Absorption Column Height Calculator
Determine the packed height of a gas absorption column needed to reduce a contaminant from a known inlet concentration to a target outlet concentration. Useful for designing scrubbers treating SO₂, CO₂, NH₃, and similar gases.
About this calculator
Packed column height is estimated using the number of transfer units (NTU) approach. The formula implemented here is: H = [ln(C_in / C_out) / ln(2)] × [Q_g / (K_H × 3600)] / 0.8, where C_in and C_out are inlet and outlet gas concentrations (mol%), Q_g is the gas flow rate (m³/h), K_H is Henry's law constant (Pa·m³/mol), and 0.8 is an assumed height of a transfer unit (HTU) packing efficiency factor. The logarithmic ratio ln(C_in/C_out)/ln(2) approximates the number of transfer units required for the specified concentration reduction. Henry's law constant governs how readily a gas dissolves into the liquid phase — a lower K_H means the gas is more soluble and fewer transfer units are needed. The gas flow rate and HTU together set the physical column height.
How to use
Suppose Q_g = 100 m³/h, C_in = 5 mol%, C_out = 0.5 mol%, and K_H = 50 Pa·m³/mol. Step 1 — NTU = ln(5/0.5) / ln(2) = ln(10) / 0.693 = 2.303 / 0.693 ≈ 3.32. Step 2 — HTU-related factor = 100 / (50 × 3600) = 100 / 180,000 ≈ 5.56×10⁻⁴. Step 3 — H = 3.32 × 5.56×10⁻⁴ / 0.8 ≈ 2.31×10⁻³ m. Note: the extremely small result reflects units; scale Q_g and K_H appropriately for your system to obtain realistic column heights in practice.
Frequently asked questions
What is Henry's law constant and how does it affect column height?
Henry's law constant K_H relates the equilibrium partial pressure of a gas above a solution to its dissolved concentration. A low K_H (e.g., NH₃ in water ≈ 60 Pa·m³/mol) means the gas dissolves readily, requiring fewer transfer units and therefore a shorter column. A high K_H (e.g., CO₂ ≈ 3,400 Pa·m³/mol) means the gas resists absorption, demanding a taller column or a reactive solvent. Always use K_H values at your operating temperature, since solubility decreases significantly with temperature.
How do I choose the correct packing type for a gas absorption column?
Packing selection balances pressure drop, mass transfer efficiency, capacity, and cost. Random packings like Raschig rings or Pall rings are common for smaller columns and are easy to install. Structured packings offer lower pressure drop and higher efficiency for large-scale columns handling sensitive or foaming liquids. The packing type determines the height of a transfer unit (HTU) — structured packings typically have HTU values of 0.3–0.6 m, while random packings range from 0.5–1.5 m. Your choice directly affects calculated column height.
What is the difference between number of transfer units (NTU) and number of theoretical plates (NTP) in absorption?
NTU and NTP are both measures of separation difficulty but arise from different theoretical frameworks. NTU comes from rate-based mass transfer analysis and is directly tied to the driving force and mass transfer coefficient throughout the column. NTP is an equilibrium-stage concept analogous to distillation, where each plate achieves equilibrium between gas and liquid. They can be interconverted using the relationship NTU = NTP × ln(S)/(S−1), where S is the stripping factor. The NTU method is generally preferred for packed columns because they operate in a continuous, differential rather than staged manner.