Problem 2
Hydrogen stored in a vessel diffuses through the steel wall of 20 mm thickness. The molar concentration at the inner surface is 2 kg mol/m3 . At the other surface it is zero. Assuming plane wall condition and Dab = 0.26×10−12m2/s, determine the mass of hydrogen diffused per1m2 .
Problem 3
Hydrogen gas is maintained at 4 bar and 1 bar on the opposite sides of a membrane of 0.5 mm thickness. At this temperature the diffusion coefficient is 8.7×10−8m2/s. The solubility of hydrogen in the material which depends on the pressure is 1.5×103 m2/s bar. Determine the mass diffusion rate of hydrogen through the membrane.
Problem 4
Consider the diffusion of hydrogen (species A) in air, liquid water, or iron (species B) at T=293 K. Calculate the species flux on both molar and mass bases if the concentration gradient at a particular location is dCA/dx=1 kmol/m3 · m. Compare the value of the mass diffusivity to the thermal diffusivity. The mole fraction of the hydrogen, xA , is much less than unity.
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Problem 5
Derive an expression for the diffusion of one component into a non diffusing (stationary) component.
Problem 6
Air at 25oC and 50% RH flows over water surface measuring 12 m × 6 m at a velocity of 2 m/s. Determine the water loss per day considering flow direction is along the 12 m side. DAB = 0.26 ×10−4 m2/s, Sc = 0.60, ν = 15.7 ×10−6 m2/s. The density of water vapour at 25oC at saturated condition, from steam tables = 0.023 kg/m3.
Problem 7
Air at 25oC and 20% RH flows through a pipe of 25 mm ID with a velocity of 5.2 m/s. The inside surface is constantly wetted with water and a thin water film is maintained throughout. Determine the water evaporated per m2 surface area.
Problem 8
Oxygen diffuses through a layer of 3 mm thickness of a mixture of hydrogen and methane in equal volumes. The total pressure is 1 bar and the temperature is 0oC. The partial pressure of oxygen at the two surfaces are 10000 Pa and 5000 Pa. Determine the rate of diffusion of oxygen. The diffusivity of O2 into H2 is 69.9×10−6m2/s and the diffusivity of oxygen into methane is 18.6×10−6m2/s.
Problem 9
Ethyl alcohol (C2H6o) is at the bottom of a glass jar of 30 mm dia at a depth of 80 mm from the top. Air flows at the top and the concentration of alcohol in the air is zero. The total pressure is 1.013 bar and the temperature is 25oC. The partial pressure of alcohol at this temperature is 0.08 bar. Determine the diffusion rate. Diffusion coefficient, D = 11.9×10-6 m2 /s. Molecular weight = 46, Rb = 8315/46 = 180.76 J/kg K.
Problem 10
Dry air at 30oC flows over a plate also at 30oC with the plate surface constantly covered with a water film. The velocity of flow is 6 m/s. The plate is 1 m long. Determine the average convective mass transfer coefficient and also the mass of water evaporated per second.