Embed Size (px)
Citation preview
CROSS-FLOW FILTRATION WITH A ROTATING MEMBRANE
By
Pengyu Ji
A THESIS
Submitted to Michigan State University
In partial fulfillment of the requirements for the degree of
Chemistry - Master of Science
2013
ABSTRACT
CROSS-FLOW FILTRATION WITH A ROTATING MEMBRANE
By
Pengyu Ji
This thesis describes a cross-flow filtration system containing a mechanically rotated
porous membrane. The unique dynamic filtration system combines the concepts of
membrane filtration and centrifugation. In principle, the system should concentrate low-
density suspended particles or liquid droplets near the center of the membrane lumen and
create large shear forces along the membrane inner surface to mitigate fouling. This is
particularly relevant to oil-water separations where low-density oil fouls the surface of
membranes in conventional cross-flow filtration. Membrane rotation should decrease flux
declines during filtration compared to the same experiment with a stationary membrane.
Unfortunately, this study did not demonstrate such a result, perhaps because the oil
droplets were too small to experience sufficient centripetal force to remove them from the
membrane surface. Model experiments with filtration of hollow glass beads (density =
0.46 g/mL), however, provide preliminary data that membrane rotation can reduce
fouling. At low cross-flow rates, membrane rotation decreases flux declines during
filtration of bead-containing solutions. Additionally, light scattering studies of solutions
containing the beads that collected on the membrane during filtration show that rotation
leads to smaller beads on the membrane surface. Centripetal force presumably moves
most larger beads away from the membrane surface. Future studies should re-exam the
performance of this system in oil-water separations with larger oil droplets.
iii
ACKNOWLEDGMENTS
I would like to thank Dr. Merlin L. Bruening and Dr. Volodymyr V. Tarabara for their
guidance throughout my graduate study. Especially, I would like to thank my advisor Dr.
Merlin L. Bruening for all time and effort he put on this thesis.
I would also want to thank Dr. Gary Blanchard and Dr. Gregory Baker for all the helpful
suggestions during my graduate study.
I want to thank all the lab members for their kind help. I especially want to think Dr.
Elodie Pasco, Christopher Crock, Hang Shi, Bin Guo and Hao Xu.
Finally, I am indebted to my parents and my fiancée Yueru Ni. Their constant love and
support for my life and study here in Michigan State University were beyond words.
iv
TABLE OF CONTENTS
LIST OF TABLES .......................................................................................................................... v
LIST OF FIGURES ....................................................................................................................... vi
KEY TO ABBREVIATIONS ...................................................................................................... viii
Chapter 1. Cross-flow Filtration with a Rotating Membrane for Oil and Water Separation .......... 1 1.1 Introduction ........................................................................................................................... 1
1.1.1 The scale of oil-water separations .................................................................................. 1 1.1.2 Current separation technologies ..................................................................................... 2
1.2 Approach ............................................................................................................................... 5 1.2.1 Separation mechanism for cross-flow filtration with a rotating membrane ................... 6 1.2.2 Flow profiles in rotating membranes with crossflow ..................................................... 8
1.3 Experimental ....................................................................................................................... 11 1.3.1 Materials ....................................................................................................................... 11 1.3.2 Characterization of oil-water dispersions and membranes ........................................... 12 1.3.3 Experiment setup and operation ................................................................................... 13
1.4 Result and discussion .......................................................................................................... 16 1.4.1 Oil droplet size distribution and rejection range .......................................................... 16 1.4.2 Control experiments: filtration of water and water with PVA .................................... 20 1.4.3 Filtration of kerosene/water dispersions through rotating and stationary membranes . 22
REFERENCES .......................................................................................................................... 25
Chapter 2. Rotating Cross-flow Filtration System to Decrease Fouling by Low-density Particles....................................................................................................................................................... 28
2.1 Introduction ......................................................................................................................... 28 2.1.1 Dynamic shear-enhanced membrane filtration ............................................................. 28 2.1.2 Rotating cylindrical membranes ................................................................................... 29 2.1.3 Rotating disk systems ................................................................................................... 30 2.1.4 Vibratory shear-enhanced processing (VSEP) ............................................................. 30
2.2 Experimental ....................................................................................................................... 31 2.2.1 Materials ....................................................................................................................... 31 2.2.2 Characterization of glass-bead dispersions ................................................................... 31 2.2.3 Experiment setup and operation ................................................................................... 32
2.3 Results and discussion ......................................................................................................... 35 2.3.1 Fouling of a rotating cross-flow membrane filtration system ...................................... 35 2.3.2 Particle size distribution in cakes formed on membrane surfaces ................................ 38 2.3.3 Force balance on beads at the membrane surface ......................................................... 41
2.4 Conclusions ......................................................................................................................... 47 REFERENCES .......................................................................................................................... 48
v
LIST OF TABLES
Table 1. Comparison of methods for treating produced water ....................................................... 4
Table 2. Experimental parameters ................................................................................................ 33
Table 3. Forces acting on beads (for average 15 µm) at the membrane wall during cross-flow filtration, and parameters employed to calculate these forces. ..................................................... 45
Table 4. Force balance on beads at the wall of a rotating membrane wall during cross-flow filtration, for beads with different diameters. ............................................................................... 46
vi
LIST OF FIGURES
Figure 1. Force balance in a conventional centrifuge (A) and a porous centrifuge (B) ................. 8
Figure 2. Flow profiles for swirling flow induced by a mixer (A) and a rotating cylinder (B). Figures B and D show flow profiles in horizontal cross sections for A and B respectively. ....... 10
Figure 3. Scanning Electron Microscope images of a TAMI TiO2 ceramic membrane (0.80 µm pore size). The images show the membrane separation skin layer (left) and a cross-section (right). ........................................................................................................................................... 13
Figure 4. Apparatus for cross-flow filtration with a rotating membrane ...................................... 15
Figure 5. Typical size distribution of oil droplets created by recirculation of a water/kerosene/poly(vinyl alcohol) mixture using a shear pump. The vertical line shows the transistion from positive to negative oil droplet velocities in a rotating membrane (see Figure 6)........................................................................................................................................................ 18
Figure 6. Calculated values of rV for oil droplets of different sizes in a membrane rotating at 1752 rpm. rV values were calculated using equation (1.5) with J=0.00053 m/s and r=R=3mm. The vertical line shows the approximate transition from positive to negative velocities. (Positive velocities are toward the center of the membrane lumen.) ........................................................... 19
Figure 7. Specific flux as a function of time during filtration of water and water containing 0.01 g/L or 0.1 g/L of PVA through a stationary 0.14 µm TAMI tubular ceramic membrane. ........... 21
Figure 8. Evolution of specific permeate flux with time during filtration of kerosene/water dispersions through rotating and stationary membranes with 0.14 mm pores. The experiments were repeated multiple times with the same membrane, and the feed solutions contained 250 ppm kerosene and 10 ppm PVA. .................................................................................................. 24
