Centrifuge Feed Compartment Centrifuge Double Vs Single

Disc Stack Centrifuge

While disc stack centrifuges are able to accept a wide range of feeds they are both mechanically complex and expensive.

From: Solid/Liquid Separation , 2007

Microalgal fatty acids—From harvesting until extraction

H.M. Amaro , ... A. Catarina Guedes , in Microalgae-Based Biofuels and Bioproducts, 2017

16.2.4.1 Disc stack centrifuges

Disc stack centrifuges are able to apply a force from 4000 to 14,000 times gravitational force ( Perry and Chilton, 1973), thus reducing separation time. These are the most common industrial centrifuges and are widely used in commercial plants for high-value microalgal products and in microalgal biofuel pilot plants (Molina-Grima et al., 2003).

Disc stack centrifuges are ideally suited for separating particles 3–30   μm in concentrations of 0.02%–0.05% of microalgal cells (Fig. 16.1). However, they generally exhibit high energy consumption (Uduman et al., 2010).

Fig. 16.1. Centrifuge application diagram, particle sizes, and concentration range

(Based on Milledge and Heaven (2013).).

As an example, a Westfalia HSB400 disc-bowl centrifuge with an intermittent self-cleaning bowl centrifugal clarifier has a maximum capacity of 95   m³  h^{−   1} but is limited to 35   m³  h^{−   1} for microalgae harvesting. The maximum power of the engine is 75   kW, but normal operating demand is probably around 50   kW, giving an energy cost for separation of 1.4   kW   h   m^{−   3}. A value of 1   kW   h   m^{−   3} has been reported for concentrating Scenedesmus from 0.1% to 12% using a Westfalia self-cleaning disk stack centrifuge (Molina-Grima et al., 2003), and an energy consumption of 1.4   kW   h   m^{−   3} has been reported for the disc bowl centrifuge harvesting of microalgae grown on pig waste (Goh, 1984). A Westfalia HSB400 centrifuge fed with a suspension of 0.02% dry weight of microalgae having an oil content of 20%, would yield the equivalent 7   kg of dry algal material per h and 1.4   kg of algal oil. However, considering an average energy density of 13   kW   h per kg of oil, to obtain 1.4   kg oil, 18.6   kW   h is necessary. Thus, to achieve this amount of oil, at least 7   kg of dry algae must be obtained, and considering the concentration of biomass of 0.02%, at least 35   m³ of culture broth have to be centrifuged, considering a 100% of recovery efficiency. Hence, this means that this process entails 49   kW   h of energy consumed, which may mean that the consumption only in the harvesting step is higher than the potential production. To improve the energy return using centrifugation, some steps should be implemented, such as preconcentration using a combination of separation techniques, use of the entire microalgal biomass rather than just the lipid fraction for energy production, or use of centrifuge to eliminate other energy-consuming unit operations in algal biofuel production process. Preconcentration to 0.5% (algal dry weight) by settlement or other low-energy methods could improve the energy balance.

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Solid/liquid separation equipment

E.S. Tarleton , R.J. Wakeman , in Solid/Liquid Separation, 2007

1.3.1.3 Disc stack

Typical uses: Clarification and thickening to produce a solids sludge.

FDS process ratings: 2 S, -, -, 6.

Typical particle size and feed concentration range: 0.1–100 μm and 0.05–2% w/w (self-cleaning and manual discharge), 0.5–10% w/w (nozzle discharge).

The disc stack centrifuge is a versatile device, which may be used for separating solid/liquid mixtures in continuous, semi-continuous and batch configurations (see Figures 1.12 and 1.13). All except some batch-operated machines are able to handle toxic, flammable and volatile feeds at throughputs up to 200 m³ h⁻¹. Liquid-liquid mixtures can be separated and with more sophisticated units a three (two liquid and one solid) phase separation is achievable. In all cases, a sufficient density difference must exist between the phases present in the feed.

Figure 1.12. Schematic representation of a nozzle discharge disc stack sedimenting centrifuge.

Figure 1.13. Photograph (left) of an ejecting, self-cleaning, disc stack centrifuge showing an external view of the disc bowl and accompanying motor drive. Also shown (right) is a typical disc stack arrangement.

