IL28749A - Purification of propylene oxide - Google Patents

Description

Purification of propylene oxide HALCOH INTiSRNATIONAL, IMC* C.21 '302 - - The present invention relates to the purification of propylene oxide and, more particularly, to the separation of propylene oxide from by-products formed in its preparation,, Propylene oxide is conventionally prepared by the epoxidation of propylene or by the dehydrohalogenation of propylene halohydrin. As an example of such preparation, there may be mentioned the reaction between propylene and oxidising agents such as peroxides, peracids or hydroperoxides, and the reaction between propylene chlorohydrin and calcium hydroxide.

A disadvantage associated with processes involving the production o.f propylene oxide is the formation of by-products which are extremely difficult to. separate from propylene oxide. These difficultly removable impurities are hydrocarbons and are believed to be propylene derivatives having from 5 to 7 carbon atoms per molecule. The compounds include hexenes, hexanes, methyl pentenes and methyl pentanes. All of these materials have boiling points sufficiently close to that of propylene oxide, about 35°C, so that they are extremely difficult to separate from propylene oxide by direct fractionation. In some cases an azeotrope is formed, and complete separation is not possible by direct fractionation.

It is, therefore, an object of the present invention to provide a method for separating propylene oxide from propylene derivatives formed in its preparation. Another object is to provide a method for separating propylene oxide from hydrocarbons which contain 5 to 7 carbon atoms per molecule. A further object is to provide an improved method for purifying propylene oxide. . These and other objects of the present invention will become It has now been ound that propylene oxide may be separated from hydrocarbons with 5 to 7 carton atoms per molecule by fractionating propylene oxide in the presence of a ·8·196θ hydrocarbon having from 8 to 20 carbon atoms per molecule* This latter hydrocarbon, herein referred to as the solvent hydrocarbon, may be an alkane, an alkene or a naphthene, or a mixture thereof. Preferably, the hydrocarbon contains from 8 to 15 carbon atoms, and most preferably, from 8 to 10 carton atoms. As specific examples of suitable alkane hydrocarbons there may be mentioned octane, nonane, decane, undecane, dodeoane^ and branched* chain alkane isomers of any of these« As specific examples of alkene hydrocarbons there may be mentioned any of the mono-unsaturated alkanes ,8.1968 compounds corresponding to the 08»C20/and in particular alkanes Examples of suitable naphthenes are dimethyl cyclohexane, ethyl cyclohexane, methylethyl cyclohexane, diethyl cyclohexane, tetramethyl cyclohexane, dicyclohexyl, or any other naphthenes ,8,1968 having 8 to 20 and in particular 8 to 15 carbon atoms. The separation of propylene oxide from difficultly removable to O hydrocarbons is accomplished by distilling the impure propylene oxide in the presence of at least one part by weight of solvent hydrocarbon fed to the dlatilldLng column per three parts by weight of propylene oxide fed to the distilling columns Preferably the amount of solvent hydrocarbon is from about one part by weight to about 20 parts by weight per part by weight of propylene oxide* 0,8,1968 The fractionation may take place at atmospheric pressure, at elevated pressure or at reduced pressure Generall β pressures in the range of from about 3 psia to 100 psia are preferred· It has been found that the presence of a suitable solvent hydrocarbon in the distilling column acts to increase relative to the volatilities of Cg to C hydrocarbons . As a result of this enhanced volatility, it is possible to recover essentially all of the propylene oxide fed to the column as an overhead stream containing negligible amounts of C5 to C7 hydrocarbons . Essentially all of the C5 -C7 hydrocarbons in the propylene oxide feed stream can be removed in the column bottoms stream along with the solvent hydrocarbon. This bottoms stream may be further processed to separate the solvent hydrocarbon from the 05-07 hydrocarbons , and the solvent hydrocarbon recovered therefrom may be recycled to the distillation column .

The follov/ing example illustrates the present invention without , however, limiting the same thereto .