Figure 9. Apparatus for cross-flow filtration with a rotating membrane. ..................................... 34
Figure 10. Concentration of hollow glass beads in the feed tank as a function of time during cross-flow filtration with and without rotating the membrane. The average cross-flow velocity in the membrane lumen was 0.36 m/s. .............................................................................................. 36
Figure 11. Concentration of hollow glass beads in the feed tank as a function of time during cross-flow filtration with and without rotating the membrane. The average cross-flow velocity in the membrane lumen was 0.06 m/s. The error bars are standard deviations of three replicate filtrations. ...................................................................................................................................... 37
Figure 12. Bead size distribution for the feed solution and membrane cakes collected during filtration with stationary and rotating membranes. The average cross-flow velocity in the membrane lumen was 0.36 m/s. Beads deposited in the cake were collected in an aqueous backflush. Similar trends occurred in three replicate filtrations. .................................................. 39
vii
Figure 13. Bead size distribution for the feed solution and membrane cakes collected during filtration with stationary and rotating membranes. The average cross-flow velocity in the membrane lumen was 0.06 m/s. Similar trends appeared in five replicate filtrations. ................. 40
Figure 14. Force balance on beads at the surface of a stationary membrane (brown box). Fx is the cross-flow drag force, FJ is the permeate drag force and f is the buoyancy force. ................. 44
viii
KEY TO ABBREVIATIONS
ω Angular velocity
θV Tangential velocity
dd Diameter of the droplet
rV Radial velocity
waterρ Density of water
oilρ Density of oil
J Water flux
R Radius of the tubular membrane
FD Frictional force
η Solution viscosity
xV Velocity due to cross flow
Δρ Difference in density between the bead and solution
r Distance from center of the particle to the wall
kB Boltzmann constant
T Temperature
1
Chapter 1. Cross-flow Filtration with a Rotating Membrane for Oil and Water Separation
1.1 Introduction
1.1.1 The scale of oil-water separations
Due to both natural water layers in oil and gas reservoirs and the injection of water to
achieve maximum oil recovery during drilling, large amounts of oil-contaminated water, termed
produced water, result from crude oil extraction. Produced water contains suspended and
dissolved crude oil (typically 100 to 5,000 mg/L [1]), which can cause widespread
environmental contamination if the produced water is not treated before discharge or re-injection
into the oil well. After gravity-based oil-water separation using oil skimming, produced water
from offshore wells still contains oil concentrations ranging from 15 to 40 mg/L, depending on
the country and specific location of the offshore platform [2]. This level is unacceptably high for
direct water discharge into the environment. Regulators all over the world have set limits on the
discharge of oil in produced water. The U.S. Environmental Protection Agency requires that
produced water discharge, as well as overboard water, must contain oil concentrations less than
29 ppm (monthly average) [3]. In China, monthly allowed average limits are 10 ppm and
100ppm for oil and chemical oxygen demand (COD), respectively, in produced water [4]. In
Europe, based on the Convention for the Protection of the Marine Environment of the North-East
Atlantic agreement, the annual average limit of crude oil in discharged produced water is 40 ppm
[5].
On the other hand, large quantities of crude oil are lost in produced water. In 2002, the
U.S. generated more than 1.6 billion m3 of produced water [6]. In the same year, oil refineries in
the European Union countries produced more than 2,000 million tons of wastewater [7], and this
2
number is increasing. For 2007, Energyfiles, an oil and gas forecasting service company,
estimated the global amount of produced water production at around 98 billion barrels [5].
Assuming the average concentration of crude oil in these 98 billion barrels is 400 ppm, simple
arithmetic shows that almost 40 million barrels (or 1680 million gallons) of crude oil worth more
than five billion dollars are wasted in produced water. Thus, separation of oil and water is
essential to today’s world, from both environmental and economic points of view.
1.1.2 Current separation technologies
Oil occurs in three forms in produced water: 1) Free oil, which exists as large droplets; 2)
dispersed oil, which includes small oil droplets with diameters usually <100 µm; and 3)
dissolved oil [8]. Removal of free oil from water is relatively easy because of the specific gravity
difference between oil and water. In large tanks, a skimmer removes the oil layer that floats on
the water, and heavy solids goes to the sludge [9]. Gas flotation is a similar separation method
with better removal efficiency. Injection of nitrogen gas with coagulant (such as ferric chloride
or aluminum sulfate) into the flotation tank facilitates skimming of the oil phase [5, 10, 11].
Compared to free oil, dispersed oil is harder to remove due to the small droplet size.
Popular separation methods for oil emulsions employ centrifuges and hydrocyclones. These two
apparati share several features: they both separate oil and water using centripetal force; they both
are energy intensive; and both centrifuges and hydrocyclones are fairly compact devices
amenable to deployment on off-shore drilling platforms. In normal water treatment, centrifuges
remove oil droplets as small as 5 µm, whereas hydrocyclones only remove oil droplets that are
larger than 20 µm [12]. Nevertheless, hydrocyclones are attractive because they consumes less
energy than centrifuges[13]. In industry, hydrocyclones are actually more commonly used as
3
solid-liquid separators. Thus, in some integrated oil/water separation systems, hydrocyclones are
replacing gravity separators for the first separation step to decrease retention times and device
footprints. Gravity separators (skimmers) are large and have high rentention times.
For dissolved oil and oil droplets (<5 µm), membrane filtration is likely the most efficient
method for oil-water separations [14]. However, fouling of the membrane with oil continues to
plague such methods. A few studies examined new coatings that may decrease oil accumulation
at the membrane surface [15-17]. Meanwhile, other studies attempted to reduce fouling using a
specific filtration configuration and high shear flow [18].
Other techniques for removing oil from water include absorption and spray freezing [5].
Table 1 compares the common oil-water separation methods. Gravity separation, air induced
flotation and absorption can process emulsions with high oil concentrations, but the process
retention time is long [5] and these methods only move big oil droplets. Centrifugation is fast,
efficient and removes small oil droplets, but it suffers from high energy costs [19]. Freeze
spraying is cheap and effective, but it requires temperature around -10 ℃ [20] which is energy
consuming.
4
Table 1. Comparison of methods for treating produced water
Technology Advantage Disadvantage Removal capacity by oil droplet size (µm)
Removal capacity by oil concentration
Gravity oil-water separator
Low cost, easy operation, durable
High retention time, need to skim off oil
layer 150
Water may contain high concentrations of oil
(>1000ppm)
Chemical assisted induced gas flotation
Low cost, easy operation, durable
High retention time, need to skim off oil
layer, need nitrogen gas 3-5
Hydrocyclone Efficient, compact module
Energy cost, no separation of solids,
high maintenance cost 20
Centrifuge Efficient, compact module
High energy cost, no separation of solids,
high maintenance cost 5
Adsorption Cheap High retention time, not
efficient for high concentration
5 Water may contain low
concentrations of oil (<1000ppm)
Spray freezing Cheap Only works at certain temperature ranges
(around -10 ℃) N/A
Membrane filter Higher recovery of fresh water, compact module
Energy cost, easy to foul N/A For trace oil
concentrations
Ref: [5], [12], [20]
5
Unfortunately, all current oil-water separation methods have assets and liabilities
and differ in their treatment capacities. No single method is sufficient for produced water
treatment. Combinations of treatment methods are essential to both achieve sufficient oil
removal and keep costs low. Most combined treatments employ a rough pre-treatment
such as gravity separation, followed by centrifugation and finally a process to remove
trace oil, most likely membrane filtration. Importantly, new hybrid technologies that
combine the concepts of existing technologies, for example apparatus combined of air
injection and hydrocyclone which developed by Zhao and Bai [21, 22], are achieving
encouraging results.
1.2 Approach
This thesis explores separations that employ cross-flow filtration with a rotating
membrane which is a novel dynamic shear enhanced filtration system. The work aims to
demonstrate that rotating flows can enhance oil-water separations and decrease
membrane fouling. The hypotheses are:
1) Large oil droplets will concentrate near the center of the membrane lumen due to
centripetal force;
2) Small oil droplets may move to the membrane surface with the permeate flow, but the
membrane will reject these droplets if they are larger than the membrane pore size;
3) High shear forces at the surface of the membrane will minimize fouling by oil.