Both photographs with permission from Alfa Laval.

Although several variants exist, the generic type is characterised by an imperforate bowl surrounding an inverted stack of 30–200 thin conical discs separated by 0.3–3 mm spacers. The disc spacing is dependent on the viscosity and solids content in the feed and needs to be fixed accordingly, lower viscosities and solids concentrations favour spacings below 1 mm. As the discs are spun on a common vertical axis the process suspension, which is fed centrally from the top, travels through the annular spaces between the discs.

Centrifugal forces up to 14000g cause particles to accumulate on the underside of the discs from where they slide down towards the outer periphery of the centrifuge bowl. In batch units the thickened solids remain in the bowl until the solids handling capacity of the centrifuge is reached. At this point rotation stops and the basket containing the trapped solids is manually replaced or a discharge valve on the periphery of the bowl is manually operated to facilitate removal of the sediment. In continuous units the solids sludge, which must be flowable, is automatically discharged, sometimes intermittently, through nozzles positioned on the outer periphery of the bowl; a typical centrifuge has between 12 and 24 nozzles. For cakes that exhibit poor flow characteristics, the 'self-ejecting' design variant allows the bottom part of the centrifuge to automatically separate at periodic intervals and discharge the accumulated solids.

While disc stack centrifuges are able to accept a wide range of feeds they are both mechanically complex and expensive. Moreover, the close stacking of conical discs means that mechanical cleaning can be difficult, and resort is often made to chemical cleaning.

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Next-Generation Process Design for Monoclonal Antibody Purification

Krunal K. Mehta , Ganesh Vedantham , in Biopharmaceutical Processing, 2018

Depth Filtration

There is a limit to the particle density that can be removed by a disk-stack centrifuge depending on the cell culture properties, centrifuge feed rate, bowl geometry, and rotational speed [14]. Because of this size limitation, further clarification by depth filtration is typically used to remove smaller solid particulates that still remain in the centrifuge product [15,16]. Commonly used depth filters consist of a thick, porous matrix of cellulose fibers with inorganic filter aids bound to them by a positively charged resin. The thick matrix provides a tortuous path to retain a range of particle sizes, and the positive charge imparts adsorptive properties to the filter. The minimum particle size that can be effectively removed solely by the sieving mechanism of a depth filter is about 0.1   μm [17]. The adsorptive mechanism, however, can remove much smaller, soluble and negatively charged impurities such as DNA and host cell proteins to further improve product quality [16]. These cellulosic depth filters release relatively high levels of water-soluble contaminants into the system. These high levels of organic and inorganic contaminants are reduced to acceptable levels through extensive preflushing prior to use [18]. In subsequently available commercial depth filters, cellulosic fibers were eliminated and water-soluble thermoset resin binders were substituted with water-soluble binders to reduce the amount of extractables, thereby lowering the risk of product contamination [18].

The use of depth filtration is not limited to the secondary harvest step; it could potentially replace disk-stack centrifugation for processes based entirely on disposables [19]. Depth filters have been effective in the removal of impurities when positioned at various stages of bioprocessing [15]. In addition to improving the product quality, depth filters aid in protecting and prolonging the life of sterilizing filters and chromatography resins [9,14–16]. The use of depth filters with relatively large pore sizes, primarily targeting retention of cells and cell debris, seems feasible for high viability and low cell density cell cultures [19]. High cell density cultures with variable cell viability have increased the risk of filter fouling, thereby making the implementation of depth filters challenging. Using depth filters for harvesting high cell density cultures may require significant filter areas and a large footprint. Since depth filtration is a single-use technology, the undesired side effect of simplifying the process by replacing the centrifuge with this approach could lead to increased manufacturing costs.

