At a Glance
Evaporation economics are dominated by energy costs. MVR and multiple-effect designs significantly reduce steam consumption. Use untangle.bio for project-specific design.
How Evaporation Works
Evaporation removes water (or solvent) from a liquid stream by supplying heat energy sufficient to vaporize the liquid at the operating pressure. Reducing pressure (vacuum operation) lowers the boiling point, enabling concentration at lower temperatures to protect heat-sensitive products. The concentrated product (liquor) exits as one stream; the condensed vapour (condensate) as the other.
Two Outputs
Concentrate / Liquor (heavy): The product-rich, water-reduced stream leaving the evaporator body. This is the target stream for downstream crystallization, drying, or formulation.
Condensate / Vapour (light): Water (or solvent) vapour that is condensed and removed. May contain volatile impurities or traces of volatile products; can be recycled or treated before disposal.
Key Parameters
- Boiling point elevation (BPE): Dissolved solutes raise the boiling point above that of pure water. BPE increases with concentration and must be accounted for in heat exchanger design. Highly concentrated sugar or salt solutions can have BPE of 5–15 °C.
- Heat-sensitive products: Proteins and other biologics denature at elevated temperatures. Vacuum evaporation (20–50 °C, <100 mbar) protects labile molecules. Residence time should be minimised with falling film or thin-film designs.
- Fouling: High-viscosity concentrates and salt crystallisation on heat exchange surfaces reduce overall heat transfer coefficient over time; CIP protocols are essential.
- Entrainment: Fine droplets can carry product into the vapour stream; mist eliminators are required to prevent product loss.
Evaporator Types
| Type | Residence Time | Best For | Limitation |
|---|---|---|---|
| Falling Film | Seconds–minutes | Heat-sensitive products; low viscosity; continuous operation | Not suitable for high-viscosity or fouling liquors |
| Forced Circulation | Minutes | Viscous liquors, salting-out systems, crystallising solutions | Higher energy consumption; more complex |
| Thin Film (Wiped) | Seconds | Very heat-sensitive; high-viscosity products near final concentration | High CAPEX; limited throughput |
| Natural Circulation (Calandria) | Minutes–hours | Low-cost commodity products; non-labile solutions | Long residence time; not suitable for biologics |
Energy Reduction Strategies
Multiple-Effect Evaporation (MEE): Steam from the first evaporator body heats the second body operating at lower pressure (lower boiling point). Typically 3–6 effects reduce steam consumption by 3–6× compared to single-effect operation, at the cost of higher CAPEX.
Mechanical Vapor Recompression (MVR): A compressor raises the pressure (and temperature) of the outlet vapour so it can be reused as the heating medium in the same evaporator body. MVR reduces steam consumption by 90–97% compared to single-effect, using only electrical energy for the compressor. MVR is the preferred technology for large continuous bioprocessing applications where steam cost is significant.
Evaporator Selection Guide
Product thermal stability and liquor viscosity are the two most important selection criteria.
| Scenario | Recommended Type | Energy Strategy |
|---|---|---|
| Thermostable commodity (ethanol, sugar, organic acids) | Forced circulation or natural circulation | Multiple-effect or MVR |
| Heat-sensitive biologics (enzymes, vitamins) | Falling film, vacuum operation 30–50 °C | Single-effect or 2-effect; short residence time priority |
| Very heat-sensitive (proteins, mAbs) | Thin film / wiped film at <40 °C | Single-effect; prefer UF concentration instead |
| Pre-crystallization concentration | Forced circulation (allows salting) | MEE + seeded crystalliser downstream |
| Solvent recovery (ethanol, acetone) | Falling film or distillation column | MVR or heat integration with fermentation |
Best Molecules for Evaporative Concentration
| Molecule | Temperature Limit | Evaporation Behavior | Application |
|---|---|---|---|
| Lactic Acid | <80 °C | Non-volatile; concentrates readily; BPE moderate | Concentrate fermentation broth 10–20× before crystallisation or spray drying |
| Citric Acid | <70 °C | Non-volatile; high solubility; BPE significant at >500 g/L | Pre-crystallization concentration to 70–80% w/w |
| Succinic Acid | <70 °C | Non-volatile; moderately soluble; concentrates well | Fermentation broth concentration before reactive crystallisation |
| Glucose | <60 °C (avoid caramelisation) | Non-volatile; high BPE at high concentration | Glucose syrup concentration; corn wet milling |
| Ethanol | Volatile (bp 78 °C) | Co-evaporates with water; requires distillation column for separation | Ethanol recovery/purification from fermentation broth |
| Acetic Acid | Volatile (bp 118 °C) | Partially co-evaporates; vacuum evaporation concentrates aqueous phase but losses occur | Vinegar concentration; acetate buffer concentration (with care) |
Cost Considerations
Capital Cost (CAPEX)
Falling film evaporators have moderate CAPEX dominated by the heat exchanger area required. MVR systems add a significant compressor investment but reduce ongoing energy costs substantially. Multiple-effect systems increase CAPEX linearly with number of effects while reducing steam consumption proportionally. For pharmaceutical GMP applications, hygienic design (316L SS, electropolished surfaces, CIP/SIP capability) significantly increases equipment cost compared to food-grade or commodity chemical equivalents.
