Ethylene hydratization

Direct Hydration of Ethylene. Hydration of ethylene to ethanol via a Hquid-phase process cataly2ed by dilute sulfuric acid was first demonstrated more than a hundred years ago (82). In 1923, the passage of an ethylene-steam mixture over alumina at 300°C was found to give a small yield of acetaldehyde, and it was inferred that this was produced via ethanol (83). Since the late 1920s, several industrial concerns have expressed interest in producing ethanol synthetically from ethylene over soHd catalysts. However, not until 1947 was the first commercial plant for the manufacture of ethanol by catalytic hydration started in the United States by Shell the same process was commerciali2ed in the United Kingdom in 1951. 

Many other acids and acidic oxides have been mentioned as catalysts for ethylene hydration (99—103) as have ion-exchange resias (104,105). 

The kinetics of the ethylene hydration reaction have been investigated for a tungstic oxide—siHca gel catalyst, and the energy of activation for the reaction deterrnined to be 125 kJ/mol (- 30 kcal/mol) (106,120). The kinetics over a phosphoric acid-siHca gel catalyst have been examined (121). By making some simplifying assumptions to Taft s mechanism, a rate equation was derived ... 

Ethylene hydration direct, 20 538-543 process of, 20 542-543, 544 reaction mechanism and kinetics of, 20 541... 

Mixed-oxide (MOX) fuel, 19 700 25 400 Mixed-phase ethylene hydration process, 10 538-539... 

For ethylene hydration [290], a correlation between the catalytic activity of cation-exchanged zeolites and the electronegativities of the cations was established. 

The several attempts, published in the literature, to describe the kinetics of vapour phase olefin (mostly ethylene) hydration can be classified into two groups according to the basic model used. One model, for reactions catalysed by phosphoric acid supported on solids, treats the kinetics as if the process were homogeneous acid catalysis and takes into account the acid strength of the supported acid. Thus, a semiempirical equation for the initial reaction rate [288]... 

Ethanol is made by both ethylene hydration and fermentation of starches and sugars. In this section the synthetic route will be discussed. The fermentation route is covered in Chapters 32 and 33. 

Direct ethylene hydration Indirect ethylene hydration ... 

The processes for manufacturing methanol by synthesis gas reduction and ethanol by ethylene hydration and fermentation are very dissimilar and contribute to their cost differentials. The embedded raw-material cost per unit volume of alcohol has been a major cost factor. For example, assuming feedstock costs for the manufacture of methanol, synthetic ethanol, and fermentation ethanol are natural gas at 3.32/GJ ( 3.50/10 Btu), ethylene at 0.485/kg ( 0.22/lb), and corn at 0.098/kg ( 2.50/bu), respectively, the corresponding cost of the feedstock at an overall yield of 60% or 100% of the theoretical alcohol yields can be estimated as shown in Table 11.12. In nominal dollars, these feedstock costs are realistic for the mid-1990s and, with the exception of corn, have held up reasonably well for several years. The selling prices of the alcohols correlate with the embedded feedstock costs. This simple analysis ignores the value of by-products, processing differences, and the economies of scale, but it emphasizes one of the major reasons why the cost of methanol is low relative to the cost of synthetic and fermentation ethanol. The embedded feedstock cost has always been low for methanol because of the low cost of natural gas. The data in Table 11.12 also indicate that fermentation ethanol for fuel applications was quite competitive with synthetic ethanol when the data in this table were tabulated in contrast to the market years ago when synthetic ethanol had lower market prices than fermentation ethanol. Other factors also... 

It was also pointed out that good dehydration catalysts are not necessarily good hydration catalysts and the important factor appears to be the presence of sites of the correct acid strength. Thus, for ethylene hydration, sites with acid strengths in the range — 8.2 —3.0 are... 

Traces of diethyl ether impurity are hydrated back to ethanol using conditions similar to those used for the ethylene hydration reaction. 

Ethylene oxidation 1.0-second contact time at 280°C, 1.0 atm Ethylene hydration 1800 hr space velocity at 300°C, 70 atm Propylene ammonoxidation, 20-second contact time at 400°C, 1 atm. 

Recent Raman studies also show that contrary to previous reports, ethane can be trapped in the small cage of Structure II hydrate at higher pressures ( 70 MPa). Similarly. Raman microprobe studies of ethylene hydrate at 95 MPa and 303.9 K show that despite ethylene s large van der Waals radius it can occupy the small cage of Structure I hydrate. ... 

Sugahara, T. Morita. K. Ohgaki, K. Stability boundaries and small hydrate-cage occupancy of ethylene hydrate system. Chem. Eng. Sci. 2000, 55, 6015-6020. 

There were also improvements in acetaldehyde and acetic anhydride manufacture. Ag based catalysts for the partial oxidation of ethanol became available around 1940. When used to oxidatively dehydrogenate ethanol [14], the conversion of ethanol to acetaldehyde was no longer equilibrium limited since the reaction was now very exothermic. Fortunately, the process still displayed excellent selectivity (ca. 93-97%) for acetaldehyde. This technology replaced the older Cu-Cr processes over the period of the 1940-1950 and made ethanol a much more attractive resource for acetaldehyde. When ethylene became available as a feedstock in the 1940 s through 1950 s, ethanol became cheaply available via ethylene hydration (as opposed to traditional fermentation). With ethanol now cheaply available from ethylene, the advent of the Ag catalyzed oxidative dehydration to acetaldehyde rapidly accelerated the shutdown of the last remaining wood distillation units. 

Hence, the mole fractions of products will increase with increasing pressure if r > q, a relationship of considerable importance in seeking high conversions in methanol synthesis and ethylene hydration for example. 

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