ERT 313 : BIOSEPARATION ENGINEERING. Mechanical - Physical Separation Process

Transcription

1 ERT 313 : BIOSEPARATION ENGINEERING Mechanical - Physical Separation Process

2 Students should be able to; APPLY and CALCULATE based on filtration principles; ANALYZE cake filtration, Constant Pressure Filtration, Continuous Filtration and Constant Rate Filtration.

3 Introduction Filtration is a solid-liquid separation where the liquid passes through a porous medium to remove fine suspended solids according to the size by flowing under a pressure differential. The main objective of filtration is to produce highquality drinking water (surface water) or highquality effluent (wastewater)

4 2 categories of filtration, which differ according to the direction of the fluid feed in relation to the filter medium. Results in a cake of solids depositing on the filter medium Minimize buildup of solids on the filter medium

5 Application of Filtration in Bio-industry Recovery of crystalline solids Recovery of cells from fermentation medium Clarification of liquid and gasses Sterilization of liquid for heat sensitive compound

6 Filtration Equipment Filtration for biological materials is generally completed using batch filtration, rotary drum filtration, or ultrafiltration methods. 1. Batch Filtration Usually performed under constant pressure with a pump that moves the broth or liquor through the filter Filter cake will build-up as filtration proceeds and resistance to broth flow will increase The filter press is the typical industrial version of a batch vacuum filter, using a plate and frame arrangement Can be used to remove cells, but does not work particularly well for animal cell debris or plant seed debris

7 Cont.Filtration Equipment 2. Rotary Drum Filtration Rotary vacuum filters can be used to efficiently remove mycelia, cells, proteins, and enzymes, though a filter aid or precoat of the septum may be necessary 3. Ultrafiltration Utilizes a membrane to separate particles that are much larger than the solvent used Successful removal occurs in the partical size range of 10 solvent molecular diameters to 0.5 μ

8 Filter Media To act as an impermeable barrier for particulate matter. Filtration media for cross-flow filtration are generally referred as MEMBRANE First and foremost, it must remove the solids to be filtered from the slurry and give a clear filtrate Also, the pores should not become plugged so that the rate of filtration becomes too slow The filter medium must allow the filter cake to be removed easily and cleanly Some widely used filter media (for conventional filtration) like filter paper, ceramics, synthetic membrane, sinterd & perforated glass, woven materials (woven polymer fiber).

9 Filter Aids Substance (solid powder)that are mixed with the feed for creating very porous cakes ( increase filtration rate very significantly) Can be added to the cake during filtration to increases the porosity of the cake and reduces resistance of the cake during filtration Can also be added directly to the feed to: i) maintain the pores in the filter cake open ii) Make the cake less compressible iii)provide faster filtration Common types of filter aid is diatomite (types of algae) and perlite. The structure of diatomite particles gives them a high intrinsic permeability

10 Filtration Principles When a slurry containing suspended solids flow against a filter medium by the application of a pressure gradient across the medium, solids begin to build up on the filter medium The buildup of solids on the filter medium is called a cake This type of filtration is sometimes referred to as dead-end filtration Darcy s law describes the flow of liquid through a porous bed of solids and can be written as follows: where V is the volume of filtrate, t is time, A is the cross-sectional area of exposed lilter medium, Δp is the pressure drop through the bed of solids (medium plus cake), µ 0 is the viscosity of the filtrate, and R is the resistance of the porous bed. In this case, R is a combination of the resistance R m of the filter medium and the resistance R c of the cake solids: It is convenient to write the cake resistance R c in terms of specific cake resistance α as follows: where ρ c is the mass of dry cake solids per volume of filtrate. Thus, the resistance increases with the volume filtered Combining Eq. (1), (2) and (3), we obtain (1) (2) (3) (4)

11 Incompressible Cake For the case of zero filtrate at time zero (before start an exp), integration of this equation yields (5) At V o K B K V A where 2 P and B R P m Y mx C (can determine specific cake resistance,α and medium resistance, Rm by plotting the graph) In a cake filtration process where a significant amount of cake is allowed to accumulate, the medium resistance, Rm become neglegible compare witn the cake 2 resistance. (Rm=0). So, V t o 2P A

