Q: Impeller tip speed classes: larger-diameter marine propellers typically operate at which rotational speed range in bioreactors?

Introduction / Context: Impeller selection balances mixing intensity, shear, and power input. For a given tip speed target, larger-diameter propellers turn more slowly than small impellers. Knowing the typical rpm range helps in preliminary design and scale-up checks. Given Data / Assumptions: Marine propellers are axial-flow impellers commonly used for low-to-moderate viscosity broths. Larger diameter implies lower rpm for a given tip speed (u_tip = π*D*N). Excessive rpm increases shear and may damage cells. Concept / Approach: To achieve practical tip speeds (for example, 2–6 m/s) in large tanks, rotational speeds are modest. Ranges of 20–150 rpm are typical for large-D propellers, whereas several hundred rpm is characteristic of small lab-scale impellers. Very high rpm (approaching 1000+) is associated with tiny impellers or non-bioprocess mixers. Step-by-Step Solution: 1) For fixed u_tip, N scales inversely with D: N = u_tip / (π*D).2) As D increases in scale-up, N must decrease to keep shear and power in check.3) Practical ranges for large propellers fall in tens to low hundreds of rpm, i.e., 20–150 rpm. Verification / Alternative check: Vendor catalogs and scale-up case studies show marine propellers in production fermenters frequently below ~120 rpm at multi-meter diameters. Why Other Options Are Wrong: 150–300 rpm and 400–800 rpm are typical of smaller D or lab systems; 1150 rpm is unrealistic for large bioreactor propellers; 5–15 rpm is too low to provide adequate mixing and O2 transfer in most cases. Common Pitfalls: Transferring lab rpm directly to plant scale without accounting for impeller diameter and tip speed constraints. Final Answer: 20 to 150 rpm

Q: Flat-blade disc turbine (Rushton) versus pitched-blade turbine: which distinguishing feature most reliably differs between these impellers in standard practice?

Introduction / Context: Bioreactor impellers strongly influence flow pattern and shear. The flat-blade disc turbine (FBDT, Rushton) is a radial-flow mixer, whereas the pitched-blade turbine (PBT) provides significant axial flow. This question asks for a robust distinguishing feature when comparing the standard versions used in practice. Given Data / Assumptions: Standard Rushton: six flat vertical blades mounted on a central disc, mainly radial discharge. Standard PBT: four pitched blades (typically 45 degrees) without a full disc, mixed axial/radial flow. We compare common commercial versions rather than all possible variants. Concept / Approach: While geometry and flow direction differ, one reliable, easy-to-verify difference is blade count: Rushtons commonly have six flat blades; PBTs often have four pitched blades. Statements about blades projecting beyond the disc or universal air requirements are not generally correct across suppliers and operating conditions. Step-by-Step Solution: 1) Identify canonical Rushton geometry → 6 flat blades on a disc.2) Identify canonical PBT geometry → 4 pitched blades, no solid disc.3) Recognize that this blade-count distinction is consistent across many standard designs. Verification / Alternative check: Vendor catalogs and mixing handbooks depict Rushtons with six blades and PBTs with four as the most common stock offerings; variations exist but are less typical. Why Other Options Are Wrong: Option A mischaracterizes geometry and curvature; Option B over-generalizes aeration needs and confounds impeller choice with gas–liquid mass transfer specifics; Option C inherits the inaccuracies of A and B; Option E is plainly false. Common Pitfalls: Assuming every supplier adheres to a single geometry; custom blade counts exist, but the 6-vs-4 distinction remains a practical rule of thumb. Final Answer: Number of blades used on the impeller (Rushton typically 6 flat blades; pitched-blade turbine often 4)

Q: Measuring oxygen at the bubble–liquid interface: how can the oxygen concentration in the bubble boundary layer be determined in an aerated bioreactor?

Introduction / Context: Gas–liquid mass transfer into cell-containing broths occurs across thin boundary layers surrounding bubbles. If we want the oxygen concentration very near the gas–liquid interface, bulk measurements are insufficient; specialized micro-probes resolve steep gradients at sub-millimeter scales. Given Data / Assumptions: Reactor is aerated and well-mixed at the bulk scale. Boundary layer around bubbles is much thinner than bulk length scales. Dissolved oxygen can vary strongly from the interface to the bulk. Concept / Approach: Mass transfer is driven by C* − C at the interface versus bulk. To characterize C near the interface, micro-electrodes positioned in the boundary layer measure local dissolved oxygen directly, revealing gradients that determine kLa effectiveness. Bulk steady-state DO cannot resolve interfacial concentrations. Step-by-Step Solution: 1) Recognize that bulk DO reflects reactor-averaged conditions, not boundary layers.2) Use micro-probes with high spatial resolution to measure DO very close to bubble surfaces.3) Interpret profiles to estimate interfacial driving forces and validate mass-transfer models. Verification / Alternative check: Micro-probe studies show steep DO gradients in viscous or cell-laden systems; these are invisible to standard bulk sensors placed in the tank wall or headplate. Why Other Options Are Wrong: Options A and B measure bulk steady state only; D and E are indirect operational metrics that do not yield boundary-layer concentration. Common Pitfalls: Assuming uniform DO throughout; in reality, bubble boundary layers dominate transfer resistance in many broths. Final Answer: By directly probing the boundary layer with a micro-electrode (micro-probe)

Q: Why do antifoams typically reduce oxygen transfer rates (kLa) when added to an aerated bioreactor?

Introduction / Context: Foam control is vital in aerobic bioprocesses, but antifoams often depress oxygen transfer performance. Understanding the mechanisms helps operators dose prudently or switch to mechanical foam breakers when possible. Given Data / Assumptions: Antifoams are surface-active agents or dispersions (e.g., silicone oils, polyethers). kLa depends on interfacial area a and mass-transfer coefficient kL. Bubble size distribution and coalescence strongly impact a and gas holdup. Concept / Approach: Antifoams alter interfacial properties: they can lower surface tension and accumulate at interfaces, stabilizing larger bubbles and suppressing bubble breakup. Increased coalescence shifts the distribution toward larger diameters, reducing specific interfacial area a and thus kLa. Interfacial films can also impede surface renewal, lowering kL. Step-by-Step Solution: 1) Add antifoam → surfactant effects change interfacial tension and film drainage.2) Bubble coalescence increases, raising mean bubble size and decreasing area per volume.3) Adsorbed antifoam films reduce interfacial renewal, lowering kL.4) Net effect: reduced kLa and O2 transfer capacity at a given aeration rate. Verification / Alternative check: kLa tests before/after antifoam dosing typically show step decreases; mechanical foam control avoids this penalty but may add shear. Why Other Options Are Wrong: Each listed mechanism contributes; selecting only one understates the overall effect. “None” conflicts with widespread empirical observations. Common Pitfalls: Overdosing antifoam or assuming it is innocuous; even small amounts can materially change bubble dynamics. Final Answer: All of the above answers

Q: Dynamic (gassing-out) method for kLa: the technique is fundamentally based on which oxygen balance principle in an aerated fermenter?

