Ethanol dehydration technology

The current ethanol dehydration technology - two-stage distillation followed by a molecular-sieve dryer, as shown in Figure 8.18(a) - uses approximately 16 000-20 000 Btu of energy/gal of ethanol produced. This is about 20% of the energy value of the ethanol produced. There is a considerable interest in membrane technology that would be lower in cost and less energy intensive. 

It should be noted that the dehydration of the ethanol by the azeotropic rectification requires considerable operational and energy expenses. Ethanol dehydration technologies using the adsorption on molecular sieves and evaporation through the membrane are less power consuming ones. However, the ethanol dehydration by the evaporation through the membrane requires significant capital investment and the smooth/uninterrupted operation of the factory. 

An approach to the production of ethylene from biomass that does not involve pyrolysis is ethanol dehydration. The catalytic conversion of syngas to ethanol from low-grade biomass (or fossil) feedstocks, and fermentation ethanol via advanced cellulose hydrolysis and fermentation methods, which make it possible to obtain high yields of ethanol from low-grade biomass feedstocks as well, are both expected to be commercialized in the United States (Chapter 11). Which technology becomes dominant in the market place has... 

Only Lummus reported a fluidized bed reactor for use in a trial ethanol dehydration study. The fluidized bed reactor operated with a temperature of about 399 C, a one-way ethanol conversion rate of greater than 99.5%, and an ethylene yield of more than 99% (before distillation Tsao and Zasloff 1979). Dehydration of ethanol to ethylene is a relatively mature technology. Different processes have been developed by many companies according to their respective catalyst (as shown in Table 2). 

Bioethylene and green PE is one of the successful biorefinery processes. To compete with PE produced from oil resources, the green PE process must be improved and developed continuously. The efficient process improvement requires much knowledge and technology so that ethanol can be manufactured at low-cost from nonfood resources improvements are especially needed in the areas of cellulose pretreatment technology, fundamental ethanol dehydration chemistry, process and equipment development, the performance enhancement of downstream products, and so on. The successful operation of green PE industrial equipment has opened up a new era for bio-based materials, and will accelerate the quick development of the biorefinery industry. The experience developed during this process will be very important for the utilization of biorenewable resources. 

Besides the methods illustrated so far in this book, there are other ways for separating azeotropes. One way is to react the azeotrope away in a reactive distillation column to form other useful products. The design and control of various reactive distillations have been extensively studied in a recent book by Luyben and Yu. Another way commonly used in ethanol dehydration is to use the hybrid distillation-adsorption process. In this process, distillation is used to purify the mixture to a composition near the ethanol-water azetrope, and then an adsorption unit (e.g., molecular sieves) is used to adsorb the remaming water so that anhydrous ethanol can be obtained. The key technology in this process is the performance of the adsorbent material in removing water from the mixture and is beyond the scope of this book. 

The production of ethylene by dehydration of ethanol is a proven technology and was demonstrated and implanented on large scale (Winter, 1976). Braskem started a full-scale plant in Brazil in 2010 (Braskem, 2012). The process consists of a dehydration reactor and several subsequent purification steps in order to obtain polymer-grade ethylene (composition 99.95 wt% ethylene, 0.05 wt% ethane, 5ppm CO and lOppm CO (Kochar et al., 1981)). Figure 4.6 iUustrates the ethanol dehydration process investigated in this study and lists the input data used for process simulation. 

Reactions of anthocyanins and flavanols take place much faster in the presence of acetaldehyde that is present in wine as a result of yeast metabolism and can also be produced through ethanol oxidation, especially in the presence of phenolic compounds, or introduced by addition of spirit in Port wine technology. The third mechanism proposed involves nucleophilic addition of the flavanol onto protonated acetaldehyde, followed by protonation and dehydration of the resulting adduct and nucleophilic addition of a second flavonoid onto the carbocation thus formed. The resulting products are anthocyanin flavanol adducts in which the flavonoid units are linked in C6 or C8 position through a methyl-methine bond, often incorrectly called ethyl-link in the literature. 

The three current applications of pervaporation are dehydration of solvents, water purification, and organic/organic separations as an alternative to distillation. Currently dehydration of solvents, in particular ethanol and isopropanol, is the only process installed on a large scale. However, as the technology develops, the other applications are expected to grow. Separation of organic mixtures, in particular, could become a major application. Each of these applications is described separately below. 

The catalytic dehydration of ethanol to ethylene in SC water may be commercially important (16). Although high quality commercial alumina catalysts exist for the vapor phase dehydration of ethanol, the commercial processes require the ethanol feedstock to be relatively free of water. Hence the ethanol must be distilled from the ethanol-water mixture which is the product of fermentation processes. By avoiding this distillation step, and securing phase separation of the ethylene product from the ethanol-water reactant, SC dehydration of ethanol could enjoy advantages over existing commercial technologies. 

Ballweg, A.H. Briischke, H.E.A. Schneider, W.H. Tusel, G.F. Boddeker, K.W. Wenzlaff, A. Pervaporation membranes. An economical method to replace conventional dehydration and rectification columns in ethanol distilleries. Fifth International Symposium on Alcohol Fuel Technology, Auckland, New Zealand, May 13-18, 1982. 

Many industrially important liquid systems are difficult or impossible to separate by simple continuous distillation because the phase behavior contains an azeotrope, a tangent pinch, or an overall low relative volatility. One solution is to combine distillation with one or more complementary separation technologies to form a hybrid. An example of such a combination is the dehydration of ethanol using a distillation-membrane hybrid, as shown in Figure 6.30. 

Most of the conunercialization of PV technology occurred in this category for dehydration of ethanol and t-propanol. 

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. 

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