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Downstream Process Design, Scale-Up Principles, and Process Modeling*

Karol M. Łącki, , ... Kjell O. Eriksson , in Biopharmaceutical Processing, 2018

Centrifugation

Separation of solids in a monoclonal purification process is limited to handling of biomass during the primary recovery of the antibody. Most industrial processes use disc stack centrifuges, as these apparatus are scalable, perform continuous operation, and have a capacity to handle a wide variety of feed stock [ 37]. Typically, secondary clarification using a depth filter after centrifugation is required prior to further downstream processing. Efficiency of the centrifugation step depends on the solids volume fraction, the effective clarifying surface (V/D), and the acceleration factor (ω ² r/g). Typically, accelerating factors of 1500   g are used for harvesting cells. The product of these two factors (ω ² rV/gD) is called the sigma factor (Σ) and is used in scale-up calculations. The sigma factor represents the equivalent area of the centrifuge and is unique for each disc stack centrifuge and the angular velocity. For continuous operation, the ratio between flow rate through the centrifuge, Q, and the sigma factor should be kept constant during the scale-up. The sigma factor can also be used to scale disc stack centrifuges from a lab bottle centrifuge, by replacing one of the flow rates in the above-mentioned ratio with centrifuged volume divided by time of the centrifugation [37]. However, keeping the ratio of Q/Σ constant may still lead to inadequate operation at large scale due to hindered settling and the presence of sub-micron particles generated during the centrifugation process itself. These particles are formed from cell and cell debris upon exposure to high shear (see discussion on the effect of mixing on bioreactor performance) and are removed from the centrifuge in the concentrate stream. In addition, shear damage can cause release of proteases that could affect stability of the antibody.

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Industry Review of Cell Separation and Product Harvesting Methods

John P. Pieracci , ... Jorg Thommes , in Biopharmaceutical Processing, 2018

Centrifugation

The most common separation technique used for inclusion body recovery is low-speed centrifugation. Both benchtop and continuous centrifugation options are amenable to inclusion body isolation. For industrial processes, common industrial disc stack centrifuges can be optimized for inclusion body recovery at large scale [ 100–102]. An approach to process development and optimization that is similar to what was described above for mammalian cell harvest applications can be leveraged for inclusion body recovery and separation. Typical centrifugation operations are performed under low × g between 5000 and 20,000   g [103]. The large density differences between inclusion bodies and cell debris provides an advantage for centrifugation based separation which have generally been found to give greater protein purity when compared to membrane based processes, although the results may be process and protein dependent [70].

In addition to removing the soluble and less dense cellular impurities, collection of the IB pellet by centrifugation provides significant volume reduction. The concentration of the IB by centrifugation can reduce tankage requirements, allow long-term frozen storage of the concentrated IB pellet, and provide an opportunity for pellet washing with appropriately designed solutions for impurity removal.

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Centrifugation

Zeki Berk , in Food Process Engineering and Technology, 2009

9.3.2 Disc-bowl Centrifuges

As explained in Section 9.2.3 above, centrifugal separation can be improved by distributing the flow to a number of parallel narrow channels between conical dishes. The disc-bowl centrifuges, also known as disc stack centrifuges, can serve both as clarifiers and as separators. The rotor (bowl) containing a stack of discs is placed on a vertical spindle and rotates inside a stationary housing. The capacity range of disc-bowl centrifuges extends to over 100 cubic meters per hour. The types of disc-bowl clarifiers described below differ in their mode of discharging the solids.

In centrifuges with a solid wall bowl (Figure 9.9), the liquid phases are discharged continuously but, just as in the tubular centrifuge, there is no outlet for the sludge. The accumulated solids are removed manually when the machine is stopped and the bowl is taken apart for cleaning. Such machines are therefore suitable mainly as separators (e.g. for separating cream from skim milk) but cannot handle suspensions with a significant concentration of solids.

Figure 9.9. Solid wall bowl centrifuge. (Courtesy of Alfa-Laval)

Nozzle centrifuges (Figure 9.10) are equipped with nozzles for the continuous discharge of solids while the machine is turning. They serve as clarifiers for suspensions containing a moderate concentration of solids (up to about 10% by volume). The bowl is of double-cone shape. The solids accumulate at the zone of maximum diameter where the centrifugal force is the highest. Narrow nozzles located on the periphery of the bowl at that zone serve as outlets for the solids. The pressure drop through the nozzles must be sufficiently high to prevent outburst of liquid from the bowl.

