2.1a. Mass-transfer coefficients in a gas absorber.
A gas absorber is used to remove benzene (C6H6) vapors from air by scrubbing the gas mixture with a nonvolatile oil at 300 K and 1 atm. At a certain point in the absorber, the benzene mole fraction in the bulk of the gas phase is 0.02, while the corresponding interfacial benzene gas-phase concentration is 0.0158. The benzene flux at that point is measured as 0.62 g/m2-s.
a) Calculate the mass-transfer coefficient in the gas phase at that point in the equipment, expressing the driving force in terms of mole fractions .
Solution
b) Calculate the mass-transfer coefficient in the gas phase at that point in the equipment, expressing the driving force in terms of molar concentrations, kmol/m3.
Solution
c) At the same place in the equipment, the benzene mole fraction in the bulk of the liquid phase is 0.125, while the corresponding interfacial benzene liquid-phase concentration is 0.158. Calculate the mass-transfer coefficient in the liquid phase, expressing the driving force in terms of mole fractions.
Solution
2.2a. Mass-transfer coefficients from naphthalene sublimation data.
In a laboratory experiment, air at 347 K and 1 atm is blown at high speed around a single naphthalene (C10H8) sphere, which sublimates partially. When the experiment begins, the diameter of the sphere is 2.0 cm. At the end of the experiment, 14.32 min later, the diameter of the sphere is 1.85 cm.
a) Estimate the mass-transfer coefficient, based on the average surface area of the particle, expressing the driving force in terms of partial pressures. The density of solid naphthalene is 1.145 g/cm3, its vapor pressure at 347 K is 670 Pa (Perry and Chilton, 1973).
Solution
b) Calculate the mass-transfer coefficient, for the driving force in terms of molar concentrations.
Solution
2.3a. Mass-transfer coefficients from acetone evaporation data.
In a laboratory experiment, air at 300 K and 1 atm is blown at high speed parallel to the surface of a rectangular shallow pan that contains liquid acetone (C3H6O), which evaporates partially. The pan is 1 m long and 50 cms wide. It is connected to a reservoir containing liquid acetone which automatically replaces the acetone evaporated, maintaining a constant liquid level in the pan. During an experimental run, it was observed that 2.0 L of acetone evaporated in 5 min. Estimate the mass-transfer coefficient. The density of liquid acetone at 300 K is 0.79 g/cm3; its vapor pressure is 27 kPa (Perry and Chilton, 1973).
Solution
2.4b. Mass-transfer coefficients from wetted-wall experimental data.
A wetted-wall experimental set-up consists of a glass pipe, 50 mm in diameter and 1.0 m long. Water at 308 K flows down the inner wall. Dry air enters the bottom of the pipe at the rate of 1.04 m3/min, measured at 308 K and 1 atm. It leaves the wetted section at 308 K and with a relative humidity of 34%. With the help of equation (2-52), estimate the average mass-transfer coefficient, with the driving force in terms of molar fractions.
Solution