Molecular Sieves

By B.N. Taber, Ph.D.

The Basics

There is generally going to be water in your biomass, which you will extract along with the your cannabinoids, terpenes, etc. When you recover your solvent, some of this water will still be in your vapor. Why not distill, say, your ethanol (EtOH)? With distillation you’re only able to get 95.57% EtOH (191 proof) because at this alcohol-by-weight the steam contains this percentage of EtOH—known as an azeotrope—and it’s not possible to reach 200-proof. But how do you remove this water so that your solvent is ready for the next run? That’s where molecular sieves come in!

Molecular Sieves

Molecular sieves are synthetic zeolites that are a long-preferred method for drying (removing water from) hydrocarbons. To dry EtOH, use a molecular sieve with 3 Å beads (and a sufficient number to absorb the mass of water needed). 4+ Å beads absorb EtOH, so only use 4 Å beads if you are drying butane and/or propane.

Molecular sieves rapidly absorb water vapor, which is a big reason why you use them in the gas phase. But how much do you need? Let’s do a back-of-the-envelope calculation! If you have 191 proof EtOH and assume that it is 95.5% EtOH, 4.5% water, then in 100 gallons you have 4.5 gallons of water, which weighs ~37.5 pounds. Molecular sieve beads typically absorb up to ~40% of their weight in water, but become less efficient after about 20% absorption. So you need between 100 and 190 pounds of 3 Å molecular sieve beads per 100 gallons of EtOH. Fortunately, you can regenerate zeolites!

Regenerating Molecular Sieve Beads

Molecular sieve beads absorb water up to 120 °C (250 °F) at atmospheric pressure, so to regenerate them you need to heat them above that temperature and/or at lower pressures (but not too high temperatures or the beads will decompose). Molecular sieve bead regeneration depends upon such factors as temperature, pressure, bead size, bed geometry, carrier gas flow rate, and adsorbate concentration. Reasonable molecular bead regeneration can occur at temperatures and pressures accessible to most commercial extraction labs.

Conclusions

Molecular sieves are an integral part of hydrocarbon solvent recovery, and we at N.B.Oler are able to construct recovery systems to fit your needs--beads and their recovery included!

References/Further Reading

Gabruś, E., Nastaj, J., Tabero, P. & Aleksandrzak, T. Experimental studies on 3A and 4A zeolite molecular sieves regeneration in TSA process: Aliphatic alcohols dewatering-water desorption. Chem. Eng. J. 259, 232–242 (2015).

Simo, M., Sivashanmugam, S., Brown, C. J. & Hlavacek, V. Adsorption/desorption of water and ethanol on 3A zeolite in near-adiabatic fixed bed. Ind. Eng. Chem. Res. 48, 9247–9260 (2009).

Wang, Y. Measurements and Modeling of Water Adsorption Isotherms of Zeolite Linde-Type A Crystals. Ind. Eng. Chem. Res. 59, 8304–8314 (2020).

Visit nboler.com or contact N.B. Oler at info@nboler.com for products, technical documents, SOPs, and more.