Figure 9.10. Continuous solid discharge (nozzle) centrifuge. (Courtesy of Alva-Laval)

Therefore, the sludge must be sufficiently thick to provide the controlled flow through the nozzle without completely plugging it. Interchangeable nozzles with different hole diameters serve to adjust the centrifuge according to the solids content of the feed.

Self-cleaning desludger centrifuges (Figure 9.11) are used as clarifiers with suspensions containing a high proportion of solids, typically 30–40% by volume. In this type, the accumulated sludge is discharged intermittently. The bowl wall is not of one piece but consists of two separate conical parts pressed together by hydraulic force. In operation, the solids accumulate in the zone of maximum diameter, about the plane separating the two cones. When the zone of accumulation is full with solids, the hydraulic system releases the bottom cone which drops slightly to leave an opening between the two halves. The solids are rapidly ejected through this opening, the bowl is closed again and a new cycle of accumulation begins. The entire operation, which may include a phase of rinsing of the accumulation chamber, is usually automated.

Figure 9.11. Intermittent solid discharge centrifuge. (Courtesy of Alva-Laval)

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Process intensification applied to microalgae-based processes and products

Rajshree Amrut Patil , ... Aniruddha Bhalchandra Pandit , in Handbook of Microalgae-Based Processes and Products, 2020

27.2.3.5 Centrifugation

The microalgae cells are retrieved from the cultivation media by applying centrifugal force. The cell separation takes place based on the difference in size and density of cells. The different types of centrifuge systems used for microalgae cell separation include disk stack centrifuge, basket centrifuge, decanter perforated basket centrifuge, and imperforated and hydro cyclones. The disk stack centrifuge and decanter centrifuge are most commonly used. Centrifugation is the preferred method for harvesting of high-value products from microalgae biomass. It offers a very high recovery rate and chemical-free biomass. However, for large-scale applications, energy consumption, treatment time, maintenance, and capital costs are very high.

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Unit Operations

Pauline M. Doran , in Bioprocess Engineering Principles (Second Edition), 2013

11.4 Centrifugation

Centrifugation is used to separate materials of different density by application of a force greater than gravity. In downstream processing, centrifugation is used to remove cells from fermentation broths, to eliminate cell debris, to collect precipitates and crystals, and to separate phases after liquid extraction. Centrifugation may also be applied in other areas of bioprocessing, such as to clarify molasses used in fermentation media and in the production of wort for brewing.

Steam-sterilisable centrifuges are applied when either the separated cells or fermentation liquid is recycled back to the fermenter, or when product contamination must be prevented. Industrial centrifuges generate large amounts of heat due to friction; it is therefore necessary to have good ventilation and cooling. Aerosols created by fast-spinning centrifuges have been known to cause infections and allergic reactions in factory workers so that isolation cabinets are required for certain applications.

Centrifugation is most effective when the density difference between the particles and liquid is great, the particles are large, and the liquid viscosity is low. It is also assisted by a large centrifuge radius and high rotational speed. However, in centrifugation of biological solids such as cells, the particles are very small, the viscosity of the medium can be relatively high, and the particle density is very similar to that of the suspending fluid. These disadvantages are overcome easily in the laboratory with small centrifuges operated at high speed. However, problems arise in industrial centrifugation when large quantities of material must be treated.

Centrifuge capacity cannot be increased by simply increasing the size of the equipment without limit; mechanical stresses in centrifuges increase in proportion to (radius)² so that safe operating speeds are substantially lower in large equipment. The need for continuous throughput of material in industrial applications also restricts practical operating speeds. To overcome these difficulties, a range of centrifuges has been developed for bioprocessing. The types of centrifuge commonly used in industrial operations are described in the next section.

