Product purification propylene oxide

Hydroperoxide Process. The hydroperoxide process to propylene oxide involves the basic steps of oxidation of an organic to its hydroperoxide, epoxidation of propylene with the hydroperoxide, purification of the propylene oxide, and conversion of the coproduct alcohol to a useful product for sale. Incorporated into the process are various purification, concentration, and recycle methods to maximize product yields and minimize operating expenses. Commercially, two processes are used. The coproducts are / fZ-butanol, which is converted to methyl tert-huty ether [1634-04-4] (MTBE), and 1-phenyl ethanol, converted to styrene [100-42-5]. The coproducts are produced in a weight ratio of 3—4 1 / fZ-butanol/propylene oxide and 2.4 1 styrene/propylene oxide, respectively. These processes use isobutane (see Hydrocarbons) and ethylbenzene (qv), respectively, to produce the hydroperoxide. Other processes have been proposed based on cyclohexane where aniline is the final coproduct, or on cumene (qv) where a-methyl styrene is the final coproduct. 

Fiaal purification of propylene oxide is accompHshed by a series of conventional and extractive distillations. Impurities ia the cmde product iaclude water, methyl formate, acetone, methanol, formaldehyde, acetaldehyde, propionaldehyde, and some heavier hydrocarbons. Conventional distillation ia one or two columns separates some of the lower boiling components overhead, while taking some of the higher boilers out the bottom of the column. The reduced level of impurities are then extractively distilled ia one or more columns to provide a purified propylene oxide product. The solvent used for extractive distillation is distilled ia a conventional column to remove the impurities and then recycled (155,156). A variety of extractive solvents have been demonstrated to be effective ia purifyiag propylene oxide, as shown ia Table 4. 

Although this process has not been commercialized, Daicel operated a 12,000-t/yr propylene oxide plant based on a peracetic acid [79-21-0] process during the 1970s. The Daicel process involved metal ion-catalyzed air oxidation of acetaldehyde in ethyl acetate solvent resulting in a 30% peracetic acid solution in ethyl acetate. Epoxidation of propylene followed by purification gives propylene oxide and acetic acid as products (197). As of this writing (ca 1995), this process is not in operation. 

Propylene oxide is a useful chemical intermediate. Additionally, it has found use for etherification of wood (qv) to provide dimensional stabiUty (255,256), for purification of mixtures of organosiUcon compounds (257), for disinfection of cmde oil and petroleum products (258), for steriliza tion of medical equipment and disinfection of foods (259,260), and for stabilization of halogenated organics (261—263). 

The stream from the reaction section is first distilled to remove unconverted propylene, whose recycle, added to the make-up, represents the feed of the first epoxidation stage. Excess propane is also removed by distillation (—50 to 60 trays) to prevent its buildup in tfie synthesis loop. The heavy end of the first column is sent to the purification train for products for which the temperatures cannot exceed 100°C to avoid undesirable degradation. On account of the boiling points at standard pressure of the components present, this makes operation under vacuum necessary. Crude propylene oxide is collected at the top of the first distillation column (50 trays), and r-butyl alcohol at the1bottom, with some hydroperoxide, the catalyst, propylene glycol, aldehydes, esters etc. This stream is sent to a r-butyl alcohol separation column (35 to 40 trays), where the alcohol is recovered at the top. 

Scheme I shows a simplified block diagram illustrating the four main stq>s of a new route to propylene oxide production. In die first step, an alkylanthrahydroquinone, propylene and air react through a series of reactors producing propylene oxide, a minor amount of solvolysis products and water. Propylene oxide is separated by distillation and recovered. In die next step, methanol, propylene glycol and its methyl ether derivatives are extracted widi water and purified. The remaining organic phase passes to the alkylanthraquinone purification/hydrogenation step and finally is fed widi methanol, back to the epoxidation reactors. The regeneration and purification of the working solution are not shown in Scheme I.

The yield of PO in the Halcon process is in the range 87-91% and more than 2 t of the co-product styrene are generated for each produced ton of propylene oxide. The investment costs for the ethylbenzene process are higher than for the tert-butanol process, because of the isolation and purification demands for polymer-grade styrene, figure 6.12.6 shows the plant design for an indirect propylene oxidation process via ethylbenzene hydroperoxide. 

The products are prepared by the alkoxylation of cetyl alcohol or stearyl alcohol using ethylene oxide or propylene oxide. In the second step, the intermediate is reacted with acryhc acid, meth-acrylic acid or a mixture from these acids. When the esterification is completed, the unreacted acid is distUled off from the product under high vacuum. An additional purification of the ester is not necessary (28). The reaction is shown in Figure 5.4. 

The purified form of cellulose, obtained from cotton linters or wood pulp, is treated with sodium hydroxide solution to produce swollen alkali cellulose that is chemically more reactive than untreated cellulose. Reaction of the alkali cellulose with chloro-methane and propylene oxide produces methyl hydroxypropyl ethers of cellulose. Further purification of the fibrous reaction product is done and is grounded to fine, uniform powder or granules. 

In the two-step process, the second-stage reactor is similar to the first-stage reactor but is packed with an optimized catalyst for aldehyde oxidation, based on Mo V oxides, and is run under different operating conditions. Care must be exercised during the separation and purification phases to avoid conditions favouring acrylic acid polymerization, e.g., by addition of a radical polymerization inhibitor such as the hydroquinone monomethyl ether. Selectivities to acrylic acid are higher than 90% at total conversion of the aldehyde. Overall yields referred to propylene are in the range 75-85%. Most acrylic acid produced is esterified for the production of acrylate esters. 

Conditions for the direct oxidation of propylene to acrolein include use of a catalyst of cuprous oxide deposited on granular silicon carbide, catalyst temperature of 375 C, feed stream composition by volume of 20 per cent propylene, 20 per cent air, and 60 per cent steam, and contact time of 1 sec. Recovery and primary purification of the acrolein from the reaction product are effected by quench scrubbing the reactor effluent with water and wth liquid propylene. The composition of the carbonylic compounds in the product is, approximately, acrolein, 90 per cent by weight acetaldehyde, 6 per cent propionaldehyde, 2 per cent and acetone, 2 per cent. At reaction temperatures of about 300 C and conversions of about 50 per cent, a selectivity to acrolein of about 40 per cent is reported for 10 per cent propylene-in-air mixtures. ... 

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