Dehydration, of ethanol

Prior to the advent of the petrochemical age, ethylene was made from ethanol by dehydration  

The figure illustrates that below 400K (120°C) the equilibrium favours the hydration of ethylene. Higher temperatures favour the dehydration of ethanol with temperatures over 200°C placing the equilibrium well to the favour of ethylene. 

62 GJ/tonne. In normal operations this would be delivered by fuel oil or gas, but in totally renewable operations this heat input may be by burning waste produced from the production of starches and sugars used in the fermenting to produce ethanol, e.g. bagasse. 

Ethanol is heated and passed to the converter where the dehydration equilibrium is established. The products are passed to a column which removes the ethylene. Then a second column separates ethanol from water. 

Because water is a product and the recycle ethanol will be wet, the ethanol feedstock need not be the highest grade, but instead the easier to produce hydrous ethanol (95% ethanol). If the reaction temperature is low, there should be no contamination from acetylene, which is a problem with higher temperature routes. 

The dehydration of all kinds of organic solvents can be carried out by pervaporation. This process is very attractive, especially in those cases where water forms an azeotrope with the solvent at low water content. Atypical case is ethanol/water with an azeotropic composition of 96% ethanol by weight. Purification of ethanol can also be achieved via a hybrid process distillation up to 96% and pervaporation to 99%, see figure VIII - 28. 

The pervaporation feed coming from the distillation unit contains no impurities and no pretreatment is necessary in this case. System design for per aporation differs from that of other membrane processes. Pervaporation is the only process where a phase transition 

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A selection of industrial appHcations of extractive distillation includes (/) the separation of the / -butane—butadiene azeotrope in mixed C -hydrocarbon streams using furfural [98-01-17, as the solvent (36) (2) the dehydration of ethanol using ethylene glycol [107-21-1] (37—39) (J)... 

Ethylene. Where ethylene is ia short supply and fermentation ethanol is made economically feasible, such as ia India and Bra2il, ethylene is manufactured by the vapor-phase dehydration of ethanol. The production of ethylene [74-85-1] from ethanol usiag naturally renewable resources is an active and useful alternative to the pyrolysis process based on nonrenewable petroleum. This route may make ethanol a significant raw material source for produciag other chemicals. 

Dehydration of ethanol has been effected over a variety of catalysts, among them synthetic and naturally occurring aluminas, siUca-aluminas, and activated alumina (315—322), hafnium and 2irconium oxides (321), and phosphoric acid on coke (323). Operating space velocity is chosen to ensure that the two consecutive reactions. 

The reaction is cataly2ed by all but the weakest acids. In the dehydration of ethanol over heterogeneous catalysts, such as alumina (342—346), ether is the main product below 260°C at higher temperatures both ether and ethylene are produced. Other catalysts used include siUca—alumina (347,348), copper sulfate, tin chloride, manganous chloride, aluminum chloride, chrome alum, and chromium sulfate (349,350). 

The vapor-phase esterification of ethanol has also been studied extensively (363,364), but it is not used commercially. The reaction can be catalyzed by siUca gel (365,366), thoria on siUca or alumina (367), zirconium dioxide (368), and by xerogels and aerogels (369). Above 300°C the dehydration of ethanol becomes appreciable. Ethyl acetate can also be produced from acetaldehyde by the Tischenko reaction (370—372) using an aluminum alkoxide catalyst and, with some difficulty, by the boron trifluoride-catalyzed direct esterification of ethylene with organic acids (373). 

Manufacture. Much of the diethyl ether manufactured is obtained as a by-product when ethanol (qv) is produced by the vapor-phase hydration of ethylene (qv) over a supported phosphoric acid catalyst. Such a process has the flexibiHty to adjust to some extent the relative amounts of ethanol and diethyl ether produced in order to meet existing market demands. Diethyl ether can be prepared directly to greater than 95% yield by the vapor-phase dehydration of ethanol in a fixed-bed reactor using an alumina catalyst (21). 

Although ethylene is produced by various methods as follows, only a few are commercially proven thermal cracking of hydrocarbons, catalytic pyrolysis, membrane dehydrogenation of ethane, oxydehydrogenation of ethane, oxidative coupling of methane, methanol to ethylene, dehydration of ethanol, ethylene from coal, disproportionation of propylene, and ethylene as a by-product. 

An elimination reaction is, in a sense, the reverse of an addition reaction. It involves the elimination of two groups from adjacent carbon atoms, converting a saturated molecule into one that is unsaturated. An example is the dehydration of ethanol, which occurs when it is heated with sulfuric acid ... 

The preferred choice of a water-selective membrane up to now has been hydrophilic membranes because of their high water affinity. However, recently Kuhn et al. reported an all-silica DDR membrane for dehydration of ethanol and methanol with high fluxes (up to 20kg m h ) and high selectivities (H20/ethanol 1500 and H20/methanol 70 at 373 K) in pervaporation operation. The separation is based on molecular sieving with water fluxes comparable to well-performing hydrophilic membranes [51]. 

P 61] For the dehydration of ethanol to ethane, electroosmotic pumping was applied for liquid transport [19], A flow rate of 0.9-1.1 pi min was applied, giving longer residence times as in [P 60]. The other details of the protocol are idenhcal with those for [P 60],... 

