Height of a Transfer Unit (HTU) characterizes the mass-transfer efficiency of packing material. Lower HTU means more efficient packing. It is typically determined from vendor data or pilot-plant tests, in units of meters or feet.

Number of Transfer Units (NTU) quantifies the difficulty of the separation — how far the actual composition must move relative to equilibrium. NTU is a dimensionless integral over the operating range.

What is the equilibrium constant m?

For dilute systems, the equilibrium relationship is often written y* = m·x (Henry's law or a linearized VLE). m is the slope of the equilibrium line on a y-x diagram.

How is NTU calculated here?

For gas-phase controlling absorption, NTU = ∫(dy / (y − y*)) from y_out to y_in. This tool evaluates the integral using Simpson's rule with 200 sub-intervals for accuracy across the full range.

What is the absorption factor A?

A = L / (m·G), where L is molar liquid rate and G is molar gas rate. A > 1 means operation is above the minimum L/G; A < 1 can lead to infeasible separation. Typical design target is A = 1.2–1.5.

Can I use mole fractions above 0.1?

The dilute-system (straight operating and equilibrium lines) assumption breaks down above roughly 10 mol%. For concentrated systems, use rigorous stage-by-stage simulation software; treat this result as an estimate.

Packed Tower Calculator

Compute HTU, NTU, and required packing height for gas absorption or liquid stripping packed towers.

Tower Parameters

Operation Type

Controlling Resistance

Key Component Compositions (mole fraction)

Gas inlet y_in

Gas outlet y_out

Liquid inlet x_in

Equilibrium constant m

Flow Rates (any consistent molar units)

Gas rate G

Liquid rate L

Packing Data

HTU_OG (m)

From vendor data or mass-transfer correlation

Results

Fill in parameters and click Calculate

NTU Integration Points (10 intervals shown)

y	y*	y − y*	1/(y − y*)

Summary

This packed tower calculator determines the height of transfer units (HTU), number of transfer units (NTU), and required packing height for gas absorption or liquid stripping columns. It applies the standard dilute-system analytical formula and a numerical integral for general cases. Enter flow rates, compositions, and the equilibrium constant to size your column packing quickly.

How it works

Select the operation type: absorption (gas into liquid) or stripping (liquid into gas).
Enter the inlet and outlet mole fractions or concentrations for the key component.
Provide the gas and liquid flow rates and the equilibrium constant (m = y*/x).
The calculator computes the operating line slope (L/G ratio), then integrates numerically to find NTU.
Enter your HTU (from mass-transfer correlations or vendor data) to get total packing height.

Use cases

Size packing height for a new gas absorption column.
Back-calculate NTU from an existing column to evaluate performance.
Compare HTU values from different packing vendors.
Estimate stripper height for VOC removal from process water.
Verify hand-calculated NTU results during design reviews.
Screen feasibility of absorption vs. adsorption for a given separation.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu