高分子概論
Membrane Distillation
班級:化材三乙
學號:49940077
姓名:李冠緯
題目:Membrane Distillation
姓名:李冠緯 學號:49940077
一.原理說明
Membrane distillation (MD) is a thermally driven process that depends on
the difference of the partial water vapor pressure across a non-wetting,
hydrophobic, porous membrane. Emerged nearly 50 years ago, no large scale
MD plants have been implemented yet for desalination. There are several
scientific and technological challenges that hamper its industrial
applications.The major barriers include MD membrane and module design,
membrane pore wetting, low permeate flow rate, and flux decay as
well as uncertain energy and economic costs. These challenges have
attracted scientists and engineers striving for the best membrane
performance, module and process design, among which the selection of
membrane materials was the most important.
A key requirement for distillation membrane is that the membrane
should not be wetted by water. Therefore, in early works,commercial
hydrophobic membranes were used because of their intrinsically
hydrophobic characteristics that resist the pore wetting of the membranes.
But these hydrophobic membranes usually give low permeability in MD
process. It is believed that hydrophobic materials may cause severe
temperature polarization and thereby lower the evaporation efficiency in
the membrane distillation process due to their good thermal conductivity.
On the other hand hydrophilic membrane may suffer less temperature
polarization and demonstrate higher evaporation efficiency because of
their high thermal resistance. In recent years, a series of works have
reported the development of dual layer membrane consists of a hydrophobic
layer and a hydrophilic support.
Khayet et al. prepared a series of hydrophobic/ hydrophilic flat-sheet
membrane for direct contact membrane distillation (DCMD) by phase
inversion. These membranes had a composite structure with a thin
hydrophobic layer (thus low resistance to water vapor diffusion) and a
thick hydrophilic sublayer (a low conductive heat loss). Although the
concept was theoretically attractive, results did not reflect significant
advantages. Qtaishat et al. continued their work on the double layer
membrane by mixing fluorinated macromolecules (SMMs) with polyetherimide
(PEI). The membranes showed a contact angle of 100◦ at the top surface
with a LEPw value of 4.7 bars. Unfortunately, the water flux in DCMD was
fairly low, about 18 L/m2 h at a feed temperature of around 65 ◦C and a
distillate temperature of 15 ◦C. A separation factor above 99% was
observed. The reasons may lie in that the structure of the hydrophobic
layer was not optimized and thereby may impart higher mass transfer
resistance.
Surface modification by plasma polymerization for the formation
of a hydrophobic layer on a hydrophilic base membrane was conducted as
well. For example, Kong et al. modified a hydrophilic microporous
cellulose nitrate membrane surface via plasma polymerization of
octafluorocyclobutane (OFCB) and vinyltrimethylsilicon/carbon
tetrafluoride (VTMS/CF4). The membrane was tested in a DCMD system using
a 0.3–0.5 M NaCl solution as feed. A water flux of 32.0 kg/m2 h was
observed at a feed and cold distillate temperature of 70/25 ◦C
respectively. Unfortunately, the membrane showed a salt rejection of only
92.1%, indicating the salt transfer across the membrane. They studied the
plasma glow discharge time on the membrane performance and found that
longer polymerization time yielded membrane with higher salt rejection
up to 99% but the flux decreased significantly possibly due to that longer
reaction time led to thicker coating layer and consequently a higher mass
transport resistance and eventually a higher salt rejection and a lower
flux. The researchers from Sirkar’s group have reported in a number of
publications on hydrophobic/hydrophilic hollow fiber membranes having a
thin layer of microporous coating of silicon fluoropolymer plasma
polymerized on the fiber outer surface. The modified hydrophobic PP
membranes showed significantly high water flux 79 kg/(m2 h) at 90 ◦C in
a cross flow module. It should be noted that all these surface modification
has been mainly focused on the one side of membrane surface, not in the
membrane matrix and the other side of the membrane.
Plasma surface modification has shown advantages in changing the
surface wettability of the materials in the nanometer scale, without
affecting the bulk properties, and has been widely used in membrane
surface modification [22–25]. However, so far, plasma treatment in
membranes has been mostly focused on improving the hydrophilicity of the
membrane for better fouling resistance. Plasma polymerization has been
widely used for the surface modification of membranes as mentioned above.
CF4 plasma has been used to improve the membrane hydrophobicity and
fluorinated membrane has been tested for gas permeation and for blood
compatibility, but using CF4 plasma without other monomers for membrane
surface modification has not yet been widely applied for membrane
distillation.
CF4 plasma treatment showed a moderate etching and a strong fluorination
effect which introduced fluorine functional groups in the material.
