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ENGR5002 / Week 10
ENGR263 2017 Q A1Past paperOld spec ENGR2632:125 marks30 min

ENGR263 Process of Mass and Heat Transfers 2017 Section A Q A1[VALID] official answers

Answer ALL parts (a) - (b).

Formulas you may need
  • Flux with bulk flow: NA,z=−cDABdyAdz+yA(NA,z+NB,z)N_{A,z} = -cD_{AB}\dfrac{dy_A}{dz} + y_A(N_{A,z} + N_{B,z}) (on the formula sheet)
  • Diffusion through a stagnant gas: NA=PDABRTδ pA1−pA2pB,lm=PDABRTδln⁡P−pA2P−pA1N_A = \dfrac{PD_{AB}}{RT\delta}\,\dfrac{p_{A1} - p_{A2}}{p_{B,lm}} = \dfrac{PD_{AB}}{RT\delta}\ln\dfrac{P - p_{A2}}{P - p_{A1}}, pB,lm=pB2−pB1ln⁡(pB2/pB1)p_{B,lm} = \dfrac{p_{B2} - p_{B1}}{\ln(p_{B2}/p_{B1})} (on the formula sheet)
  • Mass transfer coefficient: NA=kc(cA1−cA2)N_A = k_c(c_{A1} - c_{A2}), ideal gas cA=pA/(RT)c_A = p_A/(RT) (on the formula sheet)
  • Film theory: stagnant film kc=DABδ xB,lmk_c = \dfrac{D_{AB}}{\delta\,x_{B,lm}}; equimolar or dilute kc=DABδk_c = \dfrac{D_{AB}}{\delta} (learn this)
  1. (a(i))
    What arguments are used for eliminating the flux NBN_B of component B when a component A diffuses in a stagnant medium B?
    [3]Third
  2. (a(ii))
    Suggest an experimental method in which the diffusivity DABD_{AB} of component A in gaseous B could be measured.
    [3]Third
  3. (a(iii))
    Give conditions, in which the concentration of a diffusing component follows: 1. An exponential profile in the film of interphase (1 mark). 2. A linear profile in the film of interphase (1 mark).
    [2]Third
  4. (b(i))
    In a packed column, operating at approximately 1.5×105 Pa1.5 \times 10^5\ \mathrm{Pa} and 303 K, air-H2_2S mixture is scrubbed with water. It is assumed that the whole of the resistance to mass transfer may be regarded as lying within a thin laminar film on the gas side of the gas-liquid interphase. At some intermediate point, the partial pressure of H2_2S in the gas phase has been reduced to 5000 Pa while the partial pressure of H2_2S in equilibrium with the aqueous solution is 1233 Pa. Derive an expression at any position in the column for the rate of absorption of H2_2S by using the first principles of molecular diffusion.
    [5]
  5. (b(ii))
    Derive expression of mass transfer coefficient (kck_c) based on concentration. Calculate the transfer flux of absorption of H2_2S (mol m−2 s−1\mathrm{mol\,m^{-2}\,s^{-1}}), assuming that value of mass transfer coefficient is 0.96 m s−10.96\ \mathrm{m\,s^{-1}}.
    [5]
  6. (b(iii))
    Calculate the thickness of the hypothetical gas film if the diffusivity of H2_2S in air is 0.24 cm2 s−10.24\ \mathrm{cm^2\,s^{-1}}. (Data: R=8.314 J mol−1 K−1R = 8.314\ \mathrm{J\,mol^{-1}\,K^{-1}}.)
    [4]
  7. (b(iv))
    What would be the thickness of the hypothetical gas film if concentration of H2_2S was assumed negligible compared to concentration of air?
    [3]