Three-dimensional pore-scale flow and mass transport through composite asymmetric membranes

Abstract

A three-dimensional Lattice Boltzmann simulation method, is employed to investigate the pore-scale fluid flow and mass transfer in the composite asymmetric membranes. The membrane investigated is composed of three layers: a sponge-like porous layer, a finger-like hole layer, and a skin layer. The former two layers combined are called the porous support layer and they are reconstructed by a modified simulated annealing method. The permeability of the porous support layer is first studied. Then the mass transport in the composite membrane (with the skin layer) is modeled and the effective diffusivity is evaluated. A detailed description of the pore-scale vapor diffusion phenomenon in the membrane is presented. It is found that the mass transfer through the composite membrane can be enhanced by adjusting the membrane structural and chemical properties, but modifying the compositions of the skin layer is the most effective way in mass transfer intensification.

Nomenclature

c	=	speed, m/s
C	=	mass concentration/uptake, kg vapor/kg dry air, kg water/kg material
dp	=	pore diameter, m
D	=	diffusivity, m2/s
E	=	vector
E	=	energy function
f	=	distribution function for density
g	=	distribution function for concentration
H	=	height, m
K	=	permeability, 1/m2
Kp	=	partition coefficient, kg water/kg material/kg moisture/ kg dry air
L	=	length, m
p	=	pressure, Pa
q	=	mass flux, kg/m2/s
r	=	resistance/ distance between two points, s/m or m
S	=	two-point correlation function
T	=	temperature,°C
t	=	time, s
u	=	velocity, m/s
W	=	width, m

Greek letters

τ	=	relaxation time
υ	=	kinematic viscosity, m2/s
ρ	=	density, kg/m3
δ	=	thickness, m
ϵ	=	porosity
α	=	resistance ratio

Superscripts

‘	=	variables with physical units
eq	=	equilibrium
p	=	porous layer
s	=	skin layer

Subscripts

0	=	characteristic variables
e	=	effective
in	=	inlet
m	=	mass, membrane
out	=	outlet
s	=	solid, sound
tot	=	total
v	=	vapor
w	=	water