Question 1

During gas transfer from the bulk liquid to an air bubble in an aerated bioreactor, which step generally offers the greatest resistance for carbon dioxide (CO₂) leaving the liquid phase?

Accepted Answer

Introduction: Understanding where mass transfer resistances arise is crucial for designing aeration systems and predicting removal or absorption of gases. For CO₂ desorption from liquid to bubbles, the controlling resistance is typically on the liquid side near the bubble surface. Given Data / Assumptions: Aerated, well-mixed bulk liquid. Small gas bubbles rising through liquid. CO₂ transferring from liquid to gas. Concept / Approach: In two-film theory, mass transfer is controlled by thin stagnant films at phase boundaries. For sparingly soluble gases and typical bioreactor conditions, the gas-side resistance is small; the main resistance is the liquid-side film (the bubble boundary layer). Turbulence reduces, but does not eliminate, this resistance. The interface itself does not impose a separate large resistance under usual conditions; bulk liquid is well mixed and contributes little resistance compared with the liquid film. Step-by-Step Solution: 1) Decompose overall resistance into bulk, liquid film, interface, and gas film.2) In a well-mixed reactor, bulk gradients are minimal compared to films.3) The liquid film around bubbles presents the largest gradient for CO₂ escaping.4) Gas-side resistance is small due to rapid diffusion in gas and bubble renewal.5) Therefore, the bubble boundary layer is rate-limiting. Verification / Alternative check: Increased agitation (higher k_La) primarily improves liquid-side coefficients, confirming the dominance of liquid film control for CO₂ transfer in typical systems. Why Other Options Are Wrong: Interface: Usually not a significant independent barrier for CO₂ under normal conditions. Cell membrane: Relevant for intracellular transfer, not for bulk-to-bubble desorption. Bulk liquid: Well-mixed bulk presents relatively small resistance versus films. Common Pitfalls: Assuming gas-side control for CO₂ in water-like systems; gas-side control is more common for highly soluble gases absorbed into liquids under specific conditions. Final Answer: Moving through the bubble boundary layer

Question 2

According to penetration theory (Higbie model), turbulent eddies intermittently contact the interface for an exposure time t_e. Which correlation relates the liquid-side mass transfer coefficient K_L to diffusivity D_AB and exposure time?

Accepted Answer

Introduction: Penetration theory explains unsteady diffusion into fluid elements that periodically renew the interface. It is a foundational concept for estimating K_L when interfacial renewal controls mass transfer in agitated or turbulent systems. Given Data / Assumptions: Fluid elements contact the interface for time t_e before being replaced. Diffusion is unsteady during each contact. Transport is characterized by molecular diffusivity D_AB. Concept / Approach: Solving the transient diffusion equation for exposure time t_e yields K_L proportional to the square root of D_AB / t_e. The constant 2 / √π arises from the analytical solution for a semi-infinite medium initially free of solute and suddenly exposed at the interface. Step-by-Step Solution: 1) Assume elements renew at the interface every t_e seconds.2) Transient diffusion thickness grows as (D_AB * t)^0.5.3) Average flux over t_e leads to K_L ∝ (D_AB / t_e)^0.5.4) The Higbie solution gives K_L = 2 * (D_AB / (π * t_e))^0.5.5) Select the option with exponent 0.5 and coefficient form 2 / √π. Verification / Alternative check: Dimensional analysis confirms K_L units of length/time when D_AB has length^2/time and t_e is time, consistent with the square-root dependence. Why Other Options Are Wrong: Exponents 0.25 or 0.75 do not follow from the transient solution. Linear dependence on D_AB / t_e is inconsistent with diffusion scaling. Common Pitfalls: Confusing penetration theory (unsteady) with film theory (steady) or surface renewal theory; while related, they use different characteristic parameters. Final Answer: K_L = 2 * (D_AB / (π * t_e))^0.5

Question 3

Surface renewal (Danckwerts) theory predicts how the liquid-side mass transfer coefficient K_L depends on molecular diffusivity. Which statement best describes this dependence?

