ENGR263 2018 Q A3(a) (ENGR5002)

ENGR263 2018 Q A3(a)Past paperOld spec ENGR2632:215 marks18 min

ENGR263 Processes of Heat and Mass Transfers 2018 Section A Q A3[PARTIAL] Left out: Part (b) (10 marks), fitting the Lennard-Jones parameters sigma and epsilon to D(T) data for water vapour in oxygen using a collision-integral chart, is marked "unsure" in the audit (chart-based parameter fitting is not in the current slides), so it is left out.

Answer ALL parts (a) - (b). [Part (b), 10 marks, fitting Lennard-Jones parameters from diffusivity data with a collision-integral chart, is omitted.]

Formulas you may need

Wilke-Chang (liquids): $\dfrac{D_{AB}\mu_B}{T} = \dfrac{7.4 \times 10^{-8}(\Phi_BM_B)^{1/2}}{V_A^{0.6}}$ (on the formula sheet)
Effective diffusivity in a porous solid: $D_{eff} = \dfrac{\varepsilon}{\tau}D_{AB}$ ; Knudsen diffusivity $D_K = \dfrac{d_{pore}}{3}\sqrt{\dfrac{8RT}{\pi M_A}}$ (learn this)
Film theory $k_c = D_{AB}/\delta$ ; penetration theory $k_c = 2\sqrt{D_{AB}/(\pi t_e)}$ (learn this)
Unsteady diffusion into a semi-infinite medium $\dfrac{c_A - c_{A0}}{c_{As} - c_{A0}} = \mathrm{erfc}\left(\dfrac{z}{2\sqrt{D_{AB}t}}\right)$ (on the formula sheet)

(a(i))
How does the diffusion coefficient of liquid species vary with the molecular weight, temperature and viscosity of the liquid medium?
[3]
(a(ii))
How does the diffusivity vary with the porosity of a solid medium (1 mark), with the tortuosity of a solid medium (1 mark), with the diameter of the pores (1 mark)?
[3]
(a(iii))
On which side of the gas/liquid interphase boundary would the mass transfer resistance take place in the following processes? An evaporation of a liquid through a stagnant gas phase (1 mark); an absorption of a gas by a liquid of low velocity (1 mark); a continuous distillation column (1 mark).
[3]
(a(iv))
Discuss the relevance of the mass transfer coefficient for unit operations.
[3]
(a(v))
A thin liquid film is falling down one side of a vertical surface wall of an absorption column at steady-state conditions. Discuss qualitatively (no need for relevant mathematical models) the two cases of long-residence time and short-residence time and their effects on profiles of concentration through the liquid film.
[3]2:1