[membrane science and technology] membrane contactors: fundamentals, applications and potentialities...

Chapter 3. Module configurations and design

1. Introduction

In this Chapter module configurations and design for membrane contactors operations are

discussed. Different kinds of modules are presented and compared in terms of mass transfer

efficiency. Problems such as flow maldistribution and pressure drops and attempts to solve

them are reported. Being the hollow fiber module the configuration of major interest for

industrial applications, particular attention is devoted to it and to the research efforts made

worldwide for improving its performance. These include both theoretical studies, that consist

in the development of mathematical models able to describe the hollow fibers behaviour, and

new module configurations that have been conceived as alternative to the first tube-in-shell

modules with parallel flow of the streams. Modules layout is also discussed: the choice of a

particular module assembly depends on both the economic and the specific operating

conditions. The Chapter ends with information on commercial modules.

2. Modules used for membrane contactors applications

The performance of membrane contactors has been studied in different module

configurations. Modules with flat membranes are mainly used for laboratory tests, because of

106 Chapter 3

their easier building at lab scale and the simple and fast sheet replacement. Usually, a single

flat sheet is located between two plates that are equipped with the inlets/outlets of the

involved phases (Figure 1).

Inlets/Outlets

M e m b r a n e ~ ~ , N - ~ x x , x ~

Figure 1. Flat membrane contactor module.

For pilot plants as well as industrial scale applications, higher membrane area per volume

ratios are needed and the flat membranes are used in plate and frame or in spiral wound

configurations. For desalination tests a plate and frame system has been used by Andersson et

al. [1 ], whereas Gore [2] and Koschikowshi et al. [3] proposed a spiral wound configuration.

Tubular membranes have also been tested in membrane distillation with high viscous

fluids. However, at industrial scale, hollow fibers are mainly preferred, due to their high

packing density.

An important parameter to consider in the module design is the length needed to achieved

the desired results. As for conventional devices, also for membrane contactors, this length can

be expressed by:

Module Configurations and Design 107

L = HTU x NTU

where: HTU, height o f the transfer unit;

NTU, number o f transfer units.

(1)

With respect to packed towers, membrane contactors can operate also in orizontal position

and LTU (length of the transfer unit) can be used instead of HTU. LTU can be expressed as

[4]:

LTU = v/Ka (2)

where:

v, f luid velocity;

K, module-averaged overall mass transfer coefficient;

a, interfacial area.

Equation (2) states that, due to the very high interfacial area, membrane contactors lead to

lower values of LTU than conventional units.

Whatever kind of module is adopted, the principal targets to pursue for an efficient

performance are:

- to maximize the mass transfer;

- to reduce and control the fouling;

- to work with low pressure drops,

- to guarantee a constant performance of the module for all its length.

In order to limit the polarization phenomenon, several improvements of the first-type

developed modules have been proposed during years. First of all, turbulence promoters have

! ! i i �84

i �84184 U

been added to reduce the boundary layer resistances [5]. Ultrasonic treatments have been

iiii! i i �84

proposed as a mean to increase the trans-membrane flux (up to 200%) in membrane

distillation [6]. The formation of Dean vortices, which help in mixing the streams, has been

achieved by using several designs of curved membrane modules [7].

Being the hollow fiber of big interest for industrial applications, significant efforts have

been addressed to the improvement of the hollow fiber module design. When the module

design has to be carried out, several parameters have to be taken into account, such as

membrane properties (porosity, thickness, tortuosity), packing density, fibers length and

diameters, operative flowrates, pressures and concentrations, fluid physical properties,

pressure drops, breakthrough pressure. The type of flow inside the module has also to be

i i ~ ii ~III~ iili ~I;I ii~iiiil i I i~i

carefully chosen. For example, crossflow design leads to higher mass transfer coefficients

than co/counter-current one, but the pressure drops increase too. As it will be mentioned in

Chapter 4, one major limitation in the membrane contactor scale-up is the unavailability of a

general correlation for the prediction of the mass transfer coefficient at the shell side. This

lack is related to the non-uniform flow that can occur because of channeling, bypassing,

mixing, entry region phenomena, often caused by fiber deformation, non uniform fiber

distribution, polydispersity of fiber diameters, presence of stagnant zones. The first typology

of hollow fiber used was the tube-in-shell configuration with parallel flow of the involved

streams (Figure 2).

i �84

ii~ ~ii~ i~iiiiiil i iii iiiii I ii i!i~ !~ i I


I Stream 2 OUT ~ Stream 2 IN

5 3 _ Stream 1 IN ~ Stream 1 OUT

,- -= )

Figure 2. Tube-in-shell configuration with parallel flow of the involved streams.