Key CAPEX Drivers
| Factor | Impact |
|---|---|
| Evaporation duty (kg water/hr) | Primary cost driver; determines heat exchanger area and compressor size |
| Number of effects / MVR | Higher upfront cost; reduced steam consumption; payback period depends on steam price |
| Vacuum system | Required for low-temperature operation; steam ejectors or liquid ring vacuum pumps add cost |
| GMP vs. industrial grade | GMP hygienic design with CIP/SIP adds 2–3× vs. commodity chemical grade |
Operating Cost (OPEX)
Steam (or hot water) is the dominant OPEX for single-effect evaporators. MVR replaces steam with electricity, which may be cost-advantageous depending on local utility pricing. Fouling increases cleaning frequency and reduces effective uptime; anti-fouling additives or frequent CIP cycles add OPEX. Mist eliminator maintenance and vacuum system utilities are minor contributors.
Frequently Asked Questions
Why is vacuum evaporation used for heat-sensitive products?
Water boils at 100 °C at atmospheric pressure, but at 50 mbar (absolute) it boils at approximately 33 °C, and at 100 mbar at approximately 45 °C. Vacuum evaporation exploits this relationship: by reducing pressure below atmospheric, the boiling point is lowered below the temperature at which most biologics denature. Falling film evaporators operated under vacuum with short residence times (seconds to minutes) allow concentration of enzymes, vitamins, and fermentation products that would be damaged at 100 °C.
What is mechanical vapor recompression (MVR) and when is it justified?
In MVR, the water vapour leaving the evaporator is compressed by a mechanical compressor (typically a centrifugal or screw compressor), raising its temperature and pressure enough so it can be used as the heating medium on the shell side of the same heat exchanger. This recycles the latent heat of vaporization and reduces steam consumption by 90–97%. MVR is economically justified for large continuous operations (typically >5,000 kg/hr evaporation duty) where the capital cost of the compressor is offset by reduced steam costs within 2–5 years.
What is boiling point elevation and how does it affect evaporator design?
Dissolved solutes elevate the boiling point of a solution above that of pure water at the same pressure (Raoult’s law). The magnitude increases with solute concentration and molecular weight. For example, a 50% glucose syrup has a BPE of approximately 1.5 °C; a saturated salt solution may have BPE of 10–15 °C. In evaporator design, BPE reduces the effective temperature driving force across the heat exchanger (ΔT = Tsteam – Tboiling), requiring larger heat exchange area or higher-temperature steam to achieve the same evaporation rate.
Should I use ultrafiltration or evaporation to concentrate a protein solution?
Ultrafiltration is almost always the better choice for protein concentration. UF operates at ambient temperature (no thermal denaturation), uses pressure as the driving force (no phase change), and simultaneously exchanges buffer (diafiltration). Evaporation exposes proteins to heat and long residence times, increasing denaturation risk, and does not remove low-molecular-weight impurities. Evaporation is preferred for thermostable small molecules (organic acids, sugars, amino acids) where UF membranes would not retain the product or where very high concentration factors (>20×) are required before drying.
Related Separation Techniques
Add Evaporation to Your Bioprocess Flowsheet
Model your evaporation step with real mass and energy balance, connect to crystallisation or drying, and simulate the full downstream train.
Open untangle.bio