Benzene-free air flows in a tube whose inner surface is coated with pure benzene. The average mass transfer coefficient, the molar concentration of benzene in the outlet air, and the evaporation rate of benzene are to be determined.
What will be an ideal response?
Assumptions 1 The low mass flux model and thus the analogy between heat and mass transfer is applicable. 2 The flow is fully developed.
Properties Because of low mass flux conditions, we can use dry air properties for the mixture at the specified temperature of 25?C and 1 atm, for which (Table A-15). The mass diffusivity of benzene in air at 298 K is (Table 14-2). The molar mass of benzene is 78 kg/kmol.
Analysis (a) The Reynolds number of the flow is
which is greater than 10,000 and thus the flow is turbulent. The Schmidt number in this case is
Therefore, the Sherwood number in this case is determined from Table 14-13 to be
Using the definition of Sherwood number, the mass transfer coefficient is determined to be
(b) The molar concentration of benzene in the outlet air is determined as follows
(c) The evaporation rate of benzene is determined from
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