Distillation
Distillation is a thermal separation process that exploits differences in the volatility (relative volatility) of mixture components: a liquid feed is partially vaporised, the vapour enriched in the more volatile component rises and is condensed, while the less volatile component concentrates in the liquid bottoms. In a continuous distillation column, repeated vapour-liquid equilibrium stages—either trays or structured packing—progressively sharpen the separation, with the reflux ratio governing the trade-off between product purity and energy consumption. It is the most widely used separation process in the petrochemical, pharmaceutical, and food industries.
Key Formula
alpha_AB = (y_A/x_A) / (y_B/x_B) = P_A_sat / P_B_sat
LaTeX: \alpha_{AB} = \frac{y_A / x_A}{y_B / x_B} = \frac{P_A^{sat}}{P_B^{sat}}
| Symbol | Meaning | Unit |
|---|---|---|
| \alpha_{AB} | Relative volatility of A with respect to B | dimensionless |
| y_A | Mole fraction of A in vapour phase | dimensionless |
| x_A | Mole fraction of A in liquid phase | dimensionless |
| P_A^{sat} | Saturation (vapour) pressure of pure A | Pa |
| P_B^{sat} | Saturation (vapour) pressure of pure B | Pa |
Worked Example
Problem
At 78°C, the saturation pressures of ethanol and water are 101.3 kPa and 43.0 kPa respectively. Calculate the relative volatility of ethanol (A) with respect to water (B) and the vapour mole fraction of ethanol in equilibrium with a liquid containing x_A = 0.40.
Solution
Step 1: Relative volatility α = P_A_sat / P_B_sat = 101.3 / 43.0 = 2.356 Step 2: Use the equilibrium relation y_A = α·x_A / [1 + (α−1)·x_A] y_A = 2.356 × 0.40 / [1 + (2.356−1) × 0.40] y_A = 0.9424 / [1 + 0.5424] y_A = 0.9424 / 1.5424 = 0.611
Answer
α = 2.356; y_A = 0.611 (61.1 mol% ethanol in vapour)
Comparison of Distillation Column Internals
| Internal Type | Pressure Drop | Efficiency | Capacity | Cost |
|---|---|---|---|---|
| Sieve tray | Moderate | 60–80% | High | Low |
| Valve tray | Moderate | 70–85% | High | Moderate |
| Bubble-cap tray | High | 70–80% | Moderate | High |
| Random packing | Low | 75–90% | Moderate | Low |
| Structured packing | Very low | 90–98% | High | High |
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Related Terms
Mass Balance (chemical engineering)
A mass balance (also called a material balance) is the systematic application of the law of conservation of mass to a defined control volume or process unit, accounting for all mass entering, leaving, generated by reaction, and accumulating within the system. It is the foundational tool of chemical process design, enabling engineers to size equipment, determine conversion, specify recycle streams, and detect leaks or unaccounted losses. At steady state with no reaction, the balance simplifies to: mass in = mass out.
Energy Balance (chemical engineering)
An energy balance is the application of the first law of thermodynamics to a process system, tracking all energy entering, leaving, generated, and stored within a defined control volume in the forms of enthalpy, heat, work, and kinetic/potential energy. For steady-state, open flow systems (the most common case in chemical plants), the balance relates the enthalpy change of process streams to the net heat added and shaft work. Energy balances are essential for designing heat exchangers, reactors, distillation columns, and assessing process efficiency.
Absorption Column
An absorption column (absorber) is a mass-transfer device in which a gas mixture flows upward counter-currently against a descending liquid solvent, causing one or more gaseous components to dissolve into the liquid phase driven by a concentration gradient and governed by vapour-liquid equilibrium. The height of the packed or trayed column is determined by the Number of Transfer Units (NTU) and the Height of a Transfer Unit (HTU), or by the number of theoretical stages. Absorption is widely used to remove acid gases (CO₂, H₂S) from natural gas, SO₂ from flue gas, and ammonia from industrial off-gas streams.
From Latin "distillatio" (a dripping down), from "de-" (down) + "stillare" (to drip). The process was known to ancient alchemists; the word entered scientific English in the 14th century. Industrial continuous distillation developed in the early 19th century with the invention of the continuous column still by Aeneas Coffey (1831).