23-Chem-B4 Biochemical Engineering · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-Chem-B4, Biochemical Engineering — May 2014. 3 hours, Closed-Book Exam (any non-communicating calculator permitted). Six questions are printed; per the exam notes any five (5) constitute a complete paper (100 marks) and only the first five as they appear in the answer book are marked. All six are solved below for completeness.
Reference texts: Shuler & Kargi, Bioprocess Engineering: Basic Concepts, 2nd ed.; Bailey & Ollis, Biochemical Engineering Fundamentals, 2nd ed.; Madigan et al., Brock Biology of Microorganisms, 13th ed.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Approach. HTST (High-Temperature-Short-Time) continuous sterilization exploits the fact that the thermal death rate of microorganisms (first-order, rate constant rising exponentially with temperature via an Arrhenius relationship) increases much faster with temperature than the rate of nutrient degradation (typically lower activation energy). Operating briefly at a higher temperature therefore kills spores to the required probability (a very large "Del factor," ∇, the log-reduction in viable count) while degrading far less of the heat-sensitive nutrient than an equivalent low-temperature/long-time batch sterilization would. Both HTST configurations share the same three functional stages — heat, hold, cool — and differ only in how heat is delivered.
Live (culinary-grade) steam is injected directly into the flowing medium through a steam injector (sparger or venturi-type nozzle), condensing instantly on contact and releasing its full latent heat of condensation into the liquid. Because heat transfer is by direct mixing rather than across a metal wall, heating to the hold temperature (typically ≈140–150 °C) is essentially instantaneous (fractions of a second), which minimises the time nutrients spend at intermediate, nutrient-damaging temperatures. The heated stream passes through a holding tube sized to give the required residence time at Th, then is flash-cooled: throttling into a vacuum vessel instantly evaporates water (equal to the mass of condensed steam that was added), which both cools the stream (by removing the heat of vaporisation) and removes the dilution water the steam introduced.
The medium never contacts the heating medium directly; instead it flows through one side of a plate or tubular heat exchanger while steam or hot oil flows on the other side, separated by a solid metal wall. A regenerative heat-exchanger stage is standard practice: incoming cold feed is preheated by the outgoing hot, already-sterilized product (recovering energy and reducing both duty and cooling load) before a final heater brings it the rest of the way to Th. After the holding tube, the stream passes back through the regenerative exchanger (now as the hot side, giving up heat to the incoming feed) and, if needed, a final cooler.
| Aspect | Direct steam injection | Indirect heating |
|---|---|---|
| Heating rate | Essentially instantaneous (direct condensation) | Finite, limited by wall heat-transfer coefficient |
| Nutrient/product degradation | Lowest — minimal time at intermediate T | Somewhat higher — slower ramp through intermediate T |
| Product dilution | Yes — condensed steam adds water (must be removed by flash cooling) | None — no water added or removed |
| Steam quality requirement | Culinary/contaminant-free steam required (contacts product) | Ordinary plant/boiler steam acceptable (never contacts product) |
| Energy recovery | Limited (flash-cooling energy largely lost with the flashed vapour) | High — regenerative exchanger recovers most sensible heat |
| Fouling / maintenance | Low (no heat-transfer surface with the product) | Heat-exchanger surfaces can foul with proteinaceous media, need cleaning-in-place |
| Capital equipment | Simple injector + holding tube + flash vessel | More complex plate/tubular exchanger train |
The choice between them is a trade-off: direct steam injection gives the gentlest possible thermal treatment (best for highly heat-sensitive media) at the cost of diluting the product and needing food/pharma-grade steam, while indirect heating avoids dilution and recovers far more energy but degrades slightly more nutrient value because the medium ramps through the damaging intermediate-temperature range more slowly, and its heat-transfer surfaces are exposed to fouling from the medium itself.
