DCO Extraction Optimization Adapts to Changing Properties of Stillage
Centrifugal separation of corn oil from stillage is the accepted method for distillers corn oil (DCO) recovery throughout the ethanol industry. While plant operating parameters influence a plant’s oil yield and performance, the efficiency is fundamentally governed by a simple Nobel Prize-winning formula: Stokes’ Law. Understanding how normal process variables in the plant impact the factors of this equation ensures oil extraction efficiency.
Applied to DCO separation, Stoke’s Law outlines the variables that can be modified to increase separation:
• Increase the droplet size of the oil. Nonshear retention time allows oil droplets to coalesce, form larger droplets and rise.
• Increase the settling force or g-force. The two primary centrifuge technologies vary in g-force from 3,000 for a decanter centrifuge to 8,000 for a high-speed disc stack centrifuge. Most machines are running at their design maximum and cannot be increased.
• Change the density of the liquid. The density in the suspended particles of the stillage can be modified through mechanical, enzymatic or chemical forces.
• Decrease the dynamic viscosity. Variables affecting viscosity—dissolved solids including residual sugars, suspended solids, total solids or temperature—can be manipulated to increase separation efficiency. This factor is the easiest to manipulate within the ethanol process.
Corn oil extraction started around 2005 with a simple observation regarding the corn oil present at the top of whole and thin stillage tanks. The oil presented a potential problem because it could build up in the tank or, worse yet, find its way into the evaporators and cause an imbalance in steam usage. At the time, new coproducts were being sought for the animal feed market, indicating a market for corn oil, if plants could isolate it. A number of players in the marketplace, such as ICM, GreenShift and Valicor, developed technologies to separate DCO from stillage.
Two technologies emerged, both based upon the same separation principal of centrifugation: the decanter centrifuge and the high-speed disc stack centrifuge. Centrifugation g seconds is based upon three principles: residence time, g-force and surface area. In decanter centrifuge technology, the stillage is exposed to lower g-force for a longer residence time and lower surface area. High-speed disc stack centrifuges are much smaller and accomplish equivalent results by increasing the g-force and the surface area of the machine. The maximum yield in the early years was 0.4 to 0.5 pounds per bushel, which paid for the extraction technology in less than six months.
The DCO feed market expanded through trial and error into biodiesel and poultry feeding. Biodiesel facilities modified their process used for soy oil to include acid esterification to convert the high level of free fatty acids in DCO to methyl esters. Biodiesel producers also worked to reduce gums, waxes and color in DCO-based biodiesel to make a final product comparable to soy-based biodiesel. Besides the generally lower price, the new DCO feedstock scored lower on carbon intensity in life-cycle modeling, earning it financially attractive carbon credits. The two financial incentives allowed for investments in the process modifications needed to accommodate the different qualities of DCO. The poultry market expanded as feed producers started to appreciate the red color in corn oil and the benefits of the lutein and zeaxanthin content that enhance the yellow color of broiler products and egg yolks, while providing a high-energy fat source.
As the market adapted and extraction technology became commonplace, the demand for enhanced corn oil extraction efficiency increased and chemical additives were introduced. Emulsifiers such as Polysorbate80 and others were added to stillage to increase the rate of oil droplet coalescence and to liberate oil contained on the surfaces of other particles and compounds. Ethanol plants and technology providers also began modifying the placement of centrifuges and solids loading. DeltaT plant designs, for example, did not have the option of moving to the middle of the evaporation systems to lower solids loading. Vacuum distillation and increased suspended solids in the stillage, however, made them a prime candidate for improved extraction through holding material at elevated temperatures of 180 to 190 Fahrenheit, resulting in increased oil yields between 0.6 and 0.8 pounds per bushel. ICM plant designs and others opted for increased residence time in the centrifuge to optimize corn oil extraction, adding separation equipment in the middle of the evaporator train for plants running higher syrup solids. For plant design with lower syrup solids, moving the extraction unit to syrup has been a successful option to decrease the flow rate and increase residence time. Employing hold technology also increases quiescence time, which allows for coalescence of oil droplets and more efficient separation.
New Technology Challenges
New technologies seeking process gains through mechanical and chemical means influence the properties of stillage in the backend of the plant, affecting particle size and the solubility of protein and fiber, as well as introducing biochemical changes to the structure of the suspended particles.
Particle size affects separation by increasing the quantity of particles driven to the thin stillage through the decanters. The hydrophobic nature of DCO tends to bind oil to the surface area of particles and thus the DCO gets pulled into the thin stillage and is available for extraction. However, an increase in small particles also increases the viscosity of the stillage at similar solids content, and is the opposite of what is needed for efficient oil extraction, according to Stokes’ Law.
Some new technologies also increase dissolved solids high in short chains of protein and fiber in the thin stillage. The polar regions of these compounds bind to the polar region of fatty acids, and more specifically phospholipids, to create a strong emulsion. The ionic forces and hydrogen bonding in tightly bound oil emulsions require additional energy for separation when compared to the energy needed to separate free oil. When cleaving or breaking down the chemical structure of fiber or protein, the three-dimensional structure is opened up to allow bound oil to be freed from the complex protein and fiber structures. This oil is then more easily separated because it can coalesce with free oil droplets. The suspended solids matrix after oil extraction is more open as well and behaves very differently during evaporation than a noncleaved solids matrix. The rheology (flow property) changes, based upon the binding of free water and the reaction chemistry that occurs with more free reactive groups.
As new process technologies increase the amount of oil driven to the thin stillage in plants, the centrifuge technology on the back end must accommodate the changes in rheology. Valicor is correlating process changes to the behavior of stillage viscosity through a benchmarking program with a network of ethanol plants. Oil extraction efficiency data is collected and collated by plant design and operation parameters and then statistically analyzed to understand the effects of process changes on the chemistry and physics of oil separation.
Through understanding the relationship between stillage chemistry and the impacts on rheology, we have modified our centrifugation technology to efficiently extract more corn oil. For example, as the particle size decreased in stillage, the separation space between the light and heavy phase decreased and the centrifuge was modified to provide more surface area for separation with a shorter path length. In addition, selective removal of particles before the centrifuge, which has been part of Valicor technology since its first installation, has been optimized to account for changes in stillage to remove nonoil-laden particles from the process. There also is an increased chance for fouling and clogging within the system due to dissolved solids and small particles. Therefore, the particle removal and washing process has also changed to accommodate the change in solid type and size. The behavior of the particles in the evaporator has changed due to all these process changes, resulting in significant viscosity changes, which can be mitigated through adaptive technologies.
It has become evident through the years that as the ethanol industry has matured, each plant has adopted different strategies for enhancing ethanol production and corn oil yield. Through continued statistical analysis of process parameters and stillage characteristics from over 40 ethanol plants with varying plant designs, Valicor has identified key process parameters and developed analytical tools to understand factors affecting corn oil extraction efficiency. It is important to understand the efficiency of extraction throughout unit operations and not just the pounds-per-bushel ranking compared to other plants to reveal the most accurate insight into oil yield optimization. In-depth analysis of process parameters at each plant will ensure optimal corn oil extraction efficiency—the better metric for corn oil extraction.
Author: Jennifer Aurandt, PhD
R&D Program Manager, Valicor