Photobioreactor Design for Flue Gas Remediation and Biofuels | BioAlgaeSorb

This comprehensive section covers mass transfer and mixing in bioreactors. Four dimensionless numbers enable mass transfer prediction: Reynolds number (inertial/viscous force ratio), Schmidt number (momentum/mass diffusivity), Sherwood number (overall/diffusive mass transfer rate), and Grashof number (buoyancy effects). Empirical correlations for mass transfer coefficient (KL) include Moag's correlation (Sh = 0.31×Gr^(1/3)×Sc^(1/3) for small bubbles, Sh = 0.42×Gr^(1/3)×Sc^(1/2) for large bubbles) and power-based correlations (KL ∝ (P/V)^α × ug^β). Mixing objectives include blending media components, dispersing gases, suspending cells, forming emulsions, and promoting heat transfer. Mechanical systems require baffles (4-6, width 1/10 diameter) to prevent vortexing, impellers to create flow patterns, and rounded bases to eliminate stagnant zones. Stirred tank design parameters include aspect ratio 3:1-4:1, impeller/tank diameter ratio 1/2-1/4, and clearance 1.6-1.2 times impeller diameter. Pneumatic systems use gas expansion without moving parts but are limited to <500L capacity. Sparger types include porous plates, perforated pipes, nozzles, and two-phase ejectors, all requiring CIP/SIP capability. Flow patterns include rotational, radial, and axial circulation. Gas dispersion regimes include impeller flooding (high gas, low speed), impeller loading (low gas, high speed), and complete gas dispersion. Three impeller categories exist—propeller (blades on shaft, low-medium viscosity), paddle (blades on rod, simple design), and turbine (blades on disc, high turbulence). Special impellers for high viscosity include anchor, helical ribbons, and helical screws. Power requirements depend on Reynolds number (Re = ρNID²/μ) and power number (NP = P/(ρN³D⁵)). For turbulent flow, NP is constant per impeller type: Rushton turbine = 5, pitched blade = 1.3, marine propeller = 0.35. For laminar flow, NP ∝ 1/Re.

In bioreactor engineering, understanding fluid mechanics is essential for effective mixing, which involves classifying fluids as Newtonian (constant viscosity regardless of shear rate, like water) or non-Newtonian (viscosity changes with shear rate, common in fermentation broths above 10% solids concentration), calculating Reynolds number to determine flow regime (laminar, transitional, or turbulent), and applying appropriate impeller designs (turbine, propeller, helical) based on fluid viscosity; mixing functions include eliminating concentration and temperature gradients, dispersing gases for mass transfer, suspending solids, and promoting heat transfer, with mixing time and power requirements determined through empirical correlations to optimize bioreactor performance.

Mass transfer in bioprocessing occurs under concentration gradients, driven by molecular diffusion where molecules move from high to low concentration regions. In aerobic bioprocessing, oxygen must transfer from gas bulk through multiple steps to cell surfaces. Oxygen solubility in aqueous fermentation broths is very poor, making gas-liquid mass transfer enhancement crucial. Two primary theories explain mass transfer: the two-film theory assumes all resistance resides in fictitious films on either side of the interface, with transfer occurring only by molecular diffusion; the penetration theory assumes turbulent eddies remain at interfaces for constant exposure time. The general mass transfer rate expression is proportional to the mass transfer coefficient, transfer area, and concentration gradient. For gas-liquid systems, component A transfers from gas bulk to liquid bulk through two fictitious films, with concentration dropping from CAG (gas bulk) to CAi (interface equilibrium) across the gas film, then from CAi to CAL (liquid bulk) across the liquid film.

Chemical engineering divides processes into unit operations (physical operations: flow, heat transfer, mass transfer, mechanical operations) and unit processes (chemical operations: reaction kinetics, mixing). Heat transfer primarily occurs through convection, dependent on fluid flow behavior. Mass transfer similarly depends on bulk fluid movement. Residence time distribution in reactors—defining how long fluid molecules remain inside—depends entirely on velocity distribution and flow patterns, demonstrating the foundational role of fluid mechanics across all engineering disciplines.

Flue gas from thermal power plants contains several pollutants including carbon dioxide (CO2), carbon monoxide (CO), sulfur oxides (SOx), nitrogen oxides (NOx), suspended particulate matter (SPM), water vapor, and heavy metals. These pollutants form during fuel combustion when carbon, hydrogen, nitrogen, and sulfur react with oxygen. CO2 results from complete combustion and serves as an indicator of combustion efficiency, while CO forms under oxygen-deficient conditions and is highly toxic. SOx and NOx combine with atmospheric moisture to form acids causing acid rain and respiratory problems. SPM consists of fine dust and soot particles that damage human health. Strict emission limits are enforced: CO should not exceed 1000 ppm, SOx should be below 60 mg/m³, NOx below 300 mg/m³, and SPM below 50 mg/m³. Control technologies like electrostatic precipitators and bag filters help reduce these emissions to acceptable levels.

Boiler flue gases consist of multiple constituents formed during fuel combustion: Carbon Dioxide (CO2) forms when carbon in fuel reacts with oxygen (C + O2 → CO2), maintained at 10-13%; Carbon Monoxide (CO) forms during incomplete combustion with insufficient oxygen (C + ½O2 → CO), requiring maintenance between 0-50 ppm; Oxygen (O2) remains unreacted in flue gases at 2-4% concentration; Sulfur Oxide (SOx) forms when sulfur in fuel reacts with oxygen (S + O2 → SO2); Nitrogen Oxide (NOx) forms from nitrogen in air (78%) reacting with oxygen at high temperatures; Suspended Particulate Matter (SPM) consists of dust particles from fuel; Water Vapor forms from hydrogen combustion; Mercury traces exist at 0.03 mg/Nm³. Environmental standards limit SPM to 30 mg/Nm³, SOx to 100 mg/Nm³, and mercury to 0.03 mg/Nm³ to prevent atmospheric pollution.