12 Example 1 Batch Filtration A Buchner funnel 8 cm in diameter is available for testing the filtration of a cell culture suspension, which has a viscosity of 3.0 cp. The data in Table E1 were obtained with a vacuum pressure of 600 mm Hg applied to the Buchner funnel. The cell solids on the filter at the end of filtration were dried and found to weigh 14.0 g. Determine the specific cake resistance α and the medium resistance R m. Then estimate how long it would take to obtain 10,000 liters of filtrate from this cell broth on a filter with a surface area of 10 m 2 and vacuum pressure of 500 mm Hg. TABLE E1

13 Solution Example 1 According to Equation (5), we can plot t/(v/a) versus V/A and obtain α from the slope and R m from the intercept. We see that the data are reasonably close to a straight line. At V K V A B (5) Figure E1 Plot of batch filtration data for the determination of α and R m. Y mx C A linear regression of the data in this plot gives the following results (Figure E1):

14 From these values, we can calculate α and R m : Example 1 This is a typical value of R m for a large-pore (micrometer-sized) filter. To determine the time required to obtain 10,000 liters of filtrate using a filter with an area of 10 m 2, we must make the assumption that α does not change at the new pressure drop of 500 mm Hg. We use Equation (5) and solve for time: (5)

15 (5) Example 1 We calculate the two components of this equation as follows: and finally Thus, this filter is probably undersized for the volume to be filtered. In addition, from this calculation we see that at the end of the filtration, Therefore, the filter medium is contributing very little of the resistance to filtration, a typical situation in a lengthy dead-end filtration.

16 Compressible Cake Almost all cakes formed for biological material are compressible. As these cake compressed, filtration rate drop (flow become relatively more difficult as pressure increase) The pressure drop is influence by α, the specific cake resistance α can be increased if the cake is compressed The specific resistance of the cake is directly affected by Δp c, the pressure drops across the cake Studies have shown that the relationship between specific resistance and pressure drop commonly takes the form: (6) where α and s are empirical constants. The power s has been called the cake compressibility factor. (for incompressible cake, s=0 and for compressible cake, s= )

17 Cake Washing After filtration, the cake contains a significant amount of solute-rich liquid broth that usually removed by washing the cake 2 function of washing: A) displaces the solute-rich broth trapped in pores in the cake B) allows diffusion of solute out of the biomass in the cake(enhance recovery if the desired product is in the biomass)

18 It is often necessary to wash the filter cake with water or a salt solution to maximize the removal of dissolved product from the cake. Frequently, the wash must be done with more than the volume of the liquid in the cake because some of the product is in stagnant zones of the cake, and transfer into the wash liquid from these zones occurs by diffusion, which takes place at a slower rate than the convective flow of wash through the cake Data for the washing of the filter cakes has been correlated by Choudhary and Dahlstrom using the following equation: where R is the weight fraction of solute remaining in the cake after washing (on the basis that R = 1.0 prior to washing), E is the percentage wash efficiency, and n is the volume of wash liquid per volume of liquid in the unwashed cake. Assuming that the liquid viscosity and the pressure drop through the bed solids are the same during the filtration of the solids, the washing rate per cross-sectional area can be found from the filtrate flow rate per unit area given in Equation (4) at the end of the filtration Thus, for negligible filter medium resistance for filtrate volume V f at the end of time t f to form the cake, this yields (8) (7)

19 Filtration Principles If V w is the volume of wash liquid applied in time t w, then (9) Using the definition of (dv/dt) V=Vf from Eq. (8), we obtain (10) At the end of filtration, the integrated form of the filtration equation (Eq. 5), with R m neglected, can be written (11) Substituting this expression for V f /A in Eq. (10) and simplifying gives (12)

20 Filtration Principles From Eq. (11) and (12), the ration of t w to t f is It is helpful to write t w /t f in terms of n, the ration of the volume V w of wash liquid to the volume V r of residual liquid in the cake: where f is the ratio of V r to the volume V f of filtrate at the end of filtration. The ratio f can be determined by a material balance Thus, for a given cake formation time t f, a plot of wash time t w versus wash ratio n will be a straight line (13) (14)

