Propylene hydration

Manufacturing plants in the United States are beheved to use solely indirect propylene hydration. Several European companies, eg, British Petroleum, Shell, and Deutsche Texaco, also employ this older technology in plants in Europe and Japan (67). 

Rea.Ctlon Mechanism. Propylene hydration in dilute acid solution probably proceeds according to the rate-determining formation of propyl carbonium ion (115). 

According to this mechanism, the reaction rate is proportional to the concentration of hydronium ion and is independent of the associated anion, ie, rate = / [CH3Hg][H3 0 ]. However, the acid anion may play a marked role in hydration rate, eg, phosphomolybdate and phosphotungstate anions exhibit hydration rates two or three times that of sulfate or phosphate (78). Association of the polyacid anion with the propyl carbonium ion is suggested. Protonation of propylene occurs more readily than that of ethylene as a result of the formation of a more stable secondary carbonium ion. Thus higher conversions are achieved in propylene hydration. 

Table 3. Calculated Vapor—Liquid Equilibrium Composition for Propylene Hydration ...

Conversions of ca 75% are obtained for propylene hydration over cation-exchange resins in a trickle-bed reactor (102). Excess Hquid water and gaseous propylene are fed concurrentiy into a downflow, fixed-bed reactor at 400 K and 3.0—10.0 MPa (30—100 atm). Selectivity to isopropanol is ca 92%, and the product alcohol is recovered by azeotropic distillation with benzene. 

In the liquid-phase process, high pressures in the range of 80-100 atmospheres are used. A sulfonated polystyrene cation exchange resin is the catalyst commonly used at about 150°C. An isopropanol yield of 93.5% can be realized at 75% propylene conversion. The only important byproduct is diisopropyl ether (about 5%). Figure 8-4 is a flow diagram of the propylene hydration process. ... 

Tokuyama Soda commercialized a catalytic process for propylene hydration catalyzed by aqueous solutions of heteropolyacids such as H3PW12O40 (165). In Table XV, the activities of various acids are compared at a constant proton concentration. H3PW12O40 is two or three times more active than H2S04 or H3PO4. The reason for the high activity is assumed to be the stabilization of intermediate propyl cations by coordination. 

Small autoclaves, properly attended, are useful at high pressures agitation must be efficient and the vessel must attain the operating temperature rapidly or the catalyst will be damaged by side reactions. The results are at best qualitative and at their worst definitely deceptive, as is shown by the data of Table I for propylene hydration at 250° and 270 atm. The percentage of isopropanol in the product is indicated the equilibrium concentration is 11.4%. 

Concentrated 70-98% snlfuric acid was used as the catalyst at a low pressure and temperature. The reaction mechanism involved the sulfate monoester intermediate, which was then hydrolyzed to isopropanol. The proeess is similar to the production of ethanol from ethylene, although the hydration step beeomes easier as the carbon number of the olefin inereases. The acid strength reqnired for effective operation deereases from more than 90% for ethylene hydration to abont 70% for propylene hydration. 

Figure 10.3a shows a simplified fiowsheet for the production of isopropyl alcohol by the direct hydration of propylene. Different reactor technologies are available for the process, and separation and recycle systems vary, but Fig. 10.3a is representative. Propylene... 

Figure 10.3 Outline flowsheet for the production of isopropyl alcohol by direct hydration of propylene. (From Smith and Petela, Chem. Eng., 513 24, 1991 reproduced by permission of the Institution of Chemical Engineers.)...

Because huge quantities of by-product acetonitrile are generated by ammoxidation of propylene, the nitrile may be a low cost raw material for acetamide production. Copper-cataly2ed hydration gives conversions up to 83% (11), and certain bacteria can effect the same reaction at near room temperature (12). 

Worldwide, approximately 85% of acetone is produced as a coproduct with phenol. The remaining 17% is produced by on-purpose acetone processes such as the hydration of propylene to 2-propanol and the dehydrogenation of 2-propanol to acetone. The cost of production of 2-propanol sets the floor price of acetone as long as the acetone demand exceeds the coproduct acetone supply. However, there is a disparity in the growth rates of phenol and acetone, with phenol demand projected at 3.0%/yr and acetone demand at 2.0%/yr. If this continues, the coproduct supply of acetone will exceed the total acetone demand and on-purpose production of acetone will be forced to shut down the price of acetone is expected to fall below the floor price set by the on-purpose cost production. Projections indicate that such a situation might occur in the world market by 2010. To forestall such a situation, companies such as Mitsui Petrochemical and Shinnippon (Nippon Steel) have built plants without the coproduction of acetone. 

The indirect hydration, also called the sulfuric acid process, practiced by the three U.S. domestic producers, was the only process used worldwide until ICI started up the first commercial direct hydration process in 1951. Both processes use propylene and water as raw materials. Early problems of high corrosion, high energy costs, and air pollution using the indirect process led to the development of the direct hydration process in Europe. However, a high purity propylene feedstock is required. In the indirect hydration process, C -feedstock streams from refinery off-gases containing only 40—60 wt % propylene are often used in the United States. 

Indirect Hydration. Indirect hydration is based on a two-step reaction of propylene and sulfuric acid. In the first step, mixed sulfate esters, primarily isopropyl hydrogen sulfate, but also diisopropyl sulfate, form. These are then hydrolyzed, forming the alcohol and sulfuric acid. 