This configuration suffers of a maldistribution of the liquid fed at the shell side that can

occur for the reasons above exposed and that leads to a reduction of the mass transfer

efficiency. The maldistribution of flow can be limited if the feed stream is sent in the lumen

of the fibers, but this implies higher pressure drops. Based on these results, researchers

dedicated a lot of efforts to improve the module design in terms of higher mass transfer

coefficients at the shell side. For example, several authors introduced baffles that deviated the

fluid leading to a transverse flow and to a local turbulence, with consequent increase of the

mass transfer [8-9]. Figure 3 shows a baffles-containing module. Both theoretical evaluations

and experimental tests carried out worldwide are presented and discussed in sections 3 and 4,

respectively.

Stream 1 ~N

I Stream 2 OUT ~ Stream 2 IN

I I Stream 1 OUT

Figure 3. Example of a baffled module.

110 Chapter 3

3. Theoretical studies on hollow fiber modules

Several are the mathematical models developed to analyze the performance of hollow fiber

modules when a not uniform flow is established at the shell side. Works on the calculation of

the fiber and flow distribution in randomly packed fibers have been carried out by different

authors [ 10-11 ]. More recently, Wu and Chen [ 12] evaluated for the water deoxygenation the

effect of flow maldistribution on mass transfer in a randomly packed hollow fiber module,

considering a parallel flow. They used a random fiber distribution model and the L6v~que's

equation and, by comparing the theoretical predictions with experimental data, they

concluded that the model was able to well estimate the performance of the system only when

an axial laminar flow is established. The shell side mass transfer coefficient for axial flow in

hollow fiber modules has been calculated by Lipniski and Field [13] who have taken into

account the influence of the entrance effects and packing fraction. The influence of

maldistribution has been analysed by using local mass transfer coefficients. The trends they

obtained are similar to those predicted by empirical correlations over a wide range of

operating flowrates and packing densities. Lemanski and Lipscomb [ 14] analysed the effect of

the shell-side flow on the performance of hollow fiber gas separation modules. In their system

the fluid is sent across the fibers through the inlet and outlet ports, while flowing along the

fibers between the ports. In order to include both the parallel and the cross flow that are

established in the module, the analysis was two-dimensional. They also considered the effect

of fiber packing, pressure and velocity fields by using the Darcy's law. The model predictions


were in closer agreement with experimental data (obtained for a shell-fed air separator) than

one dimensional plug flow model.

The variation in fiber size is another important cause of the not uniform flow. For example,

Lemanski et al. [ 15] studied the effect of the fiber properties (size, permeance and selectivity)

on the performance of a cross-flow hollow fiber gas separation module, finding that the size

distribution had the greatest influence on the process. Wickramasinghe et al. [ 16] proposed a

correlation to take into account the difference in fiber size, assuming a Gaussian distribution

of fiber radii:

k~v = k [1-9kV/ Q r~v + 7) coe + ..... ] (3)

where: k, tube side mass transfer coefficient for a uniform distribution o f f iber radii," kay, average tube side mass transfer coefficient; V, average volume occupied by one fiber; ray, average f iber radius; eo, standard deviation o f f iber radii divided by the mean.

The costs of a membrane module are necessarily related to the membrane area packed

inside. In hollow fiber modules, the membrane area depends on the fiber size and number.

Wickramasinghe et al. [ 17] performed an analysis to determine the optimum fiber diameter

and pointed out that lower membrane costs are possible by using small fiber diameters.

However, small fibers lead to high pumping costs, thus an optimum exists and authors found

it around a few hundred microns. Table 1 summarizes the theoretical studies above reported.

112 Chapter 3

Table 1. Theoretical studies on hollow fiber modules

Studies References

Random fiber distribution model [12]

Analysis of the influence of the entrance effects and packing fraction on the shell side mass transfer coefficient for axial flow

Two-dimensional analysis of the effect of packing fraction, pressure and velocity fields on the performance of hollow fiber gas separation modules

Studies of the effect of the fiber properties on the performance of a cross-flow hollow fiber gas separation module

Analysis to determine the optimum fiber diameter

[13]

[14]

[15]

[17]

4. New module configurations

In order to improve the efficiency of membrane contactors, different types of modules have

been developed. As previously stated, one of the first attempts made was to insert baffles

inside tube-in-shell modules, with the aim to promote transversal flow and to enhance the

mass transfer coefficient. Wang and Cussler [8] used baffles in both rectangular and

cylindrical modules containing a woven fabric of fibers. The rectangular module led to a

lower mass transfer because of stagnant zones that were created between adjacent fibers.