11.4.1 Centrifuge Equipment

Centrifuge equipment is classified according to internal structure. The tubular bowl centrifuge has the simplest configuration and is employed widely in the food and pharmaceutical industries. Feed enters under pressure through a nozzle at the bottom, is accelerated to rotor speed, and moves upward through the cylindrical bowl. As the bowl rotates, particles travelling upward are spun out and collide with the walls of the bowl as illustrated schematically in Figure 11.6. Solids are removed from the liquid if they move with sufficient velocity to reach the wall of the bowl within the residence time of the liquid in the machine. As the feed rate is increased, the liquid layer moving up the wall of the centrifuge becomes thicker; this reduces the performance of the centrifuge by increasing the distance a particle must travel to reach the wall. Liquid from the feed spills over a weir at the top of the bowl; solids that have collided with the walls are collected separately. When the thickness of sediment collecting in the bowl reaches the position of the liquid-overflow weir, separation efficiency declines rapidly. This limits the capacity of the centrifuge. Tubular centrifuges are applied mainly for difficult separations requiring high centrifugal forces. Solids in tubular centrifuges are accelerated by forces between 13,000 and 16,000 times the force of gravity.

Figure 11.6. Separation of solids in a tubular bowl centrifuge.

A type of narrow tubular bowl centrifuge is the ultracentrifuge. This device is used for recovery of fine precipitates from high-density solutions, for breaking down emulsions, and for separation of colloidal particles such as ribosomes and mitochondria. It produces centrifugal forces 10⁵ to 10⁶ times the force of gravity. The bowl is usually air-driven and operated at low pressure or in an atmosphere of nitrogen to reduce the generation of frictional heat. The main commercial application of ultracentrifuges has been in the production of vaccines to separate viral particles from cell debris. Typically, ultracentrifuges are operated in batch mode, so their processing capacity is restricted by the need to empty the bowl manually. Continuous ultracentrifuges are available commercially; however safe operating speeds for these machines are not as high as for batch equipment.

An alternative to the tubular centrifuge is the disc stack bowl centrifuge . Disc stack centrifuges are common in bioprocessing. There are many types of disc centrifuge; the principal difference between them is the method used to discharge the accumulated solids. In simple disc centrifuges, solids must be removed periodically by hand. Continuous or intermittent discharge of solids is possible in a variety of disc centrifuges without reducing the bowl speed. Some centrifuges are equipped with peripheral nozzles for continuous solids removal; others have valves for intermittent discharge. Another method is to concentrate the solids in the periphery of the bowl and then discharge them at the top of the centrifuge using a paring device; the equipment configuration for this mode of operation is shown in Figure 11.7. A disadvantage of centrifuges with automatic discharge of solids is that the solids must remain sufficiently wet to flow through the machine. Extra nozzles may be provided for cleaning the bowl should blockage of the system occur.

Figure 11.7. Disc stack bowl centrifuge with continuous discharge of solids.

Disc stack centrifuges contain conical sheets of metal called discs that are stacked one on top of the other with clearances as small as 0.3   mm. The discs rotate with the bowl and their function is to split the liquid into thin layers. As shown in Figure 11.8, feed is released near the bottom of the centrifuge and travels upward through matching holes in the discs. Between the discs, heavy components of the feed are thrown outward under the influence of centrifugal forces while lighter liquid is displaced toward the centre of the bowl. As they are flung out, the solids strike the undersides of the overlying discs and slide down to the bottom edge of the bowl. At the same time, the lighter liquid flows inward over the upper surfaces of the discs to be discharged from the top of the bowl. Heavier liquid containing solids can be discharged either at the top of the centrifuge or through nozzles around the periphery of the bowl. Disc stack centrifuges used in bioprocessing typically develop forces of 5000 to 15,000 times gravity. As a guide, the minimum solid–liquid density difference for successful separation in a disc stack centrifuge is approximately 0.01 to 0.03   kg   m⁻³. In practical operations at appropriate flow rates, the minimum particle diameter separated is about 0.5 μm [3].

Figure 11.8. Mechanism of solids separation in a disc stack bowl centrifuge.

From C.J. Geankoplis, 1983, Transport Processes and Unit Operations, 2nd ed., Allyn and Bacon, Boston.

The performance characteristics of tubular bowl and disc stack centrifuges used for industrial separations are summarised in Table 11.2. In general, as the size of the centrifuge increases, the practical operating speed is reduced and the maximum centrifugal force decreases.

