Absorption Column Height Calculator
Determines the required packed bed height for a gas absorption column based on gas flow rate, inlet/outlet concentrations, and mass transfer coefficient. Used by chemical engineers designing scrubbers and stripping columns.
About this calculator
The height of a packed absorption column is determined by the number of transfer units (NTU) and the height of a transfer unit (HTU). The overall column height Z = HTU × NTU, where HTU = G / (K_ya × A) and NTU is derived from the log-mean driving force between inlet and outlet concentrations. The formula simplifies to Z = (G / K_ya) × ln(C_in / C_out), where G is the molar gas flow rate, K_ya is the overall volumetric mass transfer coefficient (kmol/m³·h), and C_in and C_out are the inlet and outlet mole fractions. A higher mass transfer coefficient or smaller required concentration change reduces the column height needed. Packing type affects K_ya and pressure drop but is used separately to look up or correct the coefficient.
How to use
Suppose you need to absorb a pollutant from a gas stream: G = 500 m³/h, inlet concentration C_in = 0.05 mol/mol, outlet concentration C_out = 0.005 mol/mol, and K_ya = 25 kmol/m³·h. Step 1: Compute the log ratio — ln(0.05 / 0.005) = ln(10) ≈ 2.303. Step 2: Compute HTU = G / K_ya = 500 / 25 = 20 m. Step 3: Column height Z = HTU × NTU = 20 × 2.303 ≈ 46.1 m. This tells you that a 46-metre packed bed is required to achieve a 90% reduction in gas-phase concentration under these operating conditions.
Frequently asked questions
What is the number of transfer units (NTU) in gas absorption and how is it calculated?
NTU is a dimensionless measure of the difficulty of the separation. It equals the integral of dC/(C − C*) over the concentration range, which for a dilute system with a straight operating line simplifies to ln(C_in / C_out). A larger NTU means a harder separation requiring a taller column. NTU is independent of equipment geometry and depends only on concentrations and equilibrium.
How does the overall mass transfer coefficient K_ya affect absorption column height?
K_ya (kmol/m³·h) combines the gas-phase and liquid-phase resistances into a single volumetric coefficient. A higher K_ya means each cubic metre of packing transfers more solute per hour, so the required column height decreases proportionally. K_ya depends strongly on packing type, liquid and gas flow rates, and the physical properties of the gas-liquid system. Structured packings typically yield higher K_ya values than random dumped packings.
When should I use a packed column versus a plate column for gas absorption?
Packed columns are preferred for corrosive systems, low-pressure-drop requirements, small-to-medium diameters, and systems prone to foaming, because the continuous liquid film minimises agitation. Plate columns are better for large-diameter towers, feeds with solids, or when side draws are needed. Packed columns also handle liquid-to-gas ratio variations more gracefully. The choice ultimately depends on throughput, fouling tendency, and capital cost trade-offs.