Therefore CF4 plasma surface modification can be used to reduce the
surface energy, enhance the material surface roughness and make the
material surface more hydrophobic. We have recently conducted CF4 plasma
enhanced chemical vapor deposition (CVD) to make superhydrophobic
composite resins. It was found that the process depends heavily on the
process parameters.At an optimal condition, a suitable etching and
fluorination yields a superhydrophobic surface. It is believed that
fluorination and deposition of fluorocarbon materials was the main reason
for the wettability change of the surface but was not elucidated clearly.
Based on our previous work on surface modification using CF4 plasma for
polyester resins, we are going to investigate the use of CF4 plasma
modification of hydrophilic base membranes for membrane distillation. The
surface modification process was investigated in order to optimize the
treatment conditions by changing the glow discharge power and treatment
time. The surface treatment effects were characterized by contact angle
measurements, liquid entry pressure and X-ray photo electron spectroscopy.
The membrane performance was evaluated in a direct contact membrane
distillation of 4% NaCl solutions. The evaporation efficiency of the
membrane distillation process was estimated and compared with literature
reports in order to assess the performance of the membrane and to shed
light on the new directions for the preparation of high performance of
membrane distillation membrane.
二.應用
Plasma treatment
Plasma treatment effect is strongly dependent on the glow discharge
power and treatment duration. In order to optimize the treatment condition,
a series of experiments were carried out with respect to these two issues,
respectively. The pretreatment was realized with argon plasma. The
purpose of pretreatment with argon was to make the membrane free of dust
particles and ready for further treatment. The condition was optimized
by varying the treatment time and glow discharge power. It was found that
at 45 W at 30 s the pretreatment was sufficient. Other plasma gases, for
example O2, should not be used because oxygen atom can be inserted in the
surface, which is not favorable for the creation of hydrophobic surface.
The surface after pretreatment was then exposed to CF4 plasma and the
influence of the glow discharge power of CF4 plasma on the membrane water
contact angle was investigated with treatment duration set for 10 min.
As shown in Fig. 3, the original membrane showed a water contact angle
of 60.0±2.0/0◦ at the top/bottom surfaces. After treated with the CF4
plasma, at 25 W for 10 min, the top surface contact angle increased
slightly to 71.0±2.0◦ while the CA of bottom surface remained unchanged.
At 50 W, the CA of both surfaces increased significantly to 115.0±2.0
◦. At 100 W,the CA of the surfaces increased slightly to 117.0±2.0◦/119.0
±2.0◦for top/bottom surfaces. Further increment of the treatment power
resulted in very slight change in the contact angle indicating the surface
modification has reached saturation. We chose 200 W for further surface
modification.
The influence of the CF4 plasma treatment time at 200 W on themembrane
contact angle was shown in Fig. 4. Similar to the trend in glow discharge
power effect, the water contact angle change showed initially significant
increase within short time and thereafter a level-off. At 5 min of
treatment, the top surface contact angle increased from 62.0◦ to 113.0
◦ and the bottom from 0◦ to 118.0◦. It appears that from 5 min to 40 min,
the contact angle at the top surface increased slightly from 113.0◦ to
125.0◦, and the contact angle of the bottom surface increased from 118.0
◦ to 124.0◦.
Water contact angle is a surface property related to the surface
composition, roughness and surface porosity]. In general, a rough and
highly porous surface normally shows a high water contact angle. Scanning
electron microscopy (SEM) images of the PES membrane structure are shown
in Fig. 5. The membrane is asymmetric in the cross section (Fig. 5a) with
a dense skin layer (Fig. 5b) and a porous support (Fig. 5c). Small pores
of 20–30 nm are observable from the top surface (Fig. 5d) while the bottom
surface shows much larger pores in the range of 1.0–4.0 _m, in agreement
with the cross section observation. The asymmetric structure might be
the reason for the difference in the water contact angles between the top
and bottom surface both before and after plasma modification . To
guarantee sufficient surface modification, we have chosen 200 W 30 min
for CF4 glow discharge and 45 W 30 s for argon pretreatment.
The characteristics of PES membranes before and after plasma treatment
are listed in Table 1. The membrane thickness, porosity, gas permeability
remained unchanged. Pore size measurement by PMI failed to give exact
bubble point due to the low limit in the set pressure, however, indicating
that the bubble pore size is below 70 nm, which agrees to the SEM
observations with the presence of pores of 20–30 nm (Fig. 5d). The
modification is more pronounced in the change of liquid entry pressure
of water (LEPw). The LEPw of the PES before and after treatment were
approximately 0.1 bar and 3.7 bar, respectively.