Table 11.2. Performance Characteristics of Tubular Bowl and Disc Stack Centrifuges

Centrifuge Type	Bowl Diameter (cm)	Speed (rpm)	Maximum Centrifugal Force (× gravity)	Liquid Throughput (l   min−1)	Solids Throughput (kg   min−1)	Motor Size (kW)
Tubular bowl	10	15,000	13,200	0.4–40		1.5
	13	15,000	15,900	0.8–80		2.2
Disc stack	18	12,000	14,300	0.4–40		0.25
	33	7500	10,400	20–200		4.5
	61	4000	5500	80–800		5.6
Disc stack with nozzle discharge	25	10,000	14,200	40–150	1.5–15	15
	41	6250	8900	100–550	7–70	30
	69	4200	6750	150–1500	15–190	95
	76	3300	4600	150–1500	15–190	95

Data from Perry's Chemical Engineers' Handbook, 1997, 7th ed., McGraw-Hill, New York.

11.4.2 Centrifugation Theory

The particle velocity achieved in a particular centrifuge compared with the settling velocity under gravity characterises the effectiveness of centrifugation. The terminal velocity during gravity settling of a small spherical particle in dilute suspension is given by Stoke's law:

(11.19) $u_{\mathrm{g}}=\frac{{\rho }_{\mathrm{p}}-{\rho }_{\mathrm{L}}}{18\mu }{D}_{\mathrm{p}}^{2}g$

where u _g is the sedimentation velocity under gravity, ρ _p is the density of the particle, ρ _L is the density of the liquid, μ is the viscosity of the liquid, D _p is the particle diameter, and g is gravitational acceleration. In a centrifuge, the corresponding terminal velocity is:

(11.20) $u_{\mathrm{c}}=\frac{{\rho }_{\mathrm{p}}-{\rho }_{\mathrm{L}}}{18\mu }{D}_{\mathrm{p}}^2{\omega }^{2}r$

where u _c is the particle velocity in the centrifuge, ω is the angular velocity of the bowl in units of rad s⁻¹, and r is the radius of the centrifuge drum. The ratio of the velocity in the centrifuge to the velocity under gravity is called the centrifuge effect or g-number, and is usually denoted Z. Therefore:

(11.21) $Z=\frac{{\omega }^{2}r}{g}$

The force developed in a centrifuge is Z times the force of gravity, and is often expressed as so many g-forces. Industrial centrifuges have Z factors up to about 16,000; for small laboratory centrifuges, Z may be up to 500,000 [4].

Sedimentation occurs in a centrifuge as particles moving away from the centre of rotation collide with the walls of the centrifuge bowl. Increasing the velocity of motion will improve the rate of sedimentation. From Eq. (11.20), the particle velocity in a given centrifuge can be increased by:

•

Increasing the centrifuge speed, ω

•

Increasing the particle diameter, D _p

•

Increasing the density difference between the particle and liquid, ρ _p −ρ _L

•

Decreasing the viscosity of the suspending fluid, μ

Whether the particles reach the walls of the bowl also depends on the time of exposure to the centrifugal force. In batch centrifuges such as those used in the laboratory, centrifuge time is increased by running the equipment longer. In continuous-flow devices such as disc stack centrifuges equipped for continuous solids discharge, the residence time is increased by decreasing the feed flow rate.

The performance of centrifuges of different size can be compared using a parameter called the sigma factor Σ. Physically, Σ represents the cross-sectional area of a gravity settler with the same sedimentation characteristics as the centrifuge. For continuous centrifuges, Σ is related to the feed rate of material as follows:

(11.22) $\mathrm{\Sigma }=\frac{Q}{2u_{\mathrm{g}}}$

where Q is the volumetric feed rate and u _g is the terminal velocity of the particles in a gravitational field as given by Eq. (11.19). If two centrifuges perform with equal effectiveness:

(11.23) $\frac{Q_1}{{\mathrm{\Sigma }}_1}=\frac{Q_2}{{\mathrm{\Sigma }}_2}$

where subscripts 1 and 2 denote the two centrifuges. Equation (11.23) can be used to scale up centrifuge equipment. Equations for evaluating Σ depend on the centrifuge design. For a disc stack bowl centrifuge [5]:

