[工学]同济化工原理课件第八章吸收2.ppt

Transfer of solute in absorption: (2) solvation at the interface; 8.3 Molecular Diffusion and Mass Transfer in a Bulk Phase molecular diffusion convection We will consider the cases at steady state. 8.3.1 Diffusion in a two-component system JA ? mass flux, kmol/m2s DAB ? diffusion coefficient, m2/s The direction of diffusion is opposite to that of concentration gradient. It can be proven that in a binary system, DAB = DBA = D (8-12) ? JA = ? JB (8-13) It is an equimolecular counter-diffusion process. In an absorption process, the mass transfer relative to a fixed reference consists of two parts: molecular diffusion diffusion induced convection For a fixed reference, the net flux of material is N = NA + NB which are all relative to the fixed reference. The bulk fluid moves with N, and molecules of A and B diffuse in the bulk fluid at the same time. Relative to the fixed reference, the flux of component A is In forced convection, the diffusion induced convection is negligible. For dilute solutions, the diffusion induced convection JA. For equimolecular counter-diffusion, NA + NB = 0 1. Diffusion of component A  through a stagnant fluid At steady state, the rate of diffusion is constant. When the diffusion induced convection is absent or negligible, B. C.: at z = 0, cA = cA1 ; at z = ?, cA = cA2 . The integration leads to Above results are accurate for equimolecular counter-diffusion and approximate for dilute solutions. rate of mass transfer the mass transfer coefficient k = D/?, and the resistance to mass transfer is ? /D. 2. Diffusion of component A through a stagnant fluid (general cases) Component B is insoluble, so that NB = 0 B. C.: at z = 0, cA = cA1; at z = ?, cA = cA2 . The integration leads to It may be written as where cM = cA