Introduction / Context: The dynamic (gassing-out) method is widely used to estimate the volumetric oxygen transfer coefficient, kLa, directly in a bioreactor under process conditions. Unlike purely chemical methods, it uses the transient dissolved-oxygen (DO) response while cells are consuming oxygen. Given Data / Assumptions: Bioreactor is equipped with a DO probe and aeration can be switched or stepped. Cells consume oxygen at a rate OUR (oxygen uptake rate). Henry’s equilibrium saturation C* is known for the operating conditions. Concept / Approach: The liquid-phase oxygen balance is dC/dt = kLa*(C* − C) − OUR. By imposing a step (e.g., turning air on/off or changing gas composition) and recording DO versus time, one can regress kLa while accounting for OUR. This distinguishes the dynamic method from chemical sulfite oxidation tests, which measure kLa in the absence of cells. Step-by-Step Solution: 1) Create a controlled change in aeration or agitation and log DO continuously.2) Model the transient using dC/dt = kLa*(C* − C) − OUR.3) Estimate kLa (and possibly OUR) from the transient profile. Verification / Alternative check: Comparing dynamic-method kLa to off-line sulfite oxidation values often shows differences due to broth rheology and cell presence; the dynamic method better reflects real process conditions. Why Other Options Are Wrong: Option B ignores OUR, a key term during growth; C describes a different chemical method; D is too vague; E lacks necessary liquid-phase information. Common Pitfalls: Neglecting probe response time and calibration, or assuming OUR is constant when it varies with biomass and conditions. Final Answer: Oxygen material balance in an aerated batch fermenter during microbial growth (kLa(C* − C) − OUR)

Q: Aeration holdup calculation: a cylindrical reactor shows a liquid height of 10 m during aeration, but 7.5 m without aeration. What is the gas holdup under these conditions?

Introduction / Context: Gas holdup (epsilon_g) is the volume fraction of gas in a gas–liquid dispersion at steady state. In tall columns and aerated reactors, holdup expands the dispersion height compared with the unaerated liquid height. This problem uses height expansion to estimate holdup quickly. Given Data / Assumptions: Height during aeration, H_aerated = 10 m. Height without aeration, H_liquid = 7.5 m. Uniform cross-section; negligible liquid compressibility; steady dispersion. Concept / Approach: For the same vessel cross-section, volume is proportional to height. Gas holdup equals gas volume divided by total dispersion volume. With heights: epsilon_g = (H_aerated − H_liquid) / H_aerated. Step-by-Step Solution: 1) Compute the height difference: ΔH = 10 − 7.5 = 2.5 m.2) Divide by aerated height: epsilon_g = 2.5 / 10 = 0.25.3) Convert to percent: 0.25 * 100 = 25%. Verification / Alternative check: As a quick consistency check, if holdup were 0% there would be no expansion (heights equal); if holdup were 100% the liquid column would be all gas—impossible. A 25% expansion is realistic for vigorously aerated systems. Why Other Options Are Wrong: 100% and 75% are far too high; 50% would require H_liquid to be half of H_aerated (5 m), not 7.5 m; 12.5% does not match the computed ratio. Common Pitfalls: Using H_liquid in the denominator or forgetting that holdup is based on total aerated volume, not the liquid-only height. Final Answer: 25%

Q: Aspect ratio 1:1 cylinder: a cylindrical reactor holds 100,000 litres (100 m^3) of liquid and has height:diameter (H:D) = 1:1. What is the approximate liquid height?

Introduction / Context: Quick geometric estimates are common in bioprocess design. For right circular cylinders, knowing volume and aspect ratio allows back-calculating height and diameter. Here, the reactor has H:D = 1:1 and contains 100,000 litres (100 m^3). Given Data / Assumptions: Volume V = 100 m^3. H:D = 1:1, so H = D. Right circular cylinder: V = π*(D/2)^2*H. Concept / Approach: With H = D, substitute into the cylinder formula: V = π*(D^2/4)*D = (π/4)*D^3. Solve for D and set H = D. Convert litres to cubic metres already done (100 m^3). Step-by-Step Solution: 1) Write V = (π/4)*D^3.2) Rearrange: D^3 = 4V/π = 400/π.3) Evaluate numerically: 400/π ≈ 127.32; cube root ≈ 5.03 m.4) Since H = D, height ≈ 5.03 m → approximately 5.0 m. Verification / Alternative check: Sanity check: With D ≈ 5 m and H ≈ 5 m, volume V ≈ π*(2.5^2)*5 ≈ 3.1416*6.25*5 ≈ 98.17 m^3, close to 100 m^3; rounding explains the slight difference. Why Other Options Are Wrong: 1.0 m and 3.0 m are far too small to enclose 100 m^3 at aspect 1:1; 7.0 m and 10.0 m would overshoot the volume for a 1:1 aspect unless diameter also increased beyond the implied value. Common Pitfalls: Using litres directly without converting to m^3, or forgetting that H = D for a 1:1 aspect ratio. Final Answer: 5.0 m

Q: Mixing and scale-up terminology: In agitator design, the power number (Np) used in impeller power correlations is also known by which classical non-dimensional name?

Introduction / Context: The power number, Np, is a dimensionless group widely used to correlate impeller power draw in stirred tanks: Np = P / (ρ * N^3 * D^5), where P is power, ρ is liquid density, N is rotational speed, and D is impeller diameter. Many dimensionless groups have historical names; recognizing equivalences is important for reading classic literature and vendor data sheets. Given Data / Assumptions: We refer to standard fluid-mechanics nomenclature. Np relates power to inertial scaling N^3 D^5. No special flow regime restrictions are added in the stem. Concept / Approach: In classical fluid dynamics, Np is called the Newton number. It quantifies the ratio of viscous dissipation power to inertial reference power for a rotating mixer, and is often presented as a function of Reynolds number Re = ρ N D^2 / μ and sometimes Froude number in gas–liquid systems. Step-by-Step Solution: Recall definition: Np = P / (ρ * N^3 * D^5).Identify historical naming: Np ↔ Newton number.Exclude other groups (Froude, Bond, Rayleigh) based on their definitions (gravity, capillarity, buoyancy/thermal driving).Select the matching option: Newton\'s number. Verification / Alternative check: Standard mixing texts and impeller vendor catalogs show Np labeled as Newton number; plotted as Np versus Re for different impeller types (Rushton, pitched blade, hydrofoil). Why Other Options Are Wrong: Froude number relates inertia to gravity (important for vortex formation and gas dispersion). Bond number compares gravity to surface tension effects. Rayleigh number is a buoyancy-driven convection parameter (thermal or compositional). Euler number concerns pressure drop relationships, not power draw scaling here. Common Pitfalls: Confusing Np with Fr in aerated systems; both can enter correlations, but they are distinct groups with different physical meanings. Final Answer: Newton\'s number

Q: Impeller hydraulics: Which of the following is best described as a radial-flow impeller used in stirred tank reactors?

