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Hollow fiber membrane modules
Norfamilabinti Che Mat, Yuecun Lou and G Glenn Lipscomb
Hollow fiber membrane modules are used in a wide range of
separation applications. Module design is critical to optimizing
process performance. The state-of-the-art in fiber bundle and
module manufacture is reviewed emphasizing industrial
practice as reflected by the patent literature. Sources of nonideal module performance are identified that arise from nonuniform module flows. Efforts to quantify these effects and
evaluate design alternatives to improve performance are
reviewed. Despite this work, gaps in understanding the
relationship between design and performance still exist.
Addresses
Chemical and Environmental Engineering Department, University of
Toledo, 2801 West Bancroft Street, Toledo, OH 43606-3390, United
States
Corresponding author: Lipscomb, G Glenn
([email protected], [email protected])
Current Opinion in Chemical Engineering 2014, 4:18–24
This review comes from a themed issue on Separation engineering
Edited by WS Winston Ho and Kang Li
2211-3398/$ – see front matter, # 2014 Elsevier Ltd. All rights
reserved.
http://dx.doi.org/10.1016/j.coche.2014.01.002
Introduction
Global need for separation processes is increasing as
society addresses grand challenges in providing water,
energy, and health care sustainably. Membrane separation processes offer unique advantages over alternatives
especially in terms of energy consumption and ability to
handle labile components.
The focus of this review is hollow fiber membrane
modules and devices as illustrated in Figure 1. These
devices are the mass transfer equivalent of a shell and
tube heat exchanger. As such they consist of a tube
bundle the ends of which are embedded in a tubesheet.
The tubesheet is created to allow separate fluid communication with the fiber interior (lumens) and exterior
(shell) spaces. The lumens are open along the tubesheet.
Upon sealing the tubesheet to the case separate manifolds
on the case exterior allow the introduction and removal of
fluid streams to the lumen and shell spaces. The feed can
be introduced to either the lumen or shell. A permeate
sweep also may be fed to the module. The sweep mixes
with the permeate and can dramatically improve module
Current Opinion in Chemical Engineering 2014, 4:18–24
performance by increasing the chemical potential difference across the membrane.
A baseline for process performance can be determined by
assuming the fibers are uniform (identical inner and outer
radii and permeances) and uniformly spaced. Additionally, baseline performance predictions assume the fluid
distribution is uniform from the external ports on the case
through the manifolds to the lumen and shell, that is,
the flow rate inside and outside each fiber in the fiber
bundle is identical. The performance of this ‘ideal’ device
can be determined by analyzing the performance of a
single fiber.
Unfortunately, actual performance is poorer than ideal
performance which has driven innovation in module
design to improve performance. The differences are
manifest in lower than expected process throughputs
and higher energy consumption. For example, in nitrogen
production from air, the retentate is the desired product
and the nitrogen composition typically is specified. Nonideality leads to lower flow rates of the desired product
and higher energy consumption. The higher energy consumption is due to a reduction in the fraction of the
compressed air fed to the process that is recovered as the
retentate product.
This review focuses on the state of the art in module
manufacture and module performance prediction. Note
that the design of unconstrained hollow fiber bundles such
as those used in membrane bioreactor applications are not
covered here as these modules and applications possess
unique features and operational characteristics not captured in the design of Figure 1.
Module manufacture
The earliest reports of hollow fiber module designs come
from the patents of Dow [1] and DuPont [2]. Dow’s
design drew most heavily on heat exchanger designs.
Modules were formed by potting the ends of individual
fibers or a small group of fibers (a fiber tow) in plugs.
Subsequently, the plugs were placed in holes arranged in
a regular array in two opposing metal plates. The fibers
were held in slight tension between the two plates to
allow uniform packing.
