Centrifugation is a separation procedure that relies on difference of the particle size as well as the density difference between the liquid and solid phases. It is typically used in the chemical, biochemical, food, and environmental industries to separate particles of size between 100 and 0.1 mm. In bioprocessing, centrifugation (mainly disk-stack) is primarily used for recovering and concentrating biomass (cell harvesting), removing cell debris particles after cell disruption, recovering inclusion bodies, and in general, separating suspended solids from a liquid solution. Centrifugation is also used to separate immiscible liquid phases with different densities (e.g., oil droplets and aqueous phase).
The technology provides advantages over microfiltration, particularly for large macromolecules such as vaccines. When large pore-size cutoffs are needed, membranes are susceptible to filter fouling by cell debris so scale-up becomes problematic due to the uncertainty induced by the fouling conditions. Centrifugation not only circumvents the problem altogether but also eliminates the cost of filter replacement.
● Centrifugation in a Decanter Procedure
● Centrifugation in a Disk-Stack Centrifuge Procedure
● Centrifugation in a Bowl Centrifuge Procedure
The material balances for the centrifugation operation are based on the user-defined removal percentages of particulate components as well as the solvent retained in the oil and solids streams. The latter can be adjusted either directly as a percentage or indirectly, by specifying the particulate concentration or the volume to volume ratio in the oil and solids streams.
The separation capability of a centrifuge is characterized by a Sigma factor (Σ), which represents the area of a gravity settler required to achieve the same separation. The maximum throughput, Q, for an efficient use of a centrifuge (i.e., complete separation of particles or droplets with diameter greater than dlim) is then expressed by:
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where:
● η is the efficiency of the centrifuge,
● ulim is the limiting sedimentation velocity,
● Σ is the sigma factor
The case where η = 1 describes the performance of a centrifuge based on the Stokes' law. In practice, factors like Newtonian rheology, hindered settling, non-laminar flow, etc. would result in lower values of η. Experience has shown that the efficiency of disk-stack separators is usually less than 50% with an average value of about 30%.
In the case of oil separation the limiting sedimentation velocity is calculated by Stoke’s law:
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eq. (A.128) |
where
● g is the gravity constant,
● dlim is the equivalent Stokes' diameter of the limiting oil globule,
● ρp is the density of the limiting oil globule,
● ρL is the density of the continuous liquid phase,
● μ is the viscosity of the continuous liquid phase
In the case of solids separation, the limiting sedimentation velocity is calculated by a general empirical rule:
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eq. (A.129) |
where
● dlim is the equivalent Stokes' diameter of the limiting solid particle,
● ρp is the density of the limiting solids particle
● b1 and n are constants the values of which are given in the table below depending on the value of the parameter K:
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eq. (A.130) |
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K |
bl |
n |
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=< 3.3 |
24.0 |
1.0 |
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> 3.3 and =< 43.6 |
18.5 |
0.6 |
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> 43.6 |
0.44 |
0.0 |
The sizing of a centrifuge in Superpro can be based either on the sigma factor or the volumetric throughput. It is advised to select the first option when physical parameter values (i.e., limiting particle diameter and density and continuous liquid phase density and viscosity) are readily available.
Sizing based on Sigma Factor
In Design Mode, the user specifies the limiting component and continuous liquid phase data and the model estimates the required Sigma factor of the centrifuge using eq. (A.127). If the calculated sigma factor exceeds the maximum, the program assumes multiple units operating in parallel with a total sigma factor equal to the calculated.
In Rating Mode, the user specifies the Sigma factor of the centrifuge (rated Sigma factor). The program will check if the calculated Sigma factor is greater than the rated Sigma factor and generate a warning accordingly.
Sizing based on Throughput
In Design Mode, the program will calculate the rated throughput required. If the rated throughput is higher than the specified maximum value, the program assumes multiple units operating in parallel.
In Rating Mode, the user specifies the rated throughput of the centrifuge. The program will check if the rated throughput is greater than the maximum value and generate a warning accordingly.
To estimate the purchase cost of a disk-stack centrifuge, the program uses a function that represents averaged cost data from two different vendors (Alfa-Laval and Westfalia).
1. Ambler, C. M. (1952). The Evaluation of Centrifuge Performance. Chem. Eng. Progress, 48, 3, 150-158.
2. Ambler, C. M. (1961). The Fundamentals of Separation, including Sharples 'Sigma value' for Predicting Equipment Performance. Industrial and Engineering Chemistry, June, 430-433.
3. Ambler, C. M. (1988). Centrifugation, Section 4.5, In Handbook of Separation Techniques for Chemical Engineers, 2nd edition, edited by P.A. Schweitzer, MacGraw-Hill.
4. Axelson, H. A. C. (1985). Centrifugation, In Comprehensive Biotechnology, 2, Moo-Young, M. (editor), Pergamon Press.
5. Frampton, G. A. (1963). Evaluating the Performance of Industrial Centrifuges, Chemical and Process Engineering, August, 402-412.
6. Murkes, J. and C-G. Carlsoon (1978). Mathematical Modeling and Optimization of Centrifugal Separation, Filtration & Separation, January, 18-21.
The interface of this operation has the following tabs:
● Oper. Cond’s, see Centrifugation: Oper. Conds Tab
● Mat. Balance, see Centrifugation: Mat. Balance Tab
● Utilities, see Centrifugation: Utilities Tab
● Labor, etc, see Operations Dialog: Labor etc. Tab
● Description, see Operations Dialog: Description Tab
● Batch Sheet, see Operations Dialog: Batch Sheet Tab
● Scheduling, see Operations Dialog: Scheduling Tab