Introduction / Context: Impellers are classified by the predominant direction of bulk flow they induce: radial, axial, or mixed. Correct identification is essential for choosing appropriate mixers for gas dispersion, solid suspension, or viscous blending. This question asks you to identify the impeller that characteristically produces radial discharge flow. Given Data / Assumptions: 45° pitched-blade impellers are common axial/mixed-flow devices. Rushton turbines are canonical radial-flow gas-dispersion impellers. Down-pumping designs and helical ribbons have axial or tangential characteristics. Concept / Approach: A Rushton turbine (flat-blade disc turbine) ejects liquid radially from the blade tips, generating strong shear near the impeller and effective bubble breakup when aerated. Pitched-blade impellers, by contrast, pump fluid axially (up or down depending on pitch), producing circulation loops that differ markedly from radial devices. Step-by-Step Solution: List candidates and their hydraulic signatures.Associate Rushton turbine → radial discharge; pitched blade → axial/mixed.Eliminate designs that intentionally push flow downward only (axial) or are meant for viscous mixing (helical ribbon).Conclude the radial-flow choice is the Rushton turbine. Verification / Alternative check: Power and flow-number charts classify Rushton as radial with high power number; pitched blades show lower Np with significant axial flow components. Why Other Options Are Wrong: 45° pitched blades: axial/mixed, not radial. Both (a) and (b): includes an axial device, so incorrect. Downward-directed axial impeller: by definition axial. Helical ribbon: used for high-viscosity, mainly tangential/axial circulation. Common Pitfalls: Assuming all turbines are radial; some modern high-efficiency turbines are mixed-flow—Rushton specifically is radial. Final Answer: A Rushton turbine (flat-blade disc turbine)

Question 1

Mixing correlations: in low-viscosity, baffled stirred tanks, when is the Froude number typically not used for power requirement calculations?

Accepted Answer

Introduction / Context: Two dimensionless groups dominate stirred-tank scale-up: the Reynolds number (inertial vs viscous forces) and the Froude number (inertial vs gravitational forces). Engineers often omit the Froude number in certain configurations. This question asks under what common condition that omission is appropriate for low-viscosity liquids. Given Data / Assumptions: Low-viscosity, turbulent regime is typical (high Reynolds number). Tanks are baffled to suppress vortex formation. Power correlations based on power number vs Reynolds are available. Concept / Approach: The Froude number becomes important when free-surface vortexing and gravity effects influence hydrodynamics, as in unbaffled tanks or at very high impeller speeds. With baffles installed, surface vortexing is damped, so gravitational effects on power draw are minor and the power number depends mainly on Reynolds, not Froude. Therefore, engineers commonly avoid Froude in such calculations. Step-by-Step Solution: 1) Identify regime: low viscosity → high Re → turbulent mixing.2) Add baffles: vortex suppression minimizes free-surface depression.3) Conclude: power requirement correlations can neglect Froude and use Po(Re) relationships. Verification / Alternative check: Empirical power curves for baffled tanks show flat power number in fully turbulent range, largely independent of Froude until extreme aeration or surface effects arise. Why Other Options Are Wrong: Option B (Re < 300) refers to laminar regime where viscous scaling dominates; Froude omission there is not the key point of the question. Option C incorrectly combines mismatched conditions. Option D ignores the standard baffling rationale. Option E is over-generalized and inaccurate. Common Pitfalls: Assuming Froude is universally required; it matters mainly with free surfaces and unbaffled vortex behavior. Final Answer: When baffled systems are used (vortexing suppressed, gravity effects minimal)

Question 2

Impeller tip speed classes: larger-diameter marine propellers typically operate at which rotational speed range in bioreactors?

Accepted Answer

Introduction / Context: Impeller selection balances mixing intensity, shear, and power input. For a given tip speed target, larger-diameter propellers turn more slowly than small impellers. Knowing the typical rpm range helps in preliminary design and scale-up checks. Given Data / Assumptions: Marine propellers are axial-flow impellers commonly used for low-to-moderate viscosity broths. Larger diameter implies lower rpm for a given tip speed (u_tip = π*D*N). Excessive rpm increases shear and may damage cells. Concept / Approach: To achieve practical tip speeds (for example, 2–6 m/s) in large tanks, rotational speeds are modest. Ranges of 20–150 rpm are typical for large-D propellers, whereas several hundred rpm is characteristic of small lab-scale impellers. Very high rpm (approaching 1000+) is associated with tiny impellers or non-bioprocess mixers. Step-by-Step Solution: 1) For fixed u_tip, N scales inversely with D: N = u_tip / (π*D).2) As D increases in scale-up, N must decrease to keep shear and power in check.3) Practical ranges for large propellers fall in tens to low hundreds of rpm, i.e., 20–150 rpm. Verification / Alternative check: Vendor catalogs and scale-up case studies show marine propellers in production fermenters frequently below ~120 rpm at multi-meter diameters. Why Other Options Are Wrong: 150–300 rpm and 400–800 rpm are typical of smaller D or lab systems; 1150 rpm is unrealistic for large bioreactor propellers; 5–15 rpm is too low to provide adequate mixing and O2 transfer in most cases. Common Pitfalls: Transferring lab rpm directly to plant scale without accounting for impeller diameter and tip speed constraints. Final Answer: 20 to 150 rpm

Question 3

Flat-blade disc turbine (Rushton) versus pitched-blade turbine: which distinguishing feature most reliably differs between these impellers in standard practice?