Process. A typical indirect hydration process is presented in Eigure 1. In the process, propylene reacts with sulfuric acid (>60 wt%) in agitated reactors or absorbers at moderate (0.7—2.8 MPa (100—400 psig)) pressure. The isopropyl sulfate esters form and are maintained in the Hquid state at 20—80°C. Low propylene concentrations, ie, 50 wt %, can be tolerated, but concentrations of 65 wt % or higher are preferred to achieve high alcohol yields. Because the reaction is exothermic, internal cooling coils or external heat exchangers are used to control the temperature. 

Direct Hydration. The acid-catalyzed direct hydration of propylene is exothermic and resembles the preparation of ethyl alcohol from ethylene (qv). 

A typical process scheme for the direct hydration of propylene is shown ia Figure 2. Turnkey plants based on this technology are available (71,81). The principal difference between the direct and iadirect processes is the much higher pressures needed to react propylene direcdy with water. Products and by-products are also similar, and refining systems are essentially the same. Under some conditions, the high pressures of the direct process can increase the production of propylene polymers. 

A Hquid-phase variation of the direct hydration was developed by Tokuyama Soda (78). The disadvantages of the gas-phase processes are largely avoided by employing a weakly acidic aqueous catalyst solution of a siHcotungstate (82). Preheated propylene, water, and recycled aqueous catalyst solution are pressurized and fed into a reaction chamber where they react in the Hquid state at 270°C and 20.3 MPa (200 atm) and form aqueous isopropyl alcohol. Propylene conversions of 60—70% per pass are obtained, and selectivity to isopropyl alcohol is 98—99 mol % of converted propylene. The catalyst is recycled and requites Htde replenishment compared to other processes. Corrosion and environmental problems are also minimized because the catalyst is a weak acid and because the system is completely closed. On account of the low gas recycle ratio, regular commercial propylene of 95% purity can be used as feedstock. 

The effects of pressure and temperature on the equihbrium concentration of alcohol ia both phases of hydration of propylene when both Hquid and vapor phases are present have been calculated and are presented ia Table 3. Low temperature reduces by-product diisopropyl ether. 

Propanol has been manufactured by hydroformylation of ethylene (qv) (see Oxo process) followed by hydrogenation of propionaldehyde or propanal and as a by-product of vapor-phase oxidation of propane (see Hydrocarbon oxidation). Celanese operated the only commercial vapor-phase oxidation faciUty at Bishop, Texas. Since this faciUty was shut down ia 1973 (5,6), hydroformylation or 0x0 technology has been the principal process for commercial manufacture of 1-propanol ia the United States and Europe. Sasol ia South Africa makes 1-propanol by Fischer-Tropsch chemistry (7). Some attempts have been made to hydrate propylene ia an anti-Markovnikoff fashion to produce 1-propanol (8—10). However, these attempts have not been commercially successful. 

Isopropyl Alcohol. Propylene may be easily hydrolyzed to isopropyl alcohol. Eady commercial processes involved the use of sulfuric acid in an indirect process (100). The disadvantage was the need to reconcentrate the sulfuric acid after hydrolysis. Direct catalytic hydration of propylene to 2-propanol followed commercialization of the sulfuric acid process and eliniinated the need for acid reconcentration, thus reducing corrosion problems, energy use, and air pollution by SO2 and organic sulfur compounds. Gas-phase hydration takes place over supported oxides of tungsten at 540 K and 25... 

Propylene is hydrated at ca 540 K under pressure. Conversions are 60—70% and selectivity to 2-propanol is 99%. 

Partially dehydrated derivatives complexed with polyethylene glycol or propylene glycol exist. In the United of hydration has been replaced by glycine are particularly popular. Aluminum zirconium tetrachlorhydrex gly antiperspinant products. 

Propylene glycol (1,2-propanediol) is produced by the hydration of propylene oxide in a manner similar to that used for ethylene oxide ... 

The production of isopropanol from propylene occurs by either a direct hydration reaction (the newer method) or by the older sulfation reaction followed by hydrolysis. 

Figure 8-4. A flow diagram for the hydration of propylene to isopropanol (1) propylene recovery column, (2) reactor, (3) residual gas separation column, (4) aqueous - isopropanol azeotropic distillation column, (5) drying column, (6) isopropyl ether separator, (7) isopropyl ether extraction.

Propanediol (1,2-Propylene glycol, 1,2-Dihydroxy propane, Methyl glycol). CH3.CHOH.CH2OH mw 76.09 colorl, viscous, stable, hygr liq bp 187.3°, d 1.0381g/cc at 20/20° RI 1.4293 at 27° fl pt (open cup) 210°F autoignition temp 780°F. Misc with w, ales, and many org solvents in all proportions. Can be prepd by hydration of propylene oxide. On nitration it yields the exp] 1,2-Propanediol Dinitrate (see below)... 

Zabor et al. (Zl) have described studies of the catalytic hydration of propylene under such conditions (temperature 279°C, pressure 3675 psig) that both liquid and vapor phases are present in the packed catalyst bed. Conversions are reported for cocurrent upflow and cocurrent downflow, it being assumed in that paper that the former mode corresponds to bubble flow and the latter to trickle-flow conditions. Trickle flow resulted in the higher conversions, and conversion was influenced by changes in bed height (for unchanged space velocity), in contrast to the case for bubble-flow operation. The differences are assumed to be effects of mass transfer or liquid distribution. 

The only data on chemical conversion in fixed-bed bubble-flow operation that have come to the author s attention are the few results referred to in Section V,A,6 obtained by Zabor et al. (Zl) for catalytic mixed-phase hydration of propylene. 

---