Absorption and stripping - Principles of Chemical Separations with Environmental Applications - page 174

and,

V
K y aA c (1

H OG =

m .

(6.31)

−

y A )

H OG can be taken out of the integral (assumed constant) if average values of V and
(1 y A ) m are used. This is normally a good assumption based on the error with correla-

tions for mass transfer coefficients.
Table 6.3 summarizes equations for HTUs and NTUs. The choice can be dictated by
the form of the mass transfer coefficient used and the phase which contributes the limiting
resistance. Each form of the mass transfer coefficient has a corresponding driving force
associated with it. The important point is that one uses the proper equation for both the
NTU and HTU terms.
We can derive a relationship between H OG , H G , and H L .With the assumption of dilute
solutions,

−

1
K y a =

1
k y a +

m
k x a

V
k y aA c =

V
k y aA c +

mV
k x aA c

V
k y aA c +

mV
L

L
k x aA c ,

=

we obtain:

mV
L

H OG = H G + H L .

Correlations

Equations (6.32) and (6.33) below can be used to estimate H G and H L for a preliminary
design [6]. More detailed correlations are available [2, 6, 7] for more accurate calculations.
The advantage of the correlations below are simplicity of use and calculation which is
often a benefit when doing an initial design calculation.

a G W G Sc 0 . G W L

H G = (6.32)

a L W L
µ

d

Sc 0 . 5
L

H L = , (6.33)

ft 2 m 2

where W L and W G are the fluxes in lb m / · hr (kg / · hr), Sc G and Sc L are the Schmidt

numbers for the gas and liquid phases, respectively, and µ is the liquid-phase viscosity in

units of lb m / hr). The values of H L and H G are computed in units of ft (m). The

constants are given in Table 6.4. The Schmidt number is calculated from Sc

ft · hr (kg / m ·

= µ/ρ D AB

where ρ is density and D AB is the diffusion coefficient of A diffusing through B .