Chemical Engineering November 2022 - 26

Facts At Your Fingertips
Crossflow Membrane Filtration Module Types
Department Editor: Scott Jenkins
I
n crossflow membrane filtration
(CMF), feed flow is parallel to the filtration
surface (rather than perpendicular
to it, as in conventional filtration).
During a given pass, a portion
of the feed permeates the membrane,
while a larger portion is retained as
retentate. Most of the retentate is returned
for multiple passes, by a circulation
pump. CMF is common across
many industry sectors, and applications
and offers advantages in its ability
to impart shear onto the filtration
surface. This reduces fouling by particulate
matter or retained molecules.
Several module configurations and
membrane types are available. This
one-page reference reviews common
filter-module configurations and
membrane materials.
Membrane materials
Modern CMF membrane materials
include polysulfone (PS), polyethersulfone
(PES), polyvinylidene fluoride
(PVDF), polyacrylonitrile (PAN),
polyamides, polytetrafluoroethylene
(PTFE) and polypropylene (PP).
Most CMF membranes are asymmetric,
with a thin selective layer on
top of a more open support layer.
The tight selective layer does the actual
filtration, while the open support
layer minimizes resistance to flow
and provides mechanical integrity.
Inorganic membranes include ceramics,
such as α-alumina, zirconia
and metal oxides (notably, titanium
dioxide), sintered stainless steel and
graphite. More robust than most
polymers, inorganic membranes can
withstand strong acids and bases, as
well as high temperatures and presFeed
Membrane
sures.
Such membranes are particularly
useful for high-fouling applications
that require aggressive cleaning.
Module configurations
Available CMF module configurations
are described here.
Hollow fiber. This module contains
a bundle of polymeric tubes (hollow
fibers), potted at both ends and
encased in a shell. The geometry is
analogous to that of a shell-and-tube
heat exchanger, and the tube wall is
the membrane. Feed enters the tubes
at the bottom, retentate leaves at the
top, and permeate passes through
the tube walls, then exits through the
shell side. A typical module is 10-20
cm in diameter by 1-1.6 m in length.
The main advantage of the hollow-fiber
configuration is the ability to tightly
pack the fibers, allowing a high membrane
area per unit volume (as high as
16,000 m2/m3). Another advantage is
the ability to backwash for more effective
cleaning.
Spiral wound. This design consists
of membrane envelopes wound
around a perforated
central
core.
Gabelman
Retentate
Permeate
FIGURE 2. In plate-and-frame modules, the
space between opposite sides provides a flow
path for permeate
26
Each envelope contains two rectangular
membrane sheets facing away
from each other, separated by a porous
spacer. These spaces provide
a path for the permeate, which flows
around the spiral to the central core,
then exits the module. The spaces
between adjacent envelopes, separated
by grids, serve as flow channels
for the feed and retentate. Each
envelope is sealed with adhesive on
three sides, and the fourth side is
attached to the central tube. Typical
channel width, module diameter
and length are 0.25-0.5 mm, 10-40
cm and 1-1.5 m, respectively. Spiralwound
modules are commonly used
for water purification. Advantages are
lower energy consumption compared
to other designs, and high area per
unit volume. Spiral-wound units are
not suitable if fibrous solids are present,
because these will clog the grids.
Tubular. Like the hollow-fiber design,
tubular modules have a shell-andtube
geometry, but the tubes are
larger, with the inner diameter ranging
from 2.5
to 25 mm
(Figure
1).
The
tubes
consist of
a
porous
support,
such as
fiberglassreinforced
epoxy,
with
the
active
layer formed
on the inside
Courtesy of Koch Membrane
Systems Inc.
FIGURE 1. Tubular modules
are useful for difficult-tohandle
feeds
surface. The shell is made from stainless
steel or a hard polymer. Tubular
modules are chosen for difficult-tohandle
feeds that are not suitable for
hollow-fiber or spiral-wound designs.
These include high-viscosity feeds
and those with high undissolved solids
content, large particles, or highly
compressible or gelatinous solids.
In addition to handling challenging
streams, other advantages of tubular
design include high resistance to fouling
and ease of cleaning. Disadvantages
are low packing density; high
energy consumption; and the inability
to backwash. Applications include
wastewater treatment, paint recovery
and juice clarification.
Plate-and-frame.
These
modules
contain a series of flat membrane
sheets arranged in a plate-and-frame
assembly. The membrane side of
each sheet faces a feed channel, with
the space between the opposite sides
providing a flow path for permeate.
This assembly, reminiscent of a traditional
filter press, is situated between
two end plates, with gaskets placed
as needed to direct flow (Figure 2).
Ceramics. A ceramic module consists
of a porous ceramic material,
called a monolith, through which parallel
flow channels have been formed.
The separation is performed by a
thin active layer of smaller particles
deposited onto the inside surface of
the channels. The active layer is usually
α-alumina, zirconia or titanium dioxide,
while the monolith is typically
α-alumina or other metal oxide. n
Editor's note: This material was adapted from the following article:
Gabelman, A., Crossflow Membrane Filtration Essentials, Chem.
Eng., April 2017, pp. 49-59.
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