21 Example 2 Rotary Vacuum Filtration It is desired to filter a cell broth at a rate of 2000 liters/h on a rotary vacuum filter at a vacuum pressure of 70 kpa. The cycle time for the drum will be 60 s, and the cake formation time (filtering time) will be 15 s. The broth to be filtered has a viscosity of 2.0 cp and a cake solids (dry basis) per volume of filtrate of 10 g/liter. From laboratory tests, the specific cake resistance has been determined to be 9 x 10 cm/g. Determine the area of the filter that is required. The resistance of the filter medium can be neglected. Solution: We can use the integrated form of the filtration equation, Equation (5), with R m = 0: We solve for A 2 to obtain In applying this equation, it is helpful to focus on the area of the drum, which is where the cake is being formed and where filtrate is being obtained.

22 Thus, A is the area of that part of the drum. We can calculate the volume of filtrate that needs to be collected during the cake formation time of 15 s: We use this volume of filtrate with t = 15 s in the equation for A 2 to obtain The area A of the entire rotary vacuum filter can be calculated from the cake formation time (15s) and the total cycle time (60s) as This is a medium-sized rotary vacuum filter, with possible dimensions of 1.0 m diameter by 1.0 m long.

23 Example 3 Washing of a Rotary Vacuum Filter Cake For the filtration in Example 2, it is desired to wash a product antibiotic out of the cake so that only 5% of the antibiotic in the cake is left after washing. We expect the washing efficiency to be 50%. Estimate the washing time per cycle that would be required. Solution; From Equation (7) for the washing efficiency of a filter cake where R is the weight fraction of solute remaining in the cake after washing (on the basis of R = 1.0 before washing), E is the percentage wash efficiency, and n is the volume of wash liquid per volume of liquid in the unwashed cake. Substituting R = 0.05 and E = 50% into this equation gives From Equation (14), the relationship between the washing time t w, and the cake formation time t f is given by

24 where f is the ratio of volume V r of residual liquid in the cake to the volume of filtrate V f after time t f. Thus, we need to estimate the volume of residual liquid in the filter cake to determine t w. At the end of the 15 s cake formation time, Assuming the cake is 70 wt% water, which is typical for filter cakes, we find Thus, Cake solids per volume of filtrate Volume of filtrate need to be collected during the cake formation time of 15s V m V m, so 194g 1000g / l 0.194l

25 Crossflow filtration as for conventional filtration, scale up of crossflow requires data from laboratory or pilot-scale units the determination of the size of a plant unit can be done by a direct scaleup of the filtration area based on the feed or output flow rate

26 continued for this scaleup, however, it is important that the following variables be kept constant: inlet and outlet pressures crossflow (or tangential) velocity flow channel sizes (height and width) Feed stream properties test slurries should be representative of the actual process streams Membrane type and configuration test data from one design cannot directly be used to design another geometry four basic modes of operation of CFF: batch concentration, diafiltration, purification and complete recycle (figure 3.11)

27 Figure 3.11: The four basic modes of operation of C/flow filtration (CF=crossflow filter)

28 continued Mode Batch concentration Diafiltration Purification Complete recycle Description The retained stream containing the product suspended particles or dissolved macromolecules is reduced in vol. The vol. of the retained stream is maintained constant by the continuous addition of water or buffer which results in the removal of low MW solutes into the permeate Commonly used when salt removal or exchange is desired A low MW product passes into the permeate and is thus separated from higher MW impurities or The product can be retained and impurities removed in the filtrate Both the retained stream and the permeate are returned to the feed tank Systems may be operated in complete recycle at start-up to reach steady state, saturate the membranes, test for leaks and blockage and adjust the feed rate

29 continued in designing a diafiltration process, a decision must be made about the concentration of retained product at which to operate as this concentration is increased, the filtration flux will decrease according to eqn. (3.16), and the total vol. of filtrate will decrease for the removal of a given percentage of a low MW solute this leads to an optimum concentration to minimize the time required which can be determined mathematically if the relation between filtrate flux and concentration in the bulk fluid (c b ) is known