The DuPont process represented a significant departure
that facilitated the formation of larger bundles and was
less labor intensive. Loops of fibers were formed by
winding individual fibers or fiber tows around a rotating
wheel and enclosed in multiple elastic socks. Tubesheets
were formed by placing a mold over the end of the fiber
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Hollow fiber membrane modules Mat, Lou and Lipscomb 19
Figure 1
External case
Permeate
Permeate Sweep
Tubesheet
Fiber bundle
Feed
Retentate
Lumen manifold
bolted to case
O-ring tubesheet-case seal
Lumen distribution manifold
Shell distribution manifold
Current Opinion in Chemical Engineering
Cut-away view of a typical hollow fiber membrane module with a lumen feed and shell sweep operating in counter-current contacting mode. Key
elements and flow paths are indicated: solid lines — lumen flow and dashed lines — shell flow. Note that the hidden portion of the module is a mirror
image of that shown.
bundle that possessed connections to introduce a tubesheet forming material (e.g. epoxy resin). The tubesheet
material was introduced while the module rotated with its
long axis parallel to the ground. The rotation kept the
tubesheet material from wicking up into the fiber bundle
due to centrifugal forces; wicking reduces the area available for mass transfer and weakens fibers. This technique
is referred to as ‘centrifugal’ potting and produces
the most uniform tubesheets among the currently available alternatives.
A modern example of module formation is provided by
Generon [3]. This process further reduces labor intensity
while striving to improve uniformity of fiber packing.
Individual fibers or fiber tows are removed from spools
produced by the spinning process and woven to form a
hollow fiber fabric. Weaving allows precise control of the
spacing between the fibers or tows. This fabric is wound
around a central tube to create a fiber bundle. The central
tube serves several functions: firstly mechanical support
of the fiber bundle, secondly contact points to facilitate
module movement through the manufacturing process,
and finally means to introduce or remove a fluid stream
from the center of the fiber bundle. Tubesheets were
formed by holding the module vertically, placing one end
into a mold, and introducing the tubesheet material. This
technique is referred to as ‘dunk’ potting and can suffer
from undesirable wicking if formation conditions are not
controlled carefully. Once rough tubesheets were formed
on both ends of the bundle, machining yielded the
desired final shape and opened fiber ends as in the
DuPont process. Subsequent improvements to the process included introduction of permeate collection channels in the fiber bundle to reduce shell side permeate
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pressure drop in lumen-fed modules [4], vacuum assisted
potting to improve tubesheet formation [5], and wrapping
of fiber tows with a support thread to improve handling in
the weaving process, in particular reduce splaying, and
thereby improve uniformity of fiber packing [6]. Wrapping fibers with one or more support threads also is used
to improve mechanical strength for high permeance fibers
that otherwise would be damaged during module manufacture [7].
Many examples of helically winding fibers to form a
structured bundle also exist. The earliest work dates back
to the late 1960s [8–10]. Helically wound modules may be
formed with a central core tube [11] or without [12,13].
The effect of wind angle on fiber length as the fiber
bundle is formed must be considered to ensure fiber
length and flow rate per fiber remain constant [14]. More
recent work suggests varying the wind angle in the axial
direction (in addition to changes with bundle diameter to
maintain constant fiber length) can produce modules with
improved flow distribution and reduced pressure drop in
the shell by forming a tapered bundle [15]. Additionally,
increasing the wind angle at the end of the bundle can
improve tubesheet strength by reducing the diameter of
the bundle and the amount of fiber enclosed in the
tubesheet [16]; pressure drop and lost membrane area
in the tubesheet are reduced as well. Note that the
weaving process also can be modified to produce a hollow
fiber fabric that will yield a tapered module [3].
Fiber bundles also can be produced from square or
circular fiber disks (wafers). Such wafers can be stacked
to form a stand-alone module [17–19] or used to produce a
module that can be placed in-line in a piping system [20].
Current Opinion in Chemical Engineering 2014, 4:18–24
20 Separation engineering
Significant effort has been devoted to adding module
design features that facilitate flow distribution in the shell
and improve mass transfer performance. These features
include the introduction of baffles to improve flow
uniformity in the shell region. The baffles may extend
radially in the form of tori [21,22] or axially in the form of
concentric sheets [23]. Flow distributors along the periphery of the fiber bundle near the shell inlet and outlet
also are used to improve uniformity [24–27]. These distributors typically consist of collars that create a distribution manifold around the fiber bundle to improve
uniformity of fluid flow from the external ports on the
case into the shell region of the fiber bundle. Recent work
describes the use of grooves and corrugated protrusions in
the distribution collar to reduce pressure drop and reduce
fiber blocking of the distribution holes in the collar [28].
