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.
Key Formula
Z = N_OG * H_OG, where N_OG = integral from y2 to y1 of dy / (y - y*)
LaTeX: Z = N_{OG} \times H_{OG}, \quad N_{OG} = \int_{y_2}^{y_1} \frac{dy}{y - y^*}
| Symbol | Meaning | Unit |
|---|---|---|
| Z | Total packing height required | m |
| N_{OG} | Number of overall gas-phase transfer units | dimensionless |
| H_{OG} | Height of an overall gas-phase transfer unit | m |
| y | Mole fraction of solute in gas phase | dimensionless |
| y^* | Equilibrium mole fraction of solute in gas | dimensionless |
Worked Example
Problem
An absorption column removes CO₂ from a gas stream. The inlet gas contains y₁ = 0.10 mol fraction CO₂ and the outlet specification is y₂ = 0.01. The equilibrium relation is y* = 1.2x. Using a dilute-system approximation (log-mean driving force), N_OG ≈ (y₁ − y₂) / ΔY_lm. Given ΔY_lm = 0.04, and H_OG = 0.5 m, find the required packing height.
Solution
Step 1: Calculate N_OG. N_OG = (y₁ − y₂) / ΔY_lm = (0.10 − 0.01) / 0.04 = 0.09 / 0.04 = 2.25 Step 2: Calculate packing height. Z = N_OG × H_OG = 2.25 × 0.5 = 1.125 m
Answer
Required packing height Z = 1.125 m ≈ 1.13 m
Common Gas-Liquid Absorption Systems in Industry
| Gas Removed | Solvent | Industry | Driving Force | Column Type |
|---|---|---|---|---|
| CO₂, H₂S | MEA / MDEA amine | Natural gas processing | Chemical reaction | Packed |
| SO₂ | Water / NaOH solution | Power plant flue gas | Physical + chemical | Packed |
| NH₃ | Water | Fertiliser plants | Physical | Trayed or packed |
| HCl | Water | Chemical manufacturing | Physical | Packed |
| CO₂ | Chilled methanol (Rectisol) | Syngas treatment | Physical | Packed |
Interactive Tools
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Related Terms
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.
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.
Liquid-Liquid Extraction
Liquid-liquid extraction (LLE), also called solvent extraction, is a separation technique that transfers one or more solutes from a feed liquid phase into an immiscible or partially miscible solvent phase based on differences in solute solubility (expressed as a distribution coefficient or partition coefficient). The process is carried out in mixer-settlers, pulse columns, rotating disc contactors, or centrifugal extractors, and is especially valuable when distillation is impractical due to azeotropes, heat-sensitive solutes, or low solute concentrations. LLE is central to hydrometallurgy, pharmaceutical purification, nuclear fuel reprocessing, and edible-oil refining.
From Latin "absorptio" (a swallowing up), from "ab-" (away) + "sorbere" (to suck in). "Column" from Latin "columna" (pillar). Industrial gas absorption columns were systematised in the late 19th century with the development of contact-process sulfuric acid plants; the transfer-unit framework was formalised by Chilton and Colburn in 1935.