Accepted Answer

Introduction: Surface renewal theory extends penetration ideas by introducing a renewal rate, describing the stochastic replacement of surface elements. It is widely used to interpret k_La trends in agitated tanks and gas–liquid contactors. Given Data / Assumptions: Random renewal of surface elements with frequency s (s^-1). Unsteady diffusion dominates during each exposure period. Diffusivity D determines how quickly concentration boundary layers develop. Concept / Approach: Danckwerts derived K_L proportional to (D * s)^0.5, so holding renewal rate s constant, K_L ∝ D^0.5. This square-root dependence reflects diffusion length scaling with time to the power 0.5 in unsteady processes, linking molecular properties with hydrodynamic renewal dynamics. Step-by-Step Solution: 1) Adopt surface renewal framework with renewal rate s.2) Solve transient diffusion for elements exposed for random times.3) Obtain K_L = (D * s)^0.5 * constant.4) Therefore, at fixed s, K_L ∝ D^0.5.5) Select the option stating direct proportionality to sqrt(D). Verification / Alternative check: Experimental correlations of K_L at different solutes (varying D) often follow a half-power law, consistent with surface renewal predictions. Why Other Options Are Wrong: Inverse or cube-root dependences are not supported by the Danckwerts derivation. Common Pitfalls: Interpreting square-root dependence as universal; hydrodynamics (s) also changes with agitation and aeration, affecting K_L beyond molecular properties alone. Final Answer: directly proportional to the square root of the molecular diffusivity

Question 4

In mixing and bioreactor design, which of the following is best described as an axial-flow impeller (i.e., one that drives liquid primarily parallel to the shaft, often for top-to-bottom circulation)?

Accepted Answer

Introduction: Axial-flow impellers are widely used in bioprocessing because they move fluid primarily along the impeller axis (parallel to the shaft), giving strong top-to-bottom circulation with relatively lower shear compared to many radial designs. This question checks recognition of design features that create axial flow patterns. Given Data / Assumptions: Axial flow means bulk liquid motion parallel to the shaft. Pitched-blade impellers at typical angles (around 30°–60°) are classic axial mixers. Down-pumping or up-pumping indicates axial directionality of the flow. Concept / Approach: Blade pitch creates an axial component of velocity. A pitched-blade turbine around 45° is a standard axial-flow choice. Descriptions that explicitly state the liquid is directed toward the base (down-pumping) also define axial flow. Large blade pitch angles approaching vertical (e.g., 75° to the vertical) are atypical for efficient axial pumping. Radial turbines (e.g., Rushton) push liquid outward from the blade tips, not along the shaft. Step-by-Step Solution: Identify axial indicators: blade pitch around 45° and explicit down-/up-pumping descriptions → axial.Exclude non-axial: Rushton-type radial turbines and extreme blade orientations that reduce axial pumping efficiency.Select the option that includes both correct axial descriptors. Verification / Alternative check: Mixing handbooks categorize pitched-blade turbines (~45°) as axial, commonly used for blending, heat transfer, and solid suspension at moderate to low shear. Why Other Options Are Wrong: An impeller with blades pitched at 75° to the vertical: Not a typical axial-flow configuration. A Rushton-type flat blade radial turbine: Produces radial flow, not axial. Common Pitfalls: Confusing radial discharge (perpendicular to shaft) with axial flow; assuming any pitched blade is axial regardless of pitch angle or installation. Final Answer: Both (a) and (b)

Question 5

Bioreactor agitation hardware: which of the following stirrer types can be used in microbiological (fermentation) processes for effective mixing and mass transfer?

Accepted Answer

Introduction: Microbiological processes rely on reliable agitation to ensure uniform mixing, gas–liquid mass transfer, and adequate heat removal. Multiple impeller families are used in practice depending on broth rheology, gas loading, and shear-sensitivity. This item tests familiarity with common stirrer types encountered in fermentation equipment. Given Data / Assumptions: MIG and InterMIG are commercial axial-flow impellers designed for gas-handling and suspension. Turbines (e.g., flat blade, pitched blade) are classical impellers in stirred tanks. Applications include aerobic fermentations with sparging. Concept / Approach: A variety of impellers are used in microbial fermenters. MIG/InterMIG promote axial circulation with good gas dispersion at lower shear than purely radial turbines. Turbine-style impellers (including pitched-blade turbines) are also standard for scale-up, offering predictable power and mass-transfer characteristics. Hence, all listed types are used in industry. Step-by-Step Solution: Recognize MIG/InterMIG as axial-flow, gas-handling impellers.Recall turbines as a broad class used in many fermenters.Select the inclusive option capturing all of these as suitable choices. Verification / Alternative check: Vendor literature and mixing texts document these impellers in microbial fermenters, often in multi-impeller stacks for tall vessels. Why Other Options Are Wrong: Each single impeller option is true but incomplete. Marine propeller only: Too restrictive; many processes use non-propeller designs. Common Pitfalls: Assuming only one impeller design suits all broths; rheology and oxygen demand drive selection. Final Answer: All of these

Question 6

Oxygen transfer in aerated fermentation: which change is most likely to increase the rate of oxygen transfer (typically expressed via k_La * (C* − C)) in a given system?

