Ethanol is dried with molecular sieve 3A. Its pore opening of about 3 angstroms (Å) is wide enough to take in water, whose kinetic diameter is near 2.8 Å, yet narrow enough to keep out the larger ethanol molecule at about 4.4 Å. The water is held inside the adsorbent and the ethanol stays in the product, so the stream is dried with little loss of ethanol.
This matters because distillation on its own cannot produce anhydrous ethanol. Ethanol and water form an azeotrope that holds ordinary distillation at about 95.6 percent ethanol by weight, and a molecular sieve bed is what removes the water that remains. Medaad supplies the 3A molecular sieve used for this duty, along with the grade and sizing advice that goes with it.
Distillation and the Ethanol Water Azeotrope
At atmospheric pressure, ethanol and water form a minimum boiling azeotrope at about 95.6 percent ethanol by weight, boiling near 78.1 °C. At that composition the vapour has the same ethanol to water ratio as the boiling liquid, so adding distillation stages does not raise the purity any further. The remaining water, roughly the last four to five percent, is removed by adsorption instead. Adsorption works by molecular size rather than by boiling point, so it is not limited by the azeotrope.
Molecular Sieve 3A Removes Water by Size Exclusion
A molecular sieve is a crystalline aluminosilicate, a synthetic zeolite, with pores of one uniform size. The separation is by size exclusion: a molecule smaller than the pore enters the crystal and is held, while a larger molecule stays in the product stream. The table shows why the 3A grade suits ethanol and other solvents.
| Molecule | Kinetic diameter | 3A pore (~3 Å) | 4A pore (~4 Å) |
|---|
| Water | ~2.8 Å | Adsorbed | Adsorbed |
| Methanol | ~3.6 Å | Excluded | Adsorbed |
| Ethanol | ~4.4 Å | Excluded | Partly adsorbed |
| Isopropanol | ~4.7 Å | Excluded | Largely excluded |
Water at 2.8 Å sits below the 3 Å opening, so it is taken up, while ethanol at 4.4 Å sits above it and is left in the product. The same reasoning applies to methanol and the larger solvents, and it is the basis for the points that follow.
4A Molecular Sieve and Ethanol Adsorption
4A does take up some ethanol, which is why it is not the grade of choice for this duty. The 4 Å opening of molecular sieve 4A is close to the size of the ethanol molecule, so some ethanol is taken up along with the water. Two problems follow:
- Ethanol is lost with the adsorbed phase, which lowers the yield.
- That ethanol can crack and leave carbon on the sieve during thermal regeneration, which shortens the life of the bed.
The narrower 3A grade keeps ethanol out and avoids both effects. 4A is still a sound general desiccant where no component has to be preserved, but for ethanol the selective 3A grade is specified. The wider comparison across grades is set out in molecular sieve 3A vs 4A vs 5A vs 13X.
Industrial Ethanol Dehydration: Vapour Phase PSA
In fuel ethanol and industrial ethanol plants, the dehydration is done in the vapour phase on a pressure swing adsorption (PSA) unit after the distillation column. Hydrous ethanol vapour passes through a bed of 3A beads, the water is held, and dry ethanol vapour leaves at 99.5 percent and above before it is condensed as product.
Two beds work in turn: one adsorbs while the other is regenerated by lowering the pressure and purging with a small part of the dry ethanol vapour, which carries the water away. The switch is made on a timed or purity basis, so dry product is made continuously. This route has largely replaced distillation with entrainers such as benzene or cyclohexane, because it uses less energy and adds nothing to the product.
Drying Ethanol and Solvents in the Laboratory
At laboratory and small batch scale, ethanol is dried over 3A beads:
- Use activated 3A beads at roughly 15 to 20 grams per litre of ethanol.
- Leave the ethanol over the beads, ideally overnight, then decant.
- For a lower residual water level, repeat over a second charge of fresh beads.
- Reactivate used beads by heating them to about 250 °C, then cool them in a sealed, dry vessel.
A 3A molecular sieve holds up to about 20 percent of its weight in water, so a modest charge treats a useful volume of solvent. The beads should be left undisturbed rather than stirred hard, since attrition produces fines that cloud the liquid.
Drying Methanol and Other Solvents
Methanol, at about 3.6 Å, is also excluded from the 3 Å pore, so a 3A molecular sieve removes water from methanol while keeping the methanol in the product. Larger polar solvents such as isopropanol and the butanols are excluded by an even wider margin, and acetonitrile is dried the same way. The 3A grade is the default wherever the solvent itself must be kept; where only bulk drying is needed and the solvent is not the product, a wider grade can be used.
Regeneration
In continuous PSA service the bed is regenerated inside the cycle, by lowering the pressure and purging with dry ethanol, with no separate heating step. Where a bed is regenerated thermally, 3A needs a lower temperature than the wider grades. Regenerating below the required temperature leaves water behind and lowers the working capacity over time, so the temperature is set from the datasheet for the grade and bead size in use.
Choosing the Grade and Bead Size
For ethanol, methanol and other polar solvents the grade is 3A. Bead size is a separate decision, because it sets the pressure drop and the rate of drying and is matched to the vessel and the flow, as covered in our guide to the molecular sieve sizes for industrial applications. 3A for this duty is supplied as beads in common ranges such as 3 to 5 mm, within the wider molecular sieve drying and purification range. The fundamentals are covered in what is a molecular sieve.
3A Molecular Sieve From Medaad
Medaad, by SorbiTech, supplies 3A molecular sieve for ethanol and solvent dehydration to customers worldwide, with technical documentation for every batch as a molecular sieve supplier. Alongside the product, our engineering and technical services cover grade selection, bead sizing and guidance on bed loading and regeneration.
Contact Medaad about your ethanol or solvent dehydration requirement.
