## *MASS TRANSFER – 4

MASS TRANSFER – 4

CONVECTIVE MASS TRANSFER AND ITS EMPIRICAL CORRELATIONS
Before discussing convective mass transfer, it is necessary to compare convective heat transfer and convective mass transfer since there is a similarity between these two. In heat transfer, there is convective heat transfer coefficient ‘ h’. Similarly there is mass transfer coefficient in mass transfer. Prandtl number (Pr = ν / α) is replaced by Schmidt number (Sc = ν / DAB) and the Nusselt number (Nu = hL/kf) is replaced by Sherwood number (Sh = kc L / DAB) and Tf is mean film temperature.
Table: Mass transfer coefficients in Forced Convection Mass Transfer

 Sr.  No. Empirical equation Object Type of flow local or average  temp Range of dimensionless number 1. Shx= 0.332 Rex1/2 Sc 1/3 Flat plate Laminar Local 0.6≤Sc ≤50 2. Shav= 0.664 Rex1/2 Sc 1/3 Flat plate Laminar Average 0.6 ≤Sc ≤50 3. Sh= 0.0296 Rex4/5 Sc 1/3 Flat plate Turbulent local Rex≤105, 0.6 ≤Sc ≤50 4. Sh= (0.037 Rex4/5 −871) Sc 1/3 Flat plate Transition Local 5×105< Rex ≤107, 0.6≤Sc ≤50 5. Sh = 0.3+ [0.62ReD1/2Sc1/3 ×[1 + (0.4/Sc)2/3]-1/4] ×[1 + (ReD/282,000)5/8]4/5 Cylinder Cross flow Average ReD Sc > 0.2 6. Sh= 2 + (0.4 ReD1/2+ 0.060 ReD2/3) Sc0.4 ×(μ/μs)1/2 Sphere Cross flow Average 3.5 < ReD< 7.6×104, 0.71 < Sc < 380 and 1.0 < (μ/μs) < 3.2

For an ideal gas

pAV =nA RT

CA=nA / V
Normally in gases, there is equimolar counter diffusion which gives
J*AZ = — DAB dCA/dz = — J*BZ = –(–) DBA dCB/dz = DBA dCB/dz
DAB = DBA
For a binary gas mixture of A and B, the diffusivity coefficient DAB=DBA in counter molar diffusion of gases.
Other parameters of equimolar counter diffusion are
J*AZ = — J*BZ or J* is constant in a steady state
dcA = –dCB
C= CA + CB
DAB = DBA

Mass transfer Coefficients

These empirical correlations are valid for low mass transfer rate (or equi-molar mass transfer) where the mole fraction of species ‘A’ is less than about 0.05. For higher mass transfer rate, corrected mass transfer coefficients, using the log mean concentration difference, must be used. Hence kc is replaced by kc/(1 − yA)lm

Where lm is logarithmic mean and is given by

(1 − yA)lm = [(1–yA) –(1–yAi)] /[ ln((1–yA) / (1–yAi))]
yA is gas mole fraction of species A = pA / pt
yAi is gas mole fraction of species A at the interface = pAi / pt
pA is the partial pressure of species A
pAi is the partial pressure of species A at the interface

Mass transfer coefficients at Macro level

When a fluid flowing outside a solid surface in forced convection motion, rate of convective molar flux is given by:

NA = kc (cL1— cLi)

NA=molar flux of species A = kmol/s m2
kc = mass transfer coefficient (m/s)
cL1 = bulk fluid concentration, kmol/m3
cLi = concentration of fluid near the solid surface, k mol/m3
kc depends on the following factors:
1. System geometry—pipe or flat plate or channel
2. Flow velocity—laminar, transition or turbulent flow
3. Fluid properties—density, viscosity, specific heat etc.
From the empirical equation of Sherwood number applicable, first mass transfer coefficient is calculated. Only then, mass flux can be known. These calculations are very similar to convective heat transfer calculations i.e.

Convective heat flux = h Δt

Where h is the convective heat transfer coefficient

Δt is the temperature difference