Bioreactor Study Guide

📖 Core Concepts Bioreactor – Engineered vessel that creates a biologically active environment for chemical processes with living organisms or enzymes. Aerobic vs. Anaerobic – Aerobic bioreactors supply O₂; anaerobic operate without O₂. Batch, Fed‑Batch, Continuous – Three basic operation modes that differ in how substrates and products are added/removed. Immobilization – Trapping or attaching cells/enzymes on a solid support so they stay in the reactor while product flows out. Key Design Variables – Temperature, pH, dissolved‑oxygen (DO), mixing/agitation, and fouling control. Typical Construction – Cylindrical stainless‑steel vessel (easy to clean, sanitize). Photobioreactor – Bioreactor equipped with a light source for photosynthetic organisms (algae, cyanobacteria). --- 📌 Must Remember Batch: all feed added once; harvest at end. Fed‑Batch: feed added continuously or intermittently during run → higher cell density. Continuous (Chemostat): constant inflow of fresh medium & outflow of product; steady‑state volume maintained. O₂ Solubility: Very low in water → must be supplied by sparging & pressurization. Baffles: Prevent vortex formation, lower power draw, improve mixing. pH Control: Small additions of acid/base keep optimal enzymatic activity. Temperature Control: Cooling jacket, internal coils, or external heat exchangers remove exothermic heat. Scale‑Down: Small‑scale reactor mimics large‑scale conditions for rapid optimization. --- 🔄 Key Processes Batch Operation Load all reactants → inoculate → run reaction → stop → harvest product. Fed‑Batch Operation Load initial batch nutrients. Begin reaction. Add nutrient feed continuously or at set intervals. Monitor growth, adjust feed rate to avoid substrate inhibition. Continuous (Chemostat) Operation Set dilution rate \(D = \frac{F{in}}{V}\) (flow in ÷ reactor volume). Pump fresh medium in, simultaneously pump culture out. Maintain steady‑state concentration of cells and metabolites. Oxygen Transfer Sparge gas → create bubbles. Agitation breaks bubbles, increases gas–liquid interfacial area. Adjust pressure or gas composition to raise dissolved O₂. Immobilized‑Cell Process Attach/entrain cells to support (e.g., packed‑bed). Pump substrate through reactor → reaction occurs on/within support. Product exits continuously; cells retained. --- 🔍 Key Comparisons Batch vs. Fed‑Batch Feed strategy: all at start vs. ongoing addition. Cell density: lower vs. higher (fed‑batch can reach >100 g L⁻¹). Fed‑Batch vs. Continuous Steady‑state: not maintained in fed‑batch; constant in continuous. Complexity: fed‑batch simpler (no outlet stream control). Stainless‑Steel vs. Alternative Materials Cleaning: stainless steel easy to sterilize; alternatives may need special protocols. Cost: stainless‑steel higher upfront but durable. Photobioreactor vs. Conventional Bioreactor Energy input: light required vs. only mechanical/thermal. Organism type: photosynthetic only vs. any microbes/enzymes. --- ⚠️ Common Misunderstandings “More agitation always means better oxygen transfer.” – Excessive speed can create shear damage to cells and increase power costs; optimal agitation balances mixing and shear. “Batch reactors are always cheaper.” – While equipment is simple, lower productivity can raise overall cost per unit product. “Immobilization eliminates the need for pH control.” – Immobilized cells still require optimal pH; fouling on supports can shift pH locally. “Chemostat = constant product concentration.” – Only at the set dilution rate; changes in feed composition or kinetic shifts alter product levels. --- 🧠 Mental Models / Intuition “Flow‑through highway” – Think of a continuous reactor as a highway where cars (substrate) enter, react along the way, and exit as finished products; the speed (dilution rate) determines how long they stay. “Bubble‑mixing handshake” – Oxygen transfer = bubble surface area × agitation “handshake” strength. More bubbles + better mixing = higher DO. “Layered cake of control” – Temperature → pH → DO → agitation: each layer builds on the previous; if the base (temperature) drifts, the upper layers become unstable. --- 🚩 Exceptions & Edge Cases O₂‑Limited Aerobic Cultures – Some high‑density cultures require pure O₂ sparging or pressurization beyond ambient air. Shear‑Sensitive Organisms – For fragile cells (e.g., mammalian), low‑shear impellers replace high‑speed Rushton turbines. Viscous Media – High‑viscosity feeds (e.g., sugar syrups) may cause plugging; use peristaltic pumps or dilute feed. Heat‑Generating Reactions – Exothermic bioprocesses may need both cooling jacket and internal coils to avoid hot spots. --- 📍 When to Use Which Batch – Early‑stage development, small batches, or when product inhibits growth. Fed‑Batch – Need high cell density, product is growth‑associated, or substrate inhibition is a risk. Continuous (Chemostat) – Desired steady‑state operation, constant product stream, or for continuous downstream integration. Photobioreactor – Working with algae, cyanobacteria, or other photosynthetic microbes; when light intensity can be controlled. Immobilized Systems – Continuous processes where cell retention is critical (e.g., wastewater treatment, enzyme reactors). --- 👀 Patterns to Recognize Declining DO trend → insufficient sparging or agitation. Sudden pH drift → accumulation of acidic metabolites or buffer exhaustion. Steady biomass increase then plateau → substrate limitation or oxygen limitation. Vortex formation on power curve → missing baffles or improper impeller speed. --- 🗂️ Exam Traps “All bioreactors are stirred tanks.” – Photobioreactors, packed‑bed reactors, and membrane reactors are also common. “Continuous operation always yields higher productivity.” – Only if dilution rate matches organism’s growth rate; otherwise washout occurs. “Higher feed rate in fed‑batch always increases product.” – Can cause substrate inhibition or overflow metabolism (e.g., acetate formation). “Stainless steel eliminates fouling.” – Fouling can still occur on heat‑exchange surfaces; cleaning protocols are still required. “pH control is only needed for bacterial cultures.” – Yeasts, mammalian cells, and enzyme reactions also need tight pH regulation. ---