30 continued the basic components in the design of a c/flow filtration sys. are shown in figure 3.12 a pump flows the feed through the filtration module to give a permeate and a retentate or retained stream the pump needs to be sized to provide the desired flow velocity and pressure the TMP is controlled by a back-pressure valve on the retentate stream exiting from the filtration module thus the TMP drop is estimated by 1 PTM 0 2 p i p pp (3.28)

31 continued where p i and p o are the retentate pressures in and out of the module respectively and p p = the pressure of the outlet permeate in designing a CFF system important to minimize the occurrence of gas-liquid interfaces, since bioproduct denaturation, especially of proteins, can occur at these interfaces in the presence of mechanical shear and turbulent flow

32 continued Figure 3.13: Comparison of (a) batch and (b) single-stage continuous (feedand-bleed) crossflow filtration systems Figure 3.12:Basic components of a crossflow filtration system.

33 continued CFF systems : operate in batch or the continuous mode (Figure 3.13) batch system : feed is pumped through the filtration module and then back to the feed tank variation mode (semibatch) for diafiltration: fluid is continuously added to the feed tank to keep the feed volume constant (3.11b) continuous mode of operation ( feed-and-bleed or retentate bleed ): feed is added to a recirculation loop by the feed pump, and concentrate exiting in the retained stream is withdrawn from the system so that the concentration factor is at the desired value

34 continued * concentration factor : i.e. conc. in the retentate divided by the conc. in the feed) when steady state achieved, the concentrate will be at its max. conc.- means that the filtration flux will be at a min. throughout the run generally, more economical to use a multistage system in a continuous process (Figure 5.14)

35 continued Figure 5.14: Multistage CFF system using the retentate bleed mode

36 continued as more stages are added, the ave. filtration flux approaches that for a batch sys., thus the total filtration area decreases refer to table 3.2 batch UF operation is compared with continuous operation using one, two, three and five stages continuous operation : economical reduced tankage preferable to batch operation for most large scale UF operations another advantage: it permits the minimization of the residence time of the product in the CFF unit (important for products that are sensitive to heat or shear)

37 Table 3.2: Comparison of Batch and Continuous UF System Batch Continuous One-stage Two-stage System b Flux (litersm -2 h -1 ) Total area (m 2 ) Three-stage Five-stage 33.1 (average) b System design for 10x conc. factor and feed rate of 5000liters/h.Flux from J = 20ln(30/c b )

38 Cell disruption / lysis is a method or process for releasing biological molecules from inside a cell (breaking / lysing cells and tissues) Biotechnological products produced by different types of cells can be intracellular or extracellular. If these are intracellular (inside the cell), the cells have to be disrupted to release these products before further separation can take place.

39 Types of Cell Need to Disruption Ease of cell breaks Bacteria ( gram gram ve) Yeast Culture (plant animal culture) Gram-positive Thick wall Gram-negative No wall (got multilayer enveloped)

40 Some Elements of Cell Structure Prokaryotic Cells Cells that do not contain a membrane-enclosed nucleus. The bacteria cell envelope consists of an inner plasma membrane that separates all contents of the cell from the outside world, a peptidoglycan cell wall, and outer membrane Bacteria cells with a very thick cell wall stain with crystal violet (Gram stain) and are called Gram positive, while those with thin cell wall stain very weakly Gram negative

41 Some Elements of Cell Structure Eukaryotic cells Eukaryotic cells (cells with nuclei and internal organelles) are considerably more complicated than prokaryotic cells, and bioproducts may have to released from intracellular particles that are themselves coated with membranes and/or consist of large macromolecular aggregates The eukraryotes includes fungi, and, of course, the higher plants and animals The cell membrane of animal cells is easily broken, whereas the cell wall of plants is strong and relatively difficult to break

42 Figure : Eukaryotic cells. Simplified diagrammatic representation of an animal cell and a plant cell.

43 Different cell disruption techniques are used. These include: Physical methods Disruption in ball mill or pebble mill Disruption using a colloid mill Disruption using French press Disruption using ultrasonic vibrations Chemical methods Disruption using detergents Disruption using enzymes e.g. lysozyme Combination of detergent and enzyme Disruption using solvents