(11.24) $\mathrm{\Sigma }=\frac{2\pi {\omega }^2(N-1)}{3g\tan \theta }(r_2^3-r_1^3)$

where ω is the angular velocity in rad s⁻¹, N is the number of discs in the stack, r ₂ is the outer radius of the discs, r ₁ is the inner radius of the discs, g is gravitational acceleration, and θ is the half-cone angle of the discs. For a tubular bowl centrifuge, the following equation is accurate to within 4% [6]:

(11.25) $\mathrm{\Sigma }=\frac{\pi {\omega }^{2}b}{2g}(3r_2^2+r_1^2)$

where b is the length of the bowl, r ₁ is the radius of the liquid surface, and r ₂ is the radius of the inner wall of the bowl (Figure 11.6). Because r ₁ and r ₂ in a tubular bowl centrifuge are about equal, Eq. (11.25) can be approximated as:

(11.26) $\mathrm{\Sigma }=\frac{2\pi {\omega }^{2}b{r}^2}{g}$

where r is an average radius roughly equal to either r ₁ or r ₂.

The equations for Σ are based on ideal operating conditions. Because different types of centrifuge deviate to varying degrees from ideal operation, Eq. (11.23) cannot generally be used to compare different centrifuge configurations. The performance of any centrifuge can deviate from that predicted theoretically due to factors such as the particle shape and size distribution, particle aggregation, nonuniform flow distribution in the centrifuge, and interaction between the particles during sedimentation. Experimental tests must be performed to account for these factors.

Example 11.2

Cell Recovery in a Disc Stack Centrifuge

A continuous disc stack centrifuge is operated at 5000   rpm for separation of bakers' yeast. At a feed rate of 60   l   min⁻¹, 50% of the cells are recovered. For operation at constant centrifuge speed, solids recovery is inversely proportional to the flow rate.

(a)

What flow rate is required to achieve 90% cell recovery if the centrifuge speed is maintained at 5000   rpm?

(b)

What operating speed is required to achieve 90% recovery at a feed rate of 60   l   min⁻¹?

Solution

(a)

If solids recovery is inversely proportional to feed rate, the flow rate required is:

$\frac{50\%}{90\%}(60\mathrm{l}{\min }^{-1})=33.3\mathrm{l}{\min }^{-1}$

(b)

Equation (11.23) relates the operating characteristics of centrifuges achieving the same separation. From (a), 90% recovery is achieved at Q ₁=33.3   l   min⁻¹ and ω ₁=5000   rpm. For Q ₂=60   l   min⁻¹, from Eq. (11.23):

$\frac{Q_1}{Q_2}=\frac{{\mathrm{\Sigma }}_1}{{\mathrm{\Sigma }}_2}=\frac{33.3\mathrm{l}{\min }^{-1}}{60\mathrm{l}{\min }^{-1}}=0.56$

Because the same centrifuge is used and all the geometric parameters are the same, from Eq. (11.24):

$\frac{{\mathrm{\Sigma }}_1}{{\mathrm{\Sigma }}_2}=\frac{{\omega }_1^2}{{\omega }_2^2}=0.56$

The ratio of ${\omega }_1^2$ to ${\omega }_2^2$ is 0.56, irrespective of the units used to express angular velocity. Therefore, using units of rpm:

${\omega }_2^2=\frac{{\omega }_1^2}{0.56}=\frac{{(5000\text{rpm})}^2}{0.56}=4.46\times {10}^7{\mathrm{rpm}}^2$

Taking the square root, ω ₂=6680   rpm.

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Engineering Fundamentals of Biotechnology

S. Chhatre , N.J. Titchener-Hooker , in Comprehensive Biotechnology (Second Edition), 2011

2.65.3.2.2 Precipitation and centrifugation

Neal et al. [42] investigated an ultra-scale-down mimic of the recovery steps in an industrial process used for the processing of a snake venom-specific antibody fragment. In the process, antibodies were precipitated from sheep serum and then recovered using a disk stack centrifuge. The size of the shear forces and the duration of exposure felt by the precipitate particles enabled definition of a time-integrated shear stress, which allowed prediction of the properties of the precipitates at full scale. An example of how close the properties of the precipitated particles between ultra-scale-down and large scale is shown in Figure 5 . By combining CFD regime analysis of the large-scale centrifugal flow field, data from the scale-down precipitation, a shear device, and operation in a laboratory centrifuge, the performance of the full-scale centrifuge was also predicted accurately by the scale-down unit. Thus, the overall performance in terms of properties such as solids content, density, viscosity, yield, and purity for the large-scale process was predicted successfully by the ultra-scale-down mimic.

Figure 5. Comparison of the physical properties of precipitated polyclonal antibodies generated by an ultra-scale-down mimic (white bars) of the recovery steps in an industrial process (black bars) for manufacturing rattlesnake antivenoms.

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The Search for Process Intensification and Simplification: Alternative Approaches versus Current Platform Processes for Monoclonal Antibodies

Robert S. Gronke , Alan Gilbert , in Biopharmaceutical Processing, 2018

Harvest

Centrifugation, in combination with depth filtration, has been the workhorse for large-scale harvest clarification since the late 1990s. Over the past 10 years, advances in bioreactor development have achieved strikingly high cell densities of over 150*10⁶  cells/mL while maintaining reasonably high harvest viabilities of >   70% [31,44]. This combination of high cell mass and associated cellular debris however has required renewed efforts in harvest technology and processing just to maintain yield, product quality, and process throughput (see Chapter 9).

Pre-treatment clarification technologies, such as acid treatment, addition of flocculants, or precipitation continue to improve such that one can now target not only the cellular debris but also process-related impurities (e.g., DNA, host cell proteins, endotoxin) or even the product itself [45]. Without pre-treatment, the centrifuge packed-cell volumes can approach 15%–20%, which approaches the limits of the centrifuge's capabilities. Shot intervals (i.e., the length of time between discharges of the concentrated cell mass) and their accompanying turbidity perturbations may be impacted. Nozzles in the centrifuge restrict the flow of the solids/concentrate stream exiting the centrifuge. In a typical disk stack centrifuge, one can achieve a flow rate of about 0.07   L/min per nozzle. Fig. 34.8 indicates that by increasing the number of nozzles from 4 to 8, the saturation flow rate is increased proportionally thereby achieving processing at packed cell volumes (PCV) up to about 16% (Panel A) within a desired flow rate range. Maximum PCVs can then be calculated for each feed flow rate, which would represent when the nozzles are fully saturated (i.e., 100% solids in the solids stream). Panel B compares the centrate turbidity profile for a traditional nozzle centrifuge (blue trace) vs. a newer continuous discharge centrifuge (pink trace). A continuous discharge centrifuge would operate at a flow rate slightly below the point of saturation but would maintain a constant baseline due to continuous removal of solids. Thus, improvements in the centrifuge itself can alleviate the throughput constraints a high-density cell culture can have on the harvest.

Fig. 34.8. Impact of nozzle number and discharge interval on centrifugation flow rate and turbidity profile. (A) Flow rates and packed solids volume required to saturate a Westfalia HFC-15 equipped with 4 or 8, 0.2   μm nozzles. (B) Impact of intermittent vs. continuous discharge type centrifuge on centrate turbidity profile.

A more recent development in harvest technology is acoustic wave separation (AWS), developed by FloDesign Sonics and licensed to Pall Corporation, for cell clarification and perfusion in the production of immunoglobulins (glycoproteins). This technology, which employs three-dimensional standing acoustic waves to trap cells and cellular debris, forces particles to cluster and settle out of solution. This technology has the potential to reduce particulates in high density cell culture to levels that are more easily manageable for simpler depth filtration. It also would allow for continuous clarification and single-use formats without the challenge of fouling physical filter membranes. Clarification efficiencies of greater than 95% with yields of greater than 85% have been reported [46,47].

With respect to product quality, a recent example of increased mAb reduction occurring during harvest intermediate hold has occurred due to increased cell culture densities. This has required developers and manufacturers to implement oxidizing strategies [18], re-introduce cold temperature downstream processing and/or implementation of charged depth filters to prevent disulfide reduction during harvest [48,49].

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