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 typically based on:
a) the user-defined split percentages of particulate components to each of the outlet streams (Solid Stream and/or Oil Stream) and
b) The solvent split is calculated based on the solvent specification for each stream; the specification can be either by setting indirectly the particulate concentration or the solvent to particulate volumetric ratio or the solvent retention as a volumetric percentage in each of the outlet streams (oil and solids streams).
Since occasionally modeling a centrifugation requires the user to specify not the split percentage of a component to a stream but rather the mass percentage of a component on a stream, or sometimes both, SuperPro offers an ‘Advanced Set of Specifications’ for a user to provide in order for the simulation engine to carry out the material balances. When you enter the advanced specifications for how to perform the centrifugation material balances, you can specify any allowable set of split percentages and/or output mass percentages and the simulation engine will adapt and generate a solution (provided one exists). For details on how this modeling approach functions consult Centrifugation: Advanced Modeling Calculations.
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.136) |
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.137) |
where
● dlimis 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.138) |
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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.135). 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).
The “Advanced Split Options” in the Centrifugation operations allows the specification of target values for particulate component mass fractions in output streams instead of or along with split percentages. More specifically, users can specify for any particulate component and in any output stream (oil, aqueous or solid) one of the following:
● - The split percentage of that component directed from the feed to an output stream
● - The mass fraction (as a percentage) of that component in an output stream
● - Both the split percentage and mass fraction of that component in an output stream.
The number of specifications is restricted by the system’s degree of freedom (DOF). More specifically, the number of specifications allowed is (n+1)(m-1) where n is the number of particulate components and m is the number of output streams (m=2 for oil or solids removal and m=3 for solids and oil removal). Normally, n(m-1) specifications are expected for particulate components and the remaining m-1 DOF are covered by the solvent-related specifications (solvent retention etc.). It is possible, however, to exceed the particulate component DOF limit by one or two but, in this case, one or two solvent-related specifications must be disabled. In other words, solvent-related specifications can be traded for extra specifications for particulates.
Aside from the constraint on the number of specifications, there exist combinations of specifications which are inadmissible. For example, it is not allowed to specify the mass fractions of all components in an output stream. In the same spirit, it is not allowed to specify the split percentage for a component in all output streams.
At solution time and if advanced split options are used, a DOF analysis precedes the solution of the centrifugation model equations. If DOF violations are detected (either in the number or the type of specifications), an error message is issued. If the solution fails (either because of DOF violations or the specified values lead to physically unrealistic results. e.g. negative flows) the centrifugation feed is directed to the aqueous stream.
For details on how to use the interface that allows you to provide advanced specifications for a centrifugation operation instead of the standard split percentages, see Centrifugation: Advanced Particulate Component Separation Specifications Dialog.
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
● Advanced Particulate Component Separation Specifications, see Centrifugation: Advanced Particulate Component Separation Specifications Dialog
● 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
