An absorber is a different animal from a still. Nothing is boiling; a solute is being washed out of a gas by a liquid that will dissolve it. Three numbers size one, in order, and the order matters.

The absorption factor is the gatekeeper: $A = \dfrac{L}{m V}$ — A equals L over m V. $L$ is the solvent molar flow and $V$ the carrier gas molar flow, both in mol/s and both molar, because $m$ is the slope of a line drawn in mole fractions. $m$ is that equilibrium slope, from Henry. $A$ is bare, and it compares the liquid's capacity to carry the solute away against the gas's capacity to deliver it.

Here is the whole lesson in one sentence. Below $A = 1$, no height of packing finishes the job. The solvent leaves saturated before it has taken up all the solute on offer, so a fraction of the feed walks straight through however tall the tower is. That is a hard limit, not a poor design, and the fix is more solvent, a colder column or a different solvent — never more packing. Above 1 it works, and the economic window is 1.2 to 2.0: past that, solvent circulation, pumping and above all regeneration duty climb without buying much separation, and regeneration is usually the dominant operating cost of the whole plant. The same group inverted, $1/A$, is the stripping factor, and a stripper is designed by requiring that to exceed 1 instead.

The count, for a dilute system with a straight equilibrium line, is Colburn's: $N_{OG} = \dfrac{\ln\left[\dfrac{y_1}{y_2}\left(1 - \dfrac{1}{A}\right) + \dfrac{1}{A}\right]}{1 - \dfrac{1}{A}}$. $y_1$ is the solute mole fraction entering at the bottom, $y_2$ the fraction leaving at the top — subscript 1 is the dirty end, 2 the clean one — and only their RATIO enters. $N_{OG}$ is the number of transfer units, a bare count of e-folds of driving force.

The metres: $Z = H_{OG} N_{OG}$, where $Z$ is the depth of packing and $H_{OG}$ the height of a transfer unit, in metres, typically 0.3 to 1 m. The split of labour is the reason it is written this way. NTU is thermodynamics — the equilibrium, the flows, the separation demanded — and no supplier can change it. HTU is hardware, and it is exactly what the supplier sells. Better packing shortens the column; a purer product lengthens it. Add height for distributors, supports and disengagement, which is not packing but is very much column — and beyond roughly 5 to 10 m the liquid has drifted to the wall and the bed needs a redistributor, so a tall answer is beds, not one pour.