Govt. Exams
Entrance Exams
Biot number = hL_c/k determines whether lumped capacitance method is applicable. Bi << 0.1 indicates uniform internal temperature.
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.
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.
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.
According to penetration theory, k_c = 2√(D_AB/(πt)), showing k_c is inversely proportional to √t. Shorter contact times give higher coefficients.
For first-order reaction in a film, the Thiele modulus φ = δ√(k₁/D_AB), which compares reaction rate to diffusion rate. It determines whether the process is reaction-controlled or diffusion-controlled.
Murphree plate efficiency accounts for non-ideal mixing on a plate, therefore it is always less than or equal to the overall efficiency which includes effects of all plates.
The vertical distance between operating and equilibrium lines represents the concentration driving force, determining whether absorption occurs and in which direction.
High solubility of A means low liquid-phase resistance for A, making the gas-phase resistance dominant. Thus, gas-phase mass transfer resistance controls the overall rate for A.