Accepted Answer

Introduction: Oxygen transfer rate (OTR) determines aerobic fermentation performance. OTR is often modeled as OTR = k_La * (C* − C), where k_La is the volumetric mass-transfer coefficient, C* is the saturation concentration of oxygen, and C is the bulk dissolved oxygen concentration. Understanding which operating changes raise OTR is central to scale-up and control. Given Data / Assumptions: Agitation influences interfacial area and turbulence, raising k_La. Antifoams generally suppress bubble dispersion and coalescence control, often decreasing k_La. Temperature increases reduce oxygen solubility (C*) even if k_La may rise modestly. Concept / Approach: Among standard levers, increasing stirrer speed reliably increases k_La by creating smaller bubbles, higher turbulence, and improved dispersion. Antifoams typically reduce interfacial area and can lower k_La. Temperature changes have competing effects (lower C* vs slightly higher k_La), and the net impact commonly reduces OTR because the solubility drop dominates in many aqueous systems. Step-by-Step Solution: Relate OTR to k_La and (C* − C).Note that agitation strongly increases k_La; select the factor with the clearest positive effect.Exclude antifoam addition due to typical k_La decrease.Exclude temperature rise because C* decreases and can offset any k_La gain. Verification / Alternative check: k_La correlations show a positive dependence on power input per volume (P/V), which rises with stirrer speed, supporting the choice. Why Other Options Are Wrong: Addition of antifoams: Often lowers k_La by reducing bubble dispersion. Increase in temperature: Decreases C*; net OTR often declines in water-like media. Both (a) and (b): Combines two changes that are not reliably beneficial. Decrease gas flow: Reduces interfacial area and typically lowers OTR. Common Pitfalls: Assuming any variable that increases turbulence helps OTR without considering oxygen solubility; ignoring antifoam effects on bubble behavior. Final Answer: Increase in stirrer speed

Question 7

Gas–liquid interfacial area in different setups: which vessel/operation has the highest surface area available for oxygen transfer under otherwise comparable conditions?

Accepted Answer

Introduction: Oxygen transfer depends strongly on gas–liquid interfacial area. Sparging directly injects gas, creating many bubbles and vastly increasing interfacial area compared with relying only on the headspace or surface agitation. This question contrasts typical setups used in labs and industry. Given Data / Assumptions: Sparging introduces bubbles throughout the liquid, maximizing area. Agitation improves dispersion and renews interfaces. Non-sparged systems rely on the top surface only, giving far less interfacial area. Concept / Approach: A sparged, agitated tank provides both bubble surface area and rapid surface renewal, resulting in the largest effective a in k_La. Shake flasks create waves and surface renewal but lack dispersed gas bubbles. Standing test tubes have minimal surface area relative to volume, and sealed bottles limit gas exchange to headspace diffusion. Step-by-Step Solution: Identify configurations with dispersed bubbles: sparged stirred tank.Recognize agitation at 400 rpm helps bubble breakup and dispersion.Select the sparged, agitated reactor as having the greatest interfacial area. Verification / Alternative check: Empirical k_La measurements are highest in sparged, well-agitated reactors versus surface-aerated vessels of similar scale. Why Other Options Are Wrong: Shake flask: Greater area than static tubes but less than sparged tanks. Standing test tube: Very low interfacial area. Non-sparged stirred tank: No bubble area, only surface renewal. Sealed bottle: Essentially no oxygen transfer by sparging. Common Pitfalls: Overestimating the effect of headspace aeration compared with direct sparging and dispersion. Final Answer: A sparged stirred tank reactor agitated at 400 rpm

Question 8

Henry’s law calculation at 25°C: if the saturation concentration of oxygen in water is 1.26 mmol/L at a partial pressure of 1 atm, what is the Henry’s law constant H using p = H * C* (report units as atm·L/mmol)?

