Anhydrous ethanol molecular sieves

Note. (1) Ethanol-free chloroform should be used (CAUTION). Ethanol present in chloroform (as stabiliser) can be removed by shaking chloroform several times with an equal volume of water, followed by drying over anhydrous calcium chloride and distilling. Alternatively stand the chloroform over a few grams of the molecular sieve... 

Dehydration Anhydrous ethanol is required for blending gasoline. It can be obtained by additional dehydration, for example, with molecular sieves or carrier-assisted distillation. 

The Matheson Company Inc. provided VF monomer, which was passed through a column packed with silica gel and molecular sieves (for removal of inhibitor and water), and distilled under vacuum. Urea (Fischer, certified ACS) was recrystallized three times from anhydrous ethanol, and 250 mmol was placed in a bulb attached to a vacuum manifold. Then 160 mmol of purified VF was added to the bulb, followed by a trace of methanol (0.2 ml) to initiate complex... 

Dehydration - To get rid of the water in the azeotrope, most ethanol plants use a molecular sieve to capture the remaining water and get anhydrous ethanol (>99.8%wt pure). 

An organic synthesis requires 50 mL of anhydrous ethanol as a solvent. Only 95 % ethanol is available. Removing the water by storing over molecular sieves or using a drying agent such as anhydrous sodium sulfate is not considered economical because too much water is present and time is short. An azeotropic distillation is decided upon. You get 150 mL of 95% ethanol and prepare the desired product. 

For the production of fuel-grade ethanol, the ethanol has to be dried . Anhydrous ethanol cannot be produced by simple distillation because ethanol forms an azeotropic mixture with water. The maximum ethanol content achievable by distillation is approximately 97.2vol.%, which is usually not sufficient for the application as fuel-ethanol. The residual water can be removed either by azeotropic distillation by the addition of, e.g., cyclohexane or by the application of molecular sieves. Today, state-of-the-art plants operate with molecular sieves which provide considerable advantages in terms of investment and operating costs. 

Many procedures have been developed for the preparation of anhydrous ethanol. In our own experience, this solvent may be purified most simply by drying with molecular sieve, followed by distillation. However, drying with metallic calcium or calcium hydride has also proved effective. 

Packaging Rectified spirit (96% ethanol by volume) is marketed directly for the manufacture of chemicals such as acetic acid, acetone, oxalic acid and absolute alcohol. Denatured ethanol for industrial and laboratory use typically contains 60-95% ethanol as well as between 1% and 5% each of methanol, isopropanol, methyl isobutyl ketone (MIBK), ethyl acetate and so forth. For beverages, the alcohol is matured and blended with malt alcohol (for manufacture of whisky) and diluted to requisite strength to obtain the desired type of liquor. This is bottled appropriately in a bottling plant. Anhydrous ethanol for fuelblending applications (power alcohol) requires concentration of the ethanol to 99.5 wt% purity. The ethanol dehydration is typically done using molecular sieves. 

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. 

Dehydration The alcohol from the top of the column will then pass through a dehydration system where the remaining water will be removed. Most ethanol plants use a molecular sieve to capture the last bit of water in the ethanol. The alcohol product at this stage is called anhydrous (pure, without water) ethanol and is approximately 200 proof. 

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