Typical van Deemter plot:

Therefore, owing to compressibility of the carrier gas, u is not constant over the whole length of the column; therefore, only a small section could operate at maximum efficiency. Here, the van Deemter equation is being presented in an expanded form. The significance of the terms is the similar as Unit 4.

The expanded edition of the van Deemter equation is:

HETP = 2λd_p + 2γD_gas/ u+ (8k ′d²f)/ (π² (1 + k ')²D_liq)/u

where,

λ = a parameter that is a measure of packing irregularities,

d_p= diameter of packing particles,

γ = a alteration factor accounting for the tortuosity of the gas channels within the column,

d_f = effective film thickness, a measure of liquid loading of packing material,

D_liq = diffusion coefficient of the solute within the liquid phase,

D_gas = diffusivity of the solute in the gas phase,

u = the linear gas velocity, and

k´= capacity factor = k (F_liq/F_gas).

Figure:  A typical van Deemter plot showing the effect of eddy diffusion (A), longitudinal diffusion (B/u) and mass transfer for both phases (Cu)