Question

Example The interfacial surface area per unit volume, a, in many types of pack- ing materials used in industrial towers is virtually impossible to measure. Both a and the mass-transfer coefficient depend on the physical geometry of the equipment and on the flow rates of the two contacting, immiscible streams Accordingly, they are normally correlated together as the volumetric mass- transfer coefficient, ka. Empirical equations for the volumetric coefficients must be obtained experimentally for each type of mass-transfer operation. Sherwood and Holloway (1940) obtained the following correlation for the liquid-film mass- transfer coefficient in packed absorption towers: 10.5 k,a G (2-107) AB LL The values of a and n to be used in equation (2-107) for various industrial packings are listed in the following table, when SI units are used exclusively. 2 0.22 0.22 0.22 0.35 0.46 0.28 0.28 0.28 0.28 Packing 50-mm Raschig ring 38-mm ring 25-mm ring 13-mm ring 9.5-mm ring 38-mm Berl saddle 25-mm saddle 13-mm saddle 76-mm spiral tiles 384 426 391 3116 731 686 502 (a) Consider the absorption of So2 with water at 294 K in a tower packed with 25-mm Raschig rings. If the liquid mass velocity is G 2.04 kg/m2-s, esti- mate the liquid-film mass-transfer coefficient. The diffusivity of SO2 in water at 294 K is 1.7 × 10-9 m3/s. Answer: 6.7 x 10-3 s1 (b) Whitney and Vivian (1949) measured rates of absorption of SO2 in water and found the following expression for 25-mm Raschig rings at 294 K k, a = 0.152(Gr)0.82 (2-108) where k,a is in kmol/m3-s and G, is in kg/m2-s. For the conditions described in part (a), estimate the liquid-film mass-transfer coefficient using equation (2-108). Compare the results

Answer 1

(a)

Sherwood- Hollouo Eauation AB ass 2. 2, 21 C St AP 2.5

(b)

0. 82 = 18 g/mol= 18 Lekmol 3-1 侃), omparis san, the vale ovs past is higher than Ital- k, a)