Additional work describes modules possessing shell
distribution manifolds that penetrate into the interior
of the fiber bundle [29] and inserts that divide the bundle
into uniform sections to facilitate fluid distribution from a
central inlet tube and reduce solid accumulation on the
membrane surface in filtration applications [30].
Module performance evaluation
Fiber bundles may be formed from multiple fiber types to
perform two separations simultaneously by spiral winding
[31]. More recent work describes a module consisting of
two concentric layers of fibers that perform the separations sequentially by controlling the shell and lumen
flows between the two layers with proper tubesheet and
internal baffle designs [32].
While these models can adequately capture real module
performance, often significant deviations exist which are
attributed to non-ideal flow behavior. This behavior can
arise from fluid distribution from manifolds into the
lumen or shell regions or from non-uniform flow within
the lumen or shell as illustrated in Figure 2. The potential
impact of these non-idealities was recognized in the
earliest module fabrication patents [2].
The performance of a hollow fiber bundle most commonly is predicted assuming all fibers possess identical
geometry and transport properties. Additionally, the
lumen and shell flow rates are assumed to be identical
for each fiber. If concentration and temperature polarization are not important, or correlations are known for fluid
boundary layer resistances, mass balances for the retentate and permeate streams can be reduced to a set of
ordinary differential equations in co-current or countercurrent contacting [37]. The correlations reported by
Lipnizki and Field [38] commonly are used in these
calculations.
One can avoid the use of mass and heat transfer correlations by rigorously solving the two-dimensional conservation equations within a domain surrounding a single
fiber. This domain can be constructed from an assumed
regular square or triangular fiber arrangement [39] or by
defining an equivalent axisymmetric free surface surrounding the fiber [40] based on Happel’s free surface
model [41].
Although not practiced commercially, two novel module
designs were proposed recently [33]. The first design
wraps a woven hollow fiber fabric simultaneously with a
commercial spiral wound module spacer to form a bundle consisting of alternating layers of fiber and spacer.
The second design requires knitting hollow fibers
within the mesh of the spacer; the fiber knitted spacer
is wrapped to form the bundle. The performance of
these novel designs was compared to a randomly packed
fiber bundle, a woven fabric fiber bundle (with
and without a central distribution tube), and a crimped
(curly or wavy) fiber bundle in direct contact membrane
distillation. The highest water fluxes were observed for
the fiber knitted spacer bundle followed closely by the
crimped fiber bundle. These results were attributed to a
transverse flow component across curved fibers that is
not present when fibers are substantially straight and
parallel.
Park and Chang [42] demonstrated that with conventional
lumen headers the lumen flow rate is highest near the
center of the bundle and then decreases rapidly, passing
through a minimum, before increasing again at the bundle
periphery. Similar observations were made recently for
hemodialyzers possessing ports that direct flow normal to
the tubesheet but flow distribution could be enhanced by
directing the flow tangentially to the tubesheet surface
[43].
Such a conclusion is consistent with previous work on the
formation of crimped fiber bundles for dialysis applications [34–36]. This literature identifies specific design
ranges for the amplitude and frequency (wavelength) of
the wavy structure to optimize performance: amplitude = 20–200% of the fiber outer diameter and wavelength = 10–500 times the fiber outer diameter.
Fluid distribution from the shell manifold into the shell
space also is non-uniform. Concentration fields within an
operating hemodialyzer determined using X-ray computed tomography are consistent with higher shell flows
closer to the external port on the shell distribution manifold [44]. The flow distribution can be altered significantly by modifying the manifold to direct flow into the
Current Opinion in Chemical Engineering 2014, 4:18–24
Fluid distribution from external manifolds into the fiber
bundle is least well understood. Ideally, fluid is introduced and removed such that the pressure drop across the
lumen of each fiber is the same and the pressure drop
between any two points in the shell, at the same radial
position on opposing tubesheets, is the same. Equal
pressure drops lead to uniform flow rates.