Accepted Answer

Introduction / Context: Bioreactor impellers strongly influence flow pattern and shear. The flat-blade disc turbine (FBDT, Rushton) is a radial-flow mixer, whereas the pitched-blade turbine (PBT) provides significant axial flow. This question asks for a robust distinguishing feature when comparing the standard versions used in practice. Given Data / Assumptions: Standard Rushton: six flat vertical blades mounted on a central disc, mainly radial discharge. Standard PBT: four pitched blades (typically 45 degrees) without a full disc, mixed axial/radial flow. We compare common commercial versions rather than all possible variants. Concept / Approach: While geometry and flow direction differ, one reliable, easy-to-verify difference is blade count: Rushtons commonly have six flat blades; PBTs often have four pitched blades. Statements about blades projecting beyond the disc or universal air requirements are not generally correct across suppliers and operating conditions. Step-by-Step Solution: 1) Identify canonical Rushton geometry → 6 flat blades on a disc.2) Identify canonical PBT geometry → 4 pitched blades, no solid disc.3) Recognize that this blade-count distinction is consistent across many standard designs. Verification / Alternative check: Vendor catalogs and mixing handbooks depict Rushtons with six blades and PBTs with four as the most common stock offerings; variations exist but are less typical. Why Other Options Are Wrong: Option A mischaracterizes geometry and curvature; Option B over-generalizes aeration needs and confounds impeller choice with gas–liquid mass transfer specifics; Option C inherits the inaccuracies of A and B; Option E is plainly false. Common Pitfalls: Assuming every supplier adheres to a single geometry; custom blade counts exist, but the 6-vs-4 distinction remains a practical rule of thumb. Final Answer: Number of blades used on the impeller (Rushton typically 6 flat blades; pitched-blade turbine often 4)

Question 4

Measuring oxygen at the bubble–liquid interface: how can the oxygen concentration in the bubble boundary layer be determined in an aerated bioreactor?

Accepted Answer

Introduction / Context: Gas–liquid mass transfer into cell-containing broths occurs across thin boundary layers surrounding bubbles. If we want the oxygen concentration very near the gas–liquid interface, bulk measurements are insufficient; specialized micro-probes resolve steep gradients at sub-millimeter scales. Given Data / Assumptions: Reactor is aerated and well-mixed at the bulk scale. Boundary layer around bubbles is much thinner than bulk length scales. Dissolved oxygen can vary strongly from the interface to the bulk. Concept / Approach: Mass transfer is driven by C* − C at the interface versus bulk. To characterize C near the interface, micro-electrodes positioned in the boundary layer measure local dissolved oxygen directly, revealing gradients that determine kLa effectiveness. Bulk steady-state DO cannot resolve interfacial concentrations. Step-by-Step Solution: 1) Recognize that bulk DO reflects reactor-averaged conditions, not boundary layers.2) Use micro-probes with high spatial resolution to measure DO very close to bubble surfaces.3) Interpret profiles to estimate interfacial driving forces and validate mass-transfer models. Verification / Alternative check: Micro-probe studies show steep DO gradients in viscous or cell-laden systems; these are invisible to standard bulk sensors placed in the tank wall or headplate. Why Other Options Are Wrong: Options A and B measure bulk steady state only; D and E are indirect operational metrics that do not yield boundary-layer concentration. Common Pitfalls: Assuming uniform DO throughout; in reality, bubble boundary layers dominate transfer resistance in many broths. Final Answer: By directly probing the boundary layer with a micro-electrode (micro-probe)

Question 5

Why do antifoams typically reduce oxygen transfer rates (kLa) when added to an aerated bioreactor?

Accepted Answer

Introduction / Context: Foam control is vital in aerobic bioprocesses, but antifoams often depress oxygen transfer performance. Understanding the mechanisms helps operators dose prudently or switch to mechanical foam breakers when possible. Given Data / Assumptions: Antifoams are surface-active agents or dispersions (e.g., silicone oils, polyethers). kLa depends on interfacial area a and mass-transfer coefficient kL. Bubble size distribution and coalescence strongly impact a and gas holdup. Concept / Approach: Antifoams alter interfacial properties: they can lower surface tension and accumulate at interfaces, stabilizing larger bubbles and suppressing bubble breakup. Increased coalescence shifts the distribution toward larger diameters, reducing specific interfacial area a and thus kLa. Interfacial films can also impede surface renewal, lowering kL. Step-by-Step Solution: 1) Add antifoam → surfactant effects change interfacial tension and film drainage.2) Bubble coalescence increases, raising mean bubble size and decreasing area per volume.3) Adsorbed antifoam films reduce interfacial renewal, lowering kL.4) Net effect: reduced kLa and O2 transfer capacity at a given aeration rate. Verification / Alternative check: kLa tests before/after antifoam dosing typically show step decreases; mechanical foam control avoids this penalty but may add shear. Why Other Options Are Wrong: Each listed mechanism contributes; selecting only one understates the overall effect. “None” conflicts with widespread empirical observations. Common Pitfalls: Overdosing antifoam or assuming it is innocuous; even small amounts can materially change bubble dynamics. Final Answer: All of the above answers

Question 6

Dynamic (gassing-out) method for kLa: the technique is fundamentally based on which oxygen balance principle in an aerated fermenter?

Accepted Answer

Introduction / Context: The dynamic (gassing-out) method is widely used to estimate the volumetric oxygen transfer coefficient, kLa, directly in a bioreactor under process conditions. Unlike purely chemical methods, it uses the transient dissolved-oxygen (DO) response while cells are consuming oxygen. Given Data / Assumptions: Bioreactor is equipped with a DO probe and aeration can be switched or stepped. Cells consume oxygen at a rate OUR (oxygen uptake rate). Henry’s equilibrium saturation C* is known for the operating conditions. Concept / Approach: The liquid-phase oxygen balance is dC/dt = kLa*(C* − C) − OUR. By imposing a step (e.g., turning air on/off or changing gas composition) and recording DO versus time, one can regress kLa while accounting for OUR. This distinguishes the dynamic method from chemical sulfite oxidation tests, which measure kLa in the absence of cells. Step-by-Step Solution: 1) Create a controlled change in aeration or agitation and log DO continuously.2) Model the transient using dC/dt = kLa*(C* − C) − OUR.3) Estimate kLa (and possibly OUR) from the transient profile. Verification / Alternative check: Comparing dynamic-method kLa to off-line sulfite oxidation values often shows differences due to broth rheology and cell presence; the dynamic method better reflects real process conditions. Why Other Options Are Wrong: Option B ignores OUR, a key term during growth; C describes a different chemical method; D is too vague; E lacks necessary liquid-phase information. Common Pitfalls: Neglecting probe response time and calibration, or assuming OUR is constant when it varies with biomass and conditions. Final Answer: Oxygen material balance in an aerated batch fermenter during microbial growth (kLa(C* − C) − OUR)

Question 7

Aeration holdup calculation: a cylindrical reactor shows a liquid height of 10 m during aeration, but 7.5 m without aeration. What is the gas holdup under these conditions?

