A Consideration on Mass Transfer in Evaporating Space of High Vacuum Evaporation

Abstract

Experiments on vacuum evaporation were made in falling-film colum in a range of residual air pressure from 1 × 10^-4 to 3.5 × 10^-1. Torr by use of di-butyl-sebacate (D.B.S.). In this work, slip-diffusion model which consists of two zones was applied to analysis of mass transfer in the evaporating space. The following results were obtained : The mechanisms of mass transfer in the evaporating space were divided by the value of P_r/P_vs.At the value of P_r/P_vs above 1.4, mass transfer in the evaporating space is controled by moecular diffusion, and then the effect of molecular-burst-evaporation on mass transfer velocity is neglected. At the value of P_r/P_vsbelow 0.3, mass transfer in evaporating space is controled by molecular-burst-evaporation, and then the effect of molecular diffusion on mass transfer velocity is neglected. At a range of P_r/P_vsfrom 0.3 to 1.4, mass transfer in evaporating space is controled by two mechanisms (molecular diffusion and molecular-burst-evaporation).
The relationship of D.B.S. between the value of P_r/P_vs and the mechanisms of mass transfer in the evaporating space described above agrees with those of di-butyl-phthalate and di-2-ethyl hexyl-phthalate. Consequently, it is considered that 0.3 and 1.4 of P_r/P_vs are the universal values.