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Downstream Processing (DSP): Disc-Stack Centrifugation
Laney Biotech
Overview
This video explains the principles and applications of disc-stack centrifugation, a key process in downstream processing for separating solids from liquids or liquids from liquids. It delves into the fundamental theory based on Stokes' Law, highlighting how particle size, density difference, and viscosity influence separation efficiency. The presentation contrasts gravity settling with centrifugal force, introducing various types of centrifuges like solid-ejecting, liquid-liquid separators, decanters, and specialized biotech designs. Practical aspects such as spin tests for evaluation, scaling up from lab to industrial machines, and cleaning procedures (CIP) are also discussed, emphasizing the importance of efficient separation in industries like biotechnology and food production.
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Chapters
- Downstream processing is crucial for separating desired products from a mixture.
- Disc-stack centrifugation is a primary method for efficient separation.
- Gustav de Laval pioneered centrifugation technology, starting with cream separation.
- Modern centrifuges utilize high rotational speeds to generate significant centrifugal force.
Understanding the historical context and basic purpose of centrifugation sets the stage for appreciating its advanced applications and the underlying scientific principles.
Gustav de Laval's initial invention to separate cream from milk using a hand-cranked separator.
- Separation efficiency is governed by Stokes' Law, which relates settling velocity to particle size, density difference, and fluid viscosity.
- Particle size is the most critical factor due to its squared relationship in the formula.
- Increasing centrifugal force (G-force) significantly enhances separation.
- Reducing viscosity, often by heating, can improve separation efficiency.
Grasping Stokes' Law is essential for predicting and optimizing separation performance by understanding which parameters can be manipulated.
Bacteria (1 micron) separate much slower than yeast cells (8-9 microns) due to the squared effect of particle size in Stokes' Law.
- Gravity settling tanks rely on natural sedimentation, which is slow and requires large surface areas.
- Adding lamella plates to gravity tanks increases surface area and reduces settling distance, improving efficiency.
- Centrifugation dramatically increases separation force, allowing for smaller equipment and higher throughput.
- Disc-stack centrifuges incorporate multiple discs to create numerous small settling areas, mimicking many gravity settlers in a compact space.
Comparing gravity settling to centrifugation highlights the power of centrifugal force in achieving rapid and efficient separations that would be impractical with gravity alone.
Lamella plates in gravity tanks reduce the distance particles need to travel to settle, similar to how discs in a centrifuge create many small settling zones.
- Clarifiers are designed to remove solids from a liquid phase.
- Liquid-liquid separators (purifiers and concentrators) separate two immiscible liquids based on density differences.
- In clarifiers, solids accumulate at the bowl wall and are periodically ejected.
- The position of holes in the discs determines the interface level for liquid-liquid separation (closer to the outside for purifiers, closer to the center for concentrators).
Different centrifuge designs are optimized for specific separation tasks, whether removing solids or separating liquids, impacting process efficiency and product recovery.
A purifier on a ship removes water from fuel, while a concentrator might remove oil from wastewater.
- Solid-ejecting centrifuges automatically discharge accumulated solids at set intervals or when turbidity reaches a threshold.
- A sliding bowl bottom, held in place by water, is actuated to eject solids.
- The ejected solids must be flowable, typically requiring 70-80% solids by volume (like a thick milkshake).
- Continuous discharge designs (nozzle or vortex) are used for higher solid loads.
Automated solid ejection allows for continuous operation and higher throughput, especially in processes with significant solid content.
A centrifuge used in baker's yeast production might eject solids periodically to maintain separation efficiency.
- Spin tests using small test tubes in a high-G centrifuge provide an initial estimate of separation performance and expected flow rates.
- Short spin times (e.g., 30-60 seconds) with clear supernatant indicate easy separation and high flow rates.
- Longer spin times (e.g., 90-120 seconds) suggest difficult separations and slower flow rates.
