Iodine is also a material that undergoes sublimation and is commonly used in che

Question

Iodine is also a material that undergoes sublimation and is commonly used in chemistry. You have a project to determine mass transfer properties of iodine, including the sublimation of iodine under different temperatures. You experiment with iodine under conditions in which air is blown past a cylinder of iodine, in which the air does not come in contact with the edges but rather only the perimeter (periphery) of the cylinder. It can also be assumed that sublimation only occurs in this region. The air is at 298 K and 1 atm, with an initial cylinder diameter of 2.0 cm. After 25 minutes under these conditions, the cylinder reaches a diameter of 1.9 cm. The length of the cylinder is 15 cm. (a) Calculate the average surface area of the cylinder exposed to the air during this time and express the driving force of sublimation in terms of partial pressures. The density of solid naphthalene is 4.93 g/cm3 and the vapor pressure of naphthalene at 298 K can be assumed as 70 Pa. Solve for the mass transfer coefficient (b) Calculate the mass transfer coefficient using molar concentrations as the driving force

Explanation / Answer

(a) Average area exposed to mass transfer = 2RavgL = 2 x 3.14 x 1.95 cm x 15 cm = 183.783 cm2

In terms of mass transfer coefficient,

Mass sublimed/Time taken = k x (Patm - VP)

Volume lost = (R12 - R22)L = 3.14 x (22 - 1.92) x 15 in cm3 = 18.369 cm3

Mass lost = volume lost x density = 18.369 x 4.93 = 90.56 g

Mass flux = mass lost/ time = 90.56g / 25 min = 3.62237 g/min

Pressure driving force = Patm - VP = 101325 Pa - 70 Pa = 101255 Pa

Thus, mass transfer coefficient k= 3.62237 g/min / 101255 Pa

k = 3.577 x 10-5 g/min-Pa = 5.9624 x 10-10 kg/Pa-s

(b) We have, PV= nRT or n/V = P/RT

Thus, driving force in pressure = RT x driving force in molar concentration

Thus, in terms of molar conc,

k= 5.9624 x 10-10 kg/Pa-s x 1/RT = 5.9624 x 10-10 / (8.314 x 298)

k = 2.406 x 10-13 kg/(mol/m3) - s

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