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Hollow fiber membrane modules Mat, Lou and Lipscomb 21
Figure 2
(a)
(b)
(c)
(d)
ΔP
ΔP
Current Opinion in Chemical Engineering
Sources of non-ideal flow distribution within hollow fiber modules: (a) flow distribution from the lumen manifold into the fiber lumens; (b) flow
distribution from the shell manifold into the shell; (c) fiber inner diameter induced lumen flow variation; and (d) inter-fiber spacing induced shell flow
variation. The length of the arrows indicates the relative magnitude of the flow rate variation anticipated for each source. (a) and (b) contain the module
elements indicated in Figure 1 while (c) and (d) are abstracted pictorial illustrations of a bundle axial cross-section.
fiber bundle as well as around the fiber bundle using a
porous distribution collar or tab [45].
Non-uniform fiber properties (e.g. size and transport
properties) can lead to flow maldistribution in the lumen
even with uniform pressure drop across each fiber. A fiber
inner diameter variation leads to a flow rate variation that
can dramatically reduce module performance, especially
in the well-developed mass transfer limit (i.e. low feed
flow rates) [46–48]. Analyses of gas separation modules
indicate variability in fiber size has a much greater effect
on performance than transport properties due to the
dependence of flow rate on the fourth power of the inner
diameter [49–51]. For sufficiently broad fiber size variations, a limit on the maximum gas purity achievable may
exist but this effect can be mitigated by module staging
[52]. Few comparisons between experiment and theory
exist for commercial size modules, but a recent study
indicates that the inclusion of the actual fiber size distribution in predictions of gas separation performance could
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not account for the performance reductions observed
experimentally relative to a module with ideal flows
[53]; the differences were attributed to poor shell flow
distribution as discussed here.
Non-uniform fiber packing affects module performance
in a manner analogous to the effect of fiber inner
diameter. Variation in the shell space between fibers will
lead to flow rate variation that can be detrimental to
performance. The variation may be so large that fluid
effectively bypasses the fiber bundle as first noted by
Noda et al. [54]. Chen and Hlavacek describe an algorithm
for representing the variation in shell void volume for
randomly packed parallel fiber bundles using Voronoi
tessellation [55]. The flow through a cross-section of
the bundle can be calculated from the hydraulic diameter
of each tessellated cell and an appropriate friction factor
relationship. This algorithm has been used to evaluate
overall effective mass transfer coefficients by flow rate
averaging individual cell mass transfer coefficients
Current Opinion in Chemical Engineering 2014, 4:18–24
22 Separation engineering
[56,57]. Mass transfer coefficients for each cell are
determined from the hydraulic diameter of the cell and
an appropriate mass transfer coefficient correlation. An
alternative algorithm represents the variation in shell void
volume by surrounding a representative distribution of
fibers with a Happel free surface of varying radius [58].
The detrimental effect of random packing in parallel fiber
bundles has been reported for a variety of membrane
separations including membrane distillation [59], membrane extraction [60], and gas–liquid contactors [61,62].
The effects of a simultaneous variation in fiber outer
radius [63] and a fractal distribution of shell void volumes
[64] also have been investigated. Reported agreement
between experiment and theory generally is good using
one or more of the shell mass transfer coefficient correlations available in the literature. Differences between
theory and experiment typically are attributed to poor
flow distribution from the shell manifold into the shell
region.
Use of Voronoi tessellation or the free surface model can
be avoided through the use of a periodic cell containing a
finite number of randomly packed fibers to represent an
infinite fiber bundle [65]. The periodic cell must contain a
sufficiently large number of fibers to provide an accurate
representation but the number required can be determined by increasing the cell size until the results are
independent of number. The computational demand of
the simulation can be reduced in the entry [66] and well
developed [67] mass transfer limits where partial analytical solutions for the concentration field are available.
Comparisons of results obtained with the periodic cell
and either Voronoi tessellation or the free surface model
are limited but the periodic cell appears to predict slightly
lower values for effective mass transfer coefficient [63].