- Pilot-scale centrifuges (like the Clara 20) are used to validate performance and gather data for scaling up to production machines.
Systematic evaluation from benchtop tests to pilot runs is crucial for accurately predicting and achieving desired separation performance at an industrial scale.
If a spin test requires 90 seconds to achieve clear liquid, the corresponding industrial centrifuge will likely have a slow flow rate.
- Biotechnology often requires gentle handling of shear-sensitive cells (e.g., CHO cells) to preserve product integrity.
- Traditional top-fed centrifuges can induce high shear and air entrainment, damaging cells.
- Low-shear designs, such as bottom-fed hermetic systems, minimize mechanical stress on cells.
- Single-use, all-plastic centrifuges are increasingly used in biotech to avoid cleaning validation issues and ensure sterility.
Specific process requirements, like cell viability in biotech, necessitate specialized centrifuge designs that minimize shear and prevent contamination.
A bottom-fed hermetic centrifuge prevents air pickup and reduces shear, protecting sensitive mammalian cells used in drug production.
- Decanter centrifuges are designed for high solid loads (20-40% by volume) and dewatering applications.
- They operate horizontally with a screw conveyor to continuously move solids to the dewatering zone.
- Decanters typically handle larger particles (above 15 microns) and produce very dry solids.
- They are often used in wastewater treatment or for products that will be dried subsequently.
Decanter centrifuges offer a solution for processes with very high solid concentrations where disc-stack centrifuges might be overloaded or inefficient.
Using a decanter centrifuge to dewater sludge in a wastewater treatment plant.
- Alpha Laval also manufactures membrane filtration systems (UF, MF, NF, RO).
- Heat exchangers, evaporators, and pasteurizers are part of their product portfolio.
- A division focuses on sanitary pumps, valves, and tank cleaning equipment.
- Small-scale test units (e.g., Explorer) are available for pilot testing membrane systems.
Centrifugation is one of several separation technologies; understanding the broader range of available equipment helps in selecting the most appropriate solution for a given process.
Spiral or flat-sheet membrane systems for ultrafiltration (UF) or reverse osmosis (RO).
Key takeaways
- Disc-stack centrifuges leverage high rotational speeds and a stack of discs to dramatically enhance separation efficiency compared to gravity settling.
- Stokes' Law is the fundamental principle governing particle separation, with particle size being the most influential factor.
- The choice between clarifiers, liquid-liquid separators, solid-ejecting centrifuges, or decanters depends on the specific separation task and the nature of the feed material.
- Proper evaluation through spin tests and pilot runs is critical for successful scale-up of centrifugation processes.
- Biotechnology applications demand specialized low-shear designs and often utilize single-use systems to maintain product integrity and simplify validation.
- Decanter centrifuges are optimized for high-solids streams and dewatering, offering a different approach than disc-stack centrifuges.
- Continuous operation is achieved through mechanisms like automatic solid ejection or continuous discharge via nozzles, enabling high throughput.
Key terms
Downstream Processing (DSP)Disc-Stack CentrifugationStokes' LawCentrifugal Force (G-force)ViscosityLamella PlatesClarifierLiquid-Liquid SeparatorSolid Ejecting CentrifugeSliding Bowl BottomNozzle SeparatorDecanter CentrifugeShear SensitivityHermetic DesignSingle-Use CentrifugeCIP (Clean-In-Place)
Test your understanding
- How does particle size influence separation efficiency according to Stokes' Law, and why is this particularly important in centrifugation?
- What are the primary differences between a clarifier and a liquid-liquid separator, and how are their disc designs optimized for these functions?
- Explain the mechanism by which a solid-ejecting centrifuge discharges accumulated solids, and what characteristic must the solids have for this to be effective?
- How can spin tests be used to predict the performance and flow rate of a large-scale industrial centrifuge?
- Why are low-shear and hermetic designs crucial for certain biotechnology applications, and what problems do they address compared to traditional designs?