Drying Ethanol for Industrial Purposes
This article will discuss the tried and true methods for drying ethanol.
Ethanol or ethyl alcohol is an organic compound with the formula C₂H₆O, generally used in fermenting alcoholic beverages of varying concentrations. It is a standard reagent and solvent for synthesizing many types of organic chemicals in the lab.
Ethanol is often found as an active ingredient in pesticides, antifreeze, cosmetics, and other household products. It is also a cleaning agent for disinfecting surfaces. Common industrial uses for alcohols are food extracts, pharmaceuticals, and automotive fuels.
Drying Ethanol: What is the ethanol distillation process?
The goal of drying ethanol is to purify and collect it from a mixture containing potential impurities to retain the expected chemical properties. Examples include the boiling point, solubility, density, vapor pressure, and molecular weight.
Ethanol acts as an intermediate in chemical reactions to produce the following:
- Acetic acid
- Acetaldehyde
- Butadiene
- Ethyl chloride
- Ethylene
The process of isolating water from ethanol is quite difficult because it’s not possible to synthesize ethanol of 100% purity, even if you repeat the distillation multiple times. But in most cases, a grade above 99% is deemed sufficient for organic solvents.
To make anhydrous ethanol, the distillation setup is fairly straightfoward: You only need to stir the alcohol with the drying agent, while boiling it in a covered container to absorb the water.
Methods for Drying Ethanol
Filtering and Absorption
In this procedure, ethanol is extracted with the help of a desiccant, choosing from a 3A molecular sieve, silica, or alumina beads to filter out the water. First, powdered KOH or MgI2 is mixed with ethanol at a concentration of 10 g/L. Then, the solvent is poured into a sealable flask that holds 10-20 g/L of molecular sieves heated to 300°C. The desiccant will gradually reduce the moisture to below 10 ppm. Let the solution cool down overnight for 24-48 hours before removing the purified ethanol.
The efficacy of drying ethanol can be measured by conducting a titration analysis inside a two-compartment glove box. The results will show that absorption is a practical drying method because it does not rely on reactive materials such as metal hydrides nor any special instruments apart from a column and a two-necked flask.
Benzene or Gasoline
For larger quantities, a better approach to recovering ethanol is adding benzene to the solution at a ratio of 2-3 times the volume of water. For instance, you could mix 2 gallons of benzene with 10 gallons of 95% alcohol, plus 88 gallons of gasoline to prepare 100 gallons of a 10% gasohol formulation.
Keep in mind that most salts will dissolve in water but not in ethanol, which means sulphates, phosphates, or calcium salt are able to absorb the water. The process involves filling a drum or container with dry salt and submerging it in the mixture, after which the ethanol can be collected through a valve at the bottom.
Dehydration with Lime
The traditional method is to simply dry ethanol with CaO (calcium oxide), also known as lime. What happens is, CaO reacts with the water to form Ca(OH)2. This is done by mixing lime in a ratio of 35 lbs per gal of water. Once it is slaked for 12-24 hours, the Ca(OH)2 will settle down since it does not dissolve in alcohol, and leave the 99.5% ethanol as the top layer.
Alternatively, you can distill the ethanol and reflux it with CaO past the condenser. Repeat that step using magnesium ethylate to purify it fully.
Azeotropic Drying
For example, in the azeotropic method, water and alcohol are combined into a binary azeotrope. If you add a hydrocarbon solvent (i.e. benzene, gasoline), it will form the third layer to be extracted. Try running a pressure-swing distillation on the azeotrope by adjusting the internal pressure: It crosses the activity coefficients of the distillates to boil out the water.
Another method is mixing trichloroethylene with the alcohol and water vapors in a rectifying column as it passes through the tube. This azeotrope should have a lower boiling point than pure ethanol at around 153°F. As a result, the water and trichloroethylene evaporate first by exiting the column, leaving the anhydrous ethanol behind at the bottom.
How Ethanol is Typically Produced
Ethanol is derived from corn and plant-based feedstocks. When locally sourced, the method will differ depending on whether the material has sugar/starch, or cellulose in it. For sugar and starch, dry milling is needed to ferment corn into ethanol whereas wet milling yields ethanol but generates by-products such as oil and carbon dioxide.
Making ethanol from cellulose is a different matter since biochemical processes are required for pretreating the sugars and breaking them down with hydrolysis. The other method is to add heat and chemicals until it yields a mixture of hydrogen and carbon monoxide, which is catalyzed into ethanol.
After the fermentation step, ethanol will be composed of roughly 6-12% water. This solution has to be purified through distillation, to comply with FDA regulations and prepare for commercial distribution. To absorb the water molecules, molecular sieves and silica-based particles are good options due to having smaller pores for diffusion.
To help extend the shelf life, ethanol must be kept away from water and certain metals to avoid ruining new batches. Sealable glassware is ideal for storing smaller quantities as a precaution against flammability.
The goal of ethanol production is to maximize water absorption and lower the cost of materials, while following the GLPs for safer handling and drying.
