For the gas absorption, the height of a transfer unit, based on the gas phase is given by (G: superficial molar gas velocity, L: superficial molar liquid velocity, FG: mass transfer co-efficient, moles/m2, a: interfacial area per unit volume of tower)

• A. G/(FG . a)
• B. FG/(G . a)
• C. (G . a)/FG
• D. L/(FG . G)