44 Mechanical Methods for Cell Lysis Sonication Ball milling Pestle homogenization Shearing devices (blender) High pressure homogenizers Bead mills

45 Bead mill Rolling beads Cascading beads Cells being disrupted Disruption takes place due to the grinding action of the rolling beads and the impact resulting from the cascading ones Bead milling can generate substantial heat Application: Yeast, animal and plant tissue Small scale: Few kilograms of yeast cells per hour Large scale: Hundreds of kilograms per hour.

46 Colloid mill Disrupted cells Rotor Cell suspension Stator Typical rotation speeds: 10,000 to 50,000 rpm Mechanism of cell disruption: High shear and turbulence Application: Tissue based material Single or multi-pass operation ERT 313/4 BIOSEPARATION ENGINEERING SEM 2 (2010/2011)

47 Separation of cells and medium Recovery of cells and/or medium (clarification) For intracellular enzyme, the cell fraction is required For extracellular enzymes, the culture medium is required On an industrial scale, cell/medium separation is almost always performed by centrifugation Industrial scale centrifuges may be batch, continuous, or continuous with desludging

48 CENTRIF UGATION

49 A centrifuge is used for separating particles from a solution according to their size, shape, density and viscosity of the medium by the application of an artificially induced gravitational field. In bioprocesses, these particles could be cells, sub cellular components, viruses and precipitated forms of proteins and nucleic acids. Centrifugation can be used to separate cells from a culture liquid, cell debris from a broth, and a group of precipitates. Centrifugation may be classified into two types: Analytical centrifugation Preparative Centrifugation

50 Industrial centrifuges Tubular Bowl Centrifuge Most useful for solid-liquid separation with enzymatic isolation Can achieve excellent separation of microbial cells and animal, plant, and most microbial cell debris in solution Disc Bowl Centrifuge Widely used for removing cells and animal debris Can partially recover microbial cell debris and protein precipitates

51 Perforate Bowl Basket Centrifuge Exception at separation of adsorbents, such as cellulose and agarose Zonal Ultracentrifuge Applied in the vaccine industry because it can easily remove cell debris from viruses Can collect fine protein precipitates Has been used experimentally to purify RNA polymerase and very fine debris in enzymes

52 Properties of industrial centrifuges Tube High centrifugal force Good dewatering Easy to clean Chamber Large solids capacity Good dewatering Bowl cooling possible Disc type Solids discharge No foaming Bowl cooling possible Limited solids capacity Difficult to recover protein No solids discharge Cleaning difficult Solids recovery difficult Poor dewatering Difficult to clean

53 Centrifugation properties of different cell types Bacteria Small cell size Resilient Yeast cells Large cells Resilient Filamentous fungi Mycelial Resilient Cultured animal cells Large cells Very fragile High speed required Low cell damage Lower speed required Low cell damage Lower speed required High water retention in pellet Very susceptible to damage

54 Forced Developed in Centrifugal Separation 1. Introductions Centrifugal separators use the common principal that an object whirled about an axis or center point a constant radial distance from the point is acted on by a force The object is constantly changing direction and is thus accelerating, even though the rotational speed is constant This centripetal force acts in a direction toward the center of rotation

55 3.2. Sedimentation in a centrifugal field Centrifugal settling or sedimentation Use of centrifuges increases the forces on particles manyfold. Hence, particles that will not settle readily or at all in gravity settlers can often be separated from fluids by centrifugal force. These high centrifugal forces do not change the relative settling velocities of small particles, but these forces do overcome the disturbing effects of Brownian motion and free convection currents. Sometimes gravity separation may be too slow because of the closeness of the densities of the particles and the fluid, or because of association forces holding the components together as in emulsions.

