purified acetone from such a terniary azeotrope by liquid-liquid extraction with water followed by hydro-extractive distillation of the aqueous phase in order to obtain a partially purified acetone fraction. 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 dipropylene glycol is lower, the size and heat requirements of the associated dipropylene glycol regenerator are reduced. U. S. Patent No. 4,140,588 discloses the use of water in an extractive distillation to remove methanol and acetone. 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. 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 dipropylene glycol 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 dipropylene glycol, 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 and acetone 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 methanol and 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 dipropylene glycol. Even more surprising is our discovery that substantially all of the acetone and most of the methanol present in the contaminated feedstock can also be removed from the propylene oxide when using dipropylene glycol as the extractive distillation agent. 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. The invention will be further described with reference to the accompanying drawing. 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 276kPa (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 dipropylene glycol 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 methanol and acetone, such as about 15 to 900 ppm of methanol and about 0.1 to 100 ppm of acetone. 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 dipropylene glycol which is recycled to distillation column 100 by line 106. Dipropylene glycol is a mixture of isomers having the following formulas: Dipropylene glycol is a colorless, hygroscopic, practically odorless liquid having a boiling point of 231.8°C., a vapor pressure of less than 0.01 mm (20°C.) and a specific gravity of 1.0252 (20/20°C.). Technical grades of dipropylene glycol will contain an appreciable amount of water (e.g., about 0.01 to 0.1 wt.%). Therefore, if fresh technical grade dipropylene glycol were introduced directly into the column 100, a substantial amount of undesired contaminating water would also be introduced. In accordance with the present invention, fresh dipropylene glycol, 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 dipropylene glycol 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. Example I Comparative distillation runs without the use of an extractive distillation solvent and with dipropylene glycol as the solvent are tabulated below. The runs were conducted in a 25 plate 2.54cm (1") Oldershaw type glass distillation column operating at atmospheric pressure. No Solvent: Component Feed wt% Solvent wt% Overhead wt% Bottoms wt% Water 0.1800 N/A 0.0784 01.3900 (WATER FREE BASIS) Lighter than MeOH/MF ND N/A ND N/An MeOH/MF 0.207 N/A 0.119 N/An PO 99.184 N/A 99.881 N/An Acetone 0.524 N/A ND N/An TBA 0.084 N/A ND N/An Heavier than TBA 0.005 N/A ND N/An Note: ND is not detected N/A is not applicable N/An is not analyzed Dipropylene Glycol Extraction Solvent: Component Feed wt% Solvent wt% Overhead wt% Bottoms wt% Water 0.1800 0.0287 0.0150 0.2990 (WATER FREE BASIS) Lighter than MeOH/MF 0.207 ND 0.2470 ND MeOH/MF 0.218 ND ND 0.343 PO 98.708 ND 99.7530 15.599 Propanol/Propenal ND ND ND 0.021 Acetone 0.754 ND 0.0010 0.724 TBA 0.112 ND ND 0.118 Heavier than TBA 0.001 0.022 ND 0.308 Dipropylene Glycol ND 99.978 ND 82.908 Note: ND is not detected Solvent:Feed ratio 0.89 (kg/kg) Example II A glass 5cm (2") diameter vacuum jacketed Oldershaw extractive distillation column having 120 actual plates was used for additional runs. The crude propylene oxide feed was introduced on tray 80 (from the top) and the dipropylene glycol extractive distillation solvent was introduced on tray 30. The solvent:feed ratio was 5:1 by weight. The tower operating pressure was 160kPa (23 psia) at the condenser. The composition of the propylene oxide feedstock is reported in Table I. Technical grade dipropylene glycol was used and contained about 260 ppm of water. The results are reported in Tables II and III, which follow. The results show substantially complete removal of water from the feed propylene oxide and also a significant reduction in the methanol content of the overheads distillation propylene oxide product. EXTRACTIVE DISTILLATION OF IMPURE PROPYLENE OXIDE WITH DIPROPYLENE GLYCOL PROPYLENE OXIDE FEED COMPOSITIONS , wt.% Run Number 1 2 3 4 5 6 Propylene Oxide 98.657 98.683 98.661 98.731 98.683 98.709 Methanol 0.221 0.211 0.222 0.212 0.218 0.220 Acetone 0.799 0.752 0.751 0.751 0.779 0.747

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 dipropylene glycol 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 dipropylene glycol, 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.

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