Environmental Engineering Reference
In-Depth Information
is such that water transport would be possible while hydrated ions, particularly anions,
would be excluded from the pore network. In practice, incorporation of nanoparticles also
lead to a reduction in membrane roughness, which would further improve performance
stability, and increased water permeability. This increased water transport rate became the
performance metric identiied and optimized for initial commercialization. TFN technology
was irst released in the form of a spiral-wound
Quantum
Flux (Qfx) seawater membrane
line in 2011, 4 years after the irst TFN publication. This chapter will discuss academic and
commercial developments related to the expanding use and potential of TFN membrane
technology in salt selective membranes.
Membranes are a widely adopted technology used to separate components of a luid
stream. Depending on the identity and phase of the components to be retained and passed
by the membrane, various technologies have been developed over the last century to treat
liquids and gases and retain components ranging from the atomic to the macroscopic. In
nearly all instances, developers of new membrane technologies are looking for ways to
increase the permeability of the membrane to the components to be passed and decrease
the permeability of the components to be rejected. This may be accomplished by altering
the morphology or chemistry of the membrane. In the ields of gas separation and RO, dif-
ferent approaches were pioneered in the 1970s to alter morphology as a means of improv-
ing membrane performance in their respective ields.
16.1.1 Mixed Matrix Membrane Development
In the ield of gas separation, composite barrier layers of polymer and inorganic particles
were reported as early as 1973 in a fundamental work looking at the transport of gases in
polymers by Paul [5]. The addition of a second phase to polymers that were known to be use-
ful as gas separation materials was an attempt to break a lux-selectivity tradeoff curve that
appeared to give an empirical maximum to a membrane's performance [6]. This approach to
membrane construction has since found increasing use in gas separation [7], pervaporation
[8], ion exchange [9], and as fuel cell electrolytes [10]. These relatively thick membranes were
made by dispersing and then casting a polymer solution containing dispersed nanoparticles
and then solidifying the membrane matrix, trapping the nanoparticles in place.
16.1.2 Thin Film Composite Membranes
In the ield of desalination, the foundations of a technology shift was set in place with the
discovery of monomers that, when used in an interfacial polymerization, resulted in an
RO membrane suitable for use in one-pass seawater desalination. The performance of such
interfacially made polyamide [2] RO membranes set in motion a multidecade shift from
thermal desalination to membrane-based RO desalination. Before that point, RO mem-
branes were made primarily from phase inversion of cellulose acetate polymers. These
new membranes were made by a different process. A support membrane was prepared by
casting a porous polysulfone layer above a polyester support. On top of this support layer,
a water-based solution of a diamine was applied, followed by a solution of triacyl chloride
in a water immiscible solvent. Immediately after contact, diamine began to diffuse into the
water-immiscible solvent and an extremely fast reaction commenced. Less than a second
after the second solution was applied, a thin polyamide layer had formed. This type of
membrane was referred to as a thin ilm composite (TFC). Early variants had better rejec-
tion than the cellulosic acetate membranes, twice the permeability of water, and improved
stability to high and low pH.
