Problem Set 8.pdf - UNIVERSITY OF ARIZONA DEPARTMENT OF...

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UNIVERSITY OF ARIZONA DEPARTMENT OF CHEMICAL AND ENVIRONMENTAL ENGINEERING CHEE 305 TRANSPORT PHENOMENA SPRING 2017 Problem Set 8 Due: Wednesday, April 26 1. A long nickel cylinder with a radius of 2.5 cm has been stored in a pure hydrogen environment at 300 kPa for a very long time (several years), and thus it contains hydrogen gas throughout uniformly. The cylinder is placed into a well-ventilated area so that the hydrogen concentration at the outer surface is zero at all times. The diffusion coefficient of hydrogen in nickel is D AB = 1.2 × 10 -11 m 2 /s. The solubility of hydrogen gas in nickel (c As ) exposed to a gas with a partial pressure of hydrogen P A is given by c As = 0.25 P A , where c As is in mol/m 3 and P A in kPa. The temperature of the gas and cylinder is 298 K at all times. (a) Determine how long it will take for the hydrogen concentration at the center of the cylinder to drop to 30 % of the initial value. (b) When the concentration at the center is 30 % of the initial value, the cylinder is isolated from the surroundings until its hydrogen concentration becomes uniform. At that point, the cylinder is
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