Accepted Answer

Introduction: Henry’s law relates the equilibrium dissolved concentration of a sparingly soluble gas to its partial pressure in the gas phase. Correct unit handling is essential because multiple H conventions exist in the literature. Here, we adopt p = H * C* (pressure proportional to concentration). Given Data / Assumptions: C* (saturation) = 1.26 mmol/L. p (oxygen partial pressure) = 1 atm. Use p = H * C* and report H in atm·L/mmol. Concept / Approach: Rearrange Henry’s law to H = p / C*. Substitute the given values using consistent units. No temperature correction is needed since all data are at 25°C and the law is applied directly. Step-by-Step Solution: Write H = p / C*.Insert numbers: H = 1 atm / 1.26 mmol·L^-1.Compute: H ≈ 0.79365 atm·L/mmol.Round sensibly to three significant figures: 0.793 atm·L/mmol. Verification / Alternative check: Dimensional check: atm divided by mmol·L^-1 yields atm·L/mmol. The magnitude is consistent with typical oxygen Henry constants at room temperature under this convention. Why Other Options Are Wrong: 0.207 or 0.126 atm·L/mmol: Arise from arithmetic or unit inversion errors. 1.26 atm·L/mmol: Numerically equal to C*, not H under p = H * C*. 8.74 atm·L/mmol: Off by an order of magnitude; likely unit mix-up. Common Pitfalls: Confusing the two common conventions (C* = k_H * p vs p = H * C*); always confirm units before computing. Final Answer: 0.793 atm·L/mmol

Question 9

Oxygen transfer rate (OTR) in bioreactors: under otherwise similar conditions, which change is most likely to increase OTR?

Accepted Answer

Introduction: OTR depends on the product k_La * (C* − C). The interfacial area a is heavily influenced by bubble size and coalescence behavior. Additives that prevent bubble coalescence generally increase a, thereby improving OTR for a given gas flow and agitation level. Given Data / Assumptions: Surfactant-like (detergent) molecules reduce surface tension and suppress bubble coalescence. Antifoams and oils often increase bubble size and reduce a. Temperature increases lower oxygen solubility C* in water-like media. Concept / Approach: Smaller, more numerous bubbles increase interfacial area and k_La. Detergent-like molecules can stabilize small bubbles, limiting coalescence and raising OTR. In contrast, many antifoams and oils promote coalescence, decreasing the interfacial area and reducing k_La. Temperature effects are mixed but typically reduce OTR due to falling C*. Step-by-Step Solution: Identify which additives decrease bubble size/coalescence → detergents.Recognize antifoams/oils commonly enlarge bubbles → reduced a, lower k_La.Consider temperature: C* decreases with T; net OTR may fall.Choose the option that most consistently raises OTR: detergent-like molecules. Verification / Alternative check: k_La measurements often rise in the presence of mild surfactants that inhibit coalescence, holding gas flow and power constant. Why Other Options Are Wrong: oil is added / antifoam is added: Typically reduce k_La by promoting coalescence. increase in the reactor temperature: Lowers C*, often counteracting any modest k_L increase. reduce gas flow rate: Decreases interfacial area and OTR. Common Pitfalls: Assuming any additive that changes surface tension improves OTR; many antifoams do the opposite of detergents. Always confirm with k_La testing. Final Answer: detergent like molecules are added

Question 10

Mixing dimensionless groups: the impeller Power number (Po) is defined as which of the following nondimensional ratios (using power P, density ρ, rotational speed N, impeller diameter D)?

Accepted Answer

Introduction: The Power number Po characterizes the relationship between power draw and flow conditions for a given impeller geometry. It is central to scale-up because it connects dimensional power to speed, fluid density, and size in a dimensionless manner. Given Data / Assumptions: P0 denotes shaft power (P). ρ is fluid density, N is rotational speed (s^-1), D is impeller diameter (m). Po is dimensionless and typically constant in fully turbulent regimes for a given impeller type. Concept / Approach: By definition, Po = P / (ρ * N^3 * D^5). This arises from dimensional analysis balancing power with inertial scaling. It enables comparison across scales and fluids using Reynolds number simultaneously. Step-by-Step Solution: Write Po = P / (ρ N^3 D^5).Match this with the listed forms.Identify the exact expression in option (b). Verification / Alternative check: Standard mixing references present Po along with Re = ρ N D^2 / μ, both widely used in agitation correlations. Why Other Options Are Wrong: Other exponents on N and D do not reduce to a dimensionless group equal to Po. ρ N D^3 / P0 is the reciprocal of part of the intended scaling and is not Po. Common Pitfalls: Confusing Power number with pumping number; ensure correct exponents for N and D. Final Answer: P0 / N3Di5ρ

Question 11

Flow pattern classification: which of the following impeller designs primarily provide radial flow (i.e., discharge perpendicular to the shaft)?

