It is important to consider the opportunity cost of wasting the byproducts of your process. It is not just a matter of expense for the distillery, but you are also losing potential revenues. For example, as of March 2024, the average national sales price for distillers wet grains (DWG) ranged from $49-$75 per wet ton. Further, the average sales prices for dried distillers grains (DDG) were approximately two to three times greater than wet products, according to the U.S. Department of Agriculture’s Agricultural Marketing Service.

Whole stillage may be approximately 10% solids (or more), so if you could successfully remove the solids, not only will you be able to sell these nutrients, but you can also reduce your wastewater volumes by up to 10% and substantially reduce the fines/fees that you pay based on the chemical composition of discharges. It is all a matter of weighing the opportunity costs.

The most common way that distilleries and ethanol plants separate solids from water is via centrifugation.

Centrifugal Separation Technologies

When using a centrifugal system to separate particles from a liquid stream, the objective is to use rotational force to accelerate the solid particles and remove them from the liquid stream flow. This is often quantified using Stoke’s Law. This equation states that the particle velocity which can be achieved in a centrifuge is a function of the density difference between the solid phase and the liquid phase, proportional to the square of particle diameter and centrifugal acceleration, but inversely proportional to the viscosity of the stillage stream. This can be written as:



Where:

  • V is particle settling velocity that can be achieved with the system (m/s)
  • d = particle diameter (m)
  • ρw = density of heavy phase (kg/m3)
  • ρo = density of light phase (kg/m3)
  • ƞ = apparent viscosity (kg/m.s)
  • rω2 = centrifugal acceleration (m/s2)

Note that the particle velocity that can be achieved will be a function of the geometry of the centrifuge, the speed at which the centrifuge operates, but also the viscosity and density of the stillage. If your mash bill changes, this will necessitate the need to alter centrifuge operations, or else this will change your separation efficiency.

Figure 1 shows examples of cutaway illustrations for two types of decanter centrifuges. There are, in fact, several styles and manufacturers available. These differ based on geometries, flowrates, and power consumption; the centrifuge appropriate for your specific application depends on your slurry’s physical characteristics, including density, viscosity, solids content, etc.

Which is best for your operations? It is not a one-size-fits-all answer. Please consult with centrifuge manufacturers to help you determine which will be best for your operations, mash bills, and whole stillage flows.

Figure 2 shows examples of integration of decanter centrifuges within the broader production process at a distillery or biofuels production plant. Note that suspended solids can either be sold as-is as distillers wet grains (also known as wet cake), or they can be dried and sold as DDG. The dissolved solids are sent to multi-effect evaporators to produce condensed distillers solubles (CDS), which can be added to the wet cake and then dried down to produce distillers dried grains with solubles (DDGS).

Of course, each distillery and biofuels plant may have tweaks and variations of these general process flows and sell modified variants on these types of coproducts.

What’s Next?

Hopefully this article has helped you gain some understanding about stillage, its characteristics, and the use of centrifugation to remove non-fermentable solids.

Future articles will further explore this topic and will discuss issues such as capital and operational costs, efficiencies, coproduct processing options, as well as stories of success at various companies. Proper solids removal is critical to operational excellence at your processing plant.

Acknowledgments
The author thanks Daniel Lakovic and the team from Flottweg USA for conversations, data, figures, and tours of various distilleries during preparation of this article.


Kurt A. Rosentrater, Ph.D., is a professor in the Department of Agricultural and Biosystems Engineering at Iowa State University, Ames. He can be contacted at karosent@iastate.edu or 515-294-4019