LEPw is an indication of the ability of a hydrophobic membrane against
wetting in the MD process. If the LEPw is low, water could be pressed easily
inside the pore of the membrane leading to pore wetting and possibly solute
leakage. Therefore, a membrane with a higher LEPw is expected to perform
better than that with a lower one. Based on the LEPw and contact angle
changes of the PES membrane, one may conclude that the hydrophilic
membrane has indeed been transformed into a hydrophobic one.
It is noted that the mechanical property of the membrane changed. In
literature, when surface modification was carried out on membrane, the
mechanical properties for example, mechanical strength is normally
strengthened [17,18]. However, the mechanical strength decreased in our
case. In order to elucidate the surface modification mechanism, we have
carried out X-ray photoelectron spectroscopy (XPS).
三.參考文獻
一.原理說明
The world’s arid and semi-arid zones are already faced with massive
water scarcity, which is prospected to increase in the upcoming decades.
Therefore the demand of desalination technologies is increasing rapidly.
Today more than 50×106m3 of fresh water is produced by industrial
desalination processes In remote areas, conventional desalination
processes cannot be applied, due to insufficient infrastructure and
energy supply. The development of medium size, autonomous and robust
desalination units is needed to establish an independent water supply in
this areas. This is the motivation for research on alternative
desalination processes. Membrane distillation (MD) seems to fit the
specific requirements very well. It is a thermal membrane process that
can be driven with any low-grade heat source. The robust membrane allows
to follow alternating operation conditions given by, e.g. regenerative
energy sources like solar energy.
In many inland regions of the arid zones, even brackish and salty water
resources are rare. One goal in the development of desalination systems
for remote areas is to optimize the utilisation of the feed water and
reduce total brine disposal, respectively. In membrane distillation,
recirculation strategies allow to discharge high concentrated brines. In
these operational modes, the influence of feed water salinity on the
process performance is of great importance. This paper presents
experimental results for desalination using membrane distillation
technology. Full scale spiral wound modules are operated at different
operational parameters, including a wide range of feed water salinities.
二.應用
Module technology
Fig. 4 shows the channel arrangement for a PGMDmodule. Cold feed water
enters the condenser channel and gains heat to approximately 73 ◦C by
internal heat recovery. An external heat source (e.g. solar collector)
heats the feed water up to 80 ◦C. The hot feed water flows through the
evaporator channel in counter current direction and exits the module with
27 ◦C at the evaporator outlet. An almost constant temperature difference
is established throughout the entire membrane surface area comparable
with the temperature profile in a counter current flow heat exchanger.
Water vapour passes through the membrane and condenses in the distillate
channel. The latent and sensible heat is transferred through the condenser
foil to preheat the feed water in the condenser channel. Pure distillate
exits the module at the distillate outlet.
Fig. 5 shows the PGMD channel arrangement transferred into a compact
spiral package. The hot zones of the channels are in the module center
and the cold zones are at the module’s shell side. Therefore even without
module insulation, only minimal heat losses to ambient occur. This module
concept goes back to a patent
of W.L. Gore from 1985
Module fabrication.
The modules are fabricated using a membrane, a condenser foil and
different spacer materials on rolls. For module fabrication a winding
machine was developed. The material coils are placed inside the machine.
The layers are aligned with guiding rollers and brought together at a motor
driven main spindle. To ensure a well-defined production process each
material spindle is equipped with a controlled brake. Here the
pretensioning of each layer can be precisely adjusted. Fig. 6 shows a
photograph of the winding machine.
Module characterization
In order to evaluate module and process performance a specialized test
facility was developed. The hydraulic schematic is given in Fig. 8. The
modules operating point can be defined by controlling the main influencing
parameters: condenser inlet temperature, evaporator inlet temperature,
feed flow rate and feed water salinity. The salty feed water is pumped
out of a 300 l-storage tank .A0.5 mm-poresize mechanical filter prevents
themodule from particles. The feed water is cooled in a heat exchanger
by using the laboratory cooling water circuit.Anyrequired condenser inlet
temperature can be set by controlling a motor valve in the cooling loop.
After passing through the modules condenser channel the preheated water
gains external heat in a heat exchanger. The evaporator inlet temperature
is controlled by a 3-phase power switch, regulating an electrical heating
element . After passing the evaporator channel the saltwater goes back
in the storage tank for recirculation. The distillate is temporarily
stored in a separate tank (7). If the filling level of the distillate tank
exceeds a certain threshold, the distillate is pumped back in the
storage automatically (8) to avoid a raising feed water salinity.
三.參考文獻