Accepted Answer

Introduction: Impellers are often categorized by the predominant discharge direction they create in a tank. Radial-flow impellers discharge fluid outward from the blade tips perpendicular to the shaft, creating strong shear and good gas dispersion but less axial circulation than axial-flow designs. Given Data / Assumptions: Paddle impellers are simple flat elements, primarily radial at moderate to high Re. Flat blade turbines and Rushton-type disk turbines are canonical radial-flow devices. Marine propellers are axial-flow and serve as a distractor. Concept / Approach: Radial-flow devices maximize shear near the impeller, enhancing dispersion of gas and breaking up aggregates; axial devices excel at bulk circulation and solids suspension. Among the options, paddles, flat blade turbines, and disk flat blade turbines all give radial discharge patterns. Step-by-Step Solution: Identify radial-flow exemplars: paddles, flat blade turbines, Rushton (disk flat blade) turbines.Exclude axial propellers from the set.Choose the inclusive option covering all listed radial devices. Verification / Alternative check: Mixing texts and vendor diagrams classify these impellers as radial-flow; propellers are axial-flow for comparison. Why Other Options Are Wrong: Marine propellers: Predominantly axial flow, not radial. Common Pitfalls: Assuming blade pitch alone defines flow; geometry and shrouding (disk) also govern discharge direction. Final Answer: All of the above

Question 12

Estimating interfacial boundary-layer concentration (C_o*): in an aerated bioreactor, how is the average C_o* around bubbles typically determined for k_La measurements?

Accepted Answer

Introduction: Determining k_La often uses clean (microbe-free) water or medium to obtain the saturation concentration C* under given aeration and agitation. This practical approach avoids biological oxygen uptake and provides a reproducible reference for mass-transfer studies. Given Data / Assumptions: Before inoculation, biological consumption is negligible. Under steady aeration/mixing, the bulk DO equals the saturation concentration C*. C_o* used in OTR expressions is taken from these baseline measurements. Concept / Approach: In practice, one measures the bulk dissolved oxygen at steady state in clean medium under the same gas and agitation conditions. This value approximates C* (or C_o* in certain notations), the driving concentration at the interface for mass transfer calculations. Direct micro-scale measurement at bubble interfaces is not common due to instrumentation limits and disturbance of the boundary layer. Step-by-Step Solution: Aerate and agitate clean medium until DO stabilizes at the saturation value.Record this steady-state bulk DO as C* for subsequent k_La testing.Use dynamic or steady-state methods post-inoculation, referencing this baseline for driving-force estimates. Verification / Alternative check: Kla determinations frequently start with water or buffer to establish C*; dynamic gassing-out methods further validate k_La using this reference. Why Other Options Are Wrong: Ultra-small probes or laser imaging: Not standard or practical at the bubble boundary layer for routine k_La work. After inoculation: DO is depressed by uptake, not equal to C*. Assuming C_o* = 0: Physically incorrect; saturation concentration is nonzero. Common Pitfalls: Confusing bulk DO under biological consumption with saturation values; always measure C* before inoculation or by re-saturating depleted broth. Final Answer: measuring the steady state concentration of oxygen in the bulk liquid prior to inoculation of the reactor

Question 13

Why are axial-flow impellers favored in crystallization and precipitation operations within chemical and bioprocess industries?

Accepted Answer

Introduction: Crystallization and precipitation processes aim to control particle size distribution for downstream separation and product quality. Agitation must suspend solids and maintain uniform supersaturation without excessive crystal breakage. Axial-flow impellers are often preferred for this balance. Given Data / Assumptions: Goal: maintain larger, more filterable particles by minimizing attrition. Need: effective bulk circulation and solids suspension at moderate power input. Constraint: avoid high shear fields that cause crystal fracture. Concept / Approach: Axial-flow impellers (e.g., pitched-blade or hydrofoil types) drive strong top-to-bottom circulation with relatively low localized shear compared to radial turbines. This promotes homogeneous supersaturation and gentle suspension, helping particles grow rather than fragment. Energy efficiency further supports scale-up in large crystallizers. Step-by-Step Solution: Identify the desired hydrodynamics: circulation with minimal shear peaks.Map axial-flow designs to these requirements: good pumping, low shear.Select the statement explicitly citing low-shear, energy-efficient mixing as the rationale. Verification / Alternative check: Crystallization literature recommends axial or mixed-flow impellers to limit attrition and secondary nucleation while ensuring suspension and heat removal. Why Other Options Are Wrong: (a) and (b) together: Contradictory; aggressive breakdown (b) is undesirable. Maximizing attrition: Opposite of typical crystallizer goals where filterability is key. Common Pitfalls: Assuming high-shear radial turbines are better for all operations; in crystallization, controlled hydrodynamics is more important than maximum shear. Final Answer: Axial flow impellers provide low shear, energy efficient mixing conditions