Accepted Answer

Introduction / Context: Gas holdup (epsilon_g) is the volume fraction of gas in a gas–liquid dispersion at steady state. In tall columns and aerated reactors, holdup expands the dispersion height compared with the unaerated liquid height. This problem uses height expansion to estimate holdup quickly. Given Data / Assumptions: Height during aeration, H_aerated = 10 m. Height without aeration, H_liquid = 7.5 m. Uniform cross-section; negligible liquid compressibility; steady dispersion. Concept / Approach: For the same vessel cross-section, volume is proportional to height. Gas holdup equals gas volume divided by total dispersion volume. With heights: epsilon_g = (H_aerated − H_liquid) / H_aerated. Step-by-Step Solution: 1) Compute the height difference: ΔH = 10 − 7.5 = 2.5 m.2) Divide by aerated height: epsilon_g = 2.5 / 10 = 0.25.3) Convert to percent: 0.25 * 100 = 25%. Verification / Alternative check: As a quick consistency check, if holdup were 0% there would be no expansion (heights equal); if holdup were 100% the liquid column would be all gas—impossible. A 25% expansion is realistic for vigorously aerated systems. Why Other Options Are Wrong: 100% and 75% are far too high; 50% would require H_liquid to be half of H_aerated (5 m), not 7.5 m; 12.5% does not match the computed ratio. Common Pitfalls: Using H_liquid in the denominator or forgetting that holdup is based on total aerated volume, not the liquid-only height. Final Answer: 25%

Question 8

Aspect ratio 1:1 cylinder: a cylindrical reactor holds 100,000 litres (100 m^3) of liquid and has height:diameter (H:D) = 1:1. What is the approximate liquid height?

Accepted Answer

Introduction / Context: Quick geometric estimates are common in bioprocess design. For right circular cylinders, knowing volume and aspect ratio allows back-calculating height and diameter. Here, the reactor has H:D = 1:1 and contains 100,000 litres (100 m^3). Given Data / Assumptions: Volume V = 100 m^3. H:D = 1:1, so H = D. Right circular cylinder: V = π*(D/2)^2*H. Concept / Approach: With H = D, substitute into the cylinder formula: V = π*(D^2/4)*D = (π/4)*D^3. Solve for D and set H = D. Convert litres to cubic metres already done (100 m^3). Step-by-Step Solution: 1) Write V = (π/4)*D^3.2) Rearrange: D^3 = 4V/π = 400/π.3) Evaluate numerically: 400/π ≈ 127.32; cube root ≈ 5.03 m.4) Since H = D, height ≈ 5.03 m → approximately 5.0 m. Verification / Alternative check: Sanity check: With D ≈ 5 m and H ≈ 5 m, volume V ≈ π*(2.5^2)*5 ≈ 3.1416*6.25*5 ≈ 98.17 m^3, close to 100 m^3; rounding explains the slight difference. Why Other Options Are Wrong: 1.0 m and 3.0 m are far too small to enclose 100 m^3 at aspect 1:1; 7.0 m and 10.0 m would overshoot the volume for a 1:1 aspect unless diameter also increased beyond the implied value. Common Pitfalls: Using litres directly without converting to m^3, or forgetting that H = D for a 1:1 aspect ratio. Final Answer: 5.0 m

Question 9

Bioreactor design insight: If the liquid height (working height) of an aerated bioreactor is increased while other operating conditions remain comparable, which mechanism most directly explains a potential increase in the oxygen transfer rate (OTR)?

Accepted Answer

Introduction / Context: Oxygen transfer rate (OTR) in aerated bioreactors depends on the overall mass transfer coefficient (KLa) and the driving force (C* - C), where C* is the saturation concentration of oxygen and C is the bulk dissolved oxygen. Changing the liquid height alters hydrostatic pressure and bubble residence time, which can influence these terms. This item asks which mechanism most directly supports an OTR increase when height is increased. Given Data / Assumptions: Liquid height is increased without changing gas type, sparger, or impeller type. Aeration is maintained; temperature is held similar. We compare mechanisms qualitatively rather than calculating exact OTR. Concept / Approach: Henry’s law implies that the saturation concentration C* of a gas in liquid increases with the partial pressure of that gas at the gas–liquid interface. In a taller column, hydrostatic pressure at the bottom is higher, increasing the local oxygen partial pressure around newly formed bubbles and thus increasing local C*. Longer bubble path length can also increase interfacial contact and gas holdup, but the most direct and universal explanation is the increased partial pressure at depth raising C* and hence the driving force (C* - C). Step-by-Step Solution: Identify OTR expression: OTR = KLa * (C* - C).Recognize that increasing height raises hydrostatic pressure at the sparger location.Higher pressure increases oxygen partial pressure → higher local C* by Henry’s law.Therefore, the driving force increases and OTR can rise, all else equal. Verification / Alternative check: Engineering correlations for bubble columns report positive effects of static head on C*; experimental profiles often show higher dissolved oxygen near the base at larger heights due to increased C* and contact time. Why Other Options Are Wrong: Decreasing saturation concentration: height does not lower C*; it tends to increase local C* at depth. Decreasing gas holdup: would usually reduce interfacial area and lower KLa. All of the above: includes incorrect statements. No effect: contradicts pressure-based increases in C* and residence time. Common Pitfalls: Assuming that only KLa changes; in reality, both KLa and C* can vary with geometry. Also, ignoring the role of temperature (higher temperature lowers C* even if height rises). Final Answer: Increasing the partial pressure of oxygen at the base of the reactor

Question 10

Mixing and scale-up terminology: In agitator design, the power number (Np) used in impeller power correlations is also known by which classical non-dimensional name?

Accepted Answer

Introduction / Context: The power number, Np, is a dimensionless group widely used to correlate impeller power draw in stirred tanks: Np = P / (ρ * N^3 * D^5), where P is power, ρ is liquid density, N is rotational speed, and D is impeller diameter. Many dimensionless groups have historical names; recognizing equivalences is important for reading classic literature and vendor data sheets. Given Data / Assumptions: We refer to standard fluid-mechanics nomenclature. Np relates power to inertial scaling N^3 D^5. No special flow regime restrictions are added in the stem. Concept / Approach: In classical fluid dynamics, Np is called the Newton number. It quantifies the ratio of viscous dissipation power to inertial reference power for a rotating mixer, and is often presented as a function of Reynolds number Re = ρ N D^2 / μ and sometimes Froude number in gas–liquid systems. Step-by-Step Solution: Recall definition: Np = P / (ρ * N^3 * D^5).Identify historical naming: Np ↔ Newton number.Exclude other groups (Froude, Bond, Rayleigh) based on their definitions (gravity, capillarity, buoyancy/thermal driving).Select the matching option: Newton\'s number. Verification / Alternative check: Standard mixing texts and impeller vendor catalogs show Np labeled as Newton number; plotted as Np versus Re for different impeller types (Rushton, pitched blade, hydrofoil). Why Other Options Are Wrong: Froude number relates inertia to gravity (important for vortex formation and gas dispersion). Bond number compares gravity to surface tension effects. Rayleigh number is a buoyancy-driven convection parameter (thermal or compositional). Euler number concerns pressure drop relationships, not power draw scaling here. Common Pitfalls: Confusing Np with Fr in aerated systems; both can enter correlations, but they are distinct groups with different physical meanings. Final Answer: Newton\'s number

Question 11

Impeller hydraulics: Which of the following is best described as a radial-flow impeller used in stirred tank reactors?

