Membrane and Desalination Technologies - TCE Moodle Website

564 J.P. Chen et al.
polyester fabric. The support layer is very porous and hence does not reject the salt. In the
following separate manufacturing step, a semipermeable membrane skin is formed on the
polysulfone substrate by interfacial polymerization of monomers containing amine and
carboxylic acid chloride functional groups.
In the 1970s, Cadotte prepared a composite membrane that consisted of a thin layer of PA
formed in situ by condensation of branched polyethyleneimine and 2,4-diisocyanate on a
porous polysulfone membrane (8). The effect of surface modification on the performance of
an aromatic PA–TFC membrane (FT-30) has been studied (9). After soaking in solutions of
various concentrations of hydrofluoric acid (HF), fluorosililic acid, and their mixtures at
controlled temperature for various times, the membrane was rinsed with deionized (DI) water
and tested for their RO performances. For membranes soaked in 15 wt.% HF solution for
7 days, the flux increased from 3.5 to 18 L/m 2 h, whereas NaCl separation increased from
94.5 to 95.3% under the same operating pressure of 250 psig. For membrane soaked in 15 wt.
% HF solution for 4 days, X-ray photoelectron spectroscopy showed that the fluorine ratio on
the membrane surface increased from 0 to 0.012 on the fresh membrane. The value increased
to 0.044 for the membrane immersed in 15 wt.% HF solution for 75 days. Thus, surface
chemical modification contributed to the dramatic increase of the flux as well as the thinning
of the selective skin layer, whereas NaCl separation remained practically unchanged.
The discovery of TF membranes is a breakthrough in achieving flows and rejections
suitable for seawater desalination. In some cases, three-layer configurations are used for
extra durability and performance. Some characteristics of the two membranes are listed in
Table 13.1.
Thin film composite membranes are subject to continuous development. Membranes with
chlorine resistance, low energy, and low fouling have been produced. Recently, about 60% of
seawater recovery has been reported for some extra high-rejection TF membranes. RO
systems were originally designed using flat CA or hollow fiber membranes that were retrofitted
with TFC membranes of polyurea and PA. The TFC PA membranes have many
advantages in comparison with CA or hollow fiber membranes. These include wider chemical
and physical tolerance ranges, higher silt density index (SDI) tolerance, feasibility of operation
at higher temperatures, and higher pH tolerance. These make pretreatment easier and less
expensive, and operation easily controlled. Furthermore, the introduction of the TFC PA
Table 13.1
Characteristics of cellulose acetate and thin-film membranes
Parameters Cellulose acetate Thin film
Temperature Up to 30 C Up to 50–90 C
Operating pressure (psi) Up to 450 typically; up to 800 occasionally 100–1,200
pH 3.0–9.0 1.0–12.0
Chlorine tolerance Fair Poor
Oxidization tolerance Good Poor
Rejection (%) Good Excellent