Industrial Evaporation
Industrial evaporation is the unit operation in which a solvent (usually water) is vaporised from a dilute solution or liquid feed to concentrate a dissolved solute, using heat supplied by steam or other heating media. Unlike drying, the product remains a concentrated liquid rather than a solid, and the process is continuous in industrial settings. It is central to the sugar industry, dairy processing (condensed milk), caustic soda production, and fruit juice concentration, and is typically performed in multiple-effect evaporator trains to maximise steam economy.
Key Formula
S (steam) related to: F×(xF − xP) = V×λ_V / λ_S
LaTeX: S = \frac{F(x_F - x_P)}{x_P - x_S}
| Symbol | Meaning | Unit |
|---|---|---|
| F | Feed flow rate | kg/h |
| x_F | Feed solute mass fraction | kg solute/kg feed |
| x_P | Product solute mass fraction | kg solute/kg product |
| V | Vapour (evaporation) rate | kg/h |
| λ | Latent heat of vaporisation | kJ/kg |
Worked Example
Problem
A single-effect evaporator concentrates a 10 wt% sugar solution to 50 wt%. Feed rate F = 5,000 kg/h at 25 °C. Steam at 110 °C (λ_S = 2,230 kJ/kg), boiling point of solution 105 °C (λ_V = 2,244 kJ/kg). Feed Cp = 3.8 kJ/(kg·K). Calculate evaporation rate V and steam consumption S.
Solution
Step 1: Mass balance on solute: F × x_F = P × x_P → P = 5,000 × 0.10 / 0.50 = 1,000 kg/h. Step 2: Evaporation rate: V = F − P = 5,000 − 1,000 = 4,000 kg/h. Step 3: Enthalpy balance: S × λ_S = F × Cp × (T_bp − T_F) + V × λ_V. Step 4: S × 2,230 = 5,000 × 3.8 × (105 − 25) + 4,000 × 2,244 = 1,520,000 + 8,976,000 = 10,496,000 kJ/h. Step 5: S = 10,496,000 / 2,230 = 4,706 kg/h. Step 6: Steam economy = V/S = 4,000/4,706 = 0.85 kg water evaporated per kg steam.
Answer
Evaporation rate V = 4,000 kg/h; steam consumption S = 4,706 kg/h; steam economy = 0.85
Steam Economy for Multiple-Effect Evaporator Systems
| Number of Effects | Steam Economy (kg evap./kg steam) | Capital Cost | Operating Cost | Typical Use |
|---|---|---|---|---|
| 1 effect | ~0.9 | Low | High | Small-scale, low steam cost |
| 2 effects | ~1.8 | Moderate | Moderate | Common in food industry |
| 3 effects | ~2.7 | Moderate–high | Lower | Sugar, paper industries |
| 4 effects | ~3.6 | High | Low | Caustic soda, pulp mills |
| 5–6 effects | ~4.5–5.4 | Very high | Very low | Large-scale, high steam cost |
Interactive Tools
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Related Terms
Industrial Drying
Industrial drying is the unit operation that removes moisture or solvent from solid, semi-solid, or liquid materials by applying thermal energy to vaporise and carry away the liquid phase. It is one of the most energy-intensive operations in the process industries, consuming an estimated 12–20% of total industrial energy use, and is critical in food processing, pharmaceuticals, ceramics, paper manufacturing, and polymer production. The design of dryers requires understanding of drying rate curves, which exhibit a constant-rate period (surface evaporation) followed by falling-rate periods (internal diffusion control).
Heat Transfer Coefficient
The heat transfer coefficient (h) is a proportionality constant that quantifies the rate of heat transfer per unit area per unit temperature difference between a surface and a fluid in contact with it. It combines the effects of conduction through the fluid boundary layer and convection driven by fluid motion, making it central to the design of heat exchangers, reactors, and process equipment. Higher values indicate more efficient heat transfer, and the coefficient depends strongly on fluid properties, flow velocity, geometry, and surface roughness.
Process Control (engineering)
Process control is the engineering discipline concerned with maintaining process variables (temperature, pressure, flow rate, composition) at desired setpoints by manipulating control variables through feedback and feedforward control strategies. A typical feedback control loop consists of a sensor, controller (commonly PID), and final control element (valve or pump) that continuously corrects deviations from setpoint. It is essential in chemical plants, oil refineries, pharmaceutical manufacturing, and food processing to ensure product quality, process safety, and energy efficiency.
"Evaporation" from Latin "evaporatio", from "evaporare" (to disperse in vapour), combining "e-" (out) + "vapor" (steam, vapour). Industrial evaporation was industrialised in the 19th century; Norbert Rillieux (1843) invented the multiple-effect evaporator, dramatically improving sugar refinery efficiency and earning him recognition as a pioneering chemical engineer.