Accepted Answer

Introduction / Context: Impellers are classified by the predominant direction of bulk flow they induce: radial, axial, or mixed. Correct identification is essential for choosing appropriate mixers for gas dispersion, solid suspension, or viscous blending. This question asks you to identify the impeller that characteristically produces radial discharge flow. Given Data / Assumptions: 45° pitched-blade impellers are common axial/mixed-flow devices. Rushton turbines are canonical radial-flow gas-dispersion impellers. Down-pumping designs and helical ribbons have axial or tangential characteristics. Concept / Approach: A Rushton turbine (flat-blade disc turbine) ejects liquid radially from the blade tips, generating strong shear near the impeller and effective bubble breakup when aerated. Pitched-blade impellers, by contrast, pump fluid axially (up or down depending on pitch), producing circulation loops that differ markedly from radial devices. Step-by-Step Solution: List candidates and their hydraulic signatures.Associate Rushton turbine → radial discharge; pitched blade → axial/mixed.Eliminate designs that intentionally push flow downward only (axial) or are meant for viscous mixing (helical ribbon).Conclude the radial-flow choice is the Rushton turbine. Verification / Alternative check: Power and flow-number charts classify Rushton as radial with high power number; pitched blades show lower Np with significant axial flow components. Why Other Options Are Wrong: 45° pitched blades: axial/mixed, not radial. Both (a) and (b): includes an axial device, so incorrect. Downward-directed axial impeller: by definition axial. Helical ribbon: used for high-viscosity, mainly tangential/axial circulation. Common Pitfalls: Assuming all turbines are radial; some modern high-efficiency turbines are mixed-flow—Rushton specifically is radial. Final Answer: A Rushton turbine (flat-blade disc turbine)

Question 12

Stirred-tank reactor internals: What is the main reason for installing baffles in a stirred tank, considering typical industrial operating speeds and common impellers?

Accepted Answer

Introduction / Context: Baffles are vertical strips mounted on the vessel wall of stirred tanks. Their presence dramatically changes flow patterns by suppressing swirl and vortex formation, thereby increasing turbulence and improving macro- and micro-mixing. This question asks for the primary reason baffles are used in practice. Given Data / Assumptions: Standard baffling is 4 baffles with width about 0.1 * tank diameter. Common impellers: Rushton (radial), pitched-blade (mixed), hydrofoil (axial). Operating speeds are sufficient to enter transitional/turbulent regimes. Concept / Approach: Without baffles, the liquid forms a central vortex and rotates as a solid body. This reduces relative velocity across the impeller, lowers turbulence, and degrades mixing and gas dispersion. Baffles interrupt tangential flow, convert it into radial/axial components, and promote top-to-bottom circulation—even with radial-flow turbines—resulting in better axial mixing and earlier onset of turbulent behavior at lower speeds. Step-by-Step Solution: Relate vortex suppression to increased turbulence intensity at lower N.Connect baffles to improved axial turnover, especially with radial impellers.Identify that both effects are the core reasons for baffles.Select the combined statement as correct. Verification / Alternative check: Mixing correlations (e.g., gas dispersion, just-suspended speed) are typically reported for baffled tanks; unbaffled data show poorer performance and strong vortexing. Why Other Options Are Wrong: (a) alone and (b) alone are true but incomplete. Baffles prevent turbulence: opposite of reality; they enhance it. Eliminating sparging: unrelated; baffles do not supply oxygen. Common Pitfalls: Equating vortex presence with “good mixing”; a deep vortex actually reduces useful turbulence and gas dispersion efficiency. Final Answer: Both (a) and (b)

Question 13

Rotational speed practice: In small propeller-driven laboratory fermenters and mixers, typical direct-drive motor speeds are closest to which of the following pairs?

Accepted Answer

Introduction / Context: Small propeller mixers are often powered by standard synchronous or induction motors. Knowledge of common base speeds helps with selecting gear ratios, estimating tip speed, and assessing shear environments for sensitive cultures. This question targets the recall of typical nominal motor speeds used for small propellers. Given Data / Assumptions: Direct-drive (no gearbox) operation. Line frequency values around 50–60 Hz. Common small propeller impeller diameters (few centimeters) in lab-scale vessels. Concept / Approach: Standard electric motors run near discrete synchronous speeds. For 60 Hz power, common unloaded speeds are approximately 1800 rpm and 1200 rpm (1750 and 1150 rpm under load for 4- and 6-pole motors). These map well to typical small propeller operations. Lower speeds like 20–300 rpm are more typical of large, geared agitators; very high speeds (6000–12000 rpm) correspond to high-shear rotor–stator devices, not standard propellers. Step-by-Step Solution: Recall standard motor pole counts and synchronous speeds.Associate 4-pole ≈ 1750 rpm (under load), 6-pole ≈ 1150 rpm.Recognize these as typical for direct-drive small propellers.Select the pair 1150 rpm or 1750 rpm. Verification / Alternative check: Equipment catalogs list direct-drive lab mixers at nominal 1150/1750 rpm with optional VFDs to adjust speed; tip speed is then calculated as π * D * N. Why Other Options Are Wrong: 20–150 rpm: characteristic of large, geared paddles or anchors. 150–300 rpm and 400–800 rpm: feasible with gear reduction; not typical direct-drive bases. 6000–12000 rpm: rotor–stator homogenizers, not propellers. Common Pitfalls: Confusing motor base speed with impeller tip speed; tip speed depends on diameter as well as rpm. Final Answer: 1150 rpm or 1750 rpm

Question 14

Oxygen limitation criterion: In a microbial suspension culture, oxygen will limit growth only under which condition for the dissolved oxygen (DO) level?

