Govt. Exams
Entrance Exams
In membrane distillation, vapor pressure difference drives transport through pores. Vacuum or pressure difference across the hydrophobic membrane promotes vapor transport while preventing liquid penetration.
L/G ratio determines the operating line slope in McCabe-Thiele diagram. Proper L/G balances absorption efficiency and prevents flooding/weeping, critical for scale-up design.
Marangoni convection and internal circulation in liquid droplets enhance mass transfer by reducing the effective liquid-film resistance, increasing overall k_c compared to rigid spheres.
RO flux J = (k_m/μ)·ΔP/Δx, where flux is directly proportional to pressure difference and inversely to membrane thickness, following modified Darcy's equation.
Lewis number Le = Sc/Pr = (ν/D_AB)/(α/ν) = ν/(D_AB·α). For air, Le ≈ 1, meaning heat and mass transfer are equally significant in coupled processes.
Selectivity = (k_l,A/k_l,B)·(H_B/H_A). While diffusivity ratio affects k_l, the Henry's constant ratio H_B/H_A determines preferential absorption, making partition coefficients critical.
When gas-film resistance is negligible, K_g ≈ k_l/H where H is Henry's constant. The liquid-phase resistance becomes dominant in overall mass transfer.
Knudsen diffusion dominates when molecular mean free path λ >> pore diameter d_p, causing molecules to collide with walls more than with other molecules, typical in microporous materials.
During constant rate drying period, surface moisture is continuously replenished from inside, so the external convective mass transfer from wet surface to air is the limiting step.
NTU accounts for changing concentrations along the column height and is more versatile for varying flow rates, while HTU is better for constant flow conditions with linear equilibrium.
