Page 354 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Membrane Processes 339

The accumulation of materials in, on, or near the membrane may have
undesirable consequences for transport across the membrane resulting
in a condition referred to as membrane fouling. For example, in water
filtration, the formation of a cake on the membrane may impede the
permeation of water under the applied driving force of pressure. The issue
of membrane fouling will be taken up later in this chapter.
Membranes can be fabricated in many shapes and sizes. Symmetric
membranes have an approximately uniform composition throughout
their entire thickness so that a slice through any layer of the mem-
brane parallel to its surface would look essentially identical. By contrast,
asymmetric membranes consist of a thin membrane skin that is respon-
sible for the separation of permeate from rejected species. The skin of
an asymmetric membrane is supported by a layer (typically much
thicker) that offers little resistance to transport and does not play a role
in membrane selectivity. The surface chemistry and composition of dif-
ferent membranes can be quite variable depending on the application.
Hydrophobic, uncharged membrane material may be desirable for some
membrane distillation applications, while high charge density mem-
branes are characteristic of polymer electrolyte membranes (PEMs)
used in fuel cells. Dense SiO 2 membranes have been used for gas sepa-
rations, while porous alumina membranes can be used to treat high
temperature brines emanating from oil and gas wells.
Membranes are packaged as several elements grouped into units
that are referred to as modules, vessels, or stacks, depending on the
type of membrane and its application. The most frequently encoun-
tered element geometries are as flat sheets, capillary fibers, hollow
fibers, tubes, or spiral wounds. The geometry of the membrane element
is critical in determining the economics of the membrane process since
it determines how much membrane area can fit into a given volume.
For example, both hollow fiber and tubular membranes share a cylin-
drical geometry, but tubular membranes have a much larger diame-
ter. Thus, the area of membrane available for mass transport per
volume of module (referred to as the packing density) will be less for
tubular membrane (Table 9.1). The cost of a membrane system tends
to decrease as the packing density of the membrane module increases,
as the costs of module housing, instrumentation and hook-up sur-
rounding the module, and space for the installation are spread out over
more membrane area. However, as the packing density increases, less
space is available within the module to allow for other functions. For
example, in water filtration, space must be provided in the module to
allow for rejected materials to circulate freely without obstructing
flow, or the feed stream to the membrane must be pretreated to remove
these materials.