Carbon capture addresses CO2 emissions from fossil fuel power stations through liquid amine solutions (current technology) or emerging solid-state adsorbents. Over 95% of research focuses on single-component CO2 studies, ignoring real-world complexity. Industrial flue gas contains 5-15% water vapor plus NOx, SOx which compete for surface sites with CO2. Direct air capture faces 1-2% humidity. Dynamic Vapor Sorption (DVS) is a gravimetric method measuring adsorption/desorption using microbalance, with modern instruments generating two gas streams for competitive studies. Experiments occur at ambient temperature/pressure since industrial conditions match these parameters.

Atmospheric air consists primarily of nitrogen (79%) and oxygen (21%), with trace gases. Fuels include liquid (diesel), solid (waste, tires), and gaseous (natural gas) types. Complete combustion converts carbon to CO2, while incomplete combustion produces toxic CO. Water vapor is a natural combustion product. Flue gases contain CO, CO2, H2O, NOx, SOx, and particulate matter. Particulate matter poses serious health risks by penetrating deep into lungs and potentially entering the bloodstream, causing cardiovascular problems.

This comprehensive section examines the environmental consequences of industrial combustion processes. Flue gas is defined as exhaust gas exiting through flues from combustion sources like furnaces, boilers, and engines. The combustion chamber is where fuel, air, and heat interact to produce fire through the basic reaction: Fuel + O₂ → CO₂ + H₂O + Heat. The video details six major flue gas components: (1) Carbon Dioxide (CO₂) - primary greenhouse gas driving climate change; (2) Nitrogen Oxides (NOₓ) - form smog and acid rain; (3) Sulfur Dioxide (SO₂) - causes acid rain and infrastructure corrosion; (4) Carbon Monoxide (CO) - toxic gas reducing blood oxygen delivery; (5) Water Vapor (H₂O) - affects emissions plumes; (6) Particulate Matter (PM) - fine particles causing respiratory and cardiovascular diseases. These pollutants result from burning fossil fuels and biomass in industrial equipment.

pH measures free hydrogen ion concentration: high hydrogen means acidic water, low hydrogen means alkaline water. Carbonate (KH) acts as a buffer system that sequesters hydrogen ions, preventing pH fluctuations. When hydrogen ions are released (from CO2 or other sources), carbonate absorbs them, maintaining pH stability. CO2 dissolved in water forms carbonic acid, which dissociates into hydrogen ions and bicarbonate, lowering pH. Low carbonate means less hydrogen absorption, causing pH to drop when hydrogen is released. High carbonate provides larger buffer capacity. Understanding this relationship is essential for maintaining stable aquarium conditions and preventing dangerous pH swings that can harm fish.

This section provides hands-on demonstrations showing how CO2 and carbonate hardness interact in real-time. The presenter conducts experiments using test kits and visual indicators to illustrate key principles: (1) Adding CO2 to soft water (0-1 dKH) causes rapid pH drops from ~7 to 6 or lower, demonstrating the direct acid-forming effect of dissolved CO2; (2) Adding carbonate hardness powder reverses these pH drops by binding excess acid, showing the buffering action; (3) Repeated acid additions deplete buffering capacity, causing increasingly severe pH crashes; (4) Without proper buffering, even minimal acid additions can cause instantaneous pH collapse to lethal levels (~pH 3). These visual demonstrations make abstract chemical concepts tangible and highlight why careful water management is essential for aquatic life safety.

Carbon dioxide dissolves in water to form carbonic acid (H2CO3), which dissociates into H+ and HCO3- ions, lowering pH. This explains why carbonated water is acidic. When CO2 escapes from a solution, the equilibrium shifts, reducing H+ concentration and causing pH to increase. Blood maintains pH around 7.5 through the carbonic acid-bicarbonate buffer system: CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-. This buffer resists pH changes when small amounts of acid or base are added, though it has limits. When breathing accelerates (hyperventilation), CO2 is expelled, shifting equilibrium leftward and reducing H+ concentration, which increases blood pH.

pH affects aquatic organisms, which are adapted to specific pH ranges. When pH changes significantly (either becoming too acidic or too basic), organisms may struggle to survive. Maintaining appropriate pH levels is essential for healthy aquatic ecosystems. CO2 affects water pH through its reaction with water to form carbonic acid. When CO2 levels decrease, carbonic acid formation decreases, causing pH to increase (become less acidic). When CO2 levels increase, carbonic acid formation increases, causing pH to decrease (become more acidic). CO2 affects water pH through its reaction with water to form carbonic acid. When CO2 levels decrease, carbonic acid formation decreases, causing pH to increase (become less acidic). When CO2 levels increase, carbonic acid formation increases, causing pH to decrease (become more acidic). CO2 affects water pH through its reaction with water to form carbonic acid. When CO2 levels decrease, carbonic acid formation decreases, causing pH to increase (become less acidic). When CO2 levels increase, carbonic acid formation increases, causing pH to decrease (become more acidic).

The carbonate buffer system involves carbonic acid (H2CO3) and bicarbonate (HCO3-). The video demonstrates calculating pH using the Henderson-Hasselbalch equation with pKa of carbonic acid. The calculation shows how the ratio of conjugate base to acid affects pH. This buffer system is important in biological systems and industrial applications where pH stability is required.