I performed initial tests of these hypotheses in the separation of kerosene and water using
rotating and stationary membranes. In principle, the rotating membrane should foul more
slowly than the stationary one.
6
1.2.1 Separation mechanism for cross-flow filtration with a rotating membrane
In a conventional centrifuge-based separation, fluid mixtures have an angular
velocity ω (dtdθω = ), which is related to their tangential velocity θV ( ωrθV = ). In
circular motion (either in a centrifuge or swirling flow), oil droplets will experience a net
force whose magnitude is the difference between the centripetal force and the buoyant
force. Thus, equation (1.1) describes the force, F, on an oil droplet undergoing circular
motion in a dispersing medium.
r
2θV
oilρdispersantρ6
3dπd
F
−= (1.1)
In this equation, dd is the diameter of the droplet, ρ represents density, and r is the
radius of the circular motion, which is large compared to the diameter of the droplet.
In a simple oil and water system, oil is less dense than water and will experience a
force pointing to the center of the circle of motion. However, when the oil droplet moves,
it will experience the frictional force, FD, described in equation (1.2), where rV is the
radial velocity.
rVdμπd3DF = (1.2)
Figure 1A shows the force balance on an oil droplet in circular motion. Equation the
forces in Equation (1.1) and (1.2) and solving for rV gives equation (1.3). Based on this
equation,
( )r
2θV
oilρwaterρμ18
2dd
rV −= (1.3)
7
larger difference in ( oilρwaterρ − ) and faster rotational velocities ( θV ) will lead to
larger droplet velocities and better separations. Similar separation principles apply to both
centrifuges and hydrocylones.
However if the wall of the centrifuge or hydrocyclone is porous, water flux
though the wall will add an additional drag force, FJ, described by equation (1.4), where J
is the water flux. Figure 1B shows the force balance in this case.
Jddfπμ3JF = (1.4)
Equating the forces in equations (1.2), (1.3), and (1.4) leads to equation (1.5) for
the radial velocity, rV . The sign and magnitude of rV depend on J, dd and the
tangential velocity
( )J
μr18
2dddρfρ2
θVrV −
−= (1.5)
8
Figure 1. Force balance in a conventional centrifuge (A) and a porous centrifuge (B). (For interpretation of the references to color in this and all other figures, the reader is referred
to the electronic version of this thesis)
( θV ). If the droplet diameter and θV are large enough and J is too small to reverse the
direction of the droplet’s movement, the droplets will move towards the center. On the
other hand, in the case of large permeate flux, J may be large enough that droplets will
move toward the porous wall. Thus, in cross-flow filtration with a rotating membrane,
large oil droplets will move towards the center of the tubular membrane, but small oil
droplets may move to the membrane surface with the permeate flow.
1.2.2 Flow profiles in rotating membranes with crossflow
When a fluid mixture rotates inside a membrane, the angular velocity ( ω ) will be
the same for all objects regardless of their position, but the tangential velocity ( ωrθV = )
of the fluid varies with the distance from the center of rotation. If the center of the
membrane is the center of flow rotation, the fluid at the center will have 0θV = because
Vr
FJ FD Vθ
Vr F
FD Vθ
F
B A
9
r=0. In contrast, the outermost layer of liquid near the membrane wall should have the
largest tangential velocity of ωR , where R is the radius of the tubular membrane.
However, if fluid rotation stems from a mixer or baffle and the cylinder wall is staying
stationary (Figure 2 A), the velocity of the outmost layer of liquid will be zero due to the
no-slip boundary condition. In this case, the tangential velocity of the fluid will initially
increase with radial distance from the center of the cross section, reach a maximum value,
and then drop to zero at the wall (Figure 2. B). In our filtration system, the tubular
membrane itself rotates, and the membrane wall induces swirls (Figure 2. C). The
tangential fluid velocity is maximum at the membrane wall and zero in the center (Figure
2. D). Thus, high value of θV at the wall can help minimize fouling, and at the same
time cause the fluid to swirl inside the membrane.
10
Figure 2. Flow profiles for swirling flow induced by a mixer (A) and a rotating cylinder (B). Figures B and D show flow profiles in horizontal cross sections for A and B respectively.
A B
C D
Vθ
Vθ
11
1.3 Experimental
1.3.1 Materials
Tubular titanium oxide ceramic membranes (FiltaniumTM, TAMI Industry,
France) with a 0.14 µm skin pore size were selected as model membranes. Each
membrane was 250 mm long with a 10 mm external diameter and a 6 mm inner diameter,
which provides an effective filtration area of 47 cm2.
Kerosene served as the model oil. The kerosene concentration in the membrane
permeate was determined through the measurement of copper concentration via atomic
absorption spectrometer. The copper was first exchanged from a water phase to an oil
phase with a reagent called Accorga (5050, Cytec Industries Inc, New Jersey). To prepare
the oil stock solution, 100 g of kerosene was gently stirred with 20.5 g of Accorga
solution for 5 min. A solution of copper sulphate was prepared by dissolving 5.6 g of
CuSO4•5H2O in 50 g of deionized water. Then, 35 g of the CuSO4 solution was added to
the oil/Accorga mixture, and this was gently stirred for another 30 min. The color of the
oil phase turned from yellowish to brown, which indicated that the copper transferred to
the oil phase. Stirring was then stopped, and the phases rapidly separated. The aqueous
phase was removed using a separating funnel. This oil was used to form emulsions (see
below), and after filtration, the copper was stripped into an aqueous solution by mixing
15 mL of emulsion with 5 mL of a pH 0.5 sulphuric acid solution. Mixing occurred by
sonication for 5 min. Flame atomic absorption (AA) spectroscopy (Varian AA-240) was
used to determine oil concentration via the measurement of copper concentration using an
absorption wavelength of 324.8 nm.
12
1.3.2 Characterization of oil-water dispersions and membranes
Oil emulsions were prepared by circulating a mixture containing 2 g of kerosene, 8 L of
distilled water and 0.01 g/L of polyvinyl alcohol (PVA, MW=25,000, Polysciences) with
a low shear screw pump (Moyno 33201, OH) inside of feed tank. Oil droplet sizes
stabilized after 30 min of circulation. The measured oil concentration in the resulting
feed solution was around 250 mg/L. A Mastersizer 2000 (Malvern, Westborough, MA)
light-scattering system was used to determine oil (refractive index = 1.4458) droplet sizes
in the feed water. Scanning electron microscope images of the model membranes were
captured using JEOL 6400 instrument (Japan Electron Optics Laboratories Ltd., Japan).
Deposition of 8 nm of sputtered gold rendered the samples conductive for imaging.
13
Figure 3. Scanning Electron Microscope images of a TAMI TiO2 ceramic membrane (0.80 µm pore size). The images show the membrane separation skin layer (left) and a
cross-section (right).
1.3.3 Experiment setup and operation
Figure 4 shows a diagram of our rotating membrane cross-flow filtration
apparatus. Oil-water separations were operated in the recycle mode (retentate was
returned to the feed tank). The feed oil-water dispersion was stored in a 15 L stainless
steel tank, which can be pressurized, and was passed through the membrane using the low
shear screw pump. The entire system was pressurized to 0.0344 mPa with a compressed
air cylinder.
2 µm
20 µm
14
The membrane was housed in a stainless steel holder (FiltaniumTM, TAMI Industry,
France) that was modified with a pulley at the top. After each filtration, the membrane
was chemically regenerated following the manufacturer’s protocol which included
washing with 1 L of 20g/L sodium hydroxide at 80 ℃ for 30 min followed by 1L of 1g/L
phosphoric acid at 50 ℃ for 15 min.
