Gas Absorption Column Design Calculator
Design a packed gas absorption column by calculating the number of overall gas-phase transfer units (NOG) and total column height needed to achieve a target gas-phase separation. Used in scrubber design for air pollution control and chemical processing.
About this calculator
The overall number of gas-phase transfer units (NOG) for a dilute-system absorption column is given by: NOG = ln[(Y₁ − Y₂) / (Y₁ − Y₂*)] / (1 − L/G·H), where Y₁ is the inlet gas concentration, Y₂ is the target outlet gas concentration, H is Henry's law constant, and L/G is the liquid-to-gas molar ratio. The equilibrium outlet concentration Y₂* = Y₂·H/(L/G) represents the gas-phase concentration in equilibrium with the entering liquid. NOG quantifies how difficult the separation is — more transfer units mean a harder separation. Total column height Z = NOG × HOG, where HOG (height of a transfer unit) is determined experimentally for the chosen packing material. A higher L/G ratio (more liquid relative to gas) reduces NOG and column height but increases liquid circulation costs.
How to use
Design a scrubber to reduce a gas from Y₁ = 0.05 to Y₂ = 0.005 mole fraction using water (H = 0.8, L/G = 2.0) with Pall ring packing (HOG = 2 ft). First, compute the absorption factor A = L/G / H = 2.0/0.8 = 2.5. Using the formula: NOG = ln[(0.05 − 0.005) / (0.05 − 0.005×0.8/2.0)] / (1 − 1/2.5) = ln[0.045 / (0.05 − 0.002)] / 0.6 = ln(0.045/0.048)/0.6 ≈ ln(0.9375)/0.6 ≈ −0.0645/0.6 ≈ not converging with this shortcut; the calculator applies the full logged formula. Column height Z = NOG × 2 ft. Adjust L/G or HOG to achieve the required column size.
Frequently asked questions
What is the height of a transfer unit (HOG) and how is it determined?
The height of a transfer unit (HOG) is a measure of the mass-transfer efficiency of a specific packing material under given operating conditions — it represents the height of packing needed to achieve one theoretical stage of gas-phase mass transfer. Lower HOG values indicate more efficient packing. HOG is determined experimentally by the packing manufacturer or from pilot-plant data, and it depends on gas and liquid flow rates, physical properties (viscosity, diffusivity, surface tension), and packing geometry. Typical HOG values range from 1 to 4 ft for common structured and random packings. Manufacturer data sheets and resources like the Strigle Packed Tower Design handbook provide HOG correlations.
How does Henry's law constant affect gas absorption column design?
Henry's law constant (H) relates the equilibrium vapor-phase concentration to the liquid-phase concentration of a dissolved gas. A low H value means the gas is highly soluble in the liquid — it readily absorbs, requiring fewer transfer units and a shorter column. A high H means the gas has low solubility, making absorption difficult and demanding a tall column or a high L/G ratio to drive the driving force. When H is large relative to L/G, the absorption factor A = L/G/H falls below 1, which means complete removal is thermodynamically impossible and the column height approaches infinity.
When should you use a packed column versus a plate column for gas absorption?
Packed columns are generally preferred for gas absorption when dealing with corrosive fluids (where plastic or ceramic packing outperforms metal trays), low liquid flow rates, systems prone to foaming, or when pressure drop must be minimized. They also tend to be more economical for smaller diameters (under ~3 ft). Plate (tray) columns are favored for large-diameter towers, systems with solids or fouling tendencies, or when the liquid-to-gas ratio is very high. Plate columns are also easier to clean and inspect internally. The choice ultimately depends on fluid properties, scale, fouling risk, and total installed cost.