Membrane and Desalination Technologies - TCE Moodle Website

92 J. Ren and R. Wang
solution at the casting window, the formation of nascent flat sheet membranes in the air gap
and solidification or vertification of flat sheet membranes in the coagulation bath.
5.2.1. Hydrodynamics of the Polymer Solution at the Casting Window
When the nonwoven fabric web was drawn at a speed of V0, the polymer solution was cast
onto the thin support through a casting window with a casting gap H. The hydrodynamics of
the polymer solution at the casting window is illustrated in Fig. 2.35. The flow field inside the
casting window is assumed to be laminal and the velocity Vz along the y direction is a linear
distribution. And the shear rate at the casting window can be described as:
dVz
dy
V0
¼ ; ð55Þ
H
where H is the height of the casting window.
A casting die exists at the exit of the casting window, where a very complicated flow field
exists. According to the transformation of flow field, the linear velocity distribution at the
casting window will be changed into an equal velocity distribution (V 0) with a nascent
membrane thickness L, which is immersed into the coagulant bath for precipitation.
The shear rate at the casting window strongly influences the membrane morphologies and
performance of flat sheet membranes (105, 106). For the fabrication of flat sheet membranes,
only shear flow exists in the casting window and no elongation flow is involved at the air gap.
He and Ulbricht (107) have reported a synergist immobilization method for surface
selective photo-grafting on porous polymer membranes. Practical applications of polymeric
membranes for air pollution control, portable water treatment, wastewater reclamation, and
hazardous industrial waste treatment can be found from the literature (108–110).
Nonwoven fabric
shear
dope solution
V
H
V0
casting blade
casting die
Casting window
nascent membrane thickness
Fig. 2.35 Hydrodynamics of the polymer solution at the casting window.
y
z
V 0
L
V 0