Volkov (1994) has given a state-of-the-art review on pervaporation. A number of industrial plants exist for dehydration of ethanol-water and (.vwpropanol-water azeotropes, dehydration of ethyl acetate, etc. There is considerable potential in removing dissolved water from benzene by pervaporation. The recovery of dis.solved organics like CH2CI2, CHCI3, CCI4, etc. from aqueous waste streams also lends itself for pervaporation and pilot plants already exist. 

Dehydration The growing use of isopropanol as a clean-rinse fluid in microelectronics produces significant quantities of an 85—90 percent isopropanol waste. Removing the water and trace contaminants is required before the alcohol can be reused. Pervaporation produces a 99.99 percent alcohol product in one step. It is subsequently polished to remove metals and organics. In Europe, dehydration of ethanol is the largest pervaporation application. For the very large ethanol plants typical of the United States, pervaporation is not competitive with thermally integrated distillation. 

Figure 12.36 Flowsheet for dehydration of ethanol using pervaporation.

Fig. 5 Free energy changes in the steam reforming, decomposition, dehydrogenation and dehydration of ethanol. The data for water-gas-shift reaction also is included.

As described in Section 3.2.3, the use of acidic supports such as A1203 favors the dehydration of ethanol to ethylene, which leads to a severe carbon deposition.66,76,78,85 Reactions with lower H20/ethanol ratio can also favor several side reactions mentioned above and result in carbon deposition on the catalyst surface. Possible strategies to reduce the carbon deposition include (i) neutralization of acidic sites responsible for ethanol dehydration to ethylene and/or modification of the support nature, including less acidic oxides or redox oxides, (ii) use of a feed containing higher H20/ethanol molar ratio, and (iii) addition of a small concentration of air or 02 in the feed. 

Ethylene is a colourless gas having boiling point 14°C. It is obtained by the dehydration of ethanol or hydrogenation of acetylene. 

In a previous section, the effect of plasma on PVA surface for pervaporation processes was also mentioned. In fact, plasma treatment is a surface-modification method to control the hydrophilicity-hydrophobicity balance of polymer materials in order to optimize their properties in various domains, such as adhesion, biocompatibility and membrane-separation techniques. Non-porous PVA membranes were prepared by the cast-evaporating method and covered with an allyl alcohol or acrylic acid plasma-polymerized layer the effect of plasma treatment on the increase of PVA membrane surface hydrophobicity was checked [37].The allyl alcohol plasma layer was weakly crosslinked, in contrast to the acrylic acid layer. The best results for the dehydration of ethanol were obtained using allyl alcohol treatment. The selectivity of treated membrane (H20 wt% in the pervaporate in the range 83-92 and a water selectivity, aH2o, of 250 at 25 °C) is higher than that of the non-treated one (aH2o = 19) as well as that of the acrylic acid treated membrane (aH2o = 22). 

Table 4. Dehydration of ethanol, using membranes PAA.HC1 (60 wt%)-PVA (35 wt%-GA (5 wt%) (aprox. 60pm thick) [33]...

Norman, W. S. Trans. Inst. Chem. Eng. 23 (1945) 66. The dehydration of ethanol by azeotropic distillation. Ibid. 89. Design calculations for azeotropic dehydration columns. 

Although the dehydration of ethanol over alumina was discovered in 1797 (1), a century elapsed before any systematic study of alcohols over this catalyst was undertaken (2, 3). Much of the experimental material is... 

Tertiary > Secondary > Primary The mechanism of dehydration of ethanol involves the following steps Mechanism... 

Write the mechanism of acid dehydration of ethanol to yield ethene. 

The presence of oxygen enhances the catalyst stability. Breen et al. [187] investigated SRE over a range of oxide-supported metal catalysts. They concluded that the support plays an important role in the reaction. In fact, they observed that alumina-supported catalysts are very active at low temperatures for dehydration of ethanol to ethylene, which at higher temperatures (550 °C) is converted into H2, CO and CO2 as major products and CH4 as a minor product. The activities of the metal decrease in the order of Rh > Pd > Ni PS Pt. Ceria/zirconia-supported catalysts are more active and exhibit 100% conversion of ethanol at high space velocity and high temperature (650 °C). 

Another fine distinction among salt catalysts was obtained by following the activity and olefin/ether selectivity of metal sulphates in the dehydration of ethanol and 1-propanol. A linear correlation between the electronegativity of the metal ion and the activity has been found, but the selectivity gave a curve with a minimum [51]. 

Kohlmann. K.L., et al.. Enhanced Enzyme Activities on Hydrated Lignocelluiosic Substrates, in Enzymatic Degradation of Insoluble Carbohydrates. ACS Symp. Ser. No. 618, J.N. Saddler and M.H. Penner. eds.. 1995. pp. 237-255. Kohlmann, K.L., et al. Enhanced Enzyme Activities on Hydrated Lignocelluiosic Substrates in Enzymatic Degradation of Insoluble Carbohydrates, ACS Synip. Ser. No. 618, J.N. Saddler and M H. Penner, eds., 1995b, pp. 238-255 Ladisch, M R and K. Dyck Dehydration of Ethanol New Approach Gives Positive Eneigy Balance, Science, 205(4409), 898- 900 (1979). 

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