56 continued gravity and centrifugal sedimentation of a single particle are illustrated in Figure 3.9 Figure 3.9: Gravity and centrifuge sedimentation of a single particle. Angular speed (ω); distance of particle to axis of rotation (r), m = mass, g = gravity

57 continued An example in the dairy industry is the separation of cream from whole milk, giving skim milk. Gravity separation takes hours, while centrifugal separation is accomplished in minutes in a cream separator. Centrifugal settling or separation is employed in many food industries, such as breweries, vegetable-oil processing, fishprotein-concentrate processing, fruit juice processing to remove cellular materials, and so on. Centrifugal separation is also used in drying crystals and for separating emulsions into their constituent liquids or solid liquid

58 continued Centrifugal filtration Centrifuges are also used in centrifugal filtration, where a centrifugal force is used instead of a pressure difference to cause the flow of slurry in a filter where a cake of solids builds up on a screen. The cake of granular solids from the slurry is deposited on a filter medium held in a rotating basket, washed, and then spun dry. Centrifuges and ordinary filters are competitive in most solid liquid separation problems.

59 3.2.2 Forces developed in centrifugal separation If the object being rotated is a cylindrical container, the contents of fluid and solids exert an equal and opposite force, called centrifugal force, outward to the walls of the container. This is the force that causes settling or sedimentation of particles through a layer of liquid or filtration of a liquid through a bed of filter cake held inside a perforated rotating chamber In Fig. 3.10a cylindrical bowl is shown rotating, with a slurry feed of solid particles and liquid being admitted at the center

60 Figure 3.10: Sketch of centrifugal separation: (a) initial slurry feed entering. (b) settling of solids from a liquid, (c) separation of two liquid fraction.

61 continued The feed enters and is immediately thrown outward to the walls of the container, as in Fig. 3.10b. The liquid and solids are now acted upon by the vertical gravitational force and the horizontal centrifugal force. The centrifugal force is usually so large that the force of gravity may he neglected. The liquid layer then assumes the equilibrium position, with the surface almost vertical. The particles settle horizontally out ward and press against the vertical bowl wall. in Fig. 3.10c two liquids having different densities are being separated by the centrifuge. The denser fluid will occupy the outer periphery since the centrifugal force on it is greater.

62 Equations for centrifugal force In Fig 3.11 a schematic of a tubular-bowl centrifuge is shown. The feed enters at the bottom, and it is assumed that all the liquid moves upward at a uniform velocity carrying solid particles with it. The particle is assumed to be moving radially at its terminal settling velocity υ t. The trajectory or path of the particle is shown in Fig A particle of a given size is removed from the liquid if sufficient residence time is available for the particle to reach the wall of the bowl where it is held. The length of the bowl is b m. At the end of the residence time of the particle in the fluid, the particle is at a distance r B m from the axis of rotation. If r B < r 2, then the particle leaves the bowl with the fluid.

63 Figure 3.11: Particle settling in sedimenting tubularbowl centrifuge.

64 continued If r B = r 2, it is deposited on the wall of the bowl and effectively removed from the liquid. In circular motion the acceleration due to the centrifugal force is where a e is the acceleration from a centrifugal force in m/s 2 (ft/s 2 ), r is radial distance from the center of rotation in m (ft), and ω is angular velocity in rad/s The centrifugal force F c in N (lb f ) acting on the particle is given by 2 F c ma e 2 mr Where g c = lb m.ft/lb f.s 2 2 a e r E3.19 (SI) F c mr g (English) E3.20 c

65 continued Since ω = υ/r, where υ = the tangential velocity of the particle in m/s 2 2 F c mr r m r Often rotational speeds are given as N rev/min and 2N 60 N 60 2r E3.21 E3.22

66 continued For settling in the Stokes law range, the terminal settling velocity at a radius r is obtained by substituting eqn. (3.19) for the acceleration g into eqn. (3.8): where υ t = settling velocity in the radial direction (m/s) D p = particle diameter (m) ρ p = particle density kg/m 3 ρ = liquid density in kg/m 3 and μ = liquid viscosity in Pa s, 2 2 rd E3.23 p p t 18

67 continued If hindered settling occurs, t gd p 2 Since υ t = dr/dt, Eqn. (3.23) becomes 2 p E p dt 2 18 dr 2 D r p p E3.25 Integrating between the limits r = r 1 at t = 0 and r = r 2 at t = t T t T 18 ln 2 r 2 2 p D p r1 E3.26

68 continued V = πb (r 2 2 r 12 ), thus, the feed volumetric flow rate Q in m 3 /s is ln 18 ln 18 r r b r r D V r r D Q p p p p E3.27 Particles having diameters smaller than that calculated from eqn. ( 3.27) will not reach the wall of the bowl and will go out with the exit liquid. Larger particles will reach the wall and be removed from the liquid.

