CHEMICAL ENGINEERING – MASS TRANSFER
1. Hydrogen gas at a pressure of 1.25 bar is contained in a thick-walled neoprene rubber sphere which has inner and outer radii of 70 mm and 80 mm, respectively. The concentration of hydrogen at the outer surface of the sphere is negligible and leakage is so small that steady-state conditions can be assumed to occur for a long time. The solubility of hydrogen in rubber is 2.37 × 10–3 kmol m–3 (bar)–1 and the diffusivity is 1.8 × 10–10 m2 s–1. Calculate the rate at which hydrogen escapes from the sphere. (Assume the inner surface of rubber is saturated with hydrogen.) 2. (a) Ammonia gas is being absorbed by water in a wetted-wall column. At one level of the column, the following data applies: gas-phase mass transfer coefficient 5.22 × 10–9 kmol m–2 s–1 Pa–1 liquid-phase mass transfer coefficient 3.88 × 10–5 m s–1 Henry’s constant 0.955 kPa (kmol m–3)–1. Estimate the overall mass transfer coefficient KL. (b) What is the ratio of the individual mass transfer resistances in Question 2 (a)? (c) Use the following additional information to find the mass transfer flux in the column: mole fraction of ammonia in liquid* 1.351 × 10–3 mole fraction of ammonia in gas* 0.065 total pressure of system 1.013 bar mole mass of ammonia 17 *Note: these values will need to be converted to concentrations in kmol m–3.
(d) What is the partial pressure of the ammonia gas at the gas/liquid interface? (e) What is the molar concentration of the ammonia in the liquid at the interface? 3. Based on your experience or from reference books, choose a real industrial example of mass transfer which uses a column as its contacting device. Suggest reasons why the particular column was used in preference to other designs.
You will need two sheets of graph paper to complete 4. (a) Assuming that Raoult’s law applies to all possible mixtures of two components, ‘A’ and ‘B’, plot a “pressure/composition” diagram at constant temperature T for the full range of mixtures from the data below.