Fluid distribution from the shell manifold across the fiber
bundle represents an additional source of flow maldistribution in the shell. Fluid must flow from the shell inlet
distribution manifold across the fiber bundle before it can
flow along the fiber bundle toward the outlet; likewise,
the fluid also must flow across the fiber bundle at the
outlet end before it can be collected in the outlet manifold. This flow introduces a cross-flow component to the
nominally counter-current flow as well as a residence time
distribution due to different flow path lengths between
the inlet and outlet distribution manifolds; the paths near
the shell outlet are illustrated in Figure 1. Attempts to
evaluate fluid distribution have relied on approximating
the shell region as a porous media within which Darcy’s
law provides the relationship between volume-average
pressure and velocity [68,69]. This work demonstrates the
residence time distribution and its effect on overall shellside mass transfer coefficient depends on the product of
the radial to axial Darcy permeability ratio and the square
of the module aspect ratio (length to radius ratio). Darcy’s
Current Opinion in Chemical Engineering 2014, 4:18–24
law has been used to describe flow in the lumen as well as
the shell to evaluate the effect of shell-side flow distribution on gas separation [70] and hemodialysis [71]. The
results indicate that the residence time distribution can
lead to large concentration gradients at the lumen and
shell outlets but these gradients have surprisingly little
effect on module performance. Recent work on shellswept modules for gas dehydration is consistent with
these observations in that large concentration gradients
exist within the module but have minimal impact on
performance [72].
Recent attempts to reduce concentration polarization in
the shell region rely on novel fiber shapes. Fibers with a
corrugated or convoluted circumference can increase the
membrane area available per unit module volume as well
as enhance fluid mixing and thereby reduce concentration
polarization [73,74,75]. Recent work also demonstrates
that use of elliptical fibers can improve performance [76]
as well as introducing baffles around individual fibers [77].
Future challenges and conclusions
The patent literature describing the formation of large
hollow fiber bundles and modules dates back nearly
50 years. Industry currently favors the manufacture of
structured bundles using hollow fiber fabrics or spiral
winding techniques that offer better shell flow distribution and lower mass transfer resistance. Fiber crimping
also has been introduced to reduce mass transfer resistances. The introduction of fluid into the shell region of
these devices has received significant attention as it can
impact performance significantly.
The analysis of hollow fiber membrane module performance most commonly relies on assuming all fibers are
identical and the flow rates in the fiber lumen and shell
region surrounding each fiber are identical. However,
variability in fiber size and packing can reduce module
performance dramatically. Well-developed algorithms
exist to assess the uniformity of shell and lumen flows
and their effect on performance.
The effect of fluid distribution into the lumen and shell
regions of the fiber bundle through external ports and
manifolds has received less attention. The impact of the
lumen manifold on lumen flow uniformity appears to be
less significant than the effect of the shell manifold on
shell flow uniformity. However, additional work is
required to clarify the influence of both, especially distribution from the shell manifold which has been invoked as
the source of unexplained performance deviations.
Theoretical analyses of modern fiber bundle designs that
utilize fiber fabrics and crimped fibers have lagged commercial implementation. The availability of simulation
tools would help guide the evolution of these designs and
foster future innovations.
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Hollow fiber membrane modules Mat, Lou and Lipscomb 23
Comparisons of theoretical performance predictions to
experimental measurements for commercial scale units
are limited. Comparisons to hand-made, lab-scale units
can help assess simulation fidelity. However, conclusions
drawn from these units may not be applicable to larger
commercial modules due to manufacturing expertise that
leads to improved fiber and fiber bundle uniformity as
well as differences in module scale, for example, fluid
distribution from the shell manifold into the shell region
is expected to depend on the ratio of fiber outer diameter
to bundle diameter for sufficiently small bundles.
As higher performance membrane materials become
available with greatly enhanced permeances, concentration polarization and pressure drops will become more
important considerations in module design. Emerging
applications in forward osmosis are especially sensitive
to concentration polarization while carbon dioxide capture applications require low pressure drop designs.
References and recommended reading
Papers of particular interest, published within the period of review,
have been highlighted as:
of special interest
of outstanding interest
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This patent clearly identifies the key issues in module manufacture and
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