Two swivels (Rotary Systems Inc., MN) were installed at the two ends of the
membrane holder allowing for the free rotation of the entire membrane module. The
module was rotated by an electric motor via a belt-and-pulley arrangement. The speed of
the motor was 1725 rpm, and the apparatus was shielded with an external plywood box
for safety.
Pressure sensors (EW-68075, Cole Parmer, IL) were installed before and after the
membrane module, and a flow meter (M101, McMillan Inc., TX) was installed to
measure the retentate stream flow rate. A bypass pipe was installed parallel to the
membrane module to reduce the cross-flow rate in the membrane. Data from the pressure
sensors and flow meter were collected with a National Instrument data acquisition system
and Labview 2010 (National Instruments, TX).
15
Screw Pump
F
Flow Meter
P
Pressure Sensor
P
Pressure Sensor
Feed Tank
Membrane Holder
Feed Solution
Rententate
Mass Balance
Membrane
Permeate
Gas Cylinder
Electric Motor
Swivels
Figure 4. Apparatus for cross-flow filtration with a rotating membrane
Flow Meter Pressure Sensor
Electric Motor
Mass Balance
Feed Tank
Gas Cylinder
Retentate Swivels
Membrane Holder
Membrane
Screw Pump Pressure Sensor
Permeate
Feed Solution
16
1.4 Result and discussion
1.4.1 Oil droplet size distribution and rejection range
Figure 5 shows the typical oil droplet size distribution obtained by recirculating a
water/kerosene/poly(vinyl alcohol) mixture through using a shear pump. The figure
shows the distribution in both number and volume percent. Because droplet volume
increases with the diameter cubed, the center of the volume percent distribution is much
larger than the center of the number percent distribution. On a number basis, most of the
droplets have diameters between 0.6 to1 µm. In contrast, on a volume basis, most of the
oil resides in droplets with diameters between 10 and 50 µm.
In experiments with rotating membranes, the motor speed of ω =1725 rpm fixes
the value of θV for a given distance from the center of the membrane, whereas J depends
on the membrane’s permeability. If most of the oil moves to the center of the membrane
due to centripetal force, less oil will approach the surface of the membrane. Figure 6
shows the values of rV (at the inner wall of the membrane) as a function of droplet size
using equation (1.5) and experimental values of the permeate flux. Droplets with a
diameter > 18 μm have a positive rV pointing towards the center of membrane, so they
should not collect at the membrane wall. Moreover, the larger the droplets, the larger the
rV . However, droplets with a diameter < 18 μm, have a negative velocity and move to
the wall of membrane. Thus these droplets may accumulate at the wall and cause fouling
to occur.
This calculation shows the importance of obtaining large oil droplets to see
advantages of rotating membranes in resisting fouling. Formation of the emulsion using
the screw pump is relatively gentle compared to sonication and high speed mixing, and
17
thus yields large droplets that should move to the center of a rotating membrane. By
volume, almost half of the oil was in droplets that were larger than 18 µm (Figure 5).
Based on the above calculations and size distribution, with the rotating membrane about
half of the droplets (by volume) should move away from the membrane wall.
18
Figure 5. Typical size distribution of oil droplets created by recirculation of a water/kerosene/poly(vinyl alcohol) mixture using a shear pump. The vertical line shows the transistion from positive to negative oil droplet velocities in a rotating membrane (see
Figure 6).
0
5
10
15
20
25
0.3 3 30 300 3000
Perc
enta
te (%
)
Diameter (µm)
Oil volume based
Oil number based
18 µm
19
Figure 6. Calculated values of rV for oil droplets of different sizes in a membrane rotating at 1725 rpm. rV values were calculated using equation (1.5) with J=0.00053 m/s
and r=R=3mm. The vertical line shows the approximate transition from positive to negative velocities. (Positive velocities are toward the center of the membrane lumen.)
-0.005
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0 20 40 60 80 100 120
Vr (
m/s
)
Diameter (µm)
18 µm
20
1.4.2 Control experiments: filtration of water and water with PVA
To obtain baseline values of membrane permeability, I determined permeate flux
during filtration of pure water and water containing solutions containing 0.1 g/L and 0.01
g/L of PVA. Figure 7 shows the evolution of specific flux (flux normalized by pressure)
with time in these experiments. The pure water flux decreased about 15% over 30 min.
Perhaps a small amount of residual oil in the system slightly fouled the membrane. The
flux of the 0.1 g/L PVA solution decreased much more dramatically, dropping 80% over
30 min. This suggests that PVA fouls the membrane, even though a polymer with a
molecular weight of 25,000 should easily pass through a membrane with 0.14 µm pores.
However, because the kerosene concentration in later filtrations was 250 ppm, I
employed only 10ppm (0.01g/L) of PVA (1/25 of the oil concentration) in the actual
kerosene-water separations. During filtration of 0.01g/L PVA, flux only decreased 25%
over 30 min, which is similar to the decrease in the pure water flux. Thus the 0.01g/L
PVA in kerosene/water dispersions should not contribute greatly to fouling.
21
Figure 7. Specific flux as a function of time during filtration of water and water containing 0.01 g/L or 0.1 g/L of PVA through a stationary 0.14 µm TAMI tubular ceramic membrane.
0
2
4
6
8
10
12
14
16
18
20
0 500 1000 1500 2000
Nor
mal
ized
Per
mea
ter F
lux
(kg/
m2 /M
Pa/h
r)
Time (s)
Pure Water
0.01g/L PVA
0.1g/L PVA
22
1.4.3 Filtration of kerosene/water dispersions through rotating and stationary membranes
To examine the effect of membrane rotation on oil removal, I filtered aqueous
emulsions containing 250 ppm of kerosene through stationary and rotating membranes.
The solutions contained 10 ppm PVA and a kerosene droplet size distribution similar to
that in Figure 6. Figure 8 shows the specific fluxes as a function of time during three
filtrations with rotating membranes and two filtrations with nonrotating membranes. I
hoped to see significantly lower declines in flux over time with the rotating membranes
due to centripetal movement of oil droplets away from the membrane wall. However,
such a trend did not occur. The second filtration through the stationary membrane showed
the least flux decline of any of the experiments. Moreover, for the first 750 s of filtration,
the flux in the first filtration with the stationary membrane was higher than the flux
during two experiments with rotating membranes. In all of the experiments, the oil
concentration was zero in permeate, indicating 100% oil rejection by the membrane.
There are several possible reasons why rotating the membrane during filtration
did not increase flux:
1) Oil droplets were not large enough for centripetal forces to move them away from the
membrane wall. Even on a volume basis, only half of the oil droplet volumes had
diameters >18 µm, the approximate threshold for movement of droplets away from the
wall. Moreover, on a number basis, most of the oil droplets had diameters around 1 µm,
and thus would move toward the membrane wall even during rotation.
2) Permeate collection was <100% during rotation. When the membrane was rotating,
permeate may have splashed out of the collection device. This flaw would give low
values for the permeate flux during experiments with rotating membranes.
23
3) PVA could increase fouling during membrane rotation. Because the density of PVA is
greater than that of water, membrane rotation actually moves PVA toward the wall.
However, the concentration of PVA was low and PVA is highly soluble in water, so this
seems unlikely. Nevertheless, PVA may increase the density of oil droplets to decrease
centripetal force on the droplets.