Woven fibers have been also tested by Wickramasinghe et al. [18], who noticed the better


performance of the system with respect to the modules built with individual fibers, because of

the more uniform fiber distribution achievable.

The transverse flow has been obtained by Bhaumik et al. [19] by means of a device

containing the fibers in a mat wrapped around a central tube (distributor of the liquid). The

authors demonstrated the efficiency of the system for the CO2 absorption in water. Figure 4

shows how this module looks like.

Hole T Stream 2 our T Stream 2 OUT

':~' 9 o q 0 " ........... o ..... Stream 2

T StreamliN ~ StreamlouT

Figure 4. Transverse flow in a device containing the fibers in a mat wrapped around a central tube (From [ 19], Copyright (1998), with permission from Elsevier).

The water deoxygenation has been carried out by Wickramasinghe et al. [16] in four

alternative configurations, all operating in cross-flow, and the results have been compared

with those achievable in a parallel flow cylindrical module. All modules led to higher removal

than the parallel flow cylindrical module (7%). The highest removal was achieved with the

114 Chapter 3

rectangular bundle module (98%), followed by the helical bundle (86%) and the cylindrical

bundle (82%). The crimped flat plate led to 72% of removal.

A three-phase hollow fiber membrane contactor with frame elements has been recently

proposed by Vladisavljevic and Mitrovic [20]. The module is made of stacks of polygonal

plates containing internal frames packed with hollow fibers. For each stack, the inlets and

outlets of the fluids flowing inside the fibers are provided on an external frame. Plates can be

monoaxial, if a two-phase contact is needed, or biaxial for allowing a three-phase contact. In

figure 5 the monoaxial and biaxial internal frames are depicted.

Iltl!lllll (a) (b)

Figure 5. Monoaxial (a) and biaxial (b) internal frames (From [20], Copyright (2001), with permission from Elsevier).

Authors claimed several advantages of their system over conventional parallel flow

modules, such as the possibility to adjust the fiber length independently on the module length,

the regular positioning of the fibers within the sheets that prevents the flow maldistribution

outside the fibers, the possibility to replace the only submodules that contain the failed fibers.


Concerning the pressure drops, their value at the shell side was lower than the tube side. In

latter case, authors found that the pressure drops were mainly related to the local obstacles in

the module rather than the resistance in the fibers.

Patents on new module designs have been recently presented.

TNO (The Netherlands) patented [21] a rectangular module containing fibers located at

well defined positions that ensure a good flow distribution. The system performs with high

mass transfer coefficients, low pressure drops, it is easy in the scale-up and has been

successfully tested in pilot plants for different applications [22]. The same Company patented

a new type of module, able to operate with gaseous streams at high pressures [23]. The

module houses hollow fiber membranes inside a pressure vessel and can be adopted for

absorbing species such as CO2 and H2S usually present in natural gas or petrochemical

streams.

A spiral-wound design for membrane contactor applications has been patented by Nitto

Denko of Japan [24]. The device consists of a central feed pipe around which membranes are

wound. Tests on water ozonation demonstrated the possibility to obtain a water with an ozone

content 10% higher than that achievable with hollow fibers.

The different kind of modules developed are reported in Table 2.

116 Chapter 3

Table 2. Module design developments ml | ,

Module design References

Baffles in both rectangular and cylindrical modules containing a woven fabric of fibers

Transverse flow by means of a device containing the fibers in a mat wrapped around a central tube

Crimped flat plate, rectangular, helical and cylindrical bundle modules operating in cross- flow

Three-phase hollow fiber membrane contactor with frame elements

Rectangular module containing well-located fibers

Module for high pressure gas treatments

Spiral-wound module with a central feed pipe

[8]

[19]

[16]

[201

[21]

[23]

[24]

5. Modules layout

In practical applications, it is usually impossible to achieve the desired target by using a

single module and a combination of different modules is often required. As other membrane

units, membrane contactors can be assembled in series or in parallel. The former type of

layout leads to higher efficiency (e.g., higher purities of the treated stream) whereas the latter

increases the system capacity. In order to reach the desired performance, both types of

assembling are sometimes necessary and a combination of parallel and series layouts is made

(Figure 6).


v I ~ j ,"-

""t ~ J

v t ~ j - -

"L 1-4 l---I 1

Figure 6. Membrane contactors assembled in a series/parallel fashion.