Accepted Answer

Introduction / Context: Cells exhibit a characteristic critical dissolved oxygen level (Ccrit). Above Ccrit, the specific growth rate μ is essentially independent of DO; below Ccrit, μ declines as oxygen becomes limiting. Recognizing this threshold guides aeration and agitation strategies in bioreactors to avoid mass-transfer-limited performance. Given Data / Assumptions: Critical concentration is organism- and condition-specific. DO is measured as concentration or % saturation at the operating temperature and pressure. We consider steady-state or quasi-steady operation. Concept / Approach: Growth limitation occurs when oxygen supply cannot meet cellular demand. Operationally, when DO < Ccrit, respiration becomes oxygen-limited, lowering growth and product formation rates. Maintaining DO above Ccrit via higher KLa (agitation, aeration, oxygen enrichment) or reduced demand avoids this limitation. Step-by-Step Solution: Define Ccrit as the DO threshold separating oxygen-sufficient from oxygen-limited regimes.If DO < Ccrit → oxygen becomes rate-limiting for metabolism.If DO ≥ Ccrit → growth is governed by kinetics or other nutrients.Therefore, limitation occurs only when DO is less than Ccrit. Verification / Alternative check: Respirometry and DO step tests show μ and OUR plateau above Ccrit and drop below it; many processes control DO at a setpoint above Ccrit (e.g., 20–40% air saturation) for safety margin. Why Other Options Are Wrong: Greater than Ccrit: non-limiting regime. Greater than saturation: physically impossible at equilibrium without overpressure/supersaturation, and still not limiting. Exactly at saturation: ensures maximum C*; no limitation solely due to oxygen. Probe calibration gas: unrelated to physiological limitation. Common Pitfalls: Confusing Ccrit with saturation concentration C*; C* is a physicochemical property, Ccrit is biological and varies with strain and conditions. Final Answer: Concentration is less than the critical concentration

Question 15

Gas–liquid mass transfer theory: Under the two-film theory, which relationship correctly links the liquid-side mass transfer coefficient KL with diffusivity DAB and effective film thickness Zf?

Accepted Answer

Introduction / Context: The two-film (double-film) theory models interphase mass transfer by assuming thin stagnant films adjacent to the interface through which transport occurs by molecular diffusion. The bulk phases are well mixed outside these films. Recognizing the proportionality between the mass transfer coefficient and diffusivity divided by film thickness is foundational in reactor and separation design. Given Data / Assumptions: Steady-state diffusion across a liquid film of thickness Zf. Fickian diffusion with diffusivity DAB. Concentration gradient is approximately linear across the film. Concept / Approach: For a species A diffusing through a stagnant film, the molar flux NA is given by NA = -DAB * dCA/dz. With a linear concentration drop ΔC across thickness Zf, NA ≈ DAB * (ΔC / Zf). By definition, NA = KL * (C* - Cb). Therefore, KL ≈ DAB / Zf when the driving force is represented by the concentration difference across the film. This captures the intuitive result: faster diffusion (higher DAB) and thinner films (smaller Zf) both increase KL. Step-by-Step Solution: Write Fick\'s law for 1-D steady diffusion.Assume linear profile: dC/dz ≈ ΔC / Zf.Relate flux to driving force: NA = KL * (C* - Cb).Identify KL = DAB / Zf from comparing expressions. Verification / Alternative check: Dimensional analysis: DAB has units of length^2/time; dividing by length gives length/time, consistent with KL units. Empirical correlations refine this ideal with turbulence-based film thickness estimates. Why Other Options Are Wrong: KL = DAB: missing film thickness; dimensionally incorrect. Zf / DAB or DAB * Zf: inverted or multiplied; wrong scaling and units. 1 / (DAB * Zf): unit mismatch; does not produce length/time. Common Pitfalls: Confusing overall Kla with liquid-side KL; a is interfacial area per volume and must not be conflated with film thickness. Final Answer: KL = DAB / Zf

Question 16

Temperature effects on oxygen transfer: How does increasing temperature typically influence oxygen transfer in bioreactors, considering both KLa and the saturation concentration C* of oxygen?

Accepted Answer

Introduction / Context: Oxygen transfer involves both transport kinetics (KLa) and thermodynamics (saturation concentration C*). Temperature influences liquid properties, gas solubility, and microbial physiology. Engineers must balance faster mass transfer kinetics with reduced gas solubility at elevated temperatures. Given Data / Assumptions: Viscosity decreases with temperature, boosting turbulence and diffusion. Gas solubility in liquids generally decreases as temperature rises. Other variables (salt, pressure, antifoams) are held constant for this comparison. Concept / Approach: KLa tends to increase with temperature because reduced viscosity enhances mixing and increases the liquid-side mass transfer coefficient KL; diffusivity also rises, shrinking effective film thickness. Conversely, oxygen solubility follows Henry-type behavior where the Henry constant increases with temperature, lowering C*. Thus higher temperature often yields higher KLa but lower C*, creating competing effects on OTR = KLa * (C* - C). Step-by-Step Solution: Relate KLa to hydrodynamics: lower μ → higher turbulence and KL → larger KLa.Relate C* to thermodynamics: higher T → lower gas solubility → smaller C*.Combine effects in OTR = KLa * (C* - C).Conclude: higher T increases KLa but decreases C*. Verification / Alternative check: Empirical kLa correlations include temperature via viscosity and diffusivity terms; gas solubility tables show monotonic decreases in oxygen solubility with rising temperature at constant pressure and salinity. Why Other Options Are Wrong: Lowering KLa but increasing C*: opposite to typical trends. Increasing both or lowering both: not supported by standard physical behavior. No effect: temperature clearly affects both properties. Common Pitfalls: Assuming an OTR increase is guaranteed with higher temperature; if C* drop dominates, net OTR can decrease despite higher KLa. Balance is system-specific. Final Answer: Increasing KLa but lowering C*o

Question 17

Geometric design rule: For a paddle agitator in a standard baffled tank, what is the typical design ratio of paddle diameter (D) to tank diameter (T) used as a starting point?

