Introduction to Heat Transfer, Evaporation & Distillation
Controlling heat transfer is fundamental to every pharmaceutical manufacturing operation. Evaporation concentrates drug solutions (syrups, extracts). Distillation purifies solvents and isolates active ingredients. Understanding these unit operations is essential for process engineers and industrial pharmacists.
Syllabus & Topics
- 1Heat Transfer: Definition; Objectives (heat sterilization, evaporation, drying, distillation).
- 2Mechanism 1 – Conduction: Heat transfer through solid by molecular vibration. Fourier’s Law: Q/t = -kA(dT/dx). k = thermal conductivity.
- 3Mechanism 2 – Convection: Heat transfer through fluid motion. Natural convection (density differences). Forced convection (pump/fan). Newton’s law of cooling: Q = hA(T-Ts).
- 4Mechanism 3 – Radiation: Heat transfer as electromagnetic waves (infrared). Stefan-Boltzmann law: Q ∝ T⁴. In IR sterilization of ampoules.
- 5Heat Interchangers: Jacketed vessels. Heat Exchangers: Shell-and-tube, Plate heat exchanger – used for pharmaceutical liquid processing.
- 6Evaporation: Definition – Removal of solvent (water) from solution by application of heat, leaving behind concentrated solute. Differences from other heat processes (drying, distillation, crystallization).
- 7Factors Influencing Evaporation: Temperature, concentration, surface area, nature of liquid (heat-sensitive?), pressure (vacuum reduces boiling point).
- 8Evaporation Equipment:
- 9Steam Jacketed Kettle: Simplest evaporator; steam in jacket heats liquid; good for viscous liquids (ointments, syrups); batch process.
- 10Horizontal Tube Evaporator: Steam inside horizontal tubes; natural circulation; suitable for non-viscous liquids; scales easily.
- 11Climbing Film (Rising Film) Evaporator: Liquid rises through vertical tubes by vapor generation; continuous; suitable for heat-sensitive products (antibiotics, milk).
- 12Falling Film Evaporator: Liquid falls as thin film down tubes; even less heat exposure; for most sensitive products.
- 13Forced Circulation Evaporator: Pump circulates liquid; handles viscous/crystallizing solutions; reduced scaling.
- 14Multiple Effect Evaporator: Steam economy. In a triple-effect: steam from 1st effect heats 2nd, steam from 2nd heats 3rd. Economy ≈ n-1 kg water evaporated per kg steam (triple effect ≈ 2.5). Pressure decreases from 1st to last effect.
- 15Economy of Multiple Effect Evaporation: Economy = kg water evaporated / kg steam used. Single effect ≈ 1; triple effect ≈ 2.5; quadruple effect ≈ 3.5.
- 16Distillation: Definition – Separation based on differences in boiling points (volatility).
- 17Simple Distillation: One-step vaporization and condensation; separates liquids with large boiling point differences (>25°C); used for purification of water.
- 18Flash Distillation: Rapid vaporization when heated liquid pressure is suddenly reduced.
- 19Fractional Distillation: Fractionating column (plates/packing) for multiple equilibration stages; separates liquids with close boiling points; used for ethanol purification, petroleum refining.
- 20Distillation Under Reduced Pressure (Vacuum Distillation): Reduces boiling point; used for heat-sensitive materials (glycerol 290°C → distilled under vacuum at lower temp).
- 21Steam Distillation: Steam passes through water-immiscible material; both components vaporize; mixture boils below 100°C; used for volatile oils (peppermint oil, turpentine from dried bitter orange peel).
- 22Molecular Distillation: Extremely high vacuum (0.001 mmHg); mean free path of molecules exceeds the distillation path; no equilibrium; used for heat-sensitive vitamins (Vitamin A, Vitamin E) and fatty acids.
Learning Objectives
Frequently Asked Questions (FAQs)
Q1. What is the Economy of Multiple Effect Evaporation?
Economy = kg of water evaporated / kg of steam used. In a triple-effect evaporator, steam generated in Effect 1 is used to heat Effect 2, and steam from Effect 2 heats Effect 3. This results in approximately 2.5–3 kg of water evaporated per kg of steam, compared to 1 kg per kg in a single-effect evaporator, significantly reducing energy costs.
Q2. Why is Climbing Film Evaporator Suitable for Heat-Sensitive Products?
In a climbing film evaporator, liquid makes a single pass through the heated tubes, resulting in very short residence time and minimal heat exposure. The rising vapor film provides agitation, improving heat transfer efficiency. It is used for heat-sensitive materials such as antibiotics, juices, and milk products where prolonged heating may cause degradation.
Q3. What is Steam Distillation and When is it Used?
Steam distillation is used for distilling water-immiscible substances that would decompose at their normal boiling points. When two immiscible liquids are present, the mixture boils at a temperature lower than the boiling point of either component (Dalton’s Law). It is commonly used to extract volatile oils from plant materials, such as peppermint oil, eucalyptus oil, and rose oil, preserving delicate aromatic compounds.
Q4. What is Molecular Distillation?
Molecular distillation is carried out under extremely high vacuum (0.001–0.01 mmHg), where the mean free path of molecules exceeds the distance between the evaporating and condensing surfaces. There is no equilibrium stage; molecules travel directly from the evaporating surface to the condenser. It is used for heat-sensitive substances such as Vitamin A (from fish liver oil), Vitamin E (tocopherol concentrates), plasticizers, and fatty acids.
Q5. What are the Three Mechanisms of Heat Transfer?
(1) Conduction: Transfer of heat through solids by molecular vibration, governed by Fourier’s law. (2) Convection: Transfer of heat through fluid motion (natural or forced), governed by Newton’s law of cooling. (3) Radiation: Transfer of heat through electromagnetic waves (infrared), requiring no medium, governed by the Stefan–Boltzmann law (Q ∝ T⁴). In pharmaceutical processes, all three mechanisms may operate simultaneously.