Approximately 14.5 billion pounds of propylene oxide are produced every year. Propylene oxide has many uses. Between 60 and 70% of all propylene oxide is converted to polyether polyols for the production of polyurethane plastics. About 20% of propylene oxide is hydrolyzed into propylene glycol, via a process which is accelerated either by thermal reaction or by acid or base catalysis. Other major products are polypropylene glycol, propylene glycols ethers, and propylene carbonate. To produce these end products, propylene oxide free of impurities is needed.
Methods of producing alkylene oxides including propylene oxide involve hydrochlorination and epoxidation of its corresponding olefins. The oxidates used in the epoxidation processes are derived from tertiary or secondary hydrocarbons by direct oxidation with molecular oxygen; hence, they contain oxygenate impurities and precursors. Additional oxygenate impurities are also generated in the step of epoxidation of olefins. Crude alkylene oxides, such as propylene oxide, particularly those produced from epoxidation with hydrocarbon oxidates contain a significant amount of oxygenated impurities difficult to separate from alkylene oxides. The impurities generally include water, acids, alcohols, aldehydes, ketones and esters. A need exists for continued improvement of systems and methods for separating propylene oxide from these impurity constituents of effluent streams of various methods of producing propylene oxide.
U.S. Pat. No. 3,338,800 teaches extractive distillation of alkylene oxides having from 3 to 18 carbon atoms with a paraffin or paraffin naphtha solvent. More particularly, this patent suggests that oxygenated impurities boiling within 5° C. of the alkylene oxide may be separated by extractive distillation using acyclic paraffinic hydrocarbons as solvents having boiling points at least 35° C. above the boiling points of the said impurities. The problem addressed by this patent is that epoxide fractions produced by the direct oxidation of ethylenically unsaturated compounds with molecular oxygen in the liquid phase contain oxygenated impurities which, because their boiling points are similar to the desired epoxide product, cannot be separated by conventional distillation techniques.
U.S. Pat. No. 3,881,996 teaches that the sequence of the fractionation steps has a major effect on the final purity of the propylene oxide obtained, particularly with regard to aldehyde content. Substantially improved results are obtained when the removal of acetaldehyde and lower boiling materials precedes the step in which propylene oxide is separated from propionaldehyde and higher boiling material. This result is highly unusual and is not in accord with customary calculable performance of fractional distillation equipment. The inventor believes that chemical reactions may be occurring during distillation which interfere with the normal mass transfer steps and thereby produce anomalous results. However, the scientific reasoning is not offered.
U.S. Pat. Nos. 3,464,897 and 3,843,488 teach using hydrocarbon solvents of 8-20 carbon atoms can effective remove C5-C7 impurities from propylene oxide in extractive distillation. U.S. Pat. No. 3,607,669 teaches a method for separating propylene oxide from water by distilling the mixture in the presence of acyclic or cyclic paraffin containing 8 to 12 carbon atoms by breaking water-propylene oxide azeotrope at elevated pressure. There are many other U.S. Patents, such as U.S. Pat. Nos. 4,140,588, 5,000,825, 5,006,206, 5,116,466, 5,116,467, 5,139,622, 5,145,561, 5,145,563, 5,154,803, 5,154,804, 5,160,587, 5,340,446, 5,620,568, 5,958,192 and 6,559,248 that reflect use of various solvents in extractive distillation operations for propylene oxide purification. U.S. Pat. Nos. 2,550,847, 2,622,060, 3,350,417, 3,477,919, 4,691,034, 4,691,035, 5,106,458 and 5,107,002 teach how to separate methyl formate from propylene oxide. Although these patents teach the removal of selected propylene oxide impurities, none address removal of aldehydes, particularly formaldehyde and acetaldehyde.
U.S. Pat. No. 6,024,840 uses methanol as extractive solvent to remove acetaldehyde from propylene. However, solvent methanol itself becomes close-boiling propylene oxide contaminant. U.S. Pat. No. 7,705,167 teaches using water wash propylene oxide followed by contacting aqueous phase with hydrocarbon extractive solvent and subsequent distillation. These teachings are impractical for the existing plant improvement. Because it is difficult to recover a propylene oxide containing total aldehydes below 50 ppm and free of formaldehyde, particularly for propylene oxide produced from tert-butyl hydroperoxide process, it is the objective of the present invention to provide a method applicable to the existing plants for recovering propylene oxide in a high state of purity low in aldehydes without substantial loss of propylene oxide product.