In defining the combinations of modules, it is important to take under control the pressure

drops inside the system. This constraint is more pronounced in membrane contactors with

respect to other membrane devices because of the breakthrough pressure limitations.

Generally, the number of parallel modules decreases as the the number of modules in series

increases, but more modules in series mean also an increase of pressure drops. Therefore, an

optimum will exist. Moreover, higher the number of modules in series lower the velocity of

the fluid through them and, thus, lower the mass transfer efficiency. The "tree-assembly" is

sometimes adopted for working with the same fluid velocity in all modules. In this layout the

fluid encounters, during its flow, a reduced cross sectional area (Figure 7). The total

membrane area is now reduced, but the pressure drops are increased.

118 Chapter 3

The module layout has, then, to be optimized depending on the specific application and on

the capital and operating costs. In Table 3 the main factors that influence the choice of a

particular module layout are summarized.

-L

j___. ,-[

Figure 7. "Tree assembly" of membrane contactors.

y

Table 3. Main factors that influence the choice of the module layout

Process capacity

Pressure drops

Membrane area

Pumping costs

Level of purity required

Mass transfer efficiency


6. Commercial modules

Several are the commercial modules that can be used for membrane contactors applications.

Some of them are produced by Companies that are not enterely devoted to membrane

contactors design. This is the case, for example, of the Microdyn Technologies, Enka

(Germany) that mainly produces modules for filtration purposes but offers also modules

equipped with polypropylene capillaries that have been used in membrane distillation

experiments [25-26].

In order to reduce problems of fouling and headlosses that can occur in membrane

contactors, Membrane Corporation (Minneapolis, MN) developed a bubble-free gas-liquid

mass transfer module containing various fiber bundles that are fluidized by the liquid flowing

outside. A 100% of gas transfer is obtained by sealing the fiber at one end. By working with

low packing densities high turbulence, no plug for suspended solids and low pressure drops

are achieved.

Modules for bubble-free ozonation of water (DISSO3LVE TM) are commercialized by W.L.

Gore&Associates (Elkton, MD). A helix arrangement of the ozone-resistant fibers in PTFE

characterizes this device.

Both hydrophobic and hydrophilic hollow fibers microporous membranes in shell-and-tube

modules are commercialized by Sepracor Inc.

GVS Spa (Italy) manufactures flat membrane contactors specifically developed for

controlling the air humidity. The membranes used are microporous super-hydrophobic (PTFE,

120 Chapter 3

PVDF or PP treated to increase the hydrophobicity) and the module has a plate and flame

configuration. Several are the advantages with respect to traditional dehumidifiers, such as up

to 50% lower capital and operating costs, low pressure drops and low noise emissions [27].

Membrana-Charlotte, a division of Celgard LLC, commercializes different hollow fiber

modules, among which the hollow fiber Liqui-Cel Extra Flow module.

The module has been developed mainly for gas-liquid applications, such as ultrapure water

production in electronical industry, water carbonation, water deareation, etc., but it has been

also successfully used for liquid-liquid extractions and osmotic distillation tests [28-29]. The

system has been designed for avoiding large pressure drops and for enhancing the mass

transport coefficient. It is characterized by a central baffle that forces the liquid stream (sent

to the shell side) to flow perpendicularly to the fibers. This implies a reduction of both mass

transport resistances and shell side bypassing with respect to parallel flow devices.

Furthermore, a more uniform fiber spacing is achieved thanks to a woven fabric of the fibers.

Figure 8 refers to the water deoxygenation process and shows the flow pattern of the liquid

stream at the shell side. The picture contains also information on the fiber size and structure

and the hollow fiber array.


Figure 8. Liqui-Cel| Extra-Flow Membrane Contactor. Liqui-Cel is a registered trademark of Membrana-Charlotte, a division of Celgard LLC (with permission).

Fotos of some commercial modules of different size are reported in Figure 9. Sengupta et

al. [30] developed for the LiquiCel devices a procedure for predicting the separation

performance of similar contactors (similar packing densities, same hollow fiber properties) of

different size.

122 Chapter 3

Figure 9. Membrane Contactors commercialized by Membrana. Liqui-Cel is a registered trademark of Membrana-Charlotte, a division of Celgard LLC (with permission).