During the second filtration with a stationary membrane, I suddenly started
rotating the membrane after 1500 s of filtration. As Figure 8 shows, when the membrane
started to rotate, flux initially increased. However this increase only lasted a few min
even though the membrane continued rotating. This result suggests that a high shear
force at the surface of the membrane during acceleration may have removed some of the
oil. However, overall this study does not demonstrate a clear benefit of combining
membrane rotation with filtration.
24
Figure 8. Evolution of specific permeate flux with time during filtration of kerosene/water dispersions through rotating and stationary membranes with 0.14 mm
pores. The experiments were repeated multiple times with the same membrane, and the feed solutions contained 250 ppm kerosene and 10 ppm PVA.
0
1
2
3
4
5
6
7
8
9
10
0 500 1000 1500 2000
Nor
mal
ized
Per
mea
ter F
lux
(kg/
m2 /M
Pa/h
r)
Time (s)
Rotating 1st
Rotating 2nd
Rotating 3rd
Stationary 1st
Stationary to rotatingStart to rotate
25
REFERENCES
26
REFERENCES
[1] J.H. Hargreaves, R.S. Silvester, Computational fluid-dynamics applied to the analysis of deoiling hydrocyclone performance, Chemical Engineering Research and Design, 68 (1990) 365-383.
[2] Ocean Studies Board and Marine Board, Oil in the sea III, National Academies Press, Washington, D.C., 2003.
[3] USEPA, http://www.epa.gov/radiation/tenorm/oilandgas.html
[4] G.T. Tellez, N. Nirmalakhandan, J.L. Gardea-Torresdey, Performance evaluation of an activated sludge system for removing petroleum hydrocarbons from oilfield produced water, Advances in Environmental Research, 6 (2002) 455-470.
[5] A. Fakhru’l-Razi, A. Pendashteh, L.C. Abdullah, D.R.A. Biak, S.S. Madaeni, Z.Z. Abidin, Review of technologies for oil and gas produced water treatment, Journal of Hazardous Materials, 170 (2009) 530-551.
[6] V. Rawn-Schatzinger, D. Arthur, and B. Langhus, Coalbed natural gas resources: water rights and treatment technologies, GasTIPS, 9 (2003) 13-18.
[7] H. Wake, Oil refineries: a review of their ecological impacts on the aquatic environment, Estuar Coast Shelf Science, 62 (2005) 131-140.
[8] J.A.P. Veil, Markus; Elcock, Deborah; Redweik, Jr., Robert, A White Paper Describing Produced Water from Production of Crude Oil, Natural Gas, and Coal Bed Methane, prepared by Argonne National Laboratory for U.S. Department of Energy, 2004.
[9] M.R. Beychok, Aqueous Wastes from Petroleum and Petrochemical Plants, John Wiley and Sons, 1967.
[10] A.A. Al-Shamrani, A. James, H. Xiao, Separation of oil from water by dissolved air flotation, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 209 (2002) 15-26.
[11] K. Bensadok, M. Belkacem, G. Nezzal, Treatment of cutting oil/water emulsion by coupling coagulation and dissolved air flotation, Desalination, 206 (2007) 440-448.
[12] N. Kharoua, L. Khezzar, Z. Nemouchi, Hydrocyclones for De-oiling Applications—A Review, Petroleum Science and Technology, 28 (2010) 738-755.
[13] W. van den Broek, R. Plat, M. van der Zande, Comparison of Plate Separator, Centrifuge and Hydrocyclone, (1998).
27
[14] P. Xu, J.E. Drewes, D. Heil, Beneficial use of co-produced water through membrane treatment: technical-economic assessment, Desalination, 225 (2008) 139-155.
[15] S. Kasemset, A. Lee, D.J. Miller, B.D. Freeman, M.M. Sharma, Effect of polydopamine deposition conditions on fouling resistance, physical properties, and permeation properties of reverse osmosis membranes in oil/water separation, Journal of Membrane Science, 425–426 (2013) 208-216.
[16] Q. Wen, J. Di, L. Jiang, J. Yu, R. Xu, Zeolite-coated mesh film for efficient oil-water separation, Chemical Science, 4 (2013) 591-595.
[17] A.K. Kota, G. Kwon, W. Choi, J.M. Mabry, A. Tuteja, Hygro-responsive membranes for effective oil–water separation, Nat Commun, 3 (2012) 1025.
[18] M. Ebrahimi, O. Schmitz, S. Kerker, F. Liebermann, P. Czermak, Dynamic cross-flow filtration of oilfield produced water by rotating ceramic filter discs, Desalination and Water Treatment, 51 (2012) 1762-1768.
[19] A. Cambiella, J.M. Benito, C. Pazos, J. Coca, Centrifugal separation efficiency in the treatment of waste emulsified oils, Chemical Engineering Research and Design, 84 (2006) 69-76.
[20] W. Gao, D.W. Smith, D.C. Sego, Treatment of pulp mill and oil sands industrial wastewaters by the partial spray freezing process, Water Research, 38 (2004) 579-584.
[21] Z. Bai, H. Wang, S. Tu, Oil–water separation using hydrocyclones enhanced by air bubbles, Chemical Engineering Research and Design, 89 (2011) 55-59.
[22] L. Zhao, M. Jiang, F. Li, Experimental study on the separation performance of air-injected de-oil hydrocyclones, Chemical Engineering Research and Design, 88 (2010) 772-778.
28
Chapter 2. Rotating Cross-flow Filtration System to Decrease Fouling by Low-density
Particles
2.1 Introduction
This chapter describes the same system in Chapter 1, a dynamic shear-enhanced
membrane filtration system, which includes a rotating cylindrical ceramic membrane and
cross-flow filtration. I tested the system using model glass beads to demonstrate the
effects of rotation on membrane fouling. Shear forces move beads away from the
membrane to reduce bead deposition on the membrane wall, and this effect is more
pronounced with larger beads. To put the work in perspective, this introduction describes
other examples of shear-enhanced membrane filtration.
2.1.1 Dynamic shear-enhanced membrane filtration
Cross-flow filtration is a membrane separation technique employed in many
applications including desalination [1], wastewater treatment [2, 3], food and juice
manufacturing [4, 5], wine and beer making [6, 7], and blood plasma separation and
dialysis [8, 9]. Cross flow creates a shear force at the membrane surface, which
effectively reduces concentration polarization or cake formation. In conventional cross-
flow membrane filtration, cross-flow along the membrane surface provides the shear
force, which increases with cross-flow velocity [10]. In a related development, several
recent studies examined dynamic or shear-enhanced filtration, where the shear force is
higher than in conventional cross-flow membrane filtration. Instead of creating liquid
flows on stationary membrane surfaces, dynamic or shear-enhanced membrane filtration
includes moving parts such as rotating or vibrating membranes [10, 11].
29
There are several advantages to dynamic shear-enhanced filtration over cross-
flow filtration, the most obvious of which is the higher shear force. Due to the no-slip
condition in conventional cross-flow membrane filtration [12], the cross-flow fluid
velocity drops to zero at the membrane surface. In contrast, in dynamic filtration with a
moving membrane the shear force is highest at the membrane wall. Additionally,
membrane rotation or vibration can create higher velocities than conventional cross flow
[10].
A second important advantage of dynamic filtration is decoupling of flow velocity
and shear force. As mentioned above, in conventional cross-flow shear is proportional to
feed cross flow velocity. However, in dynamic filtration, shear depends on the velocity of
the moving part not the inlet fluid flow rate[10].