Question 14

Agitated tank design guideline: What is the typical design ratio of liquid depth (H) in the vessel to the tank internal diameter (Dt) recommended for bioprocess stirred tanks?

Accepted Answer

Introduction / Context: Selecting an appropriate aspect ratio for a stirred bioreactor strongly influences mixing time, gas–liquid mass transfer, power draw, and scale-up strategy. The ratio H:Dt (liquid height to tank diameter) is a foundational geometric choice in process vessels used for fermentation and cell culture. Given Data / Assumptions: Standard, vertically mounted cylindrical tank with flat or dished ends. Mechanical agitation with baffling for vortex suppression. We seek a widely adopted “typical” design guideline rather than extreme or special-purpose values. Concept / Approach: Bioprocess vessels often favor a moderate-to-tall aspect ratio to improve gas residence time and promote oxygen transfer while keeping impeller power within practical limits. An H:Dt near 2:1 balances circulation loops, allows multiple impellers on a common shaft for tall liquids, and provides reasonable hydrostatic head without excessive shaft length or bearing loads. Step-by-Step Solution: Identify the aim: ensure good mixing and oxygen transfer for microbial or mammalian cultures.Use standard practice: choose H ≈ 2 * Dt to enable staged impellers and adequate axial circulation.Confirm practicality: 1:1 is common in chemical mixing, but 2:1 is more typical in aerated bioreactors.Select 2 : 1 as the representative guideline for stirred bioprocess tanks. Verification / Alternative check: Design texts and vendor datasheets commonly show laboratory to production fermenters with H:Dt ≈ 2:1 (sometimes 2–3:1), reflecting the need for vertical circulation and multi-impeller configurations that enhance kLa and reduce gradients. Why Other Options Are Wrong: 1 : 1: used in many mixing tanks, but aerated bioreactors more often adopt taller geometries for gas–liquid contact. 4 : 1: excessively tall for many installations; creates mechanical and headroom challenges. None of these: unnecessary because 2 : 1 is a well-supported standard guideline. Common Pitfalls: Assuming one ratio fits all processes; highly viscous or non-gassed systems may use 1 : 1. Ignoring headspace needs above H for foaming and gas disengagement. Final Answer: 2 : 1

Question 15

Dimensionless groups in mixing: The power number (Po) is best interpreted as the ratio of which effects in an agitated vessel?

Accepted Answer

Introduction / Context: The power number Po is a key dimensionless parameter for correlating agitator power in geometrically similar vessels across different scales and operating conditions. It connects measured power to fluid properties and impeller dimensions. Given Data / Assumptions: Standard definition: Po = P / (ρ * N^3 * D^5). P is shaft power, ρ is density, N is rotational speed, D is impeller diameter. Fully baffled tank where Po is relatively independent of Reynolds number at turbulent conditions for a given impeller type. Concept / Approach: Po is analogous to a drag coefficient for mixing: it scales the imposed hydrodynamic resistance (torque/power) against characteristic inertial scaling (ρ * N^3 * D^5). Thus, it reflects the relative magnitude of impeller-imposed forces compared with inertial forces in the flow. Step-by-Step Solution: Start from Po = P / (ρ * N^3 * D^5).Recognize ρ * N^3 * D^5 as inertial scaling for power in rotating flows.Interpret Po as a dimensionless power coefficient (imposed resistance vs. inertia).Conclude the best phrasing: imposed (drag/torque) forces to inertial forces. Verification / Alternative check: In turbulent regimes, Po is approximately constant for a given impeller geometry (e.g., ~5–6 for a Rushton turbine), consistent with a drag-like coefficient relative to inertial scaling. Why Other Options Are Wrong: Buoyant to inertial or gravitational to inertial: those relate to Froude or Archimedes numbers, not Po. Imposed to gravitational: misses the inertial basis central to Po’s definition. Common Pitfalls: Confusing Po with Reynolds or Froude numbers; each captures different physics. Using Po correlations outside their valid geometry range (blade count, pitch, baffling). Final Answer: Imposed (drag/torque) forces to inertial forces

Question 16

Agitation energy demand: On what does the power consumption of an agitated bioreactor primarily depend?