Accepted Answer

Introduction / Context: Initial sizing of impellers relies on practical geometric ratios. For flat-blade paddles, the impeller diameter relative to tank diameter affects flow pattern, power draw, and shear. A common baseline helps designers estimate motor size and hydrodynamics before detailed optimization. Given Data / Assumptions: Standard baffled, cylindrical tanks with flat-blade paddles. Moderate viscosity Newtonian fluids. Single impeller placed near the lower third of liquid height. Concept / Approach: Design handbooks typically recommend D/T around 0.2–0.33 for paddles, with 0.25 used frequently as a starting value. Larger D/T (e.g., 0.5) is typical for anchors/helical ribbons in viscous mixing; smaller D/T (0.1–0.2) is more common for propellers or multi-impeller shafts where multiple stages provide circulation. Step-by-Step Solution: List typical D/T ranges by impeller type.Identify paddle agitators fall near 0.25 baseline.Exclude values inconsistent with standard practice (too small or too large).Select 0.25 as the representative design ratio. Verification / Alternative check: Vendor catalogs and mixing texts commonly present power/flow data normalized with D/T ≈ 0.25 for paddle examples, confirming the convention. Why Other Options Are Wrong: 0.1: too small; characteristic of small propellers. 0.8: unrealistically large for paddles; near anchor scale. 0.5: large; more like anchors or scrapers. 0.02: non-physical for bulk mixing. Common Pitfalls: Assuming a single ratio fits all fluids; viscous or non-Newtonian systems may require different D/T and blade widths. Final Answer: 0.25

Question 18

Mass transfer vs kinetics diagnosis: An alkane has a maximum solubility (saturation concentration) of 5 ppm in the medium. If the measured dissolved concentration during fermentation is 1 ppm, what conclusion about growth limitation can be made from…

Accepted Answer

Introduction / Context: Diagnosing whether a bioprocess is limited by mass transfer or by intrinsic kinetics requires comparing supply and demand. Knowing only the saturation concentration (5 ppm) and the instantaneous dissolved concentration (1 ppm) is insufficient to assign a rate limitation. This question tests your ability to recognize what additional information is needed to distinguish limitations. Given Data / Assumptions: Saturation concentration C* = 5 ppm at operating T and P. Measured dissolved concentration C = 1 ppm in the bulk. No values provided for KLa, uptake kinetics (e.g., Monod constants), or critical concentration. Concept / Approach: Mass transfer limitation is assessed by comparing the maximum transfer rate OTR (or STR for substrate) = KLa * (C* - C) with the biological uptake rate OUR (or SUR). A low measured concentration might arise because cells consume substrate quickly (kinetic demand high) or because transfer is inadequate (KLa low). Without KLa and the cell uptake parameters (μ vs substrate, or specific uptake rate qS) and the critical concentration threshold, we cannot declare the dominant limitation. Step-by-Step Solution: Identify given data: C* and instantaneous C only.Recognize the missing pieces: KLa (or Kla equivalent) and cellular kinetics (qS, Ks, Ccrit).Conclude that multiple scenarios are consistent with C = 1 ppm (mass-transfer-limited or kinetically limited).Therefore, no definitive conclusion can be drawn from concentration values alone. Verification / Alternative check: Perform a step-change test in gas–liquid transfer (e.g., increase agitation/aeration). If growth improves, mass transfer is implicated; if not, intrinsic kinetics or other nutrients likely limit. Why Other Options Are Wrong: Declaring mass transfer or kinetics a priori requires rates, not just concentrations. Stating neither is limiting contradicts the possibility that either could be. Substrate inhibition cannot be inferred from a concentration below saturation without inhibition data. Common Pitfalls: Equating C < C* with mass transfer limitation automatically; the key is whether supply rate meets demand at the operating point. Final Answer: No conclusion on growth limitation can be drawn from these data alone

Question 19

In aerobic, gas-sparged bioreactors, how does the power required by a rotating impeller at a given speed typically compare with the power in the same liquid without gas (ungassed), and what is the process reason for this difference?

Accepted Answer

Introduction / Context: Power input in stirred, gas-sparged bioreactors is a key design and scale-up variable affecting mixing, oxygen transfer, shear, and heat removal. This question checks whether you know the usual qualitative effect of aeration on impeller power draw when speed is held constant. Given Data / Assumptions: Same impeller speed in both cases (gassed vs. ungassed). Same impeller and tank geometry; Newtonian broth assumed for simplicity. Operation below flooding; normal aeration rates for fermentation. Concept / Approach: Introducing gas reduces the effective density and continuity of the liquid around the blades, causing slip and blade ventilation. As a result, the impeller power number in the gassed state (Pg) is lower than the ungassed value (Pu) at the same rotational speed. This is widely observed for radial and axial impellers before flooding. Therefore, at constant rpm, Pg < Pu in most practical regimes. Step-by-Step Solution: Hold N (rpm) constant; compare P = Np * ρ * N^3 * D^5 between gassed and ungassed states.Recognize that Np,gassed < Np,ungassed due to blade ventilation and two-phase flow effects.Conclude Pg < Pu under non-flooded, typical aeration rates.Note exceptions (e.g., near flooding) are special cases and not the usual situation. Verification / Alternative check: Engineering correlations and shop tests show power reduction factors (Pg/Pu) commonly in the range 0.3–0.8 depending on gas rate, impeller type, submergence, and scale—consistent with “lesser.” Why Other Options Are Wrong: Higher: occurs only in rare, geometry-specific edge cases; not typical. Same: contradicts standard two-phase power correlations. “May be lesser or higher…”: too noncommittal; the usual trend is lower. “Negligible…”: power remains a major design load. Common Pitfalls: Confusing constant power with constant tip speed; or assuming increased oxygen transfer must mean increased power draw. Final Answer: Lesser.

Question 20

In bioreactor characterization, which standard method(s) can be used to determine the oxygen absorption (transfer) rate in a liquid culture system?

Accepted Answer

Introduction / Context: Measuring oxygen transfer is central to aerobic fermentation design and scale-up. Engineers routinely estimate volumetric mass transfer coefficients and oxygen absorption rates using chemical and biological methods. This question reviews the common laboratory/plant techniques you should recognize. Given Data / Assumptions: Well-mixed vessel; dissolved oxygen (DO) probe is available when needed. Air or oxygen-enriched gas is sparged; temperature and salinity controlled. Biological oxygen uptake may be present or purposely eliminated (chemical method). Concept / Approach: The sodium sulfite oxidation test chemically removes oxygen with a well-known stoichiometry, letting you back-calculate transfer rate. The dynamic gassing-out method strips DO to low levels and then tracks the DO rise after re-aeration to estimate kLa and absorption rate. Under steady state with known oxygen uptake rate, direct DO balance can also yield oxygen absorption/transfer rates. Step-by-Step Solution: Sulfite method: dose Na2SO3 plus catalyst; measure oxygen uptake; compute transfer.Dynamic gassing-out: deoxygenate, then fit DO(t) to 1 − exp(−kLa t) to obtain kLa and rate.Direct measurement: at steady state, set transfer rate = biological uptake rate by mass balance. Verification / Alternative check: Cross-compare estimates from chemical vs. biological methods under identical hydrodynamics; values should agree within experimental error if probe response is accounted for. Why Other Options Are Wrong: Saying only one method works ignores standard practice. “None”: contradicts decades of reactor testing methodology. Common Pitfalls: Neglecting probe lag, temperature correction, or catalyst presence in sulfite tests; misfitting DO transients. Final Answer: All of these.

Agitation and Aeration Questions