4相际传质.ppt

文档介绍：equation of rate in partial pressure difference
∵According to NA= kG(pG-pi) = kL(Ci-CL)
NA与G-phase or Liquid-p浓度和界面浓度有关。
But : ⑴This interface ~假设的;
⑵在一些特殊情况下它是存在,但很难测定。
而:G~ L主体的浓度是可以测定的。
∴类似于传热~用冷热流体的进出口温度,
传质~用主体浓度。
令:CL=(1/H)pL* and Ci=(1/H)pi 代入上式:
Equation of rate for mass transfer with convection
§4 Between g-phase and l-phase for mass transfer
Equation of rate for mass transfer with convection
总传质系数
Overall coefficient
总传质推动力(两相)Overall driving force
Individual coefficient in GP
Individual coefficient in LP

(Interpretations for table 8-4)
for mass transfer
∵传质通量(NA)
=传质系数×传质推动力
∴表达形式很多,见Table 8-4.
Equation of rate for mass transfer with convection
⑴与相有关;
⑵不同因次的表达方法有关。
[Kmol/m2·s]
Numbers for mass-transfer (see P19)
Equation of rate for mass transfer with convection
∵对流条件下的传质,且在一定温度、压力条件下
∴有关的参数:ρ、u、D、μ、d、k(对流传质系数) 类似于对流传热。
对流传质的无因次准数有两个:Sh、Sc
number
Sh =kd/D ~ 表征了对流传质与分子扩散的关系
number
Sc=μ/(ρ D) ~ 表征了流动与传质的关系
Obviously: Sh=f(Re, Sc)
目的:求出对流传质系数, k=f(ρ, u, d, D)→K
For example:

即总阻力=气膜阻力+液膜阻力
:对于一定吸收设备水溶液体系,
k L~ 10-3m/s,
k G ~ 10-3~10-4[kmol/m2·atm·s]
∴ E or H对KG有重要影响
E or H :very high ~ very low
Steps of controlling resistance for mass-transfer.
RT =rG + rL
~串联电路
the solubility of the gas is very high ~易溶气体