Accepted Answer

Introduction / Context: Power input drives mixing, suspension, and mass transfer in bioreactors. Engineers must anticipate how fluid properties, hardware geometry, and operating parameters combine to determine the required shaft power and energy per unit volume. Given Data / Assumptions: Mechanically agitated tank with baffles. Possibility of gas sparging affecting power draw. Newtonian or non-Newtonian broths depending on biomass. Concept / Approach: Power correlations use the power number Po and Reynolds number Re (and sometimes gas flow number Fl). For a given geometry, P = Po * ρ * N^3 * D^5 in ungassed conditions; Po depends on Re (via viscosity μ) and geometry, and gas sparging typically reduces P at constant N. Hence power depends on physical properties, operating conditions, and vessel/impeller geometry. Step-by-Step Solution: Relate P to Po, ρ, N, and D for ungassed operation.Include viscosity via Re = ρ * N * D^2 / μ.Account for geometry: D/T, blade type (Rushton vs. pitched–blade), clearance, and baffles set Po–Re trends.Consider operating conditions: speed N, gas flow, and baffling modify effective Po and thus P. Verification / Alternative check: Scale-up rules (constant P/V or constant tip speed) explicitly reference these variables, demonstrating that all three categories influence power consumption. Why Other Options Are Wrong: Any single category alone omits critical determinants. Accurate predictions require fluid properties, operating parameters, and geometric factors together. Common Pitfalls: Extrapolating Po from water-like fluids to viscous, shear-thinning broths. Ignoring gas-induced power reduction when sparging at fixed N. Final Answer: All of the above

Question 17

Rushton turbine design feature: Why is a flat disc used with a radial-flow (Rushton) turbine impeller in bioreactors?

Accepted Answer

Introduction / Context: The Rushton turbine is a classic radial-flow impeller for gas–liquid contacting. Its geometry includes a central flat disc with six flat blades. Understanding the role of the disc helps explain gas dispersion, shear fields, and energy distribution in aerated tanks. Given Data / Assumptions: Flat disc with radial blades in a baffled tank. Often used below a sparger for bubble dispersion. Operation in turbulent regimes for microbial fermentations. Concept / Approach: The disc reduces flow through the impeller hub region and forces liquid to accelerate around blade tips, where strong shear is created. In aerated systems, the high-shear zones efficiently fragment incoming bubbles, improving interfacial area. The disc also helps maintain a uniform discharge pattern and prevents energy loss in the low-shear core near the shaft. Step-by-Step Solution: Recognize that shear and turbulence primarily arise near blade edges.The flat disc blocks short-circuit flow through the hub to concentrate energy at the blades.High shear at the blade tips assists bubble breakup and dispersion.Therefore, the disc simultaneously promotes shear, bubble breakup, and efficient energy use at the blades. Verification / Alternative check: Power distribution and laser Doppler velocimetry studies show intense turbulent kinetic energy near blade tips and reduced recirculation through the central region when a disc is present. Why Other Options Are Wrong: Each individual statement captures only part of the function; in practice the disc achieves all of these outcomes together. Common Pitfalls: Assuming the disc is only a mounting hub; its hydrodynamic role is significant. Underestimating the disc’s importance in gas dispersion performance. Final Answer: All of the above

Question 18

Vortexing during bioreactor agitation: Why is the formation of a deep central vortex generally undesirable in biological mixing operations?