Example 1 Tertiary butyl hydroper oxide is reacted with an excess of propylene in the presence of a soluble molybdenum catalyst . The reactor effluent is flashed to remove unreacted propylene and then fractionated to produce an overhead stream of crude propylene oxide and a bottoms stream comprising mainly t-butyl alcohol along with some catalyst and other reaction products . The crude propylene oxide stream, which contains about 95 weight % propylene oxide, is analysed by gas chromatography, and the analysis shows significant amounts of C5 to C7 hydrocarbons such as 2- ethyl pentane and Cg olefins , the total concentration of such hydrocarbons being less than one weight ¾ . A sample of the crude propylene oxide is processed in an Othmer-typs recirculation still at atmospheric pressure to measure the volatilities of t e 1r^puritie« relative to the volatil ty cf propylene oxide . A second sample of the crude propylene oxide is mixed with 9 parts by weight of n-octane per part by weight of crude propylene oxide, processed similarly in an Othmer Still, and the relative volatilies are measured . A third sample of the crude propylene oxide is mixed with 9 parts by weight of 1-octene and processed in similar fashion. A fourth sample of the crude propylene oxide is mixed with 9 parts by weight of ethyl cyclohexane and processed in similar fashion. Table I shows effect of added solvent hydrocarbons on the volatilities of several impurities in the crude propylene oxide . All volatilities are measured relative to propylen oxide .

TABLE I Solvent Hydrocarbon None n-octane 1-octene ethylcyclohexan grams solvent hydrocarbon per gram of crude propylene oxide - 9 9 9 Relative volatility of component: propylene oxide 1 .00 1 .00 1 .00 1.00 2 -methyl pentane 1 .33 0.49 0.76 0.61 Cg olefin "A" 1. 12 0.63 0 .73 0.72 Cg olefin " B" 0.83 0.48 C7 paraffin 0.73 0 .27 0.48 The data in Table I show that the effect of the added solvent hydrocarbons is to increase the volatility of propylene oxide relative to that of the C6-C7 hydrocarbon impurities . For 2 -methyl pentane the volatility relative to propylene oxide is about 1 .39 at high concentrations of propylene oxide, but it is not possible to make a complete separation of propylene oxide from 2 -meth pentane by conventional fractionation because of the existence, of an aseotrope .

When a sufficient amount of a suitable solvent hydrocarbon such as n-octane is added, however, the volatility of 2-methyl pentane relative to propylene oxide is sufficiently reduced so that the azeotrope is avoided, and an essentially complete separation of 2-methyl pentane from propylene oxide can be achieved in a fractionation column. For the Cg olefin " B" , which is believed to be a methyl pentene, the volatility relative to propylene oxide is about 0.83 at high concentrations of propylene oxide, indicating that there is no azeotrope but that complete separation by conventional fractionation would be very difficult When a sufficient amount of a suitable solvent hydrocarbon such as n -octane is added, however, the volatility of the Cg olefin relative to propylene oxide is sufficiently reduced so that a complete separation can readily be obtained by fractionation.

Claims (11)

- & - 28749/2 G lMS

1. » A process for purifying propylene oxide which comprises fractionating impure propylene oxide in the presence of at least one part by weight of a hydrocarbon having from 8 to 20 carbon atoms in the molecule per three parts by weight of propylene oxide.

2. A process for purifyingpropylene oxide which comprises fractionating propylene oxide at pressures Of from about 3 psia to about 100 psla in the presence of at least one part by weight of a solvent hydrocarbon having from 8 to 15 carbon atoms per three parts by weight of propylene oxide9 the propylene oxide being recovered as overhead and the bottoms comprising the solvent hydrocarbon and hydrocarbons having 5 to 7 carbon atoms per molecule*

3. A process according to Claim 1 or 2, wherein the amount of solvent hydrocarbon is from about one part by weight to about 20 parts by weight per part by weight of propylene oxide.

4. A process according to Olai® 1 or 2, wherein the impure propylene oxide is obtained by the epoxidation of propylene·

5. A process according to Claim 4» wherein the epoxidation is effected by means of an organic hydroperoxide,

6. A process according to Claim 4» wherein the impure propylene oxide contains propylene derivatives.

7. A process according to Claim 6, wherein the propylene derivatives comprise compounds having from 5 to 7 carbon atoms in the molecule*, - 7 - 28749/2

8. A process according to Claim 2, wherein the hydrocarbon has from 8 to 10 carbon atoms in the molecule,

9. · A process according to Claim 8, wherein the impure propylene oxide s obtained by the epoxidation of propylene by means of an organic hydroperoxide, and contains propylene derivatives having from 5 to.7 carbon atoms in the molecule, the fractionation being carried out at pressures of from about 3 psia to about 100 psia.

10. A process for purifying propylene oxide according to Claim 1 , substantially as described hereinbefore.

11. A process for purifying propylene oxide according X. to Claim t substantially as exemplified hereinbefore. For the Applicants FARTHERS MC:mz