2.1.2 Rotating cylindrical membranes
The first commercialized dynamic filtration apparatus employed a rotating
cylindrical membrane [10]. They system, which relied on Taylor-Couette flow [11]
contained an inner cylinder (the membrane) rotating concentrically inside of an outer
nonporous cylinder. The rotation speed ranged from 2000 to 4000 rpm [13, 14]. Reducing
the gap between the inner and outer cylinders increased filtration performance [15], and
this system successfully separated plasma from blood [16-18]. Filtration was fast, and the
energy cost was not a problem because the size of this plasma separator was small, 600
mL in process volume [10]. In 1985, Vigo and co-workers used a rotating membrane
system to separate oil and water [15]. Their study showed that permeate flux increased as
the annular gap decreased or tangential velocity increased. My rotating membrane
30
apparatus only have one cylinder and is operated under inside-out filtration mode, while
rotating cylinder apparatus has two cylinders and operated under outside-in filtration
mode.
2.1.3 Rotating disk systems
There are two types of rotating disk membrane filtration. In one design, the
membrane is stationary, and rotating disks, which are usually made of metal, create high
shear on the membrane surface. The Optifilter CR system commercialized by Metso
Paper (Finland), which was used in more than 30 pulp and paper effluent or pigment
recovery facilities, is one example of such a system [19]. However, the performance of
this design still can be limited by no-slip condition. In another design, rotating disks are
made of membrane not metal. In order to achieve high shear, the membrane disks are
housed either inside a small chamber or overlapped with each other. The MSD system
commercialized by Westfalia Separator employs this strategy by rotating ceramic
membrane disks mounted on eight parallel shafts and membrane disks on the shafts that
next to each other were overlapped [20, 21]. Both ceramic membrane and polymeric
membrane can function in both rotating disk membrane designs [10]. Shear increases
with increasing rotation speed and decreasing gaps between disks or chamber.
2.1.4 Vibratory shear-enhanced processing (VSEP)
Vibratory shear-enhanced processing simply involves putting a membrane on a
vibrating base. Both New Logic Research and Pall Corp. produced similar types of VSEP
systems. They used flat sheet membranes, and the vibration resonant frequency was about
31
60 Hz. The systems served in biotechnological and food applications [10]. Hollow fiber
membranes can also function in VSEP systems [22, 23]. To vibrate fibers, a hollow fiber
cartridge was attached to a sliding rod connected to a rotating disk to produce a axial
oscillations [10].
This chapter describes the same cross-flow with a rotating membrane apparatus in
Chapter 1. A dispersion of hollow glass beads served as a model system to examine the
effects of rotation on membrane fouling.
2.2 Experimental
2.2.1 Materials
Tubular titanium oxide ceramic membranes (FiltaniumTM, TAMI Industry, France)
with a 0.14 µm skin pore size were selected as model filters. Each membrane was 250
mm long with a 10 mm external diameter and 6 mm inner diameter. Thus, the effective
filtration area was 47 cm2. Hollow glass beads with a density of 0.46 g/mL and a size
ranging from 5 to 27 µm were purchased from Cospheric LCC (Santa Barbara, CA).
Homogeneous bead dispersions were prepared by continuous magnetic stirring of 1.18 g
of glass beads in 4 L of deionized water. Beads sizes were large enough for complete
rejection by the membrane.
2.2.2 Characterization of glass-bead dispersions
A Mastersizer 2000 (Malvern, Westborough, MA) light scattering system was used
to determine the glass bead (refractive index =1.52) size distribution in the feed water.
32
UV-Vis spectroscopy (MultiSpec-1501, Shimazu, Japan) was used to determine glass
bead concentration via the extent of light scattering at 800 nm.
2.2.3 Experiment setup and operation
Figure 9 shows a diagram of the rotating membrane cross-flow filtration apparatus.
The feed dispersion of glass beads was stored in an open bucket with continuous
magnetic stirring, and was passed through the membrane by a diaphragm pump
(HYDRA-CELL M-03, Wanner Engineering Inc., MN). Retentate and permeate flows
were returned to the feed tank to keep the liquid volume and the bead concentration in the
feed nearly constant. The pump provided the transmembrane pressure (~ 0.1 MPa),
which was adjusted by a back-pressure valve. A pulse dampener minimized the
fluctuation of the pressure in system.
The membrane was housed in a stainless steel holder (FiltaniumTM, TAMI Industry,
France) that was modified with a pulley at the top. After each filtration, the membrane
was chemically regenerated following the manufacturer’s protocol, which included
washing with 1 L of 20g/L sodium hydroxide at 80 ℃ for 30 mins followed by 1 L of
1g/L phosphoric acid at 50 ℃ for 15 mins.
Two swivels (Rotary Systems Inc., MN) were installed at the two ends of the
membrane holder allowing for the free rotation of the entire membrane module. The
module was rotated by an electric motor at 1725 rpm via a belt-and-pulley arrangement.
The apparatus was shielded with an external plywood box for safety.
Pressure sensors (EW-68075, Cole Parmer, IL) were installed before and after the
membrane module, and a flow meter (M101, McMillan Inc., TX) monitored the retentate
33
stream flow rate. A bypass pipe was installed parallel to the membrane module to reduce
cross flow through the membrane. Data from the pressure sensors and flow meter were
collected with a National Instrument data acquisition system using Labview 2010
(National Instruments, TX). After each experiment, a back flush washed the cake off the
membrane surface with around 1 L of deionized water. Analysis of the backflux with the
Mastersizer showed which particles were retained on the membrane surface.
Table 2. Experimental parameters
Bead diameter 5-27 µm
Bead density 460 kg/m3
Dispersant water
Membrane cut-off pore size 0.14 µm
Permeate flux (initial) 3169 L/(m2h)
Cross-flow velocity (average) slow 0.06 m/s
Cross-flow velocity (average) fast 0.36 m/s
Rotation frequency 1725 rpm
Average trans-membrane pressure 0.1 mPa
34
Pump
F
Flow Meter
P
Pressure Sensor
P
Pressure Sensor
Membrane
Electric Motor
Swivels
Dampener
Membrane Holder
Bypass
Permeate
Backup Pressure Valve
Figure 9. Apparatus for cross-flow filtration with a rotating membrane.
Pressure Sensor
Flowmeter
Backup Pressure Valve
Bypass
Permeate
Pump
Feed Tank Dampener
Pressure Sensor
Swivels
Electric Motor
Membrane
Membrane Holder
35
2.3 Results and discussion
2.3.1 Fouling of a rotating cross-flow membrane filtration system
I first monitored the concentration of beads in the feed tank as a function of filtration time.
Decreases in the bead concentration should reflect bead collection at the surface of the
membrane or within the apparatus. As Figure 10 shows, at an average cross-flow velocity of 0.36
m/s in the membrane lumen, decreases in bead concentration with time were similar for rotating
and non-rotating membranes. The ~35% decrease in bead concentration over the course of the
filtration likely results from loss of beads in the pump or tubing rather than formation of a cake
on the membrane surface. A force balance on the beads (see below and Figure 14) suggests that
at this cross-flow rate, the cross-flow drag force is four times higher than permeate drag force for
average beads size of 15 µm, which should greatly limit bead deposition on the membrane. This
likely explains why rotating the membrane did not affect the change in concentration of beads in
the feed tank.