69 Notes: centrifugation is most effective when the particles to be separated are large, the liquid viscosity is low and the density difference between particles and fluid is great it is also assisted by large centrifuge radius and high rotational speed 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 the suspending fluid. These disadvantages easily overcome in the lab with small centrifuges operated at high speed

70 continued 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 equipment without limit; mechanical stress in centrifuges increases 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 industry

71 Sigma Analysis & Scale up commonly used analysis in industry is sigma analysis which uses the operation constant Σ to characterize a centrifuge into which feed flows at volumetric flow rate Q Estimation of Q the followed equation can be used: Q = {υ g } [Σ] where υ t = the sedimentation velocity at 1 x g, namely t gd 2 p 18 E3.28 E3.29 and Σ represents the geometry and speed of centrifuge and as the cross-sectional area equivalent of the centrifuge with units of length squared p

72 continued therefore, in eqn. (3.28) the accolades { } indicate properties of the particle to be separated and of the fluid in which separation is occurring and the brackets [ ] indicate properties of the centrifuge if two centrifuges perform with equal effectiveness: Q 1 1 Q 2 2 where subscripts 1 and 2 denote the two centrifuges the above equation can be used to scale-up centrifuge equipment equations for evaluating Σ depend on the centrifuge design

73 continued the above equations for Σ are based on ideal operating conditions Because different types of centrifuge deviate to varying degrees from ideal operation that equation cannot generally be used to compare different centrifuge configurations performance of any centrifuge can deviate from theoretical production due to factors such as Particle shape and size distribution Aggregation of particles Non-uniform flow distribution in the centrifuge and Interaction between particles during sedimentation Experimental tests must be performed to account for these factors

74 3.2.3 Centrifuge Equipment Figure 3.12: Common types of production centrifuge: (a) tubular bowl (b) Multichamber, (c) disk, nozzle (d) disk, intermittent discharge, (e) scroll and (f) basket. Arrows indicate the path of the liquid phase; dashed lines show where the solids accumulate

75 The Operation Steps of Centrifuge Equipment Centrifuge equipment is classified according to internal structure Tubularbowl centrifuge the simplest configuration widely employed in the food and pharmaceutical industries Feed enters under pressure through a nozzle at the bottom, is accelerated to rotor speed and moves upwards through the cylindrical bowl As the bowl rotates, particles are traveling upward are spun out and collide with the walls of the bowl as illustrated schematically in Figure 3.13 Solids are removed from the liquid if they move with sufficient velocity to reach the wall of the bowl within the residence time of liquid in the machine As the feed rate is increased the liquid layer moving up the wall of the centrifuge becomes thicker; this reduces performance of the centrifuge by increasing the distance a particle must travel to reach the wall

76 continued = R 1 = R 0 Figure 3.13: Separation of solids in a tubular-bowl centrifuge

77 continued liquid from the feed spills over a weir at the top of the bowl; solids which 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 applied mainly for difficult separations requiring high centrifugal forces solids in tubular centrifuges are accelerated by forces between and times the force of gravity

78 continued The equations of motion that give the trajectory of sedimented particles - in the radial direction from equation (E3.23) t dr dt then in the axial direction, due only to pumped flow, Q dz Q Q 2 2 dt A R 0 R where A = the cross-sectional area for liquid flow in the centrifuge. These equations of motion are combined to give the trajectory equation dr dt dz dt 2 2 rdp p 18 1 dr dz (E3.30) (E3.31) (E3.32)

