大连理工大学化工传递Summery of mass transfer and problems.ppt

Summery of mass transfer and problems for mass transfer Problems and solutions for mass transfer That’s the end of the course Thank you * mass transfer diffusion Convective mass transfer Steady state Unsteady state Diffusion with or without chemical reactions thermal diffusion, pressure diffusion, forced diffusion etc Biot number of mass, its difference from Sherwood number concentration is uniform? Bi’ =0.1 One dimensional problems Heisler’s figures Newman’s law to solve multi dimensional problems With chemical reactions Forced Natural Driving forces, expressions of Sherwood number, Gr’, Sc, Sh, et al, their physical meanings Inside pipes On plates Constant mass flux and constant wall concentration, their solutions concentration boundary layer, integral solutions transport process with momentum, heat and mass transferred simultaneously laminar turbulent characteristics and time-averaged method; the concept of eddy diffusivity, mass transfer mixing length; concentration profile in pipes, analogy models 1. The mass diffusion coefficient of chloroform in air can be measured with the Arnold diffusion chamber. The measured results include: y0=7.4cm, 10 hours later, y1=7.84cm, xAL at y=0 is 0, ρl=1485 kg/m3, vapor pressure of chloroform is 2.66*104 Pa, Tair=25℃, Pair=1.013*105 Pa. Please find DA-air. solution: Gas B A + B Liquid A t=0, y=y0 t=t1, y=y1 y=0, xAL y=y, xA0 y 2. Benzene (B) and toluene (A) are distilled inside a falling film distillator under normal pressure. At a certain section, xAg=0.147, xAl=0.3, T=86.6℃, the vapor pressure of toluene is 4.914*104 Pa. Suppose the diffusion resistance inside vapor equals that in a static gas film with 2.54 mm in thickness and DABg=5.06*10-6m2/s, please find NAy or NBy. solution: Liquid film vapor Liquid film vapor 3. The air with 0℃ and normal pressure flows over a naphthalene (A) slab with 10 mm in thickness, 0.2m in length. V∞=10 m/s, DAB=5.14*10-6 m2/s, the vapor pressure of naphthalene is 0.785 Pa, ρAs=1152 k