Solvent channeling is a result of fluid dynamics. Liquids in motion choose the path of least resistance. When solvent enters a column filled with biomass, the solvent’s path through that biomass is largely dependent upon the consistency of cross-sectional resistance/density. An uneven pack density results in uneven resistance to solvent flow which causes the solvent to channel on its way through. The solvent flows easier, and therefore heavier, through less dense regions than it does through tightly packed regions.
The biomass itself is already dense with respect to solvent. Take a bud for example; If one bud was -placed into a small cylinder without being packed and the solvent was run through the cylinder, the solvent flows around the bud where there is the least amount of resistance. If the bud was packed down into the small cylinder, more of the solvent would flow through the bud. This is why tighter packs have more potential for consistent resistance to flow, and therefore less channeling.
To take full advantage of these principles at scale, care must be taken to pack with uniform cross-sectional pressure. Every cross-section is critical since it is perpendicular to the solvent flow path. The resulting consequence of uneven and/or lower density packs is lower yields due to uneven rinsing of solute from the biomass. With tightly and uniformly packed columns, you will have less solvent channeling, higher percent yields, and higher overall yields per batch using less solvent in the process.
In addition to pack density and uniformity, channeling can be affected by other factors. Uniformity of temperature also assists in maximizing yields. A solvent when cold has a higher density than it does when it’s warm. This property will pull solvents toward regions of biomass that are lower in temperature relative to the solvent. To promote even solvent distribution, the biomass should be kept at as consistent a temperature as possible throughout. Another factor that affects yields is how the solvent enters and exits the column. When liquid enters and exits a cylinder through an orifice smaller in diameter than the cylinder, the mass flow will take a conical path at the inlet and outlet. This effect on yields can be countered by ensuring even solvent distribution throughout the entire area of the column at the entrance and exit.
Lastly, uneven pressures (typically governed by uneven temperatures) throughout the biomass can cause uneven solvent distribution. If the outlet pressure is too low with respect to the solvent’s temperature you risk turning the solvent to vapor inside the column. Since vapor will travel much easier through restrictions, and that vapor will not extract or wash the biomass, inadequate pressure at the outlet will cause lower yields.
INTERESTING NOTE- Because fluid velocity through a pipe is always 0 at the walls and maximum in the center during laminar flow, the column itself can cause an uneven mass flow through the column. Again, tightly packed biomass will optimize mass flow distribution.