Accepted Answer

Introduction / Context: In unbaffled or poorly baffled tanks, surface depression and a strong central vortex can form at elevated speeds. In bioprocessing, this is typically undesirable because it degrades mixing quality and increases mechanical stress on delicate cells. Given Data / Assumptions: Aerated or non-aerated biological suspensions. Concern for shear-sensitive cells and foaming behavior. Standard axial or radial impellers in cylindrical vessels. Concept / Approach: Vortexing draws air from the surface, entraining bubbles into the impeller zone. Bubble impingement combined with blade strikes and turbulent eddies can lyse cells. Meanwhile, circulation can collapse into a swirling pattern with poor top-to-bottom exchange, so high speed does not guarantee good macro-mixing. Finally, energy is wasted in maintaining the vortex rather than creating effective turbulent eddies throughout the vessel. Step-by-Step Solution: Identify vortex causes: inadequate baffling or off-design impeller placement.Recognize effects: air entrainment, cell damage, and poor axial circulation.Implement solutions: install baffles, reduce liquid swirl, or use off-center/angled impeller mounting.Conclude that all listed issues make vortexing undesirable in bio-mixing. Verification / Alternative check: Comparative mixing tests show reduced mixing time and improved oxygen transfer when baffles eliminate free-surface swirl versus vortexed, unbaffled conditions at the same power input. Why Other Options Are Wrong: Each single reason is valid, but focusing on only one understates the combined negative impact; all apply simultaneously in practice. Common Pitfalls: Increasing speed alone, which can deepen the vortex and worsen problems. Ignoring headspace and antifoam management when air entrainment is severe. Final Answer: All of the above

Question 19

Baffle sizing guideline for fermenters: What is the typical range for individual baffle width as a fraction of the vessel diameter (w_baffle ≈ ____ × Dt)?

Accepted Answer

Introduction / Context: Baffles suppress swirling and surface vortex formation, converting tangential flow into axial and radial circulation for efficient mixing. Correct baffle sizing is essential for predictable power consumption and scale-up in bioreactors. Given Data / Assumptions: Vertical cylindrical, baffled tank with four equally spaced baffles. Standard radial or axial impellers. Goal: minimize vortexing and stabilize flow patterns. Concept / Approach: A widely used rule-of-thumb is a baffle width around 0.1 × Dt (often 0.08–0.12 × Dt). This range offers strong swirl suppression without unduly increasing drag or causing stagnant zones behind oversized baffles. It aligns with the frequently cited “1/10 of Dt” guideline; some designs use slightly narrower baffles (≈1/12) for lower drag. Step-by-Step Solution: Choose baseline: w_baffle ≈ 0.1 × Dt.Allow practical tolerance: 0.083–0.1 × Dt (≈1/12 to 1/10).Verify that this sizing delivers minimal vortexing and robust circulation across scales.Confirm that option '1/10 to 1/12 of Dt' covers the commonly recommended range. Verification / Alternative check: Mixing handbooks and vendor guides consistently cite ~10% of Dt, with minor variations to tune power and flow regimes for specific fluids. Why Other Options Are Wrong: 1/8 to 1/10 or 1/5 to 1/8: larger widths increase drag and may create dead zones; they are not typical for standard fermenters. None of these: unnecessary because the canonical range is given. Common Pitfalls: Overlooking baffle clearance from the wall (small gap to mitigate fouling). Using full-height baffles without considering foaming and headspace effects. Final Answer: 1/10 to 1/12 of Dt

Question 20

Reactor classifications by agitation method: Which of the following are non-mechanically agitated bioreactors (i.e., mixing is achieved without a rotating impeller)?

Accepted Answer

Introduction / Context: Bioreactors can be mixed by mechanical impellers or by gas-driven circulation. Non-mechanical systems avoid moving shafts and seals, often reducing shear and maintenance while relying on gas flow for mixing and mass transfer. Given Data / Assumptions: Bubble columns rely on rising gas bubbles to induce circulation. Airlift reactors use riser and downcomer sections (with or without a draft tube) to create loop flow. Stirred tank reactors employ powered impellers. Concept / Approach: Non-mechanical agitation is defined by the absence of rotating mechanical elements inside the vessel. Bubble columns and airlifts satisfy this definition; their hydrodynamics arise from gas hold-up differences and buoyancy-driven circulation, not impeller rotation. Step-by-Step Solution: Identify STR as mechanically agitated (reject option a).Recognize bubble column mixing via gas sparging-induced eddies (accept b).Recognize airlift loop circulation via density differences (accept c).Therefore, choose Both (b) and (c). Verification / Alternative check: Design literature classifies bubble columns and airlifts as gas–liquid contactors without internal moving parts; they are widely used for shear-sensitive cultures and large-scale oxidations. Why Other Options Are Wrong: STR: explicitly mechanical, with impellers and seals. Either single choice (b) or (c) alone omits the other non-mechanical class. Common Pitfalls: Assuming airlift reactors contain impellers; the “draft tube” guides flow but is not a rotating agitator. Underestimating shear levels; gas-driven systems can still be shearing near spargers. Final Answer: Both (b) and (c)

Agitation and Aeration Questions