Membrane and Desalination Technologies - TCE Moodle Website

48 J. Ren and R. Wang
Key Words Polymeric membranes phase inversion process thermodynamics
hydrodynamics hollow fiber membranes flat sheet membranes.
1. INTRODUCTION
Simply, a membrane is a selective barrier between two phases (1), and a membrane process
should be always associated with its application according to this definition, which can range
from gas separation, pervaporation and vapor permeation, desalination, nanofiltration, ultrafiltration,
microfiltration, membrane extraction, membrane distillation, supported liquid
membrane, and so on. The term of membrane is different from filter, which is usually limited
to the structures that separate particulate suspensions larger than 1–10 mm(2). Since Loeb and
Sourirajan (3) developed the first anisotropic membranes by phase inversion technique using
cellulose acetate in the 1960s, membrane separation technology has been becoming one of
the most innovative and rapidly growing fields across science and engineering, and has been
used in a variety of applications. Depending on different applications, a large number of new
polymers are employed to make different membranes with specific morphologies and transport
properties suitable to the desired separation processes. Except for the Loeb-Sourirajan’s
phase inversion technique, some new membrane formation processes such as interfacial
polymerization, composite coating, multilayer composite casting, stretching, etc., were also
developed to make high performance membranes in the past decades. To make various
membranes with controlled morphology and desired transport properties, it is vital to
understand the principles of membrane formation. Some critical reviews have been made
to address relevant issues (2, 4–10).
This chapter mainly describes the principles of polymeric membrane formation process
with focus on the phase inversion technique. A variety of polymer materials suitable for
membrane fabrication are briefly introduced. The thermodynamic behavior of the casting
polymer solution, the membrane formation process and the fabrication of hollow fiber and flat
sheet membranes.are involved. Based on the thermodynamic description of the polymer
solution, the linearized cloud point curve correlation is presented. As for the membrane
formation process, the vitrification of the membrane morphology, the membrane surface
formation, as well as the formation of nascent porous membranes are all associated with two
macroscopic time scales of delay time and gelation time. In addition, two important parameters
for membrane fabrications, the approaching ratio of the casting solution and the
approaching coagulant ratio, are also described and discussed.
2. MEMBRANE CLASSIFICATION
Membranes are generally classified by the nature of the materials, the membrane morphology,
geometry, preparation method, separation regime and processes et al. (8, 10).
A representative membrane classification is shown in Fig. 2.1.