At a lower average cross-flow velocity in the membrane lumen (~0.06 m/s), the permeate
drag force should be higher than the cross-flow drag force (Table 2), so beads may form a cake
on the membrane. Figure 11 shows that at this cross-flow velocity, the bead concentration in the
feed solution dropped more than 60% during filtration without membrane rotation. This likely
stems in part from formation of a cake on the membrane surface. Moreover, with a rotating
membrane and the same cross-flow velocity, the bead concentration in the feed dispersion
dropped only 35% percent, which is close to the concentration drop during filtration with an
average cross-flow velocity of 0.36 m/s. This suggests that at low cross-flow rates membrane
rotation may prevent cake formation.
36
Figure 10. Concentration of hollow glass beads in the feed tank as a function of time during cross-flow filtration with and without rotating the membrane. The average cross-flow velocity in
the membrane lumen was 0.36 m/s.
0.16
0.18
0.2
0.22
0.24
0.26
0.28
0.3
0 5 10 15 20 25 30
Bea
d C
once
ntra
tion
(g/L
)
Time (min)
Rotating
Stationary
37
Figure 11. Concentration of hollow glass beads in the feed tank as a function of time during cross-flow filtration with and without rotating the membrane. The average cross-flow velocity in
the membrane lumen was 0.06 m/s. The error bars are standard deviations of three replicate filtrations.
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 5 10 15 20 25 30 35
Bea
d C
once
ntra
tion
(g/L
)
Time (min)
Stationary
Rotating
38
2.3.2 Particle size distribution in cakes formed on membrane surfaces
Using a backflush immediately after filtration, I collected beads remaining on the ceramic
membrane. Figure 12 shows the size distribution of the beads that collected on the membrane
during filtration with a high cross-flow velocity. Compared to the initial feed solution, the size
distribution shifted to smaller diameters during filtration with both a stationary and a rotating
membrane. The cross flow likely selectively moves large particles away from the membrane
surface (see the force balance discussion below). However, filtration with membrane rotation
gives the size distribution with the smallest diameters, suggesting that the centripetal force due to
membrane rotation selectively prevents the largest particles from collecting on the membrane.
Under slower cross flow average velocities of 0.06 m/s, the size distribution of beads in
the cake collected during filtration with a stationary membrane is essentially the same as the size
distribution for beads in the feed solution (Figure 13). In contrast, beads from the cake that
formed during filtration with a rotating membrane show a significant reduction in size compared
to the feed solution. For the stationary membrane, the low cross-flow rate has little effect on the
bead deposition, so the deposited beads are similar in size to those in the feed solution. However,
for the rotating membrane, the centripetal force again selectively moves the largest beads away
from the membrane surface. As noted above, the rotation also leads to less total deposition of
particles in the membrane.
39
Figure 12. Bead size distribution for the feed solution and membrane cakes collected during filtration with stationary and rotating membranes. The average cross-flow velocity in the membrane lumen was 0.36 m/s. Beads deposited in the cake were collected in an aqueous
backflush. Similar trends occurred in three replicate filtrations.
02468
101214161820
0 10 20 30 40 50
Num
ber
Perc
enta
ge (%
)
Diameter (µm)
Feed solutionStationaryRotating
40
Figure 13. Bead size distribution for the feed solution and membrane cakes collected during filtration with stationary and rotating membranes. The average cross-flow velocity in the
membrane lumen was 0.06 m/s. Similar trends appeared in five replicate filtrations.
02468
101214161820
0 10 20 30 40 50
Num
ber
Perc
enta
ge (%
)
Diameter (µm)
Feed solution
Stationary
Rotating
41
2.3.3 Force balance on beads at the membrane surface
Beads at a stationary membrane experience a permeate drag force toward the membrane:
JπηrV6JF = (2.1)
a cross flow drag force parallel to the membrane:
xπηrV6xF = (2.2)
and a buoyancy force parallel to the membrane:
Δρvgf = (2.3)
In these equations, η is the solution viscosity, JV is the velocity of the solution toward the
membrane due to permeate flow, xV is the velocity due to cross flow, Δρ is the difference in
density between the bead and solution, v is volume of the beads and g is acceleration due to
gravity. Figure 14 schematically shows the forces on a bead. Equation (2.4) describes the axial
velocity xV as a function of the distance, r, from the membrane wall [12]. In equation (2.4), r is
the distance from center of the particle to the wall (or the particle radius for particles at the wall),
LΔP is the pressure gradient along the axis of the membrane, and R is the diameter of the
tubular membrane.
)2R
2r(12RL
ΔPη4
1xV −= (2.4)
In the case of a rotating membrane, beads will have tangential velocity ωrθV = , which
introduces a centripetal velocity, rV , defined as ( )
Jμr18
2dddρfρ2
θVrV −
−= (equation (1.5)).
42
Once beads form a cake on the membrane surface, they may diffuse from the cake to the
solution. The Stokes-Einstein equation (equation 2.5) describes the diffusion constant for a small
particle, where kB is the Boltzmann constant and T is temperature.
1s2μm0.015m610151s1mkg31013.146
K2981kJ23101.38πηr6
TBkD −⋅≈
−××−⋅−⋅−×××
×−⋅−×== (2.5)
Based on this equation, the diffusion coefficient is 0.015 µm2/s for beads with a diameter of 15
µm. Based on random 3-dimensional motion, equation (2.6) describes the distance a particle
may mover during a given time, t. Every second, the beads move 0.3 µm which is a small
μm0.32μm0.0156Dt6r =×≈>=< (2.6)
distance compared to the average diameter of the beads. Thus, the back diffusion of beads from
cake to solution should be negligible. The residence time of solution in the membrane is only
around 4 seconds for an average cross flow velocity of 0.06 m/s.
Tables 2 and 3 show estimated values of the forces applied on beads as determined from
equations (1.5), (2.1), (2.2), and (2.3) (These equations assume fully developed flow, which is
only an approximation). Table 2 shows that for a bead diameter of 15 µm at a 0.06 m/s average
cross-flow velocity, the drag force is smaller than the permeate drag force. Thus, beads foul the
membrane under slow cross-flow. On the other hand, with a 0.36 m/s average cross-flow
velocity, the drag force is more than four times larger than the permeate drag force so no
significant cake deposits on the membrane surface. Moreover, the tangential drag force
( θπηrV6θF = ) when the membrane rotates is even larger than the cross-flow velocity. This
indicates that when tangential fluid velocity is fully developed, it can create a higher drag force
on membrane surface to reduce fouling. Table 3 shows that larger beads have higher drag forces
43
and more easily move away from the membrane. This is consistent with the decrease in bead size
(relative to the feed solution) in the small amount of cake formed on the rotating membrane.
44
Figure 14. Force balance on beads at the surface of a stationary membrane (brown box). Fx is the cross-flow drag force, FJ is the permeate drag force and f is the buoyancy force.
R
Fx
f
FJ
r
45
Table 3. Forces acting on beads (for average 15 µm) at the membrane wall during cross-flow filtration, and parameters employed to calculate these forces.
Bead diameter (average) 15 µm
Bead volume 1.77 x 10-15 m3
Bead density 460 kg/m3
Water viscosity 1.00 x 10-3 kg/m/s
Water density 998 kg/m3
Permeate flux (initial) 3169 L/(m2h)
Tangential velocity 0.54 m/s
Cross-flow velocity (average) slow 0.06 m/s
Cross-flow velocity (average) fast 0.36 m/s
Slow cross flow (average velocity of 0.06 m/s)
Permeate drag force 1.25 x 10-10 N
Buoyancy-gravity force 9.34 x 10-12 N
Cross-flow drag force 8.49 x 10-11 N
Buoyancy/permeate drag 0.075 Cross-flow drag/permeate drag 0.68 Fast cross flow (average velocity of 0.36 m/s)
Permeate drag force 1.25 x 10-10 N
Buoyancy-gravity force 9.34 x 10-12 N
Cross-flow drag force 5.09 x 10-10 N
Buoyancy/permeate drag 0.075 Cross-flow drag/permeate drag 4.08 Rotating
Permeate drag force 1.25 x 10-10 N
Buoyancy-gravity force 9.34 x 10-12 N
Tangential drag force 7.65 x 10-10 N
Centripetal force 9.33 x 10-11 N
Buoyancy/permeate drag 0.075
Tangential drag/permeate drag 6.14
Centripetal force/permeate drag 0.7464
46
Table 4. Force balance on beads at the wall of a rotating membrane wall during cross-flow filtration, for beads with different diameters.