79 continued substituting equation (E3.30) and (E3.31) into this ratio (E3.32), integrating dr between r 1 and r 2 and integrating dz between 0 and b and solving for Q gives Q the first factor in equation (3.33) can be multiplied by g while the second is divided by g to give, again, equation (E3.28) for Σ analysis: Q ={υ g }[Σ] D b r r p p 18 2 ln r 2 1 r 1 2 (E3.33) (E3.28)

80 continued where for a tubular bowl centrifuge, b r 2 2 r 2 1 r g ln r (E3.34)

81 Example 3.5: Complete recovery of bacterial cells in a tubular bowl centrifuge It is desired to achieve complete recovery of bacterial cells from a fermentation broth with a pilot plant scale tubular centrifuge. It has been already determined that the cells are approximately spherical with a radius of 0.5μm and have a density of 1.10g/cm 3. The speed of the centrifuge is 5000rpm, the bowl diameter is 10cm, the bowl length is 100cm and the outlet opening of the bowl has a diameter of 4cm. Estimate the maximum flow rate of the fermentation broth that can be attained.

82 Solution

83 continued Ultracentrifuge (A type of narrow tubularbowl centrifuge) Used for recovery of fine precipitates from high-density solutions, breaking down emulsions separation of colloidal particles such as ribosomes and mitochondria produces centrifugal forces times the force of gravity the bowl is usually air-driven and operated at low pressure or in an atmosphere of nitrogen to reduce generation of frictional heat a typical ultracentrifuge operates discontinuously so its processing capacity is restricted by the need to empty the bowl manually continuous ultracentrifuge are available commercially

84 continued Disc-stack bowl centrifuge many types of disc centrifuge; the principal difference between them is the method used to discharge the accumulated solid 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; figure (3.14) A disadvantage of centrifuge 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 blockages of the system occur

85 continued Figure 3.14: Disc-stack bowl centrifuge with continuous discharge of solids

86 continued Contain conical sheets of metal called discs - 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 (3.15), the feed is released near the bottom of the centrifuge and travels upward through matching holes in the discs Between the disc, heavy components of the feed are thrown outward under the influence of centrifugal forces as lighter liquid is displaced towards the center of the bowl As they are flung out, the solids strike the undersides of the discs and slide down to the bottom edge of the bowl At the same time, the lighter liquid flows in and 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

87 continued Figure 3.15: Mechanism of solids separations in a discstack bowl centrifuge

88 continued Q 2a g 2n ( R0 3g R 3 1 )cot (E3.35) Q ={υg}[σ] therefore, in a sensitivity analysis, factor depends on the cube of the bowl radius, the cotangent of the disk acute angle, the number of disks in the stack and as in the tubular centrifuge, the square of the rotor speed the disk acute angle θ made by the conical disks is typically between 35 and 45 degrees

89 A comparison of the advantages and disadvantages of the different centrifuge designs is given in table (3.2.1) System Advantages Disadvantages Tubular bowl a)high centrifugal force b)good dewatering c)easy to clean d)simple dismantling of bowl Chamber bowl Disk centrifuge a)clarification efficiency remains constant until sludge space full b)large solids holding capacity c)good dewatering d)bowl cooling possible a)solids discharge possible b)liquid discharge under pressure eliminates foaming c)bowl cooling possible a)limited solids capacity b)foaming unless special skimming or centrifugal pump used c)recovery of solids difficult a)no solids discharge b)cleaning more difficult than tubular bowl c)solids recovery difficult a)poor dewatering b)difficult to clean

90 continued System Advantages Disadvantages Scroll or decanter centrifuge Basket centrifuge a)continuous solids discharge b)high feed solids concentration a)solids can be washed well b)good dewatering c)large solids holding capacity a)low centrifugal force b)turbulence created by scroll a)not suitable for soft biological solids b)no solids discharge c)recovery of solids difficult

91 Table 3.2.2: Capabilities of tubular and disk centrifuges Type Bowl dia.(mm) Speed (rpm) Max. dimensionless acceleration G, ω 2 R/g Throughput (liters/min) Tubular bowl ,000 15,000 15,000 61,400 13,200 16, Disk with nozzle discharge ,000 6,250 4,200 3,300 14,200 8,850 6,760 4,