一,Total 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, ⑴ This 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:
8.4.1 Equation of rate for mass transfer with convection
§ 8.4 Between g-phase and l-phase for mass transfer
8.4.1 Equation of rate for mass transfer with convection
G G L
1 1 H = +
K k k
令:
*A G G LT h e n, N = K ( p - p )
GiA
G
p - p
N =
1
k
*
*L i L
A i L
L
k p - p
N = ( p - p ) =
HH
k
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
二,传质速率方程的各种表达形式
(Interpretations for table 8- 4)
1.Flux 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.
8.4.1 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]
三,Dimensionless Numbers for mass-transfer (see P19)
8.4.1 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
1.Sherwood number
Sh =kd/D ~ 表征了对流传质与分子扩散的关系
2.Schimidt 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:
a.具有串联电路的特征
即 总阻力=气膜阻力+液膜阻力
b.实验证明: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
8.4.2 Steps of controlling resistance for mass-transfer.
G G L
1 1 H=+
K k k
一,For the solubility of the gas is very high ~ 易溶气体
1.传质阻力大部分集中在气膜内
For example,NH3~ H2O, HCl ~ H2O
∵ H/ kL<< 1/kG, 而:
∴ KG = k G
This result indicates that the gas-film resistance
controls the rate of absorption,
实验表明,易溶气体, 气膜阻力 可占到 80~ 90% 。
∴ 属于 气膜控制
8.4.2 Steps of controlling resistance for mass-transfer
G G L
1 1 H = +
K k k
一,For the solubility of the gas is very high ~ 易溶气体
2.增加传质速率应采取的措施

而 KG= k G
∴ PG- PL* ≈ PG- Pi
即,PL* ≈ Pi
∴ 如果传质速率不太大的话,应该设法增加 kG。
提高气速 G → δ G ↓,增加气相湍动程度。
另一方面,若 G对 NA影响大,→ Gas film controls it.
8.4.2 Steps of controlling resistance for mass-transfer.
? ? *GA G G i GL= K ( p - p N = k p - p )
二,For solubility of the gas is very low~难溶气体
1.传质阻力大部分集中在液膜内
∵ H很大,例如,H2O~ O2,H2,and CO2
Hco2 = 2580 pa/[kmol/m3]
∴ H/kL >> 1 / kG
It Indicates that the liquid-film resistance controls
the rate of absorption,and 1/kG can be ignored,
Then,KG≈ kL/H or KL≈ kL ~ 液膜控制
8.4.2 Steps of controlling resistance for mass-transfer.
? ? *LAL GL Li = K ( C N -C=k ) C - C
2.增加传质速率应采取的措施

而 KL= k L
∴ C i- CL ≈ C*G- CL
即,C i ≈ C*G
∴ 如果传质速率不太大的话,应该设法增加 kL。
提高液速 L → δ L ↓,增加液相湍动程度。
另一方面,若 L对 NA影响大,→ Liquid film controls it.
三,For the intermediate solubility of gases ( 一般情况 )
8.4.2 Steps of controlling resistance for mass-transfer.