Compact Membrane Systems, Inc. (USA) has developed composite membranes by coating

different microporous supports (polypropylene, PVDF, polysulfone, etc.), both as hollow

fibers and flat sheets, with a thin nonporous perfluoro layer. The composite membranes are

characterized by higher contact angles than the supports and possess an excellent thermal

stability. Furthermore, the presence of the dense layer allows to operate at high pressures

without the problem of the breakthrough. The membranes have high gas fluxes and find

application in different areas such as bubbleless introduction/removal of gases, portable

oxygen for respiratory care, emissions reduction et. CMS commercializes both hollow fiber

modules and flat sheet configurations.

Gas permeable membranes made using patented hollow fiber technology are also employed

in a contactor designed for water ozonation by Mykrolis Corp. (USA).


The main characteristics of some of the commercial modules described are summarized in

Table 4.

Table 4. Main characteristics of some commercial modules

Company Membrane type Membrane area (m 2)

Pore size

(l.tm)

Microdyn Technologies (Enka)

GVS SpA

Capillary polypropylene 0.1 - 10 0.2

Flat superhydrophobic 0.5 - 5 0.02-5

Membrana-Charlotte Hollow fiber polypropylene 0.18 - 220 0.03

124 Chapter 3

References

[1] S.I. Andersonn, N. Kjellander and B. Rodesjo. Design and field tests of a new membrane

distillation desalination process. Desal., 56 (1985) 345-354

[2] D.W. Gore. Gore-Tex Membrane distillation. Proc. of the 10 th Ann. Con. Water (1982) 25-29

[3] J. Koschilowski, M. Wieghaus and M. Rommel. Solar thermal-driven desalination plants based on

membrane distillation. Desal., 156 (2003) 295-304

[4] A. Gabelman and S.T. Hwang. Hollow fiber membrane contactors. J. Membrane Sci., 159 (1999)

61-106

[5] L. Martinez-Diez, M.I. Vasquez-Gonzalez, F.J. Florido-Diaz. Study of membrane distillation using

channel spacers. J. Membrane Sci., 144 (1998) 45-56

[6] C. Zhu and G. Liu. Modelling of ultrasonic enhancement on membrane distillation. J. Membrane

Sci., 176 (2000) 31-41

[7] J.N. Ghogomu, C. Guigui, J.C. Rouch, M.J. Clifton and P. Aptel. Hollow fibre membrane module

design: comparison of different curved geometries with Dean vortices. J. Membrane Sci. 81 (2001)

71-80

[8] K.L. Wang and E.L. Cussler. Baffled membrane modules made with hollow fiber fabric. J.

Membrane Sci., 85 (1993) 265-278

[9] A.F. Seibert and J.R. Fair. Scale-up of hollow fiber extractors. Sep. Sci. Technol., 32 N.1-4 (1997)

573-583

[ 10] V. Chen and M. Hlavacek. Applications of Voronoi tessellation for modeling randomly

packed hollow fiber bundles. AIChE J., 41 (1995) 2322


[ 11 ] J.D. Rogers and R. Long. Modeling hollow fiber membrane contactors using film theory,

Voronoi tessellations and facilitation factors for systems with interface reactions. J. Membrane

Sci., 134 (1997) 1

[12] J. Wu and V. Chen. Shell-side mass transfer performance of randomly packed hollow fiber

modules. J. Membrane Sci. 172 (2000) 59-74

[13] F. Lipniski and R.W. Field. Mass transfer performance for hollow fibre modules with shell-

side axial feed flow: using an engineering approach to develop a framework. J. Membrane Sci.,

193 (2001) 195-208

[14] J. Lemanski and G.G. Lipscomb. Effect of shell-side flows on the performance ofhollow-fiber

gas separation modules. J. Membrane Sci., 195 (2001) 215-228

[ 15] J. Lemanski, B. Liu and G. G. Lipscomb. Effect of fiber variation on the performance of

cross-flow hollow fiber gas separation modules. J. Membrane Sci. 153 (1999) 33-43

[ 16] S.R. Wickramasinghe, M.J. Semmens and E.L. Cussler. Mass transfer in various hollow fiber

geometries. J. Membrane Sci., 69 (1992) 235-250

[17] S.R. Wickramasinghe, M.J. Semmens and E.L. Cussler. Better hollow fiber contactors. J.

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126 Chapter 3

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[23] TNO (The Netherlands), US 6,355,092

[24] Nitto Denko (Japan), US 6,168,648

[25] E. Drioli, A. Criscuoli and E. Curcio. Integrated membrane operations for seawater

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[membrane science and technology] membrane contactors: fundamentals, applications and potentialities...

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