Bead density 460 kg/m3
Water viscosity 1.00 x 10-11 kg/m/s
Water density 998 kg/m3
Permeate flux (initial) 3169 L/(m2h)
Tangential velocity ( wR) 0.54 m/s
Tangential velocity (25 µm beads)
Permeate drag force 2.08 x 10-10 N
Buoyancy-gravity force 4.32 x 10-11 N
Tangential drag force 2.12 x 10-9 N
Centripetal force 4.32 x 10-10 N
Buoyancy/permeate drag 0.208 Tangential drag/permeate drag 10.2 Centripetal force/permeate drag 2.07
Tangential velocity (15 µm beads)
Permeate drag force 1.25 x 10-10 N
Buoyancy-gravity force 9.34 x 10-12 N
Tangential drag force 7.65 x 10-10 N
Centripetal force 9.33 x 10-11 N
Buoyancy/permeate drag 0.075 Tangential drag/permeate drag 6.14 Centripetal force/permeate drag 0.7464
Tangential velocity (5 µm beads)
Permeate drag force 4.16 x 10-11 N
Buoyancy-gravity force 3.46 x 10-13 N
Tangential drag force 8.51 x 10-11 N
Centripetal force 3.46 x 10-12 N
Buoyancy/permeate drag 0.008
Tangential drag/permeate drag 2.05
Centripetal force/permeate drag 0.08
47
2.4 Conclusions
This study describes a cross-flow filtration apparatus with a rotating membrane. Hollow
glass beads served as a model system for evaluating the effects of membrane rotation on the
collection of beads at the membrane-solution interface. At low cross-flow velocities during
filtration, rotating the membrane leads to less loss of beads from the feed solution than filtration
with a stationary membrane. Moreover, the size distribution of particles in beads recovered from
the membrane after filtration shows lower particle diameters for filtrations performed with
membrane rotation. Presumably, centripetal force due to membrane rotation preferentially
moves larger particles away from the membrane. Force balances demonstrate that rotation and
rapid cross-flow filtration should move 15 µm-diameter particles away from the cake. However,
rotation and cross flow may be insufficient to prevent fouling with small particles. Future
studies will revisit whether rotating flow can help prevent fouling of membranes during oil-water
separations.
48
REFERENCES
49
REFERENCES
[1] H. Lee, F. He, L. Song, J. Gilron, K.K. Sirkar, Desalination with a cascade of cross-flow hollow fiber membrane distillation devices integrated with a heat exchanger, AIChE Journal, 57 (2011) 1780-1795.
[2] F. Carstensen, A. Apel, M. Wessling, In situ product recovery: Submerged membranes vs. external loop membranes, Journal of Membrane Science, 394–395 (2012) 1-36.
[3] P. Le-Clech, V. Chen, T.A.G. Fane, Fouling in membrane bioreactors used in wastewater treatment, Journal of Membrane Science, 284 (2006) 17-53.
[4] A.P. Echavarría, C. Torras, J. Pagán, A. Ibarz, Fruit Juice Processing and Membrane Technology Application, Food Engineering Reviews, 3 (2011) 136-158.
[5] K. Schreier, K. Schafroth, A. Thomet, Application of cross-flow microfiltration to semi-hard cheese production from milk retentates, Desalination, 250 (2010) 1091-1094.
[6] Y. El Rayess, C. Albasi, P. Bacchin, P. Taillandier, J. Raynal, M. Mietton-Peuchot, A. Devatine, Cross-flow microfiltration applied to oenology: A review, Journal of Membrane Science, 382 (2011) 1-19.
[7] R.G.M. van der Sman, H.M. Vollebregt, A. Mepschen, T.R. Noordman, Review of hypotheses for fouling during beer clarification using membranes, Journal of Membrane Science, 396 (2012) 22-31.
[8] R. Pörtner, H. Märkl, Dialysis cultures, Applied Microbiology and Biotechnology, 50 (1998) 403-414.
[9] C.K. Colton, Analysis of membrane processes for blood purification, Blood Purification, 5 (1987) 202-251.
[10] M.Y. Jaffrin, Dynamic shear-enhanced membrane filtration: A review of rotating disks, rotating membranes and vibrating systems, Journal of Membrane Science, 324 (2008) 7-25.
[11] M.Y. Jaffrin, Hydrodynamic Techniques to Enhance Membrane Filtration, Annual Review of Fluid Mechanics, 44 (2012) 77-96.
[12] W. Wang, X. Jia, G.A. Davies, A theoretical study of transient cross-flow filtration using force balance analysis, The Chemical Engineering Journal and the Biochemical Engineering Journal, 60 (1995) 55-62.
[13] K.H. Kroner, V. Nissinen, Dynamic filtration of microbial suspensions using an axially rotating filter, Journal of Membrane Science, 36 (1988) 85-100.
[14] G. Belfort, J.M. Pimbley, A. Greiner, K.Y. Chung, Diagnosis of membrane fouling using a rotating annular filter. 1. Cell culture media, Journal of Membrane Science, 77 (1993) 1-22.
50
[15] F. Vigo, C. Uliana, P. Lupino, The Performance of a Rotating Module in Oily Emulsions Ultrafiltration, Separation Science and Technology, 20 (1985) 213-230.
[16] A.A. Kaplan, S.E. Halley, Plasma exchange with a rotating filter, Kidney International, 38 (1990) 160-166.
[17] G. Rock, P. Tittley, N. McCombie, Plasma collection using an automated membrane device, Transfusion, 26 (1986) 269-271.
[18] G. Beaudoin, M.Y. Jaffrin, Plasma Filtration in Couette Flow Membrane Devices, Artificial Organs, 13 (1989) 43-51.
[19] M. Mänttäri, K. Viitikko, M. Nyström, Nanofiltration of biologically treated effluents from the pulp and paper industry, Journal of Membrane Science, 272 (2006) 152-160.
[20] L.H. Ding, M.Y. Jaffrin, M. Mellal, G. He, Investigation of performances of a multishaft disk (MSD) system with overlapping ceramic membranes in microfiltration of mineral suspensions, Journal of Membrane Science, 276 (2006) 232-240.
[21] G. He, L.H. Ding, P. Paullier, M.Y. Jaffrin, Experimental study of a dynamic filtration system with overlapping ceramic membranes and non-permeating disks rotating at independent speeds, Journal of Membrane Science, 300 (2007) 63-70.
[22] S.P. Beier, M. Guerra, A. Garde, G. Jonsson, Dynamic microfiltration with a vibrating hollow fiber membrane module: Filtration of yeast suspensions, Journal of Membrane Science, 281 (2006) 281-287.
[23] G. Genkin, T.D. Waite, A.G. Fane, S. Chang, The effect of vibration and coagulant addition on the filtration performance of submerged hollow fibre membranes, Journal of Membrane Science, 281 (2006) 726-734.
